Low Latitude Ground Station Research Articles

Observation of whistlers and associated ionospheric parameters during St. Patrick’s Day storm at low Latitude, Varanasi

The paper presents the analysis of a large number of whistlers recorded during the St. Patrick's Day geomagnetic storm (17–23 March 2015) at Indian low latitude ground station Varanasi (geomag. lat. 140 55/ N, geomag. long. 1530 54/ E, L = 1.078). The increased occurrence of the recorded whistlers has been explained in terms of duct formations for the propagation of whistlers during the geomagnetic storm. The whistler’s activity extended up to the 19th March 2015 during the daytime. The ducted propagation of the recorded whistlers is supported by the analysis of the World-Wide Lightning Location Network (WWLLN) data. In this paper, the result showed that the occurrence time of causative sferics of more than 80 % was in good agreement with that of WWLLN lightning discharges within the propagation time. The origins of these lightning strikes are lying within a 500 km radius of the conjugate point of the Varanasi. By analyzing recorded whistlers, we have computed different ionospheric parameters of Earth. The dispersion for the nighttime whistlers lies between 11 and 15 sec½ whereas dispersion for the daytime whistlers lies between 12 and 25 sec½. The columnar ionospheric electron contents (TECs) computed for the recorded whistlers during nighttime lie between 25.47 TECU and 42.84 TECU whereas TECs of the daytime whistlers lie between 30.08 TECU and 125.48 TECU. These calculated columnar ionospheric electron contents are found to be of the same order of magnitude as the IRI-2012 modeled TEC over Varanasi and that of conjugate of Varanasi whereas GPS-TEC are found to be comparatively 8–15 TECU lower over Varanasi.

Day time whistlers observed at low latitude Varanasi ($$L = 1.078$$)

We present results of the first time observations of whistlers during day time (sunrise) on 4th January 2017 at 01 UT ($$\hbox {UT} + 5.30 = \hbox {IST}$$) at Indian low latitude ground station Varanasi (geomag. lat. $$14{^{\circ }}55^\prime \hbox {N}$$, geomag. long. $$153{^{\circ }}54^\prime \hbox { E}$$, L.1.078). The main goal of analysis is to study the propagation characteristic of the observed whistlers during the day time (sunrise). These whistlers were observed during the quiet geomagnetic conditions (Dst-index = –8 nT). The dispersions of the observed whistlers are found between 11.16 and $$14.78\hbox { s}^{1/2}$$, which shows that the observed whistlers have propagated in the ducted mode and the whole propagation path of whistlers lies in the ionosphere. Their columnar ionospheric electron contents lie between 23.57 TECU and 39.44 TECU. The ionospheric parameters derived from whistler data at Varanasi compare well with other measurements made by other techniques.

Observation of discrete VLF emissions at Indian low-latitude ground station Srinagar (L = 1.28)

During the routine analysis of recorded very low-frequency data during the period November, 2012 to February, 2013, at an Indian low-latitude ground station Srinagar (geomag. lat., 24°10′N; L = 1.28), we observed some interesting discrete emissions during quiet period on February 5, 2013, which has been presented in the present study. The spectral analysis of the observed discrete very low-frequency emissions consisting of risers, fallers, hooks and oscillating tones has been presented. To explain the observed dynamic spectra of the discrete chorus emissions, a possible generation mechanism has been presented based on the recent nonlinear theory. On the basis of this theory, frequency sweep rate of discrete chorus emissions has been computed and compared with those of our experimentally observed values.

Simultaneous Observations of Daytime Whistlers and VLF-ELF Emissions at a Low Latitude Indian Ground Station

This paper reports an unexpected simultaneous observation of whistlers and different types of ELF/VLF emissions during daytime at a low latitude Indian ground station Jammu (geomag. lat., 22o 26/ N; L=1.17), which are (a) pulses of atmospheric bursts, (b) discrete chorus riser ELF emissions triggered from the bottom of the atmospherics, (c) whistlers, (d) whistler-triggered discrete chorus riser VLF emissions triggered from the lower end of the whistler, (e) long enduring ELF/VLF hiss emissions, (f) band limited quasi-periodic pulsing ELF/VLF hiss emissions, (g) hook and inverted hook ELF emissions, (h) oscillating tone discrete chorus riser ELF emissions. The data shown in this paper indicate that there is a strong possibility that lightning is an important source of different types of VLF/ELF emissions, at least in the embryonic sense, recorded at Jammu. The present observation is in fact the first simultaneous occurrence of different types of VLF/ELF emissions alongwith whistlers recorded on a single day during daytime at a low latitude ground station.

