Statistical pitch angle properties of substorm-injected electron clouds and their relation to dawnside energetic electron precipitation (original) (raw)
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Medium energy pitch angle distribution during substorm injected electron clouds
Geophysical Research Letters, 2005
An investigation of pitch angle resolved electron data of energy <47 keV made by the MPA instrument on LANL geosynchronous satellites reveals a signature of symmetric peaks in the pitch angle distributions, observed to expand from being nearly at perpendicular pitch angles ($90°) to nearly field aligned at lower energies. This feature is seen in almost every interval of increased fluxes at geosynchronous orbit. A closer examination reveals the expansion of the peak in pitch angle to be such that the corresponding parallel velocity (along B) is constant. In some time intervals there are simultaneous symmetric peaks in more than one energy band and the parallel energy corresponding to the symmetric peaks is seen to vary as function of time. The observations might be explained by particles interacting with whistler mode chorus through Landau resonance, diffusing particles with parallel velocity near the phase velocity of the waves.
Substorm-induced energetic electron precipitation: Morphology and prediction
Journal of Geophysical Research: Space Physics, 2015
The injection, and subsequent precipitation, of 20 to 300 keV electrons during substorms is modeled using parameters of a typical substorm found in the literature. When combined with onset timing from, for example, the SuperMAG substorm database, or the Minimal Substorm Model, it may be used to calculate substorm contributions to energetic electron precipitation in atmospheric chemistry and climate models. Here the results are compared to ground-based data from the Imaging Riometer for Ionospheric Studies riometer in Kilpisjärvi, Finland, and the narrowband subionospheric VLF receiver at Sodankylä, Finland. Qualitatively, the model reproduces the observations well when only onset timing from the SuperMAG network of magnetometers is used as an input and is capable of reproducing all four categories of substorm associated riometer spike events. The results suggest that the different types of spike event are the same phenomena observed at different locations, with each type emerging from the model results at a different local time, relative to the center of the injection region. The model's ability to reproduce the morphology of spike events more accurately than previous models is attributed to the injection of energetic electrons being concentrated specifically in the regions undergoing dipolarization, instead of uniformly across a single-injection region.
Global-scale electron precipitation features seen in UV and X rays during substorms
Journal of Geophysical Research, 1999
The Polar Ionospheric X-ray Imaging Experiment (PIXIE) and the ultraviolet imager (UVI) onboard the Polar satellite have provided the first simultaneous global-scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung and the auroral ultraviolet (UV) emissions. While the UV images respond to the total electron energy flux, which is usually dominated by electron energies below 10 keV, the PIXIE, 9.9-19.7 keV X-ray images used in this study respond only to electrons of energy above 10 keV. Previous studies by ground-based, balloon, and space observations have indicated that the patterns of energetic electron precipitation differ significantly from those found in the visible and the UV auroral oval. Because of the lack of global imaging of the energetic electron precipitation, one has not been able to establish a complete picture. In this study the development of the electron precipitation during the different phases of magnetospheric substorms is examined. Comparisons are made between the precipitation patterns of the high-energy (PIXIE) and low-energy (UVI) electron populations, correlated with ground-based observations and geosynchronous satellite data. We focus on one specific common feature in the energetic precipitation seen in almost every isolated substorm observed by PIXIE during 1996 and which differs significantly from what is seen in the UV images. Delayed relative to substorm onsets, we observe a localized maximum of X-ray emission at 5-9 magnetic local time. By identifying the location of the injection region and determining the substorm onset time it is found that this maximum most probably is caused by electrons injected in the midnight sector drifting (i.e., gradient and curvature drift) into a region in the dawnside magnetosphere where some mechanism effectively scatters the electrons into the loss cone.
Global Scale Electron Precipitation during Substorm Expansions
Astrophysics and Space Science Library, 1998
The Polar Ionospheric X-ray Imaging Experiment (PIXIE) and the Ultraviolet Imager (UVI) on the POLAR satellite have provided the first simultaneous global scale views of the patterns of electron precipitation through imaging of the atmospheric X-ray bremsstrahlung and the auroral UV emissions. While UVI responds to the total electron energy input, PIXIE responds only to the high energy (multi-keV) electron precipitation. During the substorm expansion phase, clear time delays occur between the electron injection at the nightside and the start of the precipitation on the morning/dayside. The observations are generally consistent with patterns previously deduced from ground-based and suborbital observations.
