On the Stability of Siphon Flows Confined in Coronal Loops

We use a time-dependent hydrodynamic model to study the dynamics of siphon flows triggered by differences of pressure or heat deposition asymmetries between the two footpoints of a coronal loop. We show that steady pressure driven flows in a uniformly heated loop are unstable, whereas those driven by asymmetries in the heating function may be stable. We also show that, in these cases, relatively cool loops might be filled far above their static pressure scale height. This revised version was published online in June 2006 with corrections to the Cover Date.     

Life and intelligence in the universe from nanobiological principles: A survey and budget of concepts and perspectives

The new-born bioscience called Nanobiology has tackled the problems of the possibility of existence of extraterrestrial life and intelligence and of biosystem distribution in the Universe, as such questions actually belong to the realm of Theoretical Biology. The central, and yet unanswered points of such science have been reinvestigated by attempting knowledge and control of the hard-to-determine nanoscale-level classical and quantum interactions, which would supposedly give mechanistic, definite answers, both informationally and energetically, to the vexing questions put by biosystems to science: is the “living state” a physically definible concept, and how to define it? Are nanoscale kinetics or even detailed mechanics involved in the origin of life? What about intelligence, consciousness and their nanophysical roots? Are “life” and “intelligence” engineerable properties, or is any Artificial Intelligence program bound to mere metaphors? Self-organization, studied at the thermodynamic and the hydrodynamic level, showed the possibility of chemical evolution from amino acids, probably of cometary and/or meteoritic origin, up to spatiotemporal organization, autopoiesis and biological evolution, but didn't explain the origins of life. Questioning the uniqueness of the earthly evolutionary chemistry is cardinal for the ETI dilemma, as from a budgetary appraisal of perspectives in bionanoscale chaotic undecidable dynamics, quantum gravity and quantum vacuum, both “living state” and “intelligence” look like nonlocal, spacetime-linked cosmic phenomena.     

Devising an unconventional formal logic for bioinspired spacefaring automata

The field of robotics is increasingly moving from robots confined to factory floors and assembly lines and bound to perform the same tasks over and over in an uncertainty-free, well foreseeable environment, to robots designed for operating in highly dynamic and uncertainty domains, like those of interest in space exploration. According to an idea of a “new system of formal logic less rigid than past and present formal logic” advocated by von Neumann for building a powerful theory of automata, such system should be “closer to another discipline which has been little linked in the past with logic, i.e. thermodynamics, primarily in the form it was received by Boltzmann”. Following that idea, which is particularly interesting now with the emerging computational nano-sciences, it is stressed here that a full set of isomorphisms can be established between the fundamental logical principles and the information flows, Hamiltonian or dissipative, in phase space. This form of logic, dubbed here kinetic logic, takes standard formal logic out of the field of combinatorics and into the field of the Boltzmannian form of thermodynamics, i.e. kinetics.     

Coronal heating mechanisms

As a result of the large body of data available from solar and stellar coronae, our understanding of the mechanisms responsible for the heating of coronal plasmas to temperatures of the order of ～ 10⁸ K has changed. The solar corona is highly structured by magnetic fields and the acoustic shocks which, according to early theories, should have acted as the coronal energy source have not been observed. Einstein Observatory data show moreover that coronae are present in most regions of the H-R diagram. The observed relationship between X-ray luminosity and rotational velocity in dwarf stars from spectral types F to M again suggests an active role for the magnetic fields.The basic picture which is emerging is that coronae in stellar types from F to M are produced because of the interaction of the magnetic field with the convective velocity fields generated in the photosphere resulting in MHD waves or currents which dissipate in the corona. X-ray emission in early type stars cannot be explained with this mechanism and the models which have been proposed for these stars are not yet completely satisfactory.     

Solar Weather Event Modelling and Prediction

Key drivers of solar weather and mid-term solar weather are reviewed by considering a selection of relevant physics- and statistics-based scientific models as well as a selection of related prediction models, in order to provide an updated operational scenario for space weather applications. The characteristics and outcomes of the considered scientific and prediction models indicate that they only partially cope with the complex nature of solar activity for the lack of a detailed knowledge of the underlying physics. This is indicated by the fact that, on one hand, scientific models based on chaos theory and non-linear dynamics reproduce better the observed features, and, on the other hand, that prediction models based on statistics and artificial neural networks perform better. To date, the solar weather prediction success at most time and spatial scales is far from being satisfactory, but the forthcoming ground- and space-based high-resolution observations can add fundamental tiles to the modelling and predicting frameworks as well as the application of advanced mathematical approaches in the analysis of diachronic solar observations, that are a must to provide comprehensive and homogeneous data sets.     

Interpretation of the SOHO/UVCS observations of two CME-driven shocks

We report on the analysis of two fast CME-driven shocks observed with the UltraViolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory (SOHO). The first event, detected on 2002 March 22 at 4.1 R_⊙ with the UVCS slit placed in correspondence with the flank of the expanding CME surface, represents the highest UV detection of a shock obtained so far with the UVCS instrument in the corona. The second one, detected on 2002 July 23 at 1.6 R_⊙ with the UVCS slit placed in correspondence with the front of the expanding CME surface, shows an anomalous deficiency of ion heating with respect to what observed in previous CME/shocks observed by UVCS, possibly reflecting the effect of different coronal plasma conditions over the solar cycle. From the two different sets of observations we derived an estimate for the shock compression ratio X, which turns out to be X = 2.4 ± 0.2 and X = 2.2 ± 0.1, respectively, for the first and second event. Comparison between the two events provides complementary perspectives on the dynamical evolution of CME-driven shocks.     

Stationary flows in coronal loops

We present a study of stationary flows in closed solar coronal loops. The hydrodynamic differential equations of plasma flow and energy balance are integrated with algorithms which achieve high reliability. We present here results on the detailed synthesis of loop emission in specific bands and lines, taking into account also non-equilibrium ionization.