The solar interface from the photosphere to the chromosphere and corona : contribution of eclipses and EUV filtergrams

TL;DR

This paper investigates the solar interface from the photosphere to the corona using eclipse observations and EUV imaging, revealing new insights into temperature structures, emission mechanisms, and the limitations of existing models.

Contribution

It introduces new high-cadence CCD spectra, eclipse images, and EUV data to better understand the solar interface and challenges the adequacy of 1D hydrostatic models.

Findings

Identification of temperature bifurcation and structuration at the solar limb.

Detection of low FIP emission lines and their mechanisms.

Evidence of small-scale dynamic features like spicules affecting models.

Abstract

Eclipses are very favourable for the photosphere- chromosphere and corona interface observation as the occultation takes place in space, free of parasitic light coming from the occulting disk (the Moon). Independently, EUV filtergrams of the limb region obtained in space were analyzed using one dimensional hydrostatic VAL models but this method ignores the ubiquitous magnetic field emergence phenomenon associated with the chromospheric network. A jump of temperature from 0.01 to 1 MK is observed near the 2 Mm heights and higher, further producing a permanent solar wind flow. The heating processes responsible for this temperature jump and for the flow are not yet fully understood. In this thesis, we reconsider these problems with new high cadence CCD flash spectra, white light (W-L) eclipse images and new EUV images obtained with space-borne instruments. We illustrate the mechanisms of low First Ionisation Potential (FIP) emission lines present in the low layers of the solar atmosphere, and nearby prominences. We characterize in detail the He shells and the solar interface region. Main results: i) The solar edge and the temperature bifurcation: structuration and discussion of the analysed low FIP lines and the true continuum in the 400 to 600 km heights above the limb, emission mechanisms; ii) Visible mesospheric and chromospheric emission lines in the interface regions including the Paschen alpha HeII line starting at 800 km above the limb, produced by photo-ionisation showing shells around the Sun and probing the prominence-corona interface; iii) The contribution of small scale dynamical spicules and macro-spicules starting at 1 Mm above the limb showing that the 1D hydrostatic stratified models are not adapted to the upper layers.