Field-aligned Current Region Research Articles

A theoretical model for the distribution of latitudinal extents of field-aligned electron acceleration

[1] We reassess the mathematical expression for statistical distributions of latitudinal extents of electron acceleration events; the distributions have been obtained by Newell et al. (1996, hereinafter referred to as N96) using precipitation data from all Defense Meteorological Satellite Program satellites. Paper N96 proposed a basic hypothesis that within a (given) large-scale region of upward field-aligned currents (FACs), any 1-s spectrum has a constant probability, P, of experiencing electron acceleration independently of neighboring spectra. Directly from this hypothesis, the probability of observing n consecutive accelerated spectra, denoted by Q n , is newly derived as the correction of p n-1 (1 - that was assumed in N96. Necessarily, Q n is a function of the probability P and the latitudinal width W of a large-scale FAC region; hence it is written as Q n (W, P). The occurrence frequency distribution (histogram) of latitudinal extents of observed acceleration events can be identified with a statistical superposition of a number of Q 0 (W, P) in relatively wide ranges of Wand P. For fitting to the histograms obtained in N96, 0.1° Wand 0.7 P 0.98 are appropriate. This prediction is quite in contrast with N96's supposition that P is nearly constant and that normally the width of a large-scale FAC region is sufficiently large to accommodate many accelerated spectra. (The appearance of region 1 FACs of narrow widths has already been identified both observationally and theoretically.)

Simulation of the interchange instability in a magnetospheric substorm site

Abstract. We perform modeling of the interchange instability driven by longitudinal pressure asymmetry in the region of the pressure buildup that forms in the inner magnetosphere at the substorm growth phase. The simulation refers to the dawnward side of the Harang discontinuity and times after Bz IMF turning northward. The solution for the equilibrium state indicates tailward flows associated with vortices, which is in agreement with a previous finding of Ashour-Abdalla et al. (1999, 2002). We show that in the regions of equilibrium field-aligned currents (FACs), small initial perturbations in pVγ (p is the isotropic plasma pressure, V is the unit magnetic flux tube volume, γ=5/3 the adiabatic exponent), set up as ripples inclined to azimuth, grow in time. For the background FAC of ~10-6 A/m2, the linear growth rate of the instability is ~6 min. Starting from the 12th min of evolution, the perturbations exhibit nonlinear deformations, develop undulations and front steepening. An interesting peculiarity in the distribution of the associated small-scale FACs is that they become asymmetric with time. Specifically, the downward currents are more localised, reaching densities up to 15×10-6 A/m2 at the nonlinear stage. The upward FACs are more dispersed. When large enough, these currents are likely to produce the aurora. We also run our simulation for the initial perturbations of large transverse scales in order to demonstrate that the interchange instability can be responsible for pressure and cross-tail current spatial variations of great extent.

Complex study of the auroral arc dynamics and ionospheric plasma convection in prenoon hours

The dynamics of auroral arcs, observed in the prenoon sector during the 2-h period, has been studied in the context of ionospheric convection. The appearance of an isolated arc, the poleward drift velocity of which pronouncedly exceeded the plasma drift velocity, accompanied the IMF impulse and could result from the Alfven resonance oscillations of the magnetosphere. Arcs that appeared after the northward turning of the IMF vertical component drifted poleward at a velocity close to the convection velocity. The mechanism of arc generation is related to the flute instability which develops in the region of the large-scale field-aligned current. Flute instability indications are found out in the POLAR satellite data. The study confirms the previously proposed classification criterion for dayside arcs with the source on closed field lines, based on the character of arc drift as compared to convection.

