Direct measurement results of the time lag of LOS-velocity oscillations between two heights in solar faculae and sunspots

TL;DR

This study measures the time lag of LOS velocity oscillations between different heights in solar faculae and sunspots, revealing delays that suggest wave propagation speeds exceeding sound speed, with implications for understanding solar atmospheric dynamics.

Contribution

It provides direct measurements of phase delays between chromospheric and photospheric oscillations in various solar features, highlighting the potential role of MHD waves.

Findings

Measured delays vary across different spectral line pairs.

Delays indicate wave speeds exceeding the photospheric sound speed.

Results suggest the presence of slow MHD waves in the solar atmosphere.

Abstract

We present an investigation of line-of-sight (LOS) velocity oscillations in solar faculae and sunspots. To study the phase relations between chromosphere and photosphere oscillations of the LOS velocity, we measured the time lag of the chromospheric signal relative to the photospheric one for several faculae and sunspots in a set of spectral line pairs. The measured time lags are different for different objects. The mean measured delay between the oscillations in the five-minute band in faculae is 50s for the SiI 10827{\AA}-HeI 10830{\AA} pair; for the pair FeI 6569{\AA}-H-alpha 6563{\AA} the mean delay is 20s; for the pair FeI 4551{\AA}-BaII 4554{\AA} the mean delay is 7s; for the pair SiI 8536{\AA}-CaII 8542{\AA} the mean delay is 20s. For the oscillations in the three-minute band in sunspot umbrae the mean delay is 55s for the SiI 10827{\AA}-HeI 10830{\AA} pair; for the Fe I 6569{\AA}-H-alpha 6563{\AA} pair it was not possible to determine the delay; for the FeI 4551{\AA}-BaII 4554{\AA} pair the mean delay is 6s; for the SiI 8536{\AA}-CaII 8542{\AA} pair the mean delay is 21s. Measured delays correspond to the wave propagation speed which significantly exceeds the generally adopted speed of sound in the photosphere. This raises the question of the origin of these oscillations. The possibility that we deal with slow MHD waves is not ruled out.