V.L.F. Emissions and Geomagnetic Disturbances at the Auroral Zone By Hachiroe ToKUDA Geophysical Institute, Kyoto University (Read, November 22, 1961; Received Feb. 28, 1962) Abstract Some studies of the relation between the chorus activity and geomagnetic activity have shown that the strength or the occurrence of chorus is maximum at the auroral zone on days of moderate geomagnetic activity, and that the region of maximum strength or occurrence shifts toward lower latitude on stormy days. In this paper we have examined a relation between the chorus and geomagnetic conditions at the auroral zone in details, using the chorus indices, normal-run magnetograms, and the auroral echoe indices observed at College, Alaska from August 1959 to December 1960. The result has indicated that most of the intensity increases of chorus occurred at magnetically quiet times and followed the negative bay-type magnetic disturbances. Moreover, a good correlation has been found between the magnitudes of the preceding bay-type disturbances and the rising times of strength increases of choruses. Within recent few years, considerable 1. Introduction interest has been aroused in the audio-frequency phenomenon called 'dawn chorus', consisting of rising tone, contrary to the descending tone of whistler. The correlation between the chorus activity and geomagnetic activity has been reported by Storey (1953) and Allcock (1957). Storey (1953) also found the chorus intensity varying greatly throughout the day, reaching a peak around 6 a.m. at Cambridge in England and called this phenomenon 'dawn chorus' but now the word 'dawn' has not any significance since Allcock (1957) and Pope (1957), (1960) found the latitude effect on- the diurnal maximum of chorus activity. Basing on these data, Allcock has shown that the chorus may be generated directly or indirectly by streams or bunches of high speed charged particles precipitating into the ionized exosphere in the presence of earth's magnetic field. As to the generation mechanism of V.L.F. noise in the exosphere, Gallet and Helliwell (1956, 1959) have proposed the amplification mechanism similiar to a travelling wave tube, basing on the postulation that the longitudinal interaction of the electromagnetic wave with a beam (33)
34 H. TOKUDA of ionized particles would bring about an amplification of the electromagnetic wave, provided the average velocity V of the incoming beam approximates to the phase velocity vp, of the electromagnetic wave, and assumed the input signal to be provided by themal radiation or whistler. Ellis (1957) and Ondoh (1961) have supported the Cherenkov radiation as the seed of chorus and MacArthur (1959) has suggested that chorus may be due to Dopplershifted cyclotron radiation from protons. Maeda and Kimura (1961) have theoretically treated the amplification due to the transverse interaction of the electromagnetic wave with a cyclotron mode of a proton beam in place of electron beam. Concerning the relation with magnetic activity, Crouchley and Brice (1960) and Yoshida (1961) have found that the average strength or occurrence probabilities of chorus increases with increasing value of K-index at the low latitude stations and shows a maximum at moderate value of K-index in the auroral zone. Ondoh et. al. (1961) has found that the strength of chorus has a negative correlation to that of auroral echoes during the geomagnetic storm, using the data at College in Alaska. Ellis (1961) has found, from the study of V.L.F. noise of 5Kc/s at Camden located at middle geo- magnetic storms and that a half of the noise bursts occurred simultaneously with positive bays. In this paper, we bring out 'chorus burst' defined by a sudden increase of hourly chorus indices in strength and examine its relation to geomagnetic disturbance and to auroral activity using chorus indices, auroral radar echoe indices and normal-run (1) Definition of chorus burst 2. Analysis of chorus burst Now we give the definition of 'chorus burst' with the integer index of chorus strength assigned from 0 to 5 for each hour. When the chorus index increases up to 5 from lower values, we name this increase, 'chorus burst'. Most of bursts cannot keep the maximum value no more than a few hours, but sometimes the value 5 continues more than ten hours. Naturally the chorus index depending on a subjective determination by a monitor might not be so distinct (Pope 1960) and may be affected more or less by the absorption through the ionosphere. Therefore we may lose some bursts whose peaks are not assigned for the value of 5 owing to above reasons. However, we do not adopt airy peak falling short of 5 to avoid inaccuracy. We used the data from August 1959 to December 1960, for which 94 bursts were found. The seasonal variation of the occurrence is shown in Table 1, which looks pretty different from the seasonal variation of the whole chorus activity reported by Pope (1961), in which maximum occurs in December. It seems also unlikely that no
V.L.F. Emissions and Geomagnetic Disturbances at the Auroral Zone 35 bursts occurred after August 1960 and that the chorus indices never got to even 4 after October 1960. However, we would not discuss about this question because this is not our object. The general feature of the chorus burst is shown in Fig. 1. For a few bursts the Table 1 The seasonal variation of the occurrence of bursts Fig. 1. An example of Chorus Burst and Preceding Magnetic Disturbance Fig. 2. Diurnal Variation of the Occurrence of Chorus Burst
