Diurnal & Seasonal Variation of Frequency Dependence of Ionospheric Absorption at a Tropical Latitude

119-124Multifrequency ionospheric AI-absorption data obtained at Ahmedabad (23°N, 72.6°E, mag. dip 34°N) are analyzed to study the frequency dependence of absorption for low and high solar activity epochs, and also for low and moderately high solar activity years (1976 and 1978).The exponent m in the inverse frequency dependence of measured absorption is determined for practical purposes. The diurnal variation of the index m for different months and its seasonal variation at a fixed solar zenith angle (SZA) are studied for total absorption (Ltotal) as well as for its non-deviative (Lnd) part. It is found that (Ltotal) does not obey the inverse frequency square law. During sunspot minimum, m is found to vary with time of the day showing maximum (annual average =0.71) around noon and falling towards morning and evening hours, whereas for sunspot number Rz = 100, it shows the opposite trend showing minimum (average value =0.60) around noon and increasing towards morning and evening hours. In the case of non-deviative (Lnd) absorption. the index m in 1976 and 1978behaves in a manner similar to that of Ltotal for Rz =00 and 100, but m for Lnd has higher values and range of variation, the values being clustered around the theoretical value 2 near noon, in the spring months of 1976and for most part of the day in August 1978. These results suggest that the higher value of m on the average for Ltotal during increased solar activity is due to major contribution of non-deviative absorption and only small contribution of deviative absorption to the total absorption during high solar activity. It is also verified that the value of the proportionality constant A in the assumed frequency dependence law of total absorption changes with the or SZA corresponding to the changes in the value of m

A Comparative Study of Some Aspects of Low & Middle Latitude Ionospheric Absorption

232-235Results of a study of AI method ionospheric absorption on 2.3 MHz at a midlatitude station, De Bilt (52.1°N, 5.2°E), for the years 1975-80 to find the seasonal and solar cycle variations of the index n of co sχ for diurnal variation of absorption (Ldb), delay time () in its daily maximum, and absorption at constant solar zenith angle (SZA, χ)are presented. These are compared with the results of such variations obtained at a low latitude station, Ahmedabad (23°N, 72.6°E), which is well outside the winter anomaly zone. It is found that there is, in general, an agreement in the seasonal variation of n at the two places, but not so in the case of . The time-delay at Ahmedabad is generally larger in winter than in summer varying almost in opposition to n while it has a sharp peak in winter and a shallow trough in summer at De Bilt, thus having no definite relation with n. But reasonably good anticorrelation between n and at midlatitude is also obtained if the values of these parameters for winter months are not taken into account. The range of seasonal changes in n and seems to decrease with increase in solar activity and the yearly smoothed values of rather than n also indicate a fall at the midlatitude. The absorption at constant SZA taken together for all months varies according to a linear relation with solar activity at Ahmedabad and at De Bilt; however, the linearity at the latter place is masked by the anomalous increase of absorption in winter. A curious feature noticed was that the absorption during Apr-Oct 1979 fell below that observed during the same months in 1978 although the solar activity in 1979 continued to rise. The above results have been discussed at appropriate contexts in the light of the ionizing radiations, changes in gas composition and the loss rate of ionization

Winter Anomaly in Ionospheric Absorption of Radio Waves over Half a Sunspot Cycle

219-223The winter anomaly in ionospheric absorption is studied at a mid-latitude station over half a solar cycle. It is shown that the anomalous increases of absorption on some days in winter are better correlated with 10 mb-level geopotential height than with temperature at that level. The anomaly is most marked in the month of January. It was about 168% in 1964 and 79% in 1969. The present results along with those reported by others earlier are evidences to suggest that the possible cause of winter anomaly in ionospheric absorption may be an Increase in ionization In the D and probably in the E regions of the ionosphere following disturbances in the pressure and wind circulation system at stratospheric and mesospheric levels