Artificial Periodic Inhomogeneities Research Articles

Artificial Periodic Irregularities and Temperature of the Lower Thermosphere

The results of temperature measurements in the lower thermosphere at altitudes of 90–130 km by the method of resonant scattering of radio waves on artificial periodic inhomogeneities (APIs) of the ionospheric plasma are presented. These inhomogeneities are created when the ionosphere is exposed to powerful HF radio emission. The temperature profile was obtained from measurements of the relaxation time of the API scattered signal. The data processes and the method of the temperature determination are given in detail. The height and temporal resolutions of the API technique are of the order of 1 km and 15 s, respectively, making it possible to study both fast and slow processes in the lower thermosphere. Large temperature variability at altitudes of 90–130 km during the day and from day to day, due to the propagation of atmospheric waves, has been confirmed. The temporal variations of the atmospheric parameters take place with periods from 15 min to some hours. There are often height profiles of the temperature with the wave-like variations and with the vertical scale of about 4–10 km. The irregular temperature profiles were observed above 100 km.

Study of the Mesosphere and Lower Thermosphere by the Method of Creating Artificial Periodic Irregularities of the Ionospheric Plasma

This article presented a brief review of studies of the Earth’s ionosphere at the heights of the mesosphere and lower thermosphere by a method based on the creation of artificial periodic inhomogeneities (APIs) of the ionospheric plasma by high-frequency radiation from powerful thermal installations. APIs are created by a standing wave due to the interference between upward-propagating radio waves and those reflected from the ionosphere. API studies of the ionosphere were based on Bragg scattering of probing impulse signals from an artificial periodic structure. The method makes it possible to measure the parameters of the neutral and ionized components of the Earth’s atmosphere. Note that, despite the fact that the API method assumes an artificial perturbation of the ionospheric plasma, the parameters of the mesosphere and lower thermosphere are determined at the stage of inhomogeneity relaxation and characterize the undisturbed medium. To date, periodic inhomogeneities have been observed at the heating points of Zimenki and Sura ionospheric heating facility (SURA, Vasilsursk, Russia), Gissar (Tajikistan), Arecibo (Puerto Rico, USA), High Power Auroral Stimulation Observatory (HIPAS) and High Frequency Active Auroral Research Program (HAARP, Gakona, AK, USA), and European Incoherent Scatter (EISCAT, Tromso, Norway). Most of the API studies of the ionosphere were carried out at the SURA mid-latitude heating facility (56.1° N; 46.1° E). The review presented the main results of determining the parameters of the ionosphere and neutral atmosphere at altitudes of 60–120 km and studies of the atmosphere during sunrise and sunset events and solar eclipses. In fact, the review is far from a complete illustration of the possibilities of using the API method to study the mesosphere and lower thermosphere.

Dynamics of the Mesosphere and Lower Thermosphere Based on Results of Observations on the SURA Facility

The results of studies of atmospheric dynamics in the altitude range of 60–130 km are presented based on measurements of the velocity of vertical plasma motion, the temperature and density of the neutral component, and the turbopause height. The measurements were carried via resonant scattering of radio waves by artificial periodic inhomogeneities of the ionospheric plasma. The method is based on ionospheric perturbation by powerful high-frequency radio emission, the creation of periodic inhomogeneities in the field of a standing wave, which forms upon reflection from the ionosphere of a powerful radio wave emitted into the zenith, and the location of inhomogeneities by test radio waves. The parameters of the neutral component, the turbulent velocity, and the height of the turbopause are determined from the relaxation time of the signal scattered by the inhomogeneities after the end of the ionospheric impact. The measurement of the phase of the scattered signal makes it possible to determine the rate of vertical plasma motion. The experiments were carried out on a SURA heating facility (56.15° N; 46.11° E). The paper presents and analyzes the time–altitude dependences of the vertical velocity and temperature, which are largely due to the propagation of atmospheric waves during various natural phenomena. The results of determination of the turbopause height and turbulent velocity are presented.

