Comparison of Van Allen Probes radiation belt proton data with test particle simulation for the 17 March 2015 storm

Abstract

The loss of protons in the outer part of the inner radiation belt (L = 2 to 3) during the 17 March 2015 geomagnetic storm was investigated using test particle simulations that follow full Lorentz trajectories with both magnetic and electric fields calculated from an empirical model. The simulation results presented here are compared with proton pitch angle measurements from the Van Allen Probe satellites Relativistic Electron Proton Telescope (REPT) instrument before and after the coronal mass ejection‐shock‐driven storm of 17–18 March 2015, with minimum Dst =− 223 nT, the strongest storm of Solar Cycle 24, for four different energy ranges with 30, 38, 50, and 66 MeV mean energies. Two simulations have been run, one with an inductive electric field and one without. All four energy channels show good agreement with the Van Allen Probes REPT measurements for low L (L < 2.4) in both simulations but diverge for higher L values. The inclusion of the inductive electric field, calculated from the time‐changing magnetic field, significantly improves the agreement between simulation and REPT measurements at L > 2.4. A previous study using the Highly Elliptical Orbiter 3 spacecraft also showed improved agreement when including the inductive electric field but was unable to compare effects on the pitch angle distributions.

Variability of the Proton Radiation Belt

“…Increased field-line curvature due to enhanced ring current during magnetic storms causes detrapping of protons for L ≳ 2 (Engel et al, 2015(Engel et al, , 2016. This is included in the trapped proton model with an approximate empirical estimate based on minimum storm time D st (Selesnick et al, 2013) and by the varying boundary condition at L = 2.4 that includes the effect of any losses at higher L. It should be more effective at higher E because of the larger gyroradius, but this is not accurately modeled and probably accounts for excess model proton intensity for E ≳ 50 MeV near L = 2 (Figure 3a).…”

Simulations of Electron Energization and Injection by BBFs Using High‐Resolution LFM MHD Fields

“…The conservation of can be violated when the fields vary on the time scale of the gyroperiod or when the gyroradius ( ) is comparable to either of L, R c , or L s , the magnetic gradient, curvature, and shear scale lengths. The nonadiabatic motion of charged particles in highly curved magnetic fields has been previously studied (Birmingham, 1984;Buchner & Zelenyi, 1989;Chen, 1992;Chirkov, 1978;Hudson et al, 1997;Speiser, 1991;Young et al, 2002;Engel et al, 2016). Nonadiabatic motion of particles can also exist in sheared magnetic fields (Smets, 2000;Pfefferle et al, 2015) or for short scale length Northrop (1963).…”

Characteristics of High‐Energy Proton Responses to Geomagnetic Activities in the Inner Radiation Belt Observed by the RBSP Satellite

“…Until now, some satellite observations have shown that high‐energy protons in the inner radiation belt are relatively stable in comparison with electrons and low and medium energy ions in the inner and outer radiation belt. However, significant variations of high‐energy proton fluxes are observed in the outer boundary of the inner radiation belt ( L > 2.0), which are related to geomagnetic storms and solar energetic particle events (e.g., Engel et al, , ; Lorentzen et al, ; Selesnick et al, , ). The timescales of the variations in the high‐energy proton fluxes in the inner radiation belt caused by geomagnetic storms and solar proton events vary over timescales from days to years as summarized by Lorentzen et al ().…”