Tailward Flow Research Articles

Convection Electric Field and Plasma Convection in a Twisted Magnetotail: A THEMIS Case Study 1–2 January 2009

AbstractWe investigate THEMIS satellite measurements made in a tail‐aligned constellation during a time interval on 1–2 January 2009, which has previously been attributed to an interval of an interplanetary magnetic field By‐driven magnetotail twisting. We find evidence for that the orientation of the convection electric field in the tail is twist‐mode dependent. For earthward flow and a negative twist (induced tail By< 0), the electric field is found to have northward Ez and tailward Ex components. During a positive twist (induced tail By> 0), the directions of Ez and Ex are reversed. The Ey component shows the expected dawn‐to‐dusk direction for earthward flow. The electric field components preserve their orientation across the neutral sheet, and a quasi‐collinear field is observed irrespective to the tail distance. The electric field associated with the tailward flow has an opposite direction compared to the earthward flow for the negative twist. For the positive twist, the results are less clear. The corresponding plasma convection and thus the magnetic flux transport have an opposite dawn‐dusk direction above and below the neutral sheet. The directions depend on the tail twist mode. The hemispherically asymmetric earthward plasma flows are suggested to be a manifestation of an asymmetric Dungey cycle in a twisted magnetotail. The role of tailward flows deserve further investigation.

Magnetotail Fast Flow Occurrence Rate and Dawn-Dusk Asymmetry at XGSM∼-60RE.

As a direct result of magnetic reconnection, plasma sheet fast flows act as primary transporter of mass, flux, and energy in the Earth's magnetotail. During the last decades, these flows were mainly studied within X GSM>−60R E, as observations near or beyond lunar orbit were limited. By using 5 years (2011–2015) of ARTEMIS (Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moons Interaction with the Sun) data, we statistically investigate earthward and tailward flows at around 60 R E downtail. A significant fraction of fast flows is directed earthward, comprising 43% (v x>400 km/s) to 56% (v x>100 km/s) of all observed flows. This suggests that near‐Earth and midtail reconnection are equally probable of occurring on either side of the ARTEMIS downtail distance. For fast convective flows (v ⊥ x>400 km/s), this fraction of earthward flows is reduced to about 29%, which is in line with reconnection as source of these flows and a downtail decreasing Alfvén velocity. More than 60% of tailward convective flows occur in the dusk sector (as opposed to the dawn sector), while earthward convective flows are nearly symmetrically distributed between the two sectors for low AL (>−400 nT) and asymmetrically distributed toward the dusk sector for high AL (<−400 nT). This indicates that the dawn‐dusk asymmetry is more pronounced closer to Earth and moves farther downtail during high geomagnetic activity. This is consistent with similar observations pointing to the asymmetric nature of tail reconnection as the origin of the dawn‐dusk asymmetry of flows and other related observables. We infer that near‐Earth reconnection is preferentially located at dusk, whereas midtail reconnection (X >− 60R E) is likely symmetric across the tail during weak substorms and asymmetric toward the dusk sector for strong substorms, as the dawn‐dusk asymmetric nature of reconnection onset in the near‐Earth region progresses downtail.

Magnetic reconnection in Earth's magnetotail: Energy conversion and its earthward–tailward asymmetry

Magnetic reconnection, a fundamental plasma process, releases magnetic energy and converts it to particle energy, by accelerating and heating ions and electrons. This energy conversion plays an important role in the Earth's magnetotail. A two-dimensional particle-in-cell simulation is performed to study such a conversion in a magnetotail topology, one with a nonzero Bz, and the energy conversion is found to be more efficient in the earthward outflow than in the tailward outflow. Such earthward–tailward asymmetry is manifested not only in j·E but also in Poynting flux, Hall electromagnetic fields, bulk kinetic energy flux, enthalpy flux, heat flux, bulk acceleration, heating, and suprathermal particle energization, all of which are more prevalent on the earthward side. Such asymmetries are consistent with spacecraft observations reported in the literature. Our study shows that in the magnetotail, most of the energy converted by reconnection flows predominantly toward the Earth and has the potential of being geoeffective, rather than being expelled to the solar wind by the tailward flow. The energy conversion asymmetry arises from the presence of the non-zero normal magnetic field, the stronger lobe magnetic field, and the stronger cross-tail current earthward of the reconnection site in the pre-reconnecting thin current sheet.