First artificial periodic inhomogeneity experiments at HAARP

AbstractExperiments involving the generation and detection of artificial periodic inhomogeneities have been performed at the High Frequency Active Auroral Research Program (HAARP) facility. Irregularities were created using powerful X‐mode HF emissions and then probed using short (10 μs) X‐ and O‐mode pulses. Reception was performed using a portable software‐defined receiver together with the crossed rhombic antenna from the local ionosonde. Echoes were observed reliably between about 85 and 140 km altitude with signal‐to‐noise ratios as high as about 30 dB. The Doppler shift of the echoes can be associated with the vertical neutral wind in this altitude range. Small but persistent Doppler shifts were observed. The decay time constant of the echoes is meanwhile indicative of the ambipolar diffusion coefficient which depends on the plasma temperature, composition, and neutral gas density. The measured time constants appear to be consistent with theoretical expectations and imply a methodology for measuring neutral density profiles. The significance of thermospheric vertical neutral wind and density measurements which are difficult to obtain using ground‐based instruments by other means is discussed.

Results of studying the sporadic E layer by the method of resonant scattering of radio waves by artificial periodic inhomogeneities of the ionospheric plasma

UDC 551.510.536 We report on the results of studying the lower ionosphere by a method based on the resonant scattering of radio waves by artificial periodic inhomogeneities of the ionospheric plasma. Different aspects of studying the sporadic E layer such as the influence of the vertical transfer on its formation, the possibility of examination of its ion composition, and the influence of the ionosphere heating on the layer characteristics are discussed. The results of determining the parameters of the Es layer and some characteristics of the lower ionosphere during the creation of artificial periodic inhomogeneities at two frequencies are presented.

Modification of the earth’s ionosphere by high-power HF radio emission: Artificial periodic inhomogeneities and the sporadic E layer

We present the results of new studies of the sporadic E layer in the case of heating of the ionosphere by high-power HF radio emission. The measurements were performed at the “Sura” heating facility. Ionosphere was modified by high-power radio emission from the “Sura” facility and was sounded by the probing radio waves of the same frequency and mode. The heating of the ionosphere resulted in the formation of artificial periodic inhomogeneities, and an increase in the intensity of all signals scattered by the D, E, and F regions and the sporadic E layer by 5–20 dB was observed. The increase was observed during heating of the ionosphere by each magnetoionic component, but was smaller for heating by an ordinary-mode wave. This effect was resonant and disappeared as a result of the frequency detuning down to 85 kHz. During the ionospheric modification, the signal-intensity increased due to modulation of the natural profile of the electron number density by the artificial periodic structure.

A new method for determination of the electron number density in the e region of the ionosphere from relaxation times of artificial periodic inhomogeneities

We propose a new method for determination of the electron number density in the E region of the ionosphere on the basis of scattering of radio waves from artificial periodic inhomogeneities formed by the high-power radio emission at two frequencies and having different spatial periods. The ratio of relaxation times of the artificial periodic inhomogeneities at a given altitude is determined only by the ratio of their spatial periods, which makes it possible to determine electron number density. The paper presents the corresponding calculations and the estimates of possible measurement errors.

New Results of Studying the Lower Ionosphere by the Method of Resonance Scattering of Radio Waves from Artificial Periodic Inhomogeneities

We present the results of studying the lower ionosphere in 2000–2004 at the “Sura” heating facility by the method of resonance scattering of radio waves from artificial periodic inhomogeneities of the ionospheric plasma. Experimental data on a study of the sunset–sunrise phenomena in the ionospheric D region and the possibility of determining the concentrations of atomic oxygen and excited molecular oxygen are discussed. The results of studying the sporadic layers of ionization are presented and the method for a study of ion composition of the Es layer is discussed. Data of the August 2004 experiments on a study of the influence of heating the ionosphere on the Es layer and characteristics of artificial periodic inhomogeneities are presented. Prospects for further research are discussed.

Sunset-sunrise characteristics of sporadic layers of ionization in the lower ionosphere observed by the method of resonance scattering of radio waves from artificial periodic inhomogeneities of the ionospheric plasma

We present the results of studying the sporadic layers of ionization in the lower ionosphere observed in the sunset-sunrise time during the summer months of 2000 and 2001. Measurements were performed at the “Sura” heating facility of the Radiophysical Research Institute (Nizhny Novgorod, Russia). Based on the analysis of the altitude-time dependences of the amplitude, relaxation time, and phase of the scattered signal, we obtain new data on the formation and dynamics of the evening and nighttime mid-latitude sporadic E layer, present the results of measuring the velocity of the plasma vertical motion, and estimate the relative molecular mass and total number density of metal ions whose pile-up due to the wind shear can cause the formation of the observed sporadic layers.

