James D. Lambeth

Precipitating relativistic electrons - Their long-term effect on stratospheric odd nitrogen levels

Using electron count rate data at geostationary orbit, daily energy spectra, extending from 30 keV to 15 MeV, have been developed for trapped relativistic electrons at 6.6 RE These spectra have been used to model the flux of these electrons into the atmosphere at 120 km. Energy deposition calculations permit daily sources of HOx and NOy to be calculated at auroral and subauroral latitudes due to relativistic electron precipitation (REP) for the period June 13, 1979, through June 4, 1988. Both short-term and long-term source variations are quite large over the period considered. The long-term variation of the NOy source is found to reach a maximum in late 1984 and early 1985, with significant declines thereafter. Daily Solar Backscattered Ultraviolet (SBUV) O3 data show a significant response to these precipitation events. Two-dimensional model calculations have been carried out for the period 1979 to 1990 with REP effects included through June 4, 1988. Results suggest that globally integrated NOy has increased by 35–40% from 1979 to early 1985 with declines thereafter. The largest long-term increases are found in the lower stratosphere at the high latitudes. Comparisons of Limb Infrared Monitor of the Stratosphere (LIMS), Solar Mesospheric Explorer (SME), Stratospheric Aerosol and Gas Experiment (SAGE), and SAGE II NO2 data are consistent with these calculations. The results suggest that a significant contribution to the anomalously large and unexplained global O3 declines between 1979 and 1985 has been made by the catalytic destruction of O3 by odd nitrogen in the lower stratosphere at mid to high latitudes. The results also provide evidence for a clear and strong linkage between solar variability, the state of the magnetosphere, and the chemical climatological state of the middle and lower atmosphere.

Solar atmospheric coupling by electrons (SOLACE).2. Calculated stratospheric effects of precipitating electrons, 1979-1988

An analysis has been carried out of the effects of energetic electron precipitation (EEP) on stratospheric NOy, NO2, and O3. Solar wind observations used together with precipitating electron fluxes observed aboard TIROS spacecraft show a close relationship between the long- and short-term fluctuations in the solar wind and EEP over a period of 16 years. Daily electron energy spectra for 4.25≤E≤1050 keV and energy deposition profiles are developed for both hemispheres for L≥5 and used in two-dimensional chemical transport simulations for the period January 15, 1979, through December 31, 1987. Results indicate that globally averaged column NOy (from 25 to 40 km) increases by ≈ 12% between 1979 and 1983–1985 with a rapid decline to 1979 levels between early 1985 and 1987. Day-by-day comparisons of the results with the Stratospheric Aerosol and Gas Experiment (SAGE II) column NO2 and O3 for the period October 24, 1984, and December 31, 1987, show good agreement with the inclusion of EEP in the simulations. Northern near-hemispheric decreases of column NO2 of ≈ 35% observed by SAGE II between early 1985 and 1987 are well simulated with the inclusion of EEP. Comparisons of several simulations with one another and with SAGE II NO2 data and Solar Backscattered Ultraviolet (SBUV) (V6) O3 data suggest that SOLACE represents a solar- terrestrial coupling mechanism which, for solar cycle 21, is as important to stratospheric O3 as solar UV flux variations.

Solar atmospheric coupling by electrons (SOLACE): 1. Effects of the May 12, 1997 solar event on the middle atmosphere

An analysis is carried out of the effects on middle atmospheric NO y and O 3 of a coronal mass ejection (CME) event which occurred on May 12, 1997, and which is coupled with observed solar wind fluctuations. Observations of electron fluxes by instruments aboard the SAMPEX and NOAA 12 satellites indicate large enhancements of magnetospheric electron fluxes occurring with the arrival of the high-speed solar wind. Calculations suggest that significant formation rates of NO y should occur in the mesosphere and the lower thermosphere at mid to high latitudes. Halogen Occultation Experiment (HALOE) NO observations reveal increases of more than an order of magnitude between 85 and 120 km in both hemispheres within 1-2 days after the electron flux increases. Two dimensional chemical transport simulations were carried out to assess the fate of the NO y increases. Northern hemispheric increases were lost to photochemical destruction shortly after the event ended. Southern hemispheric increases were transported in part into the stratosphere by advective descent. By October 1997, high-latitude NO y increases of 20-40% were calculated near 25 km leading to O 3 reductions of up to 20% when compared to a simulation with no electron precipitation. A solar atmospheric coupling by electrons precipitating from the outer trapping and auroral regions of the magnetosphere, and which affects middle atmospheric NO, is clearly demonstrated by the observations alone.

