Dayside auroral bifurcation sequence during By‐dominated interplanetary magnetic field: Relationship with merging and lobe convection cells

Abstract

We report observations in the cusp region of synchronous activations of latitudinally separated auroral forms (“bifurcations”) occurring in a sequence and interpret the observations in terms of an intermittent magnetic reconnection process at the dayside magnetopause proceeding in a wave‐like manner from lower to higher latitudes. The observations refer to a By‐dominated interplanetary magnetic field (IMF) orientation. Optical auroral observations are combined with radar observations of ionospheric ion drift to illustrate the association between the aurora and plasma convection in two cases representing positive and negative IMF By conditions. In the meridian photometer scans, each individual event in the sequence appeared as an initial brightening in the south (type 1), followed by a second brightening/expansion farther to the north (type 2). The events occurring during an eastward pointing (By>0) IMF are observed typically to expand westward across the 1200 magnetic local time meridian, from the postnoon to the prenoon side. The higher‐latitude, type 2 activity is associated with strong westward convection, which we identify to be part of a lobe cell, while the equatorward boundary intensifications (type 1) occur in the region of a merging cell, distorted by the prevailing IMF By condition. The auroral sequence consists of several paired activation events, typically recurring at ∼ 5‐to 10‐min intervals, and each individual event lasting ∼10 min. The ion drift observations are found to be consistent with recent MHD modeling results on IMF magnetosphere interconnection geometry and the associated composite pattern of merging and lobe convection cells in the cusp region. The ground observations reveal the intermittent nature of two components of cusp region particle precipitation and the association between the corresponding type 1 and 2 auroras and merging and lobe convection cells, respectively.