The Rise and Emergence of Untwisted Toroidal Flux Ropes on the Sun

TL;DR

This study demonstrates that untwisted toroidal magnetic flux ropes can coherently rise through the Sun's convection zone and emerge into the corona, challenging the previous assumption that twist is necessary for such processes.

Contribution

The paper introduces the novel finding that untwisted toroidal flux ropes can rise and emerge coherently, emphasizing the importance of their geometry over twist.

Findings

Untwisted toroidal flux ropes can rise coherently through the convection zone.

Emergence involves pile-up and undular instability near the photosphere.

Emerging flux ropes produce characteristic solar active region features.

Abstract

Magnetic flux ropes (MFRs) rising buoyantly through the Sun's convection zone are thought to be subject to viscous forces preventing them from rising coherently. Numerous studies have suggested that MFRs require a minimum twist in order to remain coherent during their rise. Furthermore, even MFRs that get to the photosphere may be unable to successfully emerge into the corona unless they are at least moderately twisted, since the magnetic pressure gradient needs to overcome the weight of the photospheric plasma. To date, however, no lower limit has been placed on the critical minimum twist required for an MFR to rise coherently through the convection zone or emerge through the photosphere. In this paper, we simulate an untwisted toroidal MFR which is able to rise from the convection zone and emerge through the photosphere as an active region that resembles those observed on the Sun. We show that untwisted MFRs can remain coherent during their rise and then pile-up near the photosphere, triggering the undular instability, allowing the MFR to emerge through the photosphere. We propose that the toroidal geometry of our MFR is critical for its coherent rise. Upon emerging, a pair of lobes rises into the corona which interact and reconnect, resulting in a localized high speed jet. The resulting photospheric magnetogram displays the characteristic salt-and-pepper structure often seen in observations. Our major result is that MFRs need not be twisted to rise coherently through the convection zone and emerge through the photosphere.