Doppler-shift oscillations in the H i Lyα coronal emission line: spectroscopic signature of propagating kink waves?

¹ Istituto Nazionale di Astrofisica, Osservatorio Astrofisico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy
e-mail: mancuso@oato.inaf.it
² Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MS 50, Cambridge, MA 02138, USA

Received: 1 September 2014
Accepted: 18 November 2014

Abstract

We report the first detection of long-period, slowly decaying Doppler-shift oscillations in the H i Lyα (1215.67 Å) coronal emission line with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard the Solar and Heliospheric Observatory (SOHO) satellite. The UV spectral data were collected at 1.43 R_⊙ above the eastern limb of the Sun during a special high-cadence sit-and-stare observation on 1997 December 14. Time-series analyses with different spectral techniques clearly show highly significant Doppler-shift oscillations in a portion with a size of 154′′ of the UVCS slit that lasted for several cycles. A period of P = 14.3 ± 0.4 min was established with a confidence of better than 99.9% in the Lomb-Scargle periodogram. On average, the Doppler-shift amplitude of 3.7 ± 0.7 km s^-1 was estimated for the most significant oscillations, roughly corresponding to a displacement of 800 ± 150 km. The origin of the regular H i Lyα Doppler-shift oscillations is most probably due to the excitation of propagating fast magnetoacoustic kink waves along a narrow, jet-like ejection observed higher up in the white-light corona. However, different mechanisms, such as low-amplitude coherent kink oscillations of a bundle of loops along the line of sight or quasi-periodic outflows caused by oscillatory magnetic reconnection in the low corona cannot be ruled out.