Hinode/EIS Nugget – Multi-wavelength observations of small-scale reconnection events triggered by magnetic flux emergence

by Salvo Guglielmino, Universitá di Catania
prepared by: Santiago Vargas Domínguez, MSSL



The interaction between emerging magnetic flux and pre-existing ambient field has become a "hot" topic for both numerical simulations and high-resolution observations of the solar atmosphere at different spatial scales (e.g. see previous nugget). The appearance of brightenings and surges during episodes of flux emergence is believed to be a signature of magnetic reconnection processes. In this nugget we look at the configuration and evolution of a small-scale emergence flux region (EFR) in NOAA 10971 observed simultaneously with the Hinode satellite and ground-based facilities.

Joint Observations

Data were acquired during the HOP 14, a joint campaign between the solar telescopes of the Canary Islands and Hinode. On September 30, 2007, the active region NOAA 10971 was observed with the Solar 1-m Swedish Telescope (SST) and the instruments onboard Hinode. See the video below describing the observations and the region of interest through the diverse observed spectral lines. The measurements cover a period of ~2 hours and wavelength range from visible to soft X-rays.

This is movie:
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Figure 1: Multi-wavelength observations of the EFR observed on 2007 September 30. The box indicates the location of the studied event.  Click image to see full size version
A critical step in multi-instrument observations is the co-alignment of the different datasets. Figure 1 summarizes the evolution of the EFR displaying a small-scale brightening (white box in Ca II H and Hα lines) associated with the reconnection of emerging magnetic field lines into a pre-existing ambient field.

The granulation pattern inside the white box appears disturbed and more elongated than is typical in the quiet sun. Upward velocities of 1.5 km/s associated with strong horizontal fields of 1.5 kG are found, supporting results from numerical simulations (Cheung et al, 2007, 2008).

The site of the CaII H brightening coincides with the footpoint of an X-ray loop with enhanced emission, which could be a jet. In fact, various phenomena indicating the release of energy and resulting plasma heating occurred at the EFR site, i.e. brightenings in CaII H, Ha and EUV lines, intensity enhancements in X-rays, and Ha surges.

Topology of the region

The analysis points to the emergence event as the likely trigger of the reconnection process, due to the interaction between the pre-existing arcades and the EFR field lines.

LFFF extrapolations are performed to determine the coronal connectivity above the EFR (see Figure 3). The results suggest that as the positive polarity of the EFR appears on the surface with its conjugate negative polarity, the footpoint separation increases due to the diverging motions, so the emerging lines bulge upwards. They push the overlying arcades and force reconnection thereby producing sets of long loops (surges) and small loops (brightenings).

Figure 3: on the right, Large-scale view of the potential extrapolations in the EFR site. Red lines represent field lines connecting main polarities. Thick coloured lines show selected field lines describing the topology of the EFR. 


The combination of multi-wavelength observations and detailed magnetic field extrapolations enable us to understand complex events in the solar atmosphere such as the EFR events presented in this nugget. These solar processes are difficult to interpret and by gathering information from an ample wavelength coverage we can follow the evolution and intrinsic properties of events. Observational evidence should be complemented with advanced realistic simulations to understand physical phenomena at different scales in the Sun.

This nugget is the summary of a paper in :

Astrophysical Journal (in press), by S. Guglielmino, L. R. Bellot Rubio, F. Zuccarello, G. Aulanier, S. Vargas Domínguez and S. Kamio
— click this link to download online version

For more details, please contact: Dr. Santiago Vargas Domínguez.

Last Revised: Monday, 31-Aug-2010

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