Has the new solar cycle begun?
Science nugget Dec 05, 1997
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 started? Do the fluxes show an upward
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!
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 firstname.lastname@example.org)