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Welcome to Prague. Welcome to this Congress Centre built in a close neighbourhood of the ancient seat of the first Czech dukes (Fig. 1). Its name Vyšehrad means the Upper Town. According to the oldest Czech chronicles, it was here where the legendary princess Libuše ordered her people to found the city of Prague and where she envisaged its glory touching the stars (Fig. 2). It was also here where the canon of Vyšehrad recorded in the first half of the 12th century into his chronicle some observed astronomical and meteorological phenomena.
In his book Plurality of Worlds, Steven J. Dick (1984) has chronicled the millennia of discourse about other inhabited worlds, based upon deeply held religious or philosophical belief systems. The popularity of the idea of extraterrestrial life has waxed and waned and, at its nadir, put proponents at mortal risk. The several generations of scientists now attending this General Assembly of the International Astronomical Union at the beginning of the 21st century have a marvelous opportunity to shed light on this old question of habitable worlds through observation, experimentation, and interpretation, without recourse to belief systems and without risking their lives (though some may experience rather bumpy career paths). The newly-named and funded, multi-disciplinary field of astrobiology is extremely broad in its scope and is encouraging IAU members to learn and speak the languages of previously disparate disciplines in an attempt to answer the big picture questions: ‘Where did we come from?’, ‘Where are we going?’, and ‘Are we alone?’ These are questions that the general public understand and support, and these are questions that are attracting students of all ages to science and engineering programs. These questions also push the limits of modern instrumentation to explore the cosmos remotely across space and time, as well as to examine samples of interplanetary space returned to the laboratory and samples of distant time teased out of our own Earth.
Within my personal event horizon, the other planetary systems long-predicted by theorists have been uncovered, along with many whose structures were not predicted. The ‘just-so’ conditions requisite for the comfort of astronomers have been understood to be only a very narrow subset of the conditions that nurture extremophilic, microbial life. Thus the potentially habitable real estate beyond Earth has been greatly expanded and within the next few decades it may be possible to detect the biosignatures or technosignatures of inhabitants on distant worlds, should there be any.
The Sun's magnetic field is produced throughout the solar interior; it emerges and is dispersed by surface and subsurface flows, and then expands above the surface to dominate the structure of the corona. To resolve the effects of the magnetic field it is necessary to image the interior and measure its rotation and flow systems; track the responses of the magnetic fields to flows in the surface; and to follow the evolution of structures in the corona. Because the Sun is dynamic both high spatial and temporal resolution are essential. Because the Sun's magnetic field effects encompass the entire spherical exterior, the entire surface and outer atmosphere must be mapped. And because the magnetic field is cyclic high-resolution observations must be maintained over multiple cycles.
Many similar phenomena occur in astrophysical systems with spatial and mass scales different by many orders of magnitudes. For examples, collimated outflows are produced from the Sun, proto-stellar systems, gamma-ray bursts, neutron star and black hole X-ray binaries, and supermassive black holes; various kinds of flares occur from the Sun, stellar coronae, X-ray binaries and active galactic nuclei; shocks and particle acceleration exist in supernova remnants, gamma-ray bursts, clusters of galaxies, etc. In this report I summarize briefly these phenomena and possible physical mechanisms responsible for them. I emphasize the importance of using the Sun as an astrophysical laboratory in studying these physical processes, especially the roles magnetic fields play in them; it is quite likely that magnetic activities dominate the fundamental physical processes in all of these systems.
As a case study, I show that X-ray lightcurves from solar flares, black hole binaries and gamma-ray bursts exhibit a common scaling law of non-linear dynamical properties, over a dynamical range of several orders of magnitudes in intensities, implying that many basic X-ray emission nodes or elements are inter-connected over multi-scales. A future high timing and imaging resolution solar X-ray instrument, aimed at isolating and resolving the fundamental elements of solar X-ray lightcurves, may shed new lights onto the fundamental physical mechanisms, which are common in astrophysical systems with vastly different mass and spatial scales. Using the Sun as an astrophysical laboratory, “Applied Solar Astrophysics” will deepen our understanding of many important astrophysical problems.
It is becoming ever easier to obtain first class astronomical data by working solely from one's computer terminal, the modern equivalent of the armchair. This wonderful development may tempt us to forget that astronomical discoveries and findings are first and foremost driven by hard-won progress in observational and experimental capabilities. The present article is meant to demonstrate this assertion by reviewing recent developments and future possibilities in studying the Center of the Milky Way, our best case for the existence of a (massive) black hole and a superb laboratory for studying the physical processes in the immediate vicinity of such an enigmatic object.
A remarkable variety of particle acceleration occurs in the solar system, from lightning-related acceleration of electrons to tens of MeV energy in less than a millisecond in planetary atmospheres; to acceleration of auroral and radiation belt particles in planetary magnetospheres; to acceleration at planetary bow shocks, co-rotating interplanetary region shocks, shocks driven by fast coronal mass ejections, and possibly at the heliospheric termination shock; to acceleration in magnetic reconnection regions in solar flares and at planetary magnetopause and magnetotail current sheets. These acceleration processes often occur in conjunction with transient energy releases, and some are very efficient. Unlike acceleration processes outside the solar system, the accelerated particles and the physical conditions in the acceleration region can be studied through direct in situ measurements, and/or through detailed imaging and spectroscopy. Here I review recent observations of tens of MeV electron acceleration in the Earth's atmosphere and in the Earth's radiation belts, electron and ion acceleration related to magnetic reconnection in solar flares, electron acceleration to ≥ 300 keV in magnetic reconnection regions in the Earth's deep magnetotail, and acceleration of solar energetic particles (SEPs) by shocks driven by fast coronal mass ejections (CMEs).
