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Oscillations of cometary tails: a vortex shedding phenomenon?  

Giuseppe Nistico'   Submitted: 2018-04-04 02:11

Context. During their journey to perihelion, comets may appear in the field-of-view of space-borne optical instruments, showing in some cases a nicely developed plasma tail extending from their coma and exhibiting an oscillatory behaviour. Aims. The oscillations of cometary tails may be explained in terms of vortex shedding because of the interaction of the comet with the solar wind streams. Therefore, it is possible to exploit these oscillations in order to infer the value of the Strouhal number St, which quantifies the vortex shedding phenomenon, and the physical properties of the local medium. Methods. We used the Heliospheric Imager (HI) data of the Solar TErrestrial Relations Observatory (STEREO) mission to study the oscillations of the tails of the comets 2P/Encke and C/2012 S1 (ISON) during their perihelion in Nov 2013, determining the Strouhal numbers from the estimates of the halo size, the relative speed of the solar wind flow and the period of the oscillations. Results. We found that the estimated Strouhal numbers are very small, and the typical value of St∼0.2 would be extrapolated for size of the halo larger than ∼106 km. Conclusions. Despite the vortex shedding phenomenon has not been unambiguously revealed, the findings suggest that some MHD instability process is responsible for the observed behaviour of cometary tails, which can be exploited for probing the physical conditions of the near-Sun region.

Authors: G. Nisticò, V. Vladimirov, V. M. Nakariakov, K. Battams, V. Bothmer
Projects: STEREO

Publication Status: accepted for publication in A&A
Last Modified: 2018-04-04 12:20
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Decaying and decay-less transverse oscillations of a coronal loop  

Giuseppe Nistico'   Submitted: 2013-02-12 10:50

We investigate kink oscillations of loops observed in an active region with the Atmospheric Imaging Assembly (AIA) instrument on board the Solar Dynamics Observatory (SDO) spacecraft before and after a flare. The oscillations were depicted and analysed with time-distance maps, extracted from the cuts taken parallel or perpendicular to the loop axis. Moving loops were followed in time with steadily moving slits. The period of oscillations and its time variation were determined by best-fitting harmonic functions. We show that before and well after the occurrence of the flare, the loops experience low-amplitude decayless oscillations.The flare and the coronal mass ejection associated to it trigger large-amplitude oscillations that decay exponentially in time. The periods of the kink oscillations in both regimes (about 240 s) are similar. An empirical model of the phenomenon in terms of a damped linear oscillator excited by a continuous low-amplitude harmonic driver and by an impulsive high-amplitude driver is found to be consistent with the observations.

Authors: G. Nistico', V. Nakariakov, E. Verwichte
Projects: None

Publication Status: Accepted for publication in A&A
Last Modified: 2013-02-12 10:50
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Oscillations of cometary tails: a vortex shedding phenomenon?
Decaying and decay-less transverse oscillations of a coronal loop

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