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Local Helioseismology of Emerging Active Regions: A Case Study  

Alexander Kosovichev   Submitted: 2016-07-19 18:38

Local helioseismology provides a unique opportunity to investigate the subsurface structure and dynamics of active regions and their effect on the large-scale flows and global circulation of the Sun. We use measurements of plasma flows in the upper convection zone, provided by the Time-Distance Helioseismology Pipeline developed for analysis of solar oscillation data obtained by Helioseismic and Magnetic Imager (HMI) on Solar Dynamics Observatory (SDO), to investigate the subsurface dynamics of emerging active region NOAA 11726. The active region emergence was detected in deep layers of the convection zone about 12 hours before the first bipolar magnetic structure appeared on the surface, and 2 days before the emergence of most of the magnetic flux. The speed of emergence determined by tracking the flow divergence with depth is about 1.4 km s-1, very close to the emergence speed in the deep layers. As the emerging magnetic flux becomes concentrated in sunspots local converging flows are observed beneath the forming sunspots. These flows are most prominent in the depth range 1-3 Mm, and remain converging after the formation process is completed. On the larger scale converging flows around active region appear as a diversion of the zonal shearing flows towards the active region, accompanied by formation of a large-scale vortex structure. This process occurs when a substantial amount of the magnetic flux emerged on the surface, and the converging flow pattern remains stable during the following evolution of the active region. The Carrington synoptic flow maps show that the large-scale subsurface inflows are typical for active regions. In the deeper layers (10-13 Mm) the flows become diverging, and surprisingly strong beneath some active regions. In addition, the synoptic maps reveal a complex evolving pattern of large-scale flows on the scale much larger than supergranulation.

Authors: Alexander G. Kosovichev, Junwei Zhao, and Stathis Ilonidis
Projects: SDO-HMI

Publication Status: Lecture Notes in Physics, in press
Last Modified: 2016-07-20 12:23
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Reconstruction of Solar Subsurfaces by Local Helioseismology  

Alexander Kosovichev   Submitted: 2016-07-19 18:35

Local helioseismology has opened new frontiers in our quest for understanding of the internal dynamics and dynamo on the Sun. Local helioseismology reconstructs subsurface structures and flows by extracting coherent signals of acoustic waves traveling through the interior and carrying information about subsurface perturbations and flows, from stochastic oscillations observed on the surface. The initial analysis of the subsurface flow maps reconstructed from the 5 years of SDO/HMI data by time-distance helioseismology reveals the great potential for studying and understanding of the dynamics of the quiet Sun and active regions, and the evolution with the solar cycle. In particular, our results show that the emergence and evolution of active regions are accompanied by multi-scale flow patterns, and that the meridional flows display the North-South asymmetry closely correlating with the magnetic activity. The latitudinal variations of the meridional circulation speed, which are probably related to the large-scale converging flows, are mostly confined in shallow subsurface layers. Therefore, these variations do not necessarily affect the magnetic flux transport. The North-South asymmetry is also pronounced in the variations of the differential rotation ("torsional oscillations"). The calculations of a proxy of the subsurface kinetic helicity density show that the helicity does not vary during the solar cycle, and that supergranulation is a likely source of the near-surface helicity.

Authors: Alexander G. Kosovichev and Junwei Zhao
Projects: SDO-HMI

Publication Status: Lecture Notes in Physics, vol. 914, pp. 25-41 (2016) DOI: 10.1007/978-3-319-24151-7_2
Last Modified: 2016-07-20 12:23
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Sunquakes and starquakes  

Alexander Kosovichev   Submitted: 2014-02-11 15:54

In addition to well-known mechanisms of excitation of solar and stellar oscillations by turbulent convection and instabilities, the oscillations can be excited by an impulsive localized force caused by the energy release in solar and stellar flares. Such oscillations have been observed on the Sun (`sunquakes'), and created a lot of interesting discussions about physical mechanisms of the impulsive excitation and their relationship to the flare physics. The observation and theory have shown that most of a sunquake's energy is released in high-degree, high-frequency p modes. In addition, there have been reports on helioseismic observations of low-degree modes excited by strong solar flares. Much more powerful flares observed on other stars can cause `starquakes' of substantially higher amplitude. Observations of such oscillations can provide new asteroseismic information and also constraints on mechanisms of stellar flares. The basic properties of sunquakes and initial attempts to detect flare-excited oscillations in Kepler short-cadence data are discussed.

