Tearing Instability Research Articles

Taurine exerts antioxidant and osmoprotecting activity: an in vitro and in vivo study

PurposeTo evaluate the effects of ophthalmic solutions based on sodium hyaluronate (SH) with or without taurine (TAU) in experimental dry eye disease models.MethodsRabbit corneal epithelial cells (SIRCs) were exposed to oxidative stress (1 mM H2O2) and treated with the following formulations: 0.2% SH, 0.4% SH, 0.4% SH + 0.5% TAU. Reactive oxygen species (ROS) were assessed by commercial kit (ab113851). Dry eye was induced in albino rabbits by topical application (4 times per day) of 1% atropine eye drops. Fifteen minutes after atropine instillation we treated the eyes with test formulations (0.2% SH, 0.4% SH, 0.4% SH + 0.5% TAU). The following endpoints were evaluated: tear breakup time (TBUT), Schirmer's test, ferning test, tear osmolarity. Results were compared to negative (CTR‐; normal eye) and positive control groups (CTR+; atropine‐treated eye).ResultsTaurine significantly (p < 0.01) quenched ROS production in SIRC after oxidative stress. The effect of taurine, in terms of ROS quenching, was significantly (p < 0.01) higher compared to SH‐treated cells. Topical administration of atropine in the rabbit eye significantly (p < 0.01) reduced tear volume and TBUT. Ferning test and tear osmolarity were also significantly (p < 0.01) modified by atropine treatment. All the altered parameters were significantly (p < 0.01) reversed by 0.4% SH + 0.5% TAU treatment. Furthermore, treatment with SH‐TAU formulation was more effective compared to SH formulations.ConclusionsAll together these data demonstrated that taurine has a potent antioxidant activity preventing the negative effect elicited by atropine on tear stability. Therefore, our findings support the hypothesis that the formulation containing taurine may be more useful than SH formulations in clinical practice to manage ocular surface diseases related to dry eye.

Resistive tearing instability in electron MHD: application to neutron star crusts

We study a resistive tearing instability developing in a system evolving through the combined effect of Hall drift in the Electron-MHD limit and Ohmic dissipation. We explore first the exponential growth of the instability in the linear case and we find the fastest growing mode, the corresponding eigenvalues and dispersion relation. The instability growth rate scales as $\gamma \propto B^{2/3} \sigma^{-1/3}$ where $B$ is the magnetic field and $\sigma$ the electrical conductivity. We confirm the development of the tearing resistive instability in the fully non-linear case, in a plane parallel configuration where the magnetic field polarity reverses, through simulations of systems initiating in Hall equilibrium with some superimposed perturbation. Following a transient phase, during which there is some minor rearrangement of the magnetic field, the perturbation grows exponentially. Once the instability is fully developed the magnetic field forms the characteristic islands and X-type reconnection points, where Ohmic decay is enhanced. We discuss the implications of this instability for the local magnetic field evolution in neutron stars' crusts, proposing that it can contribute to heating near the surface of the star, as suggested by models of magnetar post-burst cooling. In particular, we find that a current sheet a few meters thick, covering as little as $1\%$ of the total surface can provide $10^{42}~$erg in thermal energy within a few days. We briefly discuss applications of this instability in other systems where the Hall effect operates such as protoplanetary discs and space plasmas.

Effect of Ambient Temperature on the Human Tear Film.

During everyday life, the tear film is exposed to a wide range of ambient temperatures. This study aims to investigate the effect of ambient temperature on tear film physiology. A controlled environment chamber was used to create different ambient temperatures (5, 10, 15, 20, and 25°C) at a constant relative humidity of 40%. Subjects attended for two separate visits and were exposed to 25, 20, and 15°C at one visit and to 10 and 5°C at the other visit. The subjects were exposed to each room temperature for 10 min before investigating tear film parameters. The order of the visits was random. Tear physiology parameters assessed were tear evaporation rate, noninvasive tear break-up time (NITBUT), lipid layer thickness (LLT), and ocular surface temperature (OST). Each parameter was assessed under each condition. A threefold increase in tear evaporation rate was observed as ambient temperature increased to 25°C (P=0.00). The mean evaporation rate increased from 0.056 μL/min at 5°C to 0.17 μL/min at 25°C. The mean NITBUT increased from 7.31 sec at 5°C to 12.35 sec at 25°C (P=0.01). A significant change in LLT was also observed (P=0.00), LLT median ranged between 20 and 40 nm at 5 and 10°C and increased to 40 and 90 nm at 15, 20, and 25°C. Mean reduction of 4°C OST was observed as ambient temperature decreased from 25 to 5°C. Ambient temperature has a considerable effect on human tear film characteristics. Tear evaporation rate, tear LLT, tear stability, and OST were considerably affected by ambient temperature. Chronic exposure to low ambient temperature would likely result in symptoms of dry eye and ultimately ocular surface disorders.

