The influence of sunspot number and magnetic activity on the diurnal variation of the geomagnetic field at mid to high latitudes

The variations of the diurnal range of the geomagnetic field with sunspot number and with magnetic activity was studied at mid and high latitude stations in the northern hemisphere at different seasons. The effect of increasing sunspot number is small at lower latitudes and increases with geomagnetic latitude, while the effect of increasing magnetic activity is to increase the range at all latitudes, very greatly at the higher geomagnetic latitudes.     

The seasonal variation of the diurnal thermospheric winds over Millstone Hill during solar cycle maximum

Incoherent scatter radar measurements of the diurnal variation of the electron and ion temperatures, electron density and vertical plasma drift are used to determine the diurnal variation of neutral winds at F-layer heights over Millstone Hill (42.6°N, 71.5°W) for different seasons. The technique to derive the thermospheric winds from incoherent scatter radar data and the results for equinox conditions have been discussed by Roble et al. (1974). Data for three summer and three winter days in 1969 and 1970 are examined for seasonal characteristics during geomagnetic quiet time at solar cycle maximum conditions. The derived diurnal variation of the neutral winds for summer months shows a later morning transition from equatorward to poleward winds and an earlier evening transition from poleward to equatorward winds than during winter months. These times of transition have a pronounced influence on the ionospheric structure and are partly responsible for the characteristic ‘summer’ and ‘winter’ type behavior of the ionosphere observed over Millstone Hill. The diurnally averaged values of the thermospheric winds at 300 km also have a seasonal variation. During summer, the zonal winds are westward at about 15 m⁻¹ and are eastward at about the same velocity during the winter. The diurnally averaged meridional winds show a strong equatorward flow during summer of about 50 m s⁻¹ whereas during winter months the winds are considerably weaker and irregular, with an average equatorward flow on some days and poleward flow on others. The zonally averaged values are consistent with a mean meridional circulation from the summer hemisphere to the winter hemisphere at F-layer heights. However the irregular meridional winds in the winter indicate a weakening of this circulation near Millstone Hill.     

The vernal-autumnal asymmetry in the seasonal variation of geomagnetic activity

In intervals in which the polarity of the main solar dipole field is stabilized, a 12 month wave occurs in geomagnetic activity (indices aa, Ap, Dst) with its maximum in one of the equinoctial periods. Whether the vernal or the autumnal maximum is greater depends on the polarity of the main solar dipole; the existence of the wave may be explained by the north-south asymmetry in the main solar dipole field. The results favour the southward component of the interplanetary magnetic field as the decisive factor for geomagnetic activity.     

The day-to-day variation of atmospherics activity over the South American continent

Atmospherics activity at 5 kHz, observed during the southern summer at San Miguel, Argentina, is presented. Data are available for all directions of incidence during the time span of this investigation.The activity shows a pronounced variation from day-to-day. A spectral analysis of these data with an autocovariance function and the related power spectrum yields enhanced spectral contributions with periods at 2.4 and 7.5 days. An additional smaller peak in the power spectrum was also observed at a period of 4.7 days. These periods have the same time scale as planetary waves.     

The computation of ELF radio wave fields in the Earth-ionosphere duct

Evaluation of ELF radio wave fields using the mode theory requires the computation of Legendre functions. Four different representations of the Legendre functions in terms of hypergeometric series have been programmed for computation on a microcomputer. By an appropriate choice of series for given values of the propagation constant and distance from the source, only a few (typically eight) terms are needed to calculate the Legendre functions with a precision of six decimal digits.     

Control of diurnal and seasonal variation of particle precipitation by regular changes of the interplanetary magnetic field

Systematic changes of the position of the dipole axis of the Earth's magnetic field with respect to the solar axis induce distinct daily and seasonal variations of the vertical B_z-component in the solarmagnetospheric coordinate system (B_ZSM). Depending on the direction of the interplanetary magnetic field (IMF), negative B_ZSM- values are produced in spring by T polarity and in autumn by A polarity, whereas in the diurnal variation lowest B_ZSM-values have been calculated to occur near 23 UT for T, and near 11 UT for A polarity, respectively. In different ionospheric and geomagnetic parameters measured at high and midlatitudes increased precipitation of high energetic particles into the lower thermosphere and upper mesosphere has been detected during periods with negative B_ZSM-components. The seasonal variation of the parameters investigated, with maximum values near the equinoxes, as well as a part of their diurnal variations, can thus be explained by particle precipitation being markedly controlled by the IMF sector structure.     

On ELF transmission in the Earth-ionosphere waveguide

The propagation constant for ELF (extremely low frequency) propagation in the Earth ionosphere waveguide is determined analytically. The derivation carried out for a planar model, with the Earth's surface impedance Z_g> 0, confirms the important result obtained earlier by Greifinger and Greifinger [(1978), Radio Sci. 13, 831] in the limitZ_g = 0. The present method avoids the use of auxiliary potentials.     

