Y. Hobara

Current status of seismo-electromagnetics for short-term earthquake prediction

Short-term (timescale of hours, days and weeks) earthquake (EQ) prediction is of essential importance to mitigate EQ disasters. Short-term EQ prediction has so far been based on seismic measurements (i.e. mechanical observation of crustal movements), but it was concluded in Japan about 10 years ago that EQ prediction is impossible by means of the mechanical method. Hence, there has been an increased interest and a lot of progress in non-seismic measurement during the last decade. A new approach was developed where electromagnetic measurements provide microscopic information on the lithosphere. The present paper is intended to give a history of short-term EQ prediction, and also we hope that this paper reviews the current status of a new science field, ‘seismo-electromagnetics’. We make a general review of different phenomena taking place in the lithosphere, atmosphere and the ionosphere, but we pay more attention to the subjects of our preference including lithospheric ultra low frequency (ULF) electromagnetic emissions, and seismo-ionospheric perturbations.

Long-term periodical variations in global lightning activity deduced from the Schumann resonance monitoring

We deduce temporal variations of the level of global lightning activity from the long-term Schumann resonance (SR) monitoring performed at the Tottori observatory (35.5°N, 134.33°E). The continuous analog records of the horizontal magnetic field cover the period from August 1967 to November 1970. The data show that temporal changes of the SR amplitudes vary considerably from day to day. Meanwhile, the regular diurnal and longer-scale variations appear after averaging. We found annual, and semiannual components in variations of global thunderstorm activity between 1967 and 1970. Comparison with the long-term SR records made recently in Europe allows us to demonstrate the similarity and stability of the global thunderstorm variations on a seasonal scale. Simultaneously, it revealed a contribution from winter thunderstorms in the Japan Sea to the Tottori SR records.

Overview and early results of the Global Lightning and Sprite Measurements mission

Global Lightning and Sprite Measurements on Japanese Experiment Module (JEM-GLIMS) is a space mission to conduct the nadir observations of lightning discharges and transient luminous events (TLEs). The main objectives of this mission are to identify the horizontal distribution of TLEs and to solve the occurrence conditions determining the spatial distribution. JEM-GLIMS was successfully launched and started continuous nadir observations in 2012. The global distribution of the detected lightning events shows that most of the events occurred over continental regions in the local summer hemisphere. In some events, strong far-ultraviolet emissions have been simultaneously detected with N2 1P and 2P emissions by the spectrophotometers, which strongly suggest the occurrence of TLEs. Especially, in some of these events, no significant optical emission was measured by the narrowband filter camera, which suggests the occurrence of elves, not sprites. The VLF receiver also succeeded in detecting lightning whistlers, which show clear falling-tone frequency dispersion. Based on the optical data, the time delay from the detected lightning emission to the whistlers was identified as ∼10 ms, which can be reasonably explained by the wave propagation with the group velocity of whistlers. The VHF interferometer conducted the spaceborne interferometric observations and succeeded in detecting VHF pulses. We observed that the VHF pulses are likely to be excited by the lightning discharge possibly related with in-cloud discharges and measured with the JEM-GLIMS optical instruments. Thus, JEM-GLIMS provides the first full set of optical and electromagnetic data of lightning and TLEs obtained by nadir observations from space.

The lower ionospheric perturbation as a precursor to the 11 March 2011 Japan earthquake

It is found that clear lower ionospheric perturbations appeared as a precursor to the 11 March 2011 Japan earthquake. This study is based on the observation of two completely different phenomena: (1) subionospheric very low frequency/low frequency propagation anomaly on the NLK (Seattle, USA) – Chofu propagation path, and (2) depression of magnetospheric ultra low frequency emissions observed on the ground (Kakioka, etc.). But, both effects are suggested to be interpreted by a unified phenomenon of seismo-lower ionospheric perturbation because they occurred on the days of 5 and 6 March, 2011.

Formation of electron beams by the interaction of a whistler wave packet with radiation belt electrons

Abstract The purpose of this paper is to discuss the properties of electron beams formed by cyclotron interactions between radiation belt electrons and a quasi-monochromatic whistler wave packet from a ground-based VLF transmitter. The beams are formed due to trapping of the electrons at the forward edge of the wave packet, their acceleration inside the wave packet, the escape of the accelerated electrons from the moving backward edge of the wave packet, and their following free motion in an inhomogeneous magnetic field. A combination of these processes provides the main features of the spatial-temporal evolution of the beams which are investigated both analytically and numerically. It is shown that one or two beams can appear at one point at the same time, and that the density of the beams increases during their expansion. Motion of the pumping wave packet in the inhomogeneous magnetic field provides the variations of the initial velocity and position of the beam injection which change the spatial and temporal gradients of the parallel velocity of the beam, in contrast with the case of the pure adiabatic motion of an individual electron. Such a behaviour can be significant for the generation of secondary emissions. Numerical calculations demonstrate a wide variety of the spatio–temporal patterns of the beam parallel velocity depending on the plasma and wave packet parameters. It is shown that the most significant parameters which determine the beam characteristics are the wave packet length about the equator, its group velocity, and the initial energy and pitch angle of the electrons.

