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Found 1225 entries in the Bibliography.
Showing entries from 1201 through 1225
| 2003 |
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We perform a survey of the plasma wave and particle data from the CRRES satellite during 26 geomagnetically disturbed periods to investigate the viability of a local stochastic electron acceleration mechanism to relativistic energies driven by Doppler-shifted cyclotron resonant interactions with whistler mode chorus. Relativistic electron flux enhancements associated with moderate or strong storms may be seen over the whole outer zone (3 < L < 7), typically peaking in the range 4 < L < 5, whereas those associated with weak storms and intervals of prolonged substorm activity lacking a magnetic storm signature (PSALMSS) are typically observed further out in the regions 4 < L < 7 and 4.5 < L < 7, respectively. The most significant relativistic electron flux enhancements are seen outside of the plasmapause and are associated with periods of prolonged substorm activity with AE greater than 100 nT for a total integrated time greater than 2 days or greater than 300 nT for a total integrated time greater than 0.7 days. These events are also associated with enhanced fluxes of seed electrons and enhanced lower-band chorus wave power with integrated lower-band chorus wave intensities of greater than 500 pT2 day. No significant flux enhancements are seen unless the level of substorm activity is sufficiently high. These results are consistent with a local, stochastic, chorus-driven electron acceleration mechanism involving the energization of a seed population of electrons with energies of a few hundred keV to relativistic energies operating on a timescale of the order of days. Meredith, Nigel; Cain, Michelle; Horne, Richard; Thorne, Richard; Summers, D.; Anderson, Roger; Published by: Journal of Geophysical Research Published on: 06/2003 YEAR: 2003 DOI: 10.1029/2002JA009764 |
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We perform a survey of the plasma wave and particle data from the CRRES satellite during 26 geomagnetically disturbed periods to investigate the viability of a local stochastic electron acceleration mechanism to relativistic energies driven by Doppler-shifted cyclotron resonant interactions with whistler mode chorus. Relativistic electron flux enhancements associated with moderate or strong storms may be seen over the whole outer zone (3 < L < 7), typically peaking in the range 4 < L < 5, whereas those associated with weak storms and intervals of prolonged substorm activity lacking a magnetic storm signature (PSALMSS) are typically observed further out in the regions 4 < L < 7 and 4.5 < L < 7, respectively. The most significant relativistic electron flux enhancements are seen outside of the plasmapause and are associated with periods of prolonged substorm activity with AE greater than 100 nT for a total integrated time greater than 2 days or greater than 300 nT for a total integrated time greater than 0.7 days. These events are also associated with enhanced fluxes of seed electrons and enhanced lower-band chorus wave power with integrated lower-band chorus wave intensities of greater than 500 pT2 day. No significant flux enhancements are seen unless the level of substorm activity is sufficiently high. These results are consistent with a local, stochastic, chorus-driven electron acceleration mechanism involving the energization of a seed population of electrons with energies of a few hundred keV to relativistic energies operating on a timescale of the order of days. Meredith, Nigel; Cain, Michelle; Horne, Richard; Thorne, Richard; Summers, D.; Anderson, Roger; Published by: Journal of Geophysical Research Published on: 06/2003 YEAR: 2003 DOI: 10.1029/2002JA009764 |
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Electromagnetic ion cyclotron (EMIC) waves which propagate at frequencies below the proton gyrofrequency can undergo cyclotron resonant interactions with relativistic electrons in the outer radiation belt and cause pitch-angle scattering and electron loss to the atmosphere. Typical storm-time wave amplitudes of 1\textendash10 nT cause strong diffusion scattering which may lead to significant relativistic electron loss at energies above the minimum energy for resonance, Emin. A statistical analysis of over 800 EMIC wave events observed on the CRRES spacecraft is performed to establish whether scattering can occur at geophysically interesting energies (<=2 MeV). While Emin is well above 2 MeV for the majority of these events, it can fall below 2 MeV in localized regions of high plasma density and/or low magnetic field (fpe/fce,eq > 10) for wave frequencies just below the hydrogen or helium ion gyrofrequencies. These lower energy scattering events, which are mainly associated with resonant L-mode waves, are found within the magnetic local time range 1300 < MLT < 1800 for L > 4.5. The average wave spectral intensity of these events (4\textendash5 nT2/Hz) is sufficient to cause strong diffusion scattering. The spatial confinement of these events, together with the limited set of these waves that resonate with <=2 MeV electrons, suggest that these electrons are only subject to strong scattering over a small fraction of their drift orbit. Consequently, drift-averaged scattering lifetimes are expected to lie in the range of several hours to a day. EMIC wave scattering should therefore significantly affect relativistic electron dynamics during a storm. The waves that resonate with the \~MeV electrons are produced by low-energy (\~keV) ring current protons, which are expected to be injected into the inner magnetosphere during enhanced convection events. Published by: Journal of Geophysical Research Published on: 06/2003 YEAR: 2003 DOI: 10.1029/2002JA009700 |
