Plasma transport across the heliopause

Existing heliopause models are critically rediscussed under the new aspect of possible plasma mixing between the solar wind and the ambient ionized component of the local interstellar medium (LISM). Based on current kinetic plasma theories, effective diffusion rates across the heliopause are evaluated for several models with turbulence caused by electrostatic or electromagnetic interactions that could be envisaged in this context. Some specific cases that may lead to high diffusion rates are investigated, especially in regard to their LISM magnetic field dependence.For weak fields (less than 10^–7 G), macroscopic hydrodynamic instabilities, such as of Rayleigh-Taylor or Kelvin-Helmholtz-types, can be excited. The resulting plasma mixing rates at the heliopause may amount to 20–30% of the impinging mass flow.Recently, an unconventional new approach to the problem for the case of tangential magnetic fields at the heliopause was published in which a continuous change of the plasma properties within an extended boundary layer is described by a complete set of two-fluid plasma equations including a hybrid MHD-formulation of wave-particle interaction effects. If a neutral sheet is assumed to exist within the boundary layer, the magnetic field direction is proven to be constant for a plane-parallel geometry. Considering the electric fields and currents in the layer, an interesting relationship between the field-reconnection probability and the electric conductivity can be derived, permitting a quantitative determination of either of these quantities.An actual value for the electrical conductivity is derived here on the basis of electron distribution functions given by a superposition of Maxwellians with different temperatures. Using two-stream instability theory and retaining only the most unstable modes, an exact solution for the density, velocity, and magnetic and electric fields can be obtained. The electrical conductivity is then shown to be six orders of magnitude lower than calculated by conventional formulas. Interestingly, this leads to an acceptable value of 0.1 for the reconnection coefficient.By analogy with the case of planetary magnetopauses, it is shown here for LISM magnetic fields of the order of 10^–6 G or larger that field reconnection processes may also play an important role for the plasma mixing at the heliopause. The resulting plasma mixing rate is estimated to amount to an average value of 10% of the incident mass flow. It is suggested here that the dependence of the cosmic-ray penetration into the heliosphere on the distribution of reconnecting areas at the heliopause may provide a means of deriving the strength and orientation of the LISM field.A series of observational implications for the expected plasma mixing at the heliopause is discussed in the last part of the paper. In particular, consequences are discussed for the generation of radio noise at the heliopause, for the penetration of LISM neutrals into the heliosphere, for the propagation of cosmic rays towards the inner part of the solar system and for convective electric field mergings into the heliosphere during the course of the solar cycle, depending on the solar cycle variations. With concern to a recent detection of electrostatic plasma waves by plasma receivers on Voyagers 1 and 2, we come to an interesting alternate explanation: the heliopause, rather than the heliospheric shock front, could be responsible for the generation of these waves.     

Tracking the Frequency Translation of a Sum of Orthogonal Sinusoids

The maximum likelihood approach is used to derive a method for estimating and tracking the frequency translation of a signal consisting of a sum of orthogonal sinusoids corrupted by additive white noise. The likelihood function is reduced to an equivalent statistic expressed in terms of the squared magnitude of the finite Fourier transform of the received signal. A function that generates an error signal for a frequency translation tracking loop is derived, and a method of generating the error signal using the discrete Fourier transform (DFT) of the received signal weighted by a linear ramp is suggested. Two noise-free examples are presented.     

Modeling Random Gyro Drift Rate by Data Dependent Systems

A new modeling procedure is presented in which a mathematical model for random gyro drift rate is obtained directly from the original observations without requiring any theoretical conjectures or preprocessing of the data. The adequate model so obtained confirms the presence of a random walk component causing the "steady state" drift and a stochastic component of the nature of correlated noise. It also provides a quantitative measure of the relative contribution of each component, useful in determining the quality of the gyro in guidance over extended periods.     

