ALBERT

Description: In this study, we compare long-term simulations performed by the Versatile Electron Radiation Belt (VERB) code with observations from the MagEIS and REPT instruments on the Van Allen Probes satellites. The model takes into account radial, energy, pitch-angle and mixed diffusion, losses into the atmosphere, and magnetopause shadowing. We consider the energetic (〉100 keV), relativistic (~0.5-1 MeV) and ultra-relativistic (〉2 MeV) electrons. One year of relativistic electron measurements ( μ =700 MeV/G) from October 1, 2012 to October 1, 2013, are well reproduced by the simulation during varying levels of geomagnetic activity. However, for ultra-relativistic energies ( μ =3500 MeV/G), the VERB code simulation overestimates electron fluxes and Phase Space Density. These results indicate that an additional loss mechanism is operational and efficient for these high energies. The most likely mechanism for explaining the observed loss at ultra-relativistic energies is scattering by the Electro-Magnetic Ion Cyclotron waves.

Description: Since more than 15 years, the Cluster mission passes through Earth's radiation belts at least once every two days for several hours, measuring the electron intensity at energies from 30 to 400 keV. This data has previously been considered not usable due to contamination caused by penetrating energetic particles (protons at 〉100 keV and electrons at 〉400 keV). In this study, we assess the level of distortion of energetic electron spectra from the RAPID/IES detector, determining the efficiency of its shielding. We base our assessment on the analysis of experimental data and a radiation transport code (Geant4). In simulations, we use the incident particle energy distribution of the AE9/AP9 radiation belt models. We identify the Roederer L-values, L ⋆ , and energy channels that should be used with caution: at 3≤L ⋆ ≤4, all energy channels (40 – 400 keV) are contaminated by protons (≃230 to 630 keV and 〉600 MeV); at L ⋆ ≃1 and 4–6, the energy channels at 95 – 400 keV are contaminated by high energy electrons (〉400 keV). Comparison of the data with electron and proton observations from RBSP/MagEIS indicates that the subtraction of proton fluxes at energies ≃ 230–630 keV from the IES electron data adequately removes the proton contamination. We demonstrate the usefulness of the corrected data for scientific applications.

Description: In this study, the azimuthal evolution of stream interaction regions are investigated, with the goal of predicting the time of arrival of an interface at some later position near 1 AU. A new SIR dataset is constructed from ACE, STEREO A and STEREO B in situ measurements, and it is demonstrated that the magnetic pressure and azimuthal flow angle provide the most simple robust estimation of the interface time. This dataset was applied in the investigation. In the analysis, the geometric effects of the magnetic spiral angle, and the tilt angle of stream interfaces are considered, and it is demonstrated how they may be used to improve forecasts of the arrival time of stream interaction regions from a spacecraft located at 1 AU. The polarity of the interplanetary magnetic field, towards or away from the Sun, observed by consecutive spacecraft measurements is considered for the slow and fast streams straddling a stream interface, in order to investigate whether the geoeffectiveness of the two streams may also be forecast from 1 AU. It is found that the polarity of the magnetic field, associated with a given stream interface, is conserved when observed by two separate spacecraft at azimuthal separations of 20° or less and while in the fast wind, however, the field polarity was not always conserved when observed in the slow wind ahead of the interface. An analysis of tilt angles evolution during 2008 showed that while the azimuthal tilt angles were generally similar between observations in the same Carrington rotation and in consecutive rotations of the same CIR, the meridional tilt angles may differ significantly. The forecast analysis showed that the azimuthal evolution of a SIR at 1 AU may be predicted to within a day or two of the actual evolution time, while any discrepancy was most likely caused by changes at the coronal hole on the solar surface, leading azimuthal and radial evolution of the SIR.

