Classifications of Substorms With MLT Patterns of SMU‐SML Indices Based on Convection Properties

Abstract

Abstract During substorms, magnetospheric convection and magnetotail current sheet disturbances are two critical processes, both driving ionospheric electrojets. Dusk‐side SMU and dawn‐side SML (SuperMAG Electrojet indices) enhancements reflect two‐cell convection. Using this relation, we categorize substorms with different convection properties near onset. When primary SML enhancements occur near midnight, the presence or absence of dusk‐side SMU/dawn‐side SML activity distinguishes strong from weak convection events. Additionally, we identify “dawn‐dominant” events characterized by primary SML enhancements in the dawn sector as a separate category. “Strong convection” events feature stronger and longer southward IMF and faster solar wind speeds than “weak convection” events. “Dawn‐dominant” events share similar conditions with the “strong convection” category plus a tendency of negative B y . “Strong convection” events statistically have lower onset magnetic latitudes (MLAT), indicating current sheet instabilities closer to the Earth. Field‐aligned current (FAC) evolution reveals that the two‐cell convection pattern extends from dayside to nightside before onset, possibly contributing to flux depletion and current sheet thinning. An event with strong convection exhibits continued pressure loading, with unloading occurring only after onset. This indicates a significant contribution of open flux accumulation to current sheet thinning. The example dawn‐dominant event has strong global convection dominating the SMU/L profile, and aurora are brightened in a wide MLT range. On top of this, localized bursts in SML, aurora, dipolarizations, and flux ropes suggest that meso‐scale current sheet disturbances occur. These classifications and analyses help clarify how convection and current sheet disturbances develop and interact, potentially valuable for elucidating the descriptions of substorm processes.