Solar-atmospheric coupling by electrons (SOLACE): 3. Comparisons of simulations and observations, 1979–1997, issues and implications

Several middle atmospheric simulations have been carried out from January 1979 to December 1997 including most effects important to stratospheric O3. Results of these simulations for several species and species ratios have been compared in detail with observations made by the Halogen Occultation Experiment, the Polar Ozone and Aerosol Measurement II, the Total Ozone Mapping Spectrometer, the Jet Propulsion Laboratory Mark IV Interferometer, and during the Photochemistry of Ozone Loss in the Arctic Summer mission. For the simulation including all effects, comparisons of all species and ratios show excellent agreement. Comparisons of simulated sunset NO2 with and without the effects of energetic electron precipitation show excellent agreement with observations with the effects of the electrons included but poor agreement when they are excluded. The validated simulations indicate that the effects of a polar source of NOy must be included for an adequate simulation of stratospheric O3 and NOy. A comparison of simulations, during the 11-year solar cycle, of the relative effects on O3 of solar UV flux variations and the energetic electron precipitation has been made. For global total O3 the effects are comparable. For the global column above 25 km, the effects of energetic electron precipitation are significantly larger. The implications of, and some issues raised by, these findings are briefly discussed.

On the origin of midlatitude ozone changes: Data analysis and simulations for 1979–1993

Satellite data show large declines in global (4.5%) and midlatitude (10%) ozone in the mid-1980s and during 1992 and 1993. Analyses of ozone, temperature, and aerosol records and two-dimensional chemical transport simulations have been carried out to develop an understanding of the causes of these changes. Simulations include contemporary homogeneous and heterogeneous chemistry. Also included are the effects of trace gas increases, dilution and denitrification associated with the Antarctic ozone destruction, solar cycle effects including relativistic electron precipitation (REP), variable diabatic transport fields and temperature, and variable sulfate aerosol surface area density and acidity. Simulated global and midlatitude ozone agree very well with observations for the entire period. Mid-1980s near-global ozone declines calculated by the model were found to be due to solar cycle (including REP) effects, -1.9%; volcanic effects, -1.5%; dilution effects, -1.1%; transport and temperature effects, -1%; and trace gas effects, -0.2%. The maximum effects of these different processes occur at different times. The observed 10% reductions in midlatitude ozone are reproduced in the simulations and are primarily due to 1 to 2-year transport and temperature variations.

Temperature variations in the middle and upper stratosphere: 1979–1992

Temperature variations in the stratosphere from 1979 to 1992 are investigated using 365-day running mean of the National Meteorological Center gridded analysis temperature data. Significant variations are seen at all levels between 70 and 1 mbar. The middle stratosphere shows temperature peaks during 1982 and 1983. The upper stratosphere has significant temperature declines between 1 and 10 mbar from 1981 to 1984. Temperatures at all levels recover to near their prior values after 1984, with the 5-mbar temperatures requiring the greatest time to fully recover. The temperature declines at 1 mbar occur in both hemispheres, over all longitudes, and in every month of the year. The decreases are largest in the middle latitudes and the polar regions and during the fall and the winter months. Such temperature variations, which appear to be of natural origin, must be taken into consideration when searching for temperature trends caused by the increase of CO2 or other greenhouse gases which affect the radiative balance of the Earth-atmosphere system or stratospheric ozone.