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Variations in energy spectrum of auroral electrons detected by simultaneous observation with photometer and riometer
Planer. Space. Sci. 196.5.Voi. 13, pp. 225 to 235. Per@unon Prem Ltd. Prfntcd in Northern Irclsnd
VARIATIONS IN ENERGY SPECTRUM OF AURORAL ELECTRONS DETECTED BY SIMULTANEOUS OBSERVATION WITH PHOTOMETER AND RIOM~TER 0. E. JOHANSRN The Norwegian Institute of Cosmic Physics, Blindem, Oslo 3, Norway; also, No~e~an Defence Research Establishment, Kjeller, Norway (Received 23 November 1964)
Abstract--use auroral luminosity and aurora1 absorption am produced mairdy by electrons with different energy, s~~~~us observation with photome~r and riometer can give information about the energy spectrum of the incident electrons, and in particular be used to detect variations in this spectrum. Limited to cases with homogeneous auroral forms covering most of the sky, observations at i15577A and 27.6 MHi show that the energy spectrum can remain constant for long periods, from half an hour to several hours and whole nights. Large and rapid variations do however occur, but less frequent than one could expect from satellite observations. This indicates that many variations in electron energy q~trum observed by satellite borne instruments are variations in space rather than variations in time. An average over three winters shows a diurnal variation in the energy spectrum with a hardening in the spectrai shape between 20 and 01 local time. An unexpected high and variable light intensity corresponding to zero riometer absorption is also discussed. INTROWJCTION
Optical observations have shown that the aurora1 types most commonly occurring are produced by precipitating electrons (l*s). This conclusion has been confirmed by observations with rocket and satellite borne particle detectorPs). While the maximum aurora1 luminosity normally occurs in the ionospheric B-region, the auroral absorption of cosmic radio noise as observed with riometers, has a maximum in the D-region. The ionization which causes this abso~tion is thought to be due to direct collisions with primary and secondary electrons. There is both experimental@) and theoretical(‘) evidence indicating that “bremsstrahhmg” is of little importance to this ionization. The large variations in observed aurora1 luminosity height profiles, even in profiles with a lower border at the same height, indicate that im~~nt variations in the energy spectrum do occur (@. Measurements with satellite borne particle counters show large, rapid and very frequent changes in the electron energy spectrum(eJoJ8). Holt and OmholF and Gustafsson (6) have studied the relationship between auroral luminosity (A5577 A) and cosmic radio noise absorption (27.6 MHz). They both found a certain correlation. In fact, in view of the highly variable electron energy spectra observed by satellites, the correlation is unexpectedly high, as the two phenomena are produced by electrons of different energy. In this paper it will be shown that s~~~neous measurement with photometer and riometer can be used to obtain information about the energy spectrum of the precipitating electrons and in particular to detect variations in this spectrum. Observations will be presented which on average indicate a diurnal variation of the energy spectrum. The rapid changes in spectral shape seem, however, to be much less frequent in the aurora1 forms here investigated than one could expect from the satellite observations. z25
226
0. E. JOHANSEN LUMINOSITY
AND ABSORPTION
The
intensity of the N,f 1 NG bands is assumed to be proportional to the ionization rate of N, integrated over height. In aurora1 photometry it is, however, more convenient to observe the green [01] line 15577 A rather than the violet Ne+ I NG bands, because the former is easier to isolate with filters and is less subject to contamination by scattered light. It has been shown that the intensity ratio between the two emissions is fairly constant in aurora@). Hence, assuming the ionization cross section of Nz and of other ionizable atoms and molecules to be about the same, the height integrated intensity of 15577 A can be expressed as
Here q refers to the volume emission rate, 4 to the ionization rate, and n(N.J and n(M) to the number densities of N, and of ionizable atoms and molecules in total, respectively. Using data given by Omholt (la) the constant k has been calculated to k = DO2 photon~ion pair, the uncertainty being better than a factor two. Under special assumptions it can be shown that there exists a linear relation between the square root of the light intensity and the cosmic radio noise absorptionul): If the electron energy spectrum is constant in time, the ionization q at any height will be proportional to the intensity I. q(k 0 = c,@)~(t) (2) The constant cr is dependent on height and energy spectrum: The absorption A as measured with riometer can be expressed by the absorption coefficient K which is proportional to the number density N of free electrons. A(t) =
s
K(h, t) dh =
s
c,(h) A@, t) dh
c, being a function of altitude and riometer frequency. may be written -- ’ -Z.ZZ dN dt lfl
(3)
The continuity equation for electrons
aNz
(4)
where a is the effective recombination coefficient, and 3, the ratio between the number densities of negative ions and electrons. Assuming quasi-equilibrium, one obtains ZV(h,t) = [a(1 + J)]-l’a ql’*(h, t) = c3(h) q1/2(h, t)
(5)
Hence, by combination of equations (2), (3) and (5) Pf2(t)=
[ SC:,2~2,dh]lA(t)
=cA(t)
In this approximation the recombination coefficient, the negative ion to electron ratio, and the electron collision frequency (the last is implicit in c2) is assumed to be independent of the electron flux intensity. The ratio c thus depends only on the electron energy spectrum. The ratio c can now be computed for different energy spectra as follows-and compared with observed values. With a given electron energy spectrum the ion production rate per unit tlux at any height can be computed. The volume emission rate q and thus the integrated
VARIATION
IN SPECTRUM
OF ELECTRONS
227
intensity I can be deduced from the ionization rate height profile using equation (1). The electron number density N may be obtained from the ion production rate by means of equation (5). The absorption A is then given by the Appleton-Hartree formula, and c may be deduced using equation (6). The ionization rate profiles were computed on a Ferranti Mercury electronic computer with a program developed by Maehlum (unpublished). This program uses the energy dissipation data published by Spencerda) and computes the ionization rate for every 10 keV from 10 keV to 1000 keV for every 10 km between 60 km and 150 km. The program is developed for isotropic angular ~stribution over the upper he~sphere and arbitrary energy spectrum. The CIRA 1961’“) standard atmosphere was used in the calculations. The absorption was computed using recombination coefficients and electron collision frequencies given by HolW. Rees(16) has shown that variations in the angular distribution of the incident electrons is of minor importance to the ionization rate height profile compared with variations in the energy spectrum. Only isotropic* angular distribution will be considered here. Direct particle measurements seem to lend support to a such distribution(l’). The ratio c has been computed for different types of exponential and power law energy spectra. The no~alized distributions are given by i(E) dE: = J?&-~[exp ( -Emi,/Eo) - exp (--E,,/E,)]-l
