[1] Resonant interactions with whistler-mode chorus waves provide an important process for electron loss and acceleration during storm times. We demonstrate that wave propagation significantly affects the electron scattering rates. We show that stormtime chorus waves outside the plasmapause can scatter equatorial electrons ≤60 keV into the loss cone and accelerate trapped electrons up to ∼ MeV energies at large pitch-angles. Using ray tracing to map the waves to higher latitudes, we show that the decrease in the ratio between the electron plasma and gyro frequencies, along with the normalized chorus frequency bandwidth, enable much higher energy electrons ∼1 MeV to be scattered into the loss cone. We suggest that off equatorial pitch-angle scattering by chorus waves is responsible for relativistic micro-burst precipitation seen on SAMPEX. Off-equatorial scattering at pitch-angles well away from the loss cone also contributes to the acceleration of higher energy ≥3 MeV electrons.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003GL016973

Relativistic electron acceleration and precipitation during resonant interactions with whistler-mode chorus

An investigation has been made of processes responsible for the growth, damping, and propagation characteristics of whistler mode emissions within the earth's plasmasphere. Three-dimensional ray path calculations of the integrated wave gain in a realistic model plasmasphere have provided an explicit confirmation of the anisotropic electron cyclotron resonant generation mechanism for the maintenance of plasmaspheric hiss. Peak wave amplification occurs for field-aligned waves in the outer plasmasphere. The majority of unstable low-frequency waves follow ray trajectories that internally reflect at the plasmapause and are thus trapped within the plasmasphere. This reflection allows waves to subsequently propagate into the locally stable inner and middle plasmasphere and also migrate substantially in longitude. The combination of internal reflection at the plasmapause and magnetospheric reflection at high latitude provide an important class of wave trajectories that are recycled back to the equatorial growth region with sufficiently small wave normal angles to allow further cyclotron resonant amplification. It is unlikely that wave growth can occur from the natural incoherent emissivity level within the plasmasphere since the required gains (≈100 dB) would mandate unreasonably high energetic electron flux. An alternate, as yet unidentified, embryonic source is therefore required to initiate the growth process. But once established the observed spectrum of broad band plasmaspheric hiss can be maintained by the modest net gains anticipated along ray trajectories that are recycled back to the favorable equatorial growth region. A simulation of the spectral properties of waves arriving at selected observation points following several equatorial transits shows that the largest accumulated gain occurs for recycled waves that reflect from the plasmapause following each equatorial crossing. Furthermore, the peak gain is relatively insensitive to the location of the observation point, and waves are expected to arrive over a broad solid angle in k space consistant with observations. The ability of waves to gain access to regions far from the source is a direct result of the relatively weaker Landau damping of oblique waves along the high latitude portion of the recycled ray path in comparison to the equatorial cyclotron resonant growth. The hiss upper frequency cutoff can result from the limited anisotropy of the cyclotron resonant electrons, the more severe Landau damping at higher frequencies, or in part, from the decreased ability of the higher frequency waves to recycle and thus experience multitransit growth. The characteristic lower frequency cutoff can be due to either the decrease in flux of energetic electrons that cyclotron resonate with the lower frequency waves or from thermal electron Landau damping should the plasmaspheric electron temperature exceed several electron volts.

On the origin of plasmaspheric hiss: Ray path integrated amplification

Rapid phase perturbations of subionospherically propagating VLF/LF signals are associated with lightning-generated whistlers which propagate in the magnetosphere. The perturbations, called 'Trimpi events' or 'Trimpi effects' after their discoverer, are attributed to alterations in the earth-ionosphere waveguide caused by localized enhancements in the ionospheric D region. These enhancements are produced by bursts of high-energy electrons which are precipitated out of the earth's radiation belts by the whistler waves. The present paper has the objective to present and discuss new evidence of whistler-associated phase perturbations of subionospheric signals. Events of this type, called phase Trimpi's were first reported by Lohrey and Kaiser (1979). Attention is given to occurrence statistics in terms of the seasonal distribution and hourly rate of events, possible effects on global VLF/LF navigation, and an interpretive model.

