Bursty Bulk Flows Research Articles

Multiple overshoot and rebound of a bursty bulk flow

Chen and Wolf (1999) used a thin‐filament theory to construct a 2D model of a bursty bulk flow (BBF) motion inside the plasma sheet. The modeling revealed that the low‐entropy filament overshoots its equilibrium position and executes a heavily damped oscillation about that position. In this letter we demonstrate, for the first time, the multiple overshoot and rebound of a BBF observed by the five THEMIS probes on 17 March 2008 just after 10:22 UT. We found that the BBF oscillatory braking was accompanied by interlaced enhancements and depletions of radial pressure gradients. The earthward and tailward flow bursts caused formation of vortices with opposite sense of rotation.

Tearing stability of a multiscale magnetotail current sheet

The sufficient stability criterion of the collisionless ion tearing mode in the magnetotail current sheet, which was first obtained by Lembege and Pellat in 1982, is considered. For many conventional 2D current sheet equilibria, this criterion is satisfied within the WKB approximation, which is commonly interpreted as stability of those equilibria with respect to tearing. However, this is not necessarily the case for equilibria with more than two characteristic spatial scales. An example for substantial tearing destabilization of an equilibrium with accumulation of the magnetic flux at the tailward end of a thin current sheet is presented. Similar equilibria are reported in Geotail and THEMIS observations prior to onsets of magnetospheric substorms and dipolarization fronts associated with bursty bulk flows.

Comparison of BBFs observed by single and multi satellites

This paper, using the data of three Cluster satellites, compares the observations of Bursty Bulk Flow (BBF) by single satellite with those by multi satellites. The results indicate that there exists remarkable difference between observations of BBF by single satellite and multi satellites. The observations of BBF by a single satellite depend on its position relative to the flow channel. The difference is caused by the localization characteristics of fast flows in the plasma sheet, and can lead to diverging views about substorm and causal relations among substorm phenomena.

Vortex‐like plasma flow structures observed by Cluster at the boundary of the outer radiation belt and ring current: A link between the inner and outer magnetosphere

Two vortex‐like plasma flow structures have been observed at the outer radiation belt and/or the ring current region on 11 April 2002, from 0415 to 0635 UT, when the Cluster fleet entered (in the Southern Hemisphere) and exited (in the Northern Hemisphere) the boundary layer of the inner magnetosphere near 2130 MLT. On 11 April 2002 during the period of interest, the solar wind speed was high, and the geomagnetic activity was moderate. These two vortices have opposite rotation directions and are characterized by bipolar signatures in the flow Vx components with peak‐to‐peak amplitudes of about 40 km/s. The inflection points of the plasma flow coincide precisely with the local maxima of the duskward core flow Vy (30 km/s) which exceed the surrounding flow by 3–4 times in magnitude for both vortices. A pair of bidirectional current sheets and bipolar electric fields (Ey) are found to be closely associated with these vortices. Whereas magnetic field disturbances are observed only in Bx and By components, the magnetic magnitude stays almost unchanged. Vortices observed both inbound and outbound at the boundary of the radiation belt at nearly the same location (L shell and latitude), suggesting they may last for more than 140 min. The scale sizes of the two vortices are about 810 km and 1138 km, respectively. Interestingly, it is found that Earth's ionospheric singly charged oxygen are precipitating in the vortex dynamic process, having energies less than 1 keV and having a strong field‐aligned pitch angle distribution. These plasma flow vortices are suggested to be formed at the interface between the enhanced ionospheric outflow stream from the polar ionosphere and a sudden braking and/or azimuthal deflection of bursty bulk flows generated by the tail reconnection. These observed flow vortices provide a link among the inner magnetosphere, the tail plasma sheet, and the Earth's ionosphere by coupling magnetic shear stresses and plasma flow momentum.

Can magnetotail reconnection produce the auroral intensities observed in the conjugate ionosphere?

