Cometary ion observations at and within the cometopause-region of comet Halley

Adv. Space Res. Vol. 5, No. 12, pp.221—225, 1985 Printed in Great Britain. All rights reserved.

0273—1177/85 $0.00 + .50 Copyright © COSPAR

COMETARY ION OBSERVATIONS AT AND WITHIN THE COMETOPAUSEREGION OF COMET HALLEY A. Korth,* A. K. Richter,* K. A. Anderson,** C. W. Carlson,** D. W. Curtis,~~ R. P. Lin,~~ H. Rème,*** J. A. Sauvaud,*** C. d~Uston,*** F. Cotin,*** A. Cros*** and D. A. Mendist *Max~Planck~1nstitut für Aeronomie, D-3411 Katlenburg-Lindau, F.R.G. * * Space Sciences Laboratory, University of California, Berkeley, CA 94720, U.S.A. ***Centre d’Etude Spatiale des Rayonnements, CNRS-Paul Sabatier University, F-31029 Toulouse, France tDepartment of Electrical Engineering and Computer Sciences, University of California, San Diego, CA 92093, U.S.A. ABSTRACT Three distinct boundaries are identified from the PICCA cometary ion observations within the innermost part of th~coma of comet Halley: (1) the ‘cometopause’ at a cometocentric distance Rc 1.5x10 km, characterized by the appearance of wat~r—qroup ions well above background; (2) the ‘cold cometary plasma boundary’ at Rc 3x10 km, characterized by a sudden and simultaneous decrease in the temperatures of all cometary ions, and (3) the ‘ionopause’ at Rc 6000 km, characterized by a fast decrease in the intensity of all cometary ions by a factor 3—5. Between the first two boundaries only ions with masses less than 50 amu are present, showing distinct maximum intensities at 18, 32 and 44 amu at the second boundary. Downstream of the second boundary also ions of mass 12, 64, 76, 86 and 100 amu are detected.

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INTRODUCTION From the solar wind ion /1,2/, electron /3/, magnetic field /4,5/, and low—freauency wave /6/ observations, as obtained by the VEGA— and GIOTTO spacecraft during their inbound encounters with ~ometHalley, the existence of the cometary ‘bow shock’ at a cometocentric distance Rc 10 km was clearly demonstrated. The existence of the cometary ‘ionopause’ at Rc ‘~ 6000 km was established by the magnetic field measurements on board GIOTTO /5/. Both boundaries had been predicted by theory /7/. The existence of a third cometary boundary, the ‘cometopause’, which had not yet been foreseen theoretically~was clearly demonstrated by the VEGA plasma observations /1,8/ to occur at Rc i 1.6x10 km. Based on the cometary ion observations, performed by the PICCA instrument on board GIOTTO, the existence of the ‘cometopause’ and of the ‘ionopause’ are established, and the onsets of the various cometary ions with masses ranging from 12 to 100 atomic mass units as a function of the cometocentric distance are discussed. EXPERIMENT DESCRIPTION The PICCA (Positive Ion Cluster Composition Analyzer) instrument, which is part of the RPA—Copernic plasma Tnstrument on board GIO1TO /9/, was designed to measure the spatial distribution and the chemical composition of positive ions in the mass range from 10 to 213 aJnu (atomic mass units) in the innermost part of the cometary coma arriving from the spacecraft—comet relative velocity direction. The PICCA sensor consists of a deflection unit followed by an electrostatic analyzer. In order to determine the mass of the cometary ions we make use of the large relative fly—by velocity of the spacecraft of 68.4 km s and of the fact that the ions in the inner ~art of the coma should have small thermal and bulk velocities of the order of 1 km s and that they should be predominantly singly charged. Thus, the E/Q (energy/charqe) r~easurementscan be related directly to the mass distribution 2, where M is the mass of the ion in amu and v is the of the ions velocity by E(eV) relative — 5.lxlO— spacecraft to Mthev nucleus. The mass resolution is ~l’1 = 0.4 amu in the mass range 10-50 amu, and d’1 = 1 amu for 50-213 amu. The shortest accumulation time for one mass spectrum is 3.2 s. This applies to the last 20 minutes before encounter. Two channel electron multipliers, CEM1 and CEM2, with different geometric factors, are used to detect the ions. The field of view of the analyzer is large enough (±50) to accept all ions to be analyzed, even in case when their trajectories should differ from the r~iidirection, either due to thermal spread or to electrostatic potentials of the spacecraft.

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A. Korth at al..

