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The Sunspots  [PDF]
Angel Fierros Palacios
Journal of High Energy Physics, Gravitation and Cosmology (JHEPGC) , 2015, DOI: 10.4236/jhepgc.2015.12007
Abstract: In the present paper, the theoretical frame work of magneto hydrodynamics (MHD) is used to give a solution of the problem about the origin, persistence and disappearance of the Sunspots; as well as their tendency to appear as bipolar magnetic couples. According to the results obtained, a possible explanation about the change of polarity in both solar hemispheres is given. Heuristic but logical arguments about the periodicity of the phenomenon of the observed magnetic polarity and the tendency of couples of Sunspots to appear solely in certain latitudes that can be called tropical regions of the Sun are presented. Finally, an indirect experimental test is proposed to show the possible process that produces the polarity of the Sunspots in a given cycle, as well as the invertion of that polarity in the next solar cycle.
Magnetic Structure of Sunspots
Juan M. Borrero,Kiyoshi Ichimoto
Living Reviews in Solar Physics , 2011,
Abstract: In this review we give an overview about the current state-of-knowledge of the magnetic field in sunspots from an observational point of view. We start by offering a brief description of tools that are most commonly employed to infer the magnetic field in the solar atmosphere with emphasis in the photosphere of sunspots. We then address separately the global and local magnetic structure of sunspots, focusing on the implications of the current observations for the different sunspots models, energy transport mechanisms, extrapolations of the magnetic field towards the Corona, and other issues.
Magnetic Structure of Sunspots  [PDF]
J. M. Borrero,K. Ichimoto
Physics , 2011, DOI: 10.12942/lrsp-2011-4
Abstract: In this review we give an overview about the current state-of-knowledge of the magnetic field in sunspots from an observational point of view. We start by offering a brief description of tools that are most commonly employed to infer the magnetic field in the solar atmosphere with emphasis in the photosphere of sunspots. We then address separately the global and local magnetic structure of sunspots, focusing on the implications of the current observations for the different sunspots models, energy transport mechanisms, extrapolations of the magnetic field towards the Corona and other issues.
Structure and Dynamics of Sunspots  [PDF]
Alexandra Tritschler
Physics , 2009,
Abstract: The physics of Sunspots is a fascinating and demanding field of research in solar astronomy. Interaction of magnetic fields and plasma flows takes place in a tangled magnetic geometry and occurs on spatial scales that pose a continuous challenge for existing instrumentation and for the unambiguous interpretation of spectropolarimetric observations. Thus, the main properties of sunspots are well established but its fine structure is not yet fully understood. In this contribution we summarize the current knowledge of the magnetic and dynamic properties of sunspots at the photospheric level based on selected observations featuring the highest possible spatial and spectral resolution. We concentrate on light bridges, umbral dots, penumbral filaments and the notorious dark cores in penumbral filaments. We report on the morphology of the fine structure elements but mostly focus on observations of their line-of-sight velocities and magnetic field parameters. We briefly comment on results from recent radiative MHD simulations and more schematic model ideas that attempt to rationalize observations of the penumbra.
On the surface structure of sunspots  [PDF]
Morten Franz
Physics , 2012, DOI: 10.1002/asna.201211789
Abstract: A precise knowledge of the surface structure of sunspots is essential to construct adequate input models for helioseismic inversion tools. We summarize our recent findings about the velocity and magnetic field in and around sunspots using HINODE observation. To this end we quantize the horizontal and vertical component of the penumbral velocity field at different levels of precision and study the moat flow around sunspot. Furthermore, we find that a significant amount of the penumbral magnetic fields return below the surface within the penumbra. Finally, we explain why the related opposite polarity signals remain hidden in magnetograms constructed from measurements with limited spectral resolution.
Modeling the Subsurface Structure of Sunspots  [PDF]
H. Moradi,C. Baldner,A. C. Birch,D. Braun,R. Cameron,T. L. Duvall Jr.,L. Gizon,D. Haber,S. Hanasoge,B. W. Hindman,J. Jackiewicz,E. Khomenko,R. Komm,P. Rajaguru,M. Rempel,M. Roth,R. Schlichenmaier,H. Schunker,H. Spruit,K. Strassmeier,M. J. Thompson,S. Zharkov
Physics , 2009, DOI: 10.1007/s11207-010-9630-4
Abstract: While sunspots are easily observed at the solar surface, determining their subsurface structure is not trivial. There are two main hypotheses for the subsurface structure of sunspots: the monolithic model and the cluster model. Local helioseismology is the only means by which we can investigate subphotospheric structure. However, as current linear inversion techniques do not yet allow helioseismology to probe the internal structure with sufficient confidence to distinguish between the monolith and cluster models, the development of physically realistic sunspot models are a priority for helioseismologists. This is because they are not only important indicators of the variety of physical effects that may influence helioseismic inferences in active regions, but they also enable detailed assessments of the validity of helioseismic interpretations through numerical forward modeling. In this paper, we provide a critical review of the existing sunspot models and an overview of numerical methods employed to model wave propagation through model sunspots. We then carry out an helioseismic analysis of the sunspot in Active Region 9787 and address the serious inconsistencies uncovered by \citeauthor{gizonetal2009}~(\citeyear{gizonetal2009,gizonetal2009a}). We find that this sunspot is most probably associated with a shallow, positive wave-speed perturbation (unlike the traditional two-layer model) and that travel-time measurements are consistent with a horizontal outflow in the surrounding moat.
