Fixed-interval retrodiction approach to Bayesian IMM-MHT formaneuvering multiple targets

In a Bayesian framework, we propose a hierarchy of suboptimal retrodiction algorithms that generalize Rauch-Tung-Striebel (RTS) fixed-interval smoothing to multiple hypothesis tracking (MHT) applications employing interacting multiple model (IMM) methods (IMM-MHT). As a limiting case we obtain new simple formulae for suboptimal fixed-interval smoothing applied to Markovian switching systems. Retrodiction techniques provide uniquely interpretable and accurate trajectories from ambiguous MHT output if a certain (small) time delay is tolerated. By a simulated example with two maneuvering targets that operate closely spaced under relatively hard conditions we demonstrate the potential gain by fixed-interval retrodiction and provide a quantitative idea of the achievable track accuracy and mean time delay involved     

Cosmic Rays and Earth – A Summary

Cosmic rays provide a diagnostic tool to analyze processes in interplanetary space and at the Sun. Cosmic rays also directly affect the terrestrial environment and serve as indicators of solar variability and non anthropogenic climatic changes on Earth at present and in the distant past. After the invention of the neutron monitor by John A. Simpson in 1948, an international network of cosmic ray detectors developed in a cooperative effort to examine temporal and spatial variations in our space environment. The resulting datasets represent the longest continuous, high time resolution series of particle radiation measurement in space science. At present, the neutron monitor network is complemented by spacecraft instrumentation to study solar-terrestrial correlated phenomena. This revised version was published online in August 2006 with corrections to the Cover Date.     

From Dust to Terrestrial Planets – Introduction

Terrestrial planets are accreted in a disk orbiting a central star. The basic challenge of their formation consists of assembling micron-sized or smaller dust grains to bodies with over 10⁴ km in diameter. This formation process, ultimately based on collisions, occurs in three very different physical regimes depending upon the size of the bodies present: 1) Early on, micron- to mm-sized dust grains, chondrules and chondrites are strongly coupled to the gas. 2) As they grow larger, gravity increases the collisional cross section allowing runaway growth to occur. 3) After this runaway phase stops from exhaustion of matter in the immediate surroundings of the protoplanets, further growth occurs on timescales typical of mutual gravitational perturbations. The emphasis of this book is on the timescales corresponding to these formation phases as well as the characteristic chemical and isotopic composition of the bodies involved. This revised version was published online in June 2006 with corrections to the Cover Date.     

The Cosmic Ray Nucleonic Component: The Invention and Scientific Uses of the Neutron Monitor – (Keynote Lecture)

The invention of the neutron monitor pile for the study of cosmic-ray intensity-time and energy changes began with the discovery in 1948 that the nucleonic component cascade in the atmosphere had a huge geomagnetic latitude dependence. For example, between 0° and 60° this dependence was a ∼ 200–400% effect – depending on altitude – thus opening the opportunity to measure the intensity changes in the arriving cosmic-ray nuclei down to ∼1–2 GeV nucl⁻¹ for the first time. In these measurements the fast (high energy) neutron intensity was shown to be a surrogate for the nuclear cascade intensity in the atmosphere. The development of the neutron monitor in 1948–1951 and the first geomagnetic latitude network will be discussed. Among its early applications were: (1) to prove that there exists interplanetary solar modulation of galactic cosmic-rays (1952), and; (2) to provide the evidence for a dynamical heliosphere (1956). With the world-wide distribution of neutron monitor stations that are presently operating (∼ 50) many novel investigations are still to be carried out, especially in collaborations with spacecraft experiments. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Effect of Ground Reflections and Scattering on an Interferometer Direction Finder

The azimuth and elevation angles of an airborne beacon transmitter can be determined using a direction finder comprised of two orthogonal interferometers located near the earth's surface. In this paper we consider the reflection of the incident electromagnetic field by the earth's surface and the resultant effect on the direction-finding system. The analysis yields an upper bound or limit to the interferometer phase error and the corresponding error in bearing angle that can result because of specular reflection and diffuse scattering.     

