D. L. Hampton

Preliminary results from the Sprites94 Aircraft Campaign: 1. Red sprites

An adapter for an electrical wall fixture such as a wall light switch having a toggle or other actuator, the adapter serving to protrude the switch outside of the wall outlet box in which it is nested to the extent necessary to project the toggle through an opening in a fabric covering spaced from the wall. The standard switch includes a pair of mounting wings each having a slot therein to receive a primary screw for attaching the wing to the outlet box and a secondary screw which normally serves to hold a standard face plate over the switch to cover the outlet box. The adapter is constituted by a cover plate which overlies the outlet box and is provided with spacer elements to place the surface of the plate against the rear of the fabric covering. The cover plate has an opening to accommodate the toggle, a pair of slots which register with the wing slots and a pair of holes which register with the wing holes, whereby the switch may be secured to the rear of the cover plate by the secondary screws and the assembly of the cover plate and switch secured to the outlet box by the primary screws. The fabric covering is sandwiched between the cover plate and a transparent face plate attached thereto, the face plate having an opening therein which registers with an opening in the fabric covering to permit the toggle to project therethrough.

EPOXI at Comet Hartley 2

In situ observations show that comet Hartley 2 is an unusually hyperactive comet. Understanding how comets work—what drives their activity—is crucial to the use of comets in studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing primarily because of CO2, which drags chunks of ice out of the nucleus. It also shows substantial differences in the relative abundance of volatiles from various parts of the nucleus.

Exposed water ice deposits on the surface of comet 9P/Tempel 1

We report the direct detection of solid water ice deposits exposed on the surface of comet 9P/Tempel 1, as observed by the Deep Impact mission. Three anomalously colored areas are shown to include water ice on the basis of their near-infrared spectra, which include diagnostic water ice absorptions at wavelengths of 1.5 and 2.0 micrometers. These absorptions are well modeled as a mixture of nearby non-ice regions and 3 to 6% water ice particles 10 to 50 micrometers in diameter. These particle sizes are larger than those ejected during the impact experiment, which suggests that the surface deposits are loose aggregates. The total area of exposed water ice is substantially less than that required to support the observed ambient outgassing from the comet, which likely has additional source regions below the surface.

Optical spectral characteristics of sprites

A TV slit spectrograph was used to obtain the first optical spectra of sprites. Twenty-five events were observed over a thunderstorm on the border of Nebraska and Colorado on the night of 22 June, 1995 between 0700 and 0900 UT. For 10 of these events optical spectra were measured in the wavelength range from 540 to 840 nm. After correcting for the spectrograph response function, digitized spectrograph video images are used to measure the wavelengths of and ratios between the emissions. All emissions are found to be of the first positive bands of N2. There is no evidence of the Meinel bands of N2+ indicating that the mechanism responsible for sprites produces little or no ionization at 70 km altitude.

Observations of ‘Columniform’ sprites

Abstract This paper reports observations of a distinctive form of sprites associated with positive CG flashes carrying currents of 23 or less to about 100 kA in mesoscale thunderstorms. The sprites are characterized by long vertical columns about 10 km long, less than 1 km in diameter, and show virtually no variation in brightness along their length. Three dimensional triangulation of what we define as a ‘columniform’ sprite (c-sprite) event on the evening of 19 June 1995 showed that the individual elements had an average terminal altitude of 86.7 km and an average bottom of 76.2 km. Some show faint diffuse ‘hair’ or tendrils extending above and below the column. The sprite columns are nearly vertical, in video imagery. On some evenings, c-sprites are the dominant form of sprite activity above thunderstorms but, on other nights with many sprites, they may not be observed at all. Comparison of c-sprite forms vs National Lightning Detection Network (NLDN) positive cloud-to-ground current, shows a progression from simple thin vertical forms to brighter and more complicated forms. Theoretical explanations which predict the form and vertical structure of the classical sprites do not at present account for these different forms.

Molecular excitation in sprites

We have determined the molecular internal energy distribution in the N 2 B 3 Π g state from the fluorescence measured during the observations of sprites during 1995. Spectrally resolved data from two different instruments and three different sprites are compared with theoretical spectra to obtain excited state vibrational distributions. Energy dependent electron excitation cross-sections and laboratory data were used to estimate the energies of electrons producing the red sprite radiance. Implications for chemical production in the mesosphere and critical future measurements are discussed.

