Low-latitude Whistlers Research Articles

Rare observation of daytime whistlers at very low latitude (L = 1.08)

The source region and propagation mechanism of low latitude whistlers (Geomag. lat. <30°) have puzzled scientific community for last many decades. In view of recent reports, there is consensus on the source region of low latitude whistlers in the vicinity of the conjugate point. But the plausible conditions of ionospheric medium through which they travel are still uncertain. In addition to that, the whistlers in daytime are never observed at geomagnetic latitudes less than 20°. Here, for the first time, we present a rare observations of whistlers during sunlit hours from a very low-latitude station Allahabad (Geomag. Lat: 16.79°N, L=1.08) in India on 04 February 2011. More than 90 whistlers are recorded during 1200–1300 UT during which the whole propagation path from lightning source region to whistler observation site is under sunlit. The favorable factors that facilitated the whistlers prior to the sunset are investigated in terms of source lightning characteristics, geomagnetic and background ionospheric medium conditions. The whistler activity period was found to be geomagnetically quiet. However, a significant suppression in ionospheric total electron content (TEC) compared to its quiet day average is found. This shows that background ionospheric conditions may play a key role in low latitude whistler propagation. This study reveals that whistlers can occur under sunlit hours at latitudes as low as L=1.08 when the source lightning and ionospheric medium characteristics are optimally favorable.

First observations of low latitude whistlers using WHU ELF/VLF receiver system

The recently developed high-quality WHU ELF/VLF receiver system has been deployed in Suizhou, China (geomagnetic latitude 21.81°N, longitude 174.44°E, L=1.16) to detect low latitude extremely-low-frequency (ELF: 0.3‒3 kHz) and very-low-frequency (VLF: 3‒30 kHz) emissions originating from either natural or artificial sources since February 2016. During the first-month operation of the receiver system, a total of 3039 clear whistlers have been recorded at this low latitude station with the majority (97.0%) occurring on 28 February and 1 March 2016. Observed whistlers manifest various types including single one-hop, echo train, multi-flash, and multi-path. They tend to intensify after local midnight, reach the peak around 04‒05 LT, and then weaken quickly. Both features of lower cutoff frequencies of most whistlers below ∼1.6 kHz and almost uniform dispersion for many successive multi-flash whistlers suggest that these whistlers propagate along the geomagnetic field lines in the duct mode. The computed dispersion varies between ∼15 s1/2 and 23 s1/2 for observed one-hop whistlers and is greater than 50 s1/2 for three-hop echo train whistlers, indicating that the whistlers observed at the Suizhou station are low latitude whistlers.

Very Low Latitude Whistlers (L = 1.08):Arrival Azimuth Determination

Since last four decades and more generation and propagation mechanism of very low latitude (L < 1.4) whistlers has been studied by many workers in lowlatitude regions and especially in India. The key questions that remain unanswered include: (1) Where are the lightning discharges, the source of whistlers located? (2) Do the whistler waves at low latitudes propagate along the magnetic field lines in low latitude ionosphere? We reported that the lightning discharges that have generated whistlers are located in the conjugate region in the Indian Ocean and suggested the whistlers at these stations propagate along the magnetic field lines in the low latitude ionosphere. In this communication, we present the arrival azimuth determination technique adopted to confirm the location of the lightning discharges which generated the observed whistlers and the technique is validated with real-time lightning data. The technique adopted to determine the arrival azimuths of low latitude whistler causative lightning discharges is of significance and will help in resolving the unanswered questions of low latitude whistler phenomena.

Upper atmospheric remote sensing using ELF – VLF lightning generated tweek and whistler sferics

Tweek and whistler atmospherics (sferics) recorded at a low latitude station, Suva (18.2oS, 178.3oE, geomag. lat. 22.2oS, L = 1.17) Fiji, in the South Pacific region, during September 2003–July 2004 and March–December 2006 were analyzed to probe the nighttime D-region ionosphere and plasmasphere. From the first harmonic analysis of the selected 1063 tweeks, the path integrated ionospheric reflection height was estimated mostly between 75–97 km with median and standard deviation of 83.6 and 4.1 km, respectively. The path integrated mean electron density was estimated to vary from 24.4–31.5 cm–3 over the reflection heights. The whistler recorded on 20 September 2006, a magnetically quite day with maximum three hourly Kp value of 2o, most likely propagated in the ducted mode to this station. The whistler spectrum was used to calculate the equatorial magnetic field and the equatorial electron density at L = 1.3 which are in agreement with low latitude whistler observations.

