ITU-R P 532-1-1992 Ionospheric Effects and Operational Considerations Associated with Artificial Modification of the Ionosphere and the Radio-Wave Channel Section 6A - Ionospheric .pdf

1、CCIR RECMN*532-3 92 = 4855232 053977b 4T Rec. 532-1 RECOMMENDATION 532- 1 IONOSPHERIC EFFECTS ANI) OPERATIONAL CONSIDERATIONS ASSOCIATED WITH ARTIFICIAL MODIFICATION OF THE IONOSPHERE AND THE RADIO-WAVE CHANNEL (Question 3916) 1 (1978-1992) The CCIR, considering that artificial modification of the i

2、onosphere and the radio-wave channel can be introduced by the application a) of RF power using terrestrial (or spaceborne) transmitters; b) that ionospheric modification, particularly in the F-region, can occur as the result of high power flux-density in the ionosphere in the approximate frequency r

3、ange 2 to 12 MHz, particularly for high radiation angles and for frequencies just below the layer basic MUFs at near vertical incidence; and that such ionospheric modification may ailow propagation at frequencies up to about 400 MHz and over distances of up to 4 O00 km; C) of signals in the ionosphe

4、re is large; dl that if administrations continue to allow transmitter powers to increase, the ionosphere can be significantly altered with the result that both the services using the ionosphere as a propagation medium and VHF ground-wave services may experience deterioration of reception; e) energet

5、ic particles, and other species which will modify the natural distribution and character of the medium; 0 launches; g) that artificial modification of the medium can inuoduce new transient modes of propagation, creating the potential for increased (or decreased) coverage beyond that established by s

6、tandard radio-wave propagation prediction methods, that it has long been recognized that cross-modulation can occur at LF and MF when the power flux-density that the ionosphere can be modified by injection of chemical reagents, photo-ionizable constituents, that inadvertent or unplanned modification

7、 can be introduced by reagent processes associated with rocket recommends that in the planning and operation of radio systems utilizing the ionosphere, the following aspects should be taken into account: 1. wave transmissions, use should be made of the information contained in Annex 1; that for dete

8、rmining the modifications to the ionosphere by ionospherically propagated high-power radio- 2. use should be made of the formulations given in Annex 2; 3. should be made of the information contained in Annex 3; that for determining the effects of ionospheric modification on radio-wave transmissions

9、(cross-modulation), that for determining the modifications to the ionosphere by trans-ionospheric radio-wave transmissions, use 4. should be made of the information contained in Annex 4, that for determining the modifications to the ionosphere resulting from injection of chemical reagents, use I rec

10、ommends further 5. heights for frequencies up to approximately 12 MHz; 6. that for operational communication systems, intentional modification of the ionosphere should be discouraged due to the deleterious effects on the services of other users. that attention be paid, and measures be taken, to mini

11、mize excessive power flux-densities at ionospheric 2 CCIR RECMN*532-1 92 m q855212 0519777 306 m Rec. 532-1 ANNEX 1 Ionospheric modification by ground-based, high-power radio transmission 1. Introduction The modification of the ionospheric plasma by high-power radio transmissions divides into ohmic

12、ionospheric heating, a non-linear but classical process, and into the genedon of parametric instabilities by non-linear wave interaction processes. Most ionospheric modification activities at HF are concerned with producing changes in the upper ionosphere (150-4oO km) using purpose-built transmitter

13、s operating at frequencies close to the F-region critical frequencies. If the modifying frequency is less than the critical frequency, the modification is termed overdense; if however, the modifying frequency is greater than the critical frequency, the modification is said to be underdense. The iono

14、sphere can be appreciably modified by an oblique high-power radio emission at frequencies considerably in excess of the critical frequency of the F-region of the ionosphere. Transmitters operating over the range of VLF to UHF give rise to modifications in all regions of the ionosphere. The resulting

15、 modified region can have a significant effect on radio signals, used for communications purposes, which pass through it. 2. Ohmic heating theory Theoretical work has suggested that ionospheric heating by ohmic dissipation should produce large-scale changes in the electron temperature and as a resul

16、t in the electron density and other parameters. Many non-linear phenomena arise from the fact that the collision frequency depends on the electron temperature. Simplified theory illustrates how ohmic heating can occur. A wave with electric field E, and angular frequency o, is considered to pass thro

