Now showing items 1-20 of 57

    • Historical references to ice conditions along the Beaufort Sea coast of Alaska

      Shapiro, Lewis H.; Metzner, Ronald C.; Toovak, Kenneth (Geophysical Institute, University of Alaska, Fairbanks, 1979-09)
      The objective of this program was to extend the data base on ice hazards along the Beaufort Sea coast of Alaska backward in time by using the knowledge and understanding of ice and weather conditions of the local residents. Information for this pilot project was obtained through direct interviews with residents, or from narratives supplied by them. The results of these procedures were evaluated to provide a basis for improving similar efforts in future. Observations of particular interest obtained from these interviews and narratives include (1 ) a description of a major motion of the landfast ice off Harrison Bay in late February, (2) a description of the formation of ice push ridges and ride-up at Cape Halkett during break up, (3) reports of whales traveling inshore of Cross Island during the fall migration and of whales being taken by crews from the Prudhoe Bay area at that time of year, and 4) descriptions of conditions in the nearshore area during summer. In addition information of historical and cultural interest was obtained.
    • Low Energy Cosmic Ray Events Associated With Solar Flares

      Reid, George C.; Leinbach, Harold (Geophysical Institute at the University of Alaska, 1959-09)
    • Ice Fog: Low Temperature Air Pollution; Defined with Fairbanks, Alaska as type locality

      Benson, Carl S. (Geophysical Institute at the University of Alaska, 1965-11)
      Stable pressure systems over interior Alaska sometimes produce prolonged, extreme (below -40°C) cold spells at the surface. The meteorological conditions responsible for two such cold spells are discussed in detail in Appendix A, where it is shown that the rate of radiative cooling of the air is enhanced by suspended ice crystals which are themselves a result of the initial cooling. Radiation fogs formed during the onset of cold spells are generally of short duration because the air soon becomes desiccated. These fogs consist of supercooled water droplets until the air temperature goes below the "spontaneous freezing point” for water droplets (about -40°C); the fog then becomes an ice crystal fog, or simply "Ice Fog". During the cooling cycle water is gradually condensed out of the air until the droplets freeze. At this point there is a sharp, discontinuous decrease in the saturation vapor pressure of the air because it must be reckoned over ice rather than over water. The polluted air over Fairbanks allows droplets to begin freezing at the relatively high temperature of -35°C. Between -35 and -40°C the amount of water vapor condensed by freezing of supercooled water droplets is 3 to 5 times greater than the amount condensed by 1°C of cooling at these temperatures. This results in rapid and widespread formation of ice fog (Appendix B) which persists in the Fairbanks area as long as the cold spell lasts. The persistence of Fairbanks ice fog depends on a continual source of moisture (4.. 1 x 10^6 Kg H2O per day) from human activities within the fog. Ice fog crystals are an order of magnitude smaller than diamond dust, or cirrus cloud crystals, which in turn are an order of magnitude smaller than common snow crystals (0.01, 0.1 and 1 to 5-mm respectively). The differences in size are shown to result from differences in cooling rates over 5 orders of magnitude. Most of the ice fog crystals have settling rates which are slower than the upward velocity of air over the city center. The upward air movement is caused by convection cells driven by the 6°C "heat island" over Fairbanks. This causes a reduced precipitation rate which permits the density of ice fog in the city center to be three times greater than that in the outlying areas. The inversions which occur during cold spells over Fairbanks begin at ground level and are among the strongest and most persistent in the world. They are three times stronger than those in the inversion layer over Los Angeles. Thus, the low-lying air over Fairbanks stagnates and becomes effectively decoupled from the atmosphere above, permitting high concentrations of all pollutants. The combustion of fuel oil, gasoline, and coal provides daily inputs of: 4.1 x 10^6 kg CO2 ; 8.6 x 10^3 kg SO2 ; and 60, 46 and 20 kg of Pb, Br and Cl respectively, into a lens-like layer of air resting on the surface with a total volume less than 3 x 10^9 m^3. The air pollution over Fairbanks during cold spells couldn't be worse, because the mechanisms for cleaning the air are virtually eliminated while all activities which pollute the air are increased.
    • An HF sweep frequency study of the arctic ionosphere

