MHD‐Test Particles Simulations of Moderate CME and CIR‐Driven Geomagnetic Storms at Solar Minimum

Abstract

The Whole Heliosphere and Planetary Interactions (WHPI), an international initiative focusing on the Solar Cycle 24 solar minimum (https://whpi.hao.ucar.edu/), follows upon the Whole Sun Month in 1996 and the Whole Heliosphere Interval in 2008 (Gibson et al., 2010;Hudson et al., 2012), aiming to understand the interconnected sun-heliospheric-planetary system at solar minimum when the sun's magnetic field configuration is less complex than at solar maximum. Coronal Mass Ejections (CMEs) are large expulsions of plasma and magnetic field from the Sun's corona which emerge from magnetic active regions prominent at solar maximum (Golub & Pasachoff, 1997). Despite the relative infrequency of CMEs compared with solar maximum, there were a series of CMEs and ensuing Interplanetary Shocks (IS) observed in May 2019 from a solar active region that emerged a month earlier, providing an opportunity for comparison of the geoeffectiveness of CME-shocks with Corotating Interaction Regions (CIRs), more characteristic of solar minimum, in driving outer zone relativistic electron

Extreme Energy Spectra of Relativistic Electron Flux in the Outer Radiation Belt

“…Based on empirical plasmapause models (O’Brien & Moldwin, 2003 ), the plasmapause remained at L < 4 during this event, and Dst varied by less than 20 nT between the times at which electron fluxes measured at L * = 4.5 are displayed in Figure 6 . During this event, a growing peak of PSD of ∼2–4 MeV electrons has been observed by the Van Allen Probes at L * = 4–5, which was not reproduced by simulations including ULF wave‐driven electron inward radial diffusion without local chorus‐driven acceleration, suggesting an important effect of chorus‐driven electron acceleration (Hudson et al., 2021 ).…”

A Mosaic of the Inner Heliosphere: Three Carrington Rotations During the Whole Heliosphere and Planetary Interactions Interval

“…The radiation belt electrons (Figures 13h–13j) show significantly enhanced electron acceleration associated with the main CIR in CR2221. In fact, despite the historically moderate Sym‐H and AE for the CIR passage in CR2221, observations in CR2221 show the largest enhancement in 7.7 MeV electrons seen in the previous two years (see Hudson et al., 2021). While all three passages of the CIR drive acceleration of electrons to observable levels in the 1.8 MeV (Figure 13h) and 4.2 MeV (Figure 13i) electron measurements, only CR2221 is associated with enhancements of 7.7 MeV electrons to a detectable level (Figure 13j).…”

“…Notably, CR2221 is the only Carrington rotation to have 7.7 MeV electrons at observable levels. While the magnetospheric response to the different CIRs are moderate by historical perspectives, the 7.7 MeV electron enhancement stands out as abnormally large (see Hudson et al., 2021). A potential explanation for the significantly higher acceleration of relativistic electrons in CR2221 is related to the impact of the preceding high speed stream during this Carrington rotation.…”

Modeling Radiation Belt Electron Dropouts During Moderate Geomagnetic Storms Using Radial Diffusion Coefficients Estimated With Global MHD Simulations

“…It is still challenging to simulate the responses of the outer radiation belt to the geomagnetic conditions imposed by ICMEs and CIRs in case studies. In this context, case studies simulating electron dropouts under moderate geomagnetic storms are still scarce in the literature, with some attempts using magnetohydrodynamic (MHD)‐test particle runs (Hudson et al., 2012, 2021). Also, Yu et al.…”

“…It is still challenging to simulate the responses of the outer radiation belt to the geomagnetic conditions imposed by ICMEs and CIRs in case studies. In this context, case studies simulating electron dropouts under moderate geomagnetic storms are still scarce in the literature, with some attempts using magnetohydrodynamic (MHD)-test particle runs (Hudson et al, 2012(Hudson et al, , 2021. Also, Yu et al (2013) inserted superposed epoch conditions from 67 high-speed stream minor storms into a radial diffusion code to model relativistic electron flux dropouts, which were compared with GPS observations spanning 4 ≤ L* ≤ 7.…”