Impulsive magnetic pulsations and electrojets in the loop footpoint driven by the fast reconnection jet

The present paper studies the basic physics of the spontaneous fast reconnection model in a three-dimensional (3D) situation for different resistivity parameter values, where the threshold for occurrence of current-driven anomalous resistivity is allowed to increase with the thermal velocity (T), and the initial plasma density notably changes in space with the plasma pressure in the current sheet system. For any case, once the anomalous resistivity is ignited, the 3D fast reconnection mechanism explosively evolves as a nonlinear instability by the positive feedback between the anomalous resistivity and the reconnection flow, even if the threshold significantly increases with the thermal velocity; for the larger threshold values, the fast reconnection evolution becomes more drastic and the reconnection rate, finally attained on the nonlinear saturation phase, becomes larger. In the resulting 3D fast reconnection configuration, slow shocks stand and extend outwards in the finite extent; also, ahead of the fast reconnection jet, a large-scale 3D plasmoid swells and propagates in the central current sheet, and a vortex flow is formed near the plasmoid side boundary. In the wide range of parameter values, the basic physics of the 3D spontaneous fast reconnection evolution in the finite extent is found to be, qualitatively, consistent with the well-known two-dimensional one.

A traveling compression region (TCR) is a several‐minute long compression of the lobe magnetic field produced by a plasmoid as it moves down the tail. They are generally followed by a longer interval of southward tilting magnetic fields. This study reports the first comprehensive survey of TCRs in the distant magnetotail. A total of 116 TCRs were identified in the ISEE 3 magnetic field observations. Of this population, 37 TCRs were observed to be separated by 30 min or more from any other TCR and are termed “isolated” events. “Paired” events are defined as two TCRs separated by less than 30 min. There were 36 such TCRs corresponding to 18 paired events. “Multiple” events were also observed in which more than two TCRs occurred in a series without a gap between TCRs of more than 30 min. The 11 multiple events identified in this study had an average of about four traveling compression regions each for a total of 43 TCRs. The mean amplitude, ΔB/B, and duration, ΔT, for all TCRs were found to be 7.6% and 158 s, respectively. TCRs occurring as isolated events were the largest (ΔB/B = 8.8% and ΔT = 218 s) and those associated with multiple events were the smallest (ΔB/B = 5.6% and ΔT = 84 s). The mean duration of the period of southward tilting Bz following isolated TCRs was 12.3 min. This time interval was found to be quite similar to the average spacing between TCRs in paired and multiple events, 11.2 and 10.2 min, respectively. TCR amplitude and duration were found to be independent of location within the tail lobes suggesting that the plasmoids which cause the TCRs maintain approximately constant volume and shape as they move down the tail. Mean plasmoid dimensions estimated from TCR duration and amplitude under the assumption of a quasi‐rigid magnetopause are 35 RE (length) × 15 RE (width) × 15 RE (height). Utilizing auroral kilometric radiation, the AL index, Pi 2 pulsations at two ground stations, and energetic particle data from three geosynchronous spacecraft, it is found that over 91% of the TCR events identified in this study followed substorm onsets or intensifications. The number of TCR events identified in this study are consistent with their release in association with a new substorm onset every 4‐6 hrs. The results of this study strongly suggest that the release of plasmoids down the tail near the time of expansion phase onset is an integral step in the substorm process and an important element in the substorm energy budget.

The spontaneous fast reconnection model is applied to the earthward fast flow events observed in the near‐Earth plasma sheet. Here, the earthward fast flow events include both of bursty bulk flow events and flow burst events. In order to apply it directly to actual observations, virtual probes are located in the plasma sheet region in the three‐dimensional simulation domain so that we can directly observe the temporal variations of plasma quantities in accordance with the growth and proceeding of the fast reconnection. In this model, magnetic reconnection drastically evolves and Alfvénic fast plasma jet flows in the very restricted narrow channel, and a large‐scale plasmoid is formed ahead of the fast plasma jet. The results of virtual observation of these evolutions are found to be in good agreement with actual satellite observations. At the same time, in the lobe region, travelling compression regions (TCRs) are observed in connection with the fast flow events. The temporal profiles of magnetic fields detected by the virtual probes are also in good agreement with actual satellite observations. It is concluded that the earthward fast flow events and earthward TCR events result from the fast reconnection mechanism.

