Astronomy and Astrophysics
Aims: Our aim is to address the current lack of inverted models and diagnostics of surges, as well as to characterize the chromospheric and transition region plasma of these phenomena.
Methods: We have analyzed an episode of recurrent surges related to UV bursts observed with the Interface Region Imaging Spectrograph (IRIS) in April 2016. The mid- and low-chromosphere of the surges were unprecedentedly examined by getting their representative Mg IIh&k line profiles through the k-means algorithm and performing inversions on them using the state-of-the-art STiC code. We have studied the far-UV spectra focusing on the O IV 1399.8 Å and 1401.2 Å lines, which were previously unexplored for surges, carrying out density diagnostics to determine the transition region properties of these ejections. We have also used numerical experiments performed with the Bifrost code for comparisons.
Results: Thanks to the k-means clustering, we reduced the number of Mg IIh&k profiles to invert by a factor 43.2. The inversions of the representative profiles show that the mid- and low-chromosphere of the surges are characterized, with a high degree of reliability, by temperatures mainly around T = 6 kK at −6.0 ≤ log10(τ)≤ − 3.2. For the electronic number density, ne, and line-of-sight velocity, VLOS, the most reliable results from the inversions are within −6.0 ≤ log10(τ)≤ − 4.8, with ne ranging from ∼1.6 × 1011 cm−3 up to 1012 cm−3, and VLOS of a few km s−1. We find, for the first time, observational evidence of enhanced O IV emission within the surges, indicating that these phenomena have a considerable impact on the transition region even in the weakest far-UV lines. The O IV emitting layers of the surges have an electron number density ranging from 2.5 × 1010 cm−3 to 1012 cm−3. The numerical simulations provide theoretical support in terms of the topology and location of the O IV emission within the surges. Movie associated with Fig. 2 is available at https://www.aanda.org
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