Andrew B. Christensen

Initial observations with the Global Ultraviolet Imager (GUVI) in the NASA TIMED satellite mission

[1] The Global Ultraviolet Imager (GUVI) instrument carried aboard the NASA TIMED satellite measures the spectral radiance of the Earth’s far ultraviolet airglow in the spectral region from 120 to 180 nm using a cross-track scanning spectrometer design. Continuous operation of the instrument provides images of the Earth’s disk and limb in five selectable spectral bands. Also, spectra at fixed scanning mirror position can be obtained. Initial results demonstrate the quantitative functionality of the instrument for studies of the Earth’s dayglow, aurora, and ionosphere. Moreover, through forward modeling, the abundance of the major constituents of the thermosphere, O, N2, and O2 and thermospheric temperatures can be retrieved from observations of the limb radiance. Variations of the column O/N2 ratio can be deduced from sunlit disk observations. In regions of auroral precipitation not only can the aurora regions be geographically located and the auroral boundaries identified, but also the energy flux Q, the characteristic energy Eo, and a parameter fo that scales the abundance of neutral atomic oxygen can be derived. Radiance due to radiative recombination in the ionospheric F region is evident from both dayside and nightside observations of the Earth’s limb and disk, respectively. Regions of depleted F-region electron density are evident in the tropical Appleton anomaly regions, associated with so-called ionospheric ‘‘bubbles.’’ Access to the GUVI data is provided through the GUVI website www.timed.jhuapl.edu\guvi. INDEX TERMS: 0310 Atmospheric Composition and Structure: Airglow and aurora; 0355 Atmospheric Composition and Structure: Thermosphere—composition and chemistry; 0358 Atmospheric Composition and Structure: Thermosphere—energy deposition; 2407 Ionosphere: Auroral ionosphere (2704); KEYWORDS: airglow, aurora, ultraviolet, imaging, satellite, atmosphere

Remote Sensing of Earth's Limb by TIMED/GUVI: Retrieval of thermospheric composition and temperature

The Global Ultraviolet Imager (GUVI) onboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite senses far ultraviolet emissions from O and N2 in the thermosphere. Transformation of far ultraviolet radiances measured on the Earth limb into O, N2, and O2 number densities and temperature quantifies these responses and demonstrates the value of simultaneous altitude and geographic information. Composition and temperature variations are available from 2002 to 2007. This paper documents the extraction of these data products from the limb emission rates. We present the characteristics of the GUVI limb observations, retrievals of thermospheric neutral composition and temperature from the forward model, and the dramatic changes of the thermosphere with the solar cycle and geomagnetic activity. We examine the solar extreme ultraviolet (EUV) irradiance magnitude and trends through comparison with simultaneous Solar Extreme EUV (SEE) measurements on TIMED and find the EUV irradiance inferred from GUVI averaged (2002–2007) 30% lower magnitude than SEE version 11 and varied less with solar activity. The smaller GUVI variability is not consistent with the view that lower solar EUV radiation during the past solar minimum is the cause of historically low thermospheric mass densities. Thermospheric O and N2 densities are lower than the NRLMSISE-00 model, but O2 is consistent. We list some lessons learned from the GUVI program along with several unresolved issues.

Observations of unstable atmospheric shear layers in the lower E region in the post-midnight auroral oval

Observations of the neutral wind profiles in the post-midnight sector of the auroral oval show the development of a narrow jet near 110–120 km altitude during disturbed conditions. The shears on the bottom side of the jet are highly unstable with Richardson numbers near or below the critical value of 0.25. The unstable shears imply that turbulence and, consequently, the eddy diffusivity will be enhanced in that part of the atmosphere.

The use of far ultraviolet remote sensing to monitor space weather

Abstract This paper discusses the connection between changes in Earths thermosphere and ionosphere induced by changes in the Earths local space environment (or “space weather”) and the phenomena observed in far ultraviolet images of the Earth. Two new experiments, the Global Ultraviolet Imager (GUVI) and the Special Sensor Ultraviolet Imager (SSUSI), will provide a new capability for monitoring changes in thermospheric composition and ionospheric density as they change in response to space weather. These sensors provide a ten-fold improvement in spatial and temporal resolution and a greater than ten-fold improvement in sensitivity over that provided by sensors on the POLAR and IMAGE satellites. These sensors are expected to provide new insights into the mesoscale coupling between the ionosphere and thermosphere, as well as allowing us to develop a better specification of the high latitude convection electric field pattern.

