Techniques for detecting giant cells using spatially resolved solar velocity data

view Abstract Citations (7) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Large-Scale Flows and Solar Luminosity Variations Arendt, S. Abstract The influence of a variation of the large-scale flows present in the solar convection zone on solar luminosity is examined. The convection zone is modeled as a plane-parallel fluid layer through which heat is transported by small-scale convection as described by the mixing-length theory, and the large-scale flow is treated as a small perturbation superposed on the layer. The excess energy transported by the flow is calculated and is found to depend critically on the mixing length. The excess luminosity is found to be carried as small-scale convective heat flow rather than bulk enthalpy transport throughout all but the top few percent of the convection zone. The results are applied to the specific examples of supergranules and giant cells. Publication: The Astrophysical Journal Pub Date: April 1992 DOI: 10.1086/171217 Bibcode: 1992ApJ...389..421A Keywords: Convective Flow; Convective Heat Transfer; Solar Activity Effects; Solar Constant; Solar Interior; Stellar Luminosity; Boundary Conditions; Enthalpy; Mixing Length Flow Theory; Nimbus 7 Satellite; Solar Cycles; Solar Maximum Mission; Solar Temperature; Solar Physics; CONVECTION; SUN: ACTIVITY; SUN: GENERAL; SUN: INTERIOR full text sources ADS |

view Abstract Citations (66) References (8) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evidence for large-scale velocity features on the sun. Howard, R. Abstract Giant velocity features on the solar surface are seen in line-of-sight velocity data from the Mount Wilson magnetograph. Velocity amplitudes around 40 m/s are seen in features with dimensions about 15 deg in latitude and 30-60 deg in longitude. These features are associated with solar activity. The largest such feature accompanied the largest complex of activity in this interval, lived for about 16 rotations, and predated the first magnetic fields in the area by about two rotations. A general pattern of upward motion at the equator is seen, and motions away from the earth at higher latitudes could represent a meridional flow toward the poles of the order of 20 m/s. Publication: The Astrophysical Journal Pub Date: February 1979 DOI: 10.1086/182900 Bibcode: 1979ApJ...228L..45H Keywords: Solar Activity; Solar Atmosphere; Solar Rotation; Solar Velocity; Velocity Distribution; Earth Motion; Equatorial Orbits; Gas Flow; Meridional Flow; Meteorological Charts; Solar Granulation; Synoptic Measurement; Velocity Measurement; Solar Physics; Solar Activity; Solar Atmosphere:Motions full text sources ADS |

Strong solar flares occur in δ-spots characterized by the opposite-polarity magnetic fluxes in a single penumbra. Sunspot formation via flux emergence from the convection zone to the photosphere can be strongly affected by convective turbulent flows. It has not yet been shown how crucial convective flows are for the formation of δ-spots. The aim of this study is to reveal the impact of convective flows in the convection zone on the formation and evolution of sunspot magnetic fields. We simulated the emergence and transport of magnetic flux tubes in the convection zone using radiative magnetohydrodynamics code r2d2. We carried out 93 simulations by allocating the twisted flux tubes to different positions in the convection zone. As a result, both δ-type and β-type magnetic distributions were reproduced only by the differences in the convective flows surrounding the flux tubes. The δ-spots were formed by the collision of positive and negative magnetic fluxes on the photosphere. The unipolar and bipolar rotations of the δ-spots were driven by magnetic twist and writhe, transporting magnetic helicity from the convection zone to the corona. We detected a strong correlation between the distribution of the non-potential magnetic field in the photosphere and the position of the downflow plume in the convection zone. The correlation could be detected 20–30 h before the flux emergence. The results suggest that high free energy regions in the photosphere can be predicted even before the magnetic flux appears in the photosphere by detecting the downflow profile in the convection zone.

The objective of this review article is to critically analyze turbulence and its role in the solar atmosphere and solar wind, as well as to provide a tutorial overview of topics worth clarification. Although turbulence is a ubiquitous phenomenon in the sun and its heliosphere, many open questions exist concerning the physical mechanisms of turbulence generation in solar environment. Also, the spatial and temporal evolution of the turbulence in the solar atmosphere and solar wind are still poorly understood. We limit the scope of this paper (leaving out the solar interior and convection zone) to the magnetized plasma that reaches from the photosphere and chromosphere upwards to the corona and inner heliosphere, and place particular emphasis on the magnetic field structures and fluctuations and their role in the dynamics and radiation of the coronal plasma. To attract the attention of scientists from both the fluid-dynamics and space-science communities we give in the first two sections a phenomenological overview of turbulence-related processes, in the context of solar and heliospheric physics and with emphasis on the photosphere-corona connection and the coupling between the solar corona and solar wind. We also discuss the basic tools and standard concepts for the empirical analysis and theoretical description of turbulence. The last two sections of this paper give a concise review of selected aspects of oscillations and waves in the solar atmosphere and related fluctuations in the solar wind. We conclude with some recommendations and suggest topics for future research.

