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Title: UNDERSTANDING SOLAR TORSIONAL OSCILLATIONS FROM GLOBAL DYNAMO MODELS

Abstract

The phenomenon of solar “torsional oscillations” (TO) represents migratory zonal flows associated with the solar cycle. These flows are observed on the solar surface and, according to helioseismology, extend through the convection zone. We study the origin of the TO using results from a global MHD simulation of the solar interior that reproduces several of the observed characteristics of the mean-flows and magnetic fields. Our results indicate that the magnetic tension (MT) in the tachocline region is a key factor for the periodic changes in the angular momentum transport that causes the TO. The torque induced by the MT at the base of the convection zone is positive at the poles and negative at the equator. A rising MT torque at higher latitudes causes the poles to speed up, whereas a declining negative MT torque at the lower latitudes causes the equator to slow-down. These changes in the zonal flows propagate through the convection zone up to the surface. Additionally, our results suggest that it is the magnetic field at the tachocline that modulates the amplitude of the surface meridional flow rather than the opposite as assumed by flux-transport dynamo models of the solar cycle.

Authors:
 [1];  [2];  [3];  [4];  [5]
  1. Physics Department, Universidade Federal de Minas Gerais, Av. Antonio Carlos, 6627, Belo Horizonte, MG, 31270-901 (Brazil)
  2. European Centre for Medium-Range Weather Forecasts, Reading RG2 9AX (United Kingdom)
  3. Astronomy Department, Universidade de São Paulo, IAG-USP, Rua do Matão, 1226, São Paulo, SP, 05508-090 (Brazil)
  4. New Jersey Institute of Technology, Newark, NJ 07103 (United States)
  5. NASA, Ames Research Center, Moffett Field, Mountain View, CA 94040 (United States)
Publication Date:
OSTI Identifier:
22654234
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal Letters; Journal Volume: 828; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; AMPLITUDES; ANGULAR MOMENTUM; CONVECTION; EQUATOR; MAGNETIC FIELDS; MAGNETOHYDRODYNAMICS; OSCILLATIONS; PERIODICITY; ROTATION; SIMULATION; SOLAR CYCLE; SUN; SURFACES; TORQUE; VELOCITY

