Author: Aaron Reid, Mihalis Mathioudakis (Queen’s University Belfast), Vasco Henriques (UiO, Norway), Tanmoy Samanta (Peking University, China).

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Introduction

On-disk fan-shaped jets most often appear at light bridges ([1],[2],[3]), where the dominant magnetic field orientation changes from vertical to horizontal [4]. This has led to the theory that fan-shaped jets are formed via shearing reconnection between the vertical and horizontal photospheric magnetic field lines at the light bridge – umbra interface [5].

Three-dimensional simulations have reinforced this theory, showing fan-shaped jets being formed during shearing reconnection between horizontal magnetic fields and a vertical current sheet, with the base of the jets corresponding to the location of magnetic reconnection [6]. While the initial acceleration of the jets is due to the magnetic tension, the kinematics associated with this phenomenon are thought to be due to the gas pressure gradient created by the heating at the base of the jet [7].

In this nugget, we present evidence of recurrent magnetic reconnection driven by running penumbral waves in the photosphere. This reconnection at the penumbral edge leads to the appearance of chromospheric fan-shaped jets.

Limb observations of fan-shaped jets

We acquired Hα imaging spectroscopy data of a sunspot near the solar limb using the Swedish 1-m Solar Telescope (SST). At the sunspot edge, fan-shaped jets can be seen in the Hα line core (top-middle panel of Fig. 1). The Hα red wing shows the descent of previous jets, while in the blue wing, these events are not well captured. This is most likely due to line-of-sight effects Doppler-shifting the falling material. Using the SDO alignment tool of Rob Rutten, co-spatial and co-temporal SDO data was also created, and are also shown in Fig. 1.

The red box in Fig. 1 highlights a region where repetitive brightenings exist in the wings of Hα, indicating photospheric temperature enhancements. These brightenings have line profiles matching that of nearby Ellerman Bombs, which are caused via photospheric reconnection [8]. This agrees with the current theory of fan-shaped jet formation [6]. Light-curves were created for the acquired datasets, and it was apparent that these intensity enhancements at the base of the fan-shaped jets only occur in the 1600A and 1700A channels of AIA and the Hα line wings, and so are solely photospheric. The average time between brightenings was 214 seconds, which is similar to typical periods of running penumbral waves [9]. The peaks in the photospheric light-curves are also near-simultaneous to the appearance of new fan-shaped jets in the chromosphere. Fig. 2 shows how the photospheric brightening directly relates to the fan-shaped jets in the chromosphere. The bottom panels show how the brightening erupts material into the chromosphere, which is co-spatial and co-temporal to the appearance of a new bright-jet front in the Hα line core.

Curvilinear time slices were created for the fan-shaped jets, as highlighted by the green line in Fig. 1. The resultant output is shown in Fig. 3, and the bright front is clearly visible in the Hα line core, along with SDO AIA 304A, 171A, and 131A. Interestingly, the body of the jets appear dark in all AIA channels bar 304A (and 1600/1700A, but the jets are not seen in these channels). This darkening is most likely optically thick plasma caused by strong absorption due to an increased density of hydrogen and singly ionised helium [10].

We tried to characterise the fan-shaped jets by applying polynomial fitting to the jet front detected in the Hα red wing. This would enable a velocity and deceleration profile to be obtained for the falling material. The resultant fit is shown by the blue dashed line in Fig. 3, where the jet material appears to be accelerating back to the solar surface at a rate consistent with solar gravity. However, the initial rising of the jet fronts is clearly not decelerating in the same way. Rather, the jet fronts appear to protrude at a constant velocity of 30 km/s according to linear fitting attempted on the brightest pixels of the jet front in Hα. This suggests the presence of a driver counteracting the effects of solar gravity.

Conclusions

We have shown evidence for recurrent photospheric reconnection leading to the appearance of fan-shaped chromospheric jets at a sunspot edge. While fan-shaped jets are most commonly observed across light-bridges, the current theory for their appearance allows them to form anywhere strong vertical and horizontal fields may interact. In this case, the strong vertical fields from the umbra interact with the nearby horizontal magnetic fields, causing reconnection at the edge of the penumbra.

The fan-shaped jets appear with a bright front in the low temperature chromospheric channels, while in the hotter coronal channels, darkness is left behind in the body of the jets. This implies a relatively hot jet front (~50,000K) with cool, optically thick plasma left in the wake. Our analysis of the jet protrusion implies that there is a driver to the emanation of the jet. The Fourier power spectra of the sunspot penumbra hint that the peak frequency corresponds to the average time between photospheric eruptions at the base of the jets, implying that running penumbral waves could be such a driver.

References

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