A classification of dayside auroral forms and activities as a function of interplanetary magnetic field orientation
P. E. Sandholt, C. J. Farrugia, J. Moen, Ø. Noraberg, B. Lybekk, T. Sten, and T. Hansen
Department of Physics, University of Oslo, Norway
We present a classification of auroral forms in the dayside high-latitude ionosphere, based on ground observations from Svalbard. Having sorted the different auroral forms by magnetic local time (MLT) and morphological and optical spectral characteristics, we then study them as a function of the orientation of the interplanetary magnetic field (IMF). We find that the IMF clock angle q is a good parameter with which to order the different dayside auroras. This is illustrated by two case examples covering the whole dayside: (1) the 4-hour-long passage of the sheath region of the January 10-11, 1997, magnetic cloud and (2) a 10-hour-long interval on January 12, 1997, during passage of the corotating stream overtaking the cloud. A variety of IMF conditions were realized. We identify the following three auroral configurations in the cusp region and the IMF clock angle regimes in which they occur: (1) In the clock angle range q <45° there are auroral bands located at high latitudes (~ 78° - 79° magnetic latitude (MLAT)), (2) For q within ~ 45° - 90° , auroral bands are simultaneously present at high and low (< 75° MLAT) latitudes, (3) For q > ~90° the high-latitude aurora disappears, and only the low-latitude forms remain. These latter forms manifest themselves as quasiperiodic sequences of moving bands or band fragments within ~ 73° - 78° MLAT (called poleward moving auroral forms) or quasi-steady auroral bands with east-west moving forms at low latitudes (<73° MLAT). Strong asymmetries in auroral forms and motions are related to the east-west component (By) of the IMF. The above auroral configurations are discussed in terms of current knowledge on particle precipitation, IMF-related, field-aligned currents, and corresponding modes of solar wind-magnetosphere coupling. We find that the time history of the basic magnetopause coupling modes is manifested in the dayside aurora. We identify candidate auroral signatures of both quasi-steady and pulsed reconnection processes occurring at either low or high magnetopause latitudes. Additional auroral forms in the dawn and dusk sectors are discussed in terms of processes in a closed magnetospheric model, such as the Kelvin-Helmholtz instability.
J. Geophys. Res., Vol. 103, No. A10, 23,325-23,345, 1998