From spencer Fri Jun  3 10:40:59 1994
From spencer Fri Jun  3 10:40:59 1994
To: oleroemer
Subject: Io Volcano News: Outburst!(?)
Content-Length: 3018
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Status: RO

Apparently I had mailing list problems and not everyone got this
notice a couple of days ago, so here is a slightly updated version.
We are now even more confident that the ouburst is real, having
eliminated any uncertainty about the filter that we were using.

Greetings:

   We are fairly sure that we caught a bright outburst on Io at 5 UT
on May 30 UT, five days ago, at the IRTF using the CSHELL spectrograph
in direct imaging mode in the L (3.205-3.823 microns at half-power)
filter.  Io's L micron magnitude in eclipse was about 5.7, twice as
bright as my previous brightest L observations in eclipse at this
longitude, seen during the 1991 Loki brightening.  The
eclipsed/uneclipsed flux ratio was about 0.14, compared to 0.012 at 3.4
microns measured by Bob Howell the week before, on May 23.  Some of
this dramatic one-week increase is due to the longer effective
wavelength of the new observation, but most of it must be an intrinsic
increase in the volcanic activity.   See Bob's message below.

Observers at the IRTF were Nick Schneider, Linda Sauter, and myself.  I
stress that this observation was with an unfamiliar instrument so there
is still some small chance that the brightening is not real, but I
wanted to get the word out while the brightening might be continuing.
Follow-up observations would be very valuable- I was able to get a
rough N (10-micron) magnitude for Io's leading hemisphere in sunlight
on June 1 UT, dodging clouds, and it did not appear to be unusually
bright.

Here is Bob Howell's description of his observations the previous
week.  The point source that he noted is probably on the leading
hemisphere and is not Loki, judging by the reappearance time, but we
need to check this.  Maybe it was the precursor to this weeks'
event???

-------------------------

John

We just observed an Io in-eclipse occultation reappearance a couple hours
ago, on May 23, 1994 UT.  (This is another engineering night.  So far there 
seems to be more Io events available during the randomly scheduled engineering 
runs than during my Io observing periods.) 

I have data only in the 3.4 um (SO2C) filter.  The in-eclipse/out-of eclipse 
flux ratio is 0.012 +/- 20%, which is very similar to the value I obtained 
during the eclipse about a month ago.  I'll work on a better reduction after 
I'm back in town.  (Of course you've heard that before!)  I'm a little 
surprised the changing longitude and forshortening hasn't changed the value, 
assuming it's Loki. 

The main source reappeared at 2:44:24 UT +/- 4 seconds UT.  I'll work on a 
position when I get back, but because of the discrepancy we've had in the 
past, perhaps you'd like to see what you get. 

Since this is supposed to be an engineering run, I won't have a chance to get 
any other photometry.  Our next run will be the one in June (just before the 
ASP meeting). 

Talk with you later.

Bob

------------------------------------

Other interesting, though less urgent, Galilean satellite news will follow
shortly.

John.