Upward Currents Research Articles

Auroral phenomena related to intense electric fields observed by the Freja satellite

Electrodynamical features of the black aurora, and its optical counterpart, the aurora, are discussed on the basis of electric field, magnetic field and particle observations by Freja. Extraordinarily intense (1-2Vm-1) and small-scale (1 - 5 km) electric fields have been observed by the Freja satellite mostly in association with black aurora (vortices and east - west dark filaments) but also in association with auroral structures such as spirals and surges. The former structures are characterized by diverging electric fields, dropouts of energetic electron precipitation, precipitating or transversely energized ions and downward field-aligned currents carried by upward fluxes of ionospheric electrons, opposite to the characteristics of the aurora. The black auroral structures are associated with localized ionospheric density depletions below that of the ambient density and the field magnitude is found to be anticorrelated to the conductivity. During conditions of minimum conductivity small-scale diverging electric fields of 1Vm-1 occasionally exist down to 800 km. We suggest that the diverging electric fields observed by Freja are associated with low-altitude and narrow (≈1-2Km) potential structures similar to the auroral potential structures at higher altitude but associated with a positive space charge and a downward parallel electric field. This is supported by Freja observations of intense upward electron beams and positive potential peaks as well as by low-altitude electric field observations by the S3-3 and Viking satellites. The electric field characteristics of auroral surges and large-scale auroral spirals are very different from those of the black aurora. This will be exemplified by high-resolution electric field, magnetic field, particle and UV-imager observations from oblique Freja crossings of the eveningside auroral oval. The ambient electric field is found to intensify in the direction towards the centre of the auroral spiral which confirm previous findings that the surge and spiral head is associated with negative space charge and an intense upward field-aligned current. A pass directly through the surge head, however, reveals a very inhomogeneous and complicated picture of the surge such as narrowly structured, intense (up to 700 mV m-1) converging electric fields, intense electron precipitation and balanced field-aligned currents (up to 30μAm-2) embedded within an extended region of intense high-energy electron precipitation, weak electric fields and field-aligned currents. According to some surge models, a pronounced westward electric field component and a southward polarization electric field is expected within the entire high-conductivity region but evidence in support of this was not found in the data. Rather, these suggest that a significant part of the upward surge current is closed by distributed downward field-aligned currents from the near surroundings. The surge electric field is much more intense than previously observed or anticipated at these altitudes, having characteristics rather similar to those observed in the auroral acceleration region.

Multiple current sheets in a double auroral oval observed from the MAGION-2 and MAGION-3 satellites

Abstract. A case is described of multiple current sheets crossed by the MAGION-2 satellite in the near-midnight quieting auroral oval. The data were obtained by the magnetometer experiment onboard. Results show during a quieting period after a preceding substorm, or during an early growth phase of the next substorm, two double-sheet current bands, POLB and EQUB, located at respectively the polar and equatorial borders of the auroral oval separated by about 500 km in latitude. This is consistent with the double-oval structure during recovery introduced by Elphinstone et al. (1995). Within the POLB, the magnetic field data show simultaneous existence of several narrow parallel bipolar current sheets within the upward current branch (at 69.5–70.3° invariant latitude) with an adjacent downward current branch at its polar side at (70.5–71.3°). The EQUB was similarly stratified and located at 61.2–63.5° invariant latitude. The narrow current sheets were separated on average by about 35 km and 15 km, respectively, within the POLB and EQUB. A similar case of double-oval current bands with small-scale structuring of their upward current branches during a quieting period is found in the data from the MAGION-3 satellite. These observations contribute to the double-oval structure of the late recovery phase, and add a small-scale structuring of the upward currents producing the auroral arcs in the double- oval pattern, at least for the cases presented here. Other observations of multiple auroral current sheets and theories of auroral arc multiplicity are briefly discussed. It is suggested that multiple X-lines in the distant tail, and/or leakage of energetic particles and FA currents from a series of plasmoids formed during preceding magnetic activity, could be one cause of highly stratified upward FA currents at the polar edge of the quieting double auroral oval.

