Substorm Evolution Research Articles

Magnetotail Variability During Magnetospheric Substorms

AbstractIn this work, we present a statistical study of substorms covering a five‐year period 2016–2020. Substorm phases were identified from time series of the SuperMAG AL (SML) index using a list of 5,077 previously identified substorm onsets, the SML peak value marking transition from expansion to recovery phase, and the recovery identified as return to activity less than −100 nT in the SML index. Magnetic field observations from THEMIS, RBSP, and MMS missions were used to study the magnetotail characteristics during the substorm evolution. A superposed epoch analysis indicates that the substorm onset occurs almost simultaneously with a few minutes of uncertainty throughout the magnetotail, ranging from geostationary orbit to 20 RE. The onset in the transition region precedes the ground onset by a few minutes. The peak SML time coincides with the peak of the outer transition region ΔBZ, which suggests that the field‐aligned currents driving the SML activity arise from the outer transition region. Analysis of 2D maps of the tail magnetic field shows that the magnetotail current changes are limited to the center of the tail within |Y| < 10RE. The substorm recovery is fastest in the inner transition region and lasts longer when moving further out. We did not find major asymmetries in the substorm signatures associated with IMF BY or BZ.

Multipoint Observations of Dispersive Scale Alfvénic Field‐Line Resonances Associated With Substorm Auroral Beads

AbstractWe present a case study of the field‐aligned current (FAC) systems that transpire within the high‐altitude auroral acceleration region of an “auroral bead” initiated double oval substorm observed on 23 February 2001 by the Cluster fleet. Conjunctive Cluster measurements and auroral images from IMAGE reveal that auroral bead current system formation and evolution is a multi‐scale, injection‐mediated process. The FACs at large scales vary on substorm evolution time scales (∼minutes) in response to the injection and evolution of hotter denser magnetospheric plasma. Embedded within the large‐scale FACs are intense short‐scale (≲ few 10s of km) currents comprising dispersive scale Alfvén wave (DAW) fluctuations. The DAWs are a complex mixture of ingoing and reflected components that regularly interfere to form a broad spectrum of kinetic (dispersive) scale Alfvénic field‐line resonances (KFLRs). The Alfvénic currents appear as a nested series of upward and downward FAC densities with amplitudes reaching a few 100 nA/m2. Energized field‐aligned or counterstreaming electrons near keV energies and below are observed with parallel skews that vary in concert with variations in the DAW current sense. Positive correlations between DAW electric field energy densities and the energies of energized H+, He+, and O+ outflow are observed, indicative of ion energization within the DAW fields. Due to their L‐shell location (L ∼5.8–7.0) and associations with injections, the KFLRs are interpreted as the high‐altitude auroral zone analog of KFLRs observed in the equatorial inner magnetosphere.

Rapid Enhancements of Relativistic Electrons in the Earth's Outer Radiation Belt Caused by the Intense Substorms: A Statistical Study

AbstractUsing the data from Van Allen Probe A and B, we investigate rapid enhancements of relativistic electrons in the Earth's outer radiation belt caused by the intense substorms (AEmax > ∼900 nT) for 29 events from January 2013 to April 2015. These intense substorms may occur during the storm main phase or recovery phase. Based on the different substorm evolution characteristics, the intense substorms are divided into non‐continuous intense substorm activities and continuous intense substorm activities. In this study, we set a criterion for rapid enhancements when the electron phase space densities for μ = 1,096, 2,290, and 3,311 MeV/G increased by more than two times in 9 hr. In the time interval of 9 hr, the local acceleration by chorus waves is the dominant process for accelerating the seed populations (hundred kiloelectron volts) up to MeV energies. Our statistical results show that enhanced chorus waves and seed electrons during the intense substorms are observed in the outer radiation belt. Continuous intense substorm activities can more rapidly (<9 hr) and efficiently accelerate relativistic electrons in the outer radiation belt than non‐continuous intense substorm activities. During the intense substorms, the electric field acceleration could contribute to rapid enhancements of relativistic electrons in the outer radiation belt. Our statistical study suggests that the intense substorms during geomagnetic storms have a significant effect on the rapid variations of relativistic electron dynamics.

