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Ann. Geophys., 20, 1291-1309, 2002
www.ann-geophys.net/20/1291/2002/
© European Geosciences Union 2002
The solar wind at solar maximum: comparisons of EISCAT IPS and in situ observations
A. R. Breen1, P. Riley2, A. J. Lazarus3, A. Canals1, R. A. Fallows1, J. Linker2, and Z. Mikic2
1University of Wales Aberystwyth, Penglais, Aberystwyth, Ceredigion, Wales, UK
2Science Applications International Corporation, La Jolla, California, USA
3Massachusetts Institute for Technology, Cambridge, Massachusetts, USA
Correspondence to: A. R. Breen (azb@aber.ac.uk)
Abstract. The solar maximum solar
wind is highly structured in latitude, longitude and in time. Coronal
measurements show a very high degree of variability, with large variations that
are less apparent within in situ spacecraft measurements. Interplanetary
scintillation (IPS) observations from EISCAT, covering distances from 20 to 100
solar radii (RS), are an ideal source of information on the
inner solar wind and can be used, therefore, to cast light on its evolution
with distance from the Sun. Earlier comparisons of in situ and IPS measurements
under solar minimum conditions showed good large-scale agreement, particularly
in the fast wind. In this study we attempt a quantitative comparison of
measurements made over solar maximum by EISCAT (20–100 RS)
and the Wind and Ulysses spacecraft (at 215 RS and 300–1000
RS, respectively). The intervals studied were August–September
1999, May 2000, September 2000 and May 2001, the last-named being the period of
the second Ulysses fast latitude scan. Both ballistic and – when possible –
MHD/ballistic hybrid models were used to relate the data sets, and we compare
the results obtained from these two mapping methods. The results of this study
suggest that solar wind velocities measured in situ were less variable than
those estimated from IPS measurements closer to the Sun, with the greatest
divergence between IPS velocities and in situ measurements occurring in regions
where steep longitudinal velocity gradients were seen in situ. We suggest that
the interaction between streams of solar wind with different velocities leads
to "smoothing" of solar wind velocities between 30–60 RS
and 1 AU, and that this process continues at greater distances from the Sun.
Key words. Interplanetary physics
(solar wind plasma; sources of the solar wind; instruments and techniques)
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