Influence of a large-scale field on energy dissipation in magnetohydrodynamic turbulence [CL]

http://arxiv.org/abs/1703.03433

In magnetohydrodynamic (MHD) turbulence, the large-scale magnetic field sets a preferred local direction for the small-scale dynamics, altering the statistics of turbulence from the isotropic case. This happens even in the absence of a total magnetic flux, since MHD turbulence forms randomly oriented large-scale domains of strong magnetic field. It is therefore customary to study small-scale magnetic plasma turbulence by assuming a strong background magnetic field relative to the turbulent fluctuations. This is done, for example, in reduced models of plasmas, such as reduced MHD, reduced-dimension kinetic models, gyrokinetics, etc., which make theoretical calculations easier and numerical computations cheaper. Recently, however, it has become clear that the turbulent energy dissipation is concentrated in the regions of strong magnetic field variations. A significant fraction of the energy dissipation may be localized in very small volumes corresponding to the boundaries between strongly magnetized domains. In these regions the reduced models are not applicable. This has important implications for studies of particle heating and acceleration in magnetic plasma turbulence. The goal of this work is to systematically investigate the relationship between local magnetic field variations and magnetic energy dissipation, and to understand its implications for modeling energy dissipation in realistic turbulent plasmas.

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V. Zhdankin, S. Boldyrev and J. Mason
Mon, 13 Mar 17
8/48

Comments: 6 pages, 5 figures, to appear in Monthly Notices of the Royal Astronomical Society

On Kinetic Slow Modes, Fluid Slow Modes, and Pressure-Balanced Structures in the Solar Wind [CL]

http://arxiv.org/abs/1703.03040

Observations in the solar wind suggest that the compressive component of inertial-range solar-wind turbulence is dominated by slow modes. The low collisionality of the solar wind allows for non-thermal features to survive, which suggests the requirement of a kinetic plasma description. The least-damped kinetic slow mode is associated with the ion-acoustic (IA) wave and a non-propagating (NP) mode. We derive analytical expressions for the IA-wave dispersion relation in an anisotropic plasma in the framework of gyrokinetics and then compare them to fully-kinetic numerical calculations, results from two-fluid theory, and MHD. This comparison shows major discrepancies in the predicted wave phase speeds from MHD and kinetic theory at moderate to high $\beta$. MHD and kinetic theory also dictate that all plasma normal modes exhibit a unique signature in terms of their polarization. We quantify the relative amplitude of fluctuations in the three lowest particle velocity moments associated with IA and NP modes in the gyrokinetic limit and compare these predictions with MHD results and in-situ observations of the solar-wind turbulence. The agreement between the observations of the wave polarization and our MHD predictions is better than the kinetic predictions, suggesting that the plasma behaves more like a fluid in the solar wind than expected.

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D. Verscharen, C. Chen and R. Wicks
Fri, 10 Mar 17
48/52

Comments: 8 pages, 5 figures, submitted to ApJ

Turbulent kinetic energy in the energy balance of a solar flare [SSA]

http://arxiv.org/abs/1703.02392

The energy released in solar flares derives from a reconfiguration of magnetic fields to a lower energy state, and is manifested in several forms, including bulk kinetic energy of the coronal mass ejection, acceleration of electrons and ions, and enhanced thermal energy that is ultimately radiated away across the electromagnetic spectrum from optical to X-rays. Using an unprecedented set of coordinated observations, from a suite of instruments, we here report on a hitherto largely overlooked energy component — the kinetic energy associated with small-scale turbulent mass motions. We show that the spatial location of, and timing of the peak in, turbulent kinetic energy together provide persuasive evidence that turbulent energy may play a key role in the transfer of energy in solar flares. Although the kinetic energy of turbulent motions accounts, at any given time, for only $\sim (0.5-1)$\% of the energy released, its relatively rapid ($\sim$$1-10$~s) energization and dissipation causes the associated throughput of energy (i.e., power) to rival that of major components of the released energy in solar flares, and thus presumably in other astrophysical acceleration sites.

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E. Kontar, J. Perez, L. Harra, et. al.
Wed, 8 Mar 17
23/60

Comments: 5pages, 4 figures, to be published in Physical Review Letters

Ultrarelativistic generalized Lorentzians and the cosmic ray energy flux [HEAP]

http://arxiv.org/abs/1703.02327

We show that the rather tentative application of the ultrarelativistic generalized Lorentzian energy distribution to the spectrum of cosmic ray fluxes may provide evidence for either high TeV chemical potentials generated in the acceleration source region of the observed cosmic rays, or the presence of hypothetical particles of TeV rest mass. Such particles are not known in our accessible Universe at any accessible energies. If true they should have been produced in cosmic ray sources prior to acceleration. Conclusions of this kind depend on the validity of the generalized Lorentzian in application to cosmic rays, a hypothetical statistical mechanical equilibrium distribution occasionally encountered in observations.

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R. Treumann and W. Baumjohann
Wed, 8 Mar 17
46/60

Comments: 5 pages, 1 figure, draft prepared for submission to a meeting on cosmic rays and power law tails

Plasma turbulence at ion scales: a comparison between PIC and Eulerian hybrid-kinetic approaches [CL]

http://arxiv.org/abs/1703.02443

Kinetic-range turbulence in magnetized plasmas and, in particular, in the context of solar-wind turbulence has been extensively investigated over the past decades via numerical simulations. Among others, one of the widely adopted reduced plasma model is the so-called hybrid-kinetic model, where the ions are fully kinetic and the electrons are treated as a neutralizing (inertial or massless) fluid. Within the same model, different numerical methods and/or approaches to turbulence development have been employed. In the present work, we present a comparison between two-dimensional hybrid-kinetic simulations of plasma turbulence obtained with two complementary approaches spanning about two decades in wavenumber – from MHD inertial range to scales well below the ion gyroradius – with a state-of-the-art accuracy. One approach employs hybrid particle-in-cell (HPIC) simulations of freely-decaying Alfv\’enic turbulence, whereas the other consists of Eulerian hybrid Vlasov-Maxwell (HVM) simulations of turbulence continuously driven with partially-compressible large-scale fluctuations. Despite the completely different initialization and injection/drive at large scales, the same properties of turbulent fluctuations at $k_\perp\rho_i\gtrsim1$ are observed. The system indeed self-consistently “reprocesses” the turbulent fluctuations while they are cascading towards smaller and smaller scales, in a way which actually depends on the plasma beta parameter. Small-scale turbulence has been found to be mainly populated by kinetic Alfv\’en wave (KAW) fluctuations for $\beta\geq1$, whereas KAW fluctuations are only sub-dominant for low-$\beta$.

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S. Cerri, L. Franci, F. Califano, et. al.
Wed, 8 Mar 17
47/60

Comments: 18 pages, 4 figures, accepted for publication in J. Plasma Phys. (Collection: “The Vlasov equation: from space to laboratory plasma physics”)

Radio and the 1999 UK Total Solar Eclipse [EPA]

http://arxiv.org/abs/1703.01491

On the morning of the August 11th 1999, a total eclipse of the sun plunged Cornwall and parts of Devon into darkness. The event of the eclipse was bound to attract a great deal of scientific and media attention. Realizing that the differences in day-time/night-time propagation of VLF/LF/MF to HF bands would also apply during the darkness of the eclipse, the eclipse offered a rare PR opportunity to promote radio to the general public. At the same time the specific nature of the disturbance to the upper atmosphere and the effect on radio propagation could be examined in detail using scientific instruments at minimum cost since most instruments would not have to be moved. This would allow prediction models to be tested in a controlled fashion. Contained within this report are the details and results of the radio and ionospheric experiments conducted by the Rutherford Appleton Laboratory during the 1999 total solar eclipse. The promoting of the radio experiments with the general public produced nearly 60 appearances on local and national TV, newspapers and periodicals. Close to 1700 people responded to the general public medium wave experiment and 16 million people looked in on the general eclipse web site (part funded by RA) that included the details of the radio experiments. A large database of systematic observations across VLF to HF was collected from radio amateurs and from the RA Regional Offices allowing comparisons to be made with ITU estimates. There is a brief look at the scientific results and a forward look as to how the analysis of this disturbance might have impact on the use of ionospheric models for Space Weather tools in the future.

