Cometary kilometric radio waves and plasma waves correlated with ion pick-up effect at comet Halley

83—88, 1985 Adv. Space Res. Vol. 5, No. 12, pp. Printed in Great Britain. All rights reserved.

0273-1177/85 $0.00 + .50 Copyright ~ COSPAR

COMETARY KILOMETRIC RADIO WAVES AND PLASMA WAVES CORRELATED WITH ION PICK-UP EFFECT AT COMET HALLEY H. Oya,~A. Morioka,* W. Miyake,5 E. J. Smith~5 and B. T. Tsurutani** * *

Geophysical Institute, Tohoku University, Sendai 980, Japan *Jet Propulsion Laboratory, California Institute of Technology,

Pasadena, CA 91109, U.S.A.

ABSTRACT From the discrete spectra of the emissions from the comet in the frequency range from 30 to 195 kHz named CKR (Cometary Kilometric Radiation), movements of the bow shock at comet Halley are concluded, i.e., the observed CKR emissions can be interpreted as being generated and propagating from the moving shock. The motion of the shocks are possibly associated with time variation of the solar wind and of the cometary outgassings. By in—situ plasma waves observations using PWP (Plasma Wave Probe) onboard the Sakigake spacecraft, the characteristic spectra of the electrostatic electron plasma waves, the electron cyclotron harmonic waves, and the ion sound waves have been detected during the interval of the Halley’s comet fly—by. Compared with the results of a Faraday cup observation and a magnetometer, it is concluded that these plasma wave phenomena are the manifestation of the ion pick—up processes. The ion pick—up 6proces~esare taking place even in the remote region within a distance range from 7x10 to 10 km from the cometary nucleus. INTRODUCTION The Japanese spacecraft Sakigake was launched on January 8 in 1985 from Kagoshima Space Center to encounter Halley’s comet within a distance of 7 million kilometer. Sakigake was equipped with a magnetometer, solar wind detector, and plasma wave probe (PWP) to measure the solar wind and in—situ plasma waves in the solar wind and radiation propagating from comet. After the successful launch, the PWP experiment made observations of radio emissions from the sun, and inter—planetary plasma waves in a frequency range from 70 Hz to 195 kHz. The capability and characteristics of the instrument have been checked and calibrated during the solar wind cruise phase of the mission. Encounter observations have been made from March 10, to 13, 1986. The closest approach to Halley’s comet was at 13:05 JST, March 11, at a distance of 6.99 million kilometers. During this encounter phase, the PWP experiment detected electromagnetic wave emissions, from Halley’s comet that are attributed to the strong interaction between the incoming solar wind with cometary plasma. In the present paper, reports for the observational results of the radio emission coming from the coma region of Halley’s Comet are given, and the interpretation are given based on the concept of the moving bow shock. Data from the plasma wave observations in the frequency range from 10 kHz to 195 kHz and also in the frequency range from 70 Hz to 435 Hz show the enhancement of the locally existing plasma waves, when the Sakigake sPacgcraft pas~ed through the upstream region of the cometary nucleus within a range from 7x10 km to 10 km (/1/). The present paper also reports the confirmed ion pick—up effects based on the observed plasma wave enhancement on board the Sakigake spacecraft. ORIGIN OF CKR The spacecraft Sakigake passed through the up—stream region of the comet (see Figure 1 of /1/), where the solar wind had a velocity of 400 km/sec, a speed which allowed the plasma to arrive at the coma of the comet within 4 hours. In Figure 1, the observational data of the plasma wave spectra in the LF range are shown. We have named the emissions, in the frequency range from 30 kHz to 195 kHz, Cometary Kilometric Radiation (CKR); and we have classified the observed emissions into the three

83

H. Oya et al.

84

types: as Type—D, Type—S and Type—C (see /1/). Discrete emission Type—D indicates frequency changes from about 60 to 195 kllz during periods which is ranging from 30 to 40 minutes. This is difficult to interpret in terms of wave frequency dispersion but can be attributed to the movement of the radio source itself. It is very natural assumption that the emissions are taking place at the local plasma frequency evolved in the plasma turbulence at the shocks. Conversion of electrostatic plasma waves to the electromagnetic waves due to the turbulent effects of the plasma has long history of the studies (/2/, /3/, /4/). Basically the conversion of the electrostatic plasma waves to the electromagnetic waves takes place through the plasma turbulence without dependence on the pwP LF

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Fig. 1. Dynamic spectra of the observed LF emissions in the frequency range from 10 kHz to 195 kHz in the period of the encouter. From the panel Al to Dl, the display for the threshold level of 0.53 uV/m at 100 kHz and 5.3 uV/m at 10 kHz are displayed. For indications of the existing emissions sketches of the spectra are given in the column of the panels from A2 to D2. The indicated periods of data are therefore equal for each line of arrayed panels; i.e. Al and A2 for an example. TABLE

