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Studies on auroral hydrogen emission using an image intensifier
Journal of Atmospheric andTerrestrial Physics, 1969,Vol.31,pp.321-337.Pergamon Press.Printedin Northern Ireland
Studies on auroraI been
Epson
usingan
image
steeper
R. J. FRANCIS* and 3’. JACKA* Mawson Institute for Antarctic! Research, University of Adelaide (Received 24 June 1968)
Abs~c~-~ image intensifier and camera were used to obtain pho~~aphs of the whole sky in two wavelengths. Interference filters were used to select the wavelengths, one centred on 4859 A to pass Doppler shifted HP and one on 4879 A to assess the background emission. The equipment and data reduction methods are described in some detail. Isophotes of HP emission over a 6” wide latitude range are presented from the analysis of four nights of observations from the Australian National Antarctic Research Expeditions St&ion Mawson, Antarcticrt, in 1964. The concept of an emission zone fixed in space relative to the sun and “Invariant” poles is found to be useful but oversimplified. Usually the maximum HP intensities were about 30 Rayleigh. On one ooossion a higher intensity (approximately 70R) was observed, poleward of the usual emission zone, with sudden onset and associated with a Slowly Varying Ionospheric Absorption (SVIA) event and increased magnetic activity. This occurrence is interpreted as an aspect of the suroral sub-storm phenomenon. 1. I~T~~DuCTX~~
of spectrographic and photometric records have revealed the existence, high latitudes, of zones of emission of Doppler shifted Balmer lines of hydrogen produced by the entry of energetic protons into the upper atmosphere. EATHER and SANDFORD (1966), examining southern hemisphere data, found evidence of a poleward bulge in the zone near geomagnetic mi~ght ; this was associated with Slowly Varying Ionospheric Absorption (SVIA) of cosmic radio noise. They also found evidence of an equatorward expansion of the zone with increasing magnetic activity. The present paper reports observations made in 1964 from the Australian National Antarctic Research Expeditions station at Mawson (67*6’S, 62=9”E gg). The aim was to ~vestigate further the distribution in time and space of the HP emission. 2. INSTRUMENTATION
ANALYSES in
2.1 Image dntemijkr 2.1.1 Typo an4 parameters. The complete optical system and image intensifisr assembly is illustrated in Fig. 1. The image ~ten~er tube was & five stage tr&n~ission-secondly emission type made by “20th Century Electronics Ltd” who quoted the following basic parameters: Electron gain Photon gain (at 4500 A) Photocathode type Phosphor type Photocathode and phosphor dia. Resolution Dark emission Focussing fields
3000 106 s9 Pll 19n-Im > 15 lie p&s/mm 400 scintillations~cmS~sesec 200 gauss axial magnetic, 35 kV overall axial electric.
* Formerly with Antarctic Division, Department of External Affairs, Melbourne. 1
321
322
R. J. FRANCIS
and F. JACKA
Colllmoting lam Spherical miwor
.
9 inch f.1.
ilter and Filter
50mm. Photocathode -
-
Dynodes -
-
-
-
Phosphor
-
carriage
f/24
lens
image Intensifier
-
Solcnold
-
Camera
film
plane
Fig. 1. Sectional drawing of image intensifier assembly.
