VLF Reflection Height Research Articles

Probing the lower ionospheric electron density modulation by the 2019 solar eclipse with VLF phase studies and ion-chemistry model

During a solar eclipse, the temporary blockage of daytime solar illumination resembles a nighttime situation up to a certain degree. This effectively modulates the lower ionosphere by reducing the ionization rate and hence electron–ion density and the VLF signal responds accordingly. This study investigates the lower ionospheric response to the Annular Solar Eclipse of December 26, 2019 (ASE19) by analyzing VLF phase modulation as observed from four different locations in India during the campaign organized by the Indian Centre for Space Physics (ICSP), Kolkata. The eclipse path crossed the four signal paths between NWC (19.8 kHz) transmitter and the receivers from time to time and produced significant deviations in the detected signal amplitudes and phases. We adopt the analysis of the phase delay mechanism to obtain the perturbed electron density fluctuations over the paths and compute changes in VLF reflection heights during the eclipse. The electron density modulation during the eclipse with an in situ theoretical chemical model using the solar obscuration at five different points over all the signal paths are also calculated and compared with the results from VLF. Both the temporal VLF responses at the receiving points and the changes in derived electron densities are found to exactly follow the obscuration profile at the point of maximum obscuration for all the paths. This article, for the first time successfully compares two distinct independent models related to lower ionosphere during a solar eclipse and demonstrates that though we lack the necessary means to directly explore the lower ionospheric dynamics, our theories are more than adequate and roughly reflect the actual scenario.

Low‐Latitude VLF Radio Signal Disturbances Due to the Extremely Severe Cyclone Fani of May 2019 and Associated Mesospheric Response

AbstractWe present new results of lower ionospheric disturbances due to the extremely severe cyclonic storm Fani over northeastern part of the Indian Ocean. Very low frequency radio signal received at Kolkata, India, and mesospheric temperature and ozone concentration data from the NASA's TIMED satellite are used for this purpose. Significant wave‐like oscillations and strong amplitude anomalies in daytime and nighttime VLF signal were observed when the cyclone center was within ∼700 km from the receiver. Both the mesospheric ozone concentration and temperature showed maximum anomalies beyond 3σ during the cyclone period. Maximum ozone anomaly and maximum VLF anomaly occurred on the same day when cyclone intensity was maximum, while the maximum temperature anomaly occurred on the next day during landfall. Mesospheric temperature enhancement around VLF reflection heights indicates changes in the chemical composition and electron‐neutral balance in the D region ionosphere. Simulation of VLF signal showed that the D region reflection height decreased by 7.9 km and the D region electron density increased by ∼20 times compared to precyclone midnight values at the maximum perturbed area along the propagation path. Wavelet analysis confirmed significant enhancement of atmospheric gravity waves spectrum with periods 10 min to 2 hr around landfall. Further, a strong anticorrelation between total wavelet power in the wave band of periods 10–30 min and cyclone pressure suggests a possibility of monitoring cyclone intensity from mesospheric gravity waves using VLF radio measurements.

VLF modal interference distance and nighttime D region VLF reflection height for west‐east and east‐west propagation paths to Fiji

AbstractVery low frequency (VLF) signals from navigational transmitters propagate through the Earth‐ionosphere waveguide formed by the Earth and the lower conducting ionosphere and show the pronounced minima during solar terminator transition between transmitter and receiver. Pronounced amplitude minima observed on 19.8 kHz (NWC transmitter) and 24.8 kHz (NLK transmitter) signals recorded at Suva (18.149°S, 178.446°E), Fiji, during 2013–2014, have been used to estimate the VLF modal interference distance (DMS) and nighttime D region VLF reflection height (hN). The NWC transmitter signal propagates mostly in west‐east direction, and the NLK transmitter follows a transequatorial path propagating significantly in the east‐west direction. The values of DMS calculated using midpath terminator speed are 2103 ± 172 km and 2507 ± 373 km for these paths having west‐east and east‐west components of VLF subionospheric propagation, respectively, which agree with previously published results and within 10% with theoretical values. We have also compared the DMS estimated using a terminator time method with that calculated using terminator speed for a particular day and found both the values to be consistent. The hN values were found to be maximum during winter of Southern Hemisphere for NWC signal and winter of Northern Hemisphere for NLK signal VLF propagation paths to Suva. The hN also shows significant day‐to‐day and seasonal variabilities with a maximum of about 10 km and 23 km for NWC and NLK signal propagation paths, respectively, which could be due to the atmospheric gravity waves associated with solar terminator transition, as well as meteorological factors such as strong lightnings.