Very low latitude (L = 1.08) whistlers and correlation with lightning activity

AbstractWe present analysis of more than 2000 lightning‐generated whistlers recorded at a low‐latitude station, located at Allahabad (geographic latitude, 25.40°N; geographic longitude, 81.93°E; L = 1.081), India, during December 2010 to November 2011. The main focus of this work is on the correlation between observed low‐latitude whistlers and lightning activity detected by the World‐Wide Lightning Location Network near the conjugate point (geography 9.87°S, 83.59°E) of Allahabad. Whistler occurrence is higher in the postmidnight period as compared to the premidnight period. Whistlers were observed in the daytime only on 2 days that too before 8:30 LT (morning). Seasonally, occurrence is maximum during winter months, which is due to more lightning activity in the conjugate region and favorable ionospheric conditions. About 63% of whistlers were correlated with lightning strokes in the vicinity of the conjugate point within spatial extent of 1000 km (conjugate area). Most (about 53%) whistlers were found to be associated with lightning strokes that were offset to the southeast of the conjugate point. The results indicate that an energy range of 7.5–17.5 kJ of lightning strokes generate most of whistlers at this station. The L shell calculations show that propagation paths of the observed whistlers were embedded in the topside ionosphere. Based on these results we suggest a possibility of ducted mode of propagation even for such very low latitude whistlers.

One-to-one relationship between low latitude whistlers and conjugate source lightning discharges and their propagation characteristics

One-to-one relation with its causative lightning discharges and propagation features of night-time whistlers recorded at low-latitude station, Allahabad (geomag. lat. 16.05°N, L=1.08), India, from continuous observations made during 1–7 April, 2011 have been studied. The whistler observations were made using the Automatic Whistler Detector (AWD) system and AWESOME VLF receiver. The causative lightning strikes of whistlers were checked in data provided by World-Wide Lightning Location Network (WWLLN). A total of 32 whistlers were observed out of which 23 were correlated with their causative lightnings in and around the conjugate location (geom. lat. 9.87°S) of Allahabad. A multi-flash whistler is also observed on 1 April with dispersions 15.3, 17.5 and 13.6s1/2. About 70% (23 out of 32) whistlers were correlated with the WWLLN detected causative lightnings in the conjugate region which supports the ducted mode of propagation at low latitude. The multi-flash and short whistlers also propagated most likely in the ducted mode to this station.

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.

Study of Duct Characteristics Deduced from Low Latitude Ground Observations of Day-Time Whistler at Jammu

Propagation characteristics of low latitude whistler duct characteristics have been investigated based on day-time measurements at Jammu. The morphogical characteristics of low latitude whistlers are discussed and compared with characteristics of middle and high latitude whistlers. The Max. electron density (Nm) at the height of the ionosphere obtained from whistler dispersion comes out to be higher than that of the background which is in accordance with the characteristics of whistler duct. The equivalent width is found to be close to the satellite observations and the characteristics of whistler duct in low latitude ionosphere are similar to those in middle and high latitude ionosphere. The width of ducts estimated from the diffuseness of the whistler track observed during magnetic storm is found to lie in the range of 50 - 200 Km.

Very low latitude (L = 1.08) whistlers

For decades, whistlers observed on the ground at mid and high latitudes have been used for diagnostics of Earth's plasmasphere. Whistlers have also been observed at low latitudes however, the propagation characteristics of low latitude whistlers are poorly understood thus they have not been used effectively as a diagnostic for the low latitude ionosphere. One key limitation with past studies has been lack of knowledge of the whistler source lightning location. Here we present the first cases of low latitude ground whistlers most likely linked with their causative lightning discharges in the conjugate zone. The Global Lightning Dataset 360 (GLD360) detected lightning discharges were found to be located close to the conjugate location of the recording stations, providing direct evidence of inter‐hemispheric propagation at the low latitudes. A total of 864 whistlers were observed at Allahabad, India (Geomag. lat. 16.05°N; Geomag. long. 155.34°E; L = 1.08) during the night of 26 January 2011. Using GLD360 network data, we show the occurrence of thunderstorm activity between 200 and 450 km from the conjugate point of Allahabad. We also report the distribution of peak currents of whistler‐producing lightning, which demonstrate a cutoff at 30 kA.

The Importance of Direction Finding Technique for the Study of VLF/ELF Sferics and Whistlers

The purpose of this paper is to review the previous direction finding (DF) systems including goniometer, field-analysis method and also we demonstrate how important these DF techniques are in the general studies of VLF/ELF sferics and whistlers. We have presented some latest findings such as new results on ELF transients (charge moment change) and convincing evidence on the ducted propagation of low-latitude whistlers.

