Giotto/Cometary dust interaction event near the comet Halley ionopause

Plasma and magnetic field disturbances accompanying dust particle impacts are explained by means of creation of a secondary cloud around the spacecraft. Cold cometary ions impinging upon the cloud are scattered by atoms of the cloud. This scattering changes initial angular distribution of cometary ions. Magnetic field perturbation is created by the friction between the electron component of the cometary plasma flow and the cloud.     

等离子体彗尾动力学的数值研究

本文基于可压缩磁流体动力学模型,数值研究了尾瓣巾具有超Alfven速流动的等离子体彗尾的动力学特征。结果表明,等离子体片和尾瓣之间的剪切等离子体流动将会激发流动撕裂模不稳定性,引起彗尾等离子体片中发生磁场重联,形成磁岛和高密度的等离子体团。进而模拟了太阳风引起的局部驱动力对等离子体彗尾中磁场重联的影响,其特征时间远大于流动撕裂模。我们认为一些观测到的等离子体彗尾中的四块和彗尾截断事件可能主要与彗尾中剪切等离子体流动所引起的流动撕裂模不稳定性有关。      

The Martian magnetotail

There is evidence of a strong influence of an atmospheric (cometary) interaction on the Martian tail formation: small total magnetic flux in the tail, the existence of plasma flow of apparently planetary origin, interplanetary magnetic field control of magnetic field orientation in the tail and other evidence. At the same time the large radius of the Martian magnetotail (about 2 planetary radii) can be considered as a strong evidence for the existence of a planetary magnetic field. Plasma and magnetic field properties in the Martian tail are in many respects similar to the ones observed in the tail of Venus. The limited amount of near-Mars measurements leads to some reservations in coming to definite conclusions. A combined magnetosphere of Mars is suggested that consists of two polar-tied magnetic tubes connected to the tail and an equatorial Venus-type interaction region in-between.     

Cometary boundaries: VEGA observations at Halley

This paper describes the boundaries which have been observed by the magnetic field experiment aboard the VEGA-1 and VEGA-2 spacecraft during the Halley encounters on March 6 and 9, 1986. The outer boundaries are found to be closely related to the predicted cometary shock front. The inner boundaries near the closest approach are compared with plasma observations and with the results of MHD computer simulations in three dimensions.     

The detection of energetic cometary and solar particles by the EPONA instrument on the Giotto mission

EPONA is an energetic particle detector system incorporating totally depleted silicon surface barrier layer detectors. Active and passive background shielding will be employed and, by applying various techniques, particles of different species, including electrons, protons, alpha particles and pick-up ions of cometary origin may be detected over a wide spectrum of energies extending from the tens of KeV into the MeV range.

The instrument can operate in two modes namely (a) in a cruise phase or storage mode and (b) in a real time mode. During the real time mode, observations at high spatial (octosectoring) and temporal (0.5s) resolution in the cometary environment permit studies to be made of accelerated particles at the bow shock and/or in the tail of the comet. In conjunction with magnetic field measurements on board Giotto, observations of energetic electrons and their anisotropies can determine whether the magnetic field lines in the cometary tail are open or closed. Further, the absorption of low energy solar particles in the cometary atmosphere can be measured and such data would provide an integral value of the pertaining gas and dust distribution. Solar particle background measurements during encounter may also be used to correct the measurements of other spacecraft borne instruments potentially vulnerable to such radiation.

Solar particle flux measurements, obtained during the cruise phase will, when combined with simultaneous observations made by other spacecraft at different heliographic longitudes, provide information concerning solar particle propagation in the corona and in interplanetary space.     

Simulation of cometary jets in interaction with the solar wind

The influence of cometary jets on the solar wind interaction is studied with a 3D hybrid simulation. Anisotropic outgassing patterns were until recently not considered in cometary simulations, despite strong anisotropies found at observations. Comet 67P Churyumov–Gerasimenko, the target of the ROSETTA mission, was chosen as a case study for a simulation series. The cometary outgassing at 2.7 AU is modeled to originate from a single sun-facing jet with different levels of collimation, from isotropy to extremely thin jets. As no bow shock is present at this distance, solar wind patterns resulting from the anisotropic outgassing become more apparent. We find narrower jets to increase the standoff distance of the plasma interaction structures. Also, the Mach cone is wider and stronger for certain jet profiles. The magnetic field remains unable to propagate through the coma, resulting in strong draping patterns for narrow jets due to the increased standoff distance.     

The plasma regime at comet Giacobini-Zinner

This review of the plasma regime sampled by the encounter of the International Cometary Explorer spacecraft (ICE) with the comet Giacobini-Zinner, discusses the shock, or bow wave, ion pickup, ionization mechanisms, and the cometary plasma tail.

