Properties and origin of energetic particles at the duskside of the Earth’s magnetosheath throughout a great storm

We study an interval of 56 h on January 16 to 18, 1995, during which the GEOTAIL spacecraft traversed the duskside magnetosheath from X ≅ −15 to −40 R_E and the EPIC/ICS and EPIC/STICS sensors sporadically detected tens of energetic particle bursts. This interval coincides with the expansion and growth of a great geomagnetic storm. The flux bursts are strongly dependent on the magnetic field orientation. They switch on whenever the B_z component approaches zero (B_z ≅ 0 nT). We strongly suggest a magnetospheric origin for the energetic ions and electrons streaming along these “exodus channels”. The time profiles for energetic protons and “tracer” O⁺ ions are nearly identical, which suggests a common source. We suggest that the particles leak out of the magnetosphere all the time and that when the magnetosheath magnetic field connects the spacecraft to the magnetotail, they stream away to be observed by the GEOTAIL sensors. The energetic electron fluxes are not observed as commonly as the ions, indicating that their source is more limited in extent. In one case study the magnetosheath magnetic field lines are draped around the magnetopause within the YZ plane and a dispersed structure for peak fluxes of different species is detected and interpreted as evidence for energetic electrons leaking out from the dawn LLBL and then being channelled along the draped magnetic field lines over the magnetopause. Protons leak from the equatorial dusk LLBL and this spatial differentiation between electron and proton sources results in the observed dispersion. A gradient of energetic proton intensities toward the Z_GSM= 0 plane is inferred. There is a permanent layer of energetic particles adjacent to the magnetosheath during this interval in which the dominant component of the magnetic field was B_z.     

On the formation of a plasma pressure anisotropy in the dayside magnetosheath

We present a numerical solution for the momentum equation of the magnetosheath particles that describes the distribution of the pressure anisotropy of the magnetosheath plasma in the midday meridian plane. The pressure anisotropy is a maximum near the magnetopause subsolar point (p_⊥/p_\Vert\cong10). The pressure anisotropy is caused by two factors: particles with small pitch angles (V_\Vert>V_⊥) which travel along the magnetic field lines away from the equatorial plane of the magnetosheath; and particles, after crossing the bowshock, which reach the bulk velocity component directed along the magnetic field lines again, away from the magnetosheath equatorial plane. This velocity increases with increasing distance from the subsolar point of the bowshock, and does not permit particles with large pitch angles (V_⊥>V_\Vert) to move toward the equatorial plane.     

Quiet-time Pc 5 pulsations in the Earth’s magnetotail: IMP-8, ISEE-1 and ISEE-3 simultaneous observations

Quasi-periodic Pc 5 pulsations have been reported inside and just outside the Earth’s magnetotail during intervals of low geomagnetic activity. In order to further define their characteristics and spatial extent, we present three case studies of simultaneous magnetic field and plasma observations by IMP-8, ISEE-1 (and ISEE-2 in one case) in the Earth’s magnetotail and ISEE-3 far upstream of the bow shock, during intervals in which the spacecraft were widely separated. In the first case study, similar pulsations are observed by IMP-8 at the dawn flank of the plasma sheet and by ISEE-1 near the plasma sheet boundary layer (PSBL) near midnight local time. In the second case study, simultaneous pulsations are observed by IMP-8 in the dusk magnetosheath and by ISEE-1 and 2 in the dawn plasma sheet. In the third case study, simultaneous pulsations are observed in the north plasma sheet boundary layer and the south plasma sheet. We conclude that the pulsations occur simultaneously throughout much of the nightside magnetosphere and the surrounding magnetosheath, i.e. that they have a global character. Some additional findings are the following: (a) the observed pulsations are mixed mode compressional and transverse, where the compressional character is more apparent in the close vicinity of the plane Z_GSM=0; (b) the compressional pulsations of the magnetic field in the dusk magnetosheath show peaks that coincide (almost one-to-one) with similar peaks observed inside the dawn plasma sheet; (c) in the second case study the polarization sense of the magnetic field and the recurrent left-hand plasma vortices observed in the dawn plasma sheet are consistent with antisunward moving waves on the magneto-pause; (d) pulsation amplitudes are weaker in the PSBL(or lobe) as compared with those in the magneto-tail’s flanks, suggesting a decay with distance from the magnetopause; (e) the thickness of the plasma sheet (under extremely quiet conditions) is estimated to be \sim22 R_E at an average location of (X, Y)_GSM=(16, 17) R_E, whereas at midnight local time the thickness is \sim14 R_E. The detected pulsations are probably due to the pressure variations (recorded by ISEE-3) in the solar wind, and/or the Kelvin Helmholtz instability in the low-latitude boundary layer or the magnetopause due to a strongly northward IMF.     

