PLASMA DEPLETION LAYER MODEL FOR LOW ALFVEN MACH NUMBER - COMPARISON WITH ISEE OBSERVATIONS

Together with the magnetic shear across the magnetopause, the solar wi nd Alfven Mach number, M-A infinity plays a central role in determinin g the structure of the magnetosheath. Recent theoretical modeling has shown, in particular, that as M-A infinity decreases, the region adjac ent to the sunward side of the magnetopause where the interplanetary m agnetic field (IMF) exerts a strong influence on the flow (i.e., the s o-called ''plasma depletion layer''), is no longer confined to a thin layer similar to 0.3 Earth radii (R-E) thick but occupies an increasin gly larger fraction of the magnetosheath. Furthermore, the model predi cts the possibility of a plasma depletion layer for low M-A infinity, irrespective of the size of the magnetic shear at the magnetopause, In this paper we study three examples of low latitude ISEE 2 passes thro ugh the dayside magnetosheath near noon: December 3, 1979; October 5, 1979; and November 11, 1979, In all three examples, MA, was lower than normal. During the December 3 pass (which we treat qualitatively), we find evidence of a plasma depletion layer when the IMF was pointing s outh, On the other two passes (which we study quantitatively), the int erplanetary magnetic field was strongly northward pointing, leading to low magnetic shear at the respective magnetopause crossings, The Octo ber 5 pass was under steady interplanetary conditions and we find good agreement between theory and data, Temporal variations of the interpl anetary medium during the November 11 pass necessitated an extension o f the steady state theory to encompass piecewise steady (on average) i nterplanetary conditions, Better agreement with the data results when the theory is extended further to correct the total pressure at the su nward side of the magnetopause by integrating the magnetic tension ter m across the layer. For wide plasma depletion layers, this correction can be substantial.