Are the sunspots really vanishing?  [cached]
Clette Frédéric,Lefèvre Laure
Journal of Space Weather and Space Climate , 2012, DOI: 10.1051/swsc/2012007
Abstract: Context: The elapsed solar cycle (23) ended with an exceptionally long period of low activity and with unprecedented low levels for various series of solar irradiance and particle flux measurements. This unpredicted evolution of solar activity raised multiple questions about a future decline of the solar cycles and launched a quest for precursor signs of this possible deep solar transition over the last decade. Aim: We present here a review and overall interpretation of most current diagnostics of solar cycle 23, including the recent disagreements that appeared among solar reference indices and standard solar-based geo-indices, the indication of a changed pattern of internal torsional waves (helioseismology) or the announced fading and magnetic weakening of sunspots. Methods: Based on a statistical analysis of detailed sunspot properties over the last 24 years, we complete the picture with new evidence of a strong global deficit of the smallest sunspots starting around 2000, in order to answer the question: are all sunspots about to disappear? Results: This global scale-dependent change in sunspot properties is confirmed to be real and not due to uncontrolled biases in some of the indices. It can also explain the recent discrepancies between solar indices by their different sensitivities to small and weak magnetic elements (small spots). The International Sunspot Index Ri, based on unweighted sunspot counts, proved to be particularly sensitive to this particular small-scale solar evolution. Conclusions: Our results and interpretation show the necessity to look backwards in time, more than 80 years ago. Indeed, the Sun seems to be actually returning to a past and hardly explored activity regime ending before the 1955–1995 Grand Maximum, which probably biased our current space-age view of solar activity.
Penumbral structure and outflows in simulated sunspots  [PDF]
M. Rempel,M. Schüssler,R. H. Cameron,M. Kn?lker
Physics , 2009, DOI: 10.1126/science.1173798
Abstract: Sunspots are concentrations of magnetic field on the visible solar surface that strongly affect the convective energy transport in their interior and surroundings. The filamentary outer regions (penumbrae) of sunspots show systematic radial outward flows along channels of nearly horizontal magnetic field. These flows were discovered 100 years ago and are present in all fully developed sunspots. Using a comprehensive numerical simulation of a sunspot pair, we show that penumbral structures with such outflows form when the average magnetic field inclination to the vertical exceeds about 45 degrees. The systematic outflows are a component of the convective flows that provide the upward energy transport and result from anisotropy introduced by the presence of the inclined magnetic field.
Correlations between sunspots and their moat flows  [PDF]
Johannes L?hner-B?ttcher,Rolf Schlichenmaier
Physics , 2013, DOI: 10.1051/0004-6361/201220543
Abstract: Context. The presence of the moat flow around sunspots is intimately linked to the mere existence of sunspots. Aims. We characterize the moat flow (MF) and Evershed flow (EF) in sunspots to enhance our knowledge of sunspot structures and photospheric flow properties. Methods. We calibrated HMI synoptic Doppler maps and used them to analyze 3h time averages of 31 circular, stable, and fully developed sunspots at heliocentric angles of some 50$^{\circ}$. Assuming axially symmetrical flow fields, we infer the azimuthally averaged horizontal velocity component of the MF and EF from 51 velocity maps. We studied the MF properties (velocity and extension) and elaborate on how these components depend on sunspot parameters (sunspot size and EF velocity). To explore the weekly and monthly evolution of MFs, we compare spots rotating from the eastern to western limbs and spots that reappear on the eastern limb. Results. Our calibration procedure of HMI Doppler maps yields reliable and consistent results. In 3h averages, we find the MF decreases on average from some 1000 $\pm$ 200m/s just outside the spot boundary to 500m/s after an additional 4 Mm. The average MF extension lies at 9.2 $\pm$ 5 Mm, where the velocity drops below some 180m/s. Neither the MF velocity nor its extension depend significantly on the sunspot size or EF velocity. But, the EF velocity does show a tendency to be enhanced with sunspot size. On a time scale of a week and a month, we find decreasing MF extensions and a tendency for the MF velocity to increase for strongly decaying sunspots, whereas the changing EF velocity has no impact on the MF. Conclusions. On 3h averages, the EF velocity scales with the size of sunspots, while the MF properties show no significant correlation with the EF or with the sunspot size. This we interpret as a hint that the physical origins of EF and MF are distinct.
Statistics of Flares Sweeping across Sunspots  [PDF]
Leping Li,Jun Zhang
Physics , 2009, DOI: 10.1088/0004-637X/706/1/L17
Abstract: Flare ribbons are always dynamic, and sometimes sweep across sunspots. Examining 588 (513 M-class and 75 X-class) flare events observed by Transition Region and Coronal Explorer (TRACE) satellite and Hinode Solar Optical Telescope (SOT) from 1998 May to 2009 May, we choose the event displaying that one of the flare ribbons completely sweeps across the umbra of a main sunspot of the corresponding active region, and finally obtain 20 (7 X-class and 13 M-class) events as our sample. In each event, we define the main sunspot completely swept across by the flare ribbon as A-sunspot, and its nearby opposite polarity sunspots, B-sunspot. Observations show that the A-sunspot is a following polarity sunspot in 18 events, and displays flux emergence in 13 cases. All the B-sunspots are relatively simple, exhibiting either one main sunspot or one main sunspot and several small neighboring sunspots (pores). In two days prior to the flare occurrence, the A-sunspot rotates in all the cases, while the B-sunspot, in 19 events. The total rotating angle of the A-sunspot and B-sunspot is 193 degrees on average, and the rotating directions, are the same in 12 events. In all cases, the A-sunspot and B-sunspot manifest shear motions with an average shearing angle of 28.5 degrees, and in 14 cases, the shearing direction is opposite to the rotating direction of the A-sunspot. We suggest that the emergence, the rotation and the shear motions of the A-sunspot and B-sunspot result in the phenomenon that flare ribbons sweep across sunspots completely.
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