An Investigation of Whistlers and Chorus at High Latitudes Item Type Report Authors Pope, J. H. Publisher Geophysical Institute at the University of Alaska Download date 02/10/2021 10:35:07 Link to Item http://hdl.handle.net/11122/3562 ' { g l f f i U NO. ADJuj UNIVERSITY □ F ALASKA COLLEGE ALASKA UAG R$Q Scientific Report No. 4 AF 19(604)~1859 April, 1959 AM INVESTIGATION OF WHISTLERS AND CHORUS AT HIGH LATITUDES by J . H u Pope The research reported in this document has been sponsored by the Electronic Research Directorate of the Air Force Cambridge Respavcli Ce*»ter, Air Research and Develcpment Command. AFCRC-TN-355 916522 AST1A DOCUMENT NO. a d 2 1 6 i> ^ GEOPHYSICAL INSTITUTE of the UNIVERSITY OF ALASKA Scientific Report No. 4 Air Force Research Contract No. AF 19(604)-1859 AN INVESTIGATION OF WHISTLERS AND CHORUS AT HIGH LATITUDES by J. H. Pope Air Force Cambridge Research Center Air Research and Development Command The research reported in this document has been sponsored by the Electronic Research Directorate of the Air Force Cambridge Research Center, Air Research and Development Command, Date Submitted: Approved by; April, 1959 C. T. Elvey, Director Geophysical Institute TABLE OF CONTENTS Page Figures ii Abstract iii Introduction 1 Instrumentation 6 Observations 9 Analysis - Whistlers 11 Analysis - Chorus 20 Discussion 35 Acknowledgement 37 References 33 i LIST OF FIGURES Fig. Page 1. The Paths of Whistler Propagation 4 2. A Typical Nose Whistler Recorded at College, Alaska 5 3. Block Diagram of the Whistler Receiver 8 4. Block Diagram of the Monitor Station 8 5. Sensitivity Curve of the Preamplifier 8 6. Diurnal Variation of Whistler Occurrence at College 16 7. Average Daily Occurrence of Whistlers at College 17 8. Seasonal Variation in Occurrence of Whistlers at College 17 9. Sunlight Curve 18 10. Correlation of Daily Whistler Occurrence vs. Daily K index Sums for College by Month 19 11. Diurnal Variation of Chorus Indexes 28 12. Time of Diurnal Maximum as a Function of Geomagnetic Latitude 29 13. Primary and Secondary Diurnal Maxima vs. Month 30 14. Seasonal Variation in Occurrence of Chorus 31 15. Correlation Coefficients for Daily Chorus Indexes for College vs. Daily K Index Sums for College (A) and Daily Chorus Indexes for Washington, D.C. vs* Daily K Sums for Fredericksburg, (B). 32 16. Diurnal Variation of the Coefficient of Correlation for Chorus Indexes vs. K-Indexes 33 17. Occurrence of f 34 18. Diurnal Variation of f 34 ii ABSTRACT The whistlers and chorus received at College, Alaska during the period from December 1955 through March 1958 are studied particularly with respect to temporal variations. The diurnal curves for whistler activity show maxima after midnight local time while the seasonal variation peaks during the winter. It appears that these variations in whistler activity are in part explainable in terms of very low frequency propagation conditions. The diurnal variation of chorus shows a maximum at about 1400 hours local time. By the use of data from lower latitude stations a dependence of this time of diurnal maximum on the geomagnetic latitude of the station is shown. The coefficients of correlation for chorus activity versus magnetic activity were determined on a monthly basis. A seasonal variation in these correlations is indicated which appears to be unique for the geomagnetic latitude of College. A preliminary statistical study of one of the more easily measured characteristics of chorus is discussed. The characteristic chosen is the mid-frequency in an element of chorus. A diurnal variation in this parameter is indicated. iii INTRODUCTION It is known that a variety of natural phenomena occur in the audio frequency portion of the electromagnetic spectrum which may be observed by means of a high gain audio amplifier and an antenna. They have been called "audio atmospherics." One type of audio atmospheric may appear as a descending tone starting perhaps above 10 kc and ending at about 1 kc one or two seconds later. These descending tones, known as "whistlers" or "whistling atmospherics" have been the subject of much investiation during recent years. Whistlers were first reported by Barkhausen^^ in 1919 and a plausible theory of their nature was presented by Storey in 1952(2). According to Storey's theory, whistlers are originated by lightning discharges. Part of the radiation of the discharge may propagate in a longitudinal mode along the geomagnetic lines of force to the magnetic conjugate point of the opposite hemisphere (Fig. 1). Some of the energy is reflected to travel back along the same path. This process may be repeated several times resulting in "echoes". There can be no propagation unless an electron density of several hundred electrons per cubic centi­ meter exists everywhere