      Bates, Howard F. (Geophysical Institute at the University of Alaska, 1961-04)
      Observations made during 1958 and 1959 using a sweep frequency, HF, oblique sounder located at College, Alaska, are discussed, and selected groups of echoes are illustrated. Groundscatter is the predominant echo type observed on mid-latitude backscatter records, but this is not true in the high latitudes. The majority of the scatter echoes from soundings toward geomagnetic north were direct F region scatter of two main types - IF and constant range echoes. These same echo types were observed from the E region during magnetic disturbances (the slant Es echo corresponds to the IF echo). These echoes were centered about geomagnetic north on swept azimuth soundings and were produced by scatter near the oblique reflection point in the ionosphere. Thus, we conclude that the echoes were caused by scatter from irregularities aligned along the geomagnetic field. The IF echo branches off the first order vertical incidence F region trace and increases linearly in range with frequency. It is produced by scattered energy which is least-time focused. The scatterers are essentially randomly distributed within large regions called clouds in the F region. The height of the irregularities producing the IF echo can be computed if the vertical incidence traces are clear; heights between 200 and 350 km have been found by this technique, indicating that electron density irregularities are not confined only to the lower ionosphere. From the regularity of occurrence of the IF echo, it is highly probable that the irregularities extend throughout much of the F region. The constant range echo is produced by energy scattered from the field-aligned surface of a cloud of irregularities. The soundings were made to the north so the clouds producing the observed constant range echo extended east and west. The IF and the constant range echoes are useful research tools because they indicate the amount of cloudiness present in the F region. During the summer day, the F region is relatively free of clouds, while during the winter night, it is quite overcast. The cloud size and distribution depend primarily upon the zenith angle of the sun, the smaller the zenith angle, the smaller and fewer the clouds. Geomagnetic effects do not appear to influence the behavior or the F region clouds, although this point is somewhat uncertain. The E region clouds which produced the slant Es echo were definitely correlated with geomagnetic activity but not to solar radiation, there appeared to be no connection between the causes of the E and the F region direct scatter echoes even though they were propagated by the same general mechanisms. This study raised several new problems whose explanations might contribute greatly to ionospheric research. The sweep frequency sounding technique is a good method, but the system sensitivity must be raised greatly over that of the present equipment if meaningful results are to be obtained.
    • Distribution of Radar Auroras Over Alaska

      Leonard, Robert S. (Geophysical Institute at the University of Alaska, 1961-04)
      Analysis of data collected by five auroral radars located in Alaska shows the distribution of ionospheric disturbances as a function of time and location. The radars were operated during the IGY and were located in a nearly straight line running magnetically north-south across Alaska; these locations made it possible to observe disturbances continuously in the range, from 60 to 80 degrees geomagnetic latitude, which includes the visual auroral zone. An apparent radar auroral zone with a maximum at 67 degrees geomagnetic latitude is indicated by this study. The decrease in occurrence to the south of this maximum is verified, but the decrease to the north can not be accurately defined as the roll of aspect sensitivity is not fully understood. The radar auroral zone spreads to the south during increased magnetic disturbance, and some indication is found of a lessening of activity well north of the visual auroral zone. A conclusion is also reached that the layer causing radio wave absorption during aurora is not uniform but contains "holes" or regions of low absorption. The diurnal occurrence curves indicate two principal maxima. One is observed at all stations at times near local midnight. The time of the other maximum depends on the latitude of observation; it is later in the morning at the more northern locations. These two echoes exhibit differing degrees of aspect sensitivity, the morning echoes having a narrower scattering polar diagram.
    • Results of a Survey of IGY Patrol Spectra at College, Alaska

      Herman, L.; Belon, A. E. (Geophysical Institute at the University of Alaska, 1961-03)
      The behavior of prominent auroral emissions has been studied statistically using the IGY patrol spectrograms obtained at College, Alaska during the International Geophysical Year. Results are presented in the form of mean diurnal intensity variations and in the form of correlation tables. Both presentations suggest the occurrence of two types of auroras. One is relatively weak, has a broad intensity maximum around magnetic midnight, and contains the hydrogen Balmer emissions. The other gradually increases in intensity until magnetic midnight and thereafter maintains a high intensity level until dawn. Its spectrum is usually characterized by the presence of the first positive bands of N2. The mean diurnal intensity variation of the sodium emission suggests that the twilight enhancement of the sodium D-lines persists to much greater solar depression angles than those previously reported. Some evidence is found for a small auroral contribution to the sodium emission. This contribution is however erratic and does not correlate well with any specific auroral emission. The (OI) 6364A emission shows, as expected, a strong twilight enhancement persisting to large solar depression angles. No enhancement is observed for the (OI) 5577A emission. A weak correlation is found between the intensity of the hydrogen Balmer emissions and the occurrence of red auroras.
    • Catalogue of Huet auroral spectra 1957-1959

