A magnetic cycle of τ Bootis? The coronal and chromospheric view

τ Bootis is a late F-type main sequence star orbited by a Hot Jupiter. During the last years spectropolarimetric observations led to the hypothesis that this star may host a global magnetic field that switches its polarity once per year, indicating a very short activity cycle of only one year duration. In our ongoing observational campaign, we have collected several X-ray observations with XMM-Newton and optical spectra with TRES/FLWO in Arizona to characterize τ Boo's corona and chromosphere over the course of the supposed one-year cycle. Contrary to the spectropolarimetric reconstructions, our observations do not show indications for a short activity cycle.

Communication Nonlinearities Techniques for Analysis of Passive Intermodulation

The X-parameter based nonlinear modelling tools have been adopted as the foundation for the advanced methodology
of experimental characterisation and design of passive nonlinear devices. Based upon the formalism of the Xparameters,
it provides a unified framework for co-design of antenna beamforming networks, filters, phase shifters and
other passive and active devices of RF front-end, taking into account the effect of their nonlinearities. The equivalent
circuits of the canonical elements are readily incorporated in the models, thus enabling evaluation of PIM effect on the
performance of individual devices and their assemblies. An important advantage of the presented methodology is its
compatibility with the industry-standard established commercial RF circuit simulator Agilent ADS.
The major challenge in practical implementation of the proposed approach is concerned with experimental retrieval of the X-parameters for canonical passive circuit elements. To our best knowledge commercial PIM testers and practical laboratory test instruments are inherently narrowband and do not allow for simultaneous vector measurements at the PIM and harmonic frequencies. Alternatively, existing nonlinear vector analysers (NVNA) support X-parameter measurements in a broad frequency bands with a range of stimuli, but their dynamic range is insufficient for the PIM characterisation in practical circuits. Further opportunities for adaptation of the X-parameters methodology to the PIM
characterisation of passive devices using the existing test instruments are explored.

Ultra-high-resolution Observations of MHD Waves in Photospheric Magnetic Structures

Here we review the recent progress made in the detection, examination, characterisation and interpretation of oscillations manifesting in small-scale magnetic elements in the solar photosphere. This region of the Sun's atmosphere is especially dynamic, and importantly, permeated with an abundance of magnetic field concentrations. Such magnetic features can span diameters of hundreds to many tens of thousands of km, and are thus commonly referred to as the `building blocks' of the magnetic solar atmosphere. However, it is the smallest magnetic elements that have risen to the forefront of solar physics research in recent years. Structures, which include magnetic bright points, are often at the diffraction limit of even the largest of solar telescopes. Importantly, it is the improvements in facilities, instrumentation, imaging techniques and processing algorithms during recent years that have allowed researchers to examine the motions, dynamics and evolution of such features on the smallest spatial and temporal scales to date. It is clear that while these structures may demonstrate significant magnetic field strengths, their small sizes make them prone to the buffeting supplied by the ubiquitous surrounding convective plasma motions. Here, it is believed that magnetohydrodynamic waves can be induced, which propagate along the field lines, carrying energy upwards to the outermost extremities of the solar corona. Such wave phenomena can exist in a variety of guises, including fast and slow magneto-acoustic modes, in addition to Alfven waves. Coupled with rapid advancements in magnetohydrodynamic wave theory, we are now in an ideal position to thoroughly investigate how wave motion is generated in the solar photosphere, which oscillatory modes are most prevalent, and the role that these waves play in supplying energy to various layers of the solar atmosphere.

Introducing the CTA concept

The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project. © 2013 Elsevier B.V. All rights reserved.

Helium line intensity ratio in microwave-generated plasmas

The line intensity ratio method provides a nonintrusive diagnostic for the measurement of electron temperature in microwave-generated plasmas. For optically thin plasmas of low density, a line intensity method using He I lines can often be used, and is based on the fact that the electron impact excitation rate coefficients for helium singlet and triplet states are insensitive to electron density but differ as a function of the electron temperature. Line intensity measurements are presented from microwave-generated helium plasmas. Both steady-state corona and collision-radiative theoretical models are used to evaluate the ground and excited state populations. The line ratio versus electron temperature obtained from both of these methods are compared with the results from measurements. However, it is not possible to diagnose the electron temperature from the line ratios alone due to the presence of significant opacity and nonnegligible 1s2s S-3 metastable fraction in the plasma. (C) 2004 American Institute of Physics.

Quiet-Sun Hα Transients and Corresponding Small-scale Transition Region and Coronal Heating

Rapid blue- and redshifted excursions (RBEs and RREs) are likely to be the on-disk counterparts of Type II spicules. Recently, heating signatures from RBEs/RREs have been detected in IRIS slit-jaw images dominated by transition region (TR) lines around network patches. Additionally, signatures of Type II spicules have been observed in Atmospheric Imaging Assembly (AIA) diagnostics. The full-disk, ever-present nature of the AIA diagnostics should provide us with sufficient statistics to directly determine how important RBEs and RREs are to the heating of the TR and corona. We find, with high statistical significance, that at least 11% of the low coronal brightenings detected in a quiet-Sun region in He ii 304 Å can be attributed to either RBEs or RREs as observed in Hα, and a 6% match of Fe IX 171 Å detected events to RBEs or RREs with very similar statistics for both types of Hα features. We took a statistical approach that allows for noisy detections in the coronal channels and provides us with a lower, but statistical significant, bound. Further, we consider matches based on overlapping features in both time and space, and find strong visual indications of further correspondence between coronal events and co-evolving but non-overlapping, RBEs and RREs.