Subionospheric VLF/LF phase perturbations produced by lightning-whistler induced particle precipitation

[1] Modulation of whistler mode chorus waves, which plays an important role in driving the pulsating aurora and other processes related to energetic electron dynamics, is an interesting but a long-standing unresolved problem. Here we utilize in situ observations from the THEMIS spacecraft to investigate the role of density variations in the modulation of the chorus wave amplitude, which forms a complementary study to the modulation of chorus by compressional Pc4–5 pulsations presented in a companion paper. We show that these density variations are correlated remarkably well with modulated chorus intensity and typically occur on a timescale of a few seconds to tens of seconds. Both density depletions (DD) and density enhancements (DE) are frequently correlated with increases in chorus wave amplitudes. Furthermore, density enhancements cause a lowering of the central frequencies of the generated chorus waves and vice versa. DD events are more likely to be related to quasi-periodic chorus emissions and thus may be related to the generation of the pulsating aurora. A systematic survey of both DD and DE events shows that DD events preferentially occur between premidnight and dawn, whereas DE events dominantly occur from dawn to noon. We also evaluate the growth rates of chorus waves using linear theory for both DD and DE events and show that both density depletions and enhancements can lead to an intensification of chorus wave growth. However, other potential mechanisms for chorus intensification caused by density variations such as wave trapping by density crests and troughs cannot be excluded.

http://people.atmos.ucla.edu/jbortnik/pubs/Li_etal2011b_densityVariations.pdf

Modulation of whistler mode chorus waves: 2. Role of density variations

The previously reported Trimpi effect involves observation of amplitude perturbations in subionospherically propagating fixed-frequency VLF signals. The preturbations are coincident in time with magnetospherically propagating whistlers, and it has been inferred that the whistlers drive energetic precipitation of E>100 keV in a manner such that a localized increase in ionization occurs near 80-km altitude in the nighttime earth-ionosphere waveguide. In the present work, VLF broadband, analog chart and DF data from Palmer Station, Antarctica (Lapprox.2.3), recorded in 1978 and 1979 are used to investigate additional features of the Trimpi process. Attention was concentrated on perturbation of the signals NSS (21.4 kHz) and NLK (18.6 kHz) that have arrival bearings at Palmer of 352/sup 0/ and 318/sup 0/, respectively.

A study of whistlers correlated with bursts of electron precipitation near L = 2

Numerical ray-trace calculations have been performed to investigate the whistler ducting characteristics of various field-aligned ionization density perturbations in the earth’s plasmasphere. The results indicate that field-line curvature has a major influence on both the ducted ray trajectories and on the probability of escape during interhemispheric propagation. A specific comparison is made of the single- and multiplehop ducting ability of “bell-shaped” versus “ledge-shaped” ducts as a function of the density perturbation and invariant latitude. Modest (20%) ledge- and bell-shaped perturbations, as well as a wide variety of “hybrid” profiles are capable of effective whistler ducting in middle latitudes, with ledge-shaped profiles having considerably better multiple-hop ducting characteristics. Furthermore, for a given duct the latitude range of starting locations from which lightning generated sferics can initially be trapped is considerably broader for ledge shaped profiles. Interhemispheric propagational losses from 20% ledge-shaped profiles in middle latitudes are between 5-12 db per hop compared to 8-20 db for similar bell-shaped profiles assuming specular reflection from the ionosphere. All transmitted downgoing rays consistently become “unducted” at altitudes near a few thousand kilometers due primarily to the ambient ionospheric topside plasma density gradients and magnetic field gradients and waves received on the ground should typically be observed at several degrees lower latitude than the ducting profile.

D. J. Gorney

Papers

A comparative ray-trace study of whistler ducting processes in the Earth’s plasmasphere