Propagating Waves Transverse to the Magnetic Field in a Solar Prominence

Abstract

We report an unusual set of observations of waves in a large prominence pillar which consist of pulses propagating perpendicular to the prominence magnetic field. We observe a huge quiescent prominence with the Solar Dynamics Observatory (SDO) Atmospheric Imaging Assembly (AIA) in EUV on 2012 October 10 and only a part of it, the pillar, which is a foot or barb of the prominence, with the Hinode Solar Optical Telescope (SOT) (in Ca II and Hα lines), Sac Peak (in Hα, Hβ and Na-D lines), THEMIS ("Télescope Héliographique pour l' Etude du Magnétisme et des Instabilités Solaires") with the MTR (MulTi-Raies) spectropolarimeter (in He D 3 line). The THEMIS/MTR data indicates that the magnetic field in the pillar is essentially horizontal and the observations in the optical domain show a large number of horizontally aligned features on a much smaller scale than the pillar as a whole. The data is consistent with a model of cool prominence plasma trapped in the dips of horizontal field lines. The SOT and Sac Peak data over the 4 hour observing period show vertical oscillations appearing as wave pulses. These pulses, which include a Doppler signature, move vertically, perpendicular to the field direction, along thin quasi-vertical columns in the much broader pillar. The pulses have a velocity of propagation of about 10 km s −1 , a period about 300 sec, and a wavelength around 2000 km. We interpret these waves in terms of fast magneto-sonic waves and discuss possible wave drivers.

Spectral Characteristics of the He i D3 Line in a Quiescent Prominence Observed by THEMIS

“…A recent multi-instrumental study involving He i D3 by Levens et al (2016b) reported finding a horizontal magnetic field in the legs of a tornado-like prominence. Schmieder et al (2013) also found mostly horizontal magnetic fields by He i D3 in a prominence pillar showing signatures of magnetosonic waves propagating transverse to the magnetic field with a velocity of about 10 km s −1 , a period of about 300 s, and a wavelength of around 2 000 km. The low signal-to-noise ratio of the He i D3 polarization signatures was interpreted in Schmieder et al (2014) as a possible manifestation of a turbulent field superimposed on the macroscopic horizontal component of the prominence magnetic field.…”

“…a grid mask with three rectangular windows placed at the F1 focus of the telescope before the polarization analyzer (Semel, 1980;Briand and Ceppatelli, 2002). A rationale for its use and the necessity of 2D scanning in directions parallel (X) and perpendicular (Y) to the slit were clarified in Sainz Dalda and López Ariste (2007) and Schmieder et al (2013). The 2D scanning of the target prominence started close to the limb at 14:59:02 UT and progressed radially away from the limb untill about 17:00 UT.…”

“…One particular THEMIS spectrum at a given slit position is taken with the exposure time of 2 s, and overall, the observing procedure is the same as in Schmieder et al (2013Schmieder et al ( , 2014 and Levens et al (2016a,b). We emphasize that in our case a modulation cycle of six images, spanning the three polarization states with either positive or negative signs, is repeated ten times to increase the signal-to-noise ratio.…”

“…The primary reduction is the same as applied in Schmieder et al (2013Schmieder et al ( , 2014 and Levens et al (2016a,b), and it involves data from all ten modulation cycles. The raw THEMIS spectra are reduced using the IDL package DeepStokes, whose main characteristics are outlined in López .…”

“…The data reduction includes geometry corrections of inclination and curvature of spectral lines, dark current and bias subtraction, flat-field correction, and a careful handling of the polarization signals. Since the reduction yields Stokes Q, U, V profiles with very small amplitudes, typically 0.2 -0.4% of the Stokes I peak intensity, we repeat the reduction taking data only from five modulation cycles as in Schmieder et al (2013Schmieder et al ( , 2014 and Levens et al (2016a,b), but without improvement. Therefore, we aim the analysis only at the Stokes I profiles.…”