Kelvin–Helmholtz Instability and Alfvénic Vortex Shedding in Solar Eruptions

We report on a three-dimensional MHD numerical experiment of a small-scale coronal mass ejection (CME)-like eruption propagating though a nonmagnetized solar atmosphere. We find that the Kelvin–Helmholtz instability (KHI) develops at various but specific locations at the boundary layer between the erupting field and the background atmosphere, depending on the relative angle between the velocity and magnetic field. KHI develops at the front and at two of the four sides of the eruption. KHI is suppressed at the other two sides of the eruption. We also find the development of Alfvénic vortex shedding flows at the wake of the developing CME due to the 3D geometry of the field. Forward modeling reveals that the observational detectability of the KHI in solar eruptions is confined to a narrow ≈10° range when observing off-limb, and therefore its occurrence could be underestimated due to projection effects. The new findings can have significant implications for observations, for heating, and for particle acceleration by turbulence from flow-driven instabilities associated with solar eruptions of all scales

A cancellation nanoflare model for solar chromospheric and coronal heating. III. 3D simulations and atmospheric response

This research has made use of NASA's Astrophysics Data System. This work was supported by computational time granted from the Greek Research and Technology Network (GRNET) in the National HPC facility ARIS. P.S. acknowledges support by the ERC synergy grant "The Whole Sun."Inspired by recent observations suggesting that photospheric magnetic flux cancellation occurs much more frequently than previously thought, we analytically estimated the energy released from reconnection driven by photospheric flux cancellation, and propose that it can act as a mechanism for chromospheric and coronal heating. Using two-dimensional simulations we validated the analytical estimates and studied the resulting atmospheric response. In the present work, we set up 3D resistive MHD simulations of two canceling polarities in a stratified atmosphere with a horizontal external field to further validate and improve upon the analytical estimates. The computational evaluation of the parameters associated with the energy release are in good qualitative agreement with the analytical estimates. The computational Poynting energy flux into the current sheet is in good qualitative agreement with the analytical estimates, after correcting the analytical expression to better account for the horizontal extent of the current sheet. The atmospheric response to the cancellation is the formation of hot ejections, cool ejections, or a combination of both hot and cool ejections, which can appear with a time difference and/or be spatially offset, depending on the properties of the canceling region and the resulting height of the reconnection. Therefore, during the cancellation, a wide spectrum of ejections can be formed, which can account for the variety of multi-thermal ejections associated with Ellerman bombs, UV bursts, and IRIS bombs, and also other ejections associated with small-scale canceling regions and spicules.Publisher PDFPeer reviewe

Uncovering the birth of a coronal mass ejection from two-viewpoint SECCHI observations

We investigate the initiation and formation of Coronal Mass Ejections (CMEs) via a detailed two-viewpoint analysis of low corona observations of a relatively fast CME acquired by the SECCHI instruments aboard the STEREO mission. The event which occurred on 2 January 2008, was chosen because of several unique characteristics. It shows upward motions for at least four hours before the flare peak. Its speed and acceleration profiles exhibit a number of inflections which seem to have a direct counterpart in the GOES light curves. We detect and measure, in 3D, loops that collapse toward the erupting channel while the CME is increasing in size and accelerates. We suggest that these collapsing loops are our first evidence of magnetic evacuation behind the forming CME flux rope. We report the detection of a hot structure which becomes the core of the white light CME. We observe and measure unidirectional flows along the erupting filament channel which may be associated with the eruption process. Finally, we compare these observations to the predictions from the standard flare-CME model and find a very satisfactory agreement. We conclude that the standard flare-CME concept is a reliable representation of the initial stages of CMEs and that multi-viewpoint, high cadence EUV observations can be extremely useful in understanding the formation of CMEs.PostprintPeer reviewe

Chromospheric and coronal heating and jet acceleration due to reconnection driven by flux cancellation : I. At a three-dimensional current sheet

Funding: P.S. acknowledges support by the ERC synergy grant 'The Whole Sun'.Context. The recent discovery of much greater magnetic flux cancellation taking place at the photosphere than previously realised has led us in our previous works to suggest magnetic reconnection driven by flux cancellation as the cause of a wide range of dynamic phenomena, including jets of various kinds and solar atmospheric heating. Aims. Previously, the theory considered energy release at a two-dimensional current sheet. Here we develop the theory further by extending it to an axisymmetric current sheet in three dimensions without resorting to complex variable theory. Methods. We analytically study reconnection and treat the current sheet as a three-dimensional structure. We apply the theory to the cancellation of two fragments of equal but opposite flux that approach each another and are located in an overlying horizontal magnetic field. Results. The energy release occurs in two phases. During Phase 1, a separator is formed and reconnection is driven at it as it rises to a maximum height and then moves back down to the photosphere, heating the plasma and accelerating a plasma jet as it does so. During Phase 2 the fluxes cancel in the photosphere and accelerate a mixture of cool and hot plasma upwards.Publisher PDFPeer reviewe