The Beginning: Swift and Violent

Radiometric dating shows that the earliest steps in forming the substance of meteorites and assembling it into planetesimals occurred in a very short interval of time, 1–2 Ma. This study shows that rapid formation is also dictated by the need to use short-lived ²⁶Al (half-life T _1/2=0.74 Ma) as a heat source to metamorphose and in some cases melt the meteorite parent bodies after they accreted. The earliest events in solar system history dated by cosmochemists, formation at high temperatures of the Ca,Al-rich inclusions that occur in chondritic meteorites, probably occurred during the most energetic stage of protosolar disk evolution, as the protosun neared its present mass and infall drew to a close. This revised version was published online in June 2006 with corrections to the Cover Date.     

Multi-station observation of ionospheric disturbance of March 9 2012 and comparison with IRI-model

The magnetic storm of 9 March 2012 is a single step intense storm (Dst = −143 nT) whose main phase begins around 0100 UT and lasted for almost 11 h. The increases in NmF2 recorded 33% and 67% incidence respectively during the main and the recovery phase of the storm at the stations considered. The increase in hmF2 occurred concurrently with the increase in thickness parameter B0 between 0000 and 1100 UT, and a simultaneous decrease in the shape parameter B1 for the entire mid-latitude stations. Generally, B1 responded to the storm with a decrease away from the quiet day average, and decreased simultaneously with the increase in NmF2. B0 displays higher variability magnitude during daytime than the nighttime period. The occasional differences in the response of the ionospheric parameters to the storm event are attributed to longitudinal differences. Variation in hmF2 and NmF2 is projected to change in B1, but the rationale behind this effect on B1 is still not known and therefore left open. The two IRI options over-estimate the observed values with that of URSI higher than CCIR. The over-estimation was higher during the nighttime than the daytime for NmF2 response for the mid-latitude stations and the reverse for the equatorial station. A fairer fit of the model with the observed for all parameters over Jicamarca suggests that equatorial regions are better represented on the model. Extensive study of B1 and B0 is recommended to arrive at a better performance of IRI.     

Meteor Phenomena and Bodies

Meteoroids can be observed at collision with the Earth's atmosphere as meteors. Different methods of observing meteors are presented: besides the traditional counts of individual events, exact methods yield also data on the geometry of the atmospheric trajectory; on the dynamics and ablation of the body in the atmosphere; on radiation; on the spectral distribution of radiation; on ionization; on accompanying sounds; and also data on orbits. Theoretical models of meteoroid interaction with the atmosphere are given and applied to observational data. Attention is paid to radar observations; to spectroscopic observations; to experiments with artificial meteors and to different types of meteor sounds. The proposed composition and structure of meteoroids as well as their orbits link them to meteorites, asteroids and comets. Meteor streams can be observed as meteor showers and storms. The rate of influx of meteoroids of different sizes onto Earth is presented and potential hazards discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Product Correlator Performance for Gaussian Random Scenes

Contained herin is a derivation of two figures of merit for evaluating the performance of a product correlator and an associated discriminator curve. The general formulas are valid for both area and one-dimensional scenes. An explicit evaluation of the formulas is presented when the correlation functions are also Gaussian. In this case, the accuracy and probability of false match are found to take very simple forms.     

Review Article: MHD Wave Propagation Near Coronal Null Points of Magnetic Fields

We present a comprehensive review of MHD wave behaviour in the neighbourhood of coronal null points: locations where the magnetic field, and hence the local Alfvén speed, is zero. The behaviour of all three MHD wave modes, i.e. the Alfvén wave and the fast and slow magnetoacoustic waves, has been investigated in the neighbourhood of 2D, 2.5D and (to a certain extent) 3D magnetic null points, for a variety of assumptions, configurations and geometries. In general, it is found that the fast magnetoacoustic wave behaviour is dictated by the Alfvén-speed profile. In a ??=0 plasma, the fast wave is focused towards the null point by a refraction effect and all the wave energy, and thus current density, accumulates close to the null point. Thus, null points will be locations for preferential heating by fast waves. Independently, the Alfvén wave is found to propagate along magnetic fieldlines and is confined to the fieldlines it is generated on. As the wave approaches the null point, it spreads out due to the diverging fieldlines. Eventually, the Alfvén wave accumulates along the separatrices (in 2D) or along the spine or fan-plane (in 3D). Hence, Alfvén wave energy will be preferentially dissipated at these locations. It is clear that the magnetic field plays a fundamental role in the propagation and properties of MHD waves in the neighbourhood of coronal null points. This topic is a fundamental plasma process and results so far have also lead to critical insights into reconnection, mode-coupling, quasi-periodic pulsations and phase-mixing.