The Transition of Interplanetary Shocks through the Magnetosheath A. Szabo∗ , C. W. Smith† and R. M. Skoug∗∗ ∗ Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center, Greenbelt, MD 20771 † Bartol Research Institute, University of Delaware, Newark, DE 19716 ∗∗ Los Alamos National Laboratory, Los Alamos, NM 87545 Abstract. WIND, ACE, IMP 8 and Geotail data shows that the magnetosheath signature of IP shocks is primarily a fast-mode shock or pressure pulse with a wide ramp. No model predicted secondary discontinuities could be identified above the magnetosheath background fluctuation level. Even though the interplanetary surface geometry of IP shocks could be significantly corrugated, this study suggests that there is no significant deceleration of the pressure front in the magnetosheath. INTRODUCTION of the magnetic field on the interaction of the IP shock and bow shock by using various MHD and hybrid formulations. The one-dimensional model of Whang [10] was very successful at describing outer heliospheric observations of IP shocks. It allowed the merger of two IP shocks if they are both forward or reverse, and predicted the transmission of the two interacting shocks if one is forward and the other reverse with a tangential discontinuity (TD) forming between them. This model, however, is limited to the treatment of perpendicular shocks. Cargill [11] relaxed this requirement with the use of a one-dimensional hybrid code. He showed that while the collision between two perpendicular collisionless shocks gives rise to a TD located between the two transmitted shocks, as predicted by one-dimensional MHD theory, the collision between two oblique shocks produces a much more extensive and turbulent region between the two transmitted shocks, possibly a contact discontinuity (CD). The CD, like the TD, shows jumps in the plasma and magnetic field components and therefore should be identifiable in observational data. Moreover the transmitted shock is deflected from its original orientation that should also be observable. Aside for Zhuang et al. [12] reporting a case of ISEE 1 and 3 measurements where such a sequence of disturbances was possibly observed in the Earth’s magnetosheath, observational evidence in the literature remains rather limited and is one of the topics of this paper. Next the data sets and analytical techniques used in this study is discussed followed by the results A strong correlation of interplanetary (IP) shocks impinging on the magnetosphere and geomagnetic disturbances have been reported by many observers [1, 2, 3]. IP shocks, as all solar wind pressure events, tend to disrupt the magnetopause surface leading to magnetopause transient events [4] that, in turn, can initiate magnetic reconnection resulting in substorm onset [5]. IP shocks have also been connected to sudden commencements and auroral brightening [6]. However, before IP shocks reach the magnetopause, they have to cross the Earth’s bow shock and traverse the magnetosheath introducing both geometrical and physical modifications. Following IP shocks through the magnetosheath with in-situ observations is the topic of this paper. The interaction of IP shocks with the bow shock and its transmission through the magnetosheath to the magnetopause, has been studied mainly by gas dynamic modeling [7, 8, 9]. These models, by construction, allow the generation and propagation of only one type of wave in the magnetosheath. Therefore, it is not surprising that they find only a single, fast mode pressure pulse (or fast shock in the supersonic flanks) propagating through the magnetosheath. Interestingly the predicted disturbance geometry or shape in the magnetosheath remains nearly planar (the shape of the undisturbed IP shock) propagating with the same speed as the undisturbed IP shock [9]. This prediction is investigated in detail in this paper. Attempts have been made to include the effect CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference, edited by M. Velli, R. Bruno, and F. Malara 2003 American Institute of Physics 0-7354-0148-9/03/$20.00 782 of the interplanetary and magnetosheath shape and signatures of shocks. Finally, a short summary will be presented. DATA AND ANALYSIS TECHNIQUES During 1998–1999, twelve IP shocks have been identified that were observed by at least 2 solar wind monitors and by another spacecraft in the magnetosheath. This interval was selected because IP shocks were not very frequent and clearly observable (unlike the later solar maximum years), and there was nearly continuous coverage by WIND, ACE, IMP 8 and Geotail. Moreover, the relatively large separation between consequent IP shocks assured that there was only minimal possibility of interaction between them before reaching 1 AU keeping their surface geometry the simplest possible. In order to establish the undisturbed surface geometry of the incoming IP shocks the observed solar wind plasma and magnetic field data before and after the shocks were fitted by the non-linear least squares ”RankineHugoniot” technique originally developed by Vinas and Scudder [13] and further enhanced by Szabo [14]. This fitting