Hiroyo Ohya

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.

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

This paper presents an automated procedure to estimate apparent reflection height h (from the cutoff frequency for the first waveguide mode, fc), horizontal propagation distance d, and propagation time Tg 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, fc, and d, respectively. The difference in h(fc) 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.

Daytime tweek atmospherics

We report the first observation of daytime tweek atmospherics based on measurements at Moshiri (44.37°N, 142.27°E) and Kagoshima (31.48°N, 130.72°E), Japan, during nonsolar eclipse days for 5 months in 1980–1994. The daytime tweeks were observed on geomagnetically quiet and stormy days. The daytime tweeks had clear frequency dispersion with an average duration of 12 ms, which was shorter than that in the nighttime (~50 ms). The average occurrences of the daytime tweeks at Moshiri and Kagoshima were 0.6 and 0.1 tweeks per minute during 10:00–15:00 LT, respectively. Daytime tweeks up to the second-order mode were visible. There was no difference in the occurrence of each visible mode between storm time and magnetically quiet time. The daytime reflection heights were similar to those at night (85–100 km) but with greater variation. We evaluated the attenuation rate (αn) of tweeks by strictly taking the ionospheric reflection coefficient into account. For each frequency, αn was evaluated as a function of the electron density, electron density gradient, and ionospheric height. We found that αn had an inverse relationship with the electron density (or conductivity), electron density gradient, and ionospheric height. We suggest that the best conditions for daytime tweek observations are when the bottomside of the ionosphere is sharply defined and the ionospheric height is high.