exp (--EIE,) dE
(7)
and i(E) dE = (y - l)(E&, - E;;T$l E-” dE
(8)
respectively. Computed values of c are shown in Fig. 1. Examples of co~esponding luminosity height profiles are given in Figs. 2 and 3. The luminosity curves deviate somewhat from those obtained by ReeP), especially at high altitudes. The deviation is due to a minor difference in the basic energy dissipation data, different angular distribution*, and different energies being taken into consideration. The last difference is probably the most important. Rees has used energies between 0.4 keV and 300 keV and smaller steps in the numerical integration. The program used in this investigation was primary developed for calculation of aurora1 absorption. Low energy electrons are therefore treated somewhat roughly. A more detailed consideration of these electrons would give broader luminosity curves with longer tails towards high altitudes. The ratio c defined in equation (6) will of course be influenced by this approximation too. The greatest uncertainties are, however, introduced by the constant k given by equation (1) and the choice of recombination coefficients and electron collision frequencies. In total the uncertainty in c can be expected to be as great as a factor of two. The reasonable results obtained indicate that it actually is somewhat better. This investiga~on, however, is mainly qualitative in nature and the precise values of c given here must not be stressed too much. O~ERVA~ONS
The observations used in this investigation were performed in Tromspr (69.7’ N, 18.9’ E) during the winters 1961/62, 1962163 and 1963/X A photometer operating with an EMI 6095B photomultiplier tube and fitted with an interference filter recorded the zenith intensity of the [01] 125577A line through a 5” acceptance cone. The cosmic radio noise * Rees has used ‘isotropic’ in the meaning ‘&IXthrough a horizontal surfIn this paper ‘directional Aux independent of pitch angle’ ~31 be meant.
independent of pitch angle’.
228
0. E. JOHANSEN
keV
b
10
5
25
20
15
I c IkRp’/dE
FIG.
1.
THE RATIO c CALCULATED y
IN
EXPONENTIAL
AS FUNCTION AND
POWER
6-O
50
L-0
30
29
OF
I
THE C-FOLDING ENERGY
LAW
ENERGY
SPECTRA
E,,AND
THE EXPONENT
RESPECTIVELY.
Isotropic angular distribution. HEIGHT (km)
t-11 ~(photonslelectron FIG.
2.
COMPUTED
VOLUME
EMISSION
TO DIFFERENT
RATE
PER
EXPONENTTAL
cm)
UNIT
LAW
ELECTRON
ENERGY
Isotropic angular distribution.
FLUX
SPECTRA.
CORRESPONDING
VARIATION IN SPECTRUM OF ELECTRONS
229
absorption of 27.6 MHz was recorded with a riometer using a three-element Yagi aerial directed towards zenith, the beam width being about 30” to the 3 dB points. The signals from the photometer and the riometer were amplified loga~t~~~y and linearly respectively and recorded on identical pen recorders. Because the large difference in angle of view between the two instruments, observations must be restricted to cases when most of the sky was covered fairly homogeneously by aurora. Further, to ensure quasi-equilibrium, in order to fulfill the requirements of equation (5), cases with rapid intensity variations must be omitted. Finally, cases when the photometer observations were influenced by clouds or contaminated by scattered moonlight HEIGHT
91 ST(photons/eleclron cm) FIG. 3.
COMPUTEDVOLUMJJ EMLWON
RATE PER UNIT ELECIRON FLUX CORRJBPGNDING TODIFl%RENTpOWERL.AWENERGYSPECI-RA.
Isotropic angular distribution.
must be rejected. The selection of recordings suitable for analysis was done by means of all-sky camera films. From the three winters mentioned before, 24 nights were finally used. Typical recordings are shown in Fig. 4. To obtain the ratio c in equation (6), the square root of the intensity was plotted versus the simultaneous observed absorption. Readings were made in 5 or 10 min intervals. The intensity was corrected for ~~t~ow assuming the pure ni~t~ow intensity at A5577 A to be 150 R in zenith corresponding to the lowest in~nsity measured during zero absorption. During about half the nights studied the points thus obtained lay each night distributed fairly well along straight lines as shown in Fig. 5. These lines were determined by means of simple linear regression analysis. The slope of the regression lines and the point at which they intersect the P- axis, i.e. the intensity corresponding to zero absorption, seem, however, to be highly variable from night to night. During the other half of the nights studied the picture was somewhat more complex. The points can, however, be shown to lie distributed along two or more lines each night. For a long time, from half an hour to several hours, the points may lie distributed along one line. Then at a certain point of time the points suddenly change over to be distributed along another line. The points can remain on this line the rest of the night or they might jump back to the first line or to a third one and so on. The changes seem to occur in a very
230
0. E. JOHANSEN
2030
2110
2050
Frc.4. TYPICAL SIMWLTAN~U~
2120 MET
(?&577& MHz). Broken line refers to the riometer quiet-day curve. From the night 18-19th November 1961. AND
RECQRDINGS OF PHOT~~T~R
RIOP.BTIXR
(2%
1// X x
k0
Xx
x
I----
x
x
x
xx x
, PO
2.0
30 AkiBl
1963. Points represent readings made with 5 min intervals.
Fio.5. l%oT FROM THE NX~HT 12-13~~ OC!TOBER
VARIATION
IN SPECTRUM
OF ELECTRONS
231
short time, at the most 15 min and usually much less. The changes usually affect both the slope of the line and the point at which it intersects the P2-axis. Change in one of the quantities only, is however found to occur. Typical examples are given in Fig. 6. The points plotted in Fig. 5 are obtained between 1900 and 2300 MET (later observations were prevented by bad weather) only interrupted by short periods with active and distinct aurora1 forms. The slope of the regression line gives c = 082 f O-05(kR)‘12/dB. Assuming an exponential energy distribution this corresponds to an e-folding energy E, m 12 keV. Assuming a power law energy spectrum it gives an exponent y M 4 (Fig. 1). Because of the
x 2355-0355
MET
x
1725-0255
MET
o
MET
o
1X306-0605
MET
0400-0605
20
I.0 (4
A(dB)
2.0
1-o
A
(dB)
(b)
FIG. 6. PLOT FROMTHE NIGHTS23-24~~ NOVEMBER 1962 (a) AND 3-4~~ DECEMBER 1961 (b). Points represent readings made with 5 and 10 min intervals respectively.
