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

Abstract

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) Bz. We represent the magnetopause (for XGSE>−40RE) as an ellipsoid of revolution in solar‐wind‐aberrated coordinates and express the (p, Bz) dependence of each of the three ellipsoid parameters as a second‐order (6‐term) bivariate expansion in lnp and Bz. We define 12 overlapping bins in a normalized dimensionless (p, Bz) “control space” and fit an ellipsoid to those magnetopause crossings having (p, Bz) values within each bin. We also calculate the bivariate (lnp, Bz) 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 Bz: the aspect ratio of the ellipsoid's minor‐to‐major axes; the flank distance, radius of curvature, and flaring angle (at XGSE = 0); and the subsolar distance and radius of curvature. We confirm and quantify previous results that during periods of southward Bz the subsolar magnetopause moves inward, while at XGSE = 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 Bz that is stronger and functionally different for Bz southward as compared to Bz northward (i.e., the behavior of a “half‐wave rectifier”). In contrast, all these changes are much less sensitive to IMF Bz 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 Bz becomes more negative (although it is relatively insensitive to northward changes in Bz). 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.

Comprehensive study of the magnetospheric response to a hot flow anomaly

“…These variations must also have been generated at the bow shock, for Wind observed no significant density variations at all. However, none of the pressure variations excited a significant response at the location of GOES-8, deep within the dawn mag- Roelof and Sibeck, 1993]. It may also help explain why it is a simple matter to associate TCVs in high-latitude ground magnetograms with SI signatures at low latitudes and geosynchronous orbit [e.g., Sibeck, 1993], but it is rather more difficult to find a corresponding pressure change in the undisturbed solar wind [Sibeck and Korotova, 1996].…”

A survey of magnetopause FTEs and associated flow bursts in the polar ionosphere

“…An example of this limitation will be shown in the third case study. This problem is not a major limitation of the FTE-FCE association as it is possible to use magnetopause crossings as reference points for spacecraft position as well as empirical models of magnetopause location (Roelof and Sibeck, 1993).…”

“…The e ects of large FTEs are expected to be viewed by a spacecraft within about 1 R E of the magnetopause (Saunders et al, 1984). Proximity was determined as being within 1 R E of a model magnetopause (Roelof and Sibeck, 1993) from orbit data, or within an hour of when the Equator-S magnetometer data indicated a magnetopause crossing. Due to the location of the orbit apogee and the average magnetopause position, these criteria were functionally the same for most of the period.…”

Three‐dimensional artificial neural network model of the dayside magnetopause

“…In the study we use the dayside magnetopause crossings data set from Roelofand Sibeck [1993] We emphasize that the accuracy (o>1 RE) is similar for all four models which use different data sets (both hourly and minute averaged) and which were developed by different methods. This fact may be the result of uncertainties in the initial data sets of the models that reflect features of the magnetopause dynamics.…”

“…The model by Roelof and Sibeck [1993] (hereinafter the RS model) is the first complete empirical model based on a large statistical database of MP crossings (SW and IMF hourly averaged data from 795 crossings by 12 high-apogee satellites). For their model, Shue et al [1997] used another data set of MP from three high apogee and three geosynchronous satellites.…”