Polar Cap Boundary Research Articles

Development of parallel electric fields at the plasma sheet boundary layer

[1] Many measurements of auroral particles, in particular recent measurements from the FAST satellite, indicate that the auroral electron distribution is often broad in energy and field-aligned in pitch angle. Such electrons are seen in conjunction with strong kinetic Alfven waves with small perpendicular wavelength, and so the aurora produced by these electrons has been termed the “Alfvenic aurora.” The Alfvenic aurora is predominant at the polar cap boundary of the aurora as well as in the auroral arc that brightens during substorm onset. The process of forming parallel electric fields in Alfven waves has been investigated by means of three-dimensional two-fluid simulations. Waves with small perpendicular wavelengths can be produced by phase mixing when perpendicular gradients are present in the plasma. At lower altitudes, Alfven waves can interact with the ionospheric Alfven resonator and phase mix to scales of a few kilometers. At higher altitudes, the electron thermal speed becomes comparable or larger than the Alfven speed, and parallel electric fields due to the Landau resonance can develop. This has been modeled in the fluid code by an approximation to the plasma dispersion function used in kinetic theory. Phase mixing is most effective when there are strong gradients, such as at the plasma sheet boundary layer. Simulations indicate that these processes can produce parallel electric fields on scales of a few kilometers (in the cold plasma case) to a few tens of kilometers (in the warm plasma case), comparable to the scale sizes of auroral arcs.

Dynamic temporal evolution of polar cap tongue of ionization during magnetic storm

During a magnetic storm on 14–16 December 2006, a polar cap tongue of ionization (TOI) was detected by an all‐sky imager (ASI) at Resolute Bay, Canada (74.73°N, 265.07°E). We investigate the temporal evolution and spatial structure of the TOI in detail by combining the optical data with other observations (e.g., solar wind, GPS total electron content, SuperDARN, and DMSP and NOAA POES satellites). The TOI was observed as a bright and elongated 630 nm airglow plume for 4 h during the main phase of the storm. This interval corresponded to a period of prolonged stable large‐amplitude southward IMF during a coronal mass ejection (CME). One to one and a half hours before the appearance of TOI, the polar cap boundary expanded rapidly far equatorward, and a positive ionospheric storm occurred. This implies that both the “expansion of the high‐latitude plasma convection” and “build up of the source plasma in the midlatitudes” are necessary conditions for the formation of a TOI. Because both of them were triggered by a major southward turning of the IMF, the prolonged large‐amplitude southward IMF orientation in the trailing part of the CME was primarily responsible for the generation of TOI. After its appearance, the TOI exhibited dynamic motion in the dawn to dusk direction. Simultaneous SuperDARN data suggest that a longitudinal progression of subauroral polarization stream controlled this dynamic motion. The optical TOI was found to be a continuous stream elongated in the noon‐midnight direction although it contained some mesoscale patterns. Absence of large‐scale temporal changes in the cusp plasma flow during the stable IMF period allowed the TOI to remain continuous without being broken into polar cap patches. The mesoscale structures within the TOI were probably produced by small‐scale velocity fluctuations in the cusp plasma flow. The TOI as visualized with the all‐sky airglow imager was found to be much more dynamic and much more complicated than we ever thought. The current study indicates that such a behavior of the TOI was presumably caused by a combination of temporal variations in the global‐scale plasma circulation system, expansion and contraction of the polar cap area, and plasma density changes in the dayside low to midlatitudes.

