Masanori Nishino

Equivalent electron densities at reflection heights of tweek atmospherics in the low-middle latitude D-region ionosphere

Tweek atmospherics are ELF/VLF pulse signals with frequency dispersion characteristics that originate from lightning discharges and propagate in the Earth-ionosphere waveguide mode over long distances. In this paper, we estimate equivalent nighttime electron densities at reflection heights in D-region ionosphere at low-middle latitudes by accurately reading the first-order mode cut-off frequency of tweek atmospherics. The estimation method was applied to tweek atmospherics received simultaneously at Moshiri and Kagoshima in Japan. Equivalent electron densities ranged from 20—28 el./cm3 at ionospheric reflection heights of 80—85 km. Comparing our estimates with electron density profiles obtained from the IRI-95 model, MF radar measurements, and rocket experiments revealed almost consistent results for the lower part of the D-region ionosphere. The tweek method has the unique advantage of enabling reflection-height (equivalent electron densities) monitoring over a wide area of several thousand kilometers.

Unusual ionospheric absorption characterizing energetic electron precipitation into the South Atlantic Magnetic Anomaly

An imaging riometer (IRIS) was installed newly in the southern area of Brazil in order to investigate precipitation of energetic electrons into the South Atlantic Magnetic Anomaly (SAMA). An unusual ionospheric absorption event was observed in the nighttime (∼20 h LT) near the maximum depression (Dst ∼ −164 nT) and the following positive excursion during the strong geomagnetic storm on September 22–23, 1999. The unusual absorption that has short time-duration of 30–40 min shows two characteristic features: One feature is a sheet structure of the absorption appearing at the high-latitude part of the IRIS field-of-view, showing an eastward drift from the western to the eastern parts and subsequent retreat to the western part. Another feature is a meridionally elongated structure with a narrow longitudinal width (100–150 km) appearing from the zenith to the low-latitude part of the IRIS field-of-view, enhanced simultaneously with the sheet absorption, and is subsequently changed to a localized structure. These features likely characterize precipitation of energetic electrons into the SAMA ionosphere, associated with substorm occurrences during the strong geomagnetic storm. From the eastward drift (∼250 m/s) of the sheet absorption, precipitating electrons are estimated to be ∼20 keV energies, assuming plasmaspheric electric fields of 1.8 mV/m. However, no ionospheric effect due to the precipitating electrons was definitely detected by the ionosonde measurements at Cachoeira Paulista, separated eastward by about 1000 km from the IRIS station.

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.

Daytime ionospheric absorption features in the polar cap associated with poleward drifting F-region plasma patches

Absorption of radio waves in the polar ionosphere near the magnetic noon was observed on October 8, 1991, by the 30 MHz imaging riometer at Ny-Alesund, Svalbard (invariant latitude 76.1°). These observations showed that the initially widespread absorption features became localized and enhanced in the high-latitude sector of the field of view, and followed a poleward motion. This behavior occurred quasi-periodically and repeated every 10–20 min. Simultaneous observations by EISCAT “Polar” experiments showed that nine discrete plasma patches, with F-region electron density enhanced by an order of 106 el/cm3, drifted poleward from the polar cusp to the cap during the same period. This coincidence suggested that the ionospheric absorption was associated with F-region plasma patches in the polar cap. Theoretical absorption values of 0.14 dB, estimated using the electron densities and the electron-ion collision frequencies from the EISCAT F-region plasma data, are smaller than the observed values (<0.8 dB). This discrepancy may be related to the difference between the theoretically- and experimentally-determined collision frequencies, as indicated by Wang et al. (1994). These localized, enhanced, and poleward drifting absorption features over Ny-Alesund may be explained as F-region plasma patches produced by a magnetosheath-like particle precipitation into the cusp, and as small-scale irregularities caused by density gradients of the patches drifting into the polar cap.

Energetic particle precipitation in the Brazilian geomagnetic anomaly during the “Bastille Day storm” of July 2000

Ionospheric absorption associated with a great geomagnetic storm on July 15–16, 2000 (the “Bastille Day storm”) was observed in the Brazilian geomagnetic anomaly using a two-dimensional 4 × 4 imaging riometer (IRIS). In the afternoon of July 15, weak absorption (≈0.2 dB) was observed during the initial phase of the storm; large spatial-scale absorption exceeded the IRIS field of view (330×330 km). During the sharp magnetic decrease in the main phase of the storm, absorption was intensified (<0.5 dB) in the evening, showing a sheet structure with ≈150 km latitudinal width and >330 km longitudinal elongation. Subsequently, absorption was intensified (≈1 dB), having a small spatial-scale (≈150 km) in the background sheet structure and a pronounced westward drift (≈570 m s-1). In association with large magnetic fluctuations in the Bz component of the interplanetary magnetic field (IMF), the ground magnetic variation in the night sector showed large positive swings during the initial to main phases of the storm. With the subsequent southward turning of the IMF Bz, the ground magnetic variation in the evening sector showed rapid storm development. Particle fluxes measured by a geosynchronous satellite (L =≈6.6) demonstrated large enhancements of low-energy protons (50–400 keV) and probably electrons (50–225 keV) during the storm’s initial phase. Particle fluxes from the low-altitude NOAA satellite (≈870 km) indicated the invasion of low-energy particles into the region of L < 2 during the main phase of the storm. These results indicate that low-energy particles injected into the outer radiation belt in association with frequent and strong substorm occurrences, were transported into the inner radiation belt through direct convective access by the storm-induced electric fields during the storm’s development. These particles then precipitated into the ionosphere over the Brazilian geomagnetic anomaly. Notably, the most intense absorption could be dominantly caused by proton precipitation with energies of ≈40 keV. Key words: Bastille Day storm, Brazilian geomagnetic anomaly, energetic particle precipitation, imaging riometer.