IMP 8 and Geotail observations of traveling compression regions (TCRs) and plasmoids, respectively, are used to investigate plasmoid formation and ejection. One year of IMP 8 magnetometer measurements taken during the distant tail phase of the Geotail mission were searched for TCRs, which signal the release of plasmoids down the tail. A total of 10 such intervals were identified. Examination of the Geotail measurements showed that this spacecraft was in the magnetotail for only three of the events. However, in all three cases, clear plasmoid signatures were observed at Geotail. These plasmoids were observed at distances of X = −170 to −197 RE. The in situ plasma velocities in these plasmoids are found to exceed the time‐of‐flight speeds between IMP 8 and Geotail suggesting that some further acceleration may have taken place following release. The inferred lengths of these plasmoids, ∼27–40 RE, are comparable to the downtail distance of IMP 8. This indicates that TCR at IMP 8 can be caused by plasmoids forming not only earthward but also adjacent to or just tailward of the spacecraft. The closeness of IMP 8 to the point of plasmoid formation is confirmed by the small, ∼0–3 min, time delays between the TCR perturbation and substorm onset. In two of the plasmoid events, high‐speed earthward plasma flows and streaming energetic particles were measured in the plasma sheet boundary layer surrounding the plasmoid along with large positive Bz at the leading edge of the plasmoid suggesting that the core of the plasmoid was “snow plowing” into flux tubes recently closed at an active distant neutral line. In summary, these unique two‐point measurements clearly show plasmoid ejection near substorm onset, their rapid movement to the distant tail and their further evolution as they encounter preexisting X lines in the distant tail.

A statistical study of flux ropes and traveling compression regions (TCRs) during the Time History of Events and Macroscale Interactions during Substorms (THEMIS) second tail season has been performed. A combined total of 135 flux ropes and TCRs in the range GSM X approx -14 to -31 R(sub E) were identified, many of these occurring in series of two or more events separated by a few tens of seconds. Those occurring within 10 min of each other were combined into aggregated reconnection events. For the purposes of this survey, these are most likely the products of reconnect ion occurring simultaneously at multiple, closely spaced x-lines as opposed to statistically independent episodes of reconnection. The 135 flux ropes and TCRs were grouped into 87 reconnection events; of these, 28 were moving tailward and 59 were moving Earthward. The average location of the near-Earth x-line determined from statistical analysis of these reconnection events is (X(sub GSM), Y*(sub GSM)) = (-30R(sub E), 5R(sub E)), where Y* includes a correction for the solar aberration angle. A strong east-west asymmetry is present in the tailward events, with >80% being observed at GSM Y* > O. Our results indicate that the Earthward flows are similarly asymmetric in the midtail region, becoming more symmetric inside - 18 R(sub E). Superposed epoch analyses indicate that the occurrence of reconnection closer to the Earth, i.e., X > -20 R(sub E), is associated with elevated solar wind velocity and enhanced negative interplanetary magnetic field B(sub z). Reconnection events taking place closer to the Earth are also far more effective in producing geomagnetic activity, judged by the AL index, than reconnection initiated beyond X approx -25 R(sub E).

The spontaneous evolution of fast reconnection is studied in three dimensions by extending (in the z direction) the previous two-dimensional model that considered only the x-y plane [M. Ugai, Phys. Fluids B 4, 2953 (1992)]. It is demonstrated that the reconnection development strongly depends on three-dimensional effects; only when the central current sheet is sufficiently long in the z direction, say more than a few times larger than the current sheet width, the fast reconnection mechanism fully develops by the self-consistent coupling between the global reconnection flow and the current-driven anomalous resistivity. In this case, the reconnection flow can grow so powerfully as to enhance the current density (the current-driven resistivity) locally near an X line; otherwise, such a vital reconnection flow cannot be caused. The resulting quasisteady fast reconnection mechanism is significantly confined in the z direction, where a strong (Alfvénic) plasma jet results from standing switch-off shocks; accordingly, a large-scale plasmoid is formed and propagates in the middle of the system. It is concluded that the well-known two-dimensional spontaneous fast reconnection model can reasonably be extended to three dimensions.

