Large-scale flows and solar luminosity variations

Large-scale convective flows called giant cells were once thought to transport the Sun’s luminosity in the solar convection zone, but recent observations have called their existence into question. In place of large-scale flows, some authors have suggested the solar luminosity may instead be transported by small droplets of rapidly falling, low-entropy fluid. This “entropy rain” could propagate as dense vortex rings, analogous to rising buoyant thermals in the Earth’s atmosphere. In this work, we develop an analytical theory describing the evolution of dense, negatively buoyant thermals. We verify the theory with 2D cylindrical and 3D Cartesian simulations of laminar, axisymmetric thermals in highly stratified atmospheres. Our results show that dense thermals fall in two categories: a stalling regime in which the droplets slow down and expand, and a falling regime in which the droplets accelerate and shrink as they propagate downwards. We estimate that solar downflows are in the falling regime and maintain their entropy perturbation against diffusion until they reach the base of the convection zone. This suggests that entropy rain may be an effective nonlocal mechanism for transporting the solar luminosity.

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.

Large-scale plasma flows in the Sun's convection zone likely play a major role in solar dynamics on decadal timescales. In particular, quantifying meridional motions is a critical ingredient for understanding the solar cycle and the transport of magnetic flux. Because the signal of such features can be quite small in deep solar layers and be buried in systematics or noise, the true meridional velocity profile has remained elusive. We perform time-distance helioseismology measurements on several years worth of GONG Doppler data. A spherical harmonic decomposition technique is applied to a subset of acoustic modes to measure travel-time differences to try to obtain signatures of meridional flows throughout the solar convection zone. Center-to-limb systematics are taken into account in an intuitive, yet ad hoc manner. Travel-time differences near the surface that are consistent with a poleward flow in each hemisphere and are similar to previous work are measured. Additionally, measurements in deep layers near the base of the convection zone suggest a possible equatorward flow, as well as partial evidence of a sign change in the travel-time differences at mid-convection zone depths. This analysis on an independent data set using different measurement techniques strengthens recent conclusions that the convection zone may have multiple "cells" of meridional flow. The results may challenge the common understanding of one large conveyor belt operating in the solar convection zone. Further work with helioseismic inversions and a careful study of systematic effects are needed before firm conclusions of these large-scale flow structures can be made.

The most interesting result in solar luminosity studies in the past decade has been the detection of significant variations in the total irradiance by precision radiometers on the NIMBUS-7 and SMM spacecraft. A substantial fraction of the observed variation can be attributed to sunspot blocking. Thermal storage models indicate that the blocked flux can be stored in a slight increase of the thermal and potential energy of the convective zone. The thermal storage time is likely to far exceed one solar activity cycle, implying an 11-year modulation of the solar constant at a level of about 0.1%. Direct observations of the 11-year or longer variations are more difficult but there is some evidence for secular trends below about 0.4% amplitude over the 14-year period of modern sampling. Ongoing stellar photometric programs suggest that luminosity changes exceeding 1% may have been detected in young, chromospherically active stars.H. S. Hudson (University of California, San Diego) reviewed observations of short-term solar irradiance variations from spacecraft, commenting principally on the precision measurements of the solar constant (S) made by the Active Cavity Radiometer (ACRIM) on the Solar Maximum Mission (SMM) spacecraft (Willson et al., 1981).

One possible explanation for the failure to detect neutrinos emitted from the solar interior is that the core of the sun may be in a temporarily expanded state. If so, the solar luminosity is currently much less than its normal value, which may account for the fact that the earth is now in the middle of a major glacial period. The astrophysical background of this situation is described, and the terrestrial consequences are discussed.

The ACRIM I experiment (Active Cavity Radiometer Irradiance Monitor) on the solar maximum Mission (SMM) satellite has provided a nearly continuous record of solar total irradiance variations since early 19801. It has detected variations on time scales ranging from minutes to SMM's lifetime. The long-term variations have revealed a downward trend during the declining phase of solar cycle 21 (ref. 2) of the sunspot cycle, a flat period between mid-1985 and mid-1987, and an upturn in late 1987 which suggests a direct correlation of luminosity and solar active region population. If the upturn continues into the activity maximum of solar cycle 22, a relation between solar activity and luminosity of possible climatological significance could have been discovered. The sense of the correlation agrees with what has been predicted from the coincidence of the 'little ice age' climate anomaly in the sixteenth and seventeenth centuries and the Maunder Minimum of solar activity3. The best-fit relationship for the variation of total irradiance S, with sunspot number Rz, and 10–cm flux F10, are S=1,366.82+7.7l×10−3 Rz and S=1,366.27+8.98×10−3 F10 (W m−2). These could be used to approximate total irradiance variations over the periods for which these indices have been compiled.