The relation between ionospheric profiles and ELF propagation in the Earth-ionosphere transmission line

Based on an approximate wave solution, it is shown that, for reflexion of ELF waves from a given ionosphere described by a simple profile of ionization density, the phase-integral method may be used above a certain level, and the ionosphere may be abolished below this level. The height of ELF reflexion thus determined is independent of angle incidence, but not of frequency. The level is the one where the ionosphere passes from rapidly varying to slowly varying behavior, judged in relation to the local wavelength.An approximate solution is obtained to the mode problem in the Earth-ionosphere transmission line in terms of four crossing plane waves, one pair having O wave polarization and the other X wave polarization. The velocity of phase propagation is calculated, and also the rate of attenuation due to leakage of energy into the region above the level of reflexion. The attenuation rate due to collisional absorption below the level of reflexion is also calculated using a method similar to that employed for dielectric loss in an engineering transmission line.As the frequency descends through the ELF band, penetration of the D-region occurs in succession for the O and X waves, leading to reflexion from the E-region at the Schumann resonant frequency and penetration of the ionosphere at micropulsation frequencies. Under quiet day-time ionospheric conditions the penetration frequency-band for the D-region is around 20–60 Hz in middle and high latitudes, but around 75–100 Hz at the equator. At a frequency low enough to be reflected primarily from the E-region under quiet ionospheric conditions, an increase in D-region ionization that is just sufficient to transfer primary reflexion from the E-region to the D-region results in an increase in the rate of attenuation. On the other hand, when once reflexion is firmly established at the lower level, further increase of ionization in the D-region causes a reduction in the rate of attenuation. Similar effects are expected to occur at night in association with a sub-E-region ledge of ionization. Small variations in the ionization profile of such a ledge are the likely cause of night-time fluctuations of transmission at 45 and 75 Hz.     

Non-linear interaction between the diurnal and semidiurnal tides: terdiurnal and diurnal secondary waves

Many years of measurements obtained using French meteor radars at Garchy (Lat. 47°N) and Montpazier (Lat. 44°N) are used to show the existence of an 8 h oscillation. Some examples of the structure of this wave are displayed and compared with measurements performed at Saskatoon (Lat. 52°N) and Budrio (Lat. 45°N). This wave can be interpreted as the solar driven terdiurnal tide, or as the result of the non-linear interaction between the diurnal and semidiurnal tides. Both hypotheses are tested with numerical models. Incidentally, the possible existence of a 24 h wave resulting from this interaction is also studied.     

ELF/VLF propagation measurements in the Atlantic during 1989

The vertical electric field component was measured by a group of the Ukrainian Institute of Radio Astronomy on board the Professor Zubov scientific vessel during April 1989 at latitudes from 30°S to 50°N. Results of the amplitude measurements in the Atlantic of natural ELF radio signals and those from the VLF navigation system “Omega” at its lowest frequency of 10.2 kHz are given. Characteristics were obtained of the moving ship as the field-site for the ELF observations. Variations in the ELF radio noise amplitude recorded at tropical latitudes agree with the computed data for the model of three continental centres of lightning activity. The VLF results were obtained by the “beat” technique providing the simplest narrow-band amplitude registration. Range dependencies of the field amplitudes from A (Norway), B (Liberia) and F (Argentina) stations have been analysed. The VLF attenuation factor was estimated for the ambient day conditions along the four cardinal directions. This allowed the detection of a statistically significant attenuation difference between the east-west and west-east propagation paths. The VLF radio signal was also used as a probe to evaluate the effective height of the vertical electric antenna and to calibrate the ELF noise amplitudes.     

A modified technique to locate the sources of ELF transient events

An algorithm is proposed for the location of the sources of ELF transient events which occur in the Schumann resonance range. The procedure yields a uniformly rotating vector in the frequency domain. The vector is formed out of the recorded vertical electric and horizontal magnetic field components. The changes of this vector with frequency provide a pair of equations for the evaluation of two of the three unknown parameters: these are the distance from the source, phase and attenuation constants of the waveguide.     

A nonlinear simulation of the thermal diurnal tide

A simplified middle atmosphere general circulation model is used to investigate the nonlinear behavior of the thermal diurnal tidal waves. In the model, only a westward moving diurnal tide generated by heating with zonal wavenumber 1 is considered. The tidal wave propagation is simulated by a full nonlinear calculation with a convective adjustment scheme and a Richardson number dependent vertical eddy diffusion.The numerical results show that the growth of the diurnal tide due to the density stratification is effectively suppressed and a relatively constant amplitude distribution with height is realized by the convective adjustment in the lower thermosphere. It is also shown that mean zonal winds and mean meridional circulations are induced by the diurnal tidal waves in the region where the tidal waves are breaking by convective instability, in accordance with the wave-mean flow interaction theorem.     