Propagation modes of low- and very-low-latitude whistlers

The relative merits of ducted and non-ducted propagation of low-latitude whistlers are examined critically in the light of work done mostly in the Asian countries, India, China, and Japan since 1965. A growing consensus is found in favour of the non-ducted pro-longitudinal mode of propagation for nighttime whistlers, and the ducted mode of propagation in the presence of the equatorial anomaly for daytime whistlers. However, controversy still exists about the propagation mechanism of echo trains of whistlers observed at very low latitudes. An alternative mechanism is suggested involving a nighttime feature of the equatorial anomaly. Some recent unusual results are discussed in more detail: they include the observation of whistler triplets and the frequency and temporal fine structure of whistlers recorded for the first time in a low-latitude ground station. Lastly, further scope for work in the field is indicated.

Three‐dimensional ray‐tracing for very low latitude whistlers, taking into account the latitudinal and longitudinal gradients of the ionosphere

Propagation mechanism of very low latitude (geomagnetic latitude less than 20°) whistlers is poorly understood, and this paper examines the propagation characteristics of nonducted propagation at very low latitudes by using three‐dimensional ray‐tracing for realistic ionosphere/magnetosphere models with latitudinal and longitudinal gradients and International Geomagnetic Reference Field (IGRF) magnetic model. By assuming small possible tilts (in the latitudinal and longitudinal direction) of the initial wave normal angle in the input Southern Hemisphere as in our previous paper [Ohta et al., 1997], we have found that it is possible for us to detect, at a very low latitude position in the Northern Hemisphere, some whistler rays being able to penetrate through the ionosphere and some others that are subject to total reflection and come back to the magnetosphere. By making full use of systematic analyses of ionospheric parameters (local time (LT) dependence of latitudinal and longitudinal gradients), we conclude that the LT dependence of occurrence rate of whistlers at very low latitudes can be accounted for by means of such LT dependence of the gradient (especially latitudinal), lending to a further support to our previous nonducted propagation mechanism.

A case study of whistlers recorded at Varanasi (L = 1.07)

Large number of high dispersion whistlers recorded at low latitude station Varanasi (L = 1.07) are analysed and it is find out that they have propagated along L≃ 2.12–2.76. This is explained by considering the propagation of whistlers through the earth-ionosphere wave guide after exiting from the duct towards the equator. Using diffusive equilibrium model (DE-1), we have estimated equatorial electron density, total electron content in a flux tube and large scale convective electric fields which are in good agreement with the results reported by other workers from the analysis of mid latitude whistlers. The significance of this paper is to probe mid latitude plasmasphere using whistlers recorded at low latitudes. Further, an attempt has been made to study the propagation mechanism of low latitude whistlers.

Propagation characteristics of very low latitude whistlers by three-dimensional ray tracing

Propagation characteristics of very low latitude whistlers by three-dimensional ray tracing

Study of propagation characteristics of very low latitude whistlers by means of three‐dimensional ray‐tracing computations

The propagation mechanism of very low latitude (geomagnetic latitudes of less than 10–15°) whistlers is poorly understood. There is a controversy on their propagation; some workers using the observational facts have suggested field‐aligned propagation, but some theoretical (ray tracing) works have all indicated nonducted propagation. This paper reexamines the propagation characteristics of nonducted propagation, but we use three‐dimensional ray tracing (different from previous works) for realistic ionosphere/magnetosphere models (the electron density profile with latitudinal and longitudinal gradients and the International Geomagnetic Reference Field (IGRF) magnetic field model instead of the conventional dipole model). By assuming small possible tilts (in the latitudinal and longitudinal direction) of the initial wave normal angle in the input southern hemisphere, we have found that it is possible for us to detect simultaneously, at a very low latitude position in the northern ionosphere, one‐hop whistler rays started from slightly spaced locations in the south with different initial wave normal angles and that some of them can penetrate through the ionosphere, but some others cannot. On the basis of systematic analysis of important parameters, we come to the general conclusion that it is possible for us to find a closely spaced set of paths to reproduce the one‐hop and three‐hop whistlers in the north and to have the dispersion ratio of 1:3. The echo train whistlers, as were often observed by Hayakawa et al. [1990], are realized also by this nonducted propagation without any serious requirements.

