Publication Date Announced


Dr. Louis A. Frank's new papers on the small comet findings from NASA's Polar spacecraft appear in the Oct. 1, 1997 issue Geophysical Research Letters (Vol. 24 No. 19), published by the American Geophysical Union. Abstracts to the four papers appear below.


Transient decreases of Earth's far-ultraviolet dayglow

L. A. Frank and J. B. Sigwarth
Department of Physics and Astronomy, The University of Iowa, Iowa City

Abstract. Ten years ago transient decreases in Earth's far-ultraviolet dayglow were reported for global images acquired with the high-altitude, polar-orbiting spacecraft Dynamics Explorer 1. These decreases were observed primarily in the atomic oxygen emissions at 130.4 nm. The diameters of these dark spots, or "atmospheric holes," were in the range of 50 to 100 km. Recently a sophisticated camera for imaging Earth's far-ultraviolet dayglow, with far greater spatial and temporal resolutions than its predecessor, was launched with the Polar spacecraft. The images from this spacecraft provide irrefutable evidence that these atmospheric holes are a geophysical phenomenon.

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Detection of atomic oxygen trails of small comets in the vicinity of Earth

L. A. Frank and J. B. Sigwarth
Department of Physics and Astronomy, The University of Iowa, Iowa City

Abstract. Transient trails of emissions at far-ultraviolet wavelengths have been detected by the Earth Camera on board the Polar spacecraft. These emissions are interpreted in terms of resonantly scattered solar radiation from atomic oxygen at 130.4 nm. The temporal durations of the emission trails are typically tens of seconds. The maximum brightnesses of the shorter trails are usually lesser than those of the longer trails which indicate that the shorter trails are farther from the spacecraft. The rate of occurrence of these trails is approximately 5 to 10 each day. These events are interpreted as the signatures of the disruption and rapid dissipation of small comets in the vicinity of Earth.

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Simultaneous observations of transient decreases of Earth's far-ultraviolet dayglow with two cameras

L. A. Frank and J. B. Sigwarth
Department of Physics and Astronomy, The University of Iowa, Iowa City

Abstract. The Polar spacecraft carries two cameras which are capable of viewing Earth's far-ultraviolet dayglow. One of these two cameras was programmed into a special operating mode during 12 April and 30 July 1996 in order to obtain simultaneous images of transient decreases of dayglow emissions from atomic oxygen at 130.4 nm. During the 76 minutes of usable imaging the two cameras acquired five sets of frames for which a transient decrease was detected by each camera, and the transient decrease occurred at the same geographical position in the dayglow. These series of observations provide strong evidence for the identification of atmospheric holes as a geophysical phenomenon.

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Trails of OH emissions from small comets near Earth

L. A. Frank and J. B. Sigwarth
Department of Physics and Astronomy, The University of Iowa, Iowa City

Abstract. The results are reported for a successful search for the OH emissions associated with an influx of small comets into Earth's upper atmosphere with a camera on board the Polar spacecraft. The spatial distributions of the OH emissions are characterized by a bright core of intensities at or less than the spatial resolution of the camera which is surrounded by a larger dim region of luminosities. The Earth's shadow is employed in order to obtain a coarse determination of the altitudes of these OH trails, i.e., an altitude range < 3000 km.

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