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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.

Data from the Global Oscillation Network Group (GONG) project and other helioseismic experiments provide a test for models of stellar interiors and for the thermodynamic and radiative properties, on which the models depend, of matter under the extreme conditions found in the sun. Current models are in agreement with the helioseismic inferences, which suggests, for example, that the disagreement between the predicted and observed fluxes of neutrinos from the sun is not caused by errors in the models. However, the GONG data reveal subtle errors in the models, such as an excess in sound speed just beneath the convection zone. These discrepancies indicate effects that have so far not been correctly accounted for; for example, it is plausible that the sound-speed differences reflect weak mixing in stellar interiors, of potential importance to the overall evolution of stars and ultimately to estimates of the age of the galaxy based on stellar evolution calculations.

The splitting of the frequencies of the global resonant acoustic modes of the Sun by large-scale Ñows and rotation permits study of the variation of angular velocity ) with both radius and latitude within the turbulent convection zone and the deeper radiative interior. The nearly uninterrupted Doppler imaging observations, provided by the Solar Oscillations Investigation (SOI) using the Michelson Doppler Imager (MDI) on the Solar and Heliospheric Observatory (SOHO) spacecraft positioned at the Lagrangian point in continuous sunlight, yield oscillation power spectra with very high signal-to-L 1 noise ratios that allow frequency splittings to be determined with exceptional accuracy. This paper reports on joint helioseismic analyses of solar rotation in the convection zone and in the outer part of the radiative core. Inversions have been obtained for a medium-l mode set (involving modes of angular degree l extending to about 250) obtained from the Ðrst 144 day interval of SOI-MDI observations in 1996. Drawing inferences about the solar internal rotation from the splitting data is a subtle process. By applying more than one inversion technique to the data, we get some indication of what are the more robust and less robust features of our inversion solutions. Here we have used seven di †erent inversion methods. To test the reliability and sensitivity of these methods, we have performed a set of controlled experiments utilizing artiÐcial data. This gives us some conÐdence in the inferences we can draw from the real solar data. The inversions of SOI-MDI data have conÐrmed that the decrease of ) with latitude seen at the surface extends with little radial variation through much of the convection zone, at the base of which is an adjustment layer, called the tachocline, leading to nearly uniform rotation deeper in the radiative interior. A prominent rotational shearing layer in which ) increases just below the surface is discernible at low to mid latitudes. Using the new data, we have also been able to study the solar rotation closer to the poles than has been achieved in previous investigations. The data have revealed that the angular velocity is distinctly lower at high latitudes than the values previously extrapolated from measurements at lower latitudes based on surface Doppler observations and helioseismology. Furthermore, we have found some evidence near latitudes of 75¡ of a submerged polar jet which is rotating more rapidly than its immediate surroundings. Superposed on the relatively smooth latitudinal variation in ) are alternating zonal bands of slightly faster and slower rotation, each extending some 10¡ to 15¡ in latitude. These relatively weak banded Ñows have been followed by inversion to a depth of about 5% of the solar radius and appear to coincide with the evolving pattern of "" torsional oscillations ÏÏ reported from earlier surface Doppler studies.

Meridional flow in the solar interior plays an important role in redistributing angular momentum and transporting magnetic flux inside the Sun. Although it has long been recognized that the meridional flow is predominantly poleward at the Sun's surface and in its shallow interior, the location of the equatorward return flow and the meridional flow profile in the deeper interior remain unclear. Using the first 2 yr of continuous helioseismology observations from the Solar Dynamics Observatory/Helioseismic Magnetic Imager, we analyze travel times of acoustic waves that propagate through different depths of the solar interior carrying information about the solar interior dynamics. After removing a systematic center-to-limb effect in the helioseismic measurements and performing inversions for flow speed, we find that the poleward meridional flow of a speed of 15 m s −1 extends in depth from the photosphere to about 0.91 R . An equatorward flow of a speed of 10 m s −1 is found between 0.82 and 0.91 R in the middle of the convection zone. Our analysis also shows evidence of that the meridional flow turns poleward again below 0.82 R , indicating an existence of a second meridional circulation cell below the shallower one. This double-cell meridional circulation profile with an equatorward flow shallower than previously thought suggests a rethinking of how magnetic field is generated and redistributed inside the Sun.

By applying time-distance helioseismology measurements and inversions to Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) dynamics campaign data, we obtain synoptic maps of subsurface plasma-flow fields at a depth of 0-12 Mm for seven solar Carrington rotations, covering the years 1996-2002, from solar-activity minimum to maximum. Vorticity distribution and both zonal and meridional flows are derived from such synoptic flow maps, which contain an enormous amount of information about solar dynamics. The results for the zonal flows agree well with previous results. The meridional flows of an order of 20 m s À1 are found to remain poleward during the whole period of observations. In addition to the poleward meridional flows observed at the solar minimum, extra meridional circulation cells of flow converging toward the activity belts are found in both hemispheres, which may imply plasma downdrafts in the activity belts. These converging flow cells migrate toward the solar equator together with the activity belts as the solar cycle evolves. The vorticity distributions are largely linear with latitude, and the deviations from the vorticity caused by the mean differential rotation are presented. Patterns of large-scale flows are investigated for a large active region at different depths. Converging flows toward the center of the active region are found near the solar surface, and divergent flows in this large active region are found to be rooted much deeper than similar flows observed in individual sunspots. We conclude that the extremely rich and complicated dynamics of the upper convection zone reveal remarkable organization on the large scale, which can be correlated with the magnetic activity zones.

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The magnetic flux that is generated by dynamo processes inside the Sun emerges in the form of bipolar magnetic regions. The properties of these directly observable signatures of the dynamo can be extracted from full-disk solar magnetograms. The most homogeneous, high-quality synoptic data set of solar magnetograms has been obtained with the MDI instrument on the SOHO spacecraft during 1995-2011. We have developed an IDL program which has, when applied to the 73,838 magnetograms of the MDI data set, automatically identified 160,079 bipolar magnetic regions that span a range of scale sizes across nearly four orders of magnitude. The properties of each region have been extracted and statistically analysed, in particular with respect to the polarity orientations of the bipolar regions, including their tilt angle distributions and their violations of Hale's polarity law. The latitude variation of the average tilt angles (with respect to the E-W direction), which is known as Joy's law, is found to closely follow the relation 32.1 • × sin(latitude). There is no indication of a dependence on region size that one may expect if the tilts were produced by the Coriolis force during the buoyant rise of flux loops from the tachocline region. A few percent of all regions have orientations that violate Hale's polarity law. We show explicit examples, from different phases of the solar cycle, where well defined medium-size bipolar regions with opposite polarity orientations occur side by side in the same latitude zone in the same magnetogram. Such oppositely oriented large bipolar regions cannot be part of the same toroidal flux system, but different flux systems must coexist at any given time in the same latitude zones. These examples are incompatible with the paradigm of coherent, subsurface toroidal flux ropes as the source of sunspots, and instead show that fluctuations must play a major role at all scales for the turbulent dynamo. To confirm the profound role of fluctuations at large scales we show explicit examples where large bipolar regions differ fromthe average Joy's law orientation by an amount between 90 • and 100 • . We see no observational support for a separation of scales or a division between a global and a local dynamo, since also the smallest scales in our sample retain a nonrandom component that significantly contributes to the accumulated emergence of a north-south dipole moment that will lead to the replacement of the old global poloidal field with a new one that has the opposite orientation.