Bulk Flow Events Research Articles

Electric fields associated with small‐scale magnetic holes in the plasma sheet: Evidence for electron currents

AbstractWe report observations of magnetic holes (MHs) in the near‐Earth (8 RE to 12 RE) plasma sheet that have physical sizes perpendicular to the magnetic field (B) on the order of the ion Larmor radius (ρi) and, more importantly, have current layers less than ρi in thickness. Small‐scale MHs can have >90% depletion in |B| and are commonly associated with the braking of bursty bulk flow events. The generation of MHs is often attributed to magnetohydrodynamic (MHD) instabilities, which requires a size greater than ρi; the depletion in |B| is from an ion current consistent with a pressure gradient. Electric field (E) observations indicate a negative potential inside of small‐scale MHs that creates an outward E at the boundary, which drives an E × B electron current in a thin layer. These observations indicate that a Hall electron current is primarily responsible for the depletion of |B| in small‐scale magnetic holes, rather than the ion pressure gradient.

Magnetic forces associated with bursty bulk flows in Earth's magnetotail

AbstractWe present the first direct measurements of magnetic forces acting on bursty bulk flow plasma in the magnetotail. The magnetic forces are determined using Cluster multispacecraft measurements. We analyze 67 bursty bulk flow (BBF) events and show that the curvature part of the magnetic force is consistently positive, acting to accelerate the plasma toward Earth between approximately 10 and 20 RE geocentrical distances, while the magnetic field pressure gradient increasingly brakes the plasma as it moves toward Earth. The net result is that the magnetic force accelerates the plasma at distances greater than approximately 14 RE, while it acts to decelerate it within that distance. The magnetic force, together with the thermal pressure gradient force, will determine the dynamics of the BBFs as they propagate toward the near‐Earth tail region. The determination of the former provides an important clue to the ultimate fate of BBFs in the inner magnetosphere.

Generation of high‐frequency electric field activity by turbulence in the Earth's magnetotail

AbstractBursty bulk flow (BBF) events, frequently observed in the magnetotail, carry significant energy and mass from the tail region at distances that are often greater than 20 RE into the near‐Earth plasma sheet at ∼10 RE where the flow is slowed and/or diverted. This region at ∼10 RE is referred to as the BBF braking region. A number of possible channels are available for the transfer or dissipation of energy in BBF events including adiabatic heating of particles, the propagation of Alfvén waves out of the BBF braking region and into the auroral region, diverted flow out of the braking region, and energy dissipation within the braking region itself. This study investigates the generation of intense high‐frequency electric field activity observed within the braking region. When present, these intense electric fields have power above the ion cyclotron frequency and almost always contain nonlinear structures such as electron phase space holes and double layers, which are often associated with field‐aligned currents. A hypothesis in which the observed high‐frequency electric field activity is generated by field‐aligned currents resulting from turbulence in the BBF braking region is considered. Although linear Alfvén waves can generate field‐aligned currents, based on theoretical calculations, the required currents are likely not the result of linear waves. Observations from the Time History of Events and Macroscale Interactions during Substorms satellites support the picture of a turbulent plasma leading to the generation of nonlinear kinetic structures. This work provides a possible mechanism for energy dissipation in turbulent plasmas.

Large‐amplitude electric fields associated with bursty bulk flow braking in the Earth's plasma sheet

