Magnetopause

Introduction

Chapman and Ferraro (1931, 1932) were first to discuss the existence of a boundary to the Earth's magnetic field. During 1950's, as the concept of continuous solar wind emerged, it was obvious that such a feature should be a permanent feature of the magnetosphere. In the early 1960's, Explorer 10 and 12 provided the first measurements of this boundary (Cahill and Amazeen, 1963) that was to be called magnetopause. It plays an important role in space physics, since the coupling between the solar wind and magnetosphere occurs through it. Outside magnetopause we find the shocked solar wind region, magnetosheath, and just inside are the magnetospheric boundary layers.

Magnetopause is a direct consequence of solar wind interaction with magnetized planets. However, something similar is found also for weakly magnetized planets like Venus, although the physics differ. Then we talk about ionopauses (e.g., Luhmann, 1995).

Formation

In the first approximation, magnetopause is formed at a distance where the solar wind dynamic pressure equals the magnetic pressure of Earth's field. At this location, typically around 8 - 11 Re away on the Earth - Sun line, a large scale duskward (Chapman-Ferraro) current develops in the dayside magnetopause to cancel the Earth's field outside. At the same time, the dipole field inside is increased, being now about the two times the nominal dipole value. Similar current flows around the magnetotail, but there the direction has to be reversed in order to cancel the field outside. This current is closed via the cross-tail current. The thickness of the current layer is typically from several hundred to a thousand kilometers, which corresponds to several ion gyroradii (e.g., Berchem and Russell, 1982).

The magnetosphere presented above is of closed type. Even if simple, also it can describe some dynamic events relating to Sun-Earth connection. For example, solar wind pressure pulses push the magnetopause inside causing, e.g., SI/SSC's. However, when the effects of interplanetary magnetic field are taken into account, the magnetic reconnection complicates the physics of the magnetopause considerably by "opening" up the magnetosphere.

Models

For empirical magnetopause models, see Sibeck et al. (1991) and Petrinec and Russell (1993). Elsen and Winglee (1997) have constructed a MHD model for the magnetopause, and compared the results with the above empirical models. See also Shue et al. (1997).

References

• Berchem, J., and C. T. Russell, The thickness of the magnetopause current layer: ISEE 1 and 2 observations, J. Geophys. Res., 87, 2108-2114, 1982.
• Cahill, L. J., and P. G. Amazeen, The boundary of the geomagnetic field, J. Geophys. Res., 68, 1835, 1963.
• Chapman, S., and V. C. A. Ferraro, A new theory of magnetic storms, Part 1, The initial phase, Terrest. Magnetism and Atmospheric Elec., 36, 171-186, 1931.
• Chapman, S., and V. C. A. Ferraro, A new theory of magnetic storms, Part 1, The initial phase, Terrest. Magnetism and Atmospheric Elec., 37, 147-156, 1932.
• Elsen, R. K. and R. M. Winglee, The average shape of the magnetopause: A comparison of three-dimensional global MHD and empirical models, J. Geophys. Res., 102, 4799-4819, 1997.
• Luhmann, J. G., The magnetopause counterpart at the weakly magnetized planets: The ionopause, in Physics of the Magnetopause, Geophysical Monograph 90, American Geophysical Union, 71-79, 1995.
• Petrinec, S. M., and C. T. Russell, An empirical model of the size and shape of the near-Earth magnetopause, Geophys. Res. Lett., 20, 2695-2698, 1993.
• Shue, J.-H., J. K. Chao, H. C. Fu, C. T. Russell, P. Song, K. K. Khurana, and H. J. Singer, A new functional form to study the solar wind control of the magnetopause size and shape, J. Geophys. Res., 102, 9497-9511, 1997.
• Sibeck, D. G., R. E. Lopez, and E. C. Roelof, Solar wind control of the magnetopause shape, location, and motion, J. Geophys. Res., 96, 5489-5495, 1991.

• Space Physics Text Book of Oulu