The leading role of atomic oxygen in the collocation of elves and hydroxyl nightglow in the low‐latitude mesosphere

AbstractThe elve is the dominant type of transient luminous event (TLE) at the base of the E region ionosphere. The hydroxyl nightglow (OH* nightglow hereafter) has been reported at a similar altitude. Statistical studies show that the mean heights of elve and OH* nightglow are 87 km and 87.5 km, respectively, and that 91% of the 291 limb elves are located within ±5 km of the altitude of the brightest OH* nightglow emission and both elves and OH* nightglow show the semiannual oscillation (SAO) at low latitude. The physical reasons for the collocation of elves and OH* nightglow are the main focus of attention. A model for elve emission with the environmental‐adapted electron density profile is tested with three density profiles of atomic oxygen (O): the original profile from the NRLMSISE‐00 model and two other profiles which are shifted 5 km upward/downward from the original. For higher altitudes of the given O density profile, the peak altitudes of the elve emission layers are also increased. This result reveals the leading role of atomic oxygen in the collocation. Furthermore, the altitude variation of elves is compared with the VLF reflection height observed by DEMETER. The latter height not only shows the SAO similarity to the elve/OH* heights but also exhibits a difference between land and ocean. The possible relationship between VLF reflection height and elve/OH* height is also discussed.

A relationship between solar proton events, ionospheric uplift observed at VLF and negative ionospheric storms

Three consecutive magnetic storms during the month of September 1982 were found to be associated with solar proton events (SPE) observed over a number of high latitude VLF propagation paths. The penetration of solar protons into the auroral zone produced a marked reduction in reflection height at night for high latitude VLF paths resulting in a reduced diurnal phase shift. This effect has been known for some 50 years. However in this paper, a previously unidentified response is described consisting of an increase in the night 90km reflection height over middle latitude and transequatorial VLF paths. Solar protons do not penetrate to these latitudes and this slight increase in VLF reflection height was associated with typical negative ionospheric storm effects in the F2 region. Dynamics at the 90km base of the night ionosphere are little known and difficult to investigate except at VLF. These results are the first to suggest a response of the night ionospheric base to events leading to the well known negative ionospheric storm seen at greater heights. Such negative storms seen in the F2 region have been associated with an equatorward wind surge and change in neutral atmospheric chemistry driven by joule heating in the auroral zone produced by solar proton precipitation.

Long‐lastingD‐region ionospheric modifications, caused by intense lightning in association with elve and sprite pairs

Observations show that intense +CG lightning discharges which trigger both an elve and a sprite are associated with long‐lasting conductivity modifications in the upperD‐region ionosphere. They are observed as strong perturbations in VLF signals propagating through the disturbed region, manifested asLOng Recovery Early VLF events (LORE), which can last up to 30 minutes. These same ionospheric modifications are also responsible for step‐like changes, seen mostly in off‐storm VLF transmissions, which offset signal levels even for longer times. The evidence suggests that when a very intense positive cloud to ground lightning stroke leads to an elve and a high altitude sprite, and possibly a sprite halo as well, there is production of long lasting elevations in electron density at VLF reflection heights that cause LOREs and severe effects on VLF propagation. The present results confirm past predictions and postulations that elves may be accompanied by long‐lasting electron density perturbations in the lower ionosphere.

