THE APPEARANCE OF SPICULES IN HIGH RESOLUTION OBSERVATIONS OF Ca ii H AND Hα

Abstract

Solar spicules are chromospheric fibrils that appear everywhere on the Sun, yet their origin is not understood. Using high resolution observations of spicules obtained with the Swedish 1 m Solar Telescope, we aim to understand how spicules appear in filtergrams and Dopplergrams, how they compare in Ca II H and Hα filtergrams, and what can make them appear and disappear. We find that spicules display a rich and detailed spatial structure, and show a distribution of transverse velocities that, when aligned with the line of sight, can make them appear at different Hα wing positions. They become more abundant at positions closer to the line core, reflecting a distribution of Doppler shifts and widths. In Hα width maps they stand out as bright features both on disk and off limb, reflecting their large Doppler motions and possibly higher temperatures than in the typical Hα formation region. Spicule lifetimes measured from narrowband images at only a few positions will be an underestimate because Doppler shifts can make them disappear prematurely from such images; for such cases, width maps are a more robust tool. In Hα and Ca II H filtergrams, off-limb spicules essentially have the same properties, appearance, and evolution. We find that the sudden appearance of spicules can be explained by Doppler shifts from their transverse motions, and does not require other convoluted explanations.

Transverse Oscillations in Slender Ca ii H Fibrils Observed with Sunrise/SuFI

“…Most of these structures often share some properties and appear to be associated with photospheric concentrations of the magnetic field such as magnetic bright points and plages. These features have been mostly observed in Hα6563 Å and Ca II8542 Å images (representing the mid-to-upper chromosphere) on the solar disk (Vecchio et al 2007;Cauzzi et al 2008), and in Ca II8542Å, Ca IIH/K, and Hα images off the limb (e.g., Pasachoff et al 2009;Pereira et al 2016;seePereira et al 2014).…”

Two-fluid Numerical Simulations of Solar Spicules

“…They are best seen at the solar limb in strong chromospheric and transition-region (TR) spectral lines such as Hα, Ca ii H & K, Mg ii H & K, C ii and Si iv lines (e.g., Roberts 1945;Matsuno & Hirayama 1988;Nishikawa 1988;Suematsu et al 1995;De Pontieu et al 2007b;Suematsu et al 2008;Sterling et al 2010;Madjarska et al 2011;Pereira et al 2012;Tsiropoula et al 2012;Skogsrud et al 2014Skogsrud et al , 2015Pereira et al 2014;Rouppe van der Voort et al 2015;Pereira et al 2016;Beck et al 2016). Spicule observations have been carried out for about 130 years since they were first reported in 1877 (Secchi 1887).…”

The dynamic chromosphere: Pushing the boundaries of observations and models

“…What is observed as the quiet Sun chromosphere, at least since the advent of Hinode (Kosugi et al, 2007) and its Solar Optical Telescope (SOT, Tsuneta et al, 2008), is a sea of fib- rils or spicules (here we use the term for both disk and off-limb objects), thin elongated features that cover nearly the entire surface of the Sun. Spicules may appear sparse or dense depending on the width of the filter used, or if the filter samples the wings or core of chromospheric lines (see Pereira et al, 2016), and are rooted in the magnetic network. In Figure 1 we show examples of spicules on disk and off-limb as observed in Hα by the Swedish 1-m Solar Telescope (SST, Scharmer et al, 2003) with the CRISP interferometer (Scharmer et al, 2008).…”

“…Judge et al (2011) suggest that the spicular chromosphere is in fact a sort of optical illusion that arises from corrugated, two-dimensional sheet-like structures related to magnetic tangential discontinuities. Judge et al (2012) and Lipartito et al (2014) argue that the sudden appearance of spicules on disk is evidence of the 'sheet scenario', but Pereira et al (2016) and Shetye et al (2016) find that such appearances and disappearances at narrow filtergrams can be naturally explained by the spicular transverse motions and Doppler shifts.…”