摘要:

Thermal plasma noise spectroscopy using the ICE radio receiver yielded the parameters of the two maxwellian populations approximating the comet plasma electron distribution. Within 150,000 km around closest approach, we observe three regions : 1) the inner one coincides with the low magnetic field region or plasma sheet ; over 1200 km the density is nearly constant at 650 cm−3; the overall profile, 5000 km wide, is assymetrical and shows a narrow spike on each side of the maximum ; the temperature is 16,000 K; 2) from 9000 to 20,000 km from the nucleus, the density decreases as R−2. The distribution is slightly assymetrical, the density higher on the south side. 3) Further away, the density and temperature fluctuate widely around about solar wind values. The hot electron pressure is much smaller than that of the cold electrons except near the edges of the plasma sheet where heating processes probably oc

ISSN:0094-8276    

DOI:10.1029/GL013i003p00279

年代:1986

数据来源: WILEY

摘要:

The ICE magnetic field observations at Comet Giacobini‐Zinner have revealed the presence of a well developed magnetotail with a diameter of ∼ 10,000 km at a distance of 7,800 km downstream of the nucleus. The tail is composed of two lobes of opposite magnetic polarity separated by a ∼ 1,500 km thick plasma sheet. A ∼ 250 km diameter region of twisted, high intensity magnetic fields was observed in the plasma sheet and may correspond to a cometary “flux rope.” The magnetotail is bounded by a thin, ∼ 200 km wide magnetopause‐like current layer. The field magnitude increases by a factor of 2 between the outer portions of the lobe and the central regions where peak fields of ∼ 60 nT were measured. The magnetic field in the lobes of the near‐tail region is highly flared with the field making a mean angle of about 20° with the symmetry axis. Flaring is observed to be least in the high field regions of the central tail and greatest in the outer portions of the lobes where minimum variance analyses on the magnetopause crossings provided flare angles of 20‐40°. The cross‐tail current layer is quite broad and occupies the full width of the plasma sheet. Minimum variance analysis indicates that it was inclined to the plane of the ecliptic by 43°. This current sheet orientation is consistent with the observed direction of the IMF in the Y‐Z plane during the ICE fly‐by. It is concluded on the basis of these results that the ICE magnetic field observations downstream of Giacobini‐Zinner have confirmed the Alfven field line drapin

ISSN:0094-8276    

DOI:10.1029/GL013i003p00283

年代:1986

数据来源: WILEY

摘要:

We subject the data ICE collected in its traverse of the tail of Giacobini‐Zinner to the requirements of stress balance. The result provides estimates for unmeasured quantities. We note that unlike planetary magnetotails, the comet's tail is embedded in an ionosheath that has a static pressure nearly equal to the solar wind stagnation pressure. This leads to a large lobe field strength. Transverse pressure balance reveals a systematic variation in the ion temperature across the tail which in turn tells how the pick‐up velocity of new ions varies across the pick‐up region. We find the pick‐up velocity increases from 14 km s−1in the plasma sheet to 46 km s−1in the ionosheath. This is the same velocity shear that stretches the magnetic field to form the tail. Imposing axial stress balance leads to an expression for the strength of the lobe field which reveals that it varies weakly with axial distance. The expression also gives an estimate of 24 km s−1for the plasma sheet velocity at ICE's location. We find that the plasma takes between 7 and 13 minutes to flow through the coma to the position of ICE in the

ISSN:0094-8276    

DOI:10.1029/GL013i003p00287

年代:1986

数据来源: WILEY

摘要:

During the ICE flyby of Giacobini‐Zinner the plasma wave instrument detected numerous impulsive signals caused by dust impacts on the spacecraft. Most of the impacts occurred within 30,000 km of the comet. The impact rate averaged over the inbound and outbound legs varies approximately as 1/r², as would be expected for an isotropic constant velocity radial outflow. Small differences between the inbound and outbound legs exist which may be indicative of azimuthal variations in the dust production rate. A simple model of the impact ionization and charge collection by the antenna indicates that the particles have a mass on the order of 10−12to 10−10gm, corresponding to particles with radii of a few m

ISSN:0094-8276    

DOI:10.1029/GL013i003p00291

年代:1986

数据来源: WILEY

摘要:

A thermal‐infrared image of comet Giacobini‐Zinner has been used to determine the spatial distribution of dust grains in the Comet's coma. In the present paper we emphasize the results of this imaging which bear on the expected impact rate of dust grains on the International Cometary Explorer (ICE). the expected impact rate is calculated for a range of grain radii. For particle sizes comparable to those inferred for most comets, the impact rate is on the order of several hundred per second. This rate is in agreement with the predictions by Divine (1985), but it is more than two orders of magnitude larger than the rate observed by ICE if the observed impacts were on the spacecraft body. However, the observed impact rate is roughly comparable to that inferred from our infrared imaging if, as suggested by Gurnett et al. (this issue), the impacts were on the ICE antennae, which had a much smaller cross‐sectional area than the spacecraft

ISSN:0094-8276    

DOI:10.1029/GL013i003p00295

年代:1986

数据来源: WILEY

摘要:

Evolution of gas and dust distributions following a spatially and temporally localized comet outburst was calculated using a hybrid kinetic ‐ hydrodynamic method. It was found that a comet outburst resulted in significantly increased dust terminal velocities. As these higher terminal velocity values result in larger apex distances, therefore the outburst generates distinct dust envelopes in front of the regular dust coma. Such envelopes were observed at several comets (cf. comet Donati

ISSN:0094-8276    

DOI:10.1029/GL013i003p00299

年代:1986

数据来源: WILEY