Magnetic reconnection is widely believed to be involved in dynamical phenomena such as solar flares or magnetospheric substorms. The Sweet–Parker model of magnetic reconnection in a Y-type geometry predicts a characteristic time scale proportional to S1/2 (where S is the Lundquist number), which is too slow to account for the observed time scales. The Petschek model, in contrast, predicts a time scale proportional to ln S in an X-point geometry. Numerical magnetohydrodynamic (MHD) simulations in the high-S regime generally validate the Sweet–Parker model, unless the resistivity is enhanced in the diffusion region to large values (such that typically S<103). It is demonstrated in this paper that nonlinear reconnection dynamics in a Harris sheet driven by inward boundary flows occurs on a nonlinear time scale that is proportional to S1/5 and thus has a weaker dependence on resistivity than the Sweet–Parker time scale. The current sheet amplitude at the separatrix (spanning Y points) grows algebraically in the linear regime but is suddenly enhanced after it makes a transition to the nonlinear regime. An analytical calculation is given for both the linear and the nonlinear regimes, and supported by two-dimensional resistive MHD simulations. The features of sudden current sheet enhancement and fast reconnection, controlled by boundary flows, are relevant to the phenomena of substorm onset or the impulsive phase of flares.

Energy release in solar flares occurs during the impulsive phase, which is a period of a few to about ten minutes, during which energy is injected into the flare region in bursts with durations of various time scales, from a few tens of seconds down to 0.1 s or even shorter. Non-thermal heating is observed during a short period, not longer than a few minutes, in the very first part of the impulsive phase; in average flares, with ambient particle densities not larger than a few times 1010 cm−3 it is due to thick-target electron beam injection, causing chromospheric ablation followed by convection. In flares with larger densities the heating is due to thermal fronts (Section 1). The average energy released in chromospheric regions is a few times 1030 erg, and an average number of 1038 electrons with E ≳ 15 keV is accelerated. In subsecond pulses these values are about 1035 electrons and about 1027 erg per subsecond pulse. The total energy released in flares is larger than these values (Section 2). Energization occurs gradually, in a series of fast non-explosive flux-thread interactions, on the average at levels about 104 km above the solar photosphere, a region permeated by a large number (≳ 10) of fluxthreads, each carrying electric currents of ≈ 1010–1011 A. The energy is fed into the flare by differential motions of magnetic fields driven by photospheric-chromospheric movements (Section 3). In contrast to these are the high-energy flares, characterized by the emission of gamma-radiation and/or very high-frequency (millimeter) radiobursts. Observations of such flares, of the flare neutron emission, as well as the observation of 3He-rich interplanetary plasma clouds from flares all point to a common source, identified with shortlived (∼ 0.1 s) superhot (≳ 108 K) flare knots, situated in chromospheric levels (Section 4). Pre-flare phenomena and the existence of homologous flares prove that flare energization can occur repeatedly in the same part of an active region: the consequent conclusions are that only seldom the full energy of an active region is exhausted in one flare, or that the flare energy is generated anew between homologous flares; this latter case looks more probable (Section 5). Flare energization requires the formation of direct electric fields, in value comparable with, or somewhat smaller than the Dreicer field (Section 6). Such fields originate by current-thread reconnection in a regime in which the current sheet is thin enough to let resistive instability originate (Section 7). Particle acceleration occurs ‘by fast reconnection in magnetic fields ≳ 100 G and electric fields exceeding about 0.3 times the Dreicer field at fairly low particle densities (≈ 1010 cm−3); for larger densities plasma heating is expected to occur (Section 8). Transport of accelerated particles towards interplanetary space demands a field-line configuration open to space. Such a configuration originates mainly after the gradual gamma-ray/proton flares, and particularly after two-ribbon flares; these flares belong to the dynamic flares in Sturrock and Svestka's flare classification. Acceleration to GeV energies occurs subsequently in shock waves, probably by first-order Fermi acceleration (Section 9).

