R. R. Anderson

Substorm dependence of chorus amplitudes: Implications for the acceleration of electrons to relativistic energies

Intense interest currently exists in determining the roles played by various wave-particle interactions in the acceleration of electrons to relativistic energies during/following geomagnetic storms. Here we present a survey of wave data from the CRRES Plasma Wave Experiment for lower band (0.1-0.5f(ce)) and upper band (0.5-1.0f(ce)) chorus, f(ce) being the electron gyrofrequency, to assess whether these waves could play an important role in the acceleration of a seed population of electrons to relativistic energies during and following geomagnetic storms. Outside of the plasmapause the chorus emissions are largely substorm-dependent, and all chorus emissions are enhanced when substorm activity is enhanced. The equatorial chorus (/ lambda (m) / 300 nT) with average amplitudes typically >0.5 mV m(-1) predominantly in the region 3 15 degrees) is strongest in the lower band during active conditions, with average amplitudes typically >0.5 mV m(-1) in the region 3 < L < 7 over a range of local times on the dayside, principally in the range 0600-1500 MLT, Consistent with wave generation in the horns of the magnetosphere. An inner population of weak, substorm-independent emissions with average amplitudes generally < 0.2 mV m(-1) are seen in both bands largely inside L = 4 on the nightside during quiet (AE < 100 nT) and moderate (100 nT < AE < 300 nT) conditions. These emissions lie inside the plasmapause and are attributed to signals from lightning and ground-based VLF transmitters. We conclude that the significant increases in chorus amplitudes seen outside of the plasmapause during substorms support the theory of electron acceleration by whistler mode chorus in that region. The results suggest that electron acceleration by whistle mode chorus during/following geomagnetic storms can only be effective when there are periods of prolonged substorm activity following the main phase of the geomagnetic storm.

Statistical analysis of relativistic electron energies for cyclotron resonance with EMIC waves observed on CRRES

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 1-10 nT cause strong diffusion scattering which may lead to significant relativistic electron loss at energies above the minimum energy for resonance, E-min. 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 (less than or equal to2 MeV). While E-min 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 (f(pe)/f(ce,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 4.5. The average wave spectral intensity of these events (4-5 nT(2)/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 less than or equal to2 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 similar toMeV electrons are produced by low-energy (similar tokeV) ring current protons, which are expected to be injected into the inner magnetosphere during enhanced convection events.

An empirical plasmasphere and trough density model: CRRES observations

Combined Release and Radiation Effects Satellite (CRRES) sweep frequency receiver data were used to develop an empirical model of the plasmasphere and trough number density. The over 1000 CRRES orbits provided good statistical coverage of all local times between an L shell of 3 to 7. The CRRES density data were separated into plasmaspheric-like and trough-like by assuming a minimum density value for the plasmasphere as a function of L shell. For the plasmasphere the average number density (in cm−3) as a function of L shell (3 ≤ L ≤ 7) was found to be: np = 1390 (3/L)4.8 ± 440 (3/L)3.6. For the trough the average number density (in cm−3) as a function of L-shell (3 ≤ L ≤ 7) and magnetic local time (0 ≤ LT ≤ 24) was found to be nt = l24 (3/L)4.0 + 36(3/L)3.5 cos({LT-[7.7(3/L)2.0+12]}π/12) ± {78 (3/L)4.7 + 17 (3/L)3.7 cos[(LT - 22)π/12]}. No clear dependence on magnetic activity was found for either density model. This empirical model is an improvement over earlier models in that it is continuous in local time and can be used to track densities based on refilling history. The model standard deviations are representative of either early time or late time refilling of the trough or newly filled or saturated plasmaspheric densities.

Favored regions for chorus‐driven electron acceleration to relativistic energies in the Earth's outer radiation belt

[1] Pitch angle and energy diffusion rates for scattering by whistler-mode chorus waves are proportional to the wave magnetic field intensity and are strongly dependent on the frequency distribution of the waves and to the ratio between the electron plasma frequency (f(pe)) and the electron gyrofrequency (f(ce)). Relativistic electrons interact most readily with lower-band chorus (0.1 300 nT). Enhanced waves in these regions could play a major role in electron acceleration to relativistic energies during periods of prolonged substorm activity.

