# Multiwavelength study of 20 jets that emanate from the periphery of active regions by Sargam M. Mulay et al.*

Solar jets are transient phenomena observed in the solar atmosphere. They appear as sharp-edged, impulsive, and collimated flows of plasma that move outwards with a bright spot at the footpoint, which forms an ‘inverted-Y’ topology of magnetic field lines. They are observed throughout the atmosphere i.e. in the photosphere (Hα, Ca II K surges), chromosphere (UV), transition region (EUV) and corona (X-ray). Jets can occur in different environments such as coronal holes (CHs; Young & Muglach 2014a,b), and active regions (ARs; Innes et al. 2011, Chandra et al. 2015), where they have different manifestations.

It has been observed that jets which occur at the periphery of active regions/sunspots are mostly associated with a nonthermal type-III radio bursts [see also previous CESRA highlights on type III bursts here, here and here ]. These bursts are known to be produced by energetic electrons that gyrate along the open magnetic field lines and emit radio emission at the plasma frequency (Kundu et al. 1995, Chen et al. 2013). This fast drifting emission appears as a pillar structure in the radio dynamic spectrum which is classified as a ‘nonthermal type-III radio burst’.

Also, it has been observed that the accelerated electrons which have access to open field lines produce impulsive, electron/$^3$He-rich solar energetic particle (SEP) events in the interplanetary medium (Nitta et al. 2008; Wang et al. 2006), and the accelerated electrons which are trapped in closed field lines travel downwards, lose their energies due to collisions, and produce hard X-ray emission (HXR; Glesener et al. 2012). Therefore, the study of AR jets and their associated phenomena is important for an understanding of Sun-Earth relationship and the effect on the Earth’s environment.

We present a multiwavelength analysis of 20 EUV jets observed between August 2010 and June 2013. In this study, we included events which were observed on the solar disk within $\pm60^{\circ}$ latitude and occurred at the periphery of active regions close to sunspots. We discuss the physical parameters of the jets and their relation with other phenomena such as nonthermal type-III radio bursts and magnetic activity in the photosphere.

Observations and Results

In this section, we discuss the observation and analysis of one of the active region jets and investigate the relationship with other phenomena.

The Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) instrument onboard the Solar Dynamic Observatory (SDO) observed a jet on the solar disk on 2010 August 02 that originated from the western periphery of the active region 11092 (N13 E07). The jet activity started at 17:10 UT and ended at 17:35 UT. The first brightening at the footpoint was seen at 17:07 UT and as time progressed, a small loop started to develop between the footpoint of the jet and the edge of the umbral-penumbral region of sunspot. At 17:18:43 UT, the loop started to expand and became associated with a spire of the jet. Figure 1 (left panel) shows the AR jet observed at 17:26 UT in the AIA 193 Å channel (reverse color image). The white over-plotted box shows the field-of-view for the region shown in the right panel. The untwisting nature of the complex, multi-threaded spire was clearly observed in all AIA channels.

Figure 1 (right panel) shows the line-of-sight component of the photospheric magnetic field observed by the Helioseismic and Magnetic Imager (HMI; Scherrer et al. 2012) onboard the SDO. The jet was associated with a negative-polarity sunspot. The evolution of positive-polarity at the edge of the sunspot indicates flux emergence while the cancellation of the negative-polarity was seen at the western side of the sunspot (shown by white arrows). This observation is co-spatial with the location of the footpoint of the jet.

Fig. 1 Left panel : The jet observed in the AIA 193 Å channel (reverse colour image). The over-plotted white box represents the field-of-view shown in the right panel. Right panel : The line-of-sight (LOS) component of the photospheric magnetic field is shown before (at 16:56 UT) and after (at 17:36 UT) the evolution of the jet.

To estimate the speed of the jet, we performed the time-distance analysis using AIA 171 Å filter images (dominated by Fe IX log T [K] = 6.0). We employed an artificial slit along the direction of the jet spire (fig. 2 left panel) and calculated the plane-of-sky velocity of the jet-front (fig. 2 right panel). The velocity was found to be 236 km/s. The complex multi-threaded and untwisting nature of the jet spire is clearly visible in this plot.

Fig. 2 Left panel : jet evolution in the AIA 171 Å channel. The white line shows the artificial slit which was used to produce a time-distance plot. Right panel : time-distance plot along the jet spire. The white dashed line is used for the velocity calculation. This is found to be 236 km/s.

We compared the jet timings with observations of dynamic spectra recorded by the WAVES instrument (Bougeret et al. 1995) onboard the Wind satellite. A nonthermal type-III radio burst was observed at 17:25 UT by the RAD 1 and RAD 2 radio receivers in the frequency range from 13 MHz to 220 kHz as shown in Fig. 3 (left panel).

Figure 3 Left panel : dynamic radio spectrum indicates a nonthermal type-III radio burst observed by the WIND/WAVES at 17:25 UT. This burst is co-temporal with the evolution of the jet. Right panel : The PFSS extrapolation of the active region at 18:04 UT. The white and pink lines indicate the closed and open magnetic structures respectively in the active region and the nearby region.

We also investigated the location of these bursts using the potential field source surface (PFSS; Schatten et al. 1969) extrapolation of the photospheric magnetic field. This tool is used to visualise the solar coronal magnetic field in the active region. Figure 3 (right panel) shows the extrapolated coronal magnetic field at 18:04 UT using the PFSS analysis technique. The white lines show the closed magnetic structure that is associated with the active region and the pink lines indicate the open magnetic field lines. The source region of the jet and the open magnetic field lines share the same location. This indicates that the jet is ejected in the direction of these open field structures. The presence of open field lines confirm the source region of the nonthermal type-III radio burst in the jet.

Conclusion

In Mulay et al. (2016), we present a comprehensive study of multiwavelength observations of 20 active region jets observed during August 2010 – June 2013. We measured the general physical properties of the jets, such as lifetime and velocity and their relationship with nonthermal type-III radio burst and photospheric activity.

1) Most of the jets originated from the western periphery of active regions close to sunspots.

2) The lifetime (as noted from the AIA 193 Å channel) ranged from 5 to 39 min with an average of 18 min.

3) The velocities ranged from 87 to 532 km/s with an average of 271 km/s.

4) In 17 out of 20 events (85%), we found that the nonthermal type-III radio bursts were cotemporally associated with jets.

5) The PFSS analysis showed the presence of open magnetic field lines in the source region, which indicates that jets were ejected in the direction of this open magnetic field structure. These observations show the signature of particle acceleration.

6) Also, 10 out of 20 events showed that the jets originated in regions of flux cancellation and six in regions of flux emergence. Four events showed a flux emergence and then cancellation during the  jet evolution.

It is important to note that AR jets mostly occur in regions of open magnetic field lines that connect to the heliosphere. Therefore, these are one of the best features for future detailed space-weather studies with missions such as Solar Orbiter and Solar Probe plus suite of remote-sensing and in-situ instruments.

The high resolution imaging and spectral capabilities of Low-Frequency Array (LOFAR) instrument give us a great opportunity to study solar radio emission in greater detail in the wide range of frequencies from 10 to 240 MHz. The nonthermal type III radio burst source can be traced in the lower corona with LOFAR and Nancay Radioheliograph (which observes the Sun at ten frequencies between 150 and 450 MHz) observations. These types of coherent radio emission can give us clues about the geometry and plasma parameters near the acceleration region.

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*Sargam M. Mulay, Durgesh Tripathi, Giulio Del Zanna, and Helen Mason