Weekly Notes from the Yohkoh Soft X-Ray Telescope

(Week 49)

5-Dec-97 07:00 UT

Instrument Status

The Soft X-ray Telescope remains in fine working order. The big flares seem to have tapered off, but there had been four M-class events during the week. At the end of the week there were two fat filament cavities waiting to blow, and the large active region in the N (AR8113) appeared to be "reaching out" across the equator to a weaker region in the S. We predict that the net helicities will be found to match.

A more detailed report of this week's activities is available here.

Observation of the Week

The SXT total fluxes let us measure the soft X-ray irradiance of the quiet Sun, because they are derived from standard movie images and hence exclude large flares and CMEs (ie, those above the flare-mode trigger level of about C2). Has the new solar cycle has started? Do the fluxes show an upward trend?

The plots above give two looks at more than half an 11-year solar cycle bridging the recent minimum, covering September 1991 through October 1997. Click on the image for a close-up view. Top, linear scale; bottom, log scale; the latter shows a well-structured slow trend down and then up, with a minimum in mid-1996 as reported earlier. Superposed on this is shorter-term variability, probably including quite a bit of rotational modulation (the points in these plots are from a 50-point smoothing of the "movie" images, which corresponds to 1-2 days depending on coverage). Our recent spate of activity is at about 1/3 the solar-maximum level, judging from this plot. The linear view doesn't do much for me!

Below find images:

Science problem of the Week

One of the more puzzling things about solar soft X-ray irradiance variations is the following. We see in this plot a nice factor-of-100 variation across the years. Yet ROSAT and other X-ray astronomy observatories observe stellar coronae almost in every class of star, including solar-type, without this large a scatter in the ratio of X-ray to optical luminosity. How can this be so if other solar-type stars have solar-type magnetic cycles and solar-type (or even more exaggerated) cyclic variability of X-ray luminosity? Shouldn't the ensemble of stars observed by the X-ray astronomers represent all phases of their respective cycles?

H. Hudson 5-Dec-97 (email hudson@isass0.solar.isas.ac.jp)