Solar Mesosphere Explorer Research Articles

Implications of the comparison of ozone abundances measured by the Solar Mesosphere Explorer to model calculations

Two years of ozone measurements from the Solar Mesosphere Explorer satellite are compared to the results of a model of lower mesospheric photochemistry. The measured ozone mixing ratios are larger than those predicted by the model by as much as a factor of two at pressures near 0.1 mbar and 1.3 at 1.0 mbar using currently accepted reaction rate coefficients. The model is brought into good agreement with the measurements over a wide range of latitudes and solar zenith angles only if the efficiency of the odd hydrogen catalytic cycle which destroys odd oxygen is decreased by 30–50%.

On the mean particle size and water content of polar mesospheric clouds

The ultraviolet spectrometer on board the Solar Mesosphere Explorer (SME) satellite has been measuring the scattering of ultraviolet sunlight from optically thin cloud layers in the upper boundary of the mesosphere (85 km) since the launch of the spacecraft in October 1981. These layers are present only at high latitudes during the summer season. During Summer 1983 an observing sequence was undertaken to measure the cloud radiance at two different scattering angles—one in the forward hemisphere at 50°, the other in the backward hemisphere at 130°. The data show a pronounced tendency for the brighter clouds to exhibit greater forward-scattering behavior, indicating that particle size may be the most important factor in determining the cloud brightness. We conclude that if the particles are monodisperse water ice aggregates, their radii do not exceed 70 nm. Estimates are provided for the water content and column particle number of the clouds, depending upon the unknown shape of the particle size distribution. Characterizing the distribution by two parameters, the spherical equivalent particle radius and the width of the distribution, the bulk cloud properties are shown to be dependent upon the limb radiance and the width parameter. For narrow widths the water ice content is less than that expected for the water vapor content at mesopause heights of a few parts per million. This confirms our earlier analysis using a smaller data set and assuming the cloud particles are monodisperse. However if the size dispersion is broad, the implied water ice exceeds the static atmospheric water supply. The calculated column number for the brightest clouds exceeds our estimates for the total supply of condensation nuclei. The alternative is that the brighter clouds are limited to a fairly narrow range of particle sizes from 40 to 60 nm. This conclusion is supported by theoretical time dependent calculations by Turco et al. (1982).

The response of ozone to solar proton events during solar cycle 21: A theoretical interpretation

Substantial ozone decreases at the stratopause and in the Mesosphere have now been observed by instruments on the NIMBUS 7 solar backscattered ultraviolet (SBUV) and Solar Mesosphere Explorer (SME) satellites during solar proton events (SPE's) of solar cycle 21. Since the ozone depletion was only observed during the SPE's, we believe that increased production of short‐lived HOx species by protons is responsible for the depletion. We find that one‐dimensional time‐dependent model calculations are close to agreement with the two satellite instrument measurements in the upper stratosphere and lower Mesosphere only when the observed ozone decrease as a function of altitude is used in our model calculations. Most of the ozone depletion between 2 and 0.5 mbar (∼45 and 55 km) was caused not by direct particle effects, but by the large ozone decreases at higher altitudes allowing increased penetration of ultraviolet to lower than normal altitudes. Our calculations are qualitatively in agreement with the SBUV measurements but tend to underestimate the ozone depletion for pressure levels between 1 and 0.3 mbar (∼50 and 60 km). The calculated ozone depletions at 0.5 and 0.3 mbar (∼55 and 60 km) agree fairly well with the observations of SME. The strong solar zenith angle (SZA) dependent depletion that was observed is predicted by the model. Larger SZA's generally reflected larger ozone depletions. However, for SZA's greater than 84° during the July 13, 1982, SPE an ozone decrease less than the ozone decrease at 84° SZA and 1 mbar (∼50 km) was predicted by theory and observed in SBUV data. Similar behavior was observed in three other SPE's. As far as we know, this ozone response is the first reported evidence for the much discussed self‐healing effect.

Ozone peaks at spring equinox

Scientists from the University of Colorado, Boulder, and the National Oceanic and Atmospheric Administration (NOAA) have discovered that the density of ozone in the upper mesosphere triples each year at the spring equinox. The scientists used data from the National Aeronautics and Space Administration's Solar Mesosphere Explorer (SME) satellite operated by the university.According to authors R. J. Thomas, C. A. Barth, and S. Solomon, who reported their findings in the July 1984 Geophysical Research Letters, “Large seasonal changes occur, particularly near 80 km where ozone densities at the equinoxes are about 2–3 times those observed at the solstices… Further, the variations are highly repeatable from year to year and from hemisphere to hemisphere.”

