Quasi-biennial oscillations in the solar tachocline caused by magnetic Rossby wave instabilities

TL;DR

This paper proposes that magnetic Rossby wave instabilities in the solar tachocline, driven by differential rotation and magnetic fields, can explain the quasi-biennial oscillations observed in solar activity indices.

Contribution

It demonstrates that magnetic Rossby wave instabilities with specific magnetic field strengths can produce 2-year oscillations, linking them to observed solar activity variations.

Findings

Magnetic Rossby wave instability occurs for magnetic fields > 10^5 G.

The 2-year harmonic is antisymmetric and depends on rotation and magnetic parameters.

QBO period varies with magnetic field and differential rotation changes.

Abstract

Quasi-biennial oscillations (QBO) are frequently observed in the solar activity indices. However, no clear physical mechanism for the observed variations has been suggested so far. Here we study the stability of magnetic Rossby waves in the solar tachocline using the shallow water magnetohydrodynamic approximation. Our analysis shows that the combination of typical differential rotation and a toroidal magnetic field with a strength > 10^5 G triggers the instability of the m=1 magnetic Rossby wave harmonic with a period of 2 years. This harmonic is antisymmetric with respect to the equator and its period (and growth rate) depends on the differential rotation parameters and the magnetic field strength. The oscillations may cause a periodic magnetic flux emergence at the solar surface and consequently may lead to the observed QBO in the solar activity features. The period of QBO may change throughout the cycle, and from cycle to cycle, due to variations of the mean magnetic field and differential rotation in the tachocline.