A. C. Aikin

Temperature trends in the lower mesosphere

The largest atmospheric temperature changes due to the increase of greenhouse gases are expected in the 40 to 60 km altitude region, where enhanced infrared cooling decreases the temperature. Ten-year (1980–1990) temperature trends at 55 km and 0.4 mb, derived using data from the ground-based lidar at Haute Provence, 44°N 6°E, and the SSU channel 47X on several satellites, are presented. These data show temperature decreases that are as large and in some cases exceed predictions based on current models. At 44°N, the ground-based lidar and satellite techniques give a negative trend of −0.10±0.04% per year and −0.14 ± 0.02% per year, respectively. Agreement between these two data sets based on different measurement techniques gives confidence in the detected trends at this latitude. Further analysis of the SSU 47X satellite data between 45°S and 45°N indicates a maximum decline of 0.16% per year near 30°N. A minimum trend decrease of 0.07% per year is detected between 20° and 30°S. Based on NOAA satellite radiance observations, these long-term temperature changes are larger than changes at any of the other stratospheric levels below 55 km monitored during this period.

Variations of polar mesospheric oxygen and ozone during auroral events

Abstract Temporal solutions are given of the photochemical equations describing the distribution of ozone and atomic oxygen in an oxygen atmosphere for the polar regions. Similar calculations are applied to auroral events, and indicate a strong dependence of mesospheric atomic oxygen on the intensity and energy spectrum of auroral electrons, which dissociate molecular oxygen. It is shown that there can be no significant atmospheric ozone and atomic oxygen modifications due to soft spectrum electron events which appear to characterize most bright auroras. On the other hand, the hard spectrum auroral electrons which appear in most weak, quiet auroras should cause significant increases in the atomic oxygen and ozone concentration below 80 km, provided their flux is more than 106/cm2 sec.

Thermospheric molecular oxygen measurements using the ultraviolet spectrometer on the Solar Maximum Mission spacecraft

The technique of solar occultation has been utilized to measure molecular oxygen densities in the 140- to 220- km altitude region. The ultraviolet spectrometer/polarimeter on the Solar Maximum Mission (SMM) spacecraft was operated in the occultation mode between 1985 and the end of 1989, at wavelengths of 137.46 and 139.06 nm. This covered the period from the solar sunspot minimum in 1986 to December 1989 when solar activity was near the sunspot maximum. The observed solar area was 10×10 arc s2, which yields O2 concentrations with an altitude resolution of 0.17 km. These molecular oxygen data are compared with O2 data obtained under similar conditions of local time, location, and solar activity using other techniques. SMM data are also compared with the mass spectrometer/incoherent scatter (MSIS) - 86 model. Good agreement is found near 150 km for several O2 temporal variations including the annual cycle, local time, solar activity, and magnetic activity. Unlike the model, the measured density profiles exhibit no significant increase with increasing solar activity. The disagreement of the SMM O2 data with the MSIS-86 model increases with increasing altitude.

Influence of peroxyacetyl nitrate (PAN) on odd nitrogen in the troposphere and lower stratosphere

Abstract Nonmethane hydrocarbon breakdown in the atmosphere produces aldehydes of which a fraction are transferred into peroxyacetyl nitrates (PAN) in the presence of NO and NO2. Since ethane is destroyed photochemically primarily above 1 km, PAN can be introduced into the upper troposphere and lower stratosphere without the need to be transported from the boundary layer where most hydrocarbons are destroyed and where PAN may be lost due to thermal decomposition and heterogeneous loss. Mixing ratios of ethane in the lower troposphere increase by a factor of 4–8 from equatorial to northern mid-latitudes. This difference is directly translatable into a PAN latitude gradient. At mid-latitudes the concentration of PAN below 20 km is 0.1 ppb comparable to and in some instances larger than predicted HO2NO2 mixing ratios. Like HO2NO2 and HNO3, PAN serves as a reservoir for odd nitrogen.

Energetic particle-induced enhancements of stratospheric nitric acid

Inclusion of complete ion chemistry in the calculation of minor species production during energetic particle deposition events leads to significant enhancement in the calculated nitric acid concentration during precipitation. An ionization rate of 1.2×103 cm−3s−1 imposed for 1 day increases HNO3 from 3×105 to 6×107 cm−3 at 50 km. With an ionization rate of 600 cm−3 s−1, the maximum HNO3 is 3×107 cm−3. Calculations which neglect negative ions predict that nitric acid will fall during precipitation events. The decay time for converting HNO3 into odd nitrogen and hydrogen is more than 1 day for equinoctial periods at 70° latitude. Examination of nitric acid data should yield important information on the magnitude and frequency of charged particle events.

Equatorial ozone profiles from the solar maximum mission—a comparison with theory

Abstract The u.v. spectrometer polarimeter on the Solar Maximum Mission has been utilized to measure mesospheric ozone vs altitude profiles by the technique of solar occultation. Sunset data are presented for 1980, during the fall equinoctal period within ± 20° of the geographic equator. Mean O 3 , concentrations are 4.0 × 10 10 cm −3 at 50 km, 1.6 × 10 10 cm −3 at 55 km. 5.5 × 10 9 cm −3 at 60 km and 1.5 × 10 9 cm −3 at 65 km. Som profiles exhibit altitude structure which is wavelike. The mean ozone profile is fit best with the results of a time-dependent model if the assumed water vapor mixing ratio employed varies from 6 ppm at 50 km to 2–4 ppm at 65 km.

