Gregal Vissers, Luc Rouppe van der Voort
High-quality imaging spectroscopy in the H{\alpha} line, obtained with the
CRisp Imaging SpectroPolarimeter (CRISP) at the Swedish 1-m Solar Telescope
(SST) at La Palma and covering a small sunspot and its surroundings, are
studied. They exhibit ubiquitous flows both along fibrils making up the
chromospheric canopy away from the spot and in the superpenumbra. We term these
flows "flocculent" to describe their intermittent character, that is
morphologically reminiscent of coronal rain. The flocculent flows are
investigated further in order to determine their dynamic and morphological
properties. For the measurement of their characteristic velocities,
accelerations and sizes, we employ a new versatile analysis tool, the CRisp
SPectral EXplorer (CRISPEX), which we describe in detail. Absolute velocities
on the order of 7.2-82.4 km/s are found, with an average value of 36.5\pm5.9
km/s and slightly higher typical velocities for features moving towards the
sunspot than away. These velocities are much higher than those determined from
the shift of the line core, which shows patches around the sunspot with
velocity enhancements of up to 10-15 km/s (both red- and blueshifted).
Accelerations are determined for a subsample of features, that show clear
accelerating or decelerating behavior, yielding an average of 270\pm63 m/s^2
and 149\pm63 m/s^2 for accelerating and decelerating features, respectively.
Typical flocculent features measure 627\pm44 km in length and 304\pm30 km in
width. On average 68 features are detected per minute, with an average lifetime
of 67.7\pm8.8 s. The dynamics and phenomenology of the flocculent flows suggest
they may be driven by a siphon flow, where the flocculence could arise from a
density perturbation close to one of the footpoints or along the loop
structure.
View original:
http://arxiv.org/abs/1202.5453
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