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 2 years of ground-based Global Oscillation Network Group (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 centerto-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 Helioseismic and Magnetic Imager (HMI) data. No strong evidence of multiple circulation cells in depth or 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.

Solar Cycle Variability Induced by Tilt Angle Scatter in a Babcock–Leighton Solar Dynamo Model

“…This value is in agreement with Jiang et al (2014) who found about 30% variation for the cycle 17 in the axial dipole moment and the polar field compared to the value without tilt scatter.…”

“…While we in our 3D dynamo model and Lemerle & Charbonneau (2017) in their coupled 2D×2D model explicitly demonstrate this, the original idea was known since the work of Charbonneau & Dikpati (2000). Recently, Cameron et al (2013) demonstrated this idea using observations, while Jiang et al (2014) for the first time quantified the effect of the tilt scatter on the polar field using a surface flux transport model. Based on this idea many authors (e.g., Choudhuri & Karak 2009;Yeates et al 2008;Olemskoy & Kitchatinov 2013) modeled irregular features of the solar cycle by including fluctuations in the BL α term of their 2D flux transport dynamo models.…”

“…Because of this random component, the actual tilt angle in our model has a considerable variation. Since the poloidal field generated by the BL mechanism depends sensitively on the BMR tilt (Dasi-Espuig et al 2010;Jiang et al 2014;Hazra et al 2017), its random scatter gives rise to cycle variability. This greatly enhances the relatively modest cycle variability arising just from the random time delay (MT16).…”

“…Recent helioseismic inversions suggest that this may not be accurate but they have not yet converged on a robust determination of what the structure and amplitude may in fact be (Jackiewicz et al 2015;Rajaguru & Antia 2015;Zhao & Chen 2016). In the absence of this information and to make contact with previous BL dynamo models, we have retained the single-celled profile.…”

“…Random scatter in BMR tilt angles has been proposed by many authors as a possible mechanism to explain the irregularity of the solar cycle and it has been studied previously within the context of 2D BL dynamo models (e.g., Charbonneau & Dikpati 2000;Jiang et al 2007;Choudhuri & Karak 2009;Olemskoy & Kitchatinov 2013), surface flux transport (SFT) models (Jiang et al 2014;Hathaway & Upton 2016) and in a coupled 2D×2D BL/SFT dynamo model (Lemerle & Charbonneau 2017). However, to our knowledge, our model is the first 3D solar dynamo model to explicitly investigate the implications of tilt angle scatter with regard to solar cycle variability.…”