Sub-oval proton aurora spots: Mapping relatively to the plasmapause A.G. Yahnin a,n , T.A. Yahnina a , H. Frey b , V. Pierrard c a Polar Geophysical Institute, Apatity, Russia b Space Sciences Laboratory, University of California, Berkeley, California, USA c Belgian Institute for Space Aeronomy, Brussels, Belgium article info Article history: Received 14 April 2012 Received in revised form 15 August 2012 Accepted 15 September 2012 Available online 4 October 2012 Keywords: Proton aurora EMIC waves Plasmapause abstract Sub-oval proton auroras discovered by the IMAGE spacecraft correlate with EMIC waves (geomagnetic pulsations of the Pc1 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. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction One of the main results of the IMAGE spacecraft mission is the global imaging of the ‘‘proton aurora’’ (the Doppler-shifted emis- sion of neutral hydrogen atoms originating from precipitating protons after charge exchange). In particular, different types of the proton aurora equatorward of the main auroral oval were discovered. During the recovery phase of a magnetic storm, ‘‘proton aurora spots’’ with typical dimension of 100–300 km may appear (Frey et al., 2004). These spots have a rather long (up to few hours) duration. They stay on approximately the same geomagnetic latitude and drift eastward with the co-rotation speed. On the evening side, ‘‘proton aurora arcs’’ are observed (Immel et al., 2002; Burch et al., 2002). During magnetospheric compressions related to sharp increases of the solar wind dynamic pressure, so-called ‘‘proton aurora flashes’’ (short-lived, but relatively large-scale proton aurora forms) appear on the dayside (Hubert et al., 2003; Zhang et al., 2003; Fuselier et al., 2004). The IMAGE observations of the proton auroras including various sub-oval forms were reviewed by Frey (2007). Unlike the proton aurora oval which associates with 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 (e.g., Sergeev et al., 1983; Blockx et al., 2005), the sub-oval proton auroras are related to the localized precipitation of 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 a transverse anisotropy is favorable for the development of the ion-cyclotron (IC) instability (Cornwall, 1965; Kennel and Petschek, 1966). Since this instability leads to scattering of protons into the loss-cone, it has been considered as a candidate mechanism for the sub-oval proton auroras (e.g., Frey et al., 2004; Burch et al., 2002; Fuselier et al., 2004). The instability also leads to the growth of electro- magnetic ion-cyclotron (EMIC) waves; thus, correlation of proton auroras and EMIC waves (or geomagnetic pulsations in the Pc1 range, which are the signature of EMIC waves on the ground) is an important test to prove the mechanism of the proton aurora generation. The correlation has been indeed found for all types of the sub-oval proton aurora (e.g., Immel et al., 2005; Yahnin et al., 2007; Yahnina et al., 2008; Yahnin et al., 2009; Zhang et al., 2008). In particular, Yahnin et al. (2007) and Yahnina and Yahnin (2012) showed a close temporal and spatial relationship between the proton aurora spots and geomagnetic Pc1 pulsations. This rela- tionship strongly supports the IC instability as a mechanism of the proton precipitation responsible for the sub-oval proton aurora. The variety of sub-oval proton aurora forms as well as related types of EMIC waves evidently points to different magnetospheric Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics 1364-6826/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2012.09.018 n Corresponding author. E-mail addresses: ayahnin@gmail.com (A.G. Yahnin), hfrey@ssl.berkeley.edu (H. Frey), viviane.pierrard@oma.be (V. Pierrard). Journal of Atmospheric and Solar-Terrestrial Physics 99 (2013) 61–66