Atmosphere Explorer-C Satellite Research Articles

A statistical study of ion frictional heating observed by EISCAT

Abstract. Results of a statistical survey of F-region ion frictional heating are presented, a survey which is based on over 4000 h of common programme observations taken by the European incoherent scatter (EISCAT) UHF radar facility. The criterion adopted in this study for the identification of ion frictional heating was that defined by McCrea et al., requiring an enhancement in the F-region field-parallel ion temperature exceeding 100 K over two consecutive integration periods, which was itself based on a selection criterion for frictional heating derived for the study of high-latitude F-region ion temperature observations from the Atmospheric Explorer-C satellite. In the present study, the diurnal distribution of ion frictional heating observed by EISCAT is established and, furthermore, its dependence on geomagnetic activity and the orientation of the interplanetary magnetic field (IMF) is investigated; results are interpreted with reference to corresponding distributions of enhanced ion velocity, again derived from the extended set of EISCAT UHF common programme observations. The radar, due to its location relative to the large-scale convection pattern, observes ion frictional heating principally during the night, although preferentially during the post-midnight hours where there is reduced coupling between the ion and neutral populations. There is an increased preponderance of frictional heating during intervals of high geomagnetic activity and for a southward z component of the IMF and, moreover, evidence of asymmetries introduced by the y component of the IMF.

A statistical study of ion frictional heating observed by EISCAT

Results of a statistical survey of F-region ion frictional heating are presented, a survey which is based on over 4000 h of common programme observations taken by the European incoherent scatter (EISCAT) UHF radar facility. The criterion adopted in this study for the identification of ion frictional heating was that defined by McCrea et al., requiring an enhancement in the F-region field-parallel ion temperature exceeding 100 K over two consecutive integration periods, which was itself based on a selection criterion for frictional heating derived for the study of high-latitude F-region ion temperature observations from the Atmospheric Explorer-C satellite. In the present study, the diurnal distribution of ion frictional heating observed by EISCAT is established and, furthermore, its dependence on geomagnetic activity and the orientation of the interplanetary magnetic field (IMF) is investigated; results are interpreted with reference to corresponding distributions of enhanced ion velocity, again derived from the extended set of EISCAT UHF common programme observations. The radar, due to its location relative to the large-scale convection pattern, observes ion frictional heating principally during the night, although preferentially during the post-midnight hours where there is reduced coupling between the ion and neutral populations. There is an increased preponderance of frictional heating during intervals of high geomagnetic activity and for a southward z component of the IMF and, moreover, evidence of asymmetries introduced by the y component of the IMF.

The effect of temperature on shuttle glow

The glow on ram surfaces of the space shuttle has been reported from a series of photographic observations made during several orbiter missions1–13. These measurements have shown that the spectrum of the glow is a continuum7,13, has a spectral peak at 680 nm4,7, and the brightness decreases with altitude5. The spectrum has been tentatively identified as the nitrogen dioxide continuum and follows the interaction of adsorbed nitric oxide with ramming atmospheric oxygen7–9. One recent observation of the ram glow displayed an unusually low intensity10,13. Further investigation has led us to discover that the main difference between this, and earlier measurements was the spacecraft attitude which in turn significantly influenced the temperature of the ram surfaces observed. In this paper the variation of the glow brightness among several different shuttle flights is re-examined and it is shown that a major contributing factor to glow brightness is probably the temperature of the rammed surface. The derived temperature dependence is also consistent with the Atmospheric Explorer-C satellite ‘red’ glow intensity data.

On spacecraft‐induced optical emission:A proposed second surface luminescent continuum component

The status of observations and current theories of spacecraft‐induced optical emission (ram glow) from the Atmospheric Explorer‐C satellite to the most recent shuttle missions is reviewed. To resolve the discrepancy between the observed spectral continuum and the most likely gas‐phase emitter, the NO2 continuum, a second continuum is proposed, lying further to the red and due to true surface emission (chemiluminescence) as a consequence of atomic and molecular oxygen adsorption. An altitude variation modelled on the basis of this mechanism, using laboratory data, offers good agreement with observations by the AE‐E satellite, and is capable of producing the quantitative intensities in general.

