Conjugacy of isolated auroral arcs and nonconjugate auroral breakups

Sato, Natsuo; Nagaoka, Takayuki; Hashimoto, Kumiko; Sæmundsson, Þorsteinn

doi:10.1029/98ja00461

Cited by 48 publications

(53 citation statements)

References 31 publications

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“…Furthermore, it seems that the substorm onset occurred in the Northern Hemisphere a whole 35 min earlier than in the Southern Hemisphere. Such substorm onset timing conjugacy asymmetries have been reported before by Frank and Sigwarth [2003] and Sato et al [1998] using auroral images and Chisham et al [2000] with radar data. We believe that some of the timing difference is due to the location of the WSD station with respect to the auroral oval.…”

Section: Journal Of Geophysical Research: Space Physicsmentioning

confidence: 61%

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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Section: Journal Of Geophysical Research: Space Physicsmentioning

confidence: 61%

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

2014

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“…The Z component deflection (positive downwards) was positive at stations north of BJN and negative south of BJN, implying an average location of the westward electrojet close to BJN. Although substorms are sometimes found to occur in one hemisphere and are absent in the other hemisphere (Sato et al, 1998), our observations derived from FUV imagers in the Southern Hemisphere and from ground-based magnetometers in the north show clearly that this isolated substorm occurs simultaneously and conjugately in both hemispheres. Figure 4 shows field-aligned currents (FACs) and Hall currents (HC) derived from the magnetic field measurements on CHAMP (e.g.…”

Section: Observationsmentioning

confidence: 81%

Global manifestations of a substorm onset observed by a multi-satellite and ground station network

Wang

Lühr

et al. 2006

Ann. Geophys.

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Abstract. With a favorable constellation of spacecraft and ground stations, a study is made on the global manifestations of a substorm onset. The onset occurred simultaneously and conjugately in both hemispheres, confirmed by observations of the auroral breakup from IMAGE FUV-WIC and a sudden intensification of a westward electrojet from ground-based magnetometers. Concurrently with the onset, field-aligned and Hall currents in the auroral ionosphere are observed by CHAMP, which are consistent with the signature of a Harang discontinuity. Immediately after the onset a magnetic field dipolarization is clearly observed by Double Star TC-1, located near the central magnetotail and subsequently, by the Cluster quartet. The observations can be explained by a dawnward propagation of the substorm current wedge at a speed of about 300 km/s.

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“…We think that the preexisting ionospheric conductivity has an influence on the location of the auroral acceleration region, possibly by moving to a latitude favorable for the instability, which is thought to be an element of the acceleration process (Haerendel, 1990). Sato et al (1998) reported that ionospheric conductivity might explain why auroral features sometimes occur in one hemisphere and are absent in the other hemisphere. When accepting this concept it is no surprise that the observed FACs and electrojets in the Southern Hemisphere may deviate significantly from the conjugate point of the substorm onsets in the Northern Hemisphere due to differences in illumination conditions.…”

Section: Relation Between Onset Location and Facs And Hall Current Lamentioning

confidence: 99%

Statistical study of the substorm onset: its dependence on solar wind parameters and solar illumination

Wang

Lühr

et al. 2005

Ann. Geophys.

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Abstract. Based on 1829 well-defined substorm onsets in the Northern Hemisphere, observed during a 2-year period by the FUV Imager on board the IMAGE spacecraft, a statistical study is performed. From the combination of solar wind parameter observations by ACE and magnetic field observations by the low altitude satellite CHAMP, the location of auroral breakups in response to solar illumination and solar coupling parameters are studied. Furthermore, the correspondence of the onset location with prominent large-scale field-aligned currents and electrojets are investigated. Solar illumination and the related ionospheric conductivity have significant effects on the most probable substorm onset latitude and local time. In sunlight, substorm onsets tend to occur 1 h earlier in local time and 1.5 • more poleward than in darkness. The solar wind input, represented by the merging electric field, integrated over 1 h prior to the substorm, correlates well with the latitude of the breakup. Most poleward latitudes of the onsets are found to range around 73 • magnetic latitude during very quiet times. Field-aligned and Hall currents observed concurrently with the onset are consistent with the signature of a westward travelling surge evolving out of the Harang discontinuity. The observations suggest that the ionospheric conductivity has an influence on the location of the precipitating energetic electron which causes the auroral break-up signature.

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Conjugacy of isolated auroral arcs and nonconjugate auroral breakups

Cited by 48 publications

References 31 publications

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

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

Global manifestations of a substorm onset observed by a multi-satellite and ground station network

Statistical study of the substorm onset: its dependence on solar wind parameters and solar illumination

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