Plasma Sheet Boundary Layer Ion Research Articles

Ion beta dependence on the development of Alfvénic fluctuations in reconnection jets

AbstractThe generation of magneto‐hydro‐dynamic (MHD) to ion‐scale fluctuations in collisionless magnetic reconnection is discussed using a two‐dimensional electromagnetic hybrid code. It is shown that reconnection jets become turbulent specifically in low beta conditions, βi0<0.1–0.2 (where βi0 is the ion plasma beta in initial inflow regions). The fluctuations observed in reconnection jets consist of outgoing Alfvénic fluctuations. As probable candidates for the origin of Alfvénic fluctuations, this study focused on the dynamics in the plasma sheet boundary layer (PSBL) and a current sheet. We suggest that PSBL ion dynamics play an important part in excitation and suppression of waves. PSBL beam ions drive Alfvén waves in MHD to ion scale, kλi<0.5 (λi is ion inertial length), independent of βi0. On the other hand, because the beam temperature is highly correlated with that of inflowing ions, the waves decay by cyclotron damping as the value of the inflow ion beta increases. Local linear analysis suggests that this damping signature changes in βi0∼0.1–0.2 and suppresses the wave activity of Alfvén modes in high beta reconnection jets.

Emergence of the active magnetotail plasma sheet boundary from transient, localized ion acceleration

Observations of the Earth's magnetotail plasma sheet boundary layer (PSBL) have been typically accompanied by field‐aligned crescent‐shaped ion beams, thought to emanate at distant or mid‐tail semi‐permanent or impulsive acceleration sites. Typically such observations, and the theoretical and modeling efforts to explain them, have been disjoint from the adjacent plasma sheet properties near the equatorial projection of the observation. Thus the plasma sheet boundary layer has been thought of as a harbinger of remote, rather than local plasma sheet activity, exception of plasma sheet expansions during the recovery phase of substorms. Using case and statistical studies from THEMIS, obtained simultaneously at the near‐Earth PSBL and at its adjacent central plasma sheet (CPS), we study the transient and impulsive nature of PSBL beams and their inherent connection with CPS bursty bulk flows and associated dipolarization fronts. We show that PSBL beams typically commence a few minutes before CPS flow bursts, which in turn are seen tens of seconds ahead of the arrival of dipolarization fronts. These timing correlations, the crescent shapes of PSBL ion beams, the CPS ion flux enhancements in the earthward and dawnward directions, and other particle distribution characteristics can all be well reproduced by a simple model of ion reflection and acceleration at earthward‐propagating dipolarization fronts associated with CPS flow bursts. The emerging paradigm, therefore, unifies impulsive transport phenomena across latitudes in the near‐Earth magnetotail plasma sheet.

The relation between ion temperature anisotropy and formation of slow shocks in collisionless magnetic reconnection

We perform a two‐dimensional simulation by using an electromagnetic hybrid code to study the formation of slow‐mode shocks in collisionless magnetic reconnection in low beta plasmas, and we focus on the relation between the formation of slow shocks and the ion temperature anisotropy enhanced at the shock downstream region. It is known that as magnetic reconnection develops, the parallel temperature along the magnetic field becomes large in association with the anisotropic plasma sheet boundary layer ion beams, and this temperature anisotropy has a tendency to suppress the formation of slow shocks. On the basis of our simulation result, we found that the slow shock formation is suppressed due to the large temperature anisotropy near the X‐type region, but the ion temperature anisotropy relaxes with increasing the distance from the magnetic neutral point. As a result, two pairs of current structures, which are the strong evidence of dissipation of magnetic field in slow shocks, are formed at the distance ∣x∣ ≥ 115 λi from the neutral point.