Supernovae effects on the terrestrial atmosphere

Abstract The first effects of a nearby (∼ 10 parsec) supernova on the Earths atmosphere will be caused by ultraviolet radiation dissociating molecular oxygen. The event will be of about one months duration. Several months later nuclear gamma radiation may arrive, causing a decrease in atmospheric ozone. Cosmic radiation from the supernova remnant will not intercept the Earth for at least 1000 years at which time ozone will be seriously depleted. Supernova ultraviolet radiation increases column ozone and atomic oxygen. Atmospheric thermal structure is modified with a large temperature increase in the mesosphere and lower thermosphere and a decrease at higher altitudes caused by enhanced heat loss due to atomic oxygen radiation and conduction.

An intercomparison of mesospheric ozone profiles determined by the UVSP and SAGE II solar occultation experiments

A comparison is made of individual UVSP and SAGE II mesospheric ozone profiles between 50 and 70 km altitude as determined by the solar occultation technique. The generally good agreement between the two data sets below about 57 km leads to the conclusion that they may be considered as complementary, thus extending the effective altitude range of both. Comparison of the long-term ozone trend at 55.5 km shows a systematic difference of a few percent between the two measurements.

Spring polar ozone behavior

Abstract It has been recognized since the commencement of Antarctic ozone measurements during the IGY that spring southern polar total ozone amount is less than spring northern polar total ozone amount. More importantly, since 1980 there has been a decline in the minimum spring total ozone value, from 250 DU in 1980 to 125 DU in 1987 and below 120 in 1991. This decline occurs within the winter polar vortex, which acts as a containment vessel preventing polar ozone from escaping to lower latitudes and excluding ozone-rich air from the polar region. Ozone decrease can be explained in terms of heterogeneous reactions of chlorine and nitrogen reservoir molecules on polar stratospheric clouds. These clouds form in the lower polar stratosphere during winter when temperatures in the Antarctic are sufficiently low to create water ice clouds. Clouds involving nitric acid form at higher temperatures. Chlorine reservoirs such as HCl are converted to Cl2, which is photodissociated in the presence of sunlight. The resulting Cl reacts with O3 to form ClO. Measurements of ClO and other species give agreement of theory and experiment within the uncertainties of the measurement. Heterogeneous chemistry accounts for most of the ozone hole. A small amount of ozone loss is also observed above the polar stratospheric cloud level, implying another mechanism, either chemical or dynamical. Above 25 km, formation of ozone-destroying odd nitrogen in the upper stratosphere by energetic electrons and the existence of any trend is still an open question. There is much less ozone depletion in the Arctic. This is the result of a less stable polar vortex and warmer temperatures, which reduce polar stratospheric cloud formation. There is strong evidence that tropospheric forcing within or just outside the vortex leads to adiabatic cooling with resulting cloud formation. During such events ozone-poor tropospheric air is transported into the stratosphere. In the Arctic this can result in the transport of long-lived hydrocarbons. Subsequent reactions lead to the formation of HCl, reducing the effect of Cl. There is also production of HO2, which accelerates ozone loss due to chlorine. There are also small areas of large and rapid ozone depletion termed miniholes. Ozone-poor air from these regions can propagate to lower latitudes, as can the air from within the vortex, when it disintegrates in late spring. Data from the BUV ozone-measuring instrument on the Nimbus 4 satellite indicate the existence of October 1970 Antarctic ozone of only 250 DU. This is evidence of the existence of ozone loss with only CH3Cl and low concentrations of CFCs as chlorine sources.

Stratospheric evidence of relativistic electron precipitation

Abstract The hypothesis that relativistic electron precipitation is modifying the high-latitude southern hemisphere ozone distribution is tested by examining simultaneous electron density data as measured with a ground-based partial reflections sounder and ozone mixing ratio data in the 40 to 50 km region obtained from the sateliteborne SBUV instrument Elections with energies in the 1 to 3 MeV range are stopped in this altitude region creating ionization, which can be observed as an enhancement in electron density. The resulting nitric oxide should destroy ozone. Ionization enhancement events at 50 km are observed at least 20% of the time by the partial reflections sounder at Scott Base, Antarctica. On January 15, 1984, the electron density at 50 km was 800 cm−3 (von Biel, 1989, 1991). Since the ion-pair production function required to produce this amount of ionization was 300 cm−3 s−1 and acted over a day, the corresponding reduction in ozone is expected to be more than 40%. Examination of ozone mixing ratio data from the SBUV instrument on Nimbus 7 shows no corresponding ozone decrease in the January 15 to 17, 1984 period. Possible explanations for this failure to observe an ozone decrease include another mechanism for producing the electron density enhancement or relativisttc electron precipitation in a very limited area. The relativistic electron fluxes in the 1 to 3 MeV range required to produce the partial reflection electron density profiles are the same as observed by geosynchronously orbiting spacecraft. In addition to finding no orone decrease there are two difficulties with assuming that the partial reflection electron densities are caused by the electrons observed in orbit. The L value of the field lines containing the electrons are L = 3 to 8, white Scott Base is located at L = 33. The electron density profiles observed at Scott Base are not enhanced above 70 km indicating a lack of precipitating electrons with energies less than 1 MeV. It is suggested that a more complete study be undertaken, including many events, corresponding in-orbit electron flux changes and ground-based riometer data in addition to ozone and partial reflection data.