656 research outputs found
The shock-acoustic waves generated by earthquakes
We investigate the form and dynamics of shock-acoustic waves generated by
earthquakes. We use the method for detecting and locating the sources of
ionospheric impulsive disturbances, based on using data from a global network
of receivers of the GPS navigation system and requiring no a priori information
about the place and time of associated effects. The practical implementation of
the method is illustrated by a case study of earthquake effects in Turkey
(August 17, and November 12, 1999), in Southern Sumatera (June 4, 2000), and
off the coast of Central America (January 13, 2001). It was found that in all
instances the time period of the ionospheric response is 180-390 s, and the
amplitude exceeds by a factor of two as a minimum the standard deviation of
background fluctuations in total electron content in this range of periods
under quiet and moderate geomagnetic conditions. The elevation of the wave
vector varies through a range of 20-44 degree, and the phase velocity
(1100-1300 m/s) approaches the sound velocity at the heights of the ionospheric
F-region maximum. The calculated (by neglecting refraction corrections)
location of the source roughly corresponds to the earthquake epicenter. Our
data are consistent with the present views that shock-acoustic waves are caused
by a piston-like movement of the Earth surface in the zone of an earthquake
epicenter.Comment: EmTeX-386, 30 pages, 4 figures, 3 tabl
HF spectrum occupancy and antennas
This paper deals with the research made during the COST 296 action in the WG2, WP 2.3 in the antennas and HF spectrum management fields, focusing the Mitigation of Ionospheric Effects on Radio Systems as the subject of this COST action.info:eu-repo/semantics/publishedVersio
Atmosphere-Ionosphere Response to the M9 Tohoku Earthquake Revealed by Joined Satellite and Ground Observations. Preliminary results
The recent M9 Tohoku Japan earthquake of March 11, 2011 was the largest
recorded earthquake ever to hit this nation. We retrospectively analyzed the
temporal and spatial variations of four different physical parameters -
outgoing long wave radiation (OLR), GPS/TEC, Low-Earth orbit tomography and
critical frequency foF2. These changes characterize the state of the atmosphere
and ionosphere several days before the onset of this earthquake. Our first
results show that on March 8th a rapid increase of emitted infrared radiation
was observed from the satellite data and an anomaly developed near the
epicenter. The GPS/TEC data indicate an increase and variation in electron
density reaching a maximum value on March 8. Starting on this day in the lower
ionospheric there was also confirmed an abnormal TEC variation over the
epicenter. From March 3-11 a large increase in electron concentration was
recorded at all four Japanese ground based ionosondes, which return to normal
after the main earthquake. We found a positive correlation between the
atmospheric and ionospheric anomalies and the Tohoku earthquake. This study may
lead to a better understanding of the response of the atmosphere /ionosphere to
the Great Tohoku earthquakeComment: Preliminary results reported at EGU 2011 in Vienna, Austri
Wavelet analysis of the LF radio signals collected by the European VLF/LF network from July 2009 to April 2011
In 2008, a radio receiver that works in very low frequency (VLF; 20-60 kHz) and LF (150-300 kHz) bands was developed by an Italian factory. The
receiver can monitor 10 frequencies distributed in these bands, with the measurement for each of them of the electric field intensity. Since 2009, to
date, six of these radio receivers have been installed throughout Europe to establish a ‘European VLF/LF Network’. At present, two of these are into
operation in Italy, and the remaining four are located in Greece, Turkey, Portugal and Romania. For the present study, the LF radio data collected
over about two years were analysed. At first, the day-time data and the night-time data were separated for each radio signal. Taking into account
that the LF signals are characterized by ground-wave and sky-wave propagation modes, the day-time data are related to the ground wave and
the night-time data to the sky wave. In this framework, the effects of solar activity and storm activity were defined in the different trends. Then, the
earthquakes with M ≥5.0 that occurred over the same period were selected, as those located in a 300-km radius around each receiver/transmitter and
within the 5th Fresnel zone related to each transmitter-receiver path. Where possible, the wavelet analysis was applied on the time series of the radio
