Abstract
AbstractThe impact of radial diffusion in storm time radiation belt dynamics is well‐debated. In this study we quantify the changes and variability in radial diffusion coefficients during geomagnetic storms. A statistical analysis of Van Allen Probes data (2012–2019) is conducted to obtain measurements of the magnetic and electric power spectral densities for Ultra Low Frequency (ULF) waves, and corresponding radial diffusion coefficients. The results show global wave power enhancements occur during the storm main phase, and continue into the recovery phase. Local time asymmetries show sources of wave power are both external solar wind driving and internal sources from coupling with ring current ions and substorms. Wave power enhancements are also observed at low L values (L < 4). The accessibility of wave power to low L is attributed to a depression of the Alfvén continuum. The increased wave power drives enhancements in both the magnetic and electric field diffusion coefficients by more than an order of magnitude. Significant variability in diffusion coefficients is observed, with values ranging over several orders of magnitude. A comparison to the Kp parameterized empirical model of Ozeke et al. (2014) is conducted and indicates important differences during storm times. Although the electric field diffusion coefficient is relatively well described by the empirical model, the magnetic field diffusion coefficient is approximately ∼10 times larger than predicted. We discuss how differences could be attributed to data set limitations and assumptions. Alternative storm‐time radial diffusion coefficients are provided as a function of L* and storm phase.
Highlights
Earth's inner magnetosphere is host to a population of highly variable, highly dynamic, and highly energetic particles known as the Van Allen radiation belts (Li & Hudson, 2019; Van Allen et al, 1958, 1959)
We show that important local time dependences exist, implying significant consequences for estimating event-specific radial diffusion coefficients from single spacecraft measurements
Each row corresponds to a storm phase, as labeled, and shows samples binned for L and Magnetic Local Time (MLT)
Summary
Earth's inner magnetosphere is host to a population of highly variable, highly dynamic, and highly energetic particles known as the Van Allen radiation belts (Li & Hudson, 2019; Van Allen et al, 1958, 1959). Of particular interest is the outer radiation belt population that typically occupies radial distances greater than 3–4 RE and is host to extremely energetic MeV electrons During geomagnetic storms, this population undergoes dramatic enhancements as well as rapid flux dropouts (e.g., Baker et al, 2004; Murphy et al, 2018; Turner et al, 2012). It is a key outstanding goal of the Space Weather community to fully understand the dynamics and variability of the radiation belt population during geomagnetic storms (Fry, 2012; Schrijver et al, 2015)
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