Thomas Hartlep

Detection of Equatorward Meridional Flow and Evidence of Double-Cell Meridional Circulation inside the Sun

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 Suns 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.

Signatures of Emerging Subsurface Structures in Acoustic Power Maps of the Sun

We show that under certain conditions, subsurface structures in the solar interior can alter the average acoustic power observed at the photosphere above them. By using numerical simulations of wave propagation, we show that this effect is large enough for it to be potentially used for detecting emerging active regions before they appear on the surface. In our simulations, simplified subsurface structures are modeled as regions with enhanced or reduced acoustic wave speed. We investigate the dependence of the acoustic power above a subsurface region on the sign, depth, and strength of the wave-speed perturbation. Observations from the Solar and Heliospheric Observatory/Michelson Doppler Imager (SOHO/MDI) prior and during the emergence of NOAA active region 10488 are used to test the use of acoustic power as a potential precursor of the emergence of magnetic flux.

VALIDATING TIME-DISTANCE FAR-SIDE IMAGING OF SOLAR ACTIVE REGIONS THROUGH NUMERICAL SIMULATIONS

Far-side imaging using time-distance helioseismology methods is assessed using numerically generated artificial data. The data are generated using direct numerical simulations of acoustic oscillations in a spherical solar model. Localized variations of the sound speed in the surface and subsurface layers are used to model the perturbations associated with sunspots and active regions. The accuracy of acoustic travel-time far-side maps is shown to depend on the size and location of active regions. Potential artifacts in the far-side imaging procedure, such as those caused by the presence of active regions on the solar near side, are also investigated.

IMAGING THE SOLAR TACHOCLINE BY TIME-DISTANCE HELIOSEISMOLOGY

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.

Solar Wave-field Simulation for Testing Prospects of Helioseismic Measurements of Deep Meridional Flows

The meridional flow in the Sun is an axisymmetric flow that is generally directed poleward at the surface, and is presumed to be of fundamental importance in the generation and transport of magnetic fields. Its true shape and strength, however, are debated. We present a numerical simulation of helioseismic wave propagation in the whole solar interior in the presence of a prescribed, stationary, single-cell, deep meridional circulation serving as synthetic data for helioseismic measurement techniques. A deep-focusing time-distance helioseismology technique is applied to the synthetic data, showing that it can in fact be used to measure the effects of the meridional flow very deep in the solar convection zone. It is shown that the ray approximation that is commonly used for interpretation of helioseismology measurements remains a reasonable approximation even for very long distances between 12° and 42° corresponding to depths between 52 and 195 Mm. From the measurement noise, we extrapolate that time-resolved observations on the order of a full solar cycle may be needed to probe the flow all the way to the base of the convection zone.

Local helioseismology of sunspot regions: Comparison of ring-diagram and time-distance results

Local helioseismology provides unique information about the subsurface structure and dynamics of sunspots and active regions. However, because of complexity of sunspot regions local helioseismology diagnostics require careful analysis of systematic uncertainties and physical interpretation of the inversion results. We present new results of comparison of the ring-diagram analysis and time-distance helioseismology for active region NOAA 9787, for which a previous comparison showed significant differences in the subsurface sound-speed structure, and discuss systematic uncertainties of the measurements and inversions. Our results show that both the ring-diagram and time-distance techniques give qualitatively similar results, revealing a characteristic two-layer seismic sound-speed structure consistent with the results for other active regions. However, a quantitative comparison of the inversion results is not straightforward. It must take into account differences in the sensitivity, spatial resolution and the averaging kernels. In particular, because of the acoustic power suppression, the contribution of the sunspot seismic structure to the ring-diagram signal can be substantially reduced. We show that taking into account this effect reduces the difference in the depth of transition between the negative and positive sound-speed variations inferred by these methods. Further detailed analysis of the sensitivity, resolution and averaging properties of the local helioseismology methods is necessary for consolidation of the inversion results. It seems to be important that both methods indicate that the seismic structure of sunspots is rather deep and extends to at least 20 Mm below the surface, putting constraints on theoretical models of sunspots.

HELIOSEISMIC INVESTIGATION OF EMERGING MAGNETIC FLUX IN THE SOLAR CONVECTION ZONE

Helioseismology is capable of detecting signatures of emerging sunspot regions in the solar interior before they appear at the surface. Here we present measurements that show the rising motion of the acoustic travel-time perturbation signatures in the deep convection zone, and study the possible physical origin of these signatures using observational and numerical simulation data. Our results show that the detected signatures first appear at deeper layers and then rise, with velocities of up to 1 km s–1, to shallower regions. We find evidences that these signatures may not be caused by subsurface flows or wave-speed perturbations, but are associated with acoustic power variations and frequency shifts of the cross-covariance function measured in the emerging-flux region. We also confirm with the use of numerical simulation data that phase travel-time shifts can be associated with frequency shifts related to acoustic power variations. The results of this work reveal the rising motion of magnetic flux in the deep convection zone and explain the large amplitude of the detected perturbation signatures.

DETECTION OF FAST-MOVING WAVES PROPAGATING OUTWARD ALONG SUNSPOTS’ RADIAL DIRECTION IN THE PHOTOSPHERE

Helioseismic and magnetohydrodynamic waves are abundant in and above sunspots. Through cross-correlating oscillation signals in the photosphere observed by the SDO/HMI, we reconstruct how waves propagate away from virtual wave sources located inside a sunspot. In addition to the usual helioseismic wave, a fast-moving wave is detected traveling along the sunspots radial direction from the umbra to about 15 Mm beyond the sunspot boundary. The wave has a frequency range of 2.5 - 4.0 mHz with a phase velocity of 45.3 km/s, substantially faster than the typical speeds of Alfven and magnetoacoustic waves in the photosphere. The observed phenomenon is consistent with a scenario of that a magnetoacoustic wave is excited at approximately 5 Mm beneath the sunspot, and its wavefront travels to and sweeps across the photosphere with a speed higher than the local magnetoacoustic speed. The fast-moving wave, if truly excited beneath the sunspots surface, will help open a new window to study the internal structure and dynamics of sunspots.

Verification of the helioseismic Fourier-Legendre analysis for meridional flow measurements

Measuring the Suns internal meridional flow is one of the key issues of helioseismology. Using the Fourier-Legendre analysis is a technique for addressing this problem. We validate this technique with the help of artificial helioseismic data. The analysed data set was obtained by numerically simulating the effect of the meridional flow on the seismic wave field in the full volume of the Sun. In this way, a 51.2-hour long time series was generated. The resulting surface velocity field is then analyzed in various settings: Two

VERIFICATION OF THE HELIOSEISMOLOGY TRAVEL-TIME MEASUREMENT TECHNIQUE AND THE INVERSION PROCEDURE FOR SOUND SPEED USING ARTIFICIAL DATA

360^\circ \times 90^\circ