A Hideous daily difference image

Science Nugget: Nov 20, 1998

We have started routinely looking at daily difference images, something that we should have done long ago. The Yohkoh data come in daily sets as the Earth, carrying the Kagoshima ground station, rotates underneath its orbit. One day thus is a natural unit for difference-image searches. One motivation for this would be to spot emerging flux as it happens, in order to guide observations of new regions. An example appears below:


This particular difference pair was selected to show some interesting phenomena. It looks rather ugly (hence the title) because we have not corrected for solar rotation - thus new things or brightening things show up as white, whereas old things or dimming things show up as black. Newly emerging flux would be white. The gray-scale is saturated to show faint things; the original images have extremely large dynamic range because they are composed from pairs of long and short exposures. We show the same image below with a heliographic grid for reference, and call out several items. The grid lines are at the usual 15-degree increments; N up and W to the right.

  • (S20, W20) emerging flux! This white patch does not have a black counterpart one day's rotation earlier.
  • (N00, W00) a huge, bright transequatorial interconnecting loop system. We've done several earlier science nuggets on related systems [1], [2], [3], [4]. This one brightened overnight. See an early Yohkoh paper by S. Tsuneta, or still earlier SMM and Skylab work.
  • (N50, W05) an isolated cusp structure. Such cusps are often taken as evidence of magnetic reconnection or at least eruption.
  • (E limb) two large dark features have disappeared, implying either eruption (probably not) or cooling. Note also the faint shadows of other interconnecting loops, also indicating time variability. This is the "quadrupolar corona" shown so clearly by the LASCO C1 images.
  • The differences show many other things, and we intend to develop this tool for routine monitoring. Of course, dimming resulting from a CME occurrence would be nice to be able to sense routinely, but this effect is quite subtle and would be difficult to detect with relatively hideous images such as these.