Relationship between TEC jumps and auroral substorm in the high-latitude ionosphere

The influence of an auroral substorm on the total electron content (TEC) jumps and cycle slips on Global Positioning System (GPS) at high-latitudes is studied. For the first time, optical data from the all-sky imager, as well as interplanetary magnetic field and magnetometer data are used to complete the analysis of the slips occurrence and to monitor the substorm evolution. Two types of slips are considered: (i) instrumental slips including losses in the measured phase of the GPS signal and (ii) sharp TEC variations (TEC jumps) It is demonstrated that the jumps in TEC determined from the GPS signals are mainly related to the auroral particle precipitation that normally occurs during geomagnetic substorms in the polar ionosphere. The GPS frequency {L}_{2} is consistently subject to more slips than frequency {L}_{1} both for quiet and disturbed conditions. The probability of TEC jumps is higher than for cycle slips in phase at frequencies {L}_{1} and {L}_{2}. The maximum of TEC jumps is observed during the recovery phase of the auroral substorm. Our findings are based on a data set obtained for a particular event. A generalization of the obtained numerical estimates to other events requires additional research and further analysis.

Lower thermospheric wind variations in auroral patches during the substorm recovery phase

AbstractMeasurements of the lower thermospheric wind with a Fabry‐Perot interferometer (FPI) at Tromsø, Norway, found the largest wind variations in a night during the appearance of auroral patches at the substorm recovery phase. Taking into account magnetospheric substorm evolution of plasma energy accumulation and release, the largest wind amplitude at the recovery phase is a fascinating result. The results are the first detailed investigation of the magnetosphere‐ionosphere‐thermosphere coupled system at the substorm recovery phase using comprehensive data sets of solar wind, geomagnetic field, auroral pattern, and FPI‐derived wind. This study used three events in November 2010 and January 2012, particularly focusing on the wind signatures associated with the auroral morphology, and found three specific features: (1) wind fluctuations that were isolated at the edge and/or in the darker area of an auroral patch with the largest vertical amplitude up to about 20 m/s and with the longest oscillation period about 10 min, (2) when the convection electric field was smaller than 15 mV/m, and (3) wind fluctuations that were accompanied by pulsating aurora. This approach suggests that the energy dissipation to produce the wind fluctuations is localized in the auroral pattern. Effects of the altitudinal variation in the volume emission rate were investigated to evaluate the instrumental artifact due to vertical wind shear. The small electric field values suggest weak contributions of the Joule heating and Lorentz force processes in wind fluctuations. Other unknown mechanisms may play a principal role at the recovery phase.

Substorm evolution of auroral structures

AbstractAuroral arcs are often associated with magnetically quiet time and substorm growth phases. We have studied the evolution of auroral structures during global and local magnetic activity to investigate the occurrence rate of auroral arcs during different levels of magnetic activity. The ground‐magnetic and auroral conditions are described by the magnetometer and auroral camera data from five Magnetometers — Ionospheric radars — All‐sky cameras Large Experiment stations in Finnish and Swedish Lapland. We identified substorm growth, expansion, and recovery phases from the local electrojet index (IL) in 1996–2007 and analyzed the auroral structures during the different phases. Auroral structures were also analyzed during different global magnetic activity levels, as described by the planetary Kp index. The distribution of auroral structures for all substorm phases and Kp levels is of similar shape. About one third of all detected structures are auroral arcs. This suggests that auroral arcs occur in all conditions as the main element of the aurora. The most arc‐dominated substorm phases occur in the premidnight sector, while the least arc‐dominated substorm phases take place in the dawn sector. Arc event lifetimes and expectation times calculated for different substorm phases show that the longest arc‐dominated periods are found during growth phases, while the longest arc waiting times occur during expansion phases. Most of the arc events end when arcs evolve to more complex structures. This is true for all substorm phases. Based on the number of images of auroral arcs and the durations of substorm phases, we conclude that a randomly selected auroral arc most likely belongs to a substorm expansion phase. A small time delay, of the order of a minute, is observed between the magnetic signature of the substorm onset (i.e., the beginning of the negative bay) and the auroral breakup (i.e., the growth phase arc changing into a dynamic display). The magnetic onset was observed to precede the structural change in the auroral display. A longer delay of a few minutes was found between the beginning of the growth phase and the first detected auroral structure.

