Mappings of auroral X-ray emissions to the equatorial magnetosphere: A study of the 11 April 1997 event

Abstract

Most photons detected by PIXIE (the X-ray imager aboard GGS/Polar and the first instrument to provide global X-ray images of Earth) have resulted from precipitation of auroral electrons. Here we trace magnetic field lines (from the high-latitude ionosphere out to the equatorial magnetosphere) to learn which regions of the equatorial magnetosphere would have been populated by the corresponding electrons just before they precipitated. Since this question is model-dependent, we use both the source-surface (Schulz-McNab) model of the magnetosphere and Tsyganenko's data-based T96_01 field model. Of several mappings performed from PIXIE data in this first-ever such study based on global X-ray images, we show a representative example from the magnetic storm of 11 April 1997 (which followed the coronal mass ejection of 7 April). The most intense X-ray features during this event map to R ∼ 15 in the evening sector of Tsyganenko's model magnetosphere. The same features map almost to the inner edge of the cross-tail current sheet (i.e., almost to the boundary between closed and open field lines) in the source-surface model if this inner edge is placed about 7 R E downstream from Earth. (This inference illustrates the first use of X-ray data to evaluate an adjustable parameter of the field model.) Less intense X-ray features map to regions of decidedly closed (non-tail) field lines, especially in the pre-dawn quadrant of the equatorial magnetosphere, where diffuse auroral electrons are widely presumed to originate. Regions of least X-ray intensity around the auroral oval typically map to statistically uncertain radial distances in the early afternoon sector in both field models.