Two-point measurements of the magnetopause: Interball observations

The impulsive penetration model predicts that blobs of solar wind plasma pe netrate the magnetosphere by becoming detached from the magnetopause. If so , negative radial density gradients should be common near the magnetopause. We present the results of case and statistical studies of simultaneous Int erball 1 and Magion 4 omnidirectional plasma sensors (VDP) plasma observati ons during magnetopause passes. The spacecraft observe very similar feature s when their interspacing is small. By contrast, the spacecraft located far ther radially outward invariably observes densities greater than or equal t o those seen at the spacecraft nearer Earth when the interspacing increases . The observations are consistent with the standard interpretation in which a boundary layer of plasma with intermediate densities often lies sandwich ed between the magnetosheath and the magnetosphere proper. Blobs of magneto sheath or boundary layer plasma rarely, if ever, become detached from the m agnetopause.