Shengpan P. Zhang

Tidal influence on the oxygen and hydroxyl nightglows: Wind Imaging Interferometer observations and thermosphere/ionosphere/mesosphere electrodynamics general circulation model

Longitudinal zonally averaged Wind Imaging Interferometer (WINDII) (on UARS) night-time oxygen (O(1S)) and hydroxyl (P(3) line in the OH(8,3) Meinel band) volume emission rates exhibit dramatic spatial and temporal variations. The recently improved thermosphere/ionosphere/mesosphere electrodynamics general circulation model (TIME-GCM) produces simulations for the two airglows through the input of (1,1) upward propagating diurnal tides. The model simulations show excellent agreement with WINDII observations in both the local tune domain and the latitudinal domain between 40°S and 40°N. The influence of diurnal tides on the two airglows in strongest in the tropical region. In the local solar time domain the emission rate and peak altitude at the equator show large tidal perturbations, but they are fairly stable at midlatitude. In the latitudinal domain there is an equatorial trough in the oxygen emission rate which exists regardless of local time and season. The hydroxyl emission rate is more dependent on local time and season. At equinox it has a prominent equatorial maximum which disappears at dawn, whereas at solstice it has a very weak equatorial maximum at dusk, changing soon after midnight to an equatorial minimum. These features of emission rates are also compared to TIME-GCM simulations for meridional wind, temperature, and atomic oxygen density, [O], with and without upward propagating diurnal tides. The results are as follows: (1) The large oscillations of the two nightglows as well the atomic oxygen density in the tropical region are driven by the diurnal propagating tides. In altitude the mesosphere and lower thermosphere is divided into two type of cells, one with meridional winds converging at the equator, higher temperature, and enhanced [O] and airglow emission rates, and the other with meridional winds diverging from the equator, lower temperature, and depleted [O] and airglow emission rates; all these are essentially related to the wavelength and phase of the diurnal tides. (2) The relatively stable airglow emission rates at around 20° are related to the minimum fluctuation of temperature and stable meridional wind directions in spite of the maximized wind speed in this region. (3) The year-around equatorial depletion in the oxygen airglow and the double-peaked profile in the hydroxyl airglow are most likely produced by the diurnal tides. Further investigation is needed particularly for the absolute value of the oxygen emission rate, the interhemispheric comparison of the oxygen emission rate, why WINDII observes maximum summer emissions whereas the TIME-GCM gives maximum winter emission, and the effects of waves in addition to the diurnal tides.

Neutral winds in the lower thermosphere observed by WINDII during the April 4–5th, 1993 storm

Data from a major geomagnetic storm in April 1993 were selected for the first WINDII study of storm-induced neutral winds in the lower thermosphere between 90 and 200 km. The storm commenced at ∼1200 UT April 4th, when the Kp index increased from 1.3 to 7.7. The geomagnetic latitude range of WINDII measurements from the daytime O(¹S) emission reaches the southern geomagnetic pole. The wind exhibits a dramatic change during the storm. In the polar region the maximum velocity of ∼200 ms−1 at 140 km altitude on April 2nd increases to ∼650 ms−1 at 200 km on April 5th. Winds plotted in geomagnetic coordinates show the familiar two-cell pattern in that winds are much stronger in the dusk sector than in the dawn sector. This pattern penetrates down to ∼130 km. During the recovery period on April 8th, winds at higher altitudes return to normal while the two-cell pattern persists at lower altitudes. The thermospheric O(¹S) emission rate is severely depleted by the storm to half of its normal value.

Tides in emission rate and temperature from the O2 nightglow over Bear Lake Observatory

The mesopause oxygen rotational temperature imager, MORTI, was operated at Bear Lake Observatory (41.9°N, 111.4°W) during the period November, 1991 to May, 1993. Fluctuations of long period in both emission rate and temperature of the O2 Atmospheric (0–1) nightglow layer are evident that seem related to the diurnal and semidiurnal tides. The data show a dominant semidiurnal tidal mode in January, but a dominant diurnal component at the spring equinox. Other long-period fluctuations also appear for which a linear tidal explanation does not seem applicable.

