The problem of a polarization of magnetic pulsations generated by heated circular region on the ground for two cases is considered. For first case the analythical expression for magnetic field on the ground was obtained. For latter one the numerical solution was calculated. These case may be associated with auroral arc. Two pulsation periods were taken: 1 sec and 120 sec, for which in the case of the homogeneous ionosphere the polarization being elliptic and linear, respectively. The space distribution of the magnetic field vectors and of the equivalent current lines on the ground was obtained. The linear polarization turned out to be keep the linear tipe of polarization even in presence of the ionospheric inhomogeneity. For elliptic polarization the ellipticity changes at this case. For both tipes of the polarization the position of the polarization ellipses on the ground changes. The influence of size and parameters of the ionospheric inhomogeneity on the polarization characteristics such as ellipticity and angle between major half-axis of the magnetic field ellipse and the external ionospheric electric field was studied.
Ionospheric small - scale wave processes in the F region
initiated by HF heating facilities at N.Novgorod (56.3¸N, 49.3¸E)
have been studied by the method of field-aligned scattering of
diagnostic HF radio signals.Observations are carried out on Kiev-
N.Novgorod - S.Petersburg path by Doppler method during several
heating campaigns with heater effective radiated power ERP = 10,
20 and 100 MW.
For winter experiments the initiation of the oscillations of Doppler frequency shifts fd in the range of Pc 3-4 magnetic pulsations (20-150 s) during heating cycles is established. The obtained results show that the heater power has an impact on horisontal east-west component of electric field E, vertical component of Doppler velocity Vd and amplitude of the vertical displacement M of the heated region. The values of pointed parameters are the following: E = 1.25 mV/m, Vd = 6 m/s, M = 600-1500 m under ERP=20 MW and E = 2.5-4.5 mV/m, Vd=11-25 m/s and M = 1500-5000 m under EPR=100 MW.
The obtained results confirm the hypothesis of excitation of the Alfven resonator by powerful HF radio waves which leads to the generation of the magnetic field line oscillations in the heated region giving rise to artificial magnetic and ionospheric pulsations.
At the end of presentation future experiments using HF heating facility at Tromso are discussed.
Large-scale natural ionospheric irregularities moving in the E region produce local electric fields and currents of polarization due to the small changes of conductivity. Such random currents are the sources of noise-like magnetic perturbations. The presented theory shows that the magnitude of magnetic fluctuations depend on amplitude and dimentions of plasma irregularities, their drift velocity and integrated conductivities of the ionosphere. Changes of E region conductivity due to the heating should influence the discussed effect. Quantitative estimations of the influence of HF heating on magnetic perturbations are given.
The transformation process of electrostatic LHR oscillations on plasma striations into whistlers is discussed within stochastic ap proximation. LHR oscillations are produced via decay process of ordi nary EM pump wave into UHR and LHR modes. It is shown that artificial whistlers have rather broad frequency band and small transversal scales. Their amplitude depends strongly on the quiver velocity of electrons in the pump wave. Estimations show that for most powerful heating faci lities artificial whistlers can be measured on board of satellites.
Scope for using of SEE as a method to study temporal evolution of both low- and high-frequency artificial ionospheric turbulence are discussed. We present experimental data concerning the influence of heater-induced small-scale irregularities on SEE generation. Some results of the DSEE use as a method for diagnostics of the disturbed ionosphere are also considered. We discuss a possibility to employ SEE for studying the evolution of HF-exited plasma waves, among others involving the lifetime measurements of different SEE components depending on the relation between the pump wave frequency and electron-cyclotron harmonics. The avenues for further investigations are discussed.
Analysis of the Nov.93 data revealed small optical effect of heating
(about 1 Re changes in the auroral luminosity on 5 kRe background).
There was no information about the sign of effect - is the increasing in
the heating power produce more brightness or not.
In the oct. 94 heating campaign there was the reference heating signal on
tape (in VLF data), that gave a possibility to make a coherent detecting and
so to reveal the sign of heating effect.
It was found that for the morning auroral pulsations sign is negative -
luminosity decreases with increasing of the heating power. In the evening and
midnight hours effect is positive and surely smaller. It means that heating
effect is controlled by the effective energy of precipitating electrons.
Some methods of creating the positive feedback between heating
process and different geophysical phenomena are discussed. The goal is to
try resonantly increase heating effect. Main idea is to modulate the
heating transmitter by signal from some detectors (after special processing
and with appropriate phase shift) - photometer, looking in the area of
heating, magnetic pulsations detector, VLF signal in the different
frequency bands and so on. Possible physical effects and, mostly, technical
ideas and problems are investigated.