limitations in the method nothing can be said about the electron energy spectrum during the periods with active and distinct aurora. Case (a) in Fig. 6 demonstrates a sudden change in the slope of the regression line. Only interrupted by short periods with active and distinct aurora the points first lay distributed along one line giving c(1) = O-65 f 0.07 (kR)‘/“/dB. Later they suddenly changed over to a line giving c(2) = 0.25 f 0.02 (kR)‘12/dB. Assuming an exponential electron energy spectrum this corresponds to a change from E,,(l) = 14 keV to E,(2) = 30 keV. With a power law distribution the exponent must change from y(l) = 3.6 to y(2) = 2.8. The uncertainty in the differences E,,(l) - E,(2) and y(l) - y(2) is 3 keV and 0.1 respectively. Case (b) in Fig. 6 shows no significant change in the slope of the regression lines. The intensity corresponding to zero absorption changed, however, from 540 R to 185 R. (Total intensities, not corrected for any assumed nightglow intensity.) The uncertainty in the intensity difference is less than 10 R. The calibration uncertainty is estimated to 30 per cent. The observations available contain 24 significant examples of the sudden changes described. Some of these occurred in connection with aurora1 outbursts, but an equaI amount did not. Moreover, a lot of outbursts occurred without any change being detectable. Magnetograms have been investigated but no typical feature seems to be associated
232
0. E. JOHANSEN
with the changes observed. No particular time of the night seems to be associated with the changes. The results from all nights analysed are plotted in Figs. 7 and 8 versus local time. The average of the ratio c shows a marked diurnal variation. It must, however, be stressed that this variation is statistical in nature. There are nights with no detectable variation in c, and even nights with variations opposite to the arithmetic mean diurnal variation do occur. The average of the intensity corresponding to zero absorption shows no diurnal variation, but lies somewhat higher than one would expect from night~ow alone.
I 0.1 J
x
,
,
18
I
I
20
I
,
I
22
I
00
,
*
x
I
I
02
x
x
I
,
04
t
06
MET h3.7.
OBSERVED
ZENITH
INTENSITY PLOTD3D
AT
A55778, CORRESPGNDING
VBRSUS
LOCAL
TO ZERO ABSORPTXON
TIME.
Arithmetic mean and standard deviation are indicated.
As mentioned before the observed intensities have been corrected for nightglow. In most cases, however, the intensity corresponding to zero absorption was greater than the assumed nightglow intensity. This will of course give a minor deviation from the rectilinear form of the regression lines predicted by equation (6). Hence, using linear regression analysis small errors are introduced. If the number of points referring to high intensities compared to the zero absorption intensity are dominant, the slope of the line will be fairly good, but the intensity corresponding to zero absorption slightly too low. On the other hand, if the number of points referring to in~nsities comparable to the zero abso~tion intensity are dominant, the slope of the regression line will be slightly too low, but the intensity corresponding to zero absorption fairly good. Normally the errors introduced by this effect are of minor importance. IEXXJSSION It is
obvious that simultaneous observations with photometer and riometer can give valuable isolation about the energy spectrum of the pr~cipita~ng electrons. By using this method nothing can be said of the precise spectral shape. The energy spectrum seems, however, to be fairly independent of the intensity. Thus, choosing special types of reference spectra, for example the two types most frequently used; exponential and power law spectra, one can determine with a reasonable degree of certainty the parameter which fits
VARIATION
IN SPECTRUM
233
OF ELECTRONS
the observations best. Variations in the energy of the incident electrons can easily be detected. By referring to some spectral type the amount of change can be given with good accuracy. Small and slow variations are difficult to detect, Furthermore, the method is not applicable in cases when the aurora is excited partly or wholly by protons. Aurora1 spectrophotometry shows, however, that such cases are rare(12). Due to the instruments used, the present investigation had to be limited to homogeneous forms covering most of the sky. The applicability can be extended by use of directive aerials or by use of a system of stops and screens fitted on the photometer to simulate the riometer field of view. C
EO
(kRj?dB
(kcv)
3.0I
I
x
x
x
x
x
x
x
--6
L
c
_-5
x
i
L
L
f+ .
:
1
’
I
x
it
i i L
3i
* ; :
--4
L
Fro.8. OFSERVED SU~PEOPREGRESSIONLINEPU~~TEDVERSUS~ALTIME. Arithmetic mean and standard deviation are indicated. At right corresponding values of the e-folding energy E. and the exponent y in exponential law and power law electron energy spectra respectively are given.
It seems to be typical at least for the aurora1 forms here investigated, that the electron energy spectrum can keep constant for long periods, from half an hour to several hours and whole nights. Changes in energy spectrum do, however, occur. When it occurs, it seems to be very rapid changes, usually occurring within much less than 15 min. The changes in energy distribution seem to be much less frequent than one could expect from satellite observations. This indicates that many of the variations in energy spectrum detected by satellite borne instruments are variation in space rather than variation in time. Simultaneous observation with satellite borne particle detectors and ground based photometers and riometers seem to be a promising method of obtaining a resolution in time and space of .$hevariations observed. On average observations from the three winters available show a clear diurnal variation of the energy spectrum (cf. Fig. 8). Between 20 and 01 MET (approximately between geomagnetic and geographic midnight) there is a significant hardening of the energy spectrum reaching maximum hardness between 22 and 23 MET. There also seems to be a tendency to a hardening in the spectral shape toward the day sides. This would be in accordance with a preliminary satellite data analysis published by Sharp ct &(la) which shows a harder energy spectrum on the day side than on the night side in the north.
