Magnetic Reconnection Rate Research Articles

Extremely energetic 4B/X17.2 flare and associated phenomena

We observed 4B/X17.2 flare in Hα from super-active region NOAA 10486 at ARIES, Nainital. This is one of the largest flares of current solar cycle 23, which occurred near the Sun’s center and produced extremely energetic emission almost at all wavelengths from γ-ray to radio-waves. The flare is associated with a bright/fast full-halo earth directed CME, strong type II, type III and type IV radio bursts, an intense proton event and GLE. This flare is well observed by SOHO, RHESSI and TRACE. Our Hα observations show the stretching/de-twisting and eruption of helically twisted S shaped (sigmoid) filament in the south-west direction of the active region with bright shock front followed by rapid increase in intensity and area of the gigantic flare. The flare shows almost similar evolution in Hα, EUV and UV. We measure the speed of Hα ribbon separation and the mean value is ∼ 70 km s-1. This is used together with photospheric magnetic field to infer a magnetic reconnection rate at three HXR sources at the flare maximum. In this paper, we also discuss the energetics of active region filament, flare and associated CME.

Analysis of Solar Flare Ribbon Evolution: A Semiautomated Approach

We exploit a rare joint set of high-resolution, very high cadence TRACE UV images and high-resolution magnetograms from SOHO MDI to investigate the dynamical properties of flare ribbons in a GOES M1 class flare from NOAA active region 9236 on 2000 November 23 at 23:28 UT. Assuming that flare ribbons locate the chromospheric footpoints of magnetic field lines reconnecting in the corona and that magnetic flux is conserved, we measure the magnetic reconnection rate (in maxwells per second) by overlaying the ribbons on co-registered magnetograms and using intensity-based binary masks to track the magnetic flux swept over by the evolving ribbons, and by assumption swept up in the reconnection. In the event observed, the ribbons did not separate with time but remained stationary while they brightened, lengthened, and faded in place. Thus, the ribbons may be akin to hard X-ray flare kernels moving antiparallel to each other, which others interpret as caused by strong photospheric shear. The derived reconnection rate is noisy, with little correlation between adjacent 1.4 s samples; the peak rate for pixels summed over the ribbon is ~5 × 1018 Mx s-1; the average rise-phase rate is 10 times lower. The local rates for adjacent pixels added to the ribbon at adjacent times show correlations with 1600 A band intensities, supporting the reconnection interpretation. For simple assumptions about geometry, the reconnection appears fast (Vin ≥ 0.01VA). The peak reconnection rates, along with estimates of the current-sheet length scale suggested by measured quantities, imply peak electric fields of order 40 V cm-1. We discuss caveats to these results.

Indirect calculation of the magnetic reconnection rate from flare loops

High time resolution Transition Region And Coronal Explorer (TRACE) 171 and 195 Å observations of the evolution of flare loops on 1999 March 18 have been investigated. Given the location of the magnetic loops on the northeast solar limb and the cadence of the TRACE observations (~50 s), an estimation of the footpoint velocity due to ongoing reconnection was undertaken. This was achieved by calculating the velocity at which successive loops brighten in the emission lines during the postflare phase. A typical footpoint velocity of 1.5 km s km s-1 is obtained and a reconnection rate of ~0.001-0.03 is determined using the method outlined in Isobe et al. (2002, ApJ, 566, 528). This value for the reconnection rate is consistent with the regime outlined by Petschek's model for magnetic reconnection.

Fast magnetic reconnection with the current sheet ejection in the TS‐3 merging experiment

AbstractThe current sheet ejection phenomenon, which causes enhancement of the magnetic reconnection rate, was investigated in the TS‐3 merging experiment at the University of Tokyo. The current sheet was ejected when two coaxial torus plasmas were compressed rapidly by the external force under high‐intensity toroidal field (about 500 G). The magnetic reconnection rate γ increased sharply from 0.14/µs to 0.33/µs right after the ejection out of the X‐point region, while the reconnection rate is low in an initial pileup phase. The current sheet was separated and ejected from the diffusion region by the Lorentz force, jt×Bz. We made a new simple current sheet model on the basis of the mass conservation law, and the theoretical curve of the inflow velocity increment u accorded with the experimental one generally. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 152(3): 1–7, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20067