Modulation of mantle plumes and heat flow at the core mantle boundary by plate-scale flow: results from laboratory experiments

view Abstract Citations (11) References (5) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Techniques for detecting giant cells using spatially resolved solar velocity data Brown, T. M. ; Gilman, P. A. Abstract Whether giant cells exist in the convection zone of the sun, and what their properties might be, are matters of great importance for the understanding of the dynamics of the solar interior. So far, such cells have escaped detection, probably because of the small amplitude of their associated velocity fields and the large amplitudes of shorter-lived flows. Techniques for improving the detectability of giant cells are presented. These methods are based on the spatial extent and symmetry properties of giant cells as seen in self-consistent dynamical models of the solar convection zone. Simulations suggest that these techniques allow detection of giant cells with photospheric rms velocities of 2 m/s (0.4 m/s per longitudinal wavenumber), given observations spanning about one year. Publication: The Astrophysical Journal Pub Date: November 1984 DOI: 10.1086/162656 Bibcode: 1984ApJ...286..804B Keywords: Convective Flow; Solar Atmosphere; Solar Granulation; Solar Velocity; Power Spectra; Solar Activity; Solar Interior; Solar Physics full text sources ADS |

We use the 3D stellar structure code DJEHUTY to model the ingestion of protons into the intershell convection zone of a 1 solar mass asymptotic giant branch star of metallicity Z=10^-4. We have run two simulations: a low resolution one of around 300,000 zones, and a high resolution one consisting of 2,000,000 zones. Both simulations have been evolved for about 4 hours of stellar time. We observe the existence of fast, downward flowing plumes that are able to transport hydrogen into close proximity to the helium burning shell before burning takes place. The intershell in the 3D model is richer in protons than the 1D model by several orders of magnitude and so we obtain substantially higher hydrogen-burning luminosities -- over 10^8 solar luminosities in the high resolution simulation -- than are found in the 1D model. Convective velocities in these simulations are over 10 times greater than the predictions of mixing length theory, though the 3D simulations have greater energy generation due to the enhanced hydrogen burning. We find no evidence of the convective zone splitting into two, though this could be as a result of insufficient spatial resolution or because the models have not been evolved for long enough. We suggest that the 1D mixing length theory and particularly the use of a diffusion algorithm for mixing do not give an accurate picture of these events. An advective mixing scheme may give a better representation of the transport processes seen in the 3D models.

The solar meridional flow is an axisymmetric flow in solar meridional planes, extending through the convection zone. Here we study its solar-cycle variations in the convection zone using SOHO/MDI helioseismic data from 1996 to 2010, including two solar minima and one maximum. The travel-time difference between northward and southward acoustic waves is related to the meridional flow along the wave path. Applying the ray approximation and the SOLA inversion method to the travel-time difference measured in a previous study, we obtain the meridional flow distributions in 0.67 ≤ r ≤ 0.96R ⊙ at the minimum and maximum. At the minimum, the flow has a three-layer structure: poleward in the upper convection zone, equatorward in the middle convection zone, and poleward again in the lower convection zone. The flow speed is close to zero within the error bar near the base of the convection zone. The flow distribution changes significantly from the minimum to the maximum. The change above 0.9R ⊙ shows two phenomena: first, the poleward flow speed is reduced at the maximum; second, an additional convergent flow centered at the active latitudes is generated at the maximum. These two phenomena are consistent with the surface meridional flow reported in previous studies. The change in flow extends all the way down to the base of the convection zone, and the pattern of the change below 0.9R ⊙ is more complicated. However, it is clear that the active latitudes play a role in the flow change: the changes in flow speed below and above the active latitudes have opposite signs. This suggests that magnetic fields could be responsible for the flow change.

Recent helioseismic observations (Duvall et al.) have demonstrated how new data analysis techniques can determine local changes in the acoustic properties beneath the photosphere. The recent results provide compelling evidence of a latitudinal sound speed variation. Using results from numerical simulations, we show here how this acoustic variation has the correct form and amplitude needed to account for the previously observed solar photometric changes. In this picture, both the acoustic and irradiance changes may be caused by magnetically induced entropy fluctuations near the base of the solar convection zone.

