Solar identification of solar-wind disturbances observed at Ulysses

The Ulysses polar passages are producing a unique set of observations of solar-wind disturbances at high heliographic latitudes. In this paper we use the Yahkoh soft X-ray telescope (SXT) to locate some of these events, as defined by the Ulysses/SWICS data, in the solar corona. Of 8 events, we identify two with flares, three with front-side large arcade events, two with far-side events, and one was not seen in the Ulysses data. The arcade events generally resemble long-duration flares seen in active regions, but are larger, slower, and cooler. We present Yohkoh images of each of these events. In the large arcade events (see Alexander ct al., 1996, for a detailed look at one of them) the magnetic morphology at the location of the Yohkoh arcade is generally consistent with the development of a large system of loops. Some of the identifications are ambiguous, and we summarise the reasons for this. From the SWIGS data we have obtained ionization temperatures for several events, and find that they have no obvious pattern in relation to the X-ray temperatures; this may be expected on the basis that the interplanetary plasma cloud is physically distinct from the plasma trapped in the corona. Soft X-ray observations of the solar corona show occasional occurrences of large-scale brightenings in the form of arcades of loops. Such structures have been known since Skylab (e.g., Sturrock, 1980), and have a dear relationship with coronal mass ejections (e.g., Kahler, 1977). We now may study this phenomenon statistically with the much more comprehensive Yohkoh observations; with Yohkoh movies we can also begin to extend our knowledge to the three-dimensional development of the structures. At the same time Ulysses has sampled the latitude dependence of the interplanetary effects. With this paper we introduce this subject and provide a preliminary listing of events from the passage of Ulysses through high heliographic latitudes. The starting point of the present survey is a, list pf interplanetary plasma clouds (IPC's) derived from Ulysses/SWICS data. These are essentially the same as the events termed CMEs by Gosling ct al. (1994a, 1994b). For this identification the presence of bidirectional streaming in the suprathermal electron distribution is one of the main criteria. We note that there are no direct coronagraph observations, however. The Yohkoh observations were examined at the apparent time of origin of each Ulysses event, resulting in some clear and some less-certain identifications. We also studied the ionization temperatures of the IPC material as a beginning step to give the identifications a physical basis. There has been little study thus far of the Yohkoh soft X-ray observations in relationship to CMEs, which we believe to be closely related to the interplanetary disturbances. Hiei et al. (1993) reported the only Yohkoh event yet studied in conjunction with white-light coronagraph observations. However Klimchuk ct al. (1994) showed that X-ray eruptive phenomena with parameters similar to those of CMEs occur frequently at the limb, and there have been several studies of individual eruptive events (e.g., Watanabe ct al., 1992). Presently there is no systematic knowledge of the X-ray coronal counterparts of CMEs, and the survey represented here is part of the effort to rectify this situation.