MAGNETOTAIL SUBSTORMS OBSERVATIONS DURING SOLAR WIND MAGNETIC CLOUDS AND HIGH SPEED STREAMS

Transcription

1 MAGNETOTAIL SUBSTORMS OBSERVATIONS DURING SOLAR WIND MAGNETIC CLOUDS AND HIGH SPEED STREAMS I.V. Despirak 1, A.A. Lubchich 1, R. Koleva 2 1 Polar Geophysical Institute, RAS, Apatity, Murmansk region, , Russia, despirak@gmail.com; 2 Space Research & Technologies Institute, BAS, Sofia, Bulgaria Abstract. In this work we discuss the problem of the magnetic reconnection site location in the magnetotail during substorms associated with different solar wind streams. It was shown recently that solar cycle variations of the solar wind control the location of magnetic reconnection in the tail. A well-known fact is that solar wind high-speed streams have different nature during different phases of solar activity. During the declining phase and minima of the solar cycle predominate the high speed recurrent streams (HSS) originating from coronal magnetic holes. During solar cycle maxima the flare streams connected with coronal mass ejections prevail. We analyze the relationship between the locations of the tail magnetic reconnection site during substorms connected with solar wind magnetic clouds (MC) and recurrent streams. We use data from Geotail spacecraft in the magnetotail and solar wind parameters from Wind spacecraft observations; the auroral bulge parameters were obtained by the Ultra Violet Imager onboard Polar. We show that magnetic reconnection in the magnetotail takes place closer to Earth when substorm is observed during MC, and further in radial distance for substorms during solar wind high speed recurrent streams. Introduction Recently the authors in [Nagai et al., 2005] showed that the location of the reconnection site in the magnetotail depends on the phase of the solar activity cycle. They found out that during solar cycle maximum magnetic reconnection is observed nearer to Earth than during the minimum of solar activity. Fig.1 The occurrence of reconnection events as a function of X GSM for the two periods. Thick lines present values for solar minimum ( ), thin lines present values for solar maximum ( ). (Figure taken from [Nagai et al., 2005]). 264

2 Figure 1 shows the occurrence of reconnection events as a function of X GSM for the two periods [Nagai et al., 2005]). There are 19 solar minimum events ( ) and 15 solar maximum events ( ). In the solar minimum ( ), reconnection events are observed only beyond 21 R E, and occurrence is high beyond 25 R E. In the solar maximum ( ), reconnection events can be found inside 21 R E, and occurrence is relatively constant at X GSM = 17 to 31 R E. It is well known that solar wind is not a uniform flow; various large-scale structures and streams exist within it [e.g. Pudovkin, 1996; Yermolaev et al., 2009]. Besides during different solar activity phases prevail different solar wind flows [Krieger et al., 1973; Burlaga et al., 1982; Richardson et al., 2001]. During the declining phase and at the minimum of solar activity high speed recurrent solar wind streams (HSS) originating in coronal holes prevail while in solar activity maximum prevail flows from coronal mass ejecta which are observed as magnetic clouds (MC) near Earth. In this work we will investigate the reconnection site location in the magnetotail during substorms observed under different solar wind structures. When a reconnection site passes through a satellite in the night plasma sheet first a fast earthward plasma flow is registered and then a fast tailward flow. In one of the main substorm models NENL, the observation of a fast flow reversal is interpreted as a tailward motion of the reconnection site past the satellite [Hones, 1979; Angelopoulos et al., 1996; Petrukovich et al.; 1998; Yahnin et al., 2006]. In another substorm model, the Current Disruption model, the observation of a fast flow reversal is interpreted as a passage along the satellite the current disruption region [e.g. Lui et al., 2008]. The goal of this work is to determine the reconnection site location in the magnetotail associated with substorms developed under different solar wind streams high-speed recurrent streams (HSS), magnetic clouds (MC), and the region of compressed plasma in front of these streams (Sheaths and CIRs). For this purpose we used Geotail position at the moment of observation of an X-line moving tailward (observation of a fast flow reversal) for cases related to different solar wind streams. We combine the data of the UV Imager onboard the Polar satellite [Torr et al., 1995] with Geotail plasma measurements by LEP instrument [Mukai et al., 1994], and magnetic field measurements by MGF instrument [Kokubun et al., 1994]. The auroral disturbances are studied by Polar UVI data. It is known that auroral disturbances in the ionosphere is investigated by means of the device UVI, registering glow in the ultraviolet region of the spectrum. The main source of radiation in the UV range is the system of Lyman Birdge Hopfield Spectral bands - LBH), we used data from the emission band in LBHL ( Å). The method of determining the auroral bulge parameters was described in detail in [Despirak et al., 2009, 2011]. The solar wind and interplanetary magnetic field parameters measured by Wind spacecraft were taken from OMNI database. Sheath and CIR regions were detected by their typical showings as regions in front of recurrent streams or magnetic clouds, which are characterized by an increase of solar wind pressure, temperature and density. For high speed recurrent streams (HSS) high velocities and low values of the negative B Z component of the IMF are typical characteristic [see e.g., Tsurutani et al., 2006]. The magnetic clouds (MCs) have high magnetic field values with long interval of negative B Z component of the IMF and also increased values of plasma velocity [e.g. Burlaga et al., 1982]. The events were selected using the following criteria: 1) The auroral disturbances should be observed by the UVI onboard Polar; 2) The auroral disturbances should be observed during MC, HSS, Sheath, CIR solar wind structures; 3) The meridian of the Geotail footprint should cross the auroral bulge; 4) Geotail should be in the night plasma sheet. The criterion β > 0.1 (β is the ratio of kinetic plasma pressure to magnetic pressure) and eye inspection of ion and electron spectra are applied for the plasma sheet identification. All auroral disturbances observed by Polar during MC, HSS, Sheath, CIR for the periods ; 2000; October 2001 and December 1996 were studied. The MC, Sheath, CIR solar wind regions discussed by us are events with small duration [see for example Yermolaev et al., 2009] and hence observations of substorms during MC, Sheaths and CIRs are quite rare cases. Coincidence of Polar observations of such substorms with Geotail measurements in the plasma sheet is yet a much rarer event. During the whole period that we have analyzed ( ; 2000; October 2001 and December 1996) we have found only 6 events when Geotail was in the plasma sheet during auroral bulge formation related to Sheath regions in the solar wind; 3 events- to CIR regions; 3 events- to MC and 7 events to HSS. Below we present one example connected to solar wind magnetic cloud (MC). Data 265

