Main Objective: To quantify how the evolution of small-scale magnetic flux in the solar photosphere affects the plasma contained within an associated filament.
Scientific Justification: Filaments, composed of relatively cool, dense plasma suspended in the hot, tenuous solar corona, are a common feature in the solar atmosphere. Current models suggest that the cool filamentary material is supported by a magnetic structure known as a flux rope which consists of twisted magnetic field lines built up by continuous reconnection of the solar magnetic field. These structures can then support cool chromospheric material at heights of up to 100 Mm. However, it is not clear how this chromospheric material comes to reside in the corona or indeed how the continuous dynamic motion of the magnetic field affects the stability of the structure.
Although they have not yet been observed directly using remote-sensing observations, magnetic flux ropes are believed to be the primary component of solar eruptions. These magnetically twisted structures can form and evolve within the solar environment in a variety of ways, although their formation is primarily attributed to the reconnection of coronal magnetic field around an axial field. Despite the magnetic structure existing at coronal heights, observations have shown that cool chromospheric material is able to remain thermally isolated from the surrounding hot corona in the concave-up portions of the magnetic flux rope.
Two main mechanisms have been proposed to answer the question of how cool plasma can make its way into the hot corona. The first suggests that the plasma is directly injected into the corona by magnetic reconnection occurring low in the solar atmosphere (Priest et al., 1996; Litvinenko & Martin, 1999; Litvinenko, 2007; Okamoto et al., 2010; Litvinenko, 2015). This concept is largely motivated by the connection between the formation of filament channels and flux cancellation (van Ballegooijen & Martens, 1989; Martin, 1998; Wang & Muglach, 2007, 2013). The reconnection can occur at the ends of the flux tubes or across the polarity inversion line (Chae, 2003), and naturally produces unidirectional flows that could explain the presence of fast counter-streaming and buoyant flows in certain filament observations (Zirker, Engvold & Martin, 1998; Alexander et al, 2013; Berger et al, 2011). However, there is a lack of evidence for this mechanism being responsible for the supply of mass in quiescent filaments. The second mechanism suggests that the plasma is levitated into the corona by magnetic reconnection in association with flux cancellation which provides the change in magnetic topology required to lift plasma into the upper atmosphere (van Ballegooijen & Martens, 1989; Litvinenko & Martin, 1999). The atmospheric height at which the process of magnetic reconnection in association with flux cancellation occurs is still under debate, but there is support for reconnection occurring in both the photosphere (Yurchyshyn & Wang, 2001; Bellot Rubio & Beck, 2005) and the chromosphere (Litvinenko & Martin, 1999; Chae et al., 2001; Kim et al., 2001, Litvinenko 2015). However, the ability of this process to lift plasma to heights of ~ 100 Mm into the corona is still unclear.
Recent results by Jenkins et al. (2018) and Jenkins et al. (2019) have shown that the traditionally ignored filamentary material can play a vital role in restraining the loss-of- equilibrium of the flux rope before its eruption under quiet-Sun conditions. Hence, if the levitation mechanism supplies both material and magnetic flux to the host flux rope, such behaviour suggests that there exists a very delicate equilibrium between the amount of material added into the quiescent filament (that acts as an anchor) and the magnetic field being built up by continuous flux emergence and cancellation. In addition, for material to be el- evated to coronal heights, the magnetic field containing the concave-up topology has to be sufficiently strong to liberate plasma from the chromosphere. Under quiescent conditions, it is not immediately obvious how this would be the case. Understanding how filamentary material is added to a quiescent filament and how the local evolution of magnetic flux can affect its stability can provide a vital insight into how these quiescent filaments form and remain stable for extended periods of time. |
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