Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

Location o f th e ion-cyclotron instability region relatively to p la sm a p a u se during m agnetospheric compressions Discussion Summarizing the results presented in Figures 1 and 2 and keeping in mind that sub-oval proton auroras are the consequence of the IC interaction in the equatorial plane, we conclude that: 1) the most of the region where the IC instability develops during magnetospheric compression does not relate to the enhanced cold plasma density and 2) the preceding geomagnetic activity is the factor controlling the development of the compression-related IC instability close to the Earth and plasmapause. As already noted, the result 1) means that during compressions the cold plasma density is not so important parameter for the development of the IC instability as, say, in the case of generation of sub-oval proton spots. Relative enhancement of the anisotropy (Ai/Ao, where Ao and Ai is anisotropy before and after compression) is stronger around noon in the outer magnetosphere ( Olson and Lee, 1983) and, probably, is insignificant at and inside plasmapause. However, in some cases (first five events in Fugure 1) the mapped proton aurora flashes overlap with modeled plasmasphere. Evidently, in these cases the combination of the parameters controlling the IC instability growth rate (density of cold plasma, density of hot plasma and hot plasma anisotropy) before the compression corresponds to a marginal stability in a wide range of distances including the outer plasmasphere. Also, for such cases either A(SYM-H), characterizing the compression, or geomagnetic activity, or both have the largest values. How the geomagnetic activity can affect spatial characteristics of the IC instability region? We suggest that the activity changes radial distribution of the hot proton population. Indeed, enhanced geomagnetic activity associates with enhanced magnetospheric convection and substorm ejections. Both these processes contribute to filling the near-Earth environment with energetic particles. Thus, after long period of the activity one may expect significant increase of the hot plasma density close to the Earth (and plasmasphere). This may lead to the situations when even relatively weak compressions provoke the instability development close to the Earth. The latter is, evidently, the reason of weak dependence of L^, and Lcq-Lpp on A(SYM-H) shown in Figure 2. 10 - 8 -1 c r _ ® 6 - 4 - 9 10 — R-squared = 0.10 _ 8 - 0 0 _ _ 10 - i R-squared = 0.47 8 - ♦ * V о- ® 6 - 4 - О О о ю -1 R-squared = 0.45 0 0 о R-squared = 0.55 ♦ / Ч . О. 4 CL & 2 - 0) - J 0 - .9 l 1 l 1 l 1 l 1 2 R-squared = 0.14 л cl 4 4 ♦♦ * 2 - ♦ ♦ ♦ * 0 - о 1 [ г р г Г i | I | 2 R-squared = 0.32 4 _ . о. ♦ / * - 1 0 Ч З г J 0 - о , 1 1 1 , 1 1 1 2 _ R-squared = 0.32 л __ C l ^ > * Ч 2 - СГ Z- * " 0 - ч ч Ч ч ч R-squared = 0.46 V . v , с I ! 1 I 1 I I I - 20 40 60 80 -e Д(SYM-H) Ч Ч Ч Ч Ч 0-40-20 0 20 40 С (SYM-H )1 1 1 1 1 1 1 1 1 - 10 20 30 40 Mean Кр I г I I I 200 400 Mean AE Figure 2. The distance between the equatorial projection of the proton aurora flash and the Earth (Leq) and modeled plasmapause (Leq-Lpp) as depended on different geomagnetic parameters. From left to right: Dependence on intensity of magnetospheric compression characterizing by difference between SYM-H index values just before and after the compression; dependence on the value of SYM-H index just before the compression; dependence on average Kp for 48 hour interval before the compression; and dependence on average AE for 48 hour interval before the compression. Note, that squared values of the correlation coefficients are presented above each plot. Acknowledgements. We thank the IMAGE FUV team and staff of the observatory Lovozero for data used in this study. The information on indices of geomagnetic activity is taken from the CDAWeb database. The work is performed in frames of the basic research program № VI. 15 “Plasma processes in the solar system” of the Division of Physical Sciences of RAS. The work by T.A.Y. and T.A.P. is also supported by the RFBR grant № 1 1-02-00397. V. Pierrard thanks the European Commission’s Seventh Framework Program (FP7/2007-2013) inside the grant agreement SWIFF (project n°2633430, www.swiff.eu ~) 59