A new VLF (Very low Frequency) receiver has been developed by the Peruvian Space Agency (CONIDA) for space weather studies. The receiver has been designed based on a Red Pitaya board which performs ...an SDR (Software Defined Radio) to digitize, process and store the signal. The receiver is composed of a vertical antenna, a preamplifier to filter and amplify the incoming VLF signals from several transmitters located around the world. The receiver is able to cover a bandwidth from 1 up to 50 kHz and it has been developed in such a way as to be cost-effective, autonomous and solar-powered, making it suitable for installation in multiple locations with different geographic conditions. We show the performance of the receiver, the typical daily pattern of the lower ionosphere for the NAA VLF signal, as observed in Peru, and the first solar flares observed. The VLF amplitude curves recorded are validated by comparing them with data from SAVNET (The South American VLF Network) receiver installed in Peru. In a first effort to investigate the impact of solar flares on the lower ionosphere, we conducted a statistical analysis between VLF amplitude perturbations and 1–8 Å solar X-rays flux provided by GOES satellites, resulting in a linear relationship.
•Software Defined Radio implemented on FPGA.•A cost-effective, autonomous and solar-powered VLF receiver has been designed.•Ionospheric disturbances related to solar flares are detected as VLF amplitude enhancement.•A significant correlation between VLF amplitude enhancement and C- and M-class solar flares was found.
•Low latitude D-region ionospheric disturbances observed by a multi-station VLF network during the December 2019 solar eclipse.•Short propagation paths are used to formulate the relationship between ...solar obscuration and reduction of D-region electron density.•Based on the observations and calculation, spatial distributions of D-region electron density over the Indian sub-continent are presented.
We present the low-latitude D-region (60–90 km) ionospheric disturbances due to the December 2019 annular solar eclipse observed using the multi-station Very Low Frequency (VLF) radio network in West Bengal, India. VLF signals from communication transmitters were received from ten places using low-cost VLF radio receivers. The receivers were capable of recording the amplitudes of VLF signals mainly from the transmitters VTX, India at 18.2 kHz and NWC, Australia at 19.8 kHz. During the solar eclipse, the VTX signal experienced considerable positive, negative, and mixed-type amplitude variations depending on the propagation lengths between the transmitter and receivers, whereas the NWC signal displayed a relatively small amplitude response. We have also found that the highest VLF signal disturbances in each propagation path occurred after the time when the solar obscuration was maximum over the entire path. We used the observations of the VTX signal to formulate the relationship between solar obscuration and D-region electron density reduction during the solar eclipse. On the basis of this, we presented the electron density distribution of the D-region ionosphere during the solar eclipse across the Indian subcontinent. This study shows a useful method for integrating VLF observations and simulations to estimate the D-region electron density distribution over a vast area during a solar eclipse.
The lower ionospheric disturbances associated with the Super Cyclonic Storm Amphan over the Bay of Bengal have been presented in this paper. The disturbances were observed by sub-ionospheric Very Low ...Frequency (VLF) radio signals from two navigational transmitters VTX and NWC, received at Cooch Behar, India. Strong variations of the nighttime signal amplitudes exceeding three times their standard deviations have been observed during the peak intensity phase of the cyclone. Morning terminator time in the VLF signal was influenced just after the cyclone peak and evening terminator time was influenced significantly after landfall. The radius of the strongly perturbed ionospheric region associated with the cyclone during its peak intensity has been estimated approximately between 500 and 1000 km from VLF observations.
•First observation of VLF radio signal perturbations associated with a Super Cyclonic Storm in Indian sub-continent.•Significant changes in nighttime signal amplitudes and sunrise/sunset terminator times during the cyclone period.•Spatial size of the ionospheric disturbances associated with the cyclone has been estimated from observations.
We investigate the influence of the perturbed (by a solar X-ray flare) ionospheric D-region on the global navigation satellite systems (GNSS) and synthetic aperture radar (SAR) signals. We calculate ...a signal delay in the D-region based on the low ionospheric monitoring by very-low-frequency (VLF) radio waves. The results show that the ionospheric delay in the perturbed D-region can be important and, therefore, should be taken into account in modeling the ionospheric influence on the GNSS and SAR signal propagation and in calculations relevant for space geodesy. This conclusion is significant because numerous existing models ignore the impact of this ionospheric part on the GNSS and SAR signals due to its small electron density which is true only in quiet conditions and can result in significant errors in space geodesy during intensive ionospheric disturbances.
The Alpha navigation system is the only operating radio system based on very-low-frequency (VLF) signals that can be used to research VLF navigation, timing, and ionospheric characteristics. The ...detection of the Alpha navigation signal is the key step in the Alpha receiver; however, the received Alpha navigation signal is susceptible to noise and mutual interference, which deteriorates signal detection performance. This paper presents a multichannel orthogonal correlation method for Alpha navigation signal detection. Once the three frequency signals of the Alpha navigation system are obtained using a notch filter, station identification is realized using a multichannel orthogonal correlation method and signal format. The selection of key parameters and the detection performance under noise and mutual interference are analyzed. This method’s detection probability exceeds 90% when the signal-to-noise ratio (SNR) is greater than −10 dB. The influence of mutual interference on the signal correlation peak is less than 1% when the signal-to-interference ratio (SIR) of the mutual interference is greater than −28 dB. The proposed method is verified using an actual signal collected using an Alpha receiver. The results show that an Alpha signal can be detected at an extremely low SNR. This method has strong practicability and satisfies the application requirements of an Alpha receiver.
