A principal goal of the HMI investigation is to better characterize flows in the solar interior. Solar dynamics are a critical component of solar activity on both global scales and local scales. Of particular interest is the very difficult problem of finding the return flow (or flows) of meridional circulation. Other interesting areas of current research involve solar differential rotation, especially at high latitudes, and differences between the current solar cycle and previous ones; the characterization of the spectrum of velocities with depth; the characterization of local flow structures; and the calibration and improvement of far-side imaging techniques.
Plenary speaker: Vincent Böning (MPS)
|16:30||Statistical constraints on active region emergence from the surface motion of the polarities||Schunker, H||Oral|
| ||Hannah Schunker, Aaron Birch, Robert Cameron, Doug Braun, Laurent Gizon[1,3]|
| || Max Planck Institute for Solar System Research,  NorthWest Research Associates,  Georg-August-Universitaet Goettingen, Institut fuer Astrophysik|
| ||We measured the motion of the two main opposite polarities in 154 emerging active regions using line-of-sight magnetograms from SDO/HMI.
Our results reveal two phases of the emergence process defined by the rate of change of the separation speed as the
polarities move apart. Phase one begins when the opposite polarity pairs first appear at the surface, with an east-west alignment and an
increasing separation speed of 1.6 +/- 0.4 km/s. Phase two begins when the separation speed starts to decrease, about 0.1 days after emergence, and ends about 2.5 days after emergence when the polarities have stopped separating. This is consistent with the picture of Chen, Rempel, & Fan (2017): during phase one, the peak of a flux tube breaks through the surface and then, during phase two, the magnetic field lines are straightened by magnetic tension to eventually lie directly above their subsurface footpoints. The scatter in the location of the polarities is consistent with the length and time scales of supergranulation, supporting the idea that convection buffets the polarities as they separate. On average, the polarities emerge with an east-west orientation with the tilt angle developing over time independent of flux, in contrast to predictions from thin flux tube theory.
|16:45||Revisiting helioseismic constraints on subsurface convection||Birch, A||Oral|
| ||Aaron Birch,Tom Duvall,Laurent Gizon[1,2],Shravan Hanasoge,Bradley Hindman, Kaori Nagashima, Katepalli Sreenivasan|
| || Max-Planck-Institut für Sonnensystemforschung,  Georg-August-Universität Göttingen,  Tata Institute of Fundamental Research,  University of Colorado Boulder, Department of Physics, Courant Institute of Mathematical Sciences, and Department of Mechanical and Aerospace Engineering, New York University|
| ||There is disagreement by orders of magnitude between different helioseismic measurements of the the amplitude of subsurface convective flows.
In addition, there are enormous differences between some measurements and simulations of subsurface convection.
Further observational and theoretical work on the topic of solar subsurface convection is crucial.
Motivated by the need to establish a clear baseline for future work,
we present a uniform view of the existing results by expressing upper limits and flow
estimates as root-mean-square velocity per multiplet for all cases.
The disagreements between the upper limit of Hanasoge, Duvall, and Sreenivasan (2012), the ASH simulations
of Miesch et al. (2008), and the helioseismic analysis of Greer et al. (2015) remain, but are reduced in amplitude.
Reconciling the helioseismic measurements may involve reconsidering the assumptions about the vertical correlations
of the flow field and the methods for separating signal and noise.|
|17:00||Temporal evolution of solar meridional flow in the deep interior during 2010-2018||Chen, R||Oral|
| ||Ruizhu Chen[1,2], Junwei Zhao|
| ||Physics Department, Stanford University,  Hansen Experimental Physics Laboratory, Stanford University|
| ||Meridional flow plays an important role in solar dynamo, which drives solar cycles of magnetic field variations, and it is curious whether the meridional flow itself also shows temporal variations during difference phases of a solar cycle. Here we employ a comprehensive time-distance measurement scheme and derive the solar meridional flow using 8 years of SDO/HMI Doppler-velocity data, and explore the temporal evolution of the meridional-flow profile.
Our comprehensive measurement scheme utilizes acoustic travel-time shifts measured along all radial directions of the solar disk for all travel distances. By solving a set of linear equations, we disentangle the systematic center-to-limb effect and meridional-flow-induced travel-time shifts from the measurements, and then invert the flow-induced travel-time shifts for the meridional flow. Our 8-year-averaged meridional-flow profile shows a 3-layer structure: an equatorward flow is sandwiched between two poleward flow zones above and beneath it. Moreover, the 3-layer flow pattern is more significant when solar activity level is low, while the flow structure is more complicated during the active phase of the solar cycle, indicating that the meridional flow variation is correlated with the solar cycle variation.
