Processes responsible for the compositional structure of the thermosphere

Abstract

A diagnostic postprocessor analysis package used with runs of the National Center for Atmospheric Research thermospheric general circulation model (NCAR‐TGCM) has been extended to include the terms of the neutral composition equation that is solved by this model. The purpose of the development of this capability is to quantify the relative importance of the various physical and chemical mechanisms that force changes in neutral thermospheric composition for given geophysical situations. Such mechanisms include photodissociation of molecular oxygen and three‐body recombination of atomic oxygen, molecular and eddy diffusion and vertical and horizontal wind advection. Compositional term analysis calculations are presented using a diurnally‐reproducible TGCM run for a day of moderate geomagnetic activity (Kp = 3) near December solstice. Principal results for F region altitudes are in general agreement with previous work and include the following: (1) upper thermospheric composition is controlled by three major processes of vertical advection, horizontal advection and molecular diffusion; (2) the time scale for changes of 1/e in the mass mixing ratio for N2 and O resulting from these processes is typically ∼25–30 hours; (3) large gradients exist in the mass mixing ratios for N2 and O in the summer hemisphere high‐latitude region that, when combined with horizontal winds blowing across the magnetic pole, led to significant composition changes due to horizontal mass advection; (4) vertical advection is the major cause of the changes in mass mixing ratio of N2 and O at low and middle latitudes, with cooling and subsidence leading to decreases (increases) in the mixing ratios of N2 (O) at night, and heating and expansion leading to increase (decreases) during the daytime; (5) upward winds caused by Joule heating in the morning and evening sectors of the auroral oval cause large increases in the mixing ratio of N2 in these regions due to vertical mass advection. The principal results at ∼120 km altitude include the following: (1) photodissociation is an important term for O and O2 in this altitude region, becoming roughly commensurate with molecular diffusion, vertical and horizontal advection; (2) significant increases in the mass mixing ratio of O in the summer hemisphere occur due to photodissociation, especially in the daytime at middle latitudes; (3) downward molecular diffusion of O is important in controlling the mass mixing ratio for O, particularly in the high‐latitude winter hemisphere; (4) a complicated morphological pattern of horizontal and vertical advection maintains the mass mixing ratios at these altitudes; (5) vertical advection resulting from Joule heating in the morning and evening sectors of the auroral zone is of importance for the composition of the high‐latitude lower thermosphere; (6) typical time scales for maximum changes of 1/e in the mass mixing ratio of N2 are of the order of 60–90 hours.

The winter helium bulge revisited

“…Molecular and eddy diffusion were included in all runs. In this way we can understand how the diffusive and advective terms interact [see Burns et al, 1989] to cause the winter helium bulge. Inherently, there will be nonlinear interactions between terms in the minor species equation.…”

On the solar cycle variation of the winter anomaly

“…Hagan [1993] and Buonsanto and Witasse [1999] used Millstone Hill radar data to show that there were small changes in meridional winds in the daytime over a solar cycle. Seasonal changes in neutral composition are driven primarily by changes in the neutral winds [Mayr et al, 1978;Burns et al, 1989]. Burns et al showed that the winds that blew through a constant pressure surface, which respond to changes in the latitudinal and longitudinal gradients of the horizontal winds, were the major source of changes in neutral composition.…”

“…But the winds also control the composition. The horizontal winds are not the main driver for composition changes, rather it is the vertical winds that blow through pressure surfaces [Burns et al, 1989]. Downward winds enhance the relative abundance of the light species like O, whereas upward winds enhance heavy species like N 2 .…”

“…King [1964] and Duncan [1969] indicated that seasonal neutral composition differences could result from the summer to winter circulation. These wind-driven composition changes result primarily from the vertical winds, rather than the horizontal ones, although horizontal winds are important in the early morning hours [Burns et al, 1989]. These vertical winds have important effects on the diurnal variation of composition and should thus affect the diurnal response of the winter anomaly-a subject that needs further study.…”

The variability of Joule heating, and its effects on the ionosphere and thermosphere

“…The absolute and relative changes in T n over the values relevant to It is well-known (e.g. Burns et al, 1989) that the thermosphere becomes more (less) molecular at a fixed pressure level when T n increases (decreases). Figure 6 shows the corresponding increases in the MMM both in absolute and relative terms for the two conditions shown in Fig.…”