- Conference date: 21–26 June 2009
- Location: Saint‐Malo, (France)
We examine the distribution of directions of the heliospheric magnetic field (HMF) measured by the Ulysses spacecraft during its mid‐latitude transits of the heliosphere when it observed solar wind shears from the incursions of high‐latitude fast solar wind toward the low‐latitude slow solar wind. We look for nearly radial field orientations commonly observed in rarefaction regions (i.e. where the solar wind velocity decreases monotonically over several days). In contrast to CMEs where the HMF tends to be enhanced, the rarefaction regions with nearly radial magnetic fields tend to have extremely low magnetic field magnitude values. Within these rarefaction regions, there are “dwells” in the coronal source longitude of the measured solar wind in which the time dependence of the solar wind velocity is given to a very good approximation by where R is the heliocentric radial distance of the spacecraft. We have therefore compiled distributions within the dwells of the parameter which is the cosine of the cone angle of the field from the radial direction. We study the mid‐latitude transits that occurred during different phases of solar activity: 1992–1993 (decline), 1996–1997 (minimum), 2001–2002 (maximum), and 2005–2006 (decline). During three out of these four periods, the distributions of the HMF tended to be rather uniform in all HMF directions but during 2001–2002, when the dwells were more frequent, the distributions tended more strongly toward Such distributions were observed up to very high latitudes (65° N). We discuss these observations in the context of the models proposed to explain the underwinding of the Parker spiral in terms of both a temporal variation in the coronal solar wind velocity and/or a longitudinal velocity gradient at the eastern edge of the solar wind source region. Only the latter can explain 26‐day recurrent periods of radial magnetic field.
- Solar wind
- Magnetic fields
- Solar magnetic fields
- Magnetic field measurements
- Solar corona
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