WDS'07 Proceedings of Contributed Papers, Part II, 50–56, 2007. ISBN 978-80-7378-024-1 © MATFYZPRESS

Magnetosheath Turbulence and Low Latitude Boundary Layer (LLBL) Formation

S.S. Rossolenko, E.E.Antonova Skobeltsyn Institute of Nuclear Physics, Moscow State University, Space Research Institute RAS, Moscow, Russia.

Yu.I. Yermolaev, M.I. Verigin, I.P. Kirpichev, N.L. Borodkova Space Research Institute RAS, Moscow, Russia.

E.Yu. Budnik Centre d’Etudie des Recherchers Rayonnements, Toulouse, France.

Abstract. Simultaneous observations of plasma parameters and magnetic field in the low latitude boundary layer (LLBL) measured by INTERBALL/Tail probe and in the magnetosheath measured by Geotail satellite are analyzed for the event of March 2, 1996. The high level of magnetic field fluctuations is one of the main features of magnetosheath observations. It is shown that the amplitude of such fluctuations is higher than the value of the magnetic field in the region earthward from the magnetopause in the analyzed event. Results of observations show that fluctuations of the magnetic field in the magnetosheath can be an important factor in the conditions for the magnetopause formation. Local disruptions of magnetopause pressure balance under the influence of such fluctuations can lead to the magnetosheath plasma penetration inside the magnetosphere and formation of the LLBL. The role of the magnetopause pressure balance disruption in the formation of reconnection events is discussed.

Introduction The low latitude boundary layer (LLBL) is the region just earthward of the magnetopause where the magnetosheath-like and magnetosphere-like plasmas coexist. The solution of the problem of the LLBL formation is connected with the conditions of mass, momentum, and energy transfer between the magnetosheath and the magnetosphere. The LLBL is formed due to process of interaction between the turbulent solar wind and the Earth’s magnetic field. Despite more than 25-year history of its study, many problems of the LLBL formation continue to be unsolved (see the review of Němeček et al., 2003). The condition of stress balance on the magnetopause is not well studied till now. Therefore the problem of the magnetosheath plasma penetration through the magnetopause has no definite solution. Changes in the observed LLBL properties are commonly explained as a result of changes in the conditions of the magnetosheath plasma penetration through the magnetopause under different interplanetary magnetic field (IMF) parameters. However, the processes inside the magnetosphere can also influence the observed LLBL properties (Antonova, 2005). In this paper we examine the results of simultaneous INTERBALL/Tail probe, WIND and Geotail observations of LLBL, solar wind and magnetosheath for the event of March 2, 1996 and try to show that fluctuations of magnetic field in the magnetosheath can be the important factor of magnetosheath plasma penetration inside the magnetosphere. Preliminary results of these studies were submitted for the publication (Rossolenko et al., 2007).

Results of observations We use data obtained by the Corall low-energy ion experiment (Yermolaev et al., 1997), the Electron low-energy electron experiment (Sauvaud et al., 1997), and the MIF magnetic field instrument (Klimov et al., 1997) onboard the INTERBALL/Tail spacecraft. The plasma moments such as the ion density, bulk velocity vector, ion temperature, etc. are computed on the basis of the Corall measurements. We also analyze data of solar wind observations on the WIND spacecraft and magnetosheath observations on Geotail satellite (http://stp.isas.jaxa.jp/geotail/). Fig. 1 shows the

50 ROSSOLENKO ET AL.: MAGNETOSHEATH TURBULENCE AND LLBL FORMATION relative position of the INTERBALL/Tail probe and Geotail in special coordinate system (Verigin, 2001a,b) connected to magnetopause and shock wave. The X axis in this coordinate system is antiparallel to solar wind velocity V, calculated in the coordinate system moving with the Earth.

2 2 2 eX = (−VX ,−(VY +Ve ),−VZ ) / VX + (VY +Ve ) +VZ , (1) where VX, VY, VZ are components of the velocity V in GSE coordinate system, Ve=29,8 km/s is the orbital velocity of the Earth. The Y axis is determined by components of the IMF magnetic field vector В in GSE system, the Z axis is perpendicular to the plane in which X and Y are deposited.

