From spencer Mon Jun 10 20:59:49 1996 To: oleroemer Subject: Io Volcano News Christophe Dumas observed the eclipse and occultation of Io by Jupiter at the IRTF on June 2nd UT, with remote assistance from myself and John Stansberry. Seeing was excellent and we got some high-quality data. As this was one of our last detailed looks at Io before the G1 Galileo observations on June 28th, I'll give the preliminary results in some detail here, with the knowledge that I may be proved wrong in a few week's time. Volcanic thermal emission from the Jupiter-facing hemisphere was about as faint as it ever gets (though a few percent brighter than in late April, following a gradual fade since February), with the following broadband disk-integrated magnitudes in Io eclipse, corrected to a geocentric distance of 4.2 AU: Filter Wavelength Magnitude M 4.8 5.13 L' 3.8 7.36 L 3.5 7.82 2.3 2.3 11.1 1.7 1.7 12.9 At 3.50 microns, the L magnitude translates to a flux at Io of 19 GW mic-1 str-1, compared for instance to 70 GW mic-1 str-1 at the height of the winter 1991 Loki eruption. The combined occultation lightcurve and disk-resolved images allow identification of about 11 hot spots with varying degrees of confidence. At 3.5 microns, the Jupiter occultation wavelength, Loki is the brightest spot, and the second brightest is probably at Surt, which has turned on since late April. The spots are as follows, from west to east: 3.5 mic. Vertical Spot Latitude Longitude Flux Comments ------------------------------------------------------------ A -8 +/-10 42 +/- 4 2.7 B -11 +/- 8 36 +/- 4 2.9 Kanehekili C -25 +/-12 22 +/-12 0.9 D 19 +/-10 3 +/- 5 1.0 E 50 +/- 4 332 +/- 6 4.6 Probably Surt F -39 +/-12 350 +/-12 1.1 G 20 +/-12 326 +/- 6 1.7 H 0 +/-30 316 +/- 6 1.3 I 11 +/- 5 307 +/- 4 5.6 Loki J -36 +/- 8 307 +/- 6 3.7 K -30 +/-20 300 +/- 6 1.6 Fluxes are in GW mic-1 str-1, and are corrected for viewing angle assuming a cosine(emission angle) foreshortening factor. The longitudes are mostly constrained by occultation disapperance times and are thus better constrained than the latitudes, which come from the direct images, though the error boxes are not usually aligned exactly N/S. Loki fluxes, normally easy to determine, are complicated by the presence of the bright spot J to the south, which is occulted by Jupiter at the same time as Loki. As usual, Loki is relatively brighter than the other spots at longer wavelengths, being larger and cooler than the rest. John Spencer.