Physics of auroral phenomena : proceedings of the 40th annual seminar, Apatity, 13-17 March, 2017 / [ed. board: N. V. Semenova, A. G. Yahnin]. - Апатиты : Издательство Кольского научного центра РАН, 2017. - 143 с. : ил., табл.

Polar Geophysical Institute ON THE OCCURRENCE OF PERIODIC AURORAL ARC RESTRUCTURING BEFORE SUBSTORM ONSET “Physics of Auroral Phenomena”, Proc. XL Annual Seminar, Apatity, pp. 27-29, 2017 © Polar Geophysical Institute, 2017 T.A. Kornilova, I.V. Golovchanskaya Polar Geophysical Institute, Apatity, Russia Abstract. We show that periodic arc restructuring manifesting as repetitive poleward excursions of auroral arcs 3- 15 min before To is typically observed at the location of subsequent substorm onset by the precise optical data of Lovozero, Loparskaya, Tumanny and Gillam. This feature can be explained by an apparent latitudinal motion of the phase of oscillations inside Alfvenic field-lined resonance layer (FLR) conjugate to the onset arc. However, to claim that the occurrence of the FLR is the necessary condition of substorm initiation, one should explain the lack o f such restructuring in other events. Here, by investigating statistically the presence or absence of arc restructuring depending on substorm intensity characterized by the value of magnetic bay, as well as on the amplitude of the associated Alfvenic (flapping) fluctuations in the boundary plasma sheet, we test the possibility that for weak events the field- aligned currents inside the corresponding Alfvdn resonance layer may be not sufficient to produce the optical effect under study. A transition to substorm explosive phase is a longstanding complicated problem. Among numerous scenarios of this phenomenon, there is one in which the preonset arc is associated with the Alfven resonance [Southwood, 1974; Chen and Hasegawa, 1974; Samson, 1992; Rae etal., 2014] that occurs at the location of subsequent substorm onset. In our view, within this scenario the main optical signatures of substorm initiation can be reasonably explained. Among these features there are (i) narrowing and brightening of the preonset arc as a manifestation o f resonance narrowing and resonant Alfvdnic (flapping) oscillations growing in time [Greenwald and Walker, 1980]; (ii) periodic auroral arc restructuring 3-15 min before To (the periods are 1-3 min) as a manifestation of apparent phase propagation within the Alfven field-line resonance layer (FLR) [Wright et al., 1999]; (iii) the appearance of the fine structure in the preonset arc (auroral breakup onset) as a manifestation of resonance transfer to the nonlinear stage [Rankin et al., 1995]. Whatever is the mechanism of further substorm development [e.g., Lui and Murphree, 1998], the ultimate cause of cross-tail current diversion to the ionosphere in the suggested scenario are resonant oscillations that grow in time, reach large amplitudes and spoil the cross-tail conductance in the magnetospheric current sheet along the onset arc. In this study we investigate the above point (ii) in more detail in order to clear up if the occurrence of Alfvenic resonance is the necessary condition of subsequent explosion onset. For this purpose, we compare substorm events with and without optical signatures of apparent phase propagation within the resonance layer before To. A representative event, where periodic auroral restructuring is clearly seen ~ 3 min before To by filtered optical observations in LOZ, is shown in Fig. 1. The time To is identified as the beginning of full-scale auroral poleward expansion. 05.03.2002 Lovozero (b) (c) restructuring Figure 1. Magnetic X-component (a) from LOZ (LOZ, 64.2°, 114.4° CGM coordinates, MLT = UT + 2.6 h), original (b) and filtered (c) N-S keograms constructed by all-sky auroral observations at LOZ station for the event of 5 March, 2002; W n and To signify the moments of the restructuring and magnetic bay onset, respectively. 27