March 23, 2018, 1:00 pm - 2:00 pm

March 23, 1:00 pm - 2:00 pm

Helios observations of periodic density structures in the slow solar wind

Simone Di Matteo (University of L'Aquila)

The outflow of coronal plasma from the Sun, the solar wind, has been historically classified by means of its velocity in fast and slow streams. While the high-speed streams have been associated with coronal holes, the slow solar wind has been related to different sources: boundaries between coronal holes and streamers, current sheets near the top of helmet streamers cups, and active regions. The appearance of solar wind plasma with properties typical of the closed field regions in the corona suggest that the reconnection of magnetic field lines plays an important role in the release of coronal plasma. According to recent simulations, a particular feature of this process might be the periodic release of structures (like plasmoids, fluxropes, or similar). Signatures of a periodic release of solar wind structures have been observed at 1 AU as concomitant fluctuations of the solar wind proton and heavy ions density. Here we show a case study of events of periodic density structures observed for the first time between 0.3 AU and 0.6 AU using the Helios1 and Helios2 data. Using the photospheric field maps from Mt. Wilson Observatory as input for the WSA (Wang-Sheeley-Arge) model, we estimated the topology of the solar magnetic field and the source of the corresponding solar wind stream. These structures are of particular interest not only for the understanding of the physical process at the origin of the solar wind, but also for their influence on the Earth's magnetosphere. The periodicity of this structures, which vary from ~2 hours down to few minutes, compresses and relaxes the magnetospheric cavity in a periodic way. This 'forced breathing' manifests in the magnetosphere as ULF waves that play a fundamental role in the acceleration and losses of the radiation belt electrons.