Solar surges related to UV bursts

Characterization through k-means, inversions, and density diagnostics^⋆

¹ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
e-mail: dnobrega@iac.es
² Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain
³ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029 Blindern 0315 Oslo, Norway
⁴ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029 Blindern 0315 Oslo, Norway
⁵ Dipartimento di Fisica e Astronomia “Ettore Majorana” – Sezione Astrofisica, Università degli Studi di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁶ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁷ Lockheed Martin Solar & Astrophysics Laboratory, Palo Alto, CA 94304, USA
⁸ Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94035, USA
⁹ Stanford University, HEPL, 466 Via Ortega, Stanford, CA 94305-4085, USA

Received: 4 June 2021
Accepted: 31 August 2021

Abstract

Context. Surges are cool and dense ejections typically observed in chromospheric lines and closely related to other solar phenomena such as UV bursts or coronal jets. Even though surges have been observed for decades now, questions regarding their fundamental physical properties such as temperature and density, as well as their impact on upper layers of the solar atmosphere remain open.

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, n_e, and line-of-sight velocity, V_LOS, the most reliable results from the inversions are within −6.0 ≤ log₁₀(τ)≤ − 4.8, with n_e ranging from ∼1.6 × 10¹¹ cm⁻³ up to 10¹² cm⁻³, and V_LOS 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.