dependence of magnetopause locations on the imf ......ticipates in the magnetopause creation is...

Dependence of Magnetopause Locations on the IMF BZ

Component During the Solar Minimum

G. Granko, J. Safrankova, Z. Nemecek, K. Jelınek

Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.

Abstract. We have collected the data set of more than 7000 crossings of thesubsolar and flank low-latitude magnetopause observed by five THEMIS spacecraftin the course of the solar minimum and analyzed the deviations between observedmagnetopause radial distances and those predicted by an empirical magnetopausemodel. The empirical models usually scale a second order surface with respect to thesolar wind dynamic pressure and IMF BZ component but the crossings observed bythe spacecraft are often far away from the predicted locations. The data propagatedfrom the L1 point were used as model inputs. We discuss the influence of upstreamparameters (solar wind dynamic pressure, the IMF cone angle, IMF BZ component)on magnetopause locations and compare these effects for different local times.

Introduction

The magnetopause is a boundary layer originating from the pressure balance between thetotal pressure at the magnetosheath and on the magnetospheric side. The solar wind dynamicpressure and interplanetary magnetic field (IMF) BZ component control the magnetosphericdynamics and the shape of the magnetopause.

From this, it follows that the IMF BZ component is an important parameter of magne-topause processes. Its strength and orientation influence mainly the shape of the magnetosphere.One of mechanisms of plasma transfer between the solar wind and magnetosphere which par-ticipates in the magnetopause creation is reconnection. Reconnection is the process by whichindividual field lines lose their links and become connected to a new region. When the IMF issouthward (BZ < 0) reconnection which accelerates and heats the plasma in the boundary lay-ers takes place at the subsolar region. Consequently, the subsolar magnetopause moves inwardwhen IMF BZ is southward [Fairfield , 1971]. On the other hand, northward IMF (BZ > 0)leads to lobe reconnection with a significantly smaller reconnection rate and thus its influenceon the magnetopause location is under debate.

Prediction of the magnetopause position is important in magnetospheric physics. Differentmodels of the magnetopause shape and location have been suggested [Fairfield , 1971; Formisano,1979; Sibeck et al., 1991; Petrinec and Russell , 1996; Shue et al., 1997; Alexeev et al., 1999;Boardsen et al., 2000; Chao et al., 2002; Lin et al., 2010]. These models use usually a secondorder surface that is scaled with the solar wind dynamic pressure and IMF BZ .

We have studied the predicted locations of the magnetopause from the Shue et al. [1997]model. This model assumes a cylindrical symmetry around the aberrated Sun-Earth line andhas been obtained by fitting large set of the magnetopause crossings. Shue et al. [1997] suggestthat this model is valid over a wide range of upstream parameters and it has been scaledaccording to the solar wind dynamic pressure with a power law – 1/6.6.

We have used a large fresh set of magnetopause crossings observed by the fleet of theTHEMIS spacecraft with motivation to find the further parameters that control the magne-topause location. The known dependence of magnetopause locations on upstream parameterswas taken into account by calculation of differences between observed and predicted by model[Shue et al., 1997] locations.

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WDS'11 Proceedings of Contributed Papers, Part II, 40–44, 2011. ISBN 978-80-7378-185-9 © MATFYZPRESS

GRANKO ET AL.: DEPENDENCE OF THE MAGNETOPAUSE LOCATIONS

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(a) (b)Figure 1. A projection of the THEMIS magnetopause crossings onto the XY (a) and XZ (b)GSE planes together with the nominal location of the magnetopause [Shue et al., 1997]. Theblack points stand for crossings observed inside a region XGSE > 5,−5 < YGSE < 5,−5 <ZGSE < 5; the rest of crossings is shown by the gray points.

Data processing

We have collected crossings in selected regions of the low-latitude magnetopause observedby five THEMIS spacecraft [Angelopoulos, 2008] in the course of the 2007–2008 years and ana-lyzed the deviations of observed magnetopause locations from those predicted by the Shue

et al. [1997] model. Our database of crossings (Fig. 1a and 1b) was complemented with thecorresponding upstream parameters from Wind and ACE lagged on the propagation time. Thepresent analysis uses IMF from ACE [Smith et al., 1998] and plasma moments from Wind[Ogilvie et al., 1995] because such combination provided a best coverage of the crossings withupstream parameters. These data were propagated by the two-step approximation [Safrankova

et al., 2002] to the location of a particular THEMIS magnetopause crossing (the decelerationin the magnetosheath was omitted) and 5-minute averages were used as upstream conditions.The model used is axisymmetric around its X axis, but the magnetopause can be expected tobe rotationally symmetric around the solar wind velocity direction. For this reason, we haveconverted the locations of magnetopause crossings into aberrated coordinates [Safrankova et al.,2002].