We have analyzed ELF‐VLF receiver and search coil magnetometer data from five Antarctic stations from 1998 and 1999 to study quasiperiodic emissions (QPs) and periodic emissions (PEs), which occur as ULF‐range modulations of ELF‐VLF signals between 0.5 kHz and ∼4 kHz. QPs are modulated at frequencies of ∼20–50 mHz, and PEs are modulated at frequencies of ∼200–500 mHz. The stations used covered a range of magnetic latitudes from −62° (Halley) to −74° (South Pole Station); three automated geophysical observatories (AGOs) were located at intermediate latitudes. Consistent with earlier studies, most QPs were observed with magnetic pulsations of identical period in the Pc3 range (type I QPs). Of those QPs not observed with simultaneous magnetic pulsations (type II QPs), nearly all were accompanied by PEs. Type I QPs, PEs, and events during which both appeared together (QPPEs) were found to have different latitudinal, seasonal, and diurnal occurrence patterns: QPs of both types were more likely to occur between −65° and −70° magnetic latitude, while PEs occurred more often around −60° magnetic latitude. QPs were more common during the months of October though March, while PEs were more common during the months of May through September. QPs, whether with or without simultaneous PEs or magnetic pulsations, were predominantly a dayside phenomenon, with a broad maximum near local noon. The occurrence of QPs unaccompanied by PEs was restricted to the dayside, however, while a small number of QPPEs appeared even during nighttime hours. PEs, on the other hand, could be seen at all local times, but with latitudinally dependent diurnal patterns. Most higher‐latitude QPs were type I events (observed with magnetic pulsations), while type II QP events (without simultaneous magnetic pulsations) occurred relatively more often at lower latitudes. A case study from 1 August 1999 using wideband data from South Pole and Halley provides evidence of a transition from echoing whistler activity to PE activity and then to QP activity and suggests a causal relationship.

Computer simulations study the temporal dynamics of magnetic reconnection in a current sheet system under a continuous local plasma injection imposed on the inflow boundary. The basic reconnection process is strongly influenced by the resistivity model. For a uniform resistivity model, the field lines become flattened with time so that the resulting quasisteady configuration involves a long diffusion region and the reconnection rate strongly depends on the resistivity, in good agreement with the Sweet–Parker mechanism. For an anomalous resistivity model, magnetic reconnection drastically occurs, leading to development of a large-scale plasmoid. Once an effective resistivity is triggered, a quasisteady fast reconnection mechanism spontaneously develops even in the absence of the specified boundary condition, in good agreement with the spontaneous fast reconnection model. When the imposed plasma injection is larger than the one inherent to the spontaneous fast reconnection mechanism, the fast reconnection mechanism can no longer proceed steadily and multiple X points and plasmoids result. It is concluded that it is not the boundary condition that drives the fast reconnection mechanism; the boundary condition may simply initiate the onset of magnetic reconnection (or effective resistivity) but has no essential effect on the fast reconnection development.

The possibility of using ULF (1–20 mHz) waves to diagnose the structure of the magnetosphere has recently given new impetus to the study of Pc 4,5 magnetic pulsations. In this paper we demonstrate that the frequency spectrum of dayside Pc 4,5 pulsations near noon may be significantly altered in association with the onset of a magnetospheric substorm near midnight. The response time for the dayside Pc pulsations to a substorm onset can be as short as 2–3 min, suggesting information transfer across the magnetosphere at velocities of the order of the Alfvén speed. The characteristic response of the dayside pulsations is a marked increase in the dominant frequency at stations inside the dayside auroral oval. Our results taken together with the observations of dayside auroras by Eather et al. [1979] suggest that substorm onsets are accompanied by a sudden inward motion of the center of the partial ring current. We propose that this ring current motion causes changes in the magnetic field in the equatorial plane of the magnetosphere that result in changes in the Alfvén velocity on field lines where the magnetic pulsations are observed. Possible mechanisms for the generation of Pc 4,5 pulsations are discussed in the light of the observations reported in this paper.

