Linwood B. Callis’s research while affiliated with Hampton VA Medical Center and other places

Mesospheric nitric oxide as observed by the Halogen Occultation Experiment (HALOE) and calculated by the Langley Research Center two-dimensional chemical transport model are compared on a daily and collocated basis for the period 920101 through 971231. Results show excellent agreement when energetic electron precipitation (EEP) from the outer trapping region of the magnetosphere is included. A simulation using only an upper boundary condition derived from the HALOE, but not explicitly including EEP, is deficient in NO at altitudes above 60 km. The contribution to the stratosphere of odd nitrogen formed by EEP in the mesosphere is significant and is approximately three times the contribution due to the HALOE upper boundary condition (HUBC) which approximates auroral electron precipitation, and solar EUV and solar X-ray effects on NO formation.

Intermittent high-speed solar wind streams impinging on the Earth's magnetosphere lead to space weather events which are manifested by a variety of processes. Among these processes is the acceleration of electrons, within the outer trapping region of the magnetosphere, with energies between 1 keV and 10 Mev. The precipitation of these electrons into the mesosphere and lower thermosphere may, through the formation of odd nitrogen during energy deposition, and subsequent vertical transport, alter the chemical climatology of the middle atmosphere between 25 and 100 km in ways that may be important to global ozone. It is shown that the existence of such effects can be established with observations. We use NOAA TIROS electron data, HALOE NO and NOX data, and ISAMS nighttime NO2 data to demonstrate the various aspects of this linkage. We also present results from a contemporary 2-D chemical transport model with variable transport to illustrate the effects of such a linkage on global ozone. Effects are found to be comparable to those due to solar UV flux variations during the 11-year solar activity cycle.

Recent observational studies have demonstrated a linkage between the Sun and the Earth, driven by the tenuous high speed solar wind streams emanating from solar coronal holes. This linkage extends from the Sun through the near-space environment and the atmosphere to the lower stratosphere. It varies with time scales that range from days to decades. Observations and interpretative calculations suggest that this linkage significantly affects the global odd nitrogen and ozone budgets with the potential to have effects on tropospheric climate. The coupling is described and related observations discussed. Simulations, validated in both the mesosphere and stratosphere, are used to assess the importance of this linkage to stratospheric ozone compared to the effects of solar UV variability. Above 25 km, the present linkage is found to have a larger effect on global ozone than the variations of solar UV during the 11-year solar cycle. The significant variation of the efficacy of this linkage to produce global change in the middle atmosphere between solar cycles 21 and 22, is described. Finally, the potential of this linkage to affect tropospheric climate through ozone-related thermal perturbations of the stratosphere is discussed.

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.

Recent work calls into question the adequacy of using atmospheric models with an upper boundary near 60 km for interpretive and prognostic studies of stratospheric odd nitrogen (NOY = NO + NO2 + 2 × N2O5 + HNO3 + CINO3 + NO3) and ozone (O3). Studies have shown that the effects of energetic particle precipitation occurring within the mesosphere and the lower thermosphere (MLT, roughly 60-110 km) ultimately affect stratospheric NOY and, calculations suggest, O3. The magnitude of these effects--- which are solar cycle-dependent---on the stratosphere is calculated to be comparable with those due to solar flux variations associated with the 11-year solar activity cycle. Accounting for the effects of these precipitating particles requires models with an upper boundary no lower than 100 km. Also, the polar night observations of the species required to unambiguously characterize this solar and particle-driven coupling between atmospheric regions have not been made and are not yet planned.

Model derived sensitivities of O3 near 40 km to solar UV flux changes (11-year and 27-day) fall within the range 0.36-0.55 with S11y ≈ S27d. These sensitivities derived from observations have been reported to be 0.91 and 0.39-0.46, respectively. This discrepancy appears to be due to long-term (2-10 years) fluctuations in stratospheric NO2 in the 1980s. Such fluctuations, if not taken into account in the analysis of the data, could lead to high values of the sensitivity for the 11-year activity cycle but not for the 27-day solar rotation period. Simulations accounting for such effects suggest a consistency between theoretical values of S11y and those derived from data if NO2 effects are included. The appropriate values are approximately 0.48, a value similar to published model calculations and results from the analysis of data from the 27-day solar rotation period. Verification of this suggestion awaits an NO2 data base of sufficient quality and length to permit solar-cycle trend calculations to be made.

An analysis is carried out of the effects on middle atmospheric NOy and Oa 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 N OAA 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 N Oy should occur in the mesosphere and the lower therrnosphere at mid to high latitudes. Italogen Occultation Experiment (ItALOE) 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 NOy 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 N Oy increases of 20-40% were calculated near 25 km leading to O3 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.

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.

A geoeffective magnetic cloud impacted the Earth early on 15 May 1997. The cloud exhibited strong initial southward interplanetary magnetic field (BZ∼−25 nT), which caused intense substorm activity and an intense geomagnetic storm (Dst ∼−170 nT). SAMPEX data showed that relativistic electrons (E ≳ 1.0 MeV) appeared suddenly deep in the magnetosphere at L=3 to 4. These electrons were not directly “injected” from higher altitudes (i.e., from the magnetotail), nor did they come from an interplanetary source. The electron increase was preceded (for ∼2 hrs) by remarkably strong low-frequency wave activity as seen by CANOPUS ground stations and by the GOES-8 spacecraft at geostationary orbit. POLAR/CEPPAD measurements support the result that high-energy electrons suddenly appeared deep in the magnetosphere. Thus, these new multi-point data suggest that strong magnetospheric waves can quickly and efficiently accelerate electrons to multi-MeV energies deep in the radiation belts on timescales of tens of minutes.