Magnetopause stand-off distance in dependence on the magnetosheath and solar wind parameters

A model of the magnetosheath structure proposed in a recent paper from the authors is extended to estimate the magnetopause stand-off distance from solar wind data. For this purpose, the relationship of the magnetopause location to the magnetosheath and solar wind parameters is studied. It is shown that magnetopause erosion may be explained in terms of the magnetosheath magnetic field penetration into the magnetosphere. The coefficient of penetration (the ratio of the magnetospheric magnetic field depression to the intensity of the magnetosheath magnetic field B_mz = -B_m sin²/2, is estimated and found approximately to equal 1. It is shown that having combined a magnetosheath model presented in an earlier paper and the magnetosheath field penetration model presented in this paper, it is possible to predict the magnetopause stand-off distance from solar wind parameters.     

地球弓激波的旋转非对称性

通过对太阳风-磁层-电离层系统的全球MHD模拟,研究地球弓激波相对日地连线的旋转非对称性．模拟限于太阳风速度沿日地连线、地球磁偶极矩和行星际磁场（IMF）与日地连线垂直的简单情况．模拟结果表明,即便对于IMF强度为零的情况,弓激波相对日地连线也不具备旋转对称性质：终端面（晨昏子午面）及其向阳侧的弓激波截线的东西宽度大于南北宽度（约9%~11％）,终端面尾侧的弓激波截线东西宽度小于南北宽度（约8％）．在存在IMF的情况下,弓激波的位形同时受到磁层顶的形状和快磁声波速度各向异性的影响．磁层顶向外扩张并沿IMF方向拉伸,且其扩张和拉伸程度随IMF由北转南而增强．在磁鞘中,垂直于磁场方向的快磁声波速度高于平行方向．因此,磁层顶拉伸方向与快磁声波速度最大方向垂直,它们对弓激波位置的效应恰好相反;弓激波的最终形状取决于何种效应占据主导地位．对于终端面尾侧,快磁声波速度的各向异性起主导作用,弓激波截线沿IMF垂直方向的宽度大于平行方向．对于终端面及其向阳侧,弓激波截线的形状与IMF取向有关：在准北向或晨昏向IMF情况下,弓激波截线沿IMF垂直方向的宽度仍大于平行方向;在准南向IMF情况下,弓激波截线沿IMF垂直方向的宽度小于平行方向的．鉴于弓激波形状同IMF取向之间的密切关系,我们提议以IMF为基准方向,提取弓激波截线的平行半宽度R_b∥和垂直半宽度R_b⊥作为尺度参数．这些尺度参数和通常引入的弓激波截线的东西半宽度y_b和南北半宽度z_b相比,更为合理地表征了弓激波的几何性质．模拟结果表明,在终端面上,y_b/z_b和R_b∥/R_b⊥在IMF各向同性取向下的统计平均值均低于1,与观测得到的结论一致.     

IMP 8 magnetosheath field comparisons with models

This study presents cross-sectional vector maps of the magnetic field derived from IMP 8 magnetometer in the magnetosheath at 30 R_e behind the Earth. In addition the vector patterns of the magnetosheath field for northward, southward, and east-west interplanetary magnetic field (IMF) directions are qualitatively compared with those obtained from the Spreiter-Stahara gas dynamic (GD) and Fedder-Lyon magnetohydrodynamic models (MHD). The main purpose is to display the cross-sectional differences in relation to the dayside merging with different IMF directions, allowing the reader to make direct visual comparisons of the vector patterns. It is seen that for east-west IMF directions, the data-based and MHD-based patterns differ noticeably in a similar way from the GD model, presumably reflecting the influence of dayside magnetic merging of the Earths magnetic field with the y-component of the interplanetary magnetic field. All three northward IMF cross sections show comparable field draping patterns as expected for a closed magnetosphere. For southward IMF case, on the other hand, differences between the three cross-sectional patterns are greater as seen in the field vector sizes and directions, especially closer to the magnetopause where more disturbed magneto-spheric conditions are known to be exist. The data comparisons with the MHD and GD models show that the differences result from the magnetic field-flow coupling and that the effects of dayside reconnection are present in IMP 8 magnetic field measurements.     

