Whistler Ducts Research Articles

Latitudinal dependence of static mesospheric E fields above thunderstorms

AbstractElectrostatic fields generated by thunderclouds can significantly heat and modify the lower ionospheric electrons at altitudes of 70–80 km. These fields can map to higher altitudes along the geomagnetic field lines and have been proposed as the mechanism for generation of whistler ducts. Previous 2‐D modeling of these fields have been limited to azimuthally symmetric cases which requires a vertical magnetic field. We have developed a 3‐D model of the electrostatic thundercloud fields which allows the consideration of effects of the geomagnetic field dip angle on the mapping of the fields to high altitudes. The results show stronger electric fields at altitudes of 70–110 km with an equatorward and eastward shift of tens of kilometers at lower geomagnetic latitudes. These stronger fields are mapped into the magnetosphere and may therefore be important for whistler duct generation. The fields also indicate a more significant contribution of the quiescent heating on VLF early/fast events.

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.

Contrasting the efficiency of radiation belt losses caused by ducted and nonducted whistler‐mode waves from ground‐based transmitters

It has long been recognized that whistler‐mode waves can be trapped in plasmaspheric whistler ducts which guide the waves. For nonguided cases these waves are said to be “nonducted”, which is dominant for L < 1.6. Wave‐particle interactions are affected by the wave being ducted or nonducted. In the field‐aligned ducted case, first‐order cyclotron resonance is dominant, whereas nonducted interactions open up a much wider range of energies through equatorial and off‐equatorial resonance. There is conflicting information as to whether the most significant particle loss processes are driven by ducted or nonducted waves. In this study we use loss cone observations from the DEMETER and POES low‐altitude satellites to focus on electron losses driven by powerful VLF communications transmitters. Both satellites confirm that there are well‐defined enhancements in the flux of electrons in the drift loss cone due to ducted transmissions from the powerful transmitter with call sign NWC. Typically, ∼80% of DEMETER nighttime orbits to the east of NWC show electron flux enhancements in the drift loss cone, spanning a L range consistent with first‐order cyclotron theory, and inconsistent with nonducted resonances. In contrast, ∼1% or less of nonducted transmissions originate from NPM‐generated electron flux enhancements. While the waves originating from these two transmitters have been predicted to lead to similar levels of pitch angle scattering, we find that the enhancements from NPM are at least 50 times smaller than those from NWC. This suggests that lower‐latitude, nonducted VLF waves are much less effective in driving radiation belt pitch angle scattering.

Reconsidering the effectiveness of quasi‐static thunderstorm electric fields for whistler duct formation

It has been suggested that E × B mixing of magnetospheric plasma can lead to geomagnetic field‐aligned ionization enhancements termed whistler ducts. DC electric fields from thunderstorms have been put forward as the source of the required radial electrostatic field. Recent experimental observations have indicated that quasi‐static radial thunderstorm electric fields are not responsible for whistler duct formation. This evidence appears to be in contradiction to the current theoretical calculations. In this paper we reconsider whistler duct formation through quasi‐static thunderstorm electric fields. Both the charge distributions and ionospheric profiles previously used are based on rather dated assumptions. We find that more realistic thunderstorm charge distributions in conjunction with the earlier ionospheric profiles produce high‐altitude electric fields that are of insufficient strength, given average thunderstorm effective charge, to create ducting within a reasonable time period. This is, however, not confirmed when the same charge distributions are examined using a more modern ionospheric profile based on international standard models for the ionosphere and neutral atmosphere. In this case some charge distributions will lead to realistic whistler duct creation. Our modeling suggests that quasi‐electrostatic radial fields from thunderstorms could drive whistler duct formation in some situations.

Correction to “Are whistler ducts created by thunderstorm electrostatic fields?” by C. J. Rodger et al.

Correction to “Are whistler ducts created by thunderstorm electrostatic fields?” by C. J. Rodger et al.

