Fast Plasma Flow Research Articles

Large‐amplitude wave electric field in the inner magnetosphere during substorms

CRRES electric field data during substorms have been analyzed to investigate intense electric fields in the inner magnetosphere associated with dipolarizations. Substorm injections during 13:00–15:00 UT on 7 March 1991 exhibit many large‐scale electric fields and short‐duration electric field spikes. Large‐scale electric fields with durations of ∼1 min are identified as spatial structures associated with the region 1 field‐aligned currents. Amplitudes of electric fields reach 30 mV/m with a high correlation with variations of the magnetic field, and the Poynting fluxes are directed toward the ionosphere with magnitudes of more than 0.5 mW/m2. 16‐Hz high‐resolution data show intense electric field spikes with amplitudes of ∼100 mV/m with durations of ∼1 s. Most of the spikes are electromagnetic with Poynting fluxes of ∼0.1 mW/m2 directed toward the ionosphere. The electric field spikes are identified as right‐handed whistler waves with a size of ∼1000 km with frequencies just below the ion cyclotron frequency. A nearly simultaneous measurement by the DMSP‐F9 satellite shows intense plasma flows with durations of 1 s and inverted‐V electron precipitation. An estimation of the wave and particle energy fluxes shows that about half of the Poynting flux of the electric field spikes is consumed accelerating auroral particles, and 1% of the Poynting flux drives the fast plasma flows at the ionosphere. It is suggested that the electromagnetic spikes provide sufficient energy for auroral particle acceleration.

Cross-tail velocity component in the plasma sheet fast flows

Abstract. The flux transfer in the magnetotail plasma sheet is mainly provided by the tail-aligned fast plasma flows (Bursty Bulk Flows – BBFs). In this paper we study the events with a large cross-tail velocity component, including their occurrence and relationship to the standard BBFs. We found out that a significant part of large Vy events are a subgroup connected with the BBFs propagation. The maximal deviation of the velocity vector from the X direction (about 40–50 degrees, on average) is observed near the BBFs' leading front in the sheath, where the fast flow interacts with surrounding plasma. The average variation of the velocity direction in the vicinity of the BBF resembles a plasma vortex. Our results support the model, in which the BBF represents a polarized, bubble-like flux tube, propagating through the plasma sheet.

Substorm onset dynamics in the magnetotail as derived from joint TC-1 and Cluster data analysis

Abstract This study investigates two substorm onset events with favorable constellations of spacecraft, TC-1 and Cluster, separated by several Earth radii. The substorms have been identified in both auroral regions. One is believed to be triggered by a northward turning of the interplanetary magnetic field (IMF), while the other occurs under generally southward IMF. For both events, Cluster was located tailward of TC-1, but observed the dipolarization at earlier time for one event while at later time for the other. The timing difference of dipolarization at different positions could be explained by the earthward or tailward propagation of the field disturbances in the radial direction. The earthward dipolarization front was found in one case to bounce back and forth at TC-1. The earthward propagation was accompanied by a fast earthward plasma flow for the 21 September 2005 event. The event analysis suggests that substorms can be quite different from case to case due to differences in the solar wind condition and magnetotail configuration.

Ion heat flux and energy transport near the magnetotail neutral sheet

Ten‐year averages of energy transport rates near the neutral sheet showed that the enthalpy flux density or thermal energy term QT = (5/2)PV was the largest, where P is the isotropic pressure and V is the bulk flow velocity. The ion heat flux, qi, was the next largest term. Sorting data using a magnetic flux transport parameter showed that qi could become dominant during periods of slow flow. Both qi and the ion bulk velocity Vi were duskward on the dusk side of the neutral sheet. This relationship is characteristic of cross‐tail drift and a heat flux that can be attributed to the energy dependent gradient and curvature drifts. The qi and Vi vectors often pointed in different directions on the dawn side. The x component of qi on the dawn side pointed tailward, suggesting entry through the magnetopause of a suprathermal ion component. On the dusk side the qix plots that were sorted using a magnetic flux transport parameter showed evidence of plasma sheet reconnection. The long‐term averaged x component of QT pointed earthward almost everywhere in the neutral sheet, and was attributed to periods of very fast plasma flow. The cross‐tail component of QT was separated into two contributions. One part of QTy involved a common drift away from midnight during both earthward and tailward fast flows. This feature suggests that thermal energy and plasma flow from the outer plasma sheet toward the neutral sheet near midnight, and then toward the flanks. The other part of QTy involved a differential duskward drift during fast earthward flows and a dawnward drift during fast tailward flows. The incremental E fields that would produce such convection point tailward during the fastest earthward flows and earthward during the fastest tailward flows. The dependencies of Vi, qi and QT on the interplanetary magnetic field (IMF) clock angle also were studied. Both Vi and QT were reduced when the IMF was northward and the neutral sheet plasma became cold and dense. However, no dependence of qi on the IMF direction was seen. This shows that the generation of a cold dense plasma sheet does not substantially change the distribution of the suprathermal ions that are most important in the production of qi.

