A broad climatology of very high latitude substorms Anand K. Singh ⇑ , A.K. Sinha, Rahul Rawat, Bulusu Jayashree, B.M. Pathan, Ajay Dhar Indian Institute of Geomagnetism, Kalamboli Highway, New Panvel (W), Navi Mumbai 410218, India Received 20 December 2011; received in revised form 27 July 2012; accepted 30 July 2012 Abstract Magnetic data from a newly commissioned Indian Antarctic station Bharati (corrected geomagnetic (CGM) coordinates 74.7°S, 97.2°E) and closely-spaced IMAGE chain observatories (100° magnetic meridian in Northern hemisphere) has been analyzed to study the climatology of substorms which were localized poleward of the standard auroral oval. We considered four austral summers (year 2007–2010) when data from Bharati was available. Several very high latitude substorms were observed in this duration when the solar activity remained unexpectedly low for a long time. Various features of very high latitude substorms, e.g., local time dependence, inter- planetary state, hemispherical asymmetry and their nightside low latitude signatures are examined. Events studied here, suggested the following properties of substorms occurring at very high latitudes: (1) maximum occurrence was observed near magnetic midnight (21:00–02:00 MLT). (2) In contradiction to earlier reports, many substorms were observed even during negative IMF Bz condition. In addition, majority of substorms occurred during low or moderate solar wind streams. (3) Magnetic signatures were often pronounced in the winter hemisphere. (4) Even if widely used standard AE indices fail to monitor very high latitude substorms, their low latitude signatures are often evident. Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Substorm; AE indices; Positive bay; Pi2 pulsations 1. Introduction The solar wind continuously buffets the Earth’s magne- tosphere and deposits enormous amount of energy into the magnetotail due to the solar wind magnetosphere interac- tion. Magnetic field lines on the day side of the Earth’s magnetosphere connect with oppositely directed interplan- etary magnetic field (IMF) lines, which are subsequently dragged to the night side magnetotail region by the solar wind (Dungey, 1961). The storage of magnetic and particle flux in the magnetotail is the first stage of a substorm activ- ity, which is known as growth phase (McPherron, 1972). The stored energy is explosively released into the inner magnetosphere during next stage of a substorm called onset or expansion phase, followed by recovery phase (Akasofu, 1964). Auroral latitudes (magnetic 60°–70°) are the most severely affected region due to a substorm activity. Nevertheless, nearly all regions of the magnetosphere undergo significant magnetic and electric field changes dur- ing substorms (McPherron et al., 1973a; Sastri, 2002). A fraction of tail current is diverted into the inner mag- netosphere along the magnetic field lines during the course of a substorm, entering from the dawn and flowing back to the tail on the dusk side via night side auroral ionosphere (McPherron et al., 1973b). The strong ionospheric current flowing from the dawn to the dusk (i.e., westward) casts dramatic magnetic field changes at night side auroral lati- tudes. In addition to it, the high latitude two cell convec- tion pattern, observed as an effect of ionospheric plasma convection (Clauer and Kamide, 1985), perturbs the mag- netic field in other local times at auroral latitudes. Nor- mally, an eastward flowing ionospheric current in the dusk hours is observed at auroral latitude due to the two cell convection pattern, which in turn enhances the mag- netic field during the course of a substorm. 0273-1177/$36.00 Ó 2012 COSPAR. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.asr.2012.07.034 ⇑ Corresponding author. Tel.: +91 22 27484000; fax: +91 22 27480762. E-mail addresses: singhaaks@gmail.com (A.K. Singh), ashwini@iigs. iigm.res.in (A.K. Sinha), rahuliig@gmail.com (R. Rawat), bulusujayashree @gmail.com (B. Jayashree), bmpathan@iigs.iigm.res.in (B.M. Pathan), ajaydhar@iigs.iigm.res.in (A. Dhar). www.elsevier.com/locate/asr Available online at www.sciencedirect.com Advances in Space Research xxx (2012) xxx–xxx Please cite this article in press as: Singh, A.K., et al. A broad climatology of very high latitude substorms. J. Adv. Space Res. (2012), http:// dx.doi.org/10.1016/j.asr.2012.07.034