A Study of the LLBL Profile Using n-t Plots

Transcription

1 WDS'07 Proceedings of Contributed Papers, Part II, 42 49, ISBN MATFYZPRESS A Study of the LLBL Profile Using n-t Plots Š. Dušík, J. Šafránková, and Z. Němeček Charles University Prague, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. The structure of the magnetopause has been a subject of an intensive study for many years. At low latitudes, one can identify the low-latitude boundary layer (LLBL) on the magnetospheric side and rather often a depletion layer (DL) on the magnetosheath side of the magnetopause. The LLBL is encountered at the interface between two plasma regions the magnetosheath and plasma sheet and contains a mixture of both plasma populations. In our contribution, we analyze formation of the LLBL. We found very useful to draw the electron temperature as a function of the electron density for analysis of LLBL profiles under different upstream conditions. We discuss the results of IMF B Z and IMF B Y influences on the LLBL profile. Introduction A boundary layer called a low-latitude boundary layer (LLBL) which can be found everywhere along the magnetopause at low-latitudes has properties intermittent between the magnetosheath and the plasma sheet (e.g., Eastman et al., 1976). The magnetopause and LLBL are highly structured and depend on solar wind conditions and on interplanetary magnetic field orientations. Several mechanisms responsible for a formation of the LLBL have been suggested magnetic reconnection between the magnetospheric and magnetosheath magnetic fields (Sonnerup et al., 1981), impulsive penetration of magnetosheath plasma (Lemaire and Roth, 1978), and viscous/diffusive mixing of plasma populations at the magnetopause (Eastman and Hones, 1979). However, the last investigations show that observable LLBL features are consistent with magnetic reconnection (e.g., Nemecek et al., 2003). There is an evidence that LLBL lies at times on open field lines (magnetospheric field lines, which have one foot in the ionosphere and extend into the magnetosheath), sometimes on closed field lines. Problem of the opened/closed field lines is not clear till now (e.g., Nemecek et al., 2003). Lotko and Sonnerup (1995) presented the overview of LLBL plasma properties on closed field lines. On contrary, daytime LLBL was interpreted being on open field lines by Lyons et al. (1994). In accordance with Sauvaud et al. (1997), Interball/Tail LLBL measurements are interpreted as being on closed field lines due to the bidirectionally streaming electrons (Antonova et al., 2005). A LLBL formation on open field lines is well understood during a southward IMF (Interplanetary magnetic field) orientation. A role of reconnection during a northward IMF orientation is much less clear. Through periods of the northward IMF orientation, magnetosheath filed lines may reconnect poleward of both cusp nearly simultaneously and be appended to the magnetosphere as closed LLBL magnetic lines (Nemecek et al., 2003). n-t plots The spatial structure of the LLBL is of interest since it provides an indication as to whether or not diffusion plays a role in formation of the LLBL. While the LLBL exhibits a density gradient normal to the magnetopause at the flanks of the magnetopause, the dayside LLBL shows a density plateau, and the LLBL is one of several sub-layers of the boundary layer (e.g., Song et al., 1992). For diffusive plasma entry, one expects smooth and gradual density, temperature, and flow profiles, together with close coupling to the properties of the adjacent magnetosheath. On the other hand, sharp gradients bordering plateau profiles may be consequences of reconnection, although time-of-flight effects associated with reconnection may also give rise to gradual profiles of density and temperature (e.g., Lockwood and Hapgood, 1997). Gradual, abrupt and plateau-like profiles of density, temperature, and flow have been found in time series of magnetopause crossings. Although the plasma density and electron temperature are highly fluctuating during the LLBL crossings, it has been shown by Hapgood and Lockwood (1995) that the n-t plot is usually well organized. It led many investigators to the 42

2 conclusion that the LLBL exhibits a smooth change of both quantities across its thickness and that the observed fluctuations are a consequence of a permanent LLBL motion. This motion then sweeps the LLBL profile in irregular manners along the spacecraft. This suggests that n-t plots would be a useful tool for a study of plasma penetration to the magnetosphere and for investigations of the LLBL role in this process. Detailed study of one magnetopause crossing event using n-t plots was provided by Fear et al. (2005). In our paper, we examine several Interball-1 LLBL crossing events using n-t plots. We study dependence of the shape of these plots on upstream parameters. For this task, we divide the measured points in each time interval according to several criteria and plot them with different colors. In one case, where upstream parameters are very disturbed we divide the whole interval into several subintervals at times of the changes of the upstream density and we find some correlation between these changes and the particular n-t profile. Data set The present study is based on measurements of the Interball-1 spacecraft. We used the 3-D electron distribution measured by the ELECTRON spectrometer (Sauvaud et al., 1997). This experiment provides a 180 o x 6 o field of view with an angular resolution of 22.5 o x 6 o. Due to the spacecraft spin, the aperture scans the full 4π solid angle once per revolution. It covers the energy range 10 ev 26 kev. We use the moments of the distribution function measured by the mentioned analyzer that were computed onboard and transmitted to the telemetry with a time resolution of one spin period (~120 s). These moments include the electron density, the components of the bulk velocity vector, the pressure tensor and the heat flux vector. In our study, we use the electron density and temperature. Upstream solar wind conditions were estimated from WIND measurements. The propagation time of the solar wind features from the Wind position to the location of the particular event observed by the Interball-1 satellite was computed as a two-step approximation from the Wind solar wind velocity measurements (Safrankova et al., 2002). Magnetopause layers The n-t profiles could be often divided into three parts differing by the slope. The part with the largest temperatures can be probably attributed to the plasmasheet. The consecutive part with the Figure 1. The comparison of electron density vs. electron temperature. The left panel is an example of the typical n-t profile shape for the dawn LLBL, the right panel is an example of the typical n-t profile for the dusk LLBL. 43