234
0.
E. JOHANSEN
The diurnal variation in energy spectrum reported here, of course reflects at least some statistical properties of the electron acceleration mechanism. Existing theories for this acceleration seem, however, not to be developed to such a degree of sophistication that the observed effects can be used in testing the theories. Information about the energy spectrum could also be obtained by comparing the computed luminosity height profiles with observed luminosity curves. Ground based observations of the luminosity profiles of the aurora1 forms studied here are, however, impossible on principle. Photometer data from rockets shot through aurora is at present very scanty. The light intensity corresponding to zero absorption seems to be highly variable. In average, however, no significant diurnal variation can be detected (cf. Fig. 7). The nightglow intensity of 15577 A in zenith is usually assumed to be about 250 R(s). The observed average intensity is approximately 400 R. This high intensity may indicate that there always are some aurora present in the aurora1 zones even during apparently quiet periods. The absorption corresponding to a such fairly constant and weak aurora1 background would be included in the riometer quiet day curve and not detected as aurora1 absorption using conventional techniques. An aurora1 background could of course also be produced by low energetic electrons at great altitudes giving no detectable contribution to the absorption. From the present observations it is impossible to decide whether the high intensity observed is pure nightglow or caused by and additional more or less constant aurora1 background, or something in between. A study of the spectral composition of this light can easily settle the question. Acknowledgements-The
author is greatly indebted to Professor A. Omholt for helpful advice through all stages of this work. I would also like to express my gratitude to Dr. B. Maehlum for enlightening discussions and for putting a completely developed electronic computer program at the authors disposal. The work has been sponsored in part by the Air Force Cambridge Research Laboratories, OAR, through its European Office under Contract AF 61(052)-686 and concerning the riometer observations under Contract AF 61(052)-599. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
A. OMHOLT, Geofys. Publ. 20, No. 11 (1959). A. OHMOLT, Geofys. Publ. 21, No. 1 (1959). C. E. MCILWAIN, J. Geophys. Res. 65,272i’ (1960). B. J. O’BRIENand H. TAYLOR,J. Geophys. Res. 69,45 (1964). R. D. SHARP,J. E. EVANS,W. L. IMHOF,R. G. JOHNSON, J. B. REAGANand R. V. SMITH,J. Geophys. Res. 69,272l (1964). G. GUSTAPSSON, Planet. Space Sci. 12,195 (1964). M. H. RYES,Planet. Space Sci. 12, 1093 (1964). J. W. CHAMBERLAIN, Physics of the Aurora and Airglow. Academic Press, New York and London (1961). B. J. O’BRIEN,C. D. LAUGHLIN,J. A. VAN ALLENand L. A. FRANK,J. Geophys. Res. 67,1209 (1962). B. J. O’BRIENand C. D. LAUGHLIN,J. Geophys. Res. 67,2667 (1962). 0. HOLTand A. OMHOLT, J. Atmos. Terr. Phys. 24,467 (1962). A. OMHOLT,in Proceedings from COSPAR Conference on High Latitude Particles, AIpbach 1964 (Ed. B. Maehlum). Logos Press, London (1964). L. V. SPENCER,Nat. Bur. Stand. Monogr. NO. 1 (1959). COSPAR International Reference Atmosphere, 1961. North Holland, Amsterdam (1961). 0. HOLT,NDRE Report 46, Norwegian Defence Research Establishment (1963). M. H. Ram, Planet. Space Sci. 11, 1209 (1963). B. J. O’BRIEN,J. Geophys. Res. 69, 13 (1964). R. D. SHARP,J. E. EVANS,R. G. JOHNSON and J. B. REAGAN,Paper presented at COSPAR Ftfth Space Science Symposium, Florence (May, 1964). Pea~M+BBn~y TOl'O,YTO CBeTHMOCTb IIa6cop6qrin IIOJIRpHbIXCHRHHii IIpOkiBBOWTCH TnBBHbIM o6paaou NIBKTPOHBMH C HeOAHOpOAHOi BHBpl'KE!i,O~HOBpBMeHHOB Ha6nmJysiIle +OTOMeTPOM
H PHOMeTPOM
MOPKBT CHB6AIITb CBW&?HUR
OTHOCHTejIbHO CWKTPB
VARIATION
IN SPECTRUM OF ELECTRONS
OHeprK~~~aIo~KXO~eKTpOHOBKB~aCTHOCTH6bITbEC~OJIb~OB;UIO~~~o6Hapy~eHKrl BapHaI@i B OTOM CIIeKTpe. OrpaHHWBaHCb CJIyYaFlMK C O~HOpO~IibIMH $OpMaMEi IIOJWpHbIX CHHHEIii,8aHHMalO~KX 60JIbUIKHCTBO Ke6ecKoB IIOBepXHOCTE, Ha6JIIO~eHEFI npu 5577 A pi27.0 MHznoKaab~Ba~OT,~~~cneKTpaHepr~~~Ome? OCTaBaTbCfiHeWaYeKHblMBTe~eHHe~O~rEIX~epUO~OB,OT~O~y~aCa~OHeCKO~bKHXYaCOBH~eJIbMKHO'IaMEI. Tern He MeHee, KpyllHbIe.M 6bIcTpbIeBapHaqHR HeCOMHeHHO IIpOHCXOJViT,HO He CTOJIb YaCTO, KaK OTO MOWHO 6~10 6~ IIpe~llOJlOHUiTbHa OCHOBaHKK Ka6mo~emB HaJ( CIlyTHiiKaMK. 3TO yKaabIBaeT Ha TO 06CTOJITeJIbCTB0,YTO B CneKTpe mieprau NIeKTP~H~B MHorkie sapiiaum, HaBmo~aehme Hecomhwi cnyTmKam npK6opamu, RBBapuaqmm Bo BpeMeKK. JIFIIOTCFI CKOpee BapHaIJ~HMK B IIpOCTpaHCTBe, Hexem ~pOMe~TOrOo6cyHc~aeTcR.TaK~eHe~pe~BIl~eHHOBbICOKaFIIIHe~OCTO~HHaFIUHTeHCIIBHOCTb CBeTa,COOTBeTCTByIO~afI HyJIIOpKOMeTpHqeCKOt a6copbwm.