Agreement of noninvasive tear break-up time measurement between Tomey RT-7000 Auto Refractor-Keratometer and Oculus Keratograph 5M.

BackgroundIt is difficult to standardize assessment of dry eye in different clinical settings. Increasingly, tear stability is recognized to be important for the definition and assessment of patients with dry eye. Recently, two commercially available instruments have been made available for objectively measuring noninvasive tear break-up time (NIBUT), as an indicator of tear stability: the Tomey RT-7000 Auto Refractor-Keratometer and Oculus Keratograph (K)5M. We aim to assess the agreement of NIBUT measurements using these modalities.MethodsThis prospective cross-sectional study was carried out in a tertiary referral eye center and involved 126 consecutive dry eye patients. NIBUT assessment was performed on the right eyes of participants with both the RT-7000 and the K5M techniques, with the order of assessment randomized. The Standardized Patient Evaluation of Eye Dryness (SPEED) questionnaires were administered to assess dry eye symptoms in the 2 weeks before assessment.ResultsThe age of the participants was 56.0±14.3 years (69.84% females). Measurements for both modalities were non-normally distributed (right-skewed). The median RT-7000 and K5M readings were 4.2 (range 0.1–10.0) and 6.4 (0.1–24.9) seconds, respectively. RT-7000 and K5M readings were poorly correlated (ρ=0.061, P=0.495). Intraclass correlation coefficient between the modalities was 0.187 (95% confidence interval −0.097 to 0.406). The Bland–Altman plot showed no systematic differences between the readings with these machines. The agreement between machines was not different in different SPEED categories.ConclusionWhile there are theoretical and practical benefits of NIBUT for assessment of tear stability over dye-based methods, the agreement between the two modalities was poor. Hence, studies and trials assessing NIBUT should avoid using these modalities interchangeably for NIBUT assessment. More research is needed to improve consensus on how to determine NIBUT.

MHD simulations of three-dimensional resistive reconnection in a cylindrical plasma column

Magnetic reconnection is a plasma phenomenon where a topological rearrangement of magnetic field lines with opposite polarity results in dissipation of magnetic energy into heat, kinetic energy and particle acceleration. Such a phenomenon is considered as an efficient mechanism for energy release in laboratory and astrophysical plasmas. An important question is how to make the process fast enough to account for observed explosive energy releases. The classical model for steady state magnetic reconnection predicts reconnection times scaling as $S^{1/2}$ (where $S$ is the Lundquist number) and yields times scales several order of magnitude larger than the observed ones. Earlier two-dimensional MHD simulations showed that for large Lundquist number the reconnection time becomes independent of $S$ ("fast reconnection" regime) due to the presence of the secondary tearing instability that takes place for $S \gtrsim 1 \times 10^4$. We report on our 3D MHD simulations of magnetic reconnection in a magnetically confined cylindrical plasma column under either a pressure balanced or a force-free equilibrium and compare the results with 2D simulations of a circular current sheet. We find that the 3D instabilities acting on these configurations result in a fragmentation of the initial current sheet in small filaments, leading to enhanced dissipation rate that becomes independent of the Lundquist number already at $S \simeq 1\times 10^3$.

Short-Term Omega 3 Fatty Acids Treatment for Dry Eye in Young and Middle-Aged Visual Display Terminal Users.

