Spectropolarimetric Investigation of the Propagation of Magnetoacoustic Waves and Shock Formation in Sunspot Atmospheres

Abstract

Velocity oscillations in sunspot umbrae have been measured simultaneously in two spectral lines: the photospheric Silicon i 10827Å line and the chromospheric Helium i 10830Å multiplet. From the full Stokes inversion of temporal series of spectropolarimetric observations we retrieved, among other parameters, the line of sight velocity temporal variations at photospheric and chromospheric heights. Chromospheric velocity oscillations show a three minute period with a clear sawtooth shape typical of propagating shock wave fronts. Photospheric velocity oscillations have basically a five minute period, although the power spectrum also shows a secondary peak in the three minute band which has proven to be predecessor for its chromospheric counterpart. The derived phase spectra yield a value of the atmospheric cut-off frequency around 4 mHz and give evidence for the upward propagation of higher frequency oscillation modes. The phase spectrum has been reproduced with a simple model of linear vertical propagation of slow magneto-acoustic waves in a stratified magnetized atmosphere that accounts for radiative losses through Newton's cooling law. The model explains the main features in the phase spectrum, and allows us to compute the theoretical time delay between the photospheric and chromospheric signals, which happens to have a strong dependence on frequency. We find a very good agreement between this and the time delay obtained directly from the cross-correlation of photospheric and chromospheric velocity maps filtered around the 6 mHz band. This allows us to infer that the 3-minute power observed at chromospheric heights comes directly from the photosphere by means of linear wave propagation, rather than from non-linear interaction of 5-minute (and/or higher frequency) modes.

Small-scale chromospheric jets above a sunspot light bridge

“…One of these is umbral flashes (UFs), sudden brightenings that are observed in the core of the chromospheric Ca ii lines with a periodicity of about 3 min (Wittmann 1969). Umbral flashes are a result of waves propagating from the photosphere which turn into shocks on reaching the less dense chromosphere and are seen as blueshifted emission reversals in the core of the Ca ii 8542 Å line (Rouppe van der Voort et al Centeno et al 2006;Felipe et al 2010). Although UFs are a large-scale phenomenon, it has been shown that within a resolution element there are at least two distinct magnetic components, one that has the same polarity as the spot with zero or slightly downflowing velocity, while the other is related to the shock wave that has a line core emission reversal and strong upflows (Socas-Navarro et al 2000).…”

Bright fibrils in Ca II K

“…Yet another alternative explanation for the lowered acoustic cutoff frequency involves radiative cooling. A loose reasoning can be made where the more inhomogeneous plage region has a shorter radiative cooling time which in turn leads to a decreased acoustic cutoff frequency (Centeno et al 2006).…”

Stable umbral chromospheric structures

“…These shocks steepen from the three-minute oscillations, observed at the discovery of umbral flashes themselves (Beckers & Tallant 1969) as well as in early follow-up papers (Wittmann 1969). In turn, the threeminute oscillations are likely caused by a resonance in the umbral atmosphere itself (see early review by Thomas 1985) and are, at least partially, powered by the photospheric five-minute oscillations (Centeno et al 2006).…”