      Romick, Gerald J. (Geophysical Institute of the University of Alaska, 1961-03)
      The zenith auroral spectra at College, Alaska, obtained during the 1957-1959 observing seasons, has been assembled in catalogue form. The prime purpose of this catalogue is to present the auroral activity in a manner which can be used by others in the interpretation of aurorally associated phenomena. Prom the general appearance of the spectra and other factors, a table of daily index numbers (1 -9) is given for two observing periods. Although these numbers should not be used in themselves as correlation data they are valuable as representative indices. This point is indicated by the clear appearance of the spring maximum in activity and a general yearly decline in activity towards the minimum of the sunspot cycle.
    • A study of magnetic storms and auroras

      Akasofu, Syun-Ichi; Chapman, Sydney (Geophysical Institute of the University of Alaska, 1961-03)
      New notations for magnetic disturbance fields are proposed, based on the theoretical consideration of the electric current systems by which they are produced. A typical magnetic storm begins suddenly when the onrush of the front of the solar gas is halted by the earth's magnetic field. This effect (DCF field) is most markedly observed as a sudden increase of the horizontal component of the earth's field (the storm sudden commencement, abbreviated to ssc)— like a step function. In many cases, however, the change of the field during the ssc is more complicated, and different at different places. Such a complexity superposed on the simple increase (DCF) is ascribed to a complicated current system generated in the polar ionosphere (DP current). It is found that the changes of electromagnetic conditions in the polar regions are communicated, without delay, to lower latitudes, even down to the equatorial regions. It is inferred that the equatorial jet is affected by such a change and produces the abnormal enhancement of ssc along the magnetic dip equator. From the extensive analysis of several magnetic storms that occurred during the IGY and IGC, it is suggested that the capture of the solar particles in the outer geomagnetic field occurs when irregularities (containing tangled magnetic fields and high energy protons) embedded in the solar stream, impinge on the earth.. Thus the development of a magnetic storm depends on the distribution of such irregularities in the stream. The motions and resulting currents and magnetic fields of such "trapped" solar particles are studied in detail for a special model. It is inferred that a large decrease (DR field) must follow the initial increase; it is ascribed to the ring current produced by such motion of solar protons oi energy of order 500 Kev. It is proposed that during the storm there appears a transient 'storm-time1 belt well outside the outer radiation belt. It is predicted that the earth's magnetic field is reversed in limited regions when the ring current is appreciably enhanced. This involves the formation of neutral lines there. These may be of two kinds, called X lines or 0 lines according as they are crossed or encircled by magnetic lines of force. These may be entirely separated or may be joined to form a loop, called an OX loop. It is shown that one of them, the X line, which is connected with the auroral ionosphere by the lines of force, could be the proximate source of th<e particles that produce the aurora polaris. By postulating the existence of such X-type neutral lines at about 6 earth radii, an explanation is obtained of the detailed morphology of the aurora. This includes the auroral zones and their changes, the nighttime peak occurrence of auroras, their thin ribbon-like structure and their multiplicity, their diffuse and active forms and the transition between them (break-up) the required electron and proton flux, and the ray and wavy structures. Among the most important phenomena associated with the sudden change of the aurora from the diffuse to the active form are the simultaneous appearance of the auroral electrojet and the resulting polar magnetic disturbances (DP sub-storms). Several typical DP sub-storms are studied in detail. It is concluded that a westward auroral jet is produced by a southward electric field. It is shown that an instability of the sheetbeam issuing from along the X-type neutral line can produce a southward electric field of the required intensity. The southward electric field produces an eastward motion of the electrons in the ionosphere. This may be identified with the eastward motion of an active aurora and with the westward auroral electrojet. Besides such large changes- of the field, there often appear various quasi-sinusoidal changes of the field, much less intense. They are supposed to be hydromagnetic waves, some of which are generated in the outer atmosphere and propagated through the ionosphere, where a certain amount of their energy is dissipated. It is concluded however that Such a dissipation is not sufficient to produce any appreciable heating of the ionosphere.
    • Experiment Luxembourg, Scientific Report No. 4