Citation Formats

Guerrero, G., Smolarkiewicz, P. K., Pino, E. M. de Gouveia Dal, Kosovichev, A. G., and Mansour, N. N., E-mail: guerrero@fisica.ufmg.br, E-mail: smolar@ecmwf.int, E-mail: dalpino@astro.iag.usp.br, E-mail: alexander.g.kosovichev@njit.edu, E-mail: Nagi.N.Mansour@nasa.gov. UNDERSTANDING SOLAR TORSIONAL OSCILLATIONS FROM GLOBAL DYNAMO MODELS. United States: N. p., 2016. Web. doi:10.3847/2041-8205/828/1/L3.
Guerrero, G., Smolarkiewicz, P. K., Pino, E. M. de Gouveia Dal, Kosovichev, A. G., & Mansour, N. N., E-mail: guerrero@fisica.ufmg.br, E-mail: smolar@ecmwf.int, E-mail: dalpino@astro.iag.usp.br, E-mail: alexander.g.kosovichev@njit.edu, E-mail: Nagi.N.Mansour@nasa.gov. UNDERSTANDING SOLAR TORSIONAL OSCILLATIONS FROM GLOBAL DYNAMO MODELS. United States. doi:10.3847/2041-8205/828/1/L3.
Guerrero, G., Smolarkiewicz, P. K., Pino, E. M. de Gouveia Dal, Kosovichev, A. G., and Mansour, N. N., E-mail: guerrero@fisica.ufmg.br, E-mail: smolar@ecmwf.int, E-mail: dalpino@astro.iag.usp.br, E-mail: alexander.g.kosovichev@njit.edu, E-mail: Nagi.N.Mansour@nasa.gov. 2016. "UNDERSTANDING SOLAR TORSIONAL OSCILLATIONS FROM GLOBAL DYNAMO MODELS". United States. doi:10.3847/2041-8205/828/1/L3.
@article{osti_22654234,
title = {UNDERSTANDING SOLAR TORSIONAL OSCILLATIONS FROM GLOBAL DYNAMO MODELS},
author = {Guerrero, G. and Smolarkiewicz, P. K. and Pino, E. M. de Gouveia Dal and Kosovichev, A. G. and Mansour, N. N., E-mail: guerrero@fisica.ufmg.br, E-mail: smolar@ecmwf.int, E-mail: dalpino@astro.iag.usp.br, E-mail: alexander.g.kosovichev@njit.edu, E-mail: Nagi.N.Mansour@nasa.gov},
abstractNote = {The phenomenon of solar “torsional oscillations” (TO) represents migratory zonal flows associated with the solar cycle. These flows are observed on the solar surface and, according to helioseismology, extend through the convection zone. We study the origin of the TO using results from a global MHD simulation of the solar interior that reproduces several of the observed characteristics of the mean-flows and magnetic fields. Our results indicate that the magnetic tension (MT) in the tachocline region is a key factor for the periodic changes in the angular momentum transport that causes the TO. The torque induced by the MT at the base of the convection zone is positive at the poles and negative at the equator. A rising MT torque at higher latitudes causes the poles to speed up, whereas a declining negative MT torque at the lower latitudes causes the equator to slow-down. These changes in the zonal flows propagate through the convection zone up to the surface. Additionally, our results suggest that it is the magnetic field at the tachocline that modulates the amplitude of the surface meridional flow rather than the opposite as assumed by flux-transport dynamo models of the solar cycle.},
doi = {10.3847/2041-8205/828/1/L3},
journal = {Astrophysical Journal Letters},
number = 1,
volume = 828,
place = {United States},
year = 2016,
month = 9
}
  • We have computed frequencies of solar oscillation for normal modes described by spherical harmonics Y/sup m//sub l/(theta,phi) with values of l between 0 and 4. The frequencies for the standard solar model differ from the nearest observed frequencies by 1 to 15 ..mu..Hz. Uncertainties in the interior physics, including nuclear cross sections, equation of state, and opacities, produce an uncertainty in the calculated frequencies of about 1 ..mu..Hz. Because of uncertainties in the outer boundary condition, in the physics of the superadiabatic layer of the convention zone, and in the structure of the solar chromosphere and corona, we cannot rulemore » out change in the frequencies by as much as 10 ..mu..Hz; however, modifications in the model producing such a change will also change the spacing between the frequencies of successive eigenmodes. Thus, an error in the outer layers could produce agreement for one mode only, and the remaining modes would disagree by more than the theoretical uncertainty of 1 ..mu..Hz. We compare our frequencies to those of three independent groups and find substantial agreement, although the differences between our frequencies and theirs are larger than our estimated uncertainty. The additional range of the results of other investigators can be attributed to the use of a less accurate equation of state, less accurate opacities, or an inadequate number of mesh points. Even with these additional factors, no standard model yet computed is in agreement with the observations. We conclude that the discrepancy between the theoretical and observed frequencies represents a real failure of the standard solar model of significance comparable to the failure to predict the correct neutrino flux.« less
  • Measurements from tracers and local helioseismology indicate the existence of a meridional flow in the Sun with strength in the order of 15 m s{sup -1} near the solar surface. Different attempts were made to obtain information on the flow profile at depths up to 20 Mm below the solar surface. We propose a method using global helioseismic Doppler measurements with the prospect of inferring the meridional flow profile at greater depths. Our approach is based on the perturbation of the p-mode eigenfunctions of a solar model due to the presence of a flow. The distortion of the oscillation eigenfunctionsmore » is manifested in the mixing of p-modes, which may be measured from global solar oscillation time series. As a new helioseismic measurement quantity, we propose amplitude ratios between oscillations in the Fourier domain. We relate this quantity to the meridional flow and unify the concepts presented here for an inversion procedure to infer the meridional flow from global solar oscillations.« less
  • The 11-yr torsional oscillations of the sun may represent deep-seated natural oscillations of the surface layers, excited by convection. Estimates are obtained for the torsional-wave frequency and amplitude. Torsional modulation of the magnetic-field generation process might account for the magnetic effects that parallel the solar cycle.