On the propagation of LF whistler-mode waves deduced from conjugate measurements at low latitudes

Abstract Whistler-mode signals transmitted from three Decca stations (Biei, L = 1.54, ƒ c = 85.725 kHz; Akkeshi, 1.51, 114.300 kHz; Wakkanai, 1.61, 128.588 kHz), Japan, were measured at multiple observing points in the magnetic conjugate area around Birdsville ( L = 1.55), Australia. The whistler-mode signals observed in the conjugate area indicated a remarkable frequency dependence of the occurrence ; the signals were usually observed at 85.725 kHz at the multiple points including the conjugate point of the transmitter ( ʌ = ƒ c /ƒ Heq = 0.32 ), and not detected at 114.300 kHz (0.40), and they were not identified at 128.588 kHz around the conjugate point (0.55). No detection at 114.300 kHz may be due to the heavy attenuation of signals while penetrating the ionosphere down to the observing point (Birdsville) with wave normals at large angles with respect to the magnetic field. However, such a difference of occurrence between 85.725 and 128.588 kHz can not be understood from calculation of the transmission loss in the lower ionosphere by means of a full wave treatment, and it has demonstrated the ducted propagation in field-aligned enhancements of electron density. Also, direction finding results at 85.725 kHz may support the ducted propagation, indicating ionospheric exit almost parallel to the magnetic field through the observing point. At magnetically severely disturbed times, the whistler-mode signals appeared almost continuously during the night-time, and they were intensified by more than 20 dB. Such an intensity increase may be due to interactions with energetic electrons of 50–100 keV.

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.

Magnetic storm-related energetic electrons and magnetospheric electric fields penetrating into the low-latitude magnetosphere (L ∼ 1.5)

Abstract Energetic electrons measured by the NOAA-6 satellite are compared with LF whistler-mode signals transmitted from a Decca station (Biei, L = 1.54, fc = 85.725 kHz), Japan, and measured in the magnetic conjugate area, Australia. The simultaneous satellite measurement of energetic electrons indicated the considerable enhancement of energetic electron fluxes more than 30 keV in the low L-shell region below ∼ 2 at the maximum depression phase of Dst, and the subsequent abundant fluxes of trapped electrons more than 30 keV on 1 day and occasionally 2 days after the maximum phase. Associated with magnetic disturbances, the LF whistler-mode signals were intensified. However, the intensity increase of the signals was not so large at the maximum phase, which may be attributable to an ineffective wave growth caused by a rather isotropic pitch angle distribution of energetic electrons. The intensity increase was the largest one day after the maximum phase, due to the wave growth caused by cyclotron resonance interactions with trapped electrons. Also associated with magnetic disturbances, the frequency of enhanced LF whistlermode signals shifted, which is caused by the drift of whistler ducts due to the magnetospheric electric fields penetrating into the low-latitude magnetosphere. Therefore, the penetration of storm-related energetic electrons and magnetospheric electric fields into the low L-shell region below ∼ 2 may be deduced from ground-based conjugate measurements of resonant waves.

Doppler shifts of LF whistler-mode signal observed at a low-latitude (L = 1.54)

Abstract The whistler-mode signals transmitted from a Decca station (Biei, L = 1.54, 85.725 kHz), Japan and measured at the magnetic conjugate point, Australia usually revealed the frequency shifts: a dusk positive shift ( −0.5 Hz), being independent of magnetic activity. The Doppler shift being the change of the phase path of the signal may be caused by a combination effect of the electron density variation of a field-aligned whistler duct with downward (upward) electron flux and the drift of the whistler ducts due to westward (eastward) equatorial electric fields generated by an ionospheric dynamo process around sunset (sunrise), respectively. The occurrence (total duration per day) of the whistler-mode signal indicates the highest correlation with the magnetic activity ( σK p ) on 1-day prior to the occurrence. On magnetically severely disturbed times the whistler-mode signals appeared almost continuously during the night-time, and its frequency shifted smoothly from positive to zero. The positive frequency shift during the night-time may be attributed to the drift of the whistler-ducts due to westward convective electric fields penetrated into the low-latitude magnetosphere in association with magnetic disturbances.

A new direction finding technique for auroral VLF hiss based on the measurement of time differences of arrival at three spaced observing points

Abstract A newly developed direction finding (DF) technique for auroral hiss based on the measurement of time differences of wave arrival was carried out in 1978 at Syowa Station (geomag. lat. -70.4°), Antarctica and its two slave unmanned observing points located at about 20 km distances from Syowa. The auroral hiss signals (0.3–100 kHz) received at the two spaced points were transmitted to Syowa by a wide-band telemeter of 2 GHz. The arrival time difference of auroral hiss between Syowa and each spaced point was automatically determined by cross-correlating the waveforms of the received signals, and then the incident and azimuthal angles were measured with an accuracy of about 10°. It has been found that the new DF technique can determine localized exit regions at the ionospheric level which show rapid temporal movements. A comparison of the DF results with ground-based auroral data has shown that impulsive type auroral hiss with a wide-band frequency range has not emerged from the whole region of a bright aurora but from some localized regions of bright electron auroras at the ionospheric level, and that the arrival directions of auroral hiss change rapidly in accordance with the auroral movements.