The observations are consistent with the existence of a weak shock, which may be pulsating, but do not exclude the suggestion by Wallis and Dryer that the shock, though present around the sub-solar point, is in process of decaying to a wave on the flanks.

Pickup of cometary ions provokes, by means of several mechanisms, ion cyclotron, mirror, beam and electrostatic instabilities which cause strong turbulence in the inner coma, as indicated in the power spectra of the magnetic field in the coma and the surrounding volume. Heavy mass loading and consequent slowing down of the solar wind is observed. Acceleration of ions by a stochastic mechanism is indicated.

Ionization of cometary neutrals occurs principally by photoionization and charge exchange. Alfvens critical velocity mechanism, likely operates only in the inner coma not visited by ICE. A steep increase of nearly two orders of magnitude in electron density occurs in the tail, where electron velocity distributions show evidence of entry of electrons from the solar wind. The turbulence there is damped by the high ion density and low temperature.

In general, the vicinity of the comet is filled with plasma phenomena and a rich variety of corresponding atomic and molecular processes can be studied there. Comparison between the ICE, Giotto, and Vega observations forms a most valuable future study.     

Solar wind interaction with comet Halley

In March 6 and 9, 1986 the spacecrafts ‘Vega-1’ and ‘Vega-2’ have flown through the coma of comet Halley and have carried measurements of plasma, energetic particles, magnetic field and plasma waves along its trajectory. A short review of these measurements and its comparison with theoretical models of solar wind interaction with comets are given.

The spacecrafts ‘Vega-1’ and ‘Vega-2’ have studied the solar wind loading by cometary ions, the structure of cometary bow shock and the processes in the inner coma of comet Halley. Exactly in this sequence we discuss the results of measurements and compare them with the theory.     

Thermomechanical stresses on comets

Thermal stresses due to temperature differences between the cometary surface and the core have been calculated for different models of cometary nuclei. It is shown, that for comets on P/Halley-type orbits thermomechanical stresses exceed the (cohesive) strength of water ice near to the cometary surface. Consequently, there should be cracks on cometary surfaces. The existence of line structures on the surface of P/Halley, which might be due to thermomechanical cracks, is demonstrated on the basis of VEGA-images from the surface of P/Halley.     

Comet-solar wind interactions: A dusty point of view

Anticipating the new results from the space missions to Comet Halley and Comet Giacobini-Zinner, we make a brief review of recent theoretical and observational studies of dust-plasma environment. In order to relate different disciplines in cometary research in the context of comet-solar wind interaction, two separate issues: (a) surface processes and (b) plasma processes are considered to indicate how various kinds of observations of cometary dust comas and tails may be used to infer the conditions of solar wind - comet interaction and the corresponding plasma processes in the cometary ionospheres and ion tails (and vice-versa). In particular, it is suggested that the narrow sunward-pointing dust streamers emitted from the cometary nuclei could be related to the electrostatic transport of sub-micron dust over the nuclear surfaces at large heliocentric distances; and the striae sometimes observed in cometary dust tails at smaller heliocentric distances could be the consequence of electrostatic fragmentation of fluffy dust particles in the ion tails.     

Cometary probe of the Venera-Halley mission

Venera-Halley mission is to be launched to Venus in Dec. 1984. It will fly by Venus in June 1985. Separation of the cometary probe and Venera descend module will take place at that time. The gravitational swing-by at Venus will provide the encounter with the Halley comet in March 1986. The remote sensing of the inner coma (TV-imagery, spectrometry in the region from 1200 A to 12 μm, polarimetry) and of the nucleus, direct measurements of dust fluxes, dust composition, plasma and magnetic field are planned in the framework of multinational cooperation.     

Comet plasma tail formation — Giotto observations

The process of mass loading of the solar wind by cometary ions, which forms comet tails, has been observed throughout the coma of comet Halley. Three distinct regimes were found where the nature of the energy and momentum coupling between solar wind and cometary ions is different. Outside the bow shock, where there is little angular scattering of the freshly ionised particles, the coupling is described by the simple pickup trajectory and the energy is controlled by the angle between the flow and the magnetic field. Just inside the bow shock, there is considerable scattering accompanied by another acceleration process which raises some particle energies well above the straightforward pickup value. Finally, closer to the nucleus, the amount of scattering decreases and the coupling is once more controlled by the magnetic field direction.     