Depletion of oxygen in aurora : Evidence for a local mechanism

A ground-based and rocket investigation of the response of the neutral atmosphere to E region auroral heating has been carried out at Poker Flat, Alaska. The temporal evolution of the atomic oxygen to molecular nitrogen ratio (O/N2) in the lower thermosphere has been monitored using the optical emissions from the aurora as a diagnostic. Comparisons between the changes in the O/N2 ratio and the auroral Joule and particle heating have shown several examples of close similarity between the durations of the heating events and the depletions. Using the thermospheric winds measured during the rocket flights and the temporal structure of the depletions, the upper limit on the horizontal scale size of the depletions has been estimated at 200–400 km. Moreover, in situ rocket measurements of atomic oxygen showed significant differences at points separated horizontally by approximately 220 km. It is also concluded from the near coincidence between the depletion events and the Joule heating events that the dynamical mechanism(s) that drive the depletions were not far distant from the observing site, that is, local processes are sometimes dominant f during periods of moderate auroral activity. We suggest that the observation of a strong wind shear in the 100- to 120-km altitude region [Larsen et al., 1997] could be responsible for turbulence that contributes to the changes in minor constituent composition.

E Region neutral winds in the postmidnight diffuse aurora during the atmospheric response in Aurora 1 Rocket Campaign

Measured E region neutral winds from the Atmospheric Response in Aurora (ARIA 1) rocket campaign are compared with winds predicted by a high-resolution nonhydrostatic dynamical thermosphere model. The ARIA 1 rockets were launched into the postmidnight diffuse aurora during the recovery phase of a substorm. Simulations have shown that electrodynamical coupling between the auroral ionosphere and the thermosphere was expected to be strong during active diffuse auroral conditions (Walterscheid and Lyons, 1989). This is the first time that simulations using the time history of detailed specifications of the magnitude and latitudinal variation of the auroral forcing based on measurements have been compared to simultaneous wind measurements. Model inputs included electron densities derived from ground-based airglow measurements, precipitating electron fluxes measured by the rocket, electron densities measured on the rocket, electric fields derived from magnetometer and satellite ion drift measurements, and large-scale background winds from a thermospheric general circulation model. Our model predicted a strong jet of eastward winds at E region heights. A comparison between model predicted and observed winds showed modest agreement. Above 135 km the model predicted zonal winds with the correct sense, the correct profile shape, and the correct altitude of the peak wind. However, it overpredicted the magnitude of the eastward winds by more than a factor or 2. For the meridional winds the model predicted the general sense of the winds but was unable to predict the structure or strength of the winds seen in the observations. Uncertainties in the magnitude and latitudinal structure of the electric field and in the magnitude of the background winds are the most likely sources of error contributing to the differences between model and observed winds. Between 110 and 135 km the agreement between the model and observations was poor because of a large unmodeled jetlike feature in the observed winds (140 m s−1). Agreement between the present simulation and the earlier simulations of Walterscheid and Lyons (1989) is favorable, although the winds in the present simulation are considerably weaker for particle precipitation of similar characteristic energy and flux. The reasons for the difference were the smaller latitudinal extent of the model diffuse aurora and the weaker electric fields in our simulation. We have shown that the enhanced electron densities and electric fields associated with the postmidnight diffuse aurora provide the potential for a rapid acceleration of the zonal winds as shown by Walterscheid and Lyons (1989). However, the modeled response to the large-scale electric field is too great. This suggests that the assimilated mapping of ionospheric electrodynamics (AMIE) electric field is also too large. The actual electric field is most likely reduced locally in regions of enhanced ionization and conductivity within the diffuse aurora. In addition, we have shown that the “exotic” jetlike wind feature between 110 and 135 km is not aurorally forced. However, it may be the result of an enhancement of the Hall drag relative to the Coriolis force that modifies the geostrophic balance with the large-scale pressure gradient.

