Wednesday, May 11, 2022

HMI as a Hi-Speed Coronagraph (updated)

The time in the header of the HMI movie turns out to be off by 3 hours (the movie runs from 1320-1342 UTC). Once I heard that, it was back to Helioviewer (, which now redirects to a GSFC website) to produce an AIA 221 Å movie. I selected the area around the flare and made a movie that covers between 1300 and 1400 UTC. Now there is good agreement in the presence and shape of the filament, even the change from upright oval to a more horizontal appearance. You can compare the movies yourself to see how well the HMI filament tracks the AIA 221 Å filament. The post-flare arcade is much easier to see in this movie.

I also looked at the AIA 1600 Å passband and see a beautiful filament at 13:23:50 UTC. The filament is bright because it reflects light from the Sun towards SDO and emits its own light. Use Helioviewer to create your own movie of this passband to see the rest of the flare and filament liftoff.

Solar Cycle 25 is getting interesting!

A magnetic complex, destined to become Active Region 3006, was rotating into view on May 3, 2022, when it was the site of an X1.1 flare at 1309 UTC. Three hours later HMI observed a filament liftoff.

Junwei Zhao used all of the HMI frames to produce this movie of a filament liftoff and other evolution of the solar atmopshere above the nascent AR 3006.

This movie was made by using all of HMI’s 6 line-position intensities — not just the continuum images. Thus, this movie has an amazing 7.5-sec cadence, showing many more details than a 45-sec cadence movie. Because the data is from above the limb, Fe I 6173 is no longer an absorption line, we mostly see how it scatters light from the solar surface towards SDO. (The same is true when you look at filaments in Hα. The filaments are dark against the disk but are seen as bright prominences above the limb.) In this movie the on-disk signal is completely saturated so that the off-limb signal is more easily seen. This movie is also showing a reverse image of the off-limb pixels. The dark material would be bright in the original images. The images are 150" squares. The Sun is 1904" across on May 3, so the area seen in the movie is only 0.6% of the Sun's area.

The movie clearly shows material being ejected from the Sun. Some material falls back towards the surface, but then stopped falling and was held there for a few minutes before the light faded away.

Another interesting thing is that a post-flare arcade formed right beside the limb. Although it is small and remains close to the limb, it was undoubtedly there for a few minutes.

This is the fourth off-limb flare captured by the HMI. The first three resulted in quite a few publications in studying the polarization of the off-limb flare loops, the emission mechanisms of the off-limb white flares, and studies coupling the white-light and UV/EUV/X-ray observations. This high-cadence movie shows that we still haven't figured out all of the ways SDO data can be used to study the Sun.

You are right if you are thinking HMI was lucky. Here is an AIA 304 image of the same region from 1619 UTC of May 3, 2022. The large number of coronal loops makes it very difficult to track the filament seen in the HMI movie.The AIA 304 Å images at the corrected times are saturated and need more analysis to see the filament.

Thanks to Todd Hoeksema, Sushant Mahajan, and Junwei Zhao, all members of the HMI Team at Stanford, for discovering and providing the movies of this limb flare and filament liftoff.

I hope you enjoy this movie.