Numerical studies on three‐dimensional earthward fast plasma flows in the near‐Earth plasma sheet by the spontaneous fast reconnection model

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.

Cluster Observation and Analysis of Bifurcation Current Sheet Structure in a Quiet Time

AbstractWe analyse the vertical structure of the magnetotail current sheet during 2011~2030 UT on 19 August 2001, during which the Cluster spacecrafts crossed the current sheet in a quiet time. In the intervals, the current sheet moved slowly, and the value of the AE index was relatively small, about 40~130 nT. In the quiet time, without fast plasma flow and without significant flapping motion, we also directly observed the bifurcated current sheet. Ion density, velocity and temperature, measured by Cluster spacecrafts in the current sheet during 2011~2030 UT 19 August 2001 are discussed. It is shown that ion density was comparatively bigger near the neutral line, presented non‐symmetry distribution, but the distribution of temperature was uniform. And it is observed that in the y direction, the proton bulk velocity is opposite on both sides of the neutral line. Finally we discuss a possible mechanism for the bifurcated current sheet in a quiet time, which is nonlinear evolution of the lower‐hybrid drift instability in the current sheet. These may be important for perfecting the formation mechanism of current sheet bifurcation.

Effect of Fast Plasma Flows on Volume Recombination including MAR in Detached Divertor Plasma

The effect of fast plasma flows on volume recombination including molecular activated recombination (MAR) in detached divertor plasma is theoretically investigated by solving particle balance equat...

Bifurcated Current Sheet Structure in a Quiet Time by ClusterSpacecrafts

We analyse the vertical structure of the magnetotail current sheetfor two time intervals during which the Cluster spacecraftscrossed the neutral sheet in a quiet time. In the intervals, thecurrent sheet moved slowly, and the value of the AE index wasrelatively small, about 40–130 nT. We find two examples of currentsheets, with the current density maximum at the magnetic equator (Bx = 0), as well as an example of off-centre or bifurcated current sheets.In the quiet time, without any fast plasma flow and withoutsignificant flapping motion, we also directly observed the bifurcatedcurrent sheet. The bifurcated current sheet is probably associated withinstabilities in the current sheet. These may be important forresearching the mechanism of current sheet bifurcation.

Indirect mapping of the source of the oppositely directed fast plasma flows in the plasma sheet onto the auroral display

Abstract. Data from Polar and Geotail spacecraft are combined to investigate the relationship between locations of active auroras and the magnetotail plasma sheet region where reversed fast plasma flows are generated during substorms. Using the magnetospheric magnetic field model, it is shown that at the beginning of the tailward fast flow the ionospheric footprint of the spacecraft measuring the flow tends to be located poleward of the auroral bulge. The spacecraft within the earthward flow is mapped equatorward of the poleward edge of the auroral bulge. We conclude that a source of the fast plasma flows is conjugated with the poleward edge of the auroral bulge. Analysis of the behavior of the plasma and the magnetic field in the vicinity of the source of the diverging flows allows us to conclude that the source region, interpreted as the magnetic reconnection site, coincides with the region of the cross-tail current reduction, and the tailward propagation of the region is associated with the tailward propagation of the current disruption front.

Relationship Between Substorm Auroras and Processes in the Near-Earth Magnetotail

Although the auroral substorm has been long regarded as a manifestation of the magnetospheric substorm, a direct relation of active auroras to certain magnetospheric processes is still debatable. To investigate the relationship, we combine the data of the UV imager onboard the Polar satellite with plasma and magnetic field measurements by the Geotail spacecraft. The poleward edge of the auroral bulge, as determined from the images obtained at the LHBL passband, is found to be conjugated with the region where the oppositely directed fast plasma flows observed in the near-Earth plasma sheet during substorms are generated. We conclude that the auroras forming the bulge are due to the near-Earth reconnection process. This implies that the magnetic flux through the auroral bulge is equal to the flux dissipated in the magnetotail during the substorm. Comparison of the magnetic flux through the auroral bulge with the magnetic flux accumulated in the tail lobe during the growth phase shows that these parameters have the comparable values. This is a clear evidence of the loading–unloading scheme of substorm development. It is shown that the area of the auroral bulge developing during substorm is proportional to the total (magnetic plus plasma) pressure decrease in the magnetotail. These findings stress the importance of auroral bulge observations for monitoring of substorm intensity in terms of the magnetic flux and energy dissipation.

