Prominence signatures in the Fraunhofer G-band; Testing ionization memory with multi-line prominence diagnostics

TL;DR

This study investigates prominence emissions in the Fraunhofer G-band, identifying metal line contributions and testing for ionization memory effects, with implications for solar prominence diagnostics.

Contribution

It provides the first detailed analysis of prominence emissions in the G-band, clarifying the spectral lines involved and assessing ionization memory effects in neutral and ionized lines.

Findings

Prominence emission in the G-band mainly from Ti II and Ca I lines.

No evidence of ionization memory effects in the observed lines.

CH molecular lines do not contribute to prominence emission in this spectral region.

Abstract

The Fraunhofer G-band around 4304 {\AA} is widely used as a photospheric diagnostic and is generally not expected to show signatures of chromospheric or coronal structures. However, recent amateur observations have suggested the presence of off-limb prominence emission in this spectral region. We investigate the origin of the prominence emission in the G-band to determine if this is caused by methylene (CH) or other lines in this band. We also aim to test these lines for the presence of ionization memory effects in neutral lines. We present a case study of two prominences, one obtained with a Solar Explorer (Sol'Ex) spectroheliograph and another with the high-resolution Fast Multi-Line Universal Spectrograph (FaMuLUS) camera system at the echelle spectrograph of the German Vacuum Tower Telescope (VTT). Line widths are measured for simultaneously observed neutral and ionized metal lines, allowing a comparison of thermal and non-thermal broadening components to see if these lines exhibit any ionization memory effects. We report clear prominence emission in several metal lines within the G-band, primarily from Ti II and Ca I lines, while contributions from CH molecular lines are not observed. A comparison of the simultaneously observed ionized and neutral lines reveals no clear evidence for an ionization memory effect. Since the prominence emission does not originate from CH lines, we will not call them "G-band prominences" but rather prominences in the G-band, as they are independent of the primary diagnostic in this spectral window. In addition, the absence of a clear ionization memory effect suggests that such effects may be less pronounced for weak neutral lines.