Magnetohydrodynamic process in solar activity

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Abundances of Lithium, Beryllium, and Lead in the Solar Atmosphere. Mutschlecner, J. P. Abstract A redetermination has been made of the abundances of the elements lithium, beryllium, and lead in the solar atmosphere. The observational material consisted of numerous spectral scans obtained with the vacuum spectrograph of the McMath-Hulbert Observatory. Exact curves of growth were computed for each line using digital-computer programs. Features of the theoretical calculations include (1) the use of a newly derived semiempirical solar model atmosphere, (2) the inclusion of an empirically determined component of the continuous absorption coefficient in those regions where it was required, (3) an analysis of the influence of various uncertainties on the curve of growth and hence on the derived abundances. The resulting abundances on the silicon scale (i.e., number of atoms per million Si atoms) are: N(Li)=1.1, N(Be)=6.9, N(Pb)=1.4. An analysis of the source of errors shows that basic uncertainties in the solar model probably produce the largest abundance uncertainties. The upper limit to the combined error due to all sources is about 100% for lithium and lead and about 30% for beryllium. The beryllium and lead values are in good accord with meteoritic determinations while the solar lithium value is as little as 2% of meteoritic abundances. It is impossible at present to derive any conclusions as the depth of the solar convective zone from the lithium and beryllium abundances because of poor knowledge of the past history of the convective zone. The lead abundance is in reasonable agreement with a semi-theoretical prediction of the abundance by Clayton et al. EAnn. Phys. (N.Y.) 12, 331, 1961J based upon a detailed analysis of the s-process for nuclear synthesis. This work was done under auspices of University of Michigan Astronomy Department, Ann Arbor, Michigan, in partial fulfillment of Ph.D. ~equirements. Publication: The Astronomical Journal Pub Date: June 1963 DOI: 10.1086/109148 Bibcode: 1963AJ.....68S.287M full text sources ADS |

Meridional circulation regulates the Sun’s interior dynamics and magnetism. While it is well accepted that meridional flows are poleward at the Sun’s surface, helioseismic observations have yet to provide a definitive answer for the depth at which those flows return to the equator, or the number of circulation cells in depth. Here, we explore the observability of multiple circulation cells stacked in radius. Specifically, we examine the seismic signature of several meridional flow profiles by convolving time–distance averaging kernels with mean flows obtained from a suite of 3D hydrodynamic simulations. At mid and high latitudes, we find that weak flow structures in the deep convection zone can be obscured by signals from the much stronger surface flows. This contamination of 1–2 m s−1 is caused by extended side lobes in the averaging kernels, which produce a spurious equatorward signal with flow speeds that are 1 order of magnitude stronger than the original flow speeds in the simulations. At low latitudes, the flows in the deep layers of the simulations are stronger (>2 m s−1) and multiple cells across the convection zone can produce a sufficiently strong signal to survive the convolution process. Now that meridional flows can be measured over two decades of data, the uncertainties arising from convective noise have fallen to a level where they are comparable in magnitude to the systematic biases caused by nonlocal features in the averaging kernels. Hence, these systematic errors are beginning to influence current helioseismic deductions and need broader consideration.

If the solar dynamo operates at the bottom of the convection zone, then the magnetic flux created there has to rise to the surface. When the convection zone is regarded as passive, the rising flux is deflected by the Coriolis force to emerge at rather high latitudes, poleward of typical sunspot zones (Choudhuri and Gilman, 1987; Choudhuri, 1989). Choudhuri and D'Silva (1990) included the effects of convective turbulence on the rising flux through (a) giant cell drag and (b) momentum exchange by small-scale turbulence. The momentum exchange mechanism could enable flux tubes of radii not more than a few hundred km to emerge radially at low latitudes, but the giant cell drag mechanism required unrealistically small flux tube radii (a few meters for a reasonable giant cell upflow) to counteract the Coriolis force. We now include the additional effect of Kelvin-Helmholtz instability in a symmetrical flux ring caused by the azimuthal flow induced during its rise. The azimuthal flow crosses the threshold for the instability only if there is a giant cell upflow to drag the flux tubes appreciably. In the absence of such a drag, as in the case of a passive convection zone or in the case of momentum exchange by small-scale turbulence, the azimuthal velocity never becomes large enough to cause the instability, leaving the results of the previous calculations unaltered. The giant cell drag, aided by Kelvin-Helmholtz instability, however, becomes now a viable mechanism for curbing the Coriolis force - $10^4\hspace{2mm}G$ flux tubes with radii of a few hundred km being dragged radially by upflows of $70 ms^{-1}$.

