M. Spasojevic

Wave acceleration of electrons in the Van Allen radiation belts

The Van Allen radiation belts are two regions encircling the Earth in which energetic charged particles are trapped inside the Earths magnetic field. Their properties vary according to solar activity and they represent a hazard to satellites and humans in space. An important challenge has been to explain how the charged particles within these belts are accelerated to very high energies of several million electron volts. Here we show, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth, that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz. Wave acceleration can increase the electron flux by more than three orders of magnitude over the observed timescale of one to two days, more than sufficient to explain the new radiation belt. Wave acceleration could also be important for Jupiter, Saturn and other astrophysical objects with magnetic fields.

Identifying the plasmapause in IMAGE EUV data using IMAGE RPI in situ steep density gradients

plasmapause location observed by RPI is compared tothe location of the He + edge extracted from the closest-in-time EUVimage, a correlation coefficient of 0.83 is obtained. When the EUV He + edge location is taken as the average of two EUV measurements (one before and one after the RPI measurement), the correlation coefficient increases to 0.87. The high degree of correlation justifies the assumption that the He + edge coincides with the plasmapause. For eighteen cases inwhich the plasmasphere has no sharp outer boundary the intensity of the uncalibrated EUV images is compared with the electron number density extracted from the RPI data, and the lower sensitivity threshold of the EUV instrument is

Modeling the electromagnetic ion cyclotron wave-induced formation of detached subauroral proton arcs

[1] Detached dayside proton arcs have been recently observed at Earth with the IMAGE FUV instrument as subauroral arcs separated from the main oval and extending over several hours of local time in the afternoon sector. We investigate the mechanisms causing the proton precipitation during two subauroral arc events that occurred on 23 January 2001 and 18 June 2001. We employ our kinetic physics-based model coupled with a dynamic plasmasphere model and calculate the growth rate of electromagnetic ion cyclotron (EMIC) waves self-consistently with the evolving ring current H + ,O + , and He + ion distributions. Modeled plasmaspheric densities agree well with in situ observations from geosynchronous LANL satellites and duskside plasmapause observations from IMAGE EUV but overestimate the drainage plume extent toward noon on 18 June. Global images of precipitating H + ions are obtained and compared with IMAGE observations of proton arcs. We find that EMIC waves are preferentially excited, and proton precipitation maximizes, within regions of spatial overlap of energetic ring current protons and dayside plasmaspheric plumes and along steep density gradients at the plasmapause. The model matches very well the temporal and spatial evolution of FUV observations on 23 January. The predicted location of the proton precipitation on 18 June extends a few hours westward of the observations, and an offset of 2 hours in the convection electric field is needed to reproduce well the evolution of the proton arc. This study indicates that cyclotron resonant wave-particle interactions are a viable mechanism for the generation of subauroral proton arcs.

Activity-dependent global model of electron loss inside the plasmasphere

Using data from the CRRES plasma wave experiment, we develop quadratic fits to the mean of the wave amplitude squared for plasmaspheric hiss as a function of Kp, L, and magnetic latitude (λ) for the dayside (6 < magnetic local time (MLT) ≤ 21) and nightside (21 < MLT ≤ 6) magnetic local time sectors. The empirical model of hiss waves is used to compute quasi-linear pitch angle diffusion coefficients for energetic, relativistic, and ultrarelativistic electrons in the energy range of 1 keV to 10 MeV. In our calculations, we account for changes in hiss wave normal angle and plasma density with increasing λ. Electron lifetimes are then calculated from the diffusion coefficients and parameterized as a function of energy, Kp, and L. Coefficients for both the hiss model and the electron lifetimes are provided and can be easily incorporated into existing diffusion, convection, and particle tracing codes.

Temporal evolution of proton precipitation associated with the plasmaspheric plume

[1] The temporal evolution of afternoon sector proton precipitation observed by a global auroral imager is examined in detail for two case events. We focus on precipitation regions that are magnetically mapped to the plasmapause and plasmaspheric plume regions. The spatial and temporal variation of the plume-associated precipitation, including its relationship to the main proton oval, is dependent on the prevailing solar wind and magnetospheric driving conditions. Two contrasting events are presented here in association with 1) a substorm injection and 2) a northward IMF turning. We find that proton precipitation within the plasmaspheric plume is a persistent feature during geomagnetically disturbed periods, but the precipitation regions only appear latitudinally detached from the main proton oval under specific conditions. The evolution in both time and space of the plume-associated precipitation regions is consistent with theoretical predictions for EMIC wave scattering of protons in the ring current energy range.

