Magnetic Reconnection Rate Research Articles

Global Environmental Constraints on Magnetic Reconnection at the Magnetopause From In Situ Measurements

AbstractProgress in locating the X‐line on the magnetopause beyond the atypical due south interplanetary magnetic field (IMF) condition is hampered by the fact that the global plasma and field spatial distributions constraining where reconnection could develop on the magnetopause are poorly known. This work presents global maps of the magnetic shear, current density and reconnection rate, on the global dayside magnetopause, reconstructed from two decades of measurements from Cluster, Double Star, THEMIS and MMS missions. These maps, generated for various IMF and dipole tilt angles, offer a unique comparison point for models and observations. The magnetic shear obtained from vacuum magnetostatic draping is shown to be inconsistent with observed shear maps for IMF cone angles in 12.5° ± 2.5° ≤ |θco| ≤ 45° ± 5°. Modeled maximum magnetic shear lines fail to incline toward the equator as the IMF clock angle increases, in contrast to those from observations and MHD models. Reconnection rate and current density maps are closer together than they are from the shear maps, but this similarity vanishes for increasingly radial IMF orientations. The X‐lines maximizing the magnetic shear are the only ones to sharply turns toward and follow the anti‐parallel ridge at high latitude. We show the behavior of X‐lines with varying IMF clock and dipole tilt angles to be different as the IMF cone angle varies. Finally, we discuss a fundamental disagreement between X‐lines maximizing a given quantity on the magnetopause and predictions of local X‐line orientations.

Inferring Fundamental Properties of the Flare Current Sheet Using Flare Ribbons: Oscillations in the Reconnection Flux Rates

Magnetic reconnection is understood to be the main physical process that facilitates the transformation of magnetic energy into heat, motion, and particle acceleration during solar eruptions. Yet, observational constraints on reconnection region properties and dynamics are limited due to a lack of high-cadence and high-spatial-resolution observations. By studying the evolution and morphology of postreconnected field-lines footpoints, or flare ribbons and vector photospheric magnetic field, we estimate the magnetic reconnection flux and its rate of change with time to study the flare reconnection process and dynamics of the current sheet above. We compare high-resolution imaging data to study the evolution of the fine structure in flare ribbons as ribbons spread away from the polarity inversion line. Using data from two illustrative events (one M- and X-class flare), we explore the relationship between the ribbon-front fine structure and the temporal development of bursts in the reconnection region. Additionally, we use the RibbonDB database to perform statistical analysis of 73 (C- to X-class) flares and identify quasiperiodic pulsation (QPP) properties using the Wavelet Transform. Our main finding is the discovery of QPP signatures in the derived magnetic reconnection rates in both example events and the large flare sample. We find that the oscillation periods range from 1 to 4 minutes. Furthermore, we find nearly cotemporal bursts in Hard X-ray (HXR) emission profiles. We discuss how dynamical processes in the current sheet involving plasmoids can explain the nearly cotemporal signatures of quasiperiodicity in the reconnection rates and HXR emission.

Simultaneous observations of a breakout current sheet and a flare current sheet in a coronal jet event

ABSTRACT Previous studies have revealed that solar coronal jets triggered by the eruption of minifilaments (MFs) conform to the famous magnetic-breakout mechanism. In such a scenario, a breakout current sheet (BCS) and a flare current sheet (FCS) should be observed during the jets. With high spatial and temporal resolution data from the SDO, the NVST, the RHESSI, the Wind, and the GOES, we present observational evidence of a BCS and a FCS formation during coronal jets driven by a MF eruption occurring in the active region NOAA 11726 on 2013 April 21. Magnetic field extrapolation shows that the MF was enclosed by a fan-spine magnetic structure. The MF was activated by flux cancellation under it, and then slowly rose. A BCS formed when the magnetic fields wrapping the MF squeezed to antidirectional external open fields. Simultaneously, one thin bright jet and two bidirectional jet-like structures were observed. As the MF erupted as a blowout jet, a FCS was formed when the two distended legs inside the MF field came together. One end of the FCS connected the post-flare loops. The BCS’s peak temperature was calculated to be 2.5 MK. The FCS’s length, width, and peak temperature were calculated to be 4.35–4.93, 1.31–1.45, and 2.5 MK, respectively. The magnetic reconnection rate associated with the FCS was estimated to be from 0.266 to 0.333. This event is also related to a type III radio burst, indicating its influence on interplanetary space. These observations support the scenario of the breakout model as the trigger mechanism of coronal jets, and flux cancellation was the driver of this event.