Magnetic Reconnection Rate and Flux‐Rope Acceleration of Two‐Ribbon Flares

Forbes & Lin derived simple equations to link the properties of magnetic reconnection in the corona to observed signatures of solar flares. We measured the photospheric magnetic fields and the flare ribbon separation speeds then applied these equations to derive two physical terms for the magnetic reconnection rates: the rate of magnetic flux change rec involved in magnetic reconnection in the low corona and the electric field Erec inside the reconnecting current sheet (RCS) that is generated during magnetic reconnection. The central interest in this work is to investigate and quantify the statistical correlation between the magnetic reconnection rate and the corresponding flux-rope acceleration. From a sample of 13 well-observed two-ribbon flares, which are associated with filament eruptions or coronal mass ejections (CMEs), the acceleration of erupting filaments is found mainly in the range of 0.05-0.4 km s-2, up to 3 km s-2. Correspondingly, the maximum Erec and rec mostly occur in the range of 0.2-5 V cm-1 and 0.5-6 × 1018 Mx s-1, respectively. A positive and strong correlation is found with a cross-correlation coefficient of 0.94-0.97 between the magnetic reconnection rate and the acceleration of erupting filaments that represents the early stages of flux-rope eruptions in the low corona. However, the inferred reconnection rate is not correlated to the acceleration of CME fronts measured by the Large Angle and Spectrometric Coronagraph (LASCO) observations in the range of 2-30 solar radii (the correlation coefficient is less than 0.2). A reasonable correlation is found between the reconnection rate and the velocity of CMEs, which indicates the cumulative acceleration of CMEs from the low corona to the LASCO C2 field of view. The temporal correlation between the magnetic reconnection rate and the flare nonthermal emissions has also been verified in this paper.

Hall Magnetic Reconnection Rate

Two-dimensional Hall magnetohydrodynamic simulations are used to determine the magnetic reconnection rate in the Hall limit. The simulations are run until a steady state is achieved for four initial current sheet thicknesses: L=1,5,10, and 20c/omega(pi), where c/omega(pi) is the ion inertial length. It is found that the asymptotic (i.e., time independent) state of the system is nearly independent of the initial current sheet width. Specifically, the Hall reconnection rate is weakly dependent on the initial current layer width and is partial differential Phi/ partial differential t less, similar 0.1V(A0)B0, where Phi the reconnected flux, and V(A0) and B0 are the Alfvén velocity and magnetic field strength in the upstream region. Moreover, this rate appears to be independent of the scale length on which the electron "frozen-in" condition is broken (as long as it is <c/omega(pi)) and of the system size.

Scaling of forced magnetic reconnection in the Hall-magnetohydrodynamical Taylor problem with arbitrary guide field

Two-dimensional, nonlinear, Hall-magnetohydrodynamical (MHD) numerical simulations are used to investigate the scaling of the rate of forced magnetic reconnection in the so-called Taylor problem. In this problem, a small amplitude boundary perturbation is suddenly applied to a tearing stable, slab plasma equilibrium. The perturbation is such as to drive magnetic reconnection within the plasma. This type of reconnection, which is not due to an intrinsic plasma instability, is generally termed “forced reconnection.” Hall effects are found to greatly accelerate the rate of magnetic reconnection, relative to the well-known Sweet–Parker rate. In the nonlinear Hall-MHD regime with arbitrary guide field, the peak reconnection rate is found to be independent of the resistivity, and to scale like dψ/dt∼[β/(1+β)]3/4di3/2Ⅺ02, where β is the plasma beta calculated using the guide field, di the collisionless ion skin depth, and Ⅺ0 the amplitude of the boundary perturbation.

Traces of the Dynamic Current Sheet during a Solar Flare

High-cadence and high-resolution time sequences of far Halpha off-band images provide a unique tool to study the evolution of the fine structure of flare kernels. The fine structure contains important information on flare topology and the triggering mechanism. In this Letter, we concentrate on the rapid changes of the relative positions of two conjugate flare footpoints. In order to carry out this study with the highest physical precision, we use r(c) = Sigmar(j)I(j)/SigmaI(j) (I-j is the Halpha brightness at r(j)) to compute the centroid of an Halpha bright kernel region caused by solar flares. Using this, we probe the fine temporal structures connected to the distance between the centroids of two conjugate kernels of an M2.3 flare. The flare, which occurred on 2002 September 9 in NOAA Active Region 0105, was observed at Big Bear Solar Observatory at the far off-band center wavelength of Halpha - 1.3 Angstrom, with a cadence of similar to40 ms. The flare was also observed by RHESSI. The time profile of the separation distance shows an excellent anticorrelation to that of the hard X-ray (HXR) emissions in 25-50 keV, which exhibit a number of separate spikes ( the linear Pearson correlation coefficient is found to be similar to-0.83). The separation between the two centroids decreases at the rising periods of four HXR spikes, then it increases after the peak time of the flare to show the expected separation motion. The most obvious decreasing, which occurred during the first HXR peak, was confirmed by corresponding images. This implies that during the impulsive phases, the energy transported from the corona is deposited increasingly inwardly between the two kernels. This new and perhaps surprising tendency for the energy deposition can be explained as being caused by current sheet pinch motions, which, at the same time, enhance the magnetic energy reconnection rate to produce the observed HXR spikes.