Effects of the fast plasma sheet flow on the geosynchronous magnetic configuration: Geotail and GOES coordinated study

The present study statistically examines how (or if) the geosynchronous (GOES) magnetic field responds to fast earthward flow observed by the Geotail satellite in the plasma sheet. The change of the GOES H (north‐south) component within 15 min of the detection of fast flows, ΔH, is used as a primary measure of the geosynchronous response. It is found that following the detection of fast flows, the geosynchronous magnetic field rarely dipolarizes, but it often becomes more stretched, which is manifested by negative ΔH. This H decrease is not accompanied by any correlated variation of the D (azimuthal) component, suggesting that the associated stretching is not an edge effect of the substorm current wedge formation, but it can be attributed to the intensification of the local tail current. No systematic dependence of ΔH on the satellite separation can be found. On the other hand, the geosynchronous magnetic field tends to dipolarize if it is already stretched significantly, although the associated changes in the H and V (radial) components are not much larger than those in events that are not preconditioned. The flow intensity does not seem to be a controlling factor, either. However, caution needs to be exercised because the present study is not able to address the azimuthal structure of the fast flow. It is concluded that in most events the fast plasma flow does not reach geosynchronous orbit and that the generation of the fast plasma flow in the plasma sheet is not sufficient for causing geosynchronous dipolarization.

Motion of the dayside polar cap boundary during substorm cycles: II. Generation of poleward-moving events and polar cap patches by pulses in the magnetopause reconnection rate

Abstract. Using data from the EISCAT (European Incoherent Scatter) VHF and CUTLASS (Co-operative UK Twin-Located Auroral Sounding System) HF radars, we study the formation of ionospheric polar cap patches and their relationship to the magnetopause reconnection pulses identified in the companion paper by Lockwood et al. (2005). It is shown that the poleward-moving, high-concentration plasma patches observed in the ionosphere by EISCAT on 23 November 1999, as reported by Davies et al. (2002), were often associated with corresponding reconnection rate pulses. However, not all such pulses generated a patch and only within a limited MLT range (11:00-12:00 MLT) did a patch result from a reconnection pulse. Three proposed mechanisms for the production of patches, and of the concentration minima that separate them, are analysed and evaluated: (1) concentration enhancement within the patches by cusp/cleft precipitation; (2) plasma depletion in the minima between the patches by fast plasma flows; and (3) intermittent injection of photoionisation-enhanced plasma into the polar cap. We devise a test to distinguish between the effects of these mechanisms. Some of the events repeat too frequently to apply the test. Others have sufficiently long repeat periods and mechanism (3) is shown to be the only explanation of three of the longer-lived patches seen on this day. However, effect (2) also appears to contribute to some events. We conclude that plasma concentration gradients on the edges of the larger patches arise mainly from local time variations in the subauroral plasma, via the mechanism proposed by Lockwood et al. (2000).

Acceleration of Plasma Flows Due to Reverse Dynamo Mechanism

The "reverse dynamo" mechanism—the amplification/generation of fast plasma flows by microscale (turbulent) magnetic fields via magneto-fluid coupling—is recognized and explored. It is shown that macroscopic magnetic fields and flows are generated simultaneously and proportionately from microscopic fields and flows. The stronger the microscale driver is, the stronger the macroscale products will be. Stellar and astrophysical applications are suggested.

Auroral kilometric radiation activity during magnetically quiet periods

The behavior of auroral kilometric radiation (AKR) during magnetically quiet periods is investigated. A clear AKR onset sometimes occurs without any typical substorm signatures characterized by auroral electrojet enhancement on the ground, tail current disruption in the near‐Earth plasma sheet, and energetic particle injection from the plasma sheet. However, the onset is accompanied by auroral brightening on the contracted oval, Pi2 pulsation on the ground, and fast plasma flow in the plasma sheet. These features of quiet‐time AKR seem consistent with a contracted oval substorm, indicating that the onset of AKR is one of the elementary components of the contracted substorm. Unexpected features showing that AKR spectra do not always depend on the substorm magnitude are also presented: (1) the AKR intensity during the magnetically quiet period was sometimes comparable with the usual substorm AKR, and (2) the frequency‐bandwidth expansion of AKR at the time of onset, which is considered to be related with the vertical development of the field‐aligned potential during the substorm, was sometimes comparable between quiet‐ and disturbed‐period AKR.