|17:15||Twenty-one-year helioseismic measurement of solar meridional circulation from SOHO/MDI and SDO/HMI: Anomalous northern hemisphere during cycle 24||Liang, Z||Oral|
| ||Zhi-Chao Liang, Laurent Gizon, Aaron C. Birch, Thomas L. Duvall, Jr., S. P. Rajaguru|
| ||Max-Planck-Institut f\"ur Sonnensystemforschung, Institut f\"ur Astrophysik, Georg-August-Universit\"at G\"ottingen, Center for Space Science, New York University Abu Dhabi, Indian Institute of Astrophysics|
| ||We apply time-distance helioseismology to MDI and HMI medium-degree Dopplergrams covering May 1996--April 2017, i.e., 12-yr of cycle 23 and 9-yr of cycle 24. Our data analysis takes several systematic effects into account, including the P-angle error, surface magnetic field effects, and the center-to-limb variations. For comparison, forward-modeled travel-time differences are computed in the ray approximation for representative meridional flow models. The measured travel-time differences are similar in the southern hemisphere for cycles 23 and 24. However, they differ in the northern hemisphere between cycles 23 and 24. Except for cycle 24's northern hemisphere, the measurements favor a single-cell meridional circulation model where the poleward flows persist down to about 0.8 solar radii, accompanied by local inflows toward the activity belts in the near-surface layers. Cycle 24's northern hemisphere is found to be anomalous: travel-time differences are significantly smaller when travel distances are greater than 20 deg. This asymmetry between northern and southern hemispheres during cycle 24 was not present in previous measurements (e.g., Rajaguru & Antia 2015), which assumed a different P-angle error correction where south-north travel-time differences are shifted to zero at the equator for all travel distances. In our measurements, the travel-time differences at the equator are zero for travel distances less than about 30 deg, but they do not vanish for larger travel distances. Rather than a P-angle error, this equatorial offset for large travel distances might be caused by the asymmetrical near-surface flows around the end points of the acoustic ray paths.
|17:30||HMI Full-disk Vector Magnetograms: Products and Issues||Liu, Y||Oral|
| ||Yang Liu, HMI team|
| ||Stanford University|
| ||HMI vector magnetic field over the Sun's disk is derived from the measured Stokes parameters (I, Q, U, V) using a Milne-Eddington based inversion model. The 180 degree azimuth ambiguity is resolved using the Minimum Energy algorithm for pixels in active regions and for strong-field pixels (the field is greater than about 150 G) in quiet Sun regions. Three other methods are used for the rest of the pixels: the potential-field method, the radial-acute angle method, and the random method.
HMI vector magnetic field synoptic charts are one of the data products produced from the full-disk vector magnetograms. We evaluate the three methods for weak-field pixels, and demonstrate that the random method is the best for synoptic charts in term of low noise level and no additional artifacts being introduced to the charts.
We show two issues in the full-disk vector magnetograms: noise distribution and change sign of field. It is shown that the nooise varies over the Sun's disk and this pattern is also dependent on the orbital velocity. The east-west component of magnetic field in the quiet Sun regions changes its sign when crossing the central
meridian, though this sign change does not affect the vector field synoptic charts: the charts use observation at the central meridian.
We place a discussion here on what causes this sign change and potential methods to solve this issue.
|17:45||HMI Data Corrected for Scattered Light Compared to Hinode SOT-SP Data||Norton, A||Oral|
| ||A.A. Norton, T.L. Duvall, Jr., J. Schou, M.C.M Cheung, P.H. Scherrer, K.C. Chu, J. Sommers|
| || HEPL, Stanford University,  Max Planck Institute for Solar System Research,  Lockheed Martin Solar and Astrophysics Laboratory|
| ||In March 2018, the Helioseismic Magnetic Imager (HMI) team began providing full-disk data to the public on a daily basis that were corrected for scattered light. In addition to the intensity and magnetogram data, the improved vector magnetic field maps are also provided. The process uses a Richardson-Lucy algorithm and a known PSF. The deconvolution results in a few percent decrease in umbral intensity corresponding to a ~200 K decrease in temperature, a doubling of the intensity contrast of granulation from 3.6 to 7.2%, an increase in total field strength values (not only line-of-sight B) in plage by ~1.4, faculae brightening and network darkening, and a partial correction for the convective blue-shift. The new data series can be found in JSOC with names similar to the original but with the qualifying term ‘_dcon’ or ‘_dconS’ appended (denoting whether the deconvolution was applied to the filtergrams or Stokes images). Comparisons to near-simultaneous Hinode SOT-SP data demonstrate that the correction brings the two instruments into much better agreement, including the inverted magnetic field parameters. We compare our results to similar efforts in the literature such as work by Diaz Baso and Asensio Ramos (2018) in which HMI intensity and magnetogram data was enhanced using neural networks and super-resolution.|
|18:00||Investigation of Acoustic Halos using Multi-Height SDO Observations||Tripathy, S||Oral|
| ||S.C. Tripathy, Kiran Jain, S. Kholikov, O. Burtseva, F. Hill, P. Cally]|
| ||National Solar Observatory, Boulder, Co 80303 USA,  Monash University, Clayton, Victoria 3800, Australia|
| ||The interpretation of acoustic waves surrounding active regions has been
a challenging task since the influence of magnetic field on the incident waves
is not fully understood. As a result, structure and dynamics of active
regions beneath the surface show significant uncertainties. Recent numerical
simulations and helioseismic measurements in active regions have demonstrated
that the key to the understanding of these complex processes
requires a synergy between models and helioseismic inferences from observations.
In this context, using data from Helioseismic Magnetic Imager and Atmospheric Imaging Assembly
instruments on board the Solar Dynamics Observatory, we characterize the spatio-temporal
power distribution around active regions as a function of the height in the
solar atmosphere. We find power enhancements (acoustic halos) occur above the acoustic cutoff frequency
and extends up to 10 mHz in HMI Doppler and AIA 170 nm observations and are strong functions of
magnetic field and their inclination angle. We also examine the relative phases and cross-coherence spectra and
find different wave characteristics at different heights. |