3 strongest dependence of the temperature on the density then belongs to the inner LLBL and the last part consist of three regions - magnetosheath, the depletion layer adjacent to the magnetopause, and outer part of LLBL (Nemecek et al., 2002). We do not distinguish these parts in the present study but it must be stressed out that dividing the profiles into different magnetopause/llbl layers could bring further information (Safrankova et al., 2007). We should note that this shape of the n-t plot is typical for the dawn LLBL (left panel of Fig. 1), whereas a continuous change of the n-t plot slope is usually found through the dayside or dusk LLBL (right panel of Fig. 1). According to Dusik et al. (2006), these features are very robust and do not depend on the IMF orientation. February 6, 1997 event In this event, we investigate the influence of the upstream density on the n-t profile. The event was recorded at the dusk flank under a very stable IMF (last two panels in Fig. 2). The density changed from 9 cm -3 at the beginning to 3 cm -3 at the end of the investigated interval. The spacecraft moved from the magnetosheath, through the LLBL, to the plasmasheet. The LLBL scanning is rather long and it suggests a very thick LLBL because the magnetopause is moving outward (in an opposite direction than the spacecraft) due to a gradual decreasing upstream pressure. The enhanced LLBL thickness can be connected with the duskward and northward IMF orientation. As can be seen from the n-t plot (Fig. 3), the changes of the upstream density do not affect the electron parameters inside the LLBL. We have distinguished the region with the depressed upstream density with the light gray color (interval from 1628 to 1819 UT) in order to show that the parameters inside the LLBL are in the same ranges regardless of the upstream density. Figure 2. Observations of the Interball-1 spacecraft on February 6, From top to bottom: Electron spectra (ev); magnetic field components B X, B Y, B Z ; total magnetic field B; solar wind data from Wind: x-component of the solar wind velocity; ion density; dynamic pressure; B X, B Y, B Z components of IMF; total IMF. 44

4 Figure 3. The scatter plot of electron density vs. electron temperature from Interball-1 (dusk flank). The dark gray points represents the interval from 1430 to 1628 UT with higher solar wind particle density, the light gray points represents interval from 1528 to 1819 UT with depressed upstream particle density. September 18, 1996 event Figure 4. Observations of the Interball-1 spacecraft on September 18, From top to bottom: Electron spectra (ev); magnetic field components B X, B Y, B Z ; total magnetic field B; solar wind data: x-component of the solar wind velocity; ion density; dynamic pressure; B X, B Y, B Z components of IMF; total IMF. 45

5 According to Dusik et al. (2006), the LLBL profiles differ on dawn and dusk flanks. For this reason, our second case brings observations from the dawn flank. Upstream conditions (Fig. 4) were a little disturbed and a change of the IMF B X orientation results in the change of conditions for a local coupling of the LLBL with the adjacent magnetosheath. We have divided the measured points according to several criteria and plotted them with different colors. In the left plot in Fig. 5, we investigate the influence of the upstream density. The break point for the division was 2.5 cm -3 because it provided an equal number of points in both groups. As can be seen from the event overview (shown in Fig. 3), the ion density jumps around this point throughout the whole interval but the corresponding n-t plot does not reveal any change of LLBL parameters. Following plots investigate a possible influence of the IMF direction. The most pronounced change in our interval is a sharp turn of B x (middle panel in Fig. 5) about midnight but even such change does not affect the LLBL parameters. The same is true for the orientation of IMF B Y (right panel of Fig. 5) that changes its sign several times. Figure 5. Electron density vs. electron temperature divided by various solar wind parameters. The first panel represents a division according to higher (>2.5 cm -3 ) and lower (<2.5 cm -3 ) ion densities, the second panel represents measurements divided according to the IMF B X sign and the last panel divides measured points according to the IMF B Y sign. September 20, 1995 event Our previous examples suggest that the LLBL profile does not respond to upstream changes. For this reason, we have chosen a really disturbed interval when the upstream density varies from 9 to 22 cm -3 and the speed from 360 to 420 km/s (the overview of the event is in Fig. 7). IMF exhibits 46

