Fast Plasma Flow Research Articles

Relationship between magnetotail variations and auroral activities during substorms

We have compared magnetotail variations with auroral activities during 8 substorms using GEOTAIL and Polar UVI data. In the 1737 UT substorm event on 15 December 1996, auroral breakups and intensifications were highly correlated with fast plasma flows with the variations in the north‐south magnetic field and the total pressure in the magnetotail. GEOTAIL was located around X ∼ −21 RE, and several fast tailward flows were observed in the early expansion phase with the southward magnetic field and the total pressure enhancement, associated with plasmoids. These flows were observed simultaneously with or within about 1 min of auroral breakups or pseudobreakups. In the late expansion or recovery phase, some fast Earthward flows were observed with the northward magnetic field as well as the total pressure enhancement slightly earlier than small auroral intensifications. These observations imply that the activation and tailward retreat of the near‐Earth neutral line is intermittent. Furthermore, including the other events, we found that the total pressure decrease in the magnetotail can be correlated with auroral activity better than the fast plasma flow. The total pressure in the magnetotail significantly decreases during auroral breakups or poleward expansions of the auroral bulge but slightly decreases during pseudobreakups. The duration of the expansion and the maximum size of the auroral bulge are closely correlated with the duration and amount of total pressure decrease in the magnetotail, respectively. These results imply that the magnitude of auroral activities associated with substorms depends on that of energy dissipation in the magnetotail.

Origin of some anisotropic tailward flows in the plasma sheet

Abstract. We use a test particle model to explore anisotropy and fast flows in the central plasma sheet (CPS) that are a consequence of plasma sheet boundary layer (PSBL) ion beam dynamics. Ion distributions and flows (velocity moments) in the CPS and equatorial current sheet (CS) are compared and we find that mirroring of initially earthward beams from the PSBL, and their subsequent convection to the CS region, results in strong anisotropy throughout the CPS. At higher latitudes, velocity moments are field-aligned and feature earthward flow. Deeper in the CPS, velocity moments yield flows in the anti-earthward direction. There is no clear distinction between the PSBL and CPS, since velocity distributions with large streaming components occur throughout the model CPS, but in the CS region they are anisotropic and nongyrotropic. In the CS region velocity moments can feature anti-earthward cross field flows. These tailward flows (> 400 km/s) are observed in the CS region between X = - 20 to - 30 RE due to nonadiabatic effects. Model results suggest that fast tailward plasma flows can be obtained without necessarily appealing to magnetotail processes associated with dynamic geomagnetic activity.Key words. Magnetospheric physics (magnetospheric con-figuration and dynamics) – Space plasma physics (charge particle motion and acceleration; numerical simulation studies)

Counterstreaming ions as evidence of magnetic reconnection in the recovery phase of substorms at the kinetic level

Counterstreaming ions embedded in hot isotropic ions are found at the front of fast earthward plasma flows in the recovery phase of substorms in the Earth’s magnetotail. The counterstreaming ions are present only when the northward component of the magnetic field increases in the equatorial plane. Hybrid simulations of magnetic reconnection have been carried out. It is found that counterstreaming ions form in the leading edge of jetting plasmas produced with magnetic reconnection, where the magnetic field lines pile up due to the pre-existing stationary plasmas. These counterstreaming ions originate from cold ions on the northern and southern tail lobe field lines, and earthward transport of the reconnected field lines makes these cold ions flow into the equatorial plane. The present observations provide strong evidence of magnetic reconnection in the recovery phase of substorms at the kinetic level.

Does the braking of the fast plasma flow trigger a substorm?: A study of the August 14, 1996, event

In a substorm event of August 14, 1996, the Geotail satellite was at XGSM ∼ −10 RE aligned with the GOES 8 satellite in the X direction in the premidnight sector. Two Pi2 onsets were identified. Whereas the first onset was associated with a pseudobreakup, an auroral bulge developed following the second onset. At the pseudobreakup Geotail observed a fast (>900 km/s) plasma flow along with dipolarization, but the geosynchronous magnetic field became stretched subsequently. In contrast, for the main onset, the magnetic field dipolarized at GOES 8, although the associated total (time‐integrated) transport of magnetic flux at Geotail was no stronger than that for the pseudobreakup. It is suggested that the braking of the fast flow does not necessarily cause tail current disruption in a large scale, and there is an additional (or alternative) condition to suffice for an initial brightening to develop to a global substorm.