AbstractWe report observations of large‐amplitude (>50 mV/m) electric fields primarily associated with bursty bulk flow events. These electric fields reach ~500 mV/m, which are some of the largest electric fields (E) observed in the magnetotail. E not only has a larger than expected component perpendicular to the magnetic field but often has an intense parallel component. High time resolution waveforms reveal nonlinear structures such as electron phase‐space holes and double layers, which suggest strong field‐aligned currents or electron beams. Further examination shows that these large‐amplitude electric fields are almost always accompanied by enhanced magnetic field fluctuations. The electric fields are enhanced both above and below the ion cyclotron frequency, whereas the magnetic field fluctuations (δB) are mostly below the ion cyclotron frequency. Analysis of the wave spectra and the Poynting flux suggest that shear Alfvén waves are participating in these events. The Alfvén waves are revealed through the |δE|/|δB| ratio and strong field‐aligned Poynting flux, sometimes reaching nearly 1 mW/m2. This value, when mapped to the low‐altitude auroral region, exceeds 1 W/m2, which is an extreme value for that region. This Alfvénic activity is accompanied by evidence of compressional modes. These observations support a hypothesis whereby intense currents or electron beams, generated by kinetic Alfvénic waves that result from a turbulent cascade in bursty bulk flow (BBF) braking region, may be an energy source for large‐amplitude electric fields. The large‐amplitude electric fields may act as a dissipation mechanism and relax the highly tangled magnetic fields that result from the turbulence. Furthermore, these observations offer strong support that Alfvénic Poynting flux from the BBF braking region can be the energy source for Alfvénic aurora.

In situ observation of magnetic reconnection in the front of bursty bulk flow

AbstractUsing the Cluster observation in the magnetotail, we investigate the dynamic processes associated with a bursty bulk flow (BBF) event. The BBF is inferred to be caused by magnetic reconnection proceeding to the lobe region in its tail, called “primary reconnection.” On the BBF front, another reconnection was directly encountered by one of the four Cluster satellites, and no signatures of this reconnection were simultaneously measured by the satellite at the plasma sheet boundary. It indicates that this reconnection on the BBF front remained within the plasma sheet, called “secondary reconnection.” The secondary reconnection moved earthward and was followed by a magnetic island. A few earthward moving pulses of Bz were detected between the island and the primary reconnection site. These Bz pulses, propagating faster than the island ahead of it, would lead to a more compressed Bz magnetic field in the wake of the island. The observational scenario is in accordance to the model proposed to explain the generation of dipolarization front in simulations. Furthermore, both electrons and ions were significantly accelerated in this process. The mechanism is discussed also.

Substorm growth and expansion onset as observed with ideal ground-spacecraft THEMIS coverage

[1] We present a fortuitous case of an isolated substorm on 29 March 2009, observed by the Time History of Events and Macroscale Interaction during Substorms (THEMIS) probes clustered at ∼11–14 RE with simultaneous coverage by the THEMIS ground network. The four probes are at roughly the same radial and azimuthal location, with one probe staying near the neutral sheet during entire growth phase and during the ensuing transition to the substorm expansion phase. Prior to substorm onset, THEMIS observed the damping of earthward convection and development of an embedded tail current sheet with half thickness ≤ 0.15 RE and current density ∼20 nA/m2, while the total magnetic field closest to the neutral sheet was below 2 nT. Tail activity was observed to start prior to substorm onset tailward of the THEMIS probes (<−14 RE) with the gradual increase of earthward flow and total pressure in front of an earthward moving bursty bulk flow event (BBF). There was no evidence of an outward propagating rarefaction wave prior to BBF onset. Peak flow was accompanied by two short dipolarization pulses, followed by a sharp reduction of the total pressure (up to 30–50%) and a decrease in the lobe magnetic field. By analyzing the lobe field waveforms, we infer their reconnection origin and argue that different onset-related magnetotail phenomena discussed in past literature (BBFs, bubbles, current disruption, nightside flux transfer events, earthward traveling convection regions) are views of the same dissipative structure formed by reconnection, when it is observed from different vantage points. Although the first ionospheric signatures of the substorm were observed near the equatorward edge of the auroral oval, the adaptive magnetospheric model maps the breakup to the tail current sheet at 15–20 RE, consistent with other estimates of the substorm onset location available in our case. The event provides definite evidence in favor of tail reconnection within a thin current sheet as the primary substorm initiation process. It also demonstrates that actual changes of tail current magnitude in the magnetotail can be an order of magnitude larger, compared to the magnitude of 3-D current inferred from ground observations.

