Auroral luminosity and absorption of cosmic radio noise

Journalof AtmosphericnndTerrestrialPhysics,1962,Vol. 24pp. 467to 474. PergamonPressLtd. Printedin NorthernIreland

Aurora1 luminosity and absorption of cosmic radio noise 0. HOLT Norwegian

Defence

Research

Establishment, and A.

The Norwegian

Institute

Kjeller,

Norway

OMHOLT

of Cosmic Physics,

(Received

Lillestriim,

24 Januaq

Blindern,

Oslo, Norwq

1962)

Abstract-Riometer measurements of absorption of 27.6 MHz radio waves and simultaneous measurements of the aurora1 intensity in the zenith have been compared. There is a reasonably good correlation between the two sets of observations. There appears to be a time delay between This time delay is usually a few minutes, in outbursts of aurora and the increase in absorption. good agreement with current information on the recombination coefficients and electron densities. Occasionally the time delay is very short (less than 1 min). This appears to be associated wit,h short bursts of strong aurora. 1.

INTRODUCTIVE

the winter 1960/1961 a photometer, recording the zenith intensity of the aurora1 5577 d [01] line, and a riometer, receiving cosmic radio noise at 27.6 MHz, were both in operation at Alta (69.9”N, 23”E) in northern Norway. The photometer utilized an EM1 6095B photomultiplier tube, an interference filter to separate the 5577 A line, a logarithmic amplifier, and a pen recorder. The optical design was such that the photometer viewed a vertical cone of 5’ width. The riometer is not very different from the type described by LITTLE and LEINBACH (1959). A three-element Yagi aerial was used, the width of the beam being about 30” to the 3 dB points. The relative noise power was recorded on a linear scale by a pen recorder identical to that used for the photometer. In this paper we shall present some results from this period of observation, showing the correlation between the intensity of the aurora1 5577 A emission and the absorption of cosmic radio noise at 27.6 MHz. The interpretation of the ohservations will be briefly discussed. DURING

2.

OBSERVATIONS

From the all-sky camera films from Tromsii (69*7’N, 18.9”E) the clear nights with an aurora occurring were selected. Nights when the moonlight could possibly influence the photometer records were rejected. We were then left with only six nights from which the records were suitable for analysis. An example of records from a short period with an aurora occurring is shown in Fig. 1. The correlation between the two phenomena is obvious. According to current theory the intensity of the First Negative N, + bands in the aurora1 spectrum is proportional to the integrated production rate, Jqdh through the column viewed by the photometer, of free electrons by primary particles and secondary electrons, and to the total influx of energy into the aurora. In this case we measured the green [01] 467

468

0. HOLT

and A. OMHOLT

line rather than the violet K 2+ bands, because the intensity ratio between the two is fairly constant in aurora and the former is less subject to contamination by scattered moonlight or scattering in haze (CHAMBERLAIX, 1961). The production of free electrons in the D-region is probably not caused by direct impact of energetic electrons, but rather by X-rays generated as “bremsstrahlung” when the elect’rons are stopped at higher levels in the ionosphere. The X-ray intensity is proportional to the flux of primary_ -particles provided the energy spectrum of these does not change. In this f&t order approximation the production rate q for

4

3 2 1

\

\

Fig. 1. Example of records during aurora1 activky. These observat’ions mere made on 22 December 1960. 10 min. between vertical lines.

the electrons that cause the absorption observed by the riometer is thus proportional to the recorded light intensity I. The absorption of a radio wave passing through the ionosphere is proportional to JKdh, where K is the absorption index. K is proportional to the number density, N, of free electrons, and the ratio K/N has its maximum in t’he lower part of the D-region. Provided the relative electron distribution with height is constant, the absorption is proportional to the electron density. The relation between electron density and electron product,ion is given by the equation of conservation,

(1) where cc is the effective recombination coefficient, and 1. is the ratio between the number densities of negative ions and electrons. During quasi-equilibrium N is proportional to the square root of 4. With our first order approximations the absorption should thus be proportional to the square root of the intensity I. To study the correlation between the two phenomena more quantitatively. the observed absorption was plotted versus the simultaneously observed ratio (1/1,)1/2, where I, is a reference intensity. Readings were made in 5 or 10 min intervals. According to the simplified model outlined above this should yield a straight line. Actual plots, however, have the features shown in Fig. 2. Considering the rough approximations

Auroral luminosity and absorption

of cosmic rdio

noise

469

made, the scatter is not very surprising. The explanation may he found partly in the distribution of aurora1 luminosity over the sky, together with the difference in direction sensitivity of the photometer and the riometer. As mentioned earlier the photometer views a cone of only 5” 111 . the zenith. A bright but limited auroral display in this region would cause a large deflection on the phot,ometer, whereas the ionization in such a limited region would not cause much reduction of the noise power recorded by the riometer, which receives radiation from a muoh wider area. If ‘-bremsstrahlung” is responsible for the ionization that gives rise to the absorption. the ionized area will be broadened due to the spread-out of the X-rays, but the

3

T 2 f .

