Abstract
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 basic vector-field frame list cadence is 135 seconds, but to reduce noise the filtergrams are combined to derive data products every 720 seconds. The primary 720 s 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∘ 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. Definitive SHARP processing is completed only after the region rotates off the visible disk; quick-look products are produced in near real time. 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.
Highlights
The Helioseismic and Magnetic Imager (HMI, Schou et al, 2012b) provides the first uninterrupted time series of space-based, full-disk, vector magnetic field observations of the Sun with a 12-minute cadence
Vector observables Full-disk Very Fast Inversion of the Stokes Vector (VFISV) fd10 Milne–Eddington inversion magnetic and plasma parameters with uncertainties Disambiguated full-disk vector magnetic field Vector and scalar field HMI Active Region Patches (HARPs) + active-region indices region tracked for entire disk passage every 720 s Remapped field patches and active regions (ARs) indices cylindrical equal area projection of Br, Bθ, Bφ Velocity-dependent noise masks for vector field aA description of Joint Science Operations Center (JSOC) data series is provided in Appendix A. bSeries available in both definitive and near-real-time versions, e.g. hmi.sharp_720s_nrt
The temporal life of a definitive HARP starts when it rotates onto the visible disk or one day before the magnetic feature is first identified in the photosphere
Summary
The Helioseismic and Magnetic Imager (HMI, Schou et al, 2012b) provides the first uninterrupted time series of space-based, full-disk, vector magnetic field observations of the Sun with a 12-minute cadence. Scalar observables Level 1 Stokes-polarization images – 720 s averages 24 images: I Q U V at six wavelengths Full-disk line-of-sight magnetic field 45-second full-disk line-of-sight magnetic field Full-disk line-of-sight Doppler velocity Full-disk computed continuum intensity Ic with limb darkening removed Full-disk computed line width Full-disk computed line depth HMI active region patch (HARP) geometry information tracked bitmap computed from LoS magnetic field. Vector observables Full-disk VFISV fd Milne–Eddington inversion magnetic and plasma parameters with uncertainties Disambiguated full-disk vector magnetic field Vector and scalar field HARPs + active-region indices region tracked for entire disk passage every 720 s Remapped field patches and AR indices cylindrical equal area projection of Br , Bθ , Bφ Velocity-dependent noise masks for vector field aA description of JSOC data series is provided in Appendix A. The flux density and intensity contrast reported by HMI are lower for a variety of reasons, as may be expected, see Section 7.3.2
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