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A quasi-periodic pc 1 micropulsation emission
1786
RESEARCH
Planet. Space Sci. 1972, Vol. 20, pp. 1786 to 1787.
A QUASI-PERIODIC
NOTES
Perxamon Press. p&nted in Northern Ireland
PC 1 MICROPULSATION
EMISSION
(Received infinal form 30 March 1972) Abstract-A pc 1 micropulsation event, recorded shortly after the geomagnetic storm of 3 May 1967, displays a periodicity of spectrographic morphology which is closely analogous to that more commonly exhibited by quasi-periodic VLF emissions. It is suggested that this event is associated with the periodic bunching of the ambient plasma in the geotail, induced by a geotail cavity hy~omagnetic eigenoscillation, and its subsequent injection into the closedfield region of the night-time magnetosphere. The spectral morphology of an undispersed pc 1 hydromagnetic whistler is very similar to that of a VLF periodic emission (Helliwell 1965, 1967) and as a result Jacobs and Watanabe (1967) and Kokubun (1970) use the term periodic emission to describe this class of pc 1 micropulsation event. This note describes a particular pc 1 event which may, by analogy with a VLF quasi-periodic emission (Helliwell, ibid), be regarded as a pc 1 micropulsation ‘quasi-periodic emission’. Figure 1 shows the spectrogram of a pc 1 micropulsation event, recorded by the author in Dunedin, consisting of a set of discrete rising tones which is then repeated with a definite 15 min periodicity. The basic structure. is repeated after 17, 1.5 and 31 min. The tinal interval is, in fact, double the basic period; which implies the existence of a missing structural element. Figure 2 shows this event, in schematic form, superimposed upon a moderately strong 15 min pc which was recorded, some 150 km from Dunedin (Poletti, 1968), straight after the ‘quasi-periodic emission’. The peaks of the pc event are in almost exact time coherence with the structured elements of the ‘quasi-periodic emission’, which implies a common magnetosoheric causal factor for the two events. ‘The g&era1 level of g~ma~etic activity at the time was high, a modemtefy-ever magnetic storm havine ended some 10 hours ~foreh~d. The K., sum for the precedii~ 24 hr is 39. C&tinuous spectrographic records, covering-a total of 23*months_between February 1967 and July 1969, have been carefully searched for similar ‘quasi-periodic emission’ events; without success, It is still possible that there is a tendency for these events to occur only during, or shortly after, magnetic storms; since the spectrograms are normally heavily masked by strong, wide-band and unstructured hydromagnetic noise at these times. The local onset time for the ‘quasi-periodic emission’ was ~1945 hr which, for the prevailing level of geomagnetic activity, is approximately the time when the western edge of the neutral sheet recombination slot passes over the geomagnetic meridian through Dunedin (Offen, 1970). This fact combined with the very definite 15 min structure periodicity of the ‘quasi-periodic emission’, and the immediately following PC, provides a clue to the origin of this particular event. It is highly probable that moderately severe geomagnetic disturbances are capable of exciting resonant hydromagnetic oscillations within the quasi-cylindrical geotai! cavity. Such oscillations could then bunch the ambient geotail plasma and, concomitantly, periodically inject these bunches out of the neutral sheet ~mbination slot into the night-side closed region of the magnetosphere. The protons in these injected pfasma bunches, if they possess suf&ient pitch angle anisotropy, will be unstable with respect to the proton cyclotron instability (Kennel and Petschek, 1966; Liimohn, 1967) and will thus radiate into the LH shear Ah%& mode. This is the normal propagation mode for pc 1 hydromagnetic whistlers; events which the discrete rising tones of the repeated structural elements strongly resemble. The fact that only a few risi?g tones are observed in each ‘burst’ is to be expected, since the spatially small bunch of injected plasma ~111 soon cease to be radiation unstable as it diffuses away along the geomagnetic field. Studies of possible eigenmodes of the geotail cavity, utilising typical tail parameters, have been conducted by: McClay and Radoski (1967), who propose pure TE and TM modes with eigenperiods of 20-35 min; Pate1 (1968a), who predicts mixed transverse and compressional modes with eigenperiods of 24-32 min; and Siscoe (1969), who fluds that the geotai1 can support both symmetrical and asymmetrical compressional modes with eigenperiods of 2 and 11 min respectively. Pate1 (1968b) gives an example of a ground recorded 10-12 min pc that correlates we11with an si detected by IMP 1 in the geotail at a geocentric distance of ~17 R,. He (ibid.) also finds definite quasi-periodicities, of ~12-15 min, in the geotail field during si events. These observed 12-15 min geotail field periodicities are almost the same as the structure periodicity of the ‘quasi-periodic emission’ under discussion. It must be assumed then, that the relevant geotail eigenmode is the fundamental asymmetric compressional mode or a first harmonic of a TE, TM, or mixed, mode
22:: 2.0
I" \ Lc
l-6 l-2 0.8 0.4 0
I
1
0830
0730 UT
F1c.1. A SPECTROGRAM OF THE pcl ‘QUASI-PERIODIC EMISSION' RECORDED AT DUNEDIN (45.8”S, 170S”E, GEOGRAPHIC; -53.3” (L = 2.80), 252.7”, GEOMAGNETIC) ON 4 MAY 1967.