Current carriers near dipolarization fronts in the magnetotail: A THEMIS event study

[1] We study current carriers observed within thin current sheets ahead of and during the passage of earthward moving dipolarization fronts in the near‐Earth plasma sheet using Time History of Events and Macroscale Interactions During Substorms (THEMIS) multipoint measurements. The fronts are embedded within flow bursts at the initial stage of bursty bulk flow events. Simultaneous north‐south and radial separations between probes P3, P4, and P5 and the planar current sheet approximation enable estimation of cross‐tail current density in the current sheet ahead of and within the fronts, respectively. The cross‐tail current density increase ahead of the fronts, a substorm growth phase signature, is predominantly due to the ion diamagnetic current; at times, however, the electron pressure gradient may contribute up to 60% of the total current density. Note that in this paper we refer to the horizontal (vertical) current sheet as the cross‐tail current sheet (current sheet associated with dipolarization fronts). At the dipolarization fronts, the horizontal cross‐tail current sheet (with a current density of several nA/m 2 ) relaxes, and a vertical current sheet (with a current density of several tens of nA/m 2 ), consistent with the thin interface of the front, appears. Thus, the cross‐tail current at longitudes adjacent to the flow burst feeds into the dipolarization front’s current sheet and may be extended to higher latitudes. The vertical current density also decreases after passage of the front. The pressure gradient of 1–10 keV electrons is a dominant contributor to the current in the dipolarization fronts. In the event studied, probes P1 and P2, which were several Earth radii downtail, reveal a tailward expansion of the current reduction process at a propagation velocity ∼50 km/s, even as the bulk flow carrying the magnetic flux remains earthward. This study shows how dipolarization fronts and their current systems are building blocks of the large‐scale substorm current wedge.

Three dimensional configuration of earthward fast plasma flow in the near-Earth plasma sheet

Abstract The earthward short-term (1-min. order) fast flow event (Flow Burst) and the long-term (10-min. order) fast flow event (Bursty Bulk Flow) observed in the near-Earth plasma sheet are examined using three dimensional MHD simulations on the basis of spontaneous fast reconnection model. It is well known that these fast flow events are closely related to the magnetic substorms. On the other hand, it is considered that these fast flow events are caused by the magnetic reconnection in the near-Earth magnetotail. The time profiles of plasma quantities in these events observed by in-situ satellites are quite different in each event. Above Flow Burst and Bursty Bulk Flow events are often examined separately due to the large difference in time scale. In this paper, these differences are interpreted by the three-dimensional position of satellite relative to the X-line and the reconnection jet channel, and the simulation results are directly compared with the results of in-situ satellite observations using the virtual satellites located in simulation domain.

Observations of Double Layers in Earth’s Plasma Sheet

We report the first direct observations of parallel electric fields (E_{ parallel}) carried by double layers (DLs) in the plasma sheet of Earth's magnetosphere. The DL observations, made by the THEMIS spacecraft, have E_{ parallel} signals that are analogous to those reported in the auroral region. DLs are observed during bursty bulk flow events, in the current sheet, and in plasma sheet boundary layer, all during periods of strong magnetic fluctuations. These observations imply that DLs are a universal process and that strongly nonlinear and kinetic behavior is intrinsic to Earth's plasma sheet.