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[1] During magnetic storms, relativistic electrons execute nearly circular orbits about the Earth and traverse a spatially confined zone within the duskside plasmapause where electromagnetic ion cyclotron (EMIC) waves are preferentially excited. We examine the mechanism of electron pitch-angle diffusion by gyroresonant interaction with EMIC waves as a cause of relativistic electron precipitation loss from the outer radiation belt. Detailed calculations are carried out of electron cyclotron resonant pitch-angle diffusion coefficients Dαα for EMIC waves in a multi-ion (H+, He+, O+) plasma. A simple functional form for Dαα is used, based on quasi-linear theory that is valid for parallel-propagating, small-amplitude electromagnetic waves of general spectral density. For typical observed EMIC wave amplitudes (1\textendash10nT), the rates of resonant pitch-angle diffusion are close to the limit of \textquotedblleftstrong\textquotedblright diffusion, leading to intense electron precipitation. In order for gyroresonance to take place, electrons must possess a minimum kinetic energy Emin which depends on the value of the ratio (electron plasma frequency/electron gyrofrequency); Emin also depends on the properties of the EMIC wave spectrum and the ion composition. Geophysically interesting scattering, with Emin comparable to 1 MeV, can only occur in regions where (electron plasma frequency/electron gyrofrequency) >=10, which typically occurs within the duskside plasmapause. Under such conditions, electrons with energy >=1 MeV can be removed from the outer radiation belt by EMIC wave scattering during a magnetic storm over a time-scale of several hours to a day. Published by: Journal of Geophysical Research Published on: 04/2003 YEAR: 2003 DOI: 10.1029/2002JA009489 |
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Using the data from the NOAA and Exos-D satellites during the 3 November 1993 magnetic storm, the dynamic behavior of electrons with energies from a few tens of kiloelectronvolts to a few and its relation to plasma waves were examined. After the late main phase, relativistic electron flux started to recover from the heart of the outer radiation belt, where the cold plasma density was extremely low, and intense whistler mode chorus emissions were detected. The phase space density showed a peak in the outer belt, and the peak increased gradually. The simulation of the inward radial diffusion process could not reproduce the observed energy spectrum and phase space density variation. On the other hand, the simulated energy diffusion due to the gyroresonant electron-whistler mode wave interactions, under the assumption of the Kolmogorov turbulence spectrum, could generate the relativistic electrons without the flux transport from the outer region. The present study suggested that the seed population of relativistic electrons, which appeared in the heart of the outer radiation belt during the late main phase, was the ring current electrons injected from the plasma sheet, and that the acceleration by whistler mode chorus via gyroresonant wave\textendashparticle interactions outside the plasmapause could play an important role to generate the relativistic electrons. Published by: Journal of Geophysical Research Published on: 03/2003 YEAR: 2003 DOI: 10.1029/2001JA007542 |
| 2002 |
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X-ray observations of MeV electron precipitation with a balloon-borne germanium spectrometer The high-resolution germanium detector aboard the MAXIS (MeV Auroral X-ray Imaging and Spectroscopy) balloon payload detected nine X-ray bursts with significant flux extending above 0.5 MeV during an 18 day flight over Antarctica. These minutes-to-hours-long events are characterized by an extremely flat spectrum (\~E-2) similar to the first MeV event discovered in 1996, indicating that the bulk of parent precipitating electrons is at relativistic energies. The MeV bursts were detected between magnetic latitudes 58\textdegree\textendash68\textdegree (L-values of 3.8\textendash6.7) but only in the late afternoon/dusk sectors (14:30\textendash00:00 MLT), suggesting scattering by EMIC (electromagnetic ion cyclotron) waves as a precipitation mechanism. We estimate the average flux of precipitating E >= 0.5 MeV electrons to be \~360 cm-2s-1, corresponding to about 5 \texttimes 1025 such electrons precipitated during the eight days at L = 3.8\textendash6.7, compared to \~2 \texttimes 1025 trapped 0.5\textendash3.6 MeV electrons estimated from dosimeter measurements on a GPS spacecraft. These observations show that MeV electron precipitation events are a primary loss mechanism for outer zone relativistic electrons. Published by: Geophysical Research Letters Published on: 12/2002 YEAR: 2002 DOI: 10.1029/2002GL015922 |
| 2001 |
