Showing papers by "Akira Kadokura published in 2014"

Seasonal variation and solar activity dependence of the quiet‐time ionospheric trough

Abstract: We have conducted a statistical analysis of the ionospheric F region trough, focusing on its seasonal variation and solar activity dependence under geomagnetically quiet and moderate conditions, using plasma parameter data obtained via Common Program 3 observations performed by the European Incoherent Scatter (EISCAT) radar between 1982 and 2011. We have confirmed that there is a major difference in frictional heating between the high- and low-latitude sides of the EISCAT field of view (FOV) at ~73°0′N–60°5′N (geomagnetic latitude) at an altitude of 325 km, which is associated with trough formation. Our statistical results show that the high-latitude and midlatitude troughs occur on the high- and low-latitude sides of the FOV, respectively. Seasonal variations indicate that dissociative recombination accompanied by frictional heating is a main cause of trough formation in sunlit regions. During summer, therefore, the occurrence rate is maintained at 80–90% in the postmidnight high-latitude region owing to frictional heating by eastward return flow. Solar activity dependence on trough formation indicates that field-aligned currents modulate the occurrence rate of the trough during the winter and equinox seasons. In addition, the trough becomes deeper via dissociative recombination caused by an increased ion temperature with F10.7, at least in the equinox and summer seasons but not in winter.

Substorm onset process: Ignition of auroral acceleration and related substorm phases

Abstract: The substorm onset process was studied on the basis of the vertical evolution of auroral acceleration regions derived from auroral kilometric radiation (AKR) spectra and Pi pulsations on the ground. The field-aligned auroral acceleration at substorm onset demonstrated two distinct phases. Low-altitude acceleration (h~3000–5000 km), which accompanied auroral initial brightening, prebreakup Pi2, and direct current of ultralow frequency (DC-ULF) pulsation were first activated and played an important role (precondition) in the subsequent substorm expansion phase onset. Prebreakup Pi2 is suggestive of the ballooning mode wave generation, and negative decrease in DC-ULF suggests increasing field-aligned current (FAC). We called this stage the substorm initial phase. A few minutes after this initial phase onset, high-altitude acceleration, which accompanied auroral breakup and poleward expansion with breakup Pi1 and Pi2 pulsations, suddenly broke out in an altitude range from 8000 to 16,000 km. Thus, substorm expansion onset originated in the magnetosphere-ionosphere (M-I) coupling region, i.e., substorm ignition in the M-I coupling region. Statistical investigations revealed that about 65% of earthward flow bursts observed in the plasma sheet were accompanied by enhanced low-altitude AKR, suggesting that flow braking of bursts causes FAC and resulting low-altitude field-aligned acceleration in the M-I coupling region. On the basis of these observations, we propose a substorm onset scenario in which FAC that originated from the braking of plasma flow bursts first enhances low-altitude acceleration (substorm initial phase onset) and then the increasing FAC induces current-driven instability in the M-I coupling region, which leads to high-altitude acceleration and resulting substorm expansion phase onset.

Interrelationship between preonset auroral and magnetic signatures at a geomagnetically conjugate Iceland‐Syowa pair

Abstract: We present the initiation and development of Pi1-Pi2 band pulsations in concert with spatiotemporal evolution of preonset auroral arc during an isolated auroral substorm reported by Motoba et al. (2012), and their interhemispheric similarities/dissimilarities, based on the colocated optical and magnetic field measurements at a geomagnetically conjugate Iceland-Syowa pair. At least ~7 min prior to the auroral expansion onset, the first identifiable signature of preonset arc began with a small-scale (~30–50 km) azimuthal beading. Interestingly, the early development of the visible arc beading for ~3–4 min was not accompanied by any ground magnetic perturbation. Then the bead-like forms in the arc evolved into brighter, larger undulations, eventually developing into the poleward expansion. Such a preonset optical sequence was almost identical at both stations. In the transition from the beads into undulations, Pi2 pulsations were first identified clearly, followed by Pi1 pulsations a few minutes later. Whereas the Pi2 onset was coincident with the initiation of progressive increase of the preonset arc luminosity, the Pi1 onset was associated with a subsequent steep increase. The Pi2 wave cycles superimposed on the magnetic negative bay were well correlated with the preonset arc luminosity time series. These results imply the potential linkage between the ground Pi1-Pi2 signatures in the onset region and the preonset auroral arc processes. Although the Pi1-Pi2 features were generally similar between the two hemispheres, there was also some dissimilarity in their temporal behaviors, which could reflect a small but noteworthy interhemispheric asymmetry in the overhead conjugate preonset aurora.

Metadata Management at the Polar Data Centre of the National Institute of Polar Research, Japan

Abstract: The Polar Data Centre of the National Institute of Polar Research has had the responsibility to manage the data for Japan as a National Antarctic Data Centre for the last two decades. During the International Polar Year (IPY) 2007–2008, a considerable number of multidisciplinary metadata that mainly came from IPY-endorsed projects involving Japanese activities were compiled by the data centre. Although long-term stewardship of those amalgamated metadata falls to the data centre, the efforts are in collaboration with the Global Change Master Directory, the Polar Information Commons, and the newly established World Data System of the International Council for Science.

Fine‐scale transient arcs seen in a shock aurora

Abstract: We report, for the first time, fine-scale transient arcs that emerged successively within the initial 1–2 min evolutionary interval of a postnoon shock aurora on 14 July 2012. Data were acquired from ~2 Hz temporal resolution imaging of dayside aurora with a white light all-sky camera (ASC) at South Pole Station (magnetic latitude = −74.3°, magnetic local time = UT −3.5 h). Just after 1809:50 UT at which the initial response to an interplanetary (IP) shock was detected in the postnoon geosynchronous magnetic field, the ASC observed three successive transient arcs of which the locations shifted equatorward with an abrupt jump by ~0.2° in latitude. All of the transient arcs occurred in a closed field line region, ~1.0°–1.5° equatorward of the polar cap or open/closed field line boundary inferred from the intensity ratio of I630.0/I557.7 but just poleward of the shock-induced proton and diffuse-type electron aurorae. Each of the transient arcs had a latitudinal width of ~0.1° and a short lifetime of ~20–30 s. Although the obvious mechanism has still remained unclear, possible interpretations of the fine-scale transient arc features are discussed in terms of a local process of each of the magnetospheric origin (mode conversion) and ionospheric origin (feedback interaction) that may be induced by IP shock.