Quantification of the total ion transport in the near-Earth plasma sheet

Abstract. Recent studies strongly suggest that a majority of the observed O+ cusp outflows will eventually escape into the solar wind, rather than be transported to the plasma sheet. Therefore, an investigation of plasma sheet flows will add to these studies and give a more complete picture of magnetospheric ion dynamics. Specifically, it will provide a greater understanding of atmospheric loss. We have used Cluster spacecraft 4 to quantify the H+ and O+ total transports in the near-Earth plasma sheet, using data covering 2001–2005. The results show that both H+ and O+ have earthward net fluxes of the orders of 1026 and 1024 s−1, respectively. The O+ plasma sheet return flux is 1 order of magnitude smaller than the O+ outflows observed in the cusps, strengthening the view that most ionospheric O+ outflows do escape. The H+ return flux is approximately the same as the ionospheric outflow, suggesting a stable budget of H+ in the magnetosphere. However, low-energy H+, not detectable by the ion spectrometer, is not considered in our study, leaving the complete magnetospheric H+ circulation an open question. Studying tailward flows separately reveals a total tailward O+ flux of about 0. 5 × 1025 s−1, which can be considered as a lower limit of the nightside auroral region O+ outflow. Lower velocity flows ( &lt; 100 km s−1) contribute most to the total transports, whereas the high-velocity flows contribute very little, suggesting that bursty bulk flows are not dominant in plasma sheet mass transport.

Suprathermal electron acceleration in the near‐Earth flow rebounce region

AbstractFlux pileup regions (FPRs) are traditionally referred to the strong‐Bz bundles behind dipolarization fronts (DFs) in the Earth's magnetotail and can appear both inside earthward and tailward bursty bulk flows. It has been widely reported that suprathermal electrons (40–200 keV) can be efficiently accelerated inside earthward FPRs, leaving the electron acceleration inside tailward FPRs as an open question. In this study, we focus on the electron acceleration inside a tailward FPR that is formed due to the flow rebounce in the near‐Earth region (XGSM ≈ −12 RE) and compare it quantitatively with the acceleration inside an earthward FPR. By examining the Cluster data in 2008, we sequentially observe an earthward FPR and a tailward FPR in the near‐Earth region, with the earthward one belonging to decaying type and the tailward one belonging to growing type. Inside the earthward FPR, Fermi acceleration and betatron cooling of suprathermal electrons are found, while inside the tailward FPR, Fermi and betatron acceleration occur. Whistler‐mode waves are observed inside the tailward FPR; their generation process may still be at the early stage. We notice that the suprathermal electron fluxes inside the tailward FPR are about twice as large as those inside the earthward FPR, suggesting that the acceleration of suprathermal electrons is more efficient in the flow rebounce region. These acceleration processes have been successfully reproduced using an analytical model; they emphasize the role of flow rebounce in accelerating suprathermal electrons and further reveal how the MHD‐scale flow modulates the kinetic‐scale electron dynamics in the near‐Earth magnetotail.

Behavior of fast earthward flow near the braking region: Hall MHD simulation

Behavior of the fast earthward flow near the braking region in the magnetotail during a substorm is investigated using the Hall MHD simulation. The results indicate that the high-speed earthward plasma flow is associated with fast reconnection in the middle tail. The fast flow is mainly confined in the range . In the region of , due to intermittent magnetic reconnection, the earthward flow exhibits a fluctuating property, i.e., the flow is localized in space and is bursty in time. The pile-up of the magnetic flux and plasma in the near-Earth region leads to formation of the fast-flow braking region or dipolarization front. After colliding into the fast-flow braking region, a part of the Earth flow bounces back, and leads to an intermittent tailward flow in the near-Earth magnetotail.

