Energetics of magnetic transients in a solar active region plage

Abstract

Context. Densely packed coronal loops are rooted in photospheric plages in the vicinity of active regions on the Sun. The photospheric magnetic features underlying these plage areas are patches of mostly unidirectional magnetic field extending several arcsec on the solar surface. Aims. We aim to explore the transient nature of the magnetic field, its mixed-polarity characteristics, and the associated energetics in the active region plage using high spatial resolution observations and numerical simulations. Methods. We used photospheric Fe i 6173 Å spectropolarimetric observations of a decaying active region obtained from the Swedish 1-m Solar Telescope (SST). These data were inverted to retrieve the photospheric magnetic field underlying the plage as identified in the extreme-ultraviolet emission maps obtained from the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory (SDO). To obtain better insight into the evolution of extended unidirectional magnetic field patches on the Sun, we performed 3D radiation magnetohydrodynamic simulations of magnetoconvection using the MURaM code. Results. The observations show transient magnetic flux emergence and cancellation events within the extended predominantly unipolar patch on timescales of a few 100 s and on spatial scales comparable to granules. These transient events occur at the footpoints of active region plage loops. In one case the coronal response at the footpoints of these loops is clearly associated with the underlying transient. The numerical simulations also reveal similar magnetic flux emergence and cancellation events that extend to even smaller spatial and temporal scales. Individual simulated transient events transfer an energy flux in excess of 1 MW m −2 through the photosphere.Conclusions. We suggest that the magnetic transients could play an important role in the energetics of active region plage. Both in observations and simulations, the opposite-polarity magnetic field brought up by transient flux emergence cancels with the surrounding plage field. Magnetic reconnection associated with such transient events likely conduits magnetic energy to power the overlying chromosphere and coronal loops.

Power spectra of solar brightness variations at various inclinations

“…This explanation in reasonably, given that cancellation events have been observed on the solar surface in active regions (e.g. Green et al 2011, Tiwari et al 2019, in active region plage (Chitta et al 2019) and even in quiet Sun regions (Wang Jingxiu et al 1988).…”

Extreme-ultraviolet bursts and nanoflares in the quiet-Sun transition region and corona

“…8a). It is possible that a small-scale weak opposite-polarity magnetic element is present, but remained undetected in HMI observations as a result of a combination of the moderate spatial resolution and sensitivity of the instrument (e.g., Chitta et al 2017bChitta et al , 2019. It is also possible that the burst is triggered by a component magnetic reconnection within the unipolar magnetic patch.…”

Impulsive coronal heating during the interaction of surface magnetic fields in the lower solar atmosphere

“…Our choice of time-averaging the magnetic field only over the initial 30 min ensures that we do not misclassify unipolar cases as mixed cases in events when new flux emergence brings up persistent opposite polarity in the region after the heating event peak. At the same time we might underestimate the number of mixed cases when the opposite polarity is weak and transient in the first 30 min window (see examples of transient opposite-polarity patches in unipolar regions presented in Chitta et al 2019).…”

“…Here we conducted an extensive statistical analysis of 137 impulsive heating events from seven different active regions covering 404 h of solar evolution and found that about 73% of cases (101 out of 137) have mixed polarity magnetic fields with flux content over 10 17 Mx at the footpoints of hot loops. Due to the limited spatial resolution and sensitivity of the HMI instrument, the number of mixed events is likely to be an underestimate as shown in examples where the mixed polarity nature of the fields remains hidden to HMI observation (Chitta et al 2017a(Chitta et al , 2019.…”

“…On the Sun, flux emergence could naturally lead to such conditions when magnetic fragments of one polarity from an emerging flux feature approach and interact with the ambient magnetic field of opposite polarity (e.g. Chitta et al 2019). Initially, the fragments are so far apart that they are not linked magnetically.…”