Energy distribution and substructure formation in astrophysical MHD simulations

ABSTRACT During substructure formation in magnetized astrophysical plasma, dissipation of magnetic energy facilitated by magnetic reconnection affects the system dynamics by heating and accelerating the ejected plasmoids. Numerical simulations are a crucial tool for investigating such systems. In astrophysical simulations, the energy dissipation, reconnection rate, and substructure formation critically depend on the onset of reconnection of numerical or physical origin. In this paper, we hope to assess the reliability of the state-of-the-art numerical codes, pluto and koral by quantifying and discussing the impact of dimensionality, resolution, and code accuracy on magnetic energy dissipation, reconnection rate, and substructure formation. We quantitatively compare results obtained with relativistic and non-relativistic, resistive and non-resistive, as well as two- and three-dimensional set-ups performing the Orszag–Tang test problem. We find sufficient resolution in each model, for which numerical error is negligible and the resolution does not significantly affect the magnetic energy dissipation and reconnection rate. The non-relativistic simulations show that at sufficient resolution, magnetic and kinetic energies convert to internal energy and heat the plasma. In the relativistic system, energy components undergo mutual conversion during the simulation time, which leads to a substantial increase in magnetic energy at 20 per cent and 90 per cent of the total simulation time of 10 light-crossing times – the magnetic field is amplified by a factor of 5 due to relativistic shocks. We also show that the reconnection rate in all our simulations is higher than 0.1, indicating plasmoid-mediated regime. It is shown that in koral simulations more substructures are captured than in pluto simulations.

A Database of Magnetic and Thermodynamic Properties of Confined and Eruptive Solar Flares

Solar flares sometimes lead to coronal mass ejections that directly affect Earth's environment. However, a large fraction of flares, including on solar-type stars, are confined flares. What are the differences in physical properties between confined and eruptive flares? For the first time, we quantify the thermodynamic and magnetic properties of hundreds of confined and eruptive flares of GOES class C5.0 and above, 480 flares in total. We first analyze large flares of GOES class M1.0 and above observed by the Solar Dynamics Observatory, 216 flares in total, including 103 eruptive and 113 confined flares, from 2010 until 2016 April; we then look at the entire data set of 480 flares above class C5.0. We compare GOES X-ray thermodynamic flare properties, including peak temperature and emission measure, and active-region (AR) and flare-ribbon magnetic field properties, including reconnected magnetic flux and peak reconnection rate. We find that for fixed peak X-ray flux, confined and eruptive flares have similar reconnection fluxes; however, for fixed peak X-ray flux confined flares have on average larger peak magnetic reconnection rates, are more compact, and occur in larger ARs than eruptive flares. These findings suggest that confined flares are caused by reconnection between more compact, stronger, lower-lying magnetic fields in larger ARs that reorganizes a smaller fraction of these regions’ fields. This reconnection proceeds at faster rates and ends earlier, potentially leading to more efficient flare particle acceleration in confined flares.

Statistical analysis of equatorial electrojet responses to the transient changes of solar wind conditions

Introduction: Prior case studies have indicated that changes in solar wind conditions have a significant impact on equatorial ionospheric electrodynamics. However, there have been limited statistical studies on this topic, impairing our understanding of the coupling between solar wind, magnetosphere, and equatorial ionosphere electrodynamics.Methods: In this study, we conducted a superposed epoch analysis of long-term data from the South America equatorial electrojet (EEJ) spanning from 2001 to 2021 examining the responses of the equatorial ionospheric electric field to step-like changes in solar wind velocity, density, dynamic pressure, and interplanetary magnetic field (IMF) Bz.Result: Our study shows that step-like changes in solar wind velocity, density, and dynamic pressure can trigger changes in EEJ within ∼20–40 min. EEJ exhibits the highest sensitivity to variations in solar wind velocity while being relatively less sensitive to changes in dynamic pressure. Furthermore, the response of EEJ shows greater responsiveness to northward IMF Bz compared to southward IMF Bz.Discussion: Our work provides statistical evidence of how changes in solar wind can lead to changes in low-latitude ionospheric EEJ. We inferred that the changes in solar wind conditions cause magnetospheric deformation and changes in magnetic reconnection rates, leading to the fluctuations of the ionospheric electric field and the resultant EEJ variations.