Subionospheric VLF perturbations observed at a low latitude station Varanasi (L = 1.07)

Initial results of subionospheric very low frequency (VLF) perturbations observed on NWC (19.8kHz) and NPM (21.4kHz) transmitter signals at low latitude ground station Varanasi, India, during the period January–June 2010, are presented. Characteristics and occurrence rates of these events mainly observed during nighttime are studied. Most of the early VLF events had slow recovery with amplitude perturbations of 0.5–4.5dB and phase changes of 3–12°. Temporal variation of the events is studied. World Wide Lightning Location Network data along with the broadband VLF data are analyzed to find the location of causative lightning discharge and/or the sferics associated with these early VLF events. Lightning induced changes in D-region ionospheric conductivity are attributed to the perturbations in the VLF signals.

Observations of Rising Tone Chorus Emissions at Indian Low Latitude Ground Station and Their Generation Mechanism

Observations of Rising Tone Chorus Emissions at Indian Low Latitude Ground Station and Their Generation Mechanism

Estimation of duct-lifetimes for storm-time low latitude whistlers observed at Srinagar, India

Whistler data recorded during 5-h period at our low latitude ground station Srinagar (geomag. lat. 24°10′N; L = 1.28) are used to determine lifetime of whistler ducts. On certain occasions, whistler occurrence rate at Srinagar is found to exhibit some periodicity. Power spectrum analysis of occurrence rate yields a dominant period of about 1 hour. It is suggested that this period is an indication of the duct-lifetimes at low L-values. Dispersion analysis of these whistlers has qualitatively confirmed the existence of separate ducts during the period of observation. Further the dispersion analysis shows that these whistlers have propagated in the magnetosphere along higher L-values (L = 2.10–4.12). Power spectrum analysis may not be applicable to the whistler data corresponding to high L-values because at high latitudes the occurrence rate of whistlers is very high. Moreover a large number of ducts might simultaneously contribute to the occurrence rate.

Observations of Rising Tone Chorus Emissions at Indian Low Latitude Ground Station and Their Generation Mechanism

Observations of Rising Tone Chorus Emissions at Indian Low Latitude Ground Station and Their Generation Mechanism

Wp index: A new substorm index derived from high‐resolution geomagnetic field data at low latitude

Geomagnetic field data with high time resolution (typically 1 s) have recently become more commonly acquired by ground stations. Such high time resolution data enable identifying Pi2 pulsations which have periods of 40–150 s and irregular (damped) waveforms. It is well‐known that pulsations of this type are clearly observed at mid‐ and low‐latitude ground stations on the nightside at substorm onset. Therefore, with 1‐s data from multiple stations distributed in longitude around the Earth's circumference, substorm onset can be regularly monitored. In the present study we propose a new substorm index, the Wp index (Wave and planetary), which reflects Pi2 wave power at low‐latitude, using geomagnetic field data from 11 ground stations. We compare the Wp index with the AE and ASY indices as well as the electron flux and magnetic field data at geosynchronous altitudes for 11 March 2010. We find that significant enhancements of the Wp index mostly coincide with those of the other data. Thus the Wp index can be considered a good indicator of substorm onset. The Wp index, other geomagnetic indices, and geosynchronous satellite data are plotted in a stack for quick and easy search of substorm onset. The stack plots and digital data of the Wp index are available at the Web site (http://s‐cubed.info) for public use. These products would be useful to investigate and understand space weather events, because substorms cause injection of intense fluxes of energetic electrons into the inner magnetosphere and potentially have deleterious impacts on satellites by inducing surface charging.

THEMIS multipoint observations of Pi2 pulsations inside and outside the plasmasphere

[1] Much evidence indicates that Pi2 pulsations in the inner magnetosphere can be explained as radially trapped fast-mode waves in the plasmasphere. Most of these studies, however, are based on observations from a single spacecraft in the plasmasphere and low-latitude ground stations within several hours of local time. Here we present simultaneous observations of two Pi2 pulsations on 26 January 2009 by three THEMIS probes (P3, P4, and P5) and the Ascension Island (ASC) ground station (L ∼ 1.002, MLT ∼ 23). By using plasma density and spacecraft potential measurements, we determined that during this time P4 and P5 were located inside the plasmasphere and P3 was located outside it. It is found that fast-mode components (compressional component Bz, azimuthal component Ey) observed by P4 and P5 have a high correlation with the H component at ASC while those at P3 do not. The Bz oscillations at P4 are nearly in phase with ground Pi2 while those at P5 near the plasmapause are almost out of phase with the H component on the ground during the two Pi2 events. The Ey -H cross phases at P4 and P5 are nearly constant at −90° for both Pi2 events. These multipoint observations strongly suggest that fast-mode waves trapped in the plasmasphere are the source of the low-latitude Pi2 pulsations.