Multiple Flux Rope Events at the High-Latitude Magnetopause: Cluster/Rapid Observation on 26 January, 2001

Cluster measurements of the cusp and high latitude magnetopause boundary on 26 January, 2001 confirm that the cusp is a dynamic region full of energetic charged particles and turbulence. An energetic ion layer at high-latitudes beyond and adjacent to the duskside magnetopause exists when the Interplanetary Magnetic Field (IMF) has a southward orientation. Multiple energetic ion flux bursts were observed in the energetic ion layer. Each energetic ion flux burst was closely related to a magnetic flux rope. The axes of the flux ropes lie in the direction pointing duskward/tailward and somewhat upward. An intense axis-aligned current flows inside the ropes, with the current density reaching ∼10⁻⁸ A/m². The main components of the energetic ions are protons, helium and CNO ions, which originate from the magnetosphere, flowing out into the magnetosheath along the axis of the flux ropes. The velocity of the magnetosheath thermal plasma relative to the deHoffman-Teller (DHT) frame is found to be basically along the axis of the flux ropes also, but towards the magnetosphere. These flux ropes seem to be produced somewhere away via magnetic reconnection and move at similar DHT velocities passing over the spacecraft. These observations further confirm that the high-latitude magnetopause boundary region plays an important role in the solar wind-magnetopause coupling.     

Identification of magnetosheath mirror modes in Equator-S magnetic field data

Between December 1997 and March 1998 Equator-S made a number of excursions into the dawn-side magnetosheath, over a range of local times between 6:00 and 10:40 LT. Clear mirror-like structures, characterised by compressive fluctuations in |B| on occasion lasting for up to 5 h, were observed during a significant fraction of these orbits. During most of these passes the satellite appeared to remain close to the magnetopause (within 1–2 Re), during sustained compressions of the magnetosphere, and so the characteristics of the mirror structures are used as a diagnostic of magnetosheath structure close to the magnetopause during these orbits. It is found that in the majority of cases mirror-like activity persists, undamped, to within a few minutes of the magnetopause, with no observable ramp in |B|, irrespective of the magnetic shear across the boundary. This suggests that any plasma depletion layer is typically of narrow extent or absent at the location of the satellite, at least during the subset of orbits containing strong magnetosheath mirror-mode signatures. Power spectra for the mirror signatures show predominately field aligned power, a well defined shoulder at around 3–10 × 10⁻² Hz and decreasing power at higher frequencies. On occasions the fluctuations are more sinusoidal, leading to peaked spectra instead of a shoulder. In all cases mirror structures are found to lie approximately parallel to the observed magnetopause boundary. There is some indication that the amplitude of the compressional fluctuations tends to be greater closer to the magnetopause. This has not been previously reported in the Earth’s magnetosphere, but has been suggested in the case of other planets.     

Specific features of magnetic barrier formation near the magnetopause

The numerical three-dimensional MHD model is used to study the formation of the magnetic barrier in the inner part of the magnetosheath near the magnetopause. The set of the quasistationary solutions for several characteristic directions of the interplanetary magnetic field (IMF) has been obtained: for northward and southward IMF, for the direction along the Parker helix (at an angle of 45° with respect to the Sun-Earth line), and for the predominantly radial direction (at an angle of 20° with respect to the Sun-Earth line). The mechanism used to take into account the effect of magnetic reconnection at the magnetopause on a flow in the magnetosheath is introduced in the case of southward IMF. The results of the calculations indicate that the magnetic field absolute value in the magnetic barrier reaches its maximal value when IMF is northward. The introduction of magnetic reconnection at southward IMF can result in an insignificant decrease in the field value. However, the model predicts that a decrease in the magnetic field is much more substantial when the IMF direction is close to radial.     