      Rumi, G. C.; Benson, R. F. (Geophysical Institute at the University of Alaska, 1960-12)
      Experiment Luxembourg was designed to measure the electron density and electron collision-frequency as a function of height in the D region over College, Alaska using the technique of radio-wave interaction. A block diagram, which includes all the equipment necessary for the actual operation of the experiment, is described and illustrated in detail. The major parts of the system are: the disturbing transmitter operating on 17.5 Mc/s and using a 4x4 array of Yagi antennas, the wanted transmitter operating on ~ 5 Mc/s and using a circular polarization unit with 4 dipoles arranged in a quadrangle, a similar circular polarization unit and antenna for 5 Mc/s at the receiving site, and the delicate receiving system which detects a cross-modulation of 1x10 -4 for a one volt input signal. Some values of electron density and collision-frequency have been obtained and will be discussed in following publications.
    • Earth Current Activity at College, Alaska, July 1959

      Hessler, V. P. (Geophysical Institute at the University of Alaska, 1960-06-15)
      This report presents College N-S earth current records for the month of July 1959. The scale of 1 inch per hour permits detailed scaling directly from the reproductions in the report. Scalings of hourly range in amplitude and rapid fluctuation activity are given. The amplitude scalings are also presented as curves together with average curves for a period of several years to show the relative magnitudes of the July 1959 activity. The earth current always displays much more pronounced fine structure than the corresponding geomagnetic activity.
    • A Study of the Aurora of 1859

      Kimball, D. S. (Geophysical Institute at the University of Alaska, 1960-04)
      The two great auroral displays of August 28-29 and September 1-2, 1859 are studied from a collection of world-wide descriptive observations. Both auroras reached to unusually low latitudes. Red glows were reported as visible from within 23° of the geomagnetic equator in both north and south hemispheres during the display of September 1-2. It is shown that by using graphic symbols, descriptive reports may be used to indicate the significant features of an auroral display. A series of world-wide maps show the hourly locations and lowest latitude limits of auroral visibility and overhead aurora for the most active hours. They illustrate how the progress of an aurora may be followed throughout the night. Both auroras seen in North America reached their southern limits near local midnight. During the larger display of September 1-2 the aurora moved to lower latitudes and also covered a wide range in latitudes. This indicates that during great displays the auroral activity appears to expand in latitude until local midnight, at the same time moving towards the geomagnetic equator. Over large areas both displays were predominantly red. Magnetic records indicate that there were two distinct disturbances associated with the two displays. A tabulation of all known available auroral observations reported from August 28 to September 5, 1859 illustrates that by using a letter code, significant auroral activity may be recorded for use in auroral catalogues.
    • A Note on Harmonic Analysis of Geophysical Data with Special Reference to the Analysis of Geomagnetic Storms

      Sugiura, Masahisa (Geophysical Institute at the University of Alaska, 1960-04-18)
      Some geophysical characteristics tend to have a fixed distribution relative to the sun. An example is the distribution of air temperature on an ideal earth that is perfectly symmetrical (e.g., in its pattern of land and water) about its axis of rotation. In such a case the geophysical characteristic at any fixed station on the earth undergoes a daily variation that depends only on local time (and latitude and season). This simple pattern of daily change may be modified by intrinsic changes in the solar influences on the earth. The harmonic components of the daily variation at any station may in this case undergo phase changes, in some respects corresponding to Doppler shifts of frequency in optical or sonic phenomena. Care is then needed if the results of harmonic analysis are to be properly interpreted. Such interpretation is discussed with reference to the parts Dst and DS of the magnetic storm variations. Like caution must be observed in cases where the amplitude of a harmonic variation changes,with fixed phase.
    • Radio Properties of the Auroral ionosphere, Supplement to Final Report (Phase I)

      Reid, G. C.; Stiltner, E. (Geophysical Institute at the University of Alaska, 1960-02-01)
      The usefulness of the phase-sweep technique in interferometers designed to record radio star signals is discussed. Interferometers of this type have been built for use at frequencies of 223 and 456 Mcs., and their electronic design is explained in some detail. The report also includes a discussion of the automatic data processing system which has been designed to operate in conjunction with the interferometers in the analysis of the amplitude scintillation of radio stars.
    • An evaluation of auroral all-sky camera observations

