Plasma Sheet Thinning Research Articles

Identification of substorm onset location and preonset sequence using Reimei, THEMIS GBO, PFISR, and Geotail

We present state‐of‐the‐art multiple instrument observations of an isolated substorm on October 12, 2007. The auroral breakup was observed simultaneously by Reimei, THEMIS ASI, and PFISR. The footprint of Geotail was also near the breakup. These observations allow for detailed study of the breakup location in terms of large‐ and small‐scale auroral morphology, particle precipitation, and ionospheric convection, which has not previously been achieved. It also allows for detailed identification of the sequence leading to the breakup. We report the first spaceborne high spatial and temporal resolution images of part of a breakup arc and a wave‐like auroral enhancement captured by Reimei. Observations suggest a sudden plasma sheet thinning initiated ∼10 min before the onset. Wave‐like auroral enhancements were observed twice at the most equatorward arc ∼3 min and ∼1 min before the breakup. These enhancements are likely due to some near‐Earth instability, such as ballooning instability. Unlike the usual substorm sequence, this most equatorward arc did not develop into the breakup arc but remained almost stable until being engulfed by the auroral equatorward expansion from higher latitude after onset. The wave‐like auroral enhancement was associated with three fine inverted V arcs and embedded within energetic ion precipitation. Following this enhancement, an arc, likely a poleward boundary intensification, formed at higher latitude just adjacent to the plasma sheet boundary layer (PSBL). This arc then extended southwestward and led to the breakup arc, which was located poleward of the wavy structures. Assuming longitudinal homogeneity of ion precipitation over 1°, this breakup arc was located in a region without ion precipitation just poleward of the energetic ion precipitation. These observations suggest the possible existence of a low‐entropy flow channel associated with the arc adjacent to the PSBL, which might be associated with instability in the near‐Earth plasma sheet responsible for the auroral breakup.

Bi-layer structure of counterstreaming energetic electron fluxes: a diagnostic tool of the acceleration mechanism in the Earth's magnetotail

Abstract. For the first time we identify a bi-layer structure of energetic electron fluxes in the Earth's magnetotail and establish (using datasets mainly obtained by the Geotail Energetic Particles and Ion Composition (EPIC/ICS) instrument) that it actually provides strong evidence for a purely spatial structure. Each bi-layer event is composed of two distinct layers with counterstreaming energetic electron fluxes, parallel and antiparallel to the local ambient magnetic field lines; in particular, the tailward directed fluxes always occur in a region adjacent to the lobes. Adopting the X-line as a standard reconnection model, we determine the occurrence of bi-layer events relatively to the neutral point, in the substorm frame; four (out of the shown seven) events are observed earthward and three tailward, a result implying that four events probably occurred with the substorm's local recovery phase. We discuss the bi-layer events in terms of the X-line model; they add more constraints for any candidate electron acceleration mechanism. It should be stressed that until this time, none proposed electron acceleration mechanism has discussed or predicted these layered structures with all their properties. Then we discuss the bi-layer events in terms of the much promising "akis model", as introduced by Sarafopoulos (2008). The akis magnetic field topology is embedded in a thinned plasma sheet and is potentially causing charge separation. We assume that as the Rc curvature radius of the magnetic field line tends to become equal to the ion gyroradius rg, then the ions become non-adiabatic. At the limit Rc=rg the demagnetization process is also under way and the frozen-in magnetic field condition is violated by strong wave turbulence; hence, the ion particles in this geometry are stochastically scattered. In addition, ion diffusion probably takes place across the magnetic field, since an intense pressure gradient is directed earthward; hence, ions are ejected tailward of akis. This way, in front of akis an "ion capsule region" is formed with net positive charge. In between them a distinct region with an electric field E⊥ orthogonal to the magnetic field is emerged; E⊥ in front of akis is directed earthward. The field-aligned and highly anisotropic energetic electron populations have probably resulted via spatially separated antiparallel and field-aligned electric fields being the very heart of the acceleration source. We assume that the ultimate cause for the field-aligned electric fields are the net positive capsule charge and the net negative charge trapped at the tip of akis; both charges will be eventually neutralized through field aligned currents, but they remain unshielded for sufficient time to produce the observed events.

