D-region Electron Density Profiles Research Articles

First ever VLF monitoring of the lunar occultation of a solar flare during the 2010 annular solar eclipse and its effects on the D-region electron density profile

A ground based Very Low Frequency (VLF) radio receiver of Indian Centre for Space Physics located at Khukurdaha (22°27′N, 87°45′E) monitored the VLF signal at 19.8kHz from the NWC station during a partial solar eclipse (maximum obscuration 75%) which took place on January 15, 2010. The receiver and the transmitter were on two opposite sides of the annular eclipse belt. During the same period, a solar flare also occurred and it was partly blocked by the lunar disk. Thus the resultant signal was perturbed both by the eclipse and by the flare. The deviation of the signal from the normal value was obtained by subtracting from the average diurnal signal on days bracketing the eclipse. The deviation was analysed. We compare the data from GOES-14, HINODE and RHESSI satellites during the event. We got a clear depression in the data during the period of the partial eclipse. Most interestingly, there was also a flaring activity in the sun which reached its peak (C-type) just after the time when the eclipse was near maximum. By superposing the lunar disk on the image obtained by HINODE mission, we pin-point the time frame of blocking hard and soft X-rays. We extract the time variation of the electron density profile in the D-region of the ionosphere due to occulted solar flare from the combined effect of the eclipse and the flare. We also compare the results with a normal solar flare.

Electron density profiles in the ionospheric D-region estimated from MF radio wave absorption

Electron density measurements in the lower ionosphere were carried out more than 6 times during the period from 1975 to 1992 by using sounding rockets launched at KSC (Kagoshima Space Center in Japan). Low electron densities were estimated from the absorption of the characteristic mode of ground-based radio signals (17.4 kHz and 873 kHz) in the lower ionosphere measured by onboard receivers. Two kind of methods, i.e., VLF mode absorption and MF absorption methods were developed to estimate the D-region electron density by comparing the observed wave intensity with that calculated by a full wave treatment. In this paper, both absorption methods are introduced paying attention to the capability of low electron density measurement. In particular the S-310-18 rocket experiment is discussed in detail, in which the D-region electron density profile derived from the altitude variation of MF radio wave intensity is presented. Finally the lower ionospheric electron density profiles so far measured by those method at mid-latitude in Japan are compared with those of the IRI-95 model.

Combined ionospheric effect due to Forbush decreases and magnetospheric high energy particles at mid-latitudes

After geoactive solar flares the Forbush decrease of galactic cosmic rays and magnetospheric high energy particles can cause opposite ionization effects in the ionospheric D-region at mid-latitudes. This has been shown by the change of the ELF and VLF atmospheric radio noise level and that of the LF absorption. The frequency dependence of these variations has been interpreted by waveguide mode theory and full-wave solutions using D-region electron density profiles under conditions characteristic of these ionization disturbances.

High-resolution incoherent scatter radar measurements during electron precipitation events

The EISCAT UHF radar was used to record high-resolution measurements of D-region electron density profiles during periods of energetic electron precipitation. Two sudden onsets in precipitation are studied in detail. The events occurred on 23 August 1985 in the post-midnight and evening sectors. The electron spectra deduced from the measurements indicate that each onset is associated with a complex influx of soft and hard energy particles. Examples are given of periods when the EISCAT radar measured significant variations in electron density even when the riometer system recorded no activity.

Modelling of the lower ionosphere

The model of calculations of electron density profiles in D-region is suggested. The model includes four positive ions, four negative ions and electrons. The effective rate coefficients were received from detailed models of ionization-recombination cycle. The calculations, which were made, and the comparisons with experimental data (Ne-profiles and their variations, absorption of radiowaves) have showed, that in general the model described the basic features of D-region parameters.

Development of D-region electron and ion densities under various auroral conditions during the Energy Budget Campaign (EBC)

D-region electron density profiles and time variations were obtained during the Energy Budget Campaign 1980 by a partial reflection radar at Ramfjordmoen, Tromsø, located between the rocket ranges at Andøya and Kiruna. The observations were made under various geophysical conditions which are illustrated by riometer observations. The partial reflection measurements indicate that the rockets were launched into a relatively stable D-region on two occasions, while it was somewhat more disturbed on the third. A comparison between the electron density profiles derived by the partial reflection technique and rocket borne probes and Faraday rotation experiments does indicate fair agreement during the quiet conditions, but relatively large discrepancies during disturbed conditions. Simultaneously derived electron density profiles, by use of the Faraday technique, and ion density profiles, by gridded electrostatic spheres mounted on the rocket payload, have made it possible to estimate the negative ion to electron density ratio A versus height. These values of λ are within the range of model calculations.

