Whistlers observed at low-latitude ground-based VLF facility in Fiji

The propagation features of nighttime whistlers to low-latitude station, Suva (−18.2°, 178.3°, geomag. lat. −22.1°, geomag. long. 253.5°, L=1.15), Fiji, from preliminary observations made during the period from September 2003–2005, are reported. The observations of ELF–VLF signals commenced in September 2003 using the VLF set-up of World Wide Lightning Location Network at our station. The whistlers were observed during the severe magnetic storm of 20–22 November 2003 and moderate magnetic storm of 17–19 July 2005. A whistler with dispersion D=12.7 s^1/2 occurred on 22 November at 00:11 h LT. On 20 July at 01:00 h LT, a short whistler with dispersion D=20.9 s^1/2 and two whistler events having two-component whistlers with D=15.8, 16.7 s^1/2 and 16.7, 17.3 s^1/2 were observed. Non-ducted pro-longitudinal mode of the whistler propagation supported by negative latitudinal electron density gradients in the ionosphere that are enhanced by magnetic storms, seems most likely mode of propagation for the whistlers with dispersion of 12.7–17.3 s^1/2 to this low-latitude station.     

Hisslers observed during daytime in a low latitude ground station

The observation of hisslers during daytime at low latitude station Jammu, India, is reported. The hissler elements are quasi-periodic falling tones observed during the period of hiss activity and appear in minutelong sequences with average spacing between individual elements of the order of 0.15 s. Hissler elements exhibit almost no dispersion and no complex internal structure in slope and intensity, and successive hissler elements do not overlap in time. It seems that the reported hisslers might have propagated in prolongitudinal mode.     

A Study of Early/Slow VLF Perturbations Observed at Agra,India

We present here the results of sub-ionospheric VLF perturbations observed on NWC (19.8 kHz) transmitter signal propagating in the Earth-ionosphere waveguide, monitored at our low latitude station Agra. During the period of observation (June-December 2011), we found 89 cases of VLF perturbation, while only 73 cases showing early character associated with strong lightning discharges. Out of 73 events, 64 (~84%) of the early VLF perturbations are found to be early/slow in nature; the remaining 9 events are early/fast. The onset duration of these early/slow VLF perturbations is up to ~ 5 s. A total of 54 observed early events show amplitude change lying between ± 3.0 dB, and phase change ± 12 degree, respectively, and found to occur mainly during nighttime. One of the interesting results we found is that the events with larger recovery time lie far away from the VLF propagation path, while events with smaller duration of recovery are within the ± 50–100 km of signal path. The World Wide Lightning Location Network (WWLLN) data is analysed to find the location of causative lightning and temporal variation. The lightning discharge and associated processes that lead to early VLF events are discussed.     

VLF quarter-gyrofrequency plasmaspheric emissions as observed by intercosmos 24 and magion 2 satellites

Summary Quarter-gyrofrequency plasmaspheric emissions with spectral properties differing from those of discrete plasmaspheric emissions, usual in active intervals, have been observed by low-altitude Intercosmos 24 and Magion 2 satellites during periods in which geomagnetic activity decreases. Their occurrence in satellite records shows very good correlation with simultaneously observed subauroral electron temperature enhancements and increase of electron temperature anisotropyT _e being larger than T _e . An analysis of the observed wave characteristics is given. Propagation of the emissions within the plasmasphere is discussed. It is shown that the region where they are observed at low altitudes can be closely connected along geomagnetic field lines with the equatorial region of their origin.     

First results of simultaneous recording of VLF emissions at two closely located points at auroral latitudes

For the first time, simultaneous observations of very low frequency (VLF) emissions at auroral latitudes (L = 5.3) are carried out at two points located at close geomagnetic latitudes and spaced in longitude by 400 km: the Finnish Kannuslehto station (Φ = 64.2°) and the Russian Lovozero observatory (Φ = 64.1°). A recording equipment with similar frequency responses was used. The first results of a comparison of simultaneous observations showed that, in the overwhelming majority of cases, bursts of VLF emission appeared at both points synchronously with an identical (more often right-handed) polarization of the magnetic field of VLF waves, which can be evidence in favor of large dimensions of the ionosphere exit region of VLF waves. A simultaneous burst of quasi-periodic VLF emissions that occurred on February 02, 2013, during a substorm at 23–24 UT is discussed in detail. Additionally, VLF bursts were recorded which were observed only at one point, e.g., the appearance of left-hand polarized periodic emissions (PEs) in band 2.5–4.0 kHz with a repetition period of 3–4 s.     

