A two-dimensional model of thermospheric nitric oxide sources and their contributions to the middle atmospheric chemical balance
Introduction
Auroral activity is known to produce huge amounts of nitric oxide (NO) in the Earth’s lower thermosphere (on the order of ≈103 to ≈104 cm−3 s−1) (Townsend and Burke, 1965, Cleary, 1986, Roble et al., 1987, Siskind et al., 1989, Roble, 1992, Barth, 1992). It has been shown that auroral production of NO during geomagnetic disturbances is highly variable and of sufficient magnitude to enhance the globally averaged reservoir of thermospheric NO. During polar night, the aurorally produced NO is long-lived in the thermosphere and may be subject to downward transport into the middle atmosphere where it can couple into middle atmospheric chemistry (Solomon and Garcia, 1984, Roble, 1992, Siskind, 1994, Siskind et al., 1997).
Energetic electrons, both auroral and photoelectrons, produce atomic nitrogen in the thermosphere by ionizing and dissociating N2. Atomic nitrogen is quickly converted to NO, which may be transported to the mesosphere and stratosphere if certain conditions are present. The influence of the odd nitrogen (NOy = N, NO, NO2, NO3, N2O5, BrONO2, ClONO2, HO2NO2, and HNO3) budget in the middle atmosphere by the transport of NO from the thermosphere has been the subject of many studies (Strobel, 1971, McConnell and McElroy, 1973, Brasseur and Nicolet, 1973, Jackman et al., 1980, Solomon, 1981, Solomon et al., 1982, Frederick and Orsini, 1982, Garcia et al., 1984, Solomon and Garcia, 1984, Russell et al., 1984, Brasseur, 1984, Brasseur, 1993, Callis and Natarajan, 1986, Siskind et al., 1989, Siskind et al., 1997, Legrand et al., 1989, Garcia and Solomon, 1994). Nonetheless, the contribution of the thermosphere to the odd nitrogen budget of the middle atmosphere has still not been well determined.
Transport processes relevant to the mesosphere and lower thermosphere are not well understood. Siskind et al. (1997) suggest that planetary and gravity waves play a significant role in the transport process. For example, Siskind et al. show that model calculations which consider planetary wave mixing are in better agreement with observations than those which assume a constant horizontal diffusion coefficient. Siskind et al. also point out that these model calculations tend to show a spring time enhancement of NOy in the stratosphere which is not present in the observations. Their model shows that the aurora contributes much more to the polar stratospheric NOy budget than the sources associated with solar EUV and soft X-rays at low and middle latitudes and may be an important source of polar stratospheric NOy. In the current study, we incorporate a time series of solar and auroral activity into a two-dimensional zonally averaged photochemical transport model in order to assess the temporal consequences of including thermospheric sources of NOy on the chemical budget of the middle atmosphere. The Siskind et al. (1997) model does not incorporate such a time series. We conclude that polar mesospheric NOy is greatly enhanced during polar night when the downward transport is strongest. We find that thermospheric NOy may be transported down to ≈30–40 km during the polar night.
Section snippets
Basic two-dimensional model
We use the Goddard Space Flight Center two-dimensional zonally averaged photochemical transport model (GSFC 2D model) which is described by Jackman et al. (1996). The model was first presented by Douglass et al. (1989), but many changes to the model have since been made. The pressure level range was extended through the mesosphere by Jackman et al. (1990). Considine et al. (1994) included heterogeneous chemical processes which occur on the stratospheric sulfate aerosol layer and polar
Comparison of results to observations
Nitric oxide in the lower thermosphere and upper mesosphere was observed by the ultraviolet nitric oxide spectrometer (UVNO) experiment on the Atmosphere Explorer D (AE-D) satellite. The UVNO instrument made measurements during the time period from late November 1974 until early February 1975. This was during the minimum of solar cycle 20. Fig. 2(a) shows the NO resulting from the inversion methods described by Cravens et al. (1985).
NO calculated by a run of the 2D model with the time series
Summary and conclusions
The consequences of incorporating thermospheric sources of NO into the GSFC 2D photochemical transport model has been assessed. The enhancement of NOy in the polar middle atmosphere (produced by thermospheric inputs) is strongly dependent on season. We see an asymmetry with respect to hemispheres, with a stronger enhancement seen at the south pole. During polar night, thermospheric NO is transported down to ≈40 km at the north pole and ≈30 km at the south pole. The magnitude of the polar night
Acknowledgements
Support for the research is provided by NASA grant NGT 5-3. We thank D.S. Evans and T.J. Fuller-Rowell for the use of the TIROS auroral electron power time series and the use of the statistical auroral ionization model. Useful discussions with T.P. Armstrong and C.A. Barth are also acknowledged.
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