The
Helioseismic and Magnetic Imager
(HMI) instrument and investigation as a part of the NASA
Solar Dynamics Observatory
(SDO) is designed to study convection-zone dynamics and the solar dynamo, the ...origin and evolution of sunspots, active regions, and complexes of activity, the sources and drivers of solar magnetic activity and disturbances, links between the internal processes and dynamics of the corona and heliosphere, and precursors of solar disturbances for space-weather forecasts. A brief overview of the instrument, investigation objectives, and standard data products is presented.
NASA’s
Solar Dynamics Observatory
(SDO) spacecraft was launched 11 February 2010 with three instruments onboard, including the
Helioseismic and Magnetic Imager
(HMI). After commissioning, HMI began ...normal operations on 1 May 2010 and has subsequently observed the Sun’s entire visible disk almost continuously. HMI collects sequences of polarized filtergrams taken at a fixed cadence with two
4096
×
4096
cameras, from which are computed arcsecond-resolution maps of photospheric observables that include line-of-sight velocity and magnetic field, continuum intensity, line width, line depth, and the Stokes polarization parameters
I
,
Q
,
U
,
V
. Two processing pipelines have been implemented at the SDO Joint Science Operations Center (JSOC) at Stanford University to compute these observables from calibrated Level-1 filtergrams, one that computes line-of-sight quantities every 45 seconds and the other, primarily for the vector magnetic field, that computes averages on a 720-second cadence. Corrections are made for static and temporally changing CCD characteristics, bad pixels, image alignment and distortion, polarization irregularities, filter-element uncertainty and nonuniformity, as well as Sun–spacecraft velocity. We detail the functioning of these two pipelines, explain known issues affecting the measurements of the resulting physical quantities, and describe how regular updates to the instrument calibration impact them. We also describe how the scheme for computing the observables is optimized for actual HMI observations. Initial calibration of HMI was performed on the ground using a variety of light sources and calibration sequences. During the five years of the SDO prime mission, regular calibration sequences have been taken on orbit to improve and regularly update the instrument calibration, and to monitor changes in the HMI instrument. This has resulted in several changes in the observables processing that are detailed here. The instrument more than satisfies all of the original specifications for data quality and continuity. The procedures described here still have significant room for improvement. The most significant remaining systematic errors are associated with the spacecraft orbital velocity.
The Helioseismic and Magnetic Imager (HMI) investigation will study the solar interior using helioseismic techniques as well as the magnetic field near the solar surface. The HMI instrument is part ...of the Solar Dynamics Observatory (SDO) that was launched on 11 February 2010. The instrument is designed to measure the Doppler shift, intensity, and vector magnetic field at the solar photosphere using the 6173 Fe I absorption line. The instrument consists of a front-window filter, a telescope, a set of wave plates for polarimetry, an image-stabilization system, a blocking filter, a five-stage Lyot filter with one tunable element, two wide-field tunable Michelson interferometers, a pair of 4096(exo 2) pixel cameras with independent shutters, and associated electronics. Each camera takes a full-disk image roughly every 3.75 seconds giving an overall cadence of 45 seconds for the Doppler, intensity, and line-of-sight magnetic-field measurements and a slower cadence for the full vector magnetic field. This article describes the design of the HMI instrument and provides an overview of the pre-launch calibration efforts. Overviews of the investigation, details of the calibrations, data handling, and the science analysis are provided in accompanying articles.
We compare line-of-sight magnetograms from the
Helioseismic and Magnetic Imager
(HMI) onboard the
Solar Dynamics Observatory
(SDO) and the
Michelson Doppler Imager
(MDI) onboard the
Solar and ...Heliospheric Observatory
(SOHO). The line-of-sight magnetic signal inferred from the calibrated MDI data is greater than that derived from the HMI data by a factor of 1.40. This factor varies somewhat with center-to-limb distance. An upper bound to the random noise for the 1′′ resolution HMI 720-second magnetograms is 6.3 Mx cm
−2
, and 10.2 Mx cm
−2
for the 45-second magnetograms. Virtually no
p
-mode leakage is seen in the HMI magnetograms, but it is significant in the MDI magnetograms. 12-hour and 24-hour periodicities are detected in strong fields in the HMI magnetograms. The newly calibrated MDI full-disk magnetograms have been corrected for the zero-point offset and underestimation of the flux density. The noise is 26.4 Mx cm
−2
for the MDI one-minute full-disk magnetograms and 16.2 Mx cm
−2
for the five-minute full-disk magnetograms observed with four-arcsecond resolution. The variation of the noise over the Sun’s disk found in MDI magnetograms is likely due to the different optical distortions in the left- and right-circular analyzers, which allows the granulation and
p
-mode to leak in as noise. Saturation sometimes seen in sunspot umbrae in MDI magnetograms is caused by the low intensity and the limitation of the onboard computation. The noise in the HMI and MDI line-of-sight magnetic-field synoptic charts appears to be fairly uniform over the entire map. The noise is 2.3 Mx cm
−2
for HMI charts and 5.0 Mx cm
−2
for MDI charts. No evident periodicity is found in the HMI synoptic charts.
