Absorption Of Radio Waves Research Articles

Scintillation and Absorption of Radio Waves in the Earth's Atmosphere in Radio Occultation Experiments on the Satellite-to-Satellite Link

A radio occultation experiment where both transmitter and receiver are above troporpheric characteristic altitudes and at long distances from each other has been performed. Results from such a satellite-to-satellite link are analyzed.

Similarities in the variability of the lower ionosphere and foF2 in the period range of 2–15 days

In the mid-latitude lower ionosphere, there are several dominant quasi-periodic oscillations in the period range of 2–15 days, which are caused by planetary waves. The quasi five- and ten-day periods have usually been considered to belong to the dominant periods both in the lower ionosphere and the F2 region. With the use of data from central Europe, similarities and differences between the 5- and 10-day wave activities in foF2 and the radio wave absorption in the lower ionosphere are studied.

On the 18-day quasi-periodic oscillation in the ionosphere

The presence and persistence of an 18-day quasi-periodic oscillation in the ionospheric electron density variations were studied. The data of lower ionosphere (radio-wave absorption at equivalent frequency near 1 MHz), middle and upper ionosphere (critical frequencies f0E and f0F2) for the period 1970–1990 have been used in the analysis. Also, solar and geomagnetic activity data (the sunspot numbers Rz and solar radio flux F10.7 cm, and aN index respectively) were used to compare the time variations of the ionospheric with the solar and geomagnetic activity data. Periodogram, complex demodulation, auto- and cross-correlation analysis have been used. It was found that 18-day quasi-periodic oscillation exists and persists in the temporal variations of the ionospheric parameters under study with high level of correlation and mean period of 18–19 days. The time variation of the amplitude of the 18-day quasi-periodic oscillation in the ionosphere seems to be modulated by the long-term solar cycle variations. Such oscillations exist in some solar and geomagnetic parameters and in the planetary wave activity of the middle atmosphere. The high similarities in the amplitude modulation, long-term amplitude variation, period range between the oscillation of investigated parameters and the global activity of oscillation suggests a possible solar influence on the 18-day quasi-periodic oscillation in the ionosphere.

Modeling of profile effects for inductive helicon plasma sources

A computer code for modeling existing and new helicon sources for materials processing has been developed. The Nagoya type-III, helical, and Stix coil antennas have been modeled to study and examine plasma density and temperature profile effects on power absorption of a small fraction (n/sub fe//n/sub e//spl ap/5%) of fast electrons (T/sub e-fast//spl ap/40 eV) which provide ionization of the neutral gas in the experiment, and bulk (T/sub e-slow//spl ap/3 eV) electron distributions in an argon gas. The "ANTENA" computer code, originally written by McVey (1984) to study ion cyclotron waves, was modified and used to study and model helicon sources. A collisional model that includes radial density and temperature profiles was added to the code to study the effect of collisions on the heating mechanisms. The competing effects of collisional and Landau damping heating mechanisms have been investigated in detail, and results indicate that collisions play an important role in the plasma absorption profile at high densities (n/sub c//spl ges/10/sup 13/ cm/sup -3/). The radio frequency wave absorption profiles are sensitive to the plasma density and temperature profiles. The partial-turn helix antenna, that solely excites the m=+1 azimuthal mode, is found to be more efficient in coupling the power to an assumed plasma profile than the Nagoya type-III. The Stix coil is also found to be promising due to its on-axis peaking of the wave heating fields.

A test for the solar cycle variability of ionospheric radio wave absorption by a 1 technique

The quiet time solar cycle variability of ionospheric radio wave absorption of 2. 132 MHz at Delhi is compared with the absorption calculated by using the well known aeronomical (six-ion) and empirical (IRI-1990) models of electron density profiles and standard collision frequency distributions. It is found that the observed increase of absorption in solar cycle 21 22 is by a factor of 2.5 greater than values computed with IRI. These latter are almost satisfactory for low and moderate values of the 1–8 Å solar X-ray flux. Absorption and X-ray flux are well correlated for low and moderate, not so for high flux values.

Gravity wave activity in the lower ionosphere and in the F2 region — Similarities and differences

The gravity wave activity (T ∼ 10 min – 3 hours) is inferred from the continuous digital nighttime LF radio wave absorption measurements in the lower ionosphere and from ionosonde measurements (from rapid sequence measurements) in the F-region. All measurements have been performed at the Pru̇honice Observatory (∼ 50°N, 15°E). During solar proton events we observe a simultaneous and similar change of the gravity wave activity at both ionospheric levels (an increase of activity at shorter periods), which very probably indicates the important role of high latitudinal ionospheric/thermospheric source of gravity waves. Also spread-F conditions are associated with simultaneous changes of gravity wave activity in both regions.