Ionospheric convection signatures of the interchange cycle at small interplanetary magnetic field clock angles

The purpose of this paper is to show a “proof of the existence” of the ionospheric situation that is expected for the interchange cycle, during periods of favorable interplanetary magnetic field (IMF) and dipole tilt conditions. To do so, we present three case studies of dayside high‐latitude ionospheric convection that is observed around the equinoxes (near‐zero dipole tilt) and at small IMF clock angles (one θc ∼ −30° event and two θc ∼ 30° events, where θc ≡ Arg(BZ + iBY)), using Super Dual Auroral Radar Network (SuperDARN)/Defense Meteorological Satellite Program (DMSP)/National Oceanic and Atmospheric Administration (NOAA) data in the Northern Hemisphere and, when available, DMSP data in the Southern Hemisphere. The convection pattern exhibits twin reverse cells in both hemispheres, but the constituents of each cell are different. In the Northern Hemisphere, for θc ∼ 30° (θc ∼ −30°), the center of the dawnside (duskside) cell is located poleward of the polar cap boundary, while the center of the duskside (dawnside) cell is located equatorward of the polar cap boundary. For θc ∼ 30°, we confirmed that the above‐mentioned dawn‐dusk relation reverses in the Southern Hemisphere. The north‐south asymmetric behavior of the conjugate reverse cells, on the dawnside and duskside each, is consistent with two independent interchange cycles that result from the coupling of IMF‐lobe reconnection in one hemisphere with lobe‐closed reconnection in the opposite hemisphere.

Monopolar and bipolar auroral electric fields and their effects

Abstract. Most of the high-altitude auroral electric fields observed by CLUSTER can be classified into monopolar and bipolar structures. The observations associate monopolar electric fields with polar cap boundary arcs, while bipolar fields tend to be linked to discrete arcs within the auroral oval and to polar cap arcs. The present paper proposes an explanation for this association based on a simple model of the magnetotail configuration and kinetic model computations. The paper introduces a quasi-electrostatic model to describe the auroral current system associated with monopolar and bipolar high-altitude fields. Analytic solutions are presented. The model gives indications about the location of the up- and downward field-aligned current regions, the ionospheric and magnetospheric convection along the arc, the acceleration or deceleration of precipitating particles, and the behaviour of escaping ionospheric ions.

Polar cap flow channel events: spontaneous and driven responses

Abstract. We present two case studies of specific flow channel events appearing at the dusk and/or dawn polar cap boundary during passage at Earth of interplanetary (IP) coronal mass ejections (ICMEs) on 10 January and 25 July 2004. The channels of enhanced (>1 km/s) antisunward convection are documented by SuperDARN radars and dawn-dusk crossings of the polar cap by the DMSP F13 satellite. The relationship with Birkeland currents (C1–C2) located poleward of the traditional R1–R2 currents is demonstrated. The convection events are manifest in ground magnetic deflections obtained from the IMAGE (International Monitor for Auroral Geomagnetic Effects) Svalbard chain of ground magnetometer stations located within 71–76° MLAT. By combining the ionospheric convection data and the ground magnetograms we are able to study the temporal behaviour of the convection events. In the two ICME case studies the convection events belong to two different categories, i.e., directly driven and spontaneous events. In the 10 January case two sharp southward turnings of the ICME magnetic field excited corresponding convection events as detected by IMAGE and SuperDARN. We use this case to determine the ground magnetic signature of enhanced flow channel events (the NH-dusk/By<0 variant). In the 25 July case a several-hour-long interval of steady southwest ICME field (Bz<0; By<0) gave rise to a long series of spontaneous convection events as detected by IMAGE when the ground stations swept through the 12:00–18:00 MLT sector. From the ground-satellite conjunction on 25 July we infer the pulsed nature of the polar cap ionospheric flow channel events in this case. The typical duration of these convection enhancements in the polar cap is 10 min.

Seasonal and interplanetary magnetic field–dependent polar cap contraction during substorm expansion phase