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R. Bamford
Tue, 7 Mar 17
16/66

Comments: 41 pages, 33 Figures, government funded research final report, unclassified

FRiED: A novel three-dimensional model of coronal mass ejections [CL]

http://arxiv.org/abs/1703.01659

We present a novel three-dimensional (3D) model of coronal mass ejections (CMEs) that unifies all key evolutionary aspects of CMEs and encapsulates their 3D magnetic field configuration. This fully analytic model is capable of reproducing the global geometrical shape of a CME with all major deformations taken into account, i.e., deflection, rotation, expansion, “pancaking”, front flattening and rotational skew. Encapsulation of 3D magnetic structure allows the model to reproduce in-situ measurements of magnetic field for trajectories of spacecraft-CME encounters of any degree of complexity. As such, the model can be used single-handedly for consistent analysis of both remote and in-situ observations of CMEs at any heliocentric distance. We demonstrate the latter by successfully applying the model for analysis of two CMEs.

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A. Isavnin
Tue, 7 Mar 17
40/66

Comments: N/A

Magnetic Reconnection in Turbulent Diluted Plasmas [CL]

http://arxiv.org/abs/1703.01238

We study magnetic reconnection events in a turbulent plasma within the two-fluid theory. By identifying the diffusive regions, we measure the reconnection rates as function of the conductivity and current sheet thickness. We have found that the reconnection rate scales as the squared of the inverse of the current sheet’s thickness and is independent of the aspect ratio of the diffusive region, in contrast to other analytical, e.g. the Sweet-Parker and Petscheck, and numerical models. Furthermore, while the reconnection rates are also proportional to the square inverse of the conductivity, the aspect ratios of the diffusive regions, which exhibit values in the range of $0.1-0.9$, are not correlated to the latter. Our findings suggest a new expression for the magnetic reconnection rate, which, after experimental verification, can provide a further understanding of the magnetic reconnection process.

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N. Offeddu and M. Mendoza
Mon, 6 Mar 17
21/47

Comments: 9 Pages, 6 figures

Predictions of solar coronal mass ejections with heliospheric imagers verified with the Heliophysics System Observatory [SSA]

http://arxiv.org/abs/1703.00705

We present a major step forward towards accurately predicting the arrivals of coronal mass ejections (CMEs) on the terrestrial planets, including the Earth. For the first time, we are able to assess a CME prediction model using data over almost a full solar cycle of observations with the Heliophysics System Observatory. We validate modeling results on 1337 CMEs observed with the Solar Terrestrial Relations Observatory (STEREO) heliospheric imagers (HI) with data from 8 years of observations by 5 spacecraft in situ in the solar wind, thereby gathering over 600 independent in situ CME detections. We use the self-similar expansion model for CME fronts assuming 60 degree longitudinal width, constant speed and constant propagation direction. Using these assumptions we find that 23%-35% of all CMEs that were predicted to hit a certain spacecraft lead to clear in situ signatures, so that for 1 correct prediction, 2 to 3 false alarms would have been issued. In addition, we find that the prediction accuracy of HI does not degrade with longitudinal separation from Earth. Arrival times are predicted on average within 2.6 +/- 16.6 hours difference to the in situ arrival time, similar to analytical and numerical modeling. We also discuss various factors that may improve the accuracy of space weather forecasting using wide-angle heliospheric imager observations. These results form a first order approximated baseline of the prediction accuracy that is possible with HI and other methods used for data by an operational space weather mission at the Sun-Earth L5 point.

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C. Mostl, A. Isavnin, P. Boakes, et. al.
Fri, 3 Mar 17
14/62

Comments: 22 pages, 7 figures, 1 table, submitted to the AGU journal Space Weather on 2 March 2017

Electron dynamics surrounding the X-line in asymmetric magnetic reconnection [CL]

http://arxiv.org/abs/1702.07244

Electron dynamics surrounding the X-line in magnetopause-type asymmetric reconnection is investigated using a two-dimensional particle-in-cell simulation. We study electron properties of three characteristic regions in the vicinity of the X-line. The fluid properties, velocity distribution functions (VDFs), and orbits are studied and cross-compared. In the low-$\beta$ side of the X-line, the normal electric field enhances the electron meandering motion from the high-$\beta$ side. The motion leads to a crescent-shaped component in the electron VDF, in agreement with recent studies. In the high-$\beta$ side of the X-line, the magnetic field line is so stretched in the third dimension that its curvature radius is comparable with typical electron Larmor radius. The electron motion becomes nonadiabatic, and therefore the electron idealness is no longer expected to hold. Around the middle of the outflow regions, the electron nonidealness is coincident with the region of the nonadiabatic motion. Finally, we introduce a finite-time mixing fraction (FTMF) to evaluate electron mixing. The FTMF marks the low-$\beta$ side of the X-line, where the nonideal energy dissipation occurs.

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S. Zenitani, H. Hasegawa and T. Nagai
Fri, 24 Feb 17
10/50

Comments: Comments are welcome

Chaos Control with Ion Propulsion [CL]

http://arxiv.org/abs/1702.06581

The escape dynamics around the triangular Lagrangian point L5 in the real Sun-Earth-Moon-Spacecraft system is investigated. Appearance of the finite time chaotic behaviour suggests that widely used methods and concepts of dynamical system theory can be useful in constructing a desired mission design. Existing chaos control methods are modified in such a way that we are able to protect a test particle from escape. We introduce initial condition maps in order to have a suitable numerical method to describe the motion in high dimensional phase space. Results show that the structure of initial condition maps can be split into two well-defined domains. One of these two parts has a regular contiguous shape and is responsible for long time escape; it is a long-lived island. The other one shows a filamentary fractal structure in initial condition maps. The short time escape is governed by this object. This study focuses on a low-cost method which successfully transfers a reference trajectory between these two regions using an appropriate continuous control force. A comparison of the Earth-Moon transfer is also presented to show the efficiency of our method.

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J. Sliz, T. Kovacs and A. Suli
Thu, 23 Feb 17
20/48

Comments: 14 pages, 11 figures, accepted for publication in Astronomische Nachrichten

Sheath-Accumulating Propagation of Interplanetary Coronal Mass Ejection [SSA]

http://arxiv.org/abs/1702.06607

Fast interplanetary coronal mass ejections (interplanetary CMEs, or ICMEs) are the drivers of strongest space weather storms such as solar energetic particle events and geomagnetic storms. The connection between space weather impacting solar wind disturbances associated with fast ICMEs at Earth and the characteristics of causative energetic CMEs observed near the Sun is a key question in the study of space weather storms as well as in the development of practical space weather prediction. Such shock-driving fast ICMEs usually expand at supersonic speed during the propagation, resulting in the continuous accumulation of shocked sheath plasma ahead. In this paper, we propose the “sheath-accumulating propagation” (SAP) model that describe the coevolution of the interplanetary sheath and decelerating ICME ejecta by taking into account the process of upstream solar wind plasma accumulation within the sheath region. Based on the SAP model, we discussed (1) ICME deceleration characteristics, (2) the fundamental condition for fast ICME at Earth, (3) thickness of interplanetary sheath, (4) arrival time prediction and (5) the super-intense geomagnetic storms associated with huge solar flares. We quantitatively show that not only speed but also mass of the CME are crucial in discussing the above five points. The similarities and differences among the SAP model, the drag-based model and the`snow-plough’ model proposed by \citet{tappin2006} are also discussed.