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The great March 1991 magnetic storm and the immediately preceding solar energetic particle event (SEP) were among the largest observed during the past solar cycle, and have been the object of intense study. We investigate here, using data from eight satellites, the very large delayed buildup of relativistic electron flux in the outer zone during a 1.5-day period beginning 2 days after onset of the main phase of this storm. A notable feature of the March storm is the intense substorm activity throughout the period of the relativistic flux buildup, and the good correlation between some temporal features of the lower-energy substorm-injected electron flux and the relativistic electron flux at geosynchronous orbit. Velocity dispersion analysis of these fluxes between geosynchronous satellites near local midnight and local noon shows evidence that both classes of electrons arrive at geosynchronous nearly simultaneously within a few hours of local midnight. From this we conclude that for this storm period the substorm inductive electric field transports not only the usual (50\textendash300 keV) substorm electrons but also the relativistic (0.3 to several MeV) electrons to geosynchronous orbit. A simplified calculation of the electron ε \texttimes B and gradient/curvature drifts indicates that sufficiently strong substorm dipolarization inductive electric fields (≳ 10 mV/m) could achieve this, provided sufficient relativistic electrons are present in the source region. Consistent with this interpretation, we find that the injected relativistic electrons have a pitch angle distribution that is markedly peaked perpendicular to the magnetic field. Furthermore, the equatorial phase space density at geosynchronous orbit (L = 6.7) is greater than it is at GPS orbit at the equator (L = 4.2) throughout this buildup period, indicating that a source for the relativistic electrons lies outside geosynchronous orbit during this time. Earthward transport of the relativistic electrons by large substorm dipolarization fields, since it is unidirectional, would constitute a strong addition to the transport by radial diffusion and, when it occurs, could result in unusually strong relativistic fluxes, as is reported here for this magnetic storm. Ingraham, J.; Cayton, T.; Belian, R.; Christensen, R.; Friedel, R.; Meier, M.; Reeves, G.; Takahashi, K; Published by: Journal of Geophysical Research Published on: 11/2001 YEAR: 2001 DOI: 10.1029/2000JA000458 |
| 2000 |
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The temporal evolution of electron distributions and associated wave activity following substorm injections in the inner magnetosphere are investigated using data from the CRRES satellite. Equatorial electron distributions and concomitant wave spectra outside the plasmapause on the nightside of the Earth are studied as a function of time since injection determined from the auroral-electrojet index (AE). The electron cyclotron harmonic (ECH) wave amplitudes are shown to be very sensitive to small modeling errors in the location of the magnetic equator. They are best understood at the ECH equator, defined by the local maximum in the ECH wave activity in the vicinity of the nominal magnetic equator, suggesting that the ECH equator is a better measure of the location of the true equator. Strong ECH and whistler mode wave amplitudes are associated with the injected distributions and at the ECH equator, in the region 6.0 <= L < 7.0, exponential fits reveal wave amplitude decay time constants of 6.3\textpm1.2 and 4.6\textpm0.7 hours, respectively. Pancake electron distributions are seen to develop from injected distributions that are nearly isotropic in velocity space and, in this region, are seen to form on a similar timescale of approximately 4 hours suggesting that both wave types are involved in their production. The timescale for pancake production and wave decay is comparable with the average time interval between substorm events so that the wave-particle interactions are almost continually present in this region leading to a continual supply of electrons to power the diffuse aurora. In the region 3.8 <= L < 6.0 the timescale for wave decay at the ECH equator is 2.3 \textpm 0.6 and 1.1 \textpm 0.2 hours for ECH waves and whistler mode waves respectively, although the pancakes in this region show no clear evolution as a function of time. Meredith, Nigel; Horne, Richard; Johnstone, Alan; Anderson, Roger; Published by: Journal of Geophysical Research Published on: 06/2000 YEAR: 2000 DOI: 10.1029/2000JA900010 |
| 1998 |
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Resonant diffusion curves for electron cyclotron resonance with field-aligned electromagnetic R mode and L mode electromagnetic ion cyclotron (EMIC) waves are constructed using a fully relativistic treatment. Analytical solutions are derived for the case of a single-ion plasma, and a numerical scheme is developed for the more realistic case of a multi-ion plasma. Diffusion curves are presented, for plasma parameters representative of the Earth\textquoterights magnetosphere at locations both inside and outside the plasmapause. The results obtained indicate minimal electron energy change along the diffusion curves for resonant interaction with L mode waves. Intense storm time EMIC waves are therefore ineffective for electron stochastic acceleration, although these waves could induce rapid pitch angle scattering for ≳ 1 MeV electrons near the duskside plasmapause. In contrast, significant energy change can occur along the diffusion curves for interaction between resonant electrons and whistler (R mode) waves. The energy change is most pronounced in regions of low plasma density. This suggests that whistler mode waves could provide a viable mechanism for electron acceleration from energies near 100 keV to above 1 MeV in the region outside the plasmapause during the recovery phase of geomagnetic storms. A model is proposed to account for the observed variations in the flux and pitch angle distribution of relativistic electrons during geomagnetic storms by combining pitch angle scattering by intense EMIC waves and energy diffusion during cyclotron resonant interaction with whistler mode chorus outside the plasmasphere. Summers, D.; Thorne, Richard; Xiao, Fuliang; Published by: Journal of Geophysical Research Published on: 09/1998 YEAR: 1998 DOI: 10.1029/98JA01740 |