Recent observations from simultaneous imaging in two hemispheres have shown that the polar caps can attain considerably different shapes as the auroral ovals contract during the substorm expansion phase. We use images from 2770 substorms to study the evolution of the polar cap boundary location statistically. We show that, during the first 26 min after substorm expansion phase onset, the polar cap boundary location depends on seasons, interplanetary magnetic field (IMF) By, and IMF Bx. For different signs of By, with ∣By∣ > 3 nT, the asymmetry in polar cap boundary observed at onset increases during the expansion phase, consistent with an increase in tail reconnection of field lines with asymmetrical footpoints. When Bx > 2 nT and ∣By∣ < 2 nT, the polar cap boundary dawnward of the onset propagates slightly farther poleward compared to negative Bx conditions. In the sunlit hemisphere, the polar cap boundary evolves from a pronounced equatorward displacement at onset to an almost reversed displacement during the expansion phase, compared to substorms observed in darkness. Substorms in the dark hemisphere also have a much more pronounced bulge than substorms in the sunlit hemisphere. If the interpretation of the poleward auroral boundary as being coincident with the open/closed field line boundary (OCB) is correct, the seasonal differences in OCB locations imply seasonal differences in the ionospheric convection during the substorm expansion phase.

Vertical thermal O+ flows at 850 km in dynamic auroral boundary coordinates

Contemporary magnetosphere models now include species‐dependent dynamics. Energetic O+ has significant consequences for the energy stored in the ring current, the rate of reconnection, and perhaps the timing of substorm injections. The mechanism by which thermal O+ escapes from the top of the ionosphere and into the magnetosphere is not fully understood. Previous studies have used dynamic auroral boundary coordinates to describe the outflowing energetic O+ ions above the ionosphere. In this study we focus on the vertical flow of O+ ions at lower altitudes before they are accelerated to escape velocity. An algorithm has been devised to identify auroral zone boundaries using precipitating electron observations from the Defense Meteorological Satellite Program (DMSP) spacecraft. Vertical ion flows measured by the DMSP special sensor for ions electrons and scintillation ion drift meter and the retarding potential analyzer instruments aboard the F12 (noon‐midnight) and F13 (dawn‐dusk) spacecraft from 1997 to 1998 were projected into dynamic auroral boundary coordinates and used to investigate the dependence of Southern Hemisphere bulk flows on interplanetary magnetic field (IMF) and geomagnetic conditions. Initial results show that (1) net upward flows occur primarily in the auroral zone and net downward flows occur primarily in the polar cap, (2) there exists a strong upward flow at 9 magnetic local time (MLT) near the polar cap boundary, 3) the downward ion flow orientation is strongly dependent on IMF By, and 4) the auroral boundary does not coincide exactly with the upward/downward boundary for bulk flows.

Plasma flow during the brightening of proton aurora in the cusp

On the basis of simultaneous observations from the Super Dual Auroral Radar Network (SuperDARN), the far ultraviolet instrument on the IMAGE spacecraft, and a magnetometer installed on the east coast of Greenland, we present the characteristics of plasma flow during a westward moving proton aurora in the cusp, observed on 28 July 2000. Data with a time resolution of 12 and 25 s from SuperDARN at Þykkvibær, Iceland, show that the flow having a poleward component, which was accompanied by a quick equatorward expansion of the flow region, occurred in the early stages (approximately 1.5 min) of the brightening. Data from the magnetometer from a ground station on the Greenland east coast, which is located a few hundred kilometers west of the radar's field of view, show perturbations that are consistent with equatorward flow overhead, coincident with the expansion. The presence of flow vectors in opposite directions suggests that vortical flow was present during the proton aurora. Evidence supporting the suggested flow pattern has been found in an event recorded by the Greenland west coast magnetometer chain and in a cusp pass of the DE 2 spacecraft in the literature. We attribute the moving proton aurora to the traveling bulge at the polar cap boundary, which is the footprint of a flux transfer event, and imply that the preexisting vortical flow may be intensified when it becomes inflow to the bulge.