“Geography” of ion acceleration in the magnetotail: X‐line versus current sheet effects

Field‐aligned ion beams are often observed in the Plasma Sheet Boundary Layer (PSBL) of Earth's magnetotail. We studied two types of ion beams observed in the magnetotail at different times. Both types of ion beams are pitch angle‐collimated along the magnetic field direction but they differ significantly with the width of ion velocity distribution functions in parallel energies. The first type represents energy‐collimated ion beams with observed durations of 3–23 min. The majority of such beams have energies ≤20 keV. Together with such ion beams, isotropic electron distributions are observed. We suggest that such ion beams are accelerated at spatially localized regions located rather far from the distant X‐line where magnetic field lines are already closed. Another type of PSBL ion distributions represents wide in parallel energy ion beams with typical energies ≥ tens of keV. The registered durations of such beams are 1–6 min. They are observed together with anisotropic electron velocity distributions formed by the cold and hot counter‐streaming components. This feature is peculiar for the magnetic separatrix and indicates that the spacecraft crosses still open or very recently closed magnetic field lines. We have analyzed 987 crossings of the high‐latitude edge of the PSBL by Geotail at −220 RE < X < −20 RE. The majority of Type‐I ion beams are moving earthward at X ≥ −110 RE and so have their sources located earthward from the distant X‐line (at X < −110 RE). These ion beams were observed during quiet geomagnetic periods. More than 50% of Type‐II ion beams move tailward at X ≤ −50 RE. Thus their acceleration sources are located closer to Earth. Type‐II ion beams were observed during both quiet and disturbed periods. The energy distribution of the first type of ion beams along the dawn‐dusk direction approximately conforms to the assumption about their non‐adiabatic acceleration by the quasi‐steady dawn‐dusk electric field. On the other hand, we suggest that ion beams of the second type are generated in close proximity to the X‐line. The energy distribution of such ion beams along the Y direction indicates ion energization by a strong (presumably inductive) electric field, especially in cases of ion acceleration tailward from the X‐line.

Energy-dispersed ions in the plasma sheet boundary layer and associated phenomena: Ion heating, electron acceleration, Alfvén waves, broadband waves, perpendicular electric field spikes, and auroral emissions

Abstract. Recent Cluster studies reported properties of multiple energy-dispersed ion structures in the plasma sheet boundary layer (PSBL) that showed substructure with several well separated ion beamlets, covering energies from 3 keV up to 100 keV (Keiling et al., 2004a, b). Here we report observations from two PSBL crossings, which show a number of identified one-to-one correlations between this beamlet substructure and several plasma-field characteristics: (a) bimodal ion conics (<1 keV), (b) field-aligned electron flow (<1 keV), (c) perpendicular electric field spikes (~20 mV/m), (d) broadband electrostatic ELF wave packets (<12.5 Hz), and (e) enhanced broadband electromagnetic waves (<4 kHz). The one-to-one correlations strongly suggest that these phenomena were energetically driven by the ion beamlets, also noting that the energy flux of the ion beamlets was 1–2 orders of magnitude larger than, for example, the energy flux of the ion outflow. In addition, several more loosely associated correspondences were observed within the extended region containing the beamlets: (f) electrostatic waves (BEN) (up to 4 kHz), (g) traveling and standing ULF Alfvén waves, (h) field-aligned currents (FAC), and (i) auroral emissions on conjugate magnetic field lines. Possible generation scenarios for these phenomena are discussed. In conclusion, it is argued that the free energy of magnetotail ion beamlets drove a variety of phenomena and that the spatial fine structure of the beamlets dictated the locations of where some of these phenomena occurred. This emphasizes the notion that PSBL ion beams are important for magnetosphere-ionosphere coupling. However, it is also shown that the dissipation of electromagnetic energy flux (at altitudes below Cluster) of the simultaneously occurring Alfvén waves and FAC was larger (FAC being the largest) than the dissipation of beam kinetic energy flux, and thus these two energy carriers contributed more to the energy transport on PSBL field lines from the distant magnetotail to the ionosphere than the ion beams.

