Shelley C. Hansen | Searcharxiv

arXiv2012

Benchmarking Fast-to-Alfvén Mode Conversion in a Cold MHD Plasma. II. How to get Alfvén waves through the Solar Transition Region

Alfvén waves may be difficult to excite at the photosphere due to low ionization fraction and suffer near-total reflection at the transition region (TR). Yet they are ubiquitous in the corona and heliosphere. To overcome these difficulties, we show that they may instead be generated high in the chromosphere by conversion from reflecting fast magnetohydrodynamic waves, and that Alfvénic transition region reflection is greatly reduced if the fast reflection point is within a few scale heights of the TR. The influence of mode conversion on the phase of the reflected fast wave is also explored. This phase can potentially be misinterpreted as a travel speed perturbation, with implications for the practical seismic probing of active regions.

astro-ph.SR↗

arXiv2011

Benchmarking Fast-to-Alfven Mode Conversion in a Cold MHD Plasma

Alfvén waves may be generated via mode conversion from fast magneto-acoustic waves near their reflection level in the solar atmosphere, with implications both for coronal oscillations and for active region helio-seismology. In active regions this reflection typically occurs high enough that the Alfvén speed $a$ greatly exceeds the sound speed $c$, well above the $a=c$ level where the fast and slow modes interact. In order to focus on the fundamental characteristics of fast/Alfvén conversion, stripped of unnecessary detail, it is therefore useful to freeze out the slow mode by adopting the gravitationally stratified cold MHD model $c\to0$. This provides a benchmark for fast-to-Alfvén mode conversion in more complex atmospheres. Assuming a uniform inclined magnetic field and an exponential Alfvén speed profile with density scale height $h$, the Alfvén conversion coefficient depends on three variables only; the dimensionless transverse-to-the-stratification wavenumber $κ=kh$, the magnetic field inclination from the stratification direction $θ$, and the polarization angle $ϕ$ of the wavevector relative to the plane containing the stratification and magnetic field directions. We present an extensive exploration of mode conversion in this parameter space and conclude that near-total conversion to outward-propagating Alfvén waves typically occurs for small $θ$ and large $ϕ$ ($80^\circ$--$90^\circ$), though it is absent entirely when $θ$ is exactly zero (vertical field). For wavenumbers of helioseismic interest, the conversion region is broad enough to encompass the whole chromosphere.

astro-ph.SR↗

arXiv2009

An Exact Test of Generalized Ray Theory in Local Helioseismology

Generalized Ray Theory (GRT) provides a simple description of MHD mode transmission and conversion between magnetoacoustic fast and slow waves and is directly applicable to solar active regions. Here it is tested in a simple two-dimensional, isothermal, gravitationally-stratified model with inclined magnetic field using previously published exact solutions and found to perform very well.

astro-ph.SR↗