technique provides the best possible local shock normal direction and speed determination by a single spacecraft with the associated uncertainties. Data from the WIND Magnetic Field Investigation (MFI) [15], and Solar Wind Experiment (SWE) [16], IMP 8 magnetic field and plasma experiments [17], and ACE magnetic field experiment (MAG) [18] and solar wind plasma instrument (SWEPAM) [19] were used for the analysis. Preliminary use was also made of the Geotail magnetic field (MGF) [20] and solar wind (CPI) [21] data. FIGURE 1. Angular deviation between shock normal directions for the same event as observed by WIND and ACE (solid circles) and WIND and IMP 8 (open circles). Crosses mark those shocks that were clearly driven by magnetic clouds. served by at least two solar wind monitors during the rising phase of the previous solar cycle (1997– 1999)have been fitted and the shock normal directions compared. Figure 1 shows the angular deviation between the corresponding shock normal directions as a function of the inter-spacecraft separation perpendicular to the Sun-Earth line. Solid circles mark shocks observed by WIND and ACE, while open circles refer to WIND and IMP 8 observations (note the significantly larger associated uncertainties). Considerable deviation from planarity is apparent. Also the degree of curvature does not appear to be a simple function of the inter-spacecraft separation. In addition, those IP shocks that were clearly driven by magnetic clouds are marked by a solid cross indicating that some clouds drive shocks that are very nearly planar, on the other hand, some cloud driven shocks can be as corrugated as some of those for which the drivers are uncertain. This result complicates the analysis of the magnetosheath propagation of IP shocks as clearly planarity cannot be assumed (unlike for computer simulations) and some allowance for local curvature will have to be made. IP SHOCK GEOMETRY IN INTERPLANETARY SPACE Generally it is assumed that incoming IP shocks are planar on the scale size of the magnetosphere. Indeed, in a recent study Russell et al. [23] analyzed a single IP shock with four solar wind satellites and found that three of them were consistent with the planarity assumption. However, deviation from planarity has been reported before [24, 25]. Specifically, Szabo et al. [25] have found that IP shocks driven by small magnetic clouds have a highly corrugated surface geometry on the scale-size of the magnetosphere. To further illustrate that significant deviations from planarity is possible, 17 IP shocks ob- IP SHOCKS IN THE MAGNETOSHEATH During the time period of 1998–1999 twelve IP shocks have been identified that were observed by at least 2 solar wind monitors (WIND and ACE) to place some limit on the interplanetary curva- 783 FIGURE 3. Difference in the predicted and observed arrival times in the magnetosheath as a function of the spacecraft separation. See the text for details. eral magnetosheath background fluctuations could easily mask small variations. However, it does point out that for magnetospheric energy and momentum input the leading fast-mode pressure jump or shock is the most significant. In order to make some assessment of the magnetosheath geometry of the transmitted disturbance, the IP shock surface normal directions and speeds, fitted in the solar wind data, were used to estimate a predicted arrival time at the magnetosheath monitor. This predicted arrival time was compared to the actually observed time delay. The thus obtained difference is plotted in Figure 3. Positive difference time refer to the actual magnetosheath observation being later than predicted based on the upstream shock fit results. This would be the expected case if the pressure front is decelerated in the sheath as is the case for TDs. Negative difference times correspond to earlier than expected arrival times. This could be due to an unlikely acceleration of the front or more likely to the intrinsic curvature of the IP shock. All time differences are calculated with respect to the beginning of the sheath pressure pulse. The length of the pressure ramp is indicated by the gray bars. A time difference within the gray bars would be consistent with an unaltered shock disturbance front. The same procedure is repeated for both solar wind monitors. The results corresponding to the solar wind monitor closest to the sheath monitor perpendicular to the Sun-Earth line is plotted as a solid circle (the other result is plotted as a cross) as a function of this cross-wind separation. Just as in the case of the undisturbed solar wind IP shocks (Figure 1), there is no clear dependence on the spacecraft separation. The same data is plotted in a similar format in Figure 4 as a function of the distance of the sheath monitor behind the bow shock. The location of the bow shock was estimated using the Peredo model FIGURE 2. WIND, ACE and IMP 8 magnetic field magnitude and GSE Cartesian and spherical components on May 29, 1998. The WIND and ACE data is time shifted to line up the IP shock marked by the dashed line. ture of the incoming shock, while IMP 8 provided magnetosheath