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.

*Physics o fAuroral Phenomena”, Proc. XXXIV Annual Seminar, A patity, pp . 5 7 - 6 0 2011 / ^ \ Polar © Kola Science Centre, Russian Academy o f Science, 2011 W w Geophysical \ y y Institute LOCATION OF THE ION-CYCLOTRON INSTABILITY REGION RELATIVELY TO PLASMAPAUSE DURING MAGNETOSPHERIC COMPRESSIONS A. Yahnin1, T. Yahnina1, H. Frey2, V. Pierrard3, T, Popova1 ; Polar Geophysical Institute, Apatity, Russia 2Space Sciences Laboratory, University o f California, Berkeley, California, USA Belgian Institutefo r Space Aeronomy, Brussels, Belgium 1Polar Geophysical Institute, Apatity, Russia Abstract. Compression of the magnetosphere by a jump of the solar wind dynamic pressure produces, among other consequences, a large-scale dayside precipitation of energetic protons responsible for sub-oval proton aurora flashes. These flashes are related to a sudden appearance of geomagnetic pulsations in the Pci range. Both proton precipitation (proton aurora) and Pci manifest the development of the ion-cyclotron instability in the equatorial plane of the magnetosphere. To explore the magnetospheric domain where the instability develops we combined the projection of the equatorial edge of the proton aurora flashes observed by the IMAGE spacecraft and plasmapause location. The latter was determined using the plasmapause model. It was shown that during magnetospheric compression an ion-cyclotron interaction mostly occurs outside plasmasphere. It was also found that the location of the Earthward edge of the ion-cyclotron instability region as well as the distance between this edge and plasmapause depends on preceding geomagnetic activity. During stronger geomagnetic activity the ion-cyclotron instability region tends to locate closer to the Earth and plasmapause. Introduction Compression of the magnetosphere during an increase of the solar wind dynamic pressure leads to increased anisotropy of the ring current protons and, respectively, to increase of the ion-cyclotron (IC) instability growth rate. Maximal growth rate is expected on the dayside [Olson and Lee, 1983]. Cyclotron instability related to the magnetospheric compressions leads to generation of the electromagnetic ion-cyclotron (EMIC) waves observed in space (e.g., Anderson and Hamilton, 1993; Zhang et al., 2008). On the ground the waves are seen as geomagnetic pulsations in the Pci range (e.g., Anderson et al., 1996; Yahnina et al., 2008; Popova et al., 2010). Another manifestation of the IC instability is precipitation of energetic protons, which can produce so-called “proton aurora”. During compressions proton aurora flashes are often observed on the dayside at sub-oval latitudes visualizing the IC instability region (e.g., Hubert et al., 2003; Fuselier et al., 2004; Yahnina et al., 2008;Popova et al., 2010). Besides hot proton anisotropy, other factors influencing the IC instability growth rate are the cold plasma density and the hot proton density (e.g., Olson and Lee, 1983). For example, sub-oval proton spots and arcs, which are also results of the IC interaction (Yahnin et al., 2007; 2009), are generated at gradients of the cold plasma (e.g., Frey et al., 2004; Spasojevic and Fuselier, 2009). As to proton aurora flashes, their relation to the cold plasma is not clear. Fuselier et al. (2004) considered two cases when the comparison of proton auroras and cold plasma observations was possible. They showed that the proton aurora flashes could map both close to plasmapause and well outside it. The authors referred the latter situation to the increased hot proton intensity outside the cold plasma region. Since comparisons of proton auroras and direct observations of the cold plasma are scanty, one may use plasmapause models. Here we discuss how the proton aurora flashes (indicator of the IC instability region during magnetospheric compressions) locate relatively to the cold plasma using the plasmapause model by V. Pierrard and co-authors. We also consider how the proton aurora flash location depends on the preceding geomagnetic activity, which can be used as a measure for the intensity of the hot plasma population in the near-Earth environment. Data and analysis To select events for the study we used a database of sub-oval proton aurora flashes by Popova et al. (2010). These flashes correspond to sharp solar increases (ДР > 1 nPa) of the wind dynamic pressure. Only those proton aurora flashes were considered, which were conjugated with observations o f geomagnetic pulsations in the Pci range. Such association of proton aurora and Pci pulsations proves that the proton precipitation is due to ion-cyclotron interaction. In all, 25 events were selected. 57