⎧− B + (B,e X )e X / B − (B,e X )e X , (B,e X ) > 0, eY = ⎨ (2) ⎩ + B − (B,e X )e X / B − (B,e X )e X , (B,e X ) < 0

eZ = e X ×eY (3) Zenith angle θ is used in the coordinate system magnetosheath – interplanetary medium:

ϑ = arccos((r,e X ) / r), (4) where r is the arbitrary vector in the GSE coordinate system.

Figure 1. Satellite coordinates and scheme of the magnetopause and shock wave: the dependence of normalized distance on zenith angle for March 2, 1996 event.

The distance F from the model magnetopause is also introduced. F=0 when the satellite crosses the magnetopause location, calculated using model of Shue et al. (1998), and F=1 - the bow shock location, calculated in accordance with the model of Verigin et al. (2001a,b) at any time moment. The X axis in Fig. 1 shows “zenith” angles, the Y axis shows the distance from the model magnetopause. Points of trajectory for investigated time interval are calculated with 10 min averages. It is possible to see that Geotail crossed the shock wave and was in the magnetosheath region and the INTERBALL/Tail probe crossed the magnetopause at the same flank of the system magnetosphere- magnetosheath-shock wave during investigated interval. Fig. 2 presents the results of INTERBALL/Tail probe observations. The panel 1 shows the results of magnetic field observations by the MIF instrument. The panel 2 shows energy-time spectrogram of the Electron instrument. Density and temperature of ions are illustrated at the panels 3 and 4. The results of Corall measurements are shown at the bottom panel 5 for the channel measuring ions with the angle 65° relative to the Earth-Sun line. The intensity of the grey-scale on the spectrogram is color-coded according to the logarithm of the measured count rate per sample as shown by the color bar on the right of the figure. A lower part of the figure demonstrates the satellite coordinates in the GSM system. INTERBALL/Tail from 03.30 UT till 05.30 UT March 2, 1996 crossed the magnetosheath, LLBL and then plasma sheet. The satellite trajectory was localized at the evening flank of the magnetosphere. The satellite was in the magnetosheath till 04.07 UT, from 04.07 UT till 04.33 UT and after 05.08 UT was in the plasma sheet.

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The LLBL was not observed from 04.07 UT when satellite moved from the magnetosheath to plasma sheet at Z ≈4.5 RE. Spectra corresponding to simultaneous presence of the magnetosheath and the plasma sheet are present from 04.33 till 05.08 UT at Z ≈ 3 RE which means low-latitude boundary layer crossing. Regions where hot and cold ion populations coexist simultaneously were crossed by satellite at the intervals 04.33 - 04.43 UT and 04.55 - 05.01 UT. The analysis of the LLBL crossing gives the possibility to select jet structures. Fig. 3 shows WIND measurements of the solar wind velocity V (panel 1), density (panel 2) and IMF value (panel 3) on March 2, 1996. Solar wind velocity, as is shown in Fig. 2, was practically unchangeable and constituted ~335 km/s. Solar wind magnetic field was nearly stable and increased from 2.4 nT to 4.2 nT.

Figure 2. The results of observations on March 2, 1996 on satellite INTERBALL/Tail probe.

52 ROSSOLENKO ET AL.: MAGNETOSHEATH TURBULENCE AND LLBL FORMATION

Figure 3. Interplanetary magnetic field parameters in accordance with WIND data for March 2, 1996 event.

Fig. 4 shows the results of Geotail observations of V (panel 1), density (panel 2) and magnetic field (panel 3). The Geotail satellite was at the same flank of the magnetosphere as INTERBALL/Tail when INTERBALL/Tail probe crossed LLBL, but it was nearer to the Sun in the magnetosheath region. In Fig.4 it is possible to select typical fluctuations of the magnetosheath magnetic field, velocity and density. Satellite Geotail crossed the magnetosheath during time intervals 03.50-04.23 UT, 04.34-04.45 UT and 04.57-05.14 UT which is possible to determine from the measured values of velocity, density and magnetic field (see Fig.4).