Ionospheric F region currents at middle and low latitudes estimated from Magsat data

Two methods for determining the F region current density at satellite heights by means of a decomposition of the magnetic field into toroidal and poloidal parts are presented. The first method allows for determination of the radial current component only by neglecting the height variation of the satellite orbit. Application to Magsat data yields the well‐known field‐aligned currents of polar latitudes (average current density Jr ≲ 200 nA/m2) and the meridional current system of the equatorial electrojet (Jr ≈ 10 – 20 nA/m2) during dusk. In addition, a midlatitude interhemispheric current system could be observed for the first time. Current direction is from the winter to the summer hemisphere during dusk and in opposite direction during dawn, as predicted by three‐dimensional models of the ionospheric dynamo. Presumably, for the first time a lunar contribution to the meridional current system of the equatorial electrojet was found, too. Amplitude at the dip equator is Jr ≈ 1.8 nA/m2; time of maximum is in agreement with magnetic ground observations, ionospheric electric field measurements, and ionospheric dynamo theory. The second method allows for estimation of both the radial and the horizontal current density by expanding the radial dependence of the magnetic field in Taylor series. First results confirm the current direction of the low‐latitude meridional current system as suggested by dynamo calculations: upward currents at the dip equator and field‐aligned downward currents at low latitudes.

Space-time correlation structures of horizontal coherent vortices in compound open-channel flows by using particle-tracking velocimetry

In compound open-channel flows, it is very important to clarify an interaction mechanism between the mainchannel and flood-plains induced by coherent vortices, so-called the “intermittent upward secondary currents” and the “large-scale horizontal vortices”, as shown in Figure 1. In the present study, time-averaged structures of secondary currents were revealed experimentally by making use of a two-component fiber-optic Laser Doppler Anemometer (LDA). Next, a new quantitative visualization technique called the “Particle-Tracking Velocimetry (PTV)” was used to investigate evolutionary patterns of horizontal large-scale vortices, which are the ultimate interest in a shallow water flood-plain. By this technique, instantaneous velocity components in arbitrary horizontal plane were obtained at all grid points of laser-light-sheet (LLS) from continuous four images of one particle, and space-time correlation structures of coherent vortices between the main-channel and flood-plain were investigated by using a conditional sampling technique.

SABRE observations of Pi2 pulsations: case studies

Abstract. The characteristics of substorm-associated Pi2 pulsations observed by the SABRE coherent radar system during three separate case studies are presented. The SABRE field of view is well positioned to observe the differences between the auroral zone pulsation signature and that observed at mid-latitudes. During the first case study the SABRE field of view is initially in the eastward electrojet, equatorward and to the west of the substorm-enhanced electrojet current. As the interval progresses, the western, upward field-aligned current of the substorm current wedge moves westward across the longitudes of the radar field of view. The westward motion of the wedge is apparent in the spatial and temporal signatures of the associated Pi2 pulsation spectra and polarisation sense. During the second case study, the complex field-aligned and ionospheric currents associated with the pulsation generation region move equatorward into the SABRE field of view and then poleward out of it again after the third pulsation in the series. The spectral content of the four pulsations during the interval indicate different auroral zone and mid-latitude signatures. The final case study is from a period of low magnetic activity when SABRE observes a Pi2 pulsation signature from regions equatorward of the enhanced substorm currents. There is an apparent mode change between the signature observed by SABRE in the ionosphere and that on the ground by magnetometers at latitudes slightly equatorward of the radar field of view. The observations are discussed in terms of published theories of the generation mechanisms for this type of pulsation. Different signatures are observed by SABRE depending on the level of magnetic activity and the position of the SABRE field of view relative to the pulsation generation region. A twin source model for Pi2 pulsation generation provides the clearest explanation of the signatures observed.

