Challenging the Use of Ring Current Indices During Geomagnetic Storms

J. K. Sandhu,M.‐T. Walach,I. J. Rae

doi:10.1029/2020ja028423

J. K. Sandhu, M.‐T. Walach + Show 1 more

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https://doi.org/10.1029/2020ja028423

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Abstract

AbstractThe ring current experiences dramatic enhancements during geomagnetic storms; however, understanding the global distribution of ring current energy content is restricted by spacecraft coverage. Many studies use ring current indices as a proxy for energy content, but these indices average over spatial variations and include additional contributions. We have conducted an analysis of Van Allen Probes' data, identifying the spatial distribution and storm‐time variations of energy content. Ion observations from the HOPE and RBSPICE instruments were used to estimate energy content in L‐MLT bins. The results show large enhancements particularly in the premidnight sector during the main phase, alongside reductions in local time asymmetry and intensity during the recovery phase. A comparison with estimated energy content using the Sym‐H index was conducted. In agreement with previous results, the Sym‐H index significantly overestimates (by up to ∼4 times) the energy content, and we attribute the difference to contributions from additional current systems. A new finding is an observed temporal discrepancy, where energy content estimates from the Sym‐H index maximize 3–9 h earlier than in‐situ observations. Case studies reveal a complex relationship, where variable degrees of agreement between the Sym‐H index and in‐situ measurements are observed. The results highlight the drawbacks of ring current indices and emphasize the variability of the storm time ring current.

Highlights

Geomagnetic storms were first discovered from observations of large irregular disturbances in the global geomagnetic field (Graham, 1724; Chapman & Bartels, 1940).It was suggested that charged drifting particles in the magnetosphere generate a westward current and an associated magnetic field perturbation that opposes the background geomagnetic field (e.g., Chapman & Dyson, 1918; Chapman & Ferraro, 1930; Singer, 1957).This current is known as the ring current
Previous studies have established that the ring current can exhibit strong local time asymmetries with energy content peaking in the pre-midnight sector (e.g., Jordanova et al, 2003), and the Magnetic Local Time (MLT) bins employed in Figure 2 were chosen to centre on the regions of maximum asymmetry
The L shell dependence is observed to be relatively independent of storm phase, where the L profile shown in Figure 2b is similar for both storm times and quiet times

Summary

Introduction

It was suggested that charged drifting particles in the magnetosphere generate a westward current and an associated magnetic field perturbation that opposes the background geomagnetic field (e.g., Chapman & Dyson, 1918; Chapman & Ferraro, 1930; Singer, 1957). This current is known as the ring current. The enhanced storm time ring current, and the associated magnetic field perturbations from the westward current, play an important role in a number of magnetospheric processes

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