Meridional Flow in the Solar Convection Zone II: Helioseismic Inversions of GONG Data

Abstract

Meridional flow is thought to play a very important role in the dynamics of the solar convection zone; however, because of its relatively small amplitude, precisely measuring it poses a significant challenge. Here we present a complete time-distance helioseismic analysis of about two years of ground-based GONG Doppler data to retrieve the meridional circulation profile for modest latitudes, in an attempt to corroborate results from other studies. We use an empirical correction to the travel times due to an unknown center-to-limb systematic effect. The helioseismic inversion procedure is first tested and reasonably validated on artificial data from a large-scale numerical simulation, followed by a test to broadly recover the solar differential rotation found from global seismology. From GONG data, we measure poleward photospheric flows at all latitudes with properties that are comparable with earlier studies, and a shallow equatorward flow about $65$\,Mm beneath the surface, in agreement with recent findings from HMI data. No strong evidence of multiple circulation cells in depth nor latitude is found, yet the whole phase space has not yet been explored. Tests of mass flux conservation are then carried out on the inferred GONG and HMI flows and compared to a fiducial numerical baseline from models, and we find that the continuity equation is poorly satisfied. While the two disparate data sets do give similar results for about the outer $15\%$ of the interior radius, the total inverted circulation pattern appears to be unphysical in terms of mass conservation when interpreted over modest time scales. We can likely attribute this to both the influence of realization noise and subtle effects in the data and measurement procedure.

18 pages preprint format, 5 figures, 1 table, accepted to ApJ

References in corpus (5)

• Solar Meridional Flow in the Shallow Interior during the Rising Phase of Cycle 24
• Meridional Flow in the Solar Convection Zone I: Measurements from GONG Data
• Validating Time-Distance Helioseismology With Realistic Quiet Sun Simulations
• Full Waveform Inversion of Solar Interior Flows
• Issues with time--distance inversions for supergranular flows

Cited by in corpus (12)

• Solar Cycle Variability Induced by Tilt Angle Scatter in a Babcock--Leighton Solar Dynamo Model
• A Comprehensive Method to Measure Solar Meridional Circulation and Center-to-Limb Effect Using Time-Distance Helioseismology
• Parameter optimization for surface flux transport models
• Inversions for Deep Solar Meridional Flow Using Spherical Born Kernels
• A Babcock-Leighton-type Solar Dynamo Operating in the Bulk of the Convection Zone
• Continuous Solar Observations from the Ground -- Assessing Duty Cycle from GONG Observations
• Improved Measurements of the Sun's Meridional Flow and Torsional Oscillation from Correlation tracking on MDI \& HMI magnetograms
• Estimating the deep solar meridional circulation using magnetic observations and a dynamo model: a variational approach
• Finite-frequency sensitivity kernels in spherical geometry for time-distance helioseismology
• Probing the Solar Meridional Circulation using Fourier Legendre Decomposition
• Computing Helioseismic Sensitivity Kernels for the Sun's Large-Scale Internal Flows using Global-Scale Wave-Propagation Simulations
• Helioseismic finite-frequency sensitivity kernels for flows in spherical geometry including systematic effects