In a recent case study, Borg et al. (2007) reported that an inverted V structure, caused by a field‐aligned potential drop of 30 kV producing very strong X‐ray aurora, was found in connection with tail reconnection. However, the in situ particle measurements indicated clearly that the particles responsible for the X‐ray aurora were not accelerated by the reconnection process. In this article, we report the predicted auroral intensities of thirteen reconnection events where Cluster passed through the reconnection region. For six of the events, global auroral imaging data were available and the predicted auroral intensities could be compared with the observed intensities. Our main findings are as follows: (1) Acceleration in the reconnection region is generally not sufficient to account for the observed auroral intensities. (2) Additional acceleration between the reconnection region and the ionosphere is needed to explain the auroral intensities. Although we see signatures that point toward potential drops at the flanks of bursty bulk flows (BBFs), we also find signatures of Alfvén wave accelerated electrons at 700 km and we are not able to determine the most likely acceleration mechanism. (3) The reconnection events are observed 2–14 min after substorm onset and indicate that reconnection is an expanding process observed along the poleward boundary of the aurora.

The structure of an earthward propagating magnetic flux rope early in its evolution: comparison of methods

Abstract. We analyze a magnetic signature associated with the leading edge of a bursty bulk flow observed by Cluster at −19 RE downtail on 22 August 2001. A distinct rotation of the magnetic field was seen by all four spacecraft. This event was previously examined by Slavin et al. (2003b) using both linear force-free modeling as well as a curlometer technique. Extending this work, we apply here single- and multi-spacecraft Grad-Shafranov (GS) reconstruction techniques to the Cluster observations and find good evidence that the structure encountered is indeed a magnetic flux rope and contains helical magnetic field lines. We find that the flux rope has a diameter of approximately 1 RE, an axial field of 26.4 nT, a velocity of ≈650 km/s, a total axial current of 0.16 MA and magnetic fluxes of order 105 Wb. The field line twist is estimated as half a turn per RE. The invariant axis is inclined at 40° to the ecliptic plane and 10° to the GSM equatorial plane. The flux rope has a force-free core and non-force-free boundaries. When we compare and contrast our results with those obtained from minimum variance, single-spacecraft force-free fitting and curlometer techniques, we find in general fair agreement, but also clear differences such as a higher inclination of the axis to the ecliptic. We further conclude that single-spacecraft methods have limitations which should be kept in mind when applied to THEMIS observations, and that non-force-free GS and curlometer techniques are to be preferred in their analysis. Some properties we derived for this earthward– moving structure are similar to those inferred by Lui et al. (2007), using a different approach, for a tailward-moving flux rope observed during the expansion phase of the same substorm.

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.

Role of entropy in magnetotail dynamics

The role of entropy conservation and loss in magnetotail dynamics, particularly in relation to substorm phases, is discussed on the basis of MHD theory and simulations, using comparisons with particle‐in‐cell (PIC) simulations for validation. Entropy conservation appears to be a crucial element leading to the formation of thin embedded current sheets in the late substorm growth phase and the potential loss of equilibrium. Entropy conservation also governs the accessibility of final states of evolution and the amount of energy that may be released. Entropy loss (in the form of plasmoids) is essential in the earthward transport of flux tubes (bubbles, bursty bulk flows). Entropy loss also changes the tail stability properties and may render ballooning modes unstable and thus contribute to cross‐tail variability. We illustrate these effects through results from theory and simulations.

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>−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.

Observation of an inner magnetosphere electric field associated with a BBF-like flow and PBIs

Abstract. Themis E observed a perpendicular (to the magnetic field) electric field associated with an Earthward plasma flow at XGSM=−9.6 RE on 11 January 2008. The electric field observation resembles Cluster observations closer to Earth in the auroral region. The fast plasma flow shared some characteristics with bursty bulk flows (BBFs) but did not meet the usual criteria in maximum velocity and duration to qualify as one. Themis C observed the same flow further downtail but Themis D, separated by only 1 RE in azimuthal direction from Themis E, did not. At the time of the electric field and ion flow event, the all-sky imager and ground-based magnetometer at Rankin Inlet observed Poleward Boundary Intensifications (PBIs) and a negative bay signature. None of the other Themis ground-based observatories recorded any significant auroral or magnetic field activity, indicating that this was a localized activity. The joint Themis in situ and ground-based observations suggest that the two observations are related. This indicates that auroral electric fields can extend to regions much farther out than previously seen in Cluster observations.