Studies on aurora1hydrogen emission using an image intensifier
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2.1.2 Operatbg p&acipZe. The image intensifier consists essentially of a photocathode, a number of electron multiplying dynodes and an output phosphor. The optical image to be intensified is focussed on the pho~~thode where it is converted into a phot~lectron image. The electrons are then accelerated and focussed on a transmissionsecondary emitting dynode in the form of a thin Clm baoked by a conducting aluminum layer and supported on a 500 A thick aluminium oxide window. The acceleratingpotentials are so chosenthat the electronshave a small probability of penetrating a dynode but a high probability of releasing a number of secondary electrons from the exit face. These secondary ebotrons are then accelerated and focussed on the second dynode where the process is repeated. After five stages of multiphcation the electrons are finally fooussedon a phosphor which transforms the electron image back to an intensifiedoptical image. The eleotxonimage is focussed between stages by combined homogeneouscoaxial magnetic and electria fields. Double loop helical fooussing was used in the photocathode and phosphor stages and single loop focussing between dynodes. 2.1.3 Reoo&w eficienc7y. EMBERSOX et al. (1962), from measurement of tube parameters and from consideration of the statistics of the secondary emission m~tip~c&tion prooess, estimated that approximately 89 per cent of the photoelectronsfrom the cathode resulted in a scintillation in the phosphor. The scintillation intensitiesare also governed by the statistics of the multiplication process and show a large spread. Using the data of EMBEIWSON et al.it is estimated that the output system used in the present investigation recorded approximately 90 per cent of the phosphor scintillations. The quantum recording efficiency of the system is thus largely limited by the quantum efficiencyof the photocathode material. The system is estimated to have a recording eBieieney, at the wavelength of maximum photocathode efficiency, of 7 per cent; this should be compared with about 0.1 per cent for direct photography using the fastest available fihn at moderate light intensities. A further advantage is achieved at the light levels involved in the present study where, using direct photography, reciprocity failure lowers film speed further. The recorded intensified image is effectively a photograph of a series of bright scintillationsand does not suffer from this effect. 2.1.4 Resolutiora. The resolution of the tube itself was quoted by the manufacturers as >I5 line pairs/mm. The input optics were designed to give 10 l.p/mm. In practice 7 l.p/mm could be resolved at high f&n densities, while at lower levels this was reduced by statistical fluctuations in the spot number density. 2.1.5 Tube mounting. 7%~ encapsulated tube as supplied was a cylinder with plastic end plates, 15 in. long and 4 in. dia. This was mounted in a sheath of black polythene for further high voltage protection. This sheath was suspendedfrom a sphericalseating inside the solenoid coil. The centre of curvature of the seating coincided with the centre of the photocathode. With this ~&ngement the geometric axis of the tube could be moved into alignment with the magnetio field without defooussingor shifting of the optical image on the photocathode. The phosphor was operated at 35 kV above Earth. For high optical aoupling efficiency lensesneeded to be brought close to the phosphor. To prevent arcingto earthed systems a transparent insulating barrier was provided. One of the lenseswas mounted inside this barrier, one outside, an arrangementwhich avoided the aberrations that would have been produced by a transparent slab between a lens and its focal surface. This arr~gement, however, slightly ineressed the vignetting in the output optics. 2.2 O$ics 2.2.1 .l%lteroptiCs. The system was designed to use narrow band pass multilayer dielectric interferencefilters to select wavelengthsof interest. The filtersused had a 6 A pass band to light at normal in&dance and the peak tra~~t~d wavelengths shifted about 1 A per degree of incidence from normal. In the system used the filters accepted a cone of light of 2-5” semiangle; the pass band wss broadened to about 8 A. Over the whole diameter of the filter the axis of the oone was parallel to the optic axis of the system ensuringthat the wavelength of pack transmission and the bandwidth were the same for all parts of the field of view. For a given image diameter the maximum effective aperture of any optical system used with
324
R. J. FRANCIS and F. JAG=
