Satellite observations of reconnection between emerging and pre-existing small-scale magnetic fields

TL;DR

This study uses multi-wavelength satellite observations to investigate small-scale magnetic reconnection during flux emergence in the solar atmosphere, revealing dynamic plasma heating, jet activity, and bi-directional flows.

Contribution

It provides detailed observational evidence of magnetic reconnection processes at small scales in the solar atmosphere during flux emergence.

Findings

Detection of UV bursts and jet activity at emergence sites.

Observation of bi-directional high-velocity flows up to 100 km/s.

Correlation between Doppler velocity, line width, and skewness in UV burst profiles.

Abstract

We report multi-wavelength ultraviolet observations taken with the IRIS satellite, concerning the emergence phase in the upper chromosphere and transition region of an emerging flux region (EFR) embedded in the unipolar plage of active region NOAA 12529. The photospheric configuration of the EFR is analyzed in detail benefitting from measurements taken with the spectropolarimeter aboard the Hinode satellite, when the EFR was fully developed. In addition, these data are complemented by full-disk, simultaneous observations of the SDO satellite, relevant to the photosphere and the corona. In the photosphere, magnetic flux emergence signatures are recognized in the fuzzy granulation, with dark alignments between the emerging polarities, cospatial with highly inclined fields. In the upper atmospheric layers, we identify recurrent brightenings that resemble UV bursts, with counterparts in all coronal passbands. These occur at the edges of the EFR and in the region of the arch filament system (AFS) cospatial to the EFR. Jet activity is also found at chromospheric and coronal levels, near the AFS and the observed brightness enhancement sites. The analysis of the IRIS line profiles reveals the heating of dense plasma in the low solar atmosphere and the driving of bi-directional high-velocity flows with speeds up to 100 km/s at the same locations. Furthermore, we detect a correlation between the Doppler velocity and line width of the Si IV 1394 and 1402 \AA{} line profiles in the UV burst pixels and their skewness. Comparing these findings with previous observations and numerical models, we suggest evidence of several long-lasting, small-scale magnetic reconnection episodes between the emerging bipole and the ambient field. This process leads to the cancellation of a pre-existing photospheric flux concentration of the plage with the opposite polarity flux patch of the EFR. [...]

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Full text

1 Introduction

The advent of the IRIS satellite [1] provided overwhelming evidence of small-scale energy release events that occur in the solar atmosphere. In particular, [2] reported on intense, transient brightenings seen in ultraviolet (UV) images. They consist of compact ($\approx 2^{\prime\prime}$) knots exhibiting a factor of about $10^{3}$ intensity increase in UV lines, with a strong line broadening indicating plasma ejections of $100-200\,\mathrm{km\,s}^{-1}$. These events have been called IRIS bombs or UV bursts [3]. They are usually associated with cospatial canceling opposite-polarity magnetic flux patches in the photosphere, like similar phenomena observed at optical wavelengths, e.g., Ellerman bombs. For this reason, they are likely to occur as a result of small-scale magnetic reconnection at low atmospheric heights.

Indeed, reconnection may take place when an emerging flux region (EFR) interacts with a pre-existing flux system, such as the ambient field of an active region (AR), leading to energy release that heats locally the atmosphere and drives high-temperature plasma flows. This has been predicted in numerical simulations [4], which also show that the relative orientation between the reconnecting flux systems regulates the energetics of the process [5], with the external field acting as a guide for plasma ejections [6]. These results have been observationally confirmed at granular scale as well [7, 8].

Here, we report on recent joint observations performed by the IRIS, Hinode and SDO satellites. They revealed long-lasting brigthenings, similar to UV bursts, in the upper chromosphere, transition region and corona, with simultaneous plasma ejections occurring where an EFR emerged within the plage of AR NOAA 12529 [9, 10].

2 Observations and Data Analysis

AR NOAA 12529 was observed in April 2016 [11]. It consisted of a large, preceding sunspot, characterized by a conspicuous umbral filament [11, 12], and a diffuse following polarity, with positive sign, where an EFR appeared (Figure 1, left panel).