36 H. TOKUDA indices get to the peak rapidly, but for more than 80% of bursts the rising remains stationary at the values of 2, 3, or 4 for 1 to 5 hours and then runs up to the peak rapidly as shown in Fig. 1. Now we name these codes ta,tb,tc, and td, where to is the time of commencement of the rising, tb is the time of the second commencement, tc is the time when the chorus index reaches up to 5, and td is the starting time of decrease. Diurnal variations of to and tc are shown in Fig. 2, which indicates that 65% of 89 bursts get to the maximum value in intensity from 9h00m to 12h00m L.T., which seems to appear a little earlier than the maximum around 14h00m for the average chorus activity reported by Pope (1960). According to Pope, secondary peaks in the morning appear to be significant during the winter months, and drift to the main peaks month by month, and in July these two peaks are merged by a broad maximum. From the fact that more than 60% of our bursts were observed from Feburary to May as shown in Table 1, these bursts occurred at the time corresponding to the secondary peaks in these months. (2) Association of chorus burst with magnetic disturbance Comparing magnetograms with the chorus burst, we have found that 89 bursts were preceded by negative 'bays' in H component. Four bursts did not follow any magnetic disturbance and only one burst followed a positive 'bay'. The depths of these bays, the deviations of H component from interpolated curves connecting the one of the middle geomagnetic latitude stations and we have found that 64 disturbances in all 89 cases were observed also at Tucson as positive bays. As 30% of these preceding negative disturbances were not worldwidely typical bays, we describe them as 'negative bays' with an apostrophe in this paper. No chorus bursts except one associated with Sc were found in our data. To examine the relation between chorus bursts and preceding 'bays' more closely, we now name codes, tl, tm and tn, where tl is the time of beginning of a bay, tm is the time of its maximum decrease, and tn is the time of its ending. It is interesting that commencement of the chorus burst, ta coincides with the 'bay' phase in nearly all cases, as shown in Fig. 1 and Fig. 3. To investigate the accuracy of this coincidence, the time-lags of to after tm have been measured for 85 preceding negative bays and the result is shown in Fig. 4, which indicates that roughly 50% of ta are closing around tm. In 7 cases tq is behind the ending time of the 'bay', tn, or is simultaneous with tn, but in all cases ta never precedes the beginning time of the bay, tl. Thus it is concluded that most of chorus bursts begin to rise up immediately after the maximum decrease of bay-type magnetic disturbances, which seems to be connected with Ellis' results, in which 8 V.L.F. noise storms were observed in the main phase of magnetic storms. Then, to see how the depth of the preceeing 'bay' affects the character of the
V.L.F. Emissions and Geomagnetic Disturbances at the Auroral zone 37 Fig. 3. Examples of Chorus Bursts and Preceding Magnetic Disturbances
38 H. TUKUDA Fig. 5. Correlation between the magnitude of the preceding 'bay' and the time lag of Tc after Tm chorus burst, a relation between the depths and time-lags of chorus peaks after the maximum decrease of 'bay', tc-tm, is examined as shown in Fig. 5, which indicates that correlation coefficient between them is -0.54 where the significance at 95% is 0.05. Thus we can expect that a chorus burst following a rather big 'bay' can get to its peak quickly and that the absorption in the ionsphere owing to the preceding weak magnetic disturbance has not sufficient effect on the chorus strength, for a big 'bay' causes the much absorption and would prolong the rising time of chorus peak provided the absorption is so effective. There have been some speculation about the origin of chorus connected with auroras, but highly positive correlation between auroras and K-indices found by many researchers suggests such a conjecture unlikely. The aurora indices on an hourly basis are better measures of magnetic activity to use with chorus indices assigned at each hour than three-hourly K-indices. Therefore, we use the index of aurora radar echoes assigned from 0 to 5, to examine some relation between chorus and geomagnetic activity. High values of auroral index may surely indicate the increase of K-index and the penetration of auroral particles. But the auroral index of o may not necessarily deny these phenomena because the limitation of observation in the radar operation, and so the auroral indices do not increase sometimes when magnetic disturbances occur. Thus the correlation between chorus index and auroral index, either of which is higher than 2, is examined in the duration from 12 hours (before ta, to 12 hours) after td. As shown Table 2, the correlation coefficient is -0.62., which corresponds to the fact that most of auroral peaks (do not coincide with chorus peaks) but precede them as shown in Fig. 1 and Fig. 3.