observations. Some tentative Geotail sheath events were also identified. For some IP shocks a corresponding pressure pulse is clearly identifiable in the sheath observations. Figure 2 shows 90 minutes of observations of the magnetic field and its components by WIND, ACE and IMP 8 on May 29, 1998. The WIND and ACE data has been time shifted by 29 and 37 minutes, respectively to line up the shock observations with the sheath pressure pulse event (the higher overall field values correspond to the IMP 8 compressed sheath measurements). The sheath pressure pulse is very clear in both magnetic field and plasma observations. Also it should be noted that the nearby large field rotation corresponding to a TD has a markedly different advection time delay. This is consistent with the pressure pulse corresponding to the IP shock that travels faster than the strictly advecting TD. Such a clear sheath signature was not always apparent. Weaker and reverse shocks had significantly broader ramps, some reaching over 10 minutes. On the other hand, even for the clearest cases no other discontinuity nearby could be identified. This does not prove that the secondary discontinuities, predicted by MHD and hybrid codes, do not exist as the gen- 784 ACKNOWLEDGMENTS The authors are grateful to A. J. Lazarus, J. D. Richardson and the MIT IMP 8 and WIND team for providing plasma data, to S. Kokubun and L. A. Frank for the use of the Geotail key parameter magnetic field and plasma data. REFERENCES 1. FIGURE 4. Difference in the predicted and observed arrival times in the magnetosheath as a function of the distance of the sheath monitor behind the bow shock. See the text for details. 2. 3. [26] and the measured solar wind conditions. Clearly, better determination could be made of the sheath position of a spacecraft, but this plot serves to show that some of the better timing agreements happened for cases where the sheath monitor was deep inside the sheath, hence any systematic deceleration of the pressure front would be most noticeable. 4. 5. 6. 7. 8. 9. SUMMARY While the data set presented is very limited and the question of the transition of IP shocks through the magnetosheath complicated, at least a few preliminary assessments could be made. A clear fast-mode shock or pressure pulse with a wider ramp could be identified for most solar wind observed IP shocks. However, the model predicted secondary discontinuities were not apparent in the highly fluctuating sheath background indicating that for energetics the leading pressure pulse is the most relevant. Even though the intrinsic curvature or corrugation of IP shocks complicates the determination of the geometrical effects of the magnetosheath on the transmitted shocks, the data presented suggests that there is no systematic deceleration of the pressure front. That is, the uncertainty of the arrival time of an IP shock to the magnetopause based on upstream solar wind observations (a value that could be near 10 minutes) is not due to the effects of the magnetosheath but most likely to the unknown interplanetary geometry of the shock surface fronts. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 23. 24. 25. 26. 785 Tsurutani, B.T., et al., J. Geophys. Res. 100, 21,717(1995). Gonzales, W.D., Tsurutani, B.T., and Gonzales, C.de, Space Sci. Rev. 88, 529–562 (1999). Gonzales, W.D., and Tsurutani, B.T., ”The interplanetary causes of magnetic storms: A Review,” in Magnetic Storms, AGU Monograph 98, Washington, D.C., 1998. Sibeck, D.G., and Newell, P.T., J. Geophys. Res. 100, 21,773 (1995). Wu, C.C., J. Geophys. Res. 105, 7533 (2000). Tsurutani, B.T., et al., J. Atmos Sol.-Terr. Phys. 63, 513 (2001). Shen, W.W., and M. Dryer, J. Geophys. Res. 77, 4627 (1972). Dryer, M., Radio Sci. 8, 893 (1973). Spriter, J.R., and Stahara, S.S., ”Computer modeling of solar wind interaction with Venus and Mars,” in The Comperative Study of Venus and Mars: Atmospheres, Ionospheres and Solar Wind Interactions, AGU Monograph 66, Washington, D.C., 1992, pp. 345–383. Whang, Y.C., Space Sci. Rev. 57, 339 (1991). Cargill, P.J., J. Geophys. Res. 95, 20,731 (1990). Zhuang, H.C., Russell, C.T., Smith, E.J., and Gosling, J.T., J. Geophys. Res. 86, 5590 (1981). Vinas, A.F., and Scudder, J.D., J. Geophys. Res. 91, 39 (1986). Szabo, A., J. Geophys. Res. 99, 14,737 (1994). Lepping, R.P., et al., Space Sci. Rev. 71, 207 (1995). Ogilvie, K.W., et al., Space Sci. Rev. 71, 55 (1995). Lepping, R.P., et al., NASA/GSFC LEP int. doc., (1992). Smith, C.W., et al., Space Sci. Rev. 86, 613 (1998). McComas, D.J., et al., Space Sci. Rev. 86, 563 (1998). Kokubun, S., et al., J. Geomag. Geoelectr. 46, 7 (1994). Frank, L.A., et al., SES-TD-92-007SY, (1992). Russell, C.T., et al., J. Geophys. Res. 105, 25,143 (2000). Russell, C.T., et al., J. Geophys. Res. 88, 9941 (1983). Szabo, A., et al., ”The evolution of interplanetary shocks driven by magnetic clouds”, in Proceedings of ”Solar Encounter: The First Solar Orbiter Workshop”, ESA SP-493, The Netherlands, 2001, pp. 383–387. Peredo, M., Slavin, J.A., Mazur, E., Curtis, S.A., J, Geophys. Res. 100, 7907 (1995).
© Copyright 2024 Paperzz