Yohkoh and TRACE find "moss" during the SERTS rocket flight

Science Nugget: June 25, 1999


On June 24 1999, between 17:00 and 17:10 UT, the Solar Extreme ultraviolet Rocket Telescope and Spectrograph (SERTS) experiment was succesfully launched into a short suborbital flight from White Sands Missile Range in New Mexico (USA). Its first spectra are already on-line! (Follow the link given above.)

The experiment contains a spectrometer which allows scientists to examine radiation in the Extreme Ultraviolet (EUV) regime. These wavelengths are emitted by the Sun's hot corona at temperatures typically in the range of 1 to 3 million degrees. By making measurements at a large number of wavelengths between 300 and 355 Angstroms, information about the density, temperature, and plasma flows within individual coronal structures can be derived.

Yohkoh -SXT, TRACE, and CDS, EIT , and CELIAS/SEM onboard the SOHO spacecraft all coordinated their observations in support of the SERTS flight. In particular SERTS, SXT and TRACE focused on the same region on the Sun (N20 E30) in a string of active regions.


SXT got 11 good 128X128 pixel partial frame images in full resolution of the target area during the flight, cycling twice through its five filters, and making one dark calibration image. Exposures in the two cycles are rather different, as we had planned, so the dynamic range can be extended considerably by combining images in the same filter. We also have two good full disk images for the time of the flight. Four of the images are shown in the figure below, which you can click to enlarge


Comparison with TRACE Data

Such good overlap between full resolution SXT and TRACE images as during the SERTS flight, is not often achieved. I took advantage of this opportunity to put together the following nearly simultaneous (merely seconds apart) and perfectly co-aligned images. That this co-alignment can be done so easily now, even by amateurs like myself, is due in great part to the efforts of scientist Dominic Zarro, who wrote this marvelous package of mapping software.

The TRACE image on the left shows many very thin and clearly delineated loops, which avoid the regions where there are more diffuse loops in the SXT image on the right. That can be explained by the fact that the plasma in the TRACE loops must be close to 1 million degrees Kelvin to be visible, while the plasma inside the SXT loops is much hotter, from 3 to 5 million degrees Kelvin. So in the two different pictures one is looking at quite different temperatures. The difference in loop temperatures is probably caused by different rates of heating along the loops -- the hotter loops are heated more strongly. What causes the TRACE loops to be so thin and so clearly marked, while the SXT loops are much fatter and fuzzier, is still an open question. It is partly just a question of resolution, since TRACE has 25 pixels for each individual SXT pixel.  However, this is probably not the whole story.

If one interprets the thin loops in the TRACE image as very fine hairs, then, by analogy, much of the remainder of the image looks like hair that has been cut very short. That structure has been called ``moss" because of its lacy, textured appearance. In a string of recent papers (see below) it has been established with near certainty that moss is emission from the lower legs of the much hotter SXT loops. The two images above confirm that: the brightest moss is concentrated at the footpoints (ends) of the brightest SXT loops, as theory predicts.

The  picture below is a TRACE image in the 195 Angstrom passband at about 1.5 million degrees Kelvin. The "moss" at this temperature is clearly less textured than that seen in the cooler 171 Angstrom line, but the intensity distribution of the moss is nearly identical, again as theory would predict. So I end with the same conclusion as last week: that simultanous measurements at different wavelengths are a prerequisite for putting together the global picture.


"Moss" papers:

June 27, 1999: Piet Martens