M. W. Dunlop

The Cluster Magnetic Field Investigation

The Cluster mission provides a new opportunity to study plasma processes and structures in the near-Earth plasma environment. Four-point measurements of the magnetic field will enable the analysis of the three dimensional structure and dynamics of a range of phenomena which shape the macroscopic properties of the magnetosphere. Difference measurements of the magnetic field data will be combined to derive a range of parameters, such as the current density vector, wave vectors, and discontinuity normals and curvatures, using classical time series analysis techniques iteratively with physical models and simulation of the phenomena encountered along the Cluster orbit. The control and understanding of error sources which affect the four-point measurements are integral parts of the analysis techniques to be used. The flight instrumentation consists of two, tri-axial fluxgate magnetometers and an on-board data-processing unit on each spacecraft, built using a highly fault-tolerant architecture. High vector sample rates (up to 67 vectors s-1) at high resolution (up to 8 pT) are combined with on-board event detection software and a burst memory to capture the signature of a range of dynamic phenomena. Data-processing plans are designed to ensure rapid dissemination of magnetic-field data to underpin the collaborative analysis of magnetospheric phenomena encountered by Cluster.

Temporal evolution of the electric field accelerating electrons away from the auroral ionosphere

The bright night-time aurorae that are visible to the unaided eye are caused by electrons accelerated towards Earth by an upward-pointing electric field. On adjacent geomagnetic field lines the reverse process occurs: a downward-pointing electric field accelerates electrons away from Earth. Such magnetic-field-aligned electric fields in the collisionless plasma above the auroral ionosphere have been predicted, but how they could be maintained is still a matter for debate. The spatial and temporal behaviour of the electric fields—a knowledge of which is crucial to an understanding of their nature—cannot be resolved uniquely by single satellite measurements. Here we report on the first observations by a formation of identically instrumented satellites crossing a beam of upward-accelerated electrons. The structure of the electric potential accelerating the beam grew in magnitude and width for about 200u2009s, accompanied by a widening of the downward-current sheet, with the total current remaining constant. The 200-s timescale suggests that the evacuation of the electrons from the ionosphere contributes to the formation of the downward-pointing magnetic-field-aligned electric fields. This evolution implies a growing load in the downward leg of the current circuit, which may affect the visible discrete aurorae.

Conditions for the formation of hot flow anomalies at Earth's bow shock

Hot flow anomalies (HFAs) result from the interaction of an interplanetary current sheet with Earths bow shock and were discovered over a decade and a half ago. The deflected flow and hot interior of an HFA are consequences of ions reflected at the bow shock being channeled along the current sheet. Previous studies have shown that this requires a solar wind motional electric field pointing toward the current sheet on at least one side and that the current sheet must be a tangential discontinuity. Recent reports of a rapid displacement of the magnetopause by 5 Re as the result of an HFA have led us to explore the interplanetary conditions surrounding all reported HFAs. The kinetic aspects of HFA formation suggest that current sheets should pass relatively slowly along the bow shock; that is, their normals should have large cone angles. This hypothesis is confirmed. Individual multispacecraft case studies confirm that the underlying current sheets are tangential discontinuities, but most HFAs have relatively small jumps in field magnitude from before to after and thus would fail traditional identification tests as definite tangential discontinuities. The combination of our results suggests that HFAs should occur at a rate of several per day, and thus they may play a significant role in the solar-terrestrial dynamics.

A survey of flux transfer events recorded by the UKS spacecraft magnetometer

Abstract The UKS spacecraft operated from August 1984 through to January 1985. During that time, it made multiple crossings of the magnetopause in local time sectors extending from mid-afternoon to just behind the dawn meridian. We have surveyed the magnetometer records from these magnetopause encounters and have compiled a catalogue of flux transfer events (FTEs using criteria identical to those employed by Rijnbeek et al. (1984, J. Geophys. Res. 89, 786) in their survey of ISEE spacecraft magnetometer data. Using the catalogue, we find that FTE occurrence determined from the UKS data set is substantially less than that detected using data from the early ISEE 1 2 spacecraft orbits. The UKS data set shows a correlation between FTE occurrence and southward external magnetic field, but there are several instances of passes in which no FTEs are detected but for which the external field was unam- biguousluy southward. The passes with the largest number of events are those for which the field outside the magnetopause has a large BM component. We conclude that the lower latitude of the UKS encounters is responsible for the discrepancy with the ISEE occurrence. The most likely source region appears to be near the subsolar region.

Mirror mode structures observed in the dawn‐side magnetosheath by Equator‐S

The Equator-S satellite was ideally positioned to make magnetic field observations in the dawn-side magnetosheath, relatively close to the magnetopause. The magnetosheath data were particularly rich in compressional signatures, consistent with mirror mode structures, which occurred during ∼30% of orbits crossing into the magnetosheath. In most, although not all cases, strongly compressive signatures extended up to the magnetopause boundary, with no increase in the underlying magnetic field magnitude on the time scale of ten to thirty minutes. The proximity and character of mirror-like fluctuations near the magnetopause suggest that in the dawn-side magnetosheath the plasma depletion layer (PDL) is of narrower extent than is generally observed closer to the subsolar point, or is absent.