0I 0

i0

1 0 Absorption

1

2

j

dl3

3

Fig. 2, Plot of absorption vs. (1/1,J1j2 for the nights 11-12 December 1960 (left) am1 22-23 December 1960 (right).

electron density produced will be correspondingly smaller, so that the absorption will not be increased to the same extent as the 5577 8, intensity. It seems therefore reasonable to select periods when the aurora covers most of the sky. For both periods shown in Fig. 2, the aurora1 displays during the first part of the night were distinct and limited forms, but after around midnight t’hey rapidly spread over almost the whole sky when they occurred. If we replot the data of Fig. S!,including only these latter periods, we obtain the results shown in Fig. 3. In these cases the correlation is reasonably good. The results from t’he two nights give, however, a very different quantitative connexion between the absorption and the int’ensity. This may be due to differences in the primary electron energy spectrum on the two nights, since the efficiency in creating X-rays hhrough “bremsstrahlung” depends strongly on the electron energy.

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and A. OMEIOLT

It should be expected that the electron density, and hence the absorption, should be delayed relatively to the production (and the intensity variation), due to the recombi~lation time of the free electrons. One might therefore think that better correlation could be obtained by plotting the ratio (f/I,)1f2 versus the absorption observed a few minutes later. Inspection of the different records shows that the time-shifts vary from one short period of enhanced absorption to another during the same night. It seems therefore more profitable to study rather short periods of

Absorption

_I_,

dB

Fig. 3. Plot of absorption vs. (I/l,)1la during periodswhen the auroracovered most of the sky. Observations are from the same nights as in Fig. 2.

observation separately. In Fig. 4 we have shown results from the period 03150500 hours on 22 November 1960. Time shifts of 4-S min between corresponding peaks are found. These values are typical for several periods of observation. A time lag of the absorption of up to 10 min has been observed. In some cases the time lagappears to be absent, or at least very small (say less than 1 min). Examples of such observations are shown in Fig. 5. The increase in luminosity is seen to be more abrupt than in periods like the one shown in Fig. 4, and it is also of much shorter duration. 3.

DISCUSSION ANU

CONCLUSION

The correlation between the absorption and the aurora1 luminosity strongly supports the view that the ionization process in the D-layer during disturbed conditions is connected to the ionization and excitation in the aurora. Our results are

Aurod

luminosity and absorpt,ion of cosmic radio noise

47f

consistent with, although no proof for, the view t,hat X-rays generaDed through “bremsstrahlung” are responsible for the D-layer ionization. It could be that this low-lying ionization is caused by a few energetic particles arriving simultaneously with those stopped higher up and causing the aurora, but in this case it would be more surprising to find the correlation between absorption and luminosit,y as good as it is. The observed time delay between the luminosity (electron production rate) and t.he a.bsorption is also generally in agreement with the expected value around 60 km. The “d3rnami~” recombination coefhcient Min equation (1) is believed to be of the

+---r--T--‘..1

I

0315

I

I

0330

I

I

I

I

I

r

I

I

r

I

I

I

I

I

I

1

I

0400

0630

0500

LMT Fig. 4. Example of observations with a time delay of some minutes between increase in aurora1 luminosity and absorption of radio waves. The observations were made on 22 November 1960.

order 1O-5 cm3 se+ or less, and the electron density iv of the order lo3 or less. This should imply that the characteristic delay time r = (&?)-I is of the order of 100 see or more. This is in agreement with most of the records, but the shortest of the observed delay times is somewhat lower than expected, and we shall discuss these a little more in detail. When the period of variation is of the same order of magnitude as the characteristic delay time or less, the difference between the peaks in the observed quantities is no Ionger a good measure of the characteristic deIay time. One should rather take the difference between the “average time” of the light “pulse” and the assoEven so, the shortest delay times observed are definitely ciated absorption “pulse”.

