Spacecraft Crossings Research Articles

A survey of flux transfer events observed by Cluster during strongly northward IMF

During Cluster's annual dayside seasons (November–June) the four spacecraft cross the magnetopause at high latitudes near local noon, and at lower latitudes further along the flanks. During these crossings, observations of flux transfer events (FTEs), a signature of transient or variable‐rate magnetopause merging, are often made. We have compiled a survey of FTEs observed by Cluster in the 2002/3 dayside season. A significant number of FTEs, presented here, were observed under strongly northward IMF. Multi‐spacecraft techniques enable more accurate velocities to be calculated than previously possible. The observed velocities are consistent with a long, component merging X‐line emanating from the antiparallel merging site in the lobe, but require a relaxation of the antiparallel merging hypothesis to allow the X‐line to extend to regions of lower shear on the flank. The velocities observed at lower latitudes are not consistent with a subsolar X‐line.

Cluster observations of earthward flowing plasmoid in the tail

The energetic electrons and ions embedded in Earthward‐moving plasmoid structures have been observed. These plasmoids are associated with a rotational local Bz component (bi‐polar) signature. Energetic electrons are found to be confined in a smaller spatial region than ions inside the plasmoid. Energetic ions and electrons seem to be a good indicator for the structure boundary. The fleet of Cluster spacecraft cross the plasmoid structure in a “first entry, last out” order (Note: when spacecraft cross a planar discontinuity, e.g. magnetopause, they will be in “first entry, first out” order). This documents the fact that the plasmoid has a non‐planar nested structure. The large separation distance (around 1 RE) of the Cluster satellites in October 2002 is an advantage to provide constraints on the size and shape of the plasmoid structure of interest. In addition, the plasmoid (with closed field lines) should preserve the ion composition information where it is formed. The ion composition observed in the plasmoid shows significantly lower O and He than in the ambient plasma. This implies few heavy ions are involved in the reconnection process where the plasmoid is formed. Multiple flux ropes/plasmoids observation presented in this paper can be interpreted as strong evidence for multiple X‐lines.

The Earth's quasi-parallel bow shock: Review of observations and perspectives for Cluster

Spacecraft crossings of the Earth's quasi-parallel bow shock under typical high Mach number conditions are characterized by the presence of large-amplitude, compressive, low-frequency magnetic pulsations. Thus, instead of the fairly abrupt jump from upstream to downstream states one associates with their quasi-perpendicular counterparts, quasi-parallel shocks are more aptly described in terms of an extended transition from upstream to downstream conditions. The complex, turbulent appearance of the quasi-parallel shock diverted much of the research efforts in the field towards the quasi-perpendicular regime until about 15 years ago. This paper reviews what we have learnt about the quasi-parallel shock transition zone from spacecraft observations at the Earth's bow shock in the last decade and a half. Field and particle measurements suggest a prominent role is played by short (∼10 s) large-amplitude ( |δ B |/| B |>2 , typically 3 or more) magnetic structures that evolve out of the upstream wave field and are convected towards the shock by the supersonic solar wind. As these structures grow in amplitude, they steepen, slow down in the shock rest frame and begin to merge with similar structures, leading to the downstream state. A number of questions about the nature of the quasi-parallel shock transition remain and the possible contribution Cluster may make towards their resolution is highlighted.

Four‐point Cluster application of magnetic field analysis tools: The discontinuity analyzer

The Cluster spacecraft have collected 3‐D information on magnetic field structures at small to medium scales (100–1000 km) in the Earth's dayside magnetosphere, magnetosheath, and near solar wind. We focus here on the first application of the discontinuity analyzer analysis technique (determination of the geometry and motion of magnetic discontinuities, using interspacecraft timing and variance information) for the case of boundaries that are planar over the four‐spacecraft array. This multipoint technique uses minimum variance of the magnetic field to determine the boundary normals at each spacecraft. Identification of the four spacecraft crossings requires accurate timing of encounters at each spacecraft, which depends on key features in each time series being stationary in the frame of the boundary. The technique is tested for selected boundary crossing events (magnetopause and bow shock, and one TD in the magnetosheath), for which the independently determined normals are closely (∼1–2°) colinear. Closely aligned normals, on spatial scales of ∼600–1000 km, were found for most crossings studied. For such a planar geometry, the motion can be determined unambiguously and shows that significant acceleration (up to ∼10 km s−2) of the boundary over the spacecraft is nearly always present. This has the consequence that previous estimates of boundary scales, based on constant motion between two spacecraft, may be in error by factors of two or three and that implied variations in boundary thickness could therefore be predominantly due to variations in speed. We comment on the error introduced through assumptions of constant motion.