Latitudinal and seasonal variations of quasiperiodic and periodic VLF emissions in the outer magnetosphere

We have analyzed ELF-VLF receiver and search coil magnetometer data from five Antarctic stations from 1998 and 1999 to study quasiperiodic emissions (QPs) and periodic emissions (PEs), which occur as ULF-range modulations of ELF-VLF signals between 0.5 kHz and similar to4 kHz. QPs are modulated at frequencies of similar to20-50 mHz, and PEs are modulated at frequencies of similar to200-500 mHz. The stations used covered a range of magnetic latitudes from -62degrees (Halley) to -74degrees (South Pole Station); three automated geophysical observatories (AGOs) were located at intermediate latitudes. Consistent with earlier studies, most QPs were observed with magnetic pulsations of identical period in the Pc3 range ( type I QPs). Of those QPs not observed with simultaneous magnetic pulsations ( type II QPs), nearly all were accompanied by PEs. Type I QPs, PEs, and events during which both appeared together (QPPEs) were found to have different latitudinal, seasonal, and diurnal occurrence patterns: QPs of both types were more likely to occur between -65degrees and -70degrees magnetic latitude, while PEs occurred more often around -60degrees magnetic latitude. QPs were more common during the months of October though March, while PEs were more common during the months of May through September. QPs, whether with or without simultaneous PEs or magnetic pulsations, were predominantly a dayside phenomenon, with a broad maximum near local noon. The occurrence of QPs unaccompanied by PEs was restricted to the dayside, however, while a small number of QPPEs appeared even during nighttime hours. PEs, on the other hand, could be seen at all local times, but with latitudinally dependent diurnal patterns. Most higher-latitude QPs were type I events (observed with magnetic pulsations), while type II QP events (without simultaneous magnetic pulsations) occurred relatively more often at lower latitudes. A case study from 1 August 1999 using wideband data from South Pole and Halley provides evidence of a transition from echoing whistler activity to PE activity and then to QP activity and suggests a causal relationship.

Global empirical models of plasmaspheric hiss using Van Allen Probes

Plasmaspheric hiss is a whistler-mode emission that permeates the Earths plasmasphere and is a significant driver of energetic electron losses through cyclotron resonant pitch angle scattering. The Electric and Magnetic Field Instrument Suite and Integrated Science instrument on the Van Allen Probes mission provides vastly improved measurements of the hiss wave environment including continuous measurements of the wave magnetic field cross-spectral matrix and enhanced low-frequency coverage. Here, we develop empirical models of hiss wave intensity using two years of Van Allen Probes data. First, we describe the construction of the hiss database. Then, we compare the hiss spectral distribution and integrated wave amplitude obtained from Van Allen Probes to those previously extracted from the Combined Release and Radiation Effects Satellite mission. Next, we develop a cubic regression model of the average hiss magnetic field intensity as a function of Kp, L, magnetic latitude, and magnetic local time. We use the full regression model to explore general trends in the data and use insights from the model to develop a simplified model of wave intensity for straightforward inclusion in quasi-linear diffusion calculations of electron scattering rates.

Wave-induced loss of ultra-relativistic electrons in the Van Allen radiation belts

The dipole configuration of the Earths magnetic field allows for the trapping of highly energetic particles, which form the radiation belts. Although significant advances have been made in understanding the acceleration mechanisms in the radiation belts, the loss processes remain poorly understood. Unique observations on 17 January 2013 provide detailed information throughout the belts on the energy spectrum and pitch angle (angle between the velocity of a particle and the magnetic field) distribution of electrons up to ultra-relativistic energies. Here we show that although relativistic electrons are enhanced, ultra-relativistic electrons become depleted and distributions of particles show very clear telltale signatures of electromagnetic ion cyclotron wave-induced loss. Comparisons between observations and modelling of the evolution of the electron flux and pitch angle show that electromagnetic ion cyclotron waves provide the dominant loss mechanism at ultra-relativistic energies and produce a profound dropout of the ultra-relativistic radiation belt fluxes.

New global loss model of energetic and relativistic electrons based on Van Allen Probes measurements

The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrument on the Van Allen Probes provides a vast quantity of fully resolved wave measurements below L = 5.5, a critical region for radiation belt acceleration and loss. EMFISIS data show that plasmaspheric hiss waves can be observed at frequencies as low as 20 Hz and provide three-component magnetic field measurements that can be directly used for electron scattering calculations. Updated models of hiss properties based on statistical analysis of Van Allen Probes data were recently developed. We use these new models to compute and parameterize the lifetime of electrons as a function of kinetic energy, L shell, Kp index, and magnetic local time. We present a detailed analysis of the electron lifetime sensitivity to the model of the wave intensity and spectral distribution. We also compare the results with previous models of electron loss, which were based on single-component electric field measurements from the sweep frequency receiver on board the CRRES satellite.

Afternoon Subauroral Proton Precipitation Resulting from Ring Current—Plasmasphere Interaction

We investigate the occurrence of arcs of precipitating protons equatorward of and detached from the afternoon proton auroral oval and their relationship with the plasmasphere and electromagnetic ion cyclotron waves. In a four month study interval including sixteen events, we find that the detached proton arcs are more likely to occur during geomagnetically disturbed periods and specifically at times when enhanced energetic ion densities and temperature anisotropies are observed in the equatorial magnetosphere. The disturbance-time arcs tend to be located at lower magnetic latitudes and are consistently associated with plasmaspheric plumes. Conversely, arcs which occur during quiet times tend to be located at higher latitudes, and their relationship with regions of enhanced cold plasma density remains unclear. Wave data available for two of the detached arc events indicate the presence of strong ion cyclotron waves near the equator in the vicinity of the proton precipitation region.