Investigating the possible association between thunderclouds and plasmaspheric ducts

It has been suggested that quasi‐electrostatic electric fields from thunderclouds might produce whistler ducts by driving plasma interchange of geomagnetic flux tubes. Using lightning activity as a proxy for the presence of electrified clouds, the relation between thunderclouds and duct number was investigated through the study of two periods of 10 days, that is from June 13–22, 1993, and August 6–15, 1993. Typical duct numbers observed using NAA transmissions (24.0 kHz, Cutler, Maine) received at Faraday during the nights of June and August 1993 were ∼ 7, while the maximum duct numbers were 9–12. Duct numbers dropped down to zero or near zero during the day. Times of increased duct number could not be clearly associated with increased thunderstorm activity in the duct footprint region. Duct numbers observed on the path from NLK (24.8 kHz, Seattle, Washington) to Dunedin, New Zealand, were also studied, where very low thunderstorm activity levels imply that thundercloud‐driven duct formation should not be significant. The typical nighttime peak was ∼ 5 at about 1000 UT (23 LT). The difference between duct numbers in areas with high thunderstorm activity and quiet areas might be thought to be due to thundercloud duct formation. However, the F region absorption on the NLK footprints at 1000UT (23 LT) in June was found to be equivalent to 0000 UT (20 LT) in June on the NAA duct footprints. The typical number of ducts observable on the NAA path at 0000 UT in June is ≈ 5, very similar to the number observed on NLK at 0930 UT. The results found in this study are consistent with the idea that ionospheric absorption is a significant influence on the number of ducts observed. It does not appear that quasi‐electronstatic fields from thunderclouds play a significant role in the formation of plasmaspheric whistler ducts. The similarity in quiet time midlatitude duct nuumbers at these two different locations suggest a globally consistent duct formation mechanism, such as the interchange of magnetospheric tubes of plasma induced by spatially varying electric fields in the E region.

Pearl-type micropulsations at mid-latitude; their relation to whistlers, solar and geomagnetic activity as well as ionospheric absorption

The occurrence of pearl-type (Pc 1) micropulsations recorded at the mid-latitude station Nagycenk (Hungary) during a half solar cycle showed a quite regular variation on this long time scale. Around solar activity maximum, the number of days with Pc 1 occurrence was rather low, while it began to increase during medium solar activity rising to a maximum around solar activity minimum. Pc 1 pulsations have been analyzed in relation to further parameters and on a shorter time scale, too. Based on data of 2 years with maximum Pc 1 occurrence (around solar activity minimum in 1985 and 1986), a seasonal variation was also found. Additionally, it was confirmed that pearl-type micropulsations might frequently occur, on and after days, with geomagnetic disturbances. At Nagycenk, the selected geomagnetic disturbances were generally associated with an increased ionospheric absorption of radio waves caused by enhanced ionization due to particle precipitation from the magnetosphere into the lower ionosphere. Whistler observations carried out at Panska Veš (a station in the Czech Republic) showed a significant whistler activity connected with these geomagnetic disturbances, however, no after-effect appeared in whistler activity. One of the main goals of the present study was to find a relationship between Pc 1 pulsations and whistlers. Results revealing an increased whistler activity associated with Pc 1 occurrences confirm our previous findings rather convincingly. The latter ones hinted at the probability that certain magnetospheric configurations, e.g. geomagnetic field line shells and whistler ducts are closely connected, as similar positions of the two structures were found within the magnetosphere when characteristics of Pc 3 pulsations and whistlers were analyzed.

A quantitative estimate of the ducted whistler power within the outer plasmasphere

From a statistical study of ducted whistlers observed at Halley, Antarctica, in 1996, which had propagated on paths in the range L=2.5–4.5, we report mean occurrence rates, numbers of components per whistler, intensities, etc. for night and day conditions and in different seasons at solar minimum. We found an average whistler rate of 5 min −1 and 3 components per whistler. Received whistler amplitudes were measured as a function of frequency and were in the range 2–40 fT, typically 10 fT at 4 kHz. Combining these results with a propagation model, we estimate mean whistler duct output powers to be around 1–10 mW, (≃0.1–1 mJ per whistler in the 3–5 kHz band). Inferred typical equatorial wave fields for ducted whistlers of 0.3 pT led to estimated radiation belt lifetimes for 1–100 keV electrons due to gyroresonance with ducted whistlers of 2×10 6 days. This compares with published lifetimes due to plasmaspheric hiss of order 10 5 days or less, and we conclude that, on average, lightning which enters and propagates in magnetospheric ducts, although known to cause pitch angle scattering and precipitation of trapped electrons, does not significantly affect the radiation belt fluxes in a statistical sense. We have compared our results with those from a similar study by Burgess and Inan (J. Geophys. Res. 98 (1993) 15,643–15,665). In a separate investigation of multi-component whistlers received in winter at quiet times, using the same methodology, we have found that the duct output power generally decreases with increasing L. This is consistent with previous theoretical work and parallels a similar experimental conclusion with respect to higher-frequency whistler-mode signals from VLF transmitters.

