Solar Wind Control of Density and Temperature in the Near-Earth Plasma Sheet: WIND-GEOTAIL Collaboration. Abstract

Transcription

1 1 Geophys. Res. Letters, 24, , Solar Wind Control of Density and Temperature in the Near-Earth Plasma Sheet: WIND-GEOTAIL Collaboration T. Terasawa 1, M. Fujimoto 2, T. Mukai 3, I. Shinohara 1, Y. Saito 3, T. Yamamoto 3, S. Machida 4, S. Kokubun 5, A. J. Lazarus 6, J. T. Steinberg 6, and R. P. Lepping 7 Abstract A statistical survey of GEOTAIL observations reveals the following properties of the near-earth plasma sheet ( 15 <X GSM < 50 Re): During the periods when the northward IMF dominates, (1) the plasma sheet becomes significantly cold and dense, (2) the best correlations between the plasma sheet and the IMF parameters occur when the latter quantities are averaged over hours prior to the plasma sheet observations, and (3) temperatures diminish and densities increase near the dawn and dusk flanks of the plasma sheet. We suggest that during prolonged northward IMF periods ( several hours) there is a slow diffusive transport of the plasma from the solar wind into the plasma sheet through the the magnetotail flanks. Introduction. The dependence of magnetotail plasma sheet parameters on the solar wind is central to magnetospheric physics. About particles/s are required to maintain the plasma sheet population [e.g. Hill, 1974]. The solar wind supplies most of the plasma sheet protons, while the earth s ionosphere provides He +,O +, and other heavy ions. It is usually thought that during southward interplanetary magnetic field (IMF) periods part of the solar wind plasma and energy flows into the magnetosphere via magnetic reconnection process on the equatorial dayside magnetopause. Excess energy stored in the magnetotail is then released by magnetospheric substorms which heat the plasma sheet up to several kev or more. During periods of strongly northward IMF, magnetopause reconnection occurs at high latitudes with a much less effective transfer of the solar wind plasma and energy into the near-earth plasma sheet. Several authors [e.g., Fairfield et al., 1981; Lennartsson and Shelley, 1986; aumjohann et al., 1989; Lennartsson, 1992] have noted that the plasma sheet becomes cold and dense during geomagnetically quiet periods, which likely correspond to the periods when northward IMF dominates. However, there seems to be no consensus concerning the physical mechanism responsible for this cooling and densitythickening of the plasma sheet. This letter presents a statistical analysis of the solar wind dependence of plasma sheet parameters utilizing comprehensive data sets provided from GEOTAIL and WIND spacecraft for the period of November 1994 December Data Selection and Processing. We use GEO- TAIL MGF magnetic field [Kokubun et al., 1994] and LEP plasma ion [Mukai et al., 1994] observations. We denote plasma sheet parameters by N ion (ion density), T ion (temperature), P ion (pressure, k N ion T ion, where k is the oltzmann constant), and the sum of pressures P sum = P ion + 2 x /2µ 0. (We calculate these parameters as if all ions are protons.) The WIND MFI and E instruments provide solar wind magnetic field [Lepping et al., 1995] and plasma [Ogilvie et al., 1995] observations. We used observed solar wind velocities to estimate lag times from WIND to Earth and added these lags to the WIND data. We produced N hour averages (1 N 48) of various solar wind parameters; velocity magnitude V N and corresponding kinetic energy e N sw m p V 2 /2, density N N, latitudinal angle θ N and z component N sw,z of the IMF. From the period between November 1994 and December 1995, we identified hour intervals when GEOTAIL was in the magnetotail ( 50 X GSM 15 Re, 25 Y GSM 25 Re) and solar wind data from WIND were available. We use a modified geocentric solar magnetospheric coordinate system (GSM ) with an aberration angle of 4 degrees throughout this letter. We excluded intervals when GEOTAIL was near the magnetopause to avoid contamination to our data base with magnetosheath or low latitude boundary layer observations. The duration of these excluded intervals was 3-6 hours, which was determined by the visual inspection of the data. We then selected hour intervals of relatively steady plasma sheet by requiring the ion beta ( 2µ 0 P ion / 2 ) 1 and the standard deviation of P sum to be less than 20% of the 1-hour average of P sum. During these intervals GEOTAIL was positioned at various distances Z from the center of the plasma sheet where x = 0. To eliminate the