1786
RESEARCH
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I
I
1787
NOTES
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2100
2000
I 2200
NZSLP THE ‘QUASI-PERIODIC EMISSION’ EVENT OF 4 MAY A WNG PERIOD pC RFCORDED IMMEDIATELY AFTERWARDS. THE EMISSION WAS RF.CORDED AT DUNEDIN WHRST THE pc WAS RECORDED ~150 km SOUTH-WEST OF DUNEDIN.
FIG. 2. A SCHEMATIC COMPARISON
1967
BETWEEN
AND
The 15 min pc shown in Fig. 2 is then probably a ground-level manifestation of the geotail oscillations which had previously produced the plasma bunches responsible for the ‘qua+periodic emission’. Thus it appears that the micropulsation ‘quasi-periodic emission’ under discussion can be adequately explained; though it is indeed a pity that these interesting events are not as common as their VLF counterparts. R. J. OFFEN Department of Physics University of Otago, Dunedin, New Zealand. REFERENCES HELLIWELL,R. A. (1965). Whistlers and Related Ionospheric Phenomena (Stanford University Press). HELLIWELL, R. A. (1967). J. geophys. Res. 72,4773. KOKUBUN, S. (1970). Rept. Zonosph. Space Res. Japan 24,24. JACOBS, J. A. and WATANABE,T. (1967). Planet. Space Sci. 15,799. KENNEL, C. F. and PETSCHBK, H. E., AVCO Res. Rep. No. 259, 1966. LIEMOHN,H. B. (1967). J.geophys. Res. 72, 39. MCCLAY, J. F. and RADOSKI, H. R. (1967). J. geophys. Res. 72,452s. OPFEN,R. J. (1970). High Frequency Geomagnetic Micropulsations, Ph.D. Thesis, Otago University. PATEL, V. L. (1968a). Physics Letts 26, 596. PATEL, V. L. (1968b). J. geophys. Res. 73, 3407. POLEXTI,M. J. (1968). Private communication. SISCOE,G. L. (1969). J.geophys. Res. 74, 6482.
Planet.SpaceSci. 1972, Vol. 20, pp. 1787-1790. Pcroamon Pms.
COLLISIONLESS
PLASMA
Printed in Northern Ireland
FLOW OVER A CONDUCTING
SPHERE
(Received 10 March 1972) The complexity of phenomena involved in the interaction between a spacecraft and its environmental space plasma are well recognized. Attempts to study aspects of these phenomena and effects were undertaken in the last decade mainly from the theoretical point of view (a detailed review including an extensive bibliography is given by Gurevich et al.“‘) However, the present situation is still controversial in various respects. There is some doubt as to the relative importance of various principal plasma parameters in defining the characteristics of the disturbed zone around satellites, and there is also some controversy as to the signilican~ and reality of assumptions, both physical and mathematical, used in the numerous theories. Attempts to study the disturbed zone via in situ observations were also made by Samir”) but these observations are a priori bound to be fragmentary in nature. No space mission has ever been launched with wake studies as the principal experimental objective. The available in situ observations consist of studies made by using different plasma probes namely, planar and cylindrical Langmuir probes and multigrid retarding potential analyzer. The data was obtained from satellites of a variety of shapes and sizes which
RESEARCH
Planet. Space Sci. 1972, Vol. 20, pp. 1786 to 1787.
