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|Title:||On the energetics in the lower thermosphere|
|Authors:||Bhatnagar, V P|
|Abstract:||The resultant global mean heating rate due to solar EUV radiation in presence of the infrared cooling from the gases like carbon dioxide, nitric oxide and the atomic oxygen has been estimated in the altitude range 120 - 200 km for different solar activity and seasons at moderate latitudes utilizing MSIS-E models. The simplified energy equation leads to a peak height in the heating/cooling rates and a direct dependence of the exospheric temperature on these rates at the lower thermospheric boundary. The computations are performed mainly for quiet periods and steady state noon time maximum and nighttime minimum conditions, when the temperature variation with local time can be ignored. The increase in cooling rates in the lower thermospheric heights from increased densities tends to pull the exospheric temperature down and <i>vice versa</i>. However, during daytime the relative magnitudes of the heating and cooling rates determine the decrease in exospheric temperature. It is shown that ideally the heating/cooling rates above their peaks decrease with height exponentially [exp(– s (z - z<sub>0</sub>))], where, z, is the height; z<sub>0</sub>, the lower boundary; and s, a parameter dependent on the temperature profile and can be easily estimated from any model or observed temperatures. The case studies show that the resultant heating / cooling rates increase with increase in the solar activity / temperature irrespective of the season by a factor of about 2 from the minimum to maximum solar activity. The seasonally averaged heating rate values for daytime and nighttime at the peaks for moderate solar activity of about 0.7 × 10<sup>-7</sup> and 0.9 × 10<sup>-7</sup> (ergs cm<sup>-3</sup> s<sup>-1</sup>), respectively agree well with the existing theoretical and observed values. Above the peaks, the resultant average cooling rates during nighttime for quiet periods are larger by a factor of 2 to 8 depending on the solar activity from the observed ones just for nitric oxide by SABER instrument. This is probably due to the fact that SABER values include all local times, seasons, latitudes, longitudes and geophysical activities and does not include the main constituent, the atomic oxygen. The changes in the cooling rates from increased CO<sub>2</sub> and CH<sub>4</sub> could imply corresponding change in the atmospheric densities originating from related changes in the exospheric temperatures with resulting feedback to the cooling rates and subsequent density changes.|
|ISSN:||0975-105X (Online); 0367-8393 (Print)|
|Appears in Collections:||IJRSP Vol.39(4) [August 2010]|
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