No abstract

The interplanetary magnetic field (IMF) By‐dependent distribution of field‐aligned currents in the cleft region is studied, using the magnetic field and plasma data from 47 passes of Dynamics Explorer (DE) 2. These orbits were chosen on the conditions that cusp/cleft particles are detected and that at the same time the IMF By and Bz components satisfy the criteria |By|≥5 nT and |Bz|≤5 nT during the satellite's crossing of the relevant field‐aligned current region. When By is positive (negative) in addition to satisfying these conditions, there is a strong eastward (westward) magnetic perturbation caused by a pair of field‐aligned current sheets, consisting of an equatorward sheet with downward (upward) current and a poleward sheet having upward (downward) current. These By‐dependent field‐aligned currents in the equatorward and the poleward sheets are referred to as the low‐latitude cleft current (LCC) and the high‐latitude cleft current (HCC), respectively. The cusp/cleft electron precipitation region and the LCC region overlap with each other to a varying degree irrespective of the sign of By. For positive (negative) By, LCC has the same direction as the morning (afternoon) region 1 current or the afternoon (morning) region 2 current. Thus an interpretation has been given in the past that the LCC region is an extension of the region 1 or region 2 current system. However, in this paper we present an alternative view that the LCC region is not an extension of the region 1 or region 2 current system and that a pair of LCC and HCC constitutes the cleft field‐aligned current regime. The proposed pair of cleft field‐aligned currents is explained with a qualitative model in which this pair of currents is generated on the open field lines that have just been reconnected on the dayside magnetopause. The model assumes a quasi‐steady reconnection operating within certain longitudinal width extending to both sides of the stagnation point on the dayside magnetopause. The reconnected flux tubes move under the influences of the field tension and the magnetosheath flow. When the magnetosheath By is positive, the northern hemisphere field lines reconnected on the eastward side of the stagnation point are pulled toward higher latitudes, and the field lines reconnected on the westward side of the stagnation point are pulled along the dawnside magnetopause flank. The electric fields associated with these motions are present immediately inside the magnetopause (rotational discontinuity). This is the source region of LCC and HCC. The electric fields are transmitted along the field lines to the ionosphere, creating a poleward electric field and a pair of field‐aligned currents when By is positive; the pair of field‐aligned currents consists of a downward current at lower latitudes (LCC) and an upward current at higher latitudes (HCC). In the By negative case, the model explains the reversal of the field‐aligned current direction in the LCC and HCC regions.

Abstract. We present the first ever comprehensive statistical study of the spatiotemporal characteristics of field-aligned currents in the terrestrial magnetosphere-ionosphere system using multi point measurements. We determine how the FAC density, variability and scale size are coupled. The three ST 5 satellites were in a pearls-on-a-string formation making measurements of the magnetic field with variable interspacecraft separations ranging from a few seconds to about 10 min. More than 4700 sets of satellite passes are analyzed using a robust correlation analysis aimed at determining the variability of the FAC system as a function of scale size and satellite spacing. We find significant differences between the FAC characteristics on the dayside and on the nightside in terms of dynamics of the current systems. On the dayside the FAC characteristics are found to be independent of IMF B z and geomagnetic activity while the nightside indicates increased variability during disturbed conditions. The boundary separating highly and poorly correlated FACs can be fitted by a linear line for satellite separations shorter than 60 s (dayside) and 160 s (nightside). We interpret this as the dayside and nightside magnetospheric reconfiguration times respectively. For times exceeding this the FAC characteristics are suggested to be controlled by the solar wind (dayside) and plasma sheet (nightside) dynamics. Finally, the characteristics of FAC system with scale sizes larger than ∼200 km (at ionospheric altitude) appear to be stable and repeatable on time scales of the order of a minute (i.e. comparable to the low-altitude orbiting satellite's traverse time across the auroral belt). In this sense, our results effectively validate the Iijima and Potemra (1978) assumption that on average the large-scale currents with scale sizes of the Region1 and Correspondence to: J. W. Gjerloev (jesper.gjerloev@ift.uib.no) Region2 are quasi-persistently significant in the transport of energy and momentum between the magnetosphere and the ionosphere.

[1] The structure of the density discontinuity across the plasmapause is often based on electron and H + density profiles with the contribution of heavy ions (He + , O + etc) neglected. Electron and ion density measurements in this region may differ significantly due to the presence of heavy ions and it is important for the intercomparison of different datasets to understand these differences. Dynamics Explorer (DE-1) magnetic field and plasma composition data have been used to compare heavy ion responses across the plasmapause and to calculate the mass loaded ion density (r) profiles. To illustrate this we investigate mass loading through radial profile variations in the Alfven velocity (V A ). Results show that the gradient in r and V A across the plasmapause is modified when mass loading due to multiple heavy ion species is included, particularly in the presence of the O + torus. Application to ultra-low frequency (ULF) field line resonance is used as an example where the contribution from heavy ions smoothes out the expected ULF wave resonant frequency discontinuity at the plasmapause.

[1] Examination of observations taken by radially aligned International Solar Terrestrial Physics spacecraft in the nightside magnetosphere on 9 July 1997 has revealed close temporal correlations between earthward flow bursts in the plasma sheet and the ejection of plasmoids. A onedimensional model of plasma sheet flow is applied to these observations to determine the time and location for the initiation of lobe flux tube reconnection. For the single clear flow burst-plasmoid pair observed during the first substorm and the three pairs produced by the second substorm, lobe flux reconnection was inferred to have started at X $ À15 to À18 R E , respectively, about 2 -5 min prior to the observations of substorm expansion phase onset. These time delays and propagation speeds are shown to be consistent with the measured plasma sheet bulk flow speeds. Substorm expansion phase onset was essentially coincident with the arrival of the flow bursts at Geotail, which was located near the inner edge of the plasma sheet at X $ À9 R E . The dipolarization of the magnetic field at geosynchronous orbit, auroral kilometric radiation (AKR) emissions, Pi2 pulsations, high-latitude negative magnetic bays, and auroral breakup marking substorm expansion onset are all coincident within the ±1 min resolution of the measurements. Accordingly, it appears that earthward of the inner edge of the plasma sheet, where Geotail was located, substorm effects propagated at speeds comparable to the Alfven speed characteristic of the high-latitude inner magnetosphere, $10 3 km s À1 . In summary, the results of our investigation strongly support the modern near-Earth neutral line (NENL) model of substorms in which the onset of lobe flux tube reconnection in the near tail is followed $2-5 min later by the braking of earthward flow bursts as they encounter the inner magnetosphere and within $1 min, by Pi2s generations, current wedge development, and AKR and auroral expansion, and finally, $10 -20 min later, by the tailward retreat of the neutral line and either the development of a new NENL or the initiation of the recovery phase.