Description: The dependence of outer-radiation belt electron fluxes upon solar wind velocity and density is investigated using the OMNI solar wind database and LANL-GEO geosynchronous satellites for a period spanning over 20 years. Two dimensional probability distribution functions (PDF) of the flux-solar wind velocity (Vsw) and flux-solar wind density are calculated for electron energies in the 10's of keV to MeV range. The PDF's are normalized by Vsw and density and reveal new distinct relationships. Triangle-shaped flux-Vsw distributions become non-linear PDF's, and the most probable flux increases with Vsw. The only significant saturation of fluxes observed with an increase in Vsw occurs for the lower energy electron fluxes (31.7 keV). The low energy fluxes exhibit a positive correlation with solar wind density, while mid-to-high energy electron fluxes are anti-correlated with density. The maximum probability in the PDF's depends upon both velocity and density, the probability is higher for larger Vsw, and the maximum probability is larger for a given Vsw than for density. The results indicate that Vsw may be more important for determination of fluxes than density, especially for periods of high Vsw if suitable mixed delay times are applied to each solar wind parameter. It is shown that the source population of relativistic electrons of tens of keV exhibit a 2-D normalized flux-Vsw PDF, which is strikingly similar to that of the relativistic electrons. The findings support a model whereby solar wind velocity drives convective transport of source and seed electrons, to the inner magnetosphere, where local acceleration and subsequent radial diffusion is responsible for the enhanced fluxes. The results of this study also indicate that, statistically, ULF waves driven by dynamic pressure variations may act as a significant cause of loss for electrons in the 100's of keV to MeV range.

Description: A case of the major intensification of the subauroral polarization stream (SAPS) during the substorm recovery phase is presented. The continuous high-time-resolution Doppler velocity measurements in the subauroral and auroral regions were conducted with the Unwin HF radar, and these are analyzed in the context of the simultaneous and coincident measurements of the auroral luminosity and the total electron content (TEC) by the IMAGE and GPS satellites. Additional information was provided by other SuperDARN radars, DMSP F15 satellite crossing the fully-developed SAPS region, ground-based measurements near the location of the substorm onset, and GOES and LANL satellites in the inner magnetosphere. The association between the SAPS and TEC trough is further substantiated at short timescales and TEC is shown to exhibit two distinct types of responses to substorm onset poleward of and within SAPS, with no evidence of a TEC decrease during SAPS intensification. It is argued that the positive feedback between the electric fields and electron densities was probably not responsible for the observed SAPS intensification. Moreover, it is proposed that the strong and steady plasma acceleration within SAPS may be triggered by a burst of auroral activity, rather than accompanied by a similarly steady variation in other observed parameters either in the ionosphere or in the inner magnetosphere. It is also argued that the SAPS intensification occurring during the recovery phase is not necessarily expected from the current models of the SAPS formation and evolution, but is consistent with the observationally-based view of a fully-developed SAPS as a substorm recovery phenomenon.

Description: The response of high-energy particle precipitation to substorm onset is investigated using observations of cosmic noise absorption by a 7 × 7 beam imaging riometer in Kilpisjärvi, Finland, and substorm onset information obtained from the IMAGE satellite. A new method is developed for automatic detection of absorption responses to substorm onsets. Superposed epoch analysis shows that absorption exhibits some fading prior to and a clear response preceding substorm onset. These features are interpreted as being due to the stretching of magnetic field lines and a rapid dipolarization during the late growth phase of a substorm. A distinct dependence on substorm onset location is discovered with response delay increasing with distance from substorm onset and all responses to closest substorms (within 250 km) preceding substorm onset by 1–5 min. The absorption propagation is further examined using the average velocity from the onset to the riometer location as well as instantaneous velocities for a short period of time when substorm-enhanced absorption reaches the field of view. Comparison of the two velocity estimates shows that the absorption propagation is slower and oriented in more poleward directions away from substorm onset, with some evidence of near instantaneous expansion in zonal directions over large distances of ∼2000 km from onset followed by a slower expansion primarily in poleward and westward directions. The observations suggest that during substorms precipitating electrons in the high-energy part of the spectrum behave differently from their low-energy counterparts and that they exhibit some evidence of predictable behavior.

Description: Obtaining the global state of radiation belt electrons through reanalysis is an important step towards validating our current understanding of radiation belt dynamics, and for identification of new physical processes. In the current study, reanalysis of radiation belt electrons is achieved through data assimilation of five spacecraft with the 3D VERB code using a split-operator Kalman filter technique. The spacecraft data are cleaned for noise, saturation effects, and then intercalibrated on an individual energy channel basis, by considering phase space density conjunctions in the T96 field model. Reanalysis during the CRRES era reveals a never-before-reported 4-zone structure in the Earth's radiation belts during the March 24th 1991 shock-induced injection superstorm: 1) an inner belt, 2) the high-energy shock-injection belt, 3) a remnant outer radiation belt, and 4) a second outer radiation belt. The third belt formed near the same time as the second belt, and was later enhanced across keVto MeV energies by a second particle injection observed by CRRES and the NORSTAR riometer network. During the recovery phase of the storm, the fourth belt was created near L * =4 R E , lasting for several days. Evidence is provided that the fourth belt waslikely created by a dominant local heating process. This study outlines the necessity to consider all diffusive processes acting simultaneously, and the advantage of supporting ground-based data in quantifying the observed radiation belt dynamics. Itis demonstrated that 3D-data assimilation can resolve various non-diffusive processes, and provides a comprehensive picture of the electron radiation belts.

