• Auroral zone absorption of radio waves transmitted via the ionosphere

      Owren, Leif; Leinbach, Harold; Nichols, B.; Stark, R.; Smith, Carol (Geophysical Institute at the University of Alaska, 1956)
      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.
    • Drift Motions of Auroral Ionization

      Nichols, B. (Geophysical Institute at the University of Alaska, 1957-07)
      The primary subject of this report is the drift motions of auroral ionization. The existence of rapid motions of the ionization has been demonstrated in previous radar studies of the aurora, but neither the nature of the motions nor their explanation has been established until now. The purpose of our experimental observations was to determine the direction and speeds of the motions. In doing so, we obtained additional information concerning the general nature of the auroral ionization. Measurements were taken at College, Alaska, during the winter and spring of 1956-57* using CW transmitters. By locating the transmitters at Sielson Air Force Base, it was possible to separate the transmitters and the receivers by bZ kilometers along a ge©magnetically east-west line. The basic technique used was to examine the frequency spectra of radio echoes from the aurora at 106 Mc/s and tyl.l5 Mc/s. A comparison of the results obtained at 106 Mc/s and WL.15 Mc/s showed that the frequency shifts are proportional to the transmitted frequency, as would be expected of Doppler shifts. By measuring the spectra of the echoes received from east and west of geomagnetic north at the same time, it was possible to determine the following: (i) That the motions are generally horizontal and in the geomagnetic east-west plane; and (ii) That the speeds of the motions vary from 350 meters per second to 2,000 meters per second. On the basis of our experimental results and the published literature, we show that the electron drift motions in the aurora are of the same order of magnitude and direction as the motions of the electrons in the ionospheric current system required to explain magnetic disturbances. These electron motions produce the Doppler shifts that are responsible for the well known rapid fading of auroral radio echoes. The fading of radar auroral echoes is therefore associated with the increased electric fields which drive the currents in auroral regions. Following a review of the available information concerning general motions in the ionosphere, motions of the visible aurora, and motions inferred from magnetic storms, we show that the drift motions of auroral ionization do not constitute a separate and distinct group. Instead, they are found at the upper end of a continuous curve of increasing speed of motions with increasing magnetic disturbance. The intense ionospheric currents that produce the magnetic disturbances are found to be associated with both increased electron density and increased speed of motion. In our examination of the amplitude of VHF radio auroral echoes, the basic premises of the theory of scattering by non-isotroplc irregularities produced by turbulence [Booker, 1956] are found to be satisfactory. However, the numerical values of the parameters suggested by Booker require revision. In particular, our results indicate that the mean square fractional deviation of electron density is much greater than Booker conjectured on the basis of the then available evidence; in fact, it is greater by two to three orders of ten.