235
VARIATIONS IN ENERGY SPECTRUM OF AURORAL ELECTRONS DETECTED BY SIMULTANEOUS OBSERVATION WITH PHOTOMETER AND RIOM~TER 0. E. JOHANSRN The Norwegian Institute of Cosmic Physics, Blindem, Oslo 3, Norway; also, No~e~an Defence Research Establishment, Kjeller, Norway (Received 23 November 1964)
Abstract--use auroral luminosity and aurora1 absorption am produced mairdy by electrons with different energy, s~~~~us observation with photome~r and riometer can give information about the energy spectrum of the incident electrons, and in particular be used to detect variations in this spectrum. Limited to cases with homogeneous auroral forms covering most of the sky, observations at i15577A and 27.6 MHi show that the energy spectrum can remain constant for long periods, from half an hour to several hours and whole nights. Large and rapid variations do however occur, but less frequent than one could expect from satellite observations. This indicates that many variations in electron energy q~trum observed by satellite borne instruments are variations in space rather than variations in time. An average over three winters shows a diurnal variation in the energy spectrum with a hardening in the spectrai shape between 20 and 01 local time. An unexpected high and variable light intensity corresponding to zero riometer absorption is also discussed. INTROWJCTION
Optical observations have shown that the aurora1 types most commonly occurring are produced by precipitating electrons (l*s). This conclusion has been confirmed by observations with rocket and satellite borne particle detectorPs). While the maximum aurora1 luminosity normally occurs in the ionospheric B-region, the auroral absorption of cosmic radio noise as observed with riometers, has a maximum in the D-region. The ionization which causes this abso~tion is thought to be due to direct collisions with primary and secondary electrons. There is both experimental@) and theoretical(‘) evidence indicating that “bremsstrahhmg” is of little importance to this ionization. The large variations in observed aurora1 luminosity height profiles, even in profiles with a lower border at the same height, indicate that im~~nt variations in the energy spectrum do occur (@. Measurements with satellite borne particle counters show large, rapid and very frequent changes in the electron energy spectrum(eJoJ8). Holt and OmholF and Gustafsson (6) have studied the relationship between auroral luminosity (A5577 A) and cosmic radio noise absorption (27.6 MHz). They both found a certain correlation. In fact, in view of the highly variable electron energy spectra observed by satellites, the correlation is unexpectedly high, as the two phenomena are produced by electrons of different energy. In this paper it will be shown that s~~~neous measurement with photometer and riometer can be used to obtain information about the energy spectrum of the precipitating electrons and in particular to detect variations in this spectrum. Observations will be presented which on average indicate a diurnal variation of the energy spectrum. The rapid changes in spectral shape seem, however, to be much less frequent in the aurora1 forms here investigated than one could expect from the satellite observations. z25
226
0. E. JOHANSEN LUMINOSITY
AND ABSORPTION
The
intensity of the N,f 1 NG bands is assumed to be proportional to the ionization rate of N, integrated over height. In aurora1 photometry it is, however, more convenient to observe the green [01] line 15577 A rather than the violet Ne+ I NG bands, because the former is easier to isolate with filters and is less subject to contamination by scattered light. It has been shown that the intensity ratio between the two emissions is fairly constant in aurora@). Hence, assuming the ionization cross section of Nz and of other ionizable atoms and molecules to be about the same, the height integrated intensity of 15577 A can be expressed as
Here q refers to the volume emission rate, 4 to the ionization rate, and n(N.J and n(M) to the number densities of N, and of ionizable atoms and molecules in total, respectively. Using data given by Omholt (la) the constant k has been calculated to k = DO2 photon~ion pair, the uncertainty being better than a factor two. Under special assumptions it can be shown that there exists a linear relation between the square root of the light intensity and the cosmic radio noise absorptionul): If the electron energy spectrum is constant in time, the ionization q at any height will be proportional to the intensity I. q(k 0 = c,@)~(t) (2) The constant cr is dependent on height and energy spectrum: The absorption A as measured with riometer can be expressed by the absorption coefficient K which is proportional to the number density N of free electrons. A(t) =
s
K(h, t) dh =
s
c,(h) A@, t) dh
c, being a function of altitude and riometer frequency. may be written -- ’ -Z.ZZ dN dt lfl
(3)
The continuity equation for electrons
aNz
(4)
where a is the effective recombination coefficient, and 3, the ratio between the number densities of negative ions and electrons. Assuming quasi-equilibrium, one obtains ZV(h,t) = [a(1 + J)]-l’a ql’*(h, t) = c3(h) q1/2(h, t)
(5)
Hence, by combination of equations (2), (3) and (5) Pf2(t)=
[ SC:,2~2,dh]lA(t)
=cA(t)
In this approximation the recombination coefficient, the negative ion to electron ratio, and the electron collision frequency (the last is implicit in c2) is assumed to be independent of the electron flux intensity. The ratio c thus depends only on the electron energy spectrum. The ratio c can now be computed for different energy spectra as follows-and compared with observed values. With a given electron energy spectrum the ion production rate per unit tlux at any height can be computed. The volume emission rate q and thus the integrated
VARIATION
IN SPECTRUM
OF ELECTRONS
227
intensity I can be deduced from the ionization rate height profile using equation (1). The electron number density N may be obtained from the ion production rate by means of equation (5). The absorption A is then given by the Appleton-Hartree formula, and c may be deduced using equation (6). The ionization rate profiles were computed on a Ferranti Mercury electronic computer with a program developed by Maehlum (unpublished). This program uses the energy dissipation data published by Spencerda) and computes the ionization rate for every 10 keV from 10 keV to 1000 keV for every 10 km between 60 km and 150 km. The program is developed for isotropic angular ~stribution over the upper he~sphere and arbitrary energy spectrum. The CIRA 1961’“) standard atmosphere was used in the calculations. The absorption was computed using recombination coefficients and electron collision frequencies given by HolW. Rees(16) has shown that variations in the angular distribution of the incident electrons is of minor importance to the ionization rate height profile compared with variations in the energy spectrum. Only isotropic* angular distribution will be considered here. Direct particle measurements seem to lend support to a such distribution(l’). The ratio c has been computed for different types of exponential and power law energy spectra. The no~alized distributions are given by i(E) dE: = J?&-~[exp ( -Emi,/Eo) - exp (--E,,/E,)]-l