Evidence for chorus‐driven electron acceleration to relativistic energies from a survey of geomagnetically disturbed periods

[1] 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 pT(2) 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.

Outer zone relativistic electron acceleration associated with substorm-enhanced whistler mode chorus

[1] We present plasma wave and particle data from the CRRES satellite during three case studies to investigate the viability of a local stochastic electron acceleration mechanism to relativistic energies driven by resonant interactions with whistler mode chorus. We first consider a strong geomagnetic storm that contains prolonged substorm activity during its 3-day recovery phase. The recovery phase is characterized by electron injections at subrelativistic energies, enhanced whistler mode chorus amplitudes, and a gradual increase in the flux of relativistic electrons (E > 1 MeV) over the entire outer zone, with fluxes exceeding the prestorm level by an order of magnitude in the region 3.5 < L < 4.5. We next consider a strong geomagnetic storm that contains very little substorm activity during its 3-day recovery phase. Here the recovery phase is characterized by a lack of sustained electron injections at subrelativistic energies, a low level of chorus amplitudes, and a net reduction in the flux of relativistic electrons in the outer zone. Finally, we examine a period of prolonged substorm activity in the absence of a significant storm signature, as measured by Dst. This period is characterized by electron injections at subrelativistic energies, enhanced chorus amplitudes, and a gradual increase in the flux of relativistic electrons in the region 4 < L < 6.5. These results suggest that the gradual acceleration of electrons to relativistic energies seen on a timescale of days during geomagnetic storms can be effective only when there are periods of prolonged substorm activity following the main phase of the geomagnetic storm. Furthermore, gradual electron acceleration to relativistic energies can be obtained during periods of prolonged substorm activity in the absence of a significant storm signature as indicated by Dst. The case studies show that the acceleration mechanism is confined to the region outside of the plasmapause and occurs in the presence of enhanced chorus waves. These results suggest that a local acceleration mechanism involving the energization of a seed population of electrons with energies of the order of a few hundred keV to relativistic energies by wave-particle interactions involving whistler mode chorus contributes to the reformation of the relativistic outer zone population following prolonged substorm activity.

Energetic Outer Zone Electron Loss Timescales During Low Geomagnetic Activity

Following enhanced magnetic activity the fluxes of energetic electrons in the Earths outer radiation belt gradually decay to quiet-time levels. We use CRRES observations to estimate the energetic electron loss timescales and to identify the principal loss mechanisms. Gradual loss of energetic electrons in the region 3.0 ≤ L ≤ 5.0 occurs during quiet periods (Kp 7), indicating that the decay takes place in the plasmasphere. We compute loss timescales for pitch-angle scattering by plasmaspheric hiss using the PADIE code with wave properties based on CRRES observations. The resulting timescales suggest that pitch angle scattering by plasmaspheric hiss propagating at small or intermediate wave normal angles is responsible for electron loss over a wide range of energies and L shells. The region where hiss dominates loss is energy-dependent, ranging from 3.5 ≤ L ≤ 5.0 at 214 keV to 3.0 ≤ L ≤ 4.0 at 1.09 MeV. Plasmaspheric hiss at large wave normal angles does not contribute significantly to the loss rates. At E = 1.09 MeV the loss timescales are overestimated by a factor of ∼5 for 4.5 ≤ L ≤ 5.0. We suggest that resonant wave-particle interactions with EMIC waves, which become important at MeV energies for larger L (L > ∼4.5), may play a significant role in this region.

Initial Results from the ISEE-1 and -2 Plasma Wave Investigation

In this paper we present an initial survey of results from the plasma wave experiments on the ISEE-1 and -2 spacecraft which are in nearly identical orbits passing through the Earths magnetosphere at radial distances out to about 22.5Re. Essentially every crossing of the Earths bow shock can be associated with an intense burst of electrostatic and whistler-mode turbulence at the shock, with substantial wave intensities in both the upstream and downstream regions. Usually the electric and magnetic field spectrum at the shock are quite similar for both spacecraft, although small differences in the detailed structure are sometimes apparent upstream and downstream of the shock, probably due to changes in the motion of the shock or propagation effects. Upstream of the shock emissions are often observed at both the fundamental, f-p, and second harmonic, 2fp-, of the electron plasma frequency. In the magnetosphere high resolution spectrograms of the electric field show an extremely complex distribution of plasma and radio emissions, with numerous resonance and cutoff effects. Electron density profiles can be obtained from emissions near the local electron plasma frequency. Comparisons of high resolution spectrograms of whistler-mode emissions such as chorus detected by the two spacecraft usually show a good overall similarity but marked differences in detailed structure on time scales less than one minute. Other types of locally generated waves, such as the (n+1/2)f-gelectron cyclotron waves, show a better correspondence between the two spacecraft. High resolution spectrograms of kilometric radio emissions are also presented which show an extremely complex frequency-time structure with many closely spaced narrow-band emissions.