the filter is limited solely by the diameter of the filter and the permitted divergence of the beam through it; in the present system the effective aperture was f/2*8. 2.2.2 .F&& ~~~~~~0~. The filter optics had an 18” diameter field of view. To obtain an all-sky field the filter optics were preceded by a system which compressed a 180” field to an 18” field. It consisted of a convex spherical mirror surface which formed a virtual image of the whole sky in a plane half way between its top and oentre of curvature. This virtual image was reflected by a plane mirror above it into a collimating lens in the centre of the spherical mirror and positioned in the plane of the entrance pupil of the filter optics (labelled “aperture plane” in Fig. 1). 2.2.3 Output optics. A pair of flf.5 lenses mounted front to front focussed the phosphor of the image intensifier on 35 mm Ilford 5G91 film. Wiith this symmetrical system, vrtriations of the spacing of the lenses did not affect the focus; the camera position was thus not critical. 2.3 Operating programme 2.3.1 ~~~~e~ wed for ~~~roge~ ern~~~o~ o~$e~u~~o~~. For investigation of the hydrogen emission the sky was photographed through filters centred on 4859 11 and 4879 & alternately. The HP line wa.s selected although it has about one-third of the intensity of I&; the gain in photocathode efficiency at the wavelength of HP outweighs this. In addition the aurora1 spectrum is comparatively weak and flat in the HP region while the II& line may be seriously contaminated by overlapping aurora1 emissions. The integrated line intensity of HP is of comparable magnitude to the “background” emission passed by an 8 A b&nd~dth filter and ~bknowledge of the magnitude of this background is essential. The HP profile is centred on 4861 A when viewed in the magnetic horizon direction but is Doppler shifted and peaked near 4857 rf in the magnetic3zenith; the profile extends about 15 A on the long wavelength side of 4861 Bi and about 30 A on the short wavelength side. The “background” filter was selected to peak at 4879 .t%to pass little HD flux but to be as close as possible to the HP wavelengths. The Alter for XI@was centred on 4859 ,& to pass & contribution from both the horizon and zenith profiles. The relative positions and widths of the profiles and pass bands are shown in Fig. 2. After trials of various exposure times, an exposure of two minutes was adopted, this being the shortest that gave adequate film densities. Operation was attempted whenever there was no moon and no cloud. In all, observations were made on nineteen nights of which only four were suitable for detailed rtnalysis; the others were cut short by weather changes or marred by “experimental diB?oulties.” A pair of data, frames is reproduced in Fig. 3. 2.3.2 C~~~&ra~~~. The system was ~~~br&~d against a standard light source which had been calibrated at the National Standards Laboratory prior to leaving Au&r&e. This unit provided a series of eight known uniform snrfaoe intensities between 15.3 R per A and 1.76 kR per A at 4861 A. Exposures were made through two filters (6577 A and 4278 A) at each of the intensity levels. These provided s series for interpolation of any input intensity against film density. The 4859 A and 4879 .& filters were fitted into this calibration series by taking exposures through them at an intermediate intensity level. 3. DATA 3.1 F&n
ANALYSIS
scanning
Each frame was scanned with a recording microdensitometer in the direction of the magnetic meridian using a scanning slit 1 mm wide transverse to the scan direction and 0.2 mm wide (the largest dimension available) in the scan direction. Successive scans were separated by 1 mm across the frame and the scan pattern was the same for all frames, being fixed in orientation and starting point on each frame. Subsequently the density traces were further averaged over 1 mm intervals
Studies on aurora1 hydrogen emission using an image intensifier
325
in the scan direction to give a set of density readings for each frame from 1 mm square areas. The 16 x 16 set of density readings covering the sky image area for each frame was punched on computer cards for further data processing. 3.2 Theory of calibration procedure A uniform source was found to produce a non-uniform image principally because of vignetting in the output optics, non-uniformity of photocathode sensitivity and non-uniformity of filter transmission coefficient ; for this reason each field point was calibrated separately. /
Zenith profile
(Veisberg)
Arbitrary Units
Fig. 2. Shapes of assumed HP line profiles end filter passbands.
For field co-ordinates 5 and 7, the film density D( 5, 7) is a function of the product is some constant dependent on the field co4~5 rl)tJ WWV) dJ, where 45,q) ordinates and on the filter used. IS(I) is the source surface intensity per unit wavelength interval, T(l) the relative transmission coefficient of the filter and t is the exposure time. The photocathode efficiency is assumed independent of 3, over the bandwidth of a filter. For the calibration frames obtained with the standard light source, &‘(I) can also be taken as constant over the bandwidth of a filter. Then D(E, 7) = A,(& r)E‘ [ts,SW)
dA]
where the subscript YCdenotes the filter used. W, = j T,(l) dil is the equivalent bandwidth of the filter and may be obtained from plots of relative transmission versus wavelength. Then D(E, 7) = &(5> 9) JVX, Wnl. By establishing a series of values for .D([, 7) for a series of “exposures” (tS,W,J we may interpolate between them from any subsequent density found to obtain
Fig. 3. A pair of image intensifier photographs of the whole sky, showing the The R.H. photo is through the 14859 b (HP) filter, the scintillation structure. L.H. photo is through the I. 4879 A filter. An intense aurora1 band runs vertically on the left of each frame.
