The Solar Wind Interaction with Unmagnetized Planets: A Tutorial

  1. C. T. Russell,
  2. E. R. Priest and
  3. L. C. Lee
  1. J. G. Luhmann

Published Online: 21 MAR 2013

DOI: 10.1029/GM058p0401

Physics of Magnetic Flux Ropes

Physics of Magnetic Flux Ropes

How to Cite

Luhmann, J. G. (1990) The Solar Wind Interaction with Unmagnetized Planets: A Tutorial, in Physics of Magnetic Flux Ropes (eds C. T. Russell, E. R. Priest and L. C. Lee), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM058p0401

Author Information

  1. Institute of Geophysics and Planetary Physics, University of California, Los Angeles

Publication History

  1. Published Online: 21 MAR 2013
  2. Published Print: 1 JAN 1990

ISBN Information

Print ISBN: 9780875900261

Online ISBN: 9781118663868



  • Solar photosphere;
  • Magnetic flux;
  • Astrophysics


Venus serves as the prototype of the solar wind interaction with unmagnetized planets because we have a fairly complete observational picture relative to that of Mars. To a first approximation, Venus appears as a highly electrically conducting obstable in the supermagnetosonic flow of the solar wind. The “surface” of the obstacle, or ionopause, is located near the locus of points where the component of solar wind pressure normal to the surface is balanced by the thermal pressure of the ionospheric plasma. The subsolar location of this obstacle is typically around 300 km at solar maximum. Upstream of the obstacle, a bow shock forms to slow and divert the flow around it. Any planetary ions produced in the intervening region or magnetosheath between the shock and obstacle surface are picked up by the solar wind convection electric field. Some of these ions are swept away, while others are energized and returned to the planet. The magnetic field of the solar wind piles up and drapes around the obstacle. Inside the ionopause, the ionospheric plasma is found to be affected by the solar wind interaction in several ways. There is usually a sharp gradient in plasma density at the ionopause, and the ionospheric electron and ion temperatures indicate that there are heating processes other than those due to solar photon radiation absorption. There are also ionospheric magnetic fields that are related to the solar wind interaction.

In the limit where the ionopause is depressed below ∼240 km altitude due to an increase in the magnitude of the solar wind dynamic pressure, the ionospheric magnetic field is a large scale horizontal field with a fairly well defined altitude profile. When the ionopause is near or above its typical solar maximum position, the ubiquitous “flux rope” features are seen. The connection between these two types of ionospheric fields is uncertain. However, both phenomena require a consideration of how magnetic flux of solar wind origin is transported through the ionopause and distributed within the ionosphere. The production and evolution of the large scale magnetic field is relatively well understood in terms of diffusion and downward convection, but the behavior of flux ropes continues to present a challenge for both theorists and observationalists. The relative strength of the solar wind and ionospheric pressures at Mars suggest that if the Martian intrinsic field is insignificant, the ionosphere of Mars is likely to exhibit a large scale field instead of flux ropes.