Occultation of In-Eclipse Io by Jupiter.

July 21, 1998 10.3 microns

(JPL-Mirlin Camera at NASA-IRTF)

On July 21, 1998 Glenn Orton, Brendan Fisher, and Charles Kaminski, observing at the NASA Infrared Telescope Facility, used JPL's MIRLIN camera to obtain a "movie" of the in-eclipse Io as it disappeared behind Jupiter.  The observations were obtained at a wavelength of 10.3 microns.   At this wavelength and under these conditions, the flux from Io comes entirely from volcanic hot spots.  The occultation technique allows us to determine the brightness of the individual hot spots, as one-by-one they disappear behind the planet.    (See also a discussion of the occultation technique as applied at near-infrared wavelengths.)

At long wavelengths the technique is more difficult because Jupiter is much brighter -- but the long wavelength measurements are essential if one is to obtain the total energy output from the hot spots.

Shown below is an image of Jupiter and Io shortly before the occultation disappearance begins. Io is already in eclipse and can just barely be seen in this image adjusted to show Jupiter. However if the display is adjusted to emphasize Io it is clearly evident just to the right of Jupiter. It is also evident when one subtract from each frame an image of Jupiter obtained after Io has disappeared.

Io about to go behind Jupiter Io about to go behind Jupiter, high brightness version

 

When one processes the series of images obtained as Io disappears, then measures the brightness of Io, one obtains the following curve. It clearly shows the disappearance of one major hotspot (Loki) which contributes 2/3 of the total flux. The disappearance of the remaining flux can be modeled by a smooth decrease which would imply many faint hotspots distributed across the disk. However there is some slight evidence for a two-step character for that remaining flux, which would be consistent with the shorter-wavelength occultation lightcurve obtained from the Wyoming Infrared Observatory (WIRO) two days earlier by Robert Howell and Justin Gregg.

10 micron brightness of Io as it disappears behind Jupiter 3.44 micron brightness of Io as it disappears behind Jupiter

 

The location of the three possible hotspots on Io can be obtained by drawing on a map of Io the location of the Jupiter limb at the times of the three steps. When that is done the location of the large step clearly corresponds to Loki, which was undergoing a major eruption during the summer of 1998. The first step may correspond to Kanehekili – a persistent hotspot marked by the small square.

Map of Jupiter showing possible location of hotspots

 

The noise in the 10.3 micron occultation is caused by several factors. The primary sources are imperfect registration and subtraction of the Jupiter image and missing flux caused by "bad pixels" in the camera. One of the 16 readout channels was not functioning, causing the pattern of bad pixels seen in the images below. Because Io occupies several pixels it is possible to interpolate across the missing ones to obtain a reasonable estimate of the flux, which was done in the above images and for the above lightcurve. However the interpolation is imperfect, especially if the peak Io pixel falls on or adjacent to one of the bad pixels. That effect is probably the cause of several discrepant points in the above lightcurve.

In the following movie the first frame is shown without Jupiter subtraction, and with the bad pixels "fixed" by interpolation. In subsequent frames a Jupiter image has been centered and subtracted, and the bad pixels are shown as black to enable one to judge possible interpolation problems. The Loki disappearance occurs at frame 71, 184 seconds after the occultation disappearance began. The frames are numbered and also labeled with time after event start.

10 micron image of Io about to disappear behind Jupiter Step 02 image of Io disappearing behind Jupiter Step 03 image of Io disappearing behind Jupiter Step 04 image of Io disappearing behind Jupiter Step 05 image of Io disappearing behind Jupiter

    25: -45 sec         26: -40 sec           27: -35 sec        28: -30 sec        29: -25 sec

Step 06 image of Io disappearing behind Jupiter Step 07 image of Io disappearing behind Jupiter Step 08 image of Io disappearing behind Jupiter Step 09 image of Io disappearing behind Jupiter Step 10 image of Io disappearing behind Jupiter

    30: -20 sec         31: -15 sec           32: -10 sec        33: - 5 sec         34: 0 sec

Step 11 image of Io disappearing behind Jupiter Step 12 image of Io disappearing behind Jupiter Step 13 image of Io disappearing behind Jupiter Step 14 image of Io disappearing behind Jupiter Step 15 image of Io disappearing behind Jupiter

    35: 5 sec             36: 10 sec            37: 15 sec           39: 21 sec         39: 26 sec

Step 16 image of Io disappearing behind Jupiter Step 17 image of Io disappearing behind Jupiter Step 18 image of Io disappearing behind Jupiter Step 19 image of Io disappearing behind Jupiter Step 20 image of Io disappearing behind Jupiter

    40: 30 sec           41: 36 sec            42: 41 sec           43: 46 sec         44: 51 sec

Step 21 image of Io disappearing behind Jupiter Step 22 image of Io disappearing behind Jupiter Step 23 image of Io disappearing behind Jupiter Step 24 image of Io disappearing behind Jupiter Step 25 image of Io disappearing behind Jupiter

    45: 56 sec          46: 61 sec             47: 66 sec           48: 71 sec         49: 76 sec

Step 26 image of Io disappearing behind Jupiter Step 27 image of Io disappearing behind Jupiter Step 28 image of Io disappearing behind Jupiter Step 29 image of Io disappearing behind Jupiter Step 30 image of Io disappearing behind Jupiter

    50: 81 sec          51: 86 sec             52: 91 sec           53: 96 sec         54: 102 sec

Step 31 image of Io disappearing behind Jupiter Step 32 image of Io disappearing behind Jupiter Step 33 image of Io disappearing behind Jupiter Step 34 image of Io disappearing behind Jupiter Step 35 image of Io disappearing behind Jupiter

    55: 107 sec        56: 112 sec           57: 116 sec       58: 122 sec        59: 127 sec

Step 36 image of Io disappearing behind Jupiter Step 37 image of Io disappearing behind Jupiter Step 38 image of Io disappearing behind Jupiter Step 39 image of Io disappearing behind Jupiter Step 40 image of Io disappearing behind Jupiter

    60: 132 sec        61: 137 sec           62: 142 sec       63: 147 sec        64: 152 sec

Step 41 image of Io disappearing behind Jupiter Step 42 image of Io disappearing behind Jupiter Step 43 image of Io disappearing behind Jupiter Step 44 image of Io disappearing behind Jupiter Step 45 image of Io disappearing behind Jupiter

    65: 157 sec        66: 162 sec           67: 168 sec       68: 172 sec        69: 177 sec

Step 46 image of Io disappearing behind Jupiter Step 47 image of Io disappearing behind Jupiter Step 48 image of Io disappearing behind Jupiter Step 49 image of Io disappearing behind Jupiter Step 50 image of Io disappearing behind Jupiter

    70: 182 sec        71: 188 sec           72: 193 sec       73: 198 sec        74: 203 sec

Step 51 image of Io disappearing behind Jupiter Step 52 image of Io disappearing behind Jupiter Step 53 image of Io disappearing behind Jupiter Step 54 image of Io disappearing behind Jupiter Step 55 image of Io disappearing behind Jupiter

    75: 208 sec        76: 213 sec           77: 218 sec       78: 223 sec        79: 228 sec

Step 56 image of Io disappearing behind Jupiter Step 57 image of Io disappearing behind Jupiter Step 58 image of Io disappearing behind Jupiter Step 59 image of Io disappearing behind Jupiter Step 60 image of Io disappearing behind Jupiter

    80: 233 sec        81: 238 sec           82: 243 sec       83: 248 sec       84: 253 sec

 

Sept. 28, 1998 Occultation results

Back to Loki 1998 Brightening

Robert Howell's home page

Last revised: April 08, 1999.