Validating Time‐Distance Far‐Side Imaging of Solar Active Regions through Numerical Simulations

Hartlep, T.; Zhao, Junwei; Mansour, Nagi N.; Kosovichev, A. G.

doi:10.1086/592721

Cited by 29 publications

(24 citation statements)

References 27 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Prior to the availability of direct far-side observations, the reliability of imaging far-side ARs could only be assessed through applying the technique on numerical simulation data (Hartlep et al 2008), or comparing the detected far-side ARs with the near-side ARs before their rotation onto the far side or after their rotation onto the near side. More recently, the validity of helioseismic far-side imaging can be better evaluated by comparing with the Sun's far-side EUV observations of the Extreme UltraViolet Imager (EUVI) onboard the twin Solar TErrestrial RElations Observatory (STEREO) spacecraft (Howard et al 2008).…”

mentioning

confidence: 99%

Imaging the Sun's Far-side Active Regions by Applying Multiple Measurement Schemes on Multiskip Acoustic Waves

Zhao

Hing

Chen

et al. 2019

ApJ

Self Cite

View full text Add to dashboard Cite

Being able to image active regions on the Sun's far side is useful for modeling the global-scale magnetic field around the Sun, and for predicting the arrival of major active regions that rotate around the limb onto the near side. Helioseismic methods have already been developed to image the Sun's far-side active regions using near-side high-cadence Doppler-velocity observations; however, the existing methods primarily explore the 3-, 4-, and 5-skip helioseismic waves, leaving room for further improvement in the imaging quality by including waves with more multi-skip waves. Taking advantage of the facts that 6-skip waves have the same target-annuli geometry as 3and 4-skip waves, and that 8-skip waves have the same target-annuli geometry as 4skip waves, we further develop a time-distance helioseismic code to include a total of 14 sets of measurement schemes. We then apply the new code on the SDO/HMIobserved Dopplergrams, and find that the new code provides substantial improvements over the existing codes in mapping newly-emerged active regions and active regions near both far-side limbs. Comparing 3 months of far-side helioseismic images with the STEREO/EUVI-observed 304Å images, we find that 97.3% of the helioseismically detected far-side active regions that are larger than a certain size correspond to an observed region with strong EUV brightening. The high reliability of the new imaging tool will potentially allow us to further calibrate the far-side helioseismic images into maps of magnetic flux.

show abstract

mentioning

confidence: 99%

Imaging the Sun's Far-side Active Regions by Applying Multiple Measurement Schemes on Multiskip Acoustic Waves

Zhao

Hing

Chen

et al. 2019

ApJ

Self Cite

View full text Add to dashboard Cite

show abstract

“…Hanasoge et al (2006) replaced the near-surface layer above 0.98R ⊙ with an empirical model that satisfies convective stability while preserving hydrostatic equilibrium, allowing stable simulations to be extended over a temporal window of several days. Hartlep et al (2008) neglected the terms containing A in the momentum equation because they did not affect the frequencies in their range of investigation. Shelyag, Erdélyi, and Thompson (2006) adiabatic exponent [Γ = 5/3] of a perfect gas and then adjusted pressure and density to reach convective stability and hydrostatic equilibrium.…”

Section: Background Stabilization In Time-domain Simulationsmentioning

confidence: 99%

“…Modified background models employed by Hanasoge et al (2006), Shelyag, Erdélyi, and Thompson (2006) and Parchevsky and Kosovichev (2007) all satisfy reciprocity. By contrast, seismic reciprocity is not automatically enforced in the model of Hartlep et al (2008), which neglects the term A in the momentum equation and in the CSM solar models of Schunker et al (2011), which are not hydrostatic.…”

Section: First-order Correction To the Wave Fieldmentioning

confidence: 99%

See 1 more Smart Citation

Propagating Linear Waves in Convectively Unstable Stellar Models: A Perturbative Approach