The Role of Magnetic Shear in Reconnection-driven Flare Energy Release

Using observations from the Solar Dynamics Observatory’s Atmosphere Imaging Assembly and the Ramaty High Energy Solar Spectroscopic Imager, we present novel measurements of the shear of post-reconnection flare loops (PRFLs) in SOL20141218T21:40 and study its evolution with respect to magnetic reconnection and flare emission. Two quasi-parallel ribbons form adjacent to the magnetic polarity inversion line (PIL), spreading in time first parallel to the PIL and then mostly in a perpendicular direction. We measure the magnetic reconnection rate from the ribbon evolution, and also the shear angle of a large number of PRFLs observed in extreme ultraviolet passbands (≲1 MK). For the first time, the shear angle measurements are conducted using several complementary techniques allowing for cross validation of the results. In this flare, the total reconnection rate is much enhanced before a sharp increase in the hard X-ray emission, and the median shear decreases from 60°–70° to 20°, on a timescale of 10 minutes. We find a correlation between the shear-modulated total reconnection rate and the nonthermal electron flux. These results confirm the strong-to-weak shear evolution suggested in previous observational studies and reproduced in numerical models, and also confirm that, in this flare, reconnection is not an efficient producer of energetic nonthermal electrons during the first 10 minutes when the strongly sheared PRFLs are formed. We conclude that an intermediate shear angle, ≤40°, is needed for efficient particle acceleration via reconnection, and we propose a theoretical interpretation.

Magnetic Reconnection Rate in the M6.5 Solar Flare on 2015 June 22

Magnetic reconnection is regarded as the mechanism for the rapid release of magnetic energy stored in active regions during solar flares, and quantitative measurements of the magnetic reconnection rate are essential for understanding solar flares. In the context of the standard two-ribbon flare model, we derive the coronal magnetic reconnection rate of the M6.5 flare on 2015 June 22 in two terms, reconnection flux change rate and reconnection electric field, both of which can be obtained from observations of the flare morphology. Data used include a sequence of chromospheric Hα images with unprecedented resolution during the flare from the Visual Imaging Spectrometer of the Goode Solar Telescope (GST) at the Big Bear Solar Observatory and a preflare line-of-sight photospheric magnetogram from the GST Near-InfraRed Imaging Spectropolarimeter along with hard X-ray data from the Ramaty High Energy Solar Spectroscopic Imager. The temporal correlation between the magnetic reconnection rate and nonthermal emission is found, and the variation of the reconnection electric field is mainly determined by the ribbon speed, not by the local magnetic field encountered by the ribbon front. Spatially, the hard X-ray source overlaps with the location of the strongest electric field obtained at the same time. The ribbon motion shows abundant fine structures, including a local acceleration at the location of a light bridge with a weaker magnetic field.

Magnetic Energy Dissipation and γ-Ray Emission in Energetic Pulsars

Some of the most energetic pulsars exhibit rotation-modulated γ-ray emission in the 0.1–100 GeV band. The luminosity of this emission is typically 0.1%–10% of the pulsar spin-down power (γ-ray efficiency), implying that a significant fraction of the available electromagnetic energy is dissipated in the magnetosphere and reradiated as high-energy photons. To investigate this phenomenon we model a pulsar magnetosphere using 3D particle-in-cell simulations with strong synchrotron cooling. We particularly focus on the dynamics of the equatorial current sheet where magnetic reconnection and energy dissipation take place. Our simulations demonstrate that a fraction of the spin-down power dissipated in the magnetospheric current sheet is controlled by the rate of magnetic reconnection at microphysical plasma scales and only depends on the pulsar inclination angle. We demonstrate that the maximum energy and the distribution function of accelerated pairs is controlled by the available magnetic energy per particle near the current sheet, the magnetization parameter. The shape and the extent of the plasma distribution is imprinted in the observed synchrotron emission, in particular, in the peak and the cutoff of the observed spectrum. We study how the strength of synchrotron cooling affects the observed variety of spectral shapes. Our conclusions naturally explain why pulsars with higher spin-down power have wider spectral shapes and, as a result, lower γ-ray efficiency.

