Universal time variations of the auroral electrojet indices

Ahn, B.‐H.; Kroehl, H. W.; Kamide, Y.; Kihn, E. A.

doi:10.1029/1999ja900364

Cited by 45 publications

(53 citation statements)

References 22 publications

(31 reference statements)

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“…As a comparison, we also perform contour plots of AE index before/after SSCs under all circumstances (Figures 13a and 13b). The variation of AE index before and after SSCs don't show clear feature of semiannual variation and UT variation as the R-M effect or the equinoctial hypothesis suggested, which is likely caused by the equatorward expansion of the auroral electrojets and the longitudinal station gaps [Ahn et al, 2000]. However, even the seasonal and diurnal variation of AE index is quite distinctive, AE variation with positive/negative IMF B y still shows identical feature as the R-M effect with positive/negative IMF polarity: when IMF B y is negative, AE index is higher around the spring equinox and relatively lower around the fall equinox; when IMF B y is positive, it is higher around the fall equinox and lower around the spring equinox.…”

Section: Seasonal and Diurnal Variation Of Geomagnetic Activity Beformentioning

confidence: 99%

Seasonal and diurnal variation of geomagnetic activity: Russell‐McPherron effect during different IMF polarity and/or extreme solar wind conditions

Zhao¹,

Zong²

2012

J. Geophys. Res.

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[1] The Russell-McPherron (R-M) effect is one of the most prevailing hypotheses accounting for semiannual variation of geomagnetic activity. To validate the R-M effect and investigate the difference of geomagnetic activity variation under different interplanetary magnetic field (IMF) polarity and during extreme solar wind conditions (interplanetary shock), we have analyzed 42 years interplanetary magnetic field and geomagnetic indices data and 1270 SSC (storm sudden commencement) events from the year 1968 to 2010 by defining the R-M effect with positive/negative IMF polarity (IMF away/toward the Sun). The results obtained in this study have shown that the response of geomagnetic activity to the R-M effect with positive/negative IMF polarity are rather profound: the geomagnetic activity is much more intense around fall equinox when the direction of IMF is away the Sun, while much more intense around spring equinox when the direction of IMF is toward the Sun. The seasonal and diurnal variation of geomagnetic activity after SSCs can be attributed to both R-M effect and the equinoctial hypothesis; the R-M effect explains most part of variance of southward IMF, while the equinoctial hypothesis explains similar variance of ring current injection and geomagnetic indices as the R-M effect. However, the R-M effect with positive/negative IMF polarity explains the difference between SSCs with positive/negative IMF B y accurately, while the equinoctial hypothesis cannot explain such difference at the spring and fall equinoxes. Thus, the R-M effect with positive/negative IMF polarity is more reasonable to explain seasonal and diurnal variation of geomagnetic activity under extreme solar wind conditions. Citation: Zhao, H., and Q.-G. Zong (2012), Seasonal and diurnal variation of geomagnetic activity: Russell-McPherron effect during different IMF polarity and/or extreme solar wind conditions,

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Section: Seasonal and Diurnal Variation Of Geomagnetic Activity Beformentioning

confidence: 99%

Seasonal and diurnal variation of geomagnetic activity: Russell‐McPherron effect during different IMF polarity and/or extreme solar wind conditions

Zhao¹,

Zong²

2012

J. Geophys. Res.

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“…Similarly, Hajkowicz [1998] determined using only the WDC AE index that there was also a UT effect in the onset of auroral disturbances, which tended to maximize between 09 and 18 UT and reach a minimum between 03 and 06 UT. Ahn et al [2000] has offered a potential explanation for the minimum in the index between 03 and 06 UT. They suggested that between 02 and 08 UT, the AL index often underestimated the geomagnetic activity.…”

Section: Introductionmentioning

confidence: 99%

Auroral electrojet indices in the Northern and Southern Hemispheres: A statistical comparison