The formation of ionosphere–magnetosphere ducts over the seismic zone

A physical mechanism of whistler duct formation due to electric field growth in the ionosphere above the seismic zone is considered. The ducts are plasma irregularities, stretched along geomagnetic field, with the transverse spatial scale of the order of 10 km and with the relative plasma density variation over the background value of the order of 10%. It is shown that such structures are formed by the field-aligned currents as a result of the interaction between the ionospheric electric field and the horizontal irregularities of the ionospheric conductivity. The conductivity irregularities are excited by the instability of acoustic-gravity waves (AGW), developed in the ionosphere when the electric field exceeds a certain critical value. Since the ionosphere conductivity disturbances depend on the AGW parameters, the wave amplitude growth results in modulating the conductivity and forming the additional currents due to the ionospheric electric field. The Joule heat released by these currents amplifies the AGW amplitude and, hence, the magnitude of the conductivity variations, which resulted in a growth of the oscillations and in the exponential growth of the disturbances, associated with the acoustic-gravity wave.

Lightning induced enhancements of D-region ionisation and whistler ducts

A 3-D magnetospheric ray-tracing program has been constructed in which ray paths can be calculated for propagation in 3-D ducts having a Gaussian enhancement of electron density in both the geomagnetic meridian and geomagnetic longitude directions. Such calculations show a reduction in wave path excursions both in geomagnetic latitude (or L-value) and longitude as rays propagate upwards in a duct between low altitude (∼1000 km) and the equatorial plane. The change in wave energy flux between low altitude and the interaction region for wave–particle interactions and the cross- L extent of related precipitating electron fluxes can then be determined. The size of localised electron density enhancements in the D-region lightning induced enhancements (LIEs) which cause phase and amplitude perturbations on sub-ionosphere VLF propagation (Trimpi effect) can then also be estimated. LIE extents determined in this way appear to be generally about an order of magnitude smaller than estimates from Trimpi observations or low altitude satellite observations of LEP (lightning-induced electron precipitation). This discrepancy has been investigated by ray-tracing in a variety of 3D duct models, including ducts which vary in width and/or enhancement along their length. Calculations made for ducts with maximum enhancement and width in Δ L-value constant along their length imply that precipitation regions and LIEs would typically have smaller dimensions in L-value than the low altitude whistler duct widths measured by the spatial extent of the whistler-mode waves. Therefore, to account for this discrepancy in duct width, ray paths were also determined for ducts which vary in enhancement and/or width in Δ L-value along their length. Although a little better fit with experimental observations is obtained for such ducts, the results can still not explain significantly wider ducts in L-value in the interaction region than at low altitudes. Thus other possible explanations are considered; in particular that the non-linear resonant current in the interaction region near the equatorial plane will radiate wave power into modes with larger wavenormal angles to the geomagnetic field direction than the original trapped modes, and this will then result in a larger region from which precipitating electrons arise and hence larger LIEs than the actual duct width in the interaction region.

Are whistler ducts created by thunderstorm electrostatic fields?

Park and Helliwell [1971] suggested that whistler ducts might be caused by plasma interchange of geomagnetic flux tubes driven by thundercloud electrostatic fields. At present, this is the most favored whistler duct creation mechanism. Theoretical formulations have been developed to calculate high‐altitude electric fields due to electrified clouds, taking into account the presence of the ionosphere. Previous studies have calculated the values of these fields, making use of approximated conductivity profiles. Calculations are undertaken using more accurate representations of more modern conductivity profiles than those used previouly. Preference is given to experimental rather than theoretically derived conductivity profiles. These calculations indicate that under “typical” conductivity conditions the high‐altitude electric fields from even giant thunderclouds are too small to create a realistic whistler duct in a realistic period. This is the case for both day and night conditions.