3 Results Auroral disturbances during MC event on 13 November 1998 A magnetic cloud (MC) arrived at ~ 05:00 UT on 13 November and passed away at ~ 07 UT on 14 November 1998 (as deduced from Wind data). The Sheath was registered from ~01 UT to ~05 UT. A substorm was observed at 21:47 during the MC passage. The blue line on Fig. (2a) delimits the intervals of Sheath and MC, the interval of MC is red-crosshatched. In front of the magnetic cloud there is a region of interaction with slower streams (Sheath). This is a region with magnetic field and plasma compression. Vertical blue lines and blue crosshatched regions show the times of Sheath (Fig.2a) The black solid line shows the onset time of the substorm registered by the Polar satellite. On the top panel of Fig. (2b), the auroral bulge development from 21:47 to 22:11 UT according to Polar UVI data is shown. The onset latitude L 0 of the bulge was 53.3º CGLAT; the maximum latitude L m 72.6º CGLAT; the latitudinal size L lat ~16.7º CGLAT, the longitudinal size L long ~ 162º CGLAT, the ratio between the longitudinal and the latitudinal sizes L lat /Ll ong was equal to ~ 9.7. Keograms on the bottom panel of Fig. (2b) demonstrate the clear poleward expansion of the bulge. Geotail data for the period UT on 13 November 1998 are presented in Fig. (2c). The satellite started registering fast tailward plasma flows about 21:59 UT, a flow reversal took place at ~ 22:00 UT and a maximum of earthward flow is observed at 22:07 UT. The flow reversal is associated with a decrease of the total (magnetic plus plasma) pressure, which followed a period of a pressure increase. Solar wind and IMF Auroral disturbances development by POLAR UVI GEOTAIL data (a) (b) (c) Figure 2. Auroral disturbances during MC event - 13 November (a) From top to bottom: total magnetic field B, three magnetic field components, SW flow velocity, density, dynamic pressure; (b) Top: auroral bulge development from onset to maximal phase. Bottom: keograms in the LBHL emission (black line) and LBHS emission (blue line) at the meridian of Geotail footprint. Vertical lines indicate the times of plasma flows in magnetotail by Geotail data; (c) From top to bottom: temperature, density, MF component Bx, total pressure, plasma β, GSM X component of the velocity. 266

4 Relationship between the latitude of the poleward edge of the auroral bulge and the distance of the reconnection site from the Earth For all 19 selected cases we determined the latitude of the poleward edge of the auroral bulge at Geotail footprints meridian at the moment when Geotail registered a fast flow reversal in the magnetotail. Then we related this latitude with Geotail position in the magnetotail. The results are shown in Fig. 3, where the distance between Geotail and the Earth is evaluated by Geotail X-coordinate in earth radii. On the right panel cases observed during MCs are depicted in red, cases during HSS in blue, those observed during Sheath in green, and cases during CIRs in black. On the left panel cases are grouped according their origin: cases connected with MCs and Sheaths are depicted in red, cases of HSS and CIRs in blue. Figure 3. Latitude of the poleward edge of the aurora (seen by Polar UVI at the meridian of Geotail) as a function of the distance between Geotail and Earth at the moment of the flow reversal registration From Fig.3 it is clearly seen that the reconnection site during substorms associated with high speed recurrent solar wind streams is observed on the average at larger distances from earth X =25.7±2.8 R E than during substorms associated with magnetic clouds, the latter being at 15.±1.5 R E. Discussion In all cases considered here, during the auroral disturbances in the ionosphere fast plasma flows in the magnetotail are observed. In the literature there is enough evidence that in the course of substorm fast plasma flows in the magnetotail are observed and satellites in the near or middle tail can register a reversal of a tailward plasma flow to an earthward plasma flow. In the midtail initiation model or alternatively referred to as the near-earth neutral line model (NENL), the flows are ordered by the magnetic field tailward flows accompanied by negative Bz, earthward flows under positive Bz, the region where these flows are generated is associated with the formation of a near-earth X-line. The observation of oppositely directed flows is interpreted as tailward movement of the reconnection site [Hones, 1979; Angelopoulos et al., 1996; Runov et al., 2003], the tailward flow being inside a plasmoid. At the moment, when a maximum of tailward flows is observed, the satellite is situated inside the plasmoid; when a maximum of earthward flows is observed, the spacecraft is within the dipolarized region earthward of the X-line. In the near-earth initiation scenario oppositely directed flows are also observed, but not organized by the magnetic field; their generation evoked by current disruption (CD), the region of current disruption and dipolarization proceeds progressively down the magnetotail [e.g. Lui et al., 2008]. The earthward flow is observed in the depolarized region, the tailward tailward of the CD region. The observation of fast flow reversal by a spacecraft in the plasma sheet 267