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Substorm electron injections: Geosynchronous observations and test particle simulations We investigate electron acceleration and the flux increases associated with energetic electron injections on the basis of geosynchronous observations and test-electron orbits in the dynamic fields of a three-dimensional MHD simulation of neutral line formation and dipolarization in the magnetotail. This complements an earlier investigation of test protons [Birn et al., 1997b]. In the present paper we consider equatorial orbits only, using the gyrocenter drift approximation. It turns out that this approximation is valid for electrons prior to and during the flux rises observed in the near tail region of the model at all energies considered (\~ 100 eV to 1 MeV). The test particle model reproduces major observed characteristics: a fast flux rise, comparable to that of the ions, and the existence of five categories of dispersionless events, typical for observations at different local times. They consist of dispersionless injections of ions or electrons without accompanying injections of the other species, delayed electron injections and delayed ion injections, and simultaneous two-species injections. As postulated from observations [Birn et al., 1997a], these categories can be attributed to a dawn-dusk displacement of the ion and electron injection boundaries in combination with an earthward motion or expansion. The simulated electron injection region extends farther toward dusk at lower energies (say, below 40 keV) than at higher energies. This explains the existence of observed energetic ion injections that are accompanied by electron flux increases at the lower energies but not by an energetic electron injection at energies above 50 keV. The simulated distributions show that flux increases are limited in energy, as observed. The reason for this limitation and for the differences between the injection regions at different energies is the localization in the dawn-dusk direction of the tail collapse and the associated cross-tail electric field, in combination with a difference in the relative importance of E \texttimes B drift and gradient drifts at different energies. The results demonstrate that the collapsing field region earthward of the neutral line appears to be more significant than the neutral line itself for the acceleration of electrons, particularly for the initial rise of the fluxes and the injection boundary. This is similar to the result obtained for test ions [Birn et al., 1997b]. Birn, J.; Thomsen, M.; Borovsky, J.; Reeves, G.; McComas, D.; Belian, R.; Hesse, M.; Published by: Journal of Geophysical Research Published on: 05/1998 YEAR: 1998 DOI: 10.1029/97JA02635 |
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Substorm electron injections: Geosynchronous observations and test particle simulations We investigate electron acceleration and the flux increases associated with energetic electron injections on the basis of geosynchronous observations and test-electron orbits in the dynamic fields of a three-dimensional MHD simulation of neutral line formation and dipolarization in the magnetotail. This complements an earlier investigation of test protons [Birn et al., 1997b]. In the present paper we consider equatorial orbits only, using the gyrocenter drift approximation. It turns out that this approximation is valid for electrons prior to and during the flux rises observed in the near tail region of the model at all energies considered (\~ 100 eV to 1 MeV). The test particle model reproduces major observed characteristics: a fast flux rise, comparable to that of the ions, and the existence of five categories of dispersionless events, typical for observations at different local times. They consist of dispersionless injections of ions or electrons without accompanying injections of the other species, delayed electron injections and delayed ion injections, and simultaneous two-species injections. As postulated from observations [Birn et al., 1997a], these categories can be attributed to a dawn-dusk displacement of the ion and electron injection boundaries in combination with an earthward motion or expansion. The simulated electron injection region extends farther toward dusk at lower energies (say, below 40 keV) than at higher energies. This explains the existence of observed energetic ion injections that are accompanied by electron flux increases at the lower energies but not by an energetic electron injection at energies above 50 keV. The simulated distributions show that flux increases are limited in energy, as observed. The reason for this limitation and for the differences between the injection regions at different energies is the localization in the dawn-dusk direction of the tail collapse and the associated cross-tail electric field, in combination with a difference in the relative importance of E \texttimes B drift and gradient drifts at different energies. The results demonstrate that the collapsing field region earthward of the neutral line appears to be more significant than the neutral line itself for the acceleration of electrons, particularly for the initial rise of the fluxes and the injection boundary. This is similar to the result obtained for test ions [Birn et al., 1997b]. Birn, J.; Thomsen, M.; Borovsky, J.; Reeves, G.; McComas, D.; Belian, R.; Hesse, M.; Published by: Journal of Geophysical Research Published on: 05/1998 YEAR: 1998 DOI: 10.1029/97JA02635 |
