Plasma Sheet Convection Research Articles

Importance of the dusk-dawn interplanetary magnetic field component (IMF By) to magnetospheric convection in Earth’s magnetotail plasma sheet

The solar wind and its embedded magnetic field, the interplanetary magnetic field (IMF) together with magnetic reconnection power the large-scale plasma and magnetic flux circulation in the Earth’s magnetosphere-ionosphere system. This circulation is termed as convection and its strength is controlled by the north-south IMF component (IMF Bz). In recent years, an interest has arisen to investigate the lesser-known role of the dusk-dawn component (IMF By) in convection. It has been previously known though that prevailing nonzero IMF By can cause plasma flow asymmetries in the high-latitude ionosphere, but how the magnetospheric flows, for instance, in the magnetotail plasma sheet are affected, remains to be investigated. In this article, we introduce the recent progress and the latest achievements in the research of the influence of IMF By on tail plasma sheet convection. The research progress has been rapid and it has revealed that both fast and slow convection are affected in a manner that is in accordance with the asymmetries observed in the ionospheric convection. The results indicate the significance of the IMF By component on magnetospheric convection and they represent a major advance in the field of solar wind-magnetosphere coupling.

Properties of suprathermal electrons associated with discrete auroral arcs

AbstractWe report on the properties of suprathermal electrons observed over three discrete auroral arcs from a sounding rocket. By applying shifted kappa distributions and analyzing kappa parameters (density, temperature, and kappa), we found three novel characteristics to provide clues to understand the auroral acceleration mechanisms and magnetosphere‐ionosphere coupling processes. First, the auroral potential drop was proportional to the inverse square of kappa, consistent with previous theoretical investigations by Dors and Kletzing (). The observed dependency was slightly stronger than their calculations, suggesting additional contributions from nonlinear plasma processes. Second, the polytropic relation showed nonadiabatic (near isothermal) state of the source electrons. This can provide a restriction on the pressure balance issues in the plasma sheet convection. Third, there was a clear difference in the polytropic and kappa indices for the first arc as opposed to the second and the third arcs, suggesting different source locations in the plasma sheet for the precipitating electrons that cause these nearby arcs.

The substorm cycle as reproduced by global MHD models

AbstractRecently, Gordeev et al. (2015) suggested a method to test global MHD models against statistical empirical data. They showed that four community‐available global MHD models supported by the Community Coordinated Modeling Center (CCMC) produce a reasonable agreement with reality for those key parameters (the magnetospheric size, magnetic field, and pressure) that are directly related to the large‐scale equilibria in the outer magnetosphere. Based on the same set of simulation runs, here we investigate how the models reproduce the global loading‐unloading cycle. We found that in terms of global magnetic flux transport, three examined CCMC models display systematically different response to idealized 2 h north then 2 h south interplanetary magnetic field (IMF) Bz variation. The LFM model shows a depressed return convection and high loading rate during the growth phase as well as enhanced return convection and high unloading rate during the expansion phase, with the amount of loaded/unloaded magnetotail flux and the growth phase duration being the closest to their observed empirical values during isolated substorms. Two other models exhibit drastically different behavior. In the BATS‐R‐US model the plasma sheet convection shows a smooth transition to the steady convection regime after the IMF southward turning. In the Open GGCM a weak plasma sheet convection has comparable intensities during both the growth phase and the following slow unloading phase. We also demonstrate potential technical problem in the publicly available simulations which is related to postprocessing interpolation and could affect the accuracy of magnetic field tracing and of other related procedures.

