Physics of auroral phenomena : proceedings of the 37th Annual seminar, Apatity, 25 - 28 February, 2014 / [ed. board: A. G. Yahnin, N. V. Semenova]. - Апатиты : Изд-во Кольского научного центра РАН, 2014. - 125 с. : ил., табл.

“POLAR” AND “HIGH LATITUDE” SUBSTORMS AND SOLAR WIND CONDITIONS I.V. Despirak1, A.A. Lubchich1, N.G. Kleimenova2 1 Polar Geophysical Institute, RAS, Apatity, Murmansk region, 184200, Russia, e-mail: despirak@gmail. com; 2 Institute o f Physics o f the Earth, RAS, Moscow, Russia Abstract. All substorms observed at high latitudes can be divided into 2 types - "polar" (observed only at >70° latitudes; at <70° latitudes disturbances are absent) and "high latitude" substorms (propagating from auroral (<70°) to polar (>70°) geomagnetic latitudes). The aim of this study was to compare solar wind conditions during these two types of substorms. For this purpose, we used the data of IMAGE magnetometers and the solar wind data base (OMNI) for 1995, 2000, 2006-2011 periods. There were selected 105 "polar" and 55 "high latitude" substorms. It is shown that "polar" substorms observed at low solar wind velocity, after passing high speed stream, during late recovery phase of a geomagnetic storm. "High latitude" substorms, on the contrary, are observed at high values of the solar wind velocity, increased temperature and pressure of the solar wind, while passing by the Earth recurrent high speed stream. In addition, the variability of the solar wind parameters for the “high latitude” substorms is stronger than for the “polar” substorms. Introduction It is well known that auroras and the westward electrojet move poleward during the expansion phase of a substorm ([1], [2], [3]). Substorm disturbances propagate sometimes to extremely high geomagnetic latitudes. Substorms observed at high geomagnetic latitudes can be divided into two different types: “polar” and “high latitude” substorms. In the first type, a disturbance starts at geomagnetic latitudes higher 71° and then propagates poleward; geomagnetic disturbances are absent at latitudes below 70°. These substorm disturbances were called “polar” substorms [6]. Substorms of the second type starts at auroral latitudes, then propagate poleward, and the westward electrojet (or the westward electrojet center) moves to extremely high geomagnetic latitudes (>75°) in the substorm development maximum. These substorm disturbances were called “high latitude” substorms ([7], [8], [9]). Substorms of the first type, i.e., “polar” substorms, where all disturbances are concentrated in a narrow latitude region near the polar cap, usually occur under a low geomagnetic activity, when the auroral oval is contracted and poleward shifted [10]. Such substorm disturbances are often called “substorms on the contracted oval.” Studies have shown that “substorms on the contracted oval” do not differ from usually substorms in the parameters, both in the ionosphere and the magnetospheric tail, and usually occur when the Bz IMF component is northward directed (Bz>0) [11],[12],[13],[14]. Substorms of the second type, i.e., “high latitude” substorms, start in the auroral zone and then propagate to high latitudes [15], [16], [17], [18], [19], [20], [21], [22]. No differences in parameters were revealed between “high latitude’and ordinary substorms; however, it was shown that the solar wind velocity is a determining factor for occurrence of “high latitude” substorms. “High latitude” substorms are mainly observed during the period of a solar activity minimum, where recurrent high speed streams from coronal holes prevail [7], [23]. During a solar activity maximum, where streams related to coronal mass ejections (CMEs) prevail, “high latitude” substorms are observed seldom [9], [24]. “High latitude” substorms are also identified during compressed plasma propagation at fronts of solar wind streams, the so called Sheath and CIR regions [25]. However, these substorms contribute a little in the observation statistics of substorms at high latitudes, since the duration of Sheath and CIR regions is small as compared to the duration of recurrent high speed solar wind streams. The aim of this work is the comparison of the interplanetary conditions under which “polar” and “high latitude” substorms are observed. Data Data from the IMAGE magnetometer stations for the 1995 and 2006 2001 periods, close to the solar activity maximum, and for the period of the solar activity maximum (2000) were used. The solar wind and IMF parameters were determined from the OMNI data base. We have chosen and analyzed 160 events of substorm observation at high geomagnetic latitudes for the 1995, 2000, 2006-2011 periods: 105 of them were “polar: substorms and 55 were “high latitude” substorms. To study the latitudinal shift of the substorm westward electrojet, data from IMAGE ground based magnetometers were used, namely, the NUR-NAL (Numijarvi-Ny Alesund) meridional chain from 56.89° to 75.25° geomagnetic latitudes. To construct a latitudinal profile of the westward electrojet, the MIRACLE network was used ( http://www. “Physics o f Auroral Phenomena”, Proc. XXXVII A nnual Seminar, Apatity, pp. 10-13, 20 1 4 © Kola Science Centre, Russian Academy of Science, 2014 Polar Geophysical Institute 10