Spectroscopy at the solar limb

Abstract

Aims. We present constraints on the thermodynamical structure of the chromosphere from ground-based observations of the Ca ii H line profile near and off the solar limb. Methods. We obtained a slit-spectrograph data set of the Ca ii H line with a high signal-to-noise ratio in a field of view extending 20 across the limb. We analyzed the spectra for the characteristic properties of average and individual off-limb spectra. We used various tracers of the Doppler shifts, such as the location of the absorption core, the ratio of the two emission peaks H 2V and H 2R , and intensity images at a fixed wavelength. Results. The average off-limb profiles show a smooth variation with increasing limb distance. The line width increases up to a height of about 2 Mm above the limb. The profile shape is fairly symmetric with nearly identical H 2V and H 2R intensities; at a height of 5 Mm, it changes into a single Gaussian without emission peaks. We find that all off-limb spectra show large Doppler shifts that fluctuate on the smallest resolved spatial scales. The variation is more prominent in cuts parallel to the solar limb than on those perpendicular to it. As far as individual structures can be unequivocally identified at our spatial resolution, we find a specific relation between intensity enhancements and Doppler shifts: elongated brightenings are often flanked all along their extension by velocities in opposite directions. Conclusions. The average off-limb spectra of Ca ii H present a good opportunity to test static chromospheric atmosphere models because they lack the photospheric contribution that is present in disk-center spectra. We suggest that the observed relation between intensity enhancements and Doppler shifts could be caused by waves propagating along the surfaces of flux tubes: an intrinsic twist of the flux tubes or a wave propagation inclined to the tube axis would cause a helical shape of the Doppler excursions, visible as opposite velocity at the sides of the flux tube. Spectroscopic data allow one to distinguish this from a sausage-mode oscillation where the maximum Doppler shift and the tube axis would coincide.

The extended chromosphere of CoRoT-2A

“…For the Sun, Beck & Rezaei (2011) observed Ca H and K emission up to 5000 km above the solar limb. The largest solar prominences such as the "Grand Daddy Prominence" reach projected heights of 112 000 km as measured in Hα emission (Pettit 1946).…”

Can spicules be detected at disc centre in broad-band Ca ii H filter imaging data?

“…The offlimb intensity stems exclusively from emission in the line-core region of Ca ii H (cf. the average off-limb profiles in Beck & Rezaei 2011a) and H (cf. Appendix A) and cannot contain any contribution from photospheric radiation.…”

“…One data set contains Ca spectra at and beyond the solar limb, and is described in detail in Beck & Rezaei (2011a) and Beck et al (2011). These data are also available on-line (Beck & Rezaei 2011b).…”

Explosive Events: Swirling Transition Region Jets

“…We are not aware of any physical reason for a lower limit of spicular features, which should allow us to interpret our observation as if we were observing with a magnifying glass and to suggest that they also rotate at subresolution scales, although we have no conclusive evidence for this. Recently, Beck and Rezaei (2011) have identified in POLIS Ca ii H off-limb spectra elongated structures that are often flanked all along their extension by velocities in opposite direction. Pariat, Antiochos, and DeVore (2009) have suggested a MHD model to reproduce the ejection of a fast moving helical jet.…”