A. I. Ershkovich

Kelvin-Helmholtz instability in type-1 comet tails and associated phenomena

Selected problems of the solar wind — comet tail coupling that are currently accessible to quantitative analysis are reviewed. The model of a comet tail as a plasma cylinder separated by a tangential discontinuity surface from the solar wind is discussed in detail. This model is compatible with the well-known Alfvén mechanism of formation of the comet tail. The stability problem of the comet tail boundary (considered as a discontinuity surface) is solved. Under typical conditions a comet tail boundary can undergo the Kelvin-Helmholtz instability. With finite amplitude the stabilizing effect of the magnetic field increases, and waves become stabilized. This model supplies a detailed quantitative description of helical waves observed in type-1 comet tails. A more general model of the tail boundary as a transition layer with a continuous change of the plasma parameters within it is also considered.This theory, in principle, enables us to solve one of the fundamental problems of cometary physics: the magnetic field of the comet tail can be derived from the observations of helical waves. This field turns out to be of the order of the interplanetary field. Various other considerations, discussed in this review, also support this conclusion.

Titan's magnetic wake: Atmospheric or magnetospheric interaction

We compare the magnetic field topology in the Titan wake to an idealized picture of magnetic field lines draping about a conductive nonmagnetic obstacle. It is shown that in the inner part of the wake the magnetic field picture differs significantly from that expected for an idealized draping: The transverse magnetic field component rotates by 90° as compared with the direction of the upstream transverse magnetic field. Another difference is the existence of a deep magnetic field minima separating the inner and outer parts of the wake. Transverse magnetic field rotation can be explained neither by temporal changes in the upstream magnetic field nor by reconnection processes in the wake. We find that this behavior of the transverse magnetic field can be explained at least qualitatively if one assumes the existence of a small intrinsic magnetic field of Titan. An effective magnetic dipole of 1021 G cm3 can account for the observed topology of the Titan magnetic wake. The origin of this field may be related to a residual magnetization of Titans crust or to induction in a conducting ionosphere of the satellite. We present results of MHD simulations which support the above theoretical conclusion.

The induced magnetosphere of comet Halley: Interplanetary magnetic field during Giotto Encounter

Direction of the interplanetary magnetic field (IMF) which governs the orientation of the Halleys magnetosphere was restored by using Giotto magnetic field data. The Giotto trajectory was a twisted curve in the coordinate system rotating with IMF vector and covered rather uniformly the transverse cross section of the comet. Magnetic field vectors show a regular structure arising from the field line draping about the cometary ionosphere. Violations of the field line draping occur only at the moments of rapid changes of IMF orientation.

Plasma density in the outer Jovian magnetosphere

We assume that the dipole wobble excites Alfven waves which propagate outward along the field lines. The plasma density in the outer Jovian magnetosphere is derived from the amplitude of such diurnal magnetic field variations, as measured by Pioneer 10. The number density obtained by this method is of the same order of magnitude as that derived from pressure balance, the dynamic pressure of the outflow being neglected. This result casts some doubt on the existence of a super-Alfvenic outflow in the Jovian magnetosphere. (AIP)

The induced magnetosphere of comet Halley: 4. Comparison of in situ observations and numerical simulations

A comprehensive comparison between the results of a multiscale three-dimensional adaptive MHD model of the comet Halley magnetosphere and in situ observations by the Giotto mission is presented. It is shown how a steady state model can describe the effects of the varying IMF direction on the cometary magnetosphere. The simulation results reproduce the observed profiles of the magnetic field and plasma velocity very well. There are only two discrepancies. First, the magnitudes of both the magnetic field strength and plasma velocity are slightly but systematically lower in numerical simulation as compared with the experiment. This can be attributed to some inaccuracy in our knowledge of plasma interaction parameters (ion-neutral momentum exchange rate, photoinization and recombination rates). Second, the observed and simulated magnetic field profiles show some differences in the anticipated ion pile-up region along the outbound trajectory. This means that the ion pile-up region on the outbound leg (if it existed) differed significantly from that on the inbound leg.