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R. J. FRANCIS and F. JACKA
326
the “exposure” t j iS’(A)T(1)dil. By measuring D( [, 7) using another filter, m, and knowing t, S,, and IV,, we may calculate for it a new A,([, 7) and use the same series of calibration points in a similar way. In the investigation of the hydrogen emission the quantity we wish to measure is B, the surface intensity integrated over the line profile, i.e. B =
s
I(a, 2) dil
where I( cc,A) is the emission intensity per unit wavelength interval, viewed at a magnetic zenith angle LX,the angle between the line of sight and the magnetic field line at the emission point. In addition to the hydrogen emission there will be some “background” emission consisting of starlight and unresolved aurora1 bands and continuum. In the spectral region of interest these give an intensity per filter bandwidth of the same order as the HP line. It is assumed that this background is uniform over the region of the spectrum containing both filter pass bands, and of intensity C Rayleigh per unit wavelength. The exposure through the H, filter will give a density
and an exposure through the “background” titer will give &(E, 11)= A,([, ?;I)F [t/CT,(L)
dA] = A,(E,
IlPvc~,l
where the subscript 0 refers to the 4859 A filter and 1 refers to the 4879 L%“background” filter. By interpolation from the table of D vs. exposure for each filter, values are obtained for P = t J (1(a, A) + C)!Z’,(A)dl and Q =
tCW,;
hence s
I(a,
l)T,(A)
di2 = P -
Q;. 1
Knowing T(A) vs. il and the shape of the I( a, A) curves for various a the following integrals can be calculated:Y(a)
= a(a)fi(a,
A) 02,
X(a)
= a(a)b(a,
W’&)
dk
where a(a) is an arbitrary (normalizing) constant. Hence B = s
I(a, A) d3, = $f(P-82).
Studies on aurora1 hydrogen emission using an image intensifier
327
B can then be corrected “to the zenith” by dividing by the Van Rhijn ratio for the appropriate zenith angle. Allowance must also be made for losses in the field compressor which was not used when the calibration was performed; this is done by multiplying all the intensities obtained above by an appropriate constant. 3.3 Parametersused in analysis 3.3.1 ~~~e~o~~~ f7a?% Rbijs T&O ami fw$$t o~e~~~8~on. The difbm nature of the hydrogen emission makes height d&e rminstion by triangulation impro&icable. EATHER and JACKA (1966a), found it impracticable to determine the height from the variation of Van Rhijn ratio with zenith angle because of uncertainty in the shape of the HP line profile. Instead they matched theoretical Van Rhijn curves for reasonable emission heights against those deduced using the various published line profiles to find which profile seemed most appropriate. The best mstch was obtained assuming an emission height of about 100 km md VEISSBERU’S (1962) H@ line pro6le. EATHER and BURROWS (1966) caloulated the theoretical emission height for various combinations of bombarding proton energies and pitch angle distributions. The more probable of these combinations involved emission vers-usheight profiles which peaked sharply between 100 and 120 km altitude. Most other reliable investigations of the height profile of the hydrogen emission lead to similar estim&es of the height of emission. In the present analysis an emission height of 110 km h&s been assumed; Veissberg’s profile and Esther and Jacka’s best matching Van Rhijn curve have been used. V&s&erg published only a magnetic zenith profile for Hg. An appropriate horizon profile to match this was assumed by Esther and Jacka. Protiles for intermediate zenith angles were deduced from these by interpolation. The Doppler shift of the proiile peak is proportional to cos ccwhere 0:is the magnetic zenith angle; the intermediate HP line profiles were interpolated linearly about this peak from the zenith and horizon profiles. Figure 2 shows three of the profiles used. From these pro&s values of Y/X were calculated. They range from 2.8 in the magnetic zenith direction to 2.5 on the magnetic horizon. By oomparison, Z~ICK and SIXIGPPERD’S(1963) profiles yield vdues between 3.9 in the zenith to 2.4 on the horizon, demonstrating the sensitivity of the calibration to choice of profile shape. 3.3.2 Co-ordinates for presentation of resulta. The emission intensities derived from the film images will be displayed as s function of Eccentric Dipole time (COLE 1963, SIM~NOW 1963), and Invariant latitude R d&ned in terms of McI~w~‘s (1961) L value by L cos I = 1. Invarisnt latitude approximates closely to Eccentric Dipole latitude. In analysis of the data an emission point not on the Eeoentric Dipole meridian of Mawson has been as&bed a different emission time; longitude and time have been used interchangeably. This interchangeability is valid if the emission zone is fixed with respect to the sun and the Eccentric Dipole pole (close to Invariant pole). Its justification lies in the agreement between d&a from different frames and times processed in this way. For most of the time the assumption of a fixed zone appears valid but there are instances where t#hedata indicate a fast change in position &rid/orintensity of the emission. 3.4