Conjugate ionospheric equivalent currents during bursty bulk flows

Ionospheric equivalent currents related to bursty bulk flows (BBF) during Cluster and the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometer network conjunctions between 2001 and 2006 are studied. A geomagnetically southeast–northwest aligned, relatively narrow channel of northwestward equivalent current density with downward field‐aligned current at its northeastward flank and upward field‐aligned current at its southwestward flank is confirmed as the ionospheric signature of BBFs in the majority of cases, i.e., whenever an ionospheric signature is present. Unlike in previous event studies, all conjugate BBFs between 2001 and 2006 are analyzed and during 19 out of 22 BBFs, the channel is observed in the ionosphere. The mean duration of the BBFs observed when the footprint of Cluster is located close to the poleward boundary of the auroral oval is clearly longer (17 min) than the that of the BBFs observed close to the equatorward boundary of the oval (3 min). A superposed epoch analysis of the local IMAGE AE (IE) index reveals a stronger activity level preceding a substorm‐related than a nonsubstorm BBF. After the BBF observation, however, the activity in both cases is on the same level as that preceding the substorm BBF observation. The observation of the substorm BBF coincides with a substorm‐like disturbance of the IE, while the observation of the nonsubstorm BBF coincides with a step‐like increase of the IE.

Dipolarization fronts as a signature of transient reconnection in the magnetotail

Dipolarization fronts (DFs), characterized by a strong and steep increase of the tail magnetic field component Bz normal to the neutral plane and preceded by a much less negative dip of Bz, are reported in many observations of bursty bulk flows and substorm activations throughout the whole Earth's magnetotail. It is shown that similar structures appear in full‐particle simulations with open boundaries in a transient regime before the steady reconnection in the original Harris current sheet driven out of the equilibrium by the initial X‐line perturbation is established. Being secondary reconnection structures propagating with the Alfvén speed, DFs are different from the magnetic field pileup regions reported in earlier simulations with closed boundaries. They also differ from the secondary plasmoids with bipolar Bz changes reported in earlier fluid simulations and particle simulations with open boundaries. In spite of their transient nature, DFs are found to form when the force balance is already restored in the system, which justifies their interpretation as a nonlinear stage of the tearing instability developing in two magnetotail‐like structures on the left and on the right of the initial central X‐line. Both electrons and ions are magnetized at the front of the dipolarization wave. In contrast, in its trail, ions are unmagnetized and move slower compared to the E × B drift, whereas electrons either follow that drift being completely magnetized or move faster, forming super‐Alfvénic jets. In spite of the different motions of electrons and ions, the growth of the front is not accompanied by the corresponding growth of the electrostatic field and the energy dissipation in fronts is dominated by ions.

Reconnection in substorms and solar flares: analogies and differences

Abstract. Magnetic reconnection is the crucial process in the release of magnetic energy associated with magnetospheric substorms and with solar flares. On the basis of three-dimensional resistive MHD simulations we investigate similarities and differences between the two scenarios. We address in particular mechanisms that lead to the onset of reconnection and energy release, transport, and conversion mechanisms. Analogous processes might exist in the motion of field line footpoints on the sun and in magnetic flux addition to the magnetotail. In both cases such processes might lead to a loss of neighboring equilibrium, characterized by the formation of a very thin embedded current sheet, which acts as trigger for reconnection. We find that Joule (or ohmic) dissipation plays only a minor role in the overall energy transfer associated with reconnection. The dominant transfer of released magnetic energy occurs to electromagnetic energy (Poynting) flux and to thermal energy transport as enthalpy flux. The former dominates in low-beta, specifically initially force-free current sheets expected for the solar corona, while the latter dominates in high-beta current sheets, such as the magnetotail. In both cases the outflow from the reconnection site becomes bursty, i.e. spatially and temporally localized, yet carrying most of the outflow energy. Hence an analogy might exist between bursty bulk flows (BBFs) in the magnetotail and pulses of Poynting flux in solar flares. Further similarities might exist in the role of collapsing magnetic flux tubes, as a consequence of reconnection, in the heating and acceleration of charged particles.