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T. Takahashi and K. Shibata
Thu, 23 Feb 17
45/48

Comments: 20 pages, 5 figures, accepted for publication in ApJL

The Twist of the Draped Interstellar Magnetic Field Ahead of the Heliopause: A Magnetic Reconnection Driven Rotational Discontinuity [CL]

http://arxiv.org/abs/1702.06178

Based on the difference between the orientation of the interstellar $B_{ISM}$ and the solar magnetic fields, there was an expectation that the magnetic field direction would rotate dramatically across the heliopause (HP). However, the Voyager 1 spacecraft measured very little rotation across the HP. Previously we showed that the $B_{ISM}$ twists as it approaches the HP and acquires a strong T component (East-West). Here we establish that reconnection in the eastern flank of the heliosphere is responsible for the twist. On the eastern flank the solar magnetic field has twisted into the positive N direction and reconnects with the Southward pointing component of the $B_{ISM}$. Reconnection drives a rotational discontinuity (RD) that twists the $B_{ISM}$ into the -T direction and propagates upstream in the interstellar medium towards the nose. The consequence is that the N component of $B_{ISM}$ is reduced in a finite width band upstream of the HP. Voyager 1 currently measures angles ($\delta=sin^{-1}(B_{N}/B)$) close to solar values. We present MHD simulations to support this scenario, suppressing reconnection in the nose region while allowing it in the flanks, consistent with recent ideas about reconnection suppression from diamagnetic drifts. The jump in plasma $\beta$ (the plasma to magnetic pressure) across the nose of HP is much greater than in the flanks because the heliosheath $\beta$ is greater there than in the flanks. Large-scale reconnection is therefore suppressed in the nose but not at the flanks. Simulation data suggest that $B_{ISM}$ will return to its pristine value $10-15~AU$ past the HP.

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M. Opher, J. Drake, M. Swisdak, et. al.
Wed, 22 Feb 17
13/37

Comments: 19 pages, 5 figures, submitted

How Anomalous Resistivity Accelerates Magnetic Reconnection [CL]

http://arxiv.org/abs/1702.06109

Whether Turbulence-induced anomalous resistivity (AR) can facilitate a fast magnetic reconnection in collisionless plasma is a subject of active debate for decades. A particularly difficult problem in experimental and numerical simulation studies of the problem is how to distinguish the effects of AR from those originating from Hall-effect and other non-turbulent processes in the generalized Ohm’s. In this paper, using particle-in-cell simulations, we present a case study of how AR produced by Buneman Instability accelerates magnetic reconnection. We first show that in a thin current layer, the AR produced by Buneman instability spontaneously breaks the magnetic field lines and causes impulsive fast non-Hall magnetic line annihilation on electron-scales with a rate reaching 0.6~$V_A$. However, the electron-scale magnetic line annihilation is not a necessary condition for the dissipation of magnetic energy, but rather a result of the inhomogeneity of the AR. On the other hand, the inhomogeneous drag arising from a Buneman instability driven by the intense electron beams at the x-line in a 3D magnetic reconnection can drive in the electron diffusion region electron-scale magnetic line annihilation. The electron-scale annihilations play an essential role in accelerating the magnetic reconnection with a rate two times faster than the non-turbulent Hall-dominated 2D magnetic reconnection. The reconnection rate is enhanced around the x-line, and the coupling between the AR carried out by the reconnection outflow and the Hall effect leads to the breaking of the symmetric structure of the ion diffusion region and the enhancement of the outward Poynting flux.

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H. Che
Tue, 21 Feb 17
12/70

Comments: submitted to Physics of Plasma

A Maximum Entropy Principle for inferring the Distribution of 3D Plasmoids [HEAP]

http://arxiv.org/abs/1702.05782

The Principle of Maximum Entropy, a powerful and general method for inferring the distribution function given a set of constraints, is applied to deduce the overall distribution of plasmoids (flux ropes/tubes). The analysis is undertaken for the general 3D case, with mass, total flux and (3D) velocity serving as the variables of interest, on account of their physical and observational relevance. The distribution functions for the mass, width, total flux and helicity exhibit a power-law behavior with exponents of $-4/3$, $-2$, $-3$ and $-2$ respectively for small values, whilst all of them display an exponential falloff for large values. In contrast, the velocity distribution, as a function of $v = |{\bf v}|$, is shown to be flat for $v \rightarrow 0$, and becomes a power law with an exponent of $-7/3$ for $v \rightarrow \infty$. Most of these results exhibit a high degree of universality, as they are nearly independent of the free parameters. A preliminary comparison of our results with the observational evidence is presented, and some of the ensuing space and astrophysical implications are discussed.

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M. Lingam, L. Comisso and A. Bhattacharjee
Tue, 21 Feb 17
25/70

Comments: 15 pages, 6 figures

Experimental overview on Future Solar and Heliospheric research [CL]

http://arxiv.org/abs/1702.05091

Solar and heliospheric cosmic rays provide a unique perspective in cosmic ray research: we can observe not only the particles, but also the properties of the plasmas in which the they are accelerated and propagate, using in situ and high-resolution remote sensing instruments. The heliospheric cosmic ray observations typically require space missions, which face stern competition against planetary and astrophysics missions, and it can take up to decades from the initial concept proposal until the actual observing of the cosmic rays can commence. Therefore it is important to have continuity in the cosmic ray mission timeline. In this overview, we review the current status and the future outlook in the experimental solar and heliospheric research. We find that the current status of the available cosmic ray observations is good, but that many of the spacecraft are near the end of their feasible mission life. We describe the three missions currently being prepared for launch, and discuss the future outlook of the solar and heliospheric cosmic ray missions.

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T. Laitinen
Mon, 20 Feb 17
9/37

Comments: XXV ECRS 2016 Proceedings – eConf C16-09-04.3

Study of Parallel Shock Acceleration, the Bend-over Energy of Spectrum of Charged Energetic Particles [HEAP]

http://arxiv.org/abs/1702.04647

Shock acceleration is considered one of the most important mechanisms of astrophysical energetic particles’ acceleration. In this work, we calculate large amount of test charged particles’ trajectories accurately in a parallel shock with magnetic turbulence. We investigate energetic particles’ acceleration mechanisms by calculating particles’ energy and flux evolution with time. From simulations we obtain double power-law energy spectra with the bend-over energy increasing as a function of time. With the mean accelerating time and averaged momentum change during each cycle of particles crossing of shock from diffusive shock acceleration model, a time differential equation for the maximum of shock accelerated energy $E_{acc}$ can be approximately obtained following Drury. We assume the bend-over energy as $E_{acc}$. It is found that the model of the bend-over energy generally agrees with the simulations, and that the bend-over energy model with our non-linear diffusion theory, NLGCE-F, performs better than that with the classic quasi-linear theory, QLT.

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L. Zhang, G. Qin, P. Sun, et. al.
Thu, 16 Feb 17
6/45

Comments: 20 pages, 5 figures and 1 table

Centennial evolution of monthly solar wind speeds: Fastest monthly solar wind speeds from long-duration coronal holes [CL]

http://arxiv.org/abs/1702.03924

High speed solar wind streams (HSSs) are very efficient drivers of geomagnetic activity at high latitudes. In this paper we use a recently developed $\Delta{H}$ parameter of geomagnetic activity, calculated from the night-side hourly magnetic field measurements of the Sodankyl\”a observatory, as a proxy for solar wind (SW) speed at monthly time resolution in 1914-2014 (solar cycles 15-24). The seasonal variation in the relation between monthly $\Delta{H}$ and solar wind speed is taken into account by calculating separate regressions between $\Delta{H}$ and SW speed for each month. Thereby, we obtain a homogeneous series of proxy values for monthly solar wind speed for the last 100 years. We find that the strongest HSS-active months of each solar cycle occur in the declining phase, in years 1919, 1930, 1941, 1952, 1959, 1973, 1982, 1994 and 2003. Practically all these years are the same or adjacent to the years of annual maximum solar wind speeds. This implies that the most persistent coronal holes, lasting for several solar rotations and leading to the highest annual SW speeds, are also the sources of the highest monthly SW speeds. Accordingly, during the last 100 years, there were no coronal holes of short duration (of about one solar rotation) that would produce faster monthly (or solar rotation) averaged solar wind than the most long-living coronal holes in each solar cycle produce.