Nightside magnetospheric current circuit: Time constants of the solar wind‐magnetosphere coupling

AbstractThis study addresses the characteristics of the nightside magnetospheric current system using the analogy of an electric circuit. The modeled circuit consists of the generator (V: solar wind), inductor (L: tail lobes), capacitor (C: plasma sheet convection), and resistor (R: particle energization). The electric circuit has three time constants: τCR(=CR), , and τL/R(=L/R). Here τCR is of the order of the ion gyroperiod in the plasma sheet, τLC is a global timescale (2πτLC is several tens of minutes), and τL/R is even longer (several hours). Despite uncertainty in the estimate of each circuit element, τCR ≪ τLC ≪ τL/R holds generally for the magnetosphere, which characterizes the electric circuit as overdamped. The following implications are obtained: (1) During the substorm growth phase the cross‐tail current increases continuously even if interplanetary magnetic field (IMF) BZ does not change after southward turning; (2) the magnetotail current weakens following northward turnings if the change of IMF BZ is comparable to the preceding southward IMF BZ; otherwise it may strengthen continuously if more gradually; (3) during the early main phase of magnetospheric storms the enhancement of the lobe magnetic energy is far more prominent than the enhancements of the kinematic and kinetic energies of the plasma sheet plasma; (4) The efficiency of the solar wind‐magnetosphere coupling changes on a timescale of several hours (τL/R) through the change of the tail flaring, and so does the cross polar‐cap potential; and (5) the magnetospheric current system does not resonate to an oscillatory external driver, and therefore, the periodicity of some magnetotail phenomena reflects that of their triggers.

Statistics of plasma sheet convection

[1] Determining the characteristics of plasma sheet convection and their response to changes in various solar wind parameters is important for understanding the energy and mass transport, as well as disturbance propagation, through geospace. We use 15 years of data obtained by Geotail, Cluster, and THEMIS to study statistically the characteristics of plasma sheet flows and the effect of the interplanetary magnetic field (IMF) on the convection. We find that plasma sheet convection is dominated by slow speed ( 500 km/s are directed almost purely sunward. Both IMF By and IMF Bz are observed to penetrate the plasma sheet. During southward IMF conditions, a channel of increased Bz is created in the nightside around the aberrated midnight axis. We suggest that the channel is caused by dipolarization and magnetic flux pileup related to fast flows. The nightside region of highest mean flow speed is located more duskward during dawnward IMF conditions than during duskward IMF conditions. For plasma sheet flows with speeds higher than 100 km/s, we find that the orientation of IMF (clock angle) controls the speeds, while the magnitude of the solar wind electric field plays a minor role. The increasing speed indicates that energy transfer per unit length of the nightside X line increases as IMF turns southward.

Prompt penetration electric fields (PPEFs) and their ionospheric effects during the great magnetic storm of 30–31 October 2003

We explore the ionospheric effects of prompt penetration electric fields (PPEFs) for a variety of interplanetary magnetic field directions. We use the great magnetic storm of 30–31 October as an example of PPEF effects. For intense southward interplanetary magnetic fields (IMFs), inward plasma sheet convection occurs with the result of magnetospheric ring current formation and an intense magnetic storm. Concurrent with the above, positive phase ionospheric storms occur in the dayside, and negative phase ionospheric storms occur on the nightside, the topics of this paper. The dayside ionospheric storms due to PPEFs are characterized by transport of near‐equatorial plasma to higher altitudes and latitudes, forming a giant plasma fountain. These features are part of what is called the dayside ionospheric superfountain (DIS). For these southward IMFs, dusk and dawn plasma are predicted to be transported toward the dayside. For northward IMFs, negative phase ionospheric storms are expected on the dayside if the PPEFs indeed reach that region of space. IMF By components are expected to have weak or neglible ionospheric effects. On the basis of PPEF arguments, intervals of IMF By should not be related to geomagnetic storms (they are not). IMF By intervals should, however, cause a shearing of the magnetotail, a feature that has been previously reported in the literature.

