Portal de Periódicos da CAPES

Magnetopause shape as a bivariate function of interplanetary magnetic field B z and solar wind dynamic pressure

1993; American Geophysical Union; Volume: 98; Issue: A12 Linguagem: Inglês

10.1029/93ja02362

2156-2202

E. C. Roelof, D. G. Sibeck,

Geomagnetism and Paleomagnetism Studies

We present a new method for determining the shape of the magnetopause as a bivariate function of the hourly averaged solar wind dynamic pressure ( p ) and the north‐south component of the interplanetary magnetic field (IMF) B z . We represent the magnetopause (for X GSE >−40 R E ) as an ellipsoid of revolution in solar‐wind‐aberrated coordinates and express the ( p , B z ) dependence of each of the three ellipsoid parameters as a second‐order (6‐term) bivariate expansion in lnp and B z . We define 12 overlapping bins in a normalized dimensionless ( p , B z ) “control space” and fit an ellipsoid to those magnetopause crossings having ( p , B z ) values within each bin. We also calculate the bivariate ( lnp , B z ) moments to second order over each bin in control space. We can then calculate the six control‐space expansion coefficients for each of the three ellipsoid parameters in configuration space. From these coefficients we can derive useful diagnostics of the magnetopause shape as joint functions of p and B z : the aspect ratio of the ellipsoid's minor‐to‐major axes; the flank distance, radius of curvature, and flaring angle (at X GSE = 0); and the subsolar distance and radius of curvature. We confirm and quantify previous results that during periods of southward B z the subsolar magnetopause moves inward, while at X GSE = 0 the flank magnetopause moves outward and the flaring angle increases. These changes are most pronounced during periods of low pressure, wherein all have a dependence on B z that is stronger and functionally different for B z southward as compared to B z northward (i.e., the behavior of a “half‐wave rectifier”). In contrast, all these changes are much less sensitive to IMF B z at the highest pressures. As an application of these new results, we use a pressure balance relationship to estimate the difference between the magnetic field strength just inside the subsolar magnetopause and that of the dipole field, and we find that this difference decreases rapidly as B z becomes more negative (although it is relatively insensitive to northward changes in B z ). Quantitative comparison shows that Region 1 Birkeland currents could make the dominant contribution to this depression in the inferred magnetic field at the subsolar point.