Jean-André Sauvaud

Radiation belt electron precipitation due to VLF transmitters: Satellite observations

In the Earth’s inner magnetosphere, the distribution of nenergetic electrons is controlled by pitch-angle scattering by nwaves. A category of Whistler waves originates from npowerful ground-based VLF transmitter signals in the nfrequency range 10–25 kHz. These transmissions are nobserved in space as waves of very narrow bandwidth. nHere we examine the significance of the VLF transmitter nNWC on the inner radiation belt using DEMETER satellite nglobal observations at low altitudes. We find that nenhancements in the 100–600 keV drift-loss cone nelectron fluxes at L values between 1.4 and 1.7 are linked nto NWC operation and to ionospheric absorption. Waves nand particles interact in the vicinity of the magnetic nequatorial plane. Using Demeter passes across the drifting ncloud of electrons caused by the transmitter; we find that n300 times more 200 keV electrons are driven into the ndrift-loss cone during NWC transmission periods than nduring non-transmission periods. The correlation between nthe flux of resonant electrons and the Dst index shows that nthe electron source intensity is controlled by magnetic storm nactivity.

Magnetic storm modeling in the Earth's electron belt by the Salammbô code

The Salammbo code, which solves the three-dimensional phase-space diffusion equation for the electron radiation belt, was used to explain the dynamic conditions present during a geomagnetic storm. With a simple injection model, we have characterized the dynamic behavior for relativistic electrons in the outer belt. The particles in the range 100 keV – 500 keV are diffused throughout the belt, with a shape essentially dependent on the radial diffusion coefficients. Particles with higher energies are “created” by acceleration of slower particles near the plasmapause location. The calculated shape of the fluxes in an L versus time grid for the 1 MeV electrons looks globally like those measured aboard the CRRES satellite.

Contrasting the efficiency of radiation belt losses caused by ducted and nonducted whistler-mode waves from ground-based transmitters

It has long been recognized that whistler-mode waves can be trapped in plasmaspheric whistler ducts which guide the waves. For nonguided cases these waves are said to be nonducted, which is dominant for L < 1.6. Wave-particle interactions are affected by the wave being ducted or nonducted. In the field-aligned ducted case, first-order cyclotron resonance is dominant, whereas nonducted interactions open up a much wider range of energies through equatorial and off-equatorial resonance. There is conflicting information as to whether the most significant particle loss processes are driven by ducted or nonducted waves. In this study we use loss cone observations from the DEMETER and POES low-altitude satellites to focus on electron losses driven by powerful VLF communications transmitters. Both satellites confirm that there are well-defined enhancements in the flux of electrons in the drift loss cone due to ducted transmissions from the powerful transmitter with call sign NWC. Typically, ∼80% of DEMETER nighttime orbits to the east of NWC show electron flux enhancements in the drift loss cone, spanning a L range consistent with first-order cyclotron theory, and inconsistent with nonducted resonances. In contrast, ∼1% or less of nonducted transmissions originate from NPM-generated electron flux enhancements. While the waves originating from these two transmitters have been predicted to lead to similar levels of pitch angle scattering, we find that the enhancements from NPM are at least 50 times smaller than those from NWC. This suggests that lower-latitude, nonducted VLF waves are much less effective in driving radiation belt pitch angle scattering.

Electron and proton radiation belt dynamic simulations during storm periods: A new asymmetric convection‐diffusion model

Using a convection-diffusion theory, we give the first results from a four-dimensional model of electron and proton radiation belts. This work is based on the numerical solution of a convection-diffusion equation taking into account (1) for protons, the deceleration of protons by the free and bounded thermospheric and ionospheric electrons, the charge exchange loss process, radial and azimuthal transports, and (2) for electrons, the deceleration of electrons by the free and bounded electrons of the medium, pitch angle diffusion by Coulomb and wave-particle interactions, radial and azimuthal transport. This model allows for simulation of a magnetic storm effects by increasing convective electric field and injecting particles with keV range energies in the nightside region. Particles in the energy range 50 – 100 keV are “created” by acceleration of slower particles in the L = 4 region. Four hours are needed for ring current formation. The calculated particle distribution at 6.6 Earth radii as well as at low altitude are in good agreement with those deduced from ATS 6 measurements (the drift echo is well reproduced at this altitude) and from statistical studies of the precipitation by the DMSP satellites, respectively.

