Ionospheric observations during the annular solar eclipse of 20 May 1966—I Intensity and atmospheric absorption of hydrogen Lyman-α radiation

Journalof Atmospheric and TerrestrialPhyslce,1970,Vol.32, pp. 1849-1854. Perg8znon Press. P&&din NorthernIreland

Ionospheric observations during the annular solar eclipse of 20 May 1866-I Intensity and atmospheric absorption of hydrogen Lyman-a radiation 5. E. HALL Radio and Space Research Station, Ditton Park, Slough, Bucks., England (Received 29 June 1970) Abstra&---The intensity of Hydrogen Lymm-a wm measured before and during the solar eclipse of 20 May 1966. The intensity of the full sun was 3.6 erg ornBa 8-l rend of the 56 per cent eclipsed Sun, 2.1 erg cmBa s-l. An e&s&e has been made of the production of ion&&ion in the ®ion by Lyman-a tatting on nitric oxide. The molecular oxygen density from 70 to 85 km was deduced from the absorption of Lyman-u in the atmosphere and the height variation of electron collision frequency has been oelculated from the sir pressure. 1. INTRODUCTION

of Hydrogen Lyman-E radiation from the Sun was measured at 1151 on 15 May 1966 and at 1101 on 20 May 1966, by experiments carried on Arcas rockets (AEOl and AEO3) launched from Karystos, Greece. The second rocket was launched when the sun was 56 per cent eclipsed. The Lyman-a radiation was detected by ionization chambers with lithium fluoride windows and nitric oxide gas filling. The chambers were operated at unity gas gain using a potential of -45 V on the central electrode. The currents from the chambers were amplified and a signal proportional to current was transmitted to ground via telemetry. To increase the range of measurable Lyman-a intensities, the gains of the amplifiers were repetitively switched between two settings twenty times per second. The rockets were spin stabilized and the ionization chambers were set at such an angle to the spin axis that the normals to their windows passed close to the direction of the Sun on each rocket rotation. The angle between the normal and the Sun was measured by a simple photoelectric attitude sensor. THE INTENSITY

2. INTENSITY

~~EASUREME~TS

The inte~ity, -I, of Lyman-cc radiat,ion is related to the current produced in an ionization chamber by .

&AL!-_=; e ?A(@)

where

i hv e q

1.025 x 108-Ycm-% s-l 11A(@)

is the chamber current in amps is the energy of a photon of wavelength 1216 A is the electronic charge is the chamber efficiency defined as the ratio of ion pairs detected in the chamber to the photon fiux incident normally on the window A (0) is the effective area of the window 0 is the angle between the window normal and the direction of the Sun. Figure 1 shows the observed Lyman-a intensity variations with height calculated 1849 1

J. E.

1850

HYALL

from the above equation. The intensities above the atmosphere have been calculated on the assumption that absorption is proportional to the molecular oxygen density (see Section 3). The intensity from the full sun was 3.6 erg cm-2 s-l and from the 56 per cent eclipsed sun, 2.1 erg om-2 s-1. The main uncertainty in these results arises from possible errors in the values used for chamber efficiencies, q, which were uncertain to a factor of 1.5 and could cause a systematic error in the

I

1

I

I

t

I

75

83

85

90

1 3-

2-

I--

I_____J HEIGHT

----c km

Fig. 1. Curves I and 2 show the Lym8n.a intensity measured from rook& AEOl on the control day (15 Mety 1966) and from rocket AEO3 on the eclipse dey (20 May 1966). The second set of measurements were taken when the Sun was

56 per cent eclipsed.

ordinates of Fig. 1 of this mag~tude. The relative efficiency of the two chambers was known much more accurately and the ratio of the intensity of Lyman-K observed during the eclipse to that on the control day was O-6 with an uncertainty of 10 per cent. This ratio may be compared with the fraction 0.44, of the solar disc still visible at the time of the second flight. The difference in the two ratios may be a measure of the total Lyman-a intensity variation over a 5 day period or it may reflect the non-uniformity of the emission across the Sun’s disc. Such non-uniformity has been observed on photographs of the Sun in Lyman-a (FRIIDMAN,

1959).

3. OXYGEN DENSITY ~~ASUREM~~T The decrease in intensity of Lyman-a,

is caused by atmospheric absorption.

as it penetrates lower into the atmosphere It may be shown, using absorption data

Ionospheric observations-I

1851

given by WATANABE(1958), that absorption by oxygen molecules dominates over other absorption mechanisms in this part of the atmosphere. The number density of oxygen molecules, n(O,), may be found from the rate of absorption of Lyman-a using the expression cos x 1 dI MO,) =--yyz (2) where x is the solar zenith angle 6 is the absorption cross section of Lyman-a in oxygen molecules z is height. In deriving this expression it has been assumed that the atmosphere is horizontally stratified and that the zenith angle is sufficienfly small for terms depending on earth curvature to be neglected. Since the ionization chamber current, i, is proportional to Lyman-x intensity the determination of oxygen density is unaffected by uncertainties in the parameters occurring in equation (l), and equation (2) may be written (3) Laboratory measurements of the absorption cross section, cr have been made by a number of workers and reported by Watanabe 1958. In fhe following it will be assumed that r~= 1O-2ocm2 with an uncertainty of 15 per cent. Figure 2 gives the 90

E ;I.

80

t 5 w I 70

Fig. 2. Height d~tribut~on of oxygen molecules calculated from the absorption of Lyman-a. radiation compared with CLRA 1965. x Control day data (15 Nay 1966); 0 Eclipse day data (20 May 1986); ~ CIRA 1965.