Dynamics and local boundary properties of the dawn-side magnetopause under conditions observed by Equator-S

Magnetic field measurements, taken by the magnetometer experiment (MAM) on board the German Equator-S spacecraft, have been used to identify and categorise 131 crossings of the dawn-side magnetopause at low latitude, providing unusual, long duration coverage of the adjacent magnetospheric regions and near magnetosheath. The crossings occurred on 31 orbits, providing unbiased coverage over the full range of local magnetic shear from 06:00 to 10:40 LT. Apogee extent places the spacecraft in conditions associated with intermediate, rather than low, solar wind dynamic pressure, as it processes into the flank region. The apogee of the spacecraft remains close to the magnetopause for mean solar wind pressure. The occurrence of the magnetopause encounters are summarised and are found to compare well with predicted boundary location, where solar wind conditions are known. Most scale with solar wind pressure. Magnetopause shape is also documented and we find that the magnetopause orientation is consistently sunward of a model boundary and is not accounted for by IMF or local magnetic shear conditions. A number of well-established crossings, particularly those at high magnetic shear, or exhibiting unusually high-pressure states, were observed and have been analysed for their boundary characteristics and some details of their boundary and near magnetosheath properties are discussed. Of particular note are the occurrence of mirror-like signatures in the adjacent magnetosheath during a significant fraction of the encounters and a high number of multiple crossings over a long time period. The latter is facilitated by the spacecraft orbit which is designed to remain in the near magnetosheath for average solar wind pressure. For most encounters, a well-ordered, tangential (draped) magnetosheath field is observed and there is little evidence of large deviations in local boundary orientations. Two passes corresponding to close conjunctions of the Geotail spacecraft are analysed to confirm boundary orientation and motion. These further show evidence of an anti-sunward moving depression on the magnetopause (which is much smaller at Equator-S). The Tsyganenko model field is used routinely to assist in categorising the crossings and some comparison of models is carried out. We note that typically the T87 model fits the data better than the T89 model during conditions of low to intermediate K_p index near the magnetopause and also near the dawn-side tail current sheet in the dawnside region.     

Effect of the IMF B y component on the ionospheric flow overhead at EISCAT: observations and theory

We have analysed a database of 300 h of tristatic ionospheric velocity measurements obtained overhead at Tromsø (66.3° magnetic latitude) by the EISCAT UHF radar system, for the presence of flow effects associated with the y-component of the IMF. Since it is already known that the flow depends upon IMF B_z, a least-squares multivariate analysis has been used to determine the flow dependence on both IMF B_y and B_z simultaneously. It is found that significant flow variations with IMF B_y occur, predominantly in the midnight sector (2100/0300 MLT), but also pre-dusk (1600/1700 MLT), which are directed eastward for IMF B_y positive and westward for IMF B_y negative. The flows are of magnitude 20/30 m s^–1 nT^–1 in the midnight sector, and smaller, 10/20 m s^–1 nT^–1, pre-dusk, and are thus associated with significant changes of flow of order a few hundred m s^–1 over the usual range of IMF B_y of about ±5 nT. At other local times the IMF B_y-related perturbation flows are much smaller, less than 5 m s^–1 nT^–1, and consistent with zero within the uncertainty estimates. We have investigated whether these IMF B_y-dependent flows can be accounted for quantitatively by a theoretical model in which the equatorial flow in the inner magnetosphere is independent of IMF B_y, but where distortions of the magnetospheric magnetic field associated with a penetrating component of the IMF B_y field changes the mapping of the field to the ionosphere, and hence the ionospheric flow. We find that the principal flow perturbation produced by this effect is an east-west flow whose sense is determined by the north-south component of the unperturbed flow. Perturbations in the north-south flow are typically smaller by more than an order of magnitude, and generally negligible in terms of observations. Using equatorial flows which are determined from EISCAT data for zero IMF B_y, to which the corotation flow has been added, the theory predicts the presence of zonal perturbation flows which are generally directed eastward in the Northern Hemisphere for IMF B_y positive and westward for IMF B_y negative at all local times. However, although the day and night effects are therefore similar in principle, the model perturbation flows are much larger on the nightside than on the dayside, as observed, due to the day-night asymmetry in the unperturbed magnetospheric magnetic field. Overall, the model results are found to account well for the observed IMF B_y-related flow perturbations in the midnight sector, in terms of the sense and direction of the flow, the local time of their occurrence, as well as the magnitude of the flows (provided the magnetic model employed is not too distorted from dipolar form). At other local times the model predicts much smaller IMF B_y-related flow perturbations, and thus does not account for the effects observed in the pre-dusk sector.     

