Abstract
<p>During Solar Minimum, the Sun is perceived to be quite inactive with barely any spots emerging on the solar surface. Consequently, we observe a drop in the number of highly energetic events such as solar flares and coronal mass ejections (CMEs), which are often associated with active regions on the photosphere. However, our magnetofrictional simulations during the minimum period suggest that the solar corona could still be significantly dynamic while evolving in response to the large-scale shearing velocities on the solar surface. The non-potential evolution of the corona leads to the accumulation of magnetic free energy and helicity, which is periodically lost through eruptive events. Our study shows that these events can be categorised into two distinct classes. One set of events are caused due to full-scale eruption of low-lying coronal flux ropes and could be associated with occasional filament erupting CMEs observed during Solar Minimum. The other set of events are not driven by destabilisation of low-lying structures but rather by eruption from overlying sheared arcades. These could be linked with streamer blowouts or stealth CMEs. The two classes differ considerably in the amount of magnetic flux and helicity shed through the outer coronal boundary. We additionally investigate how other measurables such as current, open magnetic flux, free energy, coronal holes area, and the horizontal component of the magnetic field on the outer model boundary vary during the two classes of event. This study demonstrates and emphasises the importance and necessity of understanding the dynamics of the coronal magnetic field during Solar Minimum.</p>
Highlights
The magnetic-field structure in the solar corona is primarily governed by evolution of the photospheric magnetic field
We focus on the temporal evolution of global measures such as the free magnetic energy and current associated with the build-up of non-potentiality within the corona
Both quantities are strongly correlated and have a general decreasing trend beyond the initial rising phase. This initial phase arises because the initial potential coronal magnetic field requires a certain amount of “ramp time” to form non-potential structures
Summary
The magnetic-field structure in the solar corona is primarily governed by evolution of the photospheric magnetic field. Most earlier research with the magnetofriction model aimed to study the formation and evolution of non-potential structures such as flux ropes in the solar corona, and their sudden eruption. Dense chromospheric plasma, which is much cooler than the 1 MK hot corona, stays confined by the helical magnetic-field lines of the associated flux rope They appear as dark elongated structures against the bright solar disk when observed in the Hα absorption line. During solar minimum, when there is little new sunspot emergence, a significant number of filaments (as high as 200 in a year: Hao et al, 2015) can be observed at higher latitudes (> 30◦) These are known as polarcrown filaments (Parenti, 2014), and they comprise quiescent filaments forming over long neutral lines passing across the diffused and weak magnetic-field distribution.
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