Hisao Yamagishi

Validation of sprite-inducing cloud-to-ground lightning based on ELF observations at Syowa station in Antarctica

Abstract Waveform monitoring of ELF radio signals in the frequency range of 1– 400 Hz have been carried out on a routine basis at Syowa station (69.0°S, 39.6°E in geographic coordinates), Antarctica since February, 2000. The main purpose of these observations is to monitor global lightning activity and to locate lightning-induced sprites and elves. The ELF observation system consisting of two search coil sensors (geomagnetic north–south (H) and east–west (D) sensors) was installed at a remote unmanned observatory in West Ongul Island located 5 km southwest from Syowa station. As a back up system, the same system was installed near Syowa station in East Ongul Island. Signals from these sensors were digitally sampled at 1000 Hz with a GPS time code. On July 4, 2000 during the STEPS (Severe Thunderstorm Electrification and Precipitation Studies) 2000 campaign carried out over the Great Plains in the US, 57 sprite events were observed from Yucca Ridge Field Station (40.7°N, 104.9°W), Colorado, and 53 out of these sprite events had one-to-one correspondence to transient Schumann resonances (SR) detected at Syowa station. The waveforms of these SR are characterized by sharp initial pulses and following damped oscillations. The great circles representing the propagation paths are determined from the Lissojous plots of the H and D magnetic field data of the transient SR. It has been demonstrated that the minimum distance between the great circles and the locations of causative cloud-to-ground (CG) discharges is ∼240 km on average. It is thus concluded that the method to determine the propagation paths from Lissajous plots is extremely accurate when we use the Syowa ELF waveform data. Consequently, it would be possible to globally triangulate sprite-inducing CG locations by setting up at least one more observation site with the same system.

Observations of 50‐ and 12‐MHz auroral coherent echoes at the Antarctic Syowa station

Data on 50- and 12-MHz E region coherent echoes from approximately the same directions and in a broad range of azimuths are available for 1995–1997 radar observations at the Antarctic Syowa station. One event of such observations is considered to compare power and Doppler velocity of echoes at these significantly different frequencies. For the considered event, Doppler velocities of more than 600 ms−1 (as recorded at both radar frequencies) were observed. We show that even though the 50-MHz echoes exhibit strong flow angle variation of the power, the 12-MHz echoes do not. Apparent aspect angle power attenuation is found to be 10 and 2.5 dBdeg−1 at 50 and 12 MHz, respectively. Measured Doppler velocities along specific radar directions are found to be comparable in spite of the significant difference in radar frequencies, with the velocity ratio V50MHz/V12MHz being in between 1.6 and 0.9 for high velocities of more than 400 ms−1. The velocity ratio varies with slant range and azimuth of observations. Ionospheric propagation effects for radio waves and plasma physical effects for irregularities are considered in an attempt to explain several observed features, for example, the echo power and velocity ratio variations with slant range.

On the power-velocity relationship for 12- and 50-MHz auroral coherent echoes

Nearly simultaneous observations of 12- and 50-MHz coherent echoes at the Antarctic Syowa station are considered for studying the power-Doppler velocity relationship at these significantly different radar frequencies. We concentrate on postmidnight measurements when both 12- and 50-MHz echoes were seen as narrow bands (∼150–200 km) of strong backscatter at short distances of <500 km. First, we examine the slant range profiles for the power and Doppler velocity by averaging data for nine separate events in March 1997. At 50 MHz the power was found to increase with velocity. At 12 MHz a power increase was found only in the velocity range of 100–350 m s−1. For larger velocities a saturation of 12-MHz echo power and even some power decrease were seen. To further explore the effect, we restricted the database to only almost simultaneous records at 12 and 50 MHz. We show that overall measured 12- and 50-MHz velocity values are similar, with significant deviations at some moments. We again found a power increase with velocity for 50-MHz echoes. We also confirm a 12-MHz power increase for relatively low velocities, in between 50 and 450 m s−1. For larger 12-MHz velocities, data indicate clear saturation of the power and power decrease, though the latter effect is very subtle. We argue that the 12-MHz power saturation and decrease at large velocities are not due to smaller electrojet irregularity intensity at these velocities but, rather, to irregularity intensity decrease with height and 12-MHz radio wave focusing on the top of the electrojet layer.

A very large scale flow burst observed by the SuperDARN radars

We examined the dynamics of the ionospheric plasma in the dayside sector by using the HF radar data at Iceland West and at Finland from 1100 to 1230 UT on September 5, 1995. During that period, the solar wind density was high and the IMF was strongly southward. The dayside magnetopause was highly compressed nearly to the geosynchronous orbit. The two radars simultaneously detected a poleward flow burst in the noon sector which, assuming uniformity of flow in the region of the data gap (1.5 MLT) between the two radars, showed a magnetic local time extent of 5 hours. This local time extent is 2 to 3 hours wider than previous results. The maximum poleward plasma velocity of the flow burst is ∼750 m/s, and the latitudinal size of the flow burst region is ∼100 to 200 km. This flow burst region initially expanded in longitude up to 5 hours, and then shifted poleward with a phase speed of 400 to 670 m/s. The flow burst has a duration of ∼20 min. This large-scale poleward flow burst is likely to be due to large-scale reconnection occurring at the dayside magnetopause and subsequent convection as the magnetic field lines are transported across the polar cap.