OBSERVATION OF THE COMETOPAUSE The PICCA instrument started to detect the first ior~s — probably scattered solar wind protons — at a cometocentric distance Rc i 1.05x10 km~This occurred shortly after GIOTTO crossed the cometary ‘bow shock’ at Rc i 1.15x10 km /5/. Downstream of the ‘bow shock’ in the ‘cometosheath’ region the velocity of the solar wind plasma as well as of the pick-up cometary ions gradually decre 5ased /2/, while the magnetic field remained at a lower level of ~ 10 nT /5/. At Rc 1.4xlO km the magnetic field intensity suddenly increased to values of about 30 nT /5/. Almost simultaneously the PICCA instrument started to detect the first h~avycometary ions, which, according to Fig. 1, occurred at 23:27 UT or at Rc i 1.5x10 km, i.e., more or less at exactly the distance where, based on the VEGA plasma observations, the location of the ‘cometopause’ had been identified before /8/. At first, the ion distribution is rather broad, indicating that the temperature of the cometary ions is still rather hiqh. Thus, the E/Q measurements at this time cannot be directly related to a certain mass distribution. The center of these first spectra is located at about 24 amu. However, as the comet’s nucleus is approached, the ion temperature is gradually decreasing while the intensity is increasing, and the peak of the mass distribution is slowly shifting towards 18 amu. In turn, this indicates that cometary water—group ions are actually dominating the region directly behind the ‘cometopause’. OBSERVATIONS INSIDE THE COMETARY PLASMA REGION While GIOTTO is propagating further downstreaii of the ‘cometopause’ into the ‘cometary plasma region’ /8/, other mass groups recover from background (Fig. 1). So, e.g., the mass group 28—32 amu appears at Rc 7.4x1O~km, and the mass group around 44 amu at Rc 4.5xlO” km. A~,tertheir appearances their intensities increase rather rapidly. At a distance Rc i 3xlO km the spreads in the various ion distributions and thus the temperatures of the various ions drop rather abruptly, indicating that GIOTTO has now entered the ‘cold cometary plasma region’. This effect was also recorded by the neutral mass spectrometer on board GIOTTO /11/. It is after this ‘boundary’ that PICCA detects three distinct peaks in the mass range 10 to 50 amu, which can now be associated undoubtedly with the masses 18, 32 and 44 amu, respectiv~ly. Ions with masses larger than 50 amu, however, are not recorded outside of Rc i 2.7xlO km due to the sensitivity of the instrument. According to Fig. 2 it is only inside this regime that the intensities of the following ions start to recover successively from background: ions of mass 64 and of 12 amu, of 76 amu, of about 86 amu, and then of about 100 amu (a detailed mass spectrum is shown in /12/). This scenario is sumarized in the following table, where, depending on the various atomic mass units of the cometary ions, the corresponding cometocentric distances are given, at which the intensity emerges from background. Ion Mass (amu)

Cometocentric Distance of Onset (km)

18 30 44 12 & 64 76 86 100

1.5 7.4 4.5 2.7 1.8 1.4 1.2

x x x x x x x

10” 1O~ 10” 10” 10” 10”

OBSERVATION OF THE IONOPAUSE Once the intensities of the various cometary ions have emerged from background, they are continuously increasing. This overall tendency continues up to a distance of Rc t 10000 km. Further inside the intensities of the various cometary ions decrease rather simultaneously by a factor 3—5 down to a distance Rç i 6000 km. These decreases go in parallel with the drop in the magnetic field intensity from about 50 nT to nearly zero nT /5/. According to theory this latter observation indicates the crossing of the cometary ‘ionopause’. Based on the PICCA observations, it is confirmed that this boundary separates the regime of cometary ions from the region dominated by cometary neutral particles. Acknowl edgements This work was supported by the Max—Planck—Gesellschaft zur F~rderungder Wissenschaften e.V., by the Bundesministerium fUr Forschunq und Technologie under grant no. 01 OF 052, by CNES under grant no. 1212 and by NASA contract NASW-3375. The data reduction for the PICCA instrument was performed by H. Michels.

Ion Observations At and Within Cometopause

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Fig. 1. Colour-coded representation of the logarithmic normalized intensity (counts 1 s1) versus time (horizontal axis) and atomic mass units (vertical axis). The amu time interval is shown on top in spacecraft event time. Data are averaged over four spacecraft spins or 16 s. The locations of the ‘cometopause’, of the ‘cold cometary plasma boundary’, and of the ‘ionopause’ are indicated by triangles, respectively.

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Fig. 2. Same as Fig. 1, for a smaller time interval and for higher masses. Data are not averaged over four spacecraft spins but each mass is measured every 3.2 s. Therefore a spin modulation is observed, as the geometric factor is dependent on the direction of the incoming ions.

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