Magnetotail response to prolonged southward IMF Bz intervals: Loading, unloading, and continuous magnetospheric dissipation

The response of the Earth's magnetotail to prolonged southward interplanetary magnetic field (IMF) has been determined for the three Geotail magnetotail seasons from November to April, 1999–2002. We examine the total magnetotail pressure PT,tail = B2/2μ0 + NikTi because variations should be similar in this parameter in the lobes and in the plasma sheet. We found 13 events when IMF Bz remained southward for 8 hours or longer and Geotail was located within the magnetotail farther than 10 RE downstream. All 13 events were subdivided into separate intervals characterized as (1) loading, if the tail total pressure increased more than 100%; (2) unloading, if the total pressure decreased by more than 50%; and (3) what we term here continuous magnetospheric dissipation (CMD), if the tail total pressure increased by less than 100% and/or decreased less than 50% during the entire mode interval. In total, 37 loading, 37 unloading, and 28 CMD events were found. The plasma sheet magnetic flux transfer rate, ϕEarth ≈ vx · Bz, and plasma bulk velocity has been analyzed to determine the steadiness of the plasma sheet convection. Plasma sheet convection was found to be highly disturbed and intense plasma flows (BBFs and FBs) were observed during all convection states. However, the occurence rate and amplitude of plasma flows distinguish loading‐unloading and continuous dissipation periods from each other. BBFs seem to be more numerous (135) but weaker (about 500 km/s) during continuous dissipation intervals compared with BBFs existing during unloading mode (61 and 660 km/s). Finally, it was found that CMD‐type convection is more likely when the mean southward IMF Bz > −5 nT, while loading‐unloading is more likely when IMF Bz < −5 nT.

Average characteristics of the midtail plasma sheet in different dynamic regimes of the magnetosphere

Abstract. We study average characteristics of plasma sheet convection in the middle tail during different magnetospheric states (Steady Magnetospheric Convection, SMC, and substorms) using simultaneous magnetotail (Geotail, 15-35 RE downtail) and solar wind (Wind spacecraft) observations during 3.5 years. (1) A large data set allowed us to obtain the average values of the plasma sheet magnetic flux transfer rate (Ey and directly compare it with the dayside transfer rate (Emod for different magnetospheric states. The results confirm the magnetic flux imbalance model suggested by Russell and McPherron (1973), namely: during SMC periods the day-to-night flux transport rate equals the global Earthward plasma sheet convection; during the substorm growth phase the plasma sheet convection is suppressed on the average by 40%, whereas during the substorm expansion phase it twice exceeds the day-to-night global flux transfer rate. (2) Different types of substorms were revealed. About 1/3 of all substorms considered displayed very weak growth in the tail lobe magnetic field before the onset. For these events the plasma sheet transport was found to be in a balance with the day-to-night flux transfer, as in the SMC events. However, the lobe magnetic field value in these cases was as large as that in the substorms with a classic growth phase just before the onset (both values exceed the average level of the lobe field during the SMC). Also, in both groups similar configurational changes (magnetic field stretching and plasma sheet thinning) were observed before the substorm onset. (3) Superimposed epoch analysis showed that the plasma sheet during the late substorm recovery phase has the characteristics similar to those found during SMC events, the SMC could be a natural magnetospheric state following the substorm.

Energy Flux in the Earth’s Magnetosphere: Storm-Substorm Relationship

Three ways of the energy transfer in the Earth’s magnetosphere are studied. The solar wind MHD generator is an unique energy source for all magnetospheric processes. Field-aligned currents directly transport the energy and momentum of the solar wind plasma to the Earth’s ionosphere. The magnetospheric lobe and plasma sheet convection generated by the solar wind is another magnetospheric energy source. Plasma sheet particles and cold ionospheric polar wind ions are accelerated by convection electric field. After energetic particle precipitation into the upper atmosphere the solar wind energy is transferred into the ionosphere and atmosphere. This way of the energy transfer can include the tail lobe magnetic field energy storage connected with the increase of the tail current during the southward IMF. After that the magnetospheric substorm occurs. The model calculations of the magnetospheric energy give possibility to determine the ground state of the magnetosphere, and to calculate relative contributions of the tail current, ring current and field-aligned currents to the magnetospheric energy. The magnetospheric substorms and storms manifest that the permanent solar wind energy transfer ways are not enough for the covering of the solar wind energy input into the magnetosphere. Nonlinear explosive processes are necessary for the energy transmission into the ionosphere and atmosphere. For understanding a relation between substorm and storm it is necessary to take into account that they are the concurrent energy transferring ways.

