Physics of auroral phenomena : proceedings of the 35th Annual seminar, Apatity, 28 Februaru – 02 March, 2012 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2012. - 187 с. : ил., табл.

“Physics o f Auroral Phenomena", Proc. XXXV Annual Seminar, Apatity, pp. 49 - 52, 2012 © Kola Science Centre, Russian Academy of Science, 2012 MAPPING THE PROTON AURORA SPOTS INTO THE MAGNETOSPHERE A.G. Yahnin1, T.A. Yahnina1, H. Frey2, V. Pierrard 3 1Polar Geophysical Institute, Apatity, Russia 2Space Sciences Laboratory, University of California, Berkeley, California, USA 3 Belgian Institutefor Space Aeronomy, Brussels, Belgium Abstract. Sub-oval proton auroras discovered by the IMAGE spacecraft correlate with electromagnetic ion- cyclotron (EMIC) waves (geomagnetic pulsations of the Pci range). This means that a common source of the waves and proton precipitation is the ion-cyclotron (IC) instability developing in the vicinity of the equatorial plane. Different forms of the proton auroras reflect different regimes of the IC instability and different conditions in the near-Earth equatorial magnetosphere. To understand what are the conditions for the generation of the sub-oval proton aurora one may map the aurora onto the equatorial plane and compare the projection with some important magnetospheric boundaries. In this report we compare the projection of so-called “proton aurora spots” with the location of the plasmapause. The latter is determined by the plasmapause formation model based on the quasi­ interchange instability mechanism. The comparison shows that often the proton aurora spot source is located in the vicinity of the plasmapause or in the cold plasma gradient inside the plasmapause. In some events, the proton aurora spots map well outside the plasmapause. We assume that in the latter case the IC instability develops when westward drifting energetic protons interact with the cold plasma that was earlier detached from the plasmasphere. 1. Introduction One of the main results of the IMAGE spacecraft mission is the global imaging of the “proton aurora” (the Doppler- shifted emission of neutral hydrogen atoms originating from precipitating protons after the charge exchange). Different types of the proton aurora equatorward of the main auroral oval were discovered. In particular, during the recovery phase of the magnetic storm, “proton aurora spots” with typical dimension of 100-300 km may appear (Frey et al., 2004). These spots have rather long (up to few hours) duration. They stay on approximately the same latitude and drift eastward with a co-rotation speed. Unlike the proton aurora oval associates with the proton precipitation from the plasma sheet region, where the pitch-angle distribution of the protons is isotropic due to scattering in the weak equatorial magnetic field, the sub­ oval proton auroras are related to the localized precipitation of the energetic protons (LPEP; see, Yahnin and Yahnina, 2007) within the anisotropic zone where the loss-cone is typically empty and the transverse temperature of protons is higher than the field-aligned temperature. Such transverse anisotropy is favorable for the development of the ion-cyclotron (IC) instability (e.g., Cornwall, 1965). Since this instability leads to scattering of protons into the loss-cone, it has been considered as a candidate for mechanism of precipitation responsible for the sub-oval proton auroras (e.g., Frey et al., 2004; Burch et al., 2003; Fuselier et al., 2004). The instability also leads to growth of EMIC waves; thus, correlation of proton auroras and EMIC waves (or geomagnetic pulsations in the Pci range, which are the signature of EMIC waves on the ground) is an important test to prove the mechanism of the proton aurora generation. Yahnin et al. (2007) and Yahnina and Yahnin (2012) showed close temporal and spatial relationship between the proton aurora spots and geomagnetic pulsations Pci. This relationship strongly supports the IC instability 7 as a mechanism of the proton precipitation responsible for the sub-oval proton aurora spots. Although a primary source of the IC instability is the transverse anisotropy of proton temperature, other parameters (e.g., cold plasma density and hot plasma beta) are also important factors controlling the development of the instability. Cold plasma gradients are often considered as a location where the instability growth rate is maximal (e.g., Kozyra et al., 1984). To understand magnetospheric conditions in the source region, one may map the proton auroras into the magnetosphere and compare the projections with some important magnetospheric structures, for example, with the cold plasma distribution. Frey et al. (2004) compared proton aurora spot projections with plasmasphere images obtained with the IMAGE EUV instrument. They considered only two events and concluded that the proton aurora spots map into the vicinity of the cold plasma gradient (plasmapause or inner cold plasma gradient). Unfortunately, such direct comparisons of the proton auroras with EUV observations of the plasmasphere are scanty. Another way for such comparison is to use plasmasphere models. The aim of this paper is to compare projections of the proton aurora spots with the location of the plasmapause obtained from the numerical model based on the quasi-interchange instability mechanism for the plasmapause formation (see, e.g., Lemaire and Gringauz, 1998). The model calculates the position of the plasmapause for the required time interval, assuming the corotation and using the convection electric field model E5D (Mcllwain, 1986) and the associated magnetic field model М2 (Mcllwain, 1972). The electric field E5D depends on Kp-index, and for calculations the changes of Kp observed Polar Geophysical Institute