Observations of non-field-aligned auroral rays

Journal of Atmospheric and Terrestrial Physics, 1960, Vol. 18, pp. 332 to 343. Pergamon Press Ltd. Printed in Northern Ireland

RESEARCH NOTES

Observations of non-field-aligned auroral rays (Received 5 February

1960)

DVRING the auroral display of 26 October 1956, fifteen colour photographs were taken at St. Andrews Observatory, Scotland, as pa1~¢ of an investigation into the photometric value of eolour photography of the aurora (GADSDEN, 1960). The film used was Kodak Daylight Ektachrome Type E-2, and the camera was an ordinary 35 m m commercial camera with a 45 m m F/2.8 lens. The exposures necessary varied from 30 min to 4 rain. Careful examination of the photographs revealed the existence on five of them of an auroral ray of unusual appearance. (These five photographs are reproduced in monochrome in Fig. l, by the process of printing the colour transparencies in light from a neon lamp onto a panchromatic plate. The resulting negative was then printed in the normal way onto high contrast bromide paper.) Three facts were immediately obvious about the unusual rays: (a) in two cases, they were inclined at an appreciable angle to other rays appearing in the same region of sky; (b) as far as can be determined from a colour photograph, they were uniformly red from top to bottom; and (c) they were not due to spurious effects arising from local, ground level, lighting since, in the first two photographs taken {Fig. 1 a and b), the rays were obscured over part of their length by cloud. The possibility t h a t the ray is due to some defect in the emulsion of the colour film has been eliminated b y a visual observation of a similar ray (which was not photographed) during the course of the display and by a black-and-white photograph taken, by chance, of one of the rays at almost the same time, and in almost the same direction, as photograph (c) in Fig. 1. I t should be mentioned t h a t the rays are entirely different in appearance to the red rays reported by STORMER (1957) on the same evening, and also photographed from St. Andrews. The colour of the rays suggests t h a t they were at a considerable height in the atmosphere (ST6RMER, 1955) and the question arises whether this accounts for the apparent departure from field-alignment, when compared with the normal auroral rays. That is to say, whether the great range of the rays from the observer would have caused the apparent nonalignment to be due to their being aligned along magnetic field lines at a great distance from the observer. Assuming that the magnetic field of the earth m a y be represented sufficiently well by t h a t of a geocentric dipole, whose axis intersects the Earth's surface at 78-6°N, 70°W ( V E S T I N E et al., 1947), the apparent angle of intersection of a line of force with the horizon of an observer at geomagnetic latitude ¢ is given by tan0=

3d 2 .... d 2 1 ~d 2c°ta-}-I ~h~tan¢coseca

where a is the geomagnetic azimuth of the point of observation; d is the range of the line of force from the observer in the horizontal direction (in units of earth radii); and 0 is the apparent inclination of the tangent to the line of force to the observer's horizon. 332

(b)

(c)

(d)

(e) Fig. 1. Photographs of mmsu~l auroral rays taken lrom St. Andrews, Scotland, oll 26 October 1956. The times (U.T.) of the exposures are: (a) 1938-2000; (b) 2034--2054; (c) 2230-2245; (d) 2258-2302; (e) 2321-2329. The unusual rays appear as narrow strips running almost vertically upwards from closo to the bottom edges of the photographs.

332

Research notes

For small d, and a equal to 90 °, this expression reduces to tan 0 ~ 2 t a n ¢ = t a n D where D is the local angle of dip, calculated on the assumption of a central dipole field. Fig. 2 shows the variation of 0 and a as a function of d for ¢ ~ 59.4 °, the geomagnetic latitude of St. Andrews. The observed values of 0 and a, measured from the photographs, are listed below and are plotted on Fig. 2 as open circles. Time of exposure (U.T.)

0o

ao

1938-2000 2034-2054 2230-2245 2258-2302 2321-2329

90 83 76 97 86

55 E 45 E 46W 51 W 41 E

I t will be seen from Fig. 2 t h a t there are no possible values of d corresponding to these values of 0 and ~, and t h a t the observed points differ from physically realisable values of o 80

~

o o d--O

6()

oo

',,

20

~a=2

",,

o

-20

\ -40

~.-

-80 ~ 0

n 20

40

60

o;

degrees

80

Fig. 2. V a r i a t i o n o f 0 w i t h G for v a r i o u s v a l u e s o f d. T h e o p e n circles are o b s e r v e d v a l u e s of e a n d ~ for t h e non-field-aligned a u r o r a l rays.

0 b y 5 ° to 13 °. I t is unlikely, therefore, t h a t the rays were aligned along a magnetic line of force which had been distorted from the corresponding dipole line during the magnetic disturbance present on 26 October. (The K-index recorded at Eskdalemuir at the time of the exposures was 6.) I t should also be noted t h a t the local field at St. Andrews (dip angle = 70.4 °) departs from the dipole field (dip angle ~ 73.5 °) in such a way as to make the departure of the ray from field alignment even more marked. 6

333

Research notes 60

40

8, -8

20

0

iI

- ~'0

I

I -40

-60

. . . .