exp (--EIE,) dE
(7)
and i(E) dE = (y - l)(E&, - E;;T$l E-” dE
(8)
respectively. Computed values of c are shown in Fig. 1. Examples of co~esponding luminosity height profiles are given in Figs. 2 and 3. The luminosity curves deviate somewhat from those obtained by ReeP), especially at high altitudes. The deviation is due to a minor difference in the basic energy dissipation data, different angular distribution*, and different energies being taken into consideration. The last difference is probably the most important. Rees has used energies between 0.4 keV and 300 keV and smaller steps in the numerical integration. The program used in this investigation was primary developed for calculation of aurora1 absorption. Low energy electrons are therefore treated somewhat roughly. A more detailed consideration of these electrons would give broader luminosity curves with longer tails towards high altitudes. The ratio c defined in equation (6) will of course be influenced by this approximation too. The greatest uncertainties are, however, introduced by the constant k given by equation (1) and the choice of recombination coefficients and electron collision frequencies. In total the uncertainty in c can be expected to be as great as a factor of two. The reasonable results obtained indicate that it actually is somewhat better. This investiga~on, however, is mainly qualitative in nature and the precise values of c given here must not be stressed too much. O~ERVA~ONS
The observations used in this investigation were performed in Tromspr (69.7’ N, 18.9’ E) during the winters 1961/62, 1962163 and 1963/X A photometer operating with an EMI 6095B photomultiplier tube and fitted with an interference filter recorded the zenith intensity of the [01] 125577A line through a 5” acceptance cone. The cosmic radio noise * Rees has used ‘isotropic’ in the meaning ‘&IXthrough a horizontal surfIn this paper ‘directional Aux independent of pitch angle’ ~31 be meant.
independent of pitch angle’.
228
0. E. JOHANSEN
keV
b
10
5
25
20
15
I c IkRp’/dE
FIG.
1.
THE RATIO c CALCULATED y
IN
EXPONENTIAL
AS FUNCTION AND
POWER
6-O
50
L-0
30
29
OF
I
THE C-FOLDING ENERGY
LAW
ENERGY
SPECTRA
E,,AND
THE EXPONENT
RESPECTIVELY.
Isotropic angular distribution. HEIGHT (km)
t-11 ~(photonslelectron FIG.
2.
COMPUTED
VOLUME
EMISSION
TO DIFFERENT
RATE
PER
EXPONENTTAL
cm)
UNIT
LAW
ELECTRON
ENERGY
Isotropic angular distribution.
FLUX
SPECTRA.
CORRESPONDING
VARIATION IN SPECTRUM OF ELECTRONS
229
absorption of 27.6 MHz was recorded with a riometer using a three-element Yagi aerial directed towards zenith, the beam width being about 30” to the 3 dB points. The signals from the photometer and the riometer were amplified loga~t~~~y and linearly respectively and recorded on identical pen recorders. Because the large difference in angle of view between the two instruments, observations must be restricted to cases when most of the sky was covered fairly homogeneously by aurora. Further, to ensure quasi-equilibrium, in order to fulfill the requirements of equation (5), cases with rapid intensity variations must be omitted. Finally, cases when the photometer observations were influenced by clouds or contaminated by scattered moonlight HEIGHT
91 ST(photons/eleclron cm) FIG. 3.
COMPUTEDVOLUMJJ EMLWON
RATE PER UNIT ELECIRON FLUX CORRJBPGNDING TODIFl%RENTpOWERL.AWENERGYSPECI-RA.
Isotropic angular distribution.
must be rejected. The selection of recordings suitable for analysis was done by means of all-sky camera films. From the three winters mentioned before, 24 nights were finally used. Typical recordings are shown in Fig. 4. To obtain the ratio c in equation (6), the square root of the intensity was plotted versus the simultaneous observed absorption. Readings were made in 5 or 10 min intervals. The intensity was corrected for ~~t~ow assuming the pure ni~t~ow intensity at A5577 A to be 150 R in zenith corresponding to the lowest in~nsity measured during zero absorption. During about half the nights studied the points thus obtained lay each night distributed fairly well along straight lines as shown in Fig. 5. These lines were determined by means of simple linear regression analysis. The slope of the regression lines and the point at which they intersect the P- axis, i.e. the intensity corresponding to zero absorption, seem, however, to be highly variable from night to night. During the other half of the nights studied the picture was somewhat more complex. The points can, however, be shown to lie distributed along two or more lines each night. For a long time, from half an hour to several hours, the points may lie distributed along one line. Then at a certain point of time the points suddenly change over to be distributed along another line. The points can remain on this line the rest of the night or they might jump back to the first line or to a third one and so on. The changes seem to occur in a very
230
0. E. JOHANSEN
2030
2110
2050
Frc.4. TYPICAL SIMWLTAN~U~
2120 MET
(?&577& MHz). Broken line refers to the riometer quiet-day curve. From the night 18-19th November 1961. AND
RECQRDINGS OF PHOT~~T~R
RIOP.BTIXR
(2%
1// X x
k0
Xx
x
I----
x
x
x
xx x
, PO
2.0
30 AkiBl
1963. Points represent readings made with 5 min intervals.
Fio.5. l%oT FROM THE NX~HT 12-13~~ OC!TOBER
VARIATION
IN SPECTRUM
OF ELECTRONS
231
short time, at the most 15 min and usually much less. The changes usually affect both the slope of the line and the point at which it intersects the P2-axis. Change in one of the quantities only, is however found to occur. Typical examples are given in Fig. 6. The points plotted in Fig. 5 are obtained between 1900 and 2300 MET (later observations were prevented by bad weather) only interrupted by short periods with active and distinct aurora1 forms. The slope of the regression line gives c = 082 f O-05(kR)‘12/dB. Assuming an exponential energy distribution this corresponds to an e-folding energy E, m 12 keV. Assuming a power law energy spectrum it gives an exponent y M 4 (Fig. 1). Because of the
x 2355-0355
MET
x
1725-0255
MET
o
MET
o
1X306-0605
MET
0400-0605
20
I.0 (4
A(dB)
2.0
1-o
A
(dB)
(b)
FIG. 6. PLOT FROMTHE NIGHTS23-24~~ NOVEMBER 1962 (a) AND 3-4~~ DECEMBER 1961 (b). Points represent readings made with 5 and 10 min intervals respectively.