Lower hybrid drift instability in a neutral sheet withO+ions

The electromagnetic lower-hybrid drift instability (LHDI) in the intermediate-wavelength regime k_(y)sqrt[rho_(i)rho_(e)] approximately 1 , where k_(y) and rho_(e,i) are the wave vector and the electron and ion gyroradii, respectively, in a thin plasma sheet containing electrons and H+ and O+ ions is examined using kinetic theory. It is shown that the growth rate of the LHDI first decreases and then increases with increase in the O+ content and temperature, with a minimum at a moderate level of the latter. The results can be relevant to understanding magnetic reconnection in the presence of LHDI.

Dynamical Casimir effect for TE and TM modes in a resonant cavity bisected by a plasma sheet

Parametric photon creation via the dynamical Casimir effect (DCE) is evaluated numerically, in a three-dimensional rectangular resonant cavity bisected by a semiconductor diaphragm (SD), which is irradiated by a pulsed laser with frequency of GHz order. The aim of this paper is to determine some of the optimum conditions required to detect DCE photons relevant to a novel experimental detection system. We expand upon the thin plasma sheet model [Crocce et al., Phys. Rev. A 70 033811 (2004)] to estimate the number of photons for both TE and TM modes at any given SD position. Numerical calculations are performed considering up to 51 inter-mode couplings by varying the SD location, driving period and laser power without any perturbations. It is found that the number of photons created for TE modes strongly depends on SD position, where the strongest enhancement occurs at the midpoint (not near the cavity wall); while TM modes have weak dependence on SD position. Another important finding is the fact that significant photon production for TM$_{111}$ modes still takes place at the midpoint even for a low laser power of 0.01 micro J/pulse, although the number of TE$_{111}$ photons decreases almost proportionately with laser power. We also find a relatively wide tuning range for both TE and TM modes that is correlated with the frequency variation of the instantaneous mode functions caused by the interaction between the cavity photons and conduction electrons in the SD excited by a pulsed laser.

Multispacecraft and ground‐based observations of substorm timing and activations: Two case studies

Two case studies are performed to investigate substorm timing and activations based on Double Star TC1, Cluster, Polar, IMAGE, LANL geostationary satellites and ground‐based geomagnetic field measurements. In both events, an earthward flow associated with plasma sheet thinning is measured by Cluster 8–10 min ahead of the auroral breakup. A couple of minutes after the breakup, either TC1 at ∼X‐10 RE first detects plasma sheet expansion and then the LANL satellites near the midnight measure energetic electron injections at geostationary orbit or the LANL satellites first measure the electron injections and then TC1 detects the plasma sheet expansion. More than about 20 min later, Cluster at X∼16 RE and Polar (at higher latitude) successively observe plasma sheet expansion. The open magnetic flux of the polar cap, Ψ, is found to continually increase during the early substorm phase and then to rapidly fall when the IMF turns northward. When Ψ reaches its minimum value, bright and broad auroral activities start to decrease. Tailward progression of the magnetic dipolarization and a poleward expansion of auroral bulges are shown to closely map to one another. These results suggest that substorm activations start in the midtail before ground onset and then move earthward, which leads to an expansion onset in the near‐Earth tail around X∼ ‐(8–9) RE. After onset, the activations progress both earthward and tailward. Substorm onset is possibly related to plasma sheet reconnection of close field lines, while tail lobe reconnection of open field lines release more energy to support the full expansion of the substorm. In a fully developed expansion phase, an initial dipolarization in the near‐Earth may eventually evolve to enable disruption of the cross‐tail current over a wide region of the magnetotail.

Near‐Earth substorm features from multiple satellite observations

We investigate a substorm on 3 October 2004 during which 11 satellites were located in near‐Earth magnetotail (XGSM > −10 RE). Double Star 1 (TC‐1), Cluster, and LANL‐97 satellites were closely aligned in the dawn‐dusk direction (<1 RE apart) for this conjunction. After substorm expansion onset, TC‐1 observed plasma sheet thinning at X ≈ −5.5 RE and later detected signature of plasma flow shear that may be associated with an auroral arc. Analysis of the dawn‐dusk magnetic perturbations from GOES‐10 and Polar suggests that these could be caused by a substorm current system consisting of not only the azimuthal closure of field‐aligned currents (the substorm current wedge) but also the meridional closure of field‐aligned currents. The temporal sequence of substorm activity (particle injection, current disruption, and dipolarization) revealed by these satellites indicates that the substorm expansion activity was initiated close to the Earth and spread later to further downstream distances. Furthermore, TC‐1 and Cluster data show that there is no close relationship between some dipolarizations and Earthward plasma flows in the near‐Earth region. The overall development of substorm activity is in agreement with the near‐Earth initiation model for substorms. A temporal evolution of the magnetic field reconfiguration and plasma boundary motion during this substorm is constructed from these observations.

