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 ( 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.