limitations in the method nothing can be said about the electron energy spectrum during the periods with active and distinct aurora. Case (a) in Fig. 6 demonstrates a sudden change in the slope of the regression line. Only interrupted by short periods with active and distinct aurora the points first lay distributed along one line giving c(1) = O-65 f 0.07 (kR)‘/“/dB. Later they suddenly changed over to a line giving c(2) = 0.25 f 0.02 (kR)‘12/dB. Assuming an exponential electron energy spectrum this corresponds to a change from E,,(l) = 14 keV to E,(2) = 30 keV. With a power law distribution the exponent must change from y(l) = 3.6 to y(2) = 2.8. The uncertainty in the differences E,,(l) - E,(2) and y(l) - y(2) is 3 keV and 0.1 respectively. Case (b) in Fig. 6 shows no significant change in the slope of the regression lines. The intensity corresponding to zero absorption changed, however, from 540 R to 185 R. (Total intensities, not corrected for any assumed nightglow intensity.) The uncertainty in the intensity difference is less than 10 R. The calibration uncertainty is estimated to 30 per cent. The observations available contain 24 significant examples of the sudden changes described. Some of these occurred in connection with aurora1 outbursts, but an equaI amount did not. Moreover, a lot of outbursts occurred without any change being detectable. Magnetograms have been investigated but no typical feature seems to be associated
232
0. E. JOHANSEN
with the changes observed. No particular time of the night seems to be associated with the changes. The results from all nights analysed are plotted in Figs. 7 and 8 versus local time. The average of the ratio c shows a marked diurnal variation. It must, however, be stressed that this variation is statistical in nature. There are nights with no detectable variation in c, and even nights with variations opposite to the arithmetic mean diurnal variation do occur. The average of the intensity corresponding to zero absorption shows no diurnal variation, but lies somewhat higher than one would expect from night~ow alone.
I 0.1 J
x
,
,
18
I
I
20
I
,
I
22
I
00
,
*
x
I
I
02
x
x
I
,
04
t
06
MET h3.7.
OBSERVED
ZENITH
INTENSITY PLOTD3D
AT
A55778, CORRESPGNDING
VBRSUS
LOCAL
TO ZERO ABSORPTXON
TIME.
Arithmetic mean and standard deviation are indicated.
As mentioned before the observed intensities have been corrected for nightglow. In most cases, however, the intensity corresponding to zero absorption was greater than the assumed nightglow intensity. This will of course give a minor deviation from the rectilinear form of the regression lines predicted by equation (6). Hence, using linear regression analysis small errors are introduced. If the number of points referring to high intensities compared to the zero absorption intensity are dominant, the slope of the line will be fairly good, but the intensity corresponding to zero absorption slightly too low. On the other hand, if the number of points referring to in~nsities comparable to the zero abso~tion intensity are dominant, the slope of the regression line will be slightly too low, but the intensity corresponding to zero absorption fairly good. Normally the errors introduced by this effect are of minor importance. IEXXJSSION It is
obvious that simultaneous observations with photometer and riometer can give valuable isolation about the energy spectrum of the pr~cipita~ng electrons. By using this method nothing can be said of the precise spectral shape. The energy spectrum seems, however, to be fairly independent of the intensity. Thus, choosing special types of reference spectra, for example the two types most frequently used; exponential and power law spectra, one can determine with a reasonable degree of certainty the parameter which fits
VARIATION
IN SPECTRUM
233
OF ELECTRONS
the observations best. Variations in the energy of the incident electrons can easily be detected. By referring to some spectral type the amount of change can be given with good accuracy. Small and slow variations are difficult to detect, Furthermore, the method is not applicable in cases when the aurora is excited partly or wholly by protons. Aurora1 spectrophotometry shows, however, that such cases are rare(12). Due to the instruments used, the present investigation had to be limited to homogeneous forms covering most of the sky. The applicability can be extended by use of directive aerials or by use of a system of stops and screens fitted on the photometer to simulate the riometer field of view. C
EO
(kRj?dB
(kcv)
3.0I
I
x
x
x
x
x
x
x
--6
L
c
_-5
x
i
L
L
f+ .
:
1
’
I
x
it
i i L
3i
* ; :
--4
L
Fro.8. OFSERVED SU~PEOPREGRESSIONLINEPU~~TEDVERSUS~ALTIME. Arithmetic mean and standard deviation are indicated. At right corresponding values of the e-folding energy E. and the exponent y in exponential law and power law electron energy spectra respectively are given.
It seems to be typical at least for the aurora1 forms here investigated, that the electron energy spectrum can keep constant for long periods, from half an hour to several hours and whole nights. Changes in energy spectrum do, however, occur. When it occurs, it seems to be very rapid changes, usually occurring within much less than 15 min. The changes in energy distribution seem to be much less frequent than one could expect from satellite observations. This indicates that many of the variations in energy spectrum detected by satellite borne instruments are variation in space rather than variation in time. Simultaneous observation with satellite borne particle detectors and ground based photometers and riometers seem to be a promising method of obtaining a resolution in time and space of .$hevariations observed. On average observations from the three winters available show a clear diurnal variation of the energy spectrum (cf. Fig. 8). Between 20 and 01 MET (approximately between geomagnetic and geographic midnight) there is a significant hardening of the energy spectrum reaching maximum hardness between 22 and 23 MET. There also seems to be a tendency to a hardening in the spectral shape toward the day sides. This would be in accordance with a preliminary satellite data analysis published by Sharp ct &(la) which shows a harder energy spectrum on the day side than on the night side in the north.
234
0.
