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.

On the occurrence o fperiodic auroral arc restructuring before substorm onset 15.03.2009 Gillam С 8.H i c 10200 15000 10200 (a) 04:00 04:30 05:00 05:30 06:00 UT so с a = J К СЛ о и 05 (b) We note that the fluctuations in the Bx component measured by THEMIS P3 probes (Fig. 3c) are connected with the vertical (flapping) motions of the boundary plasma sheet due to gradient in the Bx component directed from the equatorial plasma sheet to the lobe inside the FLR layer. In the case of the presence o f auroral restructuring before To (Fig. 3b) there were significant magnetic oscillations in the plasma sheet (7-10 nT in the Bx component). On the contrary, in the substorm event on 15 March 2009, where no auroral restructuring before To was indicated by optical observations (Fig. 4b), there were no pronounced oscillations in the plasma sheet, as suggested by conjugate in situ observations of THEMIS P2 (THC) probe (Fig. 4c). Thus the amplitude o f associated Bx-oscillations did not exceed 2 nT (Fig. 4c). Conclusions 1. By filtered optical data it is shown that the periodic auroral arc restructuring often develops 3-15 min before To. They can be reasonably explained if one suggests that an Alfven resonance is formed at the location o f a subsequent substorm onset. 2. In the events here considered, the observed direction of apparent phase propagation across the resonance layer in the linear stage is poleward rather than equatorward. This indicates that the Alfven resonance forms in the magnetospheric region with the earthward gradient of the Alfven resonant frequency of the magnetic field lines. 3. For the events with auroral restructuring it is shown the presence of conjugate oscillations of Alfv6n (flapping) type in the plasma sheet with approximately the same period (~ 2-3 min) as the period of auroral restructuring before To. This supports the scenario, in which an Alfven resonance forms at the location of subsequent substorm onset. 4. The conjugate Alfvenic oscillations in the magnetosphere are o f smaller amplitude in the events where auroral restructuring is not observed by optical data before To. Thus, the absence of auroral restructuring before To in these cases can be explained by weak field-aligned currents inside the corresponding Alfven resonance layer, not sufficient to produce the optical effect under study. 05:00 Ш' (c) 04:20 04:30 04:40 04:50 UT Figure 4. The set of data for substorm on 15 March 2009 by (a) magnetic, (b) optical observations at GILL station and (c) conjugate magnetic field Bx-component measured by THEMIS P2 probe in the boundary plasma sheet. Acknow ledgments. This work was supported by Program 7 of the Presidium o f the Russian Academy of Sciences. References - Chen L., Hasegawa A. A theory of long-period magnetic pulsations // J. Geophys. Res. V. 79. № A7. P. 1024-1032.1974. - Greenwald R. A., andWalker A. D. M. Energetics of long period resonant hydromagnetic waves // Geophys. Res. Lett. V. 7. № 10. P. 745-748. 1980. - Kornilov I.A., Kornilova T.A., Golovchanskaya I. V. On the physical nature of auroral breakup precursors as observed in an event on March 2008, Geomagn. Aeron. (Engl. Transl), 2015, vol. 55, no.2, pp. 210-218. - Lui A. T. Y., and Muiphree J. S. A substormmodel with onset location tied to an auroral arc // Geophys. Res. Lett. V. 25. № 8. 1269-1272.1998. - Rae I. J., Murphy K. R., Watt С. E. J., Rostoker G., Rankin R., Mann I. R., Hodgson C. R., Frey H. U., Degeling A. W., Forsyth C. Field line resonances as a trigger and a tracer for substorm onset // J. Geophys. Res. V.119. P. 5343-5363, doi:10.1002/2013JA018889. 2014. - Rankin R., Frycz P, Tikhonchuk V. Т., Samson J. C. Ponderomotive saturation of magnetospheric field line resonances // Geophys. Res. Lett. V. 22. № 13. P. 1741-1744. 1995. - Samson J. C., Wallis D. D., Hughes T. J., Creutzberg F., Ruohoniemi J. М., Greenwald R. A. Substorm intensifications and field line resonances in the nightside magnetosphere // J. Geophys. Res. V. 97. №A6. P. 8495-8518.1992. - Southwood D. J. Some features of field-line resonances in the magnetosphere // Planet. Space Sci. V. 22. P. 483. 1974. -Wright A. N., Allan W., Elphinstone R.D., Cogger L.L. Phase mixing and phase motion of Alfven waves on tail-like and dipole­ like magnetic field lines // J. Geophys. Res. V. 104. №A5. P. 10159-10175.1999. 29