Magnetic Reconnection and Mass Acceleration in Flare–Coronal Mass Ejection Events

An observational relationship has been well established among magnetic reconnection, high-energy flare emissions and the rising motion of erupting flux ropes. In this paper, we verify that the rate of magnetic reconnection in the low corona is temporally correlated with the evolution of flare nonthermal emissions in hard X-rays and microwaves, all reaching their peak values during the rising phase of the soft X-ray emission. In addition, however, our new observations reveal a temporal correlation between the magnetic reconnection rate and the directly observed acceleration of the accompanying coronal mass ejection (CME) and filament in the low corona, thus establishing a correlation with the rising flux rope. These results are obtained by examining two well-observed two-ribbon flare events, for which we have good measurements of the rise motion of filament eruption and CMEs associated with the flares. By measuring the magnetic flux swept through by flare ribbons as they separate in the lower atmosphere, we infer the magnetic reconnection rate in terms of the reconnection electric field Erec inside the reconnecting current sheet (RCS) and the rate of magnetic flux convected into the diffusion region. For the X1.6 flare event, the inferred Erec is ~5.8 V cm-1 and the peak mass acceleration is ~3 km s-2, while for the M1.0 flare event Erec is ~0.5 V cm-1 and the peak mass acceleration is 0.2-0.4 km s-2.

Scaling of forced magnetic reconnection in the Hall-magnetohydrodynamic Taylor problem

Two-dimensional, incompressible, zero guide-field, nonlinear Hall-MHD (magnetohydrodynamical) simulations are used to investigate the scaling of the rate of forced magnetic reconnection in the so-called Taylor problem. In this problem, a small-amplitude boundary perturbation is suddenly applied to a tearing stable, slab plasma equilibrium; the perturbation being such as to drive magnetic reconnection within the plasma. This type of reconnection, which is not due to an intrinsic plasma instability, is generally known as “forced reconnection.” The inclusion of the Hall term in the plasma Ohm’s law is found to greatly accelerate the rate of magnetic reconnection. In the linear Hall-MHD regime, the peak instantaneous reconnection rate is found to scale like dΨ/dt∼diη1/3Ⅺ0, where Ψ is the reconnected magnetic flux, di the collisionless ion skin depth, η the resistivity, and Ⅺ0 the amplitude of the boundary perturbation. In the nonlinear Hall-MHD regime, the peak reconnection rate is found to scale like dΨ/dt∼di3/2Ⅺ02.

TS-3における電流シート放出による高速磁気リコネクション実験

The current sheet ejection phenomenon, which causes enhancement of the magnetic reconnection rate, was investigated in the TS-3 merging experiment at the University of Tokyo. The current sheet was ejected when two coaxial torus plasmas were compressed rapidly by the external force under high-intensity toroidal field (about 500 G). The magnetic reconnection rate γ increased sharply from 0.14/µs to 0.33/µs right after the ejection out of the X-point region, while the reconnection rate is low in an initial pileup phase. The current sheet was separated and ejected from the diffusion region by the Lorentz force, jt×Bz. We made a new simple current sheet model on the basis of the mass conservation law, and the theoretical curve of the inflow velocity increment u accorded with the experimental one generally. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 152(3): 1–7, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20067

Flux Rope Acceleration and Enhanced Magnetic Reconnection Rate

A physical mechanism of flares has emerged from our 2.5-dimensional resistive MHD simulations of the dynamical evolution of current sheet formation and magnetic reconnection and flux rope acceleration subject to the continuous, slow increase of magnetic shear in the arcade. With anomalous nonuniform resistivity in the current sheet the simulation results relate the flux rope's accelerated rising motion with an enhanced magnetic reconnection rate and thus an enhanced reconnection electric field in the current sheet during the flare rise phase. The simulation results provide good quantitative agreement with observations of the acceleration of flux ropes, which are manifested in the form of ejected soft X-ray plasmas, erupting filaments, or CMEs. For the X-class flare events studied in this paper the peak reconnection electric field is ~O(103 V m-1) or larger, enough to accelerate electrons to over 100 keV in a field-aligned distance of 0.1 km and produce impulsive hard X-ray emission observed during the flare rise phase.

