Low-latitude solar wind during the Fall 1998 SOHO-Ulysses quadrature

Abstract

The Fall 1998 Solar and Heliospheric Observatory (SOHO)-Ulysses quadrature occurred when Ulysses was at 5.2 AU, 17.4°S of the equator, and off the west limb of the Sun. SOHO coronal observations, at heliocentric distances of a few solar radii, showed that the line through the solar center and Ulysses crossed, over the first days of observations, a dark, weakly emitting area and through the northern edge of a streamer complex during the second half of the quadrature campaign. Ulysses in situ observations showed this transition to correspond to a decrease from higher-speed wind typical of coronal hole flow to low-speed wind. Physical parameters of the low-latitude coronal plasma sampled over the campaign are determined using constraints from what is the same plasma measured later in situ and simulating the intensities of the hydrogen Lyman-α and OVI 1032 and 1037 Å lines, measured by the Ultraviolet Coronagraph Spectrometer on SOHO. It appears that low-latitude wind from small coronal holes and polar wind have different characteristics in the corona, differences well known at interplanetary distances through in situ experiments. Small, low-latitude coronal holes have a higher expansion factor than typical polar holes, and their plasma moves at a lower speed than plasma from polar holes, reaching, at 3.5 R_sun, only about one-fifth of the terminal speed. Wind emanating from bright regions, above streamer complexes, is, at the altitudes we analyzed (i.e., 3.5 and 4.5 R_sun), about a factor 3 slower than the low-latitude coronal hole wind, implying a shift to even higher altitudes of the region where plasma gets accelerated. We surmise that open field regions, interspersed amidst closed coronal loops/streamers, may be at least partially responsible for the well-known slow wind speed variability. As in polar fast wind, OVI ions move faster than protons, over the range of altitudes we sampled, and are frozen-in at temperatures of ≈1.3-1.5 10⁶ K, depending on the site where the outflow originates. An oxygen abundance variation from a value of 8.55, in low-latitude holes, to 8.73 in bright areas, has also been inferred.