Physics of auroral phenomena : proceedings of the 36th Annual seminar, Apatity, 26 February – 01 March, 2013 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2013. - 215 с. : ил., табл.

THE STUDY OF MAGNETIC BARRIER PARAMETERS DEPENDENCE ON THE DIRECTION AND INTENSITY OF THE INTERPLANETARY MAGNETIC FIELD K.Yu. Slivka, V.S. Semenov, and N.P. Dmitriyeva (Saint-Petersburg State University) Abstract. The magnetic barrier is the region with enhanced magnetic field magnitude and depleted plasma. It is formed in the inner magnetosheath layer adjaccnt to the low-latitude dayside magnetopause. There is a general point of view now that a magnetic barrier can persist only for the northward direction of interplanetary magnetic field (IMF) while it is absent for the southward direction. We make a study to check appearance of magnetic barrier for different directions of IMF. To this end a data base consisting 63 events o f low-latitude dayside magnetopause crossings by the THEMIS satellites was created and analyzed. In order to study the variations of key plasma parameters and the magnetic field near the magnetopause in a systematic way we used a superposed epoch analysis. It turns out that the magnetic barrier is the most pronounced for the northward IMF. For the southward IMF we still were able to find events with signatures of magnetic barrier. According to the theory magnetic barrier builds up more slowly than it collapses much more. Due to reconnection we find out that the magnetic barrier field at the dayside magnetopause is about 80% of the magnetospheric value for southward IMF cases with a minimum magnetopause reconnection rate. In other hand, previous statistical studies showed that for southward IMF the magnetic barrier field is no more than 50% of the magnetospheric value. Introduction The interaction between the solar wind (SW) and the geomagnetic field defines topology and properties of the magnetosphere and the surrounding areas. The magnetic barrier in front of the magnetosphere is one of the most important elements of this interaction. The magnetic barrier is the area of the transition layer adjacent to the magnetopause with an increased magnetic field intensity and decreased plasma density. At the southern orientation of the interplanetary magnetic field (IMF) magnetopause reconnection destroys the magnetic barrier. The magnetic barrier is stronger in a case of northern IMF and completely absent while IMF is directed to the South, as it was showed by the superposed epoch analysis in [1]. This point of view is widespread in the scientific community. However, where does the energy required for magnetopause reconnection come from if IMF is oriented to the south? In this regard, we set the following objectives: to compile a database with magnetopause crossings, to divide events into couple groups by IMF orientation and investigate them; to find the individual events with a southern IMF in which there is the magnetic barrier and compare them with the events in which the magnetic barrier is destroyed by the magnetopause reconnection. Methods and materials It was used data parameters from THEMIS-B and -C satellites for the period 2007-2009. The magnetic field and plasma parameters were measured with a spin-resolution of about 3s. Details are on the THEMIS website [2]. Solar wind data with a spin-resolution of 1 min have been taken from OMNI [3]. It was selected 74 low-latitude, dayside (10-14 LT) magnetopause crossings during the period from 2007 to 2009. It can be observed multiple magnetopause crossings in many events. To weed out the plasma parameters variations associated with the oscillatory motion of the magnetopause we chosen one of the magnetopause crossing for each event: the first crossing when a satellite had come into the magnetosphere and the last one otherwise. Additional selection criterion was the condition that the satellite crossed the magnetopause at an angle greater than 45°. Further we are going to use the term «magnetic shear». Magnetic shear is the angle between the magnetosheath magnetic field and the geomagnetic field. There were appropriated 74 magnetopause crossings: 25 events with low- shear magnetopauses (according to northern IMF) and 49 ones with high-shear magnetopauses (according to southern IMF). To get an overall picture of changes in the magnetic field and plasma parameters we used the superposed epoch analysis following by [1]. At first, we determined the magnetopause crossing time which we took as the time T0(t = 0). The reference time T0 was determined by the sharp leap in the density, temperature and particle spectrum. It's worth noting that our results do not strictly depend on the error of determining the magnetopause position since the considered area is much wider than the magnetopause itself. The analysis was performed for 20 min before the magnetopause crossing and 2 min after entering into the magnetosphere. For the outbound passes we changed the timeline to combine it with the timeline for inbound passes. There were 25 crossings with low-shear magnetopauses (magnetic shear ~ 0°-40°) and 49 with high-shear magnetopauses (magnetic shear ~ 80°-180°). "Physics o fAuroral Phenomena", Proc. XXXVI Annual Seminar, Apatity, pp. 59 - 62, 2013 P o la r ©Kola Science Centre, Russian Academy of Science, 2013 Ssfflute8'08* 59