Mapping of auroral arcs during substorm growth phase

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Abstract
The substorm growth phase is manifested in the auroral oval by brightening and equatorward motion of auroral arcs, prior to the formation of the westward traveling surge at substorm onset. In the magnetosphere the cross‐tail current enhances and the current sheet becomes very thin, and at substorm onset the tail current system disrupts, connecting the magnetosphere and the ionosphere through field‐aligned currents. The relation between the auroral and tail processes is studied here by magnetic‐field‐aligned mapping from the ionosphere to the current sheet using the Tsyganenko 1989 field model. The dynamical changes in the field and current distributions are modeled by stepwise modification of the field model to include the current sheet thinning and enhancement of the tail current. In addition, the field signatures of a current meander suggested to form in the tail are demonstrated. These signatures are compared with a model where line currents are added to represent the effects of the current wedge forming at substorm onset. Using the Tsyganenko model, the mapping of a growth phase arc recorded by Finnish all‐sky cameras moves earthward from 12 RE at the beginning of the growth phase to about 6 RE by the time of the substorm onset, corresponding to the equatorward motion of the arc. If the observed motion of the arc is interpreted to be entirely due to the stretching of the tail field during the growth phase, the enhancement of the cross‐tail current and the changes in the field configuration can be estimated. In this particular case, the current sheet has to be thinned to 10% of the original value at X = −8 RE, and the cross‐tail current enhanced by a factor of 1.3 to correspond to the equatorward motion of the arc.