Simultaneous THEMIS observations in the near‐tail portion of the inner and outer plasma sheet flux tubes at substorm onset

We analyzed the measurements made by two Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes in ideal observational conditions (quiet background, near midnight, inside the substorm current wedge) during two distinct isolated substorm onsets, with probe P2 measuring the inner plasma sheet at ∼8 Re and P1 near the plasma sheet–lobe interface at 11–12 Re. The earliest onset‐related strong perturbations were observed by P1; they include the increase of both Bz (dipolarization) and Ey (a few mV/m) as well as the simultaneous drop in total pressure, indicating the unloading process. This was also accompanied by fast inward plasma motion (up to 100 km/s, toward the neutral sheet) and fast plasma sheet thinning while the poleward auroral expansion was in progress in the conjugate ionosphere. These perturbations were followed after 6–8 min by the rapid expansion of the already heated plasma sheet. While in the adjacent lobe during this thinning phase, probe P1 continued to observe intense flux transfer toward the sheet center plane. The inner probe observed intense dipolarization and inward plasma injection but with a smaller flux transfer and starting 1–2 min after the perturbations at P1, supporting the conclusion that onset instability took place tailward of 12 Re. We also demonstrate the global MHD simulations to show that a nontrivial combination of dipolarization and the plasma sheet thinning may be observed simultaneously in the outmost part of the dipolelike region during a sudden increase of the reconnection rate at the nearby active X line, staying a few Re from the observation point. These observations provide constraints for the choice of substorm onset mechanism and indicate near‐Earth magnetic reconnection as the most probable source process.

Thinning and stretching of the plasma sheet

With Cluster observations in the magnetotail we study dynamics of plasma sheet thinning and stretching during 39 intervals associated with substorm growth phases. The cross‐tail current density and normal magnetic field generally scale as Bn ∼ TpNp1/2/J0, but with frequent transient variations. Typical pre‐onset values are Bz ∼ 1–2 nT, J0 ∼ 4–8 nA/m2, thickness (Harris estimate) >3000 km. A current density increase in each particular event is not accompanied with a corresponding number density increase. About 30% of the events are characterized by a large (>5 nT) field component parallel to the current (in most of cases equal to By), implying adiabatic particle dynamics even with small Bz. Most local onsets, associated with the ends of thin sheet intervals, were accompanied with tailward plasma flows. In some cases embedded current sheet structure was detected and, therefore, estimation of thickness requires caution.

Interaction of a charge with a thin plasma sheet

The interaction of the electromagnetic field with a two-dimensional plasma sheet intended to describe the pi-electrons of a carbon nanotube or a C-60 molecule is investigated. By first integrating out the displacement field of the plasma or the electromagnetic field, different representations for quantities like the Casimir energy are derived which are shown to be consistent with one another. Starting from the covariant gauge for the electromagnetic field, it is shown that the matching conditions to which the presence of the plasma sheet can be reduced are different from the commonly used ones. The difference in the treatments does not show up in the Casimir force between two parallel sheets, but it is present in the Casimir-Polder force between a charge or a neutral atom and a sheet. At once, since the plasma sheet is a regularization of the conductor boundary conditions, this sheds light on the difference in physics found earlier in the realization of conductor boundary conditions as thin or thick boundary conditions in Phys. Rev. D 70, 085010 (2004).

The Casimir effect for thin plasma sheets and the role of the surface plasmons

We consider the Casimir force betweeen two dielectric bodies described by the plasma model and between two infinitely thin plasma sheets. In both cases in addition to the photon modes surface plasmons are present in the spectrum of the electromagnetic field. We investigate the contribution of both types of modes to the Casimir force and confirm resp. find in both models large compensations between the plasmon modes themselves and between them and the photon modes especially at large distances. Our conclusion is that the separation of the vacuum energy into plasmon and photon contributions must be handled with care except for the case of small separations.