Cluster observes the Earth’s magnetopause: coordinated four-point magnetic field measurements

Abstract. The four-spacecraft Cluster mission has provided high-time resolution measurements of the magnetic field from closely maintained separation distances (200–600 km). Four-point coverage of the Earth’s magnetopause began on the 9 and 10 November 2000 when all spacecraft first exited the dusk-side magnetosphere at about 19:00 LT, providing extensive coverage of the near flank magnetosheath and magnetopause boundary layer on re-entry to the magnetosphere. The traversals on this occasion were caused by the arrival of an intense CME at the Earth, which produced a large compression of the magnetopause and high magnetic activity. The magnetopause traversals represent an unprecedented data set, allowing detailed analysis of the local magnetic structure (gradients) and dynamics of the magnetopause boundary. By performing minimum variance analysis (MVA) on the magnetic field data from all four spacecraft, we demonstrate that the magnetopause was planar on the scale of the spacecraft separation scales and that the transverse scale size of the magnetopause boundary layer was 1000–1100 km. We also show that the motion of the boundary (defined by the magnetic shear at the current layer), is changing over the sequence of spacecraft crossings so that acceleration of the magnetopause can be very high in this region of the magnetosphere. Indeed, the magnetopause speed reaches the order of 300 km/s in response to the arrival of the interplanetary shock. Using MVA coordinates, we have identified a number of magnetospheric and magnetosheath FTE signatures, which are sampled simultaneously by all spacecraft at different distances from and on either side of the magnetopause. The signatures show a variation of scale with distance from the boundary.Key words. Magnetospheric physics (magnetopause, cusp and boundary layers) Space plasma physics (discontinuities; magnetic reconnection)

Observation of the interstellar helium cone by the NOZOMI spacecraft

‘The Japanese Mars probe, NOZOMI, is staying in the interplanetary space (1–1.5 AU) until its Mars’ orbit insertion scheduled in early 2004. Every 16 months on this interplanetary orbit the spacecraft crosses around 1 AU the ‘gravitational focusing cone’ of the interstellar helium, which are penetrating into the inner heliosphere under the solar gravity. During the first crossing of the cone in the season of March–May 2000, we observed these helium particles after the solar wind pickup process with an E/q type ion detector aboard NOZOMI. We have estimated the original temperature of the interstellar helium as 11 000 K.

The low-latitude flank magnetosheath, magnetopause and boundary layer: WIND observations

This paper presents highlights of a survey of the WIND spacecraft crossings of the low-latitude dawn and dusk magnetosheath, magnetopause (MP), and the low-latitude boundary layer (LLBL). In the magnetosheath the plasma flow direction is found to be remarkably steady, irrespective of the local magnetic field direction. In the LLBL, the flow direction in a substantial portion of the region is nearly aligned with that in the magnetosheath, while the flow tangential to the MP slows down gradually across the LLBL. This flow characteristic seems to be independent of the external magnetosheath magnetic field orientation, and may suggest that the LLBL is locally coupled to the adjacent magnetosheath, irrespective of the magnetic shear across the local MP.

Observations of a very thin collisionless shock

The structure of collisionless shocks is governed by the upstream plasma conditions. The laminar collisionless shock is well studied and has been found to have a ramp thickness on the order of an ion inertial length. The behavior of the shock at higher Mach number and β is not as thoroughly studied. However, it has been speculated that an exactly perpendicular shock behaves like the soliton wave solution from classic plasma theory. This would result in a much thinner shock ramp with a thickness on the order of electron inertial lengths. No such thin shock has been observed in the laminar regime. In this study of ISEE‐1 and ‐2 crossings of supercritical, nearly‐perpendicular bow shocks (θBn > 80°), we find that the shock velocity is highly variable between spacecraft crossings and during observations. The fine structure of high velocity shocks is poorly resolved by the ISEE‐1 and ‐2 magnetometers due to filters used to prevent aliasing of the digitized data. When we restrict our attention to slowly moving shocks, we find one example of a very thin shock with a ramp thickness of only two electron inertial lengths. We suggest that this thin structure arises when the nearly perpendicular shock moves into the perpendicular regime, but cannot confirm this conjecture because of the uncertainty and temporal variability of the normal.