First-Principles Theory of the Relativistic Magnetic Reconnection Rate in Astrophysical Pair Plasmas.

We develop a first-principles model for the relativistic magnetic reconnection rate in strongly magnetized pair plasmas. By considering the energy budget and required current density near the x-line, we analytically show that in the magnetically dominated relativistic regime, the x-line thermal pressure is significantly lower than the upstream magnetic pressure due to the extreme energy needed to sustain the current density, consistent with kinetic simulations. This causes the upstream magnetic field lines to collapse in, producing the open outflow geometry which enables fast reconnection. The result is important for understanding a wide range of extreme astrophysical environments, where fast reconnection has been evoked to explain observations such as transient flares and nonthermal particle signatures.

Evidence of oblique electron acoustic solitary waves triggered by magnetic reconnection in Earth’s magnetosphere

Motivated by the recent Magnetospheric Multiscale (MMS) observations of oblique electron acoustic waves, we addressed the generation mechanism of the observed waves by utilizing the reductive perturbation technique. A nonlinear Zakharov-Kuznetsov (ZK) equation is derived for a collisionless, magnetised plasma composed of cool inertial background electrons, cool inertial electron beam, hot inertialess suprathermal electrons; represented by a κ-distribution, and stationary ions. Moreover, the instability growth rate is derived by using the small-k perturbation expansion method. Our findings revealed that the structure of the electrostatic wave profile is significantly influenced by the external magnetic field, the unperturbed hot, cool, and electron beam densities, the obliquity angle, and the rate of superthermality. Such parameters also have an effect on the instability growth rate. This study clarifies the characteristics of the oblique electron solitary waves that may be responsible for changing the electron and ion distribution functions, which alter the magnetic reconnection process. Moreover, the increase of the growth rate with the plasma parameters could be a source of anomalous resistivity that enhances the rate of magnetic reconnection.

Magnetic Reconnection Rates in a Confined X Class Flare

Magnetic Reconnection Rates in a Confined X Class Flare

Acceleration of cold ions in magnetic reconnection

This paper describes the results of 2.5D particle-in-cell kinetic simulations of magnetic reconnection. We consider the case without cold ions (case 0) and with a cold ion flow loaded at two localized positions (cases 1 and 2). The cold ions are loaded on the lower side of the inflow region in cases 1 and 2: along the x-direction, the cold ions are centered at the x location of the X-line in case 1 and are positioned to one side in case 2. Our simulations suggest that cold ions are accelerated and heated near the upper separatrix region in both cases 1 and 2. The gyroradius of the orbit of cold ions increases in the diffusion region owing to the weak magnetic field and then enter the outflow region, where the cold ions pick up the E×B outflow. The cold ions are prevented from crossing the upper separatrix by the Hall electric field, which is negative at the upper exhaust region separatrix. The cold ions are accelerated along the negative z-direction in the upper inflow region by the nonideal electric field (E+vci×B)z for case 1. During this process, the cold ions undergo demagnetization drift association with the finite Larmor radius effect of cold ions. Yet, the reconnection rate decreases after the cold ions flow into the diffusion region. However, the magnetic reconnection rate exhibits no significant changes in case 2.

