Buoyancy Modes in a Low Entropy Bubble

Intensive correlative studies of magnetic fields and plasmas within the earth's magnetotail at geocentric radial distances of ∼23–46 RE during March-October 1974 have revealed striking new features. The instruments employed in this survey were the University of Iowa Lepedea and the Goddard Space Flight Center magnetometer. The hot tenuous plasmas within the plasma sheet were found to be in a state of almost continual flow and were threaded with northward, or closed, geomagnetic field lines. The magnetic field is referred to as ‘northward’ or ‘southward’ if the component Bz is positive or negative, respectively, regardless of the magnitudes of the other components. Proton bulk speeds were in the range 50–500 km s−1. The magnetic fields are directed northward, irrespective of whether the plasma flows are tailward or earthward. These observations show the demand for a strong persistent source of magnetic flux and hot plasmas for the plasma sheet. No characteristic proton bulk flows, e.g., strong dawn-to-dusk motions, were evident during crossings of the magnetic neutral sheet. Occasionally, the satellite encountered the region of acceleration in the magnetotail, the ‘fireball.’ This spectacular phenomenon exhibits strong jetting of plasmas in excess of 1000 km s−1, proton temperatures of ∼107 °K (kT ∼ 1 keV), disordered magnetic fields, southward magnetic fields during tailward jetting of plasmas, and northward magnetic fields for fast plasma flows toward earth. Observations of electrons are used to demonstrate that earthward plasma flows within the fireball are threaded with closed geomagnetic field lines, and open magnetic field lines are embedded in the tailward jetting plasmas. The magnetosheathlike plasmas within the boundary layers which are positioned contiguously to the plasma sheet display striking evidences of plasma heating, great changes in bulk flow velocities and acceleration of energetic electrons with E > 45 keV. Typical temperatures and bulk flow speeds within the boundary layer plasmas are ∼106 °K and ∼400 km s−1, respectively. Persistent zones of southward magnetic fields are detected, which are often positioned adjacent to the plasma sheet and within the boundary layer plasmas. Rotations of the magnetic fields from southward to northward, or vice versa, in these boundary layers are accompanied by large enhancements of energetic electron intensities, substantial heating of the low-energy electron distributions, and strong perturbations of the proton velocity distribution functions. During periods for northward directed fields the proton bulk flows decelerate to speeds of ≲200 km s−1, densities decrease, and the proton velocity distributions can be fitted with two Maxwellians with temperatures of ∼2 × 105 °K and ∼ 107 to 5 × 107 °K for specific examples. The boundary layer plasmas appear to be the primal source of hot plasmas and closed geomagnetic field lines for the plasma sheet. Qualitatively, all of the major results of the present study are consistent with macroscopic features which are expected from merging of magnetic field lines in the earth's magnetotail or, more specifically, in the magnetosheathlike boundary layer plasmas.

Stress balance and current closure in the magnetosphere are equivalent representations of the same condition. The stresses and currents in the magnetosphere that determine the steady-state convective flow are reviewed. A thinner plasma sheet implies a stronger earthward force (more-dipolar field lines) in the plasma sheet and hence a greater stress in the magnetosphere driving convective flow. Many of the observed properties of substorms can be explained by simply assuming that tail current is diverted along magnetic field lines and through the ionosphere in some local time sector without knowing the cause of the diversion. In particular: i) the current system is capable of controlling the ionospheric conductivity by precipitation associated with the current and the collapse of field lines to a more dipolar form, ii) increased ionospheric conductivity requires more-dipolar field lines near midnight in the plasma sheet, iii) the westward electrojet acts to limit the inflow, also resulting in more-dipolar field lines, iv) the current system causes a significant redistribution of plasma if tail current is due to proton drifts and field-aligned currents are carried by electrons, v) a twist must appear in flux tubes as the upward current increases. It is not clear whether the substorm energy comes from stored energy in the magnetotail, or whether it flows more or less directly from the solar wind due to an increased efficiency of the magnetosphere in extracting energy from the solar wind. The latter can occur because an increase in ionospheric conductivity leads to a distorted magnetosphere which takes more energy from the solar wind. Evidence for this is found in the thinning of the plasma sheet, in the substorm-associated motion of the dayside auroras, and in the correlation between the AE index and solar-wind parameters. Three possible macroinstabilities might be the cause of the current diversion and hence of the substorm. The first is the formation of an X-type neutral line, the second is magnetosphere-ionosphere coupling and the control of currents by precipitation (and vice versa) resulting in a feedback system, and the third is an interchange instability which is decoupled from the ionosphere by parallel electric fields. A mathematical development related predominantly to the second of the above macro instabilities gives satisfactory instability growth times and propagation velocities, and further shows that the system is normally near the instability threshold. The most likely cause of the onset of instability is an increase in electric field due to either an increase in cross-polarcap potential or a decrease in the width of the auroral oval. A model in which the macroinstability is due to ionospheremagnetosphere coupling and in which the energy flow is more or less directly from the solar wind probably gives the best fit to observations at this time.

