R. P. Lowe

Atmospheric Chemistry Experiment (ACE): Mission overview

SCISAT-1, also known as the Atmospheric Chemistry Experiment (ACE), is a Canadian satellite mission for remote sensing of the Earths atmosphere. It was launched into low Earth circular orbit (altitude 650 km, inclination 74°) on 12 Aug. 2003. The primary ACE instrument is a high spectral resolution (0.02 cm-1) Fourier Transform Spectrometer (FTS) operating from 2.2 to 13.3 μm (750-4400 cm-1). The satellite also features a dual spectrophotometer known as MAESTRO with wavelength coverage of 285-1030 nm and spectral resolution of 1-2 nm. A pair of filtered CMOS detector arrays records images of the Sun at 0.525 and 1.02 μm. Working primarily in solar occultation, the satellite provides altitude profile information (typically 10-100 km) for temperature, pressure, and the volume mixing ratios for several dozen molecules of atmospheric interest, as well as atmospheric extinction profiles over the latitudes 85°N to 85°S. This paper presents a mission overview and some of the first scientific results. Copyright 2005 by the American Geophysical Union.

Spectrometric and imaging measurements of a spectacular gravity wave event observed during the ALOHA‐93 Campaign

During the ALOHA-93 campaign coincident imaging and interferometric measurements of the near infrared and visible wavelength nightglow emissions were made from Haleakala Crater, Maui. On 10 October, 1993 a most unusual wave event was observed. This disturbance appeared as a sharp “front” followed by several conspicuous wave crests which progressed rapidly through the imagers field of view (180°). As the front passed overhead the interferometer detected a sudden jump in both the OH intensity (>50%) and its rotational temperature (∼20 K) with the temperature increase leading the intensity by almost 15 min. At the same time the imager registered a sharp decrease in the OI(557.7 nm) emission intensity. A description of this remarkable event follows.

Seasonal temperature variations in the mesopause region at mid-latitude: comparison of lidar and hydroxyl rotational temperatures using windii⧹uars oh Height profiles

Abstract Fifty-one nights of lidar data and seventy-eight nights of Fourier transform spectrometer data taken during 1993 for the purpose of temperature measurements in the mesopause region were analyzed. Observed OH∗-temperatures from the (3-1) band over Delaware Observatory (43°N) were compared statistically to (1) Na-layer temperatures, (2) temperatures at 87 km, and (3) OH∗-temperatures inferred from the WINDII observed OH∗ height profiles and lidar observed temperature profiles, all over Fort Collins (41°N). Seasonal temperature variations were deduced for two midlatitude sites and the resulting measurements were compared statistically. Very good agreement was obtained between the two sites in the magnitude of seasonal differences and variations. With marginal statistical significance, an overall 6–7 K difference between the observed and inferred seasonal averaged temperatures was detected. Other than the ∼3 K difference in the summer accountable by the latitude difference, the remaining discrepancy may be tentatively resolved by a longitudinal difference with higher temperatures near mountainous region, revealed from a comparison of temperatures observed at the same nights over different Stations. Even with substantial data set considered here, geophysical variability still dominates the measurement uncertainty in seasonal means. One useful conclusion of this study is that for many purposes, the hydroxyl rotational temperature can be used as proxy for the temperatures at 87±4 km.

Longitudinal structure in atomic oxygen concentrations observed with WINDII on UARS

WINDII, the Wind Imaging Interferometer on the Upper Atmosphere Research Satellite, began atmospheric observations on September 28, 1991 and since then has been collecting data on winds, temperatures and emissions rates from atomic, molecular and ionized oxygen species, as well as hydroxyl. The validation of winds and temperatures is not yet complete, and scientific interpretation has barely begun, but the dominant characteristic of these data so far is the remarkable structure in the emission rate from the excited species produced by the recombination of atomic oxygen. The latitudinal and temporal variability has been noted before by many others. In this preliminary report on WINDII results we draw attention to the dramatic longitudinal variations of planetary wave character in atomic oxygen concentration, as reflected in the OI 557.7 nm emission, and to similar variations seen in the Meinel hydroxyl band emission.

Mesospheric cloud observations at unusually low latitudes

Abstract Noctilucent clouds (NLC) are a beautiful, high-latitude, summertime phenomenon that was first reported over 100 years ago. They are seen during the hours of twilight by the scattering of sunlight from sub-micron-sized ice particles that form in the vicinity of the cold mesopause region. NLC are quite distinctive, often appearing silvery-blue in color. In recent years there has been a marked increase in their frequency of occurrence, possibly due to an increase in mesospheric water vapor and/or to a cooling of the mesopause region, prompting speculation that they are “harbingers” of potentially serious changes in the mesospheric climate. In concert with this trend there are also a growing number of ground-based NLC sightings at significantly lower latitudes than expected. Here we report two unusual NLC displays photographed from Logan, UT, USA (∼42°N) during June 1999, well over 10° lower in latitude than expected and implying a major, yet temporary, departure from normal mid-latitude summertime conditions. These data provide new evidence for the occasional expansion of NLC to unusually low latitudes possibly due to exceptional dynamical forcing. Alternatively, they may be an early indicator of significant long-term changes taking place in the upper mesospheric summertime environment.