Seasonal and UT variations of the position of the auroral precipitation and polar cap boundaries

The data of the DMSP F7 spacecraft are used for studying the influence of the geomagnetic dipole tilt angle on the latitudinal position of auroral precipitation boundaries in the nighttime (2100–2400 MLT) and daytime (0900–1200 MLT) sectors. It is shown that, in the nighttime sector, the high-latitude zone of soft diffuse precipitation (SDP) and the boundary of the polar cap (PC) at all levels of geomagnetic activity are located at higher and lower latitudes relative to the equinox period in winter and summer, respectively. The position of boundaries of the diffuse auroral precipitation zone (DAZ) located equatorward from the auroral oval does not depend on the season. In the daytime sector, the inverse picture is observed: the SDP precipitation zone takes the most low-latitude and high-latitude positions in the winter and summer periods, respectively. The total value of the displacements from winter to summer of both the nighttime and daytime boundaries of the PC is ∼2.5°. A diurnal wave in the latitudinal position of the nighttime precipitation boundaries is detected. The wave is most pronounced in the periods of the winter and fall seasons, is much weaker in the spring period, and is almost absent in summer. The diurnal variations of the position of the boundaries are quasi-sinusoidal oscillations with the latitude maximum and minimum at 0300–0500 and 1700–2100 UT, respectively. The total value of the diurnal displacement of the boundaries is ∼2.5° of latitude. The results obtained show that, undergoing seasonal and diurnal variations, the polar cap is shifted as a whole in the direction opposite to the changes in the tilt angle of the geomagnetic dipole. The seasonal displacements of the polar cap and its diurnal variations in the winter period occur without any substantial changes in its area.

Comparison of the open‐closed separatrix in a global magnetospheric simulation with observations: The role of the ring current

The development of global magnetospheric models, such as Space Weather Modeling Framework (SWMF), which can accurately reproduce and track space weather processes has high practical utility. We present an interval on 5 June 1998, where the location of the polar cap boundary, or open‐closed field line boundary (OCB), can be determined in the ionosphere using a combination of instruments during a period encompassing a sharp northward to southward interplanetary field turning. We present both point‐ and time‐varying comparisons of the observed and simulated boundaries in the ionosphere and find that when using solely the coupled ideal magnetohydrodynamic magnetosphere‐ionosphere model, the rate of change of the OCB to a southward turning of the interplanetary field is significantly faster than that computed from the observational data. However, when the inner magnetospheric module is incorporated, the modeling framework both qualitatively, and often quantitatively, reproduces many elements of the studied interval prior to an observed substorm onset. This result demonstrates that the physics of the inner magnetosphere is critical in shaping the boundary between open and closed field lines during periods of southward interplanetary magnetic field (IMF) and provides significant insight into the 3‐D time‐dependent behavior of the Earth's magnetosphere in response to a northward‐southward IMF turning. We assert that during periods that do not include the tens of minutes surrounding substorm expansion phase onset, the coupled SWMF model may provide a valuable and reliable tool for estimating both the OCB and magnetic field topology over a wide range of latitudes and local times.

Interhemispheric observations of emerging polar cap asymmetries

In this paper we use simultaneous global UV images of the aurora in the two hemispheres to study differences in the polar cap boundary location. We show that the northern and southern auroral ovals circumvent the same amount of magnetic flux, providing additional evidence that the poleward boundary of the aurora coincides with the open/closed field line boundary. During a period of significant flux closure, large asymmetries in the polar cap boundaries developed between the hemispheres. The asymmetry was strongest in the regions where the polar caps contracted the most, suggesting that emerging interhemispheric polar cap asymmetries is an intrinsic phenomenon during substorm expansions, when magnetic flux closes rapidly in the tail. Utilizing the prolonged surveillance of the open/closed boundary location, we show that the growing asymmetries can be accounted for by differences in the ionospheric convection in the two hemispheres. The observations suggest that the differences in convection were due to seasonal differences between the hemispheres, and that the summer hemisphere responded more promptly to changes in magnetospheric convection than the winter hemisphere.

Stratification of east-west plasma flow channels observed in the ionospheric cusp in response to IMF BYpolarity changes

[1] During a period of predominantly north-westward flow for IMF BZ negative and IMF BY positive, a sequence of three distinct negative excursions of IMF BY resulted in a train of three eastward directed flow channels, interleaved by westward flow enhancements propagating into the polar cap. The high resolution of the EISCAT Svalbard radar data enables us to track formation and movement of the flow channels, which are interpreted as a sequence of intermittent reconnection alternating between different reconnection sites. Our observations are consistent with the view that a new region of reconnected flux manifests as development of a distinct flow channel near the polar cap boundary, and that successive events stay separated while pushing each other into the polar cap. Each flow channel will remain separated from neighboring channels mapping to different reconnection sites as long as the magnetic tension force with its associated field aligned current systems is maintained.

