ULF waves in geospace and the challenge of estimating radial diffusion coefficients from in situ data.

Radial diffusion, driven by Ultra-Low Frequency (ULF) waves in the Pc4–5 band (2–25 mHz), has been established as one of the most important mechanisms that influences the dynamics of electrons in a quite broad energy range, as it can lead to both energization and loss of relativistic electrons in the outer Van Allen radiation belt. The same has been observed in other planetary magnetospheres. The dependence of ULF wave power spectral density and radial diffusion coefficients (D_LL) on solar wind parameters has been investigated by some studies, but their relationship on the various solar and interplanetary drivers is far from well-studied and understood. In this study, we use the “SafeSpace” database (https://synergasia.uoa.gr/modules/document/?course=PHYS120), which contains radial diffusion coefficients and ULF wave power spectral density, and was created using magnetic and electric field measurements from the THEMIS satellites in the 2011–2019 time-period. We conduct an extensive statistical analysis of D_LL in order to investigate the relationships between the magnetic and electric components as well as their dependence on interplanetary drivers (i.e. High Speed Streams and Interplanetary Coronal Mass Ejections). Our results reveal an energy dependence of the radial diffusion coefficients as well as significant variations of the D_LL spectral profiles as a function of Roederer’s L*. Our findings highlight statistical, as well as physical, characteristics and aspects of D_LL which are not included in most semi-empirical models typically used in radiation belt simulations, thus potentially introducing significant biases in the estimation of the outer belt relativistic electron environment. Further discussion will be devoted to the uncertainties of such efforts as well as the possible contribution of magnetosheath processes (e.g., jets and electron injections from the foreshock) and solar wind mechanisms (periodic density structures).

The work leading to this paper has received funding from the European Union’s Horizon Europe research and innovation programme under grant agreement No 101081772 for the FARBES (Forecast of Actionable Radiation Belt Scenarios) project.