Performance measure for Markovian switching systems using best-fitting Gaussian distributions

This paper considers the theoretical posterior Cramer-Rao lower bound (PCRLB) for the case of tracking a manoeuvring target with Markovian switching dynamics. In a recent article [2] it was proposed to calculate the PCRLB conditional on the manoeuvre sequence and then determine the bound as a weighted average, giving an unconditional PCRLB. However, we demonstrate that this approach can produce an overly optimistic lower bound, because the sequence of manoeuvres is implicitly assumed known. Motivated by this, we develop a general approach and derive a closed-form estimate of the PCRLB in the case of Markovian switching systems. The basis of the approach is to, at each time step, replace the multi-modal prior target probability density function (pdf) with a best-fitting Gaussian (BFG) approximation. We present a recursive formula for calculating the mean and covariance of this Gaussian distribution, and demonstrate how the covariance increases as a result of the potential manoeuvres. We are then able to calculate the PCRLB for this BFG model using an existing Riccati-like recursion. Because of the BFG approximation, we are no longer guaranteed a bound and so we refer to our estimate as an error performance measure rather than a bound. The presented approach is applied both to filtering and smoothing cases. The simulation results indicate a very close agreement between the proposed performance measure and the error performance of an interacting multiple model estimator.     

The Role of Magnetic Fields in the Interstellar Medium of the Milky Way

Synchrotron radiation is generated throughout the Milky Way. It fills the sky, and carries with it the imprint of the magnetic field at the point of origin and along the propagation path. Observations of the diffuse polarized radio emission should be able to provide information on Galactic magnetic fields with detail matching the angular resolution of the telescope. I review what has been learned from existing data, but the full potential cannot be realized from current observations because they do not adequately sample the frequency structure of the polarized emission, or they lack information on large-scale structure. I discuss three surveys, each overcoming one of these limitations, and show how use of complementary data on other ISM tracers can help elucidate the role of magnetic fields in interstellar processes. The focus of this review is on the small-scale field, on sizes comparable with the various forms of interaction of stars with their surroundings. The future is bright for this field of research as new telescopes are being built, designed for the survey mode of observation, equipped for wideband, multichannel polarization observations.     

Dust grain orbital behavior around ceres

Many asteroids show indications they have undergone impacts with meteoroid particles having radii between 0.01 m and 1 m. During such impacts, small dust grains will be ejected at the impact site. The possibility of these dust grains (with radii greater than 2.2 μm) forming a halo around a spherical asteroid (such as Ceres) is investigated using standard numerical integration techniques. The orbital elements, positions, and velocities are determined for particles with varying radii taking into account both the influence of gravity, radiation pressure, and the interplanetary magnetic field (for charged particles). Under the influence of these forces it is found that dust grains (under the appropriate conditions) can be injected into orbits with lifetimes in excess of one year. The lifetime of the orbits is shown to be highly dependent on the location of the ejection point as well as the angle between the surface normal and the ejection path. It is also shown that only particles ejected within 10° relative to the surface tangential survive more than a few hours and that the longest-lived particles originate along a line perpendicular to the Ceres-Sun line.     

The effects of magnetic fields on period changes,mass transfer and evolution of algol binaries

Variations in the magnetic pressure and flux blocking by starspots during the magnetic cycle of the cool semidetached component of an Algol binary may cause cyclic changes in the quadrupole moment and moment of inertia of the star which can cause alternate period changes. Since several different processes and timescales are involved, the orbital period changes may not correlate strongly with the indicators of magnetic activity. The structural changes in the semidetached component can also modulate the mass transfer rate. Sub-Keplerian velocities, supersonic turbulence, and high temperature regions in circumstellar material around the accreting star may all be a consequence of magnetic fields embedded in the flow. Models for the evolution of Algols which include the effects of angular momentum loss (AML) through a magnetized wind may have underestimated the AML rate by basing it on results from main sequence stars. Evolved stars appear to have higher AML rates, and there may be additional AML in a wind from the accretion disk.     

On the association of quiet-time Pi2 pulsations with IMF variations

The association of quiet-time Pi2 pulsations with the variations of the interplanetary magnetic field (IMF) has been examined by using three reported events, occurring during extremely quiet intervals, of which the first was on 10 March 1997, the second 27 December 1997, and the third 11 May 1999. For the first event, the onset time of ground Pi2s maps to the IMF structure bearing a variation cycle of north-to-south and north again as seen by Wind in the upstream region and Geotail in the magnetosheath. Likewise, the second and the third events have respectively, four and three recurrent turnings propagating to the Earth sensed by multiple satellites. The comparison of geomagnetic perturbations, auroral brightenings, and energetic particle data in the magnetotail with the IMF observations shows successive substorm-like activations accompanied by ground Pi2 onsets. For a clear variation cycle, the first Pi2 burst appears 36 ± 8 min after southward turning of the IMF and the second one follows14 ± 4 min after a northward turning. Moreover, ground Pi2 onsets recur under low IMF clock angle conditions. These observational results can be interpreted with the prevailing models of externally triggered substorm. But the solar wind coupling to the magnetosphere under quiet conditions proceeds in a less efficient way than under substorm time conditions. Consequently, we suggest that recurrent quiet-time Pi2s can be associated with IMF variations and their cause can be the same as those for substorm times.     

Trace — The transition region and coronal explorer

TRACE is a single-instrument solar mission that will be put into a Sunsynchronous polar orbit and will obtain continuous solar observations for about 8 months per year. It will collect images of solar plasmas at temperatures from 10⁴ to 10⁷ K, with 1-arcsec spatial resolution and excellent temporal resolution and continuity. With such data, we expect to gain a new understanding of many solar and stellar problems ranging from coronal heating to impulsive magnetohydrodynamic phenomena.     

The Dollars and Sense of Continuing Education

Lifetime earnings of a high school graduate of $150 000 can be increased to $725 000 with a doctorate. Continued study is required to ?stand still? with an engineering half life of ten years. These points, among others, point to the value of investment in continued education.     

Dust in and Around the Heliosphere and Astrospheres

Space Science Reviews - The ESA Swarm mission, launched on 22 November 2013, consists of three spacecraft each equipped with a Micro Advanced Stellar Compass ( $\mu $ ASC) from the Technical...