Multiple current sheets in a double auroral oval observed from the MAGION-2 and MAGION-3 satellites

A case is described of multiple current sheets crossed by the MAGION-2 satellite in the near-midnight quieting auroral oval. The data were obtained by the magnetometer experiment onboard. Results show during a quieting period after a preceding substorm, or during an early growth phase of the next substorm, two double-sheet current bands, POLB and EQUB, located at respectively the polar and equatorial borders of the auroral oval separated by about 500 km in latitude. This is consistent with the double-oval structure during recovery introduced by Elphinstone et al. (1995). Within the POLB, the magnetic field data show simultaneous existence of several narrow parallel bipolar current sheets within the upward current branch (at 69.5–70.3° invariant latitude) with an adjacent downward current branch at its polar side at (70.5–71.3°). The EQUB was similarly stratified and located at 61.2–63.5° invariant latitude. The narrow current sheets were separated on average by about 35 km and 15 km, respectively, within the POLB and EQUB. A similar case of double-oval current bands with small-scale structuring of their upward current branches during a quieting period is found in the data from the MAGION-3 satellite. These observations contribute to the double-oval structure of the late recovery phase, and add a small-scale structuring of the upward currents producing the auroral arcs in the double- oval pattern, at least for the cases presented here. Other observations of multiple auroral current sheets and theories of auroral arc multiplicity are briefly discussed. It is suggested that multiple X-lines in the distant tail, and/or leakage of energetic particles and FA currents from a series of plasmoids formed during preceding magnetic activity, could be one cause of highly stratified upward FA currents at the polar edge of the quieting double auroral oval.

SABRE observations of Pi2 pulsations: case studies

<strong class="journal-contentHeaderColor">Abstract.</strong> The characteristics of substorm-associated Pi2 pulsations observed by the SABRE coherent radar system during three separate case studies are presented. The SABRE field of view is well positioned to observe the differences between the auroral zone pulsation signature and that observed at mid-latitudes. During the first case study the SABRE field of view is initially in the eastward electrojet, equatorward and to the west of the substorm-enhanced electrojet current. As the interval progresses, the western, upward field-aligned current of the substorm current wedge moves westward across the longitudes of the radar field of view. The westward motion of the wedge is apparent in the spatial and temporal signatures of the associated Pi2 pulsation spectra and polarisation sense. During the second case study, the complex field-aligned and ionospheric currents associated with the pulsation generation region move equatorward into the SABRE field of view and then poleward out of it again after the third pulsation in the series. The spectral content of the four pulsations during the interval indicate different auroral zone and mid-latitude signatures. The final case study is from a period of low magnetic activity when SABRE observes a Pi2 pulsation signature from regions equatorward of the enhanced substorm currents. There is an apparent mode change between the signature observed by SABRE in the ionosphere and that on the ground by magnetometers at latitudes slightly equatorward of the radar field of view. The observations are discussed in terms of published theories of the generation mechanisms for this type of pulsation. Different signatures are observed by SABRE depending on the level of magnetic activity and the position of the SABRE field of view relative to the pulsation generation region. A twin source model for Pi2 pulsation generation provides the clearest explanation of the signatures observed.

On the self‐consistent description of dynamic magnetosphere‐ionosphere coupling phenomena with resolved ionosphere

A set of model equations is presented that allows for the self‐consistent description of dynamic magnetosphere‐ionosphere coupling phenomena with finite ionosphere. The model is very similar to magnetohydrodynamics (MHD), but the plasma‐neutral gas interaction in the lower ionosphere is taken into account by a frictional force between ions and neutrals and an ionization‐recombination term in the plasma transport equation. Further, the Hall term and the electron pressure term are retained in Ohm's law. It is shown that for the collision‐dominated E layer, the familiar Pedersen and Hall conductivities can be derived directly from the basic equations. The model is used to numerically simulate the dynamic formation of a magnetospheric‐ionospheric current system as the response to prescribed localized magnetospheric convection. Apart from a pair of Birkeland current sheets that are closed by Pedersen currents, associated Hall currents are generated. In addition, density irregularities in the E layer form as a direct consequence of the current closure. They are, however, not related to electron precipitation. For small length scales (≈ 10 km), these density perturbations result in considerably enhanced conductivities below the upward Birkeland currents which in turn lead to a spatial concentration of the latter compared to the downward currents. The timescale for the relaxation of the current system and the ionospheric convection toward a stationary state after the onset of magnetospheric convection is longer than the Alfvén travel time and depends on the height‐integrated Pedersen conductivity. This is in good agreement with earlier theoretical predictions [Southwood and Kivelson, 1991].