Wave driven magnetic reconnection in the Taylor problem

An improved Laplace transform theory is developed in order to investigate the initial response of a stable slab plasma equilibrium enclosed by conducting walls to a suddenly applied wall perturbation in the so-called Taylor problem. The novel feature of this theory is that it does not employ asymptotic matching. If the wall perturbation is switched on slowly compared to the Alfvén time then the plasma response eventually asymptotes to that predicted by conventional asymptotic matching theory. However, at early times there is a compressible Alfvén wave driven contribution to the magnetic reconnection rate which is not captured by asymptotic matching theory, and leads to a significant increase in the reconnection rate. If the wall perturbation is switched on rapidly compared to the Alfvén time then strongly localized compressible Alfvén wave-pulses are generated which bounce backward and forward between the walls many times. Each instance these wave-pulses cross the resonant surface they generate a transient surge in the reconnection rate. The maximum pulse driven reconnection rate can be much larger than that predicted by conventional asymptotic matching theory.

Explosive and oscillatory tearing-mode instability in gyrotropic plasmas

Linear theory of resistive tearing-mode instability in gyrotropic plasmas has predicted that for large pressure anisotropy of p⊥&amp;gt;p∥ the eigenmode is not purely exponential but contains oscillation with the overall growth rate much larger than the isotropic case. This paper shows that such solutions may develop in the nonlinear anisotropic magnetohydrodynamic model; in particular, the X-line and O-line take place alternatively with magnetic islands growing and saturated with oscillation period of about ten Alfvén transit times. The growth rate is on the order of the Alfvén time scale and the plasma flow velocity may reach the Alfvén speed. The presence of the mirror-mode wave may result in magnetic bottle structure on top of the X- and O-lines that is responsible for the enhanced growth rate and oscillation of magnetic reconnection.

Study of Ribbon Separation of a Flare Associated with a Quiescent Filament Eruption

In this paper, we present a detailed study of a two-ribbon flare in the plage region observed by Kanzelhohe Solar Observatory (KSO), which is one of the stations in our global Hα network. We select this event due to its very clear filament eruption, two-ribbon separation, and association with a fast coronal mass ejection (CME). We study the separation between the two ribbons seen in Hα as a function of time and find that the separation motion consisted of a fast stage of rapid motion at a speed of about 15 km s-1 in the first 20 minutes and a slow stage with a separation speed of about 1 km s-1 lasting for 2 hr. We then estimate the rate of the magnetic reconnection in the corona, as represented by the electric fields Ec in the reconnecting current sheet, by measuring the ribbon motion speed and the magnetic fields obtained from MDI. We find that there were two stages as well in evolution of the electric fields: Ec = 1 V cm-1 averaged over 20 minutes in the early stage, followed by Ec = 0.1 V cm-1 in the subsequent 2 hr. The two stages of the ribbon motion and electric fields coincide with the impulsive and decaying phases of the flare, respectively, yielding clear evidence that the impulsive flare energy release is governed by the fast magnetic reconnection in the corona. We also measure the projected heights of the erupting filament from KSO Hα and SOHO/EIT images. The filament started to rise 20 minutes before the flare. After the flare onset, it was accelerated quickly at a rate of 300 m s-2, and in 20 minutes, reached a speed of at least 540 km s-1, when it disappeared beyond the limb in the EIT observations. The acceleration rate of the CME is estimated to be 58 m s-2 during the decay phase of the flare. The comparison of the height and velocity profiles between the filament and CME suggests that fast acceleration of mass ejections occurred during the impulsive phase of the flare, when the magnetic reconnection rate was also large, with Ec = 1 V cm-1.