D-region processes at equatorial latitudes

Our current understanding of the ionization production and loss processes, and electron and ion density distributions in the undisturbed D-region are reviewed with special reference to the equatorial latitudes. The importance of combining ground-based measurements with rocket-borne measurements in the development of D-region electron density profile models is stressed. The need for coordinated experiments at low latitudes for investigating outstanding problems related to the quiet D-region is emphasized.

The accuracy and resolution of model ionospheres derived from VLF propagation parameters

An inversion technique is outlined for calculating the resolution and gross error associated with D-region electron density profiles obtained by modelling from a given set of VLF radiowave propagation parameters. Two profiles, representing quiet and disturbed ionospheric conditions, are examined in detail and the height resolution and accuracy obtainable from the amount of observational data used in constructing these profiles is determined.

An experimental and theoretical study of the D-region—I. Mid-latitude D-region electron density profiles from the radio wave interaction experiment

In Part I a summary of quiet D-region electron density measurements made by tlie wave interaction technique at The Pennsylvania State University for the period October 1969 through January 1971 is presented. The results include the mean features of noontime electron density profiles as well as the diurnal and seasonal variations. Detailed discussion on the methods of data analysis, including a critical evaluation of errors, is also presented. The results presented in the part form a partial basis for developing the semi-empirical D-region model presented in Part II.

Studies of the modal parameters of VLF radiowaves propagated below the night-time ionosphere

Calculations of the modal parameters of VLF radiowaves propagating to great distances at night show a marked dependence on the D-region electron density profile. The two models considered are those of Deeks (1966) and Smith etal. (1968). The parameters calculated using the latter model are similar to those obtained by other workers for exponential conductivity profiles. The variations of the mode parameters with wave frequency are described for four directions of propagation. The azimuth dependence is also investigated for a limited number of frequencies. An interesting feature of the results is the apparent interchange of identity between mode 1 and mode 2 at about 18 kHz for the Deeks profile.

Ionospheric observations during the annular solar eclipse of 20 May 1966—III Four D-region electron density profiles measured by rocket techniques

Measurements of Faraday rotation and of differential absorption on frequencies 2.2, 3.9 and 7.8 MHz have been used to determine the height distribution of collision frequency and electron concentration during the annular solar eclipse of May 20, 1966 and on a comparison day.

Comparison of A3-absorption measurements at 1.f. and rocket measured D-region profiles

The paper is concerned with the comparison of ground-based absorption results at low frequencies and available rocket measurements of D-region electron density profiles at comparable solar zenith angles in the same season. Full-wave treatment is used. In spite of considerable longitude and latitude differences between these measurements, there is reasonable agreement between computed and observed absorption, if the rocket measured electron density and collision frequency profiles are used.

D-region electron density measurements during the solar eclipse of May 20, 1966

Rocket (Underwood and Muney, 1967) and satellite observations of 2–8 Å X-ray emissions indicate that the solar eclipse of May 20, 1966, occurred during a period in which the Sun was moderately active. The Goddard Space Flight Center conducted a series of small sounding rocket experiments to measure the ionospheric D-region electron density profile during successive stages of the eclipse. Compared to the uneclipsed profile, an order of magnitude reduction is observed in the D-region profile obtained near totality. From the electron density measurements together with the X-ray spectral data of Underwood and Muney it is concluded that on May 20, 1966, 2–8 Å X-rays were not the dominant source of D-region ionization below approximately 80 km.

Growth of the D-region at sunrise

D-region electron density profiles of high accuracy (±10 per cent), and high resolution (0·1 km) are presented for each of 5 Nike Apache rockets launched at sunrise from Wallops Island, Virginia at solar zenith angles of 108, 96, 94, 94, and 85°. Orderly increases of electron density between 70 and 80 km are observed as the solar zenith angle decreases from 108 to 94 degrees. At a zenith angle of 94°, the observed electron density between 87 and 90 km is larger by a factor of 6 on a morning of high VLF absorption which commenced at 98° compared with mornings of normal VLF absorption which commenced at 94°. At a zenich angle of 85°, a “ C-layer” or local maximum electron density of about 90 cm −3 at 68 km, and a decreased electron density at 75 km are observed.