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Register a new userLarge amplitude oscillatory motion along a solar filament
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Large amplitude oscillations of solar filaments is a phenomenon known for more than half a century. Recently, a new mode of oscillations, characterized by periodical plasma motions along the filament axis, was discovered. We analyze such an event, recorded on 23 January 2002 in Big Bear Solar Observatory H$alpha$ filtergrams, in order to infer the triggering mechanism and the nature of the restoring force. Motion along the filament axis of a distinct buldge-like feature was traced, to quantify the kinematics of the oscillatory motion. The data were fitted by a damped sine function, to estimate the basic parameters of the oscillations. In order to identify the triggering mechanism, morphological changes in the vicinity of the filament were analyzed. The observed oscillations of the plasma along the filament was characterized by an initial displacement of 24 Mm, initial velocity amplitude of 51 km/s, period of 50 min, and damping time of 115 min. We interpret the trigger in terms of poloidal magnetic flux injection by magnetic reconnection at one of the filament legs. The restoring force is caused by the magnetic pressure gradient along the filament axis. The period of oscillations, derived from the linearized equation of motion (harmonic oscillator) can be expressed as $P=pisqrt{2}L/v_{Aphi}approx4.4L/v_{Aphi}$, where $v_{Aphi} =B_{phi0}/sqrt{mu_0rho}$ represents the Alfven speed based on the equilibrium poloidal field $B_{phi0}$. Combination of our measurements with some previous observations of the same kind of oscillations shows a good agreement with the proposed interpretation.
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On 20 August 2010 an energetic disturbance triggered damped large-amplitude longitudinal (LAL) oscillations in almost an entire filament. In the present work we analyze this periodic motion in the filament to characterize the damping and restoring mechanism of the oscillation. Our method involves placing slits along the axis of the filament at different angles with respect to the spine of the filament, finding the angle at which the oscillation is clearest, and fitting the resulting oscillation pattern to decaying sinusoidal and Bessel functions. These functions represent the equations of motion of a pendulum damped by mass accretion. With this method we determine the period and the decaying time of the oscillation. Our preliminary results support the theory presented by Luna and Karpen (2012) that the restoring force of LAL oscillations is solar gravity in the tubes where the threads oscillate, and the damping mechanism is the ongoing accumulation of mass onto the oscillating threads. Following an earlier paper, we have determined the magnitude and radius of curvature of the dipped magnetic flux tubes hosting a thread along the filament, as well as the mass accretion rate of the filament threads, via the fitted parameters.
On 20 August 2010 an energetic disturbance triggered large-amplitude longitudinal oscillations in a nearby filament. The triggering mechanism appears to be episodic jets connecting the energetic event with the filament threads. In the present work we analyze this periodic motion in a large fraction of the filament to characterize the underlying physics of the oscillation as well as the filament properties. The results support our previous theoretical conclusions that the restoring force of large-amplitude longitudinal oscillations is solar gravity, and the damping mechanism is the ongoing accumulation of mass onto the oscillating threads. Based on our previous work, we used the fitted parameters to determine the magnitude and radius of curvature of the dipped magnetic field along the filament, as well as the mass accretion rate onto the filament threads. These derived properties are nearly uniform along the filament, indicating a remarkable degree of cohesiveness throughout the filament channel. Moreover, the estimated mass accretion rate implies that the footpoint heating responsible for the thread formation, according to the thermal nonequilibrium model, agrees with previous coronal heating estimates. We estimate the magnitude of the energy released in the nearby event by studying the dynamic response of the filament threads, and discuss the implications of our study for filament structure and heating.
Although the behavior of fluid-filled vesicles in steady flows has been extensively studied, far less is understood regarding the shape dynamics of vesicles in time-dependent oscillatory flows. Here, we investigate the nonlinear dynamics of vesicles in large amplitude oscillatory extensional (LAOE) flows using both experiments and boundary integral (BI) simulations. Our results characterize the transient membrane deformations, dynamical regimes, and stress response of vesicles in LAOE in terms of reduced volume (vesicle asphericity), capillary number ($Ca$, dimensionless flow strength), and Deborah number ($De$, dimensionless flow frequency). Results from single vesicle experiments are found to be in good agreement with BI simulations across a wide range of parameters. Our results reveal three distinct dynamical regimes based on vesicle deformation: pulsating, reorienting, and symmetrical regimes. We construct phase diagrams characterizing the transition of vesicle shapes between pulsating, reorienting, and symmetrical regimes within the two-dimensional Pipkin space defined by $De$ and $Ca$. Contrary to observations on clean Newtonian droplets, vesicles do not reach a maximum length twice per strain rate cycle in the reorienting and pulsating regimes. The distinct dynamics observed in each regime result from a competition between the flow frequency, flow time scale, and membrane deformation timescale. By calculating the particle stresslet, we quantify the nonlinear relationship between average vesicle stress and strain rate. Additionally, we present results on tubular vesicles that undergo shape transformation over several strain cycles. Broadly, our work provides new information regarding the transient dynamics of vesicles in time-dependent flows that directly informs bulk suspension rheology.
From recent high resolution observations obtained with the Swedish 1 m Solar Telescope in La Palma, we detect swaying motions of individual filament threads in the plane of the sky. The oscillatory character of these motions are comparable with oscillatory Doppler signals obtained from corresponding filament threads. Simultaneous recordings of motions in the line of sight and in the plane of the sky give information about the orientation of the oscillatory plane. These oscillations are interpreted in the context of the magnetohydrodynamic theory. Kink magnetohydrodynamic waves supported by the thread body are proposed as an explanation of the observed thread oscillations. On the basis of this interpretation and by means of seismological arguments, we give an estimation of the thread Alfven speed and magnetic field strength by means of seismological arguments.
We examine the nonequilibrium production of topological defects -- braids -- in semiflexible filament bundles under cycles of compression and tension. During these cycles, the period of compression facilitates the thermally activated pair production of braid/anti-braid pairs, which then may separate when the bundle is under tension. As result, appropriately tuned alternating periods of compression and extension should lead to the proliferation of braid defects in a bundle so that linear density of these pairs far exceeds that expected in thermal equilibrium. Secondly, we examine the slow extension of braided bundles under tension, showing that their end-to-end length creeps nonmonotonically under a fixed force due to braid deformation and the motion of the braid pair along the bundle. We conclude with a few speculations regarding experiments on semiflexible filament bundles and their networks.
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