Cluster view of the plasma sheet boundary layer and bursty bulk flow connection

Abstract. The high-latitude boundaries of the plasma sheet (PSBL) are dynamic latitude zones of recurring and transient (minutes to tens of minutes) earthward and magnetic field-aligned bursts of plasma, each being more or less confined in longitude as well, whose ionic component is dominated by protons with flux, energies and density that are consistent with a central plasma sheet (CPS) source at varying distance (varying rates of energy time dispersion), sometimes as close as the ~19 RE Cluster apogees, or closer still. The arguably most plausible source consists of so called "bursty bulk flows" (BBFs), i.e. proton bulk flow events with large, positive and bursty GSE vx. Known mainly from CPS observations made at GSE x&gt;−30 RE, the BBF type events probably take place much further downtail as well. What makes the BBFs an especially plausible source are (1) their earthward bulk flow, which helps explain the lack of distinctive latitudinal PSBL energy dispersion, and (2) their association with a transient strong increase of the local tail Bz component ("local dipolarization"). The enhanced Bz provides intermittent access to higher latitudes for the CPS plasma, resulting in local density reductions in the tail midplane, as illustrated here by proton data from the Cluster CIS CODIF instruments. Another sign of kinship between the PSBL bursts and the BBFs is their similar spatial fine structure. The PSBL bursts have prominent filaments aligned along the magnetic field with transverse flux gradients that are often characterized by local ~10 keV proton gyroradii scale size (or even smaller), as evidenced by Cluster measurements. The same kind of fine structure is also found during Cluster near-apogee traversals of the tail midplane, as illustrated here and implied by recently published statistics on BBFs obtained with Cluster multipoint observations at varying satellite separations. Altogether, the Cluster observations described here mesh rather well with theories about so called plasma sheet "bubbles," i.e. earthward drifting closed magnetic flux tubes with reduced particle pressure and enhanced magnetic field strength at their apex. It is argued that such bubbles may be initiated by localized diamagnetic instabilities.

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.

Do BBFs contribute to inner magnetosphere dipolarizations: Concurrent Cluster and Double Star observations

We examined the relationship between bursty bulk flow (BBF) events observed by Cluster between −19 RE &lt; X &lt; −12 RE and dipolarization events observed by Double Star TC1 between −13 RE &lt; X &lt; −6 RE. TC1 observed dipolarizations for ∼33% of the cases when BBFs were observed by Cluster. During these dipolarization events the TC1 location was closer to the Cluster location and the local BZ at TC1 was smaller than during events where TC1 observed no clear dipolarization associated with BBFs at Cluster. This result suggests that (1) flow‐associated activity dissipates within a limited spatial scale, 4–8 RE, and that (2) the initial magnetic topology in the inner magnetosphere can contribute strongly to fast flow penetration toward the Earth. The fact that there were no TC1 dipolarization events at X &gt; −8 RE associated with BBFs at Cluster in our dataset suggests two possibilities: near‐geosynchronous dipolarization needs another mechanism in addition to flux pile‐up and braking, or during near‐geosynchronous dipolarization the near‐tail current sheet/plasma sheet is too thin to be observed by Cluster.

Plasma transport from multicomponent approach

Plasma measurements in the Earth's magnetotail have revealed occasional existence of multiple plasma components. In this work, we present the first quantitative comparison between single‐component and multicomponent approaches on the transport of mass and energy in the magnetotail with Geotail 3D plasma measurements. Two events are examined in detail, one during substorm expansion and the other during substorm recovery. It is found that the mass and energy transport from the plasma moment approach is usually underestimated for the dynamic hot plasma component. For the substorm expansion case, the bursty bulk flow event based on the moment approach is actually a case of a super‐Alfvénic‐speed beam passing through (BPT) a rather stationary plasma. This newly discovered BPT phenomenon should be distinguished from bursty bulk flow. Therefore, the multicomponent approach could provide a better insight than the moment approach on the actual dynamic situation.

Multipoint Analysis of the Temporal Scale of Bursty Bulk Flow Events during the Quiet Time of Magnetotail

A series of Bursty Bulk Flow events (BBFs) were observed by CLUSTER satellites during the quiet time of the magnetotail in 2001. The data of multi-satellites of CLUSTER show that the duration of BBFs observed by multi-satellites is longer than that of BBFs previously observed by single satellite. The duration of multi-satellites is about twice that of single satellite. Flapping of BBFs in the dusk-dawn direction mainly determined temporal order of observation of BBFs of three CLUSTER satellites. The localization of BBFs is also confirmed by multi-satellites observations simultaneously. The longer duration of BBFs during the quiet time of magnetotail enhances the earthward transport of mass and energy, and changes the onset of the substorm. Our study also provides a new idea to solve the Pressure Balance Inconsistency (PBI).

