[2] During relatively quiet periods of solar activity, the interplanetary magnetic field (IMF) has the so-called sector structure, when the field is directed toward the Sun during several days, then away from the Sun, and so on. In order to describe these observations, the term "magnetic sector" was introduced. Depending on the phase of the solar activity period, the number of the sectors can vary from two to six. When the Earth crosses the boundaries of magnetic sectors, the reversal of the IMF polarity can occur during approximately 24 hours and even longer time. It should be noted that the notion of the boundary and its position between magnetic sectors is somewhat conventional. In the majority of cases, crossings of a fairly wide boundary region between sectors which is filled by "small" sectors whose number is more than that of "large" sectors takes place several days. Therefore the change of the IMF polarity near the Earth can also take place when the Earth crosses the boundaries between "small" sectors. As observations at about 1 a.u. have shown, the magnetic sectors are associated with high-speed solar wind streams, which rotate together with the Sun (like magnetic sectors) and have an occurrence period of about 27 days, i.e., they are as recurrent as magnetic sectors. Each powerful high-speed stream is connected with the magnetic polarity on the Sun, which determines the magnetic sector polarity. As a rule, the high-speed stream is observed at the Earth's orbit near the leading edge of the magnetic sector. Beginning from the undisturbed level (300-400 km s-1 ), the stream velocity reaches its maximum value (600-700 km s-1 ) during approximately a day, while relaxation to the undisturbed level requires several days. In addition, a short-term, but very strong, increase (in several times) of the plasma density occurs near the leading edge of the stream, after which the density decreases. During a slow decrease in the stream velocity the density is at a lower level than in the undisturbed solar wind. In the most high-speed stream, the proton temperature increases in several times, while variations in the proton temperature are similar to variations in the stream velocity. The observed magnetic polarity, i.e., the sign of the radial field component remains typically unchanged inside each high-speed stream. The duration of a typical stream is about 5 days [Veselovskiy, 1983; Zeldovich, 1983].
Powered by TeXWeb (Win32, v.2.0).