Solar Mesosphere Explorer Ultraviolet Spectrometer: Measurements of ozone in the 1.0–0.1 mbar region

The ozone density of the earth's mesosphere in the 1.0–0.1 mbar (48 to 70 km) region has been measured at sunlit latitudes for the period from December 1981 until the present by an ultraviolet spectrometer on the Solar Mesosphere Explorer satellite. Results for 1982 are reported. The ozone mixing ratios are found to be highly variable in time and place, with maxima occurring in the winter hemispheres. The results show complex time variations at all pressure levels, with annual and semiannual variations apparent at most pressures and latitudes. A relative maximum occurs in July at the equator.

Solar Mesosphere Explorer Near‐Infrared Spectrometer: Measurements of 1.27‐μm radiances and the inference of mesospheric ozone

Ozone in the mesosphere is determined from observations made by the near‐infrared spectrometer experiment on the Solar Mesosphere Explorer satellite (SME) between 50 and 90 km over most latitudes at 3:00 P.M. local time. The spectrometer measures emission from O2(1Δg) at 1.27 μm that is primarily due to the photodissociation of ozone. The instrument consists of a parabolic telescope that limits the field of view to less than 0.1°, an Ebert‐Fastie spectrometer, and a passively cooled lead sulfide detector system. The limb radiances, measured as the spacecraft spins, are inverted, producing volume emission rate profiles from which ozone densities are inferred. The vertical resolution is better than 3.5 km. The calculation of ozone accounts for quenching and atmospheric transmission of both solar radiation and 1.27‐μm radiation. We have established the existence of a secondary maximum of ozone density near 80km. An error analysis shows that the effects of random errors in the data and in the analysis on the final ozone profile are less than 10% between 50 and 82 km.

Solar Mesosphere Explorer measurements of polar mesospheric clouds (noctilucent clouds)

The Ultraviolet Spectrometer experiment on board the Solar Mesosphere Explorer satellite has measured scattering of sunlight from the polar mesospheric cloud layer in the 0.2–0.3 μm spectral range. The layer is manifested in the limb-scanning measurements as large increases in radiance at heights near 80 km, at latitudes 60–90°N and 60–90°S and during the summer season. They are similar to noctilucent clouds with respect to their height, geometrical thickness (less than 3.5 km) and morphology. However, as shown by Donahue et al. (1972), they are much brighter and occupy the entire polar ‘cap’ area, a region largely inaccesible to ground-based observation. The measurements have revealed a forward-scattering asymmetry, which increases with layer brightness. This behaviour shows that the brighter clouds are composed of particles with radii according to Mie theory up to 0.07 μm, at least for the limited set of data studied so far. The variation of layer brightness with asymmetry factor is consistent with the layer being limited by the available water content of the atmosphere. The calculated water content of the particles, assuming them to consist of pure water ice, is about 100 μg m −2, provided the particle distribution is monodisperse. This corresponds to the total amount of atmospheric water vapor residing in a vertical column above 80 km for a water vapor mixing ratio of 1.2 ppmv. This is consistent with the amount of water vapor believed to exist at mesopause heights (a few ppmv). The large amounts of ice reported by Donahue et al. (1972) are too large by a factor ten. Their corrected values are consistent with the present analysis. A brief description is given concerning additional research topics which are being pursued using the extensive SME data base, which now consists of five complete summer seasons from 1981 to the present.

Measurements of stratospheric NO2 from the solar mesosphere explorer satellite: 2. General morphology of observed NO2 and derived N2O5

Observations of NO2 densities from 28 to 40 km as measured by the Solar Mesosphere Explorer (SME) satellite are compared to model calculations for the month of January. Low densities are obtained in the tropics and in high latitude winter, with much larger values in the summer middle and high latitude regions in both the observations and the model. The reasons for areas of agreement and disagreement between the model and the observations are explored. The observed NO2 distribution is also used to infer the N2O5 distribution based on presently accepted chemistry and suggests that very large amounts of N2O5 are present in high latitude winter.

