We investigate the nonthermal X-ray emission from Cas A, using BeppoSAX, Compton Gamma Ray Observatory OSSE, and Chandra data. For the hard X-ray continuum we test the model proposed by Laming, which invokes nonthermal bremsstrahlung from electrons accelerated by lower hybrid plasma waves. The justification for this model comes from our determination of a lower limit to the average magnetic field of B > 0.5 mG. For such high magnetic fields, the synchrotron losses are severe enough that most of the electron populations responsible for the radio emission have maximum electron energies well below the limit for which X-ray synchrotron emission is important. However, we do suggest that the rim surrounding Cas A, seen in Chandra continuum images, is X-ray synchrotron emission. The width of this rim of 15-4'' can be used to infer the magnetic field near the shock front, for which we estimate B = 0.08-0.16 mG and electron energies of ~57-40 TeV. This magnetic field strength is lower than the average magnetic field but higher than what may be expected from shocked interstellar medium, suggesting either a high magnetic field in the wind of the progenitor or rapid postshock field amplification by nonlinear growth of plasma waves. Combining the two magnetic field measurements, we have constructed a simple two-zone model. Most of the radio emission comes from inside Cas A, where the magnetic field is strong. In contrast, the inverse Compton emission is dominated by emission from near the shock front. Only for our lower limit on the magnetic field strength near the shock front is it possible to explain the recent detection of TeV emission by the High Energy Gamma Ray Astronomy experiment with inverse Compton emission, for which, in addition, we have to assume a rather high far-infrared photon density that should be twice as high as our best estimate of ~70 cm-3. Pion decay is therefore likely to be the dominant emission from Cas A at TeV energies.

https://iopscience.iop.org/article/10.1086/345832/pdf

On the Magnetic Fields and Particle Acceleration in Cassiopeia A

The magnetic fields of Mars have been measured from Phobos 2 with high temporal resolution in the tail and down to an 850-km altitude. During four successive highly elliptical orbits, the position of the bow shock as well as that of a transition layer, the 'planetopause', were identified. Subsequent circular orbits at 6000-km altitude provided the first high-resolution data in the planetary tail and indicate that the interplanetary magnetic field mainly controls the magnetic tail. Magnetic turbulence was also detected when the spacecraft crossed the orbit of Phobos, indicating the possible existence of a torus near the orbit of this moon.

Magnetic fields near Mars: First results

Ion thermalization mechanisms downstream of the quasi-perpendicular Earth's bow shock are examined by means of plasma and magnetic field data from the AMPTE/IRM spacecraft which include three-dimensional ion distributions, plasma fluid parameters derived every ∼4.3 s, and spectra of transverse and parallel magnetic fluctuations up to 16 Hz. The objects studied in detail are low-Mach number, low-β shocks in which reflected-gyrating ions are present and contribute to the downstream ion temperature but where processes beyond the ramp take place slowly, so that the basic phenomenology becomes apparent. In MHD terms, most of these shocks qualify as marginally critical. Downstream of the ramp, the initially separated core and ring ions slowly merge into a joint, less anisotropic distribution possessing a high-energy tail. The ion temperature ratio, T⊥/T∥, is high not only in the shock foot and ramp but also within some distance downstream; its speed of decline rises and the residual level lessens with increasing β. The ions diffuse about equally fast in energy and in pitch angle. An asymmetry of the distributions with respect to the field direction is present when the shock is slightly oblique. It decays only slowly, which might indicate that the pitch angle diffusion rate near zero pitch angle is reduced. Low-frequency electromagnetic waves are present below the proton gyrofrequency; they are characterized by strong left-hand-polarized emissions and a low level of parallel fluctuations except very close to the shock. The left-hand emissions are often concentrated into a narrow frequency band but sometimes they exhibit a double-humped structure. Waves and ion distributions approach a slowly varying equilibrium some distance downstream of the shock. After extending the analysis to one supercritical shock representing the majority of bow shock encounters, we conclude that our deductions are more generally valid, although the thermalization is faster, usually, and appears to involve nonlinear processes which tend to obscure most of the features noted.

Ion thermalization in quasi-perpendicular shocks involving reflected ions

The solar wind interacts very strongly with the extensive cometary coma, and the various interaction processes are initiated by the ionization of cometary neutrals. The main ionization mechanism far outside the cometary bow shock is photoionization by solar extreme ultraviolet radiation.Electron distributions measured in the vicinity of comets Halley and Giacobini-Zinner by instruments on the VEGA and ICE spacecraft, respectively, are used to calculate electron impact ionization frequencies. Ionization by electrons is of comparable importance to photoionization in the magnetosheaths of Comets Halley and Giacobini-Zinner. The ionization frequency in the inner part of the cometary plasma region of comet Halley is several times greater than the photoionization value. Tables of ionization frequencies as functions of electron temperature are presented for H2O, CO2, CO, O, N2, and H.

/pdf/electron-impact-ionization-in-the-vicinity-of-comets-3gsw3r2shy.pdf

Electron impact ionization in the vicinity of comets

The 2.5-12 μm spectrum of comet halley from the IKS-VEGA experiment

We present the first in situ observations and a description of the large-scale behaviour of comet Halley's plasma environment. The PLASMAG-1 experiment, carried aboard the spacecraft Vega 1 and Vega 2, had the following aims: (1) to study the change of plasma parameters and distributions as a function of distance from the nucleus; (2) to investigate the existence and structure of the cometary bow shock; (3) to determine the change in chemical composition of the heavily mass-loaded plasma as the spacecraft approached the comet; and (4) to measure the neutral gas distribution along the spacecraft trajectory. We observe a discontinuity (the ‘cometopause’) between the solar-wind-controlled cometosheath and heavy-ion mantle and the magnetized cometary plasma region. From the measured neutral gas density distribution we estimate a total gas production rate of 1.3×1030 molecules s−1.

First in situ plasma and neutral gas measurements at comet Halley

Plasma observations near comet Halley indicate that around 1.6x105km from the nucleus a newly discovered sharp boundary (cometopause) separates the solar wind controlled external and the heavy cometary ion dominated internal regions. Such a discontinuity was previously not predicted by theoretical models. Inside the cometopause (in the cometary plasma region) the protons and heavy ions move with different speeds: the heavy ion velocity is less than a few km/s throughout this region, while the protons decelerate from several tens of kn.s (observed near the cometopause) to a few km/s (near 1.5x10'km).

/pdf/detection-of-a-new-chemical-boundary-at-comet-halley-hpsakt2wd5.pdf

I. Apathy

Papers

First in situ plasma and neutral gas measurements at comet Halley

Detection of a new “chemical” boundary at comet Halley

The 'VEGA' probe instrument package for measuring charged particles with energies less than 25 keV

Cometary plasma region in the coma of Comet Halley: Vega-2 measurements