The
X-ray background decreased through the week, due to bright and extended
regions rotating off the west limb. They were taken over by less bright
areas. But they have exhibited something interesting, so let me describe
it. In the SXT image to the left (click to enlarge), taken on 1 September,
1999, two boxes are placed. It appears that the brightening at A (around
01 UT) disturbed a diffuse area (probably unresolved loops) to the south,
followed by the formation of a cusp-shaped loop at B by 02:30 UT. Then
by 07:30 UT or so, loops crossing the equator became quite prominent (as
indicated by an arrow). Such trans-equatorial loops have repeatedly been
taken up in the previous nuggets, such as
the
one on 19 February 1999.
The time variations are more easily seen when the images are shown as
a movie (click on the thumbnail movie strip below to run it).
This movie compares SXT images in two
thin filters (right) with EIT 195 Å images (left), which have lower
but more regular cadence (no interruption due to night). Each pair has
the images taken closest in time. The different appearances of SXT and
EIT images primarily reflect their different sensitivities to temperature,
as a result of different optics used. SXT is sensitive to a broad temperature
range above 2.5 MK, see this
plot at MSSL. The response curves of EIT are found here.
Despite multi-peak nature, they are narrow-band responses, and the peak
in the 195 Å channel is about 1.5 MK.
In the movie, EIT images more clearly show southward dimming (probably depletion of mass) associated with the flaring at area A, but SXT images show the disturbances in the form of emission, giving an impression that the corona appears more dynamic in SXT data. Moreover, neither the loops topped with a cusp (B) nor the transequatorial loops (arrow) is readily identified in the EIT 195 Å movie. This indicates that these loops are quite hot, say higher than 2.5 MK. Areas A and B may be connected by loops so cool that even EIT does not see. It is also possible that the brightening at A and the formation of a cusp structure at B may be governed by instability in these large-scale cool loops. We would need observations with better cadence to understand what is going on.
Lastly, to be fair, I will show below how well the EIT 195 Å image traces filaments or filament channels. These images are (from left to right) SXT, EIT 195 Å, EIT 304 Å and H-alpha (from Meudon, L'Observatoire de Paris). Close to the south pole, there were several filaments as seen in the H-alpha image. The middle one looks like a switchback, to which some people attribute the origin of activity, notably coronal mass ejections. But the LASCO movie for 1 September does not show a major CME, unless there was one during a long data gap (02:30-08:30).
4 September 1999: Nariaki Nitta (nitta@lmsal.com)