The electromagnetic effects of the solar wind interaction with Phobos

The electromagnetic effects of the solar wind interaction with the neutral gas halo and dust ring around Phobos are examined. The analysis is used to account for the strong correlation which arises during the Phobos mission between the local enhancement of the electron density and magnetic fluctuations and the spacecraft crossings with the Phobos orbit. The neutral gas interaction cannot account for the correlation since the outgassing rate of Phobos is too low. For the dust ring interaction however, a local enhancement of the electron density is expected in regions which are dominated by positively charged dust grains and magnetic disturbances will arise. The electrons are decoupled from the solar wind protons and evidence of this is seen from the high level of wave activity at frequencies below 100 Hz.

The electromagnetic effects of the solar wind interaction with the Phobos neutral gas halo and dust torus

The electromagnetic effects of the solar wind interaction with the neutral gas halo and the dust ring around Phobos are examined in this paper. The analysis is used to account for the strong correlation which arose during the Phobos-2 mission between the local enhancement of the electron density and magnetic fluctuations and the spacecraft crossings with the Phobos orbit. The solar wind/neutral gas interaction cannot account for the observed correlation, since the outgassing rate of Phobos is too low. For the solar wind/dust ring interaction, however, it is shown that a local enhancement of the electron density is expected in regions of the dust ring which are dominated by positively charged dust grains. These electrons become decoupled from the solar wind protons in the dust ring region, and evidence of this decoupling can be seen from the high level of wave activity which results at frequencies below 100 Hz.

The effect of hydromagnetic waves on the shape of the plasmapause boundary

We show that hydromagnetic wave electric fields could cause the fluctuations in cold plasma number density that have been observed as spacecraft cross the plasmapause. To explore the effects of a hydromagnetic wave electric field on the particles in the vicinity of the plasmapause we constructed a simple computer model of the flow of cold plasma in the equatorial plane of the magnetosphere. A hydromagnetic wave electric field which is localized in space and time is introduced. Particles are perturbed by the wave electric field. The resulting fluctuations in cold plasma number density are most evident in the dusk region and occur over both large (∼1 RE) and small (∼0.1 RE) distance scales. As these variations in the number density are similar to those observed, hydromagnetic wave electric fields could very well account for the density variations observed in the vicinity of the plasmapause boundary.

Evidence of drift waves at the plasmapause

As the Hawkeye 1 spacecraft crosses the plasmapause at high altitudes (R > 3 RE), a band of electric field noise is often detected in the frequency channels from 1.7 to 178 Hz. No corresponding magnetic field noise is detected, indicating that the noise is electrostatic (or at least quasi‐electrostatic), and the electric field is polarized perpendicular to the plasma density gradient. The noise is only detected when the scale length of the plasmapause is 0.1 RE or less, indicating that a large density gradient is required to produce the noise. These characteristics are all consistent with the interpretation that this noise consists of electrostatic waves excited by the drift mode instability. By using reasonable assumptions concerning the wavelengths of these waves the observed frequency spectrum can be explained as being due to Doppler shifts caused by spacecraft motion through the plasma.

Magnetosphere boundary observations along the Imp 7 orbit, 1. Boundary locations and wave level variations

We discuss and analyze magnetosphere boundary phenomena observed by the Imp 7 magnetic field, plasma, and plasma wave instruments in 1972 and 1973. The spacecraft crosses the dawn and dusk boundaries near 25 RE downstream, and the physical processes at the Imp 7 magnetopause appear to be intermediate between those observed over the poles and those observed at the lunar orbit. The Imp 7 orbit also traverses a downstream region near where ‘fireball’ phenomena occur, and this indicates that it is especially important to investigate the range of magnetopause phenomena detected with the plasma and fields payload complement. This report contains a brief description of the relevant Imp 7 instrumentation, a survey of the boundary locations for a 15-month period, a discussion of the different types of crossings (essentially, sharp and diffuse), and an analysis of the electromagnetic wave modes being detected in the broad low-frequency channel of the wave instrument. The wave mode discussion, based on comparison of Imp 7 data with low-frequency plasma sheet and magnetosheath observations from other spacecraft, leads to the conclusion that the broad low-frequency channel is sensitive to oscillations in the lower hybrid resonance region of the spectrum. Details of the broad diffuse boundary crossings will be discussed in a separate report.