| 1997 |
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The disappearance and reappearance of outer zone energetic electrons during the November 3\textendash4, 1993, magnetic storm is examined utilizing data from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), the Global Positioning System (GPS) series, and the Los Alamos National Laboratory (LANL) sensors onboard geosynchronous satellites. The relativistic electron flux drops during the main phase of the magnetic storm in association with the large negative interplanetary Bz and rapid solar wind pressure increase late on November 3. Outer zone electrons with E > 3 MeV measured by SAMPEX disappear for over 12 hours at the beginning of November 4. This represents a 3 orders of magnitude decrease down to the cosmic ray background of the detector. GPS and LANL sensors show similar effects, confirming that the flux drop of the energetic electrons occurs near the magnetic equator and at all pitch angles. Enhanced electron precipitation was measured by SAMPEX at L >= 3.5. The outer zone electron fluxes then recover and exceed prestorm levels within one day of the storm onset and the inner boundary of the outer zone moves inward to smaller L (<3). These multiple-satellite measurements provide a data set which is examined in detail and used to determine the mechanisms contributing to the loss and recovery of the outer zone electron flux. The loss of the inner part of the outer zone electrons is partly due to the adiabatic effects associated with the decrease of Dst, while the loss of most of the outer part (those electrons initially at L >= 4.0) are due to either precipitation into the atmosphere or drift to the magnetopause because of the strong compression of the magnetosphere by the solar wind. The recovery of the energetic electron flux is due to the adiabatic effects associated with the increase in Dst, and at lower energies (<0.5 MeV) due to rapid radial diffusion driven by the strong magnetic activity during the recovery phase of the storm. Heating of the electrons by waves may contribute to the energization of the more energetic part (>1.0 MeV) of the outer zone electrons. Li, Xinlin; Baker, D.; Temerin, M.; Cayton, T.; Reeves, E.; Christensen, R.; Blake, J.; Looper, M.; Nakamura, R.; Kanekal, S.; Published by: Journal of Geophysical Research Published on: 01/1997 YEAR: 1997 DOI: 10.1029/97JA01101 |
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The disappearance and reappearance of outer zone energetic electrons during the November 3\textendash4, 1993, magnetic storm is examined utilizing data from the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), the Global Positioning System (GPS) series, and the Los Alamos National Laboratory (LANL) sensors onboard geosynchronous satellites. The relativistic electron flux drops during the main phase of the magnetic storm in association with the large negative interplanetary Bz and rapid solar wind pressure increase late on November 3. Outer zone electrons with E > 3 MeV measured by SAMPEX disappear for over 12 hours at the beginning of November 4. This represents a 3 orders of magnitude decrease down to the cosmic ray background of the detector. GPS and LANL sensors show similar effects, confirming that the flux drop of the energetic electrons occurs near the magnetic equator and at all pitch angles. Enhanced electron precipitation was measured by SAMPEX at L >= 3.5. The outer zone electron fluxes then recover and exceed prestorm levels within one day of the storm onset and the inner boundary of the outer zone moves inward to smaller L (<3). These multiple-satellite measurements provide a data set which is examined in detail and used to determine the mechanisms contributing to the loss and recovery of the outer zone electron flux. The loss of the inner part of the outer zone electrons is partly due to the adiabatic effects associated with the decrease of Dst, while the loss of most of the outer part (those electrons initially at L >= 4.0) are due to either precipitation into the atmosphere or drift to the magnetopause because of the strong compression of the magnetosphere by the solar wind. The recovery of the energetic electron flux is due to the adiabatic effects associated with the increase in Dst, and at lower energies (<0.5 MeV) due to rapid radial diffusion driven by the strong magnetic activity during the recovery phase of the storm. Heating of the electrons by waves may contribute to the energization of the more energetic part (>1.0 MeV) of the outer zone electrons. Li, Xinlin; Baker, D.; Temerin, M.; Cayton, T.; Reeves, E.; Christensen, R.; Blake, J.; Looper, M.; Nakamura, R.; Kanekal, S.; Published by: Journal of Geophysical Research Published on: 01/1997 YEAR: 1997 DOI: 10.1029/97JA01101 |
| 1994 |
