Low-latitude Boundary Layer Research Articles

Quasiturbulent transport and LLBL properties

The results of experimental observations select two regions in the magnetospheric turbulent wake: plasma sheet and low latitude boundary layer (LLBL). Large-scale properties of LLBL are analyzed taking into account regular and turbulent plasma transport in Y direction and the high level of the observed turbulence. The application of the developed approach to the problem of the boundary layer formation gives the possibility to suggest the qualitative explanation of LLBL thickness during southward and northward interplanetary magnetic field (IMF) orientations and the dawn–dusk LLBL asymmetry.

Influence of the solar wind plasma density on the auroral precipitation characteristics

Based on the DMSP F6 and F7 satellite observations, the characteristics of precipitating particles in different auroral precipitation regions of the dayside sector have been studied depending on the solar wind plasma density. Under quiet geomagnetic conditions (|AL| < 100 nT and B z > 0), a considerable increase in the fluxes of precipitating ions is observed in the zones of structured auroral oval precipitation (AOP) and soft diffuse precipitation (SDP). A decrease in the mean energy of precipitating ions is observed simultaneously with the flux growth in these regions. The global pattern of variations in the fluxes of precipitating ions, which shows the regions of effective penetration of solar wind particles into the magnetosphere at a change in the solar wind density from 2 to 20 cm−3, has been constructed. The maximal flux variation (ΔJ i = 1.8 · 107 cm−2 s−1, i.e., 3.5% of an increase in the solar wind particle flux) is observed in the SDP region on the dayside of the Earth. The dependence of precipitating ion fluxes in the low-latitude boundary layer (LLBL), dayside polar cusp, and mantle on the solar wind density at positive and negative values of the IMF B z component has been studied. In the cusp region, an increase in the precipitating ion flux is approximately 17% of an increase in the solar wind density. The IMF southward turning does not result in an appreciable increase in the ion precipitation fluxes either in the cusp or in the mantle. This fact can indicate that the reconnection of the geomagnetic field with southward IMF is not the most effective mechanism for penetration of solar wind particles into these regions.

Spectral properties and source regions of dayside electron acceleration events

Eleven years of DMSP particle data (1984–1994) were used to investigate electron acceleration events on the dayside. Specifically, the characteristic energy (spectral flux peak), total energy flux, and probability of observing acceleration events were studied as a function of the type of dayside precipitation region (cusp, mantle, etc.). Most dayside electron acceleration events are latitudinally smaller than the events found premidnight, so it is usually possible to identify the type of precipitation which surrounds dayside events. All dayside regions contain embedded electron acceleration events, with such instances most common in the dayside boundary plasma sheet (BPS) and least common in the mantle and polar rain. Electron acceleration events in the cusp are not particularly rare, but the acceleration involved is quite mild. This is consistent with findings that large nightside potentials do not form in regions with a large ambient electron population. On the basis of the fraction of spectra which are accelerated (number of accelerated spectra divided by the total number of spectra measured), the dayside regions can be ordered as follows, with the acceleration probability in parenthesis: mantle prenoon (2.4%), mantle postnoon (3.0%), cusp (4.2%), open low‐latitude boundary layer (LLBL) (5.0%), closed LLBL prenoon (6.0%), closed LLBL postnoon (7.2%), BPS prenoon (8.2%), and finally BPS postnoon (8.6%). The total precipitating energy flux from electron acceleration has the same order; thus more dayside auroral luminosity is associated with the BPS than with any other region. When these regions are ordered by the spectral flux peak in acceleration events, the order changes, as follows: cusp (201 eV), open LLBL (234 eV), closed LLBL prenoon (390 eV), mantle prenoon (414 eV), closed LLBL postnoon (459 eV), mantle prenoon (463 eV), BPS prenoon (572 eV), and BPS postnoon (725 eV). The afternoon hot spot is found to have a relatively symmetric partner, a morningside “warm spot,” centered near 0830 MLT. Both the afternoon hot spot and the morning warm spot span the convection reversal boundary, since they include both BPS and LLBL precipitation, with the former region showing the most intense activity (both the highest number of events and the largest electron acceleration energies).