On the current-voltage relationship in auroral breakups and westwards-travelling surges

Abstract. Auroral precipitating electrons pass through an acceleration region before entering the atmosphere. Regardless of what produces it, a parallel electric field is assumed to cause the acceleration. It is well known that from kinetic theory an expression for the corresponding upward field-aligned current can be calculated, which under certain assumptions can be linearized to j∥=KV. The K constant, referred to as the Lyons-Evans-Lundin constant, depends on the source density and thermal energy of the magnetospheric electrons; it is an important parameter in magnetosphere-ionosphere coupling models. However, the K parameter is still rather unknown, and values are found in a wide range of 10–8–10–10 S m–2. In this study, we investigated how the type of auroral structure affects the K values. We look at onset and westwards-travelling surge (WTS) events and make comparisons with earlier results from observations of more stable auroral arcs. A new analysis technique for studying those magnetospheric parameters using ground-based measurements is introduced. Electron density measurements are taken with the EISCAT radar, and through an inversion technique the flux-energy spectra are calculated. Source densities, thermal energies and potential drops are estimated from fittings of accelerated Maxwellian distributions. With this radar technique we have the possibility to study the changes of the mentioned parameters during the development of onsets and the passage of surges over EISCAT. The study indicates that the linearization of the full Knight formulation holds even for the very high potential drops and thermal temperatures found in the dynamic onset and WTS events. The values of K are found to be very low, around 10–11 S m–2 in onset cases as well as WTS events. The results may establish a new technique where ionospheric measurements are used for studying the ionosphere-magnetosphere coupling processes.

Modelling flow patterns in conical dendroid graptolites

Life-sized wire models of several different morphologies of conical dendroid graptolites were tested in a flume to observe the flow patterns within the rhabdosome. Testing over a range of unidirectional current speeds from 3 to 15 cm/s showed that upward-directed currents are produced within the cone, and a significant amount of the water that enters the cone through the upstream wall exits the top opening. These upward currents were most evident at speeds of 3–10 cm/s. Shorter cones, wide cones, less porous mesh, and downstream tilting each increased the amount of water exiting the top of the cone, especially at higher speeds. These findings are consistent with a mode of feeding in which conical forms received afferent currents into the sides of the cones, where food particles were captured, and waste efferent currents went out the top opening. This does not support the hypothesis that downward-directed, ciliary feeding currents produced the force necessary to propel the earliest planktic graptolites into the water column in the evolutionary transition from a benthic to a planktic mode of life. The suggestion that differing rhabdosomal morphologies may represent adaptations to differing paleocurrent regimes is supported. Dendroid rhabdosomal morphology may prove to be a useful paleoenvironmental tool, particularly as an indicator of current strengths, especially in fine-grained sediments where other current-indicative structures may be lacking.Graptolite, dendroid, flow, feeding, paleoecology.

Numerical modelling of intermittent ion outflow events above EISCAT

EISCAT observations with the UHF and VHF radars of the dynamics of the upper ionosphere have revealed the occurrence of intermittent ion outflows with velocities reaching several hundred m s −1. It was previously shown that, during such events, the topside downward electron heat flux, inferred from the analysis of the vertical (field-aligned) structure of the electron temperature profiles, increases drastically up to values of about 10 μW m −2. The numerical models described in a companion paper are used here to simulate the effects of energy inputs driven by the magnetosphere. Three main effects are simulated separately: effects of frictional heating related to E × B drifts, effects due to topside heat flux perturbations and effects of upward field-aligned currents of a few tens of μA m −2. It is shown that field-aligned currents and topside heat flux perturbations produce very similar effects and that combining field-aligned currents with frictional heating allows us to model the overall characteristics of the perturbations of the electron density, of the electron and ion temperatures, and of the ion vertical velocity. A good agreement with observations is found.