South‐north asymmetry of field‐aligned currents in the magnetotail observed by Cluster

We statistically investigated features of the field‐aligned current (FAC) distribution in plasma sheet boundary layers between 17 and 19 RE in the magnetotail using the curlometer technique to calculate the current from four‐point magnetic field measurements taken in 2001. The results show that the FAC distribution in the plasma sheet boundary layers in the magnetotail has dusk‐dawn asymmetry, earthward‐tailward (polarity) asymmetry, and north‐south asymmetry. The occurrence and polarities of FACs in the Northern Hemisphere are different from those in the Southern Hemisphere. The average density and the standard deviation of the FACs that are most likely to be connected to the Earth are 4.90 nA m−2 and 2.55 nA m−2 in the Northern Hemisphere and 4.21 nA m−2 and 1.80 nA m−2 in the Southern Hemisphere, respectively. For investigating the mechanism of the north‐south asymmetry, we mapped the FACs along the field line into the polar region. The footprints of the FACs also show a difference between the Southern and Northern hemispheres (as a function of mapped latitude). These characteristics suggest a north‐south asymmetry of the FACs in the magnetosphere. Further investigation is needed to identify the causes of this asymmetry, although the configuration of the magnetosphere, the polar cap boundary, the conductivity in the ionosphere, or the various solar wind‐magnetosphere interaction processes all may be contributors. That the FAC densities are different between the hemispheres suggests that an important source of these currents must be a voltage generator.

Analysis of the step response function relating the interplanetary electric field to the dayside magnetospheric reconnection potential

We present a statistical analysis of the response of the magnetic reconnection rate between the interplanetary magnetic field and the magnetosphere to southward turnings of the interplanetary magnetic field. The magnetic reconnection rate is calculated from Super Dual Auroral Radar Network (SuperDARN) measurements. The polar cap boundary is identified as the offset circle (3° toward midnight from the magnetic pole) that best separates high spectral width backscatter, indicative of open magnetic field lines, from low spectral width backscatter. The electric potential on the boundary is determined from the best fit of the line‐of‐sight F region plasma velocity to an eighth‐order spherical harmonic function of ionospheric electrical potential. The reconnection rate is determined from the electric potential on the boundary and the expansion rate of the polar cap. Solar wind data (velocity and magnetic field) are obtained from Wind spacecraft measurements and propagated to the magnetopause. The relationship between interplanetary electric field and the magnetic reconnection rate is analyzed by calculating linear response functions. We determine that the convection term in the reconnection rate measurement exhibits a unimodal response to the interplanetary electric field, whereas the boundary motion term in the reconnection rate measurement exhibits a bimodal response, with a positive mode (increase in polar cap expansion rate) followed by a negative mode. The response of the dayside reconnection rate to a step change in the interplanetary electric field exhibits a transient response of one hour duration followed by a steady state response at a level that is half of the peak response.

Plasma convection jets near the poleward boundary of the nightside auroral oval and their relation to Pedersen conductivity gradients

Abstract. In this work, we have shown that the ionospheric azimuthal plasma velocity jets near the open-closed field line boundary on the nightside can be associated with the peak in the ionospheric conductivity gradient. Both model and DMSP observations have been utilized to conduct this investigation. The model tests show that when the gradient of conductivity in the poleward boundary becomes sharper, convection peaks appear around the poleward edge of the aurora. The model results have been confirmed by DMSP observations. Hundreds of large ion flow events are identified from one year DMSP observations, with flow speed larger than 500 m/s that occurred poleward of the aurora. Among them, 280 (74%) events are found to be associated with conductivity gradient peaks. Most of the convection jets occur in winter when conductivity gradients are expected to be large. The convection jets tend to occur at later local times (21:00–22:00 MLT) at 70°–72° MLat. These events are preceded by increasing of the merging electric field suggesting that they occur after the expansion of the polar cap. Both observation and model results show that the conductivity gradient at the polar cap boundary are one of the important elements in establishing the convection jets.