Formation of the flank LLBL: A case study

A formation of the low-latitude boundary layer (LLBL) on open field lines during intervals of southward oriented interplanetary magnetic field is now understood in terms of dayside reconnection. However, observations of counterstreaming particles suggest that the LLBL can exist on closed field lines over some portion of the magnetopause. The fluctuations of plasma parameters inside a layer are attributed either to a transient nature of the processes forming a layer or to sweeping of deformations of the magnetopause or inner edge of the LLBL surfaces along the spacecraft. The INTERBALL-1 and MAGION-4 crossed the LLBL near the terminator during intervals of disturbed solar wind conditions. Two-point observations allow us to distinguish the temporal and spatial changes, to estimate the LLBL thickness and its variations and to find the sources of observed plasma populations. Observations suggest that the flank LLBL is supplied by high-latitude reconnection near the cusp and that it lies on open field lines.

Reconnection at the dayside low‐latitude magnetopause and its nonrole in low‐latitude boundary layer formation during northward interplanetary magnetic field

On 2001‐12‐02 Wind crossed the dayside magnetopause (MP) at ∼15 MLT and traversed the adjacent low‐latitude boundary layer (LLBL) over a period of 2 hours. The IMF was steady (northward and dawnward) during the MP/LLBL encounter. Reconnection flows were observed in the MP that were directed 130° away from the magnetosheath flow direction. In contrast, the LLBL flow was aligned with the magnetosheath flow. The counterstreaming field‐aligned and anti‐field‐aligned electrons have different energies and their fluxes are unbalanced in the open MP whereas they are precisely balanced throughout most of the LLBL indicative of a closed LLBL. These observations indicate that reconnection occurs at the low‐latitude MP during northward IMF (with a significant By), but low‐latitude reconnection is not responsible for the creation of the LLBL. Instead, reconnection appears to be in the process of eroding a pre‐existing LLBL that was created either by diffusive entry or by non‐simultaneous double‐cusp reconnection.

Plasma sheet and magnetosheath plasma mixing in LLBL: Case study

Low latitude boundary layer (LLBL) of the magnetosphere of the Earth is formed due to the mixing of magnetosheath and plasma sheet plasma. The mixing process is studied on the base of Interball/Tail probe observations. The variations of the fluxes of ions (measured by CORALL instrument), electrons (measured by ELECTRON instrument) and particles with energies more than 25 keV (measured by DOK-2 instrument) are analyzed for the case of 21 September 1995 when the satellite intersects LLBL/plasma sheet boundary on the GSM (−4.1, −13.6, 2.2) R e distance. The variations of the magnetic field measured by MIF instrument are compared with the variations of particle fluxes. The thickness of the boundary region containing plasma from LLBL is estimated. The stress balance across the analyzed boundary is discussed. We find that the plasma pressure inside the LLBL is essential in understanding the pressure balance across the magnetopause.

Magnetosheath waves under very low solar wind dynamic pressure: Wind/Geotail observations

Abstract. The expanded bow shock on and around "the day the solar wind almost disappeared" (11 May 1999) allowed the Geotail spacecraft to make a practically uninterrupted 54-h-long magnetosheath pass near dusk (16:30-21:11 magnetic local time) at a radial distance of 24 to 30 RE (Earth radii). During most of this period, interplanetary parameters varied gradually and in such a way as to give rise to two extreme magnetosheath structures, one dominated by magnetohydrodynamic (MHD) effects and the other by gas dynamic effects. We focus attention on unusual features of electromagnetic ion wave activity in the former magnetosheath state, and compare these features with those in the latter. Magnetic fluctuations in the gas dynamic magnetosheath were dominated by compressional mirror mode waves, and left- and right-hand polarized electromagnetic ion cyclotron (EIC) waves transverse to the background field. In contrast, the MHD magnetosheath, lasting for over one day, was devoid of mirror oscillations and permeated instead by EIC waves of weak intensity. The weak wave intensity is related to the prevailing low solar wind dynamic pressures. Left-hand polarized EIC waves were replaced by bursts of right-hand polarized waves, which remained for many hours the only ion wave activity present. This activity occurred when the magnetosheath proton temperature anisotropy (= ) became negative. This was because the weakened bow shock exposed the magnetosheath directly to the (negative) temperature anisotropy of the solar wind. Unlike the normal case studied in the literature, these right-hand waves were not by-products of left-hand polarized waves but derived their energy source directly from the magnetosheath temperature anisotropy. Brief entries into the low latitude boundary layer (LLBL) and duskside magnetosphere occurred under such inflated conditions that the magnetospheric magnetic pressure was insufficient to maintain pressure balance. In these crossings, the inner edge of the LLBL was flowing sunward. The study extends our knowledge of magnetosheath ion wave properties to the very low solar wind dynamic pressure regime.