Experimental estimates of the value and temperature dependence of the air-humidity quadratic component of the atmospheric water-vapor absorption coefficient in the frequency band of 140?410 GHz

The water-vapor absorption of radiowaves with frequencies of 138, 247, 260, and 340 GHz was measured over a near-earth path with a length of 1 km. The obtained experimental data together with earlier measurements in atmospheric radio windows with centers near 220, 340, and 410 GHz (carried out at the Radiophysical Research Institute) were used to estimate the value and temperature dependence of the air-humidity quadratic absorption component with allowance for the known laboratory-experiment data on its linear component.

An imaging riometer for ionospheric studies

An imaging riometer for ionospheric studies (IRIS) is described. This phased-array system has been continuously operating in northern Finland since late 1994, and permits spatial observation of ionospheric absorption of radio waves over the surrounding area. The paper gives an introduction to the principles of riometry and ionospheric absorption; it describes the physical implementation of the system and presents some initial results. Consideration is also given to the effect of the absorption on radio links operating in the auroral regions, which can be severe.

Trends in planetary wave activity inferred from Hungarian radio wave absorption measurements

The nighttime and sunset LF radio wave absorption measured at Nagycenk, western Hungary over 1967–1991 is used to infer planetary wave activity and its long-term trend in the upper middle atmosphere (∼85 – 100 km). The very moderate positive and mostly statistically insignificant trends are consistent with the pattern provided by previous analyses of various day- and night-time absorption measurements. The trends could be of anthropogenic origin.

Observations of solar‐wind‐driven progression of interplanetary magnetic field BY‐related dayside ionospheric disturbances

Observations from August 2 and 3, 1991, of poleward progressing, dayside convection disturbances accompanied by geomagnetic perturbations and ionospheric radio wave absorption have been analyzed and compared to variations in the solar wind parameters as observed from the IMP 8 satellite. The convection disturbances appear to start at dayside cusp latitudes from where they progress antisunward to high latitudes. The reported observations have enabled calculations of the progression directions and velocities and precise estimates of the delays between solar wind variations as measured by the IMP 8 satellite and ionospheric convection changes as observed from an array of polar magnetic observatories. The progressing ionospheric disturbance events occur during intervals of southward interplanetary magnetic fields (negative interplanetary magnetic field (IMF) BZ component); they are found to be closely related to variations of the east‐west component BY of the IMF. The close coupling between the solar wind and the polar ionosphere(s) is explained in an open magnetospheric model in which the geomagnetic field extending from a localized region of the dayside polar cap merges with the southward interplanetary field. Variations in the IMF BY component are reproduced in corresponding modulations of the east‐west component of the plasma flow at the ionospheric foot points of the connecting “open” field lines. The perturbations of the plasma flow persist while the open field lines are convected with the ionospheric plasma across part of the dayside polar cap. The observed geomagnetic perturbations result from the combined effects of field‐aligned currents and horizontal ionospheric currents, notably the convection‐related Hall currents. The associated radio wave absorption events are explained as the result of E region electron heating by the horizontal electric fields associated with the convection enhancements.

Dynamics of Cusp-Latitude Absorption Events Observed by Imaging Riometers.

The recently developed imaging riometer technique has proven a valuable tool for investigations of the complex dynamics of auroral and polar radiowave absorption events. These events mostly relate to the precipitation of energetic particles into the upper atmosphere to create enhanced ionization in the D-region at low altitudes. A further mechanism which may cause enhanced ionospheric radiowave absorption is electron collision frequency enhancements due to processes like E-region heating by large electric fields. The imaging riometer observations are particularly well suited to separate spatial and temporal variations in the ionospheric phenomena causing radiowave absorption, and thus to define their extent and motions. This work will discuss the poleward expansion of substorm absorption events, and the variable daytime absorption events related to the modulated precipitation of substorm electrons. The high-latitude nightside observations support the model of substorms being a series of enhancements distributed in latitude and longitude rather than single onset, monotonously expanding events. The individual enhancements could be explained by an augmented current wedge mechanism. The dayside observations indicate that disturbances associated with a strong convection shear may cause enhanced and strongly variable precipitation of the eastward drifting cloud of high-energy electrons generated during substorms. Basis for the analysis are events observed, among other, by imaging riometers at the cusp-latitude observatories in Sdr. Stromfjord (inv. lat.= 73.7°) and Danmarkshavn (inv. lat. = 77.3°) in Greenland, and in Ny-Alesund (inv. lat. = 76.1°) at Svalbard, and at the auroral zone observatory in Tjornes (inv. lat. = 66.9).