Current sheet flapping motions in the tailward flow of magnetic reconnection

AbstractThe feature and origin of current sheet flapping motions are one of most interesting issues of magnetospheric dynamics. In this paper we report the flapping motion of the current sheet detected in the tailward flow of a magnetic reconnection event on 7 February 2009. This flapping motion with frequency about 12 mHz was accompanied by magnetic turbulence. The observations by the tail‐elongated fleet of five Time History of Events and Macroscale Interactions during Substorms probes indicate that these flapping oscillations were rather confined within the tailward flow than were due to a global process. This flapping motion could be due to the instability driven by the free energy associated with the ion temperature anisotropy in the tailward flow. Our observations indicate that the flapping motion in the tailward flow could have a different generation mechanism with that in the earthward flow.

Stepwise tailward retreat of magnetic reconnection: THEMIS observations of an auroral substorm

AbstractAuroral stepwise poleward expansions were clarified by investigating a multiple‐onset substorm that occurred on 27 February 2009. Five successive auroral brightenings were identified in all‐sky images, occurring at approximately 10 min intervals. The first brightening was a faint precursor. The second brightening had a wide longitude; thus, it represented the Akasofu substorm onset. Other brightenings expanded poleward; thus, they were interpreted to be auroral breakups. These breakups occurred stepwise; that is, later breakups were initiated at higher latitudes. Corresponding reconnection signatures were studied using Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellite observations between 8 and 24 RE down the magnetotail. The Akasofu substorm onset was not accompanied by a clear reconnection signature in the tail. In contrast, the three subsequent auroral breakups occurred simultaneously (within a few minutes) with three successive fast flows at 24 RE; thus, these were interpreted to be associated with impulsive reconnection episodes. These three fast flows consisted of a tailward flow and two subsequent earthward flows. The flow reversal at the second breakup indicated that a tailward retreat of the near‐Earth reconnection site occurred during the substorm expansion phase. In addition, the earthward flow at the third breakup was consistent with the classic tailward retreat near the end of the expansion phase; therefore, the tailward retreat is likely to have occurred in a stepwise manner. We interpreted the stepwise characteristics of the tailward retreat and poleward expansion to be potentially associated by a stepwise magnetic flux pileup.

Statistics of energetic electrons in the magnetotail reconnection

AbstractMagnetic reconnection has long been regarded as an important site for producing energetic electrons in solar terrestrial and astrophysical plasmas. The motivation of this paper is to provide the average properties of energetic electrons in reconnection region, which are crucial for understanding electron energization mechanism but are rarely known. We statistically analyzed the energetic electrons through 21 magnetotail reconnection events observed by Cluster spacecraft during the years of 2001–2005. Approximately 1200 data points with time resolution of 8 s have been collected for each spacecraft. Two parameters are examined: energetic electron rate (EER) and power law index. EER, which is defined as the ratio of the integrated energetic electron flux to the lower energy electron flux, is used to quantify the electron acceleration efficiency. We find that EER and energetic electron flux (EEF) are positively correlated with the power law index, i.e., the higher rate and flux generally corresponds to softer spectrum. This unexpected correlation is probably caused by some nonadiabatic heating/acceleration mechanisms that tend to soft the spectrum with high temperature. EER is much larger within the earthward flow than the tailward flow. It is positively correlated with the outflow speed Vx, while the correlation between EER and Bz is less clear. With the increment of earthward outflow speed, the occurrence rate of high EER also monotonically increases. We find that EER generally does not increase with the increment of perpendicular electric field |E⊥|, suggesting that adiabatic betatron and Fermi acceleration probably play minor roles in electron energization during magnetotail reconnection.

The dawn‐dusk length of the X line in the near‐Earth magnetotail: Geotail survey in 1994–2014

AbstractThe dawn‐dusk length of the X line is investigated for magnetic reconnection in association with substorms in the near‐Earth magnetotail on the basis of the 21 year Geotail plasma sheet observations. The X line is identified as a simultaneous plasma flow Vx and magnetic field Bz reversal inside the ion‐electron decoupling region of magnetic reconnection. Forty‐four X lines can be found at XGSM = −20 to −31 RE in the magnetotail. The X line length is estimated on the basis of occurrence of magnetic reconnection. Characteristics of flows and fields are investigated for magnetic reconnection and tailward flow events to verify the obtained X line length. The dawn‐dusk length of the X line is most likely 6 RE with its center in the premidnight sector for moderate substorms. Hence, the magnetic reconnection site is mapped to approximately 1 h local time sector in the auroral ionosphere. The dawn‐dusk length of the X line is extended mainly dawnward for larger substorms.

