Non-stationary solar wind structures and their influence on substorm bulge development I.V. Despirak 1, A.A. Lubchich 1, V. Guineva 2 1. Polar Geophysical Institute, Apatity, Russia 2. Solar-Terrestrial Influences Institute, Stara Zagora, Bulgaria Second workshop “Solar influences on the ionosphere and magnetosphere” 7-11 June 2010, Sozopol, Bulgaria

Introduction Solar wind streams, depending on the periods of solar activity, have different nature. There may be quasi-stationary streams: recurrent streams from coronal magnetic holes and non-stationary streams: sporadic flare streams connected with coronal mass ejections (CME), which are observed near the Earth as magnetic clouds (MC). In front of the magnetic cloud there is a region of interaction with undisturbed solar wind (Sheath). This region is characterized by increased pressure, temperature, and density of the solar wind. The interplanetary magnetic field in this region is large and very fluctuated. Fast stream Slow stream Corotating Interaction Regions (CIR) are created by the interaction of fast streams with upstream slow streams. CIRs are characterized by enhanced magnetic field magnitude and plasma density and temperatures. Solar Influences on the Ionosphere and Magnetosphere, 7-11 June 2010, Sozopol, Bulgaria

Introduction (continued) The storms are mainly generated by different types of solar wind: ICME including Sheath and body of ICME ( MC ) and CIR (Vieira et al. (2004); Huttunen and Koskinen (2004); Yermolaev et al.,(2005); Yermolaev and Yermolaev (2006)) There are differences between storms generated by Sheath, MC and CIR ( in intensity, recovery phase duration, etc.) Aim of this study: To investigate the distinctions in the development of substorms occurring during geomagnetic storms connected with the MC, Sheath and CIR. Solar Influences on the Ionosphere and Magnetosphere, 7-11 June 2010, Sozopol, Bulgaria

Data We use the optical data of substorm development from the Polar satellite L o - onset latitude L m- maximum latitude L lat, L long - latitudinal and longitudinal size of the bulge a) Substorm bulge: It was established that the substorm development goes on in the next way: the substorm expansion phase begins with the flash of one arc, usually the most equatorial one between the existing discrete auroral arcs. After this the area occupied by bright, short-lived arcs – the auroral bulge – is expanding in all directions, mostly toward the pole, to the West and to the East. At the time of maximal study of substorms development the auroral bulge reaches its maximum width and occupies a maximum area. Further, during the recovery phase, the auroral bulge begins to shrink, its polar edge moves to the equator, and the eguatorward edge – to the pole, the bright discrete arcs degenerate into irregular strips and fade. Solar Influences on the Ionosphere and Magnetosphere, 7-11 June 2010, Sozopol, Bulgaria

Figure 2. Two magnetic clouds ( 15 July 2000 (a) and 17 September 2000 (b)) and two Sheath-regions, one recurrent stream ( 27 February 1997 (c)) and CIR- region. b) Solar wind: The solar wind and interplanetary magnetic field parameters were taken from the WIND satellite data (SWE and IMF data with 1 minute resolution were used). a) b)c)

Examples of auroral bulge development during CIR- and Sheath-storms. substorm development of 28 February 1997 and of 17 September 2000 The blue curve in the upper panel delimits the bulge region on each auroral image. The bottom panel displays the Dst index value, the times of substorms observation by Polar satellite during Sheath and CIR structures are indicated by vertical lines. Solar Influences on the Ionosphere and Magnetosphere, 7-11 June 2010, Sozopol, Bulgaria

Examples of auroral bulge development during the MC-storm of 22 October The left picture shows the Dst index value, the vertical lines indicate the times of substorm observations by Polar, in the right panel of the picture the auroral bulge development by Polar UVI data during the MC is presented. The blue curve delimits the bulge region on each auroral image. Solar Influences on the Ionosphere and Magnetosphere, 7-11 June 2010, Sozopol, Bulgaria

Statistical regularities for the longitudinal and latitudinal sizes of auroral bulge of substorms occurred during different types of storms. The total number of selected substorms is insufficient for meaningful statistics (15, 17, and 5 substorms during CIR, MC, and Sheath driven storms). In order to exclude an effect of irregularities in the distribution, we have used the quartile method. Quartile is any of the three values which divide the sorted data set into four equal parts; the second quartile is median and cuts the data set in half. The interquartile range, being equal to the difference between the third and first quartiles, is a measure of statistical dispersion. In the top panel the median value, the first and third quartiles, and the interquartile range of distribution of latitudinal and longitudinal sizes of auroral bulge for the substorms during MCs, CIRs, and Sheaths are shown. In the bottom panel, one can see three quartiles and interquartile range of distribution of the ratio between longitudinal and latitudinal bulge sizes for the three samples of substorms. Solar Influences on the Ionosphere and Magnetosphere, 7-11 June 2010, Sozopol, Bulgaria

during MC-storms and during CIR- and Sheath-storms the substorm behaviour differs. This is evidenced by the difference in the auroral bulge longitudinal and latitudinal dimensions. The ratio of longitudinal to latitudinal dimensions of the bulge during MC-storms is rather stable and higher than the one during CIR- and Sheath-storms. during MC- storms the auroral bulge is confined in latitude and extended in longitude. However this effect is not observed during substorms occurring during CIR- and Sheath- storms. Discussion and conclusions It should be noted that the event October 2001 was considered recently in the work of Pulkkinen et al., 2006, and it was shown that the configuration of the geomagnetic tail is more stretched for this event; an intense thin current sheet occupies a wider MLT sector of the near Earth tail. The magnetic field lines are highly elongated in the tail not only in the night sector, but also in the evening and morning sectors. This means, that there is an intensive current sheet near the Earth in a wide longitudinal area. The formation of an intense thin current sheet provides favorable conditions for driving the magnetic reconnection in this region, which may be a cause of substorm (e.g., Yahnin et al., 2006). Therefore, the substorm can develop in a wider longitudinal sector, and for energy dissipation the propagation far down the magnetospheric tail, i.e. along latitude, is not needed. Solar Influences on the Ionosphere and Magnetosphere, 7-11 June 2010, Sozopol, Bulgaria

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