Large-Scale Waves in the Thermosphere Observed by the AE-C Satellite

Atmosphere Explorer C (AE-C) data are analyzed to study wavelike perturbations in the thermosphere at an altitude of about 260 km. The data were measured during one orbit on January 20, 1975. The examples shown are typical of many other orbits of both satellites AE-C and AE-E. Four geophysical parameters are analyzed: nitrogen and oxygen densities, electron density, and ion temperature, as measured by three different instruments. The data are processed by normalizing them to their average values and extracting their trends. Their fluctuations are obtained by passing the normalized detrended data through a high-pass filter. Strong periodicities are apparent that persist throughout the records, regardless of filter cutoff frequency. Fluctuations are compared by computing cross correlation functions. Spectra are obtained using the MEM and FFT procedures. It is demonstrated, for the first time, that relationships between ion variations and neutral variations are coherent over a wide range of scale sizes over global distances. It is also inferred from plane wave modeling studies and from the persistent periodicities over global distances that a quasi-stationary wave structure is present in the thermosphere that may slowly dissipate and be regenerated by auroral region sources. Large-scale structures are found in all four parameters with horizontal scale sizes ranging from about 400 to about 4000 km. The spectra for all parameters contain peaks at wavelength that are confirmed by the periods of the fluctuations, and decrease with decreasing wavelength with a power law type of variation.

Comments on the interpretation of 3371 Å filter-photometer observations and its implications for the AE-E photoelectron fluxes

Modeling of the dayglow spectrum in the wavelength region between 3300 and 3500 Å indicates that the N 2 second positive (0,0) band at 3371 Å is blended with the Vegard-Kaplan (0,9) band. A recent analysis of rocket observations of the dayglow shows that 20–30% of a 3371 Å narrow band filter-photometer signal is due to the VK emission (Conway and Christensen, 1983). Kopp et al. (1977) and Hernandez et al. (1983) reported analyses of 3371 Å photometer observations from the Visible Airglow Experiment on the Atmospheric Explorer-C (AE-C) satellite which did not consider the Vegard-Kaplan (VK) emission. The observations were compared to theoretical estimates of the second positive volume emission rate based on a photoelectron model and on absolute fluxes measured by the Photoelectron Spectrometer experiments on AE-C and AE-E. Inclusion of the VK band in the AE analysis would bring the reported photoelectron theory into agreement with the airglow observations. However, the overestimate of the N 2 second positive airglow predicted by the AE-E photoelectron flux measurements increases to a factor of nearly two rather than the 20–30% reported by Hernandez et al. (1983).

Particle and Joule heating of the neutral polar thermosphere in cusp region using Atmosphere Explorer-C satellite measurements

Simultaneous measurements taken by instruments on the Atmosphere Explorer - C satellite were used to compare electron and proton particle energy deposition, Joule heating, and neutral density perturbations in the region of the cusp. Altitude profiles of Joule heating, electron energy deposition, and electron density are derived using measurements taken by the satellite as input to a computer model. Electric fields are calculated using ion drift measurements. Figures are presented for a representative orbital pass. A peak Joule heating rate of 0.059 Wm −2 occurred in the cusp region with a peak of 0.025 Wm −2 in the evening auroral electrojet. Peak volume heating rates corresponding to these regions were 1.4 × 10 −6 Wm −3 and 7.10 −7 Wm −3, both occurring at an altitude of 115 km. Particle energy deposition was about an order of magnitude less than Joule heating. Large neutral density perturbations are related to regions of heating.

Zodiacal light surface brightness measurements by Atmosphere Explorer-C

Using the visible airglow photometer on the Atmosphere Explorer-C satellite, we have mapped the zodiacal light surface brightness at the wavelengths monitored by the instrument: 3371, 4278, 5200, 5577, 6300, and 7319 Å. The study constitutes a survey over this wavelength range, covering most of the celestial sphere, from altitudes above the atmospheric emissions, and free from atmospheric scattering and attenuation. The intensity variations reveal enhancements near elongations of 130°, and possibly near 60°, at all wavelengths. The intensity of the zodiacal light near the ecliptic pole is found to be ∼30 S 10. The color ratio with respect to the Sun is found to be redder than the Sun (0.7) at all elongations.

Magnetic ordering of the polar airglow

The visible airglow experiment on the Atmosphere Explorer-C satellite has gathered sufficient data over the Earth's polar regions to allow one to map the geographic distribution of particle precipitation using emissions at 3371 and 5200 Å. Both of these features exhibit large variations in space and time. The 3371 Å emission of N 2( C 3 π), excited by low energy electrons, indicates substantial energy inputs on the dayside in the vicinity of the polar cusp. More precipitation occurs in the morning than evening for the sample reported here, while the entire night sector between magnetic latitudes 65° and 77.5° is subjected to particle fluxes. Regions of enhanced 5200 Å emission from N( 2D) are larger in horizontal extent than those at 3371 Å. This smearing effect is due to ionospheric motions induced by magnetospheric convection.