Studies of ionospheric variations during geomagnetic activities at the low-latitude station, Ile-Ife, Nigeria

The dual frequency SCINDA NovAtel GSV 4004B GPS receiver installed at the Ile-Ife (low-latitude station) has been in operation since December 2009. Data records for the year 2010 were processed to obtain Total Electron Content (TEC) and S ₄ index. These were interpreted to analyze the ionospheric condition during low geomagnetic activity period (when Dst is from ?40 to 0 nT) and during geomagnetic storm events (with Dst about ?100 nT). Seasonal variations of the TEC and S ₄ index were also investigated. The occurrence of scintillations is closely linked to the peak value of TEC during the daytime; this is very evident during the equinox months when TEC ≥ 30 TECu. When the maximum TEC value is below 30 TECu, as shown by most of the days in the summer months, the scintillation phenomenon does not occur. During geomagnetic storms, the daytime segment of the TEC plot experiences fluctuations (even bifurcations) in values with the peak TEC value of about 40 TECu. From the interpreted data, the occurrence of geomagnetic storm does not necessarily suggest an increase in the level of scintillations at a low-latitude region. Also, there is a remarkable difference between the IRI 2007 model and the observed TEC values, as the daytime TEC peak differs in magnitude and time of occurrence from the observed TEC.     

Short-period VLF emissions as solitary envelope waves in a magnetospheric plasma maser

Space and ground-based experiments have shown evidence of natural short-period VLF emissions in which separate spectral elements are repeated with a periodicity of 2–7 s. Their basic morphological properties are found on the basis of original experimental data. In our opinion, excitation of such emissions is the result of quasi-linear relaxation effects that compensate for natural spectral dispersion. The quasi-linear relaxation of the energetic electron distribution function incrementally changes wave cyclotron instability and hence the VLF emission spectral forms. Some properties of the quasi-linear interaction of whistler waves with magnetospheric radiation belt electrons are studied. It is shown that quasi-linear relaxation can increase the cyclotron instability at the leading edge of an electromagnetic pulse. This effective saturation of absorption facilitates the division of VLF hiss-like emission into separate electromagnetic pulses without spectral modification from one pulse to the next. Some features and manifestations of this effective saturation of absorption are discussed. The results are important for a better understanding of temporal and spatial structures of VLF whistler-mode emissions and energetic electron fluxes.     

Earthquake effects along paths of VLF radio noise and impulsive signals as observed at Yakutsk

An analysis of amplitude variations in the noise and storm-induced impulsive VLF electromagnetic signals recorded at Yakutsk along paths above earthquakes is reported. It is shown that amplitude characteristics of storm-induced VLF signals can usefully supplement signals of radio stations as used in the radio monitoring of regions above earthquakes. The effect on these signals due to earthquakes of magnitude greater than 5 is observed as an amplitude increase within the three days following an earthquake. The pre-seismic variations in the storm-induced signal amplitude which can be regarded as precursors are generally observed as increased amplitudes (within 10 days before the earthquake) with a subsequent minimum.     

Numerical simulation of LEP Trimpis observed at Poitiers,France, on signals from VLF transmitters

Transient amplitude and phase perturbations on subionospheric VLF signals, known as the Trimpi effect, are caused by the scattering of VLF radiation from localized ionization enhancements in the nighttime D-region. The patches of ionization are due to precipitation from the radiation belts of keV electrons, that is induced by lightning-generated whistlers.This work is concerned with the numerical simulation of such VLF perturbations, termed LEP (lightning-induced electron precipitation) or classic Trimpis. Two different codes are used to compute the VLF propagation in the Earth-ionosphere waveguide in the presence of a D-region inhomogeneity. The first is based on mode theory, and the second on the FDTD (finite-difference time-domain) method. Both codes are two-dimensional and, therefore, relevant only to LEP events lying on the transmitter-receiver great circle path (TRGCP). A method of simulation is proposed to interpret quantitatively VLF amplitude and phase changes in terms of the approximate location and size of the associated ionospheric perturbation along the TRGCP. The method is applied to LEP Trimpis observed at Poitiers (L = 2) on signals from the NAA and GQD transmitters. Results are discussed in the light of the information that may be deduced from the high-resolution analysis of VLF temporal signatures. A sequence of simultaneous LEP Trimpis observed at Poitiers on three widely separated transmission paths is studied; two alternative interpretations are proposed.     