Diffuse auroral electron scattering by electron cyclotron harmonic and whistler mode waves during an isolated substorm

There are two main theories for the origin of diffuse auroral electron precipitation: precipitation by electrostatic ECH waves and precipitation by whistler mode waves. Here we analyze a case event where whistler mode hiss, chorus, and ECH waves are intensified during a weak substorm injection event to identify the source of particle precipitation. Examination of the particle data shows that there are three sources of free energy: a temperature anisotropy, a loss cone, and a pancake distribution. Instability analysis shows that the temperature anisotropy excites whistler mode hiss whereas both the temperature anisotropy and the pancake distribution contribute to the excitation of chorus. ECH waves are driven unstable by the loss cone. Wave propagation studies show that the path integrated gain of hiss and chorus is almost unaffected by changes in the depth of the loss cone, whereas ECH waves are very sensitive. Analysis of the changes in the resonant energy during propagation shows that the hiss resonates with electrons above a few keV while chorus resonates below a few hundred eV. As a result, neither hiss nor chorus are likely to cause significant electron precipitation from a few hundred eV to a few keV for this event. On the other hand, ECH waves resonate with electrons in the energy range between that for chorus and hiss. ECH waves can scatter electrons with pitch angles of up to 80 degrees into the loss cone. We conclude that ECH waves are responsible for the formation of the pancake distribution and are probably the main component of diffuse auroral precipitation during this event. We suggest that substorm-injected electrons are responsible for the intensification of hiss and ECH waves and that rapid scattering of electrons by ECH waves forms the pancake distribution which then excites chorus. We also suggest that rapid pitch angle scattering by ECH waves could be responsible for double frequency banded chorus emissions.

Electron scattering by whistler‐mode ELF hiss in plasmaspheric plumes

Nonadiabatic loss processes of radiation belt energetic electrons include precipitation loss to the atmosphere due to pitch-angle scattering by various magnetospheric plasma wave modes. Here we consider electron precipitation loss due to pitch-angle scattering by whistler-mode ELF hiss in plasmaspheric plumes. Using wave observations and inferred plasma densities from the Plasma Wave Experiment on the Combined Release and Radiation Effects Satellite (CRRES), we analyze plume intervals for which well-determined hiss spectral intensities are available. We then select 14 representative plumes for detailed study, comprising 10 duskside plumes and 4 nonduskside plumes, with local hiss amplitudes ranging from maximum values of above 300 pT to minimum values of less than 1 pT. We estimate the electron loss timescale τ loss due to pitch-angle scattering by hiss in each chosen plume as a function of L-shell and electron energy; τ loss is calculated from quasi-linear theory as the inverse of the bounce-averaged diffusion rate evaluated at the equatorial loss cone angle. We find that pitch-angle scattering by hiss in plumes can be efficient for inducing precipitation loss of outer-zone electrons with energies throughout the range 100 keV to 1 MeV, though the magnitude of τ loss can be highly dependent on wave power, L-shell, and electron energy. For 100- to 200-keV electrons, typically τ loss ∼ 1 day while the minimum loss timescale (τ loss ) min ∼ hours. For 500-keV to 1-MeV electrons, typically (τ loss ) min ∼ days, while (τ loss ) min < 1 day in the case of large wave amplitude (∼100s pT). Apart from inducing direct precipitation loss of MeV electrons, scattering by hiss in plumes may reduce the generation of MeV electrons by depleting the lower energy electron seed population. Models of the dynamical variation of the outer-zone electron flux should incorporate electron precipitation loss induced by ELF hiss scattering in plasmaspheric plumes.