Field-aligned currents observed by CHAMP during the intense 2003 geomagnetic storm events

Abstract. This study concentrates on the characteristics of field-aligned currents (FACs) in both hemispheres during the extreme storms in October and November 2003. High-resolution CHAMP magnetic data reflect the dynamics of FACs during these geomagnetic storms, which are different from normal periods. The peak intensity and most equatorward location of FACs in response to the storm phases are examined separately for both hemispheres, as well as for the dayside and nightside. The corresponding large-scale FAC peak densities are, on average, enhanced by about a factor of 5 compared to the quiet-time FACs' strengths. And the FAC densities on the dayside are, on average, 2.5 times larger in the Southern (summer) than in the Northern (winter) Hemisphere, while the observed intensities on the nightside are comparable between the two hemispheres. Solar wind dynamic pressure is correlated with the FACs strength on the dayside. However, the latitudinal variations of the FACs are compared with the variations in Dst and the interplanetary magnetic field component Bz, in order to determine how these parameters control the large-scale FACs' configuration in the polar region. We have determined that (1) the equatorward shift of FACs on the dayside is directly controlled by the southward IMF Bz and there is a saturation of the latitudinal displacement for large value of negative Bz. In the winter hemisphere this saturation occurs at higher latitudes than in the summer hemisphere. (2) The equatorward expansion of the nightside FACs is delayed with respect to the solar wind input. The poleward recovery of FACs on the nightside is slower than on the dayside. The latitudinal variations on the nightside are better described by the variations of the Dst index. (3) The latitudinal width of the FAC region on the nightside spreads over a wide range of about 25° in latitude.

Study of plasma pressure distribution in the inner magnetosphere using low-altitude satellites and its importance for the large-scale magnetospheric dynamics

Plasma pressure distribution in the inner magnetosphere is one of the key parameters for understanding main processes of the magnetospheric dynamics including geomagnetic substroms. However, the pressure profiles obtained from in situ measurements by the high-altitude satellites do not allow the tracking of the magnetospheric dynamics, because a time necessary to obtain these profiles (hours) generally exceeds the characteristic times of the main magnetospheric processes (minutes or less). On contrary, fast movement of low-altitude satellites makes it possible to obtain quasi-instantaneous profiles of plasma pressure along the satellite trajectory – radial or azimuthal, depending on the satellite orbit. Precipitating particle fluxes, measured by the low-altitude Aureol-3 satellite were used. Mapping into the equatorial plane and determination of a volume of the magnetic flux tube were made using the Tsyganenko 96 and 01 geomagnetic field models. Study of radial plasma gradients showed that the inner magnetosphere is stable for development of flute (interchange) instability. Modified interchange instability related to azimuthal plasma pressure gradients and field-aligned currents in equilibrium was proposed as a source of substorm expansion phase onset. It can develop when the density of the field-aligned current reaches a definite threshold value. The growth rate of the instability is higher in the region of upward field-aligned current, where the existing field-aligned potential drop leads to the magnetosphere–ionosphere decoupling.

Long-period magnetic disturbances or Pc5 events aboard INTERBALL-Auroral and POLAR

This paper discusses observations of dynamic magnetic field variations on auroral field lines at mid and high altitudes, utilizing a conjunction of POLAR and INTERBALL-Auroral spacecraft in the dusk magnetosphere on January 11, 1997. Both spacecraft indicate the presence of periodic magnetic variations in the Pc5 frequency range in the field-aligned current region 1. The satellites are believed to be closely on the same field line or within a flux tube (radius 130 km of its projection in the ionosphere). The polarization analysis shows an interesting event: a rotation of the main polarization axis by about 90° between POLAR and INTERBALL. When the two spacecraft are very close to a common field line, the difference between their observed magnetic fields might arise from a rotation of the field between the two altitudes. We cannot exclude different polarization properties arising from different horizontal positions (although small) of both satellite with respect to local plasma boundary. Frequency analysis of both solar wind density (WIND) and INTERBALL magnetic field showed that the solar wind pressure variation contains the same spectrum (2.4–4.4 mHz). It appears that the solar wind pressure variations are sufficient for a parametric mechanism of excitation of compressional disturbances propagating along the magnetopause and within the magnetospheric cavity.