A stochastic sea: The source of plasma sheet boundary layer ion structures observed by Cluster

On 14 February 2001 the Cluster Ion Spectrometry (CIS) experiment onboard three of the Cluster spacecraft observed velocity‐dispersed ion structures (VDIS) as the spacecraft passed from the tail lobes into the plasma sheet boundary layer. These are the first multiple spacecraft observations of the VDIS phenomenon. The Cluster 1 spacecraft (SC1) observed a dispersed ion signature with beamlets and a second structure like that expected to be produced by an echo, while Cluster 3 (SC3) observed much less pronounced structuring a few minutes later. During this same event and over an extended interval the ACE spacecraft observed an interplanetary magnetic field that was directed southward. We have inferred the sources and acceleration mechanisms of the ions in these VDIS observations by following millions of ion trajectories backward and forward in time through time‐dependent electric and magnetic fields obtained from a global MHD simulation. ACE data were used as input for the MHD model. We found that almost all of the particles comprising the first (A1) and second (A2) beamlets observed by SC1 had been nonadiabatic earlier in their history, while particles in the A3 beamlet exhibited a combination of adiabatic and nonadiabatic behavior. Beamlet A4 particles were always adiabatic. Moreover, for all of the beamlets the current sheet crossing that took place prior to their detection occurred between x = −13 RE and x = −16 RE in the tail, well earthward of the permanent stochastic “sea” from which all of the beamlets originated. Our model does not favor the multiple source scenario suggested by A. Keiling et al. Instead, it indicates that the source regions of the structures are spatially correlated. We have carried out a similar analysis of the SC3 observations. In general, SC3 beamlets have higher κ values, partly because of the depolarization of the field lines during these observations. In time forward calculations only a small fraction of ions from SC1 A structures returned to the spacecraft location. “Echoes” were more pronounced on SC3. In addition, in our calculations, some particles from SC1 A structures interacted with the current sheet and returned to the SC3 location, at the time when SC3 observed the A structures. When Cluster observations were organized by latitude instead of time, we found that all three Cluster spacecraft seemed to observe the same primary structure that persisted throughout the interval of observation.

Two types of PSBL ion beam observed by Geotail: Their relation to low frequency electromagnetic waves and cold ion energization

Two types of the plasma sheet boundary layer (PSBL) crossing during a geomagnetically active period are analyzed in detail to investigate relationship between the ion beams, low-frequency (0.01–2 Hz) electromagnetic waves, and cold ion energization in PSBL using the Geotail data. In response to changes in the solar wind conditions and auroral activity inferred from the AL index, the properties of PSBL ions changed dramatically: During the less disturbed period, we observed counter-streaming thermalized beams with no significant wave activity in the low-frequency range. The thermalized beams may be generated from the distant magnetotail and have undergone thermalization before reaching the Geotail position of X ∼ −30 R E. During the periods of enhanced auroral activity with southward IMF, collimated freshly injected earthward beams are observed together with the large-amplitude electromagnetic waves having a peak around 0.025 Hz (∼0.1 Ω ci). Cold ions are energized during the periods of large-amplitude low-frequency fluctuations. It is suggested that fresh beams from the near-Earth neutral line (NENL) play an important role in the cold ion energization.

Nature and location of the source of plasma sheet boundary layer ion beams

Onsager et al. (1991) have put forward a model of the formation of the plasma sheet boundary layer which relies on a steady source of plasma from a spatially extended plasma sheet, together with steady equatorward and earthward E×B convection of field lines due to reconnection at a downtail neutral line. This model is a synthesis of earlier proposals and it explains such features as an electron layer exterior to the ion boundary layer, ion velocity dispersion, counter streaming beams, low‐speed cutoffs in the beams. It also explains the apparent evolution of the ion beams through “kidney bean” shaped velocity‐space distributions toward quasi‐isotropic shells without invoking pitch angle scattering or energy diffusion. In this paper we explore two ramifications of the model: (1) low‐speed cutoffs observed in the ion distribution functions contain information about the distance to the neutral line downtail and (2) values of phase space density at the low‐speed cutoffs are the same as those in the distribution just earthward of the reconnection site. In principle we can map, as a function of time, the downtail neutral line distance and establish whether or not it is retreating during substorm recovery. We can also reconstruct the plasma distribution function near the neutral line to see if it is most consistent with mantle or plasma sheet plasma. We perform this analysis using ISEE Fast Plasma Experiment data for two plasma sheet recovery events, one on March 1, 1978, and the other on April 18, 1978. On March 1, 1978, we find evidence for an initial retreat from around 110 to 160 RE in the first 15 min; little further retreat occurs thereafter. On April 18, 1978, the neutral line location ranges from as little as 40 RE tailward of the satellite to as much as 200 RE, but there is no evidence for a systematic retreat. The reconstructed ion distributions for these events are most consistent with a plasma sheet origin for the March 1 case and possibly plasma mantle or low‐latitude boundary layer for the April 18 case.

