Abstract
The Helioseismic and Magnetic Imager (HMI) instrument is a major component of NASA's Solar Dynamics Observatory (SDO) spacecraft. Since commencement of full regular science operations on 1 May 2010, HMI has operated with remarkable continuity, e.g. during the more than five years of the SDO prime mission that ended 30 September 2015, HMI collected 98.4% of all possible 45-second velocity maps; minimizing gaps in these full-disk Dopplergrams is crucial for helioseismology. HMI velocity, intensity, and magnetic-field measurements are used in numerous investigations, so understanding the quality of the data is important. This article describes the calibration measurements used to track the performance of the HMI instrument, and it details trends in important instrument parameters during the prime mission. Regular calibration sequences provide information used to improve and update the calibration of HMI data. The set-point temperature of the instrument front window and optical bench is adjusted regularly to maintain instrument focus, and changes in the temperature-control scheme have been made to improve stability in the observable quantities. The exposure time has been changed to compensate for a 20% decrease in instrument throughput. Measurements of the performance of the shutter and tuning mechanisms show that they are aging as expected and continue to perform according to specification. Parameters of the tunable optical-filter elements are regularly adjusted to account for drifts in the central wavelength. Frequent measurements of changing CCD-camera characteristics, such as gain and flat field, are used to calibrate the observations. Infrequent expected events such as eclipses, transits, and spacecraft off-points interrupt regular instrument operations and provide the opportunity to perform additional calibration. Onboard instrument anomalies are rare and seem to occur quite uniformly in time. The instrument continues to perform very well.
Highlights
The Solar Dynamics Observatory (SDO) with the Helioseismic and Magnetic Imager (HMI) instrument onboard was launched 11 February 2010 to provide the observations necessary to understand the sources of solar variability and its impact on the terrestrial environment (Pesnell, Thompson, and Chamberlin, 2012; Scherrer et al, 2012)
The semiannual eclipse seasons can be seen as U-shaped dips to below 95% that extend over several weeks each Spring and Fall when the Earth comes between the spacecraft and the Sun for up to 72 minutes each day
The pronounced annual periodicity present during the first three years is due to temperature drift of the HMI instrument caused by the change in irradiance that is due to variation in the Sun–spacecraft distance
Summary
The Solar Dynamics Observatory (SDO) with the Helioseismic and Magnetic Imager (HMI) instrument onboard was launched 11 February 2010 to provide the observations necessary to understand the sources of solar variability and its impact on the terrestrial environment (Pesnell, Thompson, and Chamberlin, 2012; Scherrer et al, 2012). Since 1 May 2010, the HMI has observed the full disk of the Sun almost continuously to measure the velocity, intensity, and magnetic field in the photosphere (Schou et al, 2012a). The full-disk images, tuned to six wavelengths across the Fe I 6173.3433 Å spectral line in each of six polarization states, are downlinked and combined to determine the basic HMI observable quantities: Doppler velocity, line-of-sight (LoS) magnetic field, line width, line depth, continuum intensity, and the Stokes polarization parameters (Couvidat et al, 2016). More advanced products computed from these observables include vector magnetograms (Hoeksema et al, 2014) and subsurface-flow maps (Zhao et al, 2012)
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