We investigated a highly structured plasma sheet populated by counterstreaming beams observed by the Cluster and Double Star spacecraft on October 10, 2005, during an extended interval of northward interplanetary magnetic field (IMF). We used Wind spacecraft observations as input into our global magnetohydrodynamic (MHD) simulation of the event, and launched ions from the low‐latitude boundary layer, determined to be the source of the observed ions, in our large‐scale kinetic (LSK) particle tracing calculation that used the time‐dependent global fields from the MHD simulation. We found that a highly structured plasma sheet was formed in which the dawn and dusk halves were temporarily decoupled because of the presence of flux ropes in the center of the tail, possibly the result of localized near‐Earth reconnection. Our LSK simulation results showed good agreement with observed spectrograms and velocity distribution functions. On the dawn side, the ions launched in our simulation first formed a plasma sheet boundary layer (PSBL). Subsequent interactions of these ions with the current sheet scattered them and formed the central plasma sheet (CPS). On the dusk side, however, the topology of the magnetotail, including the formation of highly stretched field lines in the dusk flank and of a flux rope on open field lines in the center of the tail resulted in a structured plasma sheet, with counterstreaming beams present throughout the plasma sheet, including its center portion. This is very different from the classical picture of the PSBL in which layered counterstreaming beams exist only at the outer edges of the plasma sheet and do not penetrate the CPS.

Criticism of reconnection models of the magnetosphere

<p>A localized flux tube with reduced entropy (PV<sup>5/3</sup>, where P and V stand for the plasma pressure and flux tube volume, respectively) as compared to its neighbors is defined as a plasma-sheet bubble. Bubbles are susceptible to interchange instability. The interchange instability plays a key role in the transport of plasma from the magnetotail to the near-Earth region. A strong dawn-to-dusk electric field is formed inside the bubble which creates a shear flow. The <strong>E</strong>×<strong>B</strong> drift causes the bubble to grow earthward and the magnetic tension force drives it towards the equilibrium location where its total entropy matches the entropy of the surrounding area. According to the Vasyliunas equation, when the angle (α) between ∇V and ∇PV<sup>5/3 </sup>is either 0 (being parallel) or 180<sup>°</sup> (being antiparallel), the Birkeland current cannot be flown between the plasma sheet and the ionosphere. In this study, using the linear instability analysis we investigate the excitation and development of interchange modes in the low-beta plasma sheet (β<<1) when 0<α<180<sup>°</sup>. To this end, a boundary layer (with thickness δ) separating regions of higher and lower entropy is assumed in a small region of the ionosphere. The first-order electric potential (Φ) within this layer is numerically calculated based on time-sequence technique. The complete analytical solution for the temporal variation of Φ clearly shows that the unstable interchange modes with large wavelengths compared to the boundary layer (i.e., λ>>δ) can be generated.</p>

A simple theory of the magnetostatic equilibrium of the magnetospheric plasma sheet is proposed. It takes into consideration the effects of the large‐scale dawn‐to‐dusk electric field and developed medium‐scale turbulence. This makes it possible to describe the main features of the plasma sheet dynamics. The suggested model assumes that the magnetic field lines are not equipotential and that the regular velocity of a plasma parcel is much lower than the chaotic velocity. Hence the plasma is intensely mixed. One of the main consequences of such mixing is the experimentally observed temperature equalization across the plasma sheet. The obtained dependence of plasma pressure on the magnetic vector potential permits one to solve the Grad‐Shafranov equation and reconstruct, in the tail approximation, the magnetic field line configuration for the case of inhomogeneous distribution of the dawn‐to‐dusk electric field along the tail. The thickness of the plasma sheet in the model is determined by the magnitudes of the regular and stochastic electric fields, their scales, and plasma temperature. A quasi three‐dimensional model of the plasma sheet is developed. The substorm dynamics of the plasma sheet is analyzed. The model can explain the thinning of the plasma sheet without variations in the plasma pressure at the tail axis (thinning without compression) during the substorm growth phase. The plasma sheet expansion during substorm expansion phase is related to the growth of turbulent fluctuations.

