Jeffrey Baumgardner

Onset conditions for equatorial spread F

The problem of day-to-day variability in the occurrence of equatorial spread F (ESF) is addressed using multidiagnostic observations and semiempirical modeling. The observational results are derived from a two-night case study of ESF onset conditions observed at Kwajalein Atoll (Marshall Islands) using the ALTAIR incoherent scatter radar and all-sky optical imaging techniques. The major difference between nights when ESF instabilities did not occur (August 14, 1988) and did occur (August 15, 1988) in the Kwajalein sector was that the northern meridional gradient of 6300-A airglow was reduced on the night of limited ESF activity. Modeling results suggest that this unusual airglow pattern is due to equatorward neutral winds. Previous researchers have shown that transequatorial thermospheric winds can exert a control over ESF seasonal and longitudinal occurrence patterns by inhibiting Rayleigh-Taylor instability growth rates. We present evidence to suggest that this picture can be extended to far shorter time scales, namely, that “surges” in transequatorial winds acting over characteristic times of a few hours to a day can result in a stabilizing influence upon irregularity growth rates. The seemingly capricious nature of ESF onset may thus be controlled, in part, by the inherent variability of low-latitude thermospheric winds.

Investigations of thermospheric-ionospheric dynamics with 6300-Å images from the Arecibo Observatory

Pilot observations were conducted at Arecibo, Puerto Rico, using an all-sky, image-intensified CCD camera system in conjunction with radar, ionosonde, and Global Positioning System (GPS) diagnostic systems during the periods January 19–28, 1993, and February 21 to August 22, 1995. These represent the first use of campaign mode operations of an imager at Arecibo for extended periods of F region observations. The January 1993 period (the so-called “10-day run”) yielded a rich data set of gravity wave signatures, perhaps the first case of direct imaging of thermospheric wave train properties in the F region. The 6-month 1995 campaign revealed two additional optical signatures of F region dynamics. A brightness wave in 6300 A passing rapidly through the field of view (FOV) has been linked to meridional winds driven by the midnight temperature maximum (MTM) pressure bulge. On May 3, 1995, during a period of geomagnetic activity, a 6300-A airglow depletion pattern entered the Arecibo FOV. Such effects represent the optical signatures of equatorial spread F instabilities that rise above the equator to heights near 2500 km, thereby affecting Arecibos L = 1.4 flux tube.

OI 630 nm imaging observations of equatorial plasma depletions at 16° S dip latitude

Abstract Equatorial ionospheric irregularities in the F-layer have been the subject of intensive experimental and theoretical investigations during recent years. The class or irregularities which continues to receive much attention is characterized by large scale plasma depletions, generally referred to as ionospheric plumes and bubbles. The OI 630.0 nm F-region night-glow emissions arising from recombination processes can be used to observe the dynamics of transequatorial ionospheric plasma bubbles and smaller scale plasma irregularities. In a collaborative project between the Center for Space Physics of Boston University and Brazils National Institute for Space Research (INPE), an all-sky imaging system was operated at Cachoeira Paulista (22.7° S, 45.0° W, dip latitude 15.8° S), between March 1987 and October 1991. In addition to the imager, photometer and VHP polarimeter observations were conducted at Cachoeira Paulista, with ionospheric soundings carried out at both C. Paulista and Fortaleza, the latter at 3.9° S, 38.4° W, 3.7° S dip latitude. For this longitude, the observed seasonal variation of the airglow depletions shows a maximum from October through March and a very low occurrence of airglow depletions from April through September. This long series of OI 630.0 nm imaging observations has permitted us to determine that when there are extended plumes, the altitudes affected over the magnetic equator often exceed 1500 km and probably exceed 2500 km at times, the maximum projection that can be seen from Cachoeira Paulista. This holds true even during years of low solar flux.

Discovery of the distant lunar sodium tail and its enhancement following the Leonid Meteor Shower of 1998

Night-time measurements using a bare CCD all-sky imaging system have detected the presence of an extensive region of neutral sodium emission (589.1 nm) in the direction of the anti-solar/lunar points. The emission was observed to occur during the nights of 21–22 August and 18–20 November, 1998 UT, centered on the new Moon period. The Moon is the most likely source of the neutral sodium, making this the first detection of the lunar sodium tail out to a distance of hundreds of lunar radii. The greater brightness of the emission feature on 19 November is attributed to the Leonid meteor shower which peaked on 17 November, 1998, less than two days before new Moon.

Monochromatic imaging instrumentation for applications in aeronomy of the earth and planets

Monochromatic imaging instrumentation has been developed that uses narrow-band [12-A full width at half maximum (FWHP)] interference filters or plane reflection gratings for 2-D imaging and imaging spectrograph applications. By changing the optics in front of the filter or grating, the field of view of the instruments can be varied from 180 deg (all sky) to 6 deg (narrow field). In the case of the 2-D monochromatic imager, the 12-mm-diam. filtered image is formed at ~f/1 on the input photocathode of an intensified CCD camera (380 x 488 pixels). Regardless of the field of view of the instrument, none of the rays passing through the filter exceeds 4.5 deg to the filter normal. In the spectrograph mode, a slit is placed on the field lens (at the focus of the objective lens) and the 4-in. interference filter is replaced with a reflection grating. The spectrum is imaged at f/1 onto the previously described detector with the spatial dimension of 380 pixels and the wavelength dimension of 488 pixels. The sensitivities of the systems are approximately 50 to 100 rayleigh seconds (Rs) (SNR~2). Examples of data taken with both of these instruments include detection and mapping of Jupiters sodium magnetonebula and stable auroral red (SAR) arcs in the terrestrial ionosphere.