Fossat et al. recently reported detecting rotational splitting of g-modes indirectly via the interaction with p-modes observed directly by the Global Oscillations at Low Frequency (GOLF) instrument ...on the Solar and Heliospheric Observatory (SOHO). They concluded that the core of the Sun is rotating 3.8 0.1 times faster than the surrounding radiative envelope. This is startling, partly because such rapid rotation almost contradicts direct inferences from the p-mode rotational splitting inferred from the same data. Moreover, the inferred amplitudes of the g-modes appear to exceed the upper bound reported by Appourchaux et al. It is also suspect because the theory of the procedure implies that the principal modes claimed to have been measured should be undetectable. We point out that there are other interpretations: one leads to a core rotation about twice as fast as the surrounding envelope; another, to a core rotating more slowly than the envelope. Here we also report on an independent assessment of the Fossat et al. analysis by applying their procedure to different representations of the GOLF data, expanding on Schunker et al. We also analyze seismic data obtained from LOI and MDI (both also on SOHO), from HMI (on SDO), and from the ground-based BiSON and GONG, and we find the evidence reported by Fossat et al. not to be robust. We also illustrate that merely fitting model spectra to observations, which Fossat et al. do to support their g-mode detections, and as Fossat & Schmider do for extracting additional g-mode splittings, is not necessarily reliable. We are therefore led to doubt the claim.
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.
We present a new methodology for the fitting of the peaks in solar oscillation power spectra that is equally well-suited for the estimation of low-, medium, and high-degree f- and p-mode parameters ...and frequency-splitting coefficients. The method can provide accurate input data over a wide portion of the dispersion plane for both structural and rotational inversions. This method, which we call the Multiple-Peak, Tesseral-Spectrum (MPTS) method, operates directly upon of all of the modes in a multiplet (n, l) of radial order n and degree l, and employs a fitting profile that consists of the sum of numerous individual overlapping profiles whose relative amplitudes are determined by the leakage matrix appropriate to the targeted mode. Hence, 2l + 1 sets of modal parameters are obtained simultaneously for each multiplet (n, l). By fitting an appropriate polynomial to the run of the fitted frequencies versus the azimuthal order, frequency-splitting coefficients are also obtained for the same multiplet. Using power spectra obtained from the 66 day long 2010 MDI Dynamics Run, we present sample structural and rotational inversions that employed frequencies and frequency-splitting coefficients from modes in the degree range of 0-1000 and the frequency range of 965-4600 Hz. The structural inversion confirms evidence for a pronounced departure of the sound speed in the outer solar convection zone from the radial sound-speed profile contained in Model S of Christensen-Dalsgaard and his collaborators that we obtained previously using a different fitting method.
Gas on the Sun's surface has been observed to flow away from the equator towards both poles. If the same flow persists to great depths, it could play an important dynamical role in the eleven-year ...sunspot cycle, by carrying the magnetic remnants of the sunspots to high latitudes. An even deeper counterflow, which would be required to maintain mass balance, could explain why new sunspots form at lower latitudes as the cycle progresses. These deep flows would also redistribute angular momentum within the Sun, and therefore help to maintain the faster rotation of the equator relative to the poles. Here we report the detection, using helioseismic tomography, of the longitude-averaged subsurface flow in the outer 4% of the Sun. We find that the subsurface flow is approximately constant in this depth range, and that the speed is similar to that seen on the surface. This demonstrates that the surface flow penetrates deeply, so that it is likely to be an important factor in solar dynamics.
The
Helioseismic and Magnetic Imager
onboard the
Solar Dynamics Observatory
(SDO/HMI) provides continuous full-disk observations of solar oscillations. We develop a data-analysis pipeline based on ...the time–distance helioseismology method to measure acoustic travel times using HMI Doppler-shift observations, and infer solar interior properties by inverting these measurements. The pipeline is used for routine production of near-real-time full-disk maps of subsurface wave-speed perturbations and horizontal flow velocities for depths ranging from 0 to 20 Mm, every eight hours. In addition, Carrington synoptic maps for the subsurface properties are made from these full-disk maps. The pipeline can also be used for selected target areas and time periods. We explain details of the pipeline organization and procedures, including processing of the HMI Doppler observations, measurements of the travel times, inversions, and constructions of the full-disk and synoptic maps. Some initial results from the pipeline, including full-disk flow maps, sunspot subsurface flow fields, and the interior rotation and meridional flow speeds, are presented.
A new fitting methodology is presented that is equally well suited for the estimation of low-, medium-, and high-degree mode parameters from m-averaged solar oscillation power spectra of widely ...differing spectral resolution. This method, which we call the "Windowed, MuLTiple-Peak, averaged-spectrum" or WMLTP Method, constructs a theoretical profile by convolving the weighted sum of the profiles of the modes appearing in the fitting box with the power spectrum of the window function of the observing run, using weights from a leakage matrix that takes into account observational and physical effects, such as the distortion of modes by solar latitudinal differential rotation. We demonstrate that the WMLTP Method makes substantial improvements in the inferences of the properties of the solar oscillations in comparison with a previous method, which employed a single profile to represent each spectral peak. We also present an inversion for the internal solar structure, which is based upon 6366 modes that we computed using the WMLTP method on the 66 day 2010 Solar and Heliospheric Observatory/MDI Dynamics Run. To improve both the numerical stability and reliability of the inversion, we developed a new procedure for the identification and correction of outliers in a frequency dataset. We present evidence for a pronounced departure of the sound speed in the outer half of the solar convection zone and in the subsurface shear layer from the radial sound speed profile contained in Model S of Christensen-Dalsgaard and his collaborators that existed in the rising phase of Solar Cycle 24 during mid-2010.
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