Simulating Plasma Microwave Diagnostics

Computational simulation of plasma diagnostics via microwave absorption has been successfully accomplished. This simulation capability is developed from solutions to a combination of the three-dimensional Maxwell equations and the generalized Ohm's law in the time domain. As the simulation procedure developed, numerical results were obtained for a range of plasma transport properties including electrical conductivity, permittivity, and plasma frequency. The present results reveal the wave reflection at the media interface and substantial distortion of the electromagnetic field within a thin plasma sheet from a guided microwave. The present numerical simulation also accurately predicts the microwave blackout phenomenon as the wave propagates through a thick plasma sheet. The diffractions and refractions occurring at antenna apertures and passing through a plasma column are captured numerically. Finally, the numerical simulation has successfully duplicated a plasma diagnostic experiment in a hypersonic magneto-hydrodynamic channel.

Electron surfing acceleration in magnetic reconnection

We discuss that energetic electrons are generated near the X‐type magnetic reconnection region due to a surfing acceleration mechanism. In a thin plasma sheet the polarization electric fields pointing toward the neutral sheet are induced around the boundary between the lobe and plasma sheet in association with the Hall electric current. By using a particle‐in‐cell simulation, we demonstrate that the polarization electric fields are strongly enhanced in an externally driven reconnection system, and some electrons can be trapped by the electrostatic potential well of the polarization field. During the trapping phase the electrons can gain their energies from the convection/inductive reconnection electric fields. We discuss that relativistic electrons with MeV energies are quickly generated in and around the X‐type neutral region by utilizing the surfing acceleration.

Transition from substorm growth to substorm expansion phase as observed with a radial configuration of ISTP and Cluster spacecraft

Abstract. Transition from the growth phase to the substorm expansion during a well-isolated substorm with a strong growth phase is investigated using a unique radial (THEMIS-like) spacecraft constellation near midnight, including the probing of the tail current at ~16 RE with Cluster, of the transition region at ~9 RE with Geotail and Polar, and of the inner region at 6.6 RE with two LANL spacecraft. The activity development on both a global scale and near the spacecraft footpoints was monitored with global auroral images (from the IMAGE spacecraft) and the ground network. Magnetospheric models, tuned using in-situ observations, indicated a strong tail stretching and plasma sheet thinning, which included the growth of the near-Earth current (approaching 30 nA/m2) and possible formation of a local B minimum in the neutral sheet (~5 nT) at ~10–12 RE near the substorm onset. However, there were no indications that the substorm onset was initiated just in this region. We emphasize the rather weak magnetic and plasma flow perturbations observed outside the thinned plasma sheet at Cluster, which could be interpreted as the effects of localized earthward-contracting newly-reconnected plasma tubes produced by the impulsive reconnection in the midtail plasma sheet. In that case the time delays around the distinct substorm onset are consistent with the activity propagation from the midtail to the inner magnetosphere. A peculiar feature of this substorm was that 12min prior to this distinct onset, a clear soft plasma injection to the GEO orbit was recorded which has little associated effects both in the ionosphere and in the transition region at ~9 RE. This pseudo-breakup was probably due to either a localized ballooning-type activity or due to the braking of a very narrow BBF whose signatures were also recorded by Cluster. This event manifested the (previously unknown) phenomenon, a strong tail overloading (excessive storage of magnetic energy) contrasted to the modest energy dissipation and plasma acceleration, which are both discussed and interpreted as the consequences of cold/dense and thick pre-substorm plasma sheet which often occurs after the long quiet period. The lessons of using the radial spacecraft configurations in substorm onset studies are also discussed. Keywords. Magnetospheric physics (Auroral phenomena, plasma sheet, storms and substorms)

Nonadiabatic orbit features in ion distribution functions of fast flow magnetotail configurations

One of the major problems in space physics is the limitation of measurements. We present here one way to infer global characteristics of the magnetosphere from only a single satellite position. Several consistent orbit‐tracing (COT) models of fast flows in thin plasma sheets were constructed. The resulting ion distribution functions were examined to see if there are any features which can distinguish a uniform earthward flow of plasma from flows which are confined in a narrow channel in the dawn‐dusk direction. The model distribution functions during uniform flows had the appearance of an initially isotropic f(v) that had simply been shifted by the earthward flow speed. The model f(v) exhibited lima bean–shaped features when the flow was confined to a region comparable to the diameter of a cyclotron orbit based on Bz. Distribution functions observed by the Geotail satellite were examined for comparisons, and the measured distributions were found to exhibit features similar to those obtained in the COT model. We are able to demonstrate that the distributions measured by Geotail may only be reproduced in our technique if the convective electric field is confined in the dawn‐dusk direction.

