Numerical simulation of superhalo electrons generated by magnetic reconnection in the solar wind source region

Yang, Liping; Wang, Linghua; He, Jiansen; Tu, Chuanyi; Zhang, Shaohua; Zhang, Lei; Feng, Xueshang

doi:10.1088/1674-4527/15/3/005

Cited by 11 publications

(8 citation statements)

References 59 publications

(121 reference statements)

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“…Here we assume that S is similar to the total area of coronal holes that varies from 2.5% to 7.5% (5% in average) of the solar surface, measured by SOHO/EIT between 2006-2010 (Lowder et al, 2010). Thus, the number ratio η, varying from η 17% to η 50%, is consistent with the test particle simulations of electron acceleration in the solar wind source region by Yang et al (2015), but it is more than one order of magnitude larger than the number ratio (≈ 0.1% − 1%) estimated for solar HXR flares associated with solar energetic electron events (Lin, 1974;Pan et al, 1984;Krucker et al, 2007). Compared to solar flares that generally occur low in the corona, the superhalo sources likely lie high in the corona, leading to an easier escape of the accelerated electrons into the interplanetary space, i.e., a larger ratio of escaping electrons to downward-traveling electrons.…”

Section: Summary and Discussionsupporting

confidence: 64%

“…Inspired by Yang et al (2015), we propose a scenario of small interchange reconnections (that are probably related to the solar wind source or coronal heating, e.g., nanoflares) to associate the superhalo electrons in the interplanetary space with the HXR emissions in the quiet Sun (see Figure 1). In this scenario, some of the accelerated electrons, e.g., by magnetic reconnection or turbulence, travel upwards along the open magnetic field lines into the interplanetary space, to form the solar wind superhalo population, while the rest propagate downwards into the lower atmosphere.…”

Section: Simulationsmentioning

confidence: 99%

“…If the superhalo electrons originate from nonthermal processes at the Sun, then these upward-traveling electrons and their downward-traveling counterparts should collide with the solar atmosphere to generate Hard X-rays (HXRs) via nonthermal bremsstrahlung from the quiet Sun, i.e., areas free of active regions and solar flares. Based on the magnetohydrodynamics (MHD) and test particle simulations, Yang et al (2015) found that the electrons can be accelerated by the electric field generated by magnetic reconnection in the solar wind source region, to form a power-law energy spectrum with a spectral index of β ≈ 1.5-2.4 at 2-200 keV. In their simulations, about half of the accelerated electrons move upwards along the newly-opened magnetic field lines that may escape into the interplanetary space to form the superhalo population in the solar wind, while the other half move downwards to collide with the dense plasma.…”

Section: Introductionmentioning

confidence: 99%

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Simulation of Quiet-Sun Hard X-Rays Related to Solar Wind Superhalo Electrons

Wang

Krucker

et al. 2016

Sol Phys

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In this paper, we propose that the accelerated electrons in the quiet Sun could collide with the solar atmosphere to emit Hard X-rays (HXRs) via non-thermal bremsstrahlung, while some of these electrons would move upwards and escape into the interplanetary medium, to form a superhalo electron population measured in the solar wind. After considering the electron energy loss due to Coulomb collisions and the ambipolar electrostatic potential, we find that the sources of the superhalo could only occur high in the corona (at a heliocentric altitude 1.9 R ⊙ (the mean radius of the Sun)), to remain a power-law shape of electron spectrum as observed by Solar Terrestrial Relations Observatory (STEREO) at 1AU near solar minimum (Wang et al., 2012). The modeled quiet-Sun HXRs related to the superhalo electrons fit well to a powerlaw spectrum, f ∼ ε −γ in the photon energy ε, with an index γ ≈ 2.0-2.3 (3.3-3.7) at 10-100 keV, for the warm/cold thick-target (thin-target) emissions produced by the downward-traveling (upward-traveling) accelerated electrons. These simulated quiet-Sun spectra are significantly harder than the observed spectra of most solar HXR flares. Assuming that the quiet-Sun sources cover 5% of the solar surface, the modeled thin-target HXRs are more than six orders of magnitude weaker than the Reuven Ramaty High Energy Solar Spectroscopic Imager fraction of downward-traveling electrons dominates by a factor of 100 to 1000 over the escaping population.