On the remote sensing of plasma sheet from low-altitude spacecraft

Using a newly modified global magnetohydrodynamics (MHD) simulation model and test-particle method, we investigated the mechanism of plasma transfer into the near-tail plasma sheet under different interplanetary magnetic field (IMF) conditions (northward and southward). The results explain some well-known observations and clearly reveal the physical nature of the plasma transfer: particles mainly originate from the solar wind under a northward IMF and from the Earth under a southward IMF. We found solar wind particles transfer into the plasma sheet through dusk and dawn flanks under a northward IMF, and the transfer paths are consistent with merging sites on the magnetopause. When the IMF is southward, the solar-wind plasma transferring into the plasma sheet is mainly from the northern and southern sides. The average energy of the injected ions is much higher when the IMF is southward, which would result in a shorter stay-time in the plasma sheet. The tail terrestrial magnetic field lines are more open in the north-south direction when the IMF is southward, and this pattern makes it easier for terrestrial particles with higher energies to be ejected into the plasma sheet.

Considerable progress on the plasma sheet dynamics during magnetospheric substorms has been made on the basis of comprehensive studies of magnetic field and plasma flow observations in the magnetotail. Among the new findings are (1) the realization that rapid plasma flow in the plasma sheet is largely field-aligned flow rather than convective \( \mathop {\rm{E}}\limits^ \to {\rm{ x }}\mathop {\rm{B}}\limits^ \to \) drift, (2) the determination of synoptic plasma flow patterns earthward of X ≃ −30 Re at thinning and recovery of the plasma sheet during substorms, and (3) the result that there is no large-scale Bz reversal at the neutral sheet within the lunar distance in the magnetotail during substorm expansion. Assessment of observations indicates that both large-scale and localized phenomena occur in the plasma sheet during substorms. The phenomenological model which invokes a large-scale X-type magnetic neutral line at a downstream distance between 10 and 25 Re during sub storm expansion does not describe the most common substorm behavior of the magnetic field and plasma flow in the plasma sheet. Possible alternatives are the tearing mode model, which invokes multiple localized neutral lines, and the current diversion model. These two alternative models represent basically two different views on the primary substorm process. The tearing mode model expresses the view that localized phenomena are intrinsic to the primary substorm process and large-scale phenomena are consequences of localized activities, whereas the current diversion model regards large-scale phenomena as the intrinsic features and localized phenomena as secondary features which may not be directly related to the primary substorm process.KeywordsPlasma FlowPlasma SheetNeutral LineSubstorm OnsetNeutral SheetThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The linear MHD (magnetohydrodynamic) stability for high beta plasmas in the inward shifted Large Helical Device (LHD) configurations has been investigated for a wide range of magnetic Reynolds numbers $S$ using numerical simulations based on the kinetic MHD model with kinetic thermal ions where the beta is the ratio of the plasma pressure to the magnetic pressure. It is found that the dependence of the linear growth rate of the resistive ballooning modes on the $S$ number changes from $\unicode[STIX]{x1D6FE}\propto S^{-1/3}$ to $\unicode[STIX]{x1D6FE}\propto S^{-1}$ by the kinetic thermal ion effects so that the resistive ballooning modes are significantly suppressed as the $S$ number increases. For a high $S$ number comparable to experimental values, the most unstable modes are interchange modes. The kinetic thermal ion effects change the most unstable interchange mode from the ideal mode to the resistive mode. This transition of the interchange modes by kinetic thermal ion effects is consistent with the shift of the marginal stability boundary for the ideal interchange modes observed in the LHD experiments.

