arXiv subjects

Paul S. Cally

Publications and source records attributed to Paul S. Cally.

25 records · Page 2Linked to original sources

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

Analysis of 3D MHD Instability of Antisolar Latitudinal Differential Rotation in F, G and K stars

Motivated by observations that only a very few stars have been found to have antisolar differential rotation, much weaker in amplitude than that of the Sun, we analyze the stability of antisolar and solar type latitudinal differential rotations in the tachoclines of typical F,G and K stars. We employ two three-dimensional thin shell models, one for a Boussinesq but non-hydrostatic system, the other for a hydrostatic but non-Boussinesq system. We find that, in general, the combination of toroidal field band and differential rotation is more unstable, and unstable for lower toroidal fields, for antisolar than for solar-type differential rotation. In the antisolar case, the instability is always found to weaken the differential rotation, even if the primary energy source for the instability is the magnetic field. This favors surface antisolar differential rotations in stars being weaker than solar types, if the instability in the tachocline is felt at the surface of the star. This is most likely to happen in F stars, whose convection zones are much thinner than they are in G and K stars. This effect could help explain why the antisolar differential rotations that have been found are very weak compared to the Sun.

astro-ph.SR↗

arXiv2011

Mode conversion of radiatively damped magnetogravity waves in the solar chromosphere

Modelling of adiabatic gravity wave propagation in the solar atmosphere showed that mode conversion to field guided acoustic waves or Alfvén waves was possible in the presence of highly inclined magnetic fields. This work aims to extend the previous adiabatic study, exploring the consequences of radiative damping on the propagation and mode conversion of gravity waves in the solar atmosphere. We model gravity waves in a VAL-C atmosphere, subject to a uniform, and arbitrarily orientated magnetic field, using the Newton cooling approximation for radiatively damped propagation. The results indicate that the mode conversion pathways identified in the adiabatic study are maintained in the presence of damping. The wave energy fluxes are highly sensitive to the form of the height dependence of the radiative damping time. While simulations starting from 0.2 Mm result in modest flux attenuation compared to the adiabatic results, short damping times expected in the low photosphere effectively suppress gravity waves in simulations starting at the base of the photosphere. It is difficult to reconcile our results and observations of propagating gravity waves with significant energy flux at photospheric heights unless they are generated in situ, and even then, why they are observed to be propagating as low as 70 km where gravity waves should be radiatively overdamped.

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

Magnetic and Thermal Phase Shifts in the Local Helioseismology of Sunspots

Phase perturbations due to inclined surface magnetic field of active region strength are calculated numerically in quiet Sun and simple sunspot models in order to estimate and compare the direct and indirect (thermal) effects of the fields on helioseismic waves. It is found that the largest direct effects occur in highly inclined field characteristic of penumbrae, and scale roughly linearly with magnetic field strength. The combined effects of sunspot magnetic and thermal anomalies typically yield negative travel-time perturbations in penumbrae. Travel-time shifts in umbrae depend on details of how the thermal and density structure differs from the quiet Sun. The combined shifts are generally not well approximated by the sum of the thermal and magnetic effects applied separately, except at low field strengths of around 1 kG or less, or if the thermal shift is small. A useful rule-of-thumb appears to be that travel-time perturbations in umbrae are predominantly thermal, whereas in penumbrae they are mostly magnetic.

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↗

arXiv2008

Multiple scattering of waves by a pair of gravitationally stratified flux tubes

We study the near-field coupling of a pair of flux tubes embedded in a gravitationally stratified environment. The mutual induction of the near-field {\it jackets} of the two flux tubes can considerably alter the scattering properties of the system, resulting in sizable changes in the magnitudes of scattering coefficients and bizarre trends in the phases. The dominant length scale governing the induction zone turns out to be approximately half the horizontal wave length of the incident mode, a result that fits in quite pleasantly with extant theories of scattering. Higher-$β$ flux tubes are more strongly coupled than weaker ones, a consequence of the greater role that the near-field jacket modes play in the such tubes. We also comment on the importance of incorporating the effects of multiple scattering when studying the effects of mode absorption in plage and interpreting related scattering measurements. That the near-field plays such an important role in the scattering process lends encouragement to the eventual goal of observationally resolving sub-wavelength features of flux tubes using techniques of helioseismology.

astro-ph↗