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 Rsun, 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 Rsun), 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.