Analysis

The dependence of the magnetopause location on IMF BZ is still under question. Shue

et al. [1997] suggest a weak dependence on BZ but the exclusion of this term from their modeldoes not lead to worse results in a statistical sense (Fig. 2a). Our data was collected duringlast solar minimum, while the model was created using data collected without such resolution;during solar maximum and minimum. Long intervals of a small dynamic pressure during lastsolar minimum allowed us to re-evaluate the pressure dependence in the model. We suggestthat 6.6 rate coefficient used in the Shue et al. [1997] model is too weak to account for the lowersolar wind dynamic pressures and replaced it with the coefficient of 5.099 in accord with Dusik

et al. [2010]. A comparison of both models as histograms are shown in Fig. 2b. The half-widthbecomes slightly narrower and the distribution is more centered.

A test of the IMF BZ influence on the magnetopause location in a new model is shown inFig. 2c, where the model locations were computed under an assumption BZ =0. The comparisonof both figures (Fig. 2b and Fig. 2c) shows that the BZ effect is small. We will return to thispoint later.

Merka et al. [2003] reported larger magnetopause oscillations during intervals of a quasi-

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(c) (d)Figure 2. Histograms of distributions of differences between observed and predicted by Shueet al. [1997] magnetopause locations. In all panels, the black curves represent the deviationsof observed magnetopause position from the Shue et al. [1997] model, the gray curves showdeviations of observed magnetopause positions from corrected by different ways. The parametersof distributions are shown in the top of each figure.

radial IMF, similarly as Suvorova et al. [2010] and Jelinek et al. [2010] that mentioned anunusual magnetopause position through the radial IMF. Fig 3a presents the dependence ofthe differences, ROBS − RMOD on the IMF cone angle (angle between the Sun–Earth lineand IMF vector) in both subsolar (upper) and near-flank (bottom) regions. To calculation ofROBS −RMOD, the Shue et al. [1997] model was used. Comparing both panels, one can clearlysee that the crossings observed during a radial IMF are in average on about 1 RE outward fromthe Earth [Dusik et al., 2010]. Thus, as a next step, we applied a correction of the modeleddistances on the cone angle according to the equation:

RCORR MOD = RMOD − sin(cone angle) + 0.5

The results of this correction are shown in Fig 3b. In this case, the model with the coefficient of5.099 (according to Dusik et al. [2010]) was applied. The parabolic fits changed to weak lineardependences. The final evaluation of both model corrections is shown in Fig. 2d. One can seethat it gives a smaller width of the Gaussian fit.

The correction of the magnetopause model including the dynamic pressure and IMF coneangle allows us to re-check a possible influence of the IMF BZ component on the magnetopauselocations. We plotted the differences between observed and predicted (by the corrected model)magnetopause locations separately for the subsolar (YGSE, ZGSE < 5 RE) and flank subsets

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(a) (b)Figure 3. Differences between observed and predicted magnetopause locations vs. the coneangle for the susolar (upper panels) and flank (bottom panels) regions: (a) Shue et al. [1997];(b) Shue et al. [1997] with the cone angle correction.

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(a) (b)Figure 4. Dependence of the differences of observed and predicted magnetopause locations onIMF BZ for southward (a) and northward (b) IMF.

in the upper and bottom panels in Fig. 4, respectively. As it can be seen from the figure,we did not find any systematic change that could be attributed to IMF BZ . A small outwarddisplacement of the flank magnetopause suggested by the linear fit in Fig. 4a should be furtheranalyzed because the number of crossings in this subset is rather small.

Finally, we should note that this study uses a non-usual data set because THEMIS mag-netopause observations were collected through a long-lasting solar minimum. The Shue et al.

[1997] model was created using data collected throughout the whole solar cycle which couldcause a significant deviation of the magnetopause position.

Conclusion

Our analysis of THEMIS 2007–2008 observations of the subsolar and near-flank magne-topause and a comparison of these observations with predictions of the Shue et al. [1997] modelrevealed that (1) The Shue et al. [1997] model underestimates the effect of the solar wind

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dynamic pressure on the magnetopause location. (2) A replacement of the rate coefficient of6.6 with 5.099 provides better results. (3) The dependence of the magnetopause location onthe cone angle is corrected. (4) We do not find any reliable dependence of the magnetopauselocation on IMF BZ . What is the cause of lack of the IMF BZ effect?

We suppose to analyze in detail all observations in the solar wind to compare different setsof observations in various phases of the solar cycle.

Acknowledgments. The authors thank the THEMIS, WIND and ACE working teams for themagnetic field and plasma data. G. Granko and K. Jelinek thank the Charles University Grant Agency(GAUK 163810) for supports. The work was supported by Research project MSM0021620860 financedby the Ministry of Education of Czech Republic.

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dependence of magnetopause locations on the imf ......ticipates in the magnetopause creation is...

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