• Arctic Propagation Studies at Tropospheric and Ionospheric Modes of Propagation: Final Report

      Owren, Leif; Bates, H. F.; Hunsucker, R. D.; Pope, J. H.; Stark, R. A. (Geophysical Institute at the University of Alaska, 1959-10)
      Two types of direct scatter from the F region are identified on the records from the oblique incidence sweep-frequency sounder located at College, Alaska. One type of echo appears to come from randomly distributed, field-aligned irregularities in the ionosphere and the other from discrete patches of irregularities. The former is essentially a nighttime phenomenon, while the latter occurs mostly during the day. From these direct scatter modes we can obtain an estimate on the horizontal and the vertical extents of the irregularities. Analysis of the data for the past year has shown that the randomly distributed irregularities commonly occur in regions having horizontal extents of more than 1000 km. The discrete irregularities appear to extend throughout most of the lower half of the F layer. The sequence of events near sunrise and sunset on a magnetically quiet winter day indicates that solar radiation eliminates the random irregularities and accentuates the discrete irregularities. Certain phenomena frequently recorded on high latitude ionograms such as Spread F and triple splitting are probably manifestations of backscatter from ionospheric irregularities. The occurrence of Z-traces in College ionograms is studied statistically and it is concluded that the majority, if not all, of the Z-traces are produced by backscatter of the radiation obliquely incident in the direction of the magnetic zenith. Fixed frequency oblique incidence soundings on frequencies of 12, 18 and 30 mc/s made at College, Alaska show both direct backscatter from the E and F layers and F layer propagated backscatter from the ground. The 12 mc/s soundings made during 1956 have been re-scaled under this contract to extract the available information concerning direct backscatter echoes at ranges below 1000 km. The direct backscatter echo from the F layer (IF echo) has a large diurnal maximum at approximately 1800 AST and a smaller maximum at 0400 AST. IF echoes are observed at ranges from 500 to 1000 km, usually occurring at approximately one-half the range of the 2F echo. The azimuth distribution of the IF echo has a maximum centered on magnetic north. Direct backscatter from the E layer (IE echo) occurs in the range interval of 200 to 800 km with a maximum between 300 and 500 km. The azimuth distribution maximum is centered on magnetic north and the diurnal distribution shows maxima from 0000 to 0200 AST and 0300 to 0400 AST. F layer propagated backscatter from the ground (2F echo) is investigated using both the 12 mc/s 1956 soundings and soundings on 12, 18 and 30 mc/s obtained during 1958. Histograms showing the diurnal distribution of 2F echo occurrence on 12 mc/s for 1956 and 1958 are essentially the same, and illustrate solar effects on the F layer. The behaviour of the regular 2F echo on 12, 18 and 30 mc/s for a typical day in December 1958 is illustrated by a series of PPI photographs. The results obtained during an experimental investigation of the drift motions of auroral ionization are summarized, and certain properties of the luminous aurora established by photo-electric measurements reviewed. Some preliminary observations of solar radio emission at 65 mc/s are reported. A technique of estimating the electron densities of the outer ionosphere by the use of nose whistlers is described. The method involves the numerical integration of the whistler dispersion equation after first assuming a model for the distribution in density. This technique is applied to several whistlers which occurred on 19 March 1959 resulting in estimates of electron densities between four and five earth's radii. The temporal variations in the occurrence of chorus during the IGY at College and Kotzebue, Alaska are studied. The results of an investigation of the effect of latitude on the diurnal maximum of chorus indicate that it is desirable to use a latitude based on the location of the eccentric dipole rather than the usual geomagnetic latitude for the study of chorus. The mathematical theory of longitudinally propagated whistlers in a magnetic dipole field is developed. The usual method for deriving electron density distributions in the exosphere from nose whistler observations by means of assumed distribution functions is criticized and shown to be ambiguous and subjective. A systematic method which avoids subjective assumptions is described. The whistler propagation problem is reduced to an integral equation and a first order principal value solution is obtained by using an approximate form of the equation. Higher order solutions may then be derived by an indicated iterative procedure. Five short-term transpolar transmission tests conducted jointly by the Geophysical Institute and the Norwegian Defence Research Establishment during 1956-59 are described briefly. Some preliminary results of a transarctic propagation study on 12, 18 and 30 mc/s made by the Geophysical Institute in cooperation with the Kiruna Geophysical Observatory, Sweden, are reported. Simultaneous backscatter soundings of the polar region from College, Alaska and recordings c£ the forward propagated signal at Kiruna, Sweden are used to deduce the propagation conditions and modes. The 12 mc/s and 18 mc/s pulse transmissions from College were received at Kiruna over 80% of the time during the month of December 1958. Groundscatter echoes from the polar regions indicated that a three-hop mode occurred 52% of the time on 12 mc/s and 49% of the time on 18 mc/s. Similarly, a two-hop mode occurred 9% of the time on 12 mc/s and 127. of the time on 18 mc/s. A signal was recorded at Kiruna 197. of the time without any corresponding groundscatter being observed from College. This could indicate propagation by a one-hop high ray (Pedersen) mode or by a lateral mode.
    • Arctic Radio Wave Propagation

      Owren, Leif; Little, C. Gordon (Geophysical Institute at the University of Alaska, 1958-03)
      The object of this investigation is to obtain additional information concerning the effects of aurora on high frequency radio signals which is essential to a complete understanding of new modes of propagation that have tactical and strategic applications.
    • 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.
    • Auroral zone absorption of radio waves transmitted via the ionosphere

      Owren, Leif; Stark, Robert (Geophysical Institute at the University of Alaska, 1955)
      Signal intensity operations have stopped and the preliminary reduction of all field intensity records completed . The 12 me back-scatter equipment is described and operating conditions stated. Tentative interpretations of ob served echoes in terms of possible reflection mechanisms are given.