Properties of near-relativistic (E ≳ 30 keV, NR) and relativistic (E ≳ 0.3 MeV) electron events produced near the Sun and observed within 1 AU are reviewed. Observations suggest the CME-driven shocks are the sources of many events, but flares are often sources for NR events.
Starting from 2.5D MHD modelling of solar flares on a global scale we calculate (using the PIC and test-particle simulations) the radio and X-ray emissions generated in solar flare reconnection. Our results – the radio and X-ray spectra and brightness distributions, and their dynamics – are directly comparable with observations providing thus a test of particle acceleration models as well as of the ‘standard’ global flare scenario.
Observations give tight constraints on the temporal and spatial scales of particle heating in solar flares, and on the required efficiency. Electrons are accelerated into a quasi-thermal population of a few tens of keV. X- and γ-rays imply tails in electron and ion distributions reaching tens of MeV and above. Simple estimates indicate that all available electrons are accelerated at least once to moderate energies, pointing to an initial process resembling bulk heating rather than acceleration of a small or localized population. In the absence of effective collisions, wave-particle interactions are the prime candidate. Here we address the outstanding questions, (i) what process can heat the entire reconnecting plasma to the above energies, and (ii) what provides the free energy for wave-particle interactions? We propose a process in which initially the ions are heated and provide the free energy for electron heating and tail formation.
During the rising phase of the August 30, 2002 X1.5 flare a short pulse with a total duration of 8 seconds was observed. Its background-subtracted radio spectrum ranges only from 5 to 12 GHz with a maximum flux density of approximately 900 s.f.u. at 7 GHz and a steep optically thin spectral index α ≃ 8. Maximum degree of polarization at 7 GHz is around 5%. The hard X-ray pulse emission above the background in the range of 30–150 keV observed by RHESSI is coincident in time with the microwave observation. Hard X-ray images reveal very compact (∼ 10″) footpoint sources. Below 30 keV, a thermal source is observed.
I review the current status of our observational knowledge of prominent classes of particle accelerators in the Galaxy, namely shell-type supernova remnants (SNRs) and pulsar wind nebulae. I highlight in particular the contribution of the recent improvement in sensitivity of very-high-energy (VHE) γ-ray observations, which are currently the most direct probe of particle acceleration in the Galaxy up to energies of several hundreds of TeV.
Shell-type SNRs have long been proposed as sources of the Galactic cosmic rays. In recent years, X-ray observations have revealed very thin, non-thermal rims in many young SNRs, and I discuss the implications of these observations for magnetic field amplification and the maximum particle energy attainable by acceleration at the blast wave. I then review the current status of the evidence for accelerated nuclei in these objects, and summarise current uncertainties.
The most numerous class of identified Galactic VHE gamma-ray sources is currently that of pulsar wind nebulae (PWNe). The emission from these objects is generally assumed to be predominantly leptonic, and I outline the new information provided by VHE gamma-ray observations beyond what could be inferred from observations of synchrotron emission.
We review the problem of particle acceleration in relativistic shocks or shear flows. We propose a converter mechanism, which operates via continuous conversion of accelerated particles from charged into neutral state and back, and show that it is capable of producing the highest energy cosmic rays.
It is quite well established that shocks accelerate particles via the Fermi mechanism. We discuss common features of various extragalactic sources, ranging from Gamma-Ray Bursts and jets of Active Galactic Nuclei to Large-Scale Structure shocks and address how they affect particle acceleration. In particular, we address constraints on the maximum energy of ultra-high-energy cosmic rays. Interestingly, some recent studies indicate that Fermi acceleration in relativistic shocks (and GRBs, in particular) faces severe difficulties. We will address this issue and demonstrate that the ‘observed’ shock acceleration of electrons may have nothing to do with Fermi acceleration, but may rather be associated with micro-physics of collisionless shocks.
Here we suggest that efficient stochastic particle re–acceleration in galaxy clusters may be driven by compressible modes. The damping of these modes is severely dominated by the TTD–resonance with thermal electrons and protons in the ICM. However, a small energy flux of these modes may be channelled into particle re-acceleration and this gives re-acceleration times of the order of ∼ 108 yrs, sufficient to mantain GeV radiating electrons in the ICM.
Magnetic reconnection is a candidate mechanism for particle acceleration in a variety of astrophysical contexts. It is now widely accepted that reconnection plays a key role in solar flares, and reconstructions of coronal magnetic fields indicate that three-dimensional (3D) magnetic null points can be present during flares. We investigate particle acceleration during spine reconnection at a 3D magnetic null point, using a test particle numerical code. We observe efficient particle acceleration and find that two energetic populations are produced: a trapped population of particles that remain in the vicinity of the null, and an escaping population, which leave the configuration in two symmetric jets along field lines near the spine. While the parameters used in our simulation aim to represent solar coronal plasma conditions of relevance for acceleration in flares, the fact that the 3D spine reconnection configuration naturally results in energetic particle jets may be of importance in other astrophysical situations. We also compare the results obtained for the spine reconnection regime with those for the other possible mode of 3D reconnection, fan reconnection. We find that in the latter case energetic particle jets are not produced, though acceleration is observed.
Astrophysical particle acceleration involves the efficient conversion of bulk energy to individual charge particle energy through work done by electric field. The ways in which this happens are quite varied but when considered from a physics perspective, commonalities can found between acceleration in quite different sites.