Field‐aligned currents associated with spatially periodic X ray structures in the morningside auroral oval

On February 9, 1992, the Atmospheric X ray Imaging Spectrometer (AXIS) on the Upper Atmosphere Research Satellite (UARS) observed spatially periodic structures in X ray luminosity from the top of the atmosphere during a pass over the dawnside auroral oval. The patches of luminosity were several hundred kilometers in size and separated by a mean distance of about 700 km. As many as a dozen distinct patches could be identified extending from local times just after midnight to the noon meridian. The characteristics of the precipitating electrons responsible for the patches were determined from the measured X ray fluxes. Peak electron energy fluxes were between 4 and 10 ergs cm−2 s−1 with e‐folding energies of 10–25 keV. In situ magnetic field data from the onboard magnetometer were used to model the field‐aligned currents associated with two of the patches. The model that best fit the data had upward currents in the western portions of the patches and more intense downward currents confined to the eastern edges. The modeling also indicated that these small‐scale currents were embedded within the large‐scale region 2 field‐aligned current sheet. Data from a ground‐based magnetometer near the satellite ground track indicated the presence of Ps 6 pulsations. We have interpreted the observations in terms of eastward‐drifting current patch systems. Under this assumption, the patches moved at 1.7 km s−1, the drift speed of ∼30‐keV electrons. This motion, and the correlation with Ps 6 pulsations suggests that the X ray patches were associated with omega bands that are seen in the morning sector during geomagnetically active times.

On the dynamical development of the downward field‐aligned current in the substorm current wedge

Data from the Scandinavian magnetometer array, a multichannel scanning photometer, and all‐sky cameras have been combined to study the carriers of the downward field‐aligned current in the substorm current wedge during the first minutes after the substorm onset. The ions are slow and thus cannot contribute much to the downward current immediately after the substorm onset, whereas the precipitating magnetospheric electrons can quickly set up the upward current. On March 4, 1979, magnetometers and all‐sky cameras recorded a substorm onset at ∼2236 UT slightly to the west of the stations. A scanning photometer was located at the center of the downward current during the event. The observations suggest that the downward current in this event was set up less than a minute after the formation of the upward current and that at this stage the current was mainly carried by cold ionospheric electrons.

On the occurrence and characteristics of intense low-altitude electric fields observed by Freja

Abstract. High-resolution measurements by the double probe electric field instrument on the Freja satellite are presented. The observations show that extremely intense (up to 1 V m–1) and fine-structured (&lt;1 km) electric fields exist at auroral latitudes within the altitude regime explored by Freja (up to 1700 km). The intense field events typically occur within the early morning sector of the auroral oval (01–07 MLT) during times of geomagnetic activity. In contrast to the observations within the auroral acceleration region characterized by intense converging electric fields associated with electron precipitation, upward ion beams and upward field-aligned currents, the intense electric fields observed by Freja are often found to be diverging and located within regions of downward field-aligned currents outside the electron aurora. Moreover, the intense fields are observed in conjunction with precipitating and transversely energized ions of energies 0.5–1 keV and may play an important role in the ion heating. The observations suggest that the intense electric field events are associated with small-scale low-conductivity ionospheric regions void of auroral emissions such as east-west aligned dark filaments or vortex streets of black auroral curls located between or adjacent to auroral arcs within the morningside diffuse auroral region. We suggest that these intense fields also exist at ionospheric altitudes although no such observations have yet been made. This is possible since the height-integrated conductivity associated with the dark filaments may be as low as 0.1 S or less. In addition, Freja electric field data collected outside the auroral region are discussed with particular emphasis on subauroral electric fields which are observed within the 19–01 MLT sector between the equatorward edge of the auroral oval and the inner edge of the ring current.

Characteristics of ionospheric convection and field‐aligned current in the dayside cusp region