Nightglow zenith emission rate variations in O(1 S) at low latitudes from wind imaging interferometer (WINDII) observations

Volume emission rate data of the O( 1 S) nightglow layer measured by the wind imaging interferometer (WINDII) aboard the Upper Atmosphere Research Satellite (UARS) are vertically integrated to give equivalent zenith column emission rates. The data are restricted to tropical latitudes and categorized according to season. The migrating tidal signature is extracted by plotting zonally averaged data as a function of local time. The main features are a premidnight maximum at the equator during equinox intervals and a midnight maximum at 20°N and S. The two equinoxes are not equivalent, nor is the behavior identical in the two hemispheres. Winter periods show little or no well-defined tidal signature. Longitudinal variations at fixed local time were also studied. These variations are larger than the tidal signature and are geographically random, showing no consistent identification with land masses. Some of these variations could be of planetary scale, while others are probably long-wavelength gravity waves, but in any case they were seen to extend over 10° of latitude at least. Zenithal emission rate measurements exceeding 500 R occurred at the coincidence of these longitudinal wave structures and the tidal maxima. The suggestion is put forward that much of the perceived tidal variability in the tropical lower thermosphere may be explained by such wave-tide superpositions.

On the response of the atomic oxygen red line emission rates to the Sun's energy input: an empirical model deduced from WINDII/UARS global measurements

In a previous paper by Zhang and Shepherd, an empirical model for the peak volume emission rate (Vp) and the integrated volume emission rate of the O(1D) (630 nm) dayglow was deduced from more than 130,000 daytime emission rate profiles observed by the Wind Imaging Interferometer (WINDII) on the Upper Atmospheric Research Satellite (UARS) during 1991-1995. In the model, the emission rates are given as functions of the solar zenith angle (χ) and solar irradiance using the F10.7 cm flux as a proxy. This paper extends the daytime empirical model into the twilight zone and includes the height of the peak emission rate and the width of the emission layer. For a given day, the O(1D) emission layer during both daytime and twilight-time is found to be sensitive to the solar zenith angle when solar irradiance is treated as a constant. Positive linear relationships are found between the daytime emission rate and cos1/eχ at χ < 87° the twilight-time emission rate and cos(χ+0.25)1.8 at 87° less than or equal to χ less than or equal to 104.5°, and the width of the emission layer and cosχ at χ < 87°. A negative linear relationship is found between the peak emission rate and its height at χ < 104.5°. In the long-term, the emission layer varies according to the solar cycle in that both the emission rate and the height of the emission layer increase with increasing solar irradiance. The empirical model provides the peak volume emission rate and its height, and the integrated emission rate, for both daytime and twilight zones, and the width of the daytime emission layer as functions of the solar zenith angle and solar irradiance using F10.7, E10.7, and Lyman-β as proxies. The profiles of the volume emission rate and global morphology of the red line emission therefore can be constructed using the model. Effects of solar storms, and physical precesses and photochemical reactions other than that due to the direct solar energy deposition in the thermosphere can be derived by comparing to the model.

Aurora and diurnal tides in the daytime O( 1 S) emission rates from WINDII/UARS measurements

In a previous study by Zhang and Shepherd, an empirical model for the daytime (sunlit) O(1S) green line emission layer was deduced using more than 520,000 emission rate profiles observed by he Wind Imaging Interferometer (WINDII) on the Upper Atmospheric Research Satellite (UARS) during 1991-1997. In the model, the peak emission rates and their altitudes, and the widths of both the F-layer and the E-layer of the emission are given as functions of the solar zenith angle χ and solar irradiance using F10.7 as a proxy. With this model, the daytime emission rate directly related to χ and solar irradiance can be calculated and removed, resulting in the residual emission rates. In this paper, the residual emission rates are presented in both geographic and geomagnetic latitude and local time coordinates grouped by seasons and Kp values. The main results are as follows. (1) The residual emission rates show a midday enhancement at the equator and midday depletions at mid-latitudes in the E-layer. Those variations may be attributed to the diurnal tide. The midday equatorial enhancement also occurs in the F-layer. (2) There is a deep gap in the E-layer at 35°S-65°S at the June solstice, which is wider in the morning than in the afternoon when Kp is low, and vice versa when Kp is high. (3) At latitudes poleward of 50° the daytime O(1S) aurora is conspicuously displayed in geomagnetic coordinates in both layers even for days with low Kp values, peaking at 60-70° geomagnetic latitudes and in the morning sector or in the afternoon sector or both depending on seasons. The aurora is significantly enhanced when Kp is increased. (4) There is a midday (geomagnetic noon) gap at high latitudes in both layers with a width of 3-4 hours. The gap is deepened when Kp is increased. (5) The integrated volume emission rates have similar features at high latitudes to those seen in the peak volume emission rates.