First-principles theory of the rate of magnetic reconnection in magnetospheric and solar plasmas

The rate of magnetic reconnection is of the utmost importance in a variety of processes because it controls, for example, the rate energy is released in solar flares, the speed of the Dungey convection cycle in Earth’s magnetosphere, and the energy release rate in harmful geomagnetic substorms. It is known from numerical simulations and satellite observations that the rate is approximately 0.1 in normalized units, but despite years of effort, a full theoretical prediction has not been obtained. Here, we present a first-principles theory for the reconnection rate in non-relativistic electron-ion collisionless plasmas, and show that the same prediction explains why Sweet-Parker reconnection is considerably slower. The key consideration of this analysis is the pressure at the reconnection site (i.e., the x-line). We show that the Hall electromagnetic fields in antiparallel reconnection cause an energy void, equivalently a pressure depletion, at the x-line, so the reconnection exhaust opens out, enabling the fast rate of 0.1. If the energy can reach the x-line to replenish the pressure, the exhaust does not open out. In addition to heliospheric applications, these results are expected to impact reconnection studies in planetary magnetospheres, magnetically confined fusion devices, and astrophysical plasmas.

Magnetic reconnection rate during sawtooth crashes in ASDEX Upgrade

The radial velocity of the plasma core during the sawtooth crashes has been measured for the first time with electron cyclotron emission imaging diagnostic. The measurements have been compared with nonlinear two-fluid simulation. The comparison reveals good qualitative and quantitative agreement, which indicates that two-fluid effects (inertia and pressure gradient of electrons) are sufficient for the correct prediction of the experimental results. Contrarily, the crash time of the Kadomtsev model, which is based on a single-fluid picture of magnetic reconnection, disagrees with the experimental results.

Energetic Dynamics of the Inner Magnetosphere in Contact with Fast Solar Wind Currents: Case of the Period 1964-2009

The Earth’s magnetosphere is a magnetic shield that protects Earth from high-energy particles and is subject to a series of internal processes caused by jets of the solar wind (SW) that destabilize it. These disturbances affect health as well as technology and become more extreme when SW is more accelerated. Thus, to better understand the impact of high-speed solar wind (HSSW) invasion on the dynamics of the magnetospheric system, a statistical study of HSSW populations was conducted for even (20 and 22) and odd (21 and 23) solar cycles. The regression analysis using the solar-derived fields from all solar cycles, indicates three states of the inner magnetosphere: 1) the 00:00UT-15:00UT period marked by a magnetic reconnection on the day side of the Earth closest to the Sun with the interplanetary magnetic field (IMF) facing South; 2) the 15:00UT-21:00UT period where IMF changes from South to North and remains there until 21:00UT; and 3) the 21:00UT-24:00UT period where there is a reconnection on the night side with stretched field lines. Observations made at different phases of solar activity lead us to suggest that the magnetospheric electric field (EM) and the Bz component of IMF (IMF-Bz) are strongly correlated not only at a particular time scale, but at different time scales. We believe that the daily fluctuations of the electrical and magnetic effects of magnetospheric origin currents play a very important role in the dayside magnetic reconnection rate. Moreover, examination of the cycles with different parities shows important amplitudes of the solar causes for the even cycles compared to the odd solar cycles. Therefore, even solar cycles have a strong influence on our socio-economic system compared to odd cycles.

Decimetric Type-U Solar Radio Bursts and Associated EUV Phenomena on 2011 February 9

Abstract A GOES M1.9 flare took place in active region AR 11153 on 2011 February 9. With a resolution of 200 kHz and a time cadence of 80 ms, the reverse-drifting (RS) type-III bursts, intermittent sequence of type-U bursts, drifting pulsation structure (DPS), and fine structures were observed by the Yunnan Observatories Solar Radio Spectrometer (YNSRS). Combined information revealed by the multiwavelength data indicated that after the DPS was observed by YNSRS, the generation rate of type-U bursts suddenly increased to 5 times what it had been. In this event, the generation rate of type-U bursts may depend on the magnetic-reconnection rate. Our observations are consistent with previous numerical simulation results. After the first plasmoid produced (plasma instability occurred), the magnetic-reconnection rate suddenly increased by 5 to 8 times. Furthermore, after the DPS, the frequency range of the turnover frequency of type-U bursts was obviously broadened to thrice what it was before, which indicates a fluctuation amplitude of the density in the loop top. Our observations also support numerical simulations during the flare-impulsive phase. Turbulence occurs at the top of the flare loop and the plasmoids can trap nonthermal particles, causing density fluctuation at the loop top. The observations are generally consistent with the results of numerical simulations, helping us to better understand the characteristics of the whole physical process of eruption.