17、ugh a slab of ionospheric plasma with effective collision frequency v between electrons and ions or neutrals. This field acts upon the electrons, of mass m, and charge e and accelerates them. Collisions, however, retard them and energy is extracted from the wave, resulting in an increase in electron

18、 temperature. Although the electrons become hotter, they transfer only a small part of their excess energy to the ions or neutrals during collisions because the electron mass is so much smaller than the ion or neutral mass. In the F-region, the electron-ion collision frequency is S lo%, the fraction

19、al energy loss per collision is I 10-4, and the time constant for energy loss is therefore, -10 s. This low loss rate makes appreciable heating of the electrons possible. Heating of the E-region is less easy. Here the electron-neutral collision frequency is -2 x lo%, the fractional energy loss per c

20、ollision is 5 x lF and consequently the time constant for energy loss is only -1 ms. Strong absorption of the incident radio wave occurs in the region where the electron plasma frequency is near the radio frequency. This is because the wave is slowed near this natural resonance, and the electrons ha

21、ve a greater opportunity to collide with the heavy particles. The electric field needed to cause large thermal perturbation of the ionospheric plasma temperature and for w v, varies from about 3 x 10-4 f (mV/m) in the D and E-regions to about 10-4 f (mV/m) in the F-region; f is the frequency of the

22、perturbing wave (Hz). Such fields imply equivalent isotropically radiated powers of approximately 100 Mw. 3. Parametric instability theory The parametric wave-plasma instability generally involves a three-wave interaction. In the context of ionospheric modification, a high power HF electromagnetic w

23、ave provides the initial driving or pump field whose energy cascades into a lower frequency electrostatic electron plasma wave and a lower frequency ion-acoustic wave. I CCIR RECMN*532-3 92 4855232 0539778 242 = Rec. 532-1 3 I The non-linear mechanism responsible for most parametric instabilities in

24、 the ionosphere is the thermal pressure force. The electron temperature perturbations caused by wave heating give rise to an additional thermal pressure term in the electron equation of motion and lead to the generation of field-aligned ionospheric irregularities. 4. Modification effects Some of the

25、 copious modification effects caused by HF (and other frequencies) heating radio waves are described below. At altitudes of less than 200 km, electrons collide primarily with neutrals, the collision frequency increases with temperature and strong radio waves are absorbed more than weak radio waves.

26、Above 200 km, where electrons collide primarily with ions, the collision frequency decreases with temperature and strong radio waves are absorbed less than weak radio waves. Perturbations in electron density occur if heating is maintained for a sufficiently long time. At altitudes below about 200 km

27、 an increase in electron density occurs. At higher altitudes, in the F-region, high electron temperatures correspond to an increase in pressure causing plasma to stream out of the heated region along the geomagnetic field line. Electromagnetic energy is then focused into the region of reduced electr

28、on density leading to further heating and expansion. This results in large-scale irregularities in the F-region electron density, aligned along the geomagnetic field and with transverse dimensions of approximately 1 km. One result of this thermal self-focusing process is the production of artificial

29、 spread F. One of the unexpected effects of the early HF ionospheric modification experiments was the generation of small scale (approximately 1 m) field-aligned irregularities which cause back-scatter to VHF and UHF waves. These irregularities are probably generated about 200 m below the reflection

30、 height of the HF heating wave where its heating effect is greatest. For transmitted powers greater than a threshold value, received signals have been observed to decrease with an increase in e.r.p. It was also found that in the field of an obliquely incident high power wave with a frequency near to

31、 the MUF of the F2 layer a modification occm in the regular ionosphere which can have a considerable effect on the characteristics of radio signals passing through this perturbation. In addition to modification of the upper ionosphere by high power HF waves, it is possible to generate ELF and VLF wa

32、ves as a result of modification of the lower ionosphere by use of pulsed high power HF waves. Evidence of ELFNLF generation apparently due to LF and MF broadcast emissions has been observed at high latitudes. These signals can heat the auroral D or E region, modulating the auroral electrojet which t

33、hen emits ELFNLF signals. Integral harmonics of the ELF modulating frequencies can be produced non-linearly in the auroral D and E regions. Controlled injection of VLF signals from ground-based transmitters causes electron precipitation from the radiation belts which increases ionization at ionosphe

