Notes on the auroral electrojet indices
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The auroral electrojet indices (AU, AL, and AE) have served well for more than two decades as measures of magnetospheric substorm activity. However, as substorm studies have progressed considerably during the last several years, the accuracy of the present electrojet indices has become an important issue. Thus it is opportune to reexamine and evaluate the accuracy of the present electrojet indices and improve them if necessary. For a better use of the present indices and for future improvement we examine the limitations of the auroral electrojet indices as an accurate quantitative measure of the auroral electrojets and of magnetospheric substorms. Such limitations should be kept in mind in studying individual substorms, the correlation with solar wind parameters, etc., particularly because the accuracy of the AE index decreases for AE < ∼250 nT. Some of the limitations arise from the data availability and also from the present simplified scheme in deriving them, but some of them originate in the definition itself. A few suggestions are made to improve the present indices, which can be implemented efficiently when digital outputs become available from all the observatories contributing to the electrojet indices.Keywords:
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Abstract. We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°λ-78°λ. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71°λ by the time of the expansion phase onset. A westward electrojet, centred at 65.4°λ, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1°λ at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the "distant" neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.
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Abstract. Enhancements in the auroral electrojets associated with magnetospheric substorms result from those in either the electric field or the ionospheric conductivities, or both. Their relative importance varies significantly, even during a single substorm, depending on the location as well as on the substorm phases. It is predicted that different parts of the electrojets tend to respond in different ways to substorm activity. The unprecedented, unique opportunity for CLUSTER spacecraft observations of electric/magnetic fields and precipitating particles, combined with radar measurements of ionospheric quantities and with ground magnetometers, will provide us with crucial information regarding the physical nature of the separation between the "electric field-dominant'' and "conductivity-dominant'' auroral electrojets. This study also discusses the implications of these two auroral-electrojet components in terms of solar wind-magnetosphere-ionosphere interactions.
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On the basis of ground magnetometer data from 75 northern hemisphere stations and the ionospheric conductivity distribution estimated from Viking satellite observations of auroral images, various electrodynamic quantities in the polar ionosphere are calculated for the April 1, 1986, Coordinated Data Analysis Workshop (CDAW) 9 substorm. Since the Scandinavia and Russia chains of magnetometers were located in the premidnight‐midnight sector during this interval and the estimated conductivity distribution is instantaneous, our data set provides us with a unique opportunity to examine some long‐standing problems associated with the substorm expansion onset. Several important findings of this study are summarized as follows: (1) Before the expansion onset of the substorm, intensifications of ionospheric currents or the cross‐polar cap potential are very weak in this particular example. Both quantities begin to increase notably only with the initiation of the substorm expansion onset. (2) The intensified westward electrojet flows along the poleward half of the enhanced ionospheric conductivity belt in the midnight sector during the expansion phase, while its equatorward half is occupied by a weak eastward electrojet. (3) The Joule heating rate and the energy input rate of auroral particles are quite comparable preceding the expansion onset. During the expansion phase of the substorm, however, Joule heating shows a marked intensification, but the latter increases only moderately, indicating that the Joule dissipation is more effective than auroral particle energy input during substorm times. (4) The Hall currents are not completely divergence‐free. The corresponding field‐aligned currents show highly localized structures during the maximum epoch of the substorm, with the upward current being located in the region of the steepest conductivity gradient on the poleward side of the westward electrojet in the midnight sector. This is indirect evidence that the so‐called imperfect Cowling channel is effective behind the westward traveling surge.
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Abstract. We report multi-instrument observations during an isolated substorm on 17 October 1989. The EISCAT radar operated in the SP-UK-POLI mode measuring ionospheric convection at latitudes 71°λ-78°λ. SAMNET and the EISCAT Magnetometer Cross provide information on the timing of substorm expansion phase onset and subsequent intensifications, as well as the location of the field aligned and ionospheric currents associated with the substorm current wedge. IMP-8 magnetic field data are also included. Evidence of a substorm growth phase is provided by the equatorward motion of a flow reversal boundary across the EISCAT radar field of view at 2130 MLT, following a southward turning of the interplanetary magnetic field (IMF). We infer that the polar cap expanded as a result of the addition of open magnetic flux to the tail lobes during this interval. The flow reversal boundary, which is a lower limit to the polar cap boundary, reached an invariant latitude equatorward of 71°λ by the time of the expansion phase onset. A westward electrojet, centred at 65.4°λ, occurred at the onset of the expansion phase. This electrojet subsequently moved poleward to a maximum of 68.1°λ at 2000 UT and also widened. During the expansion phase, there is evidence of bursts of plasma flow which are spatially localised at longitudes within the substorm current wedge and which occurred well poleward of the westward electrojet. We conclude that the substorm onset region in the ionosphere, defined by the westward electrojet, mapped to a part of the tail radially earthward of the boundary between open and closed magnetic flux, the "distant" neutral line. Thus the substorm was not initiated at the distant neutral line, although there is evidence that it remained active during the expansion phase. It is not obvious whether the electrojet mapped to a near-Earth neutral line, but at its most poleward, the expanded electrojet does not reach the estimated latitude of the polar cap boundary.