Journal of Geophysical Research, 2008
1] Geosynchronous Los Alamos National Laboratory (LANL-97A) satellite particle data, riometer data, and radio wave data recorded at high geomagnetic latitudes in the region south of Australia and New Zealand are used to perform the first complete modeling study of the effect of substorm electron precipitation fluxes on low-frequency radio wave propagation conditions associated with dispersionless substorm injection events. We find that the precipitated electron energy spectrum is consistent with an e-folding energy of 50 keV for energies <400 keV but also contains higher fluxes of electrons from 400 to 2000 keV. To reproduce the peak subionospheric radio wave absorption signatures seen at Casey (Australian Antarctic Division), and the peak riometer absorption observed at Macquarie Island, requires the precipitation of 50-90% of the peak fluxes observed by LANL-97A. Additionally, there is a concurrent and previously unreported substorm signature at L < 2.8, observed as a substorm-associated phase advance on radio waves propagating between Australia and New Zealand. Two mechanisms are discussed to explain the phase advances. We find that the most likely mechanism is the triggering of wave-induced electron precipitation caused by waves enhanced in the plasmasphere during the substorm and that either plasmaspheric hiss waves or electromagnetic ion cyclotron waves are a potential source capable of precipitating the type of high-energy electron spectrum required. However, the presence of these waves at such low L shells has not been confirmed in this study. Citation: Clilverd, M. A., et al. (2008), Energetic electron precipitation during substorm injection events: High-latitude fluxes and an unexpected midlatitude signature,
Storm-Time Evolution of Energetic Electron Pitch Angle Distributions by Wave-Particle Interaction
Plasma Science and Technology, 2008
The quasi-pure pitch-angle scattering of energetic electrons driven by field-aligned propagating whistler mode waves during the 9∼15 October 1990 magnetic storm at L ≈ 3 ∼ 4 is studied, and numerical calculations for energetic electrons in gyroresonance with a band of frequency of whistler mode waves distributed over a standard Gaussian spectrum is performed. It is found that the whistler mode waves can efficiently drive energetic electrons from the larger pitchangles into the loss cone, and lead to a flat-top distribution during the main phase of geomagnetic storms. This result perhaps presents a feasible interpretation for observation of time evolution of the quasi-isotropic pitch-angle distribution by Combined Release and Radiation Effects Satellite (CRRES) spacecraft at L ≈ 3 ∼ 4.
Annales Geophysicae, 2006
Using the data from satellite CRRES and three geostationary LANL spacecraft, the propagation of an electron cloud from midnight to the evening sector is investigated. An electron cloud was injected during a weak isolated substorm that developed on a quiet geomagnetic background. It is found that within the local time sector from 03:00 until at least 08:00 MLT, the propagation of electrons at perpendicular pitch-angles is well described by a simple model of drift in the dipole magnetic field. The flux levels in the field-aligned electrons increase simultaneously with the flux at perpendicular pitch angles, which is attributed to the pitch angle diffusion by the whistler mode. This pitch-angle diffusion leads to precipitation of electrons from a drifting cloud and an increase in the ionospheric electron density, simultaneously observed above Tromsø, Norway, by the EISCAT UHF radar in the morning sector (04:40-05:25 MLT). The precipitation develops as quasi-periodic pulses with a period of about 100 s. We discuss the models of pulsating precipitation due to the whistler cyclotron instability and show that our observations can be explained by such a model.
Journal of Geophysical Research Space Physics, 2009
In this study, we report dynamic evolutions of 30-500 keV energetic electrons (in flux and pitch-angle distribution) in the radiation belt region with 1.6 < L < 6.2. Those evolutions were observed by the IES instrument on board the Polar spacecraft during the Halloween storm period on October 31, 2003 when the radiation belt was strongly distorted. This injection of energetic electrons into the slot region may be associated with the plasmapause movement and Hiss/Chorus enhancement. This flux enhancement is possibly associated with convective transport from the plasma sheet, enhanced radial diffusion and local wave-particle interaction acceleration. By adopting a fitting parameter of loss time τ L we solved the bounce-averaged pitch angle diffusion equation driven by field-aligned whistler-mode waves (including chorus and hiss). We show that pitch-angle scattering can account for the pitch-angle distribution evolution in 30-500 keV electrons in the innermost radiation belt near L = 1.7 (as observed by Polar satellite) and the slot region 2 < L < 3. In particular, simulated results indicate that the loss-cone region is almost empty, and outside the loss-cone region both flux and anisotropy of energetic electrons are reduced with the gyroresonant time. The obtained time scale for the pitch-angle distribution evolution is found to be approximately tens of hours, consistent with observation.
Journal of Geophysical Research, 2002
Observations from the Polar Ionospheric X-ray Imaging Experiment (PIXIE) and the Ultraviolet Imager (UVI) on board the Polar satellite have been used to derive the total energy dissipation (U A) in the Northern Hemisphere by electron precipitation in the energy range from 100 eV to 100 keV. Comparing with geomagnetic indices, we find that during substorms, U A is linearly related to the quick look AE QL 1/2 and AL QL 1/2 indices. The best correlation (0.86) is found between the energy flux above 10 keVand the AL QL index, which reflects that the energetic electron precipitation modulates the westward electrojet intensity by affecting the Hall conductance. The AU QL index which reflects the eastward electrojet intensity shows poor correlation with U A , either for soft or energetic electrons. This is consistent with an electric field dominance in the dusk sector and a minor role for auroral conductance in the eastward electrojet. On the basis of ionospheric electrodynamics, we argue that a nonlinear relation between U A and AE (and AL) is more appropriate than a linear relation. We show that the linear relations reported by others do not fit our data set and that they provide U A values which are too low. For the total electron energy flux (0.1-100 keV) we find the best fit to be U A [GW] = 4.4 AL QL 1/2 À 7.6, with a correlation coefficient of 0.83, which is slightly better than that for the AE QL index (0.77). By time-integrating the U A derived from AL QL , we obtain estimates of the total deposited energy during substorms within ±20% of the time-integrated U A derived from UV and X rays.