MHD simulation of the three-dimensional structure of the heliospheric current sheet

The existence of the radial component of the electric current flowing toward the Sun is revealed in numerical simulation. The total strength of the radial current is3 10 9 A. The only way to full the electric current continuity is to close the radial electric current by means of eld- aligned currents at the polar region of the Sun. Thus, the surface density of the closure current flowing along the solar surface can be estimated as 4 A/m, and the magnetic eld produced by this current is B 5 10 6 T, i.e. several percent of the intrinsic magnetic eld of the Sun. This seems to mean that any treatment of the solar magnetic eld generation should take into account the heliospheric current circuit as well as the currents flowing inside the Sun.

Solar wind interaction with the tail of Comet Kohoutek

Abstract Helical waves of large amplitude observed recently in the tail of Comet Kohoutek are interpreted as stable waves arising due to non-linear evolution of Kelvin-Helmholtz instability. The dispersion equation for waves of a finite amplitude shows that the phase velocity of these waves should approximately coincide with the velocity of the plasma outflow in the tail rather than with the Alfven velocity. This fact is shown to be in agreement with observations. One may estimate the magnetic field in the Comet Kohoutek tail from both the amplitude of observed helical waves and the pressure balance at the tail boundary. The field turns out to be of the order of the interplanetary magnetic field or less, i.e. ≲25 γ near ∼0.5 AU.

Magnetic field and electric current density distribution in the geomagnetic tail, based on Geotail data

Analysis of Geotail magnetic data enables us to reveal the antisunward electric current flowing along the tail axis. To our knowledge, observation of this current has not been reported yet. Distributions of the magnetic field and electric current density (along with their dependencies on the tilt angle of the Earths dipole and components of the interplanetary magnetic field) have been derived directly from Geotail data, without any ad hoc assumptions. Analysis of the electric current density distribution shows that, in addition to currents associated with the geomagnetic tail flaring, there is a current (tentatively identified as the Hall current) flowing antisunward along the tail axis. The total strength of this current is of the order of 1 MA. It closes through the midnight sector of the auroral zone resulting in field-aligned currents in the region of the Harang discontinuity.

Helical waves in type-1 comet tails

Oscillations of type-1 comet tails with plasma compressibility taken into account are studied. A comet tail is treated as a plasma cylinder separated by a tangential discontinuity surface from the solar wind. The dispersion equation obtained in the linear approximation is solved numerically with typical plasma parameters. A sufficient condition for instability of the cylindrical tangential discontinuity in the compressible fluid is obtained. The phase velocity of helical waves is shown to be approximately coincident with Alfvén speed in the tail in the reference system moving with the bulk velocity of the plasma outflow in the tail. The instability growth rate is calculated.This theory is shown to be in good agreement with observations in the tails of Comets Kohoutek, Morehouse and Arend-Roland. Hence we conclude that helical waves observed in type-1 comet tails are produced due to the Kelvin-Helmholtz instability, and the model under consideration is justified. If so, one may estimate comet tail magnetic field from the pressure balance at the tangential discontinuity; it turns out to be of the order of the interplanetary magnetic field.

On the momentum balance along the Comet Halley — Sun line during the Giotto flyby

The plasma velocity along the Sun - comet Halley line is derived by using in situ measurements of the magnetic field along the Giotto trajectory. Including this velocity profile in the momentum balance turns out to be insufficient in order to support the magnetic barrier with conventional parameters of ion-neutral interaction. The observed magnetic barrier of comet Halley is shown to be consistent with the following parameters of interaction: kin = 5.8 × 10−9 cm3 · s−1 (with Q = 6.9 × 1029 s−1) or Q = 19 × 1029 s−1 (with kin = 1.1 × 10−9 cm3 · s−1). An estimated electric field of convection enables one to find a shielding factor of 0.03 - 0.07 in the maximum of the magnetic barrier of comet Halley.