Density fluctuations measured by ISEE 1–2 in the Earth’s magnetosheath and the resultant scattering of radio waves

Radio waves undergo angular scattering when they propagate through a plasma with fluctuating density. We show how the angular scattering coefficient can be calculated as a function of the frequency spectrum of the local density fluctuations. In the Earths magnetosheath, the ISEE 1–2 propagation experiment measured the spectral power of the density fluctuations for periods in the range 300 to 1 s, which produce most of the scattering. The resultant local angular scattering coefficient can then be calculated for the first time with realistic density fluctuation spectra, which are neither Gaussian nor power laws. We present results on the variation of the local angular scattering coefficient during two crossings of the dayside magnetosheath, from the quasi-perpendicular bow shock to the magnetopause. For a radio wave at twice the local electron plasma frequency, the scattering coefficient in the major part of the magnetosheath is b(2f_p) 0.5–4 × 10^–9 rad²/m. The scattering coefficient is about ten times stronger in a thin sheet (0.1 to IR_E) just downstream of the shock ramp, and close to the magnetopause.     

High-time resolution conjugate SuperDARN radar observations of the dayside convection response to changes in IMF B y

We present data from conjugate SuperDARN radars describing the high-latitude ionospheres response to changes in the direction of IMF B_y during a period of steady IMF B_z southward and B_x positive. During this interval, the radars were operating in a special mode which gave high-time resolution data (30 s sampling period) on three adjacent beams with a full scan every 3 min. The location of the radars around magnetic local noon at the time of the event allowed detailed observations of the variations in the ionospheric convection patterns close to the cusp region as IMF B_y varied. A significant time delay was observed in the ionospheric response to the IMF By changes between the two hemispheres. This is explained as being partially a consequence of the location of the dominant merging region on the magnetopause, which is 8/12R_E closer to the northern ionosphere than to the southern ionosphere (along the magnetic field line) due to the dipole tilt of the magnetosphere and the orientation of the IMF. This interpretation supports the anti-parallel merging hypothesis and highlights the importance of the IMF B_x component in solar wind-magnetosphere coupling.     

Cusp Modeling and Observations at Low Altitude

Cusp properties have been investigated with an open-field line particle precipitation model and Defense Meteorological Satellite Program (DMSP) satellite observations. Particular emphasis is placed on the effects of IMF B_y, since previous studies focus mostly on IMF B_z. The model-data comparisons for various IMF configurations show that the model captures the large-scale features of the particle precipitation very well, not only in the cusp region, but also in other open-field line regions such as the mantle, polar rain, and open-field line low-altitude boundary layer (LLBL). When the IMF is strongly duskward/dawnward and weakly southward, the model predicts the occurrence of double cusp near noon: one cusp at lower latitude and one at higher latitude. The lower latitude cusp ions originate from the low-latitude magnetosheath whereas the higher latitude ions originate from the high-latitude magnetosheath. The lower latitude cusp is located in the region of weak azimuthal E × B drift, resulting in a dispersionless cusp. The higher latitude cusp is located in the region of strong azimuthal and poleward E × B drift. Because of a significant poleward drift, the higher latitude cusp dispersion has some resemblance to that of the typical southward IMF cusp. Occasionally, the two parts of the double cusp have such narrow latitudinal separation that they give the appearance of just one cusp with extended latitudinal width. From the 40 DMSP passes selected during periods of large (positive or negative) IMF B_y and small negative IMF B_z, 30 (75%) of the passes exhibit double cusps or cusps with extended latitudinal width. The double cusp result is consistent with the following statistical results: (1) the cusp’s latitudinal width increases with |IMF B_y| and (2) the cusp’s equatorward boundary moves to lower latitude with increasing |IMF B_y|.     

Magnetopause boundary structure deduced from the high-time resolution particle experiment on the Equator-S spacecraft

An electrostatic analyser (ESA) onboard the Equator-S spacecraft operating in coordination with a potential control device (PCD) has obtained the first accurate electron energy spectrum with energies &7 eV-100 eV in the vicinity of the magnetopause. On 8 January, 1998, a solar wind pressure increase pushed the magnetopause inward, leaving the Equator-S spacecraft in the magnetosheath. On the return into the magnetosphere approximately 80 min later, the magnetopause was observed by the ESA and the solid state telescopes (the SSTs detected electrons and ions with energies &20–300 keV). The high time resolution (3 s) data from ESA and SST show the boundary region contains of multiple plasma sources that appear to evolve in space and time. We show that electrons with energies &7 eV–100 eV permeate the outer regions of the magnetosphere, from the magnetopause to &6Re. Pitch-angle distributions of &20–300 keV electrons show the electrons travel in both directions along the magnetic field with a peak at 90° indicating a trapped configuration. The IMF during this interval was dominated by Bx and By components with a small Bz.     

