The current proposal builds upon the success of ISSI team 314 (see webpage). The aim of this proposal is to advance our understanding of the global structure of prominences and the mechanisms responsible for their destabilization. Recent studies have shown that the very common Large-Amplitude Oscillations (LAOs) in prominences open a new window into prominence structure by means of large-amplitude prominence seismology, which combines observations and theoretical modelling of LAOs. Using this technique, key physical properties of prominences can be inferred that are not accessible through other approaches. In addition, many filament eruptions are observed to be preceded or accompanied by LAOs, for reasons that remain obscure. The expected achievements of this new international Team are to understand: a) the global evolution of the morphology of solar filaments over the solar cycle, by continuing the in-progress survey and analysis of LAOs in solar cycle 24, b) the mechanisms responsible for triggering LAOs, and c) the internal processes in an eruption, by studying the relation of such processes with LAOs.
Question 1: What are LAOs telling us about filament magnetic structure over the solar cycle?
With LAOs we can infer: the orientation of the magnetic field with respect to the polarity inversion line, which measures the magnetic shear of the filament channel and, hence, the maximum energy that can be released in a solar eruption; the curvature of the field lines that support the heavy cool prominence plasma; and the minimum magnetic- field strength. Moreover, the damping mechanisms of LAOs can be related to the processes of mass accretion, resonant absorption, and aerodynamic drag.
Our team of experts will continue the ongoing survey and analysis of LAOs using data from the GONG network by focusing on the minimum (2010) and maximum (2014) years of the last solar cycle. We will extract from the LAOs the different features of the filaments at two phases of the cycle, compare the results with models of global evolution of filaments (e.g., Mackay 2015; Hao et al. 2015), and interpret the implications of the study for the formation and evolution of filament channels during the solar cycle.
Question 2: How are LAOs triggered in erupting prominences?
In some cases, the driver of the oscillation is unclear but does not appear to be external. In this situation, the oscillations could represent instabilities associated with internal changes in the prominence structure. For example, Vršnak (1990) proposed that a jump between two different metastable equilibrium configurations occurs as the prominence slowly rises, followed by oscillations around the second equilibrium position. Alternatively, the slow pre-eruptive evolution might lead the prominence to reach an unstable state manifested by growing oscillations (“overstability”). In other cases (e.g., Chen et al. 2008), enhanced activity such as repetitive jets apparently triggers prominence oscillations in the pre-erupting phase. It has also been suggested that the LAOs themselves destabilize the prominence, but it is unclear whether this mechanism is viable in such a low-b environment. Further investigation of these and other potential triggers clearly is needed.
The team will use the survey of new LAO events to find erupting prominences and analyze the observations. So far in the survey, we have identified several events where LAOs are followed by a filament eruption. The new observations also will help us refine existing theoretical models or devise new ones. The team members will discuss which models should be studied, in light of existing and new observational evidence. Then we will devise definitive analytic and numerical tests of the selected models using analytic methods and our state-of-the-art MHD codes: MPI-AMRVAC (Keppens et al. 2012), MFE (Fan 2012), MANCHA3D (Luna et al. 2016), MoLMHD (Terradas et al. 2016), and ARMS (Karpen et al. 2012). For example, a 3D prominence simulated by MPI-AMRVAC (Xia et al. 2014) could be driven to oscillate by pressure pulses of increasing strength injected at one footpoint, to determine whether the prominence structure changes, erupts, or resumes its initial equilibrium.
Question 3: How are the eruption and the associated structural changes manifested in LAOs?
Because the LAO signatures are intrinsically tied to the magnetic field and plasma properties, they can yield valuable clues about the changing morphology and perhaps the destabilization process. As discussed in the Scientific Rationale, the oscillation period and even the amplitude have been observed to change during some eruptions, which may be associated with changes in the magnetic restoring forces on the prominence structure. For events in which the changes are quasi-static, one can apply the current LAO models to infer the structural changes of the structure. However, if the structure changes rapidly enough, the LAOs will be nonlinearly coupled with the evolution of the prominence, and a more complicated approach is required. The team will carry out direct MHD simulations of the eruption of prominence hosting coronal flux ropes through various triggering mechanisms, including e.g. the onset of kink/torus instabilities, flux cancelation and tether-cutting reconnections, to study how LAOs can be induced and evolve, and how they relate to the changing structure of the destabilized flux rope.
The team members will determine the influence of the process of destabilization on the LAO period, oscillation amplitude, polarization, and other parameters through a combination of event analyses, analytical theory, and numerical simulations. As in Q2 we will simulate the eruption of prominences. Once the prominence starts to rise we will induce LAOs studying oscillation parameters during the eruption. By understanding the time-dependent effects on the LAO parameters, we can infer the structural changes of the filament. Our goal is to create a large-amplitude seismology tool capable of probing the internal morphology of erupting prominences. The team will apply the new seismological tool to the new events found in Q1 and to those already identified. In turn, we expect the results of these analyses to reveal new insights into the fundamental eruption mechanism or mechanisms, one of the top goals in contemporary heliophysics and space-weather research.