Cluster and Double Star multipoint observations of a plasma bubble

Abstract. Depleted flux tubes, or plasma bubbles, are one possible explanation of bursty bulk flows, which are transient high speed flows thought to be responsible for a large proportion of flux transport in the magnetotail. Here we report observations of one such plasma bubble, made by the four Cluster spacecraft and Double Star TC-2 around 14:00 UT on 21 September 2005, during a period of southward, but BY-dominated IMF. In particular the first direct observations of return flows around the edges of a plasma bubble, and the first observations of plasma bubble features within 8 RE of the Earth, consistent with MHD simulations (Birn et al., 2004) are presented. The implications of the presence of a strong BY in the IMF and magnetotail on the propagation of the plasma bubble and development of the associated current systems in the magnetotail and ionosphere are discussed. It is suggested that a strong BY can rotate the field aligned current systems at the edges of the plasma bubble away from its duskward and dawnward flanks.

Convective bursty flows in the near‐Earth magnetotail inside 13 RE

With the 4‐s resolution data of the magnetometer and the ion plasma analyzer on Double Star TC‐1 from June to November of each year during the period of 2004–2006, we statistically analyzed the occurrence rate of both convective and field‐aligned bursty flows (FABFs). In the region −13 RE < X < −9 RE, ∣Y∣ < 10 RE, ∣Z∣ < 4 RE, there are in total 80 convective bursty flow events and 430 FABF events with a criterion that the duration time of both V⊥x > 180 km/s and V∥x > 180 km/s exceeds 30 s, where V⊥x and V∥x are irrespectively the X component of the velocity vector perpendicular and parallel to the local magnetic field. The statistical results show that the occurrence rate of the convective bursty flow (CBF) reached a minimum around −11.5 RE; the occurrence rate of the FABF has a sudden decrease around −11.5 RE, but has no clear increase inside −11.5 RE. The spatial distribution of the all types of bursty flows shows that there had no deflection flows in the azimuthal direction related to the absence of the convective bursty flows inside 13 RE explored by TC‐1. The magnetic field analysis indicates that there exists a minimum in Bz around 12 RE. Here, we suggest that the CBFs inside −11.5 RE may not relate to the flows outside −12 RE. The CBFs outside 12 RE are likely to be the classical bursty bulk flow, originating 20–30 RE downtail and braking outside 12 RE. The minimum Bz may have close relation with the flow braking. The CBFs inside 12 RE may be produced by another mechanism. The minimum Bz may be prone to the excitation of instabilities and, consequently, initiation of the substorm expansion and/or acceleration of ions resulting in CBFs inside.

THEMIS observations of consecutive bursts of Pi2 pulsations: The 20 April 2007 event

On 20 April 2007, four Pi2 pulsation bursts occurred successively and simultaneously in the premidnight sector at the E spacecraft and ground‐based observatories for the THEMIS mission, while the AE index was less than 100 nT. Especially for the last three onsets, both ground‐based and GOES 12 magnetometers sensed magnetic perturbations, as expected from the formation of a substorm current wedge (SCW). Moreover, LANL 1994–084 detected an enhancement of energetic particle flux. Spectral analysis shows a matched wave frequency ∼6–8 mHz and another harmonic frequency ∼17 mHz for the fourth burst. The orientation of the major axis of the wave polarization hodogram points toward the SCW location. The first burst has both latitudinal and longitudinal polarization changes from counterclockwise (CCW) to clockwise (CW), in contrast to the other three that have a latitudinal reversal only. The longitudinal CW to CCW change at low latitudes signifies that hydromagnetic waves propagate westward and eastward from the longitude of the impulsive source responsible for SCW. The latitudinal CW to CCW reversal is consistent with induction by a westward moving upward field‐aligned current carried by Alfvén waves leading to field line oscillations. Consequently, they can be explained by the coupling of a fast magnetospheric cavity mode driven by fast compressional waves to field line resonances as expected from braking bursty bulk flows, resulting from magnetotail reconnection, triggered by a preceding northward interplanetary magnetic field turning. This event shows that the source mechanism of consecutive Pi2 onsets at times of weak geomagnetic activity is the same as during substorm times.

