Observations of Radiation Belt Losses Due to Cyclotron Wave-Particle Interactions

Electron loss to the atmosphere plays a critical role in driving dynamics of the Earths Van Allen radiation belts and slot region. This is a review of atmospheric loss of radiation belt electrons caused by plasma wave scattering via Doppler-shifted cyclotron resonance. In particular, the focus is on observational signatures of electron loss, which include direct measurements of precipitating electrons, measured properties of waves that drive precipitation, and variations in the trapped population resulting from loss. We discuss wave and precipitation measurements from recent missions, including simultaneous multi-payload observations, which have provided new insight into the dynamic nature of the radiation belts

Comparison of relativistic microburst activity seen by SAMPEX with ground-based wave measurements at Halley, Antarctica

Relativistic electron microbursts are a known radiation belt particle precipitation phenomenon; however, experimental evidence of their drivers in space have just begun to be observed. Recent modeling efforts have shown that two different wave modes (whistler mode chorus waves and electromagnetic ion cyclotron (EMIC) waves) are capable of causing relativistic microbursts. We use the very low frequency/extremely low frequency Logger Experiment and search coil magnetometer at Halley, Antarctica, to investigate the ground‐based wave activity at the time of the relativistic microbursts observed by the Solar Anomalous Magnetospheric Particle Explorer. We present three case studies of relativistic microburst events, which have one or both of the wave modes present in ground‐based observations at Halley. To extend and solidify our case study results, we conduct superposed epoch analyses of the wave activity present at the time of the relativistic microburst events. Increased very low frequency wave amplitude is present at the time of the relativistic microburst events, identified as whistler mode chorus wave activity. However, there is also an increase in Pc1–Pc2 wave power at the time of the relativistic microburst events, but it is identified as broadband noise and not structured EMIC emissions. We conclude that whistler mode chorus waves are, most likely, the primary drivers of relativistic microbursts. However, case studies confirm the potential of EMIC waves as an occasional driver of relativistic microbursts

Modeling the propagation characteristics of chorus using CRRES suprathermal electron fluxes

In the present paper, phase space density functions of the form f(v) = A N /v n are fitted to statistical distributions of suprathermal electron fluxes (E = 0.213–16.5 keV) from the CRRES satellite, parameterized by L-shell, Magnetic Local Time (MLT), and geomagnetic activity. The fitted distributions are used in conjunction with ray tracing to calculate the Landau damping rates of an ensemble of rays representing whistler-mode chorus waves. The modeled propagation characteristics are compared with observations of chorus wave power from the CRRES satellite, as a function of L-shell, MLT, and magnetic latitude, in various frequency bands, and under various geomagnetic conditions. It is shown that the model results are remarkably consistent with many aspects of the observed wave distributions, including frequency, L-shell, MLT, and latitudinal dependence. In addition, the MLT distribution of wave power becomes characteristically asymmetric during active geomagnetic conditions, with small propagation lengths on the nightside which increase with MLT and maximize on the dayside. This asymmetry is shown to be directly related to the dynamics of the Landau resonant suprathermal electrons which drift around the Earth whilst undergoing scattering and loss due to a variety of plasma waves. Consequently, the suprathermal electrons play an important role in radiation belt dynamics, by controlling the distribution of chorus, which in turn contributes to the acceleration and loss of relativistic electrons in the recovery phase of storms

