11 research outputs found
Using Realistic MHD Simulations for Modeling and Interpretation of Quiet-Sun Observations with the Solar Dynamics Observatory Helioseismic and Magnetic Imager
The solar atmosphere is extremely dynamic, and many important phenomena
develop on small scales that are unresolved in observations with the
Helioseismic and Magnetic Imager (HMI) instrument on the Solar Dynamics
Observatory (SDO). For correct calibration and interpretation of the
observations, it is very important to investigate the effects of small-scale
structures and dynamics on the HMI observables, such as Doppler shift,
continuum intensity, spectral line depth, and width. We use 3D radiative
hydrodynamics simulations of the upper turbulent convective layer and the
atmosphere of the Sun, and a spectro-polarimetric radiative transfer code to
study observational characteristics of the Fe I 6173A line observed by HMI in
quiet-Sun regions. We use the modeling results to investigate the sensitivity
of the line Doppler shift to plasma velocity, and also sensitivities of the
line parameters to plasma temperature and density, and determine effective line
formation heights for observations of solar regions located at different
distances from the disc center. These estimates are important for the
interpretation of helioseismology measurements. In addition, we consider
various center-to-limb effects, such as convective blue-shift, variations of
helioseismic travel-times, and the 'concave' Sun effect, and show that the
simulations can qualitatively reproduce the observed phenomena, indicating that
these effects are related to a complex interaction of the solar dynamics and
radiative transfer.Comment: 21 pages, 10 figures, accepted for publication in Ap
The Helioseismic and Magnetic Imager (HMI) Vector Magnetic Field Pipeline: Overview and Performance
The Helioseismic and Magnetic Imager (HMI) began near-continuous full-disk
solar measurements on 1 May 2010 from the Solar Dynamics Observatory (SDO). An
automated processing pipeline keeps pace with observations to produce
observable quantities, including the photospheric vector magnetic field, from
sequences of filtergrams. The primary 720s observables were released in mid
2010, including Stokes polarization parameters measured at six wavelengths as
well as intensity, Doppler velocity, and the line-of-sight magnetic field. More
advanced products, including the full vector magnetic field, are now available.
Automatically identified HMI Active Region Patches (HARPs) track the location
and shape of magnetic regions throughout their lifetime.
The vector field is computed using the Very Fast Inversion of the Stokes
Vector (VFISV) code optimized for the HMI pipeline; the remaining 180 degree
azimuth ambiguity is resolved with the Minimum Energy (ME0) code. The
Milne-Eddington inversion is performed on all full-disk HMI observations. The
disambiguation, until recently run only on HARP regions, is now implemented for
the full disk. Vector and scalar quantities in the patches are used to derive
active region indices potentially useful for forecasting; the data maps and
indices are collected in the SHARP data series, hmi.sharp_720s. Patches are
provided in both CCD and heliographic coordinates.
HMI provides continuous coverage of the vector field, but has modest spatial,
spectral, and temporal resolution. Coupled with limitations of the analysis and
interpretation techniques, effects of the orbital velocity, and instrument
performance, the resulting measurements have a certain dynamic range and
sensitivity and are subject to systematic errors and uncertainties that are
characterized in this report.Comment: 42 pages, 19 figures, accepted to Solar Physic
Solar Radius Determination from Sodism/Picard and HMI/SDO Observations of the Decrease of the Spectral Solar Radiance during the 2012 June Venus Transit
International audienceOn 2012 June 5-6, the transit of Venus provided a rare opportunity to determine the radius of the Sun using solar imagers observing a well-defined object, namely, the planet and its atmosphere, partially occulting the Sun. A new method has been developed to estimate the solar radius during a planetary transit. It is based on the estimation of the spectral solar radiance decrease in a region around the contact between the planet and the Sun at the beginning of the ingress and at the end of the egress. The extrapolation to zero of the radiance decrease versus the Sun-to-Venus apparent angular distance allows estimation of the solar radius at the time of first and fourth contacts. This method presents the advantage of being almost independent on the plate scale, the distortion, the refraction by the planetary atmosphere, and on the point-spread function of the imager. It has been applied to two space solar visible imagers, SODISM/PICARD and HMI/SDO. The found results are mutually consistent, despite their different error budgets: 959.''85 ± 0.''19 (1Ï) for SODISM at 607.1 nm and 959.''90 ± 0.''06 (1Ï) for HMI at 617.3 nm
Chromospheric and coronal observations of solar flares with the helioseismic and magnetic imager
We report observations of white-light ejecta in the low corona, for two X-class flares on 2013 May 13, using data from the Helioseismic and Magnetic Imager (HMI) of the Solar Dynamics Observatory. At least two distinct kinds of sources appeared (chromospheric and coronal), in the early and later phases of flare development, in addition to the white-light footpoint sources commonly observed in the lower atmosphere. The gradual emissions have a clear identification with the classical loop-prominence system, but are brighter than expected and possibly seen here in the continuum rather than line emission. We find the HMI flux exceeds the radio/X-ray interpolation of the bremsstrahlung produced in the flare soft X-ray sources by at least one order of magnitude. This implies the participation of cooler sources that can produce free-bound continua and possibly line emission detectable by HMI. One of the early sources dynamically resembles "coronal rain", appearing at a maximum apparent height and moving toward the photosphere at an apparent constant projected speed of 134 ± 8 km sâ1. Not much literature exists on the detection of optical continuum sources above the limb of the Sun by non-coronagraphic instruments and these observations have potential implications for our basic understanding of flare development, since visible observations can in principle provide high spatial and temporal resolution