Comparison of the open-closed field line boundary location inferred using IMAGE-FUV SI12 images and EISCAT radar observations

Abstract. We compare the location of the polar cap boundary (PCB) determined using two different techniques, and use them as proxies for the open-closed field line boundary (OCB). Electron temperatures from observations of the EISCAT radar facility are used to estimate the latitude of the PCB along the meridian of the EISCAT VHF beam. The second method utilizes global images of proton aurora obtained by the IMAGE satellite FUV SI12 instrument. These methods are applied to three different intervals. In two events, the agreement between the methods is good and the mean of the difference is within the resolution of the observations. In a third event, the PCB estimated from EISCAT data is located several degrees poleward of that obtained from the IMAGE FUV SI12 instrument. Comparison of the reconnection electric field estimated from the two methods shows that high-resolution measurements both in time and space are needed to capture the variations in reconnection electric field during substorm expansion. In addition to the two techniques introduced above to determine the PCB location, we also use a search for the location of the reversal of the east-west component of the equivalent current known as the magnetic convection reversal boundary (MCRB). The MCRB from the MIRACLE magnetometer chain mainly follows the motion of the polar cap boundary during different substorm phases, but differences arise near the Harang discontinuity.

Electrodynamic coupling in the solar wind-magnetosphere-ionosphere system

This paper presents a brief summary of our recent work based on global MHD simulations of the Solar wind-Magnetosphere-Ionosphere (SMI) system with emphasis on the electrodynamic coupling in the system. The main conclusions obtained are summarized as follows. (1) As a main dynamo of the SMI system, the bow shock contributes to both region 1 Field-Aligned Current (FAC) and cross-tail current. Under strong interplanetary driving conditions and moderate Alfven Mach numbers, the bow shock's contribution may exceed more than fifty percent of the total of either region 1 or cross-tail currents. (2) In terms of more than 100 simulation runs with due southward Interplanetary Magnetic Field (IMF), we have found a combined parameter<EM> f</EM> = <EM>E</EM><SUB>sw</SUB><EM>P</EM><SUB>sw</SUB><EM>M</EM><SUB>A</SUB>^-1/2 (<EM>E</EM><SUB>sw</SUB>, <EM>P</EM><SUB>sw</SUB>, and <EM>M</EM><SUB>A</SUB> are the solar wind electric field, ram pressure, and Alfven Mach number, respectively): both the ionospheric transpolar potential and the magnetopause reconnection voltage vary linearly with f for small f, but saturate for large <EM>f.</EM> (3) The reconnection voltage is approximately fitted by sin^3/2<EM>θ</EM><SUB>IMF</SUB>/2, where <EM>θ</EM><SUB>IMF</SUB> is the IMF clock angle. The ionospheric transpolar potential, the voltage along the polar cap boundary, and the electric fields along the merging line however defined they may be, respond differently to <EM>θ</EM><SUB>IMF</SUB>, so it is not justified to take them as substitutes for the reconnection voltage.