Role of the plasmapause in dictating the ground accessibility of ELF/VLF chorus

This study explores the manner in which the plasmapause is responsible for dictating which magnetospheric source regions of ELF/VLF chorus are able to propagate to and be received by midlatitude stations on the ground. First, we explore the effects of plasmapause extent on ground‐based observations of chorus via a 3 month study of ground‐based measurements of chorus at Palmer Station, Antarctica (L = 2.4, 50°S geomagnetic latitude), and data on the plasmapause extent from the IMAGE EUV instrument. It is found that chorus normalized occurrence peaks when the plasmapause is at L ∼ 2.6, somewhat higher than Palmer's L shell, and that this occurrence peak persists across a range of observed chorus frequencies. Next, reverse ray tracing is employed to evaluate the portion of the equatorial chorus source region, distributed in radial distance and wave normal, from which chorus is able to reach Palmer Station via propagation in a nonducted mode. The results of ray tracing are similar to those of observations, with a peak of expected occurrence when the plasmapause is at L ∼ 3. The exact location of the peak is frequency dependent. This supports the conclusion that the ability of chorus to propagate to low altitudes and the ground is a strong function of instantaneous plasmapause extent and that peak occurrence of chorus at a given ground station may occur when the L shell of the plasmapause is somewhat beyond that of the observing station. These results also suggest that chorus observed on the ground at midlatitude stations propagates predominantly in the nonducted mode

Chorus-driven resonant scattering of diffuse auroral electrons in nondipolar magnetic fields

We perform a comprehensive analysis of resonant scattering of diffuse auroral electrons by oblique nightside chorus emissions present along a field line with an equatorial crossing of 6 R(E) at 00: 00 MLT, using various nondipolar Tsyganenko magnetic field models. Bounce-averaged quasi-linear diffusion coefficients are evaluated for both moderately and actively disturbed geomagnetic conditions using the T89, T96, and T01s models. The results indicate that inclusion of nondipolar magnetic field leads to significant changes in bounce-averaged rates of both pitch angle and momentum diffusion for 200 eV to 10 keV plasma sheet electrons. Compared to the results using a dipole field, the rates of pitch angle diffusion obtained using the Tsyganenko models are enhanced at all resonant pitch angles for 200 eV electrons. In contrast, for 500 eV to 10 keV electrons the rates of pitch angle scattering are enhanced at intermediate and/or high pitch angles but tend to be considerably lower near the loss cone, thus reducing the precipitation loss compared to that in a dipole field. Upper band chorus acts as the dominant cause for scattering loss of 200 eV to 2 keV electrons, while lower band chorus scattering prevails for 5-10 keV electrons, consistent with the results using the dipole model. The first-order cyclotron resonance and the Landau resonance are mainly responsible for the net scattering rates of plasma sheet electrons by oblique chorus waves and also primarily account for the differences in bounce-averaged diffusion coefficients introduced by the use of Tsyganenko models. As the geomagnetic activity increases, the differences in scattering rates compared to the dipole results increase accordingly. Nonnegligible differences also occur particularly at high pitch angles for the diffusion rates between the Tsyganenko models, showing an increase with geomagnetic activity level and a dependence on the discrepancy between the Tsyganenko model fields. The strong dependence of bounce-averaged quasi-linear scattering rates on the adopted global magnetic field model and geomagnetic activity level demonstrates that realistic magnetic field models should be incorporated into future modeling efforts to accurately quantify the role of magnetospheric chorus in driving the diffuse auroral precipitation and the formation of electron pancake distributions

Modeling the wave power distribution and characterisitics of plamaspheric hiss

We simulate the spatial and spectral distributions of plasmaspheric hiss using a technique that involves extensive ray tracing. The rays are injected in the equatorial chorus source region outside the plasmasphere, are power weighted as a function of L-shell, frequency, and wave normal angle, so as to represent the chorus source distribution, and are propagated throughout the simulation domain until the power in each ray is effectively extinguished due to Landau damping. By setting up a large number of virtual observatories, the rays passing each observation location are counted, and a distribution is constructed. Our simulated plasmaspheric hiss spectrum reproduces the main observed features, including the lower and upper frequency cutoffs, the behavior of the bandwidth as a function of L-shell, the spatial extent, and even the two-zone structure of hiss, although the intensity is lower than observed. The wave normal distribution shows that at high latitudes, the wave normals are predominantly oblique, but near the equator, the wave normal distribution can be either predominantly field-aligned (lower L shells), or be bimodal, having a maximum in the field-aligned direction, and another maximum at very oblique angles, comprised of those rays that have broken out of their cyclical trajectories. This distribution of wave normals seems to reconcile the apparently contradictory observations that have been reported previously