Authors: Alexander G. Kosovichev
Projects: None

Publication Status: Submitted: To be published in "Precision Asteroseismology", Proceedings of IAU Symposium No. 301, 2014, J.A. Guzik, W.J. Chaplin, G. Handler & A. Pigulski, eds
Last Modified: 2014-02-12 07:44
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Sunquakes: helioseismic response to solar flares  

Alexander Kosovichev   Submitted: 2014-02-11 15:50

Sunquakes observed in the form of expanding wave ripples on the surface of the Sun during solar flares represent packets of acoustic waves excited by flare impacts and traveling through the solar interior. The excitation impacts strongly correlate with the impulsive flare phase, and are caused by the energy and momentum transported from the energy release sites. The flare energy is released in the form of energetic particles, waves, mass motions, and radiation. However, the exact mechanism of the localized hydrodynamic impacts which generate sunquakes is unknown. Solving the problem of the sunquake mechanism will substantially improve our understanding of the flare physics. In addition, sunquakes offer a unique opportunity for studying the interaction of acoustic waves with magnetic fields and flows in flaring active regions, and for developing new approaches to helioseismic acoustic tomography

Authors: Alexander G. Kosovichev
Projects: None

Publication Status: Submitted: to appear in "Extraterrestrial Seismology", Cambridge Univ. Press
Last Modified: 2014-02-11 15:51
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Helioseismic Constraints and Paradigm Shift in Solar Dynamo  

Alexander Kosovichev   Submitted: 2014-02-10 18:32

Helioseismology provides important constraints for the solar dynamo problem. However, the basic properties and even the depth of the dynamo process, which operates also in other stars, are unknown. Most of the dynamo models suggest that the toroidal magnetic field that emerges on the surface and forms sunspots is generated near the bottom of the convection zone, in the tachocline. However, there is a number of theoretical and observational problems with justifying the deep-seated dynamo models. This leads to the idea that the subsurface angular velocity shear may play an important role in the solar dynamo. Using helioseismology measurements of the internal rotation and meridional circulation, we investigate a mean-field MHD model of dynamo distributed in the bulk of the convection zone but shaped in a near-surface layer. We show that if the boundary conditions at the top of the dynamo region allow the large-scale toroidal magnetic fields to penetrate into the surface, then the dynamo wave propagates along the isosurface of angular velocity in the subsurface shear layer, forming the butterfly diagram in agreement with the Parker-Yoshimura rule and solar-cycle observations. Unlike the flux-transport dynamo models, this model does not depend on the transport of magnetic field by meridional circulation at the bottom of the convection zone, and works well when the meridional circulation forms two cells in radius, as recently indicated by deep-focus time-distance helioseismology analysis of the SDO/HMI and SOHO/MDI data. We compare the new dynamo model with various characteristics if the solar magnetic cycles, including the cycle asymmetry (Waldmeier's relations) and magnetic `butterfly' diagrams.

Authors: Alexander G. Kosovichev, Valery V. Pipin, Junwei Zhao
Projects: SDO-HMI

Publication Status: Progress in Physics of the Sun and Stars: A New Era in Helio- and Asteroseismology. Edited by H. Shibahashi and A.E. Lynas-Gray. ASP Conf. Proc. Vol. 479. 2013, p.395
Last Modified: 2014-02-11 12:20
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Advances in Global and Local Helioseismology: an Introductory Review  

Alexander Kosovichev   Submitted: 2011-03-10 02:33

Helioseismology studies the structure and dynamics of the Sun's interior by observing oscillations on the surface. These studies provide information about the physical processes that control the evolution and magnetic activity of the Sun. In recent years, helioseismology has made substantial progress towards the understanding of the physics of solar oscillations and the physical processes inside the Sun, thanks to observational, theoretical and modeling efforts. In addition to the global seismology of the Sun based on measurements of global oscillation modes, a new field of local helioseismology, which studies oscillation travel times and local frequency shifts, has been developed. It is capable of providing 3D images of the subsurface structures and flows. The basic principles, recent advances and perspectives of global and local helioseismology are reviewed in this article.