Context. Solar Rossby waves (r modes) have recently been discovered in the near-surface horizontal flow field using the techniques of granulation-tracking and ring-diagram analysis applied to six years of SDO/HMI data. Aims. Here we apply time-distance helioseismology to the combined SOHO/MDI and SDO/HMI data sets, which cover 21 years of observations from May 1996 to April 2017. The goal of this study is to provide an independent confirmation over two solar cycles and in deeper layers of the Sun. Methods. We have measured south-north helioseismic travel times along the equator, which are sensitive to subsurface north-south flows. To reduce noise, the travel times were averaged over travel distances from 6° to 30°; the mean distance corresponds to a p-mode lower turning point of 0.91 R⊙. The 21-year time series of travel-time measurements was split into three seven-year subsets and transformed to obtain power spectra in a corotating frame. Results. The power spectra all show peaks near the frequencies of the classical sectoral Rossby waves for azimuthal wavenumbers in the range 3 ≤ m ≤ 15. The mode frequencies and linewidths of the modes with m ≤ 9 are consistent with a previous study whereas modes with m ≥ 10 are shifted toward less negative frequencies by 10–20 nHz. While most of these modes have e-folding lifetimes on the order of a few months, the longest lived mode, m = 3, has an e-folding lifetime of more than one year. For each mode, the rms velocity at the equator is in the range of 1–3 m s−1, with the largest values for m ∼ 10. No evidence for the m = 2 sectoral mode is found in the power spectrum, implying that the rms velocity of this mode is below ∼0.5 m s−1. Conclusions. This work confirms the existence of equatorial global Rossby waves in the solar interior over the past two solar cycles and shows that time-distance helioseismology is a promising technique to study them deep in the convection zone.

Context. Vortices have been observed at various heights within the solar atmosphere and are suggested to play a significant role in heating the solar upper atmosphere. Multiple automated vortex detection methods have been developed and applied to identify vortices. Aims. We aim to improve the Γ-functions method for vortex identification by optimizing the value of Γ1min and the approach to calculate Γ1 and Γ2, used to determine the center and edge of the vortex. This optimization enhances detection accuracy and enables statistical studies to improve our understanding of vortex generation and evolution in the solar atmosphere. Methods. We applied the automated swirl detection algorithm (ASDA, a representative of the Γ-functions method) with different parameters to various synthetic datasets, each containing 1000 Lamb-Oseen vortices, to identify the optimal Γ1min and kernel size when calculating Γ1 and Γ2. We also compared another detection method using simulation and observational data to validate the results obtained from the synthetic datasets. Results. We achieve the best performance with the Optimized ASDA, which combines different kernel sizes (5, 7, 9, and 11) to calculate Γ1 and Γ2 with Γ1min fixed at 0.63 for vortex center detection. We find that more vortices can be detected by the optimized ASDA with improved accuracy in location, radius, and rotation speed. These results are further confirmed by comparing vortices detected by the Optimized ASDA and the SWirl Identification by Rotation-centers Localization (SWIRL) method on CO5BOLD numerical simulation data and Swedish 1-m Solar Telescope observational data.

The free parameters involved in the turbulent convection models are determined in the solar interior with the aid of helioseismology. We calculated solar p-mode oscillation l = 150 for a series of solar models described by Li & Yang, and compared the calculated frequencies with the helioseismic observations. It is found that, with appropriate choices of the turbulent parameters, solar model with both local and non-local turbulent convection models can give the calculated p-mode frequencies that are better consistent with the observations than the standard solar model does. The turbulent dissipation plays a major role in the frequency corrections, while the turbulent diffusion makes further improvements in the frequencies. Decreasing the values of parameters C-t and C-e or increasing the values of C-t1 and C-e1 helps reduce the calculated frequencies, indicating that the turbulent dissipation is weak and the turbulent diffusion is strong in the convection zone of the Sun. The parameters C-k and C-s have no obvious effect on the p-mode frequencies. The temperature is found to have a bump near the surface of the convection zone and a mild depression over the inner convection zone. It is just this temperature depression that makes the sound speed a little less, and the p modes spending a little longer time to travel in the convection zone. Combined with better values of the free parameters, our solar models with the TCMs can reduce the frequency differences between the model calculations and observation as much as 30 per cent for the modes of middle and high l. This result can give a clue to solar modelling, i.e., including turbulence in solar models is helpful to reproduce the observed solar p-mode frequencies.

