A note on the accuracy of the auroral electrojet indices
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The accuracy of the present AE(12) index is evaluated by using magnetometer data from the six IMS meridian chains and other high‐latitude stations. It is shown that the accuracy of AE(12) becomes progressively worse for lower values. There are at least a few causes for this inaccuracy, and three of them are identified. Although it is not possible to provide a single critical value, one should be cautious about the accuracy of the absolute value of AE(12) less than ∼250 nT. In particular, since 70% of AE(12) values are less than 250 nT, one must be cautious in correlative use of the AE(12) index with solar wind parameters.Keywords:
Electrojet
Meridian (astronomy)
Substorm
Abstract. The behavior of the auroral electrojet indices AU and AL during classical substorms is investigated by the use of global auroral images. A superposition of the 12 AE stations onto global auroral images and identification of the AL and AU contributing stations enable an understanding of the temporal as well as spatial behavior of the indices with respect to the substorm coordinate system and timeframe. Based on this simple technique it was found that at substorm onset the AL contributing station makes a characteristic jump from a location near the dawn terminator to the onset region, typically bypassing one or more AE stations. During the expansion phase this station typically lies at the poleward edge of the surge region. This is the location of the intense substorm current wedge electrojet in the semiempirical self-consistent substorm model of the three-dimensional current system by Gjerloev and Hoffman (2002). This current wedge is fed primarily pre-midnight by an imbalance of the Region 0 and Region 1 field-aligned currents, not from the dawnside westward electrojet. Then during the early recovery phase the AL contributing station jumps back to the dawn sector. The defining AU station does not show any similar systematic behavior. We also find that the dawn side westward electrojet seems to be unaffected by the introduction of the substorm current wedge. According to our model, much of this current is closed to the magnetosphere as it approaches midnight from dawn. Based on the characteristics of the AL station jumps, the behavior of the dawn-side electrojet, and the understanding of the three-dimensional substorm current system from our model, we provide additional experimental evidence for, and an understanding of, the concept of the two component westward electrojet, as suggested by Kamide and Kokubun (1996).
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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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We estimsted the Hall and Cowling current-caused contributions into the westward auroral electrojet (AEJ-W) during the substorm expansion phase, using data of two substorms. Between other, the data of field-aligned currents (FAC) were used, which were not available in past. As the main result, the major contributions in the unloading AEJ-W were caused by the Hall current that contradict to the conventional model. The physics behind this result is briefly discussed.
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We have produced the first series of spherical harmonic, numerical maps of the time-dependent surface perturbations in the Earth's magnetic field following the onset of substorms. Data from 124 ground magnetometer stations in the Northern Hemisphere at geomagnetic latitudes above 33° were used. Ground station data averaged over 5 min intervals covering 8 years (1998-2005) were used to construct pseudo auroral upper, auroral lower, and auroral electrojet (AU*, AL*, and AE*) indices. These indices were used to generate a list of substorms that extended from 1998 to 2005, through a combination of automated processing and visual checks. Events were sorted by interplanetary magnetic field (IMF) orientation (at the Advanced Composition Explorer (ACE) satellite), dipole tilt angle, and substorm magnitude. Within each category, the events were aligned on substorm onset. A spherical cap harmonic analysis was used to obtain a least error fit of the substorm disturbance patterns at 5 min intervals up to 90 min after onset. The fits obtained at onset time were subtracted from all subsequent fits, for each group of substorm events. Maps of the three vector components of the averaged magnetic perturbations were constructed to show the effects of substorm currents. These maps are produced for several specific ranges of values for the peak |AL*| index, IMF orientation, and dipole tilt angle. We demonstrate an influence of the dipole tilt angle on the response to substorms. Our results indicate that there are downward currents poleward and upward currents just equatorward of the peak in the substorms' westward electrojet.Show quantitative maps of ground geomagnetic perturbations due to substorms Three vector components mapped as function of time during onset and recovery Compare/contrast results for different tilt angle and sign of IMF Y-component.
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Magnetic variations processed from the AE observatories and IMP‐8 measurements of the solar wind and interplanetary magnetic field (IMF) are used to study the response of the auroral electrojets at different local times to solar wind and IMF parameters. On the basis of an interpolation technique, magnetic variations at four different local times (0000, 0600, 1200, and 1800 MLT) are estimated, which constitutes our data base for this statistical study. Forty two events in 1979 met our “isolated substorm” criteria. It is found that electrojet variations at different local times are significantly different The substorm expansion current system at midnight appears to be initiated when IMF becomes less southward, and the current systems at dawn and dusk are already active when the substorm expansion current begins and remain active even after the midnight substorm current recovers to its pre‐substorm level. During the initial stage of the recovery phase (in terms of the midnight current), the westward electrojet at 0600 MLT continues to intensify while the 1800 MLT eastward electrojet slowly decays.
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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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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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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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