Interactions of scales of convection in the Earth's mantle

We describe some of our recent work on the dynamics of flux tubes in the solar convection zone. We focus on two topics, the first being the orientation (“tilt”) of active regions, and how comparisons between observations of tilt and tilts computed from numerical simulations of flux tubes can be used to infer properties of magnetic fields deep in the solar interior. The second topic is an investigation of the kink instability of twisted flux tubes in the solar interior, and its possible relationship with flare-productive δ spot regions.

Preface. I: Theories of Solar Convection, Rotation, and Activity. Towards Understanding Solar Convection and Activity D. Gough. Fluid Dynamics and MHD of the Solar Convection Zone and Tachocline: Current Understanding and Unsolved Problems P.A. Gilman. Can We Get the Bottom B? A. Ruzmaikin. The Coupling of Solar Convection and Rotation M.S. Miesch. Realistic Solar Convection Simulations R.F. Stein, A. Nordlund. Solar Magnetoconvection N.E. Hurlburt, et al. The Solar Dynamo and Emerging Flux G.H. Fisher, et al. On the Twist of Emerging Flux Loops in the Solar Convection Zone Y. Fan, D. Gong. II: Helioseismic Tomography. Time-Distance Inversion Methods and Results A.G. Kosovichev, et al. Time-Distance Helioseismology with f Modes as a Method for Measurement of Near-Surface Flows T.S. Duvall Jr., L. Gizon. Travel Time Sensitivity Kernels A.C. Birch, A.G. Kosovichev. Calculation of the Sun's Acoustic Impulse Response by Multi-Dimensional Spectral Factorization J.E. Rickett, J.F. Claerbout. Ray Travel Time and Distance for the Planar Polytrope G.H. Price. Sensitivity Kernels for Time-Distance Inversion J.M. Jensen, et al. III: Acoustic Imaging and Holography. Acoustic Imaging of Solar Active Regions D.-Y. Chou. Basic Principles of Solar Acoustic Holography C. Lindsey, D.C. Braun. Helioseismic Holography of Active-Region Subphotospheres D.C. Braun, C. Lindsey. Phase-Sensitive Holography of Solar Activity D.C. Braun, C. Lindsey. Stochastic Seismic Emission from Acoustic Glories and the Quiet Sun A.-C. Donea, et al. IV: Ring-Diagram Analysis. Solar Shear Flow Deduced from Helioseismic Dense-Pack Samplings of RingDiagrams D.A. Haber, et al. Near-Surface Flow Fields Deduced Using Correlation Tracking and Time-Distance Analyses M. de Rosa, et al. Local Fractional Frequency Shifts Used as Tracers of Magnetic Activity B. Hindman, et al. V: Magnetic Fields and Oscillations. Sunspot Oscillations: A Review T.J. Bogdan. Modelling p-Mode Interaction with a Spreading Sunspot Field P.S. Cally. Measuring Magnetic Oscillations in the Solar Photosphere: Coordinated Observations with MDI, ASP and MWO A.A. Norton, R.K. Ulrich. Interaction between Network and Intranetwork Magnetic Fields J. Zhang, et al. VI: Solar-Cycle Variations of the Internal Structure and Rotation. Variations in Solar Sub-Surface Rotation from Gong. Data 1995-1998 R. Howe, et al. Time Variability of Rotation in Solar Convection Zone from SOI-MDI J. Toomre, et al. Possible Solar Cycle Variations in the Convection Zone S. Basu, H.M. Antia. Solar Cycle Variation in Solar f-Mode Frequencies and Radius H.M. Antia, et al. Solar Cycle Variations of Large-Scale Flows in the Sun S. Basu, H.M. Antia. Does the Tachocline Show Solar Cycle Related Changes? S. Basu, J. Schou. Empirical Estimate of p-Mode Frequency Shift for Solar Cycle 23 K. Jain, et al.

In this paper, we investigate the problem of supersonic convection caused by the application of the usual Mixing Length Theory (MLT), and give a modification to the original expression of the MLT. In the case of the usual MLT, a zone of supersonic convection exists when the stellar model goes into yellow-red supergiant phase. When the modification is applied, the models of late type supergiants possess shallower convective envelopes for a given temperature compared with the normal formalism of the MLT. Therefore the stellar models made with our new formalism move to lower effective temperature by 100-400 K depending on the luminosity of the star. Such a modification does not affect the convective envelope structure of lower luminosity late type stars, as both expressions of the MLT give the same result. in these stars.