Quiet time magnetotail dynamics and their implications for the substorm trigger

The present study seeks to systematically examine the fast plasma flow in the plasma sheet at geomagnetically quiet time. The study uses plasma measurements made by the Geotail satellite in the midnight sector at x > −50 RE during quiet intervals with a total duration of 446 hours. Comparison with the results of previous studies suggests that the occurrence frequency of the perpendicular flow velocity depends more clearly on the radial distance than on geomagnetic activity. Two extreme events were selected for detailed studies; they occurred on 23 November and 10–11 December 1994. In the 23 November event, Geotail was located ∼37 RE from Earth and observed a fast tailward flow ( ∼ −1250 km s−1) with a strongly southward magnetic field (BZ ∼ −8.9 nT). The signature indicates that a near‐Earth neutral line was formed earthward of the satellite and the reconnection reached the lobe magnetic field. On the ground, however, only weak (<100 nT) magnetic disturbances were observed at high (∼75°) latitudes but not at auroral zone stations. The result strongly suggests that lobe reconnection is not sufficient for the global development of a substorm. The 10–11 December 1994 event is very similar to the 23 November 1994 event except that Geotail observed a fast ( ∼ +1500 km s−1) earthward flow rather than a tailward flow, along with the dipolarization of the local magnetic field. It is asserted that the near‐Earth substorm process, that is, tail current disruption, controls the development of a substorm. The fast plasma flow may set a favorable condition for this process to proceed, and therefore the substorm may tend to develop following generation of the fast flow, but the result of the present study indicates that near‐Earth reconnection does not necessarily trigger the global substorm.

Ferroelectric plasma cathode with a control grid

Experimental results of high-current electron beam generation in a planar diode with a ferroelectric plasma cathode and a control grid are presented. The plasma is formed as a result of noncomplete discharge on the surface of a BaTiO3 sample when a driving pulse is applied to its rear electrode. This noncomplete surface discharge leads also to the appearance of fast plasma flows which fill the anode–cathode gap prior to the application of the accelerating electric field. The measured plasma densities and flow velocities allow us to estimate the bias potential of the control grid which is needed for the suppression of the plasma flows. The dependence of the plasma density on the grid potential is presented. Using the control grid, operation of the diode with the ferroelectric plasma cathode is achieved with negligible plasma prefilling of the anode–cathode gap.

IONOSPHERIC SIGNATURES OF BURSTY BULK FLOWS

While the plasma convection in the Earth's magnetosphere was for a long time considered to consist mostly of laminar flows with wide spatial extents, about a decade ago the phenomenon of bursty bulk flows (BBFs), which now could be understood as long, but narrow channels of fast earthward plasma flow in the central plasma sheet of the magnetospheric tail, was discovered. Soon after this it became clear that such events are not exceptional, but a large portion of the earthward plasma flow in the inner part of the magnetospheric tail is organised in this bursty, intermittent mode. Since the Earth's magnetosphere is connected by highly conducting magnetic field lines with its ionosphere, the next logical step was the search for the ionospheric signatures of BBFs. We review the results obtained so far in this young field of Space Physics, in terms of the auroral and ground magnetic signatures caused by BBFs, the particle precipitation to the ionosphere, as well as of the ionospheric electrodynamics of the processes associated with the BBFs in the magnetosphere. Finally, we briefly review some models of ionosphere-magnetosphere coupling with respect to their ability to explain the ionospheric signatures observed.

The coronal hole creation: theory and simulation

The possibility, that sufficiently fast plasma flows may be able to create channels (coronal holes, hereafter - CH) for their escape from the solar magnetic field network is investigated. Using a dissipative two-fluid code in which the flows are treated at par with the currents, we have studied the expected CH formation for representative test cases. We give the simulation results for: 1) the interaction of primary flows with 2 neighboring arcade-like ambient magnetic field structures, and 2) the primary flow interaction with 4 neighboring arcade-like structures. For the former case, though there is some dissipation of flow energy, the coronal hole is cold and practically empty compared to the closed coronal structures. In the latter case, the combined effect of the structure-structure and flow-structure interaction creates a highly divergent, and multiple looped stretched 2-dimensional coronal hole with a hot base: the velocity field is maximum in the central region of CH at some distance away from the base unlike the coronal hole formed out of the two arcade structure.