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Section: Summary and Discussionsupporting

confidence: 64%

Section: Simulationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Simulation of Quiet-Sun Hard X-Rays Related to Solar Wind Superhalo Electrons

Wang

Krucker

et al. 2016

Sol Phys

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“…Indeed, energetic electron distributions in the Hermean magnetotail observed by MESSENGER often show power-law indices between 1.5 and 4 [15][16][17] . Turbulence contributes to the electron energization via the EMF − → ε M , in a similar way as a localized anomalous resistivity 55,56 . On the other hand, for weak turbulence (negligible or small EMF − → ε M , here Cases A and B), the rate of the change of the magnetic-field (∂B/∂t) dominates the electron energization, but the electrons just become heated and not significantly accelerated.…”

Section: Acceleration Sitesmentioning

confidence: 98%

Electron acceleration by turbulent plasmoid reconnection

et al. 2018

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In space and astrophysical plasmas, like in planetary magnetospheres, as that of Mercury,energetic electrons are often found near current sheets (CSs), which hints at electron acceleration by magnetic reconnection. Unfortunately, electron acceleration by reconnection is not well understood, yet. In particular, acceleration by turbulent plasmoid reconnection. We have investigated electron acceleration by turbulent plasmoid reconnection, described by MHD simulations, via test particle calculations.In order to avoid resolving all relevant turbulence scales down to the dissipation scales, a mean-field turbulence model is used to describe the turbulence of sub-grid scales (SGS) and their effects via a turbulent electromotive force (EMF). The meanfield model describes the turbulent EMF as a function of the mean values of current density, vorticity, magnetic field as well as of the energy, cross-helicity and residual helicity of the turbulence. We found that, mainly around X-points of turbulent reconnection, strongly enhanced localized EMFs most efficiently accelerated electrons and caused the formation of power-law spectra. Magnetic-field-aligned EMFs, caused by the turbulence, dominate the electron acceleration process. Scaling the acceleration processes to parameters of the Hermean magnetotail, electron energies up to 60 keV can be reached by turbulent plasmoid reconnection through the thermal plasma.

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“…Grady et al (2012) also showed that collapsing magnetic traps may lead to accelerating particles to high energies during solar flares. Meanwhile, features of particle acceleration have been investigated in different X-type magnetic field configurations (Browning & Vekstein 2001;Heerikhuisen et al 2002;Zharkova & Gordovskyy 2005;Hannah & Fletcher 2006;Petkaki & MacKinnon 2007;Liu et al 2009;Yang et al 2015;Xia & Zharkova 2018) as well. In addition, Dalla & Browing (2006, 2008 considered the fundamental properties of particle acceleration in a 3D current sheet including a spine and span and showed that in the spine reconnection particle escape takes place in two symmetric jets along the spine, in the fan reconnection no jet is produced, and particles escape in the fan plane.…”

Section: Introductionmentioning

confidence: 99%

Particle Accelerations in a 2.5-dimensional Reconnecting Current Sheet in Turbulence

et al. 2022

ApJ

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Electric field induced in magnetic reconnection is an efficient mechanism for generating energetic particles, but the detailed role it plays is still an open question in solar flares. In this work, accelerations of particles in an evolving reconnecting current sheet are investigated via the test-particle approach, and the electromagnetic field is taken in a self-consistent fashion from a 2.5D numerical experiment for the magnetic reconnection process in the corona. The plasma instabilities like the tearing mode in the current sheet produce magnetic islands in the sheet, and island merging occurs as well. For the motion of the magnetic island, it yields the occurrence of the opposite electric field at both endpoints of the island; hence, tracking the accelerated particles around magnetic islands suggests that the parallel acceleration does not apparently impact the energy gain of particles, but the perpendicular acceleration does. Furthermore, our results indicate that the impact of the guide field on the trajectory of accelerated particles in a more realistic electromagnetic configuration works only on those particles that are energetic enough. The energy spectra of both species show a single power-law shape. The higher-energy component of the power-law spectrum results from the particles that are trapped in the current sheet, while the escaped and partly trapped particles contribute to the lower-energy component of the spectrum. The evolution of the spectrum shows a soft-hard-soft pattern that has been observed in flares.

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Numerical simulation of superhalo electrons generated by magnetic reconnection in the solar wind source region

Cited by 11 publications

References 59 publications

Simulation of Quiet-Sun Hard X-Rays Related to Solar Wind Superhalo Electrons

Simulation of Quiet-Sun Hard X-Rays Related to Solar Wind Superhalo Electrons

Electron acceleration by turbulent plasmoid reconnection

Particle Accelerations in a 2.5-dimensional Reconnecting Current Sheet in Turbulence

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