6 significant variations of the magnitude, as well as the direction. The electron spectra show several regimes of the LLBL and we try to identify these regimes in n-t plots. We have divided the whole interval into 6 subintervals at times of about changes of the upstream density. Both panels in Fig. 6 exhibit an overall profile typical for the dawn flank but three different plasma regimes are clearly seen. One can find stepwise changes of the LLBL density that follows the upstream density. However, although our subintervals cover a large range of upstream densities, their change does not affect the particular n-t profile. Figure 6. The comparison of electron density vs. electron temperature. The whole interval is divided into 6 subintervals at times of about changes of the upstream density. Figure 7. Observations of the Interball-1 spacecraft on September 20, From top to bottom: Electron spectra (ev); magnetic field components B X, B Y, B Z ; total magnetic field B; solar wind data: x-component of the solar wind velocity; ion density; dynamic pressure; B X, B Y, B Z components of IMF; total IMF. 47

7 Discussion and conclusion Conclusions that follow from our brief study of three events are rather contradictory. The first event shows that the n-t plot is not affected by the upstream density but we identified clear changes of the LLBL density and temperature immediately following abrupt changes of the upstream density. These changes are similar to those found by Safrankova et al. (2007). In this paper, the LLBL density changed in accord with upstream density steps and the authors suggested a direct coupling of the LLBL and magnetosheath. Our study reveals that the problem is more complicated because the LLBL parameters can remain unchanged even during large changes of the upstream density if these changes are slow enough. It suggests that the flank LLBL is generally on closed field lines and thus, it does not respond to upstream variations. During abrupt changes of the upstream parameters, the LLBL becomes open for a time and new plasma is injected. The opening mechanism is under question because the flank LLBL is formed by high-latitude reconnection (Nemecek et al. (2002, 2003)) and this process is controlled by the IMF direction and the density changes could play a minor role. From our preliminary study follows that to confirmation of our results we need to analyze more magnetopause crossings in different locations (e.g., dawn, dusk) and during variable upstream parameters (steady and disturbed solar wind conditions, influence of jumps in upstream parameters on the LLBL formation). Acknowledgments. The authors thank the Czech Grant Agency (Contracts 205/05/0170 and 205/06/0875) for a financial support. References Antonova, E.E., The structure of the magnetospheric boundary layers and the magnetospheric turbulence, Planetary and Space Science 53, , Dusik, S., Safrankova J., Nemecek Z., and Prech L., A Study of the Low-Latitude Boundary Layer, WDS 06 Proceedings of Contributed Papers, Part II, 21-27, Eastman T.E., Hones E.W., Bame S.J., et al., Magnetospheric Boundary-Layer - Site of plasma, momentum and energy-transfer from magnetosheath into magnetosphere, Geophys. Res. Let. 3 (11): Eastman, T.E., Hones E.W.Jr., Characteristics of the magnetospheric boundary layer and magnetopause layer as observed by IMP 6, J. Geophys. Res., 84, 2019, Fear, R.C., Fazakerley A.N., Owen C.J., Lahiff A.D., Lucek E.A., Balogh A., Kistler L.M., Mouikis C., and Reme H., Cluster observations of boundary layer structure and a flux transfer event near the cusp, Ann. Geophys., 23, , Hapgood, M.A., Lockwood M., Rapid changes in LLBL thickness, Geophys. Res. Lett., 22, 77-80, Lemaire, J., and Roth M., Penetration of solar wind plasma elements into the magnetosphere, J. Atm. Terr. Phys., 40, 331, Lockwod, M., Hapgood M.A., How the magnetopause transition parameter works, Geophys. Res. Lett., 24, , Lotko, W., and Sonnerup, B.U.O., The low-latitude boundary layer on closed field lines In Song, P., Sonnerup B.U.O., Thomsen M.F. (Eds), Physics of the magnetopause, AGU Monograph 90, Washington, D.C., , Nemecek, Z., J. Safrankova, L. Prech, and J.-A. Sauvaud, The structure of magnetopause layers at low latitudes: INTERBALL contribution to the topic, Geophysical Monograph, AGU, Geophysical Monograph Series, Volume 133, AGU, Earth's Low-latitude Boundary Layer, ed. by Patrick T. Newell and Terry Onsager, 71-82, Nemecek, Z., Safrankova J., Prech L., Simunek J., Sauvaud J.-A., Fedorov A., Stenuit H., Fuselier S.A., Savin S., Zelenyi L., and Berchem J., Structure of the outer cusp and sources of the cusp precipitation during intervals of a horizontal IMF, J. Geophys. Res. 108, A12, 1420, Safrankova, J., Nemecek Z., Dusik S., Prech L., Sibeck D.G., Borodkova N.N., The magnetopause shape and location: A comparison of the INTERBALL and GEOTAIL observations with models., Ann. Geophys.,20, No. 3, , Safrankova, J., Nemecek Z., Prech L., Simunek J., Sibeck D., Sauvaud J.-A., Variations of the flank LLBL thickness as response to the solar wind dynamic pressure and IMF orientation., J. Geophys. Res, 112, A07201, doi: /2006ja011889,

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