No.of Case

Time Interval

(mm) 1

2 3 4

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1

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of

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Shock

Average Speed Ionization rate Ionization rate of Moving ~iock 1=1.0 1= 0.5 Starting Ending Relative to Consents Position Position Nucleus Starting Ending Starting Ending Position Position Position Position (10°kn~ (l0°knm) Gomu/sec) (10°km) (10°km) (10°kn~ (10°ka) 1. 00 2. 43

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81

0. 96

0. 49

0. 71

0. 67 0. 67

0. 41 0. 50 0. 41

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0. 57 2. 00

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0. 73 0. 45

1. 38 0. 43

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0. 75

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1. 15

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73 204

0. 48 0. 54

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0. 34

5 6

47 56

12 36 0. 80 1. 13 16 0. 08 1. • No. of case is corresponding •~Results are given in Nature

0. 70 0. 48 0. 70 15 162 0. 82 1. 06 15 364 0. 82 1. 06 to the exaicples given in Table 2 of Oya et (Oya et al.,1986)

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Cometary km Radio Waves and Plasma Waves

85

conditions whether the plasma contains the magnetic field or not. For this case it may be appropriated not to consider the nonlinear regime for the wave mode conversion process because the turbulence is not as intense as the case of emissions of the solar type II bursts, where very intense shock waves generate the second harmonic waves associated with the fundamental plasma frequency emissions. The plasma density distribution in the coma ~ 6gion of the comet Halley is estimated for the molecular production rate of 1.3 xlO /sec(/5/); the estimated values provide the moving source regions of the emitted radiations. MOVING BOW SHOCKS The results of the deduced moving shock positions are given in Table 1. The data source is basically same with that given in prev~6us report (/1/) but, the correction is made for the more accurate emission rate of 1.3 xlO /sec (/5/). Since there is assumption for the ionization rate, corrections are also made for the previous results (/1/) has also been given in Table 1 (the case of ‘( =0.5). The monotonously decreasing frequency of Type_D~ emission is therefore indicating the movement of the shock within 2 million km mostly from 1 million to 0.4 million km with the moving velocity ranging from 60 to 335 km/sec. The movement of the shock position towards upstream of the solar wind is also revealed from the Type_DR emissions. That is, the radio sources associated with the shock wavgs move from the inside region of the coma with distance ranging from 0.45 to O.8OxlO km towards the upstream until the shocks arrive at the region with distance of about one million km from the nucleus. The solar wind conditions, when the Sakigake spacecraft moved across the upstream region of the coma, contained fairly large disturbance because the neutral sheet regions of the heliomagnetosphere passed through the Halley’s comet. Several remarkable discontinuities of the magnetic field with the plasma density variations have been observed during March 11. On March 12, the enhancement of the radio emissions were observed to be associated with the encounter of the turbulent regions of the heliomagnetosphere with the Halley comet. That is, after the turbulent regions passed over Sakigake (see /6/, and /7/), 3 and half hour, later when the turbulence reached the cometary bow shocks, the plasma wave emissions are increased substantially. This delay time is reasonable as the transport time of the turbulence to the coma region of the comet. The possible interpretation of the observed Type_Dr and Type_DR emissions are thus inferred to be the result of generation at the moving shock in the coma region of the comet corresponding the time dependent variations of the solar wind conditions. In addition to the discrete frequency shift which shows motions of the bow shock, the dynamic spectra in

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Fig. 2. Intensity of the observed LF emission integrated over a frequency range from 10 kHz to 195 kHz. The results are indicated for the two diffirent threshold as given in panel (A) above the relative level of 25, and in panel (B) above the relative level of 5. The high level data in the panel (A) show the general tendency of the occurrence of the plasma waves associated with the comet and the low level data given in the panel (B) show that the enhancement of the local plasma waves coinciding with the magnetic field turbulences indicated by MD—i and MD—2.

