Deducing Electron Properties from Hard X-ray Observations

X-radiation from energetic electrons is the prime diagnostic of flare-accelerated electrons. The observed X-ray flux (and polarization state) is fundamentally a convolution of the cross-section for the hard X-ray emission process(es) in question with the electron distribution function, which is in turn a function of energy, direction, spatial location and time. To address the problems of particle propagation and acceleration one needs to infer as much information as possible on this electron distribution function, through a deconvolution of this fundamental relationship. This review presents recent progress toward this goal using spectroscopic, imaging and polarization measurements, primarily from the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI). Previous conclusions regarding the energy, angular (pitch angle) and spatial distributions of energetic electrons in solar flares are critically reviewed. We discuss the role and the observational evidence of several radiation processes: free-free electron-ion, free-free electron-electron, free-bound electron-ion, photoelectric absorption and Compton backscatter (albedo), using both spectroscopic and imaging techniques. This unprecedented quality of data allows for the first time inference of the angular distributions of the X-ray-emitting electrons and improved model-independent inference of electron energy spectra and emission measures of thermal plasma. Moreover, imaging spectroscopy has revealed hitherto unknown details of solar flare morphology and detailed spectroscopy of coronal, footpoint and extended sources in flaring regions. Additional attempts to measure hard X-ray polarization were not sufficient to put constraints on the degree of anisotropy of electrons, but point to the importance of obtaining good quality polarization data in the future.     

Delta-electron emission in fast heavy ion atom collisions.

Biological damages such as mutations, chromosomal aberrations etc. are a consequence of biochemical changes mostly in the DNA. With ionizing radiation, these chemical changes are due to primary ionization events and secondary ionization effects caused by the primarily produced electrons. Differences in the biological response of densely ionizing radiation, like heavy charged particles, in comparison to sparsely ionizing radiation, such as X- or gamma-rays, are mainly due to the differences in the production of the so called delta-electrons. Therefore, the emission process of electrons i.e. the cross section for the primary ionization event as well as the energy and angular distribution of the emitted electrons should be understood in detail. The delta-electron emission processes occuring in fast heavy ion atom collisions are explained qualitatively. The different spectral structures of electron emission arising from either the target or the projectile are explained in terms of simple models of the kinetics of momentum transfer induced by the COULOMB forces. In collisions of very heavy ions with matter, high nuclear COULOMB forces are created. These forces lead to a strong polarization of the electronic states of the participated electrons. The effects of this polarization are discussed.     

INTEGRAL: Science Highlights and Future Prospects

ESA??s hard X-ray and soft gamma-ray observatory INTEGRAL is covering the 3 keV to 10 MeV energy band, with excellent sensitivity during long and uninterrupted observations of a large field of view (??100 square degrees), with ms time resolution and keV energy resolution. It links the energy band of pointed soft X-ray missions such as XMM-Newton with that of high-energy gamma-ray space missions such as Fermi and ground based TeV observatories. Key results obtained so far include the first sky map in the light of the 511 keV annihilation emission, the discovery of a new class of high mass X-ray binaries and detection of polarization in cosmic high energy radiation. For the foreseeable future, INTEGRAL will remain the only observatory allowing the study of nucleosynthesis in our Galaxy, including the long overdue next nearby supernova, through high-resolution gamma-ray line spectroscopy. Science results to date and expected for the coming mission years span a wide range of high-energy astrophysics, including studies of the distribution of positrons in the Galaxy; reflection of gamma-rays off clouds in the interstellar medium near the Galactic Centre; studies of black holes and neutron stars particularly in high- mass systems; gamma-ray polarization measurements for X-ray binaries and gamma-ray bursts, and sensitive detection capabilities for obscured active galaxies with more than 1000 expected to be found until 2014. This paper summarizes scientific highlights obtained since INTEGRAL??s launch in 2002, and outlines prospects for the INTEGRAL mission.     

Decentralized formation pose estimation for spacecraft swarms

For spacecraft swarms, the multi-agent localization algorithm must scale well with the number of spacecraft and adapt to time-varying communication and relative sensing networks. In this paper, we present a decentralized, scalable algorithm for swarm localization, called the Decentralized Pose Estimation (DPE) algorithm. The DPE considers both communication and relative sensing graphs and defines an observable local formation. Each spacecraft jointly localizes its local subset of spacecraft using direct and communicated measurements. Since the algorithm is local, the algorithm complexity does not grow with the number of spacecraft in the swarm. As part of the DPE, we present the Swarm Reference Frame Estimation (SRFE) algorithm, a distributed consensus algorithm to co-estimate a common Local-Vertical, Local-Horizontal (LVLH) frame. The DPE combined with the SRFE provides a scalable, fully-decentralized navigation solution that can be used for swarm control and motion planning. Numerical simulations and experiments using Caltech’s robotic spacecraft simulators are presented to validate the effectiveness and scalability of the DPE algorithm.     