Propagation of whistler waves driven by fine structured ion beams in the magnetotail

In a previous paper, which examined the propagation of low‐frequency whistler waves generated by ion beams in the Earth's plasma sheet boundary layer (PSBL), it was found that whistler waves driven in the PSBL are focused toward the central plasma sheet due to the global magnetotail inhomogeneities; this finding may help explain the observations of magnetic noise bursts in the tail (Burinskaya et al., 1993). In this paper the same phenomenon is examined, but this time a much more realistic model is used for the ion beams in the PSBL. While the PSBL has been modeled as a solid, homogeneous ion beam with a width of one Earth radius, observations and theoretical considerations have shown that PSBL ion beams actually have a decreasing velocity profile toward the plasma sheet and that the density of the beams within the PSBL can vary locally. We consider again the propagation and generation of electromagnetic waves but in the presence of fine structured ion beams in the PSBL. Our results show that whistler waves, generated quasi‐parallel to the background magnetic field, can be trapped locally within small spatial regions where the ion beam density is enhanced compared to the density of the adjacent PSBL region. Wave spectra and nonlinear saturation mechanisms are discussed.

Ion mixing in the plasma sheet boundary layer by drift instabilities

The linear stability properties of collisionless drift instabilities are analyzed in a Harris equilibrium model B0x tanh (z/L) of the plasma sheet boundary layer (PSBL). The streaming ions with drift type instabilities driven by υ0∥(z) ≃ υ(z) in the PSBL are considered. The fluid approximation leads to growth from ∂T/∂z and ∂υ0∥/∂z but predicts that the mode width Δz approaches the gyroradius ρi of the energetic ions. Thus an integral equation theory for the modes is developed taking into account that in the PSBL the curvature drift is weak compared with the grad‐B drift υD B. The exact wave particle resonance ω = kx υx + ky υD B (υ⊥²) is kept in the nonlocal response functions. Plasma density, temperature, and magnetic gradient drift motions are taken into account. The drift modes driven by ∂υx/∂z produce an anomalous cross‐field momentum transport mixing the PSBL ions on the time scale of tens of seconds. A nonlinear simulation is performed which shows the coalescence of the small scale, fast growing modes into large‐scale vortices. The relation between these collective modes and plasma sheet transport phenomena is discussed including the comparison with the competing plasma mixing from single‐particle stochasticity.

Ion diffusion coefficients from resonant interactions with broadband turbulence in the magnetotail

