A case study of electron precipitation fluxes due to plasmaspheric hiss (original) (raw)
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Radiation belt electron precipitation into the atmosphere: Recovery from a geomagnetic storm
Journal of Geophysical Research, 2007
1] Large geomagnetic storms are associated with electron population changes in the outer radiation belt and the slot region, often leading to significant increases in the relativistic electron population. The increased population decays in part through the loss, that is, precipitation from the bounce loss cone, of highly energized electrons into the middle and upper atmosphere (30-90 km). However, direct satellite observations of energetic electrons in the bounce loss cone are very rare due to its small angular width. In this study we have analyzed ground-based subionospheric radio wave observations of electrons from the bounce loss cone at L = 3.2 during and after a geomagnetic disturbance which occurred in September 2005. Relativistic electron precipitation into the atmosphere leads to large changes in observed subionospheric amplitudes. Satellite-observed energy spectra from the CRRES and DEMETER spacecraft were used as an input to an ionospheric chemistry and subionospheric propagation model, describing the ionospheric ionization modifications caused by precipitating electrons. We find that the peak precipitated fluxes of >150 keV electrons into the atmosphere were 3500 ± 300 el cm À2 s À1 at midday and 185 ± 15 el cm À2 s À1 at midnight. For 6 d following the storm onset the midday precipitated fluxes are approximately 20 times larger than observed at midnight, consistent with observed day/night patterns of plasmaspheric hiss intensities. The variation in DEMETER observed wave power at L = 3.2 in the plasmaspheric hiss frequency band shows similar time variation to that seen in the precipitating particles. Consequently, plasmaspheric hiss with frequencies below $500 Hz appears to be the principal loss mechanism for energetic electrons in the inner zone of the outer radiation belts during the nonstorm time periods of this study, although off-equatorial chorus waves could contribute when the plasmapause is L < 3.0. (2007), Radiation belt electron precipitation into the atmosphere: Recovery from a geomagnetic storm,
Electron precipitation from EMIC waves: a case study from 31 May 2013
Journal of Geophysical Research: Space Physics, 2015
On 31 May 2013 several rising tone electromagnetic ion cyclotron (EMIC) waves with intervals of pulsations of diminishing periods were observed in the magnetic local time afternoon and evening sectors during the onset of a moderate/large geomagnetic storm. The waves were sequentially observed in Finland, Antarctica, and western Canada. Coincident electron precipitation by a network of ground-based Antarctic Arctic Radiation-belt Dynamic Deposition VLF Atmospheric Research Konsortia and riometer instruments, as well as the Polar-orbiting Operational Environmental Satellite (POES) electron telescopes, was also observed. At the same time, POES detected 30-80 keV proton precipitation drifting westward at locations that were consistent with the ground-based observations, indicating substorm injection. Through detailed modeling of the combination of ground and satellite observations, the characteristics of the EMIC-induced electron precipitation were identified as latitudinal width of 2-3°or ΔL = 1 R e , longitudinal width 50°or 3 h magnetic local time, lower cutoff energy 280 keV, typical flux 1 × 10 4 el cm À2 sr À1 s À1 > 300 keV. The lower cutoff energy of the most clearly defined EMIC rising tone in this study confirms the identification of a class of EMIC-induced precipitation events with unexpectedly low-energy cutoffs of <400 keV. Energetic electron precipitation has been associated with a subset of EMIC waves defined as intervals of pulsations with diminishing periods (IPDP). IPDP are observed in the evening sector during geomagnetically disturbed periods [Yahnina et al., 2003, and references therein]. Yahnina et al. [2003] showed that the IPDP generation mechanism operates when newly injected protons drift westward, meeting a boundary of the dense plasmasphere such as the plasmapause or the plasmaspheric bulge region. The IPDP events were preceded by the injections of energetic protons (~100 keV) and were thus found to be related to substorm activity. The duration of IPDP events is typically shorter than other Pc1 wave types, with the duration being a few tens of minutes. NOAA Polar-orbiting Operational Environmental Satellite (POES) Space Environment Monitor-1 (SEM-1) satellite observations of precipitating electrons from EMIC-IPDP waves showed enhanced fluxes in the >30 keV channel [Yahnina et al., 2003], although we note that in an integral channel, this may be caused by energies significantly higher than CLILVERD ET AL.