Fluid and kinetics signatures of reconnection at the dawn tail magnetopause: Wind observations

We report an accelerated plasma flow event detected by Wind at the low‐latitude dawn tail magnetopause (xGSE≈−10 RE) when the local magnetic shear across the magnetopause was high (∼180°) and the GSM y component of the interplanetary magnetic field was positive and much larger than the z component. High time resolution (3 s) three‐dimensional ion and electron distributions were obtained for this event. We have performed rigorous tests of fluid and particle predictions of reconnection. Consistent with reconnection, we observed at the magnetopause (1) jetting of plasma, with a flow speed, measured in the deHoffmann‐Teller frame, at 98% of the Alfvén speed; (2) mixing of a nearly isotropic hot plasma sheet ion distribution with a field‐aligned magnetosheath distribution having a low‐energy cutoff at the predicted deHoffmann‐Teller velocity; (3) opposite streaming of magnetosheath and plasma sheet electrons consistent with open field topology; and (4) a finite inward (earthward) directed normal magnetic field (BN<0) at and an associated earthward plasma flow (at 94% of the normal Alfvén speed) across the magnetopause. The combined fluid and particle signatures provide a comprehensive set of evidence for reconnection at the magnetopause. These reconnection signatures were observed in the tail flank magnetopause, where the presence of fast plasma flow (at almost twice the local Alfvén speed) in the adjacent magnetosheath has been predicted to suppress reconnection. The sense of the flow enhancement, the direction of the electron heat flux, and the polarity of BN are all consistent with each other and with the spacecraft being located tailward and northward of the reconnection site. Our analysis places the reconnection site ∼7 Earth radii south of the magnetic equator. The dimensionless reconnection rate at this flank magnetopause is estimated to be in the range of 0.1 to 0.2, which is similar to values reported for the subsolar region. In essence, the present event provides unambiguous evidence for reconnection and shows that reconnection signatures at the tail flank magnetopause are not noticeably different from those predicted and observed in the subsolar region.

Are earthward bursty bulk flows convective or field‐aligned?

We use five years of Geotail and AMPTE/IRM measurements in the near‐Earth magnetotail to compute angles between vectors of fast earthward plasma flow and the local magnetic field. In the low‐β parts of the magnetotail, fast flows were found to be nearly field‐aligned with an average angle of ∼20°. In the high‐β plasma sheet the average angle was larger than 45°. The existence of a substantial convective component in the plasma sheet confirms the importance of the bursty bulk flow phenomenon for the magnetotail convection process.

High‐Beta Disruption in the Solar Atmosphere

The outer layers of X-ray loops in solar flares are known to be more active: they are hotter than the lower loops and above the loop-top hard X-ray sources are formed there. These phenomena are interpreted as the result of the reconnection above the loop, which converts magnetic energy into thermal and nonthermal energy of plasma. However, little direct evidence for the reconnection has been presented so far. This paper interprets the activity in the outer layer of flaring loops on a different scenario. Coronal loops filled with hot and dense plasma (high beta) or with fast plasma flow, surrounded by the low-beta corona, are unstable at their outer boundary, where the curvature is convex outward and the density gradient is inward. The centrifugal force acting upward on the plasma in the loop can exceed that of gravity. This condition is favorable for localized interchange instability called ballooning instability, and the plasma in the loop is ejected when the instability has developed into a nonlinear phase (high-beta disruption). This is a natural consequence of the high-beta (and/or the high-velocity) plasma confined in a curved coronal loop. The high-beta disruption has many elements common to solar flares. In this paper, importance of the high-beta plasma is stressed, which is a neglected part of the solar activity so far.

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.

Substorm Trigger Processes in the Magnetotail: Recent Observations and Outstanding Issues

The present article reviews recent studies about near-Earth substorm processes. A focus is placed on the relationship between two fundamental processes, that is, tail current disruption (TCD) and the formation of a near-Earth neutral line (NENL). The former is inferred to cause dipolarization, and the latter is often associated with the fast plasma flow in the plasma sheet. Whereas it is inferred from the directions of fast plasma flows that the NENL is formed at 20–30 RE from the Earth, dipolarization is most manifest in the near-Earth (6.6–12 RE) region. The observation of the fast plasma flow prior to substorm (Pi2) onsets favors the idea that the NENL is formed first and dipolarization is the effect of the pile-up of magnetic flux convected earthward from the NENL, which is called the pile-up model. The present paper addresses several outstanding issues regarding this model, including (1) the interpretation of plasma flow deceleration in terms of the flux pile up, (2) highly irregular magnetic fluctuations observed in the near- Earth region, (3) the spatial coherency of the fast plasma flow, (4) the spatial structure and expansion of dipolarization region, and (5) the explosive growth phase. The paper also proposes the possibility that TCD is an independent process, but the formation of the NENL sets a favorable condition for it.