      Davis, T. N.; Deehr, C. S.; Leinbach, H. (Geophysical Institute of the University of Alaska, 1960-03)
      From photometric, all-sky camera, and visual observations of a moderate auroral display, it is found that the all-sky camera compares favorably with the visual observer in detecting and recording auroral forms. The visual observer can make instantaneous observations and so can detect rapid changes and auroral forms lasting only a few seconds, whereas the poorer time resolution of the all-sky camera prevents it from recording very short-lived phenonema. However, the ability of the all-sky camera to accurately record the shape and intensity of the majority of auroral forms allows it to yield more precise and complete information about these aspects of auroral morphology than is normally obtained through visual observation.
    • Experiment Luxembourg, Scientific Report No. 3

      Rumi, G. C. (Geophysical Institute at the University of Alaska, 1960-02)
    • Radiation Information from 1958 δ2

      Basler, R. P.; DeWitt, R. N.; Reid, G. C. (Geophysical Institute at the University of Alaska, 1960-01)
      The telemetered radiation information from the satellite 1958 δ2 (Sputnik III) has been analyzed for sixty-two separate passes recorded in College, Alaska. The data indicate a dependence of radiation intensity on altitude in the range 250-500 km. Both the high and low energy components apparently contribute to the overall increase of intensity with altitude, but the presence of a continuous afterglow in the scintillating crystal prevented detailed interpretation of the results.
    • Incidence of Auroras and Their North-South Motions in the Northern Auroral Zone

      Davis, T. Neil; Kimball, Donald S. (Geophysical Institute at the University of Alaska, 1960-01)
      Studies of the incidence of auroral forms and their north and south motions are made by using a close-spaced array of all-sky cameras located in the northern auroral zone at the approximate geomagnetic longitude 250°E. It is found that during the observing season 1957-58 the peak of the average auroral zone occurred at 66-67° geomagnetic latitude. Although the southern extent of auroras retreats northward after local magnetic midnight, the southward motion of the individual forms, observed at the southern edge of the auroral zone, predominates over the northward motion throughout most of the night. The data indicate the existence on any given night of a latitude position near which many auroral forms occur. The first motion of auroras incident north of this position tends to be northward, and the first motion of auroras incident south of this position tends to be southward. A curve showing the occurrence of auroral forms peaks at, and is nearly symmetrical about, local geographic midnight, but the intensity of auroral emissions measured over the celestial hemisphere remains at a high level after midnight.
    • Air Force Contract No. AF 19(604)-1089

      Geophysical Institute at the University of Alaska, 1954
      This report briefly describes the progress made at the Geophysical Institute of the University of Alaska during the past three months, in the study of arctic radio wave propagation under the Air Force Contract No. AF 19(604)-1089.
    • The measurement of ionospheric absorption using extraterrestrial radio waves

      Little, C. G. (Geophysical Institute at the University of Alaska, 1957-01-28)
      Introduction: The discovery by Jansky in 1932 of the presence of radio waves incident upon the earth from outer space has led to several new methods of studying the earth's upper atmosphere. This report describes the manner in which these extraterrestrial radio waves may be used to measure the radio absorption characteristics of the ionosphere. It opens with a brief discussion of the theory of ionospheric absorption; this is followed by a description of the basic principles involved in this new technique. Two different types of equipment which may be used for this type of absorption measurement are then discussed. The report concludes with a brief summary of three types of ionospheric absorption phenomena which have been studied at various latitudes with such equipments.
    • Radio wave propagation in the arctic