Effects of partial ionization on magnetic flux emergence in the Sun

We have performed 3-D numerical simulations to investigate the effect of partial ionization on the process of magnetic flux emergence. In our study, we have modified the single-fluid MHD equations to include the presence of neutrals and have performed two basic experiments: one that assumes a fully ionized plasma (FI case) and one that assumes a partially ionized plasma (PI case). We find that the PI case brings less dense plasma to and above the solar surface. Furthermore, we find that partial ionization alters the emerging magnetic field structure, leading to a different shape of the polarities in the emerged bipolar regions compared to the FI case. The amount of emerging flux into the solar atmosphere is larger in the PI case, which has the same initial plasma beta as the FI case, but a larger initial magnetic field strength. The expansion of the field above the photosphere occurs relatively earlier in the PI case, and we confirm that the inclusion of partial ionization reduces cooling due to adiabatic expansion. However, it does not appear to work as a heating mechanism for the atmospheric plasma. The performance of these experiments in three dimensions shows that PI does not prevent the formation of unstable magnetic structures, which erupt into the outer solar atmosphere

Kelvin–Helmholtz instability and Alfvénic vortex shedding in solar eruptions

Funding: UK STFC Ernest Rutherford Fellowship (No. ST/R004285/1) (P.A.). ERC synergy grant “The Whole Sun” (P.S.).We report on a three-dimensional MHD numerical experiment of a small-scale coronal mass ejection (CME)-like eruption propagating though a nonmagnetized solar atmosphere. We find that the Kelvin–Helmholtz instability (KHI) develops at various but specific locations at the boundary layer between the erupting field and the background atmosphere, depending on the relative angle between the velocity and magnetic field. KHI develops at the front and at two of the four sides of the eruption. KHI is suppressed at the other two sides of the eruption. We also find the development of Alfvénic vortex shedding flows at the wake of the developing CME due to the 3D geometry of the field. Forward modeling reveals that the observational detectability of the KHI in solar eruptions is confined to a narrow ≈10° range when observing off-limb, and therefore its occurrence could be underestimated due to projection effects. The new findings can have significant implications for observations, for heating, and for particle acceleration by turbulence from flow-driven instabilities associated with solar eruptions of all scales.Publisher PDFPeer reviewe

The spectroscopic imprint of the pre-eruptive configuration resulting into two major coronal mass ejections

P.S acknowledges financial support from the programme Aristotelis/SIEMENS at the NOA.Aims: We present a spectroscopic analysis of the pre-eruptive configuration of active region NOAA 11429, prior to two very fast coronal mass ejections (CMEs) on March 7, 2012 that are associated with this active region. We study the thermal components and the dynamics associated with the ejected flux ropes. Methods: Using differential emission measure (DEM) analysis of Hinode/EIS and SDO/AIA observations, we identify the emission components of both the flux rope and the host active region. We then follow the time evolution of the flux rope emission components by using AIA observations. The plasma density and the Doppler and non-thermal velocities associated with the flux ropes are also calculated from the EIS data. Results: The eastern and western parts of the active region, in which the two different fast CMEs originated during two X-class flares, were studied separately. In both regions we identified an emission component in the temperature range of log T = 6.8-7.1 associated with the presence of flux ropes. The time evolution of the eastern region showed an increase in the mean DEM in this temperature range by an order of magnitude, 5 h prior to the first CME. This was associated with a gradual rise and heating of the flux rope as manifested by blue-shifts and increased non-thermal velocities in Ca xv 200.97 Å, respectively. An overall upward motion of the flux ropes was measured (relative blue-shifts of ~12 km s-1). The measured electron density was found to be 4× 109-2 × 1010 cm-3 (using the ratio of Ca xv 181.90 Å over Ca xv 200.97 Å). We compare our findings with other works on the same AR to provide a unified picture of its evolution.PostprintPeer reviewe

Recurrent CME-like eruptions in emerging flux regions. II. Scaling of energy and collision of successive eruptions