Dynamical and chemical aspects of the mesospheric Na “wall” event on October 9,1993 during the Airborne Lidar and Observations of Hawaiian Airglow (ALOHA) campaign

On October 9, 1993, observations were made from the National Center for Atmospheric Research Electra aircraft during a flight from Maui, Hawaii, toward a low-pressure system NW of the island, a flight of 7 hours in total. The leading edge (wall) of a bright airglow layer was observed 900 km NW of Maui at 0815 UT, which was traveling at 75 m s−1 toward the SE, reaching Haleakala, Maui, about 3.25 hours later [see Swenson and Espy, 1995]. An intriguing feature associated with the event was the large increase in the mesospheric Na column density at the wall (∼180%). The enhancement was distributed over a broad region of altitude and was accompanied by significant perturbations in the Meinel (OH) and Na D line airglow emission intensities, as well as the temperature. This paper describes an investigation of the combined measurements from the aircraft and at Haleakala, including an analysis of the event using a gravity wave dynamic model. The modeled atmospheric variations associated with the leading edge of the “wall” wave are then applied to models of the neutral and ionic chemistry of sodium in order to establish whether the enhancement was caused by the release of atomic Na from a local reservoir species, as opposed to redistribution by horizontal convection. The most likely explanation for the Na release was the neutralization of Na+ ions in a sporadic E layer that was first transported downward by a large amplitude (≈10%) atmospheric gravity wave and then vertically mixed as the wave pushed the atmosphere into a super adiabatic state with associated convective instabilities and overturning.

Airglow imaging of gravity waves: 1. Results from a small network of OH nightglow scanning imagers

Images of the variations of OH nightglow intensities have been recorded by a three-station network of scanning radiometers in southwestern Ontario during the period from May until September 1998. Each radiometer performs a 16 by 16 point raster scan of the night sky every minute, turning itself on and off when the solar depression angle is <6°. The horizontal phase velocity, horizontal wavelength, and frequency of the internal gravity waves are derived from the OH intensity measurements using joint time-frequency analysis techniques, including the S transform, localized cross-spectral analysis, and generalized instantaneous frequency. The ensemble of gravity waves seen in all three instruments are analyzed and observed to have a strong dependence in their propagation direction with the majority of waves seen at each of the three sites having a horizontal wave vector in the northeast direction. The most common parameters observed for horizontal phase speed is 45 m/s, horizontal wavelength is 25 km, and period is 10 min.

Formation characteristics of sporadic Na layers observed simultaneously by lidar and airglow instruments during ALOHA-90

Sporadic Na (Nas) layers were observed by the airborne lidar during ALOHA-90 on the 22, 25 and 27 March flight missions. Perturbations in the O2 and OH nighttime airglow emission intensities and temperatures were also observed by instruments on the aircraft and at Haleakala Crater (20.8°N, 156.2°W) during these events. The most striking correlation between the airglow and lidar measurements occurred during the northbound flight leg of the 25 March mission. When the Nas layer formed at 90.7 km, while the Electra aircraft was between 750 and 500 km south of Haleakala, the O2 temperatures near 95 km above the Electra and Haleakala increased by approximately 45 K. The data for this night suggest a connection between Nas and a large-scale wave, and suggest that the wave is tidal in nature. The data also suggest that some Nas layers can form very quickly over large geographic areas. Fast chemical processes are required to generate the large amounts of atomic Na involved in some of these events.

WINDII/UARS observation of twilight behaviour of the hydroxyl airglow, at mid-latitude equinox

Abstract Measurements are presented of the height profile of the volume emission rate of the P 1 (3) line of the (8-3) band of the hydroxyl airglow at latitudes of 40° N and 40° S as observed with the WINDII instrument on board the UARS satellite during the northern spring equinox in 1993. The emission layer peaks near 88 km during most of the night, with a half width of 6–8 km. The profile is slightly asymmetric with a more rapid decrease on the bottom side. During the early hours after sunset, the volume emission rate on the bottom side of the layer appears to decay exponentially with a time constant which varies from 1.1 h at 78 km to 2.9 h at 82 km. These decay rates are faster by a factor of three or four than those expected on the basis of the removal of atomic oxygen by three-body recombination. As a result it is concluded that the observed decay results from a combination of tidal modulation of the emission rate along with chemical removal of atomic oxygen.

Comparison of ALOHA‐93, ANLC‐93 and ALOHA‐90 observations of the hydroxyl rotational temperature and gravity wave activity

Hydroxyl airglow measurements during the ALOHA-93, ALOHA-90 and ANLC-93 campaigns are used to evaluate features of the temperature variation of the mesopause region. Each campaign shows a persistent pattern in the averaged local time variation of temperature with peak-to-peak amplitudes of 5 K to 15 K, suggestive of the presence of tidal modulation. The nightly temperature variance, after detrending to remove low frequency variations due to tides, is shown to be significantly less during the Hawaiian campaigns than during the ANLC-93 and other campaigns at sites where significant orographic excitation of waves might be expected.