Auroral zone absorption of radio waves transmitted via the ionosphere

Abstract

TASK A: TRANSMISSION OF HIGH FREQUENCY RADIO WAVES VIA THE ARCTIC IONOSPHERE The experimental data collected from June, 1949, through October, 1955, under "Experiment Aurora" are summarized in tables and diagrams, and the results discussed. The monthly percentage of signal in-time is tabulated for all frequencies and paths» and depicted in diagrams which allow a comparison of the values for East-West and South-North propagation at each frequency. The average monthly percentage of signal in-time for the duration of the 6-year experiment is tabulated for each frequency and path. The seasonal variation in signal in-tim e over short and long paths is shown in diagrams. The relationship found between ionospheric absorption, as measured with a vertical incidence sounder, and signal outtime is summarized. The average diurnal variation in the hourly median signal strength during the different seasons of the year 1954-55 is given for all frequencies on both short and long paths in the East-West as well as the South-North direction. The diurnal variation in signal strength on the 4 me short paths and the 12 me long paths is compared for a year of high solar activity (1949-50) and a year of low solar activity (1954-55). The discussion of the data reveals that a statistically significant difference in signal in-time for the East-West and South-North paths exists only for the 12 me short paths. The larger percentage of signal in-time found in the East-West direction is believed to be due to a preferential orientation of sporadic ionization along parallels to the auroral zone. A study of the critical frequencies observed for the E and F -layers shows that the difference in daytime variation of median signal strength between the years 1949-50 and 1954-55 may be explained in terms of the normal changes in F -layer ionization and D -layer absorption in course of a sunspot cycle. The results indicate that in Alaska there will generally be F2 propagation during daytime of 4 me signals over 350 km paths throughout the solar cycle. Regular daytime F2 propagation of 12 me signals over 1100 km paths may be expected in years of reasonably high solar activity only. TASK B: PULSE TECHNIQUES. BACK-SCATTER AT 12 MC A 12 me radar has been constructed and operated using A -scope and PPI displays. Experimental results obtained during several months of continuous operation are reviewed and discussed. Both direct backscatter and ground back-scatter echoes, as well as possible combinations of these modes, have been observed. The echoes are classified in two groups according to their fading rates, those fading rapidly being associated with aurora. Figures show the diurnal, range and range-azimuth distribution of the observed auroral echoes as well as some special types of echoes recorded. The direct back-scatter echoes at 12 me associated with aurora show characteristics consistent with those observed at YHF when allowance is made for the frequency difference. At 12 me the fading rate is proportionally less than at higher frequencies; and aspect sensitivity, although weaker, still exists. The diurnal variation is similar to that found at VHF. Several types of echoes not observed at VHF are mentioned. TASK B: VISUAL OBSERVATIONS OF THE AURORA Analysis is made of the visual auroral data obtained at five stations in Alaska during the observing period of 1954-55. Graphs giving the percentage occurrence of aurora at each station as a function of latitude and time of day are presented. Graphs showing the variation of auroral occurrence with geomagnetic latitude as a function of magnetic K index are also given. The conclusions drawn from the 1954-55 data are substantially the same as those based on the 1953-54 data discussed in an earlier report.