Relationship between the Radio Wave Absorption and Horizontal Winds in the Lower Ionosphere over East Siberia.

The horizontal wind field in the ionospheric D-region belongs to important parameter related to ionospheric radio wave absorption. The analysis of continuous ionospheric vertical radio sounding and wind measurements by the spaced receiver method in low-frequency range (Irkutsk, East Siberia 1975-1991) is applicated to define the possible coupling of absorption and dynamics in their seasonal and year-to-year dependence. For the analysis we used parameter dfmin =fmin -fmin median, prevailing winds, amplitudes and phases of semidiumal tides for zonal and meridional winds. For some seasons there are significant but changeable correlations between dfmin and prevailing wind. In most cases especially in winter the changes of wind outstrip the changes of absorption. Quasi-periodic fluctuations with planetary wave periods (18-30 days) exist in winter for both. The periodicity close to 27 days (26-30 days) exists for all seasons and all parameters. Application of correloperiodogram analysis reveals the response of ionospheric absorption to the dynamical processes, quantitatively depending on the solar and geomagnetic activity.

Rocket electron density profiles over Volgograd, Heiss Island and Esrange during the DYANA campaign

The results of the determination of electron density ( N e ) profiles at Volgograd, Heiss Island and Esrange ranges are discussed. Some of these profiles were obtained at solar zenith angles <90°, but at times near the sunset, and others at night-time. The profiles obtained during the DYANA campaign were compared with others obtained in the past, with solar and geomagnetic indices as well as with the temperature and atmospheric circulation in the stratosphere and mesosphere. The daytime electron densities below 85 km over Volgograd do not show any appreciable connection to the temperature. An anomalous period of low electron densities observed from February to the beginning of March rather seems to be connected with the meridional and vertical components of the atmospheric wind. We can speculate that the high night-time N e -values at middle latitude may be considered as a manifestation of a post-storm effect. The relationship of the N e -values at higher altitudes (>80 km) and radio wave absorption seems to be connected with the atmospheric circulation and geomagnetic activity. The N e -values in the polar cap region (Heiss Island) are below those at the same altitude in the auroral zone (Esrange), but higher than at middle latitudes (Volgograd). The N e -values over Heiss Island seem to be connected with the geomagnetic activity and the meridional component of atmospheric circulation; the thermal control of the D-region cannot be excluded.

Lower ionosphere at middle latitudes: its morphology and response to meteorological and solar-terrestrial activity during the DYANA campaign 1990

A large number of radio wave absorption, ƒ min , and IPHA data measured in Europe and East Asia are investigated over the DYANA campaign period (1 January–31 March 1990) from the point of view of their response to meteorological and solar/geomagnetic forcing, of winter anomaly, and of a brief description of the state of the lower ionosphere on rocket salvo days. European mid-latitudinal measurements of electron density are briefly mentioned. The meteorological/solar control of the mid-latitudinal lower ionosphere during DYANA is considerably weaker than during MAP/WINE (winter 1983/84), but is evidently stronger than during MAP/SINE (summer 1987). The most distinct feature of the DYANA winter is a quite different response of the mid-latitudinal lower ionosphere to meteorological variability (namely to the 10 hPa North Pole temperature— indirect characteristics of a global state of the polar vortex) in the first and second halves of the DYANA period. The winter anomaly displays an expected magnitude and pattern during the DYANA winter. There is an expected weaker response of the lower ionosphere to the strong minor stratospheric warming of the first half of February. All three rocket salvos at Biscarosse (CEL) were carried out when the state of the lower ionosphere was within the range of normal (non-extremal) conditions. Various rocket salvos at Andoya took place on days of considerably different conditions in the lower ionosphere at higher mid-latitudes. Partial reflection measurements at Nizhny Novgorod show a clear tendency to a decrease of the electron density gradient between 70 and 80 km during March 1990.

Gravity-wave activity and its relation with prevailing winds during DYANA

The DYANA campaign in early 1990 gave a unique opportunity to study the gravity wave (GW) activity in the middle atmosphere in relation to the general circulation. Several instruments, including Rayleigh lidars, ME and meteor radars, OH and O 2 spectrometers and radio-wave absorption receivers provided data at various locations in Europe and Canada to study the GW activity during the campaign. These sets of data indicate that the GW activity is in general enhanced during periods of strong westerly prevailing winds or high planetary-wave activity in the middle atmosphere. When two sets of data are compared, they show a rather fast decorrelation as a function of altitude difference or horizontal distance. This study confirms that the level of GW activity in the middle atmosphere is very sensitive to filtering by the mean wind, but does not exclude its sensitivity to the sources of GW.

