Ca II K line profile of the truly quiet Sun

Sivaraman, K. R.; Gupta, Siddharth; Kariyappa, R.

doi:10.1007/bf00165458

Cited by 5 publications

(2 citation statements)

References 13 publications

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“…This finally gives a FWHM at disk center of 53 pm, corresponding to 40 km s −1 (logarithmic value of 1.6). Wilson & Vainu Bappu (1957) give a range of 33-39 km s −1 for Ca spectra in plage where the emission core is more pronounced and can be separated from the wing contribution (see also Teplitskaja & Efendieva 1973;Jebsen & Mitchell 1978;Oranje 1983;Sivaraman et al 1996). The prediction of Wilson & Vainu Bappu (1957) …”

Section: Average Off-limb Spectramentioning

confidence: 95%

Spectroscopy at the solar limb

Beck

Rezaei²

2011

A&A

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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.

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Section: Average Off-limb Spectramentioning

confidence: 95%

Spectroscopy at the solar limb

Beck

Rezaei²

2011

A&A

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“…This finally gives a FWHM at disk center of 53 pm, corresponding to 40 kms −1 (logarithmic value of 1.6). Wilson & Vainu Bappu (1957) give a range of 33-39 kms −1 for Ca spectra in plage where the emission core is more pronounced and can be separated from the wing contribution (see also Teplitskaja & Efendieva 1973;Jebsen & Mitchell 1978;Oranje 1983;Sivaraman et al 1996). The prediction of Wilson & Vainu Bappu (1957) for the Sun as a star line width of Ca ii H or K is 1.56, derived from their curve of what later was termed the Wilson-Bappu effect (cf.…”

Section: Average Off-limb Spectramentioning

confidence: 99%

Spectroscopy at the solar limb: I. Average off-limb profiles and Doppler shifts of Ca II H

Beck,

Rezaei

2011

Preprint

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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 excursion, 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.

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4.1.1.3 The solar energy spectrum

Wilhelm

2009

Solar System

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Ca II K line profile of the truly quiet Sun

Cited by 5 publications

References 13 publications

Spectroscopy at the solar limb

Spectroscopy at the solar limb

Spectroscopy at the solar limb: I. Average off-limb profiles and Doppler shifts of Ca II H

4.1.1.3 The solar energy spectrum

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