The calculated and observed ionospheric properties during Atmospheric Explorer-C satellite crossings over Millstone Hill

The Atmospheric Explorer-C (AE-C) satellite passed almost directly over the Millstone Hill incoherent scatter radar station on 14 February 1974 and passed within the near vicinity of the station on 15 February 1974. Measurements of ionospheric and atmospheric properties were made simultaneously by the incoherent scatter radar and the AE-C satellite instruments. The incoherent scatter radar measured vertical profiles of the electron and ion temperatures and electron density and these data were used to derive a neutral gas temperature profile. The AE-C satellite measured the electron and ion densities and electron and ion temperatures, neutral gas composition, solar EUV flux, photoelectron spectra, the 6300 Å volume emission rate profile and the distribution of NO along the satellite path. These simultaneous measurements provide a consistent set of data to examine current F-region theory in the daytime ionosphere. We used a time-dependent coupled model of the ionospheric E- and F-region to calculate the ionospheric properties over Millstone Hill at the times of the AE-C crossings and then compared the calculated structure to the observed structure. The results show good agreement between the incoherent scatter radar measurements and the model calculations. There is also good agreement among satellite and incoherent scatter radar measurements and model calculations for the altitude of the satellite crossing, 161 km. The satellite measurements along the orbital path, however, reveal considerable horizontal gradients in the measured ionospheric properties.

Charge exchange of metastable 2D oxygen ions with N 2 in the thermosphere

Measurements of N 2 + and supporting data made on the Atmosphere Explorer-C satellite in the ionosphere are used to study the charge exchange process O +( 2 D)+ N 2 → k N + 2+ O The equality k = (5 ± 1.7) × 10 −10cm 3 s −1. This value lies close to the lower limit of experimental uncertainty of the rate coefficient determined in the laboratory. We have also investigated atomic oxygen quenching of O +( 2D) and find that the rate coefficient is 2 × 10 −11 cm 3s −1 to within approximately a factor of two.

Electron and ion temperatures-A comparison of ground-based incoherent scatter and AE-C satellite measurements

The Atmosphere Explorer-C satellite (AE-C) is uniquely suited for correlative studies with ground-based stations because its on-board propulsion system enables a desired ground station overflight condition to be maintained for a period of several weeks. It also provides the first low-altitude (below 260 km) comparison of satellite and incoherent scatter electron and ion temperatures. More than 40 comparisons of remote and in situ measurements were made by using data from AE-C and four incoherent scatter stations (Arecibo, Chatanika, Millstone Hill, and St. Santin). The results indicate very good agreement between satellite and ground measurements of the ion temperature, the average satellite retarding potential analyzer temperatures differing from the average incoherent scatter temperatures by −2% at St. Santin, +3% at Millstone Hill, and +2% at Arecibo. The electron temperatures also agree well, the average satellite temperatures exceeding the average incoherent scatter temperatures by 3% at St. Santin, 2% at Arecibo, and 11% at Millstone Hill. Several temperature comparisons were made between AE-C and Chatanika. In spite of the highly variable ionosphere often encountered at this high-latitude location, good agreement was obtained between the in situ and remote measurements of electron and ion temperatures. Longitudinal variations are found to be very important in the comparisons of electron temperature in some locations. The agreement between the electron temperatures is considerably better than that found in some earlier comparisons involving satellites at higher altitudes.

Direct measurements of neutral wave characteristics in the thermosphere

The elliptical and circular phases of the Atmosphere Explorer-C satellite have provided the basis for a study of the neutral wave characteristics in the thermosphere using data collected by the open source mass spectrometer used to measure both reactive (O,N) and nonreactive (O2, N2, He, Ar) constituents. The phase relationships between the constituents are discussed and the results of a wave occurrence and amplitude survey covering 338 despun orbits in which the local time and latitude characteristics of the waves are presented are discussed. Conclusions based on this survey are tested in a study of waves measured at high latitudes during a geomagnetic storm.

