Recent findings on VLF/ELF sferics

Hayakawa, Masashi; Ohta, Kenji; Shimakura, S.; Baba, Kiyohide

doi:10.1016/0021-9169(94)00074-x

Cited by 22 publications

(23 citation statements)

References 7 publications

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“…The technique is based on the fact that the firstorder mode cut-off frequency of tweek atmospherics is a significant indicator of tweek reflection heights (equivalent electron densities) in the D-region ionosphere. Tweeks are reflected by the D-and lower E-region ionosphere at altitudes below 100 km where the electron density becomes 20-28 cm −3 (Shvets and Hayakawa, 1998;Ohya et al, 2003) and propagate long distances (a few thousand kilometers) in the Earth-ionosphere waveguide (Outsu, 1960;Yamashita, 1978;Hayakawa et al, 1994Hayakawa et al, , 1995Shvets and Hayakawa, 1998). As for the polarization of the tweeks, the zero-order mode of tweeks is expected to be TM 0 (transverse magnetic) mode (Wait, 1972), in agreement with the observations by Hayakawa et al (1994).…”

Section: Introductionsupporting

confidence: 77%

Development of an automatic procedure to estimate the reflection height of tweek atmospherics

2008

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This paper presents an automated procedure to estimate apparent reflection height h (from the cutoff frequency for the first waveguide mode, f c ), horizontal propagation distance d, and propagation time T g of tweek atmospherics. Tweek data recorded at the Kagoshima Observatory (31.48• N, 130.72• E), Japan, were used to evaluate the procedure by comparing the results estimated by the automatic method to those read manually by an operator. The two types of results showed differences (automatic−manual) of +0.58 km, −9.9 Hz, and +3058.9 km for mean h, f c , and d, respectively. The difference in h( f c ) was less than the resolution of the fast Fourier transform used to obtain the tweek spectra. These comparisons indicate that the automatic estimation procedure of tweek parameters developed in this paper performs well and is a useful tool for studying long-term height variations of the ionospheric D and lower E regions using very low frequency (VLF) and extremely low frequency (ELF) records observed in Japan over the past 30 years.

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Section: Introductionsupporting

confidence: 77%

Development of an automatic procedure to estimate the reflection height of tweek atmospherics

2008

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“…Passive measurement of VLF/ELF tweeks is much easier and cheaper than measurement by active radar. The first-order mode in the Earth-ionosphere waveguide propagates with very little attenuation of ∼3 dB per 1000 km [Davies, 1969;Yamashita, 1978], so that tweeks propagate long distances (several thousand kilometers) in the Earth-ionosphere waveguide [Burton and Boardman, 1933a;Outsu, 1960;Lynn and Crouchley, 1967;Hayakawa et al, 1994Hayakawa et al, , 1995Kishore et al, 2005]. Lightning occurs continuously at low latitudes [Christian et al, 2003], and therefore many tweeks can be observed at night throughout a year at low-latitude stations.…”

Section: Introductionmentioning

confidence: 99%

Long-term variations in tweek reflection height in the D and lowerEregions of the ionosphere

2011

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[1] We investigated, for the first time, long-term variations in reflection heights of tweek atmospherics based on very low frequency (VLF) observations at Kagoshima, Japan. The results revealed the effects of the solar cycle on the nighttime lower ionosphere at low to middle latitudes. The tweek reflection heights on geomagnetically quiet days were analyzed every month over three solar cycles by using an automated spectral fitting procedure to estimate the cutoff frequency. The average and standard deviation of the reflection height were 95.9 km and ±3.1 km, respectively. Typical periods of time variation for the reflection height were 13.3, 3.2, 1.3, 1.0, 0.6, and 0.5 years. The variations in tweek reflection heights did not show simple anticorrelation with solar activity. The correlation coefficient between tweek reflection height and sunspot number was 0.03 throughout the three solar cycles. Hilbert-Huang transform analysis successfully indicated the presence of 0.5-1.5 year and ∼10 year variations as intrinsic mode functions (IMF). The decomposed IMF with the ∼10 year variation had a positive correlation with sunspot numbers and a negative correlation with galactic cosmic rays (GCRs). We hypothesize that these variations in tweek reflection heights could be caused by coupling of several ionization effects at the D and lower E regions, effects such as geocorona, GCRs, particle precipitation, and variations in neutral density in the lower thermosphere. Among these processes, the geocorona and particle precipitation could show negative correlation, while the GCRs and neutral density could show positive correlation with solar activities.Citation: Ohya, H., K. Shiokawa, and Y. Miyoshi (2011), Long-term variations in tweek reflection height in the D and lower E regions of the ionosphere,

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“…Estimation is performed with Eqs. (6) and (7) for the reason that the sferics propagating along the ground surface becomes a surface ground wave with a velocity less than the speed of light [12]. When the lightning location is estimated by the present system, the distance is 258.8 km and the direction is 49.1° clockwise from magnetic north.…”

Section: Results Of Evaluation Of Lightning Locationmentioning

confidence: 99%

“…In tweeks, the frequency components near the low-frequency cutoff (1 to 2 kHz) of the broadband impulse wave over several kilohertz are observed late in time. Also, several whisker-shaped responses appear at harmonic frequencies corresponding to the cutoff frequency if the waveguide is determined by the reflection altitude of the ionosphere [5][6][7]. From the theoretical analysis of Miyamura and colleagues [8], it is known that tweek discharges appear at relatively short distances (several hundred kilometers).…”

Section: Introductionmentioning

confidence: 99%

Estimation of lightning location from single station observations of sferics

Nagano

Yagitani

Ozaki

et al. 2006

Electron. Comm. Jpn. Pt. I

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SUMMARYA system for lightning location from single station observations has been developed for localizing lightning (distance and direction) at rather short distances (several hundred kilometers). In this system, a vertical component of the electric field and two horizontal components of the magnetic field of the VLF wave of the sferics are observed. The sferics are usually observed as a pulse train. The first pulse is used for estimation of the direction. The second and subsequent pulses are multiple reflections between the ionosphere and the ground. The differences in arrival time of these reflected waves are determined by the propagation distance between the ground and the ionosphere. The reflection altitude of the ionosphere and the horizontal distance to the lightning location are estimated from the arrival time difference of more than two pulses. In this research, the electromagnetic pulses radiated from the lightning are first analyzed by the full wave method, considering the ionosphere, free space, and the ground, in order to evaluate the location estimation method. The lightning location is estimated by calculating the space discharge sferics waveform at the observation point. Observations were also carried out with the lightning location system for estimation of lightning locations.

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Recent findings on VLF/ELF sferics

Cited by 22 publications

References 7 publications

Development of an automatic procedure to estimate the reflection height of tweek atmospherics

Development of an automatic procedure to estimate the reflection height of tweek atmospherics

Long-term variations in tweek reflection height in the D and lowerEregions of the ionosphere

Estimation of lightning location from single station observations of sferics

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