Context. Remote sensing of weak and small-scale solar magnetic fields is of utmost relevance when attempting to respond to a number of important open questions in solar physics. This requires the acquisition of spectropolarimetric data with high spatial resolution (∼10 −1 arcsec) and low noise (10 −3 to 10 −5 of the continuum intensity). The main limitations to obtain these measurements from the ground, are the degradation of the image resolution produced by atmospheric seeing and the seeing-induced crosstalk (SIC). Aims. We introduce the prototype of the Fast Solar Polarimeter (FSP), a new ground-based, high-cadence polarimeter that tackles the above-mentioned limitations by producing data that are optimally suited for the application of post-facto image restoration, and by operating at a modulation frequency of 100 Hz to reduce SIC. Methods. We describe the instrument in depth, including the fast pnCCD camera employed, the achromatic modulator package, the main calibration steps, the effects of the modulation frequency on the levels of seeing-induced spurious signals, and the effect of the camera properties on the image restoration quality. Results. The pnCCD camera reaches 400 fps while keeping a high duty cycle (98.6%) and very low noise (4.94 e − rms). The modulator is optimized to have high (>80%) total polarimetric efficiency in the visible spectral range. This allows FSP to acquire 100 photon-noise-limited, full-Stokes measurements per second. We found that the seeing induced signals that are present in narrowband, non-modulated, quiet-sun measurements are (a) lower than the noise (7 × 10 −5 ) after integrating 7.66 min, (b) lower than the noise (2.3 × 10 −4 ) after integrating 1.16 min and (c) slightly above the noise (4 × 10 −3 ) after restoring case (b) by means of a multiobject multi-frame blind deconvolution. In addition, we demonstrate that by using only narrow-band images (with low S/N of 13.9) of an active region, we can obtain one complete set of high-quality restored measurements about every 2 s.
Aims. Understanding how magnetic fields are structured within coronal mass ejections (CMEs), and how they evolve from the low corona into the heliosphere, is a major challenge for space weather forecasting and for solar physics. The study of CME morphology is a particularly auspicious approach to this problem, given that it holds a close relationship with the CME magnetic field configuration. Although earlier studies have suggested an asymmetry in the width of CMEs in orthogonal directions, this has not been inspected using multi-viewpoint observations. Methods. The improved spatial, temporal, and spectral resolution, added to the multiple vantage points offered by missions of the Heliophysics System Observatory, constitute a unique opportunity to gain insight into this regard. We inspect the early evolution (below ten solar radii) of the morphology of a dozen CMEs occurring under specific conditions of observing spacecraft location and CME trajectory, favorable to reduce uncertainties typically involved in the 3D reconstruction used here. These events are carefully reconstructed by means of a forward modeling tool using simultaneous observations of the Solar-Terrestrial Relations Observatory (STEREO) Extreme Ultraviolet Imager (EUVI) and the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) as input when originating low in the corona, and followed up in the outer fields of view of the STEREO and the Solar and Heliospheric Observatory (SOHO) coronagraphs. We then examine the height evolution of the morphological parameters arising from the reconstructions.Results. The multi-viewpoint analysis of this set of CMEs revealed that their initial expansion -below three solar radii-is considerably asymmetric and non-self-similar. Both angular widths, namely along the main axes of CMEs (AW L ) and in the orthogonal direction (AW D , representative of the flux rope diameter), exhibit much steeper change rates below this height, with the growth rate of AW L found to be larger than that of AW D , also below that height. Angular widths along the main axes of CMEs are on average ≈ 1.8 times larger than widths in the orthogonal direction AW D . The ratios of the two expansion speeds, namely in the directions of CMEs main axes and in their orthogonal, are nearly constant in time after ∼ 4 solar radii, with an average ratio ≈ 1.6. Heights at which the width change rate is defined to stabilize are greater for AW L than for AW D .
Motivated by improving predictions of arrival times at Earth of shocks driven by coronal mass ejections (CMEs), we have analyzed 71 Earth-directed events in different stages of their propagation. The study is primarily based on approximated locations of interplanetary (IP) shocks derived from Type-II radio emissions detected by the Wind/WAVES experiment during 1997 -2007. Distance-time diagrams resulting from the combination of white-light corona, IP Type-II radio, and in-situ data lead to the formulation of descriptive profiles of each CME's journey toward Earth. Furthermore, two different methods for tracking and predicting the location of CME-driven IP shocks are presented. The linear method, solely based on Wind/WAVES data, arises after key modifications to a pre-existing technique 6 Annapolis, MD, USA H. Cremades et al. that linearly projects the drifting low-frequency Type-II emissions to 1 AU. This upgraded method improves forecasts of shock-arrival times by almost 50 %. The second predictive method is proposed on the basis of information derived from the descriptive profiles and relies on a single CME height-time point and on low-frequency Type-II radio emissions to obtain an approximate value of the shock arrival time at Earth. In addition, we discuss results on CME-radio emission associations, characteristics of IP propagation, and the relative success of the forecasting methods.
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