Venus' nighttime horizontal plasma flow, ‘magnetic congestion’, and ionospheric hole production

Abstract

A persistent feature of the nightside Venus ionosphere is the existence of localized magnetic field aligned columns of depleted ionization—i.e., ‘holes’—in which the radial component of the field is enhanced. By analogy to the earth's auroral regions it has been inferred that parallel electric fields can rapidly deplete the ionization. Another, complementary plasma loss mechanism at night is downward transport into the lower ionosphere where chemical depletion occurs. A simple rectilinear two dimensional MHD model is used to explore the effects of field aligned plasma loss and cooling on a dense plasma convecting across a weak magnetic field. By parameterizing field aligned variations and explicitly solving for cross variations, it is shown that the abrupt horizontal enhancements of the vertical magnetic field as well as abrupt decreases of the plasma density to very low values—the two characteristics of a hole—can be produced in the presence of field aligned losses. The buildup of the magnetic field occurs in conjunction with a deceleration of the horizontal flow. The horizontal decrease in plasma density occurs abruptly when the magnetic field intensifies to such a level that the associated horizontal transport time becomes smaller than the field aligned loss time; this process is referred to as ‘magnetic congestion’.