Effects of Plasma Phenomena on Neutrino Mass Measurements Process Using a Gaseous Tritium β-Source

In the paper the ionization state of molecular tritium and electric potentials in a Windowless Gas Tritium Source (WGTS) of tritium β-decay experiment KATRIN are considered. The ionization processes in WGTS are sustained by β-electrons and so-called “secondary electrons”, arising from inelastic and ionization collisions of “primary” β-electrons with tritium molecules. As a result in the WGTS tritium gas volume acts as a low-temperature and slightly ionized gas steady state close to quasi neutrality (the Debye length is small in comparison with the setup characteristical sizes). On the basis of an one-dimensional self-consistent model the WGTS plasma steady state is described and the influence of plasma phenomena on neutrino mass measuring process is discussed. It is found that electric potentials in a main plasma volume can not significantly make worse the measurement process. At the same time the nonequilibrium electron spectrum and fast plasma flow at the end of the tube can result in instabilities which are able to spoil slightly the β-electron spectrum endpoint. This problem must be carefully investigated further. For more reliable conclusions more detailed consideration is required that will include kinetic effects in the WGTS plasma.

How typical are atypical current sheets?

Using multi‐satellite Cluster magnetic field data, we statistically examined how often an atypical, i.e., non‐Harris type, current sheet is formed. We found that a cross‐tail current sheet with an off‐equatorial current density maximum, indicating bifurcation, is frequently formed in thin current sheets with a half thickness of ∼1500 km. The occurrence of off‐equatorial current density maxima is correlated with the occurrence of fast plasma flows, namely if fast plasma flows occur nearby, one is more likely to observe off‐equatorial current density maxima. We also found that many of the center‐peaked current sheets exhibit a feature different from a normal Harris sheet: namely a very intense current concentrated near the equatorial plane, embedded in a broader current sheet with lower current density.

On the propagation of bubbles in the geomagnetic tail

Abstract. Using three-dimensional magnetohydrodynamic simulations, we investigate the propagation of low-entropy magnetic flux tubes ("bubbles") in the magnetotail. Our simulations address fundamental properties of the propagation and dynamics of such flux tubes rather than the actual formation process. We find that the early evolution, after a sudden reduction of pressure and entropy on a localized flux tube, is governed by re-establishing the balance of the total pressure in the dawn-dusk and north-south directions through compression on a time scale less than about 20s for the typical magnetotail. The compression returns the equatorial pressure to its original unperturbed value, due to the fact that the magnetic field contributes only little to the total pressure, while farther away from the equatorial plane the magnetic field compression dominates. As a consequence the pressure is no longer constant along a flux tube. The subsequent evolution is characterized by earthward propagation at speeds of the order of 200-400km/s, depending on the initial amount of depletion and the cross-tail extent of a bubble. Simple acceleration without depletion does not lead to significant earthward propagation. It hence seems that both the entropy reduction and the plasma acceleration play an important role in the generation of fast plasma flows and their propagation into the near tail. Earthward moving bubbles are found to be associated with field-aligned current systems, directed earthward on the dawnward edge and tailward on the duskward edge. This is consistent with current systems attributed to observed bursty bulk flows and their auroral effects.Key words. Magnetospheric physics (magnetospheric configuration and dynamics; magnetotail; plasma sheet)nguage:

Temporal structure of the fast convective flow in the plasma sheet: Comparison between observations and two‐fluid simulations

