The "SafeSpace" Radial Diffusion Coefficients Database: Dependencies and application to simulations

Radial diffusion has been established as one of the most important mechanisms contributing to both the acceleration and loss of relativistic electrons in the outer radiation belt. In the framework of the SafeSpace project we have used 9 years (2011–2019) of multi-point magnetic and electric field measurements from THEMIS A, D and E satellites to create a database of accurately calculated radial diffusion coefficients (DLL) spanning an L* range from 3 to 8. In this work we investigate the dependence of the DLL on the various solar wind parameters, geomagnetic indices and coupling functions, and moreover, on the spatial parameters L* and Magnetic Local Time (MLT), during the solar cycle 24. The spatial distribution of the DLL reveals important MLT dependence rising from the various Ultra Low Frequency (ULF) wave generation mechanisms. Furthermore, we investigate via a superposed analysis, the dependence of the DLL on solar wind drivers. We show that the Interplanetary Coronal Mass Ejections (ICME) driven disturbances accompanied by high solar wind pressure values combined with intense magnetospheric compression produce DLLB values comparable or even greater than the ones of DLLE. This feature cannot be captured by semi-empirical models and introduces a significant energy dependence on the DLL. Finally, we show the advantages of the use of accurately calculated DLL by means of numerical simulations of relativistic electron fluxes performed with the Salammbô code and significant deviations of several semi-empirical model predictions depending on the level of geomagnetic activity and L-shell.

Electron Microbursts Induced by Nonducted Chorus Waves

Microbursts, short-lived but intense electron precipitation observed by low-Earth-orbiting satellites, may contribute significantly to the losses of energetic electrons in the outer radiation belt. Their origin is likely due to whistler mode chorus waves, as evidenced by a strong overlap in spatial correlation of the two. Despite previous efforts on modeling bursty electron precipitation induced by chorus waves, most, if not all, rely on the assumption that chorus waves are ducted along the field line with zero wave normal angle. Such ducting is limited to cases when fine-scale plasma density irregularities are present. In contrast, chorus waves propagate in a nonducted way in plasmas with smoothly varying density, allowing wave normals to gradually refract away from the magnetic field line. In this study, the interaction of ducted and nonducted chorus waves with energetic electrons is investigated using test particle simulation. Substantial differences in electron transport are found between the two different scenarios, and resultant electron precipitation patterns are compared. Such a comparison is valuable for interpreting low Earth-orbiting satellite observations of electron flux variation in response to the interaction with magnetospheric chorus waves.

EVIDENCE OF THE EARTH’S INNER RADIATION BELTS DURING THE LOW SOLAR AND GEOMAGNETIC ACTIVITY OBTAINED WITH THE STEP-F INSTRUMENT

Purpose: The subject of research is the spatio-temporal charged particles in the Earth’s magnetosphere outside the South Atlantic magnetic Anomaly during the 11-year cycle of solar activity minimum. The work aims at searching for and clarifying the sustained and unstable new spatial zones of enhanced subrelativistic electron fluxes at the altitudes of the low Earth orbit satellites. Design/methodology/approach: Finding and ascertainment of new radiation belts of the Earth were made by using the data analysis from the D1e channel of recording the electrons of energies of ΔEe=180–510 keV and protons of energies of ΔEp=3.5–3.7 MeV of the satellite telescope of electrons and protons (STEP-F) aboard the “CORONAS-Photon” Earth low-orbit satellite. For the analysis, the data array with the 2 s time resolution normalized onto the active area of the position-sensitive silicon matrix detector and onto the solid angle of view of the detector head of the instrument was used. Findings: A sustained structure of three electron radiation belts in the Earth’s magnetosphere was found at the low solar and geomagnetic activity in May 2009. The two belts are known since the beginning of the space age as the Van Allen radiation belts, another additional permanent layer is formed around the drift shell with the McIlwaine parameter of L = 1.65±0.05. On some days in May 2009, the new two inner radiation belts were observed simultaneously, one of those latter being recorded between the investigated sustained belt at L≈1.65 and the Van Allen inner belt at L≈2.52. Increased particle fluxes in this unstable belt have been formed with the drift shell L≈2.06±0.14. Conclusions: The new found inner radiation belts are recorded in a wide range of geographic longitudes λ, both at the ascending and descending nodes of the satellite orbit, from λ1≈150° to λ2≈290°. Separately in the Northern or in the Southern hemispheres, outside the outer edge of the outer radiation belt, at L≥7–8, there are cases of enhanced particle fl ux density in wide range of L-shells. These shells correspond to the high-latitude region of quasi-trapped energetic charged particles. Increased particle fluxes have been recorded up to the bow shock wave border of the Earth’s magnetosphere (L≈10-12). Key words: radiation belt, STEP-F instrument, electrons, magnetosphere, drift L-shell, particle flux density