VLF-LF Radio Bursts Observed at Local Onsets of Magnetospheric Substorms

Observations of VLF hiss narrow-band intensities (5.0±0.2 kHz, 12.0±0.5 kHz, and 50.0±1.0 kHz), horizontal component (H) of the geomagnetic field, and the ionospheric absorption of cosmic radio noises of 30 MHz have been carried out at Syowa Station, Antarctica, since May, 1972. Characteristics of VLF-LF radio emissions associated with 59 magnetospheric substorms which occurred for six months in 1973 have been statistically analyzed by using the above correlation data. Wide-band VLF-LF radio bursts are often found at all channels of 5 kHz, 12 kHz, and 50 kHz within 5 minutes of local onsets of magnetospheric substorms. Small absorption (less than about 0.5 dB) of the cosmic radio noises at 30 MHz suggests that precipitations of energetic electrons above 10 keV into the auroral-zone ionosphere have little relation with the generation of VLF-LF radio bursts. The cerenkov-type wide-band emissions from auroral electrons below lOkeV may generate VLF-LF radio bursts, since the auroral breakup occurs simultaneously with the local onset of magnetospheric substorm. In the growth phase of the magnetospheric substorm, intensification of the westward electric field in the neutral sheet associated with thinning of the plasma sheet causes the inward plasma flow in the magnetotail. The electrostatic plasma instability causing the pitch angle diffusion of electrons occurs near the inner edge of the plasma sheet (L<10) as the plasma convection develops, and then the plasma sheet electrons (0.1-10 keV) precipitate, due to the pitch angle diffusion, into the auroral zone along the geomagnetic field line. Thus, the cerenkov-type emissions generated by the auroral electrons precipitating from the plasma sheet may produce the wide-band VLF-LF radio bursts at the local onsets of magnetospheric substorms

Spectral characteristics of radio noise at low and medium frequencies in the Antarctic topside ionosphere

The ISIS topside sounder data obtained at Syowa Sation, Antarctica, for the period from April 1976 to November 1977 are examined with emphasis on the noise spectra appearing on the Automatic Gain Control (AGC) data and on the ionograms. The noise events were observed on 16 out of 88 ISIS-1 passes and on 8 out of 138 ISIS-2 passes. At high altitudes near ISIS-1 apogee, almost all of the noise events are due to auroral kilometric radiations (AKR). A special event of AKR observed in the dayside ionosphere is investigated in detail. The result shows that this cusp-associated AKR occurred in a large scale region of electron density depletion where the ratio of electron plasma frequency, f_N to electron gyrofrequency, f_H ranges from 0.1 to 0.05. At altitudes below 2900km, two types of noise were observed; the whistler mode noise and the noise band appearing between the local f_N and f_T (upper hybrid resonance frequency). These noises are examined in connection with the local characteristic frequencies. The dependence of these noise intensities on the relationship between f_N and f_H is found to be in a qualitative agreement to Maggs\u27 power flux calculation of the electrostatic noise using the plausible auroral electron beam models

ジキケン アラシ カイシジ ノ VLF LF デンパ バースト

南極昭和基地で観測したVLF放射狭帯域強度(5.0土0.2kHz, 12.0士0.5kHz, 50.0士1.0kHz),地磁気水平成分,リオメーター30MHz の同時記録データを用いて,磁気圏嵐の局所的開始に伴うVLF電波バーストの解析を行った.1973年の6ヵ月間に起こった59個の磁気圏嵐に伴って発生したVLF電波バーストを統計処理した結果,磁気圏嵐の局所的開始から5分間以内に,5kHz, 12kHz, 50kHz の各周波数において,広帯域な電波バーストが頻発していることが明らかになった.このVLF-LF電波バーストに伴うリオメーター30MHzの電離層吸収ぱ小さく,D層まで侵入するような数10keV以上の電子は,VLF-LF電波バーストの発生とあまり関係しないことを示している.磁気圏嵐の局所的開始が,オーロラのbreakupと一致していることからも,10keV以下のオーロラ電子からの広帯域なセレンコフ型放射が,VLF-LFバーストの発生機構と思われる.磁気圏嵐の成長期にはプラズマシートの薄化が起こり,磁気中性シートの西向き電場が強化され磁気圏尾部での内向きプラズマ対流が発達する.プラズマ対流の発達に伴い,プラズマシート内端近くで静電気的プラズマ不安定が発生し,プラズマシート電子はピッチ角拡散によって,磁力線に沿って極光帯へ降下する.このオーロラ電子からのセレンコフ型放射が,磁気圏嵐開始時の広帯域VLF-LF電波バーストを発生すると推測される.VLF放射,地磁気活動,極光現象,電子降下現象のcorrelation dataは,極光帯電離層,磁気圏尾部の物理的機構の解明に重要なもので,今後とも続行すべきものである.Observations of VLF hiss narrow-band intensities (5.0±0.2 kHz, 12.0±0.5 kHz, and 50.0±1.0 kHz), horizontal component (H) of the geomagnetic field, and the ionospheric absorption of cosmic radio noises of 30 MHz have been carried out at Syowa Station, Antarctica, since May, 1972. Characteristics of VLF-LF radio emissions associated with 59 magnetospheric substorms which occurred for six months in 1973 have been statistically analyzed by using the above correlation data. Wide-band VLF-LF radio bursts are often found at all channels of 5 kHz, 12 kHz, and 50 kHz within 5 minutes of local onsets of magnetospheric substorms. Small absorption (less than about 0.5 dB) of the cosmic radio noises at 30 MHz suggests that precipitations of energetic electrons above 10 keV into the auroral-zone ionosphere have little relation with the generation of VLF-LF radio bursts. The cerenkov-type wide-band emissions from auroral electrons below lOkeV may generate VLF-LF radio bursts, since the auroral breakup occurs simultaneously with the local onset of magnetospheric substorm. In the growth phase of the magnetospheric substorm, intensification of the westward electric field in the neutral sheet associated with thinning of the plasma sheet causes the inward plasma flow in the magnetotail. The electrostatic plasma instability causing the pitch angle diffusion of electrons occurs near the inner edge of the plasma sheet (L<10) as the plasma convection develops, and then the plasma sheet electrons (0.1-10 keV) precipitate, due to the pitch angle diffusion, into the auroral zone along the geomagnetic field line. Thus, the cerenkov-type emissions generated by the auroral electrons precipitating from the plasma sheet may produce the wide-band VLF-LF radio bursts at the local onsets of magnetospheric substorms