A numerical model of the dayside aurora

A time‐independent model of the dayside auroras is presented within the framework of the pressure‐gradient‐driven mechanism for the generation of the region 1 field‐aligned current (FAC) in the low‐latitude boundary layer (LLBL). Either of two types of the dayside aurora, the bright spots (BSs) or the radially aligned arcs (RAs), arises from spatial modulation of the inner boundary of an LLBL particle population, but the respective source populations have different contents. BSs are produced when the boundary of a majority of the LLBL particle population is modulated. RAs are produced by large‐amplitude modulation occurring on a sharp boundary of a small fraction of the LLBL particle population. The required flux tube content, δN, of the source population of RAs is roughly 10% of that of the ambient population, if the scale length for ∇δN at the inner boundary is comparable to the magnetopause thickness. Using the 1989 Tsyganenko model, the FAC densities from the perturbed inner boundary of the particle population are evaluated and the energy fluxes of electrons precipitating to the ionosphere are then calculated on the basis of the Knight current‐voltage relation. This model can demonstrate the characteristic patterns of the RAs as well as the BSs. It also predicts that (1) both types of aurora with high luminosity as well as inverted‐V structures appear more frequently in the afternoon than in the morning, (2) this morning‐afternoon asymmetry is more pronounced for the case of BSs than RAs, and (3) during periods of northward interplanetary magnetic field, RAs can be distributed over a large area of the polar cap. Now the MLT‐dependent features of the dayside auroras, the appearances of “hot spot” and “warm spot,” and the large‐scale region 1 FAC generation can be understood systematically in view of the distortion of the (average) inner boundary of an LLBL particle population (i.e., nonalignment of the boundary with the average magnetic drift direction).

Energetic ion observations of the Earth’s magnetic cusps during an extended geomagnetically quiescent period in April 2001 using detectors on S/C ISTP/Polar

We present a method of polar cusp identification using high charge state heavy ions. There is reason to think that these ions would constitute shocked solar wind ions such as doubly charged helium ions, He 2+, and multiply charged oxygen ions, O (3−8)+, that appear to have relatively easy access to one or both of the polar cusps essentially at all times. The observations made with the ISTP/POLAR spacecraft were mostly obtained during geomagnetically non-storm times in April 2001 when the spacecraft orbit allowed northern cusp encounters with low trajectory angle between the spacecraft velocity vector and the overall cusp magnetic field orientation. We also show one extended cusp crossing during the modest magnetic storm of April 22, 2001 (with D st ∼−103 nT). It is found that during the study period, 1–10 keV high charge state ions are present in the cusp region at virtually all times, during storm or quiet. We also found that what appears to be the interior of the northern polar magnetic cusp region can be rather turbulent both in the local magnetic field direction and in the energetic charged particle kinetic flow characteristics. There is some evidence for temporally varying turbulent acceleration processes, either within the cusp domain itself or in the surrounding magnetosphere. In terms of the present ion observations, there is little to distinguish the dayside low latitude boundary layer (LB) ion signatures from those of the interior of the polar cusp region (high charge state ions, abundant 1–10 keV ion fluxes).

Accelerated electrons in the LLBL as observed by Interball on February 15, 1996

We analyze electron observations in the low latitude boundary layer (LLBL) with the Interball Tail satellite under a southward–duskward magnetosheath magnetic field. It is well known that the LLBL is characterized by bi-directional electron velocity distributions, which leads to an enhancement of the electron temperature. It is also well known that the electron temperature within the LLBL increases with a decrease of the LLBL number density. We found that the parallel electron velocity distribution changes self-consistently with varying number density. Namely, the shape of the parallel velocity distribution does not change in a log–log scale indicating that electron parallel velocities increase proportionately as the number density decreases. This suggests a possible role for the Fermi acceleration mechanism. We also discuss the LLBL topology that is provided by electron velocity distributions.