A QUASI-PERIODIC
NOTES
Perxamon Press. p&nted in Northern Ireland
PC 1 MICROPULSATION
EMISSION
(Received infinal form 30 March 1972) Abstract-A pc 1 micropulsation event, recorded shortly after the geomagnetic storm of 3 May 1967, displays a periodicity of spectrographic morphology which is closely analogous to that more commonly exhibited by quasi-periodic VLF emissions. It is suggested that this event is associated with the periodic bunching of the ambient plasma in the geotail, induced by a geotail cavity hy~omagnetic eigenoscillation, and its subsequent injection into the closedfield region of the night-time magnetosphere. The spectral morphology of an undispersed pc 1 hydromagnetic whistler is very similar to that of a VLF periodic emission (Helliwell 1965, 1967) and as a result Jacobs and Watanabe (1967) and Kokubun (1970) use the term periodic emission to describe this class of pc 1 micropulsation event. This note describes a particular pc 1 event which may, by analogy with a VLF quasi-periodic emission (Helliwell, ibid), be regarded as a pc 1 micropulsation ‘quasi-periodic emission’. Figure 1 shows the spectrogram of a pc 1 micropulsation event, recorded by the author in Dunedin, consisting of a set of discrete rising tones which is then repeated with a definite 15 min periodicity. The basic structure. is repeated after 17, 1.5 and 31 min. The tinal interval is, in fact, double the basic period; which implies the existence of a missing structural element. Figure 2 shows this event, in schematic form, superimposed upon a moderately strong 15 min pc which was recorded, some 150 km from Dunedin (Poletti, 1968), straight after the ‘quasi-periodic emission’. The peaks of the pc event are in almost exact time coherence with the structured elements of the ‘quasi-periodic emission’, which implies a common magnetosoheric causal factor for the two events. ‘The g&era1 level of g~ma~etic activity at the time was high, a modemtefy-ever magnetic storm havine ended some 10 hours ~foreh~d. The K., sum for the precedii~ 24 hr is 39. C&tinuous spectrographic records, covering-a total of 23*months_between February 1967 and July 1969, have been carefully searched for similar ‘quasi-periodic emission’ events; without success, It is still possible that there is a tendency for these events to occur only during, or shortly after, magnetic storms; since the spectrograms are normally heavily masked by strong, wide-band and unstructured hydromagnetic noise at these times. The local onset time for the ‘quasi-periodic emission’ was ~1945 hr which, for the prevailing level of geomagnetic activity, is approximately the time when the western edge of the neutral sheet recombination slot passes over the geomagnetic meridian through Dunedin (Offen, 1970). This fact combined with the very definite 15 min structure periodicity of the ‘quasi-periodic emission’, and the immediately following PC, provides a clue to the origin of this particular event. It is highly probable that moderately severe geomagnetic disturbances are capable of exciting resonant hydromagnetic oscillations within the quasi-cylindrical geotai! cavity. Such oscillations could then bunch the ambient geotail plasma and, concomitantly, periodically inject these bunches out of the neutral sheet ~mbination slot into the night-side closed region of the magnetosphere. The protons in these injected pfasma bunches, if they possess suf&ient pitch angle anisotropy, will be unstable with respect to the proton cyclotron instability (Kennel and Petschek, 1966; Liimohn, 1967) and will thus radiate into the LH shear Ah%& mode. This is the normal propagation mode for pc 1 hydromagnetic whistlers; events which the discrete rising tones of the repeated structural elements strongly resemble. The fact that only a few risi?g tones are observed in each ‘burst’ is to be expected, since the spatially small bunch of injected plasma ~111 soon cease to be radiation unstable as it diffuses away along the geomagnetic field. Studies of possible eigenmodes of the geotail cavity, utilising typical tail parameters, have been conducted by: McClay and Radoski (1967), who propose pure TE and TM modes with eigenperiods of 20-35 min; Pate1 (1968a), who predicts mixed transverse and compressional modes with eigenperiods of 24-32 min; and Siscoe (1969), who fluds that the geotai1 can support both symmetrical and asymmetrical compressional modes with eigenperiods of 2 and 11 min respectively. Pate1 (1968b) gives an example of a ground recorded 10-12 min pc that correlates we11with an si detected by IMP 1 in the geotail at a geocentric distance of ~17 R,. He (ibid.) also finds definite quasi-periodicities, of ~12-15 min, in the geotail field during si events. These observed 12-15 min geotail field periodicities are almost the same as the structure periodicity of the ‘quasi-periodic emission’ under discussion. It must be assumed then, that the relevant geotail eigenmode is the fundamental asymmetric compressional mode or a first harmonic of a TE, TM, or mixed, mode
22:: 2.0
I" \ Lc
l-6 l-2 0.8 0.4 0
I
1
0830
0730 UT
F1c.1. A SPECTROGRAM OF THE pcl ‘QUASI-PERIODIC EMISSION' RECORDED AT DUNEDIN (45.8”S, 170S”E, GEOGRAPHIC; -53.3” (L = 2.80), 252.7”, GEOMAGNETIC) ON 4 MAY 1967.
