Reflection Height Research Articles

Low-frequency ionospheric sounding with Narrow Bipolar Event lightning radio emissions: regular variabilities and solar-X-ray responses

Abstract. We present refinements of a method of ionospheric D-region sounding that makes opportunistic use of powerful (109–1011 W) broadband lightning radio emissions in the low-frequency (LF; 30–300 kHz) band. Such emissions are from "Narrow Bipolar Event" (NBE) lightning, and they are characterized by a narrow (10-μs), simple emission waveform. These pulses can be used to perform time-delay reflectometry (or "sounding") of the D-region underside, at an effective LF radiated power exceeding by orders-of-magnitude that from man-made sounders. We use this opportunistic sounder to retrieve instantaneous LF ionospheric-reflection height whenever a suitable lightning radio pulse from a located NBE is recorded. We show how to correct for three sources of "regular" variability, namely solar zenith angle, radio-propagation range, and radio-propagation azimuth. The residual median magnitude of the noise in reflection height, after applying the regression corrections for the three regular variabilities, is on the order of 1 km. This noise level allows us to retrieve the D-region-reflector-height variation with solar X-ray flux density for intensity levels at and above an M-1 flare. The instantaneous time response is limited by the occurrence rate of NBEs, and the noise level in the height determination is typically in the range ±1 km.

Connection between winter mesopause region temperatures and diurnal LF reflection height variations measured at Collm

Scale height, H, estimates are calculated from the decrease/increase of ionospheric virtual reflection heights of low-frequency (LF) radio waves at oblique incidence in suitably defined morning intervals around sunrise during winter months. The day-to-day variations of H qualitatively agree with daily mean temperature variations around 90 km from meteor radar measurements. Since mesospheric long-period temperature variations are generally accepted to be the signature of atmospheric planetary waves, this shows that LF reflection height measurements can be used for monitoring the dynamics of the upper middle atmosphere. The long-term variations of monthly mean H estimates have also been analysed. There is no significant trend, which is in agreement with other measurements of mesopause region temperature trends.

Effect of Moon phases in riometer absorption and in the ionospheric and geomagnetic parameters

Variations in the frequency of occurrence of riometer absorption, minimum frequency of reflection of the ionospheric F layer, minimum height, and height of maximum electron density of the ionospheric F layer near the solar minimum have been studied. Application of the superposed epoch technique has detected the Moon phase effect on these ionospheric parameters. This effect was: three events per day in the occurrence of riometer absorption, 0.056 MHz in the minimum frequency of reflection of the F layer, and 2.6 and 6.7 km, in the change of the minimum height of reflection and height of reflection from the region with maximum electron density of the ionospheric F layer, respectively. The lunar tide action changes the ionospheric conductivity and, thus, influences the current systems of the magnetosphere. Through changes of magnetospheric currents, the Moon phase effect is exhibited in the Ap and Dst indices and is 4.3 and 4.25 nT, respectively.

“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.

Long-period upper mesosphere temperature and plasma scale height variations derived from VHF meteor radar and LF absolute reflection height measurements

Abstract. The change of ionospheric absolute reflection heights h of low-frequency (LF) radio waves at oblique incidence in the course of the day is measured at Collm Observatory (51.3° N, 13.0° E) using 1.8 kHz sideband phase comparisons between the sky-wave and the ground wave of a commercial 177 kHz transmitter (Zehlendorf, reflection point at 52.1° N, 13.2° E). Plasma scale height estimates H are calculated from the decrease/increase of h in the morning/evening. The day-to-day variations of H are compared with those of daily mean temperatures at 90 km, measured with a VHF meteor radar (36.2 MHz) at Collm and using the amplitude decay of meteor reflections. A good qualitative correspondence is found between the two data sets. Since mesospheric long-period temperature variations are generally accepted to be the signature of atmospheric planetary waves, this shows that LF reflection height measurements can be used for monitoring the dynamics of the upper middle atmosphere.

Differentiation of α-cristobalite from opals in bentonites from Turkey

Because of the similarity of the XRD-patterns, well-ordered opal-C and opal-CT have been misidentified as α-cristobalite in many publications. This is important, because flying α-cristobalite is probably a human carcinogen and the opals are not. The evidence of α-cristobalite in bentonites is hazardous for the human health and reduces their marketing. To distinguish the opal-C and opal-CT from α-cristobalite, the XRD-patterns of bentonites before and after H 3PO 4-digestion (240 °C, 15 min) and heat treatment (1050 °C, 24 h) were evaluated in details. It was seen that the most characteristic XRD-reflection (hkl:101) centered near 0.40 nm for crystalline α-cristobalite and paracrystalline opals disappeared after the digestion and sharpened after the heat treatment. Since the crystallinity of α-cristobalite was not affected from the digestion and the heat treatment, it was concluded that the bentonites contain opal-CT or opal-C in amorphous opal-A matrix, but do not contain α-cristobalite. Since the paracrystallinity increases in order opal-CT and opal-C, the narrowing in full width at half-maximum reflection height (FWHM) 101 must be more for opal-CT than opal-C by heat treatment. Therefore, these opals were distinguished approximately from each other depending on the narrowing in the FWHM of 101 XRD-reflection by heating. Based on this result, it would not be decided that whether bentonites or other clays contain α-cristobalite or not, without performing the H 3PO 4-digestion and heat treatment.

