Sources of the solar wind at solar activity maximum

Abstract

The photospheric sources of solar wind observed by the Ulysses and ACE spacecraft from 1998 to early 2001 are determined through a two‐step mapping process. Solar wind speed measured at the spacecraft is used in a ballistic model to determine a foot point on a source surface at a solar distance of 2.5 solar radii. A potential‐field source‐surface model is then used to trace the field and flow from the source surface to the photosphere. Comparison of the polarity of the measured interplanetary field with the polarity of the photospheric source region shows good agreement for spacecraft latitudes equatorward of 60°. At higher southern latitudes, the mapping predicts that Ulysses should have observed only outward directed magnetic fields, whereas both polarities were observed. A detailed analysis is performed on four of the solar rotations for which the mapped and observed polarities were in generally good agreement. For those rotations, the solar wind mapped to both coronal holes and active regions. These findings for a period of high solar activity differ from the findings of a similar study of the solar wind in 1994–1995 when solar activity was very low. At solar minimum the fastest wind mapped to the interior of large polar coronal holes while slower wind mapped to the boundaries of those holes or to smaller low‐latitude coronal holes. For the data examined in the present study, neither spacecraft detected wind from the small polar coronal holes when they existed and the speed was never as high as that observed by Ulysses at solar minimum. The principal difference between the solar wind from coronal holes and from active regions is that the O7+/O6+ ion ratio is lower for the coronal hole flow, but not as low as in the polar coronal hole flow at solar minimum. Furthermore, the active‐region flows appear to be organized into several substreams unlike the more monolithic structure of flows from coronal holes. The boundaries between plasma flows from neighboring sources are marked by large magnetic holes, plasma sheets, and low entropy, independent of whether the sources have the same or opposite magnetic polarities. The evolution of solar wind properties between 1 AU and the 1.6–5.4 AU solar distance of Ulysses is also briefly discussed.

The Solar Wind and Its Magnetic Sources at Sunspot Maximum

“…Data gaps are indicated by black pixels. concentrated in the large polar holes, covers $20% of the Sun's surface, and has a mean footpoint field strength of $5 G. At sunspot maximum, the polar holes disappear and the open flux is rooted in the active region latitudes, where it occupies only $5% of the solar surface but has a mean strength of $20 G. Since the average areal expansion undergone by a flux tube increases by a factor of $4, the empirical v p -f exp relationship implies a dramatic decrease in the heliospheric wind speeds between sunspot minimum and maximum (see ), a prediction that has been confirmed by Ulysses observations (e.g., Neugebauer et al 2002). Given that the energy flux density at the coronal base, and thus the coronal heating rate, are increasing functions of the footpoint field strength, it might seem paradoxical that this reduction in wind speed should be accompanied by a fourfold increase in the mean value of B 0 .…”

Source Regions of the Interplanetary Magnetic Field and Variability in Heavy-Ion Elemental Composition in Gradual Solar Energetic Particle Events

“…Nevertheless, the PFSS model has proven very successful in identifying the open-field regions at the Sun and the largescale heliospheric structures such as magnetic sectors (e.g., Neugebauer et al 1998Neugebauer et al , 2002Wang & Sheeley 2006;Jian et al 2009;Lee et al 2009). More stringent tests of the PFSS model have come from mapping studies of impulsive SEP events (Nitta et al 2006;Nitta & DeRosa 2008;Rust et al 2008;MacNeice et al 2011).…”

Evolution of active region outflows throughout an active region lifetime

“…The so-called AR sources have been identified as slow wind contributors from the study of Ulysses and ACE in situ observations (see, e.g., Neugebauer et al 2002;Liewer et al 2004). According to the analysis made by Neugebauer et al 2002, the AR outflows can be comprised of different substreams, originating from multiple sites in the AR, and the distribution of open magnetic field lines can change with time.…”