Multiple intensifications inside the auroral bulge and their association with plasma sheet activities

In this coordinated ground and space study, we report multiprobe measurements from Time History of Events and Macroscale Interactions during Substorms (THEMIS), LANL‐97A, Polar, and ground observatories for a substorm that occurred on 23 March 2007. The THEMIS fleet and LANL‐97A were located in the premidnight, near‐Earth plasma sheet in the radial range from 6.6 to 13 RE, placing the spacecraft into different plasma environments which were subject to different activities. Simultaneous global Polar Ultraviolet Imager images of the aurora revealed a fine structure in the auroral bulge in the form of several time‐delayed regions of brightening. We demonstrate a correspondence between this fine structure and the spatially separated plasma sheet activities (substorm injections with energies >100 keV) by showing that both executed periodic (100–150 s) one‐to‐one correlated modulations. Additionally, the different auroral brightening regions were modulated approximately out of phase to one another, as were the separated plasma sheet activities. The periodic plasma sheet and optical modulations were also one‐to‐one correlated with large‐amplitude (δH ∼ 150 nT) ground Pi2 pulsations. In contrast to the most energetic ions (>100 keV), the lower‐energetic plasma sheet ions executed separate oscillations during the development of the substorm, including the preintensification phase, and showed the following properties. (1) The oscillation periods were different at different spacecraft locations and had a tendency to increase during the evolution of the substorm. During the preintensification phase, multiple (possibly harmonic) spectral components existed. (2) The oscillations were coupled to westward moving perturbations of an energized plasma boundary. The boundary perturbations were likely conjugate to azimuthally spaced auroral forms (“beads”) observed by Polar‐UVI during the preintensification phase and could play a role in the onset of the substorm intensification. (3) The oscillations of the lower‐energetic ions were also one‐to‐one correlated with smaller‐amplitude ground Pi2 pulsations (<15 nT). In conclusion, the combination of these observations allowed us to construct a 3‐D picture of low‐frequency, near‐Earth plasma sheet phenomena associated with a substorm and their connection to aurora and the ground. It appeared that not only one substorm current wedge, but additional current structures existed which started at different times, pulsated out of phase, and mapped from different active regions into the ionosphere. The active space regions appeared to be coupled and transferring energy from one region to the other while pulsating. We propose that the wave‐like structures in the plasma sheet, observed before and during the substorm/intensification phase, and their demonstrated properties support a wave phenomenon (such as a ballooning‐type mode) for the onset and development of the substorm/intensification, rather than directly driven periodic bursty bulk flow activations.

Near-Earth bursty bulk flows and AE index

With the 4-s resolution data of the magnetometer and the ion plasma analyzer on TC-1 from June to November of each year during the period of 2004-2006, we statistically analyzed the occurrence rate of both convective and field-aligned bursty flows (FABFs). A near-Earth bursty bulk flow (NEBBF) occurred during both the quiet time and substorm process. In general, the magnetic field and the plasma density began oscillating with the appearance of the NEBBF associated with a distinct increase of the AE index. The increase of AE index during the NEBBF was more than 100 nT in both quiet time and substorm process. The statistical analysis indicated that the occurrence rates of the FABFs were nearly the same in the different stages of the AE index, but the occurrence rate of the NEBBFs was much higher in the growth stage of the AE index, indicating that the NEBBFs were directly related to the growth and expansion phases of the substorm. The observations suggested that the quite large number of BBFs from the mid magnetotail could enter into the near-Earth tail and play important role in triggering the substorm onset.

Convective high-speed flow and field-aligned high-speed flows explored by TC-1

From June 1, 2004 to October 31, 2006, a total 465 high-speed flow events are observed by the TC-1 satellite in the near-Earth region (−13 RE < x < −9 RE, |Y|<10 RE, |Z|<5 RE). Based on the angle between the flow and the magnetic field, the high-speed flow events are further divided into two types, that is, field-aligned high-speed flow (FAHF) in the plasma sheet boundary and convective bursty bulk flow (BBF) in the center plasma sheet. Among the total 465 high-speed flow events, there are 371 FAHFs, and 94 BBFs. The CHF are mainly concentrated in the plasma sheet, the intersection angle between the flow and the magnetic field is larger, the magnetic field intensity is relatively weak. The FHF are mainly distributed near the boundary layer of the plasma sheet, the intersection angle between the flow and magnetic field is smaller, and the magnetic field intensity is relatively strong. The convective BBFs have an important effect on the substorm.