Global Sausage Oscillation of Solar Flare Loops Detected by the Interface Region Imaging Spectrograph

Abstract

An observation from the Interface Region Imaging Spectrograph reveals coherent oscillations in the loops of an M1.6 flare on 2015 March 12. Both the intensity and Doppler shift of Fe xxi 1354.08Å show clear oscillations with a period of ∼25 seconds. Remarkably similar oscillations were also detected in the soft X-ray flux recorded by the Geostationary Operational Environmental Satellites (GOES). With an estimated phase speed of ∼2420 km s −1 and a derived electron density of at least 5.4×10 10 cm −3 , the observed short-period oscillation is most likely the global fast sausage mode of a hot flare loop. We find a phase shift of ∼π/2 (1/4 period) between the Doppler shift oscillation and the intensity/GOES oscillations, which is consistent with a recent forward modeling study of the sausage mode. The observed oscillation requires a density contrast between the flare loop and coronal background of a factor 42. The estimated phase speed of the global mode provides an lower limit of the Alfvén speed outside the flare loop. We also find an increase of the oscillation period, which might be caused by the separation of the loop footpoints with time.

Detection and Interpretation of Long-lived X-Ray Quasi-periodic Pulsations in the X-class Solar Flare on 2013 May 14

“…It is not clear if that can be achieved with the current observations although the measurement of Doppler shift oscillations with IRIS (Tian et al 2016) certainly helps. At present, only constraints can be applied based on evaluating various assumed oscillating modes and comparing predicted QPP properties with observations.…”

“…However, the sausage mode is not ruled out as the source of the measured oscillations. It needs to be further explored, especially in light of the recent IRIS observations reported by Tian et al (2016) that strongly support the sausage mode interpretation.…”

“…Their characteristics would depend on the type of MHD waves (e.g. kink modes, fast sausage modes (Guo et al 2016;Tian et al 2016), or slow modes) and on the properties of magnetic loops in which they propagate, their length, density, magnetic field strength, etc. A combination of these two scenarios is also possible but it might be expected that the time scales and relevant length scales would be quite different in the two cases -multiple bursts of energy released in a current sheet are expected to be on shorter (sub-second to second) time scales and (sub-arcsecond) length scales than the characteristic scales of MHD oscillations in a magnetic loop (e.g.…”

Quasi-Periodic Pulsations During the Impulsive and Decay Phases of an X-Class Flare

“…Recent studies have attributed persistent SXR QPPs to that of compressive MHD modes such as the global fast sausage mode (Tian et al 2016) and vertical kink mode (Dennis et al 2016). Both of these compressive modes would result in the observed pulsations in the decay phase of this flare.…”

Impulsively Generated Wave Trains in Coronal Structures. I. Effects of Transverse Structuring on Sausage Waves in Pressureless Tubes

“…A substantial fraction of these measurements were made in optical passbands at total eclipses, and the measured quasiperiods range from 0.5 − 4 seconds (e.g., Pasachoff & Landman 1984;Pasachoff & Ladd 1987) to 4 − 7 seconds (Williams et al 2001(Williams et al , 2002Katsiyannis et al 2003) to 6 − 25 seconds (Samanta et al 2016). In other passbands, quasi-periodic pulsations (QPPs) with similar periods have been found in the lightcurves of solar flares as measured with such instruments as the Nobeyama Radioheliograph (NoRH, e.g., Asai et al 2001;Nakariakov et al 2003;Melnikov et al 2005;Kupriyanova et al 2013), SDO/AIA (e.g., Su et al 2012), and more recently with the Interface Region Imaging Spectrograph (IRIS, Tian et al 2016). Much of the past and ongoing efforts in observing and understanding QPPs is summarized in the recent reviews by Nakariakov & Melnikov (2009) and Van Doorsselaere et al (2016).…”