Authors: Alexander Kosovichev
Projects: SDO-HMI

Publication Status: Lecture Notes in Physics v.831 (in press)
Last Modified: 2011-03-10 07:24
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The Subsurface-shear-shaped Solar αΩ Dynamo  

Alexander Kosovichev   Submitted: 2011-02-23 14:23

We propose a solar dynamo model distributed in the bulk of the convection zone with toroidal magnetic-field flux concentrated in a near-surface layer. We show that if the boundary conditions at the top of the dynamo region allow the large-scale toroidal magnetic fields to penetrate close to the surface, then the modeled butterfly diagram for the toroidal magnetic field in the upper convection zone is formed by the subsurface rotational shear layer. The model is in agreement with observed properties of the magnetic solar cycle.

Authors: V.V. Pipin and A.G. Kosovichev
Projects: None

Publication Status: The Astrophysical Journal Letters, Volume 727, Issue 2, article id. L45 (2011)
Last Modified: 2011-02-24 08:40
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Local Helioseismology of Sunspots: Current Status and Perspectives (Invited Review)  

Alexander Kosovichev   Submitted: 2011-02-23 14:19

Mechanisms of the formation and stability of sunspots are among the longest-standing and intriguing puzzles of solar physics and astrophysics. Sunspots are controlled by subsurface dynamics hidden from direct observations. Recently, substantial progress in our understanding of the physics of the turbulent magnetized plasma in strong-field regions has been made by using numerical simulations and local helioseismology. Both the simulations and helioseismic measurements are extremely challenging, but it becomes clear that the key to understanding the enigma of sunspots is a synergy between models and observations. Recent observations and radiative MHD numerical models have provided a convincing explanation to the Evershed flows in sunspot penumbrae. Also, they lead to the understanding of sunspots as self-organized magnetic structures in the turbulent plasma of the upper convection zone, which are maintained by a large-scale dynamics. Local helioseismic diagnostics of sunspots still have many uncertainties, some of which are discussed in this review. However, there have been significant achievements in resolving these uncertainties, verifying the basic results by new high-resolution observations, testing the helioseismic techniques by numerical simulations, and comparing results obtained by different methods. For instance, a recent analysis of helioseismology data from the Hinode space mission has successfully resolved several uncertainties and concerns (such as the inclined-field and phase-speed filtering effects) that might affect the inferences of the subsurface wave-speed structure of sunspots and the flow pattern. It becomes clear that for the understanding of the phenomenon of sunspots it is important to further improve the helioseismology methods and investigate the whole life cycle of active regions, from magnetic-flux emergence to dissipation.

Authors: A.G. Kosovichev
Projects: GONG,Hinode/SOT,SoHO-MDI

Publication Status: submitted to Solar Physics
Last Modified: 2011-02-24 08:40
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Analysis of SOHO/MDI and TRACE Observations of Sunspot Torsional Oscillation in AR10421  

Alexander Kosovichev   Submitted: 2011-02-23 14:14

Rotation of the leading sunspot of active region NOAA 10421 was investigated using magnetograms and Dopplergrams from the MDI instrument of the Solar and Heliospheric Observatory, and white-light images from the Transition Region and Coronal Explorer (TRACE). The vertical, radial, and azimuthal axisymmetrical components of both magnetic and velocity field vectors were reconstructed for the sunspot umbra and penumbra. All three components of both vectors in the umbra and penumbra show torsional oscillations with the same rotational period of about 3.8 days. The TRACE white-light data also show that the sunspot umbra and penumbra are torsionally rotating with the same period. Possible mechanisms of sunspot torsional motions are discussed.