High-resolution In-situ Observations of Electron Precipitation-Causing EMIC Waves
Geophysical Research Letters, 2015
Electromagnetic ion cyclotron (EMIC) waves are thought to be important drivers of energetic electron losses from the outer radiation belt through precipitation into the atmosphere. While the theoretical possibility of pitch angle scattering-driven losses from these waves has been recognized for more than four decades, there have been limited experimental precipitation observations to support this concept. We have combined satellite-based observations of the characteristics of EMIC waves, with satellite and ground-based observations of the EMIC-induced electron precipitation. In a detailed case study, supplemented by an additional four examples, we are able to constrain for the first time the location, size, and energy range of EMIC-induced electron precipitation inferred from coincident precipitation data and relate them to the EMIC wave frequency, wave power, and ion band of the wave as measured in situ by the Van Allen Probes. These observations will better constrain modeling into the importance of EMIC wave-particle interactions. However, there are many examples in the literature where EMIC waves are observed on the ground or in space for which there appears to be no electron precipitation occurring, even when the measurements are available [e.g., Usanova et al., 2014; Engebretson et al., 2015]. There is also a growing recent experimental evidence which suggests that EMIC waves may precipitate electrons with energies as low as a few hundred RODGER ET AL.
Journal of Geophysical Research: Space Physics
A detailed comparison is undertaken of the energetic electron spectra and fluxes of two precipitation events that were observed in 18/19 January 2013. A novel but powerful technique of combining simultaneous ground-based subionospheric radio wave data and riometer absorption measurements with X-ray fluxes from a Balloon Array for Relativistic Radiation-belt Electron Losses (BARREL) balloon is used for the first time as an example of the analysis procedure. The two precipitation events are observed by all three instruments, and the relative timing is used to provide information/insight into the spatial extent and evolution of the precipitation regions. The two regions were found to be moving westward with drift periods of 5-11 h and with longitudinal dimensions of~20°and~70°(1.5-3.5 h of magnetic local time). The electron precipitation spectra during the events can be best represented by a peaked energy spectrum, with the peak in flux occurring at~1-1.2 MeV. This suggests that the radiation belt loss mechanism occurring is an energy-selective process, rather than one that precipitates the ambient trapped population. The motion, size, and energy spectra of the patches are consistent with electromagnetic ion cyclotron-induced electron precipitation driven by injected 10-100 keV protons. Radio wave modeling calculations applying the balloon-based fluxes were used for the first time and successfully reproduced the ground-based subionospheric radio wave and riometer observations, thus finding strong agreement between the observations and the BARREL measurements.
Radiation belt electron precipitation due to VLF transmitters: Satellite observations
Geophysical Research Letters, 2008
1] In the Earth's inner magnetosphere, the distribution of energetic electrons is controlled by pitch-angle scattering by waves. A category of Whistler waves originates from powerful ground-based VLF transmitter signals in the frequency range 10 -25 kHz. These transmissions are observed in space as waves of very narrow bandwidth.
Journal of Geophysical Research, 2006
1] We analyze wave and particle data from the CRRES satellite to determine the variability of plasmaspheric hiss (0.1 < f < 2 kHz) with respect to substorm activity as measured by AE*, defined as the maximum value of the AE index in the previous 3 hours. The study is relevant to modeling the acceleration and loss of relativistic electrons during storms and understanding the origin of the waves. The plasmaspheric hiss amplitudes depend on spatial location and susbtorm activity, with the largest waves being observed during high levels of substorm activity. Our survey of the global distribution of hiss indicates a strong day-night asymmetry with two distinct latitudinal zones of peak wave activity primarily on the dayside. Equatorial hiss (jl m j < 15°) is strongest during active conditions (AE* > 500 nT), with an average amplitude of 40 ± 1 pT observed in the region 2 < L < 4 from 0600 to 2100 MLT. Midlatitude (jl m j > 15°) hiss is strongest during active conditions with an average amplitude of 47 ± 2 pT in the region 2 < L < 4 from 0800 to 1800 MLT but extending out beyond L = 6 from 1200 to 1500 MLT. Equatorial hiss at 600 Hz has minimum cyclotron resonant energies ranging from 20keVatL=6to20 keV at L = 6 to 20keVatL=6to1 MeV at L = 2, whereas midlatitude hiss at 600 Hz has minimum resonant energies ranging from 50keVatL=6to50 keV at L = 6 to 50keVatL=6to2 MeV at L = 2. The enhanced equatorial and midlatitude hiss emissions are associated with electron flux enhancements in the energy range of tens to hundreds of keV, suggesting that these electrons are the most likely source of plasmaspheric hiss. The enhanced levels of plasmaspheric hiss during substorm activity will lead to increased pitch-angle scattering of energetic electrons and may play an important role in relativistic electron dynamics during storms.
