J. Dombeck

Cluster observations of electron holes in association with magnetotail reconnection and comparison to simulations

Cluster observations of electron holes in association with magnetotail reconnection and comparison to simulations

Comparisons of Polar satellite observations of solitary wave velocities in the plasma sheet boundary and the high altitude cusp to those in the auroral zone

Characteristics of solitary waves observed by Polar in the high altitude cusp, polar cap and plasma sheet boundary are reported and compared to observations in the auroral zone. The study presented herein shows that, at high altitudes, the solitary waves are positive potential structures (electron holes), with scale sizes of the order of 10s of Debye lengths, which usually propagate with velocities of a few thousand km/s. At the plasma sheet boundary, the direction of propagation can be either upward or downward; whereas at the leading edge of high altitude cusp energetic particle injections, it is downward. For these high altitude events, explanations based on ion modes and on electron modes are both examined, and the electron mode interpretation is shown to be more consistent with observations.

Alfvén waves and Poynting flux observed simultaneously by Polar and FAST in the plasma sheet boundary layer

We present the first simultaneous observations of Alfven waves at Polar and FAST altitudes, ∼7 R E geocentric and ∼3500 km, respectively, at ∼23 MLT in the main phase of a major geomagnetic storm on 22 October 1999. We compare the Poynting flux for these waves and the electron energy flux at the two spacecraft. We also present a new method of Alfven wave analysis, examining Poynting flux magnitude and directionality along with the perturbation electric to magnetic field ratio of these waves as a function of wave temporal scale (frequency). The results of this analysis are compared with those expected from kinetic Alfven wave models. There is a mean net loss of ∼2.1 ergs cm -2 s -1 (mW m -2 ) in earthward Poynting flux over the altitude region between Polar and FAST, a mean net increase in earthward electron energy flux of up to ∼ 1.2 ergs cm -2 s -1 over the same region, frequency characteristics consistent with a mixture of Alfven waves obeying the kinetic Alfven wave dispersion relation mixed with some coupling to the ionosphere, and high-frequency kinetic Alfven wave generation between Polar and FAST. Current models are found to be generally consistent with the study results but are not yet sufficiently well formulated to account for the details, including evidence for temporal and/or spatial modulation of reflectivity.

Observed trends in auroral zone ion mode solitary wave structure characteristics using data from Polar

High-resolution (8000 sample s−1) data from the Polar Electric Field Instrument are analyzed for a study of ion mode solitary waves in upward current regions of the auroral zone. The primary focus of this study is the relations between velocity, maximum potential amplitude, and parallel structure width of these solitary waves (SWs). The observed SW velocities consistently lie, within error bars, between those of the H+ and O+ beams observed simultaneously by the Toroidal Imaging Mass-Angle Spectrograph (TIMAS) instrument. In addition, there is a trend that SW amplitudes are smaller when SW velocities are near the O+ beam velocity and larger when SW velocities are near the H+ beam velocity. These results are consistent with the observed ion mode SWs being a mechanism for the transfer of energy from the H+ beam to the O+ beam. A clear trend is also observed indicating larger amplitude with larger parallel spatial width. The results suggest that the observed solitary waves are a rarefactive ion mode associated with the ion two-stream instability.

THEMIS observations of the magnetopause electron diffusion region: Large amplitude waves and heated electrons

Received 14 April 2013; revised 9 May 2013; accepted 14 May 2013; published 18 June 2013. [1 ]W e present the first observations of large amplitude waves in a well-defined electron diffusion region based on the criteria described byScudderet al.[2012]atthe subsolarmagnetopause using data from one Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite. These waves identified as whistler mode waves, electrostatic solitary waves, lower hybrid waves, and electrostatic electron cyclotron waves, are observed in the same 12 s waveform capture and in association with signatures of active magnetic reconnection. The large amplitude waves in the electron diffusion region are coincident with abrupt increases in electron parallel temperature suggesting strong wave heating. The whistler mode waves, which are at the electron scale and which enable us to probe electron dynamics in the diffusion region were analyzed in detail. The energetic electrons (~30keV) within the electron diffusion region have anisotropic distributions with Te! /Tek > 1t hat may provide the free energy for the whistler mode waves. The energetic anisotropic electrons may be produced during the reconnection process. The whistler mode waves propagate away from the center of the “X-line” along magnetic field lines, suggesting that the electron diffusion region is a possible source region of the whistler mode waves. Citation: Tang, X., C. Cattell, J. Dombeck, L. Dai, L. B. Wilson III, A. Breneman, and A. Hupach (2013), THEMIS observations of the magnetopause electron diffusion region: Large amplitude waves and heated electrons, Geophys. Res. Lett., 40 ,2 884‐2890, doi:10.1002/ grl.50565.