The structure of the magnetospheric boundary layers and the magnetospheric turbulence

The concept of the formation of the turbulent wake under the obstacle gives the possibility to explain many features of the plasma dynamics in the magnetosphere of the Earth. The formation of the plasma sheet and tail lobes in accordance with this concept is explained as the result of the interactions of large-scale (the dawn–dusk electric field) and medium-scale modes of magnetospheric turbulence. The application of the developed approach to the problem of the boundary layer formation gives the possibility to explain the low latitude boundary layer (LLBL) thickness during southward and northward interplanetary magnetic field orientations and the dawn–dusk LLBL asymmetry. Theory predictions are compared with the results of experimental observations.

Cusp and LLBL as sources of the isolated dayside auroral feature during northward IMF

An intense dayside proton aurora was observed by Imager for Magnetopause‐to‐Aurora Global Exploration Far Ultra‐Violet imager (IMAGE FUV) for an extensive period of northward interplanetary magnetic field (IMF) on 17 and 18 September 2000. This aurora partially coincided with the auroral oval and intruded farther poleward into the polar cap, and it showed longitudinal motions in response to IMF By variation. Intense magnetosheath‐like electron and ion precipitations have been simultaneously detected by Defense Meteorological Satellite Program (DMSP) above the poleward portion of the high‐latitude dayside aurora. They resemble the typical plasmas observed in the low‐altitude cusp. However, less intense electrons and more energetic ions were detected over the equatorward part of the aurora. These plasmas are closer to the low‐latitude boundary layer (LLBL) plasmas. Under strongly northward IMF, global ionospheric convection derived from Super Dual Auroral Radar Network (SuperDARN) radar measurements showed a four‐cell pattern with sunward convection in the middle of the dayside polar cap and the dayside aurora corresponded to two different convection cells. This result further supports two source regions for the aurora. The cusp proton aurora is on open magnetic field lines convecting sunward whereas the LLBL proton aurora is on closed field lines convecting antisunward. These IMAGE, DMSP, and SuperDARN observations reveal the structure and dynamics of the aurora and provide strong evidence for magnetic merging occurring at the high‐latitude magnetopause poleward from the cusp. This merging process was very likely quasi‐stationary.

Modeling the transition of the inner plasma sheet from weak to enhanced convection

We seek to determine whether the adiabatic plasma transport and energization resulting from electric and magnetic drift can quantitatively account for the plasma sheet under weak and enhanced convection observed by Geotail presented in the companion paper [Wang et al., 2004]. We use a modified Magnetospheric Specification Model to simulate the dynamics and distributions of protons originating from the deep tail and low‐latitude boundary layer (LLBL) under an assigned, slowly increasing convection electric field. The magnetic field is Tsyganenko 96 model, modified so that force balance is maintained along the midnight meridian. Our simulation results reproduce well the observed radial profiles and magnitudes of pressure and magnetic field. The changes of these parameters with convection strength are also well reproduced, indicating that the electric and magnetic drift control the large‐scale structure of the plasma sheet. The plasma flows near midnight are diverted toward dusk by diamagnetic drift. We obtain a steady state plasma sheet under strong and steady convection, showing that magnetic drift and field line stretching bring the plasma sheet away from possible convection disruption. The protons from the LLBL strongly affect the plasma sheet density and temperature during quiet times but not during enhanced convection. For the same cross–polar cap potential, stronger shielding of the convection electric field results in smaller energization. The penetration electric field is important in moving the plasma sheet to smaller geocentric radial distance. Our results suggest that the frozen‐in condition E = −v × B is not valid in the inner plasma sheet because of strong diamagnetic drift.

