Anisotropic magnetosheath: Comparison of theory with Wind observations near the stagnation streamline

C. J. Farrugia, N. V. Erkaev, D. F. Vogl, H. K. Biernat, M. Oieroset, R. P. Lin, and R. P. Lepping

Space Science Center, University of New Hampshire, Durham, NH 03824

Abstract:

We carry out a first comparison with spacecraft measurements of our recent three-dimensional, one-fluid magnetohydrodynamic (MHD) model for the anisotropic magnetosheath (Erkaev et al., 1999), using data acquired by the Wind spacecraft on an inbound magnetosheath pass on December 24, 1994. The spacecraft trajectory was very close to the stagnation streamline, being displaced by less than 1/2 hour from noon and passing at low southern magnetic latitudes (~4.5 ). A wide plasma depletion layer (PDL) adjacent to the low-shear magnetopause is observed on this pass, constituting ~1/3 of the magnetosheath thickness in the Earth-Sun direction. Its outer edge is identified well by the criterion b p =1. All quantities downstream of the bow shock are obtained by solving the Rankine-Hugoniot equations taking the pressure anisotropy into account. In this application of our model, we close the MHD equations by a 'bounded anisotropy' ansatz using for this purpose the inverse correlation between the proton temperature anisotropy, Ap (≡ Tp^ /T p ), and the proton plasma beta parallel to the magnetic field b p , observed in the magnetosheath on this pass when conditions are steady. For all quantities studied, we find very good agreement between the predicted and the observed profiles, indicating that the bounded anisotropy method of closing the magnetosheath equations, first suggested by Denton et al. (1994), is valid and reflects the physics of the magnetosheath well. We assess how sensitive our model results are to different parameters in the Ap = a0 b p -a1(a1 > 0) relation, taking for a1 the two limiting values (0.4, 0.5) resulting from the 2-dimensional hybrid simulations of Gary et al. (1997), and varying a0 in the range 0.6 - 0.8. Input solar wind conditions are as measured on this pass. In general, the model profiles depend more strongly on a0 than on a1. In particular, decreasing a0 narrows PDL width and widens the mirror stable region. For the lowest value of a0, the mirror stable region extends sunward of the outer edge of the PDL. For the other two values of a0, and regardless of the value of a1, it is contained within the PDL. Finally, we also study phenomenological bi-polytropic laws, and find polytropic indices g ^ 1, and g 1.5. These results agree well with those of another study in the same approach, also from the near-subsolar magnetosheath.

submitted, J. Geophys. Res., 2001