摘要:

While the ICE magnetometer measurements at Comet Giacobini‐Zinner led to a number of unexpected discoveries such as the intense hydromagnetic turbulence, the large‐scale structure of the induced cometary magnetosphere that it delineated was close to theoretical predictions. While the strong draping of the magnetic field lines to form a magnetotail was anticipated, the structure of this magnetotail also indicated the existence of other predicted features. These included a weak shock or compression wave, an ionopause which at least partially impeded the penetration of the solar wind into the cometary ionosphere, and a magnetic barrier region where the magnetic field was piled up ahead of the ionopause. The inferred positions of these features enabled the estimation of the production rate of the neutrals from the nucleus (≈ 4×1028mols/sec), as well as the strength of the coupling between the inflowing solar wind ions and the outflowing cometary ne

ISSN:0094-8276    

DOI:10.1029/GL013i003p00239

年代:1986

数据来源: WILEY

摘要:

Fitting of a Mach 2 shock surface to the ICE magnetic field data obtained near comet Giacobini‐Zinner has provided subsolar bow wave distances that infer neutral gas outflow rates comparable to previous measurements, and orientations of the bow wave symmetry axis that are consistent with the plasma measurements and motion of the comet relative to the solar wind. Mach values of 1.5‐2 and transition thicknesses of the order of 104km are inferred when the field magnitude and variance data are compared. Cross spectra of the transverse field components in and near the bow wave exhibit a peak near 10−2Hz, or near the cyclotron frequency of ions from the water group. However, the level of turbulence is not consistent with that observed for similar configurations at planetary bow s

ISSN:0094-8276    

DOI:10.1029/GL013i003p00243

年代:1986

数据来源: WILEY

摘要:

Electron heat flux observations and associated plasma phenomena upstream from comet Giacobini‐Zinner show many similarities to observations upstream from the Earth's magnetosheath. Two similarities are discussed. Heat flux events are used to compute the transition region boundary location. The computed subsolar standoff distance of 4×104km is commensurate with a comet gas production rate, G, of 3×1028molecules/s (mol/s), in accord with ground‐based determinations

ISSN:0094-8276    

DOI:10.1029/GL013i003p00247

年代:1986

数据来源: WILEY

摘要:

In‐situ measurements of the rest‐frame distribution functions of cometary pick‐up water‐group ions are used to derive the number and energy density of these ions as a function of distance from the Comet Giacobini‐Zinner. The observed density profiles are found to be the same in the inbound and outbound passes and their shape is consistent with predictions of a simple model of pick‐up ion production from neutrals of a given type which flow radially outward at ∼1 km/s and are lost at a rate λ. Comparing predictions of this model with the measured density profiles we find that λ = 2·10−6sec−1and that the production rate of water‐group cometary molecules is 2.6·1028/sec. The measured distribution functions of the pick‐up water‐group ions may be represented by a two‐segment power‐law with a break at an energy EB∼20 keV, corresponding to a speed, VB= 438(E/M)1/2, of ∼ 500 km/s at distances greater than ∼105km. The shapes of the distribution functions indicate that the interaction process of the solar wind with come

ISSN:0094-8276    

DOI:10.1029/GL013i003p00251

年代:1986

数据来源: WILEY

摘要:

The plasma flow in the cometary coma is described by a simple relation which results from the balance of magnetic stresses and drag forces, the latter being mainly due to ion mass loading. Inertial and pressure effects are not explicitly considered, but lumped into a correcting factor of order unity. The relation is used to infer the ionization rate due to the critical velocity effect. The limits, rcritand rlim, between which the effect dominates over photoionization are evaluated. Critical velocity ionization stabilizes the plasma flow speed at η−1/2vcrit, where η is the overall efficiency of energy transfer to the ionizing electrons. This speed is estimated to be close to 20 km/s. At the outer limit, rlim, there is a strong rise of plasma density as seen from outside. Inside rcrit, recombination and ion‐neutral friction come into play. The contact surface is found to be largely determined by ion‐neutral collisions. A second neutral component of the order of 10² cm−3is predicted to originate from recombination inside rlim. It is characterized by an antisunward flow with ∼ 20 km/s. Numerical values for the radial profile of niand of the critical boundaries (except the bow shock) are given. The high plasma density found by ICE at the closest approach to comet Giacobini‐Zinner is interpreted as a result of the critical v