Statistical visualization of Earth's magnetotail during substorms by means of multidimensional superposed epoch analysis with Geotail data

A multidimensional superposed epoch analysis of plasma and magnetic field data from the Geotail spacecraft was used to visualize the time evolution of plasmoids in the Earth's magnetotail statistically. The substorm events were identified by Pi2 events at midlatitude ground stations when Geotail was in the downtail region from −30 RE to −200 RE. Plasma density, velocity, temperature, and magnetic field data were sorted and analyzed according to the Pi2 onset time. We selected 156 substorm events that were either a single onset or the first onset of multiple onsets. In the statistical visualization, we found that plasma acceleration does not take place in a small region around X ∼ −30 RE, but occurs in a more widely spread region extending from X ∼ −30 RE down to X ∼ −90 RE. The spatial structure of the plasmoid is characterized by a bilatitudinal structure where the fast plasma flow in the plasma sheet boundary stretches from the equator and a relatively slow plasma sheet flow is encountered with preexisting plasma sheet populations. Finite By fields are created in the inner plasma sheet boundary, and the total pressure is enhanced inside the plasmoid. The evolution of the plasma and magnetic fields as well as the deduced parameters, such as the number flux of plasma or energy fluxes, agreed well with a magnetic reconnection model.

Auroral electrojet activity associated with fast plasma flows in the magnetotail

We report here a case study of simultaneous observations from Geotail in the mid‐tail region and from ground CANOPUS stations to determine the auroral electrojet activity associated with the detection of fast plasma flows in the magnetotail. Ground observations from the meridian scanning photometers indicate the occurrence of a detached auroral feature in association with a transient fast plasma flow in the mid‐tail. Ground magnetometer observations suggest that the detached auroral feature and the transient fast flow in the mid‐tail correspond to a new segment of the auroral electrojet (probably limited in the east‐west extent) moving equatorward, with the original electrojet anchoring at the poleward boundary of the auroral bulge. Based on this result, we propose a possible resolution to the controversy on whether or not transient fast flows in the tail or north‐south auroral structures are substorm features.

Substorm onset timing: The December 31, 1995, event

The objective of the present study is to examine the timing of various onset‐associated signatures and address the cause‐and‐effect relationship between the formation of a near‐Earth neutral line (NENL) and the trigger of tail current disruption. An event selected for this study took place on December 31, 1995. In this event the Geotail satellite was located at X = −30.3 RE in the midnight sector at a local time between the GOES 8 and 9 geosynchronous satellites. The timing of the Geotail observation of a fast (950‐km/s) tailward convection flow accompanied with southward Bz (< −10 nT) indicates that the near‐Earth reconnection process started at least 4 min before the ground substorm onset, which was identified by various signatures such as an auroral expansion, a Pi2 onset, a positive bay onset, and a negative bay onset. Both GOES satellites observed dipolarization. GOES 9 was located closer to the onset meridian and observed a sudden recovery (dipolarization) of the local magnetic field but with a noticeable (≈1 min) delay from the ground onset. This delay can be interpreted in terms of the earthward expansion of tail current disruption initiated outside of geosynchronous orbit. The timing of all these features is consistent with the idea that dipolarization is a pileup of magnetic flux conveyed from the NENL. However, a sharp decrease in the H component at GOES 9 prior to the local dipolarization onset and the sudden start of a substorm are difficult to explain in terms of this idea. It is asserted that tail current disruption is a unique process rather than a direct consequence of the NENL formation, although it is possible that the reconnection process sets up a favorable condition for triggering tail current disruption. The fast plasma flow in the plasma sheet ceased soon after the substorm onset, suggesting that during the expansion phase, the tail current disruption took over the near‐Earth reconnection process as a major role in the substorm dynamics.