Discussion Analyzing Fig. 2-4 it is possible to see that solar wind conditions were comparatively stable: fluctuations of solar wind velocity do not exceed 5 km/s and fluctuations of the magnetic field do not exceed 1 nT. Density of the solar wind fluctuated with amplitude of 1 cm-3. Simultaneous variations of magnetosheath velocity components were ± 100 km/s and amplitudes of magnetic field fluctuations were ~30 nT (see Fig. 4). Density of plasma measured on Geotail satellite had fluctuations of about 20 cm-3. Values of magnetic field measured by MIF on INTERBALL/Tail probe in the magnetosheath were 6-12 nT which corresponds to values downstream from the shock wave. The magnetic field is increased up to ~15-24 nT in LLBL region. The amplitudes of fluctuations of magnetic field and plasma flow parameters in magnetosheath exceed the same fluctuations in the solar wind. Plasma jets, formed in these conditions, should penetrate the magnetosphere and produce substantial distortion of the magnetic field in the magnetopause region. The amplitude of magnetic field fluctuations in the magnetosheath can exceed the value of the magnetic field in the region earthward from the magnetopause in the near cusp area (see scheme in Fig. 5), what is seen in the analyzed event. The configuration of plasma in the magnetosheath varies all the time even under stable solar wind conditions. Magnetopause position, as it is well known, is

53 ROSSOLENKO ET AL.: MAGNETOSHEATH TURBULENCE AND LLBL FORMATION determined from the condition of plasma and magnetic field pressure balance inside and outside magnetosphere. Magnetic field in the magnetosheath and its fluctuations obviously are the important factors determining the conditions of plasma penetration inside the magnetosphere when the value of magnetic field in the region earthward from the magnetopause is comparable with the value of magnetic field and its fluctuations in the magnetosheath. Jets of magnetosheath plasma can penetrate in different places of the magnetopause inside the magnetosphere due to the local pressure disbalance. These jets form LLBL and plasma mantle. The reconnection processes will appear as a consequence of the discussed penetration. If the level of fluctuations in magnetopause is high, plasma forming the LLBL can consist of jets situating at closed or opened magnetic field lines. The common theory of reconnection of Earth’s magnetic field and IMF doesn’t explain the dependence of LLBL thickness on IMF orientation. According to this theory, the northward IMF orientation leads to the reconnection poleward from the cusp, the southward IMF leads to the reconnection equatorward from the cusp. In accordance with this theory therefore, the thick LLBL should be formed under southward IMF orientation and thin – under northward one, that disagree with the experiment. The observed regularity can be explained taking into account the plasma transport inside the magnetopause and its dependence on IMF orientation, as it was done in (Antonova, 2005). The results of the work (Antonova, 2005) and our analysis can help to explain the formation of LLBL and its thickness dependence on the IMF orientation.

Figure 4. Magnetosheath plasma parameters in accordance with Geotail data for March 2, 1996 event.

Conclusion In this work we analyzed (The conducted analysis of the results of) simultaneous observations on satellites INTERBALL/Tail probe, WIND and Geotail satellites. Analysis of March 2, 1996 event showed the (supports the results obtained earlier on the existence of) high level of fluctuations of parameters of plasma and magnetic field in the magnetosheath under comparatively stable solar wind parameters, that agrees with the result obtained earlier (Shevyrev et al., 2003). This fact requires the development of a new model of LLBL formation. Local sporadic violation of balance of magnetic and plasma pressure can lead to the formation of plasma jets inside the magnetosphere. This process can explain the observation of structures in LLBL.

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Observed variations of plasma and magnetic field parameter in the magnetosheath considerably differs from the picture of plasma and magnetic field distribution near the magnetopause postulated in models of magnetic reconnection at the magnetopause. The amplitude of magnetic field fluctuations in the magnetosheath can exceed the value of the magnetic field in the region earthward from the magnetopause in the near cusp region (see Fig. 5). The violation of pressure balance can appear at different points of magnetopause under the described conditions. That is why the existence of such fluctuations is necessary to take into account in the analysis of the processes of magnetopause formation.

Figure 5. Scheme illustrating the high level of magnetosheath magnetic field fluctuations in the case of stable solar wind parameters and the possibility of the magnetopause pressure balance destruction due to the magnetic pressure in the magnetosheath.

Acknowledgements. The work is supported by RFBR grants 07-02-00042, 05-05-64394a and partially by program P-16 and OFN-16 RAS.

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