Jupiter has a strong magnetic field, and a huge magnetosphere is formed through the solar wind‐Jupiter interaction. The generated magnetosphere–ionosphere system is reproduced based on the 9‐component Magnetohydrodynamics (MHD) and the current conservation in the ionosphere. Assuming Io plasma emission rate 1.4 t/sec, this paper reproduces self‐consistently global magnetic configuration, generations of the field‐aligned current (FAC) and aurora, formation of the Io plasma disk at 8–20 RJ, plasma corotation, instability in the plasma disk, transition from the Io plasma disk to the plasma sheet at 20–150 RJ, and the plasmoid ejection. The rotating Io plasma in the disk forms instabilities that promotes radial diffusion. H+ is supplied from the ionosphere along high‐latitude magnetic field lines and mixed with heavy ions around 15–20 RJ. Beyond 20 RJ, mixed plasma diffuses further outward by the centrifugal force that can exceed magnetic tension. In the ionosphere, the main oval occurs at 13.7°–15.5° colatitude. The Io disk is inner side of magnetic field lines traced from the low‐latitude edge of the main oval. Along magnetic field lines, the main oval is mapped from the outer edge of the Io disk to the entire plasma sheet accompanying rotation delay. Due to the corotation limit, convection is accompanied by plasmoid ejection. Back reaction of plasmoid ejection affects even transport process in the Io disk. The downward FAC occurs in the polar cap showing variability. The region of externally driven Dungey convection seems quite narrow.

This paper attempts to summarize the current understanding of the storm/substorm relationship by clearing up a considerable amount of controversy and by addressing the question of how solar wind energy is deposited into and is dissipated in the constituent elements that are critical to magnetospheric and ionospheric processes during magnetic storms. (1) Four mechanisms are identified and discussed as the primary causes of enhanced electric fields in the interplanetary medium responsible for geomagnetic storms. It is pointed out that in reality, these four mechanisms, which are not mutually exclusive, but interdependent, interact differently from event to event. Interplanetary coronal mass ejections (ICMEs) and corotating interaction regions (CIRs) are found to be the primary phenomena responsible for the main phase of geomagnetic storms. The other two mechanisms, i.e., HILDCAA (high‐intensity, long‐duration, continuous auroral electrojet activity) and the so‐called Russell‐McPherron effect, work to make the ICME and CIR phenomena more geoeffective. The solar cycle dependence of the various sources in creating magnetic storms has yet to be quantitatively understood. (2) A serious controversy exists as to whether the successive occurrence of intense substorms plays a direct role in the energization of ring current particles or whether the enhanced electric field associated with southward IMF enhances the effect of substorm expansions. While most of the Dst variance during magnetic storms can be solely reproduced by changes in the large‐scale electric field in the solar wind and the residuals are uncorrelated with substorms, recent satellite observations of the ring current constituents during the main phase of magnetic storms show the importance of ionospheric ions. This implies that ionospheric ions, which are associated with the frequent occurrence of intense substorms, are accelerated upward along magnetic field lines, contributing to the energy density of the storm‐time ring current. An apparently new controversy regarding the relative importance of the two processes is thus created. It is important to identify the role of substorm occurrence in the large‐scale enhancement of magnetospheric convection driven by solar wind electric fields. (3) Numerical schemes for predicting geomagnetic activity indices on the basis of solar/solar wind/interplanetary magnetic field parameters continue to be upgraded, ensuring reliable techniques for forecasting magnetic storms under real‐time conditions. There is a need to evaluate the prediction capability of geomagnetic indices on the basis of physical processes that occur during storm time substorms. (4) It is crucial to differentiate between storms and nonstorm time substorms in terms of energy transfer/conversion processes, i.e., mechanical energy from the solar wind, electromagnetic energy in the magnetotail, and again, mechanical energy of particles in the plasma sheet, ring current, and aurora. To help answer the question of the role of substorms in energizing ring current particles, it is crucial to find efficient magnetospheric processes that heat ions up to some minimal energies so that they can have an effect on the strength of the storm time ring current. (5) The question of whether the Dst index is an accurate and effective measure of the storm time ring current is also controversial. In particular, it is demonstrated that the dipolarization effect associated with substorm expansion acts to reduce the Dst magnitude, even though the ring current may still be growing.