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The paper reports studies of the three-dimensional magnetospheric—ionospheric current systems which produced polar magnetic substorms on 1974 September 7 and September 18. The data were magnetic perturbation fields observed with a two-dimensional array of 23 three-component magnetometers located in western Canada beneath the auroral oval. In an earlier study of a substorm of September 11 (Bannister & Gough) the fields fitted calculated field for a Boström Type 1 current loop with field-aligned currents at east and west ends of the ionospheric segment, and with uniform current density across the width. The substorms here reported could not be modelled with uniform current density. An inverse method due to Oldenburg was therefore used to estimate current density distributions, and satisfactory fits of calculated to observed field resulted. Each substorm was modelled at six representative epochs. In general the principal ionospheric current seem by the array was westward. At four epochs of the September 7 substorm and throughout the September 18 substorm, significant eastward ionospheric current (or its equivalent in terms of the fields produced) was observed north of the westward electrojet. Northwestward bends in the ionospheric current segments were found at four epochs on September 7 and at three epochs on September 18. As in the September 11 substorm (Paper 1), these bends were either west of or close to magnetic midnight. In some cases the bends may follow the auroral oval, but in others they are sharper and may be associated with the Harang discontinuity. East of geomagnetic the ionospheric currents tend to run in a constant geomagnetic midnight latitude range. The developments of the three substorms, of September 7, 11 (Paper 1) and 18, are compared. They showed a variety of shifts in longitude, though all moved eastward relative to magnetic midnight.
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Characteristics of the field‐aligned current system in the nighttime sector during auroral substorms
Fujii et al. (1994) obtained characteristics of the electrodynamic parameters, that is, field‐aligned currents, electric fields, and electron precipitation, which are associated with auroral substorm events in the nighttime sector, through a unique analysis that places the ionospheric measurements of these parameters into the context of a generic substorm determined from global auroral images. In this paper we investigate in considerably more detail the characteristics of the field‐aligned currents using data from the same set of passes as the previous study. We show for the first time that the net upward field‐aligned currents throughout the surge and surge horn are sufficient to account for most if not all of the converging currents of the auroral electrojets. Current densities are largest in the surge and surge horn. Current region continuity does not appear to exist across the substorm bulge region. Much of the auroral substorm field‐aligned current is composed of filamentary currents and finite current segments at large angles to each other. The westward electrojet may contain large gradients in intensity both in local time and latitude due to sets of localized field‐aligned currents. The net downward current for several hours to the west of the surge is insufficient to account for the eastward electrojet, consistent with the concept that this electrojet originates primarily on the dayside. Our pattern of field‐aligned currents associated with the surge has common features and also differs significantly from the patterns previously derived from data from radars and ground‐based magnetometer arrays. Our pattern is considerably more complex, probably due to the much higher resolution in latitude of the satellite data. It is also larger in area, since our average substorm is much larger than those pertaining to the previous patterns, giving a substorm wedge considerably wider than that obtained from the radar and array data.
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During the last few years, the study of both temporal and spatial variations of substorm fields has rapidly expanded, mainly because of the relationships which exist between polar magnetic substorms and magnetospheric phenomena. Also during these years, proposed current systems believed to be responsible for substorm variations have evolved into complex three-dimensional systems with field-aligned and magnetospheric currents coupled to the eastward and westward electrojets. Recent model studies show that substorm variations in and near the auroral zone can easily be modelled using both two and three-dimensional current systems. In these studies, induction effects were simulated by assuming the Earth to be infinitely conducting at sonic depth below the surface. The use of magnetometers distributed along magnetic meridians has resulted in a better understanding of the complex current patterns making up the electrojets. For example, during the expansive phase of substorms, the westward and poleward progression of the overall westward electrojet was discovered to take place through the sequential development of a series of westward electrojets.
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Electrojet
Expansive
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Abstract. Enhancements in the auroral electrojets associated with magnetospheric substorms result from those in either the electric field or the ionospheric conductivities, or both. Their relative importance varies significantly, even during a single substorm, depending on the location as well as on the substorm phases. It is predicted that different parts of the electrojets tend to respond in different ways to substorm activity. The unprecedented, unique opportunity for CLUSTER spacecraft observations of electric/magnetic fields and precipitating particles, combined with radar measurements of ionospheric quantities and with ground magnetometers, will provide us with crucial information regarding the physical nature of the separation between the "electric field-dominant'' and "conductivity-dominant'' auroral electrojets. This study also discusses the implications of these two auroral-electrojet components in terms of solar wind-magnetosphere-ionosphere interactions.
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Electrojet
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This paper attempts to synthesize the diverse number of observations of electric fields and currents in the high‐latitude ionosphere during substorms. By demonstrating that there are often spatial shifts among regions of high ionospheric conductivity, large electric fields and intense currents in the auroral electrojet, it is shown that substorm time variations of the current patterns over the entire polar region consist of two basic components. The first is related to the two‐cell convection pattern and the second to the westward electrojet in the dark sector, which is in turn related to the three‐dimensional wedge current system. These two components result from the relative strength of electric fields and conductivities in the intensification of the auroral electrojet and are identified as the signatures for directly driven and the unloading components in solar wind‐magnetosphere interactions. We contend that disturbed intervals do not necessitate the presence of substorm expansion‐phase activity and that the vast number of earlier complex results concerning the auroral electrojet can be ascertained from the high degree of variability of the two components, depending on substorm events, substorm phases, and their own spatial/temporal scale sizes. It is demonstrated that several major issues that have remained controversial are now accounted for reasonably well in terms of this two‐component electrojet model. We also predict specific features of the substorm auroral electrojet that have not yet been observed.
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Equatorial electrojet
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