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Electric and magnetic fields were measured by the CRRES spacecraft at an L-value of 2.2 to 2.6 near 0300 magnetic local time during a strong storm sudden commencement (SSC) on March 24, 1991. The electric field signature at the spacecraft at the time of the SSC was characterized by a large amplitude oscillation (80 mV/m peak to peak) with a period corresponding to the 150 second drift echo period of the simultaneously observed 15 MeV electrons. Considerations of previous statistical studies of the magnitude of SSC electric and magnetic fields versus local time and analysis of the energization and cross-L transport of the particles imply the existence of 200 to 300 mV/m electric fields over much of the dayside magnetosphere. These observations also suggest that the 15 MeV drift echo electrons were selectively energized because their gradient drift velocity allowed them to reside in the region of strong electric fields for the duration of the accelerating phase of the electric field. Wygant, J.; Mozer, F.; Temerin, M.; Blake, J.; Maynard, N.; Singer, H.; Smiddy, M.; Published by: Geophysical Research Letters Published on: 08/1994 YEAR: 1994 DOI: 10.1029/94GL00375 Shock-Induced Transport. Slot Refilling and Formation of New Belts. |
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Electric and magnetic fields were measured by the CRRES spacecraft at an L-value of 2.2 to 2.6 near 0300 magnetic local time during a strong storm sudden commencement (SSC) on March 24, 1991. The electric field signature at the spacecraft at the time of the SSC was characterized by a large amplitude oscillation (80 mV/m peak to peak) with a period corresponding to the 150 second drift echo period of the simultaneously observed 15 MeV electrons. Considerations of previous statistical studies of the magnitude of SSC electric and magnetic fields versus local time and analysis of the energization and cross-L transport of the particles imply the existence of 200 to 300 mV/m electric fields over much of the dayside magnetosphere. These observations also suggest that the 15 MeV drift echo electrons were selectively energized because their gradient drift velocity allowed them to reside in the region of strong electric fields for the duration of the accelerating phase of the electric field. Wygant, J.; Mozer, F.; Temerin, M.; Blake, J.; Maynard, N.; Singer, H.; Smiddy, M.; Published by: Geophysical Research Letters Published on: 08/1994 YEAR: 1994 DOI: 10.1029/94GL00375 Shock-Induced Transport. Slot Refilling and Formation of New Belts. |
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Electric and magnetic fields were measured by the CRRES spacecraft at an L-value of 2.2 to 2.6 near 0300 magnetic local time during a strong storm sudden commencement (SSC) on March 24, 1991. The electric field signature at the spacecraft at the time of the SSC was characterized by a large amplitude oscillation (80 mV/m peak to peak) with a period corresponding to the 150 second drift echo period of the simultaneously observed 15 MeV electrons. Considerations of previous statistical studies of the magnitude of SSC electric and magnetic fields versus local time and analysis of the energization and cross-L transport of the particles imply the existence of 200 to 300 mV/m electric fields over much of the dayside magnetosphere. These observations also suggest that the 15 MeV drift echo electrons were selectively energized because their gradient drift velocity allowed them to reside in the region of strong electric fields for the duration of the accelerating phase of the electric field. Wygant, J.; Mozer, F.; Temerin, M.; Blake, J.; Maynard, N.; Singer, H.; Smiddy, M.; Published by: Geophysical Research Letters Published on: 08/1994 YEAR: 1994 DOI: 10.1029/94GL00375 Shock-Induced Transport. Slot Refilling and Formation of New Belts. |
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Electric and magnetic fields were measured by the CRRES spacecraft at an L-value of 2.2 to 2.6 near 0300 magnetic local time during a strong storm sudden commencement (SSC) on March 24, 1991. The electric field signature at the spacecraft at the time of the SSC was characterized by a large amplitude oscillation (80 mV/m peak to peak) with a period corresponding to the 150 second drift echo period of the simultaneously observed 15 MeV electrons. Considerations of previous statistical studies of the magnitude of SSC electric and magnetic fields versus local time and analysis of the energization and cross-L transport of the particles imply the existence of 200 to 300 mV/m electric fields over much of the dayside magnetosphere. These observations also suggest that the 15 MeV drift echo electrons were selectively energized because their gradient drift velocity allowed them to reside in the region of strong electric fields for the duration of the accelerating phase of the electric field. Wygant, J.; Mozer, F.; Temerin, M.; Blake, J.; Maynard, N.; Singer, H.; Smiddy, M.; Published by: Geophysical Research Letters Published on: 08/1994 YEAR: 1994 DOI: 10.1029/94GL00375 Shock-Induced Transport. Slot Refilling and Formation of New Belts. |
| 1993 |
