The response of the upper atmospheric temperature to changes in solar EUV radiation and geomagnetic activity

Abstract

The upper atmospheric neutral temperature derived from satellite drag is analyzed with a view to determine the relative importance of solar EUV and magnetic activity on heating of the upper atmosphere. As the indices of solar EUV radiation, the radio fluxes obtained from the ground based observations in the frequency range of 200–9400 Mc/s are used. Also for the period March 7–May 15, 1962, a direct comparison is made with the EUV flux obtained from the OSO 1 Satellite. As the index of magnetic activity the planetary magnetic index K p , is used. The data have been analyzed by separating the long period components of 27 days and above from the short term components by using a 5 day running mean filter. A cross correlation study between the various parameters shows that the long term variations in temperature are strongly correlated with the similar variations in the radio flux in the frequency range of 1000–3750 Mc/s, the correlation being maximum at 1000 Mc/s. The short term variations in temperature, on the other hand are strongly correlated with those in ∑ K p , the daily sum of K p indices and not with the radio flux at any of the frequencies considered. The correlation between temperature and EUV flux in the short term components is somewhat better though not as striking as the long term components. An empirical relation between temperature and the changes in radio fluxes and ∑ K p based on the above correlation study shows that T N = 524 + 2.73 S 10.7 + 1.77 (∑K p − ∑K p ) or T N = 470 + 6.53 S 30 + 1.77 (∑K p − ∑K p ) where T N is the nighttime minimum temperature; S 10.7, S 30 and ∑K p are the 5-day running means of 2800 Mc/s radio flux, 1000 Mc/s radio flux and ∑ K p respectively. The present investigation leads to the conclusion that short term variations in temperature are closely associated with the changes in magnetic activity represented by K p , and all the systematic variations temperature including the so called semi-annual variation can be fully accounted for in terms of the similar variations in solar flux.