On the generation of enhanced sunward convection and transpolar aurora in the high‐latitude ionosphere by magnetic merging

The IMAGE Wideband Imaging Camera (WIC) instrument observed the duskside development of an oval‐aligned transpolar auroral arc (TPA) in the Northern Hemisphere (NH) on 16 December 2001 during strong IMF ∣B∣ ∼ 18 nT and a generally steady ∼56° clock angle (positive IMF By and Bz). Observational evidence suggests that the dayside part of the duskside TPA formed due to quasi‐continuous merging between the IMF and the lobe magnetic field tailward of the cusp while the nightside part is associated with the Harang discontinuity. The low‐altitude CHAMP satellite confirms an upward northward IMF Bz (NBZ) field‐aligned current (FAC) over the dayside TPA while associating a downward NBZ current with the region of diminished WIC emissions in between the auroral oval and the TPA. DMSP F14 suggests that the dayside region of the downward NBZ current coincides with precipitating magnetosheath‐like ions of reversed energy‐latitude dispersion consistent with high‐latitude reconnection. SuperDARN observes enhanced ionospheric sunward flows generally centered between the oppositely directed NBZ currents. We associate these flows with a clockwise lobe convection vortex and the dayside part of the TPA. The nightside TPA, however, is related to stagnant or antisunward flow and the upward FAC region of the Harang discontinuity. Cluster observations confirm the simultaneous presence of rotational discontinuities across the duskside magnetopause with changes in the magnetosheath plasma velocity that indicate an active merging region poleward of Cluster. A global MHD simulation generates sunward flow between a pair of opposite FACs on either side of a lobe reconnection site near (X, Y, Z)GSM = (−4.7, 5.4, 10.2) RE thus conforming with Cluster and SuperDARN expectations. The sense of these FACs agrees with the low‐altitude NBZ observations.

Localized fast flow disturbance observed in the plasma sheet and in the ionosphere

Abstract. An isolated plasma sheet flow burst took place at 22:02 UT, 1 September 2002, when the Cluster footpoint was located within the area covered by the Magnetometers-Ionospheric Radars-All-sky Cameras Large Experiment (MIRACLE). The event was associated with a clear but weak ionospheric disturbance and took place during a steady southward IMF interval, about 1h preceding a major substorm onset. Multipoint observations, both in space and from the ground, allow us to discuss the temporal and spatial scale of the disturbance both in the magnetosphere and ionosphere. Based on measurements from four Cluster spacecraft it is inferred that Cluster observed the dusk side part of a localized flow channel in the plasma sheet with a flow shear at the front, suggesting a field-aligned current out from the ionosphere. In the ionosphere the equivalent current pattern and possible field-aligned current location show a pattern similar to the auroral streamers previously obtained during an active period, except for its spatial scale and amplitude. It is inferred that the footpoint of Cluster was located in the region of an upward field-aligned current, consistent with the magnetospheric observations. The entire disturbance in the ionosphere lasted about 10min, consistent with the time scale of the current sheet disturbance in the magnetosphere. The plasma sheet bulk flow, on the other hand, had a time scale of about 2min, corresponding to the time scale of an equatorward excursion of the enhanced electrojet. These observations confirm that localized enhanced convection in the magnetosphere and associated changes in the current sheet structure produce a signature with consistent temporal and spatial scale at the conjugate ionosphere.

Solar wind-magnetosphere–ionosphere coupling at Jupiter

A fundamental property of planetary magnetospheres, which varies from planet to planet, is the dynamical influence of either the solar wind and its embedded interplanetary magnetic field (IMF) or of the rotation of the planet. Jovian magnetospheric dynamics are mainly dominated by the combination of rapid planetary rotation and the outflow of material originating from the moon Io, orbiting deep within the magnetospheric cavity. The main auroral oval in Jupiter’s polar ionosphere appears fixed with respect to the planet, an indication of planetary control, and is understood to be associated with large-scale magnetosphere–ionosphere coupling, and the transfer of angular momentum from the ionosphere to the middle magnetosphere plasma. We will first discuss the origin of this main auroral oval, and the theoretical model which followed. We will also summarise recent developments of the theoretical model to include the effects of precipitation-induced enhancements of the ionospheric Pedersen conductivity in regions of upward field-aligned currents. Jupiter’s polar auroral emissions, which include all auroral emission lying poleward of the main auroral oval, are ordered by magnetic local time, indicating potential external control by the solar wind. We have recently considered from a theoretical standpoint what the effects of pulsed dayside magnetic reconnection will be at Jupiter. This will generate a twin-vortical flow pattern in the ionosphere across the open-closed field line boundary, associated with a bi-polar (i.e. upward and downward) field-aligned current pair. Here, we discuss the conditions under which such currents may be carried in either magnetospheric or cusp plasmas, and summarise the consequences for auroral emissions at UV and X-ray wavelengths.