To evaluate the effect of an omega 3 fatty acid (O3FA) oral supplement (2,400 mg/day) for 45 days on dry eye symptoms, tear production, stability, and conjunctival cytology in young and middle-aged visual display terminal (VDT) users. Institutional review board approval was obtained, and a randomized, double-blind, interventional study was done; eyes of 256 VDT users were randomized to receive 4 capsules twice daily for 45 days (O3FA group), each containing 180 mg of eicosapentaenoic acid and 120 mg docosahexaenoic acid. The O3FA group was compared with another group (n=266) who received 8 capsules of a placebo (olive oil). Patients were evaluated at baseline, 30 days, and 45 days. The primary outcome measure was an improvement in dry eye symptoms. Secondary outcome measures were improvement in the Nelson grade on conjunctival impression cytology, Schirmer test values, and tear film breakup time (TBUT). Means of groups (pretreatment, day 30, and day 45) were compared with repeated-measure analysis of variance. The relation between the outcome variables and VDT time was evaluated using linear regression. In the O3FA group, the mean symptom score differed significantly (P<0.005) (pretreatment, 30 days, and 45 days); the TBUT and Nelson grade also improved significantly but only after 45 days of intervention. Schirmer test values did not differ significantly after adjustment for multiple comparisons (P=0.010). The change was not significant in the placebo group. Consumption of 2,400 mg/day of O3FA supplement improves symptoms, tear stability, and conjunctival cytology but not tear production in symptomatic VDT users.

Photorefractive keratectomy for patients with preoperative low Schirmer test value

PurposeTo compare dry eye signs and symptoms between patients with preoperative low and normal Schirmer test after Photorefractive keratectomy (PRK). MethodsIn this prospective, nonrandomized, comparative case series, 76 eyes of 76 patients were preoperatively categorized into two groups according to selected criteria for characterization of tear film status: the low Schirmer test value (STV) group and the normal STV group. For the tear function assessment, we performed a Schirmer test with and without anesthesia, tear break-up time (TBUT) test, and measurement dry eye symptoms using the Farsi translation of Ocular Surface Disease Index (OSDI) questionnaire pre- and 3 months post-operation. ResultsPostoperatively, the Schirmer and TBUT values were significantly lower in both groups than preoperatively (all p < 0.05). Deterioration in tear secretion was significantly greater in the low STV group (p = 0.012), but tear stability was more compromised in the normal STV group (p = 0.021). The changes in OSDI score were not significant between the two groups. ConclusionThese results demonstrated that tear function deteriorates after PRK. Therefore, patients with low preoperative Schirmer test values should be thoroughly assessed for dry eye before proceeding with refractive surgery to eliminate postoperative complication.

Fast reconnection in relativistic plasmas: the magnetohydrodynamics tearing instability revisited

Fast reconnection operating in magnetically dominated plasmas is often invoked in models for magnetar giant flares, for magnetic dissipation in pulsar winds, or to explain the gamma-ray flares observed in the Crab nebula, hence its investigation is of paramount importance in high-energy astrophysics. Here we study, by means of two dimensional numerical simulations, the linear phase and the subsequent nonlinear evolution of the tearing instability within the framework of relativistic resistive magnetohydrodynamics, as appropriate in situations where the Alfven velocity approaches the speed of light. It is found that the linear phase of the instability closely matches the analysis in classical MHD, where the growth rate scales with the Lundquist number S as S^-1/2, with the only exception of an enhanced inertial term due to the thermal and magnetic energy contributions. In addition, when thin current sheets of inverse aspect ratio scaling as S^-1/3 are considered, the so-called "ideal" tearing regime is retrieved, with modes growing independently on S and extremely fast, on only a few light crossing times of the sheet length. The overall growth of fluctuations is seen to solely depend on the value of the background Alfven velocity. In the fully nonlinear stage we observe an inverse cascade towards the fundamental mode, with Petschek-type supersonic jets propagating at the external Alfven speed from the X-point, and a fast reconnection rate at the predicted value R~(ln S)^-1.

The influence of rigid gas permeable lens wear on the concentrations of dinucleotides in tears and the effect on dry eye signs and symptoms in keratoconus.

