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 (B_y) 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