On the fine structure of dipolarization fronts

Measurements from the closely spaced Cluster spacecraft are used to study the structure of the magnetic and electric fields within the magnetic ramp of dipolarization fronts (DF) observed close to the neutral sheet and the midnight meridian (YGSM<3RE). The spacecraft separation was small enough (<300 km) to treat the magnetic ramp of the DF front as a planar structure as indicated from variance analysis. The finite value of the magnetic field along the minimum variance direction for the events studied indicates that the dipolarization front structure was distinct from a tangential discontinuity. In addition to the main increase of the magnetic field in the maximum variance component, strong oscillations were observed in the intermediate component. The presence of this oscillatory structure results in an expansion of the region in which a change of magnetic pressure occurs, the size of which is typically an ion Larmor radius or greater. This widening is important in maintaining the pressure balance at the edge of the DF. This phenomenon resembles observations of intense current sheets in the magnetotail and also laboratory experiments of current sheet formation, in which a similar widening of the ramp region has been observed. In this paper we argue against the idea that an electron temperature anisotropy, resulting in electron curvature currents, can explain the formation of the oscillatory structures observed at DFs. These oscillations can be explained as eigenmode waves of the plasma that propagate away from the disturbance (DF) that is moving at subsonic speeds. Oscillations observed within the magnetic ramp indicate field-aligned currents that are expected to be associated with DF.

Cyclotron amplification of whistler waves by electron beams in an inhomogeneous magnetic field

Cyclotron wave-particle interactions in the case of well-organized distributions of energetic electrons under an inhomogeneous magnetic field are studied. Step-like and δ function distributions over the field-aligned velocity are considered. The one-hop amplification of whistler waves is calculated by simple analytical solution and numerical computation based on strict approach. The strict consideration, taking into account third-order expansion of the spatial dependence of the electron phase with respect to the wave, reveals sonic new important features of the one-hop amplification Γ as a function of frequency and electron beam parameters. The main result is that Γ exhibits a quasi-periodic structure as a function of wave frequency or characteristic electron parallel velocity, remaining always positive in the case of the step-like distribution but being sign alternative for δ-function. Dependence of Γ on the parameters of energetic electrons such as their total energy, characteristic parallel velocity, position of the injection point in relation to the equator, and dispersion in parallel velocity is discussed.

The origin of spectral resonance structures of the ionospheric Alfvén resonator. Single high‐altitude reflection or resonant cavity excitation?

A common view is that spectral maxima in observed spectral resonance structures (SRS) of ionospheric Alfven resonator (IAR) at frequencies f<5 Hz are the signature of resonance frequencies of the IAR. We have studied not only spectra but also waveforms of magnetic fluctuations at IAR frequencies registered at Moshiri station (Japan) and have found that there exist two kinds of signals. The dominant type of signal is a pair of pulses which is caused by an initial exciting impulse and accompanied by a single reflection from the top boundary of the IAR. In the absence of reflection from the lower ionosphere, such signals are not resonant and hence are not caused by IAR excitation. The minority of cases are trains of three or more pulses separated by a nearly constant time interval reflected from both IAR boundaries. We have found that different kinds of signals in time domain may correspond to similar comb-shaped Fourier spectra. So different kinds of signals in time domain practically cannot be distinguished on the basis of their Fourier spectra. We have calculated waveforms and SRS structures of the magnetic field oscillations generated by a model lightning discharge and IAR resonant frequencies. Calculated IAR resonance frequencies can be in disagreement with those of spectral maxima of pulse trains. Then, an analysis of signal waveforms in time domain is highly required to estimate IAR resonance frequencies.

A role of the second-order cyclotron resonance effect in a self-consistent approach to triggered VLF emissions

A role of the second-order cyclotron resonance effect in the self-consistent approach to the problem of triggered ELF/VLF emissions is estimated. The self-consistency includes the calculation of energetic electron beams produced by a quasi-monochromatic whistler wave packet and the analysis of secondary whistler wave generation by this beam with the magnetic field inhomogeneity taken into account. The theory allows one to estimate the maximum amplification of the secondary waves, as well as the dynamic frequency spectrum of a triggered emission. A simple computer program has been realized to find dynamic spectrograms of triggered VLF signals in particular cases with realistic dependences of electron beam velocity and geomagnetic field on the spatial coordinate and time. The main and general conclusions are summarized as follows: (1) The second-order cyclotron resonance effects give an important contribution to the formation of the dynamic frequency spectrum of triggered ELF/VLF emissions. (2) The short pump whistler wave packets (with pulse durations 0.5-0.7 s in our case) generate fallers, while the long packets (pulse durations 0.9-1.3 s) generate predominantly rising tones. (3) There are critical maximal and minimal values for the pump wave pulse duration, when triggered emissions are not generated in the absence of an external electron beam.

Spatio-temporal characteristics of sub-ionospheric perturbations associated with annular solar eclipse over Japan: Network observations and modeling

The UECs VLF/LF transmitter observation network has been operating years in order to monitor the ionospheric perturbations caused by various physical phenomena. A solar eclipse is one of the factors that produce disturbances in the lower ionosphere. In this paper, we analyze VLF amplitude data from our network associated with annular solar eclipse in 2012. Clear temporal dependences of the VLF/LF amplitude were identified at various VLF/LF receivers. Numerical computations of VLF/LF signals with the ionospheric perturbations due to the solar eclipse were carried out by using 2D-FDTD method. As a result, temporal variations of the VLF/LF amplitude are in rather good agreement with those from the numerical modeling.