Localized convection flows and field‐aligned current generation in a kinetic model of the near‐Earth plasma sheet

Three‐dimensional (3‐D) particle simulations are used to determine the characteristic field‐aligned currents produced by an interchange‐driven, localized plasma jet impinging on the near‐Earth plasma sheet. The diversion of the plasma flow produces a region 2 sense current which enhances the cross‐tail current, while the primary flow provides a current to the ionosphere whose signature is consistent with the auroral streamers. The high beta interchange mode and localized flows resulting from a tailward gradient in the equatorial magnetic field provide a physical realization of the plasma bubble model of plasma sheet convection.

Particle-in-cell simulations of magnetosphere electrodynamics

This paper reviews the basic principles and techniques involved in formulating particle-in-cell (PIC) simulation models which can be used to address medium- and large-scale problems in magnetosphere electrodynamics. The limitations imposed by the underlying kinetic physics of a plasma are emphasized, and representative algorithms are described for full particle and hybrid (particle ions, fluid electrons) models. Issues related to the choice of initial and boundary conditions and the implementation of PIC models on massively parallel computers are discussed. Explicit examples involving the diffusion region in collisionless reconnection, plasma sheet convection, and large scale structure in magnetic reconnection are presented to illustrate the current capabilities of PIC models.

Two spacecraft observation of plasma sheet convection jet during continuous external driving

During ∼18 hours of continuous activity caused by long interval of southward IMF, the INTERBALL/Tail Probe and GEOTAIL spacecraft monitored the plasma sheet in dusk and dawn sectors of the midtail plasma sheet at 25–30 RE. While bursty bulk flows were persistently observed at both spacecraft, there was a systematic difference between the hourly averages of the Earthward flux transport. During a 3‐hour interval the average flux transport rate (Ey) on GEOTAIL (dawn plasma sheet) was ∼3 times larger than both the average Ey on INTERBALL (dusk plasma sheet), and the cross‐tail Ey0 expected for the existing IMF conditions. This implies that the most intense bursty bulk flows tend to be confined within a limited longitudinal sector of the magnetotail (X < −30 RE) forming the observed average convection jet feature.

Solar wind induced processes in the magnetotail

Solar wind/IMF parameters and their variations influence the state and dynamics of the magnetosphere in several different ways, and the plasma sheet plays its own active role to form the magnetotail’s rssponse to external driving. This field is still quite conrtoversial; key unsolved issues are those which probably involve the nonsteady, nonequilibrium and nonlinear character of the system. This paper discusses an interesting development of concepts concerning (1) plasma sheet convection, (2) mechanisms which initiate the substorm onset, and (3) variability of tail dynamics. A remarkable example is a recognition of Bursty Bulk Flows as a basic way for the plasma sheet to sustain the convective transport of plasma, energy and magnetic flux via transient mesoscale dynamic structures. As concerns the substorm onset, the sharp change from the magnetic reconnection (NEL) model to processes in the dipolar-like near tail is now moving into a synthetic stage. Hear non-linear models of different coupled instabilities are actively explored to find a way to excite the tearing mode starting from a singular thin current sheet configuration which seems to form in the near tail prior to breakup. Variable responses to external drivers and the variability of substorms compel a recognition of the magnetotail as a very complicated nonlinear open system which includes a heirarchy of coupled process of different scales. A unique fleet of magnetospheric spacecraft, simultaneously probing different domains and supported by extensive ground observations, global imaging and solar wind monitoring, noe provides a real chance to understand the magnetotail as a global dynamic system.