Authors: O.S. Gopasyuk and A.G. Kosovichev
Projects: SoHO-MDI,TRACE

Publication Status: The Astrophysical Journal, Volume 729, Issue 2, article id. 95 (2011)
Last Modified: 2011-02-24 08:40
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First Sunquake of Solar Cycle 24 Observed by Solar Dynamics Observatory  

Alexander Kosovichev   Submitted: 2011-02-23 14:11

The X2.2-class solar flare of February 15, 2011, produced a powerful `sunquake' event, representing a seismic response to the flare impact. The impulsively excited seismic waves formed a compact wavepacket traveling through the solar interior and appeared on the surface as expanding wave ripples. The Helioseismic and Magnetic Imager (HMI), instrument on SDO, observes variations of intensity, magnetic field and plasma velocity (Dopplergrams) on the surface of Sun almost uninterruptedly with high resolution (0.5 arcsec/pixel) and high cadence (45 sec). The flare impact on the solar surface was observed in the form of compact and rapid variations of the HMI observables (Doppler velocity, line-of-sight magnetic field and continuum intensity). These variations, caused by the impact of high-energy particles in the photosphere, formed a typical two-ribbon flare structure. The sunquake can be easily seen in the raw Dopplergram differences without any special data processing. The source of this quake was located near the outer boundary of a very complicated complicated sunspot region, NOAA 1158, in a sunspot penumbra and at the penumbra boundary. This caused an interesting plasma dynamics in the impact region. I present some preliminary results of analysis of the near-real-time data from HMI, and discuss properties of the sunquake and the flare impact sources.

Authors: A.G. Kosovichev
Projects: SDO-HMI

Publication Status: prepared for RHESSI Science Nuggets
Last Modified: 2011-02-24 08:40
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Investigation of a Sunspot Complex by Helioseismology  

Alexander Kosovichev   Submitted: 2011-02-23 14:08

Sunspot regions often form complexes of activity that may live for several solar rotations, and represent a major component of the Sun's magnetic activity. It had been suggested that the close appearance of active regions in space and time might be related to common subsurface roots, or ''nests'' of activity. EUV images show that the active regions are magnetically connected in the corona, but subsurface connections have not been established. We investigate the subsurface structure and dynamics of a large complex of activity, NOAA 10987-10989, observed during the SOHO/MDI Dynamics run in March-April 2008, which was a part of the Whole Heliospheric Interval (WHI) campaign. The active regions in this complex appeared in a narrow latitudinal range, probably representing a subsurface toroidal flux tube. We use the MDI full-disk Dopplergrams to measure perturbations of travel times of acoustic waves traveling to various depths by using time-distance helioseismology, and obtain sound-speed and flow maps by inversion of the travel times. The subsurface flow maps show an interesting dynamics of decaying active regions with persistent shearing flows, which may be important for driving the flaring and CME activity, observed during the WHI campaign. Our analysis, including the seismic sound-speed inversion results and the distribution of deep-focus travel-time anomalies, gave indications of diverging roots of the magnetic structures, as could be expected from Ω-loop structures. However, no clear connection in the depth range of 0-48 Mm among the three active regions in this complex of activity was detected.

Authors: A. G. Kosovichev and T.L. Duvall, Jr
Projects: SoHO-MDI

Publication Status: to appear in Proc. IAU Symposium 273, Physics of Sun and Star Spots, Ventura, California 22-26 August 2010
Last Modified: 2011-02-24 08:40
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Subject will be restored when possible  

Alexander Kosovichev   Submitted: 2007-10-10 14:35

Observations of a large solar flare of December 13, 2006, using Solar Optical Telescope (SOT) on Hinode spacecraft revealed high-frequency oscillations excited by the flare in the sunspot chromosphere. These oscillations are observed in the region of strong magnetic field of the sunspot umbra, and may provide a new diagnostic tool for probing the structure of sunspots and understanding physical processes in solar flares.

