The instability of an electron beam with a finite radius in a boundless plasma

Within the framework of a linear theory, the instability of an electron beam with a finite radius in a cold magnetised boundless plasma is considered. It is shown that a finite beam dimension influences the generation of quasi-potential waves in two aspects: the perpendicular wave number is quantised so that the frequencies of the waves are subjected to strong selection; a new kind of instability appears due to wave energy losses by emission into surrounding space. Growth rate dependence of wave numbers and frequencies is investigated for typical parameters of experiments with electron beams in space.     

Evidence and Implications of Pressure Fluctuations in the ISM

A recent survey of the fine-structure excitation of neutral carbonreveals that the interstellar medium in the Galactic plane exhibits athermal pressure, nT/k, that ranges from about 10³to 10⁴cm^-3K from one location to the next, with occasional excursions in excess of about 10⁵ cm^-3K. The large excitations for small amounts of gas indicate that some regions are either subjected to shocks or must be pressurized within time scales much shorter than the time needed to reach thermal equilibrium. These rapidfluctuations probably arise from the cascade of macroscopic mechanical energyto small scales through a turbulent cascade. One consequence of thiseffect is that changes in gas temperature can arise from near adiabaticcompressions and expansions, and this may explain why investigations of21-cm emission and absorption reveal the presence of hydrogen attemperatures well below the expected values derived from the balance ofvarious known heating and cooling processes.     

An iterative scheme for use in MT 2D forward modelling — to speed up production of the Jacobian and to improve accuracy of the inverse matrix

An important stage in two-dimensional magnetotelluric modelling is the calculation of the Earth's response functions for an assumed conductivity model and the calculation of the associated Jacobian relating those response functions to the model parameters. The efficiency of the calculation of the Jacobian will affect the efficiency of the inversion modelling. Rodi (1976) produced all the Jacobian elements by inverting a single matrix and using an approximate first-order algorithm. Since only one inverse matrix required calculation the procedure speeded up the inversion. An iterative scheme to improve the approximation to the Jacobian information is presented in this paper. While this scheme takes a little longer than Rodi's algorithm, it enables a more accurate determination of the Jacobian information. It is found that the Jacobian elements can be produced in 10% of the time required to calculate an inverse matrix or to calculate a 2D starting model. A modification of the algorithm can further be used to improve the accuracy of the original inverse matrix calculated in a 2D finite difference program and hence the solution this program produces. The convergence of the iteration scheme is found to be related both to the originally calculated inverse matrix and to the change in the newly formed matrix arising from perturbation of the model parameter. A ridge regression inverse algorithm is used in conjunction with the iterative scheme for forward modelling described in this paper to produce a 2D conductivity section from field data.     

Rosetta mission mutual impedance probe modelling: the short and long Debye length plasma cases

The Rosetta spacecraft (S/C), which is planned to meet comet 46P/Wirtanen in 2011, will carry a set of five wave and plasma instruments (i.e. the Rosetta Plasma Consortium). This is to measure the cometary plasma properties from the minimum value of activity of the comet to its maximum value at perihelion. The mutual impedance probe, MIP, is one of those (Trotignon et al., 1999) five. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dynamical friction force exerted on spherical bodies

We present a rigorous calculation of the dynamical friction force exerted on a spherical massive perturber moving through an infinite homogenous system of field stars. By calculating the shape and mass of the polarization cloud induced by the perturber in the background system, which decelerates the motion of the perturber, we recover Chandrasekhar's drag force law with a modified Coulomb logarithm. As concrete examples, we calculate the drag force exerted on a Plummer sphere or a sphere with the density distribution of a Hernquist profile. It is shown that the shape of the perturber only affects the exact form of the Coulomb logarithm. The latter converges on small scales because encounters of the test and field stars with impact parameters less than the size of the massive perturber become inefficient. We confirm in this way the earlier results based on the impulse approximation of small angle scatterings.     

Oceanography and marine biology, 10: Harold Barnes (editor). George Allen and Unwin,London, 1972, 565 pp., £10.50

A bathymetric survey of the Gilliss Seamount, in the northwest Atlantic Basin, using a multi-beam sonar array system reveals an extremely complex morphologic character of this feature. A new chart provides the most detailed topographic presentation of an Atlantic seamount published to date and highlights the similarity of the Gilliss Seamount with terrestrial strata-volcanoes. Bottom photographs and samples reveal pillow-Iava formation. Seismic profiles show that the volcanic basement is irregularly covered by acoustically transparent deposits that are as much as 668 m thick. Volcanic debris and sediments locally are displaced down the flanks of the seamount. Bottom photographs and cores indicate that the transparent layer has accumulated slowly by deposition from suspensate-rich (mostly clay and planktonic foraminifera) water masses that flow around the mid to lower sectors of this submarine volcano. Bottom-current activity also modifies the abyssal plain turbidite-hemipelagic sequence surrounding the seamount.