H. Breuillard

Electron pitch‐angle diffusion in radiation belts: The effects of whistler wave oblique propagation

[1] We calculated the electron pitch-angle diffusion coefficients in the outer radiation belt for L-shell

Whistler mode waves and Hall fields detected by MMS during a dayside magnetopause crossing

4.5 taking into account the effects of oblique whistler wave propagation. The dependence of the distribution of the angle q between the whistler wave vector and the background magnetic field on magnetic latitude is modeled after statistical results of Cluster wave angle observations. According to in-situ observations , the mean value and the variance of the q distribution rapidly increase with magnetic latitude. We found that inclusion of oblique whistler wave propagation led to a significant increase in pitch-angle diffusion rates over those calculated under the assumption of parallel whistler wave propagation. The effect was pronounced for electrons with small equatorial pitch-angles close to the loss cone and could result in as much as an order of magnitude decrease of the electron lifetimes. We show that the intensification of pitch-angle diffusion can be explained by the contribution of higher order cyclotron resonances. By comparing the results of calculations obtained from two models of electron density distribution along field lines, we show that the effect of the intensification of pitch-angle diffusion is stronger when electron density does not vary along field lines. The intensification of pitch-angle diffusion and corresponding decrease of energetic electron lifetime result in significant modification of the rate of electron losses and should have an impact on formation and dynamics of the outer radiation belt. Citation: Artemyev, A., O. Agapitov, H. Breuillard, V. Krasnoselskikh, and G. Rolland (2012), Electron pitch-angle diffusion in radiation belts: The effects of whistler wave oblique propagation, Geophys.

MMS Observations of Ion-scale Magnetic Island in the Magnetosheath Turbulent Plasma

We present Magnetospheric Multiscale (MMS) mission measurements during a full magnetopause crossing associated with an enhanced southward ion flow. A quasi-steady magnetospheric whistler mode wave ...

Multispacecraft analysis of dipolarization fronts and associated whistler wave emissions using MMS data

In this letter, first observations of ion-scale magnetic island from the Magnetospheric Multiscale mission in the magnetosheath turbulent plasma are presented. The magnetic island is characterized ...

Statistical model of electron pitch angle diffusion in the outer radiation belt

Dipolarization fronts (DFs), embedded in bursty bulk flows, play a crucial role in Earths plasma sheet dynamics because the energy input from the solar wind is partly dissipated in their vicinity. This dissipation is in the form of strong low-frequency waves that can heat and accelerate energetic electrons up to the high-latitude plasma sheet. However, the dynamics of DF propagation and associated low-frequency waves in the magnetotail are still under debate due to instrumental limitations and spacecraft separation distances. In May 2015 the Magnetospheric Multiscale (MMS) mission was in a string-of-pearls configuration with an average intersatellite distance of 160u2009km, which allows us to study in detail the microphysics of DFs. Thus, in this letter we employ MMS data to investigate the properties of dipolarization fronts propagating earthward and associated whistler mode wave emissions. We show that the spatial dynamics of DFs are below the ion gyroradius scale in this region (∼500u2009km), which can modify the dynamics of ions in the vicinity of the DF (e.g., making their motion nonadiabatic). We also show that whistler wave dynamics have a temporal scale of the order of the ion gyroperiod (a few seconds), indicating that the perpendicular temperature anisotropy can vary on such time scales.

Electron Heating at Kinetic Scales in Magnetosheath Turbulence

[1]xa0We calculated the bounce averaged electron pitch angle diffusion coefficients using the statistical characteristics of lower band chorus activity collected by the Cluster mission from 2001–2009. Nine years of Cluster observations provide the distributions of the θ angle between wave vectors and the background magnetic field; and the distributions of the wave total intensity Bw2 for relatively wide ranges of the magnetic latitude λ, the magnetic local times, and the Kp indices. According to Cluster observations, the probability of observing a larger Bw2 increases with λ and depends upon the magnetic local time and Kp. We compared the obtained results with the diffusion coefficients that were calculated under an assumption of parallel whistler wave propagation with a constant intensity Bw2 = 104 pT2. The last calculations substantially underestimated pitch angle diffusion for the small equatorial pitch angles, αeq, but likely overestimates for αeq > 60°. An important increase in for αeq 15°. We took the probability density distribution of the wave mean amplitude into consideration instead of the averaged value. The obtained distribution of the diffusion coefficients indicated that approximately 20% of the most intense waves can provide the main portion of pitch angle diffusion for the dawn/day sector. For the dusk/night sector, wave intensity was significantly weaker and the relative importance of intense waves was not clearly pronounced.