Energetic electron precipitation from the outer radiation belt during geomagnetic storms
Geophysical Research Letters, 2009
Relativistic electron precipitation changes the chemistry of the upper atmosphere and depletes ozone, but the spatial and temporal distributions are poorly known. Here we survey more than 9 years of data from low altitude satellites for different phases of geomagnetic storms. We find that for the outer radiation belt, electron precipitation >300 keV peaks during the main phase of storms whereas that >1 MeV peaks during the recovery phase. Precipitation >300 keV can occur at all geographic longitudes in both hemispheres whereas that >1 MeV occurs mainly poleward of the South Atlantic anomaly (SAA) region. The data suggest that wave‐particle interactions are strong enough to precipitate >300 keV electrons into the bounce loss cone, but precipitate >1 MeV electrons into the drift loss cone. We find that whistler mode chorus waves alone cannot account for the higher MeV precipitation flux during the recovery phase. We suggest that whistler mode chorus waves accelerate ...
Journal of Geophysical Research: Space Physics, 2014
Using five spacecraft in geosynchronous orbit, plasmaspheric drainage plumes are located in the dayside magnetosphere and the measured pitch angle anisotropies of radiation belt electrons are compared duskward and dawnward of the plumes. Two hundred twenty-six plume crossings are analyzed. It is found that the radiation belt anisotropy is systematically greater dawnward of plumes (before the electrons cross the plumes) than it is duskward of plumes (after the electrons have crossed the plumes). This change in anisotropy is attributed to pitch angle scattering of the radiation belt electrons during their passage through the plumes. A test database in the absence of plumes finds no equivalent change in the radiation belt anisotropy. The amount of pitch angle scattering by the plume is quantified, scattering times are estimated, and effective pitch angle diffusion coefficients within the plume are estimated. The pitch angle diffusion coefficients obtained from the scattering measurements are of the same magnitude as expected values for electromagnetic ion cyclotron (EMIC) waves at high electron energies (1.5 MeV); however, expected EMIC diffusion coefficients do not extend to pitch angles of 90°and would have difficulties explaining the observed isotropization of electrons. The pitch angle diffusion coefficients obtained from the scattering measurements are of the same magnitude as expected values for whistler mode hiss at lower electron energies (150 keV). Outward radial transport of the radiation belt caused by the pitch angle scattering in the plume is discussed.
Longitudinal differences in electron precipitation near L = 4
Journal of Geophysical Research, 1988
The differences in the electron precipitation characteristics as seen above Siple Station, Antarctica, and the Kerguelen Islands have been studied. These two sites are both in the southern hemisphere at nearly the same magnetic latitude (L=4). The two stations are at longitudes that place them roughly equal distances east and west of the center of the South Atlantic magnetic anomaly at this L value. With respect to the direction of electron drift, Siple is upstream of the anomaly, and Kerguelen is downstream. The primary data used in the study were counting rates from rocket-borne, parachutedeployed, sodium iodide scintillation counters at altitudes of 80 to 30 km and VLF wave data from both ground-based and rocket-borne receivers. Nine rocket flights are involved, five from Kerguelen and four from Siple. The flights were made under a range of geomagnetic and VLF activity conditions. The data differ in two major respects. First, the precipitation background at Kerguelen is very low, with high levels of wave activity being required to produce any detectable precipitation, an observation which indicates that there is a range of longitude and/or magnetic local time in which the electron pitch angle diffusion rate is 2 orders of magnitude lower than the long-term global average rate. Second, X ray microbursts were found to be essentially absent at Kerguelen and very common at Siple. This observation appears to support models of the microburst generation process Which predict maximum pitch angle scatterings of only a few tenths of a degree.