86

H. Oya et al.

4.0. Closest Approach

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FrequencY (Hz) the LF range show that there are simultaneously formed two or multiple shocks. Several contradictory points are also remarkable among the in—situ observation results made by two Vega spacecrafts (/5/), Giotto (/8/) and Suisei (/9/). These discrepancy of the observation position can be understood as caused by the difference of observation times for the moving multiple shocks when we use the concept of the moving bow shock established by the CKR data. ENHANCEMENT OF PLASMA WAVES

6 km, During the remarkable closest enhancements approach of Sakigake of the plasma spacecraft waves towere comet detected Halley both with indistance LF (4 kHz of to 6.99x1O 195 kHz) and ELF (70 Hz to 3 kHz) frequency ranges (/1/) using 10 m tip to tip electric field antenna. In addition to the radiation coming from the shock waves in the coma region, excitation of the local plasma waves associated with the comet were clearly detected. In Figure 2, the observed plasma waves in LF range integrated over 10kHz to 195kHz are plotted versus observation time. The plasma waves in ELF range are also remarkably enhanced in the period of the closest approach as has been indicated in Figure 3. Though the CKR phenomena (/1/) were not able to be observed whey the spacecraft moved in the long distance range from the nucleus with distance more than 10 km, because of the threshold of the receiver, the in— situ plasma waves which show the plasma turbulence had always been observed through all of the observation period during three days. Among these phenomena, our studies are focused

1986.3110206

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FREQUENCY (kHz)

Fig. 4. Spectrum of the plasma waves in LF frequency range observed on March 11,1986,at 02:06 UT. When the values are devided by 33, the value in diagram are tansformed to pV/m Hz unit.

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Cometary km Radio Waves and Plasma Waves

87

on the phenomena around 02:00 UT, 04:00 UT and 06:20 UT on March 11, 1986 where the remarkable enhancement of the plasma waves were detected. a) Enhancement around 02:00 UT For the Case of the plasma wave enhancement around 02:00 UT, the plasma wave spectra are characterized by double peaks near 18kHz and 25 — 27kHz, with intensity of 80 pVolt/m (see Figure 4). These two peaks can be attributed to two components of electron plasma frequency of the electron plasma waves which correspond to the plasma density of 4/cc and 9/cc. When we see the solar wind data (see Figure 2 of /6/) at 02:06 UT there was abrupt change of the plasma parameters, that is, 1) change of the flow direction about 5 , within a few minutes, 2) change of the flow speed from 450km/sec to 420km/sec, 3) change of the density from 9/cc to 4/cc wit~ifla few minutes, and 4) change of the plasma temperature from 10 K to SxlO K, again within a few minutes. Also coinciding with all these changes in the plasma wave spectra, and solar wind parameters, the magnetic field data also indicated very interesting change (see Figure 4 of /7/); i.e. very clear enhancement of the ion cyclotron waves at the period of 150+sec + co~respondingto the cyclotron motion of the water group molecular ions such as 0 , OH and OH Enhancement around 04:10 UT The plasma wave enhancement for the period starting from 04:16 UT is characterized by generation of plasma waves in the low frequency range as has been given in Figure 5. The increment of the emissions around 100 — 130Hz and 210Hz are attributed to the enhancement of the electron cyclotron harmonic waves for the magnetic field intensity of 4nT. The unique features of the plasma have also been observed coinciding with the excitement of the electron cyclotron harmonic waves. That is, the plasma density changes abruptly from 5.5/cc to 9/cc (/6/) with decreasing magnetic field from 6.SnT to 4nT. c) Enhancement around 06:20 UT The spectra obtained in this period are indicated by the results given in Figure 6, very sharp increments of the electron plasma oscillation at 26 — 27kHz corresponding to the plasma density of 9/cc are observed. No significant change was observed for the observation of the solar wind plasma and also for the magnetic field observations. d) Enhancement around 23:50 UT As has been given in the example of ELF spectrum in Figure 5, the very large enhancement of the plasma waves in the low frequency side less than 70Hz is identified. These spectra become very remarkable and continued during March 12 and 13, as has been indicated by the spectra given in Figure 3.

ORIGIN OF THE PLASMA WAVE AND ION PICK—UP REGIONS One of the wave modes existing in the ambient plasma is the electrostatic plasma waves near the plasma frequency, oscillation as have been indicated by spectra in Figures 4 and 6 where remarkable peak in the spectra near 30kHz was enhanced corresponding to the plasma density about 4 and 8/cc. With increments of the plasma wave intensity and very rapid change of the ion density, the change of the plasma flow direction is als~asso~iatedw~ththe MI-ID turbulence that is enhanced at the ion cyclotron frequency of 0 , OH or H 2O ions (/7/). In addition to the plasma waves near the plasma frequency, the electron cyclotron harmonic waves enhanced near the harmonics of the electron cyclotron frequency has been confirmed as has been given in Figure 5. These electron cyclotron waves in the region of the solar wind trapped plasma could be attributed to the cometary origin as has been identified in the case of the plasma wave enhancement around 04:10 UT. The enhancement a~ound+23:5O~T is characterized by ion sound waves associated with the dynamics of the 0 , OH or OH2 ions (see Figure 5). This is also caused by interaction of the cometary plasma with the solar wind plasma which produces the electrostatic ion sound waves, in parallel to the electrostatic electron plasma waves. Investigating the observed spectra, it is also concluded that ion sound waves are not in the linear regime but in nonlinear processes due to highly developed plasma turbulences. All of these phenomena detected in the period of the Halley comet fly—by are associated with comet and we have concluded as the result of ion—pick up phenomena,i.e. the development of the plasma turbulence is not caused only by plasma flow of the solar wind itself, but caused by interactions of the solar wind with the cometary ions. CONCLUSION From the discovered cometary kilometric radiations which show the’ discrete spectra of rising and falling nature of the frequency changes versus the time, the existence of moving shocks at the comet are concluded. The shock positions are determined from the observed CKR frequencies, assuming that the frequency is corresponding to the electrostatic plasma waves