Testing the formulation of Park and Dejnakarintra to calculate thunderstorm dc electric fields

Park and Dejnakarintra [1973] presented a formulation to calculate high‐altitude electric fields due to electrified clouds, taking into account the presence of the ionosphere. Calculation of these fields is necessary to test the hypothesis that whistler ducts might be caused by plasma interchange of geomagnetic flux tubes driven by thundercloud electrostatic fields. To test the accuracy of the formulation, the calculated dc electric fields are compared with those measured during a thunderstorm campaign reported by Holzworth et al. [1985]. Using a conductivity profile measured during their campaign, it is shown that the formulation of Park and Dejnakarintra produces electric field predictions which are consistent with the measurements of Holzworth et al. This indicates the formulation developed by Park and Dejnakarintra is valid. The comparisons agree with measurements which show that large negative screening charges commonly exist around the top of thunderclouds, reducing the high altitude fields. These calculations indicate that the effective thunderstorm charge used in such calculations needs to be significantly reduced to account for screening charges. Such reductions lead to serious increases (up to a factor of 10) in the time required for high‐altitude thunderstorm electric fields to produce whistler ducts.

Whistler ducts and geomagnetic pulsation resonant field line shells near L = 2—are they identical?

Abstract Data from the processing of about 1700 whistlers recorded in Tihany ( L -value of propagation, equatorial electron density and tube content) are compared with geomagnetic pulsation characteristics (mainly of periods) as recorded at the nearby Nagycenk Observatory ( L ∼2). In addition, the number of whistlers recorded at Panska Ves (Czech Republic) for more than ten years (1971–1979, 1987, 1990) are compared with the Nagycenk pulsation activity. Both results can be reconciled with the supposition that whistler ducts and geomagnetic field line shells are closely connected with each other as they appear simultaneously with enhanced probability, and the position of these structures is similar within the magnetosphere.

Two and three component direction-finding computations on whistlers using the Matched Filtering and Parameter Estimation method

A detailed frequency analysis of the azimuthal bearing of ground-observed whistlers by the modified MFPE method (Lichtenberger et al., 1987) based on two and three component digital recordings is presented. Theoretical model calculations by Strangeways (1980) and Strangeways and Rycroft (1980) on systematic azimuthal bearing errors of direction-finding systems are confirmed. Further, directionfinding systems (based on three measured components that do not suffer from polarization error) have no advantage in practice, and are inferior to simple crossed-loop goniometer systems. The MFPE method simulating the latter can be used as a tool for investigating whistler ducts and hence plasmaspheric drifts. An example of this is presented.

Probing properties of the magnetospheric hot plasma distribution by whistler mode wave injection at multiple frequencies: Evidence of spatial as well as temporal wave growth

This is the second of two papers on the use of whistler mode wave injection to investigate properties of the magnetospheric hot plasma. Paper 1, [Sonwalkar et al., this issue] emphasized the use of signals at a single frequency to identify longitudinal structures ranging from 100 to 25,000 km in extent in ∼1–10 keV electrons drifting azimuthally through whistler ducts. This short paper discusses and illustrates the use of wave injection at multiple discrete frequencies to study temporal changes in magnetospheric hot electrons with parallel (gyroresonant) velocities in various nonoverlapping ranges. As in paper 1, the data studied were acquired during a special 9‐hour period of 1.9 – 2.9 kHz VLF transmissions from Siple Station, Antarctica, to Lake Mistissini, Canada, on January 23–24, 1988. The amplitudes of the leading edges of constant frequency pulses at 1900, 2150, and 2400 Hz were found to vary independently with time. This is interpreted as evidence of a spatial amplification process that accompanied the well known and more readily identifiable phenomena of exponential temporal growth to a saturation level. Evidence of wave‐hot plasma interactions showed a dependence on df/dt of the input signal frequency versus time format; in general, the slow frequency ramps showed the highest amplitudes and the fast ramps and parabolas the lowest, in agreement with past work.

