Hot coronal loop oscillations observed with SUMER: Examples and statistics

Astronomy & Astrophysics, Aug 2003

We give an extensive overview of Doppler shift oscillations in hot active region loops obtained with SUMER. The oscillations have been detected in loops sampled 50-100 arcsec off the limb of the Sun in ultraviolet lines, mainly  and , with formation temperature greater than 6 MK. The spectra were recorded along a 300 arcsec slit placed at a fixed position in the corona above the active regions. Oscillations are usually seen along an extended section of the slit and often appear to be from several different portions of the loops (or from different loops). Different portions are sometimes in phase, sometimes out of phase and sometimes show phase shifts along the slit. We measure physical parameters of 54 Doppler shift oscillations in 27 flare-like events and give geometric parameters of the associated hot loops when soft X-ray (SXR) images are available. The oscillations have periods in the range 7-31 min, with decay times 5.7-36.8 min, and show an initial large Doppler shift pulse with peak velocities up to 200 km s-1. The oscillation periods are on average a factor of three longer than the TRACE transverse loop oscillations. The damping times and velocity amplitude are roughly the same, but the derived displacement amplitude is four or five times larger than the transverse oscillation amplitude measured in TRACE images. Unlike TRACE oscillations, only a small fraction of them are triggered by large flares, and they often recur 2-3 times within a couple of hours. All recurring events show initial shifts of the same sign. These data provide the following evidence to support the conclusion that these oscillations are slow magnetoacoustic standing waves in hot loops: (1) the phase speeds derived from observed periods and loop lengths roughly agree with the sound speed; (2) the intensity fluctuation lags the Doppler shifts by 1/4 period; (3) The scaling of the dissipation time of slow waves with period agrees with the observed scaling for 49 cases. They seem to be triggered by micro- or subflares near a footpoint, as revealed in one example with SXR image observations. However other mechanisms cannot as yet be ruled out. Some oscillations showed phase propagation along the slit in one or both directions with apparent speeds in the range of 8-102 km s-1, together with distinctly different intensity and line width distributions along the slit. These features can be explained by the excitation of the oscillation at a footpoint of an inhomogeneous coronal loop, e.g. a loop with fine structure.

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Hot coronal loop oscillations observed with SUMER: Examples and statistics

Hot coronal loop oscillations observed with SUMER: Examples and statistics T. J. Wang 1 2 S. K. Solanki 1 2 W. Curdt 1 2 D. E. Innes 1 2 I. E. Dammasch 1 2 B. Kliem 0 1 0 Astrophysikalisches Institut Potsdam , An der Sternwarte 16, 14482 Potsdam , Germany 1 Send offprint requests to: T. J. Wang 2 Max-Planck Institut fu ̈r Aeronomie , 37191 Katlenburg-Lindau , Germany We give an extensive overview of Doppler shift oscillations in hot active region loops obtained with SUMER. The oscillations have been detected in loops sampled 50−100 arcsec off the limb of the Sun in ultraviolet lines, mainly Fe  and Fe , with formation temperature greater than 6 MK. The spectra were recorded along a 300 arcsec slit placed at a fixed position in the corona above the active regions. Oscillations are usually seen along an extended section of the slit and often appear to be from several different portions of the loops (or from different loops). Different portions are sometimes in phase, sometimes out of phase and sometimes show phase shifts along the slit. We measure physical parameters of 54 Doppler shift oscillations in 27 flare-like events and give geometric parameters of the associated hot loops when soft X-ray (SXR) images are available. The oscillations have periods in the range 7−31 min, with decay times 5.7−36.8 min, and show an initial large Doppler shift pulse with peak velocities up to 200 km s−1. The oscillation periods are on average a factor of three longer than the TRACE transverse loop oscillations. The damping times and velocity amplitude are roughly the same, but the derived displacement amplitude is four or five times larger than the transverse oscillation amplitude measured in TRACE images. Unlike TRACE oscillations, only a small fraction of them are triggered by large flares, and they often recur 2−3 times within a couple of hours. All recurring events show initial shifts of the same sign. These data provide the following evidence to support the conclusion that these oscillations are slow magnetoacoustic standing waves in hot loops: (1) the phase speeds derived from observed periods and loop lengths roughly agree with the sound speed; (2) the intensity fluctuation lags the Doppler shifts by 1/4 period; (3) The scaling of the dissipation time of slow waves with period agrees with the observed scaling for 49 cases. They seem to be triggered by micro- or subflares near a footpoint, as revealed in one example with SXR image observations. However other mechanisms cannot as yet be ruled out. Some oscillations showed phase propagation along the slit in one or both directions with apparent speeds in the range of 8−102 km s−1, together with distinctly different intensity and line width distributions along the slit. These features can be explained by the excitation of the oscillation at a footpoint of an inhomogeneous coronal loop, e.g. a loop with fine structure. Sun; activity - Sun; corona - Sun; oscillations - Sun; UV radiation - Sun; X-rays; gamma rays 1. Introduction MHD waves are believed to play an important role in the solar corona, e.g. as a possible source for heating of coronal loops. The waves can also be used as a tool to diagnose the physical parameters of the coronal plasma (e.g. Roberts et al. 1984; Nakariakov & Ofman 2001) . Various periodic and quasiperiodic oscillations in radio, visible, EUV, and soft X-ray (SXR) radiation have been observed for decades (e.g. reviews by Aschwanden 1987, 2002; Roberts 2000) . Recently, temporally and spatially resolved transverse and longitudinal oscillations have been detected in coronal loops by the Solar and Heliospheric Observatory (SOHO) and the Transition Region and Coronal Explorer (TRACE). For example, signatures of propagating compressive waves were first observed in coronal loops by the EUV Imaging Telescope (EIT) (Berghmans & Clette 1999) , later confirmed by TRACE observations (De Moortel et al. 2000, 2002a,b,c; Robbrecht et al. 2001) , and identified as propagating slow waves (Nakariakov et al. 2000; Tsiklauri & Nakariakov 2001) . Kink mode oscillations excited by flares in coronal loops were for the first time detected by TRACE in EUV radiation (Aschwanden et al. 1999b; Nakariakov et al. 1999) . An extensive overview and analysis of transverse loop oscillations was presented by Schrijver et al. (2002) and a detailed discussion of the parameters obtained from these observations can be found in Aschwanden et al. (2002) . The observed rapid damping of these transverse oscillations has been explained by anomalously high viscosity or resistivity due to resonant absorption (Nakariakov et al. 1999), or phase mixing (Ofman 2002; Ofman & Aschwanden 2002) . Schrijver & Brown (2000) proposed an alternative mechanism: loop oscillations are caused by rocking motions of the photospheric plasma associated with flares if the loop lies near magnetic nullpoints or separators. They attribute the rapid decay of the oscillation to photospheric (...truncated)


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T. J. Wang, S. K. Solanki, W. Curdt, D. E. Innes, I. E. Dammasch, B. Kliem. Hot coronal loop oscillations observed with SUMER: Examples and statistics, Astronomy & Astrophysics, 2003, pp. 1105-1121, Volume 406, Issue 3, DOI: 10.1051/0004-6361:20030858