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The question of radio emission by the ionosphere
Journalof Atmospheric andTerrestrial Physics,1959, Vol. 15,pp.51to53.Pergamon Press Ltd. Printedin NorthernIreland
The question of radio emission by the ionosphere J. L. PAWSEY IT is natural for a radio astronomer to ask if the ionosphere emits radio waves. Much of the radiation with which he is familiar originates in ionized gases, some in thermal processes and some in non-thermal processes on the nature of which be can only speculate. Thermal emission from the ionosphere is known. Non-thermal emission might well originate in some forms of disturbance, e.g. the aurora, and if it, exists it might offer a reasonable chance of observing conditions at the source and possibly of elucidating mechanisms. The probability of recognition of such radiation is suggested by the traditional reports of “radio noise from the aurora”. But a search of the literature does not substantiate this tradition; evidence is extremely meagre. It is the purpose of this note to point this out and to emphasize the importance which non-thermal emission, if it occurs, would have in ionospheric and astronomical studies. It has been conclusively demonstrated (PAWSEP et al., 1951; GARDNER: 1954) that thermal emission from the D-region can, under favourable conditions, be observed on about 2 MC/S throughout a large part of the daylight hours in temperate This implies the absence of regular or frequent intense non-thermal latitudes. components. It, does not preclude occasional bursts which might have been mistaken for interference, nor intense emission from the B-layer which would be cut off by the E-layer. On metre wavelengths (25-110 MC/S), intense noise from all over the sky has and u~lp~~blislled Canadian reports, been reported (COTTONY, 1950, ~ashington~ Otta,wa). On 10 cm COVINGTOR (1947, 1950) has reported bursts of radiation from the sky which coincided in time with magnetic disturbances or aurorae, and on 480 MC/S REBER reported, on the occasion of a severe solar noise storm, “swishes” which continued, though with much reduced intensity, through the night. FORSYT:HE et al. (1949), also on 10 cm, reported series of short-duration (microseconds) pulses at the time of aurorae, and HARTZ et al. (1956) observing on v.h.f, frequencies reported noise at such times. Of these observations, those of CovINGro~ and REBER appear highly reliable but, until they can be reliably reproduced, must be regarded as clues rather than evidence. Brief increases of noise level on a frequency of 33 Me/s coinciding with meteors have been reported by MCKINLEY and MILLMAN (1949) but they were unable to tell if the noise originated in the meteor or was simply due to an enhancement of propagation from a distant noise source by the meteor trail. There are three more recent points worth mentioning. The first is a negative result found by R. L. DOWDEN (unpublished) at Macquarie Island. He repeated the thermal noise measurements at 2 MO/S and found, at this point which is in the aurora1 zone, results very like those found in temperate latitudes. He still found 51
J. L. PAU'SEY
no evidence for non-thermal emission. The second is speculation; it has been suggested by ALCOCK (in press) that the “dawn chorus” is due to excitation of the outer atmosphere by some sort of matter ejected from the sun. The third is argument by analogy. It has been found by SHAIN and GARDNER (unpublished) that the emission from Jupiter is in bursts or groups of bursts, the bursts never being of shorter duration than about i sec. In this the bursts contrast with the very short duration impulses occurring in terrestrial atmospherics but appear analogous to solar bursts. The emission also appears to show a maximum around 20 MC/S. On this evidence the authors suggest the most plausible origin is in the Jovian ionosphere. These results are sufficient to provoke curiosity but none appeared sufficiently definite to justify launching a systematic investigation. But the results reported by Gallet at this meeting, in which he gives reasons for supposing the “dawn chorus” is indeed generated in the outer reaches of the ionosphere, are of a different order of plausibility. Here are clues well worthy of investigation.
REFERENCES ALCOCK G.McC. COTTONY H. V. COVINGTONA.E. COVINGTONA.E. FORSYTHE P. A.,PETRIE W.and CURRIE B.W. GARDNERF.F. HARTZ T.R.,REID G.C. and VOGAN E.L. MCKINLEY D.W.R.~~~MILLMAN P.M. PAXVSEY J.L.,McCREADY L.L.and GARDNERF.F. REBER G.