The transport of plasma sheet material from the distant tail to geosynchronous orbit

Several aspects of mass transport in the Earth's plasma sheet are examined. The evolution of plasma sheet material as it moves earthward is examined by statistically comparing plasma sheet properties at three different downtail distances: near‐Earth plasma sheet properties obtained from measurements by 1989‐046 near the geomagnetic equator near midnight at 6.6 RE, midtail plasma sheet properties obtained from ISEE 2 measurements during 333 encounters with the neutral sheet, and distant‐plasma sheet properties obtained from ISEE 2 measurements during 53 encounters with the interface between the plasma sheet and the plasma sheet boundary layer. Examination of the evolution of the plasma sheet through pressure‐density space shows that the transport is nearly adiabatic (γ = 1.52), with a loss of entropy observed in the near‐Earth region. The estimated pressure loss from the plasma‐sheet associated with the aurora is able to account for the observed decrease in entropy. The near‐Earth plasma sheet plasma is also found to be compressed much less than would be expected from magnetic field models. Examination of the evolution of the plasma sheet through density‐flux tube‐volume space (with the aid of the T89c magnetic field model) indicates that there is a substantial loss of mass from plasma sheet flux tubes. Global magnetic reconnection during substorms and patchy reconnection at other times is invoked to account (1) for the required mass loss, (2) for the related lack of compression, and (3) for an observed disconnection between ionospheric convection and plasma sheet convection. This reconnection must occur closer than 20 RE downtail. Selective transport is examined by statistically analyzing the ISEE 2 neutral sheet crossing data set: strong transport is found to be associated with low densities, with weak Bz, and with large flux tube volume. A correlation between the direction of the flow in the plasma sheet and the solar wind velocity indicates that earth‐ward transport is stronger when the solar wind velocity is lower. An examination of near‐Earth and of midtail plasma sheet densities, temperatures, and entropies shows that the plasma sheet is usually spatially homogeneous, contrary to a “bubbles and blobs” picture of transport. Several new points of view about plasma sheet transport are discussed, including the dominant role of near‐Earth reconnection, the importance of auroral zone pressure loss, the control of the plasma sheet properties by the density and speed of the solar wind, and the disconnection of the ionospheric and plasma sheet flow patterns.

Formation of thin current sheets during plasma sheet convection

Fully kinetic simulations in two spatial dimensions are used to demonstrate that the attempt to establish convection in the near‐Earth magnetotail can drive the cusp or hinge region to form a thin current sheet which is embedded within a thicker plasma sheet. The simulation model incorporates a realistic near‐Earth magnetic field configuration including dipolelike and taillike regions and allows plasma to precipitate at the earthward boundary. The imposition of an electrostatic convection field leads to a slow earthward convection of plasma and a tailward transfer of magnetic flux, with the inductive electric field opposing and almost canceling the external field. This flux transfer leads to the formation of a deep minimum in the equatorial Bz field in the near‐Earth plasma sheet. As a consequence of the difference in dynamics between electrons and ions, the central plasma sheet in the cusp region becomes negatively charged, which leads to the formation of a strong polarization electric field (Ez). The growth of the polarization field causes the electrons to carry the dominant current in the thin current sheet, whose characteristic half thickness is of the order of the local ion gyroradius and whose current density is an order of magnitude larger than for the usual tail current sheet. The negative space charge electric field associated with the thin current sheet and the related field‐aligned currents are consistent with the polarization Harang equatorward electric field.

Convection and the formation of thin current sheets in the near‐Earth plasma sheet

Particle simulations are used to investigate plasma sheet convection in a realistic near‐Earth magnetic field model including dipolelike and taillike regions. Convection leads to the formation of a thin current region characterized by a strong electrostatic potential and a depressed equatorial magnetic field component in which the cross tail current is carried predominantly by the electrons as a result of their E × B and diamagnetic drifts. Implications for the thin current sheets observed in the near‐Earth magnetotail during substorm growth phase are discussed.

