Alin Razvan Paraschiv 1, Alina Donea1, and K.D. Leka2,3
1. School of Mathematical Sciences, Monash University, 9 Rainforest Walk, Clayton, Victoria 3800, Australia
2. NorthWest Research Associates, 3380 Mitchell Lane, Boulder, Colorado, USA
3. Nagoya University / ISEE, Furo-cho Chikusa-ku, Nagoya, Aichi 464-9601 JAPAN
Solar active region jets are small-scale collimated plasma eruptions that are linked to magnetic sites embedded in sunspot penumbras. We report the discovery of a set of recurrent coronal jets that have comparable coronal morphological features, recur at the same extreme ultraviolet (EUV) location, and follow the same direction of propagation. The EUV observations suggest a common trigger mechanism. Multiple such trigger mechanisms for recurrent jets are under debate.
In this work we use vector magnetic field data from SDO/HMI observations to analyze a prolific photospheric configuration, identified in EUV as a ‘Coronal Geyser’. We defined Coronal Geysers[1,2] as small-scale penumbral active region structures that have long lifetimes, an open field coronal connectivity, and are prolific generators of recurrent jet eruptions. There, we address whether coronal geysers are sources of non-thermal particles and radio bursts, establish that they are classified from an energetic point of view as impulsive microflare sites, can contain filamentary structures, and can be subject to helicity conservation.
We focus on interpreting the magnetic fields of this small-scale site aiming to understand the processes that govern the recurrent jet eruptions. We selected an observation period of 24 hr data and location and temporally track the region of interest (ROI) in both EUV and vector magnetic field observations. Figure 1 shows the tracking of the small ROI, where the microflaring processes that generate all eruptions can be isolated. Ten Geyser recurrent jets along with additional minor flaring are detected.
Figure 1| A total integrated EUV flux of a 13″x17″ ROI. The individual J1-J10 jets are recovered from the total flux along with the additional minor flares (O1-O7).
The retrieval of magnetic vector fields outside a disk central position and the small-scale nature of the magnetic fields require a cautious interpretation. Projection effects manifest as the geyser was located at mid solar latitudes. SDO/HMI was the only magnetic field imager that continuously observed our ROI. To understand the observation limits, we perform a custom interpretation of the HMI data as follows. The transversal component of the magnetic fields requires cautions scrutiny. We report that a transversal magnetic field detection limit is found to be compatible with the moderate strength magnetic fields involved in generating the jets. We transform the magnetic vectors and project them on a solar plane and then compute the confidence map of the processed vertical magnetic fields corresponding to our ROI. Additionally, we develop a custom disambiguation of the azimuthal angle that is tailored to our specific science case. We report that for our particular region, the field orientation prove stable after this re-calculation of the disambiguation.
When probing the penumbral magnetic fields, it is observed that the underlying magnetic configuration plays a crucial role in continuously triggering the coronal jets. The magnetic configuration is described via the analysis of the vertical magnetic fields, to identify the process that is responsible for driving the jets. Our initial goal was to reveal if either flux emergence, or alternatively, flux cancellation can account as a photospheric trigger for our complete set of recurrent jets. Currently, the two main magnetic trigger scenarios discussed in this paper have been each attributed to jets.
We report that the two widely debated magnetic trigger processes, namely magnetic flux cancellation and magnetic flux emergence, appear to be responsible on a case by case basis for generating each eruption in our set. A combination of penumbral magnetic moving features (MMFs) and small dipole emergences are involved in producing the recurrent jets. The small MMFs can be tracked for intervals in the order of hours before canceling. Alternatively, the flux emergence associated jets were seen to manifest concurrently with the EUV eruptions. We find that 4 of 10 jets were due to flux cancellation while the rest were clearly not, and were more likely due to flux emergence. A similar ratio is qualitatively found to hold for the 7 smaller flaring events.
Figure 2| A slit based temporal tracking of MMFs and flux emergence is presented. Right panel: context view of example slit positions. Left panel: the two slit timeseries where MMF cancellation and flux emergence occur simultaneously with EUV jets.
Figure 2 shows a slit based example of how both mechanisms are acting on the same region, giving rise to jets that appear almost homologous from the EUV point of view, leading us to conclude that not all self-similar recurrent jets are homologous though they appear so. This has implications for the triggers pursued in theoretical studies of jets and/or small scale reconnection. Additionally, we report that our jets resulted from both trigger scenarios support a microfilament eruption hypothesis, although our analysis cannot discriminate if this is indeed the case for part or all of our events.
The work is accepted for publication in The Astrophysical Journal. A preprint is available at https://arxiv.org/abs/2002.11819 .
 Paraschiv, A. R. 2018, PhD Thesis, doi: 10.26180/5bc9d76627396
 Paraschiv, A. R., & Donea, A. 2019, ApJ, 873, 110