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Solar Meridional Flow in the Shallow Interior during the Rising Phase of Cycle 24  

Junwei Zhao   Submitted: 2014-06-13 11:20

Solar subsurface zonal and meridional-flow profiles during the rising phase of solar cycle 24 are studied using time-distance helioseismology technique. The faster zonal bands in the torsional-oscillation pattern show strong hemispheric asymmetries and temporal variations in both width and speed. The faster band in the northern hemisphere is located closer to the equator than the band in the southern hemisphere, and migrates past the equator when the magnetic activity in the southern hemisphere is reaching maximum. The meridional-flow speed decreases substantially with the increase of magnetic activity, and the flow profile shows two zonal structures in each hemisphere. The residual meridional flow, after subtracting a mean meridional-flow profile, converges toward the activity belts and shows faster and slower bands like the torsional-oscillation pattern. More interestingly, the meridional-flow speed above latitude 30 degree shows an anti-correlation with the poleward-transporting magnetic flux, slower when the following-polarity flux is transported and faster when the leading-polarity flux is transported. It is expected that this phenomenon slows the process of magnetic cancellation and polarity reversal in the high-latitude areas.

Authors: Junwei Zhao, A. G. Kosovichev, R. S. Bogart
Projects: SDO-HMI

Publication Status: published in ApJ Letters: ApJ Lett, 2014, 789, L7
Last Modified: 2014-06-16 10:57
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Analysis of Helioseismic Power-Spectrum Diagram of A Sunspot  

Junwei Zhao   Submitted: 2012-10-30 11:37

The continuous high spatial-resolution Doppler observation of the Sun by Solar Dynamics Observatory / Helioseismic and Magnetic Imager allows us to compute helioseismic k-omega power-spectrum diagram using only oscillations inside a sunspot. Individual modal ridges can be clearly seen with reduced power in the k-omega diagram constructed by use of 40-hour observation of a stable and round sunspot. Comparing with the k-omega diagram obtained from a quiet-Sun region, inside the sunspot the f-mode ridge gets more power reduction than p-mode ridges, especially at high wavenumber. The p-mode ridges all shift toward lower-wavenumber (or higher-frequency) areas for a given frequency (or wavenumber), implying an increase of phase velocity beneath the sunspot. This probably results from acoustic waves' travel across the inclined magnetic field of the sunspot penumbra. Line-profile asymmetries exhibited in the p-mode ridges are more significant in the sunspot than in quiet Sun. Convection inside the sunspot is also highly suppressed, and its characteristic spatial scale is substantially larger than the typical convection scale of quiet Sun. These observational facts demand a better understanding of magnetoconvection and interactions of helioseismic waves with magnetic field.

Authors: Junwei Zhao & Dean-Yi Chou
Projects: SDO-HMI

Publication Status: Accepted by Solar Physics
Last Modified: 2012-10-30 19:46
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Systematic Center-to-Limb Variation in Measured Helioseismic Travel Times and Its Effect on Inferences of Solar Interior Meridional Flows  

Junwei Zhao   Submitted: 2012-03-09 00:05

We report on a systematic center-to-limb variation in measured helioseismic travel times, which must be taken into account for an accurate determination of solar interior meridional flows. The systematic variation, found in time-distance helioseismology analysis using SDO/HMI and SDO/AIA observations, is different in both travel-time magnitude and variation trend for different observables. It is not clear what causes this systematic effect. Subtracting the longitude-dependent east-west travel times, obtained along the equatorial area, from the latitude-dependent north-south travel times, obtained along the central meridian area, gives remarkably similar results for different observables. We suggest this as an effective procedure for removing the systematic center-to-limb variation. The subsurface meridional flows obtained from inversion of the corrected travel times are approximately 10 m/s slower than those obtained without removing the systematic effect. The detected center-to-limb variation may have important implications in the derivation of meridional flows in the deep interior, and needs a better understanding.

Authors: Junwei Zhao, Kaori Nagashima, R. S. Bogart, A. G. Kosovichev, T. L. Duvall Jr
Projects: SDO-HMI

Publication Status: accepted by ApJ Letters
Last Modified: 2012-03-09 21:05
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Time-Distance Helioseismology Data Analysis Pipeline for Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory (SDO/HMI) and Its Initial Results  

Junwei Zhao   Submitted: 2011-03-23 16:59

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.