Molecular oxygen abundances in the thermosphere from Atmosphere Explorer-C ion composition measurements

We present a scheme for the daytime ion chemistry of O2+ in the thermosphere and use it to derive molecular oxygen abundances from ion composition measurements on board the Atmosphere Explorer-C satellite. The molecular oxygen abundance varies from about 1.0 × 108 to 5.0 × 108 cm−3 at 200 km. No seasonal variation is found, but a significant increase in mean O2 density occurs at high geomagnetic latitudes. We also find a small variation of the O2 density with local time.

Photometer calibration error using extended standard sources

As part of a project to compare measurements of the night airglow made by the visible airglow experiment on the Atmospheric Explorer-C satellite, the standard light sources of several airglow observatories were compared with the standard source used in the absolute calibration of the satellite photometer. In the course of the comparison, it has been found that serious calibration errors (up to a factor of two) can arise when a calibration source with a reflecting surface is placed close to an interference filter. For reliable absolute calibration, the source should be located at a distance of at least five filter radii from the interference filter.

Atomic and molecular oxygen densities in the lower thermosphere

The Atmospheric Explorer-C satellite used a neutral mass spectrometer containing a quasi-open ion source operating either in normal mode or in one that utilizes the satellite velocity to distinguish between incoming ambient particles and ambient particles which have struck instrument surfaces and become accommodated. An analysis was made of data from the mass spectrometer for those orbits when the open ion source operated either partially or entirely in the fly-through mode. A comparison of N2 density, total O density and O2/N2 density values at 200 km was made. The average N2 and O2 density values agree well with previous rocket soundings and other studies. It is believed that the individual absolute number densities obtained in the normal mode of operation are accurate to within 20% while those deduced from fly-through data may be accurate to only 30%.

Seasonal variation of neutral thermospheric constituents in the Northern Hemisphere

The Atmosphere Explorer-C satellite carries a large number of experiments including an open source neutral mass spectrometer. Among the measurements obtained with this instrument were ones of particle densities of N2, O, and He at northern latitudes in early February and late June 1974. The orbital geometry of the satellite was such that comparisons could be made between the winter and summer values at the same altitudes and latitudes and at similar local solar times. The helium density shows in midmorning a winter to summer enhancement of a factor of 29 at 400 km. The enhancement at other altitudes, both higher and lower, is less. At 250 km the winter/summer oxygen ratio is 1.7, whereas the winter/summer nitrogen ratio is 0.6. Both ratios decrease with increasing altitudes. The analysis identifies an oxygen enhancement at low altitudes in winter and at high altitudes in summer.

Phase and amplitude relationships of wave structure observed in the lower thermosphere

Data from the Atmosphere Explorer-C satellite clearly exhibit wavelike variations in neutral composition, ion density, and electron temperature, which appear to be a general feature of the atmosphere. The neutral constituents do not exhibit uniform wave characteristics since the density variations of argon are approximately twice those of molecular nitrogen and helium structure has about one-half the amplitude of the N2 variation. The waves evident in the ion density are nearly in phase with the heavy neutrals, while the electron-temperature variations are predominantly out of phase with those in the ion density. A simple model is suggested to explain the neutral composition results, wherein the enhancements in the major gas densities are in phase with the vertical component of the perturbation velocity of the gas. The vertical velocity modifies the composition by transporting parcels of air to higher or lower regions where the composition is different. The phase relationship between density and velocity implies phase velocities (assuming that these are gravitational waves) of the order of 500 meters per sec.

An auroral F-region study using in situ measurements by the Atmosphere Explorer-C satellite

On 14 July 1974 the Atmosphere Explorer-C satellite flew through an aurora at F-region altitudes just after local midnight. The effects of the particle influx are clearly evident in the ion densities, the 6300 Å airglow, and the electron and ion temperatures. This event provided an opportunity to study the agreement between the observed ion densities and those calculated from photochemical theory using in situ measurements of such atmospheric parameters as the neutral densities and the differential electron energy spectra obtained along the satellite track. Good agreement is obtained for the ions O 2 +, NO + and N 2 + using photochemical theory and measured rate constants and electron impact cross sections. Atomic nitrogen densities are calculated from the observed [ NO +] [ O 2 +] ratio. In the region of most intense electron fluxes (20 erg cm −2 sec −1) at ∼280 km, the N density is found to be between 2 and 7 × 10 7 cm −3. The resulting N densities are found to account for approx. 60% of the production of N + through electron impact on N and the resonant charge exchange of O +( 2 P) with N( 4 S) . This reaction also provides a significant source of O( 1S) in the aurora at F-region altitudes. In the region of intense fast electron influx, the reaction with atomic nitrogen is found to be the main loss of O +( 2 P) .