Temporal evolution of two auroral arcs as measured by the Cluster satellite and coordinated ground-based instruments

Abstract. The four Cluster s/c passed over Northern Scandinavia on 6 February 2001 from south-east to north-west at a radial distance of about 4.4 RE in the post-midnight sector. When mapped along geomagnetic field lines, the separation of the spacecraft in the ionosphere was confined to within 110km in latitude and 50km in longitude. This constellation allowed us to study the temporal evolution of plasma with a time scale of a few minutes. Ground-based instrumentation used involved two all-sky cameras, magnetometers and the EISCAT radar. The main findings were as follows. Two auroral arcs were located close to the equatorward and poleward edge of a large-scale density cavity, respectively. These arcs showed a different kind of a temporal evolution. (1) As a response to a pseudo-breakup onset, both the up- and downward field-aligned current (FAC) sheets associated with the equatorward arc widened and the total amount of FAC doubled in a time scale of 1–2min. (2) In the poleward arc, a density cavity formed in the ionosphere in the return (downward) current region. As a result of ionospheric feedback, a strongly enhanced ionospheric southward electric field developed in the region of decreased Pedersen conductance. Furthermore, the acceleration potential of ionospheric electrons, carrying the return current, increased from 200 to 1000eV in 70s, and the return current region widened in order to supply a constant amount of return current to the arc current circuit. Evidence of local acceleration of the electron population by dispersive Alfvén waves was obtained in the upward FAC region of the poleward arc. However, the downward accelerated suprathermal electrons must be further energised below Cluster in order to be able to produce the observed visible aurora. Both of the auroral arcs were associated with broad-band ULF/ELF (BBELF) waves, but they were highly localised in space and time. The most intense BBELF waves were confined typically to the return current regions adjacent to the visual arc, but in one case also to a weak upward FAC region. BBELF waves could appear/disappear between s/c crossings of the same arc separated by about 1min. Key words. Ionosphere (electric fields and currents) – Magnetospheric physics (auroral phenomena; magnetosphereionosphere interactions)

Characteristics of quasi-static potential structures observed in the auroral return current region by Cluster

Abstract. Temporal and spatial characteristics of intense quasi-static electric fields and associated electric potential structures in the return current region are discussed using Cluster observations at geocentric distances of about 5 Earth radii. Results are presented from four Cluster encounters with such acceleration structures to illustrate common as well as different features of such structures. The electric field structures are characterized by (all values are projected to 100 km altitude) peak amplitudes of ≈1V/m, bipolar or unipolar profiles, perpendicular scale sizes of ≈10km, occurrence at auroral plasma boundaries associated with plasma density gradients, downward field-aligned currents of ≈10µA/m2, and upward electron beams with characteristic energies of a few hundred eV to a fewkeV. Two events illustrate the temporal evolution of bipolar, diverging electric field structures, indicative of positive U-shaped potentials increasing in magnitude from less than 1kV to a few kV on a few 100s time scale. This is also the typical formation time for ionospheric plasma cavities, which are connected to the potential structure and suggested to evolve hand-in-hand with these. In one of these events an energy decay of inverted-V ions was observed in the upward field-aligned current region prior to the acceleration potential increase in the adjacent downward current region, possibly suggesting that a potential redistribution took place between the two current branches. The other two events were characterized by intense unipolar electric fields, indicative of S-shaped potential contours and were encountered at the polar cap boundary. The total observation time for these events was typically 10-20s, too short for monitoring the evolution of the structure, but yet of interest for revealing their short term stability. The locations of the two bipolar events at the poleward boundary of the central plasma sheet and of the two unipolar events at the polar cap boundary, suggest that the special profile shape depends on whether plasma populations, dense enough to support upward field-aligned currents and closure of the return current, exist on both sides, or on one side only, of the boundary.