The Effects Of Kinetic Instabilities On Small-Scale Turbulence In Earth's Magnetosheath

We present a statistical study of coherent structures at kinetic scales, using data from the Magnetospheric Multiscale mission in the Earths magnetosheath. We implemented the multi-spacecraft part ...

Poynting Vector and Wave Vector Directions of Equatorial Chorus

The Earths magnetosheath is the region delimited by the bow shock and the magnetopause. It is characterized by highly turbulent fluctuations covering all scales from MHD down to kinetic scales. Turbulence is thought to play a fundamental role in key processes such as energy transport and dissipation in plasma. In addition to turbulence, different plasma instabilities are generated in the magnetosheath because of the large anisotropies in plasma temperature introduced by its boundaries. In this study we use high-quality magnetic field measurements from Cluster spacecraft to investigate the effects of such instabilities on the small-scale turbulence (from ion down to electron scales). We show that the steepening of the power spectrum of magnetic field fluctuations in the magnetosheath occurs at the largest characteristic ion scale. However, the spectrum can be modified by the presence of waves/structures at ion scales, shifting the onset of the small-scale turbulent cascade toward the smallest ion scale. This cascade is therefore highly dependent on the presence of kinetic instabilities, waves, and local plasma parameters. Here we show that in the absence of strong waves the small-scale turbulence is quasi-isotropic and has a spectral index alpha approximate to 2.8. When transverse or compressive waves are present, we observe an anisotropy in the magnetic field components and a decrease in the absolute value of alpha. Slab/2D turbulence also develops in the presence of transverse/compressive waves, resulting in gyrotropy/non-gyrotropy of small-scale fluctuations. The presence of both types of waves reduces the anisotropy in the amplitude of fluctuations in the small-scale range.

A statistical study of kinetic‐size magnetic holes in turbulent magnetosheath: MMS observations

We present new results on wave vectors and Poynting vectors of chorus rising and falling tones on the basis of 6 years of THEMIS (Time History of Events and Macroscale Interactions during Substorms) observations. The majority of wave vectors is closely aligned with the direction of the ambient magnetic field (B0). Oblique wave vectors are confined to the magnetic meridional plane, pointing away from Earth. Poynting vectors are found to be almost parallel to B0. We show, for the first time, that slightly oblique Poynting vectors are directed away from Earth for rising tones and towards Earth for falling tones. For the majority of lower band chorus elements, the mutual orientation between Poynting vectors and wave vectors can be explained by whistler mode dispersion in a homogeneous collisionless cold plasma. Upper band chorus seems to require inclusion of collisional processes or taking into account azimuthal anisotropies in the propagation medium. The latitudinal extension of the equatorial source region can be limited to ±6∘ around the B0-minimum, or approximately ±5000 km along magnetic field lines. We find increasing Poynting flux and focusing of Poynting vectors on the B0-direction with increasing latitude. Also wave vectors become most often more field-aligned. A smaller group of chorus generated with very oblique wave normals tends to stay close to the whistler mode resonance cone. This suggests that close to the equatorial source region (within ∼20∘ latitude), a wave guidance mechanism is relevant, for example in ducts of depleted or enhanced plasma density.

Magnetic Reconnection at a Thin Current Sheet Separating Two Interlaced Flux Tubes at the Earth's Magnetopause

Kinetic-size magnetic holes (KSMHs) in the turbulent magnetosheath are statistically investigated using high time resolution data from the MMS mission. The KSMHs with short duration (i.e., < 0.5 s) have their cross section smaller than the ion gyro-radius. Superposed epoch analysis of all events reveals that an increase in the electron density and total temperature, significantly increase (resp. decrease) the electron perpendicular (resp. parallel) temperature, and an electron vortex inside KSMHs. Electron fluxes at ~ 90° pitch angles with selective energies increase in the KSMHs, are trapped inside KSMHs and form the electron vortex due to their collective motion. All these features are consistent with the electron vortex magnetic holes obtained in 2D and 3D particle-in-cell simulations, indicating that the observed KSMHs seem to be best explained as electron vortex magnetic holes. It is furthermore shown that KSMHs are likely to heat and accelerate the electrons.