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Register a new userThe dynamics of AR 12700 in its early emerging phase I: interchange reconnection
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The emergence of active regions (ARs) leads to various dynamic activities. Using high-resolution and long-lasting H$alpha$ observations from the New Vacuum Solar Telescope, we report the dynamics of NOAA AR 12700 in its emerging phase on 26 February 2018 in detail. In this AR, constant interchange reconnections between emerging fibrils and preexisting ones were detected. Driven by the flux emergence, small-scale fibrils observed in H$alpha$ wavelength continuously emerged at the center of the AR and reconnected with the ambient preexisting fibrils, forming new longer fibrils. We investigate three scenarios of such interchange reconnection in two hours. Specially, the third scenario of reconnection resulted in the formation of longer fibrils that show pronounced rotation motion. To derive the evolution of the magnetic structure during the reconnections, we perform nonlinear force-free field extrapolations. The extrapolated three-dimensional magnetic fields clearly depict a set of almost potential emerging loops, two preexisting flux ropes at 03:00 UT before the second reconnection scenario, and a set of newly formed loops with less twist at 03:48 UT after the third reconnection scenario. All of these extrapolated structures are consistent with the fibrils detected in H$alpha$ wavelength. The aforementioned observations and extrapolation results suggest that the constant interchange reconnections resulted in that the magnetic twist was redistributed from preexisting flux ropes towards the newly-formed system with longer magnetic structure and weaker twist.
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The emergence of active regions (ARs) closely relates to the solar dynamo and the dynamical atmospheric phenomena.With high-resolution and long-lasting observations from the New Vacuum Solar Telescope, we report a new dynamic activity phenomenon named fan-shaped activity (FSA) in the emerging phase of NOAA AR 12700. The FSAs are clearly observed at Ha wavelength and are closely related to the dynamics of the adjacent arch filament system (AFS), including threads deformation and materials downward motions. On 2018 February 26, the two most representative FSAs appeared around 05:21 UT and 06:03 UT, respectively, and they firstly ascended and then decayed in around 10 minutes. At the ascending phase, accompanied by the uplifting of an adjacent AFS, each FSA launches up at one end of the AFS and extends for up to 11 Mm. At the decaying phase, the FSA gradually vanishes, and materials downflows towards the other end of the AFS are detected. After checking the evolution of the magnetic fields of AR 12700, we find that each FSA is located between the end of an AFS and an adjacent magnetic patch with the same polarity and launches at the onset of the collision and compression between these two magnetic patches. We propose that the collision lifts up the AFS, and then the initially compact AFS laterally expands, resulting in the formation of FSA. A cartoon model is proposed to depict the activities.
Magnetic reconnection at the interface between coronal holes and loops, so-called interchange reconnection, can release the hotter, denser plasma from magnetically confined regions into the heliosphere, contributing to the formation of the highly variable slow solar wind. The interchange process is often thought to develop at the apex of streamers or pseudo-streamers, near Y and X-type neutral points, but slow streams with loop composition have been recently observed along fanlike open field lines adjacent to closed regions, far from the apex. However, coronal heating models, with magnetic field lines shuffled by convective motions, show that reconnection can occur continuously in unipolar magnetic field regions with no neutral points: photospheric motions induce a magnetohydrodynamic turbulent cascade in the coronal field that creates the necessary small scales, where a sheared magnetic field component orthogonal to the strong axial field is created locally and can reconnect. We propose that a similar mechanism operates near and around boundaries between open and closed regions inducing a continual stochastic rearrangement of connectivity. We examine a reduced magnetohydrodynamic model of a simplified interface region between open and closed corona threaded by a strong unipolar magnetic field. This boundary is not stationary, becomes fractal, and field lines change connectivity continuously, becoming alternatively open and closed. This model suggests that slow wind may originate everywhere along loop-coronal hole boundary regions, and can account naturally and simply for outflows at and adjacent to such boundaries and for the observed diffusion of slow wind around the heliospheric current sheet.
Using extreme-ultraviolet images, we recently proposed a new and alternative formation mechanism for coronal rain along magnetically open field lines due to interchange magnetic reconnection. In this paper we report coronal rain at chromospheric and transition region temperatures originating from the coronal condensations facilitated by reconnection between open and closed coronal loops. For this, we employ the Interface Region Imaging Spectrograph (IRIS) and the Atmospheric Imaging Assembly (AIA) of the Solar Dynamics Observatory (SDO). Around 2013 October 19, a coronal rain along curved paths was recorded by IRIS over the southeastern solar limb. Related to this, we found reconnection between a system of higher-lying open features and lower-lying closed loops that occurs repeatedly in AIA images. In this process, the higher-lying features form magnetic dips. In response, two sets of newly reconnected loops appear and retract away from the reconnection region. In the dips, seven events of cooling and condensation of coronal plasma repeatedly occur due to thermal instability over several days, from October 18 to 20. The condensations flow downward to the surface as coronal rain, with a mean interval between condensations of 6.6 hr. In the cases where IRIS data were available we found the condensations to cool all the way down to chromospheric temperatures. Based on our observations we suggest that some of the coronal rain events observed at chromospheric temperatures could be explained by the new and alternative scenario for the formation of coronal rain, where the condensation is facilitated by interchange reconnection.
The cold-dense plasma is occasionally detected in the solar wind with in situ data, but the source of the cold-dense plasma remains illusive. Interchange reconnections (IRs) between closed fields and nearby open fields are well known to contribute to the formation of solar winds. We present a confined filament eruption associated with a puff-like coronal mass ejection (CME) on 2014 December 24. The filament underwent successive activations and finally erupted, due to continuous magnetic flux cancellations and emergences. The confined erupting filament showed a clear untwist motion, and most of the filament material fell back. During the eruption, some tiny blobs escaped from the confined filament body, along newly-formed open field lines rooted around the south end of the filament, and some bright plasma flowed from the north end of the filament to remote sites at nearby open fields. The newly-formed open field lines shifted southward with multiple branches. The puff-like CME also showed multiple bright fronts and a clear southward shift. All the results indicate an intermittent IR existed between closed fields of the confined erupting filament and nearby open fields, which released a portion of filament material (blobs) to form the puff-like CME. We suggest that the IR provides a possible source of cold-dense plasma in the solar wind.
We present a database of 3137 solar flare ribbon events corresponding to every flare of GOES class C1.0 and greater within 45 degrees from the central meridian, from April 2010 until April 2016, observed by the emph{Solar Dynamics Observatory}. For every event in the database, we compare the GOES peak X-ray flux with corresponding active-region and flare-ribbon properties. We find that while the peak X-ray flux is not correlated with the active region unsigned magnetic flux, it is strongly correlated with the flare ribbon reconnection flux, flare ribbon area, and the fraction of active region flux that undergoes reconnection. We find the relationship between the peak X-ray flux and the flare ribbon reconnection flux to be $I_mathrm{X,peak} propto Phi_mathrm{ribbon}^{1.5}$. This scaling law is consistent with earlier hydrodynamic simulations of impulsively heated flare loops. Using the flare reconnection flux as a proxy for the total released flare energy $E$, we find that the occurrence frequency of flare energies follows a power-law dependence: $dN/dE propto E^{-1.6}$ for $10^{31}<E<10^{33}$ erg, consistent with earlier studies of solar and stellar flares. The database is available online and can be used for future quantitative studies of flares.
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