Magnetosphere-ionosphere coupling currents in Jupiter's middle magnetosphere: effect of precipitation-induced enhancement of the ionospheric Pedersen conductivity

Abstract. We consider the effect of precipitation-induced enhancement of the Jovian ionospheric Pedersen conductivity on the magnetosphere-ionosphere coupling current system which is associated with the breakdown of the corotation of iogenic plasma in Jupiter's middle magnetosphere. In previous studies the Pedersen conductivity has been taken to be simply a constant, while it is expected to be significantly enhanced in the regions of upward-directed auroral field-aligned current, implying downward precipitating electrons. We develop an empirical model of the modulation of the Pedersen conductivity with field-aligned current density based on the modelling results of Millward et al. and compute the currents flowing in the system with the conductivity self-consistently dependent on the auroral precipitation. In addition, we consider two simplified models of the conductivity which provide an insight into the behaviour of the solutions. We compare the results to those obtained when the conductivity is taken to be constant, and find that the empirical conductivity model helps resolve some outstanding discrepancies between theory and observation of the plasma angular velocity and current system. Specifically, we find that the field-aligned current is concentrated in a peak of magnitude ~0.25µAm-2 in the inner region of the middle magnetosphere at ~20 RJ, rather than being more uniformly distributed as found with constant conductivity models. This peak maps to ~17° in the ionosphere, and is consistent with the position of the main oval auroras. The energy flux associated with the field-aligned current is ~10mWm-2 (corresponding to a UV luminosity of ~100kR), in a region ~0.6° in width, and the Pedersen conductivity is elevated from a background of ~0.05mho to ~0.7mho. Correspondingly, the total equatorial radial current increases greatly in the region of peak field-aligned current, and plateaus with increasing distance thereafter. This form is consistent with the observed profile of the current derived from Galileo magnetic field data. In addition, we find that the solutions using the empirical conductivity model produce an angular velocity profile which maintains the plasma near to rigid corotation out to much further distances than the constant conductivity model would suggest. Again, this is consistent with observations. Our results therefore suggest that, while the constant conductivity solutions provide an important indication that the main oval is indeed a result of the breakdown of the corotation of iogenic plasma, they do not explain the details of the observations. In order to resolve some of these discrepancies, one must take into account the elevation of the Pedersen conductivity as a result of auroral electron precipitation.Key words. Magnetospheric physics (current systems, magnetosphere-ionosphere interactions, planetary magnetospheres)70d

Morphology of the ring current derived from magnetic field observations

Abstract. Our examination of the 20 years of magnetospheric magnetic field data from ISEE, AMPTE/CCE and Polar missions has allowed us to quantify how the ring current flows and closes in the magnetosphere at a variety of disturbance levels. Using intercalibrated magnetic field data from the three spacecraft, we are able to construct the statistical magnetic field maps and derive 3-dimensional current density by the simple device of taking the curl of the statistically determined magnetic field. The results show that there are two ring currents, an inner one that flows eastward at ~3 RE and a main westward ring current at ~4–7 RE for all levels of geomagnetic disturbances. In general, the in-situ observations show that the ring current varies as the Dst index decreases, as we would expect it to change. An unexpected result is how asymmetric it is in local time. Some current clearly circles the magnetosphere but much of the energetic plasma stays in the night hemisphere. These energetic particles appear not to be able to readily convect into the dayside magnetosphere. During quiet times, the symmetric and partial ring currents are similar in strength (~0.5MA) and the peak of the westward ring current is close to local midnight. It is the partial ring current that exhibits most drastic intensification as the level of disturbances increases. Under the condition of moderate magnetic storms, the total partial ring current reaches ~3MA, whereas the total symmetric ring current is ~1MA. Thus, the partial ring current contributes dominantly to the decrease in the Dst index. As the ring current strengthens the peak of the partial ring current shifts duskward to the pre-midnight sector. The partial ring current is closed by a meridional current system through the ionosphere, mainly the field-aligned current, which maximizes at local times near the dawn and dusk. The closure currents flow in the sense of region-2 field-aligned currents, downward into the ionosphere near the dusk and upward out of the ionosphere near the dawn. Key words. Magnetospheric physics (current systems; storms and substorms; magnetospheric configuration and dynamics)

Radial plasma pressure gradients in the high latitude magnetosphere as sources of instabilities leading to the substorm onset

