ALMA and IRIS Observations of the Solar Chromosphere II: Structure and Dynamics of Chromospheric Plage

TL;DR

This study introduces a new empirical method to identify chromospheric plage regions, revealing that 1.25 mm emission forms in the upper chromosphere and highlighting similarities and discrepancies between ALMA and IRIS observations, with evidence of shock heating.

Contribution

A novel empirical approach for isolating chromospheric plage regions and insights into the formation height of millimeter emission and chromospheric heating mechanisms.

Findings

1.25 mm emission in plage forms in the upper chromosphere, not the low chromosphere.

High similarity between ALMA/Band6 and IRIS/Si IV features.

Evidence of shock-induced chromospheric heating.

Abstract

We propose and employ a novel empirical method for determining chromospheric plage regions, which seems to better isolate plage from its surrounding regions compared to other methods commonly used. We caution that isolating plage from its immediate surroundings must be done with care in order to successfully mitigate statistical biases that, for instance, can impact quantitative comparisons between different chromospheric observables. Using this methodology, our analysis suggests that 1.25 mm wavelength free-free emission in plage regions observed with ALMA/Band6 may not form in the low chromosphere as previously thought, but rather in the upper chromospheric parts of dynamic plage features (such as spicules and other bright structures), i.e., near geometric heights of transition region temperatures. We investigate the high degree of similarity between chromospheric plage features observed in ALMA/Band6 (at 1.25 mm wavelength) and IRIS/Si IV at 1393\r{A}. We also show that IRIS/Mg II h and k is not as well correlated with ALMA/Band6 as was previously thought, and we discuss the discrepancies with previous works. Lastly, we report indications for chromospheric heating due to propagating shocks supported by the ALMA/Band6 observations.