To evaluate the signs and symptoms of dry eye and dinucleotide secretion in tears of keratoconus patients (KC) and the potential effect of rigid gas permeable (RGP) contact lens wear. Twenty-three KC patients and forty control subjects were enrolled in this study. Signs of dry eye including tear volume, tear stability and corneal staining along with symptoms were assessed using the McMonnies questionnaire. Tears were collected using Schirmer strips, and dinucleotide concentrations in collected tears measured using high pressure liquid chromatography. Values obtained in KC and controls were compared. The effect of contact lens wear in KC was also assessed. KC eyes showed a significantly lower tear volume compared to controls, shorter tear break up time (TBUT), higher corneal staining and higher McMonnies dry eye questionnaire scores (p<0.05). When compared with non-wearers, KC contact lens wearers showed significantly higher symptoms, lower Schirmer and TBUT values (p<0.05). Concentration of Ap4A (0.695±0.304μM vs. 0.185±0.178μM) and Ap5A (0.132±0.128μM vs. 0.045±0.036μM) were higher in KC compared to controls (p<0.001) and only Ap4A was statistically higher in RGP wearers compared to non-wearers (0.794±0.478μM vs. 0.417±0.313μM) (p<0.05). Signs and symptoms of dry eye as well as concentrations of Ap4A and Ap5A were markedly increased in KC patients compared to controls. Moreover, Ap4A and symptoms of dry eye were statistically higher in RGP wearers compared to non-wearers. This seems to indicate that factors such as RGP contact lens wear might exacerbate the clinical condition of dry eye.

Design of the power supply system for the plasma current modulation on J-TEXT tokamak

In order to further study the influence of current modulation parameters on suppressing tearing instability, the plasma current should be modulated in a wider range. So a modification scheme is designed to improve the performance of ohmic heating power supply system on J-TEXT tokamak. A multilevel cascade circuit with carrier phase-shifted PWM technique has been proposed. Coupled inductors are connected in the form of cyclic symmetry to restrain the circulating current caused by multiple paralleled branches. The simulation proves this proposed current modulation power supply system matches output requirement and achieves good current sharing effect. Finally, a prototype is designed, and the experiment results can verify the correctness of the simulation model well.

The ideal tearing mode: theory and resistive MHD simulations

Classical MHD reconnection theories, both the stationary Sweet-Parker model and the tearing instability, are known to provide rates which are too slow to explain the observations. However, a recent analysis has shown that there exists a critical threshold on current sheet's thickness, namely a/L ∼ S-1/3, beyond which the tearing modes evolve on fast macroscopic Alfvénic timescales, provided the Lunquist number S is high enough, as invariably found in solar and astrophysical plasmas. Therefore, the classical Sweet-Parker scenario, for which the diffusive region scales as a/L ∼ S-1/2 and thus can be up to ∼ 100 times thinner than the critical value, is likely to be never realized in nature, as the current sheet itself disrupts in the elongation process. We present here two-dimensional, compressible, resistive MHD simulations, with S ranging from 105 to 107, that fully confirm the linear analysis. Moreover, we show that a secondary plasmoid instability always occurs when the same critical scaling is reached on the local, smaller scale, leading to a cascading explosive process, reminiscent of the flaring activity.

MULTIPLE CURRENT SHEET SYSTEMS IN THE OUTER HELIOSPHERE: ENERGY RELEASE AND TURBULENCE

ABSTRACT In the outer heliosphere, beyond the solar wind termination shock, it is expected that the warped heliospheric current sheet forms a region of closely packed, multiple, thin current sheets. Such a system may be subject to the ion-kinetic tearing instability, and hence may generate magnetic islands and hot populations of ions associated with magnetic reconnection. Reconnection processes in this environment have important implications for local particle transport, and for particle acceleration at reconnection sites and in turbulence. We study this complex environment by means of three-dimensional hybrid simulations over long timescales, in order to capture the evolution from linear growth of the tearing instability to a fully developed turbulent state at late times. The final state develops from the highly ordered initial state via both forward and inverse cascades. Component and spectral anisotropy in the magnetic fluctuations is present when a guide field is included. The inclusion of a population of newborn interstellar pickup protons does not strongly affect these results. Finally, we conclude that reconnection between multiple current sheets can act as an important source of turbulence in the outer heliosphere, with implications for energetic particle acceleration and propagation.

Effects of wearing a daily disposable lens on tear film: a randomised controlled trial