Quasi-continuous reconnection accompanied by FTEs during IMF Bz ≈ 0 nT observed by Double Star TC-1 at the dawnside magnetopause

During a one-hour interval of interplanetary magnetic field (IMF) Bz≈0nT, the equatorial spacecraft Double Star TC-1 encountered the dawn flank magnetopause many times at the magnetic local time (MLT) of about 08:00 and the latitude of about −27°. During each encounter, reconnection jets were observed with their velocities up to more than 500km/s, significantly higher than the background flow in the magnetosheath. The fast flows match the theoretical prediction of Alfvénic acceleration well. The medium temperature and density of ions in the boundary layer indicate the open magnetic field topology inside this layer. The mainly southward and tailward flows of the plasma jets alongside with the negative slopes of the Walén test indicate that the spacecraft was located south of the reconnection site, consistent with both anti-parallel and component reconnection models. The accelerated flows were observed lasting for about one hour, with some modulations by the oscillations of the magnetopause, but no reversals in the direction of Vz were found during the interval. The significantly enhanced flows in the boundary layer compared to the adjacent magnetosheath indicate that the reconnection was quasi-continuously active at the magnetopause northward of the spacecraft under such IMF conditions. At the same time, the bipolar signatures in BN with enhancements of the magnetic field indicate the occurrence of the Flux Transfer Events (FTEs). The observed reconnection was quasi-continuous, whereas the simultaneously accompanied FTEs were time-dependent under the IMF Bz≈0nT. For this event, however, it is not possible to identify whether the reconnection was anti-parallel or component because the TC-1 was far away from the reconnection site.

Statistical characteristics of slow earthward and tailward flows in the plasma sheet

AbstractIn this paper, we statistically analyzed and compared the earthward flow (EF) and the tailward flow (TF) in the plasma sheet. It is found that the properties of the EF/TF in the central plasma sheet (CPS) of β &gt; 1 and the outer plasma sheet (OPS) of 0.1 &lt; β &lt; 1 are distinctly different. The main conclusions include that (1) the EFs occur in both the CPS and the OPS while the TFs mainly occur in the OPS, (2) both flows are dominantly convective in the CPS and parallel in the OPS, (3) in the CPS, the EF and the TF have similar characteristics, including their bulk velocities and ion densities and Ey components. Both flows tend to have isotropic temperatures; (4) in the OPS, the EFs tend to have higher ion velocity, density, and Ey than the TF. The EFs tend to have anisotropic temperatures, while the TFs tend to have more isotropic temperatures. As a whole, combined characteristics of the EF and the TF are consistent with (1) reflection at the “magnetic mirror point” near the Earth for parallel flows in the OPS and (2) bouncing off/back from the dipolar field closer to the Earth for convective flows in the CPS.

Solar-wind control of plasma sheet dynamics

Abstract. The purpose of this study is to quantify how solar-wind conditions affect the energy and plasma transport in the geomagnetic tail and its large-scale configuration. To identify the role of various effects, the magnetospheric data were sorted according to different solar-wind plasma and interplanetary magnetic field (IMF) parameters: speed, dynamic pressure, IMF north–south component, epsilon parameter, Auroral Electrojet (AE) index and IMF ultra low-frequency (ULF) fluctuation power. We study variations in the average flow speed pattern and the occurrence rate of fast flow bursts in the magnetotail during different solar-wind conditions using magnetospheric data from five Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission spacecraft and solar-wind data from NASA's OMNIWeb. The time interval covers the years from 2008 to 2011 during the deep solar minimum between cycles 23 and 24 and the relatively quiet rising phase of cycle 24. Hence, we investigate magnetospheric processes and solar-wind–magnetospheric coupling during a relatively quiet state of the magnetosphere. We show that the occurrence rate of the fast (|Vtail| &gt; 100 km s−1) sunward flows varies under different solar-wind conditions more than the occurrence of the fast tailward flows. The occurrence frequency of the fast tailward flows does not change much with the solar-wind conditions. We also note that the sign of the IMF BZ has the most visible effect on the occurrence rate and pattern of the fast sunward flows. High-speed flow bursts are more common during the slow than fast solar-wind conditions.