Fast earthward plasma flows observed in the mid/distant tail under quiet conditions: Relation to substorms

Measurements of the magnetic field and low energy plasma by the GEOTAIL spacecraft have been used to study fast earthward plasma flows in the distant (130–200 RE) and middle (40–80 RE) tail and their relationship to substorms. Results of the analysis show that high‐speed earthward ion flows are often observed in the mid/distant tail under conditions of low magnetic activity (AE ∼ 100 nT), when the IMF BZ component is northward or oscillates to about zero. A considerable portion of the fast earthward flows (29% in the distant tail and 53% in the middle tail) seems to be a precursor to substorm expansion onsets on the Earth. These flows occur at closed magnetic field lines. The cause and effect relationship between fast earthward plasma flows in the distant tail and substorm activity suggests that a neutral line can form at distances far beyond 100 RE during quiet conditions.

Measurement of the muzzle dynamic pressure of the plasma dynamic accelerator

The plasma dynamic accelerator comprises a coaxial railgun-like first stage that accelerates a plasma to very high velocities. The second stage, the compressor coil, compresses this plasma into a fast and dense plasma flow. A series of experiments was conducted to measure the dynamic pressure in front of the muzzle. Kistler Type 6157 BA pressure probes served as pressure gauges. The evaluation of these measurements was assisted by measurement of the current in the coil ground loop, the inductance of the system and by high speed photography three different coil shapes were used to evaluate the influence of the coil shape on the compression. A non-conducting coil with a shape identical to one of these coils was tested to determine the contribution of aerodynamic forces to the plasma compression. Additional tests were performed with a non-conducting conical tube to see whether a coil has aerodynamic benefits compared to closed contour. Finally, the coil was completely omitted. Only the coil holder, attached to the ground loop, remained in the system. This configuration forced a mainly axial discharge from the tip of the coaxial accelerator's center electrode to the holder and allowed the investigation of the contribution of the z-pinch to compression. This paper describes the configurations used in these experiments and presents the results of the measurements. These results are compared to each other and conclusions are drawn on how to improve the coil.

GEOTAIL observations of magnetosheath flow properties, with simultaneous observations of the solar wind by the WIND spacecraft

Observations from the GEOTAIL spacecraft are used to examine the plasma flow and magnetic field within the magnetosheath at intermediate downtail distances. Simultaneous measurements of the solar wind condition as measured by the WIND spacecraft are also used for this study. Close to the downtail magnetopause, we find that under appropriate solar wind conditions, the plasma flow velocity of the magnetosheath is slowest when the local magnetic field and velocity vectors are aligned, but are largest when the two vectors are perpendicular to one another, though the magnetosheath flow speed remains slower than or equal to the solar wind velocity. A simple model utilizing the magnetic field line tension and Alfvén phase speed is used to explain the observations. Occasionally, close to the magnetopause we find magnetosheath flows which are much greater than the solar wind velocity, but only for certain orientations of the local magnetic field. The source of these fast plasma flows is the low latitude boundary layer (LLBL), and is not the shocked solar wind plasma. Possible reasons for the observation of very fast flows near the magnetotail magnetopause are explored.

Evidence of Two Active Reconnection Sites in the Distant Magnetotail

Prior to the observation of fast tailward plasma flows, earthward plasma flows are often observed in the distant magnetotail. By studying the evolution of the ion distribution functions for the transition flow from earthward to tailward flow, we find a multi-layer structure of beam-like plasma flows in the plasma sheet boundary layer, PSBL: 1) earthward, anisotropic beam plasma can be observed in the outer region of PSBL, i.e., the plasma lobe side of the PSBL; and 2) the counterstreaming ions, which consist of earthward and tailward beam-like plasma flows, are observed in the inner region of the PSBL; i.e., the plasma sheet side. Inside the plasma sheet, an isotropic, hot ion plasma can be observed. These characteristics are similar to those measured in the PSBL closer to the Earth, and the tailward anisotropic ion beams observed in the near-Earth magnetotail are thought to be the mirrored counterpart of the earthward ion beam. In the distant magnetotail, however, it is not easy to explain the tailward beam as the mirrored counterpart, because the reflected ion beams at the Earth will be absorbed into the plasma sheet before the reflected ions can travel to the distant magnetotail. We propose another model of two active magnetic reconnection sites. The tailward ion beams come from a near-Earth reconnection site which is embedded in the distant magnetotail reconnection site ejecting the earthward flow.