Energetic Electrons as a Field Line Topology Tracer in the High Latitude Boundary/CUSP Region: Cluster Rapid Observations

Energetic electrons (e.g., 50 keV) travel along field lines with a high speed of around 20 R_Es⁻¹. These swift electrons trace out field lines in the magnetosphere in a rather short time, and therefore can provide nearly instantaneous information about the changes in the field configuration in regions of geospace. The energetic electrons in the high latitude boundary regions (including the cusp) have been examined in detail by using Cluster/RAPID data for four consecutive high latitude/cusp crossings between 16 March and 19 March 2001. Energetic electrons with high and stable fluxes were observed in the time interval when the IMF had a predominately positive B_z component. These electrons appeared to be associated with a lower plasma density exhibiting no obvious tailward plasma flow (<20 keV). On the other hand, no electrons or only spike-like electron events have been observed in the cusp region during southward IMF. At that time, the plasma density was as high as that in the magnetosheath and was associated with a clear tailward flow. The fact that no stable energetic electron fluxes were observed during southward IMF indicates that the cusp has an open field line geometry. The observations indicate that both the South and North high latitude magnetospheric boundary regions (including both North and South cusp) can be energetic particle trapping regions. The energetic electron observations provide new ways to investigate the dynamic cusp processes. Finally, trajectory tracing of test particles has been performed using the Tsyganenko 96 model; this demonstrates that energetic particles (both ions and electrons) may be indeed trapped in the high latitude magnetosphere.     

Observations of polar patches generated by solar wind Alfvén wave coupling to the dayside magnetosphere

A long series of polar patches was observed by ionosondes and an all-sky imager during a disturbed period (K_p = 7- and IMF B_z <0). The ionosondes measured electron densities of up to 9 × 10¹¹ m⁻³ in the patch center, an increase above the density minimum between patches by a factor of ≈4.5. Bands of F-region irregularities generated at the equatorward edge of the patches were tracked by HF radars. The backscatter bands were swept northward and eastward across the polar cap in a fan-like formation as the afternoon convection cell expanded due to the IMF B_y > 0. Near the north magnetic pole, an all-sky imager observed the 630-nm emission patches of a distinctly band-like shape drifting northeastward to eastward. The 630-nm emission patches were associated with the density patches and backscatter bands. The patches originated in, or near, the cusp footprint where they were formed by convection bursts (flow channel events, FCEs) structuring the solar EUV-produced photoionization and the particle-produced auroral/cusp ionization by segmenting it into elongated patches. Just equatorward of the cusp footprint Pc5 field line resonances (FLRs) were observed by magnetometers, riometers and VHF/HF radars. The AC electric field associated with the FLRs resulted in a poleward-progressing zonal flow pattern and backscatter bands. The VHF radar Doppler spectra indicated the presence of steep electron density gradients which, through the gradient drift instability, can lead to the generation of the ionospheric irregularities found in patches. The FLRs and FCEs were associated with poleward-progressing DPY currents (Hall currents modulated by the IMF B_y) and riometer absorption enhancements. The temporal and spatial characteristics of the VHF backscatter and associated riometer absorptions closely resembled those of poleward moving auroral forms (PMAFs). In the solar wind, IMP 8 observed large amplitude Alfvén waves that were correlated with Pc5 pulsations observed by the ground magnetometers, riometers and radars. It is concluded that the FLRs and FCEs that produced patches were driven by solar wind Alfvén waves coupling to the dayside magnetosphere. During a period of southward IMF the dawn-dusk electric field associated with the Alfvén waves modulated the subsolar magnetic reconnection into pulses that resulted in convection flow bursts mapping to the ionospheric footprint of the cusp.     

