Auroral forms that extend equatorward from the persistent midday aurora during geomagnetically quiet periods

Auroral forms (“crewcuts”) that extend equatorward from the geomagnetically east–west aligned persistent midday aurora have been characterized based on a survey of Svalbard 630.0-nm all-sky images from eight December solstice seasons. The 630.0-nm forms are directly related to a mix of precipitating magnetosheath and plasma sheet electrons embedded in a population of boundary layer ions, as observed by NOAA 6. The associated midday aurora lies at the equatorward edge of poleward velocity-dispersed cusp ‘ion plumes’ observed by DMSP F7 at 1000 magnetic local time. In this survey, crewcuts are found to occur for Kp≤3. Distinct in many salient ways (including dynamics, orientation and associated IMF polarity) from the poleward-moving auroral forms (PMAFs) associated with transient dayside merging, crewcuts represent a solar wind-magnetosphere interaction that is more characteristic of quiet geomagnetic conditions. Crewcuts are present on 44% of the days for which all-sky images are complete and observing conditions are clear ±1.5 h around geomagnetic noon. During these periods, crewcuts are observed during 10.6% of the minutes for which IMF data from ISEE 2 or IMP 8 are available. We derive from the data a simple set of IMF “search criteria” that maximize the likelihood of observing crewcuts around the December solstice. The IMF magnitude range for which crewcuts are most commonly observed is 3.5±0.5 nT (36%). The IMF GSM component ranges in which crewcuts are most commonly observed are: Bx=−3.5±0.5 nT (27%), By=−1.5±0.5 nT (25%), and Bz=0.5±0.5 nT (26%). The IMF clock angular ranges for which crewcuts are most commonly observed are −70°±10° and 90°±10° (both 21%). The IMF cone angular range for which crewcuts are most commonly observed is 12.5°±2.5° (28%). Considering these observations in the context of magnetic merging topologies, we suggest crewcuts observed at Svalbard are most likely to lie at the foot of overdraped field lines recently opened by spatially patchy, quasi-steady merging in the southern hemisphere.

► Auroral forms grow equatorward from the midday aurora and persist for ≤25 min.
► They occur under quiet geomagnetic conditions (Kp≤3).
► They are signatures of precipitating magnetosheath and plasmasheet electrons.
► They are most commonly observed for dominant IMF Bx (usually negative).