Can coronal hole spicules reach coronal temperatures?

TL;DR

This study investigates whether coronal hole spicules reach coronal temperatures by combining multi-instrument observations, finding that large, dynamic spicules do not show evidence of temperatures above 300,000 K, thus likely not reaching coronal temperatures.

Contribution

The paper provides observational evidence that large coronal hole spicules do not attain coronal temperatures, clarifying their thermal properties and challenging previous interpretations based on EUV imaging.

Findings

Spicules show velocities from 50 to 250 km/s.

No spectral lines formed above 300,000 K detected in spicules.

Transition region emissions can be misinterpreted as coronal signatures.

Abstract

We aim with the present study to provide observational evidences on whether coronal hole spicules reach coronal temperatures. We combine multi-instrument co-observations obtained with the SUMER/SoHO and with the EIS/SOT/XRT/Hinode. The analysed three large spicules were found to be comprised of numerous thin spicules which rise, rotate and descend simultaneously forming a bush-like feature. Their rotation resembles the untwisting of a large flux rope. They show velocities ranging from 50 to 250 km/s. We clearly associated the red- and blue-shifted emissions in transition region lines with rotating but also with rising and descending plasmas, respectively. Our main result is that these spicules although very large and dynamic, show no presence in spectral lines formed at temperatures above 300 000 K. The present paper brings out the analysis of three Ca II H large spicules which are composed of numerous dynamic thin spicules but appear as macrospicules in EUV lower resolution images. We found no coronal counterpart of these and smaller spicules. We believe that the identification of phenomena which have very different origins as macrospicules is due to the interpretation of the transition region emission, and especially the He II emission, wherein both chromospheric large spicules and coronal X-ray jets are present. We suggest that the recent observation of spicules in the coronal AIA/SDO 171 A and 211 A channels is probably due to the existence of transition region emission there.