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R. Lukianova, L. Holappa and K. Mursula
Wed, 15 Feb 17
16/47

Comments: N/A

Electric current filamentation induced by 3D plasma flows in the solar corona [SSA]

http://arxiv.org/abs/1702.03986

Many magnetic structures in the solar atmosphere evolve rather slowly so that they can be assumed as (quasi-)static or (quasi-)stationary and represented via magneto-hydrostatic (MHS) or stationary magneto-hydrodynamic (MHD) equilibria, respectively. While exact 3D solutions would be desired, they are extremely difficult to find in stationary MHD. We construct solutions with magnetic and flow vector fields that have three components depending on all three coordinates. We show that the non-canonical transformation method produces quasi-3D solutions of stationary MHD by mapping 2D or 2.5D MHS equilibria to corresponding stationary MHD states, i.e., states that display the same field line structure as the original MHS equilibria. These stationary MHD states exist on magnetic flux surfaces of the original 2D MHS states. Although the flux surfaces and therefore also the equilibria have a 2D character, these stationary MHD states depend on all three coordinates and display highly complex currents. The existence of geometrically complex 3D currents within symmetric field-line structures provide the base for efficient dissipation of the magnetic energy in the solar corona by Ohmic heating. We also discuss the possibility of maintaining an important subset of non-linear MHS states, namely force-free fields, by stationary flows. We find that force-free fields with non-linear flows only arise under severe restrictions of the field-line geometry and of the magnetic flux density distribution.

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D. Nickeler, T. Wiegelmann, M. Karlicky, et. al.
Wed, 15 Feb 17
21/47

Comments: 14 pages, 5 figures, accepted to ApJ

Is Proxima Centauri b habitable? — A study of atmospheric loss [EPA]

http://arxiv.org/abs/1702.04089

We address the important question of whether the newly discovered exoplanet, Proxima Centauri b (PCb), is capable of retaining an atmosphere over long periods of time. This is done by adapting a sophisticated multi-species MHD model originally developed for Venus and Mars, and computing the ion escape losses from PCb. The results suggest that the ion escape rates are about two orders of magnitude higher than the terrestrial planets of our Solar system if PCb is unmagnetized. In contrast, if the planet does have an intrinsic dipole magnetic field, the rates are lowered for certain values of the stellar wind dynamic pressure, but they are still higher than the observed values for our Solar system’s terrestrial planets.

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C. Dong, M. Lingam, Y. Ma, et. al.
Wed, 15 Feb 17
34/47

Comments: 7 pages, 2 figures, submitted to ApJL

Propagation Characteristics of Two Coronal Mass Ejections From the Sun Far into Interplanetary Space [SSA]

http://arxiv.org/abs/1702.04122

Propagation of coronal mass ejections (CMEs) from the Sun far into interplanetary space is not well understood due to limited observations. In this study we examine the propagation characteristics of two geo-effective CMEs, which occurred on 2005 May 6 and 13, respectively. Significant heliospheric consequences associated with the two CMEs are observed, including interplanetary CMEs (ICMEs) at the Earth and Ulysses, interplanetary shocks, a long-duration type II radio burst, and intense geomagnetic storms. We use coronagraph observations from SOHO/LASCO, frequency drift of the long-duration type II burst, in situ measurements at the Earth and Ulysses, and magnetohydrodynamic (MHD) propagation of the observed solar wind disturbances at 1 AU to track the CMEs from the Sun far into interplanetary space. We find that both of the two CMEs underwent a major deceleration within 1 AU and thereafter a gradual deceleration when they propagated from the Earth to deep interplanetary space due to interactions with the ambient solar wind. The results also reveal that the two CMEs interacted with each other in the distant interplanetary space even though their launch times on the Sun were well separated. The intense geomagnetic storm for each case was caused by the southward magnetic fields ahead of the CME, stressing the critical role of the sheath region in geomagnetic storm generation, although for the first case there is a corotating interaction region involved.

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X. Zhao, Y. Liu, H. Hu, et. al.
Wed, 15 Feb 17
38/47

Comments: accepted for publication in ApJ

A VLA Search for Radio Signals from M31 and M33 [EPA]

http://arxiv.org/abs/1702.03301

Observing nearby galaxies would facilitate the search for artificial radio signals by sampling many billions of stars simultaneously, but few efforts have been made to exploit this opportunity. An added attraction is that the Milky Way is the second-largest member of the Local Group, so our galaxy might be a probable target for hypothetical broadcasters in nearby galaxies. We present the first relatively high spectral resolution (<1 kHz) 21 cm band search for intelligent radio signals of complete galaxies in the Local Group with the Jansky VLA, observing the galaxies M31 (Andromeda) and M33 (Triangulum) – the first and third largest members of the group respectively – sampling more stars than any prior search of this kind. We used 122 Hz channels over a 1 MHz spectral window in the target galaxy velocity frame of reference, and 15 Hz channels over a 125 kHz window in our local standard of rest. No narrowband signals were detected above a signal-to-noise ratio of 7, suggesting the absence of continuous narrowband flux greater than approximately 0.24 Jy and 1.33 Jy in the respective spectral windows illuminating our part of the Milky Way during our observations in December 2014 and January 2015. This is also the first study in which the upgraded VLA has been used for SETI.

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R. Gray and K. Mooley
Tue, 14 Feb 17
11/71

Comments: 14 pages, 9 figures, 5 tables. Accepted for publication in the Astronomical Journal

On the origin of the crescent-shaped distributions observed by MMS at the magnetopause [CL]

http://arxiv.org/abs/1702.03550

MMS observations recently confirmed that crescent-shaped electron velocity distributions in the plane perpendicular to the magnetic field occur in the electron diffusion region near reconnection sites at Earth’s magnetopause. In this paper, we re-examine the origin of the crescent-shaped distributions in the light of our new finding that ions and electrons are drifting in opposite directions when displayed in magnetopause boundary-normal coordinates. Therefore, ExB drifts cannot cause the crescent shapes. We performed a high-resolution multi-scale simulation capturing sub-electron skin depth scales. The results suggest that the crescent-shaped distributions are caused by meandering orbits without necessarily requiring any additional processes found at the magnetopause such as the highly asymmetric magnetopause ambipolar electric field. We use an adiabatic Hamiltonian model of particle motion to confirm that conservation of canonical momentum in the presence of magnetic field gradients causes the formation of crescent shapes without invoking asymmetries or the presence of an ExB drift. An important consequence of this finding is that we expect crescent-shaped distributions also to be observed in the magnetotail, a prediction that MMS will soon be able to test.

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G. Lapenta, J. Berchem, M. Zhou, et. al.
Tue, 14 Feb 17
15/71

Comments: to appear on J. Geophys. Res

Evolving waves and turbulence in the outer corona and inner heliosphere: the accelerating expanding box [SSA]

http://arxiv.org/abs/1702.03014

Alfv\’enic fluctuations in the solar wind display many properties reflecting an ongoing nonlinear cascade, e.g. a well-defined spectrum in frequency, together with some characteristics more commonly associated with the linear propagation of waves from the Sun, such as the variation of fluctuation amplitude with distance, dominated by solar wind expansion effects. Therefore both nonlinearities and expansion must be included simultaneously in any successful model of solar wind turbulence evolution. Because of the disparate spatial scales involved, direct numerical simulations of turbulence in the solar wind represent an arduous task, especially if one wants to go beyond the incompressible approximation. Indeed, most simulations neglect solar wind expansion effects entirely. Here we develop a numerical model to simulate turbulent fluctuations from the outer corona to 1 AU and beyond, including the sub-Alfv\’enic corona. The accelerating expanding box (AEB) extends the validity of previous expanding box models by taking into account both the acceleration of the solar wind and the inhomogeneity of background density and magnetic field. Our method incorporates a background accelerating wind within a magnetic field that naturally follows the Parker spiral evolution using a two-scale analysis in which the macroscopic spatial effect coupling fluctuations with background gradients becomes a time-dependent coupling term in a homogeneous box. In this paper we describe the AEB model in detail and discuss its main properties, illustrating its validity by studying Alfv\’en wave propagation across the Alfv\’en critical point.

