Observations of bow shock and magnetopause crossings from geosynchronous orbit on 31 March 2001

On 31 March 2001, significant enhancements in both the strength of the interplanetary magnetic field and the solar wind dynamic pressure pushed the magnetopause and bow shock inside of geosynchronous orbit. We present here observations of the unshocked solar wind observed with the Los Alamos magnetospheric plasma analyzer on geosynchronous satellite 1994-084 near local noon. The magnetosheath was observed intermittently at geosynchronous satellites 1991-080, 1994-084, and LANL-01A, across much of the dayside magnetosphere. Comparisons with theory of the observed bow shock and magnetopause locations at geosynchronous orbit near noon are in reasonable agreement. Using nearly simultaneous observations of the magnetopause and bow shock locations at 0451 UT, we have tested models for the thickness of the magnetosheath and flaring of the magnetopause at that time. Using the least flared of the magnetopause models, the estimated thickness of the magnetosheath would be thicker than would be predicted by either the gas dynamic solution of Spreiter et al. [1966] or the MHD simulations of Cairns and Lyon [1996]. Alternatively, placing the location of the magnetopause at the nose of the magnetosphere in conformance with the thickness of the magnetosheath from Cairns and Lyon [1996] requires less flaring on the flanks of the magnetosphere than currently present in the empirical models. The models also have difficulty predicting the other observed magnetopause crossings far from local noon apparently due to excessive flaring of the models for the conditions on this day.