An Abbreviated Guide for
Small Comet Discoveries
With the Cameras on the Polar Spacecraft

Current Total: 15 Discoveries

(All discoveries by the imaging research group at The University of Iowa)

• VIS is the Visible Imaging System of The University of Iowa, one ultraviolet camera and two visible cameras. (Current library: about 1,700,000 atmospheric holes)
• UVI is the Ultraviolet Imager of The University of Washington, one ultraviolet camera.

1. Detection of atmospheric holes in Earth's dayglow as clusters of darkened pixels. Hole sizes and darknesses much greater than those expected from sensor noise in the VIS camera. Dark cluster populations in the range of 5 to 15 pixels with diameters of about 30 to 120 kilometers. Global rate varies from about 5 to 30 atmospheric holes per minute. The lifetime of these events is about 100 seconds. Same diameters and event rates as reported for Dynamics Explorer 1 in 1986.
2. Detection of atmospheric holes in consecutive images with the VIS camera. Apparent speed of the holes is in the range of 5 to 15 kilometers per second.
3. VIS determinations of the expansion speed of atmospheric holes, that is, the diameters increase at the rate of about 1 kilometer per second as expected for the vaporization of cometary water snow.
4. Wobble signatures (double vision) in the VIS images of atmospheric holes due to the motion of the camera platform as expected for a real object in the field-of-view.
5. Wobble signatures of atmospheric holes in the UVI images due to the platform motion as expected for a real object.
6. Semi-darkened rings of pixels around the dark cores of VIS atmospheric holes as expected for a real object and the angular resolution of the VIS camera.
7. Similar semi-darkened rings of pixels around the dark cores of the UVI atmospheric holes as expected for a real object and the angular resolution of the UVI camera.
8. Simultaneous images of atmospheric holes (same place and time) with both the VIS and UVI cameras.
9. Detection of the persistent and organized motion of atmospheric holes from dusk to dawn across the dayside Earth's atmosphere. That is, the atmospheric holes move in the direction of Earth's orbital motion and near the ecliptic plane, but about 10 kilometers per second faster than Earth's motion.
10. Discovery of atomic oxygen trails due to the occasional disruption of small comets at distances above Earth's surface of about 5,000 to 30,000 kilometers.

Ultraviolet, 1304 Å

1. First detection of the OH emissions at 3085 Å from the small comet water clouds. The visible camera was calibrated with Comet Hale-Bopp. These emissions are the standard proxy for the determination of water amounts in large and small comets.
2. Direct determination of the amount of water in the small comet water clouds by accurate determination of the OH emissions. This amount of water in each cloud is typically in the range of 20 to 40 tons and sufficiently large to account for the atmospheric holes in Earth's dayglow.
3. The occurrence frequency of the OH trails is similar to that for atmospheric holes. This is the final element of total observational closure between atmospheric holes and the existence of small comets.
4. The Earth's shadow is used to determine that the break-up altitude of small comets is in the altitude range of about 2,000 to 3,000 kilometers because the OH emissions are due to fluorescence in the sunlight and thus are not excited in the shadow.
5. Comparisons of VIS observations of visible emissions from Comet Hale-Bopp and the small comet clouds find that the surface brightnesses of OH are similar but that sodium and dust are much less for the small comets relative to the large comets.

References

Frank, L. A. and J. B. Sigwarth, Atmospheric holes and small comets, Rev. Geophys., 31, 1-28, 1993.

Frank, L. A. and J. B. Sigwarth, Transient decreases of Earth's far-ultraviolet dayglow, Geophys. Res. Lett., 24, 2423-2426, 1997.

Frank, L. A. and J. B. Sigwarth, Simultaneous observations of transient decreases of Earth's far-ultraviolet dayglow with two cameras, Geophys. Res. Lett., 24, 2427-2430, 1997.

Frank, L. A. and J. B. Sigwarth, Detection of atomic oxygen trails of small comets in the vicinity of Earth, Geophys. Res. Lett., 24, 2431-2434, 1997.

Frank, L. A. and J. B. Sigwarth, Trails of OH emissions from small comets near Earth, Geophys. Res. Lett., 24, 2435-2438, 1997.

Frank, L. A. and J. B. Sigwarth, Observations of transient decreases of Earth's dayglow with the Ultraviolet Imager on the Polar spacecraft, Geophys. Res. Lett., (submitted for publication), 1997.

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