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A. Tenerani and M. Velli
Mon, 13 Feb 17
16/57

Comments: 19 pages, 6 Figures. Submitted to the ApJ

Generalized phase mixing: Turbulence-like behaviour from unidirectionally propagating MHD waves [SSA]

http://arxiv.org/abs/1702.02346

We present the results of three-dimensional (3D) ideal magnetohydrodynamics (MHD) simulations on the dynamics of a perpendicularly inhomogeneous plasma disturbed by propagating Alfv\’enic waves. Simpler versions of this scenario have been extensively studied as the phenomenon of phase mixing. We show that, by generalizing the textbook version of phase mixing, interesting phenomena are obtained, such as turbulence-like behavior and complex current-sheet structure, a novelty in longitudinally homogeneous plasma excited by unidirectionally propagating waves. This constitutes an important finding for turbulence-related phenomena in astrophysics in general, relaxing the conditions that have to be fulfilled in order to generate turbulent behavior.

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N. Magyar, T. Doorsselaere and M. Goossens
Thu, 9 Feb 17
29/67

Comments: N/A

Solar Energetic Particle Acceleration by a Shock Wave Accompanying a Coronal Mass Ejection in the Solar Atmosphere [HEAP]

http://arxiv.org/abs/1702.02316

Solar energetic particles acceleration by a shock wave accompanying a coronal mass ejection (CME) is studied. The description of the accelerated particle spectrum evolution is based on the numerical calculation of the diffusive transport equation with a set of realistic parameters. The relation between the CME and the shock speeds, which depend on the initial CME radius, is determined. Depending on the initial CME radius, its speed, and the magnetic energy of the scattering Alfven waves, the accelerated particle spectrum is established during 10-60 minutes from the beginning of CME motion. The maximum energies of particles reach 0.1-10 GeV. The CME radii of 3-5 $R_\odot$ and the shock radii of 5-10 $R_\odot$ agree with observations. The calculated particle spectra agree with the observed ones in events registered by ground-based detectors if the turbulence spectrum in the solar corona significantly differs from the Kolmogorov one.

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A. Petukhova, I. Petukhov, S. Petukhov, et. al.
Thu, 9 Feb 17
65/67

Comments: 11 pages, 14 figures, published in ApJ

Preferential Heating and Acceleration of Heavy Ions in Impulsive Solar Flares [CL]

http://arxiv.org/abs/1702.01789

We simulate decaying turbulence in a homogeneous pair plasma using three dimensional electromagnetic particle-in-cell (PIC) method. A uniform background magnetic field permeates the plasma such that the magnetic pressure is three times larger than the thermal pressure and the turbulence is generated by counter-propagating shear Alfv\’en waves. The energy predominately cascades transverse to the background magnetic field, rendering the turbulence anisotropic at smaller scales. We simultaneously move several ion species of varying charge to mass ratios in our simulation and show that the particles of smaller charge to mass ratios are heated and accelerated to non-thermal energies at a faster rate, in accordance with the enhancement of heavy ions and non-thermal tail in their energy spectrum observed in the impulsive solar flares. We further show that the heavy ions are energized mostly in the direction perpendicular to the background magnetic field with a rate consistent with our analytical estimate of the rate of heating due to cyclotron resonance with the Alfv\’en waves of which a large fraction is due to obliquely propagating waves.

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R. Kumar, D. Eichler, M. Gaspari, et. al.
Wed, 8 Feb 17
58/65

Comments: 6 pages, 7 figures

The formation of magnetic depletions and flux annihilation due to reconnection in the heliosheath [CL]

http://arxiv.org/abs/1702.01697

The misalignment of the solar rotation axis and the magnetic axis of the Sun produces a periodic reversal of the Parker spiral magnetic field and the sectored solar wind. The compression of the sectors is expected to lead to reconnection in the heliosheath (HS). We present particle-in-cell simulations of the sectored HS that reflect the plasma environment along the Voyager 1 and 2 trajectories, specifically including unequal positive and negative azimuthal magnetic flux as seen in the Voyager data \citep{Burlaga03}. Reconnection proceeds on individual current sheets until islands on adjacent current layers merge. At late time bands of the dominant flux survive, separated by bands of deep magnetic field depletion. The ambient plasma pressure supports the strong magnetic pressure variation so that pressure is anti-correlated with magnetic field strength. There is little variation in the magnetic field direction across the boundaries of the magnetic depressions. At irregular intervals within the magnetic depressions are long-lived pairs of magnetic islands where the magnetic field direction reverses so that spacecraft data would reveal sharp magnetic field depressions with only occasional crossings with jumps in magnetic field direction. This is typical of the magnetic field data from the Voyager spacecraft \citep{Burlaga11,Burlaga16}. Voyager 2 data reveals that fluctuations in the density and magnetic field strength are anti-correlated in the sector zone as expected from reconnection but not in unipolar regions. The consequence of the annihilation of subdominant flux is a sharp reduction in the “number of sectors” and a loss in magnetic flux as documented from the Voyager 1 magnetic field and flow data \citep{Richardson13}.

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J. Drake, M. Swisdak, M. Opher, et. al.
Tue, 7 Feb 17
60/64

Comments: N/A

Particle acceleration model for the broadband baseline spectrum of the Crab nebula [HEAP]

http://arxiv.org/abs/1702.00816

We develop a simple one-zone model of the steady-state Crab nebula spectrum encompassing both the radio/soft $X$-ray and the GeV/multi-TeV observations. We determine analytically the photon differential energy spectrum as originated by an electron distribution evolved from a log-parabola injection spectrum: we find an impressive agreement with the synchrotron region observations whereas synchrotron self-Compton accommodates the previously unsolved origin of the broad $200$ GeV peak that matches the Fermi/LAT data beyond $1$ GeV with the MAGIC data. We determine the parameters of the log-parabola electron distribution, ruling out a simple power-law. The scale of the acceleration region is found to be $ \sim 3.8 \times 10^{-4}$ pc. The resulting photon differential spectrum provides a natural interpretation of the deviation from power-law customarily fit with empirical broken power-laws. Our model can be applied to the radio-to-multi-TeV spectrum of a variety of astrophysical sources of relativistic flows as well as to fast interplanetary shocks.

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F. Fraschetti and M. Pohl
Mon, 6 Feb 17
4/43

Comments: 8 pages, 7 figures. Submitted. Comments welcome

How Electron Two-Stream Instability Drives Cyclic Langmuir Collapse and Continuous Coherent Emission [CL]

http://arxiv.org/abs/1702.00784

Continuous plasma coherent emission is maintained by repetitive Langmuir collapse driven by the nonlinear evolution of a strong electron two-stream instability. The Langmuir waves are modulated by solitary waves in the linear stage, and by electrostatic whistler waves in the nonlinear stage. Modulational instability leads to Langmuir collapse and electron heating that fills in cavitons. The high pressure is released via excitation of a short wavelength ion acoustic mode that is damped by electrons and that re-excites small-scale Langmuir waves—this process closes a feedback loop that maintains the continuous coherent emission.

