Prominence Visibility in Hinode/XRT images

TL;DR

This study analyzes Hinode/XRT soft X-ray images of a solar prominence, concluding that the observed darkening is due to cool prominence plasma within the hot corona, not X-ray absorption.

Contribution

It demonstrates that prominence darkening in X-ray images results from emissivity deficits caused by cool plasma, not absorption, and estimates the prominence spine's extension along the line-of-sight.

Findings

Darkening corresponds with prominence structure in EUV images.

Absorption effects are negligible in causing darkening.

Prominence spine extends approximately 10^5 km along the line-of-sight.

Abstract

In this paper we study the soft X-ray (SXR) signatures of one particular prominence. The X-ray observations used here were made by the Hinode/XRT instrument using two different filters. Both of them have a pronounced peak of the response function around 10 A. One of them has a secondary smaller peak around 170 A, which leads to a contamination of SXR images. The observed darkening in both of these filters has a very large vertical extension. The position and shape of the darkening corresponds nicely with the prominence structure seen in SDO/AIA images. First we have investigated the possibility that the darkening is caused by X-ray absorption. But detailed calculations of the optical thickness in this spectral range show clearly that this effect is completely negligible. Therefore the alternative is the presence of an extended region with a large emissivity deficit which can be caused by the presence of cool prominence plasmas within otherwise hot corona. To reproduce the observed darkening one needs a very large extension along the line-of-sight of the region amounting to around 10$^5$ km. We interpret this region as the prominence spine, which is also consistent with SDO/AIA observations in EUV.