First comparisons of local ion measurements in the inner magnetosphere with energetic neutral atom magnetospheric image inversions: Cluster‐CIS and IMAGE‐HENA observations

[1]xa0Data provided by the CIS (Cluster Ion Spectrometry) instruments on board the Cluster spacecraft are used to survey recent crossings of the inner magnetosphere and ring current. CIS is capable of obtaining full three-dimensional ion distributions (about 0 to 40 keV/q) with one spacecraft spin time resolution and with mass-per-charge composition determination. Events are selected for which the Cluster spacecraft are within the field of view of the HENA (high-energy neutral atom) imager on board IMAGE. HENA provides energetic neutral atom images with a high geometric factor and with a 120° × 360° field of view over the spin. The H+ ion distribution functions obtained in situ by CIS are then compared to the ones deduced by inverting the HENA hydrogen neutral atom images for the overlapping energy range of the two instruments (27–39 keV). This analysis concerns events obtained both during well-developed ring current conditions (e.g., 18 April 2002 event) and during quiet magnetospheric conditions (e.g., 9 August 2001 event). The results show the consistency between the ion fluxes deduced from energetic neutral atom (ENA) image inversions and the fluxes measured locally. They thus show the complementarity of the two approaches. The locally measured fluxes provide the “ground truth,” and they give the detailed ion distributions. ENA images allow to situate local measurements into a global context and to position them with respect to the ring current large-scale structure. Our results also show the limitations of the ion fluxes deduced from the ENA image inversions for images taken from a single vantage point, with a substantial scatter of the inversion fluxes with respect to the in situ measured ones and a more limited dynamic range.

Ground‐based estimates of outer radiation belt energetic electron precipitation fluxes into the atmosphere

[1] AARDDVARK data from a radio wave receiver in Sodankyla, Finland have been used to monitor transmissions across the auroral oval and just into the polar cap from the very low frequency communications transmitter, call sign NAA (24.0 kHz, 44°N, 67°W, L = 2.9), in Maine, USA, since 2004. The transmissions are influenced by outer radiation belt (L = 3–7) energetic electron precipitation. In this study, we have been able to show that the observed transmission amplitude variations can be used to determine routinely the flux of energetic electrons entering the upper atmosphere along the total path and between 30 and 90 km. Our analysis of the NAA observations shows that electron precipitation fluxes can vary by 3 orders of magnitude during geomagnetic storms. Typically when averaging over L = 3–7 we find that the >100 keV POES trapped fluxes peak at about 10 6 el. cm −2 s −1 sr −1 during geomagnetic storms, with the DEMETER >100 keV drift loss cone showing peak fluxes of 10 5 el. cm −2 s −1 sr −1 , and both the POES >100 keV loss fluxes and the NAA ground‐based >100 keV precipitation fluxes showing peaks of ∼10 4 el. cm −2 s −1 sr −1. During a geomagnetic storm in July 2005, there were systematic MLT variations in the fluxes observed: electron precipitation flux in the midnight sector (22–06 MLT) exceeded the fluxes from the morning side (0330–1130 MLT) and also from the afternoon sector (1130–1930 MLT). The analysis of NAA amplitude variability has the potential of providing a detailed, near real‐time, picture of energetic electron precipitation fluxes from the outer radiation belts.

The Solar Origin of Small Interplanetary Transients

In this paper, we present evidence for magnetic transients with small radial extents ranging from 0.025 to 0.118 AU measured in situ by the Solar-Terrestrial Relations Observatory (STEREO) and the near-Earth Advanced Composition Explorer (ACE) and Wind spacecraft. The transients considered in this study are much smaller (<0.12 AU) than the typical sizes of magnetic clouds measured near 1 AU ({approx}0.23 AU). They are marked by low plasma beta values, generally lower magnetic field variance, short timescale magnetic field rotations, and are all entrained by high-speed streams by the time they reach 1 AU. We use this entrainment to trace the origin of these small interplanetary transients in coronagraph images. We demonstrate that these magnetic field structures originate as either small or large mass ejecta. The small mass ejecta often appear from the tip of helmet streamers as arch-like structures and other poorly defined white-light features (the so-called blobs). However, we have found a case of a small magnetic transient tracing back to a small and narrow mass ejection erupting from below helmet streamers. Surprisingly, one of the small magnetic structures traces back to a large mass ejection; in this case, we show that the central axis of the coronalmorexa0» mass ejection is along a different latitude and longitude to that of the in situ spacecraft. The small size of the transient is related to the in situ measurements being taken on the edges or periphery of a larger magnetic structure. In the last part of the paper, an ejection with an arch-like aspect is tracked continuously to 1 AU in the STEREO images. The associated in situ signature is not that of a magnetic field rotation but rather of a temporary reversal of the magnetic field direction. Due to its open-field topology, we speculate that this structure is partly formed near helmet streamers due to reconnection between closed and open magnetic field lines. The implications of these observations for our understanding of the variability of the slow solar wind are discussed.«xa0less

Spatial‐Temporal characteristics of ion beamlets in the plasma sheet boundary layer of magnetotail

