A classification of dayside auroral forms and activities as a function of interplanetary magnetic field orientation


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 θ 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 θ< 45° there are auroral bands located at high latitudes (∼ 78° – 79° magnetic latitude (MLAT)), (2) For θ within ∼ 45° – 90°, auroral bands are simultaneously present at high and low (< 75°MLAT) latitudes, (3) For θ > ∼ 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.

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JGR: Space Physics



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