2014

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The auroral electrojet (AE) index is traditionally derived from about 12 ground magnetometer observatories located around the average northern auroral oval location. The AE index calculation has only been performed with Northern Hemisphere data, because similar coverage in the Southern Hemisphere does not exist. In this study, eight southern auroral ground magnetometers and their near conjugate Northern Hemisphere counterparts are used to calculate conjugate AE indices for 274 days covering all four seasons from 2005 to 2010. The correlation coefficient between the northern and southern AE indices for many of the intervals is 0.65 indicating strong asymmetries between the two hemispheres. We compare our conjugate AE indices with the standard AE index and find a number of asymmetries because of station coverage gaps in the southern and northern arrays. The mean difference between the southern and northern AE indices is largest during northern summer season, and the smallest mean difference occurs in the spring. The mean differences between the southern and conjugate northern AE indices are about 31 nT with the largest differences occurring in the midnight magnetic local time (MLT) sector. We suggest that these differences may be a function of seasonal, MLT, and ionospheric effects. We also find a difference in the southern and northern AE related to UT and believe that this pertains to the distribution of the magnetometers. The fact that a difference between the southern and northern AE indices exists indicates the importance of examining geomagnetic activity in both hemispheres when considering magnetospheric phenomena.

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“…It is well known that, generally, the strongest contribution to the A L index is in the early morning hours, around 03:00 MLT, and the main contribution to the A U index is in the dusk region (around 18:00 MLT) (e.g. Kauristie et al, 1996;Ahn et al, 2000). Consequently, one might think that the orbit of the satellites for the results shown here is particularly well suited for obtaining good correlation with the ground indices.…”

Section: Figmentioning

confidence: 84%

“…As was discussed right from the beginning, however, they are flawed with obvious problems related to the limited coverage of the AEstations both longitudinal and latitudinal (e.g. Davis and Sugiura, 1966;Rostoker, 1972;Kamide and Akasofu, 1983;Ahn et al, 2000). Nevertheless, they continue to play a large role as a quantitative parameter describing various aspects of the solar-terrestrial environment (e.g.…”

Section: The Current Indicesmentioning

confidence: 99%

Investigating the auroral electrojets with low altitude polar orbiting satellites

Moretto

Olsen²,

Ritter³

et al. 2002

Ann. Geophys.

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Abstract. Three geomagnetic satellite missions currently provide high precision magnetic field measurements from low altitude polar orbiting spacecraft. We demonstrate how these data can be used to determine the intensity and location of the horizontal currents that flow in the ionosphere, predominantly in the auroral electrojets. First, we examine the results during a recent geomagnetic storm. The currents derived from two satellites at different altitudes are in very good agreement, which verifies good stability of the method. Further, a very high degree of correlation (correlation coefficients of 0.8-0.9) is observed between the amplitudes of the derived currents and the commonly used auroral electrojet indices based on magnetic measurements at ground. This points to the potential of defining an auroral activity index based on the satellite observations, which could be useful for space weather monitoring. A specific advantage of the satellite observations over the ground-based magnetic measurements is their coverage of the Southern Hemisphere, as well as the Northern. We utilize this in an investigation of the ionospheric currents observed in both polar regions during a period of unusually steady interplanetary magnetic field with a large negative Y -component. A pronounced asymmetry is found between the currents in the two hemispheres, which indicates real inter-hemispheric differences beyond the mirrorasymmetry between hemispheres that earlier studies have revealed. The method is also applied to another event for which the combined measurements of the three satellites provide a comprehensive view of the current systems. The analysis hereof reveals some surprising results concerning the connection between solar wind driver and the resulting ionospheric currents. Specifically, preconditioning of the magnetosphere (history of the interplanetary magnetic field) is seen to play an important role, and in the winther hemisphere, it seems to be harder to drive currents on the nightside than on the dayside. Key words. Ionosphere (electric fields and currents) -Correspondence to: T. Moretto (Therese.Moretto@gsfc.nasa.gov) Magnetospheric physics (current systems; magnetosphereionosphere interactions)

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Universal time variations of the auroral electrojet indices

Cited by 45 publications

References 22 publications

Seasonal and diurnal variation of geomagnetic activity: Russell‐McPherron effect during different IMF polarity and/or extreme solar wind conditions

Seasonal and diurnal variation of geomagnetic activity: Russell‐McPherron effect during different IMF polarity and/or extreme solar wind conditions

Auroral electrojet indices in the Northern and Southern Hemispheres: A statistical comparison

Investigating the auroral electrojets with low altitude polar orbiting satellites

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