H. Oya at al.

88

at the plasma frequency associated with the shocks. 6 to 2x1O6 The positions that the shocks moveresults in the of range O.45x10 km, obtained that can shock consistently giveshow interpretation to the the from in—~ituobservatigns of the shock position which reported in different ways ranging from O.4x10 km to l.lxlO km as results of the difference of observation times for moving shock waves. Th~plasma wave probe detected intense in—situ plasma waves in the region apart 7x1O6 to 10 km from the nucleus. The in—situ plasma waves indicate the characteristic spectra corresponding to the electrostatic electron plasma waves near the plasma frequency 1 the electron cyclotron harmonic waves, and the ion sound waves. All of the detected plasma waves show that there are existing the two stream type instabilities or the anti—losscone type instabilities during the pick—up processes of the cometary plasma due to merging of the solar wind plasma with the cometary plasma which is basically fixed to the cometary nucleus. It is also concluded that the regions of ion pick—up phenomena of Halley’s comet are expanded even in the range of 7 to 10 million kilometers. ACKNOWLEDGMENT The comet Halley observations by the Sakigake spacecraft were made as a part of the ISAS Planet—A projects. We thank Professor M.Oda, director of ISAS, K.Hirao and T.Itoh, project managers, and all members of the project team for their extensive support. The activities of E.J.Smith and B.T.Tsurutani were supported by NASA. REFERENCES 1. 2.

3. 4. 5.

6 7. 8.

9.

-

Oya,H., A.Morioka, W.Miyake, E.J.Smith and B.T.Tsurutani, Discovery of cometary kilometric radiations and plasma waves at comet Halley, Nature, 321, 307—310, 1986 Oya,H.,, Conversion of electrostatic plasma waves into the electromagnetic waves:numerical calculation of the dispersion relation for all wavelengths, Radio Sci., 6,1131—1141, 1971 Jones,D., Source of terrestrial nonthermal radiation, Nature, 260, 686—689, 1976 Kruth,W.S., J.D.Craven, L.A.Frank and D.A.Gurnett, Intense electrostatic waves near the upper hybrid resonance frequency,J.Gec~phys. Reg., 84, 4145—4164, 1979 Gringautz,K.I., T.I.Gombosi, A.P.Remizov, I.Apthy, I.Szemerey, M.I.Verigin,L.I. Denchikova, A.V.Dyachkov, E.Keppler, I.N.Klimonko, A.K.Richter, A.J.Somogyi, K.Szego, S.Szendro, M.Tatrallyay, A.Varga and G.A.Vladimirova, First in—situ plasma and neutral gas measurements at comet Halley,Nature. 321, 282—285, 1986 Oyama,K—I., K.Hirao, K.Yumoto and T.Saito , Was the solar wind decelerated by comet H alley ? , Nature.1 321, 310—313, 1986 Saito,T., K.Yumbto, K.Hirao, T.Nakagawa and K.Saito,Interaction between comet Halley and the interplanetary magnetic field observed by Sakigake, Natuye, 321,303—307 ,1986 Johnstone,A.A., A.Coates, S.Kellock, B.Wilken, K.Jockers,H.Rosenbauer, W.Studemann1 W.Weiss, V.Formisano, E.Amata,R.Cerulli—Irelli, M.Dobrowolny, R.Terenzi, A.Egidi, H.Borg,B.Hiltquist, J.Winningham, C.Gurgiolo, D.Bryant, T. Edwards, W.Feldman, M.Thomsen, M.K.Wallis, L.Biermann, H.Schmidt,R.Lust. G.Haerendel and G.Paschmann, Ion flow at comet Halley, Nature, 3211 344—347, 1986 Mukai,T., W.Miyake, T.Terasawa, M.Kitayama and K.Hirao, Plasma observation by Suisei of solar—wind interaction with comet Halley, Nature, 321, 299—303, 1986