Wind survey of high‐speed bulk flows and field‐aligned beams in the near‐Earth plasma sheet

We have surveyed all high‐speed (&gt;250 km/s) flows detected by Wind during its 17 perigee passes across the near‐Earth (xGSE = −25 to 0 RE) plasma sheet in the period between 1995 and 1997. By classifying high‐speed flow events based on their ion distribution characteristics rather than their plasma moments or the regions in which these flows were detected, we found that most (if not all) high‐speed flow ion distributions fall into two distinct categories: bulk flows and field‐aligned beams. We show that bulk flows are not simply the low‐latitude counterparts of field‐aligned beams. Field‐aligned beams have sharp cutoffs at low energies, occur within relatively steady magnetic field and plasma conditions, and are detected more often away from the neutral sheet. On the other hand, high‐speed bulk flows are well represented by a single drifting population and have their peak occurrence rate at the neutral sheet. Bulk flows are generally accompanied by large magnetic field fluctuations and sudden increase of energetic (up to 0.5 MeV) particle fluxes. They are often (but not always) associated with magnetic field dipolarization and plasma temperature enhancements. Little or no temperature enhancements are observed in cases where the spacecraft resides near the neutral sheet before the arrival of bursty bulk flows, suggesting that temperature enhancements seen in other bulk flow events may in part be a spatial effect instead of true heating of the plasma. Bulk flows are perpendicular to the magnetic field when detected at the neutral sheet but have a large field‐aligned component at higher magnetic latitudes. Field‐aligned bulk flows and field‐aligned beams are similar in terms of their velocity moments and may occur at the same magnetic latitudes but are easily distinguishable based on their ion distributions. We have used our categorization of bulk flow and field‐aligned beam events to search for the optimal moment‐based selection criteria to distinguish bulk flows from beams. We found that no single moment‐based parameter or threshold can serve to cleanly separate beam from bulk flow distributions because a range of values of these parameters exists where both types of fast flows are observed. We found perpendicular flow speed v⟂ &gt; 250 km/s and plasma βxy (based on the x and y components of the magnetic field) &gt; 2 to be the optimal moment‐based bulk flow selection criteria because they eliminate ∼95% of beam events while retaining ∼60% of bulk flow events. Finally, a previously reported dawn–dusk asymmetry in the occurrence of bursty bulk flows, with most events occurring in the premidnight sector, is confirmed by our survey. Field‐aligned beams, on the other hand, have no dawn–dusk bias.

Kinetic Characterization of Plasma Sheet Dynamics

The Wind spacecraft made 26 perigee passes through the near-earth plasma sheet region during 1994 to 1997. Nearly all of these passes obtained plasma data from substorms and bursty bulk flow (BBF) events. New features of ion distributions have been observed in both the plasma sheet boundary layer (PSBL) and the central plasma sheet (CPS) in the vicinity of the current sheet that are relevant for understanding the structure of the PSBL and the mechanisms of particle acceleration to MeV energies associated with the BBF events. Kinetic processes are key to understanding these new observations that are not adequately explained by existing magnetohydrodynamics (MHD) models and theories. This article will feature the phase space distribution functions as the primary data product. The main purpose of this article is to establish an observational framework for new improved models and theories. The new observations should challenge modelers and theorists.

Kinetic properties of bursty bulk flow events

Particle distributions of bursty bulk flow (BBF) events in the central plasma sheet regions show that the high earthward mean velocity comes from a combination of energetic ion flux increases (&gt;few keV) in the earthward direction and decreases of similar energy fluxes in the tailward direction. The energetic ion increases are observed to ∼few MeV during intense events, indicating the responsible mechanisms efficiently accelerate particles. The BBF events are intimately tied to the dynamics of tail current sheet reconfiguration and for the two events discussed here, they occurred 45 minutes and 5 minutes after the expansion onsets. The perception offered by our analysis differs from the earlier studies which described BBF events using MHD approach.

