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.List of Figures – List of Tables – Section I Purposes – Section II Abstract – Section III Publications, Reports and Conferences – Section IV Factual Data : 1 Task A. Transmission of High Frequency of Radio Waves Via the Arctic Ionosphere ; 2 Task B. Pulse Techniques Back-Scatter at 12 Mc. ; 3 Task B Visual Observations of the Aurora – Section V Conclusions – Section VI Recommendations – Section VII PersonnelYe