2013

View full text Add to dashboard Cite

Linear time-domain simulations of acoustic oscillations are unstable in the stellar convection zone. To overcome this problem it is customary to compute the oscillations of a stabilized background stellar model. The stabilization, however, affects the result. Here we propose to use a perturbative approach (running the simulation twice) to approximately recover the acoustic wave field, while preserving seismic reciprocity. To test the method we considered a 1D standard solar model. We found that the mode frequencies of the (unstable) standard solar model are well approximated by the perturbative approach within 1 µHz for low-degree modes with frequencies near 3 mHz. We also show that the perturbative approach is appropriate for correcting rotational-frequency kernels. Finally, we comment that the method can be generalized to wave propagation in 3D magnetized stellar interiors because the magnetic fields have stabilizing effects on convection.Keywords: Stellar models · Helioseismology · Magnetic fields · Numerical methods Time-Domain Simulations of Linear OscillationsHelioseismology is used to study complex phenomena in the solar interior and atmosphere, such as flows and magnetic heterogeneities that cover many temporal and spatial scales. Numerical simulations of wave propagation are a crucial tool for modeling and interpreting helioseismic observations. The same simulations should find applications in the study of stellar oscillations as well (low-degree modes).Acoustic waves in the Sun have very low amplitudes compared with those of the background (Christensen-Dalsgaard, 2002) and thus can be treated as weak perturbations with respect to a background reference model.

show abstract

“…Local helioseismology has been successful in detecting large active regions on the far-side of the Sun, and this technique is becoming more robust (Hartlep et al 2008). However, observations of emerging magnetic field below the surface are very difficult, because helioseismic signals for deep perturbations are weak, and in the upper convection zone the magnetic flux is emerging very fast, not giving enough time to accumulate a sufficient signalto-noise ratio in the measurements.…”

Section: Outstanding Problems Of Helioseismologymentioning

confidence: 99%

The future of helioseismology

Kosovichev¹

2009

Proc. IAU

Self Cite

View full text Add to dashboard Cite

Abstract. Helioseismology has provided us with the unique knowledge of the interior structure and dynamics of the Sun, and the variations with the solar cycle. However, the basic mechanisms of solar magnetic activity, formation of sunspots and active regions are still unknown. Determining the physical properties of the solar dynamo, detecting emerging active regions and observing the subsurface dynamics of sunspots are among the most important and challenging problems. The current status and perspectives of helioseismology are briefly discussed.Keywords. Sun: helioseismology, activity, interior magnetic fields, oscillations, sunspots Outstanding problems of helioseismologyOne of the most important unsolved problems of solar physics and astrophysics is understanding of the physical mechanism of the dynamo operating inside the Sun and producing 11-year activity cycles. Despite a significant progress in theoretical modeling of dynamo processes and successful laboratory experiments the basic understanding of the solar dynamo is still missing. The current theories assume that the poloidal component of magnetic field, which represents the global dipole field, is generated by helical turbulence at the bottom of the convection zonetachocline, and that the toroidal component, which is a source of bipolar sunspot regions, is produced by stretching the poloidal field by the differential rotation (e.g. Jouve, et al. 2008). However, so far helioseismology observations were not able to confirm these models. If the magnetic field is generated by turbulence it is expected that when the field becomes sufficiently strong it suppresses the turbulence and affects the turbulent stresses that maintain the differential rotation. However, the helioseismic observations during a whole solar cycle were not able to detect significant variations of the rotation rate in the tachocline. They found a weak evidence of variations with a period of ∼1.3 years but no changes on the 11-year scale. In addition, to explain the emergence of magnetic fields at mid-and low latitudes the strength of the toroidal magnetic field in the tachocline must be 60-100 kG. This exceeds the energy equipartition value, and requires a dynamic compression, e.g. by back reaction of updraft motions associated with emerging magnetic flux (Parker, 2009). Such motions have not been detected by helioseismology. Alternatively, the magnetic fields can be generated in the bulk of the convection zone and shaped by the subsurface shear layer (Brandenburg, 2009). The equator-ward migration of dynamo waves in this shear layer could explain the sunspot butterfly diagram. The migrating zone of sunspot formations is associated with zonal flows ("torsional oscillations"). The magnetic flux that forms sunspots tend to emerge in the shear layer between slower and faster flows. Helioseismology has established that these flows are quite deep and occupy at least the upper 30% of the convection zone. They are particularly strong at high latitudes, where they migrate towards the poles, and pe...

show abstract

Validating Time‐Distance Far‐Side Imaging of Solar Active Regions through Numerical Simulations

Cited by 29 publications

References 27 publications

Imaging the Sun's Far-side Active Regions by Applying Multiple Measurement Schemes on Multiskip Acoustic Waves

Imaging the Sun's Far-side Active Regions by Applying Multiple Measurement Schemes on Multiskip Acoustic Waves

Propagating Linear Waves in Convectively Unstable Stellar Models: A Perturbative Approach

The future of helioseismology

Contact Info

Product

Resources

About