Semi-empirical estimation of ground motion using observed records at a site in Shikoku, Japan

A semi-empirical approach using fore- or after-shockrecords as Green's functions is applicable to thesimulation of strong ground motion, however suchrecords are obviously not available for predictionpurposes. Thus we have predicted ground motion fora hypothetical large earthquake from other minorevents by adopting a distance correction based ongeometrical spreading. Another difficulty inprediction is fault modeling. Surface traces weresimplified as fault models 27, 46, 55, and 77 km inlength. Further, the actual fault rupture may beinhomogeneous, so an asperity distribution isassumed. This asperity model assumes thatdislocation and stress drop are double than theaverage values. Although, the near field term isneglected in our simulation, no significantdifference was seen in the motions estimated byindividual models for periods up to 2.0 seconds. This indicates that the dependence of source size issmall for strong motion, perhaps as a result of therandom summation of high-frequency phases.     

潮州台地下水化学异常与地震的对应关系

根据潮州台6个地下水测项20年来的观测资料，对潮州台水化学异常与地震的对应关系作了简要讨论，结果表明，在距台250km范围内发生的中强地震前，除Cl^-外，其余测项均观测至短期异常变化。     

昌平台钻孔应变固体潮调合分析

介绍了体应变观测潮汐因子和勒夫数 h2 之间的线性关系 ,h2 的变化幅度较体应变观测潮汐因子震前异常变化大 7.5倍。同时 ,昌平台剪应变观测潮汐因子 O1波与体应变观测潮汐因子M2 波震前异常有较好的一致性 ,研究钻孔应变观测潮汐因子的变化有可能是取得地震前兆的有效途径之一。     

Ultra-low-frequency (ULF) and total electron content (TEC) anomalies observed at Agra and their association with regional earthquakes

Employing a three-component search coil magnetometer (f = 0.01–30 Hz) the ultra-low-frequency (ULF) magnetic field observations have been taken in a noise free rural site at Bichpuri, Agra, India since 20 April 2007. We have analysed 7 months of night time data from 01 May to 30 November 2007. The data show seven occurrences of ULF bursts with periods ranging from a few minutes to an hour in a wide frequency range up to 15 Hz. The occurrence of such ULF bursts has been statistically analysed. The polarization parameter analysis shows that two of these bursts occurred from sources in the ionosphere and magnetosphere (Z/X < 1). It is further seen that four bursts correlated very well with some regional earthquakes (M > 4.5), which occurred at distances less than 628 km from the observation site at Agra. In some cases the bursts occurred 1–3 days prior to the main shocks.Since earthquakes are also known to perturb the ionosphere, we have analysed total electron content (TEC) data obtained from a GPS receiver stationed at Agra since June 2006. Analysis of the data for the same period as the magnetic field observations shows that there existed TEC anomalies on all the days when ULF bursts occurred. The TEC anomalies show enhancement and depletion in the data in all cases. While the mechanisms of the ULF and TEC anomalies are not well known, it is expected that the ULF bursts occurred due to microfracturing of the earth's crust during stress accumulation and that the TEC anomalies occurred as a result of the electric field induced in the ionosphere during such processes.     