Various types of instabilities have been proposed as sources of inner magnetosphere substorm expansion phase onset. Distribution of plasma pressure in the plasma sheet, β parameter, volume of magnetic flux tube are the key parameters for understanding the processes leading the instability development. Here, the radial distribution of plasma pressure was recovered using the low-altitude polar-orbiting Aureol-3 precipitating particle data. Corresponding variations in the magnetic pressure and in the volume of magnetic flux tube were estimated using Tsyganenko 96 magnetic field model. It is shown that development of an instability responsible for the substorm expansion phase onset takes place in the regions of low β or at the boundary of the regions of high and low β. A possible role of the flute-like (interchange-like) instabilities is investigated. It is shown that the obtained profiles of radial plasma pressure gradients apparently are not sharp enough to develop the instability related to radial plasma pressure gradients. The role of the instability related to the existence of the azimuthal plasma pressure gradients is discussed. Such an instability can develop when the density of the field-aligned current reaches a definite threshold value. The instability develops faster in the region of upward field-aligned current, where the existing field-aligned potential drop leads to the magnetosphere–ionosphere decoupling.

Transient production of F-region irregularities associated with TCV passage

Abstract. Transient production of F-region plasma irregularities due to traveling convection vortices (TCVs) was investigated using the Super Dual Auroral Radar Network (SuperDARN) combined with ground magnetometer networks and the POLAR ultraviolet imager. We selected two large-amplitude (100–200 nT) TCV events that occurred on 22 May 1996 and 24 July 1996. It is found that the TCV-associated HF backscatter arises in blobs with spatial scale of a few hundreds km. They traveled following tailward bulk motion of the TCV across the three fields-of-view of the SuperDARN HF radars in the prenoon sector. The spectra in the blobs showed unidirectional Doppler velocities of typically 400–600 m/s, with flow directions away from the radar. These unidirectional velocities correspond to the poleward and/or eastward convective flow near the leading edge of upward field-aligned current. The backscatter blobs overlapped the poleward and westward part of the TCV-related transient aurora. It is likely that the transient backscatter blobs are produced by the three-dimensional gradient drift instabilities in the three-dimensional current system of the TCV. In this case, nonlinear rapid evolution of irregularities would occur in the upward field-aligned current region. The spectral width of the backscatter blob is typically distributed between 50 and 300 m/s, but sometimes it is over 400 m/s. This suggests that the temporal broad spectra over 400 m/s are produced by Pc1–2 bursts, while the background spectral width of 50–300 m/s are produced by the velocity gradient structure of convection vortices themselves.Key words. Ionosphere (Electric fields and currents; Ionospheric irregularities; Plasma convection)

Investigations of the hot plasma pressure gradients and the configuration of magnetospheric currents from interball

One of the main aspects of the multi satellite INTERBALL program is connected with the experimental determination of the hot plasma pressure distribution inside the magnetosphere and the solution of the well known problem of the inner magnetosphere substorm onset. The main results of the experimental observations, the structure of high latitude currents and the modeling of the magnetospheric processes are discussed. Observations of particles in a wide energy range from tens of eV to Mev energies give the possibility to construct the full distribution function and to calculate plasma pressure. Quiet time plasma pressure profiles show the regularity and the increase of the pressure with the decrease of the geocentric distance. The observed region of plasma pressure plateau means the existence of nearly zero transverse and field-aligned current regions. The existence of such a region gives the possibility to explain the observed gap between Region 1 and Region 2 currents. Investigations of the fluctuations of the plasma bulk velocity inside the plasma sheet support the existence of plasma sheet turbulence. The existence of plasma sheet turbulence explains the inner magnetosphere substorm onset as only stable before substorm plasma region can become unstable. The introduction of the turbulent transport in the model of the plasma sheet and the theory of the plasma sheet with medium-scale turbulence are discussed

The influence of the energetic tails of ion distribution function on the main parameter of the theory of field-aligned current splitting and intercosmos-Bulgaria-1300 observations

The theory of hot plasma stratification in the region of upward field-aligned current suggests the condition of the magnetostatic equilibrium and plasma pressure variation directly related to magnetospheric potential variation. This theory was developed using a Maxwellian shape for the plasma distribution function and the assumption of a definite temperature. Such an approximation underestimate the contribution of energetic particles to the plasma pressure. The use of kappa distributions, alternatively, results in a more correct value of the main parameter of the theory of hot plasma splitting. The theoretical analysis is compared with the results of experimental observations published by Antonova et al., 1998.