      Little, C.G.; Dyce, R.B.; Hessler, V. P.; Leonard, R. S.; Owren, L.; Roof, R. B.; Sugiura, M.; Swenson, G. W. Jr (Geophysical Institute at the University of Alaska, 1956)
      The main body of this report is divided into eight sections, corresponding to the eight aspects of Arctic radio wave propagation listed in Section I, Purposes of the Contract, In cases where the work has already been fully described in Interim Scientific Report No. 1 (AFCRC-TN- 55-579, here in after referred to as R(l)), brief summaries only are given. The progress in these eight fields is summarized as follows: Task No. 1 Sweep-Frequency Ionospheric Backscatter Because of lack of equipment, no progress was made on this task. Task No. 2 Auroral and Meteor Echoes Three frequencies were used in this work: (a) At 50 me A low“power9 50 me radar equipments specially designed and built for auroral radar research, was operated with a steerable antenna to monitor both auroral and meteor activity,, The results showed that the diurnal distribution of meteor activity is similar at College to that observed elsewhere, and that the meteor echo rates observed on this equipment are not affected by the presence of aurora. (b) At 106 me The 106 me SCR 270 DA radar was used for two main experiments, as described in R(l). First, the aspect sensitivity of the auroral echoes was investigated. The results showed clearly that the auroral ionization giving rise to VHF auroral radar echoes is aligned along the earth's magnetic lines of force3 in that the auroral radar echoes are strongest when the radio waves are traveling perpendicularly to the magnetic lines of force through the aurora. Second, the relationship between visual and radar aurora was investigated; this work showed that the auroral radar echoes are often closely associated in range and azimuth with visual aurora, although the strength of the echoes is not proportional to the visual brightness of the auroral forms„ (c) At 210 me The 210 me SA-2 radar was installed in a trailer and tested without modification, It was then modified by the building of a steerable 16-Yagi array, by increasing the pulse length, and by reducing the receiver bandwidth„ Simultaneous operation of the 50 me and the improved 210 me equipment resulted in the detection of many auroral echoes at the lower frequency; no auroral (or meteor) echoes were obtained on the 210 me equipment during the contract period although good mountain * echoes were obtained at ranges up to 250 km„ Task No„ 3 Investigation of Microwave Link As explained in R(l), the experimental observations carried out on this link showed the absence of significant tropospheric refraction effects, and the work was terminated at the end of the first year of the contract. Task No. 4 Prediction of Auroral and Ionospheric Storms Several types of work were undertaken in order to improve our understanding of auroral and ionospheric storms; these storms are two aspects of the bombardment of the upper atmosphere by particles from the sun, In particular, a solar radio interferometer was set up to monitor the solar radio emissions at 65 me As described in R(l), an all sky camera and a photoelectric photometer were developed for the monitoring of the visual auroral activity. An investigation of earth potentials has shown that they provide a simple method of monitoring magnetic activity; some tests were also made using a rapid-response electronic magnetometer. Some of the results obtained with these equipments are discussed in the report. A study of the form of the front surface of a neutral corpuscular stream advancing into a magnetic field similar to the earth's magnetic field is presented. This study shows the presence of equatorial and polar forbidden zones and the fact that only the particles arriving near the border between these forbidden zones can reach the earth's upper atmosphere. An equatorial motion of the zone of bombardment could be produced by an increase either in particle density or in particle velocity. Task No. 5 Whistlers A new type of whistler has been discovered that has simultaneous rising and descending components. Analysis of data obtained during the contract period indicates a diurnal variation in the rate of occurrence of whistlers that appears to be correlated with ionospheric heights. A correlation between the day-to- day occurrence of the dawn chorus and the daily K-index sums is also found. Task No. 6 Diffraction and Scatter of Radio Waves by Mountains (a) Diffraction The diffraction of VHF radio waves by mountains has been investigated over three diffraction paths. The results show that the experimentally observed signal strengths are in fair agreement with the values calculated theoretically using knife-edge approximations. One important observation, which has not been reported previously, is the variability of the diffracted signal strength from point to point across the ground. Also, although diffracted signals are normally described as being very constant in amplitude, slow fades lasting some hours and occurring over a relatively narrow frequency band were observed over one 200- mile path. (b) Mountain scatter Observations of mountain scatter were made using the SCR-270 DA radar and a mobile receiving equipment. The results imply that detectable scattered signals can be obtained over a very wide range of azimuths (greater than + 135°) relative to the line joining the transmitter and the mountain. It was found that the scattered signals were considerably broadened in pulse length.Task No. 7 Ionospheric Absorption The work done in connection with ionospheric absorption under this contract has been described previously in R(l). Undertaken at the request of the 58th Weather Reconnaissance Squadron, USAF, this study demonstrated convincingly that their communication failures were caused by ionospheric absorption phenomena, rather than by equipment or personnel failures. Task No. 8 Assistance to the Alaska Air Command on Problems of Radio Propagation As described in R(l), an investigation of a VHF radio link was made at the request of the Alaska Air Command. Continuous records of received signal strength at each end of the link revealed that the communication failures were caused by tropospheric refraction effects. A low-noise preamplifier, built and operated in parallel with a normal equipment, was found to reduce the number of fade-outs. A one-day symposium on Arctic radio wave propagation was held at the Geophysical Institute on January 26, 1956, for the benefit of communications personnel in the territory,, Approximately fifty visitors attended these meetings. The Geophysical Institute has also assisted the Alaska Air Command by the loan of electronic equipment and pen recorders as well as by supplying specialists who have acted in an advisory capacity on problems of radio wave propagation.