Resistive relativistic magnetohydrodynamic (RRMHD) simulations are applied to investigate the system evolution of relativistic magnetic reconnection. A time-split Harten--Lan--van Leer method is employed. Under a localized resistivity, the system exhibits a fast reconnection jet with an Alfv\'{e}nic Lorentz factor inside a narrow Petschek-type exhaust. Various shock structures are resolved in and around the plasmoid such as the post-plasmoid vertical shocks and the "diamond-chain" structure due to multiple shock reflections. Under a uniform resistivity, Sweet--Parker-type reconnection slowly evolves. Under a current-dependent resistivity, plasmoids are repeatedly formed in an elongated current sheet. It is concluded that the resistivity model is of critical importance for RRMHD modeling of relativistic magnetic reconnection.

While in the lobes of the distant magnetotail, ISEE‐3 encountered regions of compressed magnetic field, δB/Bo=0.3‐0.1, at a rate of several per day. The duration of these events was 5 to 20 minutes and they were observed 10 to 30 minutes following the onset of substorm activity near the earth. During each event, the lobe magnetic field tilted first northward and then southward with the inflection point near the time of peak field strength. Following the compression events, the lobe field weakened and retained a southward component for 20 to 40 minutes. It is suggested that these traveling compression regions (TCR’s) are the lobe signatures of plasmoids moving rapidly down the tail in the plasma sheet. Comparison of ISEE‐3 compression event times with substorm onset times yielded propagation speeds of 350 to 750 km/s.

<p>Among the most unpredictable phenomena within the near-Earth space are substorms, periods of energy loading and explosive release within the magnetospheric tail. Substorms are global, as energy is extracted from the solar wind via dayside reconnection, while the tail energy release takes place in a vast domain within a few tens of seconds. Due to the scarcity of space-borne observations, it has been difficult to conclusively separate between the onset scenarios that include magnetic reconnection and various ion-kinetic instabilities, which occur at mesoscales, and small scales. Another decades-long investigation concerns the flapping of the plasma sheet, occurring within a large area favouring the substorm growth phase, although it has been observed at other times as well. Mechanisms to explain the flapping are presently unknown. Modelling efforts have failed to explain the substorm onset either because all the required physics has not been included in the simulation, or the simulation does not cover the entire domain, thus possibly missing important drivers. Vlasiator is a<span>  </span>model describing the global magnetosphere accurately at ion-kinetic scales, including the ion-kinetic effects that are absent in the fluid descriptions. Unlike many other kinetic simulations, Vlasiator extends the simulation domain to global scales and accurately represents the Earth’s unscaled magnetosphere from the dayside to the tail, in six dimensions including the 3D real space and 3D velocity space without noise that is present in the alternative PIC method. We present the first global 6D simulation encompassing the entire near-Earth space to simulate ion-kinetic magnetospheric dynamics self-consistently. We determine reconnection, ion-kinetic instabilities, plasma sheet flapping, and bursty bulk flows in the simulation domain, and show how they all contribute to the whole and work in concert in developing the substorm onset. Our results help to understand spacecraft measurements and the overall substorm process, which will significantly improve understanding space physics and eventually space weather. Our results can also be used in strategies to design a mission, which will finally and conclusively capture the substorm onset with in situ measurements.</p>

Theoretical studies concerning MHD source waves of long-period (T=10-600s) magnetic pulsations are reviewed. The source waves can be classified into two groups: One is external origins, that are upstream waves driven by the ion beam instabilities in the earth's foreshock, surface waves excited by the Kelvin-Helmholtz instabilities in the magnetospheric boundary, and sudden impulses caused by an interplanetary shock and dayside reconnection. The other is internal origins, e. g., drift mirror instabilities in ring current hot ions and sudden changes during substorm onset in the nightside magnetosphere. The upstream waves, the surface waves, and the drift waves associated with a finite energy distribution and a finite thickness of layered plasma show a narrow-band frequency, while the sudden impulses have a broad-band frequency. These source waves with a nearly monochromatic frequency and a broadband frequency can couple into standing field-line oscillations in the inner magnetosphere through the field-line resonance mechanism.