19

_ 20

i[

I

21

22 ! ! 7

23

24

!-,,

Fig. 3. T h e o b s e r v e d g e o m a g n e t i c a z i m u t h s ~i of t h e non-field-aligned a u r o r a l rays, p l o t t e d a g a i n s t t h e t i m e of o b s e r v a t i o n .

Fig. 3 shows a plot of the observed azimuth of the rays as a function of the time of observation. I t is obvious that the rays observed were unlikely to be a single ray showing systematic movement during the evening. This is supported by the small width of the r a y - any rapid movement in azimuth of the ray would have caused it to be broadened appreciably during the long exposures made. The conclusions about these rays may be summarized as follows: {i) The rays were probably high in the atmosphere, since they appeared to be uniformly red from top to horizon. (ii) They appear to have been isolated from normal auroral activity, although this may be an effect of range, in that activity close to their bases could have been below the observer's horizon. (iii) They appeared to be very much longer than they were wide. Tile ray appearing in the exposure from 2321 to 2329 U.T. extended right across the photograph (a distance of approximately 30°), and was less than 1° wide at all elevations. (iv) The rays were not field-aligned, as was the case with the normal auroral rays observed on the same evening. (v) They appear to be fairly rare. (There is no mention of similar rays by STORMER, 1955). A watch has been made from Invercargill, New Zealand, for their appearance during the last two years, without any observed occurrences. Until more observations have been secured of similar rays~ it is fl'uitless to speculate on their origin. I t seems worthwhile to make every effort to observe the occurrence of similar rays, with emphasis on obtaining parallactic photographs of them. Since they may be very high in the atmosphere, a very long base-line would be necessary for successful measurement.

Acknowledgements--One of us (M. G.) wishes to thank Professor W. D. WRmHT of the Imperial College Technical Optics Section for his encouragement and advice on a research project in the course of which the photographs were obtained. The work described is part of the research programme of the Dominion Physical Laboratory Auroral Station, New Zealand, and stems in part from a project sponsored by the Air research and Development Command, U.S.A.F. DPL Auroral Station, Awarua Radio M. GADSDEN Invercargill, New Zealand C . J . LOUOHNA_~ 334

Resea~h n o t e s REFERENCES GADSDEI~ M. STORMER C.

1960 1955

J. Atmosph. Terr. Phys. 17, 347. The Polar Aurora p. 142. Clarendon Press,

STORMER C. VESTINg. E. H., LAPORTE L., L~'~CE I. and SCOTt W. E.

1957 1947

Nature, Lond. 179, 623. The Geomagnetic Field, Its Description and Analysis. Publication No. 580, Carnegie

Oxford.

Institute, Washington.

Photoionization of atomic oxygen and atomic nitrogen (Received 9 April 1960)

I ~ connection with another problem it was necessary to compute photoionization crosssections of atomic oxygen and atomic nitrogen for incident radiation down to very short wavelengths. Because of their importance in the interpretation of solar-terrestrial effects and because they differ both qualitatively and quantitatively from the values currently available (NICOLET, 1952a), we present a brief description of the results. The calculations were carried out initially using the approximate treatment described b y BATES (1946a), which is based upon the dipole length formulation. However, a comparison of the derived polarizabilities with those computed more accurately b y other methods (DAI~ARNO and PARKINSO~, 1959) showed t h a t the dipole length values were much too large at short wavelengths and the approximate treatment b y BATES was therefore extended to the dipole velocity formulation. The derived values yielded much improved polarizabilities and the results shown in Figs. 1 and 2 refer to the dipole velocity cross-sections. The initial bound state wave functions were taken as the Hartre~-Fock functions computed for oxygen by HARTREE et al. (1940) and for nitrogen by HARTREE and HARTREE (1948) and the final continuum functions were taken as hydrogenic in a field of unit nuclear charge. All the parameters required have been tabulated b y BATES (1946b). The qualitative differences from the calculations of NICOLET concern the locations of the discontinuities of the cross-sections associated with the ejection of the inner electrons. The most recent observations of the K-absorption spectra of O and N have apparently been made by MACNUSSO~ (1938) and of the K-emission spectra by TYR~N (1940). NICOLET (1952b) has analysed the data and concludes t h a t the edge of the K-absorption continuum for O occurs at 21.5 A and for N at 29-6/~ We have preferred to adopt the values of 22.8 A for O and 30.3 A for N suggested by the H a r t r e e - F o c k calculations, but the differences are not significant; it should be noted t h a t for both 0 and N there are actually two K-absorption edges, which we do not distinguish between. For the locations of the L x edges, NICOLET suggests 125 A for 0 and 175 A for N. There are, in fact, two L I edges for O corresponding to final states O+(4P) and O+(zP) and two for N corresponding to final states N+(ss) and N+(3S). F r o m the tables of MOORE (1949). it appears t h a t the edges lie at 435/~ and 310 A for O and at 608/~ and 367/~ for N (the averages of these two pairs of edges are in h a r m o n y with those suggested b y the H a r t r e e - F o c k calculations). Since the resonance line of He + is located at 304 A, the new proposed limits m a y have important consequences. The ionization limits corresponding to ejection of a 2p electron are well-known and do not require discussion. I t should be remarked t h a t m a n y other discontinuities must occur. They m a y be associated with transitions in which the ion is left in an excited state or with 335