Magnetic cloud and magnetosphere — Ionosphere response to the 6 November 1997 CME

In the present work we analyse the magnetic cloud (MC) at 1 AU on 9 November 1997. The appearance of a hotter and dense part (dense filament), with radial extent 10 6 km, immediately behind the frontal part of the MC, is a distinctive feature of the event. The INTERBALL - Aurora] Probe had a chance to observe field-aligned currents in the mid - altitude magnetosphere during the substorm expansion phase intensification related to the dense filament. We emphasise the appearance of unusual “N”- shape magnetic structure, duration 3 min, amplitude 50 nT between field-aligned current region 1 and the magnetosphere lobe in the late evening hours. To explain some of the MC features we refer to a model scenario of the 6 November 1997 coronal mass ejection.

Modulation of Jupiter's main auroral oval emissions by solar wind induced expansions and compressions of the magnetosphere

It has recently been suggested that the jovian ‘main auroral oval’ is associated with the region of upward-directed field-aligned currents in the circuit formed through angular momentum exchange between the atmosphere–ionosphere and the equatorial iogenic magnetospheric plasma. It has also been suggested that the luminosity of these emissions should be modulated primarily by the dynamic pressure of the solar wind, which causes expansions and compressions of the magnetosphere and thereby changes the angular velocity of the magnetospheric plasma and the strength of the coupling currents. Here we present a quantitative demonstration of these effects. We consider an initial empirically based middle magnetosphere field and plasma configuration extending to 50 R J, which expands outwards to 70 R J, or is compressed inwards to 30 R J, due e.g. to sudden changes in the dynamic pressure of the solar wind. Changes in the angular velocity of the plasma are computed from the flux-preserving motions of the field lines using conservation of angular momentum. The initial configuration produces a band of aurora in which the luminosity falls over ∼1° latitude from ∼200 kR at the poleward boundary to ∼20 kR . Expansion of the current sheet to 70 R J produces a significant increase in luminosity over this range, with the largest effect occurring at the poleward boundary where a thin (∼50– 100 km ) arc-like structure of MR intensity is formed. The initial emission is reduced by about an order of magnitude for a compression to 40 R J, and essentially to zero for a compression to 35 R J, when the middle magnetosphere plasma is brought to near rigid corotation over the whole radial range such that the coupling current system is essentially switched off. Further compression to 30 R J then induces significant super-corotation, with a reversal in sense of the magnetosphere–ionosphere coupling current system. In this case the middle magnetosphere field-aligned currents reverse to downward throughout, such that we would not expect them to be associated with bright electron-induced auroras. However, upward-directed ‘return’ currents then flow in the poleward region outside of the middle magnetosphere, which may be associated with an auroral oval lying poleward of the usual ‘main oval’ location. A ‘Hill-type’ equilibrium will re-form on time scales of a few tens of hours, and with it, the main oval auroras will re-emerge, corresponding to the changed size of the magnetospheric cavity.

Conjugate observations of traveling convection vortices: The field‐aligned current system

Analysis of a single traveling convection vortex (TCV) event that combines magnetometer, auroral imager, and riometer observations from conjugate hemispheres provides a complete view of the field‐aligned current (FAC) system associated with the TCV. Conjugate observations clearly show that the TCV is a pair of FACs that flow alternatively out of and then into both ionospheres. Imaging observations at 427.8 nm indicate that the upward FAC region is associated with a narrow arc 50–100 km wide and at least 600 km long. Mapping the imaging observations made in the Southern Hemisphere to the Northern Hemisphere shows excellent collocation of the precipitation with the flow vortex associated with the upward FAC as inferred from a large two‐dimensional network of magnetometers. Also consistent with the flows inferred from the magnetometers, the arc is found to be skewed with respect to the direction of motion of the transient. Low‐altitude spacecraft passes near the time of the event suggest that the FAC is coincident with, and possibly confined to, a relatively narrow region (less than 3° in latitude) in which 1–10 keV electrons characteristic of the central plasma sheet are present.