Absorption Of Radio Waves Research Articles

On the fine structure of medium energy electron fluxes in the auroral zone and related effects in the ionospheric D-region

Abstract. This study is based on measurements of trapped and precipitated electrons of energy >30 keV and >100 keV observed by polar orbiting environmental satellites during overpasses of the imaging riometer at Kilpisjärvi, Finland. The satellites are in sun-synchronous orbits of about 850 km altitude, recording the electron fluxes at 2-s time resolution. The riometer measures the radiowave absorption at 38.2 MHz, showing the spatial pattern within a 240 km field of view. The analysis has focussed on two areas. Having found a close correlation between the radiowave absorption and the medium-energy electron fluxes during satellite overpasses, empirical relationships are derived, enabling one quantity to be predicted from the other for three sectors of local time. It is shown that small-scale variations observed during a pass are essentially spatial rather than temporal. Other properties, such as the spectra and the relation between precipitated and trapped components, are also considered in the light of the theory of pitch angle scattering by VLF waves. It is found that the properties and behaviour depend strongly on the time of day. In the noon sector, the precipitated and trapped fluxes are highly correlated through a square law relationship.

Vitaly Lazarevich Ginzburg

Vitaly Lazarevich Ginzburg

Impact of the Mt. Pinatubo volcaniceruption on the lower ionosphere andatmospheric waves over Central Europe

The very strong volcanic eruption of Mt. Pinatubo in June 1991 directly affected the troposphere and lower and middle stratosphere. Here we look at its effects in the mesopause region as revealed by the radio wave absorption measurements in the lower ionosphere over Central Europe and inferred planetary and gravity wave activity. The gravity wave activity inferred from the nighttime LF radio wave absorption displays an evident enhancement for waves of periods of about 2-3 h coinciding with regional measurements of the optical depth of (volcanic) aerosols, while there is no detectable effect for short period waves (T < 1 h). There is no detectable effect in the planetary wave activity inferred from the daytime HF radio wave absorption. As for the absorption itself, the results on the impact of the Mt. Pinatubo eruption do not provide an observable effect.

Stratospheric aerosol dynamics over Kamchatka and its association with geophysical processes

The dynamics of aerosol layers in comparison with geomagnetic and ionospheric data has been studied based on the nighttime single-frequency lidar sounding of the atmosphere over Kamchatka at altitudes of 10 to 90 km. The relation of the aerosol density to solar, magnetic, and ionospheric activity has been studied, and the stratospheric aerosol formation mechanisms have been considered. It has been indicated that variations in the aerosol density correlate with radiowave absorption, perturbations of the ionospheric parameters, and geomagnetic characteristics. The spatial and time scales of aerosol layers have been estimated. The role of stratospheric aerosol as an indicator of geophysical processes is discussed.

Eikonal acceleration technique for studying of the Earth and planetary atmospheres by radio occultation method

[1] Lately introduced a phase (eikonal) acceleration technique extends an applicable domain of radio occultation (RO) method and can be applied for identification of the plasma layers in the lower ionosphere. This technique can also convert the eikonal excess or Doppler frequency changes measured in RO experiments into the refractive attenuation variations. From these derived refractive attenuation and amplitude data one can estimate the integral absorption of radio waves. This is important for study of the radio wave propagation effects in the atmospheric telecommunication links and for remote sensing of the atmosphere and ionosphere. The advantages of the eikonal acceleration technique are validated by analyzing the RO data from the Challenging Minisatellite Payload (CHAMP) and Venus missions.

Phase acceleration: a new important parameter in GPS occultation technology

Based on 40 years of radio-occultation (RO) experiments, it is now recognized that the phase acceleration of radio waves (equal to the time derivative of the Doppler shift), derived from analysis of high-stability Global Positioning System (GPS) RO signals, is as important as the Doppler frequency. The phase acceleration technique allows one to convert the phase and Doppler frequency changes into refractive attenuation variations. From such derived refractive attenuation and amplitude data, one can estimate the integral absorption of radio waves. This is important for future RO missions when measuring water vapor and minor atmospheric gas constituents, because the difficulty of removing the refractive attenuation effect from the amplitude data can be avoided. The phase acceleration technique can be applied also for determining the location and inclination of sharp layered plasma structures (including sporadic E s layers) in the ionosphere. The advantages of the phase acceleration technique are validated by analyzing RO data from the Challenging Minisatellite Payload (CHAMP) and the FORMOSA Satellite Constellation Observing Systems for Meteorology, Ionosphere, and Climate missions (FORMOSAT-3/COSMIC).

