Atmospheric holes: Instrumental and geophysical effects

Abstract

Abstract. Global images of far-ultraviolet dayglow with the Earth Camera on board the Polar spacecraft were used for a comprehensive study of transient, localized decreases of intensities, also known as atmospheric holes. An automated determination for detection of an atmospheric hole was developed and employed in this survey of the characteristics of this phenomenon, which avoided the possibility of aliasing by visual inspection of the images. An extensive investigation of possible instrumental effects included (1) evaluation of random rates for a period in mid-January for which there were no atmospheric holes, thus providing an excellent inflight calibration of the instrument's noise, (2) the effects in the images due to energetic electrons in the outer radiation zone, (3) the nonuniform sensitivities for the pixels of the sensor, or "ot spots,"and (4) the contributions of longer wavelength radiation from the atmosphere to the camera's responses. For the straightforward selection criteria for the identification of atmospheric holes the instrumental effects were not major contributors to the occurrence frequencies of this phenomenon. The subsequent search for geophysical effects revealed (1) a strong altitude dependence of the frequency of atmospheric holes, (2) a substantial local-time variation of rates which favored locations in the local-morning sectors of the atmosphere relative to those in the evening, (3) increasing sizes of the atmospheric holes as the spacecraft altitude decreases, and (4) large seasonal variations in the hole rates during the period November 1997 through late January 1998 which were remarkably similar to those observed during the same months but 16 years earlier with Dynamics Explorer 1.

Comment on “Detection of small comets with a ground‐based telescope” by L. A. Frank and J. B. Sigwarth

“…An independent test of the small comet hypothesis is whether the integral number density of detections versus magnitude is consistent with the model distribution published in previous small comet papers [e.g., Frank and Sigwarth , 1999] and is illustrated in Figure 7 of FS. In the visual magnitude range fainter than 18.4 (at a range 137,000 km), the model distribution is approximately a power law of the form where N 0 = 3 × 10 −20 m −3 , V 0 = 18.4, and α = 1.7.…”

Far Ultraviolet Imaging from the Image Spacecraft: 1. System Design

“…For instance, (1) the response to solar flares or other sudden changes in the EUV output of the Sun, (2) the effect of conjugate region precipitation of photoelectrons, (3) recombination of O + ions in the tropical nightglow, and even (4) searches for the signatures of small comet impacts (cf., Frank and Sigwarth, 1999). Many of these phenomena can be well studied by IMAGE, either with nadir viewing at apogee or by limb viewing at perigee.…”

Comment on “‘Atmospheric holes’ are not small comets” by A. W. Harris

“…The interested reader is also referred to (1) the confirmation of the seasonal variations of atmospheric holes [Frank and Sigwarth, 1999], (2) the simultaneous sightings of the same atmospheric hole with two cameras [Frank and Sigwarth, 1997a], (3) the discovery of atomic oxygen trails at high altitudes above Earth's atmosphere [Frank and Sigwarth, 1997b], (4) the discovery of OH emissions during the impact of small comets into the atmosphere [Frank and Sigwarth, 1997c], and (5) the ground-based telescopic sightings of intact small comets at large distances from Earth [Yeates, 1989;Frank et al, 1990].…”