The assimilative mapping of ionospheric electrodynamics (AMIE) technique has been used to estimate global distributions of high‐latitude ionospheric convection and field‐aligned current by combining data obtained nearly simultaneously both from ground and from space. Therefore, unlike the statistical patterns, the “snapshot” distributions derived by AMIE allow us to examine in more detail the distinctions between field‐aligned current systems associated with separate magnetospheric processes, especially in the dayside cusp region. By comparing the field‐aligned current and ionospheric convection patterns with the corresponding spectrograms of precipitating particles, the following signatures have been identified: (1) For the three cases studied, which all had an IMF with negative y and z components, the cusp precipitation was encountered by the DMSP satellites in the postnoon sector in the northern hemisphere and in the prenoon sector in the southern hemisphere. The equatorward part of the cusp in both hemispheres is in the sunward flow region and marks the beginning of the flow rotation from sunward to antisunward. (2) The pair of field‐aligned currents near local noon, i.e., the cusp/mantle currents, are coincident with the cusp or mantle particle precipitation. In distinction, the field‐aligned currents on the dawnside and duskside, i.e., the normal region 1 currents, are usually associated with the plasma sheet particle precipitation. Thus the cusp/mantle currents are generated on open field lines and the region 1 currents mainly on closed field lines. (3) Topologically, the cusp/mantle currents appear as an expansion of the region 1 currents from the dawnside and duskside and they overlap near local noon. When By is negative, in the northern hemisphere the downward field‐aligned current is located poleward of the upward current; whereas in the southern hemisphere the upward current is located poleward of the downward current. (4) Under the assumption of quasi‐steady state reconnection, the location of the separatrix in the ionosphere is estimated and the reconnection velocity is calculated to be between 400 and 550 m/s. The dayside separatrix lies equatorward of the dayside convection throat in the two cases examined.

Visualization and Analysis of Air Currents in the Plant Tissue Culture Vessel

Air currents, and consequent diffusion processes, in the plant tissue culture vessel are slow and limit photosynthesis, transpiration, and uptake of water and nutrients by in vitro plantlets. Air currents in Magenta-type and Mayonnaise bottle-type culture vessels were visualized using fine particles of feathery crystal of metaldehyde [(CH3CHO)4] as tracers. Pictures of visualized air currents were recorded by the video camera. Air current speeds in the culture vessel were determined by analyzing the changes in video pictures with time. Air current speed around the culture vessel was set at 0.1 and 1.0 m–s–1. Shortwave radiation flux density on the shelf was varied between 0 and 34 W–m–2. Shortwave absorptivity of the medium was 44% for agar medium and 97% for agar medium containing activated charcoal. Under a shortwave radiation flux density of 10–34 W–m–2, the upward air currents were observed at the central part of the culture vessel and downward air currents were observed near the inside walls of the culture vessel. The upward air current speed was affected by air current speed around the culture vessel, shortwave radiation flux density, and shortwave absorptivity of the medium. Under shortwave radiation flux density of 34 W–m–2, the upward air current speed in the culture vessel was ≈4 times greater with 1.0 m–s–1 air current speed around the culture vessel than with 0.1 m–s–1.

Midnight velocity shear zone and the concept of Harang discontinuity

We discuss the concept of Harang discontinuity based on ionospheric observations of electrodynamic parameters, magnetic field mapping from the ionosphere to the magnetosphere, and theoretical reasoning. We consider the Harang discontinuity as synonymous to the midnight‐premidnight sector velocity shear zone in the auroral ionosphere, arguing that its main importance is related to the dynamic development of magnetospheric conditions, especially substorms. We show that typical two‐dimensional ionospheric observations are not quite consistent with the statistical Heppner‐Maynard convection models and this inconsistency becomes even more apparent when mapped into the equatorial magnetotail. The upward field‐aligned current from the velocity shear zone is one of the major problems in performing a realistic mapping procedure. Because the auroral arcs close to the Harang discontinuity are often found to activate and break up at the time of the substorm onset, this same upward current is probably connected with the substorm initiation mechanism in the magnetotail. We discuss the electrodynamics of all parts of this current circuit, i.e., the ionosphere, the magnetosphere, and the connecting field lines. We conclude that surprisingly few observationally proven facts are known of this circuit outside the ionosphere, and even there the great variability of the physical parameter values makes this region very difficult to study in detail.