Design of the HEAO Main Bus Shunt Regulator

The High Energy Astronomy Observatory (HEAO) main bus regulator contains a redundant error amplifier (REA) and a 12-section sequenced shunt transconductance amplifier (STA) in which not more than three and not less than one shunt section are in an active control mode regardless of a failure in any one of the twelve sections. The part of the work concerned with the shaping of the frequency response of the REA and STA minor feedback loops and of the teolator major feedback loop is described. Emphasis is on the application of a general design technique embodying a "design-analyze-measure iteration loop," in which a first-cut design and analysis is corrected ard improved after experimental observation of discrepancies and deficiencies. In particular, an incipient high-frequency oscillation was thereby reliably eliminated. It is demonstrated that analysis techniques involving a minimum of algebra can provide full quantitative information on an efficient and reliable system design.     

Microprocessor-Controlled Communications in an Advanced Instrument Landing System

The application of 4-bit microprocessors to the communication of the control and status signals found in an advanced aircraft instrument landing system is presented. The advantages as well as some of the peculiarities of such an approach are described. Among the benefits, one must include the possibility of a very convenient and understandable human interface.     

Streamer Evaporation

Streamer evaporation is the consequence of heating in ideal MHD models because plasma is weakly contained by the magnetic field. Heating causes inflation, opening of field lines, and release of solar wind. It was discovered in simulations and, due to the absence of loss mechanisms, the ultimate end point is the complete evaporation of the streamer. Of course streamers do not behave in this way because of losses by thermal conduction and radiation. Heating is also expected to depend on ambient conditions. We use a global MHD model with thermal conduction to examine the effect of changing the heating scale height. We also extend an analytic model of streamers developed by Pneuman (1968) to show that steady streamers are unable to contain plasma for temperatures near the cusp greater than ∼ 2 × 10⁶ K. This revised version was published online in June 2006 with corrections to the Cover Date.     

Small satellite access of the Space Network

Small satellites have been perceived as having limited access to NASA's Space Network (SN). The potential for satellite access of the Space Network when the design utilizes a fixed antenna configuration and low-power, coded transmission is analyzed. From the analysis, satellites using this configuration in high-inclination orbits are shown to have a daily data throughput in the 100 to 1000 Mbit range using the multiple access communications service     

UVCS/SOHO Observations of Spectral Line Profiles in Polar Coronal Holes

Ultraviolet emission line profiles have been measured on 15-29 September 1997 for H I 1216 Å, O VI 1032, 1037 Å and Mg X 625 Å in a polar coronal hole, at heliographic heights ? (in solar radii) between 1.34 and 2.0. Observations of H I 1216 Å and the O VI doublet from January 1997 for ? = 1.5 to 3.0 are provided for comparison. Mg X 625 Å is observed to have a narrow component at ? = 1.34 which accounts for only a small fraction of the observed spectral radiance, and a broad component that exists at all observed heights. The widths of O VI broad components are only slightly larger than those for H I at ? = 1.34, but are significantly larger at ? = 1.5 and much larger for ? > 1.75. In contrast, the Mg X values are less than those of H I up to 1.75 and then increase rapidly up to at least ? = 2.0, but never reach the values of O VI.     

Studies of Comets in the Ultraviolet: The Past and the Future

The remote sensing of comets in the ultraviolet bandpass has been a valuable tool for studying the structure, composition, variability, and physical processes at work in cometary comae. By extension, these studies of comae have revealed key insights into the composition of cometary nuclei. Here we briefly review the ultraviolet studies of comets, and then take a look toward the future of such work as anticipated by the advent of several key new instruments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Advanced materials for next generation NiMH portable, HEV and EVbatteries

While Ovonic NiMH batteries are already in high volume commercial production for portable applications, advances in materials technology have enabled performance improvements in specific energy (100 Wh/kg), specific power (600-1000 W/kg), high temperature operation, charge retention, and voltage stability. Concurrent with technology advances, Ovonic NiMH batteries have established performance and commercial milestones in electric vehicles, hybrid electric vehicles, as well as scooter, motorcycle and bicycle applications. As important as these advances, significant manufacturing cost reductions have also occurred which allow continued growth of NiMH technology. In this paper, advances in performance, applications and cost reduction are discussed with particular emphasis on the improved proprietary metal hydride and nickel hydroxide materials that make such advances possible