The artificial ULF signal amplitude due to heating the ionosphere
by powerful amplitude-modulated radio wave is determined by the
initial electron density and electron temperature profiles in the
D- and E- regions, the ionosphere electric field and the heating
radio wave parameters: power, mode of wave, wave frequency and
modulation frequency. For single case of the ULF generation it
is impossible to separate contributions of the initial electron
density and temperature profile shapes and ionospheric electric
field value into the ULF amplitude. Observations of artificial
ULF signals for several parameters of the heating radio wave do
this separation to be possible and allow thus to determine these
Preliminary results of determination of the D-region parameters and the ionospheric electric field value from the artificial ULF signal measurements on the base of numerical model for passage of powerful radio wave through the ionosphere are presented. The proposed method is based on measurement of the generated ULF signal for several effective radiated powers of the heating radio wave. The development of the method together with the possibility of scanning by heating beam on the ionosphere may open the opportunity to obtain the spatial distribution of the D-region parameters and ionospheric electric field.
In ionospheric radio heating experiments, high-frequency electrostatic turbulence is generated in a height interval from the O-mode reflection layer wo = wpe (wo applied frequency, wpe electron plasma frequency) and downwards to some distance below the upper hybrid layer. A current theoretical point of view and modelling of these processes will be described. Both the ponderomotive Langmuir turbulence, which is the origin of the enhanced spectra measured by the EISCAT radars, and the formation of small-scale magnetic field aligned striations, will be discussed from a common point of view. Particular emphasis will be put on the role of cavitation, i.e. self-induced trapped wave (or cavity) resonance, which is expected to play a role in both of the above mentioned phenomena. The predicted signature this hass in SEE generation, will be discussed.
During Heating Campaign 1993 in the Finnish ULF-VLF
Experiment a sine-wave modulation scheme has been used.
Artificial signals in VLF frequency range sometimes was
accompanied by a strong signal which frequency has been
double of the modulation one. Its intensity reached up
to 50% of this on the modulation frequency.
Numerical modelling of the time-dependent electron temperature and ionospheric conductivity disturbances is used to get an explanation of this peculiarity. The energy balance equations has been integrated for the condition similar to those which was during experiment.
A rather unexpected behavior of the height integrated ionospheric conductivity and artificial VLF signal intensity has been obtained. The conductivity and VLF signal intensity at first increase with effective radiated power (ERP) of the heating radio wave but for very large ERP they begin to decrease due to large loss of the heating wave power in the lower ionosphere.
As a result for sine-form modulation regime the artificial VLF signal intensity has no maximum but drop at the ERP maximum that leads to large value of second harmonics in VLF signal spectrum. Another interesting feature is that second harmonics is not in phase with the ERP of heating wave but shifts almost in quarter of period whereas the first harmonics remains to be in phase with ERP.
Two conditions for generation of the harmonics should be satisfied: a rather developed D-region and sine-wave modulation scheme. In this case the amplitude of second harmonics of the VLF signal may reach to 50% from the first one. Note however that if one use square modulation scheme for the heating radio wave, the efficiency of second harmonic generation in artificial VLF signal becomes very weak.
For the 16th of November 1993 experiment significant decrease of the artificial VLF response with the increase of the signal/harmonic ratio was observed. It is possible to explain this fact by growth of the D-region electron density due to energetic electron precipitation.
Value of disturbance of height-integrated ionospheric
conductivity usually is considered as a measure of amplitude
of artificial VLF/ELF/ULF signals generated during heating
experiments. This disturbance consists of two terms which
differ as on their time behaviors and on altitude of their
The first term related with increase of the electron-neutral collision frequency follows rather fast the pump wave power. For the power value which is used at the present in heating experiments the conductivity disturbance maximum is located up to 80 km. Most value of the disturbances of the height-integrated conductivity are observed when electron number density at this altitude are significant. Kuo and Lee (Geophys. Res. Lett, 1993, v.20, pp. 189 - 192) have concluded that efficiency of artificial emission generation increases when the loss of the pump wave energy in the D-region is minimized. Our calculations show that in this case the electron temperature disturbance in E-region is great but disturbance of the conductivity is not significant. Decrease of the electron-neutral collision frequency with height didn't taking into account in paper by Kuo and Lee may explain this discrepancy.
The term related with the electron density increase due to temperature dependence of the recombination coefficients produces very slow variations of the ionospheric conductivity. This process is essential for the heating wave modulation period more than few seconds. The E-region electron heating leads to very effective artificial magnetic pulsation generation.