LAO seismology is one of the best tools to understand the fundamental physics behind prominence structure and the associated plasma distribution. The goals of the proposed team are to advance our understanding of how LAOs are excited, driven, and damped, and to utilize this knowledge to refine and apply diagnostics that reveal prominence properties and behaviour inaccessible to other methods. The specific questions to be addressed, and the methodologies to be used, are described below. Our team is composed of experts on different aspects of observational and theoretical solar physics, all of which are needed to cover the complexity of the problems being investigated. The proposed work relies heavily on close collaboration between observers and theorists: the observers will provide “ground truth” to the theorists through the detailed observed features of the oscillating threads and their environment, while the theorists will provide the LAOs’ characteristic signatures and definitive model tests for the observers to search for.
Question 1: What are the observed characteristics of LAOs?
The number of reported observations of transverse and longitudinal LAOs is relatively small in comparison with the reported cases of small-amplitude oscillations. In order to better characterize and understand the LAO events, we clearly need to increase the number of identified and analysed events to form a statistically significant dataset. We are encouraged by our preliminary explorations of data from the Atmospheric Imaging Assembly (AIA) instrument on-board the SDO, which show that several LAO events can be found on the same day.
The first task to be addressed by the team is defining a protocol to identify and classify the LAOs. Large quantities of high-cadence, high-resolution data are available from both ground- and space-based telescopes such as AIA/SDO. However, the emission pattern of the corona is very complex and detecting periodic motions can be difficult. Therefore we will use full-disk data from the Hα filter telescopes in the international GONG network to constrain the areas and time spans to analyse. The filaments are clearly seen in Hα and detecting motions is easy; however, the GONG spatial resolution and cadence are low. Once the oscillating prominences are identified in the Hα observations, we will analyse the LAOs with the AIA/SDO images. The analysis of the data will give final answer to the oscillation properties, trigger locations, etc. The data of such a telescopes are available for all the public and the data are accessible online.
Question 2: Which is the restoring force and damping mechanism of LAOs?
The periods and damping times are the most important parameters of the observed oscillations because they reflect the physics responsible for driving and stopping the motions. The restoring force of the system determines essentially the period. Several possible restoring forces are described in the literature, including magnetic tension, magnetic pressure, gas pressure, and gravity. In the few LALO events analysed thus far, the damping is very strong; however, our expanded database will tell us whether or not this is a common characteristic.
We will review all relevant restoring forces and damping mechanisms, whether previously published or new, and study their applicability to both types of LAOs. We will evaluate each candidate with numerical simulations in 2D and 3D scenarios and with analytical models to determine the dominant restoring force and damping mechanism. The results will be tested by comparison with the observational data. Note that the LAOs involve motions with velocities larger than the characteristic speeds, such as the sound speed. Therefore, in order to ensure that the models and simulations include the important physical processes, we need to establish whether the LAOs are predominantly in the linear or in the non-linear regime.
Once the dominant restoring force and damping mechanism are determined, we will relate the period and damping time of the oscillation with key physical properties of the filament channel and prominence plasma such as the geometry of the supporting magnetic field, prominence plasma density, and magnetic field strength. This is a key objective of prominence seismology.
Question 3: How and why are LAOs triggered?
The excitation of LATOs is associated with EIT/Moreton waves produced by distant flares or CMEs, whereas the LALOs appear to be excited by jets produced by microflares close to the filament. Understanding how these disturbances propagate and reach the filaments is crucial to understanding why the different disturbances excite different LAO types Furthermore, the mechanisms by which the remotely released energy is transferred to kinetic energy of prominence threads is far from clear.
To investigate these issues, we need to combine our knowledge of the spatial and temporal evolution of selected LAO events (Question 1) with detailed estimates of the 3D coronal magnetic field connecting the triggers with the oscillating prominences. We will use photospheric magnetic-field observations from the Heliospheric Magnetic Imager (HMI) instrument on-board SDO as the basis for non-linear force free field extrapolations (NLFFF) of the coronal magnetic field. The team members will jointly select a few well-observed LATO and LALO events for this task. This will allow us to establish the magnetic context, and to understand how the EIT/Moreton waves and jets (or other triggers) are channeled to the filaments and produce the subsequent LAO. The NLFFF extrapolation will enable us to model the oscillating magnetic structure and its surroundings, which will add further realism to the calculations for Question 1. In addition, the magnetic field properties inferred from prominence seismology, as described above, will be compared with the results of the magnetic extrapolations, to validate or rule out theoretical models.