Abstract
We make an attempt to quantify directly the latitudinal profile of the solar wind velocity around equatorial low‐speed region, based on observations made by Sakigake, Suisei, and IMP 8 between March 1986 and April 1987, without using the assumption that the location of the minimum velocity just coincides with that of the neutral sheet. We use the formula V(km/s) = A sin² (ϕ–ϕA) + B, to parametarize the latitudinal profile. A and B are found to be 1500–11,000 km/s and 340–400 km/s, respectively, which means that the angular width of the low‐speed region below 500 km/s is about 17°. These values also indicate the latitudinal velocity gradient of 30 km/s/deg at the portion of velocity of 500 km/s. The latitude of the minimum velocity, ϕA, oscillated sinusoidally around the solar equator as a function of longitude. The amplitude of the sinusoidal oscillation was 4°–9° in 1986 and 1°–2° in early 1987. These amplitudes are smaller than the tilt angle of the neutral line calculated from the photospheric magnetic field.
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