Persistent enhancement of the HF pump-induced plasma line measured with a UHF diagnostic radar at HAARP

[1] Plasma lines excited by a powerful, high-frequency (HF) radio wave are studied using data obtained with an ultrahigh frequency (UHF) radar at HAARP (High Frequency Active Auroral Research Program) from 3 to 5 February 2005. Of particular interest is persistent enhancement of the radar backscatter power during HF on at several HF frequencies. The persistent enhancement is induced with the HF frequency slightly lower than foF2 by a few hundred kHz; by contrast the persistent enhancement does not appear when the HF frequency is equal to and higher than foF2 or lower than foF2 by more than 500 kHz. When persistent enhancements of the radar backscatter power appear, two case studies show that the local plasma frequency at the reflection height of the O-mode polarization wave is close to the second or third electron gyroharmonic frequencies, but one case study shows that the local plasma frequency at the reflection height is significantly different from the third electron gyroharmonic frequency.

Using tweek atmospherics to measure the response of the low-middle latitude D-region ionosphere to a magnetic storm

Tweek atmospherics are pulse signals from ELF/VLF electromagnetic waves caused by lightning discharges; these signals have characteristic cut-off frequencies due to long-distance propagation by the Earth–ionosphere waveguide mode. We developed a method of estimating tweek reflection heights (equivalent electron densities) in the low-middle latitude D-region ionosphere by accurate measurement of their first-order mode cut-off frequency (∼1.8 kHz). This research is the first to show reflection-height measurements of the D-region ionosphere's response to the great geomagnetic storm in October 2000 using tweek atmospherics observations. We found transient lowering/rising in the small bay D st-variation and rising near the maximum depression of the major bay D st-variation. Comparing this response in tweek reflection heights with the intensities of 40 kHz radio-wave signals, electron density profiles from the IRI-2001 model and measured by MF radars, and the ionospheric F-region parameter ( h′ F) from ionosonde data, we determined two cases where the lowering/rising of the D-region was coupled with vertical motion of the F-region and one where it was not. Based on simultaneous total electron content (TEC) perturbation data, we suggest the following possible mechanisms for these two results: E × B vertical plasma drift due to ionospheric electric fields, and the propagation of large-scale traveling ionospheric disturbances (LSTIDs) from the auroral to low-latitude ionosphere, respectively.

A synoptic study of VLF sudden phase anomalies recorded at Visakhapatnam

Abstract A synoptic survey of Sudden Phase Anomalies (SPAs) observed in the phase variation measurements of 16 kHz VLF transmissions from Rugby (England) made at Visakhapatnam (India) has been carried out. These Sudden Ionospheric Disturbances caused during solar flares are examined in relation to flare time enhancements of X-ray radiation fluxes. It is found that nearly 81% of SPAs recorded have accompanying X-ray enhancements and in 80% of the cases Hα flares and microwave bursts also occur concurrently. Using SPA magnitudes and flare time X-ray flux values, the threshold level of X-ray flux to induce an SPA has been estimated as 1.5⊙×10−3 ergs/cm2/s. In majority of the events, the change in reflection height during the flare is observed to be less than 4 kms. Other SPA characteristics like onset times, growth and relaxation times etc. have also been studied. These results are consistent with those obtained for other propagation paths.

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.

Long-term Measurements of Nighttime LF Radio Wave Reflection Heights over Central Europe

Abstract. The nighttime ionospheric absolute reflection height of low-frequency (LF) radio waves at oblique incidence has been measured continuously since late 1982 using 1.8kHz sideband phase comparisons between the sky wave and the ground wave of a commercial 177kHz LF transmitter. The dataset allows the analysis of long-term trends and other regular variations of the reflection height. Beside the clear signal of the 11-year solar cycle a quasi-biennial oscillation is visible in LF reflection heights, which is correlated to the equatorial stratospheric wind field. A long-term decreasing reflection height trend is found, confirming results from other measurements and theoretical estimations. The results can be interpreted as a long-term decrease of the height levels of fixed electron density in the lower E region, reflecting a long-term cooling trend of the middle atmosphere.

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.

