16 January 2018 - Mission Day: 8082 - DOY: 016

Stormy Regions' Journey on the Far Side

SWAN view of the whole sky on 13 November. In this projection, active regions stay in one spot as the Sun rotates. See the MDI maps below with the same projection for the location of active regions.
Also: animated GIF.

MDI near-side (top) and far-side (bottom) images of regions 10484, 10486 and 10488.

Solid line: SWAN estimate of the MgII solar index as it would have been seen by an observer rotating above the area facing Earth on 28 October 2003. Dotted line: Actual MgII index as observed at Earth.

"Halloween Storms of 2003":

X-whatever Flare! (X 28)
Sunspots on Parade
X 17.2 + 10.0 Flares!

Scientists using the SWAN instrument on SOHO spacecraft have monitored the activity of regions 10484 and 10486 and 10488 (of recent fame: see this, this, and this Hot Shot), while they were hidden on the far side of the Sun, rotating around to face the Earth once again.

Principal Investigator Jean-Loup Bertaux and colleague Eric Quémerais of the Service d'Aeronomie found that the activity of the regions first increased until 7 November, then decreased dramatically while on the far side of the Sun. As the regions were rotating into view from Earth, however, activity increased once more.

SWAN monitors the whole sky for ultraviolet light from the Sun, as it is reflected off neutral hydrogen seeping into the solar system from the outside. Since active regions are brighter than quiet regions, parts of the sky facing an active region is brighter than those facing a quiet region. Just as a rotating lighthouse beam will illuminate different patches of fog, the Sun's rotation produces a changing pattern of illumination on the sky behind the Sun's far side.

The SWAN scientists found that the Lyman-alpha brightness could be correlated with the "Mg II index", a commonly used measure of activity for the whole visible disk as seen from Earth. With SWAN, however, the index could be estimated for an observer at any point on the sky.

The SWAN team used their data to estimate what the Mg II index would be for an observer rotating with the Sun, and always facing a given active region during the solar rotation. The MgII index estimated from SWAN data increased up to November 7, but then began rapidly decreasing. The corresponding decrease of the solar Lyman-alpha brightness found by SWAN was 20%, an indication that the activity of the two active regions decreased significantly since their stunning performance on the near side of the Sun. This method should prove of value to space weather forecasters, who are just as interested in predicting "clear" days as they are in forecasting storms from the Sun.

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