[1]xa0The processes of nonadiabatic ion acceleration occurring in the vicinity of magnetic neutral lines produce highly accelerated (up to 2500 km/s) field-aligned ion beams (beamlets) with transient appearance streaming earthward in the plasma sheet boundary layer (PSBL) of the Earth’s magnetotail. Previous studies of these phenomena based on single spacecraft (s/c) missions supported the view that beamlets are temporal transients, since the typical time of a beamlet observation at a given s/c very rarely exceeds ∼1–2 min. Now multipoint Cluster observations have led to a new understanding of these phenomena with a spatial rather than a temporal structure. On the basis of 3-year Cluster measurements made in the PSBL, we present statistical evaluation of the beamlet duration (at least 5–15 min) and confirm well-manifested localization of the beamlet along Z and in some cases along Y directions, i.e., approximately across the lobe magnetic field. Earlier results reporting shorter beamlet observations could be understood by invoking not only PSBL flapping motions but also of an additional effect revealed by Cluster: earthward propagation of kink-like perturbations along the beamlet filaments. Phase velocity of these perturbations is of the order of the local Alfven velocity (V ∼ 600–1400 km/s) and related fast flappings of localized beamlet structures in the Y-Z direction significantly decreasing the time of their observation at a given spacecraft. Multipoint observations of beamlets revealed that they represent long-living (∼5–15 min) plasma filaments elongated along the lobe magnetic field (∼60–100RE) and strongly localized in direction perpendicular to the PSBL-lobe boundary (∼0.2–0.7RE). In some cases, it was also possible to estimate the width of beamlet in dawn-dusk direction which was of the order of fractions of RE.

Radiation belt electron precipitation due to geomagnetic storms: Significance to middle atmosphere ozone chemistry

[1] Geomagnetic storms triggered by coronal mass ejections and high‐speed solar wind streams can lead to enhanced losses of energetic electrons from the radiation belts into the atmosphere, both during the storm itself and also through the poststorm relaxation of enhanced radiation belt fluxes. In this study we have analyzed the impact of electron precipitation on atmospheric chemistry (30–90 km altitudes) as a result of a single geomagnetic storm. The study conditions were chosen such that there was no influence of solar proton precipitation, and thus we were able to determine the storm‐induced outer radiation belt electron precipitation fluxes. We use ground‐based subionospheric radio wave observations to infer the electron precipitation fluxes at L = 3.2 during a geomagnetic disturbance which occurred in September 2005. Through application of the Sodankyla Ion and Neutral Chemistry model, we examine the significance of this particular period of electron precipitation to neutral atmospheric chemistry. Building on an earlier study, we refine the quantification of the electron precipitation flux into the atmosphere by using a time‐varying energy spectrum determined from the DEMETER satellite. We show that the large increases in odd nitrogen (NO x) and odd hydrogen (HO x) caused by the electron precipitation do not lead to significant in situ ozone depletion in September in the Northern Hemisphere. However, had the same precipitation been deposited into the polar winter atmosphere, it would have led to >20% in situ decreases in O 3 at 65–80 km altitudes through catalytic HO x cycles, with possible additional stratospheric O 3 depletion from descending NO x beyond the model simulation period. Citation: Rodger, C. J., M. A. Clilverd, A. Seppala, N. R. Thomson, R. J. Gamble, M. Parrot, J.‐A. Sauvaud, and T. Ulich (2010), Radiation belt electron precipitation due to geomagnetic storms: Significance to middle atmosphere ozone chemistry,

Determining the spectra of radiation belt electron losses: Fitting DEMETER electron flux observations for typical and storm times

[1]xa0The energy spectra of energetic electron precipitation from the radiation belts are studied in order to improve our understanding of the influence of radiation belt processes. The Detection of Electromagnetic Emissions Transmitted from Earthquake Regions (DEMETER) microsatellite electron flux instrument is comparatively unusual in that it has very high energy resolution (128 channels with 17.9 keV widths in normal survey mode), which lends itself to this type of spectral analysis. Here electron spectra from DEMETER have been analyzed from all six years of its operation, and three fit types (power law, exponential, and kappa-type) have been applied to the precipitating flux observations. We show that the power law fit consistently provides the best representation of the flux and that the kappa-type is rarely valid. We also provide estimated uncertainties in the flux for this instrument as a function of energy. Average power law gradients for nontrapped particles have been determined for geomagnetically nondisturbed periods to get a typical global behavior of the spectra in the inner radiation belt, slot region, and outer radiation belt. Power law spectral gradients in the outer belt are typically −2.5 during quiet periods, changing to a softer spectrum of ∼−3.5 during geomagnetic storms. The inner belt does the opposite, hardening from −4 during quiet times to ∼−3 during storms. Typical outer belt e-folding values are ∼200 keV, dropping to ∼150 keV during geomagnetic storms, while the inner belt e-folding values change from ∼120 keV to >200 keV. Analysis of geomagnetic storm periods show that the precipitating flux enhancements evident from such storms take approximately 13 days to return to normal values for the outer belt and slot region and approximately 10 days for the inner belt.