On the relationship between bursty flows, current disruption and substorms

A current disruption (CD) event was observed by the Geotail spacecraft at (X, Y)GSM=(−10.0, 1.9) RE within 1 min from the onset of a substorm. The event was selected as one of four near‐Earth current disruption events identified in a search of three years of Geotail data. Fast ion flows accompanied the magnetic field dipolarization, rendering this event a near‐Earth bursty bulk flow (BBF) event. The gradient anisotropy of the 30–44 keV ions at the onset of the flow is consistent with an Earthward motion of the heated plasma and agrees with the direction of flux and energy transport. The aurora and the associated electrojet moved from low to high latitudes during substorm expansion. Our observations show that CDs and BBFs are qualitatively similar phenomena in the near‐Earth tail, both associated with poleward‐moving (classical) auroral substorms.

Are north‐south aligned auroral structures an ionospheric manifestation of bursty bulk flows?

Bursty Bulk Flow (BBF) events are an important means of plasma transport in the Earth's magnetotail during substorms. While several studies have been performed using in‐situ plasma and field data to determine the characteristics of BBFs, remarkably little attention has been paid to the question of whether or not these events also manifest themselves in the auroral ionosphere. In this paper, we present observations from the Viking UV imager, and the Los Alamos National Laboratory geosynchronous energetic particle detectors that strongly suggest that the north‐south aligned structures formed impulsively and repetitively during the expansion phase of substorms may be an ionospheric manifestation of BBFs.

Central plasma sheet ion properties as inferred from ionospheric observations

A method of inferring central plasma sheet (CPS) temperature, density, and pressure from ionospheric observations is developed. The advantage of this method over in situ measurements is that the CPS can be studied in its entirety, rather than only in fragments. As a result, for the first time, comprehensive two‐dimensional equatorial maps of CPS pressure, density, and temperature within the isotropic plasma sheet are produced. These particle properties are calculated from data taken by the Special Sensor for Precipitating Particles, version 4 (SSJ4) particle instruments onboard DMSP F8, F9, F10, and F11 satellites during the entire year of 1992. Ion spectra occurring in conjunction with electron acceleration events are specifically excluded. Because of the variability of magnetotail stretching, the mapping to the plasma sheet is done using a modified Tsyganenko [1989] magnetic field model (T89) adjusted to agree with the actual magnetotail stretch at observation time. The latter is inferred with a high degree of accuracy (correlation coefficient ∼0.9) from the latitude of the DMSP b2i boundary (equivalent to the ion isotropy boundary). The results show that temperature, pressure, and density all exhibit dawn‐dusk asymmetries unresolved with previous measurements. The ion temperature peaks near the midnight meridian. This peak, which has been associated with bursty bulk flow events, widens in the Y direction with increased activity. The temperature is higher at dusk than at dawn, and this asymmetry increases with decreasing distance from the Earth. In contrast, the density is higher at dawn than at dusk, and there appears to be a density enhancement in the low‐latitude boundary layer regions which increases with decreasing magnetic activity. In the near‐Earth regions, the pressure is higher at dusk than at dawn, but this asymmetry weakens with increasing distance from the Earth and may even reverse so that at distances X &lt; ∼−10 to −12 RE depending on magnetic activity, the dawn sector has slightly higher pressure. The temperature and density asymmetries in the near‐Earth region are consistent with the ion westward gradient/curvature drift as the ions E×B convect earthward. When the solar wind dynamic pressure increases, CPS density and pressure appear to increase, but the temperature remains relatively constant. Comparison with previously published work indicates good agreement between the inferred pressure, temperature, and density and those obtained from in situ data. This new method should provide a continuous mechanism to monitor the pressure, temperature, and density in the magnetotail with unprecedented comprehensiveness.