Variations in the polar cap area during two substorm cycles

Abstract. This study employs observations from several sources to determine the location of the polar cap boundary, or open/closed field line boundary, at all local times, allowing the amount of open flux in the magnetosphere to be quantified. These data sources include global auroral images from the Ultraviolet Imager (UVI) instrument on board the Polar spacecraft, SuperDARN HF radar measurements of the convection flow, and low altitude particle measurements from Defense Meteorological Satellite Program (DMSP) and National Oceanographic and Atmospheric Administration (NOAA) satellites, and the Fast Auroral SnapshoT (FAST) spacecraft. Changes in the open flux content of the magnetosphere are related to the rate of magnetic reconnection occurring at the magnetopause and in the magnetotail, allowing us to estimate the day- and nightside reconnection voltages during two substorm cycles. Specifically, increases in the polar cap area are found to be consistent with open flux being created when the IMF is oriented southwards and low-latitude magnetopause reconnection is ongoing, and decreases in area correspond to open flux being destroyed at substorm breakup. The polar cap area can continue to decrease for 100 min following the onset of substorm breakup, continuing even after substorm-associated auroral features have died away. An estimate of the dayside reconnection voltage, determined from plasma drift measurements in the ionosphere, indicates that reconnection can take place at all local times along the dayside portion of the polar cap boundary, and hence presumably across the majority of the dayside magnetopause. The observation of ionospheric signatures of bursty reconnection over a wide extent of local times supports this finding.Key words. Ionosphere (plasma convection; polar ionosphere) – Magnetospheric physics (magnetospheric configuration and dynamics)

Experimental Analysis of the Fast Magnetic Reconnection with the Current Sheet Ejection

We confirmed significant increase in the magnetic reconnection rate caused by a new phenomenon called the current sheet ejection. We focused on the relation between the shape distortion of the current sheet and the reconnection rate (inflow velocity), and concluded that a new simple current sheet model explain our experimental results qualitatively using mass conservation law.

Solar wind effects on ionospheric convection: a review

The solar wind, magnetosphere, and ionosphere are intrinsically coupled through magnetic field lines. The electrodynamic state of the high-latitude ionosphere is controlled by several geophysical processes, such as the location and rate of magnetic reconnection at the magnetopause and in the magnetotail, and the energisation and precipitation of solar wind and magnetospheric plasmas. Amongst the most observed ionospheric manifestation of solar wind/magnetospheric processes are the convection bursts associated with the so-called flux transfer events (FTEs), magnetic impulse events (MIEs), and travelling convection vortices (TCVs). Furthermore, the large-scale ionospheric convection configuration has also demonstrated a strong correspondence to variations in the interplanetary medium and substorm activity. This report briefly discusses the progress made over the past decade in studies of these transient convection phenomena and outlines some unsettled questions as well as future research directions.

The topology of guiding center orbits in a linear magnetic cusp

Particle orbits for a linear magnetic cusp configuration, characteristic for magnetic reconnection, are analyzed within the guiding center approximation. A class of particle orbits encircling the magnetic X-line is identified and the orbit phase space partition is obtained and characterized as a function of dimensionless parameters controlling the orbit topology. The large majority of particles are found to be trapped in the direction of the X-line implying large neoclassical corrections to the plasma resistivity. This may be the key in explaining the high rates of collisionless magnetic reconnection observed in nature.

Incoherent scatter radar identification of the dayside magnetic separatrix and measurement of magnetic reconnection

Incoherent scatter (IS) radar measurements of the ionospheric electron density are analyzed for the signature of the magnetic separatrix in the dayside ionosphere by comparison with separatrix locations determined using precipitating electron data from DMSP spacecraft within 1.5 hours in magnetic local time (MLT) of the Sondrestrom radar. A model of photoionization is used to remove its effects from the measured electron density to study the ionization by precipitating particles. The altitude of peak ionization and the peak ionization rate are used to determine whether a magnetic field line is open or closed. We observe that on closed field lines the peak ionization rate is high (&gt; 2200 cm−3 s−1) or the peak is lower than 140 km in altitude, and on open field lines the peak ionization is low (&lt; 900 cm−3 s−1) or the peak is higher than 140 km. These rules are used to identify the separatrix with an accuracy of 0.36° and a precision of ±0.39°. This signature of the separatrix is only apparent in the prenoon and noon sectors (0600 to 1300 MLT). In the postnoon sector (1630–1800 MLT) the altitude of peak ionization and the peak ionization rate do not show any systematic difference between the open and closed field line regions. Finally, an example is presented in which the data on the location and motion of the separatrix and IS radar measurements of F region plasma velocity are used to measure the magnetic reconnection rate as a function of the interplanetary magnetic field clock angle.