signal intensity, and some anomalies related to previous earthquakes were revealed. Except for some doubt in one case, success appears to have been obtained in all of the cases related to the 300 km circles in for the ground waves and the sky waves. For the Fresnel cases, success in two cases and one
failure were seen in analysing the sky waves. The failure occurred in August/September, and might be related to the disturbed conditions of the ionosphere in summer
Severe magnetic storm effects in the ionosphere over Istanbul:a case study
The present study concentrates on the effects on the ionosphere of an individual severe magnetic Storm of the Sudden Commencement (SSC)type,with SSC taking place in the daytime hours.The storm started on 29 October 1968 and went on to 2 November 1968 with geomagnetic 3-hourly magnetic activity index reaching values of Kp •7. Interplanetary magnetic field polarities included A (field polarity Away from the solar wind)positive and T (field polarity Towards the solar wind)negative polarities.In these conditions,the local response of true height of F-region (hF) ionization fails nonlinearly from fixed plasma densities.The interplanetary B z ,the magnetic field H - component and K p were examined for the 5 days following the sudden commencement.Due to intensive geomagnetic SC effects,the lower and upper limits of the F -regions were unbalanced because of gravity relaxation and solar wind pressure effects,until normal levels were restored.In the F -layer 70%deviations of critical frequencies (f 0 F )from median values,as well as hF level variations reaching hundreds of kilometers,were observed and were responsible for the destruction of communication channels.With a plasmapause location value L =1.6 and with K p •7 the protonosphere reservoir should take 1.1 days for its replenishment;one day was in fact insufficient for a full plasma recovery
Atmosphere-Ionosphere Response to the M9 Tohoku Earthquake Revealed by Multi-Instrument Space-Borne and Ground Observations. Preliminary Results
We retrospectively analyzed the temporal and spatial variations of four different physical parameters characterizing the state of the atmosphere and ionosphere several days before the M9 Tohoku Japan earthquake of March 11, 2011. Data include outgoing long wave radiation (OLR), GPS/TEC, Low-Earth orbit ionospheric tomography and critical frequency foF2. Our first results show that on March 8th a rapid increase of emitted infrared radiation was observed from the satellite data and an anomaly developed near the epicenter. The GPS/TEC data indicate an increase and variation in electron density reaching a maximum value on March 8. Starting on this day in the lower ionospheric there was also confirmed an abnormal TEC variation over the epicenter. From March 3-11 a large increase in electron concentration was recorded at all four Japanese ground based ionosondes, which returned to normal after the main earthquake The joined preliminary analysis of atmospheric and ionospheric parameters during the M9 Tohoku Japan earthquake has revealed the presence of related variations of these parameters implying their connection with the earthquake process. This study may lead to a better understanding of the response of the atmosphere/ionosphere to the Great Tohoku earthquake
Study of TEC variations using permanent stations GNSS data in relation with seismic events. Application on Samothrace earthquake of 24 May 2014
This study investigates the ionospheric total electron content (TEC) variations prior to the earthquake (MW = 6.9) of 24 May 2014 in Samothraki island of north Aegean Sea in Greece. TEC estimates were analyzed using data from GNSS (GPS+Glonass) permanent networks with the aim to detect possible ionospheric anomalies associate with the seismic event. The test period covers one week of data, 4 days before and two days after the event. Selected GNSS stations are scattered around seismic epicenter of distances from 16 up to 1375 km. TEC values estimated for every hour using PPP technique with Bernese GPS software. A comparison with global TEC estimates derived from CODE and JPL institute confirms the validation of results. It is found that a significant decrease 1-day prior to earthquake occurs at all of the selected stations. This result is not obvious when standard ionospheric model is performed for the estimation of TEC. Therefore, in such cases the use of dedicated GNSS processing data scenario is mandatory. A spatial analysis on TEC estimates with geometrical properties shows that the 1-day decrement is related with the EQ shock and may point the location area of the Earthquake. Finally, we conclude that the lithosphere-atmosphere-ionosphere coupling (LAIC) mechanism through acoustic or gravity waves has a key role for this phenomenology
- …