Toward an Understanding of the Systematic Uncertainties in Deriving the Primordial Helium Abundance from H II Region Observations

Recent determinations of the primordial He abundance have given significantly different results. We are attempting to identify some of the causes of these differences and propose observational solutions. Here we identify a systematic difference in how the data are interpreted (differences in corrections for the presence of neutral helium) and the importance of a systematic bias towards lower derived helium abundances (underestimating the presence of underlying stellar absorption).     

Exospheres and Energetic Neutral Atoms of Mars, Venus and Titan

Our understanding of the upper atmosphere of unmagnetized bodies such as Mars, Venus and Titan has improved significantly in this decade. Recent observations by in situ and remote sensing instruments on board Mars Express, Venus Express and Cassini have revealed characteristics of the neutral upper atmospheres (exospheres) and of energetic neutral atoms (ENAs). The ENA environment in the vicinity of the bodies is by itself a significant study field, but ENAs are also used as a diagnostic tool for the exosphere and the interaction with the upstream plasmas. Synergy between theoretical and modeling work has also improved considerably. In this review, we summarize the recent progress of our understanding of the neutral environment in the vicinity of unmagnetized planets.     

Viability of barley seeds after long-term exposure to outer side of international space station

Barley seeds were exposed to outer space for 13 months in a vented metal container without a climate control system to assess the risk of physiological and genetic mutation during long-term storage in space. The space-stored seeds (S0 generation), with an 82% germination rate in 50 seeds, lost about 20% of their weight after the exposure. The germinated seeds showed normal growth, heading, and ripening. The harvested seeds (S1 generation) also germinated and reproduced (S2 generation) as did the ground-stored seeds. The culm length, ear length, number of seed, grain weight, and fertility of the plants from the space-stored seeds were not significantly different from those of the ground-stored seeds in each of the S0 and S1 generation. Furthermore, the S1 and S2 space-stored seeds respectively showed similar β-glucan content to those of the ground-stored seeds. Amplified fragment length polymorphism analysis with 16 primer combinations showed no specific fragment that appears or disappears significantly in the DNA isolated from the barley grown from the space-stored seeds. Though these data are derived from nine S0 space-stored seeds in a single exposure experiment, the results demonstrate the preservation of barley seeds in outer space for 13 months without phenotypic or genotypic changes and with healthy and vigorous growth in space.     

Magnetic Fields, Relativistic Particles, and Shock Waves in Cluster Outskirts

It is only now, with low-frequency radio telescopes, long exposures with high-resolution X-ray satellites and γ-ray telescopes, that we are beginning to learn about the physics in the periphery of galaxy clusters. In the coming years, Sunyaev-Zel’dovich telescopes are going to deliver further great insights into the plasma physics of these special regions in the Universe. The last years have already shown tremendous progress with detections of shocks, estimates of magnetic field strengths and constraints on the particle acceleration efficiency. X-ray observations have revealed shock fronts in cluster outskirts which have allowed inferences about the microphysical structure of shocks fronts in such extreme environments. The best indications for magnetic fields and relativistic particles in cluster outskirts come from observations of so-called radio relics, which are megaparsec-sized regions of radio emission from the edges of galaxy clusters. As these are difficult to detect due to their low surface brightness, only few of these objects are known. But they have provided unprecedented evidence for the acceleration of relativistic particles at shock fronts and the existence of μG strength fields as far out as the virial radius of clusters. In this review we summarise the observational and theoretical state of our knowledge of magnetic fields, relativistic particles and shocks in cluster outskirts.     

Calculation of the Initial Magnetic Field for Mercury’s Magnetosphere Hybrid Model

Several types of numerical models are used to analyze the interactions of the solar wind flow with Mercury’s magnetosphere, including kinetic models that determine magnetic and electric fields based on the spatial distribution of charges and currents, magnetohydrodynamic models that describe plasma as a conductive liquid, and hybrid models that describe ions kinetically in collisionless mode and represent electrons as a massless neutralizing liquid. The structure of resulting solutions is determined not only by the chosen set of equations that govern the behavior of plasma, but also by the initial and boundary conditions; i.e., their effects are not limited to the amount of computational work required to achieve a quasi-stationary solution. In this work, we have proposed using the magnetic field computed by the paraboloid model of Mercury’s magnetosphere as the initial condition for subsequent hybrid modeling. The results of the model have been compared to measurements performed by the Messenger spacecraft during a single crossing of the magnetosheath and the magnetosphere. The selected orbit lies in the terminator plane, which allows us to observe two crossings of the bow shock and the magnetopause. In our calculations, we have defined the initial parameters of the global magnetospheric current systems in a way that allows us to minimize paraboloid magnetic field deviation along the trajectory of the Messenger from the experimental data. We have shown that the optimal initial field parameters include setting the penetration of a partial interplanetary magnetic field into the magnetosphere with a penetration coefficient of 0.2.