Computational analysis of the near‐Earth magnetospheric current system during two‐phase decay storms

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Abstract
Several two‐phase decay magnetic storms are examined using a kinetic transport model to find the spatial and temporal distribution of the perpendicular and field‐aligned currents in the inner magnetosphere. The global morphology of these currents in the calculational domain (inside of geosynchronous orbit) is discussed as a function of storm epoch, obtaining good comparison between the numerically derived features and observed values of stormtime currents in this region. The model results are also consistent with quiet time plasma observations showing an increasing pressure in to L = 3 or 4, including a pressure maximum near midnight for the generation of region 2 Birkeland currents in the proper direction. A detailed analysis of the characteristic features of these currents is also presented and discussed. It is found that most of the ring current (>90%) during the main phase and early recovery phase is partial rather than symmetric, closing mostly (up to 90%) through field‐aligned currents into the ionosphere. Conversely, the quiet time ring current is largely (>60%) symmetric, with most of the asymmetry produced by minor injections of near‐Earth plasma sheet material. In general, the peak asymmetric current (which occurs during the main phase) is 2‐3 times larger than the peak symmetric current (which occurs during the recovery phase) for any particular two‐phase decay event. This is the case for all of the events studied, regardless of storm size, solar wind parameters, or solar cycle. The maximum azimuthal current (integrated over a local time slice) reaches 5 to 20 MA, compared with <2 MA of symmetric current at quiet times.