Time–Distance Helioseismology Data-Analysis Pipeline for Helioseismic and Magnetic Imager Onboard Solar Dynamics Observatory (SDO/HMI) and Its Initial Results

J. Zhao 0 1 S. Couvidat 0 1 R.S. Bogart 0 1 K.V. Parchevsky 0 1 A.C. Birch 0 1 T.L. Duvall Jr. 0 1 J.G. Beck 0 1 A.G. Kosovichev 0 1 P.H. Scherrer 0 1 0 T.L. Duvall Jr. Laboratory for Astronomy and Solar Physics, NASA Goddard Space Flight Center , Greenbelt, MD 20771, USA 1 A.C. Birch NorthWest Research Associates, CoRA Division , 3380 Mitchell Lane, Boulder, CO 80301, USA 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. The Solar Dynamics Observatory Guest Editors: W. Dean Pesnell, Phillip C. Chamberlin, and Barbara J. Thompson. 1. Introduction The Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO/ HMI: Schou et al., 2011) observes the solar full-disk intensity, Doppler velocity, and vector magnetic field of the photosphere with high spatial resolution and high temporal cadence. Similar to the Michelson Doppler Imager (MDI: Scherrer et al., 1995), an instrument onboard the Solar and Heliospheric Observatory (SOHO), the HMI Dopplergrams are primarily used for helioseismic analysis to investigate the interior structure and dynamics of the Sun. Helioseismology data-analysis pipelines are planned for near-real-time analyses of the observations in order to provide the analysis results to the helioseismology and solar physics communities. The timedistance analysis pipeline is one of the pipelines for local helioseismology studies, and other pipelines include ring-diagram analysis and far-side active region imaging. The timedistance pipeline is designed for the routine production of nearly full-disk subsurface wave-speed perturbations and horizontal flow fields every eight hours, as well as synoptic flow maps for every Carrington rotation. It can also be used to analyze specific target areas and time periods. Timedistance helioseismology was first introduced by Duvall et al. (1993, 1996), and it has developed rapidly since then. Different inversion techniques were introduced and tested. The LSQR algorithm, introduced by Kosovichev (1996) and used later by Zhao, Kosovichev, and Duvall (2001), solves the inversion problem in the least-squares sense in the spatial domain by an iterative approach. The Multi-Channel Deconvolution (MCD) method, introduced by Jacobsen et al. (1999) and widely used in later studies (e.g. Couvidat et al., 2004), solves the least-squares problems in the Fourier domain. Later, Couvidat et al. (2005) applied a horizontal-regularization procedure for this inversion technique. More recently, an optimally localized averaging (OLA) inversion scheme was introduced to study the solar subsurface flow fields (Jackiewicz, Gizon, and Birch, 2008). Different types of sensitivity kernels, which describe the relationship between the travel times and interior properties, were also introduced and used in the timedistance inversion problems. Kosovichev (1996) first used ray-path approximation kernels, Jensen, Jacobsen, and Christensen-Dalsgaard (2000) introduced Fresnel-zone kernels; and Birch and Kosovichev (2000), Birch, Kosovichev, and Duvall (2004), and Birch and Gizon (2007) investigated Born-approximation kernels for both sound-speed structures and flow fields. Couvidat, Birch, and Kosovichev (2006) compared subsurface sound-speed perturbation structures inferred from these different types of kernels, and found that the inversion results obtained with the different kernels were basically consistent. Important results on the solar interior properties have been obtained from the time distance studies as well as from other local helioseismology techniques (e.g., Komm et al., 2004; Lindsey and Braun, 2000). The introduction that follows is limited to only timedistance results due to the scope of this paper. On global scales, poleward meridional flows were found below the photosphere (Giles et al., 1997), and solar-cycle dependent meridional flow variations were also investigated and discussed (Chou and Dai, 2001; Beck, Gizon, and Duvall, 2002; Zhao and Kosovichev, 2004). On local scales, subsurface sound-speed perturbations and flow fields were derived for supergranulation (Kosovichev and Duvall, 1997; Duvall et al., 1997; Duvall and Gizon, 2000; Sekii et al., 2007; Jackiewicz, Gizon, and Birch, 2008) and for sunspots (Kosovichev, Duvall, and Scherrer, 2000; Gizon, Duvall, and Larsen, 2000; Zhao, Kosovichev, and Duvall, 2001; Couvidat, Birch, and Kosovichev, 2006; Zhao, Kosovichev, and Sekii, 2010). Additionally, timedistance helioseismology was used to detect the emergence of active regions before their appearances in the photosphere (Kosovichev, Duvall, and Scherrer, 2000; Jensen et al., 2001; Zharkov and Thompson, 2008), to image large active regions on the far side of the Sun (Zhao, 2007; Ilonidis, Zhao, and Hartlep, 2009), and to measure sound-speed perturbations in the tachocline (Zhao et al., 2009). These results are important for space-weather forecasting and understanding the mechanisms for the generation of solar magnetism. The timedistance helioseismology pipeline analyses, based on the high spatial-resolution and high temporal-cadence observations from HMI, will greatly advance our knowledge of the interior processes and their connections with solar activity above the photosphere. However, one should keep in mind that the physics of solar oscillations in the turbulent magnetized plasma is very complicated, and that the helioseismic techniques are still in the process of being developed. Because of limited knowledge of the wave physics and complexity of the MHD turbulence, there may be systematic uncertainties in the local helioseismology inferences, particularly in strong magnetic-field regions of sunspots. For example, Lindsey and Braun (2005a, 2005b) argued that the outgoing and ingoing travel-time asymmetries observed in sunspot areas might be caused by a shower-glass effect. Schunker et al. (2005) found that the inclined magnetic field in (...truncated)