The present study examines the temporal structure of the fast flow in the plasma sheet using both observations and simulations. The data analysis part adopts the strictest criterion ever for the satellite location so that selected flows are mostly convective. From Geotail measurements at X > −31 RE, 818 earthward‐flow and 290 tailward‐flow events are selected. Superposed epoch analyses are conducted with two different reference times: the start of the fast flow and the time of a sharp change in the Bz component. The results are summarized as follows: (1) The magnetic field becomes dipolar in the course of the fast earthward flow; (2) Sharp dipolarization tends to be preceded by a transient decrease in BZ, which starts along with the fast flow and is accompanied by an increase in the plasma density; (3) The corresponding signatures, albeit less clear, can also be found for the tailward flow; (4) Whereas the plasma density decreases in association with the fast flow irrespective of the flow direction (though, more gradually for the tailward flow), the ion temperature increases for the earthward flow and decreases for the tailward flow; (5) The plasma and total pressures decrease in the course of the fast flow, suggesting the reduction of the lobe field strength; (6) In general, magnetic field and plasma parameters change more gradually in time for the tailward flow than for the earthward flow. Those characteristics of the fast flow can be found irrespective of the X distance, even though the ambient magnetic field and plasma vary significantly between X = −5 and −31 RE. The near‐Earth reconnection is inferred to be the responsible mechanism for most, if not all, flow events, and the difference between the earthward and tailward flows presumably reflects difference in downstream conditions. On the earthward side of the reconnection site, the flow needs to proceed against the rigid terrestrial magnetic field, whereas on the tailward side the flow does not have any obstruction once reconnection reaches the lobe magnetic field. This idea is consistent with the change of the magnetic inclination, which suggests that the plasma sheet becomes thicker and thinner in the course of the earthward and tailward flows, respectively. These observational results are compared with fast plasma flows modeled by two‐fluid simulations of magnetic reconnection. A focus is placed on the reduction of BZ prior to dipolarization for the earthward flow (the precursory BZ increase for the tailward flow) since this is the new finding owing to our strict condition for the convective flow. It is found that the fragmentation of the current sheet and the formation of multiple neutral lines create signatures similar to the satellite observations. After multiple X lines form, one of them dominates and establishes the overall flow pattern associated with reconnection. Magnetic islands formed between the X lines are swept downstream by the reconnection process. The signature of this earthward convection of a magnetic island past a satellite at rest in the magnetotail is a strongly bipolar signature in Bz with a sudden enhancement in the density: Bz spikes negative and then positive in rapid succession, with a maximum in the density between these two spikes. It is therefore suggested that the temporal structure of the observed fast plasma flows contains information directly linked to their genesis.

Flow Bursts in the Plasma Sheet and Auroral Substorm Onset: Observational Constraints on Connection Between Midtail and Near-earth Substorm Processes

Causality between near-Earth and midtail substorm processes is one of the most controversial issues about the substorm trigger mechanism. The currently most popular model, the outside-in model, assumes that near-Earth reconnection is initiated in the midtail region before substorm onset and that the associated flow burst causes tail current disruption in the near-Earth region. However, there remain some outstanding issues that may serve as critical tests of this model. The present article reviews recent satellite and ground observations addressing three such critical issues with a focus on substorm-related auroral features. First, near-Earth reconnection, even if it reaches the lobe magnetic field, does not necessarily trigger a global substorm, but it is often related to a pseudobreakup. This fact suggests that there is an additional or alternative condition for substorm development. Secondly, although there appears to be one-to-one correspondence between flow bursts in the plasma sheet and equatorward-moving auroral structures (auroral streamers), no such auroral feature that can be associated with the fast plasma flow can be identified prior to auroral breakups. On the other hand, the flow burst is widely regarded as a manifestation of reconnection and therefore, according to the outside-in model, should be created in the near-Earth plasma sheet before substorm onset. Finally, auroral arcs poleward of a breakup arc are not affected until the front of auroral intensification reaches those arcs. The last two points suggest that if substorm is triggered as the outside-in model describes, the ionosphere is electromagnetically detached from the magnetosphere, which, however, has not been addressed theoretically. Thus, it should be crucial for a better understanding of the substorm trigger process to implement the magnetosphere-ionosphere coupling in future modeling efforts and to address those basic issues as a guide for critically evaluating each model.

磁気浮上内部導体装置Ｍｉｎｉ−ＲＴの全体設計

Mahajan-Yoshida proposed an idea for confining high-beta plasma utilizing a fast plasma flow in a toroidal direction. In addition, Hasegawa considered a dipole fusion reactor. Here, the authors introduce a “Mini-RT" internal coil device, in which a superconducting coil is levitated in a vacuum vessel and plasma is confined by a dipole magnetic field, to explore a new high-beta plasma for fusion research. The conductor of the floating coil (RC= 0.15 m, IC= 50 kA) is Bi-2223 tape that is cooled by cold helium gas to 20 K. By controlling the coil current of the levitation coil located at the top of the vacuum chamber, a few hours of levitation can be obtained for plasma experiments. It is possible to produce various magnetic configurations with a combination of vertical field coils. Several issues related to interaction with plasma produced around the floating coil have been taken into account for designing the floating coil; for example, clearance of the magnetic surface at the torus inner region, heat load to the floating coil caused by the plasma and so on.