2,870 research outputs found
Calibration Challenges for Future Radio Telescopes
Instruments for radio astronomical observations have come a long way. While
the first telescopes were based on very large dishes and 2-antenna
interferometers, current instruments consist of dozens of steerable dishes,
whereas future instruments will be even larger distributed sensor arrays with a
hierarchy of phased array elements. For such arrays to provide meaningful
output (images), accurate calibration is of critical importance. Calibration
must solve for the unknown antenna gains and phases, as well as the unknown
atmospheric and ionospheric disturbances. Future telescopes will have a large
number of elements and a large field of view. In this case the parameters are
strongly direction dependent, resulting in a large number of unknown parameters
even if appropriately constrained physical or phenomenological descriptions are
used. This makes calibration a daunting parameter estimation task, that is
reviewed from a signal processing perspective in this article.Comment: 12 pages, 7 figures, 20 subfigures The title quoted in the meta-data
is the title after release / final editing
Cosmic Rays and Climate
Among the most puzzling questions in climate change is that of solar-climate
variability, which has attracted the attention of scientists for more than two
centuries. Until recently, even the existence of solar-climate variability has
been controversial - perhaps because the observations had largely involved
temporary correlations between climate and the sunspot cycle. Over the last few
years, however, diverse reconstructions of past climate change have revealed
clear associations with cosmic ray variations recorded in cosmogenic isotope
archives, providing persuasive evidence for solar or cosmic ray forcing of the
climate. However, despite the increasing evidence of its importance, solar
climate variability is likely to remain controversial until a physical
mechanism is established. Although this remains a mystery, observations suggest
that cloud cover may be influenced by cosmic rays, which are modulated by the
solar wind and, on longer time scales, by the geomagnetic field and by the
galactic environment of Earth. Two different classes of microphysical
mechanisms have been proposed to connect cosmic rays with clouds: firstly, an
influence of cosmic rays on the production of cloud condensation nuclei and,
secondly, an influence of cosmic rays on the global electrical circuit in the
atmosphere and, in turn, on ice nucleation and other cloud microphysical
processes. Considerable progress on understanding ion-aerosol-cloud processes
has been made in recent years, and the results are suggestive of a physically-
plausible link between cosmic rays, clouds and climate. However, a concerted
effort is now required to carry out definitive laboratory measurements of the
fundamental physical and chemical processes involved, and to evaluate their
climatic significance with dedicated field observations and modelling studies.Comment: 42 pages, 19 figure
Global MHD simulation of flux transfer events at the high-latitude magnetopause observed by the cluster spacecraft and the SuperDARN radar system
A global magnetohydrodynamic numerical simulation is used to study the large-scale structure and formation location of flux transfer events (FTEs) in synergy with in situ spacecraft and ground-based observations. During the main period of interest on the 14 February 2001 from 0930 to 1100 UT the Cluster spacecraft were approaching the Northern Hemisphere high-latitude magnetopause in the postnoon sector on an outbound trajectory. Throughout this period the magnetic field, electron, and ion sensors on board Cluster observed characteristic signatures of FTEs. A few minutes delayed to these observations the Super Dual Auroral Radar Network (SuperDARN) system indicated flow disturbances in the conjugate ionospheres. These “two-point” observations on the ground and in space were closely correlated and were caused by ongoing unsteady reconnection in the vicinity of the spacecraft. The three-dimensional structures and dynamics of the observed FTEs and the associated reconnection sites are studied by using the Block-Adaptive-Tree-Solarwind-Roe-Upwind-Scheme (BATS-R-US) MHD code in combination with a simple open flux tube motion model (Cooling). Using these two models the spatial and temporal evolution of the FTEs is estimated. The models fill the gaps left by measurements and allow a “point-to-point” mapping between the instruments in order to investigate the global structure of the phenomenon. The modeled results presented are in good correlation with previous theoretical and observational studies addressing individual features of FTEs
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