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H. Che, M. Goldstein, P. Diamond, et. al.
Mon, 6 Feb 17
9/43

Comments: 1/30/2017, published in Proceedings of the National Academy of Sciences of the United States of America

A propagation tool to connect remote-sensing observations with in-situ measurements of heliospheric structures [CL]

http://arxiv.org/abs/1702.00399

The remoteness of the Sun and the harsh conditions prevailing in the solar corona have so far limited the observational data used in the study of solar physics to remote-sensing observations taken either from the ground or from space. In contrast, the `solar wind laboratory’ is directly measured in situ by a fleet of spacecraft measuring the properties of the plasma and magnetic fields at specific points in space. Since 2007, the solar-terrestrial relations observatory (STEREO) has been providing images of the solar wind that flows between the solar corona and spacecraft making in-situ measurements. This has allowed scientists to directly connect processes imaged near the Sun with the subsequent effects measured in the solar wind. This new capability prompted the development of a series of tools and techniques to track heliospheric structures through space. This article presents one of these tools, a web-based interface called the ‘Propagation Tool’ that offers an integrated research environment to study the evolution of coronal and solar wind structures, such as Coronal Mass Ejections (CMEs), Corotating Interaction Regions (CIRs) and Solar Energetic Particles (SEPs). These structures can be propagated from the Sun outwards to or alternatively inwards from planets and spacecraft situated in the inner and outer heliosphere. In this paper, we present the global architecture of the tool, discuss some of the assumptions made to simulate the evolution of the structures and show how the tool connects to different databases.

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A. Rouillard, B. Lavraud, V. Genot, et. al.
Fri, 3 Feb 17
11/55

Comments: 22 pages, 10 figures, submitted to Planetary and Space Science

A deterministic model for forecasting long-term solar activity [SSA]

http://arxiv.org/abs/1702.00641

A phenomenological model is presented for the quantitative description of the evolution of solar cycles in terms of the number of M-class flares. The determining factor of the model is based on the relative ecliptic longitude of the planets Jupiter and Saturn. Using as input the temporal distribution of flares during cycle 21, results in notable agreement with the observations are obtained for cycles 22-24 and predictions are provided for the evolution of solar activity in the next years.

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E. Petrakou
Fri, 3 Feb 17
45/55

Comments: 26 pages, 16 figures

The optimisation of low-acceleration interstellar relativistic rocket trajectories using genetic algorithms [CL]

http://arxiv.org/abs/1702.00030

A vast wealth of literature exists on the topic of rocket trajectory optimisation, particularly in the area of interplanetary trajectories due to its relevance today. Studies on optimising interstellar and intergalactic trajectories are usually performed in flat spacetime using an analytical approach, with very little focus on optimising interstellar trajectories in a general relativistic framework. This paper examines the use of low-acceleration rockets to reach galactic destinations in the least possible time, with a genetic algorithm being employed for the optimisation process. The fuel required for each journey was calculated for various types of propulsion systems to determine the viability of low-acceleration rockets to colonise the Milky Way. The results showed that to limit the amount of fuel carried on board, an antimatter propulsion system would likely be the minimum technological requirement to reach star systems tens of thousands of light years away. However, using a low-acceleration rocket would require several hundreds of thousands of years to reach these star systems, with minimal time dilation effects since maximum velocities only reached about 0.2c. Such transit times are clearly impractical, and thus, any kind of colonisation using low acceleration rockets would be difficult. High accelerations, on the order of 1g, are likely required to complete interstellar journeys within a reasonable time frame, though they may require prohibitively large amounts of fuel. So for now, it appears that humanity’s ultimate goal of a galactic empire may only be possible at significantly higher accelerations, though the propulsion technology requirement for a journey that uses realistic amounts of fuel remains to be determined.

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K. Fung, G. Lewis and X. Wu
Thu, 2 Feb 17
7/52

Comments: 32 pages, 16 figures, Accepted for publication in Acta Astronautica

The Formation of Heliospheric Arcs of Slow Solar Wind [CL]

http://arxiv.org/abs/1701.08797

A major challenge in solar and heliospheric physics is understanding how highly localized regions, far smaller than 1 degree at the Sun, are the source of solar-wind structures spanning more than 20 degrees near Earth. The Sun’s atmosphere is divided into magnetically open regions, coronal holes, where solar-wind plasma streams out freely and fills the solar system, and closed regions, where the plasma is confined to coronal loops. The boundary between these regions extends outward as the heliospheric current sheet (HCS). Measurements of plasma composition imply that the solar wind near the HCS, the so-called slow solar wind, originates in closed regions, presumably by the processes of field-line opening or interchange reconnection. Mysteriously, however, slow wind is also often seen far from the HCS. We use high-resolution, three-dimensional magnetohydrodynamic simulations to calculate the dynamics of a coronal hole whose geometry includes a narrow corridor flanked by closed field and which is driven by supergranule-like flows at the coronal-hole boundary. We find that these dynamics result in the formation of giant arcs of closed-field plasma that extend far from the HCS and span tens of degrees in latitude and longitude at Earth, accounting for the slow solar wind observations.

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A. Higginson, S. Antiochos, C. DeVore, et. al.
Wed, 1 Feb 17
15/67

Comments: N/A

Solar signatures and eruption mechanism of the 2010 August 14 CME [SSA]

http://arxiv.org/abs/1701.08814

On 2010 August 14, a wide-angled coronal mass ejection (CME) was observed. This solar eruption originated from a destabilized filament that connected two active regions and the unwinding of this filament gave the eruption an untwisting motion that drew the attention of many observers. In addition to the erupting filament and the associated CME, several other low-coronal signatures that typically indicate the occurrence of a solar eruption were associated to this event. However, contrary to what is expected, the fast CME ($\mathrm{v}>900~\mathrm{km}~\mathrm{s}^{-1}$) was accompanied by only a weak C4.4 flare.
We investigate the various eruption signatures that were observed for this event and focus on the kinematic evolution of the filament in order to determine its eruption mechanism. Had this solar eruption occurred just a few days earlier, it could have been a significant event for space weather. The risk to underestimate the strength of this eruption based solely on the C4.4 flare illustrates the need to include all eruption signatures in event analyses in order to obtain a complete picture of a solar eruption and assess its possible space weather impact.

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E. DHuys, D. Seaton, A. Groof, et. al.
Wed, 1 Feb 17
29/67

Comments: N/A

Planar magnetic structures in coronal mass ejection-driven sheath regions [SSA]

http://arxiv.org/abs/1701.08739

Planar magnetic structures (PMSs) are periods in the solar wind during which interplanetary magnetic field vectors are nearly parallel to a single plane. One of the specific regions where PMSs have been reported are coronal mass ejection (CME)-driven sheaths. We use here an automated method to identify PMSs in 95 CME sheath regions observed in-situ by the Wind and ACE spacecraft between 1997 and 2015. The occurrence and location of the PMSs are related to various shock, sheath and CME properties. We find that PMSs are ubiquitous in CME sheaths; 85% of the studied sheath regions had PMSs with the mean duration of 6.0 hours. In about one-third of the cases the magnetic field vectors followed a single PMS plane that covered a significant part (at least 67%) of the sheath region. Our analysis gives strong support for two suggested PMS formation mechanisms: the amplification and alignment of solar wind discontinuities near the CME-driven shock and the draping of the magnetic field lines around the CME ejecta. For example, we found that the shock and PMS plane normals generally coincided for the events where the PMSs occurred near the shock (68% of the PMS plane normals near the shock were separated by less than 20{\deg} from the shock normal), while deviations were clearly larger when PMSs occurred close to the ejecta leading edge. In addition, PMSs near the shock were generally associated with lower upstream plasma beta than the cases where PMSs occurred near the leading edge of the CME. We also demonstrate that the planar parts of the sheath contain a higher amount of strongly southward magnetic field than the non-planar parts, suggesting that planar sheaths are more likely to drive magnetospheric activity.

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E. Palmerio, E. Kilpua and N. Savani
Tue, 31 Jan 17
47/58

Comments: 10 pages, 7 figures, accepted for publication in Annales Geophysicae

Radio occultations of the Io plasma tours by Juno are feasible [EPA]

http://arxiv.org/abs/1701.07435

The flow of material from Io’s volcanoes into the Io plasma torus, out into the magnetosphere, and along field lines into Jupiter’s upper atmosphere is not adequately understood. The lack of observations of spatial and temporal variations in the Io plasma torus impedes attempts to understand the system as a whole. Here we propose that radio occultations of the Io plasma torus by the Juno spacecraft can measure plasma densities in the Io plasma torus. We find that the line-of-sight column density of plasma in each of the three regions of the Io plasma torus (cold torus, ribbon, and warm torus) can be measured with uncertainties of 10%. We also find that scale heights describing the spatial variation in plasma density in each of these three regions can be measured with similar uncertainties. Such observations will be sufficiently accurate to support system-scale studies of the flow of plasma through the magnetosphere of Jupiter.