Response of low latitude D-region ionosphere to the total solar eclipse of 22 July 2009 deduced from ELF/VLF analysis

Response of the D-region of the ionosphere to the total solar eclipse of 22 July 2009 at low latitude, Varanasi (Geog. lat., 25.27° N; Geog. long., 82.98° E; Geomag. lat.=14° 55’ N) was investigated using ELF/VLF radio signal. Tweeks, a naturally occurring VLF signal and radio signals from various VLF navigational transmitters are first time used simultaneously to study the effect of total solar eclipse (TSE). Tweeks occurrence is a nighttime phenomena but the obscuration of solar disc during TSE in early morning leads to tweek occurrence. The changes in D-region ionospheric VLF reflection heights (h) and electron density (ne: 22.6–24.6cm−3) during eclipse have been estimated from tweek analysis. The reflection height increased from ∼89km from the first occurrence of tweek to about ∼93km at the totality and then decreased to ∼88km at the end of the eclipse, suggesting significant increase in tweek reflection height of about 5.5km during the eclipse. The reflection heights at the time of totality during TSE are found to be less by 2–3km as compared to the usual nighttime tweek reflection heights. This is due to partial nighttime condition created by TSE. A significant increase of 3dB in the strength of the amplitude of VLF signal of 22.2kHz transmitted from JJI-Japan is observed around the time of the total solar eclipse (TSE) as compared to a normal day. The modeled electron density height profile of the lower ionosphere depicts linear variation in the electron density with respect to solar radiation as observed by tweek analysis also. These low latitude ionospheric perturbations on the eclipse day are discussed and compared with other normal days.

D-region ionosphere response to the total solar eclipse of 22 July 2009 deduced from ELF-VLF tweek observations in the Indian sector

[1] Observations of tweeks with higher harmonics (n > 1) at low latitude stations Allahabad and Nainital, in the Indian sector, during the total solar eclipse on 22 July 2009, are presented. Allahabad and Nainital stations were in 100% and 85% of the totality paths. Observations suggest that about 30–40% obscuration of solar disc can lead to the tweeks occurrence which otherwise occur only in nighttime. A total of 148 tweeks at Allahabad and 20 tweeks at Nainital were recorded with some of them up to 3rd harmonics. The World Wide Lightning Location Network data indicated that tweeks observed were generated by lightning's located in the partial eclipse area of Asia-Oceania region. The changes in D-region ionospheric VLF reflection height and electron density (∼22–23 cm−3) during eclipse have been estimated from the first cut-off frequency of the tweeks. The reflection height increased from ∼89 km from the first occurrence of tweek to about 91–92 km at the totality and then decreased to ∼87 km at the end of the eclipse, suggesting a change of about 5 km in the reflection height during eclipse. The reflection heights are lower by 2–3 km as compared to normal nighttime tweek reflection heights. The above increase in the reflection height indicate that the partial nighttime condition is created during eclipse, as the main D-region ionizing radiation Lyman α is blocked but solar soft X-ray and EUV radiations originating from the limb solar corona are not totally blocked which produce some of ionization in the D-region.

Higher harmonic tweek sferics observed at low latitude: estimation of VLF reflection heights and tweek propagation distance

Abstract. Lightning generated signals recorded at a low-latitude station, Suva (18.2° S, 178.3° E) Fiji, in the South Pacific region, during September 2003–July 2004, are used to study the propagation features and the reflection heights of tweek atmospherics in the waveguide formed by the Earth's surface and the lower ionosphere. Tweeks are observed only during the local night and the maximum harmonic (n) recorded is six. The occurrence of tweeks with higher n progressively decreases as n increases. The dispersed part of tweeks decreases as n increases. The attenuation factor has been calculated for tweeks with n=1–3. The ionospheric reflection heights obtained assuming the transverse magnetic mode of propagation for tweek signals vary from 83–92 km. A higher harmonic of the same tweek is reflected from about 2.0 km higher than the lower harmonic. For 90% of tweeks, propagation distances are estimated to be between 1000–5000 km. Tweeks with lower n propagate longer distances than the tweeks with higher n.