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HOLT

and A. OMHOLT

less than 1 min, but the accuracy in the observations does not permit a lower limit to the delay time. In this discussion we shall adopt the reaction rates quoted by NAWROCKI and PAPA (1961). It is then easy to see that below about 70 km attachment and detachment are so rapid that il, the negative ion to electron ratio, can be considered

l

2330 LMT

2400

+

Fig. 5. Example of observations with very short time delay between increase in aurora1 luminosity and absorption of radio waves. The observations were made on 2% December 1960.

to be constant when considering also our time variations. At 60 km 2 should be about 50, but it must be stressed that the uncertainties in 1 are very large. Further, it turns out that ionic recombination (X- + I’+ -+ X + I’) is likely to be the most important process for removing the electron-ion content. The recombination coefficient for this process, ui, is thought to be of the order of 1O-7 cm3 see-l. The cheracteristic life-time of the electron-ion content, 7 = (qz(X-))-l = (c&V)-1 = (crN)-1, is then about 200 set or more, if we again take the electron

Aurora1 luminosity

and absorption

of cosmic radio noise

473

density N I lo3 cm-3. Our observations imply that this value for 7 is common, but that it occasionally is an order of magnitude lower. With the uncertainties in the basic data in mind, this is not very surprising. But if the absorption takes place mostly between 60 and 80 km the variation in delay times may not be expected to be as great as observed. The argument for this is as follows: The absorption index h” is proportional to the electron density N and to the electron collision frequency v. It is thus proportional to N . n(H) when time 7 is inn(M) is the total particle density. The characteristic recombination versely proportional to Nil. Since in this region 1 is proportional to n(X) (threebody attachment and two-body detachment) 7 should be inversely proportional to LVn(BI), or to K. The same value of E=should then give the same value of 7. Apparently this is not the case. In the few cases with r < 1 min the absorption is not any higher than in cases with T = 10 min. However, the light intensity is much higher coefficient is higher when 7 is very short. Thus it seems as if the recombination compared t*o the absorption index. If the absorption in these cases takes place mostly below 60 km this may well be so, for going downwards from this level the absorption index per electron for the frequency in question (27.6 MHz) is no longer increasing in proportion to the atmospheric density, owing to the fact t,hat the collision frequency for the electrons here approaches and eventually exceeds the angular frequency of the radio waves. It is of interest to see what electron densities are necessary to give the observed absorption, and to compare these with the delay times. In most cases, like the period shown in Fig. 3(a), the absorption does not exceed 1 dB, but sometimes it is 3d B or even more (Fig. 3b). An electron density distribution with 1.5 x lo2 electrons per cm3 at 50 km, 6 x lo2 at 60 km, and increasing to lo3 at 80 km, will give an absorption of about 1 dB in this region at 27.6 MHz. More than 75 per cent of this absorption is imposed on the wave between 60 and 80 km. These electron densities are in good agreement with densities deduced by cross modulation and partial reflection observations during periods of aurora1 absorption (HOLT et cd., 1961). These experiments, however, could not be made dusing condit,ions of very strong absorption, a’nd it is conceivable that the electron densities then may exceed the numbers quoted above by a factor of 2 or 3. Additional absorption may also be due to free electrons above SO km. There is thought, therefore, to be reasonable agreement between the size of the absorption and the observed’delay times of some minutes between the absorption and the aurora1 intensity. The cases when the delay time is very short may be explained, as pointed out above. by assuming that most of the absorption takes place below 60 km. This would require electron densities of about lo3 down to 50 km, and relatively few electrons above 60 km. Such a distribution seems strange, but may be connect#ed to the fact that the increase in luminosity in these cases has the character of a sharp “burst”, perhaps due to the sudden impact of very energetic particles. The uncertainity in the basic data for the possible processes prohibits a more detailed discussion. Also, the relevant coefficients may be strongly dependent upon temperature.

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0. HOLT and A. OMHOLT

Acknowledge4nelzts--The work reported here has been sponsored in part by the Electronics Research Directorate of the Cambridge Research Center, Air Research and Development Command, US Air Force, through its European Office, under contracts AF 61(052)-08 and AF 61(052)-526. Thanks are due to Mr. B. PETTERSEN for help with the reduction of the recordings. Mr. B. BJELLAND has been responsible for the construction of the riometer. We also wish to express our thanks to Mr. J. TR~~IM and Mr. L. LORNTZSEN for designing and building the electronics for the photometer in a very short time, and to Mr. R. LARSEN for his assistance with construction of the photometer. REFERENCES CHAMBERLAINJ. W.

1961

HOLT O., LANDMARK B. and LIED F. LITTLE C. G. and LEINBACH H. NAWROCKI P. J. and PAPA R.

1961 1959 1961

Physics of the Aurora and Airglow. Academic Press, New York. NDRE Report No 35, Norwegian Defence Research Establishment. Proc. I.R.E. 47, 315. Atmospheric America,

Processes. Geophysics Bedford, Massachusetts.

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of