2 2 Zdependence of N ion and T ion, we performed a linear regression analysis on each hourly set of 1-minute parameters, ( x, N ion ) and ( x, T ion ), from which we estimated the density and temperature at the center of the plasma sheet, N ion (0) and T ion (0). We then use those center values as our 1-hour average plasma sheet parameters. Figure 1 shows an example of this analysis. correlation between the two quantities (cross correlation coefficient = 0.46), there is large scatter of the data points. Figure 2 (b) compares the plasma sheet densities N ion (0) with the solar wind densities N 1. Although the correlation between these two densities (cross correlation coefficient = 0.66) is better than that in Figure 2 (a), the scatter remains quite large. Figure 1: Variations of ( x, T ion ):left and ( x, N ion ):right during the 1-hour interval of UT on 21 December 1994, when GEOTAIL was in the plasma sheet around (-46.3, 4.7, -6.2) GSM Re. Each dot shows the 1- min average values of x, T ion, and N ion. Dashed lines show the results of linear regression analyses, from which we estimate T ion and N ion at the center of the plasma sheet ( x = 0). Since there is no a priori reason to assume a linear dependence of N ion and T ion on x, we have checked the validity of this assumption by comparing the products k N ion (0)T ion (0) with P sum obtained for the same 1-hour intervals. We then select hour intervals for which k N ion (0)T ion (0) and P sum agree within a 20 % error. elow we follow Spence et al. [1989] in assuming that the electron contribution to the total pressure and the effect of ion pressure anisotropies are negligible. From preliminary invesitigations of three-dimensional ion distribution functions and low energy electron data ( < 100 ev) we have confirmed these assumptions. Assumption that all ions are protons can be also justified since our main concern in this letter is with geomagnetically quiet periods, during which the O + /proton ratio is at most several percent [Lennartsson and Shelley, 1986]. Observation. Figure 2 (a) presents the plasma sheet ion temperatures T ion (0) obtained through the procedure described above plotted versus the solar wind kinetic energies e 1 sw. oth quantities are for the same 1-hour intervals. While there is a positive Figure 2: (a) The estimated ion temperatures T ion at the center of the plasma sheet (T ion(0)) are plotted against the 1-hour average solar wind kinetic energies, e 1 sw. (b) The estimated ion densities N ion at the center of the plasma sheet (N ion(0)) are plotted against the 1-hour average solar wind densities, N 1. (c) The plasma sheet densities normalized by the solar wind densities, N ion(0)/n 1, are plotted against the latitudinal angle of the IMF, θ 1. Color codes in panels (a) and (b) show θ 1, and that in panel (c) shows T ion(0)/e 1 sw. Figure 3: The same as Figure 2, except that we use solar wind parameters averaged over 9 hours prior to the 1-hour intervals of the plasma sheet observation. To see the origin of this scatter, we classify the data points in Figure 2 (a) and (b) with color codes according to θ 1 : Yellow-red-purple colors correspond to the southward IMF (θ 1 < 0), and green-blue-black to the northward IMF (θ 1 > 0). There is a weak tendency for the southward IMF points to cluster in the regions of high plasma sheet temperatures and low densities,

3 3 whereas the northward IMF points are more evenly distributed. Figure 2 (c) presents normalized plasma sheet densities N ion (0)/N 1 plotted versus θ 1. Colors indicates the normalized plasma sheet temperature, k T ion (0)/e 1 sw : lue to red colors correspond to low to high temperatures. The highest densities and coldest temperatures only appear during northward IMF intervals. Figure 4: Cross correlations (solid [dotted] curves) between the normalized plasma sheet densities [temperatures] and the IMF parameters, θ N and N sw,z (see text). To reduce the scatter in the correlation, we have tried to make various choices of solar wind parameters. The best correlation is found for solar wind parameters averaged over several hours. For comparison with the plasma sheet parameters obtained during the interval of from t 1tot hours, we use N N, θ N, e N sw calculated for the interval from t N to t hours. In Figure 3 (a) and (b), T ion (0) and N ion (0) are plotted against e 9 sw and N 9, respectively. (Owing to data gaps the number of points reduces to 479.) We see that there are clearer separations of the points than in Figure 2 (a) and (b): Red points (θ 9 < 0) accumulate in the higher temperature and lower density sides, and blue points (θ 9 > 0) gather in the lower temperature and higher density sides. Figure 3 (c) plots the normalized plasma sheet densities, N ion (0)/N 9, versus θ 9. We have found that the cross correlation coefficient can be as high as 0.65 between the logarithm of N ion (0)/N 9 and θ 9. (Only the points with θ 9 > 0 are included in the correlation analysis.) Solid curves in Figure 4 show the dependence on N (hours) of the cross correlation coefficients between the logarithm of N ion (0)/N N and the IMF parameters, θ N and N sw,z. oth of these two curves reach their maxima around N =6 12 hours with a broad peak at 9 hours. This positive correlation indicates that the density in the plasma sheet increases during dominantly-northward IMF periods. Dotted curves in Figure 4 show the dependence on N of the cross correlation coefficients between the logarithm of T ion (0)/e N sw and the IMF parameters. The negative sign (i.e., anti-correlation) of these dotted curves indicates that the plasma sheet cools during dominantly-northward IMF periods. The strongest anti-correlation is seen at N = 5 10 hours, slightly shorter than for the density-imf correlation. Note that the above long time scales are reminiscent of the observation by Nakai et al. [1986], who found that the contraction of the auroral oval has a time scale of 8 hours. Figure 5 and 6 show the normalized temperatures T ion (0)/e 9 sw and densities N ion (0)/N 9 plotted against (a) X GSM and (b) Y GSM of the observation points. (We used 9-hour averaged solar wind parameters.) Stepwise lines in these panels show the average values of these quantities in 5-Re bins of X GSM or Y GSM taken for θ 9 > 15 o (blue) and θ 9 < 15 o (red), respectively. We draw these averages only for the bins where the numbers of corresponding data points exceed 4. We first note in Fig. 5a and 6a that both temperature and density tend to increase toward the earth irrespective of the IMF condition. We see then in Fig. 5b and 6b that for northward IMF periods the temperature reduces and the density increases toward the dawn and dusk edges of the plasma sheet. For southward IMF periods, red lines show no systematic decrease of temperature, or increase of density toward the dawn flank of the plasma sheet. (The small number of the data points for southward IMF in the dusk side makes the result inconclusive there.) These characteristic features in Fig. 5 and 6 are consistent with the statistical results of Lennartsson and Shelley [1986], who used the AE index instead of the IMF parameters.

