Chorus emissions are the most common form of very low frequency (VLF) emissions in the Earth’s magnetosphere which typically consist of a series of rising tones generated near the magnetic equator, excited by energetic electrons injected into the inner magnetosphere. In the present study, observation of chorus emissions recorded at Indian low latitude ground station Jammu (geomag. lat., 190 26/ N; L = 1.17) during a geomagnetic quiet period on 24 February, 1999 is reported. The spectral analysis of recorded chorus emissions shows that each chorus element originates from the upper edge of the underlying hiss band. The observed mean chorus element parameters are as follows: lower band frequency fmin = 1.2 kHz, upper band frequency fUB = 1.96 kHz, frequency sweep rate df/dt = 1.14 kHz/s and repetition period T = 2.5 s. To explain the observed dynamic spectra of these chorus emissions, a possible generation mechanism is presented based on the recent nonlinear theory. It is observed that the seeds of chorus emissions grow from the saturation level of the whistler-mode instability at the equator and then propagate away from the equator as a result of a nonlinear growth mechanism that depends on the wave amplitude. On the basis of this theory, frequency sweep rate of chorus emission is computed and compared with that of our experimentally observed values, which shows, in general, a good agreement.
The high-altitude ionospheric perturbations around the equatorial anomalies (EA) are often observed by DEMETER spacecraft on the nighttime orbits. The ionospheric perturbations are recognized as an enhancement of the electric field in the ELF frequency range in the satellite coordinate. Clear perturbations are mostly recognized on the dayside of the satellite orbit and the peak perturbation intensity tends to increase with increasing magnetic latitude based on around 1000 perturbation events observed in the year of 2005. The perturbation intensities are examined in relation with major seismic activities. As a result, most importantly higher perturbation intensity persists for the time periods around the occurrence days of the land earthquakes rather than those for the sea earthquakes and without major earthquakes nearby. However, the difference of the observed perturbation intensity between above-mentioned three cases are rather small due to relatively large variability indicative of the weak correlation of seismicity with the high altitude ionospheric perturbations near EA.
Subsurface VLF electric field changes have been monitored at Chaumuhan, Mathura (Geographic Lat. 27.5°N, Geographic Long. 77.68°E), India at the frequency of 3.012 kHz employing borehole and vertical antennas since 24 March 2011. Initial data for the period of 15 days are analysed statistically and it has been found that anomalous enhancements occurred in the VLF amplitudes observed by both the antennas. These anomalous enhancements are examined in the light of magnetic storms, local lightning, and earthquakes and are positively correlated with the devastating India-Nepal border earthquake (M=5.3) occurred on 4 April 2011. The precursory period for borehole data ranges between 3-7 days while for vertical antenna data it is 3 days. The generation and propagation mechanism of ELF/VLF emissions in the crustal region have also been discussed.
The depression (reduction in amplitude) of ULF magnetic field variations of magnetospheric origin is studied at various distances from the epicenter of the strongest earthquake (EQ) which occurred in Japan on March 11, 2011. For this purpose, we have used the ULF data in Japan observed by fluxgate magnetometers at three places located at distances of ~300 km to ~1300 km from the epicenter of the main shock. The period of data analysis is from December 1, 2010 to May 31, 2011. We have found a sharp increase in depression of the horizontal ULF magnetic field component at the frequency of 0.03 - 0.05 Hz (30-50 mHz) at all of three Japanese observatories (Kakioka, Memambetsu and Kanoya) three days before the first strong foreshock (Mw=7.5) and five days before the main shock (Mw = 9). This peak in depression is found to be several times greater than all previous values, but the depression seems to be most enhanced at Kakioka, the station nearest to the EQ epicenter. So that it is likely that this phenomenon could be a possible precursor to the huge 3.11 EQ.
One major candidate of lithosphere-atmosphere-ionosphere (LAI) coupling mechanism is through atmospheric oscillations triggered near Earth’s surface due to some pre-earthquake (EQ) effect, and this channel has been extensively proved by using meteorological disturbances much more easily treated than pre-EQ effects (Korepanov et al., 2009). In the present paper this channel is challengingly studied, for the first time, for pre-EQ phenomena, and we take a rather strong EQ named Niigata-chuetsu EQ on 23 October, 2004 (with magnitude of 6.8 and with depth of 13 km) for which we already know that the ionospheric perturbation did take place prior to the EQ (Hayakawa et al., 2006). In this paper the LAI coupling has been intensively studied by means of coordinated observational data (surface atmospheric pressure data as an indicator of atmospheric gravity waves (AGWs), our own subionospheric VLF/LF data as a measure of ionospheric perturbations and the ground-based ULF data as a measure to monitor the modulation in the ionospheric dynamic region). The wavelet analyses for these parameters in different spatial regions have all indicated the enhancements of fluctuations in the wave frequency of 10 ~ 100 min (in the frequency range of AGWs). The correlation of wavelet spectra between the atmospheric pressure and VLF/LF amplitude has yielded a high value with the delay of a few hours, while there is nearly no distinct delay of the wavelet spectra between the ionospheric perturbation and the ground-based ULF fluctuation. These observational facts are compared with the theoretical estimation of AGW hypothesis, which may provide a convincing support to the AGW channel of the LAI coupling.
The purpose of this paper is to try to find any correlation between the ionospheric perturbation as detected by subionospheric VLF/LF propagation and ground motions as detected by wideband seismic observations (so-called F-net). Two huge earthquakes are chosen for our analysis; (1) Niigata-chuetsu-oki earthquake on July 16, 2007 and (2) Iwate-nairiku-nambu earthquake on June 13, 2008. The AW(acoustic wave, period=1-10 min) and AGW(atmospheric gravity wave, period=10-100 min) components in the ground motions are compared with the ionospheric perturbation. It is found that the ground motions in the AGW and AW ranges are enhanced as compared with the backgrounds when the lower ionosphere is perturbed, which might be a possible source of ionospheric perturbation. Though the number of events is only two, this correlation may lend a support to the atmospheric oscillation hypothesis of the lithosphereionosphere coupling due to some precursory ground effect.