PurposeContact lens‐induced dry eye is commonly encountered, although its extent is not well documented with daily disposable lenses. A novel type of contact lens system incorporating moisturising agent (alginic acid) has been developed. The aim of this study was to evaluate the effect of wearing daily 2‐hydroxyethyl methacrylate disposable contact lenses for seven days on tear stability, conjunctival and limbal redness and dry eye symptoms. Then, we aimed to determine whether lens solutions containing alginic acid had any influence on tear parameters.MethodsThis was a seven‐day parallel group double‐masked clinical trial of previous contact lens wearers, where participants were randomly assigned to wearing SEED 1dayPure moisture contact lenses with (n = 15) or without alginic acid (n = 15). Tear lipid layer thickness (LLT), non‐invasive tear break‐up time (NIBUT), conjunctival redness, corneal fluorescein staining, tear break‐up time and Schirmer I readings were measured. Symptom severity and frequency were evaluated and combined using a global score from visual analogue scales.ResultsThe mean age and standard deviation of the participants was 25 ± 3.8 years. There were 24 females and six males. After reintroduction of contact lens wear for one week, there was significant improvement in the global symptom score; however, this may not be clinically significant. There were small and clinically insignificant changes in limbal and conjunctival hyperaemia but no significant changes in lipid layer thickness, Schirmer I and NIBUT in overall participants. The use of alginic acid in lens solutions did not affect these parameters compared to control lenses (p > 0.05).ConclusionsAfter seven days of wearing a modern daily disposable lens, there was no significant deterioration of tear function in a group of young contact lens wearers. In this short‐term study, there was no evidence of significant benefit of lens solutions containing alginic acid used with HEMA lenses.

Bifurcations of axisymmetric plasma equilibrium in a tokamak

Bifurcation of solutions to the Grad–Shafranov-type equation for helically symmetric plasma near the threshold for tearing instability are analyzed. Quadratic and cubic nonlinearities were added to the linear dependence of the current density on the helical flux. Depending on the character of nonlinearity, two types of bifurcation can be observed, the “small” and the “large” ones. The small bifurcation is typical of cubic nonlinearity and reveals itself in the growth of the magnetic island from zero as the profile parameter increases above the instability threshold. The large bifurcation is typical of quadratic nonlinearity and causes jumplike formation of a large-scale magnetic island upon exceeding the instability threshold. As the profile parameter decreases below the instability threshold, the large-scale island continues to persist for some time (the hysteresis effect) and then suddenly disappears.

Multi-scale structures of turbulent magnetic reconnection

We have analyzed data from a series of 3D fully kinetic simulations of turbulent magnetic reconnection with a guide field. A new concept of the guide filed reconnection process has recently been proposed, in which the secondary tearing instability and the resulting formation of oblique, small scale flux ropes largely disturb the structure of the primary reconnection layer and lead to 3D turbulent features [W. Daughton et al., Nat. Phys. 7, 539 (2011)]. In this paper, we further investigate the multi-scale physics in this turbulent, guide field reconnection process by introducing a wave number band-pass filter (k-BPF) technique in which modes for the small scale (less than ion scale) fluctuations and the background large scale (more than ion scale) variations are separately reconstructed from the wave number domain to the spatial domain in the inverse Fourier transform process. Combining with the Fourier based analyses in the wave number domain, we successfully identify spatial and temporal development of the multi-scale structures in the turbulent reconnection process. When considering a strong guide field, the small scale tearing mode and the resulting flux ropes develop over a specific range of oblique angles mainly along the edge of the primary ion scale flux ropes and reconnection separatrix. The rapid merging of these small scale modes leads to a smooth energy spectrum connecting ion and electron scales. When the guide field is sufficiently weak, the background current sheet is strongly kinked and oblique angles for the small scale modes are widely scattered at the kinked regions. Similar approaches handling both the wave number and spatial domains will be applicable to the data from multipoint, high-resolution spacecraft observations such as the NASA magnetospheric multiscale (MMS) mission.

The non-thermal origin of the tokamak low-density stability limit

DIII-D plasmas at very low density exhibit the onset of n = 1 error field (EF) penetration (the ‘low-density locked mode’) not at a critical density or EF, but instead at a critical level of runaway electron (RE) intensity. Raising the density during a discharge does not avoid EF penetration, so long as RE growth proceeds to the critical level. Penetration is preceded by non-thermalization of the electron cyclotron emission, anisotropization of the total pressure, synchrotron emission shape changes, as well as decreases in the loop voltage and bulk thermal electron temperature. The same phenomena occur despite various types of optimal EF correction, and in some cases modes are born rotating. Similar phenomena are also found at the low-density limit in JET. These results stand in contrast to the conventional interpretation of the low-density stability limit as being due to residual EFs and demonstrate a new pathway to EF penetration instability due to REs. Existing scaling laws for penetration project to increasing EF sensitivity as bulk temperatures decrease, though other possible mechanisms include classical tearing instability, thermo-resistive instability, and pressure-anisotropy driven instability. Regardless of the first-principles mechanism, known scaling laws for Ohmic energy confinement combined with theoretical RE production rates allow rough extrapolation of the RE criticality condition, and thus the low-density limit, to other tokamaks. The extrapolated low-density limit by this pathway decreases with increasing machine size and is considerably below expected operating conditions for ITER. While likely unimportant for ITER, this effect can explain the low-density limit of existing tokamaks operating with small residual EFs.