GEOTAIL observation of tilted X-line formation during flux transfer events (FTEs) in the dayside magnetospheric boundary layers

Abstract. The magnetic field and plasma structures during two successive crossings of the subsolar magnetospheric boundary layers (i.e. MagnetoPause Current Layer (MPCL) and Low-Latitude Boundary Layer (LLBL)) under the southward-dawnward IMF are examined on the basis of the data obtained by the GEOTAIL spacecraft. A significant and interesting feature is found, that is, Flux Transfer Events (FTEs) occur in association with the formation of the tilted X-line. During the first inbound MPCL/LLBL crossing, the ion velocity enhancement (in particular, the Vl component negatively increases) can be observed in association with simultaneous typical bipolar signature (positive followed by negative) in the Bn component. In addition, a clear D-shaped ion distribution whose origin is the magnetosheath can also be found in the dawnward direction. A few minutes later, the satellite experiences outbound MPCL crossing. The negative enhancement of the Vm component can be found as well as the positive enhancement of the Vl component. Simultaneously, a typical bipolar signature with the polarity (negative followed by positive) opposite that observed in the first encounter can also be observed. The ions from the magnetosheath flow predominantly in the duskward direction, although the D-shaped ion distribution cannot be observed. These results indicate that the satellite initially observes one part of a reconnected flux tube formed by FTEs whose magnetospheric side is anchored to the Southern Hemisphere. The ions confined in this partial flux tube are flowing in the south-dawnward direction. Then, the satellite observes the other part of the reconnected flux tube whose magnetospheric side is anchored to the Northern Hemisphere. The ions confined in this flux tube flow dominantly in the north-duskward direction. Furthermore, it can be considered that the second MPCL crossing is a direct cut through the diffusion region of FTEs because the LLBL is absent in the vicinity of the MPCL. On the basis of these results, it can be concluded that the satellite was passing near the tilted X-line. The information obtained through this study is expected to be of great use in discriminating between the anti-parallel (steady-state) reconnection and tilted X-line models on the dayside MPCL.

A possible origin of dayside Pc1 magnetic pulsations observed at high latitudes

Abstract. Induction magnetometer observations of dayside Pc1 activity at Barentsburg (BAB, Spitsbergen archipelago, 78.05°N, 14.12°E) are combined with data from two magnetometers located in Scandinavia and the Kola peninsula. Seven events with very large negative IMF Bz components were considered. For all of the events, the cusp location was expected to be significantly shifted equatorward from the statistical position such that the BAB magnetometer was located well inside the polar cap. The DMSP particle data indicated that the BAB magnetometer was indeed inside the polar cap, whereas other magnetometers were collocated with the ionospheric projections of the cusp, the low-latitude boundary layer or the boundary plasma sheet. Pc1 magnetic pulsations were observed only at BAB. In three cases, for which SuperDARN convection data were available, the Pc1 activity correlated with intervals of large-scale convection reconfiguration, such that the plasma flow crossing the BAB location was associated with newly-reconnected magnetic flux tubes drifting tailward. The convection reconfigurations were in response to a decrease in the IMF By component. We argue that the source of the observed Pc1 pulsations is anisotropic plasma of the depletion layer within the magnetosheath. The plasma anisotropy supports the excitation of electromagnetic ion cyclotron waves that are detectable with a ground-based magnetometer when the flux tubes containing the unstable plasma become connected to the Earth's ionosphere in the course of the dayside reconnection processes.

Reconstruction of the magnetopause and low-latitude boundary layer topology using Cluster multi-point measurements

Abstract. Multi-spacecraft data from Cluster allow for a more detailed magnetopause and boundary layer structure determination than ever before. Reconstruction methods, in which the time variability observed during a pass is interpreted as being due to boundary motion and/or spatial structure, are particularly well suited for this task. Such methods rely on the availability of plasma and field data and adopt the tangential discontinuity hypothesis to determine the motion, acceleration, boundary structure, boundary curvature and surface wave speed over an extended time interval. In this paper one- and two-dimensional reconstruction methods are applied to multi-spacecraft data for the first time.

Solar wind—magnetosphere–ionosphere coupling: an event study based on Freja data

Freja data are used to study the relative contributions from the high-latitude (reconnection/direct entry) and low-latitude (viscous interaction) dynamos to the cross-polar potential drop. Convection streamlines which are connected to the high-latitude dynamo may be identified from dispersed magnetosheath ions not only in the cusp/cleft region itself but also several degrees poleward of it. This fact, together with Freja's orbital geometry allows us to infer the potential drop from the high-latitude dynamo as well as to obtain a lower limit to the potential drop from the low-latitude dynamo for dayside Freja passes. All cases studied here are for active magnetospheric conditions. The Freja data suggest that under these conditions at least one third of the potential is generated in the low-latitude dynamo. These observations are consistent with earlier observations of the potential across the low-latitude boundary layer if we assume that the low-latitude dynamo region extends over several tens of Earth radii in the antisunward direction along the tail flanks, and that the majority of the potential drop derives from the sun-aligned component of the electric field rather than from its cross-boundary component, or equivalently, that the centre of the dynamo region is located quite far down tail. A possible dynamo geometry is illustrated.