Antarctic HF radar observations of irregularities associated with polar patches and auroral blobs: A case study

We report a case study of decameter-scale electron density irregularities associated with polar cap patches and auroral (boundary) blobs in the southern high-latitude F region ionosphere. The observations were carried out on July 14, 1995, with the Antarctic Super Dual Auroral Radar Network HF radars located at Syowa Station and Halley. On that day, 17 irregularity events associated with the patches were identified in the polar cap. The time distribution of these events is consistent with previous model calculations of patch formation and transportation in the northern hemisphere for southward interplanetary magnetic field (IMF) conditions (Bz 0). These patches seem to have been transported into the polar cap from the dayside cusp where the patches had been generated under negative Bz conditions. The striated radar echo patterns due to a series of auroral blobs, clearly observed at Halley in the evening auroral zone, are well explained by previous simulations that calculated the time evolution and transportation of a patch initially located in the polar cap.

Localized activation of the distant tail neutral line just prior to substorm onsets

We have found flow burst features in the nightside ionosphere that are thought to be the ionospheric signature of distant tail reconnection. These are observed to form just prior to substorm onsets. Simultaneous observations by the Goose Bay-Stokkseyri dual HF radars and DMSP satellites provide the data. Our conclusions are based on equatorward flow bursts on the nightside during two isolated substorms that followed a long period of magnetospheric inactivity associated with a northward interplanetary magnetic field. Both flow bursts start ∼60 min after the growth phase onset and last ∼10–20 min until the expansion phase onset, migrating equatorward with time. Simultaneous DMSP observations of precipitating particles show that the flow burst occurs at the polar cap boundary, suggesting that the equatorward migration corresponds to the expansion of the polar cap during the growth phase. For one event, the reconnection electric field at 400 km altitude was 14 mV/m and its longitudinal scale was 290 km, which is equivalent to a reconnection voltage of 4.1 kV. For the other event, these values were 11 mV/m (reconnection electric field), 380 km (longitudinal scale), and 4.0 kV (reconnection voltage). In addition to the reconnection signatures, we discuss implications for substorm dynamics during the final stage of the substorm growth phase. The morphology indicates that the distant tail neutral line is activated ∼1 hour after the growth phase onset and at the same time the nightside separatrix starts to move equatorward much faster than during the preceding early and middle growth phases. The 1-hour time lag would correspond to the timescale on which slow rarefaction waves from both northern and southern tail lobes converge in the equatorial magnetotail. The fast-moving separatrix on the nightside implies a rapid change of magnetotail configuration resulting from nonlinear enhancement and/or earthward movement of the cross-tail current for the last 10–20 min prior to the expansion phase onset.

Development of a high-resolution imaging riometer for the middle and upper atmosphere observation program at Poker Flat, Alaska

Abstract An imaging riometer equipped with a 16 × 16-element square array antenna was developed and was installed at Poker Flat, Alaska, to obtain high-resolution two-dimensional images of the cosmic radio noise absorption at 38.2 MHz. This system scans the viewing region of a 200 × 200 km 2 square of the radio sky at the 90 km altitude every second, using 208 available antenna beams among 256 antenna phasing patterns. The maximum horizontal spatial resolution is 11 km at 90 km near the zenith over the antenna, which will enable us to resolve fine structures of auroral arcs with widths of the order of tens of kilometers or less, leading to more detailed investigation of auroral absorption dynamics than the existing imaging riometers. After the system check with the full system that was temporarily installed at the CRL Yamagawa observatory, Japan, in 1994, the final installation was completed and the operation started at Poker Flat, Alaska in September 1995.

CONJUGATE FEATURES OF AURORAS OBSERVED BY TV CAMERAS AND IMAGING RIOMETERS AT AURORAL ZONE AND POlAR CAP CONJUGATE-PAIR STATIONS

For the study of magnetospheric phenomena, geomagnetically conjugate point observations in northern and southern polar regions provide unique opportunities to study whether the symmetry of magnetospheric configuration is maintained, or breaks down at the time of auroral substorms. In addition, such studies can be used to study asymmetries between the sunlit and the dark polar ionosphere.

Location, spatial scale and motion of radio wave absorption in the cusp-latitude ionosphere observed by imaging riometers

Abstract Characteristic examples of the location, spatial scale and motion of radio wave absorption events in the cusp-latitude ionosphere are obtained from daytime observations on 18 September and 17 October, 1992, by imaging riometers in the Arctic region. One case observed near local magnetic noon at Ny-Alesund, Svalbard (invariant lat. = 76.1 °) displays small-scale absorption events of 100–200 km in extent superposed on large-scale absorption features extending at least 700 km in longitude toward the prenoon sector. Many of the small-scale absorption events show quasi-periodic variations with repetition periods of 3–5 min which correlate well with local magnetic variations. Short-lived, impulsive absorption events (1–3 min duration) found among the quasi-periodic variations corresponded to impulsive magnetic variations observed over a wide range of magnetometer stations. Some of these impulsive events showed northward or northeastward motions. This case is interpreted in terms of the variable precipitation of high-energy substorm electrons. Another characteristic case observed in the noon sector at cusp-latitudes is an event of slowly varying absorption intensities associated with magnetic bays in the cusp and polar cap regions during conditions of strongly negative IMF- B y component ( B z ≈0). An interesting feature of this event is the observed antisunward motion of the front-like absorption features extending over 700 km in longitude. From these characteristics the slowly varying absorption intensities are interpreted in terms of E-region ionospheric disturbances related to the east-west oriented DPY currents in the cusp and polar cap.

Small-scale structure of electron density in the D region during onset phases of an auroral absorption substorm

Abstract Small-scale structure of the electron density in the D region during the preceding and onset phases of auroral absorption substorms classified as unloading processes was studied on the bases of imaging riometer measurements made at Tjornes, Iceland, and broad -beam riometer data collected in the Nordic Countries. The precipitation during the onset phase may be localized, covering only 10 km in both north-south and east-west directions. After the substorm onset ionospheric absorption was observed to pulsate with a period of 1 – 12 minutes. Possibly this pulsation is related to the magnetohydrodynamic, field line resonances in the nightside magnetosphere and to the near-Earth neutral line substorm model.