What Sets the Magnetic Field Strength and Cycle Period in Solar-type Stars? |
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Alexander Kosovichev Submitted: 2019-08-14 12:43
Two fundamental properties of stellar magnetic fields have been determined by observations for solar-like stars with different Rossby numbers (Ro), namely, the magnetic field strength and the magnetic cycle period. The field strength exhibits two regimes: (1) for fast rotation, it is independent of Ro, and (2) for slow rotation, it decays with Ro following a power law. For the magnetic cycle period, two regimes of activity, the active and inactive branches, have also been identified. For both of them, the longer the rotation period, the longer the activity cycle. Using global dynamo simulations of solar-like stars with Rossby numbers between ∼0.4 and ∼2, this paper explores the relevance of rotational shear layers in determining these observational properties. Our results, consistent with nonlinear α^2Ω dynamos, show that the total magnetic field strength is independent of the rotation period. Yet at surface levels, the origin of the magnetic field is determined by Ro. While for Ro≲1, it is generated in the convection zone, for Ro≳ 1, strong toroidal fields are generated at the tachocline and rapidly emerge toward the surface. In agreement with the observations, the magnetic cycle period increases with the rotational period. However, a bifurcation is observed for Ro∼1, separating a regime where oscillatory dynamos operate mainly in the convection zone from the regime where the tachocline has a predominant role. In the latter, the cycles are believed to result from the periodic energy exchange between the dynamo and the magneto-shear instabilities developing in the tachocline and the radiative interior.
Authors: Guerrero, G.; Zaire, B.; Smolarkiewicz, P. K.; de Gouveia Dal Pino, E. M.; Kosovichev, A. G.; Mansour, N. N.
Projects: SDO-HMI
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Publication Status: Astrophysical Journal, Volume 880, Issue 1, article id. 6, 20 pp. (2019).
Last Modified: 2019-08-15 16:45
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Onset of Photospheric Impacts and Helioseismic Waves in X9.3 Solar Flare of September 6, 2017 |
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Alexander Kosovichev Submitted: 2018-05-25 11:59
The X9.3 flare of September 6, 2017, was the most powerful flare of Solar Cycle 24. It generated strong white-light emission and multiple helioseismic waves (sunquakes). By using data from Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) as well as hard X-ray data from KONUS instrument onboard WIND spacecraft, and Anti-Coincidence System (ACS) onboard the INTERGRAL space observatory, we investigate spatio-temporal dynamics of photospheric emission sources, identify sources of helioseismic waves and compare the flare photospheric dynamics with the hard X-ray (HXR) temporal profiles. The results show that the photospheric flare impacts started to develop in compact regions in close vicinity of the magnetic polarity inversion line (PIL) in the pre-impulsive phase before detection of the HXR emission. The initial photospheric disturbances were localized in the region of strong horizontal magnetic field of the PIL, and, thus, are likely associated with a compact sheared magnetic structure elongated along the PIL. The acoustic egression power maps revealed two primary sources of generation of sunquakes, which were associated with places of the strongest photospheric impacts in the pre-impulsive phase and the early impulsive phase. This can explain the two types of helioseismic waves observed in this flare. Analysis of the high-cadence HMI filtergrams suggests that the flare energy release developed in the form of sequential involvement of compact low-lying magnetic loops that were sheared along the PIL.
Authors: Ivan N. Sharykin, Alexander G. Kosovichev
Projects: GOES X-rays,INTEGRAL-ACS,SDO-HMI,Wind
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Publication Status: submitted to ApJ
Last Modified: 2018-05-26 17:25
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Solar Cycle Variations of Rotation and Asphericity in the Near-Surface Shear Layer |
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Alexander Kosovichev Submitted: 2018-05-25 11:55
The precise shape of the Sun is sensitive to the influence of gravity, differential rotation, local turbulence and magnetic fields. It has been previously shown that the solar shape exhibits asphericity that evolves with the 11-year cycle. Thanks to the capability of the SoHO/MDI and SDO/HMI instruments to observe with an unprecedented accuracy the surface gravity oscillation (f) modes, it is possible to extract information concerning the coefficients of rotational frequency splitting, a1, a3 and a5, that measure the differential rotation, together with the a2, a4 and a6 asphericity coefficients. Analysis of these helioseismology data for almost two solar cycles, from 1996 to 2017, reveals a close correlation of the a1 and a5 coefficients with the solar activity, whilst a3 exhibits a long-term trend and a weak correlation in the current cycle indicating a substantial change of the global rotation, potentially associated with a long-term evolution of the solar cycles. Looking in more details, the asphericity coefficients, a2, a4 and a6 are more strongly associated with the solar cycle when applying a time lag of respectively 0.1, 1.6 and -1.6 years. The magnitude of a6-coefficient varies in phase with the sunspot number (SN), but its amplitude is ahead of the SN variation. The last measurements made in mid 2017 indicate that the magnitude of a6-coefficient has probably reached its minimum; therefore, the next solar minimum can be expected by the end of 2018 or in the beginning of 2019. The so-called seismic radius in the range of f-mode angular degree: l=137-299 exhibits a temporal variability in anti-phase with the solar activity; its relative value decreased by 2.3E-05 in Solar Cycle 23 and 1.7E-05 in Cycle 24. Such results will be useful for better understanding the physical mechanisms which act inside the Sun, and so, better constrain dynamo models for forecasting the solar cycles
Authors: A. G. Kosovichev, J. -P. Rozelot
Projects: SDO-HMI,SoHO-MDI
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Publication Status: to appear in Journal of Atmospheric and Solar-Terrestrial Physics (JASTP)
Last Modified: 2018-05-26 17:25
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Local Helioseismology of Emerging Active Regions: A Case Study |
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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
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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 |
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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
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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 |
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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
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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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Helioseismic Constraints and Paradigm Shift in Solar Dynamo |
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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
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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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Local Helioseismology of Sunspots: Current Status and Perspectives (Invited Review) |
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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
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Publication Status: submitted to Solar Physics
Last Modified: 2011-02-24 08:40
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First Sunquake of Solar Cycle 24 Observed by Solar Dynamics Observatory |
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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
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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 |
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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
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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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Properties of Flares-Generated Seismic Waves on the Sun |
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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
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Publication Status: astro-ph/0601006
Last Modified: 2006-03-29 11:17
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