Large-Amplitude Longitudial (LAL) Oscillations
High-cadence Hα observations of large-amplitude longitudinal (LAL) oscillations in solar filaments were first reported by Jing et al. 2003. These oscillations consist of rapid motions of the plasma along the filament, with displacements comparable to the filament length. Since then, a few more events have been identified Jing et al. 2006, Vršnak et al 2007, Li & Zhang 2012, Zhang et al 2012, 2013. In all of the observed oscillations the period ranges from 0.7 to 2.7 hours, with velocity amplitudes from 30 to 100 km/s. The accelerations are considerable, in many cases comparable to the solar gravitational acceleration. In addition, the oscillations are always triggered by a small energetic event close to the filament.
Several questions remains opened:
- What is the restoring force of the oscillations?
- What is the damping mechanism?
- What initiates the oscillation?
20 August 2010 LAL Observation
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 LAL oscillation event is clearly visible in Hα and the AIA/SDO filters 171 Å, 193 Å, and 131 Å but harder to discern in the 304 Å passband. The ground-based Hα images are useful for context, but they have lower spatial resolution and cadence than the AIA/SDO data. Therefore we identified and parameterized the oscillations from the EUV images, placing slits centered along the filament at different angles with respect to it. We have chosen this method because it is not possible to identify and track individual threads or features from the images.
We analyzed the oscillations by making time-distance diagrams for angles ranging from 0° to 40° at 1° intervals, for the first 28 positions along the filament. For the remaining 8 positions it was necessary to increase the angular range to 0° - 90° in order to observe the oscillations. From the resulting 1840 time-distance diagrams, we selected the best slit direction for every position along the filament, by choosing the strongest oscillation pattern: a clear dark band sandwiched between 2 distinct bright emission regions, the longest possible time interval over which the oscillations were measurable, and the maximum possible displacement amplitude.
In this movie you can see how the oscillation pattern changes in the time-distance diagrams.
LAL oscillation events
Seems that the LAL event are very common. Below there are several examples:
1-May-2012: In this movie we detect three LAL events the same day at three different filaments in Hα