“Early/slow” events: A new category of VLF perturbations observed in relation with sprites

Analysis of subionospheric VLF transmissions, observed in relation with sprites, has led to the identification of a new category of VLF perturbations caused by the direct effects of tropospheric lightning on the overlying lower ionosphere. They constitute a large subset of the so‐called “early/fast” events where now the term “fast,” which implies rapid onset durations less than ∼20 ms, does not apply. In contrast with early/fast, the perturbations have a gradual growth and thus “slow” onset durations ranging from about 0.5 to 2.5 s; thus these events are labeled herein as “early/slow.” They are indicative of a new physical process at work which, following a sprite‐causative cloud‐to‐ground discharge, leads to a gradual buildup of conductivity changes in the lower ionosphere which must be responsible for the long onset durations of the observed perturbations. Analysis of broadband VLF sferic recordings, made with a two‐channel receiver near the sprite producing storms, shows that the growth phase of an early/slow event coincides with the occurrence of complex and dynamic lightning action. This is composed of a few sequential cloud‐to‐ground lightning strokes and clusters (bursts) of sferics which are attributable to intracloud lightning. We postulate that the long onset durations are due to secondary ionization buildup in the upper D region below the nighttime VLF reflection heights, caused mainly by the impact on sprite‐produced electrons of sequential electromagnetic pulses radiated upward from horizontal in‐cloud discharges.

Large solar flares and their ionospheric D region enhancements

On 4 November 2003, the largest solar flare ever recorded saturated the GOES satellite X‐ray detectors, making an assessment of its size difficult. However, VLF radio phase advances effectively recorded the lowering of the VLF reflection height and hence the lowest edge of the Earth's ionosphere. Previously, these phase advances were used to extrapolate the GOES 0.1–0.8 nm (“XL”) fluxes from saturation at X17 to give a peak magnitude of X45 ± 5 for this great flare. Here it is shown that a similar extrapolation, but using the other GOES X‐ray band, 0.05–0.4 nm (“XS”), is also consistent with a magnitude of X45. Also reported here are VLF phase measurements from two paths near dawn: “Omega Australia” to Dunedin, New Zealand (only just all sunlit) and NPM, Hawaii, to Ny Alesund, Svalbard (only partly sunlit), which also give remarkably good extrapolations of the flare flux, suggesting that VLF paths monitoring flares do not necessarily need to be in full daylight. D region electron densities are modeled as functions of X‐ray flux up to the level of the great X45 flare by using flare‐induced VLF amplitudes together with the VLF phase changes. During this great flare, the “Wait” reflection height, H′, was found to have been lowered to ∼53 km or ∼17 km below the normal midday value of ∼70 km. Finally, XL/XS ratios are examined during some large flares, including the great flare. Plots of such ratios against XL can give quite good estimates of the great flare's size (X45) but without use of VLF measurements.

Experimental evidence for solar cycle and long-term change in the low-latitude MLT region

There is a great deal of interest in long-term change in the upper atmosphere and its possible relationship with human activities, but it is clear that we do not yet have a consistent picture of solar cycle and long-term trends in the MLT region. The long series of rocket measurements made from the early 1960s through to the mid-1990s, over a wide range of latitudes, have been interpreted by some as showing strong long-term trends, but there are considerable doubts as to the reliability of these measurements for heights above about 50 km. Measurements made by various other techniques, such as airglow rotational temperature, VLF reflection height, and satellite-borne radiometers, provide conflicting results. At low latitudes, published results from satellite and sodium lidar measurements indicate negligible trends, but new results from OH rotational temperature measurements, made a 23 ∘ S , show consistent solar cycle variations in very close synchronization with the F10.7 flux, although with a linear trend much larger than expected.