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P. Phipps and P. Withers
Fri, 27 Jan 17
26/54

Comments: 53 pages (unformatted manuscript), 10 figures, 3 tables, accepted for publication in JGR Space Physics

Radiometric Actuators for Spacecraft Attitude Control [CL]

http://arxiv.org/abs/1701.07545

CubeSats and small satellites are emerging as low-cost tools to perform astronomy, exoplanet searches and earth observation. These satellites can be dedicated to pointing at targets for weeks or months at a time. This is typically not possible on larger missions where usage is shared. Current satellites use reaction wheels and where possible magneto-torquers to control attitude. However, these actuators can induce jitter due to various sources. In this work, we introduce a new class of actuators that exploit radiometric forces induced by gasses on surface with a thermal gradient. Our work shows that a CubeSat or small spacecraft mounted with radiometric actuators can achieve precise pointing of few arc-seconds or less and avoid the jitter problem. The actuator is entirely solid-state, containing no moving mechanical components. This ensures high-reliability and long-life in space. A preliminary design for these actuators is proposed, followed by feasibility analysis of the actuator performance.

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R. Nallapu, A. Tallapragada and J. Thangavelautham
Fri, 27 Jan 17
27/54

Comments: 7 pages, 11 figures in Proceedings of the IEEE Aerospace Conference 2017

Linking Fluid and Kinetic Scales in Solar Wind Turbulence [CL]

http://arxiv.org/abs/1701.07353

We investigate possible links between the large-scale and small-scale features of solar wind fluctuations across the frequency break separating fluid and kinetic regimes. The aim is to correlate the magnetic field fluctuations polarization at dissipative scales with the particular state of turbulence within the inertial range of fluctuations. We found clear correlations between each type of polarization within the kinetic regime and fluid parameters within the inertial range. Moreover, for the first time in literature, we showed that left-handed and right-handed polarized fluctuations occupy different areas of the plasma instabilities-temperature anisotropy plot, as expected for Alfv$\acute{\textrm{e}}$n Ion Cyclotron and Kinetic Alfv$\acute{\textrm{e}}$n waves, respectively.

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D. Telloni and R. Bruno
Thu, 26 Jan 17
35/68

Comments: 5 pages, 4 figures

Electron cyclotron maser instability (ECMI) in strong magnetic guide field reconnection [CL]

http://arxiv.org/abs/1701.06961

Reconnection in strong current-aligned magnetic guide fields allows for the excitation of the electron-cyclotron-maser instability and emission of electromagnetic radiationfrom the electron exhaust at the {\sf X} point. The electrons in the guide field remain magnetized, with reconnection barely affected. The guide field is responsible for the asymmetric properties of the {\sf X} point and exhaust. Asymmetry in the electron population results in conditions favorable for ECMI. Fundamental mission beneath the guide field cyclotron is similar to electron hole emission discussed elsewhere. It can be treated in the proper exhaust frame, and maps the local magnetic field when moving together with the exhaust along the guide field. Many applications of this mechanism can be imagined. We propose an outline of the mechanism and discuss some of its advantages and prospects. Among potential applications are AKR in auroral physics, various types of solar radio emissions during flares, planetary emissions and several astrophysical scenarios involving the presence of strong fields and field-aligned currents. Escape of radiation from {\sf X} is no problem. However, observation from remote requires traversing the stop-band of X modes and implies source displacements to weaker fields.
Keywords: Electron cyclotron maser, radio emissions, radio bursts, reconnection, auroral physics, AKR, solar radiation, pulsars

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R. Treumann and W. Baumjohann
Wed, 25 Jan 17
57/74

Comments: 14 pages, 6 figures

Suprathermal electron strahl widths in the presence of narrow-band whistler waves in the solar wind [CL]

http://arxiv.org/abs/1701.04819

We perform the first statistical study of the effects of the interaction of suprathermal electrons with narrow-band whistler mode waves in the solar wind. We show that this interaction does occur and that it is associated with enhanced widths of the so called strahl component. The latter is directed along the inter- planetary magnetic field away from the Sun. We do the study by comparing the strahl pitch angle widths in the solar wind at 1AU in the absence of large scale discontinuities and transient structures, such as interplanetary shocks, interplanetary coronal mass ejections, stream interaction regions, etc. during times when the whistler mode waves were present and when they were absent. This is done by using the data from two Cluster instruments: STAFF data in frequency range between ~0.1 Hz and ~200 Hz were used for determining the wave properties and PEACE datasets at twelve central energies between ~57 eV (equivalent to ~10 typical electron thermal energies in the solar wind, E_T ) and ~676 eV (~113 E_T ) for pitch angle measurements. Statistical analysis shows that during the inter- vals with the whistler waves the strahl component on average exhibits pitch angle widths between 2 and 12 degrees larger than during the intervals when these waves are not present. The largest difference is obtained for the electron central energy of ~344 eV (~57 E_T ).

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P. Kajdic, O. Alexandrova, M. Maksimovic, et. al.
Thu, 19 Jan 17
37/42

Comments: Published in ApJ

Conductivity spectrum and dispersion relation in solar wind turbulence [CL]

http://arxiv.org/abs/1701.05163

Magnetic turbulence in the solar wind is treated from the point of view of electrodynamics. This can be done based on the use of Poynting’s theorem attributing all turbulent dynamics to the spectrum of turbulent conductivity. For two directions of propagation of the turbulent fluctuations of the electromagnetic field with respect to the mean plus external magnetic fields an expression is constructed for the spectrum of turbulent dissipation. Use of solar wind observations of electromagnetic power spectral densities in the inertial subrange then allows determination of the conductivity spectrum, the dissipative response function, in this range. It requires observations of the complete electromagnetic spectral energy densities including electric power spectral densities. The dissipative response function and dispersion relation of solar wind inertial range magnetic turbulence are obtained. The dispersion relation indicates the spatial scale decay with increasing frequency providing independent support for the use of Taylor’s hypothesis. The dissipation function indicates an approximate shot-noise spectrum of turbulent resistivity in the inertial range suggesting progressive structure formation in the inertial range which hints on the presence of discrete mode turbulence and nonlinear resonances.

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R. Treumann and W. Baumjohann
Thu, 19 Jan 17
39/42

Comments: 7 pages, no figures, preprint

Evidence for the Stochastic Acceleration of Secondary Antiprotons by Supernova Remnants [HEAP]

http://arxiv.org/abs/1701.04406

The antiproton-to-proton ratio in the cosmic-ray spectrum is a sensitive probe of new physics. Using recent measurements of the cosmic-ray antiproton and proton fluxes in the energy range of 1-1000 GeV, we study the contribution to the $\bar{p}/p$ ratio from secondary antiprotons that are produced and subsequently accelerated within individual supernova remnants. We consider several well-motivated models for cosmic-ray propagation in the interstellar medium and marginalize our results over the uncertainties related to the antiproton production cross section and the time-, charge-, and energy-dependent effects of solar modulation. We find that the increase in the $\bar{p}/p$ ratio observed at rigidities above $\sim$ 100 GV cannot be accounted for within the context of conventional cosmic-ray propagation models, but is consistent with scenarios in which cosmic-ray antiprotons are produced and subsequently accelerated by shocks within a given supernova remnant. In light of this, the acceleration of secondary cosmic rays in supernova remnants is predicted to substantially contribute to the cosmic-ray positron spectrum, accounting for a significant fraction of the observed positron excess.