E. JOHANSEN
The diurnal variation in energy spectrum reported here, of course reflects at least some statistical properties of the electron acceleration mechanism. Existing theories for this acceleration seem, however, not to be developed to such a degree of sophistication that the observed effects can be used in testing the theories. Information about the energy spectrum could also be obtained by comparing the computed luminosity height profiles with observed luminosity curves. Ground based observations of the luminosity profiles of the aurora1 forms studied here are, however, impossible on principle. Photometer data from rockets shot through aurora is at present very scanty. The light intensity corresponding to zero absorption seems to be highly variable. In average, however, no significant diurnal variation can be detected (cf. Fig. 7). The nightglow intensity of 15577 A in zenith is usually assumed to be about 250 R(s). The observed average intensity is approximately 400 R. This high intensity may indicate that there always are some aurora present in the aurora1 zones even during apparently quiet periods. The absorption corresponding to a such fairly constant and weak aurora1 background would be included in the riometer quiet day curve and not detected as aurora1 absorption using conventional techniques. An aurora1 background could of course also be produced by low energetic electrons at great altitudes giving no detectable contribution to the absorption. From the present observations it is impossible to decide whether the high intensity observed is pure nightglow or caused by and additional more or less constant aurora1 background, or something in between. A study of the spectral composition of this light can easily settle the question. Acknowledgements-The
author is greatly indebted to Professor A. Omholt for helpful advice through all stages of this work. I would also like to express my gratitude to Dr. B. Maehlum for enlightening discussions and for putting a completely developed electronic computer program at the authors disposal. The work has been sponsored in part by the Air Force Cambridge Research Laboratories, OAR, through its European Office under Contract AF 61(052)-686 and concerning the riometer observations under Contract AF 61(052)-599. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
A. OMHOLT, Geofys. Publ. 20, No. 11 (1959). A. OHMOLT, Geofys. Publ. 21, No. 1 (1959). C. E. MCILWAIN, J. Geophys. Res. 65,272i’ (1960). B. J. O’BRIENand H. TAYLOR,J. Geophys. Res. 69,45 (1964). R. D. SHARP,J. E. EVANS,W. L. IMHOF,R. G. JOHNSON, J. B. REAGANand R. V. SMITH,J. Geophys. Res. 69,272l (1964). G. GUSTAPSSON, Planet. Space Sci. 12,195 (1964). M. H. RYES,Planet. Space Sci. 12, 1093 (1964). J. W. CHAMBERLAIN, Physics of the Aurora and Airglow. Academic Press, New York and London (1961). B. J. O’BRIEN,C. D. LAUGHLIN,J. A. VAN ALLENand L. A. FRANK,J. Geophys. Res. 67,1209 (1962). B. J. O’BRIENand C. D. LAUGHLIN,J. Geophys. Res. 67,2667 (1962). 0. HOLTand A. OMHOLT, J. Atmos. Terr. Phys. 24,467 (1962). A. OMHOLT,in Proceedings from COSPAR Conference on High Latitude Particles, AIpbach 1964 (Ed. B. Maehlum). Logos Press, London (1964). L. V. SPENCER,Nat. Bur. Stand. Monogr. NO. 1 (1959). COSPAR International Reference Atmosphere, 1961. North Holland, Amsterdam (1961). 0. HOLT,NDRE Report 46, Norwegian Defence Research Establishment (1963). M. H. Ram, Planet. Space Sci. 11, 1209 (1963). B. J. O’BRIEN,J. Geophys. Res. 69, 13 (1964). R. D. SHARP,J. E. EVANS,R. G. JOHNSON and J. B. REAGAN,Paper presented at COSPAR Ftfth Space Science Symposium, Florence (May, 1964). Pea~M+BBn~y TOl'O,YTO CBeTHMOCTb IIa6cop6qrin IIOJIRpHbIXCHRHHii IIpOkiBBOWTCH TnBBHbIM o6paaou NIBKTPOHBMH C HeOAHOpOAHOi BHBpl'KE!i,O~HOBpBMeHHOB Ha6nmJysiIle +OTOMeTPOM
H PHOMeTPOM
MOPKBT CHB6AIITb CBW&?HUR
OTHOCHTejIbHO CWKTPB
VARIATION
IN SPECTRUM OF ELECTRONS
OHeprK~~~aIo~KXO~eKTpOHOBKB~aCTHOCTH6bITbEC~OJIb~OB;UIO~~~o6Hapy~eHKrl BapHaI@i B OTOM CIIeKTpe. OrpaHHWBaHCb CJIyYaFlMK C O~HOpO~IibIMH $OpMaMEi IIOJWpHbIX CHHHEIii,8aHHMalO~KX 60JIbUIKHCTBO Ke6ecKoB IIOBepXHOCTE, Ha6JIIO~eHEFI npu 5577 A pi27.0 MHznoKaab~Ba~OT,~~~cneKTpaHepr~~~Ome? OCTaBaTbCfiHeWaYeKHblMBTe~eHHe~O~rEIX~epUO~OB,OT~O~y~aCa~OHeCKO~bKHXYaCOBH~eJIbMKHO'IaMEI. Tern He MeHee, KpyllHbIe.M 6bIcTpbIeBapHaqHR HeCOMHeHHO IIpOHCXOJViT,HO He CTOJIb YaCTO, KaK OTO MOWHO 6~10 6~ IIpe~llOJlOHUiTbHa OCHOBaHKK Ka6mo~emB HaJ( CIlyTHiiKaMK. 3TO yKaabIBaeT Ha TO 06CTOJITeJIbCTB0,YTO B CneKTpe mieprau NIeKTP~H~B MHorkie sapiiaum, HaBmo~aehme Hecomhwi cnyTmKam npK6opamu, RBBapuaqmm Bo BpeMeKK. JIFIIOTCFI CKOpee BapHaIJ~HMK B IIpOCTpaHCTBe, Hexem ~pOMe~TOrOo6cyHc~aeTcR.TaK~eHe~pe~BIl~eHHOBbICOKaFIIIHe~OCTO~HHaFIUHTeHCIIBHOCTb CBeTa,COOTBeTCTByIO~afI HyJIIOpKOMeTpHqeCKOt a6copbwm.
235