Earthward and tailward flows in the plasma sheet

AbstractUtilizing C3/Cluster satellite observations from the year of 2001 to 2006, we investigated the earthward flow (EF) and tailward flow (TF) at Bz &gt; 0 in the plasma sheet. We found that the EF and the TF have similar spatial distributions. Both characteristics are independent of the distance beyond 14 RE. Both flows are deflected while closer to the Earth. Statistical results further showed that the EF/TF occur in the central plasma sheet as well as the plasma sheet boundary layer and can be observed during quiet times and periods of geomagnetic activity. A typical event reveals that the EF and the TF have different plasma population. A transition region (TR) can be formed at the interface between the EF and TF. Very significant duskward components appeared in bulk velocities for both populations. It appears that the vortical‐like structure can be formed near the TR. The magnetic field within the TR is twisted and strongly fluctuates. No clear magnetic flux pileups are observed inside the TR.

Modeling the interaction between the solar wind and Saturn's magnetosphere by the AMR‐CESE‐MHD method

AbstractIn this paper, the space‐time conservation element and solution element (CESE) method in general curvilinear coordinates is successfully applied to the three‐dimensional magnetohydrodynamic (MHD) simulations of the interaction between the solar wind and Saturn's magnetosphere on a six‐component grid system. As a new numerical model modified for the study of the interaction between the solar wind and Saturn's magnetosphere, we obtain the large‐scale configurations of Saturn's magnetosphere under the steady solar wind with due southward interplanetary magnetic field (IMF) conditions. The numerical results clearly indicate that the global structure of Saturn's magnetosphere is strongly affected by the rotation of Saturn as well as by the solar wind. The subsolar standoff distances of the magnetopause and the bow shock in our model are consistent with those predicted by the data‐based empirical models. Our MHD results also show that a plasmoid forms in the magnetotail under the effect of the fast planetary rotation. However, somewhat differently from the previous models, we find that there are two flow vortices generated on the duskside under due southward IMF at Saturn. On the duskside, the clockwise one closer to the planet is excited by the velocity shear between the rotational flows and the sunward flows, while the anticlockwise one is generated from the velocity shear between the tailward flows along the magnetopause and the sunward flows.

Study of plasma flow reversal in the near-Earth plasma sheet using numerical computations

Tailward flow in the near-Earth plasma sheet associated with a rebound of the earthward bursty bulk flow (BBF) is investigated using three-dimensional magnetohydrodynamics simulations of magnetic reconnection in the magnetotail on the basis of the spontaneous fast reconnection model. In order to investigate the properties of this tailward flow, virtual satellites are located at different positions in the plasma sheet within the simulation region, so that we can directly observe the temporal variations of plasma quantities in accordance with the growth and preceding the flow reversal associated with the magnetic reconnection. The time profile of the plasma flow velocity in the course of the BBF depends on the satellite position. Furthermore, the time profile of the magnetic field strength in the course of the reverse flow depends on the satellite position in the dawn-dusk direction. As a result of the rebound of the earthward flow, the accumulation of the plasma density and the plasma pressure is observed at any position in the plasma sheet during the interval between the BBF and the reverse flow.

Observation of large‐amplitude magnetosonic waves at dipolarization fronts

AbstractVarious plasma waves have been observed in the vicinity of dipolarization fronts (DFs) and the rarefaction regions behind them. It was suggested that these waves not only play crucial roles in regulating particle kinetics at the DFs but also may potentially affect the large‐scale dynamics of the magnetotail. In this paper, we present the observations of large‐amplitude electromagnetic waves at DFs that occurred during magnetospheric substorms. The DFs were embedded in either the tailward or earthward flows in the near‐Earth magnetotail. The wave frequencies were near the local proton cyclotron frequency. The waves propagated at highly oblique angles with respect to the ambient magnetic field (~80°–100°). Their corresponding wavelengths were on the order of the local ion gyroradii. The major magnetic field fluctuations were along the background magnetic field, while the electric field fluctuations were predominantly perpendicular to the background magnetic field. The waves were compressional waves as there was an anticorrelation between the plasma density and the wave magnetic field strength. The electric potential associated with the waves reached to more than half of the electron temperature, indicating the waves are nonlinear. We suggest that the waves were magnetosonic or ion Bernstein mode waves driven by the ion ring distribution. The waves were able to provide significant anomalous resistivity at the front, with major contributions from the electric field fluctuations. The effects of these waves on the electron pitch angle scattering and energy diffusion are also discussed.