1786
RESEARCH
I
I
I
I
I
I
1787
NOTES
I
I
I
2100
2000
I 2200
NZSLP THE ‘QUASI-PERIODIC EMISSION’ EVENT OF 4 MAY A WNG PERIOD pC RFCORDED IMMEDIATELY AFTERWARDS. THE EMISSION WAS RF.CORDED AT DUNEDIN WHRST THE pc WAS RECORDED ~150 km SOUTH-WEST OF DUNEDIN.
FIG. 2. A SCHEMATIC COMPARISON
1967
BETWEEN
AND
The 15 min pc shown in Fig. 2 is then probably a ground-level manifestation of the geotail oscillations which had previously produced the plasma bunches responsible for the ‘qua+periodic emission’. Thus it appears that the micropulsation ‘quasi-periodic emission’ under discussion can be adequately explained; though it is indeed a pity that these interesting events are not as common as their VLF counterparts. R. J. OFFEN Department of Physics University of Otago, Dunedin, New Zealand. REFERENCES HELLIWELL,R. A. (1965). Whistlers and Related Ionospheric Phenomena (Stanford University Press). HELLIWELL, R. A. (1967). J. geophys. Res. 72,4773. KOKUBUN, S. (1970). Rept. Zonosph. Space Res. Japan 24,24. JACOBS, J. A. and WATANABE,T. (1967). Planet. Space Sci. 15,799. KENNEL, C. F. and PETSCHBK, H. E., AVCO Res. Rep. No. 259, 1966. LIEMOHN,H. B. (1967). J.geophys. Res. 72, 39. MCCLAY, J. F. and RADOSKI, H. R. (1967). J. geophys. Res. 72,452s. OPFEN,R. J. (1970). High Frequency Geomagnetic Micropulsations, Ph.D. Thesis, Otago University. PATEL, V. L. (1968a). Physics Letts 26, 596. PATEL, V. L. (1968b). J. geophys. Res. 73, 3407. POLEXTI,M. J. (1968). Private communication. SISCOE,G. L. (1969). J.geophys. Res. 74, 6482.
Planet.SpaceSci. 1972, Vol. 20, pp. 1787-1790. Pcroamon Pms.
COLLISIONLESS
PLASMA
Printed in Northern Ireland
FLOW OVER A CONDUCTING
SPHERE
(Received 10 March 1972) The complexity of phenomena involved in the interaction between a spacecraft and its environmental space plasma are well recognized. Attempts to study aspects of these phenomena and effects were undertaken in the last decade mainly from the theoretical point of view (a detailed review including an extensive bibliography is given by Gurevich et al.“‘) However, the present situation is still controversial in various respects. There is some doubt as to the relative importance of various principal plasma parameters in defining the characteristics of the disturbed zone around satellites, and there is also some controversy as to the signilican~ and reality of assumptions, both physical and mathematical, used in the numerous theories. Attempts to study the disturbed zone via in situ observations were also made by Samir”) but these observations are a priori bound to be fragmentary in nature. No space mission has ever been launched with wake studies as the principal experimental objective. The available in situ observations consist of studies made by using different plasma probes namely, planar and cylindrical Langmuir probes and multigrid retarding potential analyzer. The data was obtained from satellites of a variety of shapes and sizes which