Casimir effects for a flat plasma sheet: I. Energies

We study a fluid model of an infinitesimally thin plasma sheet occupying the xy plane, loosely imitating a single base plane from graphite. In terms of the fluid charge e/a2 and mass m/a2 per unit area, the crucial parameters are q ? 2?e2/mc2a2, a Debye-type cutoff on surface-parallel normal-mode wavenumbers k, and X ? K/q. The cohesive energy ? per unit area is determined from the zero-point energies of the exact normal modes of the plasma coupled to the Maxwell field, namely TE and TM photon modes, plus bound modes decaying exponentially with |z|. Odd-parity modes (with Ex,y(z = 0) = 0) are unaffected by the sheet except for their overall phases, and are irrelevant to ?, although the following paper shows that they are essential to the fields (e.g. to their vacuum expectation values), and to the stresses on the sheet. Realistically one has X 1, the result ? ~ cq1/2K5/2 is nonrelativistic, and it comes from the surface modes. By contrast, X 1 (nearing the limit of perfect reflection) would entail ? ~ ?cqK2log(1/X): contrary to folklore, the surface energy of perfect reflectors is divergent rather than zero. An appendix spells out the relation, for given k, between bound modes and photon phase-shifts. It is very different from Levinson's theorem for 1D potential theory: cursory analogies between TM and potential scattering are apt to mislead.

Average characteristics of the midtail plasma sheet in different dynamic regimes of the magnetosphere

Abstract. We study average characteristics of plasma sheet convection in the middle tail during different magnetospheric states (Steady Magnetospheric Convection, SMC, and substorms) using simultaneous magnetotail (Geotail, 15-35 RE downtail) and solar wind (Wind spacecraft) observations during 3.5 years. (1) A large data set allowed us to obtain the average values of the plasma sheet magnetic flux transfer rate (Ey and directly compare it with the dayside transfer rate (Emod for different magnetospheric states. The results confirm the magnetic flux imbalance model suggested by Russell and McPherron (1973), namely: during SMC periods the day-to-night flux transport rate equals the global Earthward plasma sheet convection; during the substorm growth phase the plasma sheet convection is suppressed on the average by 40%, whereas during the substorm expansion phase it twice exceeds the day-to-night global flux transfer rate. (2) Different types of substorms were revealed. About 1/3 of all substorms considered displayed very weak growth in the tail lobe magnetic field before the onset. For these events the plasma sheet transport was found to be in a balance with the day-to-night flux transfer, as in the SMC events. However, the lobe magnetic field value in these cases was as large as that in the substorms with a classic growth phase just before the onset (both values exceed the average level of the lobe field during the SMC). Also, in both groups similar configurational changes (magnetic field stretching and plasma sheet thinning) were observed before the substorm onset. (3) Superimposed epoch analysis showed that the plasma sheet during the late substorm recovery phase has the characteristics similar to those found during SMC events, the SMC could be a natural magnetospheric state following the substorm.

On a summer maximum in the occurrence frequency of 150 km (F1) radar echoes over Pohnpei

Using three and a half years of 50 MHz radar data collected from Pohnpei, Federated States of Micronesia (6.96°N, 158.19°E geographic, 0.3° magnetic dip latitude), we show for the first time that the occurrence frequency of 150 km (or F1) echoes has a broad but conspicuous maximum during northern summer in this longitude sector with minimal activity in other months. Given a seasonal similarity to sporadic E (Es), we suggest that an Es‐layer instability [Cosgrove and Tsunoda, 2002] generates a polarization electric field (), which maps along geomagnetic field lines to the F1 region. There, forms thin plasma sheets that provide gradients to excite 3‐m‐scale plasma waves via an interchange process.

