A PSP Perihelion

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Number: 369
1st Author: Jessie Duncan
2nd Author: and Hugh Hudson
Published: 20 January 2020
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The Parker Solar Probe is edging its way into the innermost heliosphere, at breakneck speed, one perihelion at a time. This Nugget deals with planning for these occasions, which may produce the major breakthrough of an actual identification of the magnetic connectivity between a 2D point on the surface of the Sun with a point in the 3D space that the spacecraft traverses. It might be an actual *event*, of the sort long sought for by RHESSI in the form of a match between SEP electrons and flare ejecta. But more likely it will have to do with the *strahl* component of the solar wind.

In either case we need to proceed from our existing understanding of the coronal magnetic field (limited) to make a best guess about where to look. This is vitally important for imaging instruments with small fields of view. The imminent perihelion is the first really good one, with the footpoint on the visible hemisphere, but there will be a continuous set of future perihelia with the same issue, as we proceed into Cycle 25.

Identifying the likely footpoint

An expert team (WHPI, the Whole Heliosphere and Planetary Interactions) group has tackled this problem, with a helpful Web site at


in which one can see updating maps providing target information. The example map at the time of writing this Nugget is here as Figure 1.

Figure 1: The "consensus target" as predicted from contemporaneous databases, marked with yellow squares. The heliographic current sheet (red) shows the property expected for solar minimum: it hugs the equator. The PSP trajectory (in heliographic coordinates, not connectvity) is the thick grey line that exhibits a loop-de-loop at perihelion itself.

North or South?

As the figure shows, the PSP trajectory is close to the ecliptic, and some of the perihelion passage will see the connection north of the current sheet, and other parts south of it. These produce alternations in the dominant polarity of the heliospheric field, either "out" or "in", and correspond to the famous [sector structure]. As also shown, the latitude difference between the two footpoint domains is quite large, actually many tens of heliographic degrees and definitely greater than the field of view of many important solar telescopes, all of which should be studying this location at this time. The "consensus target" thus has a finite probability of being quite wrong.

What should observers do? If all focus on one target, that is good in the sense of creating a rich database that has broad coverage in general. But it is bad in the sense that everybody might miss the action. Perhaps in a future development of the WHPI Web page a best-guess probability for the choice between north and south alternatives. In any case, the whole-Sun imagers such as those on SDO will catch both alternative footprint sites.

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