Studies of ion solitary waves using simulations including hydrogen and oxygen beams

Particle-in-cell simulations of solitary waves have been performed using a 2 spatial and 3 velocity dimension electrostatic code with one electron and two ion species. Data from the Fast Auroral Snapshot (FAST) and Polar spacecraft are used to provide input parameters, and on the basis of these observations, no cold plasma was included in contrast to earlier simulations. Simulations containing both oxygen and hydrogen beams are compared to simulations that contain only hydrogen to examine the effects of the oxygen on the behavior of the solitary waves. In both cases the solitary wave speeds are less than the hydrogen beam speed, and they are also greater than the oxygen beam speed for the cases including oxygen. The simulated solitary waves have spatial scales of the order of 10 λD and potential amplitudes of the order of 0.1 eϕ/kTe, which are consistent with Polar spacecraft observations in the low-altitude auroral zone.

Observations of large amplitude parallel electric field wave packets at the plasma sheet boundary

We report the first observation of large amplitude electric fields parallel to the earths magnetic field at radial distances of ∼6 RE on magnetic field lines linking the plasma sheet boundary to the auroral zone. The parallel fields, with magnitudes up to 40 mV/m, occur in three different electrostatic structures: unipolar wave packets with a net potential drop, bipolar wave packets, and solitary waves. Examples from one boundary crossing are presented which show that the wave packets have durations of <1s, are traveling at velocities of ≤100 k/s, and are consistent with ion acoustic waves. The waves can contribute to the acceleration of auroral electrons and modify the pitch angle distributions. The wave packets occur in a region of large-scale field-aligned currents and are often associated with perpendicular waves near the lower hybrid frequency with strongly modulated amplitudes. The velocities determined for the solitary waves are faster than those of the wave packets.

Polar observations of solitary waves at high and low altitudes and comparison to theory

Abstract Solitary waves with large electric fields (up to 100 mV/m) are often observed by Polar in the low altitude auroral zone and, at high altitudes (∼ 4–8 RE), during crossings of the plasma sheet boundary and cusp. Electron solitary waves are ubiquitous, and are observed for wide range of fce/fpe. In contrast, to date, ion solitary waves have only been observed in the auroral zone at low altitudes in the region where fce/fpe>>1. We describe some results of statistical studies of ion solitary waves at low altitudes and electron solitary waves at high altitudes. Ion solitary waves, observed in regions of upward field-aligned currents and ion beams, are negative potential structures and have velocities between the speeds of the associated O+ and H+ beams, scale sizes of approximately 10λD, and normalized amplitudes, eφ/kTe, of order 0.1 because the electron temperatures are large (plasma sheet values). In addition, the amplitude increases with both the velocity and the scale size which is inconsistent with the predictions of ion acoustic soliton theory. The observations are well modeled by the simulations of Crumley et al. (2001) which include only the plasma sheet electrons and the beam ions. Both observations and the simulations are consistent with an ion hole mode associated with the ion two stream instability. The high altitude electron solitary waves have velocities from ∼ 1000km/s to >2500 km/s. Observed scale sizes are on the order of 1–10λD with eφ/kTe up to O(1). For these solitary waves also, the amplitude increases with both the velocity and the scale size, consistent with electron hole modes as was observed at low altitudes. Even the very large amplitude solitary waves are stable based on the criterion developed by Muschietti et al.(1999).

Dayside response of the magnetosphere to a small shock compression: Van Allen Probes, Magnetospheric MultiScale, and GOES‐13

Abstract Observations from Magnetospheric MultiScale (~8 Re) and Van Allen Probes (~5 and 4 Re) show that the initial dayside response to a small interplanetary shock is a double‐peaked dawnward electric field, which is distinctly different from the usual bipolar (dawnward and then duskward) signature reported for large shocks. The associated E × B flow is radially inward. The shock compressed the magnetopause to inside 8 Re, as observed by Magnetospheric MultiScale (MMS), with a speed that is comparable to the E × B flow. The magnetopause speed and the E × B speeds were significantly less than the propagation speed of the pulse from MMS to the Van Allen Probes and GOES‐13, which is consistent with the MHD fast mode. There were increased fluxes of energetic electrons up to several MeV. Signatures of drift echoes and response to ULF waves also were seen. These observations demonstrate that even very weak shocks can have significant impact on the radiation belts.