Fast Prograde Flows in Solar Active Regions

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Revision as of 07:16, 28 July 2022

Introduction

Numerous basic features of solar activity remain to be explored for the first time, in spite of mankind's lengthy history of solar study. Although we have a glorious array of spacecraft and observatories making unprecedented observations of the Sun all the time now, it is easy to see huge holes in the observable parameter space. The new missions cannot explore all of these nooks and crannies, some of which are obviously present and just not accessible. One such would be the "sub-telescopic" features just mentioned in the preceding Nugget; much effort goes into improving the angular resolving power of solar telescopes to deal with this particular "known unknown." Another corner of parameter space would be the simple sensitivity limit for &\gamma;-ray detection. RHESSI could only detect a few of the most powerful events.

This Nugget describes a puzzling finding in the relatively minor parameter space of Sun-as-a-star EUV observations, very simple in principle: The SDO/EVE experiment quietly achieved the unprecedented spectroscopic stability and coverage that have revealed this puzzle.

What's the Puzzle?

EVE observes the Sun as a star, integrating the entire disk. An entire solar image feeds spectrographs that measure the solar EUV spectral irradiance nominally every 10 s. Because EVE is on board SDO and situated at the great altitede of a geosynchronous orbit, it has a very stable thermal environment. This is important for delicate measurements; for example the Doppler noise level for EVE in the bright 30.4 nm line of He II (the analog of [Lyman alpha] is measured in km/s. This means time-series wavelength errors in the range Δλ/λ = 10^-5 - not incredibly good by exoplanet search standards, but still well able to resolve flows due to solar activity.

The puzzle is that the first time EVE Doppler shifts have been characterized on active-region time scales of days to weeks, the high-temperature lines from the cores of bright active regions invariably show high-speed flows, greater than 100 km/s (Ref. [1]). Also, surprisingly, these flows are essentially horizontal and in the prograde sense. Figure 1 provides some of the proof of these remarkable properties, remarkable because they had not been predicted nor previously observed with high-resolution spectrographs.

Figure 1: Two weeks of Doppler data from MEGS-B in late 2010. The upper panel shows Doppler velocities for four lines, with a clear gradation in temperature of formation. Lyman-γ shows only the few km/s expected from SDO's orbital motion, whereas the hotter lines have very high Doppler speeds. The images show that these high speeds are horizontal and prograde because of their production in active regions at the W and E limbs.

The flows are ubiquitous, but of course best seen by Sun-as-a-star data when isolated active regions appear. Otherwise multiple sources at different longitudes compete and diminish the Doppler signal. Dilution by global coronal emissions will also reduce the signal, and so the Doppler speeds seen in Figure 1 are in fact lower limits. Figure 2 provides further support for this picture of fast prograde flows from 2018 data, a time in which multiple active regions could dilute the Doppler speeds. Ref. [1] also notes that the effect appears in both N and S hemisphere as well as in Cycle 25, not just Cycle 24.

Figure 2: Two weeks of Doppler data from MEGS-B in late 2010. The panels on the left show time series of flux and Doppler measures for a Si XII line at 52.1 nm, for which the nominal formation temperature is (logT) = 6.29. The panel on the right shows the rotational phase relationship between these measures as two loops, each in the sense expected from the prograde flow direction.

Significance

The fast flows appear to relate one-to-one to high temperatures, elevating the problem of coronal heating (for active regions) to a problem of heating and flowing - a classic chicken-and-egg conundrum? The asymmetry of the flow direction (ie, prograde) matches the asymmetry of leading and following magnetic polarities, which may be a clue. In any case no mention of this novelty seem to be in the literature, and in fact a well-designed search in Fe XIV by Ref. [2] actually did not see the flows, though not in a strictly contradictory manner.

Acknowledgements

Ref. [1] is the work of HH plus Sargam M. Mulay, Lyndsay Fletcher, Jennifer Docherty, Jimmy Fitzpatrick, Eleanor Pike, Morven Strong, Phillip C. Chamberlin, and Thomas N. Woods, with extra kibitzing by Karel Schrijver.

References

[1] "Fast Prograde Coronal Flows in Solar Active Regions"

[2] "Line profile analysis of an active region corona observed successively at the east and west limb"