Helioseismology has taught us a great deal about the stratification and kinematics of the solar interior, sufficient for us to embark upon dynamical studies more detailed than have been possible before. The most sophisticated studies to date have been the very impressive numerical simulations of the convection zone, from which, especially in recent years, a great deal has been learnt. Those simulations, and the seismological evidence with which they are being confronted, are reviewed elsewhere in this volume. Our understanding of the global dynamics of the radiative interior of the Sun is in a much more primitive state. Nevertheless, some progress has been made, and seismological inference has provided us with evidence of more to come. Some of that I summarize here, mentioning in passing hints that are pointing the way to the future.

Preliminary solar envelope models have been computed using the single-mode anelastic equations as a description of turbulent convection. This approach provides estimates for the variation with depth of the largest convective cellular flows, akin to giant cells, with horizontal sizes comparable to the total depth of the convection zone. These modal nonlinear treatments are capable of describing compressible motions occurring over many density scale heights. Single-mode anelastic solutions have been constructed for a solar envelope whose mean stratification is nearly adiabatic over most of its vertical extent because of the enthalpy (or convective) flux explicitly carried by the big cell; a sub-grid scale representation of turbulent heat transport is incorporated into the treatment near the surface. The single-mode equations admit two solutions for the same horizontal wavelength, and these are distinguished by the sense of the vertical velocity at the center of the three-dimensional cell. It is striking that the upward directed flows experience large pressure effects when they penetrate into regions where the vertical scale height has become small compared to their horizontal scale. The fluctuating pressure can modify the density fluctuations so that the sense of the buoyancy force is changed, with buoyancy braking actually achieved near the top of the convection zone. The pressure and buoyancy work in the shallow but unstable H+ and He+ ionization regions can serve to decelerate the vertical motions and deflect them laterally, leading to strong horizontal shearing motions. It appears that such dynamical processes may explain why the amplitudes of flows related to the largest scales of convection are so feeble in the solar atmosphere.

We present a new model of large-scale multilayer convection in solar type stars. This model allows us to understand such self-similar structures observed at solar surface as granulation, supergranulation and giant cells. We study the slow-rotated hydrogen star without magnetic field with the spherically-symmetric convective zone. The photons flux comes to the convective zone from the central thermonuclear zone of the star. The interaction of these photons with the fully ionized hydrogen plasma with T> 105K is carried out by the Thompson scattering of photon flux on protons and electrons. Under these conditions plasma is optically thick relative to the Thompson scattering. We find the stationary solution of the convective zone structure. This solution describes the convective layers responsible to the formation of the structures on the stars surface.KeywordsSolitary WaveConvective ZoneConvective CellPolytropic IndexConvective LayerThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Using a developing delta-class solar active region (NOAA 12673) as a case study, we analyzed the distribution of solar vector magnetic fields and derived the corresponding magnetic energy and helicity densities from vector magnetograms to characterize the relationship between free magnetic energy and potential energy. Our findings indicate that the accumulation of magnetic free energy density and the dynamic evolution of the current helicity primarily occur near the magnetic neutral line associated with the magnetic flux emergence. The free magnetic energy density is found to originate from two sources: the nonpotential energy density ( B n 2 / 8 π ) and the interactive magnetic energy density ( B n · B p /4 π ), where B n represents the nonpotential component and B p the potential component of the magnetic field. Notably, the interactive term B n · B p also provides a significant contribution to the free energy in the solar atmosphere. Furthermore, we examined correlations between physical parameters, such as nonpotential magnetic energy and current helicity, and flare activity during the energy accumulation phase over the long-term evolution to avoid limitations in studies of solar eruptive activity caused by the lack of magnetic field observations in the upper solar atmosphere. It is found that the energy derived from the photospheric transverse magnetic field persists in accumulating even as the apparent magnetic field gradually weakens, until the onset of the X-class flares. Our analysis established reference thresholds for predicting M-class and X-class flares in newly emerging magnetically active regions. Understanding how magnetic energy accumulates to trigger solar flares and coronal mass ejections is crucial for advancing our knowledge of solar activity and space weather forecasting.