We have studied the three‐dimensional structure of substorm‐associated variations in the magnetotail with GEOTAIL data. For this study we selected 342 substorm events from the Pi2 pulsation and applied the method of superposed epoch analysis. We divided the data into those in the plasma sheet, the plasma sheet boundary layer, and the lobe by the ion β. It was found that the fast tailward flows start to develop in the premidnight plasma sheet around X ∼ −28 RE (GSM) about 0 ∼ 2 min before onset, associated with the plasmoid formation. Immediately after onset, the fast tailward flows develop further, and the magnetic field substantially increases southward. Simultaneously, the northward magnetic field increases around X ∼ −10 RE, corresponding to the dipolarization. In the lobe, the perpendicular plasma flow toward the plasma sheet, as well as the dawn‐dusk electric field, first starts to be enhanced around (X, Y) ∼ (−20,7) RE about O ∼ 2 min before onset and then in the surrounding regions successively. The total pressure decrease first occurs around (X, Y) ∼ (−18,7) RE about 0 ∼ 2 min before onset, and then propagates to the surrounding regions successively. The dawn‐dusk electric fields both calculated with E = −V × B and measured directly by the double probe, simultaneously develop around X ∼ −10 RE and X ∼ −28 RE immediately after onset, while those in the plasma sheet around.(X, Y) ∼ (−20,5) RE do not develop much even after onset. These observational results strongly suggest that an efficient magnetic reconnection takes place at least about 0 ∼ 2 min earlier than the Pi2 onset, that the substantial plasmoid evolution and the dipolarization occur simultaneously immediately after onset, and that, on average, the center of the energy release, where the near‐Earth neutral line (i.e., the diffusion region) is possibly created, is initially located around (X, Y) ∼ (−19,6) RE. These features are consistent with a thin‐current reconnection model.

Plasmoids are closed magnetic‐loop structures with entrained hot plasma which are inferred to occur on large spatial scales in space plasma systems. In this paper we present a small‐scale plasmoid interpretation of one of the magnetotail barium releases which occurred during the Active Magnetospheric Particle Tracer Explorers (AMPTE) program. Using information for the May 13, 1985, event, it is shown that the AMPTE barium release at 0517 UT occurred on closed magnetic field lines of the thinned plasma sheet. A magnetospheric substorm sequence was in progress at this time. The barium rapidly ionized and presumably heated over the course of the next half hour, loading the plasma sheet flux tubes locally. Direct ground‐based imaging, Doppler, and Fabry‐Perot triangulation data show that ∼33 min after the release, the Ba+ cloud brightened and moved off down the tail (antisunward) at high speed (≥200 km/s). We offer an explanation in terms of a two‐step process in which the Ba+ ions expanding along the plasma sheet field lines cause a stretching of the field due to a mass loading effect. Then, we suggest the development of an enhanced ion tearing mode due to the local presence of the heavy (Ba+) ions. The result of these processes would be the formation of a small plasmoid structure in the thin, residual plasma sheet late in the substorm sequence. As is the case for larger plasmoids, the small‐scale plasmoid would move down the tail at high speed impelled by tensional forces in the open field lines enveloping it. These results thus suggest that a plasmoid was directly observed in the AMPTE experiment by means of the optical emissions of entrained tracer ions contained in the plasma bubble.

The plasma sheet horns are the low‐altitude extensions of the plasma sheet that lie poleward of the plasmasphere and equatorward of the tail lobes. Within the horns, magnetic field lines of increasing geomagnetic latitudes map to increasing distances into the downtail plasma sheet. Plasma data from the fast plasma experiment on ISEE 2 have been analyzed for 11 outbound crossings of the horns in the premidnight sector of the magnetosphere at typical altitudes of ∼2–4 RE. These crossings typically occurred on time scales of less than 1 hour, providing almost instantaneous snapshots of plasma gradients within the horns. Ion temperatures observed during these crossings generally decreased by a factor of 4–8 as magnetic field lines of increasing geomagnetic latitude were traversed. If we make the reasonable assumption that the ion temperatures are constant along the field lines within the plasma sheet, then this result implies that the ion temperatures in the downtail plasma sheet also commonly decrease by this same factor over the radial range extending from the inner edge of the plasma sheet at the plasmapause boundary to the outer edge at a neutral line in the distant tail.