Magnetic Reconnection Onset via Disruption of a Forming Current Sheet by the Tearing Instability.

The recent realization that Sweet-Parker current sheets are violently unstable to the secondary tearing (plasmoid) instability implies that such current sheets cannot occur in real systems. This suggests that, in order to understand the onset of magnetic reconnection, one needs to consider the growth of the tearing instability in a current layer as it is being formed. Such an analysis is performed here in the context of nonlinear resistive magnetohydrodynamics for a generic time-dependent equilibrium representing a gradually forming current sheet. It is shown that two onset regimes, single-island and multi-island, are possible, depending on the rate of current sheet formation. A simple model is used to compute the criterion for transition between these two regimes, as well as the reconnection onset time and the current sheet parameters at that moment. For typical solar corona parameters, this model yields results consistent with observations.

Relativistic tearing and drift-kink instabilities in two-fluid simulations

The stability of current sheets in collisionless relativistic pair plasma was studied via two-dimensional two-fluid relativistic magnetohydrodynamic simulations with vanishing internal friction between fluids. In particular, we investigated the linear growth of the tearing and drift-kink modes in the current sheets both with and without the guide field and obtained the growth rates which are very similar to what has been found in the corresponding PIC simulations. This suggests that the two-fluid simulations can be useful in studying the large-scale dynamics of astrophysical relativistic plasmas in problems involving magnetic reconnection.

Relationship of Corneal Pain Sensitivity With Dry Eye Symptoms in Dry Eye With Short Tear Break-Up Time.

The purpose of this prospective comparative study was to investigate corneal sensitivity in subjects with unstable tear film, with and without dry eye (DE) symptoms. Forty-one eyes of 41 volunteers (mean age: 45.1 ± 9.4 years; age range, 23-57 years), with normal tear function and ocular surface except for tear stability, were studied. The eyes were divided into two groups depending on the presence or absence of DE symptoms: 21 eyes with DE symptoms (symptomatic group); and 20 eyes without DE symptoms (asymptomatic group). Three types of corneal sensitivity values were measured using a Cochet-Bonnet esthesiometer: the sensitivity for perception of touch (S-touch), the sensitivity for blinking (S-blink), and the sensitivity for pain (S-pain). Mean S-blink and S-pain were significantly higher in the symptomatic group than in the asymptomatic group (P < 0.05), whereas there was no significant difference in mean S-touch between these groups (P > 0.05). Corneal sensitivity for blinking and pain evoked by increased stimuli was higher in the symptomatic group (subjects with short break-up time DE) compared with subjects who have no DE symptoms despite decreased tear stability. The presence of both tear instability and hyperesthesia, rather than tear instability alone, may contribute to DE pathogenesis.

“Ideal” tearing and the transition to fast reconnection in the weakly collisional MHD and EMHD regimes

AbstractThis paper discusses the transition to fast growth of the tearing instability in thin current sheets in the collisionless limit where electron inertia drives the reconnection process. It has been previously suggested that in resistive MHD there is a natural maximum aspect ratio (ratio of sheet length and breadth to thickness) which may be reached for current sheets with a macroscopic length L, the limit being provided by the fact that the tearing mode growth time becomes of the same order as the Alfvén time calculated on the macroscopic scale. For current sheets with a smaller aspect ratio than critical the normalized growth rate tends to zero with increasing Lundquist number S, while for current sheets with an aspect ratio greater than critical the growth rate diverges with S. Here we carry out a similar analysis but with electron inertia as the term violating magnetic flux conservation: previously found scalings of critical current sheet aspect ratios with the Lundquist number are generalized to include the dependence on the ratio , where de is the electron skin depth, and it is shown that there are limiting scalings which, as in the resistive case, result in reconnecting modes growing on ideal time scales. Finite Larmor radius effects are then included, and the rescaling argument at the basis of “ideal” reconnection is proposed to explain secondary fast reconnection regimes naturally appearing in numerical simulations of current sheet evolution.