On the possibility of observing aurorae in the daytime

Research notes

On the possibility of observing aurorae in the daytime (Received 29 July 1957)

THE importance of aurora1 observations during the I.G.Y. makes highly desirable a eonsideration of suitable conditions for detecting activity during hours of daylight, especially since radar echoes have been obtained from the aurora1 regions. A lead in this direction is given by stellar sightings from high flying aircraft. At an altitude of 40,000-50,000 ft,, &a,rs of first magnitude are visible for approximat,el~ ~2 hr a.fter dawn and 2 hr before dusk. Frmn St. Andrews during the night of 18-19 April 1957, streamers from an aurora were seen against a dawn sky about 2 hr before sunrise when stars fainter than second or t,hird magnit,ude were not visible. A special aircraft flight was made during the solar eclipse of 30 June 1954, to detect, aurora1 activity during the total phase. Thiswas done on the suggestion of Prof. S. CHAPMAH. However, no display was seen on this occasion. Scattering of light from the sky is the decisive factor when considering t,he question of aurora1 visibility during the daytime; the only feasible method would seem to be the det’ection of the aurora1 green line at 5577 A. Assuming isotropic scattering and no polarization ue have, if I is the intensity of sunlight and w the angular area of the sun, then I’, the zenith sky intensit,y, is given by:

where CTis the scattering eoe&ient. Substitution of the appropriate quantit,ies (ALLEN, 1955) gives I’ = l-5 ergs sterad-’ see-l cnl-2 A-i at 5500 A. At’ an altitude of 80,000 ft (approximately 25 km) we see that the scattering per at,mosphere is equal to the product of the scattering coefficient for unit volume and the scale height. The variation of refra,ctive index n with density p is represented by the Lore&z -TJorenz formula : n2 - 1 ___.- = Constant. (n2 + 2)~ For a height of 25 km we find that n = 1.0000123. Xow the Rayleigh scattering coefficient, fs, is given by: 32rr3(n - 1)2 o =

-

3NP

where N is the number of particles per unit volume; hence for a wavelength of 5500 8, o = 4.87 x 10-s. Therefore the scattering per atmosphere is 3.17 x 10-3, since the scale height is 6.5 x lo5 cm. Hence: l’25km

=

496

x 10e2 ergs sterad-l

set-1 cm-2 A-l,

In this simple treatment secondary scattering has been ignored and at tohis altitude dust and water vapour will be absent. The intensity of the 5577 line (HUNTEN, 1955) in a class II to III display is approximately 5 x lo9 quanta sterad-l see-i cm-2, i.e. 1% x lOem ergs sterad-i set-1 cm-s per equivalent width. The equivalent width corresponding to a Doppler broadening at about 220°K (the tenlperature in the lower aurora1 region) is 0.2 A. The fact that the values for I’ and I’251inl were calculated for a bandwidth of 1 _& and that the intensity of the aurora1 line is less than I’s5km means that under these conditions the aurora will be invisible. 295

Research notes

However, if the polarization of the sky is taken into account, then observations made at 90” to the direction of the sun would benefit by a reduction in sky brightness of perhaps ten times (VAN DE HULST, 1952). Unfortunately the aurora1 light would suffer a depletion of 50 per cent, giving a net gain of five. A very serious obstacle is the scattering from the ozone layer. At 25 km this amounts to roughly half the ozone scattering at sea level. This implies an increase in scattering from the sky by a factor of 6 at 25 km. A particularly intense aurora might be ten times as bright as the one we have considered. Even so, there seems to be little chance of detecting it unless observations can be made from an even greater altitude. A. H. JARRETT P. L. BYARD

Uwiversity Observatory St. Andrews, Scotland REFERENCES ALLEN VAN

c. w. DE

HUNTEN

HULST

D. M.

H. C.

1955 1952

Astrophysical &zu&ities. University

of London. The Atmospheres of the Earth and Planets p. 79. University of Chicago Press. J. Atmosph. Terr. Phys. 7, 141.

1955

The coefficient of diffusion of ions in the F2 regions (Received

2 September

1957)

THE writer (FERRARO, 1945) derived an expression for the coefficient of diffusion, D,,, of ions in the ionosphere based on Sutherland’s molecular model. In this the molecules of a gas are treated as smooth rigid elastic spheres surrounded by a weak attractive field of force which for diffusion of ions in an otherwise neutral gas, is due to the attraction between the ion and the charge induced by it on a neutral molecule. Treating this as a conducting sphere the law of force is that of the inverse fifth power of the distance. The value of D,, derived by using Sutherland’s model was found to be b/n, where n is the molecular density of the neutral gas and b is a function of the temperature T and of the molecular weight of the gas which is equal to b =

3.9

x

101’T3”/(T

+ 187)

(1)

for a mean molecular weight of 25. At a temperature of about lOOO”, such as obtains in the F2 region, b is of the order of 10lg, and for this value of the coefficient of diffusion it appears difficult for a stable bank of ionization to persist for long at the level of the F2 region unless the molecular density there is of the order of lOlo mol/ cm3 at least. This conclusion is difficult to reconcile with recent rocket estimates of the molecular density in the F2 layer, which range between 5 x log and 5 x IO* mol/cm3. Because of this discrepancy the writer has reconsidered the assumptions made in deriving equation (1) and finds that the Sutherla,nd model, though not suitable in this case 296