4 4 Figure 5: The normalized plasma sheet temperatures, T ion(0)/e 9 sw, are plotted against (a) the X GSM and (b) Y GSM positions of the observations. Colors indicate θ 9. Figure 6: The normalized plasma sheet densities, N ion(0)/n 9, are plotted against (a) the X GSM and (b) Y GSM positions of the observations. Colors indicate θ 9. Discussion. We have shown that the near-earth magnetotail plasma sheet becomes relatively cold and dense during dominantly-northward IMF periods. Further observation that the plasma is colder and denser near the dawn and dusk flanks of the plasma sheet than in the central region (Figure 5b and 6b) suggests that these plasmas come across the flank magnetopause. The idea that the flank region plays the primary role in transporting the solar wind plasma into the plasma sheet has been proposed by several authors [Eastman et al., 1985; Lennartsson, 1992; Fujimoto et al., 1996]. Recently, several authors have presented evidence of the anomalous magnetotail behavior during strongly northward IMF periods [Chen et al., 1993; Fairfield, 1993; Fairfield et al., 1996; Fujimoto et al., in preparation, 1996]. Their observations suggest that the closed field region between the plasma sheet proper and the magnetosheath becomes quite turbulent. Our observations suggest that the plasma supply is due to the slow (diffusive?) transport through this turbulent region. Consistent with this view, we note that both x and y components of the plasma sheet velocity diminish to low ( km/s) fluctuating levels during northward IMF intervals (not shown). The time scale of hours, which is found for the best correlations (positive or negative) between the plasma sheet parameters (density and temperature) and the IMF parameters (θ and sw,z ), is much longer than the well-known substorm time scale for southward IMF periods (= 1 2 hours [e.g., McPherron, 1991]). We consider that this scale of hours gives a minimum estimate for the characteristic time associated with the slow transport process, τ slow : We have calculated the autocorrelation functions of sw,z and θ for the entire period of Nov Dec 1995, and found that they behave approximately as exp( t /τ ) for t < 18 hours with an autocorrelation time τ 6 hours. (eyond t 18 hours, the autocorrelation functions reach the noise level.) Suppose that τ slow is much longer than 6 hours. When the slow transport process starts with a northward turning of the IMF, it is quite often interrupted by southward IMF periods (say, after several hours), during which the plasma in the plasma sheet is made hot and tenuous (or replaced with a hot and tenuous plasma) within 1 2 hours. ecause our correlation analysis must make use of an interrupted time sequence, the resultant correlation time is likely much shorter than τ slow itself. For the better estimation of true τ slow, what is needed is a deconvolution procedure. Further statistical study of the plasma sheet property during northward IMF periods is now under way. Acknowledgments. We are grateful to M. Scholer, K. Maezawa, M. Hoshino, A. Nishida, T. Nagai, Y. Kamide, and K. W. Ogilvie for valuable discussions and comments. This work was jointly supported by Grantin-Aid for Scientific Research, the Ministry of Education, Science, and Culture of Japan (No ), and Japan-Germany Cooperative Research Project ( ), Japan Society for the Promotion of Science. References aumjohann, W., G. Paschmann, and C. A. Cattell, Average plasma properties in the central plasma sheet, J. Geophys. Res. 94, , Chen, S.-H., et al., Anomalous aspects of magnetosheath

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