Mesospheric gravity wave imaging at a subauroral site: First results from Millstone Hill

Optical detections of short-period quasi-monochromatic gravity waves in the mesospheric nightglow emissions of OH (695–950 nm), O (557.7 nm), and Na (589.3 nm) have been obtained for the first time from Millstone Hill, Massachusetts (42.6°N, 71.5°W). Using a new all-sky bare charge-coupled device (CCD) imaging system at this subauroral site, we obtained 12 nights of observations during the period February 1 to July 21, 1998. Wave structure was observed during ∼55% of the total observing time, and 35 events were identified and analyzed. Two types of wave structure were observed: extensive, long-lasting (∼1–5 hours) waves known as bands and small-scale, short-lived (∼5–40 min) features called ripples. The band events are believed to be due to freely propagating or ducted internal gravity waves. The mean measured parameter values of the band events were horizontal wavelength (λh) = 21±7 km, phase speed (cob) = 47±20 m s−1, and observed period (τob) = 8.5±4.6 min. The intrinsic wave parameters (cin and τin) and the vertical wavelength (λz) of the band events were calculated using available meteor radar winds for nine of the events. Of the deduced λz values, five were evanescent, and the remaining four ranged from 12 to 21 km, with a mean of 17±4 km. Calculated energy and momentum fluxes for two case study events yielded 4–14 × 10−3 W m−2 and 5–30 m2 s−2, respectively. Possible sources of the waves were investigated using additional data obtained during 1999. The majority of the events exhibited poleward propagation, even during periods of visible auroral activity. This indicates that although the thermosphere above Millstone Hill exhibits subauroral behavior during enhanced levels of geomagnetic activity (Kp>3), the propagation characteristics and frequency of occurrence of mesospheric gravity waves at this location differ very little from other middle- and low-latitude locations.

Imaging observations of the extended sodium atmosphere of the Moon

Pilot ground-based optical imaging observations of the extended lunar environment were conducted at McDonald Observatory in February 1991. One set of images taken on February 20 revealed emission at the combined D1 (5896 A) and D2 (5890 A) lines of neutral sodium extending out to about 5 lunar radii (RM) from the center of the Moon on the sunward side and much fainter emission extending out to about 15–20 RM on the antisunward side. Peak emission (D1+D2) above the Moons limb at a distance of 1.8 RM along the lunisolar axis was measured to be 900 Rayleighs (R), decreasing with distance as r−4. The morphology of the emitting region is qualitatively that of a comet, i.e., a bright coma centered on the Moon and an extended tail trailing away in the antisunward direction. The tail exhibits a decrease in intensity coincident with the shadow cast by the moon. The sunward extent of the sodium is used to calculate a surface ejection speed of about 2 km s−1, consistent with recent estimates of a meteor impact surface vaporization source. The data agree well with previous observations made within 1 lunar radius above the surface. Modeling yields a surface density (52 atoms cm−3) that decreases with distance as r−5 in the solar direction.

Imaging science contributions to equatorial aeronomy: Initial results from the MISETA program

Optical diagnostic techniques have been applied to studies of equatorial aeronomy for nearly half a century. The use of scanning photometers and two-dimensional imagers to observe the spatial patterns associated with low-latitude emissions has been an increasingly used approach during the last few decades. In this paper we review the rationale for the use of all-sky cameras at equatorial latitudes, with particular emphasis on the quantitative information about structures and dynamics that can be extracted from airglow images. The new imaging science instrument at Arequipa, Peru, constructed for the MISETA Program is used as a case study for such techniques. Results derived include: (a) occurrence patterns of several emission features, (b) onset and growth characteristics of airglow depletions associated with equatorial spread-F, (c) zonal plasma drifts and their altitude dependence, (d) gravity waves in the mesosphere, and (e) transient optical signatures of thermospheric dynamics associated with the midnight temperature maximum at low latitudes.

Brightening of 630.0 nm equatorial spread‐F airglow depletions

[1] Observations from the Boston University all-sky imaging system at Arecibo, Puerto Rico (18.3°N, 66.7°W, 28°N mag), show an unusual behavior of nighttime 630.0-nm airglow depletions. Associated with equatorial spread-F (ESF), these structures move eastward before reversing their motion and become airglow enhancements. Few other cases have been found, all during December solstices. For the case study presented here, data from the Arecibo incoherent scatter radar and the Republic of China Scientific Satellite (ROCSAT-1) provide supporting information. The radar shows that around local midnight the background zonal and meridional plasma motions reverse to westward and southward, respectively. ROCSAT-1 shows enhanced ion density, i.e., a low-latitude plasma blob, above the bright feature recorded by the all-sky imager, indicating a possible connection between both phenomena. Drifts parallel to the magnetic field are observed only in the region where the enhancement occurs. One possible interpretation of this change in the brightness of the depleted structure involves the influence of northward meridional winds and a reversal in the zonal drift motion, most likely caused by a zonal wind reversal.

Modeling an enhancement of the lunar sodium tail during the Leonid Meteor Shower of 1998

A region of non-terrestrial sodium emission seen in the sky on the nights of November 18–20, 1998, has been interpreted as the Moons distant sodium tail, possibly enhanced by micrometeor impact vaporization of the lunar regolith by the Leonid meteor shower. We show that the location and morphology of the spot can be explained by standard steady-state models of the Moons sodium atmosphere. Moreover, using a new time-dependent simulation of the lunar atmosphere, we find that the Na escape rate from the Moon increased to 2 or 3 times its normal level during the most intense period of the 1998 Leonid meteor shower on November 16th and 17th.