Precursory effects in the subionospheric VLF signals for the Kobe earthquake

The subionospheric VLF Omega signal transmitted from Tsushima, Japan (geographic coordinates: 34°37′N, 129°27′E) was continuously received at Inubo (35°42′N, 140°52′E). This data was analyzed during an 8-month period centered on the great Hyogo-ken Nambu (Kobe) earthquake, M g=7.2 on 17 January 1995, the epicenter located inside of Fresnel zone of the VLF path. To clarify the possible effect, we developed the special TT (terminator time) method of data processing, which was useful for our short VLF path (distance ∼1000 km). We discovered a statistically significant change of TT characteristics, which began a few days before the main shock and probably continued a few weeks after it as a transient oscillation with period ∼10 days. By simple modelling, it was shown that TT changes could be caused by the decrease of the VLF reflection height by ∼2 km. The possible underlying mechanisms of the effect are not defined; however, an increase of the regular electric field due to radon exhalation before the earthquake or an intensification of planetary waves by seismically influenced atmospheric turbulence might be considered.

Quasiperiodic ∼5–60 s fluctuations of VLF signals propagating in the Earth‐ionosphere waveguide: A result of pulsating auroral particle precipitation?

Subionospheric very low frequency and low‐frequency (VLF/LF) transmitter signals received at middle‐latitude ground stations at nighttime were found to exhibit pulsating behavior with periods that were typically in the ∼5–60 s range but sometimes reached ∼100 s. The amplitude versus time shape of the pulsations was often triangular or zigzag‐like, hence the term “zigzag effect.” Variations in the envelope shape were usually in the direction of faster development than recovery. Episodes of zigzag activity at Siple, Antarctica (L ∼4.3), and Saskatoon, Canada (L ∼4.2), were found to occur widely during the predawn hours and were not observed during geomagnetically quiet periods. The fluctuations appeared to be caused by ionospheric perturbations at the ∼ 85 km nighttime VLF reflection height in regions poleward of the plasmapause. We infer that in the case of the Saskatoon and Siple data, the perturbations were centered within ∼500 km of the stations and within ∼ 100–200 km of the affected signal paths. Their horizontal extent is inferred to have been in the range ∼50–200 km. The assembled evidence, supported by Corcuffs [1996] recent research at Kerguelen (L ∼3.7), suggests that the underlying cause of the effect was pulsating auroral precipitation. The means by which that precipitation produces ionospheric perturbations at 85 km is not yet clear. Candidate mechanisms include (1) acoustic waves that propagate downward from precipitation regions above the ∼ 85 km VLF reflection level; (2) quasi‐static perturbation electric fields that give rise to E×B drifts of the bottomside ionosphere; (3) secondary ionization production and subsequent decay at or below 85 km. Those zigzag fluctuations exhibiting notably faster development than recovery probably originated in secondary ionization produced near 85 km by the more energetic (E >40 keV) electrons in the incident electron spectrum.

Anomalies in the Sub-ionospheric VLF Signals for the 1995 Hyogo-ken Nanbu Earthquake.

The sub-ionospheric VLF Omega signal transmitted from Tsushima, Japan (geographic coordinates: 34°37'N, 129°27'E) is continuously received at Inubo (35°42'N, 140°52'E). The signal's propagation characteristics (especially phase) exhibited abnormal behavior (especially around the sunrise and sunset local times) a few days before the main shock of the 1995 Hyogo-ken Nanbu earthquake. We found from the computer simulation of VLF signal propagation that the observed effect can be explained by a decrease of about 0.7 km of VLF reflection height at the atmosphere-ionosphere boundary.

First observations of Trimpi Events at Durban (L = 1.69) using an OMSKI Receiver

We report observations of transient ionospheric disturbances at VLF reflection heights, manifested as perturbations on sub‐ionospherically propagating VLF transmitter signals: “Trimpi events.” Four VLF transmitter signals were monitored at Durban using an OMSKI receiver. Both traditional Trimpi events and a new “dome” shaped event were observed at Durban. Occurrence of Trimpis at low L values can be interpreted as the precipitation of very high energy electrons which are capable of producing events during daylight, or as evidence of the “fast” Trimpi mechanism which could dominate at low L. The average winter event rate observed at Durban was quite low (7 per day on four transmitter signals) but during one very active period on the NAA‐Durban signal 88 Trimpi events were observed in 2 hours. This activity was linked to an isolated thunderstorm occurring over the signal path over South Africa.