Long duration Pc 5 compressional pulsations inside the Earth’s magnetotail lobes

Pc 5-type magnetic field pulsations are detected by the IMP-8 spacecraft well inside the Earth’s magnetotail lobes. The three studied events with an average duration of 3 h and mean amplitude of B/B=6.6% show a strong longitudinal oscillation. The clockwise polarization sense of the magnetic field arrowheads in the north lobe (as well as the counterclockwise in the south lobe) on the XZ plane is consistent with that expected when periodic solar wind lateral pressures squeeze the magnetotail axisymmetrically while moving tailward. In the two case studies, the latter property has been found to concur with quasi-periodic upstream density fluctuations detected by ISEE-3 and/or ISSE-1. The lobe magnetic field oscillations are classified in two distinct modes. The manifestations of the first mode are tailward-travelling waves detectable along the B_y and B_z magnetic field traces (i.e., with regard to the B_z the spacecraft encounters constantly the same conspicuous signature of south-then-north tilting of field lines around each local compression region). The second mode is associated with prolonged periods of extremely low geomagnetic activity and exhibits a signature along the B_y component inconsistent with travelling waves. Thus, the maxima of compressions occur simultaneously with the maxima of B_y excursions: a feature that is explained in terms of tail-aligned current density flowing at the boundary which separates the stable magnetic field in the tail lobe from the very irregular in the magnetosheath. In this case, the spacecraft was located in the vicinity of the high-latitude tail boundary and the observed B_y excursions are consistent with those anticipated by the tail-aligned current polarity, which is determined by the dominant B_y-component of the interplanetary magnetic field (IMF). On the plane YZ we observe an almost linear and circular polarization sense of the vector magnetic field for the first and second mode, respectively.     

A flux transfer event observed at the magnetopause by the Equator-S spacecraft and in the ionosphere by the CUTLASS HF radar

Observations of a flux transfer event (FTE) have been made simultaneously by the Equator-S spacecraft near the dayside magnetopause whilst corresponding transient plasma flows were seen in the near-conjugate polar ionosphere by the CUTLASS Finland HF radar. Prior to the occurrence of the FTE, the magnetometer on the WIND spacecraft ≈226 R_E upstream of the Earth in the solar wind detected a southward turning of the interplanetary magnetic field (IMF) which is estimated to have reached the subsolar magnetopause ≈77 min later. Shortly afterwards the Equator-S magnetometer observed a typical bipolar FTE signature in the magnetic field component normal to the magnetopause, just inside the magnetosphere. Almost simultaneously the CUTLASS Finland radar observed a strong transient flow in the F region plasma between 78° and 83° magnetic latitude, near the ionospheric region predicted to map along geomagnetic field lines to the spacecraft. The flow signature (and the data set as a whole) is found to be fully consistent with the view that the FTE was formed by a burst of magnetopause reconnection.     

Magnetosheath Interaction with the High Latitude Magnetopause

We present both statistical and case studies of magnetosheath interaction with the high-latitude magnetopause on the basis of Interball-1 and other ISTP spacecraft data. We discuss those data along with recently published results on the topology of cusp-magnetosheath transition and the roles of nonlinear disturbances in mass and energy transfer across the high-latitude magnetopause. For sunward dipole tilts, a cusp throat is magnetically open for direct interaction with the incident flow that results in the creation of a turbulent boundary layer (TBL) over an indented magnetopause and downstream of the cusp. For antisunward tilts, the cusp throat is closed by a smooth magnetopause; demagnetized ‘plasma balls’ (with scale ∼ few R_E, an occurrence rate of ∼25% and trapped energetic particles) present a major magnetosheath plasma channel just inside the cusp. The flow interacts with the ‘plasma balls’ via reflected waves, which trigger a chaotization of up to 40% of the upstream kinetic energy. These waves propagate upstream of the TBL and initiate amplification of the existing magnetosheath waves and their cascade-like decays during downstream passage throughout the TBL. The most striking feature of the nonlinear interaction is the appearance of magnetosonic jets, accelerated up to an Alfvenic Mach number of 3. The characteristic impulsive local momentum loss is followed by decelerated Alfvenic flows and modulated by the TBL waves; momentum balance is conserved only on time scales of the Alfvenic flows (1/f_A ∼12 min). Wave trains at f_A∼1.3 mHz are capable of synchronizing interactions throughout the outer and inner boundary layers. The sonic/Alfvenic flows, bounded by current sheets, control the TBL spectral shape and result in non-Gaussian statistical characteristics of the disturbances, indicating the fluctuation intermittency. We suggest that the multi-scale TBL processes play at least a comparable role to that of macro-reconnection (remote from or in the cusp) in solar wind energy transformation and population of the magnetosphere by the magnetosheath plasma. Secondary micro-reconnection constitutes a necessary chain at the small-scale (∼ion gyroradius) edge of the TBL cascades. The thick TBL transforms the flow energy, including deceleration and heating of the flow in the open throat, ‘plasma ball’ and the region downstream of the cusp.