Abstract
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 ï¬uxes performed with the Salammbô code and significant deviations of several semi-empirical model predictions depending on the level of geomagnetic activity and L-shell.
Highlights
The dynamics of the outer radiation belt are driven by a complex interplay between acceleration and loss mechanisms (Reeves et al, 2003; Reeves and Daglis, 2016; Daglis et al, 2019) leading to a broad energy range of energetic electrons
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 5 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 on the spatial parameters L* and Magnetic Local Time (MLT), during the solar cycle 24
Summary
The dynamics of the outer radiation belt are driven by a complex interplay between acceleration and loss mechanisms (Reeves et al, 2003; Reeves and Daglis, 2016; Daglis et al, 2019) leading to a broad energy range of energetic electrons (a few hundreds of keV to several MeV). Falthammar (1965) dis tinguished the contribution of single-mode fluctuations in Earth’s magnetic field and induced electric fields (DM LL) and perturbations in convection electric fields (DELL) to derive a mathematical formulation for DLL He indicated that this formulation is valid for sub-relativistic particles, only. Fei et al (2006) assumed radial diffusion coefficients as the sum of the effects of perturbations in the azimuthal electric field and the parallel magnetic field: DLL = DLBL + DLEL (1). Note that DBLL 45 includes contributions only from the magnetic field oscillations, while DELL contains contributions from the total (inductive and convective) electric field It is clear, from the aforementioned formulation, that in order to have accurate estimations of the radial diffusion coefficients we need accurate magnetic and electric field measurements, which are not always available. These empirical models have the advantage of providing estimations/predictions of the
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