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The Solar Wind


The idea of the presence of a ``torrent or flying cloud of charged atoms or ions'' from sunspots, i.e. matter leaving the sun and streaming out into space, was first hypothesized by Fitzgerald [Fit92] and again by Fitzgerald [Fit00] and Lodge [Lod00]. A refined description of the solar wind was later described by Chapman and Ferraro [CF31a] as an idealized model of supersonic expansion of the solar corona. They also discussed the interaction of the solar wind with the earth's magnetic field. Chapman and Ferraro proposed that a current system would flow on the frontside of the earth's magnetic field. They also proposed the earth's magnetic field would carve out a ``hollow'' in the solar stream. This feature was eventually named the magnetosphere by Gold [Gol59]. Only in the early 1960s was the existence of the solar wind verified by observations with Russian and American space probes. The name ``solar wind'' and the correct theoretical basis were due to Parker [Par63]. Measurements of the positive ions by plasma probes indicated that the flux of particles with energies exceeding 25 eV is between tex2html_wrap_inline4498 and tex2html_wrap_inline4500 particles tex2html_wrap_inline4502 , with an equal number of electrons present for electrical neutrality. At the distance of the earth's orbit around the sun, namely 1 AU (or tex2html_wrap_inline4506 ), the speed of the solar wind is usually between 200 and 800 tex2html_wrap_inline4508 ; the flow is highly supersonic. The solar wind carries with it a weak magnetic field amounting to a few nanoTeslas; this is called the Interplanetary Magnetic Field (IMF), which was discovered in 1958 with the Pioneer I satellite as described by Sonett et al. [SSS60]. It is oriented in a direction nearly parallel to the ecliptic plane but at an angle of approximately tex2html_wrap_inline4510 to a line from the sun to the observer at 1 AU. The plasma of the solar wind with an imbedded IMF, is frequently analyzed using magnetohydrodynamis (MHD). More simply, the field is said to be ``frozen in'' to the plasma because the electrical conductivity of the plasma is very large, so that relative motion between plasma and the magnetic field becomes virtually impossible. This can be shown in a short proof using Maxwell's equations, Ohm's law and some vector identities [Hes68].
The equations of Maxwell



and Ohm's law


can be solved for tex2html_wrap_inline4512 and combined to yield:


Using the vector identity


and Maxwell's equation


we get


If the plasma is at rest ( tex2html_wrap_inline4514 ) Equation 2.7 is reduced to a diffusion equation with diffusion coefficient tex2html_wrap_inline4516 . If the conductor occupies a space characterized by length L, the time it takes for a magnetic field to enter and leave it, is approximately tex2html_wrap_inline4520 . For times smaller than tex2html_wrap_inline4464 the field and the plasma can be considered to move together; this is the case for solar-terrestrial parameters.gif

On the other hand, for times very much shorter compared to tex2html_wrap_inline4464 Equation 2.7 becomes:


We consider the rate of change of magnetic flux tex2html_wrap_inline4538 through a moving contour which is given by:


Using the Curl Theorem, the second term can be converted into a surface integral:


which can be combined with the first term to yield


Since Equation 2.8 holds in this case we must have


which means that magnetic field lines are frozen in the plasma and move with it.

The solar wind has a major influence on the ionosphere through interactions of both the plasma and the IMF with the earth's magnetic field. The monitoring of solar wind parameters is a vital part of ionospheric experiments - especially since the solar wind has time-dependent disturbances associated with it. One such important disturbance is a coronal mass ejection (CME), in which massive amounts of solar plasma jet out from the corona at higher-than-average speeds and densities [GHM tex2html_wrap_inline4466 74]. Gosling et al. [GBMP90] showed that fast CMEs, carrying plasma moving faster than the solar wind in front, produce shocks at the leading edge of the CME and these shocks are associated with large geomagnetic storms. Another solar wind disturbance in which the IMF varies slowly and regularly with time over many hours is called a magnetic cloud [BSMS81, Bur88].gif In general, the plasma and IMF conditions in the solar wind should always be considered when analyzing ionospheric events; this will be done for the times of the events in this thesis. [Har92, KR95]

next up previous
Next: The Geomagnetic Cavity Up: Basic Properties of the Previous: Basic Properties of the

Andreas Schiffler
Wed Oct 9 10:05:17 CST 1996