Local time resolved dynamics of field‐aligned currents and their response to solar wind variability

AbstractUsing 10 years of CHAMP measurements condensed into the empirical model of field‐aligned currents through empirical orthogonal function analysis, the dynamics of field‐aligned currents (FACs) is modeled and studied in separate magnetic local time (MLT) sectors. We investigate the distributions of FAC intensity and latitude and evaluate their predictability in terms of geospace parameters which are ranked according to their relative importance measured by a multivariate regression procedure. The response time to changes in solar wind variables is studied in detail and found to be much shorter for dayside FACs than on the nightside (15–25 min versus 35–95 min). Furthermore, dayside FACs can be parameterized more accurately: R2 values maximize greater than 0.7 for FAC latitude and greater than 0.3 for FAC intensity, whereas the corresponding values on the nightside are smaller than 0.3 and 0.15, respectively. The results support the separation between directly driven coupling processes acting on the dayside and unloading processes controlling the nightside. In addition, the MLT‐resolved standardized regression coefficients suggest that (1) FAC latitude is affected most significantly by the transpolar potential, substorm evolution, solar activity as represented by the F10.7 index and its square, and the dipole tilt; (2) Region‐1/2 current intensity is controlled most efficiently by substorm evolution, IMF Bz and IMF By; and (3) cusp current intensity is influenced by conductivity, IMF By and their cross item.

Effect of substorms in the Earth’s nightside sector on variations in the surface atmospheric electric field at polar and equatorial latitudes

We performed an analysis of mean daily variations in the ΔEz atmospheric electric field at the Hornsund (located near the polar cap boundary) and Kakioka (located at near-equatorial latitudes) observatories under magnetically quiet and weakly disturbed conditions. At both observatories, the mean daily variations in ΔEz were found to be mainly controlled by the location of the observation point with respect to the focuses of the convective vortices of the DP0 system. The substorm evolution in the nightside of the magnetosphere (a sharp burst in the AE index) was shown to lead to negative variations in ΔEz in the dayside sector at polar latitudes (the Hornsund observatory) and positive deviations in ΔEz at premidnight time at equatorial latitudes (the Kakioka observatory). It is concluded that variations in ΔEz at the Kakioka observatory are largely controlled by the equatorial electrojet, which is maximal during day-time hours, and at the Hornsund observatory these variations are controlled by the auroral electrojet, which is maximal at night-time and early morning hours of local time.

Comment on “The importance of auroral features in the search for substorm onset process” by Syun-Ichi Akasofu, A. T. Y. Lui, and C.-I. Meng

Comment on “The importance of auroral features in the search for substorm onset process” by Syun-Ichi Akasofu, A. T. Y. Lui, and C.-I. Meng

Substorm evolution as revealed by THEMIS satellites and a global MHD simulation

The major substorm that occurred on 1 March 2008 had excellent spacecraft coverage by the THEMIS spacecraft in the magnetotail, GOES 11, and GOES 12 at geosynchronous orbit and Geotail in the dayside magnetosheath. A global magnetohydrodynamic simulation of this substorm, driven by Wind solar wind observations, accurately reproduced the magnetospheric observations. The simulation revealed the complexity of magnetotail dynamics during the substorm, in particular, in the near‐Earth plasma sheet. Reconnection began prior to the substorm on closed field lines and a flux rope formed there. Around substorm onset, the simulation exhibited flow vortices near the locations of THEMIS P3 and P4, in agreement with observations at P3 and P4. These vortices were associated with a duskside neutral line that formed early in the substorm. Six minutes later, another neutral line formed on the dawnside of the tail. These neutral lines then merged to form a single large reconnection region that extended across the tail and greatly expanded the flux rope. The least active part of the tail was the region around midnight. Strong flows were seen in the observations and in the simulation during the two intensifications of this substorm; in particular, tailward flows were seen at THEMIS P1 and P2. Reconnection on closed field lines, vortices in the near‐Earth region, a channel of strong tailward flow, and enhanced precipitation into the ionosphere all contributed to substorm development.