Study on solar sources and polar cap absorption events recorded in Antarctica

Particularly intense events occurred on the Sun in a period around minimum of solar activity during cycle 23. We investigated the characteristics of September 2005 and December 2006 events and the properties of the correlated observations of ionospheric absorption, obtained by a 30 MHz riometer installed at Mario Zucchelli Station (MZS-Antarctica), and of geomagnetic activity recorded at Scott Base (Antarctica). Solar events are studied using the characteristics of CMEs measured with SoHO/LASCO coronagraphs and the temporal evolution of solar energetic protons in different energy ranges measured by GOES 11 spacecraft. Analysing these data, we have determined how these effects are finally observed on the Earth’s surface not only in the ionospheric absorption of radio waves and in the intense geomagnetic activity, but also as significant variations of cosmic ray modulation, even at high energies.

Влияние высоты расположения передатчика на формирование трасс коротких радиоволн в ионосфере

The article presents calculations for ray traces and the absorption of short radio waves in the ionosphere. The multi-jump traces are examined on the basis of the altitude of the transmitter and geophysical conditions.

Global mapping of ionospheric HF/VHF radio wave absorption due to solar energetic protons

Simple, one‐parameter algorithms are applied to the observed energetic proton flux as provided by instruments aboard the GOES series of satellites to yield estimates of the high‐latitude HF and VHF radio wave absorption for day and night, respectively. These results are extended to full daily coverage by treating the effects of solar illumination, geomagnetic cutoff variation, and frequency dependence over the entire earth. Validation calculations of the polar cap absorption of HF radio waves have been performed for 11 larger solar energetic particle events during the period from 1992 to 2002 and the results are compared to observations of 30 MHz riometers operated by the Air Force Geophysics Laboratory and located at Thule, Greenland. Prediction of the minimum event duration from current flux level is also obtained, and a specimen presentation of the north and south polar caps illustrates the graphical output of the model for the peak of the 6 December 2006 solar proton event.

Intermittency of the turbulent processes in the Earth's magnetosphere detected from the ground-based measurements

Turbulent processes in the Earth's magnetosphere are reflected in the dynamical behavior of the geomagnetic indices and other parameters determined from ground based observations. Intermittent properties of one minute Polar Cap (PC) index and auroral radio wave absorption are studied using 1995-2000 data sets. It was found that the probability distribution functions (PDFs) of both PC-index and absorption fluctuations display a strong non-Gaussian shape. This indicates that they are not characterized by a global time self-similarity but rather exhibit intermittency, as previously reported for solar wind velocity and auroral electrojet index values. In the case of the auroral absorption it was also found that intermittency strongly depends on the magnetic local time, being largest in the nighttime sector. This shows that the acceleration of precipitating particles is intermittent, especially near the substorm eye, where the level of turbulence increases. Application of the Local Intermittency Measure (LIM) technique confirms the aforementioned results to a better precision.

Lower ionosphere response to external forcing: A brief review

There are two ways of external forcing of the lower ionosphere, the region below an altitude of about 100 km: (1) From above, which is directly or indirectly of solar origin. (2) From below, which is directly or indirectly of atmospheric origin. The external forcing of solar origin consists of two general factors – solar ionizing radiation variability and space weather. The solar ionization variability consist mainly from the 11-year solar cycle, the 27-day solar rotation and solar flares, strong flares being very important phenomenon in the daytime lower ionosphere due to the enormous increase of the solar X-ray flux resulting in temporal terminating of MF and partly LF and HF radio wave propagation due to heavy absorption of radio waves. Monitoring of the sudden ionospheric disturbances (SIDs – effects of solar flares in the lower ionosphere) served in the past as an important tool of monitoring the solar activity and its impacts on the ionosphere. Space weather effects on the lower ionosphere consist of many different but often inter-related phenomena, which govern the lower ionosphere variability at high latitudes, particularly at night. The most important space weather phenomenon for the lower ionosphere is strong geomagnetic storms, which affect substantially both the high- and mid-latitude lower ionosphere. As for forcing from below, it is caused mainly by waves in the neutral atmosphere, i.e. planetary, tidal, gravity and infrasonic waves. The most important and most studied waves are planetary and gravity waves. Another channel of the troposphere coupling to the lower ionosphere is through lightning-related processes leading to sprites, blue jets etc. and their ionospheric counterparts. These phenomena occur on very short time scales. The external forcing of the lower ionosphere has observationally been studied using predominantly ground-based methods exploiting in various ways the radio wave propagation, and by sporadic rocket soundings. All the above phenomena are briefly mentioned and some of them are treated in more detail.