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I. Cholis, D. Hooper and T. Linden
Wed, 18 Jan 17
30/61

Comments: 5 pages, 3 figures and 4 tables

Comment to: The interaction of relativistic spacecrafts with the interstellar medium [GA]

http://arxiv.org/abs/1701.04319

Recently, Hoang et al. (arXiv:1608.05284) reported analysis of the interaction of relativistic spacecrafts with interstellar medium (ISM, i.e. gas atoms and dust particles) relevant for the Breakthrough starshot initiative (https://breakthroughinitiatives.org/Initiative/3). The main conclusion is that dust pose much greater threat to the starship than gas atoms. However, analysis used to treat interaction of the spaceship with gas atoms is based on the incorrect use of the Szenes model. Only by proper treatment of the Szenes model can be found if the conclusion remains valid – or not. In the following, the main comments we have raised about the paper are listed. Present text is based on the v2 version of the above mentioned paper [0] that was accepted for publication in Astophysical Journal.

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M. Karlusic
Tue, 17 Jan 17
34/81

Comments: working paper, comment to arXiv:1608.05284

Solar Energetic Particle transport near a Heliospheric Current Sheet [CL]

http://arxiv.org/abs/1701.04286

Solar Energetic Particles (SEPs), a major component of space weather, propagate through the interplanetary medium strongly guided by the Interplanetary Magnetic Field (IMF). In this work, we analyse the implications a flat Heliospheric Current Sheet (HCS) has on proton propagation from SEP release sites to the Earth. We simulate proton propagation by integrating fully 3-D trajectories near an analytically defined flat current sheet, collecting comprehensive statistics into histograms, fluence maps and virtual observer time profiles within an energy range of 1–800 MeV. We show that protons experience significant current sheet drift to distant longitudes, causing time profiles to exhibit multiple components, which are a potential source of confusing interpretation of observations. We find that variation of current sheet thickness within a realistic parameter range has little effect on particle propagation. We show that IMF configuration strongly affects deceleration of protons. We show that in our model, the presence of a flat equatorial HCS in the inner heliosphere limits the crossing of protons into the opposite hemisphere.

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M. Battarbee, S. Dalla and M. Marsh
Tue, 17 Jan 17
65/81

Comments: 16 pages, 15 figures, accepted for publication in The Astrophysical Journal

The Solar Orbiter Mission: an Energetic Particle Perspective [IMA]

http://arxiv.org/abs/1701.04057

Solar Orbiter is a joint ESA-NASA mission planed for launch in October 2018. The science payload includes remote-sensing and in-situ instrumentation designed with the primary goal of understanding how the Sun creates and controls the heliosphere. The spacecraft will follow an elliptical orbit around the Sun, with perihelion as close as 0.28 AU. During the late orbit phase the orbital plane will reach inclinations above 30 degrees, allowing direct observations of the solar polar regions. The Energetic Particle Detector (EPD) is an instrument suite consisting of several sensors measuring electrons, protons and ions over a broad energy interval (2 keV to 15 MeV for electrons, 3 keV to 100 MeV for protons and few tens of keV/nuc to 450 MeV/nuc for ions), providing composition, spectra, timing and anisotropy information. We present an overview of Solar Orbiter from the energetic particle perspective, summarizing the capabilities of EPD and the opportunities that these new observations will provide for understanding how energetic particles are accelerated during solar eruptions and how they propagate through the Heliosphere.

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R. Gomez-Herrero, J. Rodriguez-Pacheco, R. Wimmer-Schweingruber, et. al.
Tue, 17 Jan 17
67/81

Comments: XXV ECRS 2016 Proceedings – eConf C16-09-04.3

Spatial and temporal variations of high-energy electron flux in the outer radiation belt [HEAP]

http://arxiv.org/abs/1701.03703

The results of observation of short-term variations of high-energy electron flux in the outer radiation belt, obtained in ARINA satellite experiment (2006 – 2016), are presented. Scintillation spectrometer ARINA on board the Resurs-DK1 Russian satellite has been developed in MEPhI. The instrument carried out continuous measurements of high-energy electron flux and its energy spectrum in low-Earth orbits in the range 3-30 MeV with 10 15% energy resolution. A time profile of electron flux at different L – shells has been studied in detail on the example of March 2012, and analysis of experimental data on high-energy (4-6 MeV) electrons in the outer radiation belt zone (L~3-7) was fulfilled. It was shown a large variability of flux of such electrons there. The sharp effects in electron flux (as rise and as fall) in magnetosphere interrelated with geomagnetic storms caused by solar flares and coronal mass ejections have been observed.

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S. Koldashov, S. Aleksandrin and N. Eremina
Mon, 16 Jan 17
25/55

Comments: XXV ECRS 2016 Proceedings – eConf C16-09-04.3

Characterizing Fluid and Kinetic Instabilities using Field-Particle Correlations on Single-Point Time Series [CL]

http://arxiv.org/abs/1701.03687

A recently proposed technique correlating electric fields and particle velocity distributions is applied to single-point time series extracted from linearly unstable, electrostatic numerical simulations. The form of the correlation, which measures the transfer of phase-space energy density between the electric field and plasma distributions and had previously been applied to damped electrostatic systems, is modified to include the effects of drifting equilibrium distributions of the type that drive counter-streaming and bump-on-tail instabilities. By using single-point time series, the correlation is ideal for diagnosing dynamics in systems where access to integrated quantities, such as energy, is observationally infeasible. The velocity-space structure of the field-particle correlation is shown to characterize the underlying physical mechanisms driving unstable systems. The use of this correlation in simple systems will assist in its eventual application to turbulent, magnetized plasmas, with the ultimate goal of characterizing the nature of mechanisms that damp turbulent fluctuations in the solar wind.

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K. Klein
Mon, 16 Jan 17
29/55

Comments: 9 pages, 6 figures, accepted for publication in Physics of Plasmas

Proton fire hose instabilities in the expanding solar wind [CL]

http://arxiv.org/abs/1701.03665

Using two-dimensional hybrid expanding box simulations we study the competition between the continuously driven parallel proton temperature anisotropy and fire hose instabilities in collisionless homogeneous plasmas. For quasi radial ambient magnetic field the expansion drives $T_{\mathrm{p}\|}>T_{\mathrm{p}\perp}$ and the system becomes eventually unstable with respect to the dominant parallel fire hose instability. This instability is generally unable to counteract the induced anisotropization and the system typically becomes unstable with respect to the oblique fire hose instability later on. The oblique instability efficiently reduces the anisotropy and the system rapidly stabilizes while a significant part of the generated electromagnetic fluctuations is damped to protons. As long as the magnetic field is in the quasi radial direction, this evolution repeats itself and the electromagnetic fluctuations accumulate. For sufficiently oblique magnetic field the expansion drives $T_{\mathrm{p}\perp}>T_{\mathrm{p}\|}$ and brings the system to the stable region with respect to the fire hose instabilities.

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P. Hellinger
Mon, 16 Jan 17
49/55

Comments: Journal of Plasma Physics, 14 pages, 9 figures

Amplitude limits and nonlinear damping of shear-Alfvén waves in high-beta low-collisionality plasmas [CL]

http://arxiv.org/abs/1701.03175

This work, which extends Squire et al. [ApJL, 830 L25 (2016)], explores the effect of self-generated pressure anisotropy on linearly polarized shear-Alfv\’en fluctuations in low-collisionality plasmas. Such anisotropies lead to stringent limits on the amplitude of magnetic perturbations in high-beta plasmas, above which a fluctuation can destabilize itself through the parallel firehose instability. This causes the wave frequency to approach zero, “interrupting” the wave and stopping its oscillation. These effects are explored in detail in the collisionless and weakly collisional “Braginskii” regime, for both standing and traveling waves. The focus is on simplified models in one dimension, on scales much larger than the ion gyroradius. The effect has interesting implications for the physics of magnetized turbulence in the high-beta conditions that are prevalent in many astrophysical plasmas.

Read this paper on arXiv…

J. Squire, A. Schekochihin and E. Quataert
Fri, 13 Jan 17
1/44

Comments: N/A