The double auroral oval in the dusk‐midnight sector: Formation, mapping and dynamics

This is the authors' abstract. We don't add key points for this paper.

The present study observationally examines the double auroral oval structure in the evening‐premidnight sector. The results are summarized as follows; (1) The poleward auroral branch can be attributed to accelerated auroral precipitation, occasionally accompanied by Alfvénic precipitation, and it is collocated with the upward R1 current; (2) The equatorward auroral branch, which is embedded in the downward R2 current, very often if not always consists of the diffuse precipitation of energetic electrons and extends mostly poleward of the b2i boundary collocated with energetic ion precipitation; (3) The associated ionospheric convection is consistent with the two‐cell pattern, and the poleward branch is located at the zonal flow shear. It is inferred that the equatorward branch is mapped to the geosynchronous region and outside, whereas the poleward branch is mapped farther down the tail to the source region of the R1 system. It is suggested that the intense ring current and injection of energetic electrons are necessary for the equatorward branch to be distinct, but the substorm expansion phase is not favorable for the formation of the emission gap. This explains why the double oval often takes place during magnetospheric storms and during the recovery phase of intense substorms. The equatorial magnetic field is expected to be strong in the mid‐tail region, which might be related to the auroral gap region. It is also found that the poleward branch occasionally intensifies in an extended MLT sector in a few minutes, which may be explained in terms of the sudden enhancement of the return (sunward) convection flow.