Variation Of Ionospheric Absorption Research Articles

Examination of the relationship between riometer-derived absorption and the integral proton flux in the context of modeling polar cap absorption

Energetic protons can penetrate into the ionosphere increasing ionization in the D-region causing polar cap absorption that may potentially block high frequency radio communications for transpolar flights. The protons are guided by the geomagnetic field into the high-latitude polar cap region. Riometers monitor variations in ionospheric absorption by observing the level of background cosmic radio noise. Current polar cap absorption modelling techniques are based on the linear relationship between absorption and the square root of the integral proton flux, which has previously only been demonstrated using data from a single high-latitude polar station. The proportionality constant describing this relationship is evaluated for two different polar cap absorption events occurring 07-11 March 2012 and 23 January 2012 to 01 February 2012. Examination of the proportionality constant using data from riometers distributed between 60° and 90° magnetic latitude reveals a previously unreported latitudinal dependence for data at magnetic latitudes of ≤66.8° on the dayside and ≤70.8° on the nightside. Incorporating the latitudinal dependence into the current D-Region Absorption Prediction (D-RAP) absorption model improves the agreement between measurement-derived and modelled parameters by increasing the correlation coefficient between data sets, reducing the root mean square error, and reducing the bias.

Determination of solar cycle variations of midlatitude ELF/VLF chorus and hiss via automated signal detection

[1] An automated algorithm for detecting chorus and hiss emissions in ground-based extremely low frequency/very low frequency (ELF/VLF) wave receiver data is developed and applied to 10 years of data collected at Palmer Station, Antarctica (L = 2.4, 50°S invariant latitude). The algorithm consists of three major processing steps. First, sferics and power line hum are removed from the broadband data. Second, individual events are detected and a set of 19 scalar event parameters are determined. Finally, on the basis of the parameters, detected events are categorized by a sequential pair of neural networks as either chorus, hiss, or noise. The detector runs on a modern 8-core computer at a speed of 350x real time. Results of training indicate that the neural networks are capable of differentiating between noise and emissions with a 92% success rate and between chorus and hiss with an 84% success rate. Data collected at Palmer from May 2000 to May 2010 were processed, and yearly and seasonal trends of chorus and hiss are analyzed. Yearly occurrence rates of chorus and hiss are strongly dependent on the geomagnetic disturbance level, as measured by Kp and AE, whereas seasonal occurrence rates are more strongly dependent on variations of the day/night terminator and associated variations in ionospheric absorption.

Long-period fluctuations in ionospheric absorption and their relation with planetary activity in the stratosphere

By investigating the quasi-regular variations in ionospheric absorption along three distant radio paths with approximately one and the same equivalent frequency during the winters 1983/84 and 1984/85, an attempt is made to study the stratospheric-mesospheric relations at typical geographic mid latitudes. It is shown that the quasi-regular oscillations of ionospheric absorption, investigated in this paper, reach maximum development in each time series at one and the same time. This means, that if they are provoked by an external source when it starts acting, a spectrum of frequencies is generated rather than a definite fixed frequency. By studying the variation of the instantaneous amplitudes of the quasi-regular oscillations in ionospheric absorption with time, their connection with the amplification of the quasi-stationary planetary waves in the stratosphere with a zonal wave number 2 during the winter periods under investigation is shown.

Impulsive, quasi-periodic variations in ionospheric absorption of cosmic radio noise.

Cosmic radio noise data, acquired with a fast-response 30MHZ riometer at Siple Station, Antarctica (L≈4), show evidence of weak impulsive quasiperiodic variations in ionospheric absorption. Significant amplitude variations approaching 0.02dB, approximately the statistical noise limit of the measurement, have been measured. An event typically will consist of 3 to 5 cycles of oscillation (-60 to 90sec period) and can occur during locally disturbed or quiet geomagnetic conditions. Usually the oscillations are not related closely to magnetic field fluctuations as measured on the ground at Siple or in the conjugate area. Conjugate bremsstrahlung X-ray data, available for one event, suggest that these absorption variations are caused by the impact on the ionosphere of a modulated flux of energetic electrons from the magnetosphere. Limited statistics on the location of the plasmapause suggest that these events occur close to, but on either side of, the boundary of the plasmasphere. These absorption oscillations may be another manifestation of the process that produces short-term intensity and phase variations in fixedfrequency sub-ionospheric LF and VLF wave propagation. Further studies of this unique modulated precipitation/absorption phenomenon and its relationship to hydromagnetic wave disturbances in the magnetosphere will be conducted with a set of riometer and magnetometer stations appropriately sited in a latitude array.

On the role of atmospheric pressure and density variations in ionospheric absorption of radio waves

Under the assumption that the height distribution of NO does not change between summer and winter a height region was found where the neutral atmosphere density, pressure and temperature changes can significantly affect the radio-wave absorption. This is caused by the different behaviour of Lyman-α ionization with respect to other major ionization sources and by the dependence of the effective recombination coefficient on temperature. This phenomenon can qualitatively explain the regular winter increase of non-anomalous absorption ( Schwentek, 1971).

Lunar tidal variations in ionospheric absorption of radio waves at Calcutta

The lunar tidal variation in ionospheric absorption of radiowaves covering a large number of frequencies has been investigated. The maximum lunar tidal effects are observed for frequencies near foE. For all the frequencies, however, the semidiurnal component is always observed to be predominant. Moreover, the seasonal variation shows a minimum during the equinoctial months. The possibility of the dependence of lunar tidal variation of absorption on the similar variations in ionospheric temperatures has been invoked.

Wave-like structures in the variation of ionospheric absorption

In winter 1971/1972, a very clear wave-like structure with a period of about 7 days was found in the day-to-day variation of ionospheric absorption L ( cos χ = 0.2) obtained from recordings of the diurnal variation of absorption on the circuit Norddeich-Lindau (2.61 MHz; 52.6°N; 8.7°E); χ solar zenith angle. It is suggested that this particular pattern, which did not appear clearly in earlier winters, was due to special conditions in the lower atmosphere in winter 1971/1972. A similar and nearly coinciding pattern appeared also in the data obtained from the circuit Aranjuez-Balerma in Spain (2.83 MHz; 38.6°N; 5.2°W) some 1900 km from Lindau. Periods of 20–25 days were also found in the data of some European stations, and a period of 25 days in the data of Japanese stations.

On the lunar semidiurnal variation of theD andF2 layers

The lunar semidiurnal variation of ionospheric absorption at Freiburg and of thef0F2 at, Genova, Freiburg and Leopoldville has been deduced. The results are compared with those obtained by other Authors. Magnetic dip, rather than geomagnetic latitude, is found to be a parameter controlling the morphology of the lunar semidiurnal variation of theF2 layer. Finally, a graphical representation of the amplitude of lunar semidiurnal variation off0F2 as function of the magnetic dip is also given.

On the variation of ionospheric absorption at different stations

The possibility that the day-to-day changes in absorption at Slough, Swansea, and Freiburg are identical is examined statistically. After allowing for known sources of error, it is considered that the residual differences in summer months are too small to bo significant, but that it is probable that real differences occur in winter. In both cases the minimum probable correlation for the Swansea-Slough comparison is very high—about 0.92.