Southern high-latitude Digisonde observations of ionosphere E-region Bragg scatter during intense lacuna conditions

Abstract. During summer months at solar cycle minimum, F-region lacuna and slant-Es conditions (SEC) are common features of daytime ionograms recorded around local magnetic noon at Casey, Antarctica. Digisonde measurements of drift velocity height profiles show that the occurrence of lacuna prevents the determination of F-region drift velocities and also affects E-region drift velocity measurements. Unique E-region spectral features revealed as intervals of Bragg scatter superimposed on typical background E-region reflection were observed in Digisonde Doppler spectra during intense lacuna conditions. Daytime E-region Doppler spectra recorded at carrier frequencies from 1.5 to 2.7MHz, below the E-region critical frequency foE, have two side-peaks corresponding to Bragg scatter at approximately ±1-2Hz symmetrically located on each side of a central-peak corresponding to near-zenith total reflections. Angle-of-arrival information and ray-tracing simulations show that echo returns are coming from oblique directions most likely resulting from direct backscatter from just below the total reflection height for each sounding frequency. The Bragg backscatter events are shown to manifest during polar lacuna conditions, and to affect the determination of E-region background drift velocities, and as such must be considered when using standard Doppler-sorted interferometry (DSI) techniques to estimate ionospheric drift velocities. Given the Doppler and spatial separation of the echoes determined from high-resolution Doppler measurements, we are able to estimate the Bragg scatter phase velocity independently from the bulk E-region motion. The phase velocity coincides with the ExB direction derived from in situ fluxgate magnetometer records. When ionospheric refraction is considered, the phase velocity amplitudes deduced from DSI are comparable to the ion-acoustic speed expected in the E-region. We briefly consider the plausibility that these previously unreported polar cap E-region Bragg scatter Doppler spectral signatures, observed at Casey in December 1996 during SEC/lacuna conditions may be linked to ionosphere irregularities. These irregularities may possibly be generated by primary plasma waves triggered by current-driven instabilities, that is to say, a hybrid of the "modified two-stream" and "gradient drift" instability mechanisms.

Probing the speed of light with radio waves at extremely low frequencies.

The speed of light, a fundamental physical constant and thought to be independent of frequency, is tested here with naturally occurring radio waves in the atmosphere at extremely low frequencies. It is shown that the speed of light in the frequency range 5-50 Hz is known with an accuracy determined by perturbations of the ionospheric reflection height associated with space weather phenomena, which place an upper limit on the photon rest mass mgamma < or approximately 4 x 10(-52) kg to date.

Study of the Excitation Conditions and Characteristics of Ionospheric Plasma Turbulence at the Development Stage of the Ponderomotive Parametric Instability

We present the results of studying the development features of nonlinear effects at the initial stage of interaction of powerful HF radio waves with the plasma in the ionospheric F region. Experimental measurements were performed at the “Sura” heating facility for a wide pump frequency range (4.5–9.0 MHz) and a variety of pulse durations (0.3–100 ms) and effective radiated powers (1–30 MW) at various times of a day. The performed measurements allow us to study the excitation thresholds and time–amplitude characteristics of the ponderomotive self-action of a pump wave as well as the relaxation characteristics of the stimulated electromagnetic emission of the ionosphere as functions of the pump parameters and ionospheric conditions. The measured development features of the ponderomotive parametric instability in the ionospheric plasma are compared with the calculation results. The instability threshold fields (Eth≈ 220 mV/m) and the damping rates (γe≈ 450 s-1) of plasma waves, measured under evening-time conditions, are close to the estimates obtained on the assumption of collisional damping of Langmuir turbulence. A significant increase in the threshold field and the damping rate (by factors of up to 3 and 6, respectively) was observed under daytime conditions. In this case, the minimum values of these quantities (Eth≈ 350 mV/m and γe≈ 600 s-1) were observed for pump-wave reflection heights of about z≈ 230 km. The measurement and simulation results are indicative of the dominant effect of photoelectrons on the development features of ionospheric plasma turbulence under daytime conditions. We discuss the possibilities of using the developed method for comprehensive monitoring of the parameters of Langmuir turbulence and the background ionospheric plasma.

A method for determining intracloud lightning and ionospheric heights from VLF/LF electric field records