Unmanned magnetometer network observation in the 44th Japanese Antarctic Research Expedition: Initial results and an event study on auroral substorm evolution

In the 44th Japanese Antarctic Research Expedition (JARE-44) during 2003 to 2004, four unmanned magnetometers were deployed in Antarctica to establish a observation network for studying ionospheric and magnetospheric phenomena. Three of them were set about 80 km from Syowa Station, and the fourth was set at Dome Fuji Camp, about 800 km from Syowa. Observations were carried out continuously with a maximum sampling rate of 1 Hz. The purpose of the close network around Syowa was to observe the two-dimensional distribution of the ionospheric equivalent current in a localized area within the field-of-view of the all-sky auroral imager operated at Syowa. Dome Fuji is located at the higher-latitude edge of the auroral zone, while Syowa is located in the middle of it. Auroral observations using the all-sky imager were also carried out at Dome Fuji in 2003. The simultaneous auroral and magnetic observations both at Dome Fuji and Syowa enabled us to study auroral activities in a larger area. This paper describes, the deployment and performance of the unmanned magnetometers in the JARE-44. One auroral substorm event is analyzed in detail to show the usefulness the unmanned magnetometer network.

The magnetospheric boundary motion during the main phase of the moderate magnetic storm (case study)

As a result of ICME interaction with magnetosphere the negative sudden impulse Si and substorm event were followed. The simultaneous plasma and magnetic field measurements near the magnetospheric boundary (INTERBALL-1), in the nearest magnetotail (GOES 8, 9), in the middle tail (GEOTAIL), in solar wind (WIND), and ground based data were compared to trace the time sequence of events. It was shown that negative sudden impulse Si was observed by ground stations and at geosynchronous altitudes. The character of magnetic field variations that were observed by GOES 9 permit to suggest that compression/expansion wave was propagated inside the magnetosphere. The time sequence of events observed during substorm development pointed that substorm has begun from auroral breakup. The cross current disruption in the nearest tail and neutral line formation in the middle tail occurred almost simultaneously, but some time after auroral breakup. It is possible that both processes associated with releasing of stored energy during substorm were occurred independently in this substorm event. By using an empirical magnetopause model [Shue et al., 1998. Magnetopause location under extreme solar wind conditions. J. Geophys. Res. 103, 17691], the amplitude of magnetopause motion was evaluated. During substorm evolution the amplitude of the magnetopause motion was ∼ 2 –3 Re. The excitation of Kelvin-Helmholtz instability can explain only a few magnetopause crossings.

Substorm topology in the ionosphere and magnetosphere during a flux rope event in the magnetotail

Abstract. On 13 August 2002, at ~23:00 UT, about 10 min after a substorm intensification, Cluster observes a flux rope in the central magnetotail, followed by a localised fast flow event about oneminute later. Associated with the flux rope event, a traveling compression region (TCR) is seen by those Cluster spacecraft which reside in the lobe. In the conjugate ionospheric region in Northern Scandinavia, the MIRACLE network observes the ionospheric equivalent currents, and the electron densities and electric fields are measured by the EISCAT radar along a meridional scanning profile. Further, the auroral evolution is observed with the Wideband Imaging Camera (WIC) on the IMAGE satellite. We compare in detail the substorm evolution as observed in the ionosphere and in the magnetosphere, and examine whether topological correspondences to the flux rope event exist in the ionospheric signatures. The large-scale mapping of both the location and the direction of the flux rope to the ionosphere shows an excellent correspondence to a lens-shaped region of an auroral emission minimum. This region is bracketed by an auroral region equatorward of it which was preexisting to the substorm intensification, and a substorm-related auroral region poleward of it. It is characterised by reduced ionospheric conductances with respect to its environment, and downward field-aligned current (FAC) observed both in the magnetosphere and in the ionosphere. As determined from the ionospheric data, this downward FAC area is moving eastward with a speed of ~2 km s-1, in good agreement with the mapped plasma bulk velocity measured at the Cluster satellite closest to that area. Further southwestward to this leading downward FAC area, a trailing upward FAC area is observed that moves eastward with the same speed. The direction of the ionospheric electric field permits a current closure between these two FAC areas through the ionosphere. We speculate that these FAC areas may correspond to the ends of the flux rope in its symmetry direction.