1959 1950 1947 1950 1949
Suet. J. Phys. In press. Science 3, 41. Terr. Magn. Atmos. Elect. 52, 1947. J. Geophys. Res. 55, 33. Nature, Lond. 164, 453.
1954 1956
J. Atmosph. Terr. Phys. Canad. J. Ph,ys. 34, 728.
5, 298.
1949 1951
Proc. Inst. Radio Engrs., J. Atmosph. Terr. Phys.
N.Y. 37, 364. 1, 261.
1946
Nature,
Lond.
158, 945.
DISCUSSION
DR. J. W. CHAMBERLAIN-I have investigated the possibility of a dispersion in velocities of the protons that are incident on the atmosphere during aurora1 arcs. A velocity dispersion, in the incident flux of particles, varying approximately as v-2 over a wide range of velocities could give a consistent explanation to the luminThe angular dispersion is probably osity curves and hydrogen-line profiles. proportional to cos2 8. There seems to be no way to explain all the observations The data are inadequate to show whether protons with mono-energetic particles. alone are the cause of aurora1 arcs or whether additional processes (e.g. fast incident electrons, electric fields) need be invoked. Insofar as a theory of primary aurora1 particles is concerned, it appears that an acceleration mechanism close to the earth is required. (Complete details are given in AstropQs. J., 127, 1957). PROFESSOR CHAPMAN-DO the particles spiral in? Presumably near the end of their path they will be deflected by collisions. DR. CHAMBERLAIN-Yes they do spiral in-they will be almost stopped before they are deflected. 52
The question of radio emission by the ionosphere
DR. MENZEL-IS the maximum near 500 km explained if the electrons travel High-energy protons entering the earth’s atmosphere would with the protons? have difficulty in picking up electrons. DR. CHAMBERLAIN-NO this is just the assumption that would place the maximum at 1000 km and not 500 km. When the particles are going at a speed comparable with that of the Bohr electron it captures them.
53
The question of radio emission by the ionosphere J. L. PAWSEY IT is natural for a radio astronomer to ask if the ionosphere emits radio waves. Much of the radiation with which he is familiar originates in ionized gases, some in thermal processes and some in non-thermal processes on the nature of which be can only speculate. Thermal emission from the ionosphere is known. Non-thermal emission might well originate in some forms of disturbance, e.g. the aurora, and if it, exists it might offer a reasonable chance of observing conditions at the source and possibly of elucidating mechanisms. The probability of recognition of such radiation is suggested by the traditional reports of “radio noise from the aurora”. But a search of the literature does not substantiate this tradition; evidence is extremely meagre. It is the purpose of this note to point this out and to emphasize the importance which non-thermal emission, if it occurs, would have in ionospheric and astronomical studies. It has been conclusively demonstrated (PAWSEP et al., 1951; GARDNER: 1954) that thermal emission from the D-region can, under favourable conditions, be observed on about 2 MC/S throughout a large part of the daylight hours in temperate This implies the absence of regular or frequent intense non-thermal latitudes. components. It, does not preclude occasional bursts which might have been mistaken for interference, nor intense emission from the B-layer which would be cut off by the E-layer. On metre wavelengths (25-110 MC/S), intense noise from all over the sky has and u~lp~~blislled Canadian reports, been reported (COTTONY, 1950, ~ashington~ Otta,wa). On 10 cm COVINGTOR (1947, 1950) has reported bursts of radiation from the sky which coincided in time with magnetic disturbances or aurorae, and on 480 MC/S REBER reported, on the occasion of a severe solar noise storm, “swishes” which continued, though with much reduced intensity, through the night. FORSYT:HE et al. (1949), also on 10 cm, reported series of short-duration (microseconds) pulses at the time of aurorae, and HARTZ et al. (1956) observing on v.h.f, frequencies reported noise at such times. Of