Trends in planetary wave activity in the upper middle atmosphere inferred from the nighttime LF radio wave absorption measurements

The nighttime LF radio wave absorption in the lower ionosphere measured at two frequencies in central Europe over 1963–1985 is used to infer planetary wave activity and its long-term trend in the upper middle atmosphere (∼90–100 km). The observed positive trend is roughly consistent with results based on daytime absorption. Nighttime results are less pronounced and less statistically significant probably due to perturbing effects of geomagnetic activity. The observed trends, which are probably of anthropogenic origin, are together with the daytime results [3,4] the first evidence of long-term trends in planetary wave activity in the upper middle atmosphere.

The role of natural E-region plasma turbulence in the enhanced absorption of HF radio waves in the auroral ionosphere:Implications for RF heating of the auroral electrojet

Abstract. Physical processes which affect the absorption of radio waves passing through the auroral E-region when Farley-Buneman irregularities are present are examined. In particular, the question of whether or not it is legitimate to include the anomalous wave-enhanced collision frequency, which has been used successfully to account for the heating effects of Farley-Buneman waves in the auroral E-region, in the usual expression for the radio-wave absorption coefficient is addressed. Effects also considered are those due to wave coupling between electromagnetic waves and high-frequency electrostatic waves in the presence of Farley-Buneman irregularities. The implications for radio-wave heating of the auroral electrojet of these processes are also discussed. In particular, a new theoretical model for calculating the effects of high-power radio-wave heating on the electron temperature in an electrojet containing Farley-Buneman turbulence is presented.

On the transformation of planetary waves of tropospheric origin into waves in radio wave absorption in the lower ionosphere

Calculations are carried out of upward propagation of a tropospherically forced 10-day planetary wave into the upper middle atmosphere with the use of the COMMA-R model of the University of Cologne, of its transformation into a wave in electron density by means of the model of the Comenius University, and of its final transformation into a wave in radio wave absorption in the lower ionosphere applying the computer code of the Geophysical Institute. The calculations show that the absorption may be used for investigating the planetary wave activity, particularly of its long-term trends. The possibility of propagation of planetary waves from the winter hemisphere to the summer hemisphere is illustrated, which could contribute to explanation of the occurrence of travelling planetary waves in the mesosphere in summer.

The role of natural E-region plasma turbulence in the enhanced absorption of HF radio waves in the auroral ionosphere:Implications for RF heating of the auroral electrojet

<strong class="journal-contentHeaderColor">Abstract.</strong> Physical processes which affect the absorption of radio waves passing through the auroral E-region when Farley-Buneman irregularities are present are examined. In particular, the question of whether or not it is legitimate to include the anomalous wave-enhanced collision frequency, which has been used successfully to account for the heating effects of Farley-Buneman waves in the auroral E-region, in the usual expression for the radio-wave absorption coefficient is addressed. Effects also considered are those due to wave coupling between electromagnetic waves and high-frequency electrostatic waves in the presence of Farley-Buneman irregularities. The implications for radio-wave heating of the auroral electrojet of these processes are also discussed. In particular, a new theoretical model for calculating the effects of high-power radio-wave heating on the electron temperature in an electrojet containing Farley-Buneman turbulence is presented.

Interpretation of basal ice conditions from radio-echo soundings in the eastern Heimefrontfjella and the southern Vestfjella mountain ranges, East Antarctica

Differences in radio-echoes received from the base of the ice sheet covering two different mountain areas in Dronning Maud Land, East Antarctica, are herein interpreted as differences in the physical properties in the ice and differences in basal conditions. An abnormally strong signal can be caused either by unusually little absorption of radio waves within the ice or by water at the base of the glacier. One of the areas studied is totally covered by a 400–1400 m thick ice sheet. The altitude of the surface is about 2800 m a.s.l. The second area is a nunatak area situated relatively closer to the coast. The ice thickness in the valleys is about 500 m and the altitude of the surface is about 400 m. The annual mean surface temperature for the areas is about −30° and −17°C respectively. In both cases ice depths are moderate and flow rates low, and it is thus probable that the ice is frozen to the bed. However, in the colder locality I believe the strong echo is caused by low or negligible rate of mass flux. In the lower area strong bottom echoes are believed to be a cause of negative mass balance at the surface of the blue ice areas. The negative surface mass balance reverses the vertical component of ice flow and it also changes the temperature distribution within the ice. Basal melting conditions may thus occur at locations where the ablation rate is high. This process may also have important implications on the development of land forms under cold ice sheets.