Boundary layer polarization and voltage in the 14 MLT region

Viking midaltitude observations of ions and electrons in the postnoon auroral region show that field‐aligned acceleration of electrons and ions with energies up to a few kiloelectron volts takes place. The characteristics of the upgoing ion beams and the local transverse electric field observed by Viking indicate that parallel ion acceleration is primarily due to a quasi‐electrostatic field‐aligned acceleration process below Viking altitudes, i.e., below 10,000–13,500 km. A good correlation is found between the maximum upgoing ion beam energy and the depth of the local potential well determined by the Viking electric field experiment within dayside “ion inverted Vs.” The total transverse potential throughout the entire region near the ion inverted Vs is generally much higher than the field‐aligned potential and may reach well above 10 kV. However, the detailed mapping of the transverse potential out to the boundary layer, a fundamental issue which remains controversial, was not attempted here. An important finding in this study is the strong correlation between the maximum upgoing ion beam energy of dayside ion inverted Vs and the solar wind velocity. This suggests a direct coupling of the solar wind plasma dynamo/voltage generator to the region of field‐aligned particle acceleration. The fact that the center of dayside ion inverted Vs coincide with convection reversals/flow stagnation and upward Birkeland currents on what appears to be closed field lines (Woch et al., 1993), suggests that field‐aligned potential structures connect to the inner part of an MHD dynamo in the low‐latitude boundary layer. Thus the Viking observations substantiate the idea of a solar wind induced boundary layer polarization where negatively charged perturbations in the postnoon sector persistently develops along the magnetic field lines, establishing accelerating potential drops along the geomagnetic field lines in the 0.5–10 kV range.

Four large‐scale field‐aligned current systems in the dayside high‐latitude region

A system of four current sheets of large‐scale field‐aligned currents (FACs) was discovered in the data set of simultaneous Viking and DMSP‐F7 crossings of the dayside high‐latitude region. This paper reports four examples of this system that were observed in the prenoon sector. The flow polarities of FACs are upward, downward, upward, and downward, from equatorward to poleward. The lowest‐latitude upward current is flowing mostly in the CPS precipitation region, often overlapping with the BPS at its poleward edge, and is interpreted as a region 2 current. The pair of downward and upward FACs in the middle of the structure are collocated with structured electron precipitation. The precipitation of high‐energy (&gt;1 keV) electrons is more intense in the lower‐latitude downward current sheet. The highest‐latitude downward flowing current sheet is located in a weak, low‐energy particle precipitation region, suggesting that this current is flowing on open field lines. Simultaneous observations in the postnoon local time sector reveal the standard three‐sheet structure of FACs, sometimes described as region 2, region 1, and mantle (referred to the midday region 0) currents. A high correlation was found between the occurrence of the four FAC sheet structure and negative interplanetary magnetic field (IMF) By. We discuss the FAC structure in terms of three types of convection cells: the merging, viscous, and lobe cells. During strongly negative IMF By, two convection reversals exist in the prenoon sector; one is inside the viscous cell, and the other is between the viscous cell and the lobe cell. This structure of convection flow is supported by the Viking electric field and auroral UV image data. Based on the convection pattern, the four FAC sheet structure is interpreted as the latitudinal overlap of midday and morning FAC systems. We suggest that the four‐current sheet structure is common in a certain prenoon local time sector during strongly negative IMF By.

On the occurrence and characteristics of intense low-altitude electric fields observed by Freja

High-resolution measurements by the double probe electric field instrument on the Freja satellite are presented. The observations show that extremely intense (up to 1 V m–1) and fine-structured (<1 km) electric fields exist at auroral latitudes within the altitude regime explored by Freja (up to 1700 km). The intense field events typically occur within the early morning sector of the auroral oval (01–07 MLT) during times of geomagnetic activity. In contrast to the observations within the auroral acceleration region characterized by intense converging electric fields associated with electron precipitation, upward ion beams and upward field-aligned currents, the intense electric fields observed by Freja are often found to be diverging and located within regions of downward field-aligned currents outside the electron aurora. Moreover, the intense fields are observed in conjunction with precipitating and transversely energized ions of energies 0.5–1 keV and may play an important role in the ion heating. The observations suggest that the intense electric field events are associated with small-scale low-conductivity ionospheric regions void of auroral emissions such as east-west aligned dark filaments or vortex streets of black auroral curls located between or adjacent to auroral arcs within the morningside diffuse auroral region. We suggest that these intense fields also exist at ionospheric altitudes although no such observations have yet been made. This is possible since the height-integrated conductivity associated with the dark filaments may be as low as 0.1 S or less. In addition, Freja electric field data collected outside the auroral region are discussed with particular emphasis on subauroral electric fields which are observed within the 19–01 MLT sector between the equatorward edge of the auroral oval and the inner edge of the ring current.