Magnetic reconnection and the Kelvin-Helmholtz instability in the solar corona

Context. The magnetic Kelvin-Helmholtz instability (KHI) has been proposed as a means of generating magnetohydrodynamic turbulence and encouraging wave energy dissipation in the solar corona, particularly within transversely oscillating loops. Aims. Our goal is to determine whether the KHI encourages magnetic reconnection in oscillating flux tubes in the solar corona. This will establish whether the instability enhances the dissipation rate of energy stored in the magnetic field. Methods. We conducted a series of three-dimensional magnetohydrodynamic simulations of the KHI excited by an oscillating velocity shear. We investigated the effects of numerical resolution, field line length, and background currents on the growth rate of the KHI and on the subsequent rate of magnetic reconnection. Results. The KHI is able to trigger magnetic reconnection in all cases, with the highest rates occurring during the initial growth phase. Reconnection is found to occur preferentially along the boundaries of Kelvin-Helmholtz vortices, where the shear in the velocity and magnetic fields is greatest. The estimated rate of reconnection is found to be lowest in simulations where the KHI growth rate is reduced. For example, this is the case for shorter field lines or due to shear in the background field. Conclusions. In non-ideal regimes, the onset of the instability causes the local reconnection of magnetic field lines and enhances the rate of coronal wave heating. However, we found that if the equilibrium magnetic field is sheared across the Kelvin-Helmholtz mixing layer, the instability does not significantly enhance the rate of reconnection of the background field, despite the free energy associated with the non-potential field.

Electric resistivity of partially ionized plasma in the lower solar atmosphere

The lower solar atmosphere is a gravitationally stratified layer of partially ionized plasma. We calculate the electric resistivity in the solar photosphere and chromosphere, which is the key parameter that controls the rate of magnetic reconnection in a Sweet-Parker current sheet. The calculation takes into account the collisions between ions and hydrogen atoms as well as the electron-ion collisions and the electron-hydrogen atom collisions. We find that under the typical conditions of the quiet Sun, electric resistivity is determined mostly by the electron-hydrogen atom collisions in the photosphere, and mostly by the ion-hydrogen collisions, i.e. ambipolar diffusion, in the chromosphere. In magnetic reconnection events with strong magnetic fields, the ambipolar diffusion, however, may be insignificant because the heating by the reconnection itself may lead to the full ionization of hydrogen atoms. We conclude that ambipolar diffusion may be the most important source of electric resistivity responsible for the magnetic flux cancelation and energy release in chromospheric current sheets that can keep a significant fraction of neutral hydrogen atoms.

Dynamical and Thermal Manifestations of the Region above the Top of the Post-flare Loops: MHD Simulations

Abstract Observations proved that a distributed structure named a supra-arcade fan (SAF) exists above post-flare loops in solar eruptions. The locations of the SAF are spatially consistent with various emission sources. Termination shocks (TSs) that are often regarded as an efficient driver for particle acceleration possibly exist in the SAF. We performed the numerical simulations of solar flares based on the standard flare model to study the dynamical and thermal manifestations of the SAF, as well as the possibility of detecting TSs in extreme-ultraviolet (EUV) images. In the simulations, the SAF and TSs can be clearly identified. The motion history and temperature evolution of plasmas inside the SAF indicate that the mass of the SAF comes from the corona and the plasmas are heated in the current sheet. The height of the SAF decreases with the speed of about 64.6 km s−1 when the rate of magnetic reconnection quickly increases, and then increases with a slightly lower velocity of about 50.5 km s−1 after the peak of the rate of magnetic reconnection. The descent−ascent path of the SAF is due to the unbalance of the Lorentz force and the pressure force inside the magnetic loops. In synthetic EUV images, emission intensity variations in the area surrounding TSs are significant, indicating that, depending on the viewing angle, TSs could be identifiable in EUV observations. The results of numerical simulations are generally consistent with observations, helping us to better understand the characteristics of the SAF and the physical natures behind it.