34、ric heights. Naturally occurring electron precipitation varies greatly from much lower to much higher than observed artificially produced precipitation. 5. Scattering of radio signals from artificially tnduced irregularities With an e.i.r.p. of 0.5 MW or greater, large scale and small scale irregula

35、rities of electron density aligned with the Earths magnetic field develop within seconds of the transmitter turn-on, as a result of ohmic heating and development of parametric instabilities and plasma waves. The consequence to radio signals passing through the disturbed region for paths with both te

36、rminals on the ground as well as Earth-space paths is that both the depth and rate of fading increase. In addition, because of the field-aligned irregularities, an effective reflector of large radar cross section (= 105 to 109 m2) at altitudes of 250 to 300 km in the ionosphere is produced. These ef

37、fects are produced when the heating transmitter frequency is below the critical frequency of the F-region (I 12 MHz) but on a frequency which matches the plasma frequency at some height in the ionosphere. CCIR RECMN*532-L 92 m 4855232 05l.19779 389 m 4 Rec. 532-1 The scatterhg properties of the fiel

38、d-aligned irregularities have been used to transmit voice, teletype, facsimile and pulsed transmissions between ground terminais separated by thousands of kilometres and using frequencies, ranging from HF to UHF, which would not otherwise have been useful for these paths. A fairly high degree of asp

39、ect sensitivity is associated with the F-region scattering. Thus, the locations on the Earth at which signais are received by this scattering mechanism depend in part upon the geomagnetic position and the altitude of the modified ionospheric region. In general, the signals can be received in an area

40、 on the equatorial side of the modified region which has a large East-West extent, ranging up to about 4000 km, but only 200 to 500 km in North-South extent. A strong scattering region near 110 km altitude in the E-region can also be produced when the heating transmitter is operating on frequencies

41、below the E-region critical frequency. Fewer observations have been made of the E-region scattering during modification, but the limited evidence suggests that the scattering is less aspect sensitive than that from the F-region and, thus, signals may be received on the ground in areas having a great

42、er North-South extent than that found for the F-region. From the evidence thus far obtained it appears that there is a potential for increased interference due to signals scattered from intended or unintended modified regions, at frequencies ranging from HF to UHF. It may also be expected that under

43、 proper conditions, interference between earth terminals and satellites could exist, since scattering occurs in ail directions defined by the scattering cone and thus an earth transmitter will have energy scattered into space, and vice versa, by the irregularities in the modified region. ANNEX 2 Ion

44、ospheric cross-modulation 1. Introduction The propagation of strong modulated waves through a plasma produces perturbations in the plasma which cause changes to take place in the electron temperatures which in turn affect the collision frequency, ion chemistry and electron density; and therefore the

45、 conductivity and permittivity of the medium. The result of these changes in the medium produced by one modulated intense radio wave is the superimposition of its modulation on the carrier of another wave propagating through the same region. Because of the large number of transmissions using the HF,

46、 MF and LF bands which propagate through the D and E regions, this wave interaction or ionospheric cross-modulation is difficult to distinguish from Co-channel interference and even more difficult to measure. Measurements made in the MFLF bands at middle latitudes show cross-modulation depths less t

47、han 7%. The measurements are shown in Recommendation 498. 2. The simple theory of cross-modulation For the communications engineer concerned with estimating the interference caused by cross-modulation, the main features of the phenomenon are presented below. 2.1 The electron collisional process The

48、free electrons which are mainly responsible for the reaction of the ionosphere on radio waves are located in D and lower E regions and may be regarded as a gaseous constituent statistically in thermal equilibrium with the far more numerous molecules in the atmosphere. Each electron may be considered

49、 to have a tiiermal energy Qo and a velocity VO related to the temperature 00 of the atmosphere by the gas equation: 1 3 Qo = 5 m$ = 2 k0o where m is the mass of the electron (9.1 x 10-31 kg), k is the Boltzmanns constant (1.37 x 10-23 joules per Kelvin) and 00 is in Kelvin, Qo and Vo being in MKS units. CCIR RECMN*532-1 92 4855212 0519780 9TO Rec. 532-1 5 If at the pint under consideration the equilibrium is disturbed by increasing the velocity of .e electrons to V, Qo and 80 change to Q and 8 in accordance with (i), the temperature of the surrounding atmosphere being unchanged. There is