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Alfvénic velocity spikes and rotational flows in the near-Sun solar wind
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Alfvénic velocity spikes and rotational flows in the near-Sun solar wind
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Alfvénic velocity spikes and rotational flows in the near-Sun solar wind
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Journal Article
Alfvénic velocity spikes and rotational flows in the near-Sun solar wind
2019
Overview
The prediction of a supersonic solar wind
1
was first confirmed by spacecraft near Earth
2
,
3
and later by spacecraft at heliocentric distances as small as 62 solar radii
4
. These missions showed that plasma accelerates as it emerges from the corona, aided by unidentified processes that transport energy outwards from the Sun before depositing it in the wind. Alfvénic fluctuations are a promising candidate for such a process because they are seen in the corona and solar wind and contain considerable energy
5
–
7
. Magnetic tension forces the corona to co-rotate with the Sun, but any residual rotation far from the Sun reported until now has been much smaller than the amplitude of waves and deflections from interacting wind streams
8
. Here we report observations of solar-wind plasma at heliocentric distances of about 35 solar radii
9
–
11
, well within the distance at which stream interactions become important. We find that Alfvén waves organize into structured velocity spikes with duration of up to minutes, which are associated with propagating S-like bends in the magnetic-field lines. We detect an increasing rotational component to the flow velocity of the solar wind around the Sun, peaking at 35 to 50 kilometres per second—considerably above the amplitude of the waves. These flows exceed classical velocity predictions of a few kilometres per second, challenging models of circulation in the corona and calling into question our understanding of how stars lose angular momentum and spin down as they age
12
–
14
.
Data collected by the Parker Solar Probe in the solar corona are used to determine the organization of Alfvén waves, revealing an increasing flow velocity peaking at 35–50 km s
−1
.