Transient ULF pulsations: time dependence of magnetic fields observed at the ground

The continuum oscillation of a latitudinal range of closed geomagnetic field lines or shells appears to be a basic feature of the magnetosphere. Such oscillations are observed at the ground, and have been termed transient ULF pulsations. Earlier modelling showed that the apparent mean damping rate at the ground should be much greater than that in the magnetosphere. This modelling is extended to examine the time dependence of the magnetic field of transient pulsations as seen by a latitudinal chain of magnetometers. It is found that there should be significant temporal variation of both period and damping decrement observed at a given latitude, which could help to identify transient events even when the period variation with latitude is not obvious. Time-frequency analysis and analytical signal analysis do not seem to be effective in determining temporal parameter variation for the short, highly damped data segments typical of transient events. Least squares fitting of two decaying sinusoids gives surprisingly good results, but seems to have no physical basis, is difficult to interpret, and may be misleading. Least squares fitting of a single sinusoid with time-varying period and damping rate gives reasonably good fits. The resulting parameter variations with latitude may help to determine the structures of ionospheric current systems associated with transient ULF events. In particular, the time change of the period at a single station can determine where that station is relative to the ionospheric current maximum.     

Generation of VLF emissions during the substorm preliminary phase and the auroral dynamics at the auroral oval northern edge

About 100 breakups of different types and intensities are studied on the basis of Lovozero Observatory data. Magnetic pulsations in different frequency ranges, VLF emissions, and auroral activity are analyzed using the TV data. It is found that magnetic pulsations in all frequency ranges lag behind the moment of breakup by 0.5–2.0 min, and bursts of low-intensity broadband VLF emission hiss are observed 3–10 min before breakup. Hiss leading breakup corresponds to feeble auroras located northward of a pre-breakup arc.     

Mismatch between variations of solar indices, stratospheric ozone and UV-B observed at ground

In solar cycles 22–23, all solar indices showed maxima near 1990 and 2000 and minima in 1996. The maximum to minimum variation was only 1–2% in the UV range 240–350 nm. Dobson ozone intensities did not show any clear relationship with solar cycle and ozone variations were less than 10%. The UV-B (295–325 nm) observed at ground by Brewer spectrophotometers at some locations had variations of 50–100% for 295–300 nm, and 20–50% for 305–325 nm. The maxima were in different years at different locations (even with separations of only 300 km), did not match with the solar cycle, and were far too large to be explained on the basis of ozone changes (1% decrease of ozone is expected to cause 2% increase of UV-B). Thus, if the data are not bad, the UV-B changes do not match with solar activity or ozone changes and must be mostly due to other local effects (clouds, etc.?). When data are averaged over wide geographical regions, UV-B variation ranges are smaller (10–20%, probably because localised, highly varying cloud effects get filtered out), and are roughly as expected from ozone variations.     

GPS phase scintillation observed over a high-latitude Antarctic station during solar minimum

Radio wave scintillation due to the presence of ionospheric irregularities can cause fading and phase variation of L-band navigation signals such as those used by the Global Positioning System (GPS). The high-latitude regions experience increased scintillation events under geomagnetically disturbed conditions, particularly during solar maximum. This paper presents phase scintillation measurements observed at the South African Antarctic polar research station during solar minimum conditions. In addition, a multi-instrument approach is shown in which the scintillation events are associated with auroral electron precipitation. Furthermore, it is shown that external energy injection into the ionosphere is an important factor in the development of irregularities producing scintillation.     

Anomalous hydrogen emissions from the San Andreas fault observed at the Cienega Winery,central California

We began continuous monitoring of H₂ concentration in soil along the San Andreas and Calaveras faults in central California in December 1980, using small H₂/O₂ fuel-cell sensors. Ten monitoring stations deployed to date have shown that anomalous H₂ emissions take place occasionally in addition to diurnal changes. Among the ten sites, the Cienega Winery site has produced data that are characterized by very small diurnal changes, a stable baseline, and remarkably distinct spike-like H₂ anomalies since its installation in July 1982. A major peak appeared on 1–10 November 1982, and another on 3 April 1983, and a medium peak on 1 November 1983. The occurrences of these peaks coincided with periods of very low seismicity within a radius of 50 km from the site. In order to methodically assess how these peaks are related to earthquakes, three H₂ degassing models were examined. A plausible correlational pattern was obtained by using a model that (1) adopts a hemicircular spreading pattern of H₂ along an incipient fracture plane from the hypocenter of an earthquake, (2) relies on the FeO–H₂O reaction for H₂ generation, and (3) relates the accumulated amount of H₂ to the mass of serpentinization of underlying ophiolitic rocks; the mass was tentatively assumed to be proportional to the seismic energy of the earthquake.