Authors: Zhao,J., Couvidat, S., Bogart, R.S., Parchevsky, K.V., Birch, A.C., Duvall, T.L., Beck, J.G., Kosovichev, A. G., Scherrer, P.H.
Projects: SDO-HMI

Publication Status: Accepted by Solar Physics
Last Modified: 2011-03-24 02:58
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Towards Waveform Heliotomography: Observing Interactions of Helioseismic Waves with a Sunspot  

Junwei Zhao   Submitted: 2010-11-16 14:59

We investigate how helioseismic waves that originate from effective point sources interact with a sunspot. These waves are reconstructed from observed stochastic wavefields on the Sun by cross-correlating photospheric Doppler-velocity signals. We select the wave sources at different locations relative to the sunspot, and investigate the p- and f-mode waves separately. The results reveal a complicated picture of waveform perturbations caused by the wave interaction with the sunspot. In particular, it is found that for waves originating from outside of the sunspot, p-mode waves travel across the sunspot with a small amplitude reduction and slightly higher speed, and wave amplitude and phase get mostly restored to the quiet-Sun values after passing the sunspot. The f-mode wave experiences some amplitude reduction passing through the sunspot, and the reduced amplitude is not recovered after that. The wave-propagation speed does not change before encountering the sunspot and inside the sunspot, but the wavefront becomes faster than the reference wave after passing through the sunspot. For waves originating from inside the sunspot umbra, both f- and p-mode waves show significant amplitude reductions and faster speed during all courses of propagation. A comparison of positive and negative time lags of cross-correlation functions shows an apparent asymmetry in the waveform changes for both the f- and p-mode waves. We suggest that the waveform variations of the helioseismic waves interacting with a sunspot found in this article can be used for developing a method of waveform heliotomography, similar to the waveform tomography of the Earth.

Authors: Junwei Zhao, Alexander G. Kosovichev, and Stathis Ilonidis
Projects: SoHO-MDI

Publication Status: Accepted by Solar Physics
Last Modified: 2010-11-17 11:37
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Imaging the Solar Tachocline by Time-Distance Helioseismology  

Junwei Zhao   Submitted: 2009-07-13 13:13

The solar tachocline at the bottom of the convection zone is an important region for the dynamics of the Sun and the solar dynamo. In this region, the sound speed inferred by global helioseismology exhibits a bump of approximately 0.4% relative to the standard solar model. Global helioseismology does not provide any information on possible latitudinal variations or asymmetries between the Northern and Southern hemisphere. Here, we develop a time-distance helioseismology technique, including surface- and deep-focusing measurement schemes and a combination of both, for two-dimensional tomographic imaging of the solar tachocline that infers radial and latitudinal variations in the sound speed. We test the technique using artificial solar oscillation data obtained from numerical simulations. The technique successfully recovers major features of the simplified tachocline models. The technique is then applied to SOHO/MDI medium-l data and provides for the first time a full two-dimensional sound-speed perturbation image of the solar tachocline. The one-dimensional radial profile obtained by latitudinal averaging of the image is in good agreement with the previous global helioseismology result. It is found that the amplitude of the sound-speed perturbation at the tachocline varies with latitude, but it is not clear whether this is in part or fully an effect of instrumental distortion. Our initial results demonstrate that time-distance helioseismology can be used to probe the deep interior structure of the Sun, including the solar tachocline.

Authors: Junwei Zhao, Thomas Hartlep, A. G. Kosovichev, and N. N. Mansour
Projects: None

Publication Status: Accepted by ApJ
Last Modified: 2009-07-13 13:21
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Time-Distance Imaging of Solar Far-Side Active Regions  

Junwei Zhao   Submitted: 2007-06-19 16:50

It is of great importance to monitor large solar active regions in the far-side of the Sun for space weather forecast, in particular, to predict their appearance before they rotate into our view from the solar east limb. Local helioseismology techniques, including helioseismic holography and time-distance, have successfully imaged solar far-side active regions. In this Letter, we further explore the possibility of imaging and improving the image quality of solar far-side active regions by use of time-distance helioseismology. In addition to the previously used scheme with four acoustic signal skips, a five-skip scheme is also included in this newly developed technique. The combination of both four- and five-skip far-side images significantly enhances the signal-to-noise ratio in the far-side images, and reduces spurious signals. The accuracy of the far-side active region imaging is also assessed using one whole year?s solar observation.

Authors: Junwei Zhao
Projects: SoHO-MDI

Publication Status: ApJ Letters (in press)
Last Modified: 2007-06-20 08:38
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Abstracts by Author
Solar Meridional Flow in the Shallow Interior during the Rising Phase of Cycle 24
Analysis of Helioseismic Power-Spectrum Diagram of A Sunspot
Systematic Center-to-Limb Variation in Measured Helioseismic Travel Times and Its Effect on Inferences of Solar Interior Meridional Flows
Time--Distance Helioseismology Data Analysis Pipeline for Helioseismic and Magnetic Imager onboard Solar Dynamics Observatory (SDO/HMI) and Its Initial Results
Towards Waveform Heliotomography: Observing Interactions of Helioseismic Waves with a Sunspot
Imaging the Solar Tachocline by Time-Distance Helioseismology
Time-Distance Imaging of Solar Far-Side Active Regions

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