J. E. HALL

1862

height variation of the number density of oxygen molecules calculated using equation (3). The error bars indicated on the points for the eclipse day, which are also applicable to the points for the control day, are made up partly from the systematic error in 0 and partly from the errors in measuring i. A comparison of the results for the two days shows that the oxygen densities were slightly lower on the eclipse day but the difference is barely significant and both sets of data fit the standard atmosphere (CIRA 1965 corrected to mid-May and to the latitude of Greece) to better than the experimental errors. 4.

PRODUCTIONOF IONIZATIONBY LYMAN-~

The most probable sources of ionization in the D-region at the latitude of Greece are ionization of predominantly oxygen and nitrogen by X-rays of wavelengths shorter than 10 8, ionization of nitric oxide by Lyman-cc and, in the lower D-region, ionization by cosmic rays. Recently it has been suggested by HUNTEN and MCELROY(1968), that ionization of excited oxygen molecules by U.V.shorter than 1118 A could be significant. The relative importance of these mechanisms will be diseussed in Part V of this paper. Here the production of ionization by Lyman-a will be considered. The main uncertainty in calculating the rate of production lies in the value of nitric oxide concentration. A wide range of theoretical estimates have been made (NICOLET and AIKIN, 1960; CLYNEand THRUSH, 1961; AIKEN, KANE and TROIM, 1963; and NICOLET,1965) which cover a ratio of 300: 1. The most recent, and the largest, of these estimates gives a mixing ratio n(NO)/n(M) = 3 x 10-s for the region around 80 km (NICOLET,1965) where 1z(N0) q,_ FOR

qL

FOR

AE03

AEOl

l~rn-~s-‘)

km-35-‘l

Fig. 3. Production of ionization by Lyman-cc acting on nitric oxide. The upper scale is for the eclipse day when the Sun was 56 per cent obscured and the lower scale is for the control day. The two curves have been calculated using nitric oxide concentrations given by Nicolet (X) and by Pearce (P).

Ionospheric observations-I

1853

is the concentration of nitric oxide and rt(M) is the total concentration of all atmospheric particles. This upper limit of theoretical estimates of a(N0) is still an order of magnitude smaller than concentrations deduced from airglow observations (BARTH, 1966; PHABCE, 1968). Pearce has found that the distribution of nitric oxide with height matches the total atmospheric distribution from 75 to 90 km and that the mixing ratio a(NO)/n(M) = 8 x 10-T. The production of ionization by Lyman-a acting on nitric oxide, qL, is shown in Fig. 3, for the values of mixing ratio given by Nicofet {curve X) and by Pearce (curve P). Values of S(M) have been taken from Fig. 2 by assuming that oxygen molecules make up 21 per eent of the atmosphere. Use has been made of the cross section for ionization, i.ri= SO2 x 1O-18cm2 given by ~A~A~A~~ (1958). The Lyman-a intensity has been taken from Fig. 1 and the upper scale in Fig. 3 is applicable to the eclipse observation whiIe the lower scaIe is for the control day. The rate of production of ionization at 75 km is exceptiona~y large, if Pearce’s data are used, and leads to very large values of effective loss coefficient. These problems are discussed in Part V.

It has been shown that the mono-energetic ebctron collision frequency is proportional to ambient pressure and, absolute values of the cross section in nitrogen

Fig. 4. ~on~-ener~tic collision frequency, FM, as ~bfm~~tiou of height. The broken curve is derived from CfRA 1965 pressuredate,iandthe points art3calculated from the radio experiment described in PeartIII.

1854

J. E.

NALL

(PACKand PHELPS,1961) and the cross section in oxygen (PHELPS,1960), when applied to D-region conditions reduce to I’M = 6.6 x 104 p s-l

(4)

where p is the total pressure in dyn cmm2. Figure 2 shows that CIRA 1965 oxygen densities are in substantial agreement with the Lyman-a observations, therefore pressure data from the standard atmosphere (adjusted to mid-May and the Iatitude of Greece) has been substituted in equation (4) to give the collision frequency curve in Fig. 4. Collision frequency points deduced from the ratio propagation data in Part III are shown on the same figure. The average difference, amounting to about 20 per cent, is probably insignificant when errors are taken into account. However, this, 20 per cent discrepancy, if it is significant, implies that the constant used in equation (4) should be 8 x lo* which lies between the value 9 x IO* quoted by BAYBULATOVand KRASNUSHKIN(1966) and the value 7 x 104 quoted by MECHTLYet al. (1969). Achmowledgemelzls-This part of the paper is published by permission of the Director of Radio snd Space Research of the Science Research Council.

REFERENCES AIEINA.C.,=NE J. A.and !I-ROIM J. BARTH CA. BAYBULATOV R. B. and KRAsr~uslr~rw P-YE. BOWUNQT.S.,NOR~N K.and U’ILLMOREA.P. CLYBE M. A. A. and THRUSH B. A. COSPAR INTERNATIONAL REFERENCE ATMOSPHERE FRIEDMANH. HUNTEND.M. and MCELROY M.B. MECETLYE.A.,SEINO K. GUNSMITH

1964 1966 1966

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1967

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1961 1965

Proc. R. Sot. iW31, 259. North-Holland, Amsterdam.

1959 1968 1969

J. geophys. Res. 64, 1751. J. geophqa. Res. 13, 2421. Radio Sci. 4, 37 1.

NICOLET M. andAx~r~A.f.2. NICOLETM. PACKJ.L.~;~~PHELPSA.V. PEARCE J.B. PEELPSA.V. WATANABE K.

1960 I965 1961 1969 1960 1958

J. geophgs. res. 65, 1469. J. geophgs. Res. 70, 681. Php. Rec. 1i%1,798. J. gopher. Res. 74, 853. J. appl. F&g. 31,1723, Adv. cfeophqs. 5, 155.

L. G.