Artificial periodic inhomogeneities of the ionosperic plasma as a promising line in the ionospheric research

This paper is dedicated to a new method of ionospheric studies, developed at the Radiophysical Research Institute (NIRFI) and based on the creation of artificial periodic inhomogeneities (APIs) of the ionospheric plasma. We review the techniques and present the results of determination of the basic parameters of the ionosphere and atmosphere.

Study of the Twilight D Region of the Ionosphere using Artificial Periodic Inhomogeneities

We analyze the behavior of the ionospheric D region during sunset and sunrise using the results of measurements performed in August 2000 at the “Sura” facility by the method of artificial periodic inhomogeneities. The measured altitude profiles of the relaxation time and the scattered-signal amplitude are interpreted within the framework of a model with single negative-ion species O2-. Variations in the number densities of atomic and excited molecular oxygen and the altitude profiles of the electron density are found.

EFFECT OF ACOUSTIC GRAVITY WAVES ON VARIATIONS IN THE LOWER- IONOSPHERE PARAMETERS AS OBSERVED USING ARTIFICIAL PERIODIC INHOMOGENEITIES

We study the effect of acoustic gravity waves on variations in the atmospheric parameters of the lower ionosphere. The observations were carried out by the method of radio-wave scattering on artificial periodic inhomogeneities of the ionospheric plasma, induced by powerful radio-wave heating of the ionosphere. Measuring the altitude profile of the relaxation time of the scattered signal allowed us to determine the atmospheric temperature and density at heights 95 to 120 km, while recording the signal phase made it possible to obtain the vertical velocity of the plasma. The joint analysis of variations in the vertical velocity and the atmospheric temperature and density showed the simultaneous existence of oscillations with the same periods ranging from 5–10 min to a few hours. The amplitudes of these oscillations were, respectively, 1.5 to 4 m s-1 for the vertical velocity and 6–20% for the temperature and density. We simulate the characteristics of acoustic gravity waves using the linear theory of their free propagation in an unbounded isothermal undisturbed atmosphere. Based on the polarization relations for low-frequency waves, we calculate the corresponding relative amplitudes of variations in the atmospheric temperature and density with periods from 15 min to 4 h using the measured amplitudes of the vertical velocity. Comparison of the calculation results with the measured values shows their good agreement for waves with periods 15–30 min.

Studies of the Irregular Structure of the Ionosphere using Scattering of Radio Waves on Artificial Periodic Inhomogeneities

We present new results of our studies of the irregular structure of the ionosphere using artificial periodic inhomogeneities (APIs) of the ionospheric plasma. The observations were carried out from 9:00 to 17:00 in August 10–12, 1999 with a height step of 0.7 km and digital registration and real-time processing of the signal quadratures. It is shown that in many cases, the amplitude of the scattered signal is determined by the interference of radio waves scattered on APIs and on natural ionospheric formations including sporadic layers and large-scale natural irregularities. This allows one to study the irregular structure of the lower ionosphere by analyzing height-time dependences of the amplitude and phase of the scattered signal.

Artificial periodic inhomogeneities and a model for the lower part of theD region

The measured height profiles of the relaxation time and amplitudes of artificial periodic inhomogeneities (API) in the ionospheric D region are compared with the model. It is shown that the height dependences of the relaxation time of APIs and the amplitude of the scattered signal can be explained taking into account the hieght profile of the atmosphere density. Most probably, the upper boundary of APIs in the D region is due to the increase in the atomic oxygen density and can serve as an altitude indicator.

Vertical motions in the lower ionosphere and a sporadicE-layer

We propose a calculation technique for the main characteristics of a sporadic E-layer, including the effective recombination coefficient, the relative content of meteor and atmospheric ions in the layer, and the time of its evolution. This technique is based on measurements of vertical plasma velocities by the method of resonance scattering of radio waves by artificial periodic inhomogeneities of the electron density. The contribution of internal gravity waves and turbulent motions to the formation of sporadic layers is estimated. The characteristic values of the turbulent velocity measured by this method at the heights of the turbopause are presented. The possible mechanisms of mid-latitude sporadic E-layer formation at heights of 90 to 120 km are considered. Experimental studies confirmed the main statements of the wind shear theory concerning the determining role of the redistribution of ionization under the action of atmospheric winds with vertical gradient of velocity in the formation of Es.