along the path. Thus the wave pocket originating from the discharge propagates a long distance through a dispersive medium, where the group velocity and hence the time of propagation depends on the frequency. Since the length of the geomagnetic lines of force is a function of the observer's latitude, it might be supposed that the characteristics of whistlers change with latitude. Until 1955 whistlers had been observed only in intermediate latitudes, with a highest latitude of observation of 58°, and it was clearly important to ascertain (a) whether whistlers occurred in high latitudes and in the affirmative case (b) how their char­ acteristics differed from those observed in intermediate latitudes. Con­ sequently, a whistler recording station was established at College (geo­ magnetic latitude 64.5°) in 1955 to seek the answers to these questions. The first question about the occurrence could soon be given an affirmative answer. Spectral analysis at Stanford University of some examples of whistlers which were recorded at College on July 10, 1955 revealed a new and unusual property of the high latitude whistlers. As noted above, the usual whistler begins at some frequency and descends in tone. The high latitude whistlers begin at some intermediate frequency (3-4 kc) and both rise and descend in tone. As plotted by an audio spectrograph, the shape is approximately a parabola with its axis aligned along the time coordinate; hence the name "nose" whistler (Fig. 2). On the basis of these results (3) a theory was developed by a research group at Stanford University and independently by E l l i s ^ which indicates that an approximation made by Eckersley in the development of his dispersion theory is not sufficient for high latitude whistlers. The corrected dispersion theory permits one to infer that the frequency of maximum group velocity is of the order of one quarter of the gyromag- netic frequency along the path. The magnetic field lines at the latitude of College, Alaska causes the disturbance to propagate to a point about 30,000 km from the earth. At this distance the magnetic field strength is such that the gyromagnetic frequency is of the order 12 kc. Thus the fre­ quency of maximum group velocity is about 3 kc. This theory predicts that 2 the frequency of maximum velocity will be at about 30 kc for some of the low latitude whistler stations. Other atmospherics such as the "chorus", which are received on the same equipment can also be studied. Chorus can be described as a multitude of quickly rising tones. This phenomenon has been referred to as "dawn chorus" for historical reasons but the word "dawn" conveys no information and is in fact misleading since whether it occurs at dawn or not has been shown to depend on the geomagnetic latitude of the station. Thus it is probably desirable to use only the word "chorus", although objections have been tnade.^^ The origin of this phenomenon is unknown but there is evi­ dence which indicates a connection with solar particle activity. A syste­ matic study of chorus in high latitudes should contribute to a theory of origin. 3 Fig . 1. The Paths of Whistler Propagation. o - r I TIME IN SECONDS FREQUENCY FREQUENCY IN KC. TIME IN SECONDS Fig. 2. A Typical Nose Whistler Recorded at College, Alaska. INSTRUMENTATION Fig. 3 is a block diagram showing the whistler receiver. The signal is received on a loop antenna in the form of a triangle having a height of 30 feet and a base of 60 feet. Since the loop has only one turn, it is necessary to use a specially built transformer to couple the very low impe­ dance of the loop to the preamplifier. The preamplifier, which has a volt­ age gain of 120 db, is connected through a 1,000 ft. cable to a tape re­ corder. A programming unit is provided to turn the detector on at pre­ arranged intervals. This unit is capable of turning the device on each hour for an integral number of minutes, and has generally been set for one minute to give an hourly sample but after June 1957 the interval has been changed to two minutes to conform to IGY standards. This sampling tech­ nique permits an estimate of the diurnal variation and makes it possible to undertake correlation studies with other phenomena such as magnetic activity. The bandwidth of the system is essentially determined by the frequency response of the preamplifier which is shown in Fig. 5. At times high pass filters have been used to eliminate a.c. harmonics as high as 2 kc. To obtain the data from the tapes a monitor station has been set up. This monitor station is provided with a tape recorder and an adjustable filter to reduce interfering power line harmonics. A second tape recorder is used to copy interesting events. For frequency versus time analysis of an event a sound spectrograph is provided.