Energetic electron precipitation during substorm injection events: High‐latitude fluxes and an unexpected midlatitude signature

Geosynchronous Los Alamos National Laboratory (LANL‐97A) satellite particle data, riometer data, and radio wave data recorded at high geomagnetic latitudes in the region south of Australia and New Zealand are used to perform the first complete modeling study of the effect of substorm electron precipitation fluxes on low‐frequency radio wave propagation conditions associated with dispersionless substorm injection events. We find that the precipitated electron energy spectrum is consistent with an e‐folding energy of 50 keV for energies &lt;400 keV but also contains higher fluxes of electrons from 400 to 2000 keV. To reproduce the peak subionospheric radio wave absorption signatures seen at Casey (Australian Antarctic Division), and the peak riometer absorption observed at Macquarie Island, requires the precipitation of 50–90% of the peak fluxes observed by LANL‐97A. Additionally, there is a concurrent and previously unreported substorm signature at L &lt; 2.8, observed as a substorm‐associated phase advance on radio waves propagating between Australia and New Zealand. Two mechanisms are discussed to explain the phase advances. We find that the most likely mechanism is the triggering of wave‐induced electron precipitation caused by waves enhanced in the plasmasphere during the substorm and that either plasmaspheric hiss waves or electromagnetic ion cyclotron waves are a potential source capable of precipitating the type of high‐energy electron spectrum required. However, the presence of these waves at such low L shells has not been confirmed in this study.

Auroral radio emission and absorption of medium frequency radio waves observed in Iceland

Abstract In order to study the generation and propagation processes of MF auroral radio emissions (referred to as auroral roar and MF burst) in the polar ionosphere, an Auroral Radio Spectrograph (ARS) system was installed at Husafell station in Iceland (invariant latitude: 65.3°). Data analysis of man-made transmissions also provides useful information for the ionosphere study as well as an investigation of auroral radio emissions since the propagation character of MF radio waves changes depending on electron-neutral collisions in the bottomside ionosphere. Thus, ionospheric absorption is examined in comparison with the solar zenith angle and auroral phenomena. The results indicate that the ARS data can be used to detect ionization occurring at distant regions. In late 2006, the ARS detected one auroral roar and twoMF bursts, which were identified as left-handed polarized waves. Results of data analysis, including other auroral data and particle spectra observed by the DMSP satellite, suggest that the MF bursts are generated by electrons with an average energy of several keV associated with auroral breakup. On the other hand, the auroral roar is generated as upper hybrid waves by relatively low-energy electrons over the observation site and propagates downward, being converted into L-O mode electromagnetic waves.

Anomalous absorption of radiowaves by small-scale cylindrical irregularities stretched along the magnetic field

Results are presented from calculations of the anomalous absorption of an ordinary radiowave in the ionospheric plasma by a system of small-scale cylindrical density irregularities such that δN < 0. Account is taken of the effect of density perturbations on the propagation of excited cold waves and their conversion into plasma waves. It is shown that this effect attenuates the anomalous absorption of the radiowave. However, for sufficiently large density perturbations (about 6% or more), the absorption is substantial even for irregularities having comparatively large transverse sizes (about 5 m). The fluxes of cold and plasma waves excited by a radiowave in the anomalous absorption process are determined.

