Discrete modes and turbulence in a wave‐driven strongly magnetized plasma

Abstract

[1] We show the coexistence of discrete modes and t u r b u l e n c e i n n u m e r i c a l e x p e r i m e n t s o f magnetohydrodynamics with a strong background magnetic field. The particular system we consider is a model of an open magnetic region which we have previously used in coronal heating studies. In the simulations shown here, Alfven waves at discrete frequencies with values corresponding to solar g-modes and p-modes are injected at the boundary (although the question of how those modes could generate Alfven waves is not addressed). Numerical probes are used to compute the magnetic field fluctuations at the output. The frequency spectra obtained from those probes are broadband, but with peaks at the driving frequencies. The perpendicular (to the mean magnetic field) wavenumber energy spectrum is of a turbulent Kolmogorov-like type, and formation of small scale structures is observed. This shows, numerically, the possibility of persistence of discrete modes in a turbulent system.

Origin of the turbulent spectra in the high-altitude cusp: Cluster spacecraft observations

“…Indeed, this is shown to be possible in simulations corresponding to the open coronal magnetic field lines (Dmitruk et al, 2004). We have considered two mechanisms for the generation of the spectral breakpoint and double sloped spectra:…”

“…The Cluster trajectory is perpendicular to the ambient magnetic field direction, so that the observed broad band spectra may be a result of the turbulent cascade. Numerical MHD simulations (designed for open solar coronal magnetic field region) by Dmitruk et al (2004) show the coexistence of discrete modes and turbulence in a presence of a strong background magnetic field. It therefore may be possible that both energy cascade and linear modes coexist in the cusp geometry (with magnetic field of 100-60 nT and ion beta of 0.02-0.4).…”

Waves and turbulence in magnetohydrodynamic direct numerical simulations

“…A similar procedure was employed in Ref. 15 to study the presence of discrete modes within a turbulent system. Long time series of the single point data are obtained ͑2000 unit times duration͒ to compute the Eulerian frequency power spectra at the position of each probe.…”

“…This case we examined earlier through reduced MHD simulations, which are weakly three dimensional but support propagating Alfvén wave modes. 15,24 The context of this type of wave driving at the boundary has often been coronal heating driven at the base by Alfvén wave injection. The basic result, not reproduced here, 15 is that the signature in the frequency domain of the wave driving at the base can survive to the top of a coronal model.…”

“…15,24 The context of this type of wave driving at the boundary has often been coronal heating driven at the base by Alfvén wave injection. The basic result, not reproduced here, 15 is that the signature in the frequency domain of the wave driving at the base can survive to the top of a coronal model. This occurs even though the spatial information becomes completely scrambled as a Kolmogorov-like cascade is established.…”