Authors: A.G. Kosovichev and T. Sekii
Projects: Hinode

Publication Status: ApJL (in press)
Last Modified: 2007-10-11 09:41
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Subject will be restored when possible  

Alexander Kosovichev   Submitted: 2007-10-10 14:32

Analysis of the hydrodynamic and helioseismic effects in the photosphere during the solar flare of July 23, 2002, observed by Michelson Doppler Imager (MDI) on SOHO, and high-energy images from RHESSI shows that these effects are closely associated with sources of the hard X-ray emission, and that there are no such effects in the centroid region of the flare gamma-ray emission. These results demonstrate that contrary to expectations the hydrodynamic and helioseismic responses (''sunquakes'') are more likely to be caused by accelerated electrons than by high-energy protons. A series of multiple impulses of high-energy electrons forms a hydrodynamic source moving in the photosphere with a supersonic speed. The moving source plays a critical role in the formation of the anisotropic wave front of sunquakes.

Authors: A.G. Kosovichev
Projects: RHESSI,SoHO-MDI

Publication Status: ApJL (in press)
Last Modified: 2007-10-11 09:41
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Properties of Flares-Generated Seismic Waves on the Sun  

Alexander Kosovichev   Submitted: 2006-03-28 13:25

The solar seismic waves excited by solar flares (``sunquakes'') are observed as circular expanding waves on the Sun's surface. The first sunquake was observed for a flare of July 9, 1996, from the Solar and Heliospheric Observatory (SOHO) space mission. However, when the new solar cycle started in 1997, the observations of solar flares from SOHO did not show the seismic waves, similar to the 1996 event, even for large X-class flares during the solar maximum in 2000-2002. The first evidence of the seismic flare signal in this solar cycle was obtained for the 2003 ``Halloween'' events, through acoustic ``egression power'' by Donea and Lindsey. After these several other strong sunquakes have been observed. Here, I present a detailed analysis of the basic properties of the helioseismic waves generated by three solar flares in 2003-2005. For two of these flares, X17 flare of October 28, 2003, and X1.2 flare of January 15, 2005, the helioseismology observations are compared with simultaneous observations of flare X-ray fluxes measured from the RHESSI satellite. These observations show a close association between the flare seismic waves and the hard X-ray source, indicating that high-energy electrons accelerated during the flare impulsive phase produced strong compression waves in the photosphere, causing the sunquake. The results also reveal new physical properties such as strong anisotropy of the seismic waves, the amplitude of which varies significantly with the direction of propagation. The waves travel through surrounding sunspot regions to large distances, up to 120 Mm, without significant decay. These observations open new perspectives for helioseismic diagnostics of flaring active regions on the Sun and for understanding the mechanisms of the energy release and transport in solar flares.

Authors: A.G.Kosovichev
Projects: RHESSI,SoHO-MDI

Publication Status: astro-ph/0601006
Last Modified: 2006-03-29 11:17
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Abstracts by Author
Local Helioseismology of Emerging Active Regions: A Case Study
Reconstruction of Solar Subsurfaces by Local Helioseismology
Sunquakes and starquakes
Sunquakes: helioseismic response to solar flares
Helioseismic Constraints and Paradigm Shift in Solar Dynamo
Advances in Global and Local Helioseismology: an Introductory Review
The Subsurface-shear-shaped Solar αΩ Dynamo
Local Helioseismology of Sunspots: Current Status and Perspectives (Invited Review)
Analysis of SOHO/MDI and TRACE Observations of Sunspot Torsional Oscillation in AR10421
First Sunquake of Solar Cycle 24 Observed by Solar Dynamics Observatory
Investigation of a Sunspot Complex by Helioseismology
Subject will be restored when possible
Subject will be restored when possible
Properties of Flares-Generated Seismic Waves on the Sun

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