Studies of the ionosphere and neutral atmosphere using artificial periodic inhomogeneities in the ionospheric plasma

This paper is devoted to a ground‐based method for diagnostics of the ionosphere and neutral atmosphere. This method uses one of the numerous physical phenomena observed in the ionosphere illuminated by a high‐power radio wave, namely, the generation of artificial periodic inhomogeneities. New results on the ionospheric and atmospheric parameter measurements at E region heights are given.

Some results of measurement of atmospheric temperature and density using artificial periodic inhomogeneities of the ionospheric plasma

We present the preliminary results of measurement of atmospheric temperature and density at the E-region heights near Nizhny Novgorod in the 1990–1991 period (λ=56.15°N, ϕ=44.3°E). The monthly mean temperature and density are given in the height interval 100–110 km for September and October 1990. The temporal variations of temperature and density, associated with the propagation of acoustic-gravity and internal waves, are presented. The influence of natural turbulence on the measurement accuracy has been analyzed. The method is based on the generation of artificial periodic inhomogeneities (APIs) by a powerful radio wave and registration of the amplitudes of pulse probe radio waves backscattered by APIs.

Diagnostics of the ionosphere and neutral atmosphere at E-region heights using artificial periodic inhomogeneities

Abstract An overview of studies of the ionospheric E -region using artificial periodic inhomogeneities (API) is presented. Generation of the electron density API is considered theoretically. Methods of measurements of the electron density profile and neutral atmosphere temperature, density and vertical velocity are outlined and illustrated by experimental results. Some suggestions concerning investigations of the atmospheric turbulence using API are presented.

Observations of intensity variations and harmonics of heater induced VLF waves

Abstract Very low-frequency (VLF) radio waves have been generated in the ionosphere by the heating facility near Tromso. VLF signals have been received at Porojarvi (69.17 ° N, 21.47 ° E, L = 6.1), in Northern Finland, about 100 km from the heating transmitter. Three heating experiments were performed under different geophysical conditions and different results were obtained. Signal intensities have been compared with magnetic variations. Correlations and anti-correlations were observed. Several harmonics of the generated signal have generally been found. In some cases when the intensity of the detected VLF signal was low the first harmonic was as intensive as the fundamental frequency. VLF response from Artificial Periodic Inhomogeneities (API) experiment was also observed.

Measurements of artificial periodic inhomogeneities at HIPAS Observatory

Results are presented from recent experiments that employ high‐power, high‐frequency (HF) radio waves to probe the mesosphere and lower thermosphere. The measurements were made at the High‐Power Auroral Stimulation (HIPAS) Observatory located near Fairbanks, Alaska. One objective of the study was to determine the feasibility of using artificial electron density perturbations created in the auroral environment to measure the properties of the background neutral gas between ∼50 to ∼120 km altitude. The observing technique relies on the production of so‐called “artificial periodic inhomogeneities” (API) in the altitude region(s) of interest. These induced irregularities are believed to be horizontally stratified and conform to the standing wave pattern produced by the reflection of the powerful HF wave in the ionosphere. In the D region above HIPAS, API decay curves are strictly exponential and the phase histories are strictly linear. On occasion, echoes are detected at very low altitudes (∼45 km) in the vicinity of the polar stratopause. The API backscatter at HIPAS is often superimposed on regions of partial reflection, auroral E, and sporadic E. Information about ambipolar diffusion rates and electron attachment to O2 is obtained by measuring the relaxation time of the induced irregularities. In general, API phase velocities below ∼95 km altitude appear to be related to vertical neutral motions. However, detailed validation studies throughout the mesosphere have not yet been performed at HIPAS or any other API facility. At altitudes between ∼45 and ∼80 km, high‐resolution observations at HIPAS reveal the presence of sharply defined bands of API scatter 15–20 km in altitude extent. The existence of such bands and their fluctuation in altitude cannot be explained within the context of existing theory. Large variations in API backscatter power (10–20 dB) are typically observed in the D region over timescales of 30 s or less. Most likely this is caused by fading in the ionospherically reflected component of the standing wave pattern. Finally, power stepping studies reveal a roughly linear relationship between D region backscatter power and HF power. Fundamental questions related to the horizontal dimension of the API patch and its spatial structure remain to be addressed in future experiments.