Characteristics of mid‐latitude whistler ducts as deduced from ground‐based measurements

Propagation characteristics of mid‐latitude whistlers, especially whistler duct characteristics, have been investigated based on measurements in August, 1994 at Dunedin, New Zealand(L=2.78) and in August, 1989 at Ceduna, Australia(L=1.93), both during local midnight. Polarization analyses have enabled us to locate whistlers which exited the ionosphere just above the observing station (LDF is defined in this way). The nose extension method was also applied to these whistlers (their Ln is estimated by this method). The following findings have emerged from the analyses; LDF ≃ Ln (at Dunedin) and LDF ≃ Ln‐0.43(at Ceduna). Our analysis suggests that mid‐latitude ducts are likely to extend down to the ionosphere at L ≈ 2.8. Ray tracing studies for realistic density profiles indicates that a whistler duct terminates at an altitude of about 3,500 ∼ 5,500 km at an L value of ∼1.9 with its enhancement factor being a few percent at least. These results may imply a strong variability of the latitudinal or temporal variations of mid‐latitude whistler ducts.

Doppler shift pulsations on whistler mode signals from a VLF transmitter

Whistler mode signals from the NAA transmitter (24 kHz) received at Faraday, Antarctica are processed to obtain the Doppler shift at a much higher time resolution than has previously been possible. This has allowed the observation of pulsations of about 13 mHz frequency which are believed to be associated with hydromagnetic waves in the magnetosphere. The pulsations are observed separately on signals with a number of discrete group delay features that can be interpreted as individual whistler ducts. Using the measured pulsation phase over the array of ducts the phase velocity and wave normal direction of the hydromagnetic wave in the equatorial plane are estimated. The direction of propagation is consistent with a source on the dayside magnetopause. The association between whistler mode Doppler shifts and hydromagnetic waves has been reported before but not, as far as we are aware, using an experimental technique that allows measurements on individual ducts in order to determine the direction of propagation of the hydromagnetic wave.

Are VLF rapid onset, rapid decay perturbations produced by scattering off sprite plasma?

Rapid onset, rapid decay perturbations (RORDs) of subionospheric VLF propagation require highly localized or laterally structured plasma at low altitudes to explain the wide angle scattering observed and the rapid decay. Simultaneous occurrence of RORDs and red sprites, illustrated by a single event here, together with VLF phase and group delay measurements from a pair of spaced receivers suggest that RORDs are produced by scattering from conducting columns at the position and with the lateral shape of the sprite. The sprite luminosity decays much faster than the RORDs which depend on the sprite conductivity and so plasma density. Plasma is also produced near the sprite plasma by energetic electrons precipitated from the magnetosphere by ducted whistlers and after the expected whistler and electron propagation delay. This whistler‐induced electron precipitation (WEP) plasma produces wide angle VLF scattering similar to that by sprite plasma, implying similar lateral fine structure. This suggests that the processes leading to sprites also produce whistler ducts in the magnetosphere.

Characteristics of localized ionospheric disturbances inferred from VLF measurements at two closely spaced receivers

The very low frequency (VLF) NPM signal from Hawaii was recorded at two closely spaced (∼50 km) receivers (located at Palmer and Faraday stations) on the Antarctic Peninsula. Measurements of characteristic amplitude and phase signatures of lightning‐induced electron precipitation (LEP) events were made on three different days in March 1992. Both amplitude and, for the first time, phase measurements are quantitatively interpreted using a three‐dimensional model of VLF propagation in the Earth‐ionosphere waveguide in the presence of lower ionospheric disturbances. This is the first time such a study has been undertaken with mirrored precipitation. Differences between the amplitude and phase changes at the two sites are accounted for by the location of the LEP ionospheric disturbance transverse to the VLF signal propagation paths. The change in these differences is explained by the horizontal movement of the disturbance region and, therefore, the causative whistler duct footprint across the transmitter‐receiver paths. Trends in the amplitude and phase changes on a timescale of order 1 hour are found to be encompassed by the modeling of the passage of the day‐night terminator along the paths.