Coronal hole boundaries at small scales: III. EIS and SUMER views

TL;DR

This study investigates small-scale transient plasma events in the Sun's quiet regions and coronal holes, revealing their dynamic nature, plasma properties, and the mechanisms behind jet formation using combined spectroscopic and imaging data.

Contribution

It provides new detailed measurements of plasma densities, temperatures, and flow velocities of small-scale solar transient events, especially X-ray jets, using combined SUMER and EIS observations.

Findings

Coronal hole transient events produce high-density outflows and jets.

Most jets reach temperatures up to 2.5 MK, with some exceeding 12 MK at footpoints.

Quiet Sun brightenings do not produce jet-like flows, indicating different plasma dynamics.

Abstract

We report on the plasma properties of small-scale transient events identified in the quiet Sun, coronal holes and their boundaries. We use spectroscopic co-observations from SUMER/SoHO and EIS/Hinode combined with high cadence imaging data from XRT/Hinode. We measure Doppler shifts using single and multiple Gauss fits of transition region and coronal lines as well as electron densities and temperatures. We combine co-temporal imaging and spectroscopy to separate brightening expansions from plasma flows. The transient brightening events in coronal holes and their boundaries were found to be very dynamical producing high density outflows at large speeds. Most of these events represent X-ray jets from pre-existing or newly emerging coronal bright points at X-ray temperatures. The average electron density of the jets is logNe ~ 8.76 cm^-3 while in the flaring site it is logNe ~ 9.51 cm^-3. The jet temperatures reach a maximum of 2.5 MK but in the majority of the cases the temperatures do not exceed 1.6 MK. The footpoints of jets have temperatures of a maximum of 2.5 MK though in a single event scanned a minute after the flaring the measured temperature was 12 MK. The jets are produced by multiple microflaring in the transition region and corona. Chromospheric emission was only detected in their footpoints and was only associated with downflows. The Doppler shift measurements in the quiet Sun transient brightenings confirmed that these events do not produce jet-like phenomena. The plasma flows in these phenomena remain trapped in closed loops.