5 indicates passage of the reconnection site around the spacecraft in the NENL model [e.g. Yahnin et al. 2006] or passage of the current disruption region in the CD model. The latitudinal profiles in Figure 2b show that the oppositely directed flows and the flow reversals are observed during poleward aurora expansion. The relation of fast flows observation in the plasma sheet with the poleward expansion of the aurora is reported in many works [e.g. Fairfield et al. (1999); Borodkova et al. (2002); Perraut et al., 2003; Yanin et al. 2006]. In particular authors in Borodkova et al. (2002) and Yanin et al. (2006) demonstrated that the flows were directed tailward when the auroral bulge developed equatorward of the spacecraft ionospheric footprint. Earthward fast flow bursts were observed when active auroras moved poleward of the spacecraft ionospheric footprint. As seen in Figure 2c, in the magnetotail at Geotail location a sharp decrease of the total pressure following the interval of pressure increase was observed. The total pressure decrease (TPD) is associated with the time of plasma flow direction change. From the pressure balance condition, which is suggested to held across the plasma sheet and tail lobe boundary, the total pressure (P = nkt + B 2 /8π) in the plasma sheet must be equal to the lobe magnetic pressure. The decrease below the lobe magnetic pressure characterizes that part of the magnetic energy stored in the tail, which dissipates during the substorm. As is well known during substorm growth phese, as a result of reconnection at the magnetopause magnetic flux is stored in the magnetotail lobes, the volume of the flux stored depending on solar wind parameters and interplanetary magnetic field [e.g. Shukhtina et al., 2005]. However it is possible that during the substorm not all the stored flux but only a part of it is dissipated in the ionosphere. The area of the auroral bulge could be a measure of the dissipated magnetic flux [e.g. Miyashita et al., 2003]. So the total pressure increase followed by a decrease observed during the substorm-related fast flows is one of the signatures of substorm development in the magnetotail. Thus, in all 19 cases we have analyzed, in the course of auroral disturbances development in the ionosphere, in the magnetotail typical substorm signatures - fast plasma flows (tailward/earthward flows) and a sharp decrease of the total pressure following the interval of pressure increase are observed. Conclusions In this work we discuss the problem of the magnetic reconnection site location in the magnetotail during substorms associated with different solar wind streams. It is shown that magnetic reconnection in the magnetotail takes place closer to Earth when substorm is observed during MC, and further in the tail for substorms during solar wind high speed recurrent streams (HSS). In this way our results confirm the results of Nagai et al. (2005). However using one-point data it is difficult to separate space and time variations. We plan to continue the investigations on reconnection site location for substorms driven by different solar wind structures using CLUSTER and THEMIS data. Acknowledgements WIND data used in this study were taken from OMNI through Polar UVI data were downloaded from UVI official site We are grateful to K. Ogilvie and R. Lepping, PIs of the experiments conducted with these instruments. We thank G. Parks and other members of the Polar UVI team who made the data of the UV Imager available to the community. Data from the MFI and LEP instruments onboard Geotail were retrieved from the DARTS database. We are thankful to S. Kokubun and T. Mukai for providing the data. The paper was supported by the RFBR Grants and Program No 22 of the Presidium of the Russian Academy of Sciences (RAS) Fundamental problems of the Solar system exploration. The study is part of a joint Russian - Bulgarian Project The influence of solar activity and solar wind streams on the magnetospheric disturbances, particle precipitations and auroral emissions of PGI RAS and IKIT-BAS under the Fundamental Space Research Program between RAS and BAS. References Angelopoulos, V., D.G. Mitchell, R.W. McEntire, D.J. Williams, A.T.Y. Lui, S.M. Krimigis, R.B. Decker, S.P. Christon, S. Kokubun, T. Yamamoto, Y. Saito, T. Mukai, F.S. Mozer, K. Tsuruda, G.D. Reeves, W.J. Hughes, E. Friis-Christensen, O. Troshichev (1996), Tailward progression of the magnetotail acceleration center: Relationship to substorm current wedge. J. Geophys. Res., 101,

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