Characteristics of nighttime VLF/ELF emissions with frequency drift and estimation of the large-scale electric field from the frequency drift observed at a low latitude Indian ground station, Jammu

Characteristics of nighttime VLF/ELF emissions are examined on the basis of the data obtained at our low latitude ground station Jammu (geomagnetic latitude, 22°26′ N; L = 1.17), India during our VLF/ELF campaign. From the detailed analysis of a huge amount of acquired VLF/ELF data at Jammu we have found three remarkable events which clearly exhibit a rise in their frequency in pre and past midnight sectors during magnetically quiet and substorm periods. Our analysis shows that the frequency drift in VLF/ELF emissions seems to be a rare phenomenon at low latitudes during magnetically quiet and substorm periods in pre and post midnight sectors. This property of temporal frequency drift (regular frequency increases with time) in VLF/ELF emissions observed at our station Jammu are interpreted in terms of a quasi-linear electron cyclotron instability model for wave excitation. The initial frequency increase is believed to be due to a combind effect of L-shell drift of energetic electrons. Further, the frequency drifts in VLF/ELF emissions observed at Jammu have been used to estimate the large scale electric field during quiet and substorm periods in pre and post midnight sectors. This investigation would be most useful for the study of the wave-particle interaction processes, magnetospheric plasma structure and particle dynamics, especially during quiet periods in premidnight sector at low latitudes.

Simultaneous observation of whistlers and emissions during a geomagnetically quiet period at low latitude

A unique night-time natural electromagnetic disturbances in the VLF/ELF range received during a magnetically quite period at a low latitude Indian ground station, Jammu (geomag. lat. 19°26′ N, L=1.17) has been reported. During the routine observation of VLF waves at Jammu, whistlers and different types of VLF/ELF emissions such as whistlers of varying dispersion confined to a small band limited frequency range, hisslers, pulsing hiss, discrete chorus emissions of rising and falling tones with multiple bands, oscillating tone discrete emission, whistler-triggered hook and discrete chorus risers emissions, etc. have been observed simultaneously during the quiet period on a single night. Such type of unique simultaneous observations has never been reported from any of the low latitude ground stations and this is the first observation of its kind. The results are discussed in the light of recorded features of whistlers and emissions. Generation and propagation mechanism are discussed briefly. Plasma parameters are further derived from the dispersion analysis of nighttime whistlers and emissions recorded simultaneously during magnetically quiet periods.

Drifting ELF/VLF Emissions Observed at Low Latitude Ground Station During Geomagnetic Storm

We have reported for the first time total seven strong events of drifting ELF/VLF discrete emissions observed on 28th–29th April, 1990 in the pre-midnight sector at Varanasi (Geomag. lat. 14°55′N, long. 154°E, L = 1.07). The events exhibit a regular increasing as well as decreasing frequency drifts and are mainly discrete periodic emissions of riser, faller and hook types observed during a geomagnetic storm period, with minimum Dst-index −98 nT and Kp-index ≥ 5. The frequency drift in ELF/VLF emissions at low latitudes seems to be a rare phenomenon. The repetition period and the frequency drift rate have been evaluated for all the recorded events. The frequency drifts have been interpreted in terms of a combined effect of L-shell drift of interacting energetic electrons and the change in convection electric fields during the storm developments. The computed maximum spectral power density \( \left\langle {B_{f}^{2} } \right\rangle_{\max } \)of the wave varies between 1.8 × 10−21 and 4.08 × 10−22 Gauss2/Hz, whereas frequency drift rates are in agreement with the observed values.

Daytime hiss-triggered chorus emissions observed at low latitude ground station Srinagar, India

The first observation of hiss-triggered discrete chorus riser emissions recorded during daytime at our newly setup low latitude ground station Srinagar (geomag. lat., 24° 10′ N; L = 1.28), India on March 30, 2009 are reported. From the spectral analysis of these emissions, it is found that the chorus is hiss-triggered and each chorus element has the tendency to originate from the hiss band. A possible generation mechanism of these hiss-triggered chorus riser emissions is proposed.

Observation and modeling of quasi-periodic scintillations observed at low latitude

Quasi-periodic scintillations are characterized as primary deep fade-out infield strength, associated with regular ringing patterns before and after it. In this paper, observations of quasi-periodic scintillation using geostationary satellite (FLEETSAT) transmissions operating at frequency 250 MHz at low latitude ground station, Varanasi (geomag. lat 14° 55' N, long. 154°E) are reported. The results indicate that the quasi-periodic scintillations are most likely produced by plasma blobs/bubbles present in the E and F-region of the ionosphere. The various characteristics features of the quasi periodic scintillations are discussed after the autocorrelation, power spectrum and scintillation index analysis. The computed horizontal scale size of the quasi periodic scintillation producing irregularity varies from 100 m to 1300 m which shows that the irregularities are of intermediate-scale sizes. The spectral index obtained from the slopes of power spectrum varies from -2 to -8. The observed fading patterns, especially the modulation of the diffraction pattern (fading envelope) can be explained by considering an obstacle called radio lens in the ionosphere elongated in one direction. We have simulated successfully the amplitude versus time plot of quasi periodic scintillation patches and found that our theoretical and experimental results of quasi periodic scintillation patches compares well with each other and also with the earlier published works.

