From spencer@lowell.edu Sat Aug 26 22:54:06 1995
From spencer@lowell.edu Sat Aug 26 22:54:06 1995
From: spencer@lowell.edu (John R. Spencer)
Date: Sat, 26 Aug 1995 22:52:26 -0700
To: oleroemer@lowell.edu
Subject: Io Volcano News
Cc: spencer@lowell.edu
Content-Length: 2713
X-Lines: 45
Status: RO

After a day of analysis of the data that Christophe Dumas and
I obtained on the August 26 UT eclipse and occultation of Io
by Jupiter at the IRTF, I can give a rather more coherent account
of what is currently happening on Io's Jupiter-facing hemisphere.

The combination of the eclipse reappearance lightcurve and the
disk-resolved imaging allows us to identify six active hot spots,
as follows:
                             Flux ratio,               Position
              Fraction 3.5   Aug. 26 /                Uncertainty,  First
Hot Spot       micron flux    Jul. 25     Lat.  Lon.  +/- degrees   seen
-----------------------------------------------------------------------------
Loki              0.41         3.0       15 N   310 W    5         1979/03/05
"Late July"       0.18         0.4       22 N   351 W    5         1995/07/20
"Mid August"    c.0.17        >5.0       28 N    12 W    8         1995/08/19
"Faint South"   c.0.06          ?        35 S     0 W  >10             ?
"Kanehekili"    c.0.10         1.0       10 S    40 W   10         1989/12/24
"Hi`iaka"       c.0.04         1.5?       0 N    70 W   20         1990/03/21

The new observations confirm the identification of the "Late July"
spot with a prominent but unnamed caldera in the Voyager images.  The
"Mid August" spot is is near an "isthmus" connecting an "peninsula" of
bright material to Media Regio, but no prominent caldera is seen there
in the Voyager images.  Loki contributes a large fraction of the
in-eclipse flux despite its closeness to the limb, and is easily seen
on the sunlit disk before occultation disappearance at 3.8 microns.
Loki contributed only 15% of the 2.3 micron flux at occultation
reappearance a week ago: either it is much cooler than the other hot
spots (and this was not immediately apparent in our August 26th data)
or, more likely, Loki has done much of its brightening in the past
week.  The identification of the last two hot spots with "Kanehekili"
and "Hi`iaka", seen in previous years, is plausible but not proven.

The "flux ratio" column gives the approximate change in absolute
3.5 micron flux at occultation reappearance since July 25th.
The three brightest spots have all experienced major changes in
the past month, in remarkable contrast to the normal pattern
of only minor changes for months on end, punctuated by occasional
rapid changes at one site, usually Loki.

Rough photometry of Io in eclipse on August 26th gives the following
magnitudes: M, 5.3; L', 6.8; L, 7.2; 2.3 microns, 9.9; 1.7 microns,
11.4.  This is bright, but not exceptionally so.  Disk-integrated
brightness is similar to late July, although it is very differently
distributed between individual hot spots.