Deep meridional circulation below the solar convective envelope

TL;DR

This paper analytically models deep meridional flows below the solar convective envelope, revealing their penetration depth, impact on magnetic fields, and implications for solar dynamo theories and lithium burning.

Contribution

It provides an analytical solution for meridional flow velocity below the solar convective envelope, highlighting its penetration depth and effects on magnetic fields and lithium burning.

Findings

Meridional flow penetrates deep below the convective envelope.

Deep flow velocity is reduced by strong magnetic fields and density stratification.

Return flow likely does not reach the surface within the solar cycle period.

Abstract

With reasonable assumptions and approximations, we compute the velocity of the meridional flow $U$ in the convective envelope by modified Chandrasekhar's (1956) MHD equations. The analytical solution of such a modified equation is found to be $U(x,\mu) = \sum_{n=0}^\infty \bigl[u1_n x^n + u2_n x^{-(n+3)}\bigr] C_n^{3/2}(\mu)$, where $x$ is non-dimensional radius, $\mu = cos{\vartheta}$, ${\vartheta}$ is the co-latitude, $C_n^{3/2} {(\mu)}$ are the Gegenbaur polynomials of order 3/2, $u1_n$ and $u2_n$ are the unknown constants. The results show that meridional velocity flow from the surface appears to penetrates deep below base of the convective envelope and at outer part of the radiative zone. With such a deep flow velocity below the convective envelope and a very high density stratification in the outer part of the radiative zone with likely existence of a strong ($\sim$ $10^{4}$ G) toroidal magnetic field structure, the velocity of transport of meridional flow is considerably reduced. Hence, it is very unlikely that the return flow will reach the surface (with a period of solar cycle) as required by some of the flux transport dynamo models. On the other hand, deep meridional flow is required for burning of Lithium at outer part of the radiative zone supporting the observed Lithium deficiency at the surface.