V. L. F. Emissions and Geomagnetic Disturbances at the.auroral Zone 39 Table 2. The correlation between chorus indices and auroral indices around chorus bursts. r=-0.62 r for significance at 95%=0.09 To examine the genenal state of the time-lags of chorus peaks after auroral peaks closely, we selected 26 chorus bursts around which auroral peaks of the index of 4 or 5 appear, and a lag cross-correlogram with delays up to 8 hours between chorus indices from to to td and auroral indices preceding chorus bursts is calculated for 26 chorus bursts, and the result is shown in Table 3. As the time-lags of chosus peaks after auroral peaks take various values and the auroral indices are almost 0 except for the region of its sharp peaks, these correlations are a little weak but the negative correlation with nearly simultaneous aurora indices and positive correlation with the auroral indices preceding 8 hours the chorus bursts have been indicated, which corresponds to the fact that the maximum decrease of the 'bay' precedes a chorus burst averagely 8 hours as shown in Fg. 5. Table 3. The lag correlogram between chorus indices and preceding auroral indices The condition for the coupling between the beam modes of charged particles and the electromagnetic waves of whistler mode is expressed as velocity of charged particles, n, the local refractive indices, c, the velocity of light. (Gallet 1959) The local refractive index is given by where is the plasma frequency, f, the radio wave frequency, electron gyrof requency, N, the electron density, B, the flux density of the geomagnetic field, e, the electronic charge and m, the mass of an electron. (Storey 1953) From the above result, we can deduce that the condition for the coupling is not satisfied along the lines of force intersecting the earth's surface at the auroral zone during the geomagnetic disturbance at the auroral zone, because the local refractive
40 H. TOKUDA, index is affected by the changes of B and N as shown above. In the recovery phase of the magnetic disturbance, however, the charged particles may be accelerated as the result of changing magnetic field in the exosphere and so the local refractive index may satisfy the coupling condition, which increases the chorus strength after the disturbance. By examing the simultaneous chorus data at the middle geomagnetic latitude stations to see whether the low latitude shifting of the maximum occurrence or strength occurs in a magnetic bay-type disturbance, we shall be able to get more sufficient knowledge about the chorus generation. 3. Conclusions Nearly all of the chorus bursts occurred under relatively calm geomagnetic conditions and followed negative bay-type magnetic disturbances at the auroral zone. The magnitudes of the preceding negative bay-type magnetic disturbances have some effect on the rising times of chorus bursts at the auroral zone, that is, the larger the magnitude of the disturbance is, the shorter the rising time of chorus burst is. No chorus bursts except one associated with the sudden commencements were observed at College in Alaska during August, 1959 to December, 1960. Acknowledgements The author wishes to express his hearty thanks to prof. Y. Tamura and Dr. H. Maeda for their kind directions and advices and to prof. K. Maeda, Dr. I. Kitamura and Mr. T. Ondoh for their valuable discussions. References Allcock G.Mck (1957) Australian J. Phys. 10, 286. Ellis G. R. A. (1957) J. Atom. Terr. Phys. 10, 302. Ellis G. R. A. (1960) J. Geophys. Res. 65, 1705. Gallet R. M. (1959) Proc. IRE. 47, 211. Helliwell R. A. (1956) Studies of Low Frequency Propagation. Stanford Univ. MacArthur J. W. (1959) Phys. Rev. Letters 2, 491. Maeda K. and Kimura I. (1961) Rep. Ionos. Space Res. Japan. In press. Ondoh T. (1960) J. Geomag. Geoelec. Kyoto 12, 77. Ondoh T., Kitamura T. and Maeda H. (1961) J. Geomag. Geoelec. Kyoto 12, 115. Pope J. H. (1957) Nature 180, 443. Pope J. H. (1959) Geophys. Inst., Univ. Alaska Publ. AFCR-TN-355. Storey L. R. O. (1953) Phil. Trans. Poy. Soc. A 246, 113. Yoshida S. (1960) Uchusen-Kenkyu 5, 397.