Dense and stagnant ions in the low‐latitude boundary region under northward interplanetary magnetic field

The ion behavior in the low‐latitude boundary region is studied based on Geotail data accumulated over several years, toward understanding the formation mechanism of the cold‐dense plasma sheet under prolonged northward interplanetary magnetic field (IMF). A statistical survey shows that, during extended northward IMF, (1) dense magnetosheath‐like ions appear far more often, especially on the flanks, (2) the dense ions are mostly stagnant, in contrast to those in the classical low‐latitude boundary layer (LLBL), (3) a substantial fraction of the dense and stagnant ions is spatially mixed with hot magnetospheric ions, and (4) the mixed ion population has magnetic local time dependence in ion mixing state in energy space. Based on these findings, we argue that, under extended northward IMF, a significant transfer of the solar wind/LLBL ions onto the magnetospheric field lines occurs on the flanks, and the transport/heating process of the entrant ions is different for different local times.

The stability of the pristine magnetopause

A MHD theory of combined Kelvin–Helmholtz (KH) and Rayleigh–Taylor (RT) instabilities for a transition layer with two different scale lengths ( Δ and δ for the variation of velocity/magnetic fields and density, respectively) is presented. The study is motivated by reports of magnetopauses with no low latitude boundary layer, in which a sharp density drop over a distance δ⪡ Δ is observed (“ pristine” magnetopauses (J. Geophys. Res. 101 (1996) 49). The theory ignores compressibility effects and applies to subsonic regions of the dayside magnetopause. The RT effect is included to account for temporary periods of acceleration of the magnetopause, caused by sudden changes of the solar wind dynamic pressure. For small wavelengths λ, such that δ⪡ λ⪡ Δ, a WKB solution shows that the velocity gradient operates, together with magnetic tensions, to attenuate or even stabilize the Rayleigh–Taylor instability within a certain wavelength range. An exact dispersion relation for flute modes, valid for all λ, in the form of a fourth order polynomial for the complex frequency ω, is obtained from a model with a constant velocity gradient, d V/d y within Δ, and with δ→0. Flute modes are possible because of the existence of bands of very small magnetic shear on the dayside magnetopause (J. Geophys. Res. 103 (1998) 6703). The exact solution allows for a study of the change of the action of the velocity gradient with λ from the long- λ range where d V/d y is KH destabilizing to the short- λ range where d V/d y produces a stabilizing effect. Both, the WKB approximation and the well known tangential discontinuity model ( Δ→0) are recovered as limiting cases of the exact solution. Properties of the KH and RT instabilities, for different density ratios on either side of the magnetopause, are described. For flute modes, at very small λ the RT instability grows faster and becomes the dominant effect. However, it is shown that the growth rate remains bounded at a finite value as λ→0, when a theory with a finite δ model is considered. To study configurations with finite, arbitrary, δ/ Δ ratios, the MHD perturbation equations are solved numerically, using hyperbolic tangent functions for both the density and velocity transitions across the magnetopause. To examine the influence of different δ/ Δ ratios on the growth rates of KH and RT, calculations are performed for different δ/ Δ, with and without acceleration, and for two different density ratios. It is found that the general features exhibited by the constant d V/d y model, are confirmed by these numerical solutions. The stability of pristine magnetopauses, and the possibility of observing some theoretical predictions during magnetopause crossings in ongoing missions, are discussed.

Some theoretical models for solitary structures of boundary layer waves

Abstract. Solitary electrostatic structures have been observed in and around auroral zone field lines at various altitudes ranging from close to the ionosphere, to the magnetopause low-latitude boundary layer (LLBL) and cusp on the dayside, and to the plasma sheet boundary layer on the night-side. In this review, various models based on solitons/double layers and BGK modes or phase space holes are discussed in order to explain the characteristics of these solitary structures.