This project has received funding from the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1. The authors acknowledge support by the Royal Society. This work was supported by computational time granted from the Greek Research & Technology Network (GRNET) in the National HPC facility—ARIS.We present results of three-dimensional MHD simulations of recurrent eruptions in emerging flux regions. The initial numerical setup is the same as that in the work by Syntelis et al. Here, we perform a parametric study on the magnetic field strength (B 0) of the emerging field. The kinetic energy of the produced ejective eruptions in the emerging flux region ranges from 1026 to 1028 erg, reaching up to the energies of small coronal mass ejections. The kinetic and magnetic energies of the eruptions scale linearly in a logarithmic plot. We find that the eruptions are triggered earlier for higher B 0 and that B 0 is not directly correlated to the frequency of occurrence of the eruptions. Using large numerical domains, we show the initial stage of the partial merging of two colliding erupting fields. The partial merging occurs partly by the reconnection between the field lines of the following and the leading eruption at the interface between them. We also find that tether-cutting reconnection of the field lines of the leading eruption underneath the following eruption magnetically links the two eruptions. Shocks develop inside the leading eruption during the collision.Publisher PDFPeer reviewe

Successful and failed flux tube emergence in the solar interior

This project has received funding from the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1. The authors acknowledge support by the Royal Society. This work was supported by computational time granted from the Greek Research & Technology Network (GRNET) in the National HPC facility—ARIS.We report on our 3D magnetohydrodynamic simulations of cylindrical weakly twisted flux tubes emerging from 18 Mm below the photosphere. We perform a parametric study by varying the initial magnetic field strength (B 0), radius (R), twist (α), and length of the emerging part of the flux tube (λ) to investigate how these parameters affect the transfer of the magnetic field from the convection zone to the photosphere. We show that the efficiency of emergence at the photosphere (i.e., how strong the photospheric field will be in comparison to B 0) depends not only on B 0, but also on the morphology of the emerging field and on the twist. We show that parameters such as B 0 and magnetic flux alone cannot determine whether a flux tube will emerge to the solar surface. For instance, high-B 0 (weak-B 0) fields may fail (succeed) to emerge at the photosphere, depending on their geometrical properties. We also show that the photospheric magnetic field strength can vary greatly for flux tubes with the same B 0 but different geometric properties. Moreover, in some cases we have found scaling laws, whereby the magnetic field strength scales with the local density as B ∝ ρ κ , where κ ≈ 1 deeper in the convection zone and κ < 1 close to the photosphere. The transition between the two values occurs approximately when the local pressure scale (H p ) becomes comparable to the diameter of the flux tube (H p  ≈ 2R). We derive forms to explain how and when these scaling laws appear and compare them with the numerical simulations.Publisher PDFPeer reviewe

Recurrent CME-like eruptions in emerging flux regions. I. On the mechanism of eruptions

This project has received funding from the Science and Technology Facilities Council (UK) through the consolidated grant ST/N000609/1. This research has been co-financed by the European Union (European Social Fund—ESF) and Greek national funds through the Operational Program “Education and Lifelong Learning” of the National Strategic Reference Framework (NSRF)—Research Funding Program: Thales Investing in Knowledge Society through the European Social Fund. The authors acknowledge support by the Royal Society.We report on three-dimensional (3D) magnetohydrodynamic (MHD) simulations of recurrent eruptions in emerging flux regions. We find that reconnection of sheared field lines, along the polarity inversion line of an emerging bipolar region, leads to the formation of a new magnetic structure, which adopts the shape of a magnetic flux rope (FR) during its rising motion. Initially, the FR undergoes a slow-rise phase and, eventually, it experiences a fast-rise phase and ejective eruption toward the outer solar atmosphere. In total, four eruptions occur during the evolution of the system. For the first eruption, our analysis indicates that the torus instability initiates the eruption and that tether-cutting reconnection of the field lines, which envelop the FR, triggers the rapid acceleration of the eruptive field. For the following eruptions, we conjecture that it is the interplay between tether-cutting reconnection and torus instability that causes the onset of the various phases. We show the 3D shape of the erupting fields, focusing more on how magnetic field lines reconnect during the eruptions. We find that when the envelope field lines reconnect mainly with themselves, hot and dense plasma is transferred closer to the core of the erupting FR. The same area appears to be cooler and less dense when the envelope field lines reconnect with neighboring sheared field lines. The plasma density and temperature distribution, together with the rising speeds, energies, and size of the erupting fields, indicate that they may account for small-scale (mini) coronal mass ejections.Publisher PDFPeer reviewe