Azimuthal magnetic fields in Saturn’s magnetosphere: effects associated with plasma sub-corotation and the magnetopause-tail current system

Abstract. We calculate the azimuthal magnetic fields expected to be present in Saturn’s magnetosphere associated with two physical effects, and compare them with the fields observed during the flybys of the two Voyager spacecraft. The first effect is associated with the magnetosphere-ionosphere coupling currents which result from the sub-corotation of the magnetospheric plasma. This is calculated from empirical models of the plasma flow and magnetic field based on Voyager data, with the effective Pedersen conductivity of Saturn’s ionosphere being treated as an essentially free parameter. This mechanism results in a ‘lagging’ field configuration at all local times. The second effect is due to the day-night asymmetric confinement of the magnetosphere by the solar wind (i.e. the magnetopause and tail current system), which we have estimated empirically by scaling a model of the Earth’s magnetosphere to Saturn. This effect produces ‘leading’ fields in the dusk magnetosphere, and ‘lagging’ fields at dawn. Our results show that the azimuthal fields observed in the inner regions can be reasonably well accounted for by plasma sub-corotation, given a value of the effective ionospheric Pedersen conductivity of ~ 1–2 mho. This statement applies to field lines mapping to the equator within ~ 8 RS (1 RS is taken to be 60 330 km) of the planet on the dayside inbound passes, where the plasma distribution is dominated by a thin equatorial heavy-ion plasma sheet, and to field lines mapping to the equator within ~ 15 RS on the dawn side outbound passes. The contributions of the magnetopause-tail currents are estimated to be much smaller than the observed fields in these regions. If, however, we assume that the azimuthal fields observed in these regions are not due to sub-corotation but to some other process, then the above effective conductivities define an upper limit, such that values above ~ 2 mho can definitely be ruled out. Outside of this inner region the spacecraft observed both ‘lagging’ and ‘leading’ fields in the post-noon dayside magnetosphere during the inbound passes, with ‘leading’ fields being observed both adjacent to the magnetopause and in the ring current region, and ‘lagging’ fields being observed between. The observed ‘lagging’ fields are consistent in magnitude with the sub-corotation effect with an effective ionospheric conductivity of ~ 1–2 mho, while the ‘leading’ fields are considerably larger than those estimated for the magnetopause-tail currents, and appear to be indicative of the presence of another dynamical process. No ‘leading’ fields were observed outside the inner region on the dawn side outbound passes, with the azimuthal fields first falling below those expected for sub-corotation, before increasing, to exceed these values at radial distances beyond ~ 15–20 RS , where the effect of the magnetopause-tail currents becomes significant. As a by-product, our investigation also indicates that modification and scaling of terrestrial magnetic field models may represent a useful approach to modelling the three-dimensional magnetic field at Saturn.Key words. Magnetospheric physics (current systems; magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)

Strong electron heating and non-Maxwellian behavior in magnetic reconnection

We discuss the electron heating in the course of magnetic reconnection by using both the Geotail observation and the particle-in-cell simulation. Geotail observes several unique non-Maxwellian velocity distribution functions during the plasma sheet crossing in association with a fast plasma flow. We find that the observed distributions can be classified into four different types depending on the position in the plasma sheet. In the boundary between the lobe and the plasma sheet, the distribution consists of the cold plasma flowing toward the X-type region and the hot plasma escaping from the X-type region along the magnetic field. In the plasma sheet side of the boundary, the distribution becomes bi-Maxwellian distribution with T∥ > T⊥. Inside the plasma sheet, the distribution is deformed into a hot and isotropic distribution. We discuss the physical mechanism responsible for those electron heating in a thin plasma sheet by using the kinetic reconnection simulation. We find that the dawn-dusk reconnection electric field as well as the turbulent waves excited by the strong Hall electric currents play an important role on the strong electron heating and acceleration.

Application of an MHD simulation to the study of substorms

The substorm mechanism is studied by the numerical solutions obtained from a resistive magnetohydrodynamic (MHD) simulation. After a southward turning of the interplanetary magnetic field (IMF), the simulation results reproduce observed features of the growth phase. The numerical solutions show that the plasma sheet thinning during the growth phase is formed under the dynamic balance between the flux pileup from the midtail and the flux removal toward the dayside controlled by the convection in the magnetosphere-ionosphere (M-I) coupling system. After the growth phase, dipolarization is generated in the near-earth tail accompanied by a plasma injection into the inner magnetosphere, the formation of plasmoid in the midtail, and the enhancement of the nightside field-aligned currents (FACs). The direct cause of this onset is the state (phase space) transition of the convection system from a thinned state to a dipolarized state associated with a self-organization in the nonlinear system.