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We model the rapid (\~ 1 min) formation of a new electron radiation belt at L ≃ 2.5 that resulted from the Storm Sudden Commencement (SSC) of March 24, 1991 as observed by the CRRES satellite. Guided by the observed electric and magnetic fields, we represent the time-dependent magnetospheric electric field during the SSC by an asymmetric bipolar pulse that is associated with the compression and relaxation of the Earth\textquoterights magnetic field. We follow the electrons using a relativistic guiding center code. The test-particle simulations show that electrons with energies of a few MeV at L > 6 were energized up to 40 MeV and transported to L ≃ 2.5 during a fraction of their drift period. The energization process conserves the first adiabatic invariant and is enhanced due to resonance of the electron drift motion with the time-varying electric field. Our simulation results, with an initial W-8 energy flux spectra, reproduce the observed electron drift echoes and show that the interplanetary shock impacted the magnetosphere between 1500 and 1800 MLT. Li, Xinlin; Roth, I.; Temerin, M.; Wygant, J.; Hudson, M.; Blake, J.; Published by: Geophysical Research Letters Published on: 11/1993 YEAR: 1993 DOI: 10.1029/93GL02701 Shock-Induced Transport. Slot Refilling and Formation of New Belts. |
| 1979 |
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Direct Evaluation of the Radial Diffusion Coefficient near L = 6 Due to Electric Field Fluctuations The radial diffusion coefficient for radiation belt particles near L=6 has been calculated from the measured electric field fluctuations. Simultaneous balloon flights in August 1974 from six auroral zone sites ranging 180\textdegree in magnetic longitude produced the electric field data. The large scale slowly varying ionospheric electric fields from these flights have been mapped to the equator during the quiet magnetic conditions of this campaign. These mapped equatorial electric fields were then Fourier transformed in space and time to produce power spectra of the first two terms of the global azimuthal electric field. From these power spectra the radial diffusion coefficient has been calculated. Published by: Journal of Geophysical Research Published on: 06/1979 YEAR: 1979 DOI: 10.1029/JA084iA06p02559 |
| 1973 |
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Measurements at conjugate points on the ground near L = 4 of the power spectra of magnetic-field fluctuations in the frequency range 0.5 to 20 mHz are used as a means of estimating daily values for the relativistic-electron radial-diffusion coefficient DLL for two periods in December 1971 and January 1972. The values deduced for L-10 DLL show a strong variation with magnetic activity, as measured by the Fredricksburg magnetic index KFR. The radial-diffusion coefficient typically increases by a factor of \~10 for a unit increase in KFR. When KFR ≲ 2, it is generally found that DLL ≲ 2 \texttimes 10-9 L10 day-1 for equatorially mirroring electrons having a first invariant M = 750 Mev/gauss; a value of DLL \~4 \texttimes 10-7 L10 day-1 is deduced for one day on which the mean KFR was 4.5. The quantity L-10 DLL theoretically depends on energy and L as (L/M)(s-2)/2 for relativistic particles, where s is the logarithmic slope of the power-law spectrum of magnetic fluctuations observed on the ground. For the time period analyzed, s typically had values between 1 and 3. Lanzerotti, L.; Morgan, Caroline; Published by: Journal of Geophysical Research Published on: 08/1973 YEAR: 1973 DOI: 10.1029/JA078i022p04600 |
| 1972 |
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Inner-Zone Energetic-Electron Repopulation by Radial Diffusion A quantitative study of the intrusion of natural electrons into the inner radiation zone during and after the geomagnetic storm of September 2, 1966, shows that the transport is consistent with a radial-diffusion mechanism in which the first two invariants are conserved. Except for the 3-day period of the storm main phase when data were missing, the radial-diffusion coefficient is D = 2.7 \texttimes 10-5 L7.9 μ-0.5 day-1 in the range 1.7 <= L <= 2.6 and 13.3 <= μ <= 27.4 Mev gauss-1. This value could be produced by variation of a large-scale electric field across the magnetosphere having an amplitude of 0.28 mv / m and a period of 1600 sec. Electric fields having approximately these characteristics have been inferred from previous observations of the motion of whistler ducts within the plasmapause. If fields of this amplitude and period exist throughout the magnetosphere, the radial diffusion of all geomagnetically trapped particles except the high-energy inner-zone protons is strongly influenced by electric-field variations. A comprehensive review of previously reported radial-diffusion coefficients shows reasonable agreement for L less than about 3.0, but serious discrepancies among reported values exist for determinations made in the outer zone. These discrepancies cannot be explained by the simple theory of radial diffusion due to variation of large-scale electric or magnetic fields. Tomassian, Albert; Farley, Thomas; Vampola, Alfred; Published by: Journal of Geophysical Research Published on: 07/1972 YEAR: 1972 DOI: 10.1029/JA077i019p03441 |