On the occurrence of auroral westward flow channels and substorm phase

Auroral westward flow channels (AWFCs) are intense, narrow channels of westward drift overlapping the equatorward edge of the auroral oval in the pre-magnetic midnight sector. They are a close relative of the sub-auroral polarisation stream which encompasses polarisation jets, a phenomenon also known as sub-auroral ion drift events. Recent observations made with the Tasman Geospace Environment Radar (TIGER) (147.2°E, 43.4°S Geodetic; 55.0° Geomagnetic) have revealed close associations between the appearance of AWFCs and substorm onset, and their subsequent decay toward the end of recovery phase. In fact, in terms of electric field strength, they are the strongest signatures of substorms in the ionospheric convection (>50 mV m −1). In terms of electric potential difference (>10 kV), they also represent a substantial fraction of the total potential difference generated during substorms. The AWFCs exhibit a diverse range of behaviour, there being no typical event. The radar observations show that radial polarisation fields sometimes oscillate towards and away from the Earth, and bifurcate, within regions of closed flux in the magnetotail throughout substorm evolution. We have identified every AWFC observed by TIGER during the first year of operation, 2000. Simple statistical arguments imply that one, if not more, AWFC probably occurs during every substorm. AWFCs are a fundamental aspect of substorm evolution.

Report from scientific sessions at the 35 th COSPAR scientific assembly, Paris 2004

As a result of ICME interaction with magnetosphere the negative sudden impulse Si and substorm event were followed. The simultaneous plasma and magnetic field measurements near the magnetospheric boundary (INTERBALL-1), in the nearest magnetotail (GOES 8, 9), in the middle tail (GEOTAIL), in solar wind (WIND), and ground based data were compared to trace the time sequence of events. It was shown that negative sudden impulse Si was observed by ground stations and at geosynchronous altitudes. The character of magnetic field variations that were observed by GOES 9 permit to suggest that compression/expansion wave was propagated inside the magnetosphere. The time sequence of events observed during substorm development pointed that substorm has begun from auroral breakup. The cross current disruption in the nearest tail and neutral line formation in the middle tail occurred almost simultaneously, but some time after auroral breakup. It is possible that both processes associated with releasing of stored energy during substorm were occurred independently in this substorm event. By using an empirical magnetopause model [Shue et al., 1998. Magnetopause location under extreme solar wind conditions. J. Geophys. Res. 103, 17691], the amplitude of magnetopause motion was evaluated. During substorm evolution the amplitude of the magnetopause motion was ∼2–3 Re. The excitation of Kelvin-Helmholtz instability can explain only a few magnetopause crossings.

Investigation of subauroral ion drifts and related field-aligned currents and ionospheric Pedersen conductivity distribution

Abstract. Based on Astrid-2 satellite data, results are presented from a statistical study on subauroral ion drift (SAID) occurrence. SAID is a subauroral phenomenon characterized by a westward ionospheric ion drift with velocity greater than 1000m/s, or equivalently, by a poleward-directed electric field with intensity greater than 30mV/m. SAID events occur predominantly in the premidnight sector, with a maximum probability located within the 20:00 to 23:00 MLT sector, where the most rapid SAID events are also found. They are substorm related, and show first an increase in intensity and a decrease in latitudinal width during the expansion phase, followed by a weakening and widening of the SAID structures during the recovery phase. The potential drop across a SAID structure is seen to remain roughly constant during the recovery phase. The field-aligned current density and the height-integrated Pedersen conductivity distribution associated with the SAID events were calculated. The results reveal that the strongest SAID electric field peaks are associated with the lowest Pedersen conductivity minimum values. Clear modifications are seen in the ionospheric Pedersen conductivity distribution associated with the SAID structure as time evolves: the SAID peak is located on the poleward side of the corresponding region of reduced Pedersen conductivity; the shape of the regions of reduced conductivity is asymmetric, with a steeper poleward edge and a more rounded equatorward edge; the SAID structure becomes less intense and widens with evolution of the substorm recovery phase. From the analysis of the SAID occurrence relative to the mid-latitude trough position, SAID peaks are seen to occur relatively close to the corresponding mid-latitude trough minimum. Both these features show a similar response to magnetospheric disturbances, but on different time scales - with increasing magnetic activity, the SAID structure shows a faster movement towards lower latitudes than that of the mid-latitude trough. From the combined analysis of these results, we conclude that the SAID generation mechanism cannot be regarded either as a pure voltage generator or as a pure current generator, applied to the ionosphere. While the anti-correlation between the width and the peak intensity of the SAID structures with substorm evolution indicates a magnetospheric source acting as a constant voltage generator, the ionospheric modifications and, in particular the reduction in the conductivity for intense SAID structures, are indicative of a constant current system closing through the ionosphere. The ionospheric feedback mechanisms are seen to be of major importance for sustaining and regulating the SAID structures. Key words. Ionosphere (mid-latitude ionosphere; electric fields and currents; ionosphere-magnetosphere interactions)