Dependence of seasonal variation of absorption on solar activity in the equatorial ionosphere

From data for the absorption of radio waves at oblique incidence in Lagos, and at vertical incidence at Colombo, the seasonal variation of absorption at the two sites are examined. It is shown that, if subsolar absorption be assumed to depend upon sunspot number, the cos X law gives the same index for both the diurnal and the seasonal variation of absorption.     

Characteristics of ionospheric ELF radiation generated by HF heating

This paper describes new observations of the characteristics of ELF generation produced by modulation of the dynamo current system from HF heating of the ionospheric D-region. A model of the ELF antenna structure embedded in the D-region is described and stepped ELF frequency observations are shown to support the model assumptions. Presented are data on the phase height of the ELF ionospheric antenna versus ELF frequency, polarization of the downgoing wave and relationship to the dynamo current direction, correlation of ELF field strength with per cent cross-modulation, power linearity tests and duty cycle results. All observations used the high power heater facility of the Arecibo Observatory.     

A note on the diurnal averaging of aeronomical models

We have developed an approximate technique for diurnally time-averaging atmospheric photochemical-dynamical models which eliminates the need for a detailed numerical resolution of sunrise and sunset transitions. In its application, our scheme is equivalent to scaling certain chemical rate constants and photodissociation coefficients by appropriate aeronomical factors. To calculate the scaling factors, we parameterize diurnal variations with a step-function behavior, assuming that each species has a constant day-time and night-time concentration whose ratio we can determine by analyzing the chemical interactions occurring after sunset. Our solution accounts for the effects of night-time reactions on the 24 h average values of species abundances and on the average daily rates of the catalytic processes consuming ozone in the stratosphere. We demonstrate the accuracy of our technique by comparing its predictions to those of a full diurnal simulation; typically, the precision is better than 10%. By contrast, we show that the use of some other well-known computational schemes can result in significantly larger predictive errors—up to an order of magnitude for the N₂O₅ concentration as an extreme example.     

ELF emission in the outer ionosphere stimulated by short fractional hop whistlers

Results of a detailed study by digital methods of the energy spectra and the frequency-time characteristics of electromagnetic radiation stimulated by short fractional hop whistlers (S.W.) observed on Intercosmos 14 are presented. The phenomenon was observed at frequencies below I kHz in the ionosphere at altitudes from 360 to 450 km during summer night. It has been shown that the intensity of the stimulated emission is greater by an order of magnitude or more than that of the natural noise background and of the same order of magnitude or a little lower than the intensity of the triggering S.W. The duration of the stimulated emission varies from 0.27 to 0.64 s. The energy spectra of this emission have two maxima at frequencies of 730+ 35 Hz and 940+ 35 Hz, with a bandwidth ΔF ≃ 150 to 300 Hz.     

A diurnal study of the electrical structure of the equatorial middle atmosphere

Electrical parameters measured from 115 km down to below 20 km during the Project Condor campaign at the Punta Lobos Rocket Range near Lima, Peru, are presented Ten rocket-launched payloads measured electrical conductivity. A strong diurnal influence due to solar ultraviolet radiation is shown. Nine of the payloads also measured electric fields. No large mesospheric vertical electric fields are found in the data. A calculation of the d.c. global conduction current density at 18 km is smaller than previously measured at low latitudes and does not show the conventional diumal variation.     

Interplanetary magnetic field structure and the variations of the ELF and VLF emissions in the topside ionosphere

The Intercosmos-13 data obtained when measuring ELF and VLF emission amplitudes during the vernal equinox of 1975 at auroral latitudes and over the polar caps are compared with certain IMF parameters [the polarity of the sector structure, the signs and magnitudes of the B_y, and B_z, components of the IMF as defined in the Solar Ecliptic coordinate system (Nishida, 1978)]. The comparison shows that:

• 1.(i) the positive polarity of the IMF sector structure (when IMF vector is directed toward the Earth) involves an enhanced probability of the detection of larger emission field intensities (>25–30dB);
• 2.(ii) the emission median intensity is ~20dB higher at B_y > 0 compared with B_y < 0;
• 3.(iii) the 0.72 kHz emission median intensity in the polar caps and at night-side auroral latitudes is lower when B_z > 0 as compared with B_z < 0;
• 4.(iv) at vernal equinox there is no north-south asymmetry in the dependence of ELF and VLF emission intensity on the IMF parameters.

     

ELF and VLF wave generation by modulated HF heating of the current carrying lower ionosphere

This paper presents further experimental results on ionospheric current modulation, using powerful amplitude modulated HF waves produced by the new heating facility at Ramfjordmoen near Tromsø, Norway. As a result of the current modulation, waves in the ULF, ELF and VLF range can be efficiently generated. The experiments discussed here cover the range from low ELF up to 7 kHz. The observed signal strengths are of the order 1 pT. Decomposition of the received ELF/VLF waves into R- and L-mode shows that both modes are usually of comparable strength. The signal strength as a function of modulation frequency shows pronounced maxima at multiples of approximately 2 kHz. The paper also presents a brief theoretical discussion of the processes involved in the generation of ELF/VLF waves by HF induced current modulation.