Using proton temperature anisotropy as an in-situ diagnostic for solar wind origin
Abstract
Using a reprocessing of the Helios 3D ion distribution functions, we present a large statistical analysis of the radial evolution of protons in the inner heliosphere. Although traditionally the solar wind is split into slow and fast components, the solar wind speed distribution is not bimodal, making the slow/fast distinction arbitrary. In contrast, we show that the temperature anisotropy distribution is bimodal inside 0.8 AU, with clearly identifiable anisotropic and isotropic populations. At 0.3 AU all fast solar wind is anisotropic, but there is also a comparable amount of slow solar wind that is anisotropic, agreeing with previous studies showing the existence of a 'slow Alfvénic' wind with the same properties as the fast wind. We show for the first time that all of the anisotropic wind is highly Alfvénic, suggesting that regardless of speed it originates from magnetically open coronal hole field lines on the Sun. In contrast isotropic wind consists solely of slow speeds and is evenly split between Alfvénic a an Alfvénic component, which we conclude originates in or near active regions, and a non-Alfvénic component, which we conclude consists of small transient structures. Our results demonstrate one way in which information on the origins of the solar wind is lost by the time it has travelled to 1 AU, justifying the need for in-situ measurements in the inner heliosphere. We discuss how new measurements from Parker Solar Probe and Solar Orbiter will be used alongside these results to further constrain sources of the solar wind.
- Publication:
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Solar Heliospheric and INterplanetary Environment (SHINE 2018)
- Pub Date:
- July 2018
- Bibcode:
- 2018shin.confE..54S