Substorm classification with the WINDMI model

Abstract. The results of a genetic algorithm optimization of the WINDMI model using the Blanchard-McPherron substorm data set is presented. A key result from the large-scale computations used to search for convergence in the predictions over the database is the finding that there are three distinct types of vx Bs -AL waveforms characterizing substorms. Type I and III substorms are given by the internally-triggered WINDMI model. The analysis reveals an additional type of event, called a type II substorm, that requires an external trigger as in the northward turning of the IMF model of Lyons (1995). We show that incorporating an external trigger, initiated by a fast northward turning of the IMF, into WINDMI, a low-dimensional model of substorms, yields improved predictions of substorm evolution in terms of the AL index. Intrinsic database uncertainties in the timing between the ground-based AL electrojet signal and the arrival time at the magnetopause of the IMF data measured by spacecraft in the solar wind prevent a sharp division between type I and II events. However, within these timing limitations we find that the fraction of events is roughly 40% type I, 40% type II, and 20% type III.

Statistical behavior of proton and electron auroras during substorms

The substorm evolution was studied by performing a superposed epoch analysis of 91 substorms using IMAGE FUV data. Onset locations and times were determined from images produced by the Wideband Imaging Camera (WIC) usually dominated by electron aurora. Images taken by WIC and by the Spectrographic Imager SI12 channel, responsive to proton aurora, were transformed into rectangular magnetic latitude (ML) and magnetic local time (MLT) coordinates and plotted on a timescale related to the time of substorm onset (T = 0) and to a Relative MLT (RMLT) normalized to onset MLT. A double Gaussian was fitted to the auroral intensity data as a function of ML at RMLT of −4, −2, 0, +2, and +4. From the Gaussian coefficients the means of several parameters were plotted as a function of time. Presubstorm, there was an equatorward motion of the mean low‐latitude boundary of the electron and proton aurora. There was no evidence of preonset auroral fading in the mean intensity. At onset the proton auroral peak intensity increased only by a factor of 2 compared with ∼5 for the electrons. At RMLT = 0, rapid poleward expansion of the proton aurora after onset occurred. The proton expansion slowed down after the first few minutes, while the electron surge continued toward higher latitudes. The mean poleward expansion of the electron aurora reached about 3.5° in 5 min and reached a total expansion of 5.5° in an hour. The protons expanded about 2.5° in 5 min and expanded about 3° one hour after onset. The latitude width of the aurora increased at onset due to both a large poleward and a moderate equatorward expansion. There appeared to be stronger substorm‐related activity in the local time range toward dawn than toward dusk. In the RMLT sector duskward of onset the proton auroras were located equatorward of the electrons and poleward in the dawnward RMLT sector.

Do we understand the inner magnetotail dynamics during substorms?

Substorm evolution of the near-Earth (|X|<15 R E) plasma sheet has been emphasized recently because the inner tail is thought to link closely to the substorm auroral activity in the ionosphere during the early stage of substorms. In this paper, we discuss how the inner tail substorm phenomena during the late substorm growth phase and early expansion phase are accounted for by the two prevailing substorm models, namely, the near-Earth neutral line model and the current disruption model. We find that the late growth phase features are more satisfactorily accounted for by the current disruption model than by the near-Earth neutral line model. In addition, detailed observations on current disruption show evidence inconsistent with the proposed idea of dipolarization being due to plasma flow braking from reconnection in the mid-tail region, which poses a difficulty to the near-Earth neutral line model as well.

The current sheet disruption model for substorm expansion phase onset: Background, arguments, and relationship to global substorm evolution

The onset and evolution of substorms is viewed by many researchers to be a central question in magnetospheric physics. The most popular model for substorms is the Near-Earth Neutral Line model. This model asserts that the formation of a new, near-Earth reconnection region is the central feature of the onset of the expansion phase and that all other phenomena flow from this event. In recent years the Current Sheet Disruption model has been proposed as an alternative. This model asserts that the initiation of the substorm expansion phase is triggered by the disruption of the thin near-Earth current sheet that forms at and just beyond geosynchronous orbit in the late growth phase, and that the formation of a reconnection region proceeds from this event. In this paper we will review the history of and evidence for the Current Sheet Disruption model, especially with regard to recent evidence in favor of the NENL model. We will also review recent global MHD simulation results that provide an interesting perspective on the relationship between these two models.