This communication reports dynamic spectra of different ELF/VLF emissions such as hiss, chorus, hiss-triggered chorus and whistler-triggered emissions. We also discuss their various observed ...features. It is argued that most of the emissions are generated during Doppler-shifted cyclotron resonance interaction between the whistler mode wave and energetic electrons. Resonance energy of the participating electron and interaction length are evaluated to explain the generation mechanism of some of these emissions observed at the Indian station Srinagar.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
Very Low Frequency (VLF) waves radiated from ground-based transmitters are crucial for long-distance communication and underwater navigation. These waves can reflect between the Earth’s surface and ...the ionosphere for Earth–ionosphere waveguide propagation. Additionally, they can penetrate not only the ionosphere but also the magnetosphere, where they interact with high-energy particles in the radiation belt. Therefore, studying the spatial and temporal distribution of VLF radio signals holds significant importance. Such research enables us to understand the propagation characteristics of VLF signals, their interaction with radiation belt particles, and their response to space weather and lithospheric activity events. In this paper, we investigate the seasonal variations in the intensity of the Northwest Cape (NWC) transmitter (19.8 kHz) radio signals at satellite altitude and the displacement of the electric field’s peak center. Our analysis is based on the nightly China Seismo-Electromagnetic Satellite (CSES) data from 2019 to 2021. The results reveal the following: (1) There is no significant seasonal variation in the electric field strength within a small area (2.5° radius) around the NWC transmitter. However, a clear seasonal variation in the electric field strength is observed within a larger area (15° radius), with higher strength during winter compared with summer. (2) The power spectral density of the electric field remains constant within the peak central area (approximately 1~2° radius), but it decays with distance outside this region, showing a north–south asymmetry. Moreover, the decay rate of the radiation electric field is slower in the northern direction than in the southern direction. (3) The center of the electric field moves northward from summer to winter and southward from winter to summer. (4) In winter, VLF waves radiated by the NWC transmitter may predominantly propagate by being ducted toward the conjugate hemisphere.
The amplitude and phase of VLF/LF radio signals are sensitive to changes in electrical conductivity of the lower ionosphere which imprints its signature on the Earth–ionosphere waveguide. This ...characteristic makes it useful in studying sudden ionospheric disturbances, especially those related to prompt X-ray flux output from solar flares and gamma ray bursts (GRBs). However, strong geomagnetic disturbance and storm conditions are known to produce large and global ionospheric disturbances, which can significantly affect VLF radio propagation in the D region of the ionosphere. In this paper, using the data of three propagation paths at mid-latitudes (40–54°), we analyse the trend in variation of aspects of VLF diurnal signal under varying solar and geomagnetic space environmental conditions in order to identify possible geomagnetic footprints on the D region characteristics. We found that the trend of variations generally reflected the prevailing space weather conditions in various time scales. In particular, the ‘dipping’ of mid-day signal amplitude peak (MDP) occurs after significant geomagnetic perturbed or storm conditions in the time scale of 1–2days. The mean signal amplitude before sunrise (MBSR) and mean signal amplitude after sunset (MASS) also exhibit storm-induced dipping, but they appear to be influenced by event's exact occurrence time and the highly variable conditions of dusk-to-dawn ionosphere. We also observed few cases of the signals rise (e.g., MDP, MBSR or MASS) following a significant geomagnetic event. This effect may be related to storms associated phenomena or effects arising from sources other than solar origin. The magnitude of induced dipping (or rise) significantly depends on the intensity and duration of event(s), as well as the propagation path of the signal. The post-storm day signal (following a main event, with lesser or significantly reduced geomagnetic activity) exhibited a tendency of recovery to pre-storm day level. In the present analysis, we do not see a well-defined trend in the variation of the post-storm sunrise amplitude terminator (SRT) and sunset terminator (SST). The SRT and SST signals show more dipping in GQD-A118 propagation path but generally an increase along DHO-A118 propagation path. Thus the result could be propagation path dependent and detailed modelling is required to understand these phenomena.
•Dipping of the midday signal amplitude occurs after storm in time scale of 1–2 days.•Dipping extent depends on storm intensity, duration and signal path characteristics.•Post-storm day signal level with reduced perturbations, exhibited recovery tendency.
In Črna Jama, which is part of Postojnska Jama, underground measurement of VLF (Very Low Frequency) radio signals was periodically carried out in 2012 and 2013 for detection of possible pre-seismic ...and/or other anomalies and to ascertain suitability of VLF monitoring in a natural cave environment. The modulation of the VLF signal was connected with a powerful atmospheric front and changes of the precipitation level. VLF data showed day and night changes and also significant changes between dry and rainy periods when water from the surface reached the cave chamber through about 30 m of limestone roof. During VLF monitoring we did not receive earthquake precursor signals. VLF monitoring in a karst cave could be an option for future research in understanding pre-seismic and other anomalies. On-line connection with other VLF surface or cave monitoring sites in Europe is necessary in future VLF registration. Results of preliminary VLF monitoring showed Črna Jama to be a suitable place for future studies. Key words: VLF radio signals, noise sounds, pre-seismic anomalies, Črna Jama, Slovenia.
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