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Register a new userFormation of Large Scale Coronal Loops Interconnecting Two Active Regions Through Gradual Magnetic Reconnection and Associated Heating Process
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Coronal loops interconnecting two active regions, called as interconnecting loops (ILs), are prominent large-scale structures in the solar atmosphere. They carry a significant amount of magnetic flux, therefore are considered to be an important element of the solar dynamo process. Earlier observations show that eruptions of ILs are an important source of CMEs. It is generally believed that ILs are formed through magnetic reconnection in the high corona (>150-200), and several scenarios have been proposed to explain their brightening in soft X-rays (SXRs). Yet, the detailed IL formation process has not been fully explored and the associated energy release in the corona still remains unresolved. Here we report the complete formation process of a set of ILs connecting two nearby active regions, with successive observations by STEREO-A on the far side of the Sun and SDO and Hinode on the Earth side. We conclude that ILs are formed by gradual reconnection high in the corona, in line with earlier postulations. In addition, we show evidence supporting that ILs become brightened in SXRs and EUVs through heating at or close to the reconnection site in the corona (i.e., through direct heating process of reconnection), a process that has been largely overlooked in earlier studies on ILs
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Coronal loops are building blocks of solar active regions. However, their formation mechanism is still not well understood. Here we present direct observational evidence for the formation of coronal loops through magnetic reconnection as new magnetic fluxes emerge into the solar atmosphere. Extreme-ultraviolet observations of the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) clearly show the newly formed loops following magnetic reconnection within a plasma sheet. Formation of the loops is also seen in the h{alpha} line-core images taken by the New Vacuum Solar Telescope. Observations from the Helioseismic and Magnetic Imager onboard SDO show that a positive-polarity flux concentration moves towards a negative-polarity one with a speed of ~0.4 km/s, before the formation of coronal loops. During the loop formation process, we found signatures of flux cancellation and subsequent enhancement of the transverse field between the two polarities. The three-dimensional magnetic field structure reconstructed through a magnetohydrostatic model shows field lines consistent with the loops in AIA images. Numerous bright blobs with an average width of 1.37 Mm appear intermittently in the plasma sheet and move upward with a projected velocity of ~114 km/s. The temperature, emission measure and density of these blobs are about 3 MK, 2.0x10^(28) cm^(-5) and 1.2x10^(10) cm^(-3), respectively. A power spectral analysis of these blobs indicates that the observed reconnection is likely not dominated by a turbulent process. We have also identified flows with a velocity of 20 to 50 km/s towards the footpoints of the newly formed coronal loops.
It is clear that the solar corona is being heated and that coronal magnetic fields undergo reconnection all the time. Here we attempt to show that these two facts are in fact related - i.e. coronal reconnection generates heat. This attempt must address the fact that topological change of field lines does not automatically generate heat. We present one case of flux emergence where we have measured the rate of coronal magnetic reconnection and the rate of energy dissipation in the corona. The ratio of these two, $P/dot{Phi}$, is a current comparable to the amount of current expected to flow along the boundary separating the emerged flux from the pre-existing flux overlying it. We can generalize this relation to the overall corona in quiet Sun or in active regions. Doing so yields estimates for the contribution to corona heating from magnetic reconnection. These estimated rates are comparable to the amount required to maintain the corona at its observed temperature.
When magnetic flux emerges from beneath the photosphere it displaces the preexisting field in the corona, and a current sheet generally forms at the boundary between the old and new magnetic domains. Reconnection in the current sheet relaxes this highly stressed configuration to a lower energy state. This scenario is most familiar, and most often studied, in flares, where the flux transfer is rapid. We present here a study of steady, quiescent flux transfer occurring at a rate three orders of magnitude below that in a large flare. In particular we quantify the reconnection rate, and related energy release, occurring as new polarity emerges to form Active Region 11112 (2010-10-16T00:S18W14) within a region of preexisting flux. A bright, low lying kernel of coronal loops above the emerging polarity, observed with AIA onboard SDO and XRT onboard Hinode, originally shows magnetic connectivity only between regions of newly emerged flux when overlaid on magnetograms from HMI. Over the course of several days, this bright kernel advances into the preexisting flux. The advancement of an easily visible boundary into the old flux regions allows measurement of the rate of reconnection between old and new magnetic domains. We compare the reconnection rate to the inferred heating of the coronal plasma. To our knowledge, this is the first measurement of steady, quiescent heating related to reconnection. We determine that the newly emerged flux reconnects at a fairly steady rate of 0.38e16 Mx/s over two days, while the radiated power varies between 2~8e25 erg/s over the same time. We find that as much as 40% of the total emerged flux at any given time may have reconnected. The total amount of transferred flux (1e21 Mx) and radiated energy (7.2e30 ergs) are comparable to that of a large M- or small X-class flare, but are stretched out over 45 hours.
Employing Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) multi-wavelength images, we report the coronal condensation during the magnetic reconnection (MR) between a system of open and closed coronal loops. Higher-lying magnetically open structures, observed in AIA 171 A images above the solar limb, move downward and interact with the lower-lying closed loops, resulting in the formation of dips in the former. An X-type structure forms at the interface. The interacting loops reconnect and disappear. Two sets of newly-reconnected loops then form and recede from the MR region. During the MR process, bright emission appears sequentially in the AIA 131 A and 304 A channels repeatedly in the dips of higher-lying open structures. This indicates the cooling and condensation process of hotter plasma from ~0.9 MK down to ~0.6 MK, and then to ~0.05 MK, also supported by the light curves of the AIA 171 A, 131 A, and 304 A channels. The part of higher-lying open structures supporting the condensations participate in the successive MR. The condensations without support by underlying loops then rain back to the solar surface along the newly-reconnected loops. Our results suggest that the MR between coronal loops leads to the condensation of hotter coronal plasma and its downflows. MR thus plays an active role in the mass cycle of coronal plasma because it can initiate the catastrophic cooling and condensation. This underlines that the magnetic and thermal evolution has to be treated together and cannot be separated, even in the case of catastrophic cooling.
We investigate the relaxation of braided magnetic loops in order to find out how the type of braiding via footpoint motions affects resultant heating of the loop. Two magnetic loops, braided in different ways, are used as initial conditions in resistive MHD simulations and their subsequent evolution is studied. The fields both undergo a resistive relaxation in which current sheets form and fragment and the system evolves towards a state of lower energy. In one case this relaxation is very efficient with current sheets filling the volume and homogeneous heating of the loop occurring. In the other case fewer current sheets develop, less magnetic energy is released in the process and a patchy heating of the loop results. The two cases, although very similar in their setup, can be distinguished by the mixing properties of the photospheric driver. The mixing can be measured by the topological entropy of the plasma flow, an observable quantity.
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