The flux history of the EFR (Figure 1, right panel, black line) is obtained from SDO/HMI line-of-sight field measurements in the subFOV indicated in Figure 1 (left panel). IRIS observations caught the rising phase of the EFR. The light curve in the IRIS 1400 Å passband (Figure 1, right panel, orange line), which refers to the transition region, shows bursty brightness enhancements throughout the observing sequence. Simultaneous brightenings are observed in the SDO/AIA 1600 Å (upper photosphere) and 193 Å (low corona) channels (Figure 1, right panel, green and blue lines).

The sequence of SDO/HMI observations shows that a flux cancellation event occurred in the EFR area: a pore with positive polarity, corresponding to a pre-existing magnetic flux concentration of the plage, disappeared being replaced by a new pore with negative polarity, owing to the piling up of the emerging negative field of the EFR. Hinode spectropolarimetric measurements, performed at the EFR flux peak (Figure 1, right panel, magenta area), clearly confirm the emergence of new magnetic field and the formation of a new pore. Hinode observations also reveal the fine structure of the emerging bipole: mixed-polarity fields, with enhanced linear polarization signal, are found in the region between the opposite emerging polarities, where the granulation looks fuzzy.

Figure 2 provides a synoptic view of simultaneous, multi-wavelength measurements in the EFR area. Strong, compact brightenings are seen in the IRIS UV lines, from the chromosphere up to the coronal Fe XII 1349 Å line. Cospatial brightness enhancements are observed in the IRIS 1400 and 2796 Å slit-jaw images and in all of the SDO/AIA channels. In the middle chromosphere (O I 1356 Å line), we find the development of an arch filament system (AFS) appearing in emission, which is cospatial to the serpentine field. At higher chromospheric heights (e.g., Mg II 2796 Å and C II 1335 Å lines), we observe that plasma ejections are adjacent to the brightenings, taking place above the AFS: those are identified as surges. Notably, the Si IV 1402 Å radiance contours indicate that the areas with enhanced intensity are located near the contact region between the negative emerging polarity of the EFR and the positive, pre-existing field of the plage.

Analyzing the profiles in the brightening region, we uncovered high-velocity flows ($\approx 100\,\mathrm{km\,s}^{-1}$) in blueshifted Mg II 2796 Å, C II 1335 Å and Si IV 1394/1402 Å lines.

3 Concluding remarks

The photospheric signatures observed during the evolution of the EFR fit well with the classical scenario of flux emergence [13]. Moreover, there are strong suggestions that a flux cancellation episode occurred as a result of the interaction (reconnection) between the pre-existing field of the plage of the AR and the newly emerging EFR. This event led to energy release from the chromosphere to the corona above the EFR, as pointed out by the brightenings and plasma ejections observed in all the upper atmospheric layers.

Comparing our results with numerical MHD simulations of flux emergence [14, 15, 16], it appears that the magnetic topology overlying UV bursts determines the impact on the upper atmospheric levels. Indeed, some spectral features identified in the brightening site cospatial to the EFR indicate that the reconnection occurs higher in the atmosphere than usually observed in IRIS bombs, thus explaining the coronal intensity enhancements. That extends the results of earlier observations of flux emergence at granular scale [17] and needs to be further explored with future measurements at higher spatial resolution.

Acknowledgements.

The authors acknowledge support by the Istituto Nazionale di Astrofisica (PRIN-INAF 2014), by the Italian MIUR-PRIN grant 2012P2HRCR on “The active Sun and its effects on space and Earth climate”, by Space Weather Italian COmmunity (SWICO) Research Program, and by the Università degli Studi di Catania (Piano per la Ricerca Università di Catania 2016-2018 – Linea di intervento 1 “Chance”; Linea di intervento 2 “Dotazione ordinaria”). PRY acknowledges funding from NASA grant NNX15AF48G, and he thanks ISSI Bern for supporting the International Team Meeting “Solar UV bursts – a new insight to magnetic reconnection”. The research leading to these results has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 739500 (PRE-EST project).