Model predictions of subionospheric VLF signal perturbations resulting from localized, electron precipitation‐induced ionization enhancement regions

Whistler‐induced precipitation of energetic electrons produces transient ionospheric conductivity variations that perturb the amplitude and phase of VLF signals propagating in the earth‐ionosphere waveguide (i.e., Trimpi events). This study uses a waveguide mode theoretic propagation model to predict the effect of localized ionization enhancement (IE) regions on nighttime long‐path VLF signals from the U.S. Navy navigational transmitters NSS, NAA, NLK and NPM to Antarctic receiving stations, principally the U.S. station at Palmer. These predictions are then compared with the observed signal behavior in Trimpi events from which inferences can be drawn about an IE region's approximate location, size, and particle characteristics. The propagation model considers multiple modes, variable ground conductivity, and mode conversion. IE regions of variable length (typically 50–150 km) and distance from the receiver (up to 600 km) were considered. The conductivity change within an IE region was modeled by exponential electron density profiles modified by precipitated monoenergetic fluxes of 50‐ to 150‐keV electrons. Our calculations show that some signal paths are more sensitive than others to perturbation by IE regions and that the size of an IE region is less important than its distance from the receiver. For a specified flux, higher‐energy electrons produce larger effects than lower energy electrons; for a specified energy, larger fluxes will produce larger signal changes. For the particular case of 150‐keV electrons precipitating into an ionosphere with VLF reflection height of 87 km, the received signal was largely insensitive to increasing the electron flux above ∼10−5 erg cm−2 s−1. This flux is equivalent to ∼3×10−3 erg cm−2 s−1 for E > 40 keV and an exponential energy spectrum with e‐folding energy in the range 40–100 keV. The measured electron fluxes precipitated by lightning are of the same order. Higher saturation flux levels would apply in situations where the normal VLF reflection height was lower over that portion of a path where the whistler‐induced precipitation was occurring. This study also examines the effects of transmitter‐induced electron precipitation on the VLF signals. Such precipitation is known to occur and, as shown here, may on occasion have important effects on certain signals.

D-region characteristics deduced from pulsed ionospheric heating under auroral electrojet conditions

Characteristic times for conductivity changes in the auroral D-region caused by HF heating are deduced from the VLF/ELF signals recorded on the ground underneath the heated region. The VLF/ELF signals are caused by a heating-induced redistribution of the horizontal ionospheric currents when d.c. electric fields exist in the ionosphere. Measurements were made in the late afternoon with a dark, moderately disturbed ionosphere. The time constant for the Hall conductivity to change due to heating with 240 MW of effective radiated power is approximately 70 μs, while the time constant for cooling is approximately 120 μs. These results agree with model calculations. Delay times of the VLF/ELF pulses and their subsequent echoes result in a virtual source height of approximately 88 km and a VLF reflection height of approximately 75 km.

Winter anomalous effects of mode conversion observed in mid‐latitude VLF transmission

Mode conversion effects were observed on a long‐distance VLF propagation path at mid‐latitudes during two winters: 1974/75 and 1975/76. The occurrence of amplitude minima as a result of interference between modal components was found to be delayed in the sunrise transition during the D region winter anomaly. At the same time the depth of amplitude fading and the magnitude of phase step decreased and, sometimes, became indiscernibly small. These anomalous effects of mode conversion are interpreted as being due to the lowering of the nighttime VLF reflection height in excellent agreement with the high absorption of HF radio waves in the daytime. The implication is that the electron density in the D region is enhanced day and night during winter anomalous days. Dynamical processes in the middle atmosphere are likely to play an important role in the day‐long enhancement.

Polar D‐region electron density profiles during a solar proton event

Polar D‐region electron density profiles during a moderate solar proton event are calculated using a full‐wave computer solution on VLF step frequency sounder data from Byrd Station, Antarctica. VLF reflection height variation with solar proton flux, particle precipitation from the magnetosphere, and solar zenith angle are shown. It is suggested that the mesospheric pressure during this event is less than that extrapolated from an atmospheric model.