97. Jet formation and evolution due to 3D magnetic reconnection

December 16, 2018, from uksp_nug_ed

Author: Viktor Fedun, Gary Verth at the The University of Sheffield, J. J. González-Avilés at the IGUM-UNAM, F. S. Guzmán at the IFM-UMSNH, S. Shelyag at the Deakin University and S. Regnier at the Northumbria University.

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Introduction

There is evidence that Type II spicules are due to magnetic reconnection [1-4], oscillatory reconnection [5,6] or more recently it was shown in [7] that spicules occur when magnetic tension is amplified and transported upward. In this work, using 3D numerical simulation of the reconnection process in the photosphere-corona region, we show the formation of a jet with characteristics of Type II spicules.

In our model we assume (i) a completely ionised solar atmosphere which is governed by the resistive MHD equations subject to a constant gravitational field, and (ii) the solar atmosphere based on the C7 model in combination with a 3D potential magnetic field, extrapolated up to 10 Mm above the photosphere, with photospheric values taken from a large-scale, high-resolution, self-consistent simulation of solar magneto-convection with MURaM [8,9]. With these ingredients at the initial time we evolve the system using the resistive MHD equations on the numerical domain x∈[0,6], y∈[0,6], z∈[0,10] Mm, covered with 240×240×400 grid cells.

Jet formation and evolution

The 3D evolution of the magnetic field and the temperature on the plane x=0.1 Mm are shown in Figure 1. The corresponding 2D slice at the plane x=0.1 Mm is plotted in Figure 2. Notice that at time t=15 s the jet starts to develop at the transition region level z≈2.1 Mm where there is a strong current density, which is an indication of reconnection happening. By t=60 s the jet has features of a Type II spicule, with a base located at z≈2 Mm and a height of about z≈7 Mm measured from the transition region (see Figure 2). This in agreement with the observed heights of the Type II spicules, between 3-9 Mm. Finally at time t=90 s the spicule reaches the 9 Mm height.

Figure 1. The 3D representation of jet formation. Snapshots of the logarithm of temperature in Kelvin and magnetic field lines in 3D at times 15, 60 and 90 s. On the bottom plane we display the magnitude of the magnetic field in the x-y plane at z=0.1 Mm. The color bars represent the magnitude of the magnetic field in Gauss and Temperature in Kelvin.

In order to locate regions where magnetic reconnection has occurred, we show a 2D slice of the evolution of |J|(A m-2) and temperature contours in Figure 3. At time t=60 s, when the spicule is well formed, a strong current is located around (y,z)∼(2,2) Mm, at the basis of the spicule, which is consistent with the results in Figure 2. At t=90 s, the regions of strong current are still located at the bottom of the spicule. This analysis shows that magnetic reconnection mainly happens at the chromosphere and the transition region.

Figure 2. Snapshots of the logarithm of temperature (K) and magnetic field lines in the plane x=0.1 Mm at times 15, 60 and 90 s are shown.

We compare the forces due to the magnetic field and gas pressure. The results of the evolution of the ratio |J×B|/|∇p| and temperature contours (K) are shown in Figure 4 in the plane x=0.1 Mm of the 3D domain. Notice that at time t=15 s, which is the time when the spicule starts to form, the Lorentz force dominates. At time t=60 s, the Lorentz force is stronger exactly where the spicule forms. This dominance is still clear at t=90 s. This analysis shows that Lorentz force is an important ingredient of the jet evolution.

Figure 3. Snapshots of |J|(A m-2) and temperature contours (K) in the cross cut at the plane x=0.1 Mm at various times.
Figure 4. Snapshots of the ratio |J×B|/|∇p| and temperature contours (K) in the cross cut at the plane x=0.1 Mm at various times. A comparison with Figure 2 indicates that the Lorentz force dominates in the region where the spicule forms.

Conclusions

Based on a 3D numerical simulation of a small region in the solar atmosphere, which spans from the photosphere to the corona, we show the formation of a jet structure with characteristics of a Type II spicule, specifically the morphology, upward velocity range and time-scale of formation. This result provides a simple explanation and is in contrast with that in [7], where out of 2D simulations the formation of spicules is explained in terms of the amplification of the magnetic tension and the interaction between ions and neutrals. In our simulation we show that the full Lorentz force is important in the process of formation, which is consistent with the results obtained in the simulations of twisted magnetic flux tubes [10] and in the formation of solar chromospheric jets [11]. Our findings also show that vorticity motions are important for jet formation. By analyzing the velocity field in a specific cross-cut of the spicule we can track the circular displacement of plasma that eventually can be identified with blue-red shifts. We invite you to read [12] to review the analysis in more detail.

References

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