D- and E-region effects in the auroral zone during a moderately active 24-h period in July 2005

Abstract. The effects of energetic electron precipitation into the auroral region at a time of enhanced solar wind have been investigated during a continuous period of 24 h, using the European Incoherent Scatter (EISCAT) radar, an imaging riometer, and particle measurements on an orbiting satellite. The relative effects in the E region (120 km) and D region (90 km) are found to vary during the day, consistent with a gradual hardening of the incoming electron spectrum from pre-midnight to morning. Whereas the night spectra are single peaked, the daytime spectra are found to be double peaked, suggesting the presence of two distinct populations. A comparison between the radiowave absorption observed with the riometer and values estimated from the radar data shows generally good agreement, but with some discrepancies suggesting the occurrence of some small-scale features. The height and thickness of the absorbing region are estimated. Two periods of enhanced precipitation and the related radio absorption, one near magnetic midnight and one in the early morning, are studied in detail, including their horizontal structure and movement of the absorption patches. A sharp reduction of electron flux recorded on a POES satellite is related to the edge of an absorption region delineated by the imaging riometer. The observed particle flux is compared with a value deduced from the radar data during the overpass, and found to be in general agreement.

The wideband attenuation of short radio waves on mid-latitude paths during X-ray flares in January 2005

We present the results of the ionosphere oblique chirp sounding on the Cyprus—Nizhny Novgorod, Cyprus—Rostov-on-Don, and Moscow—Rostov-on-Don mid-latitude paths during X-ray flares in January 17, 19, and 20, 2005. It is found that during strong flares the blackout of short radio waves was observed over the entire frequency range of chirp sounding on the Cyprus—Nizhny Novgorod and Cyprus—Rostov-on-Don paths. Modeling of the electron-density profiles in the lower ionosphere based on absorption of short radio waves on the Moscow—Rostov-on-Don path at different stages of the decay of the X-ray radiation intensity is carried out. It is shown that at the instant corresponding to the maximum value of the flare radiation flux, the electron density in the lower ionosphere at altitudes 60–80 km increased by a factor of about 10 and 100 for flares with radiation flux densities 5·10−2 and 3·10−1 erg/(cm 2·s) in the wavelength range 0.5–4.0 A which took place in January 19 and 20, respectively.

Absorption and reflection of radio waves in the Martian ionosphere

Radio wave absorption in the Martian ionosphere has been predicted and tested against MARSIS radar observations. Models of the ionosphere densities and of absorption in a CO 2 neutral atmosphere were used. The appearance of ground reflections in the MARSIS observations is shown to be consistent with predictions of reflection and absorption of radio waves in the ionosphere. It is concluded that the secondary density maximum, known to be typically present below the primary density peak, contributes considerably to the absorption and thus to the appearance of ground reflections. It is the first time predicted radio wave absorption in a CO 2 planetary atmosphere has been tested against actual observations.

Behavior of the polar lower ionosphere during the solar flares in January 2005 according to the data of the partial reflections method

The results of the observations, using partial reflections, of the lower ionosphere over Tumannyi station in the Murmansk region (69.0° N, 35.7° E) during strong solar flares on January 15–20, 2005, are presented. The structure of the D region of the polar ionosphere and the effects of X-ray flares and fluxes of high energy protons on this region are considered. The anomalous values of electron density in the lower part of the D region, unusually low values of the height of the lower ionospheric boundary, complete or partial absorption of short and medium radiowaves, and bursts of the meter cosmic radio emission were detected during solar disturbances.

Variations in the radio-wave absorption in the ionosphere due to internal gravity waves

We estimate the influence of internal gravity waves on the radio-wave absorption in the Earth’s atmosphere. It is shown that the internal gravity waves can lead to significant spatio-temporal variations in the absorption. We conclude that riometric measurements can be used for the diagnostics of internal gravity waves in the Earth’s atmosphere.

Intermittency of riometer auroral absorption observed at South Pole

Auroral radio-wave absorption values measured at South Pole for 3 years using a riometer are analyzed in order to test whether they show evidence of intermittency. The properties of the parameters of the probability density functions determined for several magnetic local time sectors are found to be significantly different. The probability density functions for the pre-midnight sector show the typical shape associated to intermittency. No results are given for the afternoon sector because few auroral absorption events meet the selection criteria to give statistically significant results. It is suggested that if the precipitating particle population responsible for the riometer auroral absorption shared the intermittency features of the absorption then the present results would allow the study of the properties of the induction component of magnetospheric turbulence.