Cometary Volatiles and the Origin of Comets

We describe recent results on the CO/CO2/H2O composition of comets together with a survey of older literature (primarily for CO/H2O) and compare these with models of the protoplanetary disk. Even with the currently small sample, there is a wide dispersion in abundance ratios and little if any systematic difference between Jupiter-family comets (JFCs) and long-period and Halley-type comets (LPCs and HTCs). We argue that the cometary observations require reactions on grain surfaces to convert CO to CO2 and also require formation of all types of comets in largely, but not entirely, overlapping regions, probably between the CO and CO2 snow lines. Any difference in the regions of formation is in the opposite direction from the classical picture with the JFCs having formed closer to the Sun than the LPCs. In the classical picture, the LPCs formed in the region of the giant planets and the JFCs formed in the Kuiper Belt. However, these data suggest, consistent with suggestions on dynamical grounds, that the JFCs and LPCs formed in largely overlapping regions where the giant planets are today and with JFCs on average forming slightly closer to the Sun than did the LPCs. Presumably at least the JFCs passed through the scattered disk on their way to their present dynamical family.

BLUE JETS : THEIR RELATIONSHIP TO LIGHTNING AND VERY LARGE HAILFALL, AND THEIR PHYSICAL MECHANISMS FOR THEIR PRODUCTION

Abstract Blue jets are narrow cones of blue light that appear to propagate upward from the cloud tops at speeds of about 100 km/s to terminal altitudes of about 40 km ( Wescott et al. 1995 ). In this paper, we present the results of a refined analysis of these optical phenomena and their relationship to cloud-to-ground (CG) and intracloud lightning, and to very large hailfall, their apparent color, and possible mechanisms for their production. In a thunderstorm where more than 50 of these events were observed from aircraft on the night of 1 July 1994, about half of the blue jets occurred in a cluster near Foreman, Arkansas, and the rest in an area near Texarkana, (Texas/Arkansas). Hail 7 cm in diameter fell in those two storm cells at the time of the blue jet occurrences. One other blue jet was observed over an intense multi cell storm in Kansas on the night of 3 July 1994. Comparison to cloud-to-ground (CG) lightning strokes revealed that blue jets were not coincident with either positive or negative CG strokes, but they occurred in the same general area as negative CG strokes and large hail, and that cumulative distributions of the negative CG strokes in ±5 s before and after the jet and within a radius of 15 km showed a significant reduction in the flash rate for 2 s following the event. From an analysis of color TV signal levels and calculations of quenching and atmospheric transmission, we conclude that significant ionization is present in the jets. Theoretical work by others suggests that the mechanism for their production is a streamer, but there remain discrepancies between these theories and the observations.

N2(B3Πg) and N2+(A2Πu) vibrational distributions observed in sprites

Abstract A pair of spectra taken simultaneously by two different ground-based instruments has been analyzed by our group. As with previous observations, the spectra are composed primarily of the N 2 first positive group (1PG) ( B 3 Π g − A 3 Σ u + ). In a previous study, we compared the N 2 ( B ) vibrational distributions from the spectral analysis with those resulting from a time-dependent kinetic model of N 2 triplet excited state populations. Both spectra reflect emission between 50 and 60 km . The higher-altitude spectrum is primarily 1PG but also shows the presence of features which appear to be N 2 + Meinel ( A 2 Π u − X 2 Σ g + ). The lower-altitude spectrum shows little or none of the apparent Meinel emission but has an N 2 ( B ) vibrational distribution similar to ones observed in laboratory afterglows. In this paper we discuss the apparent presence of the Meinel emission and present the observed N 2 ( B ) vibrational distributions.

Invited Article: Deep Impact instrument calibration

Calibration of NASAs Deep Impact spacecraft instruments allows reliable scientific interpretation of the images and spectra returned from comet Tempel 1. Calibrations of the four onboard remote sensing imaging instruments have been performed in the areas of geometric calibration, spatial resolution, spectral resolution, and radiometric response. Error sources such as noise (random, coherent, encoding, data compression), detector readout artifacts, scattered light, and radiation interactions have been quantified. The point spread functions (PSFs) of the medium resolution instrument and its twin impactor targeting sensor are near the theoretical minimum [ approximately 1.7 pixels full width at half maximum (FWHM)]. However, the high resolution instrument camera was found to be out of focus with a PSF FWHM of approximately 9 pixels. The charge coupled device (CCD) read noise is approximately 1 DN. Electrical cross-talk between the CCD detector quadrants is correctable to <2 DN. The IR spectrometer response nonlinearity is correctable to approximately 1%. Spectrometer read noise is approximately 2 DN. The variation in zero-exposure signal level with time and spectrometer temperature is not fully characterized; currently corrections are good to approximately 10 DN at best. Wavelength mapping onto the detector is known within 1 pixel; spectral lines have a FWHM of approximately 2 pixels. About 1% of the IR detector pixels behave badly and remain uncalibrated. The spectrometer exhibits a faint ghost image from reflection off a beamsplitter. Instrument absolute radiometric calibration accuracies were determined generally to <10% using star imaging. Flat-field calibration reduces pixel-to-pixel response differences to approximately 0.5% for the cameras and <2% for the spectrometer. A standard calibration image processing pipeline is used to produce archival image files for analysis by researchers.