Global features of lower ionospheric radio wave absorption in relation to middle atmospheric temperature variation and wind circulation

Abstract

The A1 absorption data for a large number of stations have been utilized to study the global features of the lower ionospheric radio wave absorption. The seasonal index ns (defined in the relation L = Bs (cos χ)ns, where L is the monthly median value of absorption, and χ, the midmonth solar zenith angle), is found to decrease rapidly and smoothly with increasing latitude and also with increasing solar activity for all stations. The constant cos χ plot of seasonal values of absorption versus month shows summer as well as winter enhancements when considered on a global scale. Such plots versus latitude show maximum absorption values at around the tropical region, an increase in the absorption values at high latitudes above 50°N, with a minimum around 40°–50°N, and a fall toward the equator below 23°N during sunspot minimum. A secondary maximum with a strong peak is observed around 50°N with a minimum around 45°N during sunspot maximum. The plots of absorption values as well as temperatures for 30 and 50 km heights versus cos χ show loop‐like structuring, indicating a considerable time dependence between them. Absorption and temperature at 30 km show a similar seasonal trend, whereas temperatures at 30 and 50 km show an opposite trend in their seasonal variations. The direction of looping of both temperature and absorption changes near about 35°N between low and high latitudes. Both temperature and absorption show large variabilities during winter periods. These findings have been explained in terms of the seasonal variation of the middle atmospheric wind circulation, and the importance of the dynamics of the lower ionosphere in controlling the aeronomy has been demonstrated.

Geomagnetic control of the winter anomaly in absorption of radio waves at mid‐latitudes

“…Sechrist [1967] and Offermann [1977], however, have pointed out that temperature increases in the mesosphere are correlated well to this phenomenon. Samanta [1976] and Rees et al [1979] have reported that wind circulations and the associated temperature variations have a corre- far larger than the area estimated in the past by several au-The fmin data are also used for analysis assuming that fmin _ thors (-•1000 kmin diameter by Thomas[1961] and-2000 km enhancement (FME) is a reliable measure of occurrence of by Schwentek [1974]) and that the winter anomaly is closely absorption enhancement (see part l). The fmin data are taken correlated to occurrence of geomagnetic storm.…”