Kinetic study of the mirror mode

The linear Vlasov dispersion is solved to reveal the kinetic properties of the mirror mode. The existence of the torsional component of both the magnetic and velocity perturbations are kinetic features that are not explained by MHD theory. A parameter study is then employed to clarify the behavior of these particular components under different plasma conditions. The key parameters are the ion temperature anisotropy and the plasma to magnetic pressure ratio which act in similar ways to control the value of the torsional (or Alfvenic) components. Data from the Active Magnetospheric Particle Tracer Explorers mission show that under most magnetosheath plasma conditions the magnetic torsional component is negligible, whereas the velocity one is always larger or comparable to the other components. This experimental illustration also shows that there exist elliptically polarized mirror modes, contrary to what our, and all previous, analytical treatments predict.

Multisatellite observations of large magnetic depressions in the solar wind

Two large depressions in the magnitude of the interplanetary magnetic field, lasting ∼10-20 min, have been observed in the solar wind just upstream of the Earths bow shock by three spacecraft (Active Magnetospheric Particle Tracer Explorer UK Subsatellite (AMPTE UKS), AMPTE Ion Release Module (IRM), and ISEE 1). The multiple satellite observations show that the depressions are convecting with the ambient solar wind. Analysis of the depression boundaries shows that they are tangential discontinuities with normals aligned approximately parallel to the GSE x direction. The electron distributions measured within the structures are remarkably isotropic when compared to the more anisotropic distributions found in the ambient solar wind. There is also a reduction in plasma wave activity during the depressions. The depressions exhibit characteristics similar to those of solar wind magnetic holes but are much larger than has been typically observed. The depressions also display similarities with encounters of the heliospheric plasma sheet and heat flux dropouts, both of which are typically observed near sector boundaries, close to the heliospheric current sheet. The nature of these depressions is discussed in the context of magnetic hole and heliospheric plasma sheet observations. A large magnetic hole structure formed from a conglomeration of small holes appears the more likely scenario for the observed depressions.

Correspondence between field aligned currents observed by Ulysses and HST auroral emission

Abstract Auroral emission is direct evidence for the magnetophere/ionosphere coupling which exists for a magnetised planet such as Jupiter. This coupling takes place by way of momentum transfer which occurs via field aligned currents. The driving mechanism for the auroral emission is charged particle precipitation along the field lines. During the Ulysses flyby of Jupiter in February 1992 high latitude auroral events were recorded by the Faint Object Camera on the Hubble Space Telescope using direct imaging of UV emissions. Measurements of magnetospheric field aligned current signatures were recorded by the dual fluxgate/vector helium magnetometer onboard the Ulysses spacecraft on the same day as the auroral, emissions. A correspondence is found to occur between the locus of auroral emission on Jupiter and the oval traced onto the auroral region by projection of the observed interval of field aligned currents. The field line mapping used to establish this correspondence is performed using a magnetospheric field model tailored to the conditions observed during the Ulysses flyby.

Ulysses observations of the magnetic field structure within CIRs

Ulysses encountered two separate sequences of corotating interaction regions (CIRs) in 1992–3 and 1996–7 when the spacecraft was in the southern and northern solar hemispheres, respectively. We have examined magnetometer data from one event observed in each hemisphere. Using two separate analytical methods we provide evidence for the magnetic field vectors within these CIRs being strongly confined to a series of near-parallel planes. The distribution of the field within the interaction region is characteristic of that observed in planar magnetic structures. This planar order is evident over both small (3-hour) and large (whole event) scales. The CIRs display opposed north-south tilts that are consistent with previous authors analysis of solar wind plasma data. These results are in direct agreement with the tilted dipole model of three-dimensional corotating stream interactions.

Ulysses observations of field-perpendicular plasma flows in the Jovian magnetosphere: comparison of ExB velocity vectors derived from energetic ion and thermal electron data

Abstract The E x B velocity vectors which correspond to the field-perpendicular flow of the low-energy plasma have been independently derived for the Ulysses flyby of Jupiter using energetic ion and thermal electron data. In this paper these measurements are compared. It is shown that reasonable quantitative agreement exists during the prenoon inbound pass, though with some significant differences. Both data sets indicate the presence of slow field-perpendicular flows in the dayside outer magnetosphere (∼80–110 R J ), which are directed azimuthally opposite to corotation with the planet and radially inwards, with magnitudes of ∼100 km s −1 in each component. Any variations in this flow during the outer magnetosphere traversal are not resolved within the ∼±100 km s −1 uncertainties in the individual 35 min-averaged data. Similar flows, but of somewhat smaller magnitude, are also found in both data sets in the higher-latitude region of the inbound middle magnetosphere flanking the plasma sheet (∼45–70 R J ). It is inferred that these field lines map equatorially into the outer magnetosphere at larger distances. Flows within the middle magnetosphere plasma sheet in this region are on average in the sense of planetary rotation, but the average azimuthal velocity determined from the ATs data (∼20 km s −1 , essentially consistent with zero) is significantly smaller than that determined from the SWOOPS data (∼100 km s −1 ). A systematic effect thus seems to be present within the current sheet, possibly associated with additional (usually small) terms in the expression for the energetic ion anisotropy which have not been taken into account in the analysis procedure. Analysis of the inbound data overall, however, indicates no consistent velocity offset between these data sets to within a few tens of km s −1 , and a unit gradient between them within a factor of ∼1.5. Due to the large uncertainties in individual ∼35-min velocity values, however, and the possible current sheet effects mentioned above, the cross-correlation coefficient between the data sets is low, with an overall value of 0.23 for the principal azimuthal component observed in magnetospheric regions over a 4-day interval on the inbound pass. The probability of this degree of correlation appearing by chance, however, is only about one in 500. On the outbound pass, we find that the velocity estimates determined from the two data sets do not agree, even qualitatively. We believe that this is due to a complicated and anisotropic background in the electron data which we have been unable to fully remove.