IRIS and ALMA Observations Uncovering a Type-II Spicule and the Dynamic Nature of a Chromospheric Plage Region

We present observations of the solar chromosphere obtained simultaneously with the Atacama Large Millimeter/submillimeter Array (\emph{ALMA}) and the Interface Region Imaging Spectrograph (\emph{IRIS}). The observatories targeted a chromospheric plage region of which the spatial distribution (split between strongly and weakly magnetized regions) allowed the study of linear-like structures in isolation, free of contamination from background emission. Using these observations in conjunction with a radiative magnetohydrodynamic 2.5D model covering the upper convection zone all the way to the corona that considers non-equilibrium ionization effects, we report the detection of an on-disk chromospheric spicule with \emph{ALMA} and confirm its multithermal nature. In addition, we discuss the strikingly high degree of similarity between chromospheric plage features observed in \emph{ALMA}/Band6 and \emph{IRIS}/\ion{Si}{4} (also reproduced in our model) suggesting that \emph{ALMA}/Band6 does not observe in the low chromosphere as previously thought but rather observes the upper chromospheric parts of structures such as spicules and other bright structures above plage at geometric heights near transition region temperatures. We also show that \emph{IRIS}/\ion{Mg}{2} is not as well correlated with \emph{ALMA}/Band6 as was previously thought. For these comparisons, we propose and employ a novel empirical method for the determination of plage regions, which seems to better isolate plage from its surrounding regions as compared to other methods commonly used. We caution that isolating plage from its immediate surroundings must be done with care to mitigate stastistical bias in quantitative comparisons between different chromospheric observables. Lastly, we report indications for chromospheric heating due to traveling shocks supported by the \emph{ALMA}/Band6 observations.

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