Energetics of the magnetospheric superstorm on November 20, 2003

The magnetospheric storm on November 20, 2003 was one of two greatest events in 1957–2003. The Dst* index reached −472 nT, the polar cap potential drop exceeded 200 kV, the polar cap boundary expanded up to Φ = 60°, the plasma layer density in the synchronous orbit reached 5 cm−3, and the inner edge of the plasma sheet penetrated up to L ∼ 1.5RE. The sequence of disturbance modes including some previously unknown is described. The distribution of the total power input into the magnetosphere between the ionosphere (power Qi) and the ring current (QDR), as well as the relative roles of the spontaneous substorms and the driven disturbances in the creation of the DR current, is analyzed. The values of the parameter α = QDR/Qi are calculated with a step of 5 min. It is shown that intervals with α ≪ 1 and with maximums α ≫ 1 were observed in the events under consideration. These results contradict the dominant opinion that the energy input into the magnetosphere during disturbances is primarily dissipated in the ionosphere. The two types of α maximums are observed: one in the mode of a prevailing spontaneous substorm and the other in the mixed mode of the substorm and driven disturbance. It is concluded that both types of the maximums and corresponding enhancements of the DR current appeared due to the plasma turbulization processes in the substorm current wedge. The parameter α appears to slowly increase from α ≪ 1 to α > 1 with increasing activity level; this trend supports the driven model of creating the DR current due to an increase in the electric field of the solar wind.

On high-altitude electric fields in the auroral downward current region and their coupling to ionospheric electric fields

The downward field-aligned current region plays an active role in magnetosphere–ionosphere coupling processes associated with aurora. A quasi-static electric field structure with a downward parallel electric field forms at altitudes between 800 km and 5000 km, accelerating ionospheric electrons upward, away from the auroral ionosphere. Other phenomena including energetic ion conics, electron solitary waves, low-frequency wave activity, and plasma density cavities occur in this region, which also acts as a source region for VLF saucers. Results are presented from high-altitude Cluster observations with particular emphasis on the characteristics and dynamics of quasi-static electric field structures. These, extending up to altitudes of at least 4–5 Earth radii, appear commonly as monopolar or bipolar electric fields. The former occur at sharp boundaries, such as the polar cap boundary whereas the bipolar fields occur at softer boundaries within the plasma sheet. The temporal evolution of quasi-static electric field structures, as captured by the pearls-on-a-string configuration of the Cluster spacecraft, indicates that the formation of electric field structures and of ionospheric plasma density cavities are closely coupled processes. A related feature of the downward current is a broadening of the current sheet with time, possibly related to the depletion process. Preliminary studies of the coupling of electric fields in the downward current region, show that small-scale structures are typically decoupled from the ionosphere, similar to what has been found for the upward current region. However, exceptions are also found where small-scale electric fields couple perfectly between the ionosphere and Cluster altitudes. Recent FAST results indicate that the degree of coupling differs between sheet-like and curved structures, and that it is typically partial. The electric field coupling further depends on the current–voltage relationship, which is highly non-linear in the downward current region, and still unrevealed, as to its specific form.

Magnetic merging line and reconnection voltage versus IMF clock angle: Results from global MHD simulations

This paper proposes diagnosis methods to trace the magnetic merging line and to calculate the electric potential along it for Earth's magnetospheric magnetic fields obtained by global magnetohydrodynamic (MHD) simulations of the solar wind‐magnetosphere‐ionosphere (SMI) system. The points with minimum magnetic field strength along last closed magnetic field lines and properly selected closed field lines are combined to trace the whole merging line, and the radial ray integration of convectional electric field is used to calculate the electric potential on the merging line. The diagnosis methods are then applied to magnetospheric magnetic fields associated with different interplanetary magnetic field (IMF) clock angles, and a preliminary analysis is presented on the clock angle response of the geometry of the merging line and the associated reconnection voltage. The merging line is found to be similar in geometry to that of the compound field superposed by the Earth's dipole field and the IMF, whereas the reconnection voltage is approximately fitted by sin3/2(θIMF/2) for its response to the IMF clock angle θIMF. The ionospheric transpolar potential and the voltage along the polar cap boundary show different dependence from that of the reconnection voltage, so it is not justified to take them as substitutes for the reconnection voltage. The length of the sunward merging line between the two peaks of reconnection potential shows a nonmonotonic variation in response to θIMF, peaked at θIMF = 90°, so it is also not justified to take electric fields along the merging line, however defined they may be, to characterize the total reconnection rate and the coupling strength between the solar wind and the magnetosphere. The reconnection nearby magnetic nulls closest to the subsolar point is found to be negligible, which gives support to the component reconnection hypothesis for dayside reconnection of quasi‐steady states of the SMI system.