The Los Alamos Sferic Array (LASA) recorded VLF/LF electric‐field‐change signals from over ten million lightning discharges during the period from 1998 to 2001. Using the differential‐times‐of‐arrival of lightning sferics recorded by three or more stations, the latitudes and longitudes of the source discharges were determined. Under conditions of favorable geometry and ionospheric propagation, sensors obtained ionospherically reflected skywave signals from the lightning discharges in addition to the standard groundwave sferics. In approximately 1% of all waveforms, automated processing identified two 1‐hop skywave reflection paths with delays indicative of an intracloud (height greater than 5 km) lightning source origin. For these events it was possible to determine both the height of the source above ground and the virtual reflection height of the ionosphere. Ionosphere heights agreed well with published values of 60 to 95 km with an expected diurnal variation. Source height determinations for 100,000+ intracloud lightning events ranged from 7 to 20 km AGL with negative‐polarity events occurring above ∼15 km and positive‐polarity events occurring below ∼15 km. The negative‐polarity events are at a suprisingly high altitude and may be associated with discharges between the upper charge layer of a storm and a screening layer of charge above the storm. Approximately 100 of the intracloud events with LASA height determinations were also recorded by VHF receivers on the FORTE satellite. Independent FORTE source height estimates based on delays between direct and ground‐reflected radio emissions showed excellent correlation with the VLF/LF estimates, but with a +1 km bias for the VLF/LF height determinations.

An Overview of Long-Term Trends in the Lower Ionosphere Below 120 km

The increasing concentration of greenhouse gases in the atmosphere is expectedalso to modify the mesosphere and lower thermosphere (MLT region). However,the greenhouse cooling – instead of heating – at these heights is revealed by modelsand generally confirmed by observations. This should more or less affect variousionospheric parameters at these heights. The spatial and temporal structure oftemperature trends in the MLT region is quite complex and, therefore, such structureshould occur for trends in the lower ionosphere as well. In the lower part of theionosphere below about 90 km, the rocket measurements of electron density, theindirect phase reflection height measurements and the A3 radio wave absorptionmeasurements reveal trends corresponding to cooling and shrinking of the mesosphere,while riometric measurements of cosmic noise absorption provide inconclusive results.The radio wave absorption and rocket electron density measurements clearly display asubstantial dependence of trends on height. Ionosonde data show that there is amodel-expected trend in the maximum electron concentration of the E region ionosphere;foE is slightly increasing. On the other hand, the height of the normal E layer, h'E, isslightly decreasing. The nighttime LF radio wave reflection height measurements near95 km support an idea of increasing electron density. However, rather scarce rocketmeasurements display a negative trend in electron density at 90–120 km. The role ofthe solar cycle and other longer-term variability of natural origin in the determinationof observational trends must not be neglected. In spite of the general qualitativeagreement with model expectations, there is still some controversy between variousobservational trend results (hopefully, apparent rather than real), which needs to beclarified.

Equivalent electron densities at reflection heights of tweek atmospherics in the low-middle latitude D-region ionosphere

Tweek atmospherics are ELF/VLF pulse signals with frequency dispersion characteristics that originate from lightning discharges and propagate in the Earth-ionosphere waveguide mode over long distances. In this paper, we estimate equivalent nighttime electron densities at reflection heights in D-region ionosphere at low-middle latitudes by accurately reading the first-order mode cut-off frequency of tweek atmospherics. The estimation method was applied to tweek atmospherics received simultaneously at Moshiri and Kagoshima in Japan. Equivalent electron densities ranged from 20—28 el./cm3 at ionospheric reflection heights of 80—85 km. Comparing our estimates with electron density profiles obtained from the IRI-95 model, MF radar measurements, and rocket experiments revealed almost consistent results for the lower part of the D-region ionosphere. The tweek method has the unique advantage of enabling reflection-height (equivalent electron densities) monitoring over a wide area of several thousand kilometers.

In situ observations during an HF heating experiment at Arecibo: Evidence for Z‐mode and electron cyclotron harmonic effects

On 11 March 1998 the Langmuir Turbulence sounding rocket was launched through the Arecibo heater beam during an experiment to measure electric fields and plasma densities in the heater interaction region. In spite of a serious degradation of the Arecibo heater, the rocket data has provided evidence of Z mode waves and field aligned striations above the O mode reflection height. These observations give credence to the theory that Z mode wave interactions with field‐aligned striations may be, in part, responsible for some of the features of the reflected HF spectrum observed in heating experiments. We also find strong low‐frequency electric fields where the local plasma frequency matches an electron cyclotron harmonic. This suggests that electron Bernstein modes must be generated in the interaction process and can propagate to regions where they are severely damped.

Quasi-biennial and decadal variability obtained from long-term measurements of nighttime radio wave reflection heights over Central Europe

The nighttime reflection height of low-frequency (LF) radio waves at oblique incidence over Central Europe is measured using 1.8 kHz sideband phase comparisons between the sky wave and the ground wave of a commercial 177 kHz LF transmitter. The measurements have been carried out continuously since September 1982, now justifying the analysis of trends and regular variations of the reflection height. In the time series is found a) a long-term negative trend and b) a solar cycle dependence, both confirming earlier measurements and theoretical estimations. Moreover, a significant oscillation of quasi-biennial period is visible in LF reflection heights, indicating a reaction of the midlatitude mesosphere/lower thermosphere region on the equatorial quasi-biennial oscillation.