Statistical visualization of the Earth's magnetotail and the implied mechanism of substorm triggering based on superposed-epoch analysis of THEMIS data

Abstract. To investigate the physical mechanism responsible for substorm triggering, we performed a superposed-epoch analysis using plasma and magnetic-field data from THEMIS probes. Substorm onset timing was determined based on auroral breakups detected by all-sky imagers at the THEMIS ground-based observatories. We found earthward flows associated with north–south auroral streamers during the substorm growth phase. At around X = −12 Earth radii (RE), the northward magnetic field and its elevation angle decreased markedly approximately 4 min before substorm onset. Moreover, a northward magnetic-field increase associated with pre-onset earthward flows was found at around X = −17 RE. This variation indicates that local dipolarization occurs. Interestingly, in the region earthwards of X = −18 RE, earthward flows in the central plasma sheet (CPS) reduced significantly approximately 3 min before substorm onset, which was followed by a weakening of dawn-/duskward plasma-sheet boundary-layer flows (subject to a 1 min time lag). Subsequently, approximately 1 min before substorm onset, earthward flows in the CPS were enhanced again and at the onset, tailward flows started at around X = −20 RE. Following substorm onset, an increase in the northward magnetic field caused by dipolarization was found in the near-Earth region. Synthesizing these results, we confirm our previous results based on GEOTAIL data, which implied that significant variations start earlier than both current disruption and magnetic reconnection, at approximately 4 min before substorm onset roughly halfway between the two regions of interest; i.e. in the catapult current sheet.

MHD and kinetic analysis of flow bursts in the Earth’s plasma sheet

In an attempt to study the flow bursts in the Earth’s plasma sheet we select an event that took place on August 7, 2004 in the expansion phase of a substorm, using data from the geomagnetic index, solar wind data, plasma and magnetic field observations from C1 Cluster satellite (the Cluster mission has 4 satellites) and from Double Star TC-1 satellite. In MHD approach, TC-1 firstly observed the tailward flow, then the earthward, and finally the flow alternated in two directions. C1 firstly observed the earthward plasma flow, and then the tailward plasma flow. Before flow bursts are observed by TC-1 and C1, there are disturbances in local entropy with their tailward local entropy larger than those of the earthward. The kinetic features of the plasma flow observed by C1 are similar to those in MHD. However, kinetic characteristics of the plasma flow observed by TC-1 are far more than the description in MHD. The inadequacy mainly exists in two cases: (i) the firstly enhanced tailward flows given in MHD are found without significant increase of the energetic tailward flux; (ii) the almost stagnant flow in MHD is composed of the enhanced energetic ion flux in both earthward and tailward directions. The earthward flow burst observed by TC-1 might be multiple overshoots and rebounds. The earthward flow burst observed by C1 might be simply rebounded in the near-Earth. The pulsation observed by C1 is earlier than that observed by TC-1 with the former intensity less than that of the latter. After the energetic ion flux in the tailward direction is significantly enhanced, the power spectrum intensity of the ULF wave commences to increase obviously, which may suggest that the stream instability is closely correlated with ULF pulsations.

Plasma sheet magnetic fields and flows during steady magnetospheric convection events

Inner magnetosphere magnetic field and plasma flow data are examined during 228 steady magnetospheric convection events. We find that the BZ component of the magnetic field around geostationary orbit is weaker than during average conditions and the plasma flow speeds are higher than average in the dusk sector just beyond geostationary orbit. The steady magnetospheric convection periods include more enhanced earthward and tailward flow intervals than during average conditions. The steady convection period magnetic field is not steady: The near‐geostationary nightside field grows increasingly taillike throughout the steady convection period. In the midtail, earthward flows are enhanced in a wide region around the midnight sector, which leads to enhanced magnetic flux transport toward the Earth during the steady convection periods. Compared to well‐known characteristics during magnetospheric substorms, the inner tail evolution resembles that during the substorm growth phase, while the midtail flow characteristics during steady convection periods are similar to those found during substorm recovery phases.