Energetic positive ions in the cometary “foreshock” region

Ions produced by ionization of the cometary neutrals interact with the solar wind protons to produce large amplitude oscillations of the ambient magnetic field. Such oscillations are convected towards the comet at the unperturbed solar wind speed far from the shock and at a lower speed closer to the shock (due to the solar wind mass loading); hence, they can energize the incoming ions by Fermi acceleration. The spatial extension of the acceleration region is of the order of 10⁶ km and the resulting energy spectrum is harder than in the Earth's bow shock case. The energization of cometary ions produces an additional deceleration of the solar wind. It is suggested that Comet Halley may be the most efficient “cosmic ray shock” in the solar system.     

Thermal model and thermo-mechanical stresses in cometary nuclei

Spherically symmetric radial temperature profiles of cometary nuclei have been determined numerically (and for simplified models analytically) in dependence on the orbital position of the periodic comet Halley. These temperature fields in the nucleus are connected with thermal stress fields which have been calculated with the assumption of elastic properties of cometary matter. The remarkable result is the possible existence of stresses, strong enough to cause internal cracking of the nucleus and break-ups of the cometary surface. This may be essential understanding normal cometary activity as well as outbursts and splits.     

COSMIC PLASMA DYNAMO

A new dynamo model based on the polarization of plasma is presented in this paper. From the Maxwell equations in a moving medium, a magnetization vector can be caused with Rongon current. The steady solar magnetic field is solved from the equations. On the assumption that the meridianal flow is ignored, the distribution of magnetic field is put out. In the model, there is no additional parameter considered. The intensity of magnetic field inside the sun ranges from 1-6 T. The surface magnetic field around the pole is in the order of 1 ×10-3 T, at low latitude the calculated surface magnetic field has the order of 1×10-2 T. The maximum magnetic field is around 30° in latitude.     

中子星吸积柱中荷电粒子密度的分布及电场的形成

本文从刘维定理出发,通过相空间求平均,建立了中子星吸积柱中粒子流连续方程和动量迁移方程。并在静力学平衡下求出其解;建立了吸积柱中的荷电粒子分布;引出了某些有趣的新结果。      

膨胀磁场和太阳风粒子相互作用三维数值模拟

采用三维模型,使用混合网格质点法HPIC(Hybrid Particle-in-Cell)对膨胀的磁场和太阳风相互作用过程进行数值模拟.研究了线圈产生的偶极子磁场在注入等离子体后和太阳风粒子的相互作用过程,并对以不同速度入射的等离子体引起的太阳风粒子的变化和磁场变化进行了比较.研究结果表明,偶极子磁场和太阳风作用时会产生弓形激波,此时磁压等于太阳风粒子的动压,当向线圈产生的偶极子磁场中注入高能等离子体时引起磁场膨胀,膨胀的磁场将会排斥太阳风粒子向外运动,从而引起弓形激波的变化,增大与太阳风相互作用的面积,并且粒子入射速度越大,磁场膨胀越明显,与太阳风相互作用愈强.      

Cometary magnetospheres: a tutorial

The nucleus of an active comet, such as comet Halley near its perihelion, produces large quantities of gas and dust. The resulting cometary atmosphere, or coma, extends more than a million kilometers into space, where it interacts with the solar wind. An “induced” cometary magnetosphere is a consequence of this interaction. Cometary ion pick-up and mass loading of the solar wind starts to take place at very large cometocentric distances. Eventually this mass loading leads to the formation of a weak cometary bow shock. Even closer to the nucleus, collisional processes, such as ion-neutral chemistry, become important. Other features of the magnetosphere of an active comet include a magnetic barrier, a magnetotail, and a diamagnetic cavity near the nucleus. X-ray emission from comets is produced by the interaction of the solar wind with cometary neutrals and this topic is also discussed. A broad review of the cometary magnetosphere will be given in this paper.     

Evolution of magnetic fields and cosmic ray acceleration in supernova remnants

Observations show that the magnetic field in young supernova remnants (SNRs) is significantly stronger than can be expected from the compression of the circumstellar medium (CSM) by a factor of four expected for strong blast waves. Additionally, the polarization is mainly radial, which is also contrary to expectation from compression of the CSM magnetic field. Cosmic rays (CRs) may help to explain these two observed features. They can increase the compression ratio to factors well over those of regular strong shocks by adding a relativistic plasma component to the pressure, and by draining the shock of energy when CRs escape from the region. The higher compression ratio will also allow for the contact discontinuity, which is subject to the Rayleigh–Taylor (R–T) instability, to reach much further out to the forward shock. This could create a preferred radial polarization of the magnetic field. With an Adaptive Mesh Refinement MHD code (AMRVAC), we simulate the evolution of SNRs with three different configurations of the initial CSM magnetic field, and look at two different equations of state in order to look at the possible influence of a CR plasma component. The spectrum of CRs can be simulated using test particles, of which we also show some preliminary results that agree well with available analytical solutions.