these observations, those of CovINGro~ and REBER appear highly reliable but, until they can be reliably reproduced, must be regarded as clues rather than evidence. Brief increases of noise level on a frequency of 33 Me/s coinciding with meteors have been reported by MCKINLEY and MILLMAN (1949) but they were unable to tell if the noise originated in the meteor or was simply due to an enhancement of propagation from a distant noise source by the meteor trail. There are three more recent points worth mentioning. The first is a negative result found by R. L. DOWDEN (unpublished) at Macquarie Island. He repeated the thermal noise measurements at 2 MO/S and found, at this point which is in the aurora1 zone, results very like those found in temperate latitudes. He still found 51
J. L. PAU'SEY
no evidence for non-thermal emission. The second is speculation; it has been suggested by ALCOCK (in press) that the “dawn chorus” is due to excitation of the outer atmosphere by some sort of matter ejected from the sun. The third is argument by analogy. It has been found by SHAIN and GARDNER (unpublished) that the emission from Jupiter is in bursts or groups of bursts, the bursts never being of shorter duration than about i sec. In this the bursts contrast with the very short duration impulses occurring in terrestrial atmospherics but appear analogous to solar bursts. The emission also appears to show a maximum around 20 MC/S. On this evidence the authors suggest the most plausible origin is in the Jovian ionosphere. These results are sufficient to provoke curiosity but none appeared sufficiently definite to justify launching a systematic investigation. But the results reported by Gallet at this meeting, in which he gives reasons for supposing the “dawn chorus” is indeed generated in the outer reaches of the ionosphere, are of a different order of plausibility. Here are clues well worthy of investigation.
REFERENCES ALCOCK G.McC. COTTONY H. V. COVINGTONA.E. COVINGTONA.E. FORSYTHE P. A.,PETRIE W.and CURRIE B.W. GARDNERF.F. HARTZ T.R.,REID G.C. and VOGAN E.L. MCKINLEY D.W.R.~~~MILLMAN P.M. PAXVSEY J.L.,McCREADY L.L.and GARDNERF.F. REBER G.
1959 1950 1947 1950 1949
Suet. J. Phys. In press. Science 3, 41. Terr. Magn. Atmos. Elect. 52, 1947. J. Geophys. Res. 55, 33. Nature, Lond. 164, 453.
1954 1956
J. Atmosph. Terr. Phys. Canad. J. Ph,ys. 34, 728.
5, 298.
1949 1951
Proc. Inst. Radio Engrs., J. Atmosph. Terr. Phys.
N.Y. 37, 364. 1, 261.
1946
Nature,
Lond.
158, 945.
DISCUSSION
DR. J. W. CHAMBERLAIN-I have investigated the possibility of a dispersion in velocities of the protons that are incident on the atmosphere during aurora1 arcs. A velocity dispersion, in the incident flux of particles, varying approximately as v-2 over a wide range of velocities could give a consistent explanation to the luminThe angular dispersion is probably osity curves and hydrogen-line profiles. proportional to cos2 8. There seems to be no way to explain all the observations The data are inadequate to show whether protons with mono-energetic particles. alone are the cause of aurora1 arcs or whether additional processes (e.g. fast incident electrons, electric fields) need be invoked. Insofar as a theory of primary aurora1 particles is concerned, it appears that an acceleration mechanism close to the earth is required. (Complete details are given in AstropQs. J., 127, 1957). PROFESSOR CHAPMAN-DO the particles spiral in? Presumably near the end of their path they will be deflected by collisions. DR. CHAMBERLAIN-Yes they do spiral in-they will be almost stopped before they are deflected. 52
The question of radio emission by the ionosphere
DR. MENZEL-IS the maximum near 500 km explained if the electrons travel High-energy protons entering the earth’s atmosphere would with the protons? have difficulty in picking up electrons. DR. CHAMBERLAIN-NO this is just the assumption that would place the maximum at 1000 km and not 500 km. When the particles are going at a speed comparable with that of the Bohr electron it captures them.
53