Contributions of the low‐latitude boundary layer to the finite width magnetotail convection model

Convection of plasma within the terrestrial nightside plasma sheet contributes to the structure and, possibly, the dynamical evolution of the magnetotail. In order to characterize the steady state convection process, we have extended the finite tail width model of magnetotail plasma sheet convection. The model assumes uniform plasma sources and accounts for both the duskward gradient/curvature drift and the earthward E × B drift of ions in a two‐dimensional magnetic geometry. During periods of slow convection (i.e., when the cross‐tail electric potential energy is small relative to the source plasma's thermal energy), there is a significant net duskward displacement of the pressure‐bearing ions. The electrons are assumed to be cold, and we argue that this assumption is appropriate for plasma sheet parameters. We generalize solutions previously obtained along the midnight meridian to describe the variation of the plasma pressure and number density across the width of the tail. For a uniform deep‐tail source of particles, the plasma pressure and number density are unrealistically low along the near‐tail dawn flank. We therefore add a secondary source of plasma originating from the dawnside low‐latitude boundary layer (LLBL). The dual plasma sources contribute to the plasma pressure and number density throughout the magnetic equatorial plane. Model results indicate that the LLBL may be a significant source of near‐tail central plasma sheet plasma during periods of weak convection. The model predicts a cross‐tail pressure gradient from dawn to dusk in the near magnetotail. We suggest that the plasma pressure gradient is balanced in part by an oppositely directed magnetic pressure gradient for which there is observational evidence. Finally, the pressure to number density ratio is used to define the plasma “temperature.” We stress that such quantities as temperature and polytropic index must be interpreted with care as they lose their nominal physical significance in regions where the two‐source plasmas intermix appreciably and the distributions become non‐Maxwellian.

Anisotropic magnetotail equilibrium and convection

This paper reports on self‐consistent two‐dimensional equilibria with anisotropic plasma pressure for the Earth's magnetotail. These configurations are obtained by numerically solving the generalized Grad‐Shafranov equation, describing anisotropic plasmas with p∥ ≠ p⟂, including the Earth's dipolar field. Consistency between these new equilibria and the assumption of steady‐state, sunward convection, described by the double‐adiabatic laws, is examined. As for the case of isotropic pressure [Erickson and Wolf, 1980], there exists a discrepancy between typical quiet‐time magnetic field models and the assumption of steady‐state double‐adiabatic lossless plasma sheet convection. However, unlike that case, this inconsistency cannot be removed by the presence of a weak equatorial normal magnetic field strength in the near tail region: magnetic field configurations of this type produce unreasonably large pressure anisotropies, p∥ > p⟂, in the plasma sheet.

Interaction of reflected ions with the firehose marginally stable current sheet: Implications for plasma sheet convection

The firehose marginally stable current sheet, which may model the flow away from the distant reconnection neutral line, assumes that the accelerated particles escape and never return to re‐encounter the current region. This assumption fails on the earthward side where the accelerated ions mirror in the geomagnetic dipole field and return to the current sheet at distances up to about 30RE down the tail. Two‐dimensional particle simulations are used to demonstrate that the reflected ions drive a “shock‐like” structure in which the incoming flow is decelerated and the Bz field is highly compressed. These effects are similar to those produced by adiabatic choking of steady convection. Possible implications of this interaction for the dynamics of the tail are considered.

A quasi‐static magnetospheric convection model in two dimensions

A self‐consistent, two‐dimensional model of subsonic, plasma sheet convection is presented. Specifically, time sequences of static equilibrium solutions for the two‐dimensional magnetospheric magnetic field are constructed consistent with adiabatic convection. This model self‐consistently includes a dipole field and a reasonable accounting for the effects of inner magnetospheric shielding. Starting from a relaxed magnetospheric equilibrium, the earthward convection of plasma sheet flux tubes results in the stretching of inner plasma sheet field lines, the development of a local minimum in the equatorial magnetic field Be in the near‐Earth plasma sheet, and an increasing lobe magnetic field. This evolution in time occurs generally, independent of the specific magnetopause or far‐tail boundary conditions, provided the plasma sheet is not at marginal interchange stability. This behavior results solely from the convection of flux tubes of increasing entropy into the near‐Earth plasma sheet. We discuss these results in the general context of earthward convection in Earth's plasma sheet and in the specific context of magnetospheric substorms.