Challenging the Use of Ring Current Indices During Geomagnetic Storms

Abstract

The 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.

ULF Wave Driven Radial Diffusion During Geomagnetic Storms: A Statistical Analysis of Van Allen Probes Observations

“…The main phase has a typical duration of a few hours (Walach & Grocott, 2019). The final storm phase is the recovery phase, where the Sym‐H index gradually recovers and increases to quiet time levels as the ring current decays (e.g., Sandhu et al., 2021). The recovery phase typically lasts several days (Walach & Grocott, 2019).…”

“…The initial phase is evident from a positive perturbation in the Sym‐H index due to enhancements in the magnetopause currents and typically lasts ∼1 day (Walach & Grocott, 2019). The inital phase is followed by the main phase, which is characterized by a rapid negative change in the Sym‐H index due to significant and dramatic enhancements in the ring current population (e.g., Sandhu et al., 2021). The main phase has a typical duration of a few hours (Walach & Grocott, 2019).…”

The Roles of the Magnetopause and Plasmapause in Storm‐Time ULF Wave Power Enhancements

“…Solar wind structures, such as coronal mass ejections and corotating interaction regions are associated with strong enhancements in ULF wave power (Hudson et al., 2014; Simms et al., 2010; Zong et al., 2009). Externally driven ULF waves have a broadband nature and an occurrence peaking across the dayside sector (Liu et al., 2009; Nosé et al., 1995; Nykyri, 2013; Pahud et al., 2009; Takahashi et al., 2016; Sandhu, Rae, Wygant, et al., 2021). Internal sources are associated with enhanced substorm activity and coupling to injected particle distributions (Baddeley et al., 2005; Engebretson & Cahill, 1981; Hughes, 1983; James et al., 2016; Nosé et al., 1998; Woch et al., 1990).…”

Concurrent Empirical Magnetic Reconstruction of Storm and Substorm Spatial Scales Using Data Mining and Virtual Spacecraft

“…The most notable inconsistencies appear in Sym-H * | and D Sym-H * |, which is a result of mixing parameters of different smoothing scales. Sym-H * and its derivative are smoothed over storm time frames of st = 12 h while the AL equivalents are instead smoothed over substorm scales sst = 2 h. This also demonstrates that the pressure-corrected storm indices contain contributions from substorm current systems and is not representative of just a pure ring current as has been discussed in previous studies [79,80]. Other deviations appear in particularly when AL| it reaches its minimum.…”