Description: Ground based riometers provide an inexpensive means to continuously remote sense the precipitation of electrons in the dynamic auroral region of Earth's ionosphere. The energy-dependent relationship between riometer absorption and precipitating electrons is thus of great importance for understanding the loss of electrons from the Earth's magnetosphere. In this study, statistical and event-based analyses are applied to determine the energy of electrons to which riometers chiefly respond. Time-lagged correlation analysis of trapped to precipitating fluxes shows that daily averaged absorption best correlates with ~ 60 keV trapped electron flux at zero-time lag, although large variability is observed across different phases of the solar cycle. High-time resolution statistical cross-correlation analysis between signatures observed by riometer stations, and assuming electron motion due to gradient and curvature drift, results in inferred energies of 10-100 keV, with a clear maximum in occurrence for 40-60 keV electrons. One event is considered in detail utilizing riometer absorption signatures obtained from several stations. The mean inferred energies for the initial rise time and peak of the absorption after correction for electric field effects were ~70 keV, and ~60 keV, respectively. The analyses presented provide a means to characterize the energy of electrons to which riometers are responding in both a statistical sense, and during the evolution of individual events.

Description: Abstract Ring current electrons (1–100 keV) have received significant attention in recent decades, but many questions regarding their major transport and loss mechanisms remain open. In this study, we use the four‐dimensional Versatile Electron Radiation Belt code to model the enhancement of phase space density that occurred during the 17 March 2013 storm. Our model includes global convection, radial diffusion, and scattering into the Earth's atmosphere driven by whistler‐mode hiss and chorus waves. We study the sensitivity of the model to the boundary conditions, global electric field, the electric field associated with subauroral polarization streams, electron loss rates, and radial diffusion coefficients. The results of the code are almost insensitive to the model parameters above 4.5 RERE, which indicates that the general dynamics of the electrons between 4.5 RE and the geostationary orbit can be explained by global convection. We found that the major discrepancies between the model and data can stem from the inaccurate electric field model and uncertainties in lifetimes. We show that additional mechanisms that are responsible for radial transport are required to explain the dynamics of ≥40‐keV electrons, and the inclusion of the radial diffusion rates that are typically assumed in radiation belt studies leads to a better agreement with the data. The overall effect of subauroral polarization streams on the electron phase space density profiles seems to be smaller than the uncertainties in other input parameters. This study is an initial step toward understanding the dynamics of these particles inside the geostationary orbit.

Description: [1]  Accurate forecast models of the near-Earth radiation belt environment are of great importance to satellite operators and engineers, as the charged particles can be damaging to satellite hardware. The Geosynchronous Relativistic Electron Empirical Prediction (GREEP) model is presented in this study, and utilized to predict the daily average flux of 1.8-3.5 MeV relativistic electron fluxes at geosynchronous orbit (GEO), based upon propagated solar wind velocity (Vsw) and solar wind number density (n) measurements. The occurrence distribution of Vsw and n are used to normalize the distribution of flux with Vsw and n, respectively. A power-law fit is applied to each probability distribution function (PDF), and the fitted values are dynamically combined with recent flux measurements from GEO, and with fluxes from previous solar rotations to predict the future radiation belt environment at GEO. The model outperforms recent models in terms of forecast score for 1-day predictions, and it is found that the distribution of flux as a function of Vsw generally provides better predictions during the descending phase of the solar cycle, than the distribution of flux as a function of density. The latter performs better near solar maximum. TheGREEP model performs best in terms of forecast score also during the declining phase, and it provides further evidence of the importance of solar wind velocity in controlling high-energy electron flux. During the descending phase, relativistic fluxes are typically the highest, and thus most damaging to space-borne equipment, and pose a greater risk to humans in space.