Masahide Nishihashi

Seismo-ionospheric anomalies of the GPS-TEC appear before the 12 May 2008 magnitude 8.0 Wenchuan Earthquake

In this paper, the total electron content (TEC) of Global Ionosphere Maps (GIMs) is normalized and employed to study the seismo-ionospheric anomalies at the time of the 12 May 2008 M 8.0 Wenchuan Earthquake. The space weather conditions are taken into account. It is found that remarkable reductions appear locally around the epicentre and their conjugate points during the daytime of 29 April and 6–10 May 2008. A global study and a strict criterion are applied to detect anomalies. Results show that the anomalies on 29 April and 6 and 7 May 2008, which are respectively days 13, 6 and 5 before, are possibly related to the earthquake. The conjugate signature implies the seismo-generated electric field is essential.

Three-dimensional lightning characteristics relative to reflectivity and airflow structure in winter thunderstorm

A winter thunderstorm was observed in the Shonai area in the northern part of Japan on 30 November 2010. Data from three-dimensional lightning mapping system and two X-band Doppler radars were used to analyze the spatial-temporal relationship between winter lightning channel, reflectivity core, and airflow structure in the thunderclouds. A lightning leader propagating from a rim of echo region to the echo region with high reflectivity involving large vertical vorticity was visualized in 3D. This result indicates that strong updraft caused by airflow convergence in the precipitation system contributed to accumulate positive charges around -10°C level and enhance vertical vorticity by stretching on the convergence line.

Ionospheric anomalies possibly associated with M ≥ 6 earthquakes in Japan during 1998–2011: Case studies and statistical study

In this paper, we examine pre-earthquake ionospheric anomalies in time series and perform a statistical test by using total electron content (TEC) derived from global ionosphere maps (GIM). The normalized GIM-TEC (GIM-TEC∗), which is computed based on 15 days backward running mean of GIM-TEC, have been investigated for minimizing possible confounding effects of consecutive earthquakes and identify the abnormal signals. Meanwhile, to reduce the effect of strong geomagnetic activities such as geomagnetic storms, a criterion for removing the GIM-TEC data have been adapted; that is Dst index exceeds −60nT. Temporal variations of GIM-TEC∗ for large and destructive earthquakes in Japan have been studied; which are the 2004 mid-Niigata Prefecture Earthquake (M6.8), its aftershock (M6.1), the 2007 offshore mid-Niigata Earthquake (M6.8), and the 2008 Iwate-Miyagi Nairiku Earthquake (M7.2). Although there are some positive and negative TEC anomalies before and after the four earthquakes, there is a tendency that positive TEC anomalies appear 1–5 days before all the above earthquakes even in the quiet geomagnetic condition. Superposed epoch analysis has been performed for statistical analysis of TEC anomalies associated with M ≥ 6.0 Earthquakes during the 12-year period of May 1998–May 2010. The statistical result indicates the significance of the positive TEC anomalies 1–5 days before earthquakes within 1000 km from the epicenter around Japan.

Neural network based tomographic approach to detecting the ionospheric anomalies prior to the 2007 Southern Sumatra earthquake

In this paper, neural network based ionospheric tomography was performed to investigate the detailed structure that may be associated with earthquakes. The 2007 Southern Sumatra earthquake (M8.5) is selected because significant decreases in the Total Electron Content (TEC) have been confirmed by GPS data analysis. With respect to the analyzed earthquake, we detected significant decreases at heights of 250–400 km, especially at 300 km. The global tendency is that the decreased region expands to the east with increasing altitude and concentrated in the Southern hemisphere over the epicenter. Furthermore, obtained results are consistent with other satellite observation.