Takasi Oguti

Equatorward and poleward expansion of the auroras during auroral substorms

Using all-sky TV auroral data with high spatial and temporal resolution, the formation of the auroral bulge is investigated. We show how the discrete auroral structures within the poleward expanding bulge develop systematically toward the west, the east, and also equatorward from a localized breakup region. Auroral structure at the western end of the bulge (a surge) develops with clockwise rotation as viewed along the magnetic field direction. At the eastern part of the bulge, thin auroral features propagate eastward from the breakup region. Around the central meridian of the bulge, auroral features expand equatorward and become north-south aligned (the N-S aurora). The N-S aurora and the eastward propagating aurora develop into diffuse and pulsating aurora after the expansion. We suggest that these discrete auroral structures in the bulge develop along the plasma streamlines in a localized distorted two-cell equipotential distribution; the negative potential peak is concentrated at the western part of the bulge and the positive potential region is spread poleward of the eastward propagating aurora. Equatorward expansion of auroral structures would be attributed in this scenario to the subsequent earthward transport of plasma from the onset region. Energetic particles would also be injected at the N-S auroral region as well as to the east of the region and would trigger the pulsating auroral activity. Poleward expansion, on the other hand, would be produced by the tailward motion of the onset region in accordance with the tailward moving current disruption region.

Relationships between quasi-periodic VLF emission and geomagnetic pulsation

Abstract Relationships between quasi-periodic VLF emissions (QP) and geomagnetic pulsations are examined, based on observations at Syowa Station, Antarctica. It is confirmed that QP emissions can be classified into two types according to whether or not QP emissions are accompanied by geomagnetic pulsations with approximately the same period. One type, associated with concurrent pulsations, is a daytime phenomenon and often occurs in moderate magnetic activity. The QP period ranges mostly from 20 to 60 s. The spectral form depends significantly on the modulation period. When the period is longer than 30–40 s, the upper frequency limit of the emission band varies almost synchronously with the change in signal strength and the pulsation wave form. The rising characteristic of the QP component which comprises diffuse noises occasionally with discrete emissions, is observed when the period is shorter than 30–40 s. It is very likely that the QP emission of this type is caused by the interaction between whistler turbulence and compressional hydromagnetic waves in the outer magnetosphere. Another type, without any corresponding pulsation has a broad maximum of occurrence in the afternoon-evening and is observed in quiet geomagnetic conditions. The QP component, consists of either diffuse noises or discrete periodic emissions or a combination of both, and generally shows a midfrequency rise. These characteristics are very similar to those of QP emissions observed at Eights (L ≅ 4). It is suggested that this type of QP emission is a phenomenon occurring near the plasmapause and that is closely related to the bulge or detached regions of the plasmasphere.

Correlations between the very low frequency chorus and pulsating aurora observed by low-light-level television at

A good correlation was found between the very low frequency chorus and pulsating aurora observed at Park Site (L ~ 4.4), Saskatchewan, Canada, during the recovery phase of a substorm on January 28, 1980. The activity of the chorus was low during the periods when the pulsating aurora was active over the station. However, seven chorus events were identified during that period and six of them showed one-to-one correlation with the brightening of a pulsating patch. The brightening of the patch leads chorus events by ~0.1–0.2 s. The corresponding patch detected by a low-light-level TV camera has an oval shape 75 km in the north–south direction and 150 km in the east–west direction at the ionospheric altitudes. The energy of precipitated electrons was inferred to lie between 20 and 90 keV.

Quasi-periodic poleward propagation of on-off switching aurora and associated geomagnetic pulsations in the dawn

Abstract On the basis of TV records of auroras and induction magnetograms obtained in the southern auroral zone, geomagnetic pulsations with periods of a few seconds to several tens of second which often occur in the dawn sector during a post-breakup phase were found to be coherently related to auroral pulsations (or on-off switching) and typically to a quasi-periodic poleward propagation of the on-off switching aurora. The poleward propagation of the aurora here is similar to the behaviour of the radar aurora associated with a Pc -5, and consequently they are likely to be attributable to a common generation mechanism despite the difference in period. The quasi-periodic appearance near the low latitude border of an auroral activity and poleward propagation of this kind of aurora indicate firstly that the concurrent geomagnetic pulsation is likely to be due to the propagating mode rather than a standing oscillation, and secondly that its periodicity is primarly determined by the periodicity (or quasi-periodicity) of triggering waves at the inner boundary of the injected energetic electrons in the magnetosphere, which precipitate electrons while propagating outward.

Quasi-periodic poleward motions of Sun-aligned auroral arcs in the high-latitude morning sector: A case study

This is the first paper which reports the characteristics of quasi-periodic poleward motions of Sun-aligned auroral arcs in the high-latitude morning sector. The moving arcs are observed from ground-based stations at magnetic latitudes (MLAT) of 78° and 84° during magnetically quiet intervals (interplanetary magnetic field Bz ∼ 0 or > 0). The arcs move poleward repeatedly with a period of several minutes and a velocity of ∼400–500 m/s and disappear at around 85° MLAT. For the event observed at 78° MLAT, the arcs are repeatedly detached from a stable aurora which is located at the equatorward of the arcs. The moving arcs correspond to accelerated electrons observed by the Exos D satellite. The stable aurora corresponds to continuous precipitation of high-energy electrons which probably originate from the inner part of the plasma sheet. The ion drift data from the DMSP-F11 satellite show that the poleward moving arcs are located around the boundary of the large-scale sunward flowing region at lower latitudes and the antisunward flowing region at higher latitudes. From these results, we conclude that the arcs are connected to the boundary region between the plasma sheet and the low-latitude boundary layer in the morningside tail flank. Several mechanisms which can produce the observed motions of the arcs are discussed.

