Interplanetary scintillation observations of interaction regions in

Co-rotating interaction regions (CIRs) between fast and slow streams of plasma are a prominent feature of the solar wind. Measurements of interplanetary scintillation (IPS) using the three widely separated antennas of the EISCAT facility have been used to detect the compression regions at the leading edges of interaction regions and to determine the location and velocity of the structure. Observations show that interaction regions have developed as close to the Sun as 25–30 solar radii, a result supported by theoretical modelling which shows that the conditions needed for CIRs to develop exist inside 30 solar radii.


Introduction
Two-station measurements of interplanetary scintillation (IPS), in which simultaneous observations are made from widely separated sites (Armstrong and Coles, 1972), provide a powerful technique for determining the velocity of the solar wind at a wide range of latitudes and distances from the Sun (Rickett and Coles, 1991;Coles, 1995). The accuracy to which the velocity can be measured improves as the baseline between the sites increases (Bourgois et al., Breen et al., 1996a).
IPS measurements contain contributions from the whole ray-path from the source to the receivers, but as the``scintillation potential'' ± the ability of the medium to produce scintillation ± is proportional to 1/R 4 (where R is the distance from the Sun), the observations are dominated by contributions from the portion of the raypath closest to the Sun. IPS observations are dominated by contributions from the portion of the ray-path (Fig 3), so the ®rst estimate of velocity includes a cos(h) factor. If the location of fast and slow components of ow across the ray-path cannot be determined, then the only velocity estimate which can be quoted is thè`a pparent velocity'', containing cos(h)-weighted contributions from the whole of the line of sight (e.g. Breen et al., 1996b).
The ®rst IPS observations from EISCAT used relatively short baselines between the sites and the results were interpreted as showing a single mean velocity with signi®cant RMS variations parallel to and perpendicular to the direction of¯ow. However, in 1993 a series of observations was made in connection with the climb to high latitudes of the Ulysses spacecraft, and the geometry required by these observations demanded much longer baselines than had hitherto been used. The cross-correlation functions between scintillations received at the two sites in these observations showed two clear peaks, corresponding to distinct fast and slow velocities across the ray-path. Subsequent analysis of the EISCAT data set using a two-dimensional weak scattering model (Klinglesmith, 1997) showed that observations were best interpreted in terms of a solar wind containing two distinct components ± a fast wind at 700±800 km s A1 and a denser slow wind at 300±400 km s A1 (Grall et al., 1996;Breen et al., 1996b), a result consistent with measurements made by the particle instrument on Ulysses (Phillips et al., 1994).
Provided that the scattering volume is suciently far from the Sun for the phase variations introduced by irregularities to be small (``weak scattering''), then the scattering process can be treated as a succession of thinscreen events. The scintillation pattern observed at the receivers can then be assumed to be the sum of the patterns produced by each scattering event along the ray-path. Under these conditions, provided that regions of fast and slow wind can be located in the ray-path, it is possible to distinguish the contributions to the observed scintillation pattern made by fast and slow streams.
EISCAT observations are generally in weak scattering outside 14±15 solar radii (R) in the fast wind and 23± 30 R for the denser slow wind (systems such as MERLIN and the VLBA, which observe at higher frequencies, can penetrate even closer to the Sun).
Measurements from spacecraft (e.g. Snyder and Neugebauer, 1966;Krieger et al., 1973;Neupert and Pizzo, 1974;Nolte et al., 1977) have shown that the fast solar wind is clearly associated with coronal holes, so white-light or soft X-ray images of the corona can be used to locate the source regions of the fast wind (e.g. Snyder and Neugebauer, 1966;Schwenn, 1990;Grall et al., 1996). By projecting the IPS ray-path down to the corona the portions immersed in fast¯ow can be identi®ed, allowing the velocities, densities and locations of the fast and slow winds across the ray-path to be determined accurately, independently and separately (Breen et al., 1996b(Breen et al., , 1997. Figure 1 shows the white-light intensity maps of the corona at 1.7 R calculated from measurements made by the HAO Mk.3 coronagraph on Mauna Loa. The white line overlaid on the plot is the IPS ray-path projected down to 1.7 R with an assumed velocity of 750 km s A1 (Fig 1a) or 325 km s A1 (Fig 1b). The triangle marks the closest passage of the ray-path to the Sun and the crossed circle the position of the Earth. The dots are at intervals of 10°arc along the ray-path, corresponding to 10°of latitude when projected down onto the corona. The correlation functions calculated from the IPS observations corresponding to the white-light maps are shown in Fig. 2 a, b In each case the average of the autocorrelation functions observed at the two sites and the observed cross-correlation function are shown as broken lines. The solid lines denote the auto-and crosscorrelation functions calculated using a two-dimension- Fig. 1. a Map of white-light intensity at 1.7 R, calculated from HAO Mk.3 coronagraph measurements centred on 9 June 1997. Line 1 represents the IPS ray-path for measurements of the radio source 0521+166 on 9 June 1997, projected down to 1.7 R with an assumed velocity of 750 km s A1 . The triangle marks the closest approach of the ray-path to the Sun and the crossed circle denotes the position of the Earth. Almost all of the ray-path lies above the southern polar coronal hole. b Map of white-light intensity at 1.7 R centred on 9 September 1995. Line 1 represents the IPS ray-path for measurements of the radio source 1150-003 on 9 September 1995, projected down to 1.7 R with an assumed velocity of 325 km s A1 . The whole of the raypath lies above the bright equatorial corona