Investigations of the auroral luminosity distribution and the dynamics of discrete auroral forms in a historical retrospective

摘要

Research results about planetary-scaleauroral distributions are presented in a historical retrospective, beginningwith the first "maps of isochasms" – lines of equal visibility of aurorasin the firmament (Fig. 2) – up to "isoaurora maps" – lines ofequal occurrence frequency of auroras in the zenith (Fig. 4). Theexploration of auroras in Russia from Lomonosov in the 18th century(Fig. 1) until the start of the International Geophysical Year (IGY)in 1957 is shortly summed up. A generalised pattern of discrete auroral formsalong the auroral oval during geomagnetically very quiet intervals ispresented in Fig. 5. The changes of discrete auroral forms versuslocal time exhibit a fixed pattern with respect to the sun. The auroral formscomprise rays near noon, homogeneous arcs during the evening, and rayed arcsand bands during the night and in the morning. This fixed auroral pattern isunsettled during disturbances, which occur sometimes even during very quietintervals. The azimuths of extended auroral forms vary with local time. Suchvariations in the orientation of extended forms above stations in the auroralzone have been used by various investigators to determine the position of theauroral oval (Fig. 9). Auroral luminosity of the daytime andnighttime sectors differ owing to different luminosity forms, directions ofmotion of the discrete forms, the height of the luminescent layers, and thespectral composition (predominant red emissions during daytime and greenemissions during the night). Schemes that summarise principal peculiaritiesof daytime luminosity, its structure in MLT (magnetic localtime) and MLat (magnetic latitude) coordinates, and thespectral composition of the luminosity are presented in Figs. 15 and19. We discuss in detail the daytime sector dynamics of individualdiscrete forms for both quiet conditions and auroral substorms. The mostimportant auroral changes during substorms occur in the nighttime sector. Wepresent the evolution of conceptions about the succession of discrete auroralforms and their dynamics during disturbance intervals. This ranges fromBirkeland's polar elementary storms, over the prospect of a fixed auroralpattern up to the auroral substorm model. The classic schemes of the spatialdistribution and motion of discrete auroral forms during single substorms areshown in Fig. 20 (expansive and recovery phases) andFig. 21 (creation, expansive and recovery phases). In this review wediscuss various models of bulge formation, in particular as a result of new formation of arcs about 50–100 km poleward of previously existing auroralstructures (Fig. 24). Discrete steps in the development of anexpanding bulge are separated by 1–3 min from each other. The model ofsuccessive activations confines only to a ~40° longitudinalportion of the magnetotail (Fig. 28). We consider differences in thedevelopment of single substorms and substorms during magnetic storms. Thestructure and dynamics of auroras during steady magnetospheric convection(SMC) periods are dealt with in Sect. 8. A generalised schemeof the auroral distribution during SMC periods is shown in Fig. 34.Separate sections describe discrete auroras in the polar cap(Sect. 5), and the diffuse luminosity equatorward of the auroraloval (Sect. 9). Visual observations of diffuse auroral formsat midlatitudes suggest that the whole latitudinal interval between theauroral oval and the stable auroral red (SAR) arc is filled up with diffuseluminosity. SAR arcs with intensities of several tens of Rayleigh enclosesystematically the region of diffuse luminosity; they are positioned at theborder of the plasmasphere.