Application of lightning discharge generated radio atmospherics/tweeks in lower ionospheric plasma diagnostics

Lightning discharges during thunderstorm are the significant natural source of electromagnetic waves. They generate electromagnetic pulses, which vary from few Hz to tens of MHz, but the maximum radiated energy is confined in extremely low (ELF: 3-3000Hz) and very low (VLF: 3-30 KHz) frequency band. These pulsed signals with frequency dispersion are known as radio atmospherics or tweeks. These waves propagate through the process of multiple reflections in the earth-ionosphere waveguide over long distances with very low attenuation (2-3 dB/1000km). Since these waves are reflected by lower boundary of ionosphere, these are used extensively for probing the D-region ionosphere. D-region is important to the space weather, as well as the submarine communication and navigational aid. In this perspective the measurement of electron density profiles of the D-region is undoubtedly of great interest to both the development of reliable models and radio wave propagation. Earlier work on the tweeks is mainly focused to the theoretical considerations related to polarization, waveform analysis, and occurrence time and propagation mechanism. In this study we investigate tweeks to determine the equivalent night time electron densities at reflection height of the D-region. Distance traveled by the VLF waves from the causative lightning discharges to the receiving station has also been calculated. Tweeks recorded at a low latitude ground station of Allahabad (Geomag. lat. 16.050 N) during the night of 23 March 2007 have been used in the present analysis. Based on the analysis of the fundamental cut-off frequency of tweeks, the estimated equivalent electron density of the D-region has been found to be in the range of ~20 to 25 el/cm3 at ionospheric reflection height of ~80 to 95 km respectively. Propagation distance in Earth-Ionosphere wave guide (EIWG) from causative lightning source to experimental site varies from ~1500 to 8000 km.

A generation mechanism of chorus emissions using BWO theory

In this paper, discrete VLF chorus emissions recorded at low latitude ground station Jammu (geomag. Lat. = 22° 26/ N, L = 1.17) are reported and their characteristics based on complete spectral analysis have been carried out. These discrete chorus emissions are generated during a strong geomagnetic storm period of 2-7 May, 1998. We have computed the sweep rate, repetition period, source region, and drift rate of the individual chorus elements. It is observed that the sweep rate increases with time. To explain the various temporal and spectral features of these emissions, a possible generation mechanism has been presented based on the backward wave oscillator (BWO) regime in the magnetospheric cyclotron maser. On the basis of this model, we have computed some discrete chorus emission parameters as well as magnetospheric parameters relevant to the generation process. A comparison of the computed and observed magnetospheric parameters has been presented. These results show a good agreement with the BWO model.

Characteristics of discrete VLF falling-tone chorus emissions observed at low latitude ground station Jammu

Abstract The discrete VLF chorus emissions recorded at low latitude ground station Jammu (geomag. lat. = 22°26′N, L = 1.17) are reported and their characteristics based on spectral analysis have been carried out. These discrete chorus emissions are falling tone emissions which are rarely observed at low latitude ground stations. These discrete chorus emissions are generated during a strong geomagnetic storm period of 2–7 May, 1998. We have computed the frequency sweep rate, repetition period and source region of the individual chorus elements. It is observed that the sweep rate increases with time.

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.

Features of discrete VLF emissions observed at Gulmarg, India during the magnetic storm of 6–7 March, 1986

During the analysis of archived VLF data from Indian low latitude ground stations, some discrete VLF emissions recorded at the low latitude ground station Gulmarg (geomagnetic latitude 24°26′N; geomagnetic longitude 147°09′E, L = 1.28) during moderate magnetic storm activity (Σ K − P = 32, K P index varies from 4 to 6 during the observation period) on 6/7 March, 1986 are presented in this paper. The dynamic spectra of these discrete VLF emissions were observed along with tweeks and its harmonics, which is interesting and complex to explain. In most of the events the harmonic frequency of tweeks correlates with the starting frequency of harmonics of discrete emissions. In order to explain the observed features of discrete VLF emissions, we propose cyclotron resonance interaction between whistler mode wave and energetic electrons of inner radiation belt as possible generation mechanism. An attempt is also made to determine parallel energy, anisotropy and wave growth relevant to the generation process of VLF emissions.