Seasonal variations of ozone in the upper mesosphere and gravity waves

Ozone densities in the upper mesosphere have been measured as a function of time and latitude over a two‐year period (1982‐1983) by the Solar Mesosphere Explorer (SME) satellite. Large seasonal changes occur, particularly near 80 km where ozone densities at the equinoxes are about 2‐3 times those observed at the solstices. The structure of the ozone secondary maximum also fluctuates substantially during the year, with extremes near equinox and solstice. Further, the variations are highly repeatable from year to year and from hemisphere to hemisphere. We propose that the seasonal variability in ozone is produced by the variation of gravity‐wave‐induced transport in the mesosphere which, in turn, results from the seasonal modulation of the propagation and breaking of small‐scale gravity waves.

Measurements of stratospheric NO2 from the Solar Mesosphere Explorer satellite: 1. An overview of the results

The visible light spectrometer on board the Solar Mesosphere Explorer spacecraft measures stratospheric NO2 in the 20–40 km altitude region and provides accurate daytime NO2 density profiles with nearly complete latitudinal coverage over an extended period of time. The instrument and data analysis are discussed in detail, and NO2 results for winter/spring 1982 are presented and compared to current theoretical models. Agreement with other measurements is good, and comparison with NOx models indicates that although the overall agreement is acceptable, improvements in the models are required before good agreement is reached at all latitudes. The data indicate that NO2 has a strong memory of the physical conditions present in the stratosphere over a time period of several days.

18 months of UV irradiance observations from the Solar Mesosphere Explorer

Daily solar irradiance measurements in the spectral interval 120‐305 nm have been made since 6 October 1981 with an instrument on the Solar Mesosphere Explorer. The instrument operates with a spectral resolution of about 0.75 nm. Analysis of the observed data for the period 6 December 1981 to 3 June 1983 (20 solar rotations) shows that during this period there was an apparent decrease in irradiance at all wavelengths observed (−19.7% ± 9.7% at Ly‐α) but the decrease was not significantly different from zero at wavelengths longer than 210 nm. The cross correlations between daily values of the solar irradiance and 10.7 cm flux varied from 0.7 (Ly‐α) to 0.5 (210‐215 nm) and ∼0 (290‐295nm). Calculations of the % range (i.e., highest to lowest value) of the irradiance within each solar rotation showed that for Ly‐α the range varied between 6% and 30% over the 20 solar rotations studied. At longer wavelengths the % range was smaller—about 7% at 180 nm and about 2% beyond 240 nm. The percent range values indicate representative variations useful as input data for model calculations of stratosphere/mesosphere responses to short period solar variability.

Pioneer 10 and Voyager Observations of the Interstellar Medium in Scattered Emission of the He 584 A and H Lyα 1216 A Lines

AbstractThe combination of Pioneer photometric and Voyager spectrometric observations of EUV interstellar-interplanetary emissions in the region beyond 5 AU have been applied to a determination of atomic hydrogen and helium densities. These density estimates obtained from direct measurement of scattered radiation depend on absolute calibration of the instruments, in the same way as other earlier determinations based on the same method. However, we have combined the spacecraft data with daily full sun averages of the H Lya 1216 A line obtained by the Solar Mesospheric Explorer (SME) satellite, to obtain a measure of atomic hydrogen density independent of instrument absolute calibration. The method depends on observations of long and short term temporal variability of the solar line over a 1 year period, and the fact that the ISM is optically thick. The density estimates from preliminary work on these observations are [H] = 0,12 cm-3 and [He] = .016 cm-3, giving a density ratio close to the cosmic abundance value, in contrast to some earlier results indicating a depletion of atomin hydrogen. We have obtained estimates of galactic background emissions in the signals of both spacecraft.

Formation of the El Chichon aerosol cloud

Thermal emission at 6.8 µm and particle‐scattered radiation at 1.9 µm from the El Chichon aerosol cloud were measured by instruments on board the Solar Mesosphere Explorer satellite. The cloud moved westward circling the globe in twenty‐one days. During its initial formation the cloud was centered at an altitude of 27 km and was confined to the latitude band between the equator and 30°N. At the 27 km level, the maximum density was reached eight to nine weeks after the eruption. Following that time, the maximum in the density gradually moved lower in altitude.