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Inner-Zone Energetic-Electron Repopulation by Radial Diffusion A quantitative study of the intrusion of natural electrons into the inner radiation zone during and after the geomagnetic storm of September 2, 1966, shows that the transport is consistent with a radial-diffusion mechanism in which the first two invariants are conserved. Except for the 3-day period of the storm main phase when data were missing, the radial-diffusion coefficient is D = 2.7 \texttimes 10-5 L7.9 μ-0.5 day-1 in the range 1.7 <= L <= 2.6 and 13.3 <= μ <= 27.4 Mev gauss-1. This value could be produced by variation of a large-scale electric field across the magnetosphere having an amplitude of 0.28 mv / m and a period of 1600 sec. Electric fields having approximately these characteristics have been inferred from previous observations of the motion of whistler ducts within the plasmapause. If fields of this amplitude and period exist throughout the magnetosphere, the radial diffusion of all geomagnetically trapped particles except the high-energy inner-zone protons is strongly influenced by electric-field variations. A comprehensive review of previously reported radial-diffusion coefficients shows reasonable agreement for L less than about 3.0, but serious discrepancies among reported values exist for determinations made in the outer zone. These discrepancies cannot be explained by the simple theory of radial diffusion due to variation of large-scale electric or magnetic fields. Tomassian, Albert; Farley, Thomas; Vampola, Alfred; Published by: Journal of Geophysical Research Published on: 07/1972 YEAR: 1972 DOI: 10.1029/JA077i019p03441 |
| 1970 |
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Radial Diffusion of Outer-Zone Electrons: An Empirical Approach to Third-Invariant Violation The near-equatorial fluxes of outer-zone electrons (E>0.5 Mev and E>1.9 Mev) measured by an instrument on the satellite Explorer 15 following the geomagnetic storm of December 17\textendash18, 1962, are used to determine the electron radial diffusion coefficients and electron lifetimes as functions of L for selected values of the conserved first invariant \textmu. For each value of \textmu, the diffusion coefficient is assumed to be time-independent and representable in the form D = DnLn. The diffusion coefficients and lifetimes are then simultaneously obtained by requiring that the L-dependent reciprocal electron lifetime, as determined from the Fokker-Planck equation, deviate minimally from a constant in time. Applied to the data, these few assumptions yield a value of D that is smaller by approximately a factor of 10 than the value recently found by Newkirk and Walt in a separate analysis of 1.6-Mev electron data obtained during the same time period on another satellite. The electron lifetimes are found to be strong functions of L, with 4- to 6-day lifetimes observed at the higher L values (4.6\textendash4.8). Lanzerotti, L.; Maclennan, C.; Schulz, Michael; Published by: Journal of Geophysical Research Published on: 10/1970 YEAR: 1970 DOI: 10.1029/JA075i028p05351 |
| 1969 |
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Convection Electric Fields and the Diffusion of Trapped Magnetospheric Radiation We explore here the possible importance of time-dependent convection electric fields as an agent for diffusing trapped magnetospheric radiation inward toward the earth. By using a formalism (Birmingham, Northrop, and Fälthammar, 1967) based on first principles, and by adopting a simple model for the magnetosphere and its electric field, we succeed in deriving a one-dimensional diffusion equation to describe statistically the loss-free motion of mirroring particles with arbitrary but conserved values of the first two adiabatic invariants M and J. Solution of this equation bears out the fact that reasonable electric field strengths, correlated in time for no longer than the azimuthal drift period of an average particle, move particles toward the earth at a rate at least an order of magnitude faster than electric fields whose source is a fluctuating current on the magnetopause. Published by: Journal of Geophysical Research Published on: 05/1969 YEAR: 1969 DOI: 10.1029/JA074i009p02169 |
| 1965 |
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Effects of time-dependent electric fields on geomagnetically trapped radiation. Large-scale electric potential fields in the magnetosphere are generally invoked in theories of the aurora. It is shown in the present article that irregular fluctuations of such fields cause a random radial motion of trapped energetic particles by violating the third adiabatic invariant. When the first and second invariants are conserved, any radial motion of the particles is associated with a corresponding energy change. Some particles move outward and others inward; but, if there is a source in the outer magnetosphere and a sink farther in, there will be a net inward transport and an associated net energy gain. This mechanism supplements that of particle transport by magnetic disturbances, which has already been discussed in the literature. The transport and acceleration of energetic particles by fluctuating electric potential fields have a formal similarity to the so-called stochastic mode of acceleration in synchrocyclotrons. In the magnetosphere, the rate of displacement of trapped particles is found to depend on the spectral power density of the fluctuating electric fields at the azimuthal drift frequency and its harmonics. Which of these frequencies is most important depends on the spatial structure of the fluctuations. The observational data needed for numerical evaluation of the rate of transport are still lacking, but the formulas derived serve the purpose of indicating what properties of the fields are important and ought to be measured experimentally. The effects of magnetic time variations, which have been discussed in the literature under special assumptions, are considered in a more general way. A first-order result is given, which applies not only to initial phases of magnetic storms but also to other types of magnetic time variations. Published by: Journal of Geophysical Research Published on: 06/1965 YEAR: 1965 DOI: 10.1029/JZ070i011p02503 |