Forecasting of DST index from auroral electrojet indices using time-delay neural network + particle swarm optimization
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In this study, an artificial neural network was optimized with particle swarm algorithm and trained to predict the geomagmetic DST index one hour ahead using the past values of DST and auroral electrojet indices. The results show that the proposed neural network model can be properly trained for predicting of DST(t + 1) with acceptable accuracy, and that the geomagnetic indices used have influential effects on the good training and predicting capabilities of the chosen network.Keywords:
Electrojet
The study of the semiannual variation in both geomagnetic activity indices and raw geomagnetic field data at a large number of geomagnetic observatories showed a dramatic decrease in the relative magnitude of this variation from low to high latitudes. While the semiannual variation in the Dst index at low latitudes is about 3 times, for the midlatitude Ap and Am indices it is only about 20–30%, and the auroral electrojet AE index and polar cap PCN index show a very small or no semiannual variation. The study of auroral electrojet AU and AL indices showed that the semiannual variation is evident in AL index and not observed in AU index. Although relative magnitudes of the semiannual variation in geomagnetic activity indices are strongly reduced at high latitudes, absolute magnitudes of these variations in the Dst , Ap , Am , and AL indices are approximately the same. The semiannual variation in geomagnetic field H component consists of two equinoctial minima probably caused by the increasing westward ring current and substorm westward auroral electrojet in equinoctial months. The additional analysis of raw geomagnetic field data from geomagnetic observatories in two hemispheres confirmed the results inferred from the study of geomagnetic indices. Meanwhile in the polar caps, the semiannual variation in raw geomagnetic data showing a strong decrease in geomagnetic field H component reveals a strong increase in the Z component which also might be caused by increasing the ring current and westward auroral electrojet during enhanced geomagnetic activity occurring in equinoctial months. It was also found that the amplitude of the semiannual variation at all latitudes increased significantly with increasing geomagnetic activity.
Electrojet
Geomagnetic secular variation
Ring current
Equatorial electrojet
Geomagnetic latitude
Variation (astronomy)
Substorm
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The High Frequency Active Auroral Research Program (HAARP) facility is used to generate waves in the extremely low frequency (ELF) range via modulated HF heating of the ionosphere. This HF heating modulates the electron temperature in the D region ionosphere and leads to modulated conductivity and a time‐varying current which then radiates at the modulation frequency. We investigate the relationship between the intensity of the HAARP‐generated ELF signal and the strength of the east‐west component of the auroral electrojet as measured by a ground‐based magnetometer. We find that under all magnetic conditions, HAARP can generate ELF radiation detectable 37 km away with 73% of tones having an amplitude exceeding 0.15 pT. While strong ELF amplitudes (>1.5 pT) were most common during an enhanced electrojet, a weak electrojet can also support equally high ELF amplitudes. The relative change in ELF amplitude per unit change in electrojet current strength is inversely proportional to the absolute current strength. We interpret the dynamic relationship between ELF amplitude and electrojet current strength in terms of the time‐variable ionospheric parameters and HF heating efficiency.
Electrojet
Equatorial electrojet
Extremely low frequency
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Data from the University of Alberta meridian line of magnetometers are utilized to define the poleward and equatorward borders of the eastward electrojet in the evening sector. Soft particle spectrometer data from the Isis 2 polar‐orbiting satellite are organized in the framework of the eastward electrojet for cases where the satellite orbital path took it close to the meridian line of ground‐based magnetometer stations. It is shown that in the late evening hours the equatorward border of the eastward electrojet coincides with the equatorward edge of the central plasma sheet (cps) as marked by electrons of E >1 keV. The boundary plasma sheet (bps) spans the poleward portion of the eastward electrojet and a region up to a few degrees poleward of the eastward electrojet where the westward electrojet is known to penetrate on average. A level shift in the Y′ component of the ground magnetometer data exists across the portion of the electrojet region marked by bps; this level shift is interpreted as net upward field‐aligned current flow. In the hours between noon and dusk the correlation between the equatorward border of the eastward electrojet and the equatorward edge of the cps deteriorates completely, and the magnetic latitude profiles exhibit anomalous behavior. This problem will be discussed in paper 2 of this series. On the basis of this study we conclude that in the evening sector the cps is threaded by field lines carrying current flowing into the ionosphere, while the bps is threaded by field lines carrying current flowing out of the ionosphere. The electric field transition from the auroral oval to the polar cap must occur in the heart of the bps under average conditions.
Electrojet
Field line
Equatorial electrojet
Plasma sheet
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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.
Electrojet
Substorm
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Electrojet
Substorm
Equatorial electrojet
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Electrojet
Polar cap
Substorm
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Based on the data of the Bloxham Jackson Geomagnetic model(BJ for short) and the International Geomagnetic Reference Field(IGRF),the features of the geomagnetic main field periodicity are discussed.The results indicate that there is a periodicity about 60 years for the change rates of geomagnetic parameters,including geomagnetic moment,the spatial power spectrum and the average drift rate of spherical harmonic coefficients with different orders of the non-dipole magnetic field.And a 30-year periodicity is commonly available for the geomagnetic parameters and the average western drift rate considering the weight factors and the terms that related to n=2.Consequently,it can be confirmed that 60-year and 30-year are the main characteristic periods of geomagnetic secular variation.
Geomagnetic secular variation
Secular Variation
Ionospheric dynamo region
Ring current
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The auroral electrojet index is an important index in monitoring and predicting substorms. A substorms usually includes auroral breakup, auroral electrojet event marked by AE increase, energetic particle injection at geosynchronous orbit, mid-low latitude Pi2, etc. However the question whether an auroral electrojet event corresponds to a substorm remains unanswered. Using the auroral electrojet index in 2004, we analyzed five auroral electrojet events and studied their relation with substorms. The results show that there are three kinds of auroral electrojet events: (1) simultaneous rapid increase of westward auroral electrojet and eastward auroral electrojet; (2) rapid increase of westward auroral electrojet and almost unchangeable eastward auroral electrojet; (3) rapid increase of eastward auroral electrojet and almost unchangeable westward auroral electrojet. Most of auroral electrojet events correspond to substorms. However a few auroral electrojet events are not accompanied by substorms. This situation most often occurs for the auroral electrojet event in which eastward auroral electrojet dominates.
Electrojet
Substorm
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Abstract We investigate the effects of solar illumination and the substorm process on the auroral electrojets using CHAMP high‐resolution magnetic field data obtained over a period of 10 years. The eastward electrojet was found to be proportional to the ionospheric conductance induced by solar irradiation over the whole day. More notably, a stronger westward current was detected in times of lower fluxtube‐integrated conductance in both hemispheres. The relative spatial distribution and evolution of the westward electrojet and field‐aligned current (FAC) during the substorm periods were also investigated. The position of the westward current coincided with that of the upward FAC in 2000–2200 magnetic local time (MLT), in keeping with the structure of a westward traveling surge. Furthermore, the upward FAC in the local time sector of the poleward of the westward electrojet was supplied mostly by the remote closure current. The downward FACs in the poleward and upward FACs in the equatorward of the westward electrojet existed in the MLT sectors after 2200 h. Finally, the ratio of the peak intensity of the westward electrojet to that of the meridional Pedersen current, in the 2000–2200 MLT sector increased by about 250%, and the value in the 2200–0200 MLT increased by about 10%–20%. These results provide observational evidence of the increased efficiency of the Cowling channel in the night‐time during substorm periods and contribute to our understanding of the formation mechanism of the auroral electrojet.
Electrojet
Substorm
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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.
Substorm
Electrojet
Equatorial electrojet
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