Characteristics of whistler ducts recorded at Gulmarg

Morphological features of whistlers recorded at low latitude ground station Gulmarg (geomag. lat., 24‡ 26′N) are studied to deduce information about ducts. The morphological characteristics of low latitude whistlers are discussed and compared with the characteristics of middle and high latitude whistlers. The maximum electron density (N m ) at the height of the ionosphere obtained from whistler dispersion comes out to be higher than that of the background, which is in accordance with the characteristics of the whistler duct. The equivalent width of the whistler duct at the maximum height of its path is found to be close to the value obtained from satellite observations. The characteristics of whistler ducts in low latitude ionosphere are similar to those in middle and high latitude ionosphere. The width of ducts estimated from the diffuseness of the whistler track observed during magnetic storm is found to lie in the range of 50–200 km.

Recent findings on the propagation of low latitude whistlers

The purpose of this paper is to review our recent findings concerning the propagation of low latitude whistlers. The first subject is the simultaneous location of low-latitude whistlers and their causative sferics, which would be important for the study of the whole propagation mechanism of whistlers from the source to the receiver. The second point is the investigation of the subionospheric propagation of whistlers on the basis of the field-analysis direction finding. This would be useful for the study of the ionospheric transmission mechanism of downgoing whistlers and their coupling to Earth-ionosphere waveguide modes.

The effect of subionospheric propagation on whistlers as deduced from direction finding measurements

The characteristics of subionospheric propagation of magnetospheric whistlers have been investigated by means of both digital spectral analysis with high resolution and our field‐analysis direction finding measurement for whistlers observed in South China. It is found that very low latitude whistlers, propagated in the Earth‐ionosphere waveguide over a distance of the order of 1000km, exhibit very clear additional dispersion effects near the cutoff frequencies of the subionospheric 1st‐ and higher‐order modes. The additional traces are also found to be left‐handed polarized, and when the frequency approaches the cutoff frequencies, the wave polarization becomes very close to exactly left‐handed circular and the incident angle becomes nearly vertical. While, the whistler components at frequencies away from the cutoff frequencies are linearly polarized. Finally, we emphasize the important use of the subionospheric propagation characteristics in the study of the whistler duct structure, ionospheric transmission of a downgoing whistler and its excitation of subionospheric modes.

On the possible influence of seismic activity on the propagation of magnetospheric whistlers at low latitudes

The purpose of this paper is to examine the possible seismic influence on the propagation of magnetospheric whistlers at low latitudes on the basis of long-term data observed at Sugadaira (geomagnetic latitude 25°N) during 1970–1978 and at Sakushima (24°) during 1980–1982. It is found from statistical analysis of the Sugadaira data that anomalous whistlers whose dispersion value is greater than twice the typical value, exhibit a substantial increase in occurrence during seismic activity in the geographic longitude sector of 100°E–160°E, and this association becomes more enhanced when we take the occurrence of any whistlers (short, long and/or anomalous) during the summer time (June–August). Finally, with increasing anomaly (that is, anomalous dispersions and summertime), we have found an exceedingly high statistical correlation between the summertime anomalous whistlers at Sakushima and the seismic activity satisfying the above condition. We have studied whistler characteristics for all earthquakes. These studies have indicated that a considerable number of cases of earthquakes are of a type such that several earthquakes occur successively; and, correspondingly, we observed an enhanced occurrence of anomalous whistlers during such prolonged seismic activities. Several isolated and intense earthquakes are characterized by the appearance of anomalous whistlers prior to and after the earthquake. In conclusion, we claim that whistlers with anomalous dispersions are likely to be closely correlated with earthquakes occurring in the relevant longitude range, but we cannot make any definite statement on the use of low-latitude whistlers as precursors of earthquakes and further detailed studies are obviously required on this subject. Some possible mechanisms of the earthquake-associated effect causing whistler propagation are discussed.

Propagation characteristics of very low latitude whistlers by ray tracing

AbstractSimultaneous measurement made in South China in January 1988 has already indicated that the nighttime whistlers observed at very low latitudes (geomagnetic latitudes, 10° to 14°) are attributed to field‐aligned propagation. On the other hand, a few researchers have suggested, based on the ray‐tracing computations, that nighttime whistlers at very low latitudes are due to nonducted propagation.To solve the discrepancy between the authors' experimental results and the previous ray‐tracing studies, extensive two‐dimensional ray‐tracing calculations have been made for the nonducted propagation of whistlers in the inner plasmasphere in a wide frequency range (1 to 10 kHz). The conditions for nonducted whistlers to penetrate through the ionosphere onto the ground have been discussed to clarify the difference from the experimental results reported here.