Satellite limb‐scanning thermal infrared observations of the El Chichon stratospheric aerosol: First results

The Infrared Radiometer experiment on the Solar Mesosphere Explorer satellite has been continuously measuring the 6.8‐µm thermal emission from the stratospheric aerosol from the El Chichon volcano since the time of eruption in early April, 1982. Inversion results for the zonally‐averaged infrared extinction coefficient in height, latitude, and time show that the aerosol increased in mass to a maximum of 8 Tg(8×1012gm) about 15 weeks after the April 4 eruption. It descended in height with an average speed consistent with the gravitational settling time of particles with a diameter of about 1.4 µm.

Balloon and aircraft measurements of stratospheric sulfate mixing ratio following the El Chichon eruption

Profiles of sulfate mixing ratio versus altitude over the range 15‐28 km were obtained from filter samples collected aboard balloons and U‐2 aircraft. Observations were made in the summer, fall, and winter following the March‐April, 1982, El Chichon eruption. Observed sulfate mixing ratios were approximately two orders of magnitude greater than typical background (not volcanically augmented) mixing ratios. The sulfate mixing ratios obtained from the aircraft filter measurements were in good agreement with the results from a wire impactor and a particle counter flown simultaneously. Computations of the amount of sulfate contained in a vertical column of 1 m² were in agreement with similar calculations from airborne lidar, balloon‐borne particle counters, and Solar Mesosphere Explorer satellite radiance retrievals.

Bit error rate evaluation of GSTDN/TDRSS communication links

Statistical studies of telemetry errors were made on data from the Solar Mesosphere Explorer Satellite. An examination of frame sync words within the data stream, as received at the ground station, indicated a wide spread of bit error rates among stations. A study of the distribution of errors per station pass revealed a tendency for the station software to add an even number of spurious errors to the court. Ground station reporting of observed errors was found to fall into one of three major types of error distributions discovered. A detailed examination of instrument science data, using a fourth-order wild point algorithm rejecting dropouts and other system errors, yielded on average a random bit error rate of 3.1x10 6 with 99% confidence limits of (2.63.8) X10 ~ 6. The system errors were typically found to be from 5 to 100 times more frequent than the identified random errors.

Comparison of mesospheric ozone abundances measured by the Solar Mesosphere Explorer and model calculations

Ozone observations in the mesosphere obtained by the near infrared and ultraviolet spectrometers onboard the Solar Mesosphere Explorer (SME) satellite are compared to two dimensional model calculations for the month of January. In general, the model calculated abundances are somewhat smaller than those measured, but exhibit similar trends with respect to altitude and latitude. The possible causes of discrepancies include the mesospheric H2O content and photochemical reaction rates, particularly the rate of O2 photolysis.

Mesospheric ozone depletion during the Solar Proton Event of July 13, 1982 Part I Measurement

The near infrared spectrometer and the ultraviolet spectrometer on the Solar Mesosphere Explorer (SME) observed the ozone density as a function of latitude and altitude during the solar proton event of July 13, 1982. Airglow at 1.27 µm was observed at the earth's limb. The altitude profiles of the emission were inverted providing ozone densities. The ozone densities observed showed a clear decrease during the event. The maximum depletion seen was 70%. The decrease was observed in the northern high latitudes at mesospheric altitudes. The decrease was very short lived, lasting less than a day.

Mesospheric ozone depletion during the Solar Proton Event of July 13, 1982 Part II. Comparison between theory and measurements

The solar proton event of July 13, 1982 was the largest to date in the current solar cycle. Proton fluxes observed by the NOAA‐6 satellite have been used to calculate ionization rates during the event, which have been found to be almost as large as those of the August, 1972 event near 70 km, but much smaller at lower altitudes. This ionization leads to the production of odd hydrogen radicals (H+OH+HO2) which catalytically destroy odd oxygen in the mesosphere and stratosphere. A one‐dimensional time‐dependent model has been used to calculate the percentage change in ozone resulting from this event. The calculated ozone depletion is compared to that observed by the Solar Mesosphere Explorer (SME) satellite.

Solar Mesosphere Explorer: Scientific objectives and results

Instruments on the Solar Mesosphere Explorer simultaneously measure ozone density, temperature, and solar ultraviolet flux. Results from six months of observations show that ozone density in the mesosphere changes from day‐to‐day and with the seasons and that the principal cause of these changes is the variation in atmospheric temperature. The dependence between ozone density and temperature is inverse, with a decrease in temperature producing an increase in ozone density. This dependence is observable in the seasonal patterns and also in orbit‐to‐orbit observations during dramatic atmosphere changes such as stratospheric warmings.