Solar Flare 5th May 2007
Subtraction images

NOAA 10953 - S10W58, GOES C4.2

Pre-flare subtraction

(These subtraction images are 2D - do not use anaglyph glasses)

Before the solar flare began (as defined by the rapid rise in X-ray output detected by the GOES satellite network), the active region was flickering. This was visible on the STEREO images, when viewed as a movie.

A subtraction image, with 5 minutes between the two images, shows the flickering over the southern part of the sunspot (yellow). Simultaneously, there is a change to the south of the spot, as shown by the diffuse bright area (green).

The green zone is like plage in H alpha or faculae in white light, adjacent to a sunspot. Since these images are Ultraviolet (171 Angstrom) the bright area needs yet another name and I propose "suburb."

The suburb is connected to the central, active area, by magnetic field lines, rather like a metropolitan suburb is connected to the central business district of a city by roads and railway lines. Only the south part of the central active region was flickering and only the southern suburb is brightening on the subtraction image. The rest of the sun also shows local changes in 5 minutes, but not over such a large area as the suburb.

During the flare, subtraction from pre-flare phase

Now the solar flare is in full cry, as shown by the bright south part of the central area. The suburb is also bright, but not much more than it was with a 5 minute gap during the pre-flare flicker. Probably the magnetic field lines are re-arranging themselves, rather than putting out more energy in the suburb.

Any change, either movement or brightening, is picked up by the subtraction process (just as Digital Subtraction Angiography detects contrast agent flow in arteries). Electrons and protons accelerated by the flare might spiral along the magnetic flux lines to end up in the suburb and contribute to the brightening as they crash into ions there. But the glow is so faint (not detectable on the original images shown on page 1) that re-arrangement of flux tubes seems to me a more likely explanation for the subtraction findings.

EIT crinkles (Stirling and Moore, 2001)
"Crinkles" may be the same thing as my "suburb."
"Flare associated moss" ( as distinct from quiet moss, which covers the sun on TRACE images).
"Remote flare brightenings" is yet another term quoted in the above link.

The 1998 flares analysed by Stirling and Moore had the crinkles closer to the flaring sunspot and its sigmoid loop than shown here. They say the crinkles are moving slowly (~20km/s). They try to analyse if the flare is due to magnetic reconnection or tether breaking - but I am not clear they reach a conclusion.

Stirling and Moore do not mention any twinkling of the flare origin site, as was happening before this 2007 flare, and so the mechanism of suburb brightening may be different from crinkles?

Late, post-flare loop phase. Preflare minus late flare.

Now the solar flare is fading and the post-flare loop has developed (the so called inflation phase of a solar flare). The subtraction interval is 55 minutes and since the post flare loop is a very bright difference from the pre-flare situation, it shows as a yellow glow. (Fainter differences are blue -> green).

The suburb is now dominated by the tsunami following the shock wave heading south, through the suburb into the "countryside."

It is interesting that the "solar tsunami" is heading through the very suburb that flickered in the pre-flare phase. This shows the direction was set by magnetic field lines radiating south from that part of the central active area that dominated the solar flare.

The tsunami seems to be an erupting filament which lay in the sunspot complex as can be seen on several amateur H alpha images published on the internet, taken both before and after the flare.

If you look closely, there is also lesser shock in other directions from the flare, but the major disturbance was in a sector to the south.

Now all this can be seen on a movie I have made, but the movie is such a big file for internet use that I have shortened and converted it into a GIF file.

Note how the subtraction image fits in with the MDI magnetogram

I suspect the same features might show up on H alpha subtraction images, as taken by amateur astronomers. So far I have not been able to lay hands on H alpha filters to take these myself and I do not know of any web site which publishes H alpha images of good quality with a 5 minute or so cadence (H-alpha Patrol images on the internet are rapid but not as good quality as achieved by many amateurs using advanced but time-consuming image processing methods). So it is up to those of you with the equipment to see if this subtraction technique works in H Alpha too.

Four types of flare shock waves

1. Moreton waves are fast (500 to 2,000km/s and spread out concentrically from a solar flare, like the ripples on a pond when a stone is tossed in, reaching around 5x10 to the 5th km from the flare. They have been known for years from H alpha images.
   1. X-ray waves, from much higher in the corona, also occur at Moreton Wave speeds. They were described from Yohkoh x-ray satellite images.
   2. Moreton waves are also visible in ultraviolet (195 Å). So it seems Moreton waves occur at all levels of the inner corona.
   3. Some say Moreton waves start higher than the chromosphere, where magnetic reconnection produces the short-circuit which initiates a solar flare and also sets off a Moreton wave.
2. EIT waves are slower (about 1/3 any associated Moreton wave velocity, circa 250km/s).
   1. They start before any associated CME and may be set off by sideways expansion of a ribbon flare.
   2. They usually go in a restricted direction.
   3. The wave front compresses and brightens the plasma, followed by decompression as the wave passes, causing a dimming phase.
   4. EIT waves are said to have a blurred shock front on images, while Moreton waves are sharper.
   5. EIT waves cannot be seen in H alpha, which only shows the chromosphere.
   6. The term "EIT wave" arose when they were identified on SOHO images, taken in ultraviolet, by the EIT telescope.
      1. Chen, Wu, Shibata and Fang, wrote that EIT waves are not real waves, but are the propagation of perturbation sources as the flux ropes open and close in sequence. They are the evolution of magnetic field lines associated with separatrices.
      2. Since there are no flux ropes to open in coronal holes, EIT waves do not enter holes.
      3. EIT waves do not cross active regions containing a separatrix since the field lines will not open there.
3. Eruptive prominence / filament: sometimes a filament is incorporated in a sun-spot complex and sits in the magnetic neutral line between two different directions of magnetic fields. (Separatrix). This filament may erupt during a solar flare. The tsunami seen on flare illustrated here seems to be an eruptive filament.
4. CME: coronal mass ejections associated with solar flares. Perhaps an eruptive prominence from a flare site which manages to leave the sun's gravitational field causes loop type, flare associated, CME's?
   1.  "Tsunami" has been applied to both EIT and Moreton waves (and also filament eruptions).

Flare associations

1. Type II radio bursts from flares correlate with the Moreton wave, and are more energetic the faster the Moreton waves travels. Radio bursts do not correlate with the slow EIT waves.
2. CME (Coronal Mass Ejection) means a volume of corona is ejected into space. There are at least 2 versions of a CME:
   1. CME occurs, separately from solar flares, as ejection of a helmet streamer. This version of CME is associated with eruption of a solar prominence. A prominence or filament is a structure included in the base of a helmet streamer, in a magnetic hammock. (The French call this event a disparition brusque, or sudden loss of a solar filament). (Bubble type CME)
   2. CME can occur as a result of a solar flare, in which case it seems to be an EIT shock wave, or a spot related erupting  filament going fast enough to escape from the sun's gravity. (Loop type CME).

   

2006/12/06: National Solar Observatory. This H-alpha image sequence of a flare shock wave had a circular wave front and turned filaments off as it passed over them. It traveled fast and is a Moreton Wave. Movie

Moreton waves are not the same as EIT waves.

Tsunamis are normally "short lived, narrowly directed and seen in the super-hot outer corona.. " (rather than in than in the chromosphere) (Alexei Pevstoc, NASA).