The ARIA I Rocket Campaign

The Atmospheric Response in Aurora (ARIA) I rocket experiment was designed to measure the energy and momentum forcing of the atmosphere during auroral disturbances and the resultant compositional and dynamical changes. It consisted of one instrumented rocket, three trimethyl aluminum chemical release rockets, and various ground-based optical instruments. The rockets were launched from Poker Flat Research Range, Alaska, in March 1992. The instrumented payload included a set of eight instruments for measuring various atmospheric and ionospheric quantities. This paper describes the contents of the program and the results of electrodynamic modeling and measurements. A substorm onset occurred approximately 4 hours before launch of the instrumented payload, giving rise to both particle and Joule heating in the vicinity of Poker Flat. By launch time, the substorm was well into recovery. We used optical measurements, electron density measurements from the Langmuir probe instrument, and model results from the Strickland electron transport code to specify latitudinal profiles of the height-integrated Pedersen conductivity. Comparison with assimilated mapping of ionospheric electrodynamics (AMIE) calculations of the Pedersen conductivities for this event indicated that AMIE located the enhanced auroral conductivity region well. However, the magnitudes of the AMIE conductivities in the enhanced region were considerably less than the measurements due to localized substorm-related particle precipitation enhancements not accounted for by AMIE. Our conductivity profiles were used in conjunction with electric field values produced by the AMIE routine to examine the atmospheric heating rates associated with the substorm. The latitudinally integrated Joule heating rate was initially less than the particle heating rate, but rapidly increased to its maximum value at the time of the substorm maximum while the particle heating rate peaked prior to substorm maximum. The particle and Joule heating were collocated during the expansion and maximum phase, but as the substorm recovered, the Joule heating moved to higher latitudes, so that by the time of launch, the two heating regions were completely separated by several degrees. The analysis indicates that the rocket was launched directly into the atmospheric region where the maximum heating had occurred.

Instrumentation on the Remote Atmospheric and Ionospheric Detection System Experiment: extreme-ultraviolet spectrometer, photometer, and near-infrared spectrometer

The Remote Atmospheric and Ionospheric Detection System experiment consists of eight instruments spanning the wavelength range from the extreme ultraviolet (55 nm) to the near infrared (800 nm) oriented to view the Earths limb from the National Oceanic and Atmospheric Administration TIROS-J spacecraft to be launched into a circular orbit in 1993. Through measurements of the natural optical emissions and scattered sunlight originating in the upper atmosphere including the mesosphere and thermosphere, state variables such as temperature, composition, density, and ion concentration of this region will be inferred. The subset of instruments fabricated or otherwise provided by the Space and Environment Technology Center (formerly Space Sciences Laboratory) at The Aerospace Corporation are described.

EUV branching ratios for ionized nitrogen and oxygen emissions

Abstract Branching ratios of singly ionized atomic nitrogen and oxygen EUV emissions that terminate on the metastable states of the respective ions are presented. The four NII ratios and the 482/515 A ratio in OII reported represent the first measured values. Details of the wavelength calibration procedures adopted and a brief discussion of two modeling implications of the measured ratios are included.

Simultaneous observations of lower thermospheric composition change during moderate auroral activity from Kangerlussuaq and Narsarsuaq, Greenland

To obtain information regarding the spatial scales involved in lower thermospheric composition changes in the auroral zone, simultaneous observations were carried out using photometers at Kangerlussuaq and Narsarsuaq, Greenland, two sites separated by ∼750 km in distance. At night, Kangerlussuaq, located at 67°N geographic is most often at the northern edge of the auroral oval, while Narsarsuaq, at the southern tip of Greenland, is more often in or at the southern edge of the auroral oval. On January 29–30, 1998, moderate auroral activity was recorded from both sites. Besides the photometer systems, observations were obtained during part or all of this period from Polar satellite instruments, from the Greenland magnetometer chain, from the CANOPUS chain of magnetometers and photometers, from DMSP, and from the Sondrestrom incoherent scatter radar. The analysis of these data show the necessity of considering both nonlocal and local sources in modeling aurorally induced changes in lower thermospheric composition.