Detailed analysis of a substorm event on 6 and 7 June 1989: 2. Stepwise auroral bulge evolution during expansion phase

[1] We have analyzed in detail the auroral bulge evolution during the expansion phase of an isolated substorm, which was observed by the UV imager aboard the Akebono satellite. It was found that there were three distinct stages in the evolution. Stage 1 was characterized by rapid poleward and azimuthal ( predominantly westward) expansions in a short time (about 2 min). Stage 2 was characterized by a very slow poleward and slower and continuous azimuthal expansions. There was a certain period for transition between stage 1 and stage 2, and it was characterized by a very slow poleward and rapid eastward expansions. Stage 3 started about 11 min after the onset and was characterized by a sudden reactivation of the rapid poleward and azimuthal expansions. The reactivation started around the initial onset meridian and then spread both eastward and westward. At the azimuthal front, the expansion first occurred at the lowest latitudes, spread poleward to around the highest latitudes of stage 1, and then spread further poleward after a brief interval. Hence, the local expansion also had three distinct stages similar to the global one. The ground-based observations showed that the highest latitude of the local first stage was very close to the latitude of auroral activity that appeared near the ionospheric plasma sheet boundary layer (PSBL) region a few minutes before the onset. The further poleward expansion during the local third stage started with a significant intensification of the poleward-most auroral activity. During the local third stage, the bright electron auroral region was bifurcated into a poleward expanding part and an equatorward moving part. The proton auroral emission coexisted in the bulge during the local first and second stages and almost disappeared soon after the bifurcation during the local third stage. Based on these observations, we discuss the evolution in the magnetosphere during the expansion phase.

A statistical study of the motions of auroral arcs in the high-latitude morning sector

Motions of auroral arcs in the high-latitude morning sector (0300-0900 magnetic local time) have been studied statistically for magnetically quiet periods, using the data from all-sky images for two winters from two ground-based stations at magnetic latitudes (MLAT) of 84° and 78°. Most of the observed arcs are in a sum-aligned direction. Most of the arcs observed at 84° MLAT move duskward with a typical velocity of ∼500 m/s. A similar duskward motion of the arcs with a slightly higher velocity is dominant at 78° MLAT, though some arcs move antisunward at this latitude. It is suggested that the observed duskward auroral motion is related to poleward shrinkage of the morningside oval on closed field lines during magnetically quiet conditions.

Hiss emitting auroral activity

Abstract Real-time auroral records on a video tape obtained by use of a highly sensitive TV camera, and simultaneous records of VLF waves on the sound track of the same video tape were analyzed and specific small scale auroral activities were found to be associated with specific burst-like hiss enhancements with durations of 0.1–1 s. The hiss emitting auroral activities were identified by cross correlation analysis between the temporal variations in luminosity of auroral structures and the temporal variations of hiss intensities. The hiss emitting aurora were long-rayed sheet fragments or folds of a few km to several tens of km in horizontal scale which rapidly changed in luminosity, and displayed rapid motions such as splittings and rotations. The enhancements of local auroral electron precipitations responsible for the local auroral activity and of hiss emissions are concluded to be simultaneous within a time accuracy of 200 ms at worst and within a few tens of ms at best.

Similarity between global auroral deformations in DAPP photographs and small scale deformations observed by a TV camera

Abstract A striking resemblance has been found between the global patterns of S-shaped structures in the dusk sector seen in DAPP photographs (westward travelling surge and auroral bulge) and small-scale S-patterns observed by a highly sensitive TV camera. A remarkable similarity has also been found between flame-like structures of small-scale aurora and that of global scale. Another important point is a rotational symmetry between the formation of an S-structure and the formation of a flame-structure. Both the similarity between the deformation processes of large-scale aurora and those of smallscale aurora, and the rotational symmetry between the formation processes of an S-structure and those of a flame-structure, together indicate that auroral deformation is attributable to the general dynamics of an electron sheet or a cloud of electrons in a magnetic field, regardless of both the specific configuration and condition of the magnetic field and the plasma distribution around the Earth.

Relationships Between Auroral and Magnetic Activity in the Polar Cusp/Cleft

Both large- and small-scale dynamics of the cusp aurora and cusp electrodynamics are discussed in connection with electric currents, electric field and ground magnetic perturbations. Sources of the field-aligned electric currents in the dayside polar cusp are most likely located within the boundary layers, both the frontside low latitude and the magnetotai1, where induction currents due to relative motion between the sheath plasma and the magnetosphere produces space charges. The importance of the magnetospheric magnetic field model in the study of the cusp phenomena, especially their magnetospheric source regions, is noted.