Turbulence in a Coronal Loop Excited by Photospheric Motions

Giuseppina Nigro,Pierluigi Veltri,Vincenzo Carbone,Antonio Vecchio,Francesco Malara,Francesca Di Mare,Leonardo Primavera

doi:10.3390/atmos11040409

Abstract

Photospheric motions are believed to be the source of coronal heating and of velocity fluctuations detected in the solar corona. A numerical model, based on the shell technique applied on reduced magnetohydrodynamics equations, is used to represent energy injection due to footpoint motions, storage and dissipation of energy in a coronal loop. Motions at the loop bases are simulated by random signals whose frequency-wavenumber spectrum reproduces features of photospheric motions: the p-mode peak and the low-frequency continuum. Results indicate that a turbulent state develops, dominated by magnetic energy, where dissipation takes place in an intermittent fashion. The nonlinear cascade is mainly controlled by velocity fluctuations, where resonant modes are dominant at high frequencies. Low frequency fluctuations present a power-law spectra and a bump at p-mode frequency; similar features are observed in velocity spectra detected in the corona. For typical loop parameters the energy input flux is comparable with that necessary to heat the quiet-Sun corona.

Highlights

IntroductionThe solar corona is the most external, rarefied and hottest part of the Sun’s atmosphere, which is formed by a plasma at an average temperature of the order of or larger than T ∼ 106 K
The solar corona is the most external, rarefied and hottest part of the Sun’s atmosphere, which is formed by a plasma at an average temperature of the order of or larger than T ∼ 106 K.It is permeated by a strongly inhomogeneous magnetic field which is generated by the dynamo mechanism [1,2,3,4,5,6,7] operating in the solar convection zone
Due to the velocity imposed at the boundaries, energy is continuously injected into the domain in form of velocity and magnetic field fluctuations

Summary

Introduction

The solar corona is the most external, rarefied and hottest part of the Sun’s atmosphere, which is formed by a plasma at an average temperature of the order of or larger than T ∼ 106 K. It is permeated by a strongly inhomogeneous magnetic field which is generated by the dynamo mechanism [1,2,3,4,5,6,7] operating in the solar convection zone. One of the possible sources of energy for the corona is represented by mechanical motions in the photosphere, which is the lowest and most dense part of the solar atmosphere. Two important issues still under debate are the following: (i) in which form and at which timescales does the energy of photospheric motions reaches the corona? (ii) How is this energy moved to the very small scales where it can be dissipated?

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Results

Conclusion