ISSN:0094-8276    

DOI:10.1029/GL013i003p00255

年代:1986

数据来源: WILEY

摘要:

The turbulence surrounding comet Giacobini‐Zinner reached intensities three orders of magnitudes above that of “intermediate” solar wind conditions, was characterized by Δ B/Boalmost equal to one, and extended to cometocentric distances beyond 106km. The strong spectral peak at f ∼ 10−2Hz and the general intensity profile of the turbulence (a lack of emissions close to the nucleus [106km) are in excellent agreement with generation by plasma instabilities associated with the pickup of H2O+group ions.The total energy of the magnetic turbulence near f ∼ 10−2Hz is estimated to be ∼ 5 × 1015joules and that of ion energization due to solar wind pickup ∼6 × 1013watts. It thus takes a minimum of ∼ 100 s to build up the magnetic turbulence. This scale is approximately the linear growth rate of the ion pickup instability, giving a consistent picture of the energy s

ISSN:0094-8276    

DOI:10.1029/GL013i003p00259

年代:1986

数据来源: WILEY

摘要:

The hydromagnetic turbulence surrounding comet Giacobini‐Zinner is characterized by two dominent components: large amplitude, linearly polarized waves with periods near 10² s and circularly polarized wave packets with periods near 3 s. The latter appear in association with variations in field magnitude produced by the long period waves. A third, commonly observed feature is identified, consisting of a partial rotation of the magnetic field (a hook‐like structure) which also appears to be triggered by the long period waves. The possibility that the long period waves are generated by the pickup of freshly ionized cometary neutrals has been investigated. A resonant instability identified by Wu and Davidson (1972) appears able to account for the long period waves in terms of the pickup of cometary ions of the water group. Application of the theory also indicates that the growth rate of proton cyclotron waves, as a consequence of hydrogen ion pickup, is somewhat lower, due to both higher neutral velocities and longer ionization times. Several significant features of the wave packets and hooks are presently unexplained and will require further s

ISSN:0094-8276    

DOI:10.1029/GL013i003p00263

年代:1986

数据来源: WILEY

摘要:

Very large amplitude fluctuations in electron density, temperature, and flow velocity were a prominent aspect of the solar wind interaction with comet Giacobini‐Zinner during the ICE encounter in September 1985. These fluctuations were detected at a distance of at least 9 × 105km and grew in amplitude as ICE approached the comet, peaking in amplitude 6 × 104km from closest approach. A typical period associated with the fluctuations was ∼2 min, which corresponds to a scale length in the solar wind frame of ∼5 × 104km. For the most part these fluctuations appear to be a product of one or more plasma instabilities associated with the solar wind pick up of comet

ISSN:0094-8276    

DOI:10.1029/GL013i003p00267

年代:1986

数据来源: WILEY

摘要:

Low‐pass filtered data from the ICE electron spectrometer are used to examine relationships between plasma densities, flow speeds, flow directions, and temperatures at distances within ∼105km of the Comet Giacobini‐Zinner nucleus. The sheath/transition region is characterized by electron temperatures of ∼4 × 105K but with flow speeds and densities that commonly vary by factors of three or more. The largest amplitude density spikes often have significant flow changes associated with them, but a consistent pattern (as might accompany cometary rays, for example) is not found. Power spectral analyses in and near the sheath/transition regions show that density fluctuation levels are enhanced at all detectable frequencies from ∼0 to 21 mHz, consistent with strong density fluctuations on all measured time scales. Marginally significant power peaks are seen at periods of ∼4 min on the inbound pass and at ∼2 min on the outbound pass. Mechanisms such as amplification of convected ion pick‐up waves and cometary rays for producing the large plasma variations are discussed. It is also suggested that a Rayleigh‐Taylor driven mixing mechanism at a mass loading boundary ∼105km from the nucl

ISSN:0094-8276    

DOI:10.1029/GL013i003p00271

年代:1986

数据来源: WILEY

摘要:

Photoionization of neutral molecules in the coma surrounding a comet produces heavy ions which contaminate and mass‐load the solar wind. Cometary ion distribution functions in the vicinity of comet Giacobini‐Zinner (G/Z) are calculated using a Monte Carlo method. The distribution function calculated behind the shock has both cold (≈ 2 keV) and hot (≈ 40 keV) com

ISSN:0094-8276    

DOI:10.1029/GL013i003p00275

年代:1986

数据来源: WILEY