A potentially important source of hot ions within the central plasma sheet (CPS) is warm plasma sheet boundary layer (PSBL) ions which interact with broadband electrostatic waves and electric field drift toward the CPS. In the PSBL, the local interaction between the waves and particles can be assumed to be unmagnetized. The effect of the magnetic field is to organize wave and particle distributions in velocity space. Under these conditions, particle diffusion is, in general, two‐dimensional and is similar to magnetized diffusion. The general quasi‐linear equations describing such diffusion are presented assuming that a spectrum of waves is excited similar to the waves observed in the boundary layer. In order to apply the general quasi‐linear diffusion coefficients, two particle distribution models are assumed based on PSBL observations. One is for the outer edge and one is for the inner edge of the boundary. An expression for the unmagnetized dielectric function is given and evaluated for wave frequency and growth rate for the assumed particle distribution models. It is found that slow and fast mode ion‐sound waves can be unstable for the range of plasma parameters considered. The diffusion coefficients are then evaluated for resonant warm PSBL ion interactions with ion‐sound waves. The results illustrate how resonant ion diffusion rates vary with pitch angle and speed, and how the diffusion rates depend upon the distribution of wave energy in k space. For the model of the outer edge of the PSBL, pitch angle diffusion is found to dominate energy diffusion, whereas both types of diffusion can be important for the model of the inner edge of the PSBL. It is found that the characteristic time for resonant warm boundary layer ions to diffuse in velocity space is ∼10 min (for a wave electric field of 1 mV/m), which is approximately an ion bounce period. In addition, significant pitch angle and energy diffusion should occur resulting in isotropization of the warm PSBL ion beams to form the hot isotropic ion component in the CPS.

Convective growth of broadband turbulence in the plasma sheet boundary layer

Convective growth of slow and fast beam acoustic waves in the plasma sheet boundary layer (PSBL) is investigated. It has been shown previously that a cold ion population must be present in order to excite beam acoustic waves in the PSBL. However, growth rates are significantly enhanced when warm plasma sheet boundary layer ions are present. Net wave growth along a ray path is determined by convective growth. This quantity is calculated for particle distribution models consistent with the PSBL where the intensity of broadband turbulence is observed to peak. Total number density dependence on beam acoustic convective growth is evaluated, and it is found that even for low density conditions of ∼0.01 cm−3, a measurable level of broadband turbulence is expected. Relative drift effects between cold and warm ion populations are also considered. In particular, it is found that slow mode convective growth can be enhanced when slowly streaming cold ions are present, compared to fast ion streams.

Processes in the Magnetotail Neutral Sheet

This paper reviews neutral sheet observations and theories emphasizing particle acceleration or "inertial dissipation". The existence of the central current sheet and cross-tail electric field in the magnetotail neutral sheet implies an energy dissipation which is roughly a few percent of the incident solar wind power on the entire magnetosphere. This is an important - if not the dominant - energy dissipation in the magnetosphere. Earthward streaming ions in the plasma sheet boundary layer can result from simple single particle acceleration in the current sheet and these beams carry sufficient energy to account for this dissipation. Although stochastic processes need not be invoked, "inertial conductivity" is inferred from the time the particles spend undergoing acceleration within the current sheet. The particle motion thus leads to simple consistency "equilibrium states" and the motion has been used as the basis of an explosive reconnection model by Coroniti. Finally, observations and theories of broadband electrostatic noise, which may also provide dissipation, are reviewed. Particle distributions which result from neutral sheet energization will likely be unstable to the growth of plasma waves. Broadband electrostatic noise is the most intense and frequently occurring plasma wave mode in the magnetotail and the intensity is observed to peak in the plasma sheet boundary layer. It is thought to be excited by the ion beam acoustic instability. Resonant heating of plasma sheet boundary layer ions may be responsible for the hot ion component of the central plasma sheet.

The generation of electrostatic noise in the plasma sheet boundary layer

The one and two ion beam instability is considered as a possible explanation for the observations of broadband electrostatic noise in the plasma sheet region of the geomagnetic tail. When only hot streaming plasma sheet boundary layer ions are present, no broadband waves are excited. Cold, streaming ionospheric ions can generate electrostatic broadband waves propagating in the slow beam acoustic mode, but the growth rates of the waves are significantly enhanced when hot boundary layer ions are present. (Both the slow and fast beam acoustic modes can be excited, depending on the relative ion drift.) This model predicts that the wave intensity of the broad band noise should peak in the plasma sheet boundary layer. Observations of less intense electrostatic waves in the lobes and plasma sheet are likely a result of the absence of hot ion beams or large ion temperatures, respectively, which result in smaller growth rates. The ion beam instability may play an important role in the formation of the central plasma sheet.