Physics of auroral phenomena : proceedings of the 33rd Annual seminar, Apatity, 02 - 05 March, 2010 / [ed.: A.G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 206 с. : ил.

“Physics o f Auroral Phenomena”, Proc. XXXIIIAnnual Seminar, Apatity, pp. 1 0 3 -1 0 5 , 2011 Polar © Kola Science Centre, Russian Academy of Science, 2011 Geophysical \ V J Institute MAGNETOTAIL STRETCHING UNDER DIFFERENT SOLAR WIND CONDITIONS T.A. Y ahnina and A.G. Yahnin (Polar Geophysical Institute KSC RAS, Apatity). Abstract. We investigate how the latitudinal location of proton precipitation boundary called b2i (which is a proxy for the boundary o f isotropic precipitation) relates to the type o f solar wind stream. It is known that the isotropy boundary latitude is an indicator o f the magnetosphere magnetic field stretching. We found that the strongest stretching is observed during the streams containing so-called “magnetic clouds” with southward magnetic field. This agrees with increased intensity o f geomagnetic disturbances during such solar wind streams. Also, this explains why substroms during magnetic clouds occur at lower latitudes and have larger longitudinal dimension in comparison with substorms taking place during other solar wind streams. Introduction It has been demonstrated (e.g., Sergeev et a l, 1993; Newell et a l, 1998) that the latitude o f the isotropy boundary (IB) of energetic protons (equatorward boundary of isotropic proton fluxes at the ionosphere level corresponding to the boundary between regions o f chaotic and adiabatic motion of particles in the magnetotail) correlates with elevation o f the magnetic field at geostationary orbit in the nightside equatorial magnetosphere. The lower the isotropy boundary latitude at midnight, the smaller the magnetic field elevation angle. Smaller elevation of the magnetic field lines means their stronger deviation from the dipole configuration, that is, stronger tailward stretching of the magnetic field in the near-Earth magnetosphere. Newell et al. (1998) also showed that decrease of the midnight IB latitude is accompanied by the decrease of the vertical component (Bz) of magnetic filed near the equatorial plane of magnetosphere. The longitudinal extent o f the reduced Bz increases significantly and occupies the whole night sector when the IB latitude shifts down up to -60° (from 18 up to 06 MLT, see, Fig. 5 in paper by Newell et a l, 1998). Evidently, this signals that when the IB latitude is low, the cross-tail current is intensified and close to the Earth not only in the midnight, but also in the evening and morning sectors. Interaction of the magnetosphere with the solar wind determines the shape o f magnetosphere and brings influence on the configuration o f the magnetospheric magnetic field (and, correspondingly, on the IB latitude). In some ways, the dependence o f the IB latitude on solar wind parameters has been studied by Lvova et al. (2005) and Yahnina et al. (2005). The strongest influence on the magnetotail stretching has been found for solar wind electric field and Bz component o f the interplanetary magnetic field. Also, the latitude of IB varies in the course of the solar activity cycle. During solar activity minimum (maximal) the IB latitude is maximal (minimal), that is, during solar activity maximum (minimum) the tailward stretching of the magnetic field lines is larger (smaller). During solar activity maximum the majority o f geoeffective solar activity consists o f coronal mass ejections related (as shown by Burlaga et a l, 1982) to the magnetic clouds, and during solar minimum the coronal holes prevail generating recurrent fast solar wind streams. Thus, it is reasonable to investigate how the different solar wind streams affect the magnetotail stretching. Data selection and treatment For this study we used the IB proxy derived from the low-orbiting DMSP satellite particle measurements, namely, the latitude o f the maximal flux o f protons with energy E>3 keV. This latitude called b2i (Newell et a l, 1996) nicely, like the IB derived from >30 keV proton measurements onboard NOAA satellites (Sergeev et a l, 1993), correlate with the elevation angle of the magnetic field in the near-Earth magnetotail (see, Newell et a l, 1998). The b2i values along with other precipitation boundaries are routinely determined from the DMSP particle data by the Auroral Particles and Imagery Group at JHU/APL. The database containing b2i values since 1984 till present time can be found at the website o f this group. The list of magnetic cloud events observed by the Wind spacecraft for 1995-2007 was retrieved from the web. page of the WIND Magnetic Field Instrument team (http://wind.nasa.gov/mfi_instrument/mfi/team_science.html) . The information on the recurrent high speed solar wind streams was taken from the High Speed Streams Catalog (1996-2007) by O. Maris and G. Maris published at http://www.spacescience.ro/newl/HSS_Catalogue.html. We selected several high-speed recurrent streams for 1996 and 2001. These two years represent, respectively, minimum and maximum of the solar activity. To increase the statistics o f the IB measurements during magnetic clouds, the well-defined events were selected for intervals 1995- 1997 and 2000-2002 (centered at 1996 and 2001). All the events were re-examined using the 5-min resolution OMNI database. As result, the intervals of southward and northward magnetic field within the magnetic clouds where selected as well as intervals within recurrent streams with the stream velocity greater than 400 km/s. In addition, the intervals of enhanced plasma density at fronts of the magnetic clouds and fast recurrent streams called, respectively, as “Sheath” and CIR (corotating interaction regions) were determined. 103