Computations
A computer was used to convert film densities and positions to emission zone intensities and co-ordinates (Invariant latitude and Eccentric Dipole time). Each data frame was entered as a 16 x 16 matrix of density readings. Each reading was compared with the eight calibration levels appropriate to the field point and an intensity interpolated for it. Background frames were then subtracted from emission frames after allowance for the filter bandwidths. The differences were corrected according to the magnetic zenith angle (affecting the portion of the emission line
328
R. J. FRANCIS and F. JACKA
transmitted by the filter) and zenith angle (Van Rhijn effect). Those points near the field edge with zenith angles greater than 73’ were rejected since the Van Rhijn correction becomes highly inaccurate beyond this angle. The resultant set of integrated emission line intensities were printed out together with the Invariant latitude and Eccentric Dipole time ascribed to each field point. The latter co-ordinates were derived from the latitude of the observation point and time of observation after calculating the azimuth and distance (assuming an emission height of 110 km) of the field emission point. The field points correspond to the centres of each of the 1 mm square areas over which the film density was integrated. The field points for each frame were separated into the Invariant latitude groups half a degree wide in the range 67.3"to 73.2"and the individual points were plotted on intensity/Eccentric Dipole time grids. A set of background intensities for each observation period was also obtained to check that variations of emission intensity were not caused by a change in gain of the recording system. These showed that the gain was quite stable. From these intensity/time plots the isolines of constant intensity were derived and plotted on a latitude/Eccentric Dipole time grid. This was considered the most lucid way of displaying the data. 3.5 Continuity and correction factors Examination of successive sets of zone intensities derived from pairs of data frames by the above computations accepted at their face value leads to the conclusion that the emission zone within the field of view was most intense in a region of the sky to the north west and that the intensity always decreased near the horizon by up to a factor of two. That the emission should show preferential brightening in a particular region of the sky which is small in relation to the whole extent of the emission, and that the emission should at all times decrease toward the horizon around what is essentially an arbitrary observing point, leads to the suspicion that these are characteristics not of the emission zone but of the data reduction procedure. To accept the conclusion of this examination one would be obliged to abandon the concept of a zone fixed in space and its consequence that successive frames plotted on an intensity/time grid should show continuity. Factors that would cause a lack of continuity are examined below. 1. Vignetting in the jilter optics. This source must be included in the possibilities since the small collimator lens in the field compressor must be imaged accurately on the aperture of the 50 mm lens above the photocathode to avoid vignetting. Any misalignment would result in asymmetric vignetting in addition to that already observed on the calibration frames which were obtained with the field compressor removed and a relatively large area uniform source placed above the filter optics entrance pupil. 2. Vignetting changes in the output optics. This would arise if the camera mount was not replaced in exactly the same position whenever it was shifted, causing misalignment of the output optics and a consequent asymmetry in the vignetting. A shift of 2 or 3 mm between the time of the observations were made and the calibration performed is required to explain the observed asymmetry; this is physically possible but unlikely to have occurred.
Studies on aurora1 hydrogen emission using an image intensifier
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3. Overestimation of Van Rhijn ratio. The effect observed is of the same order of magnitude as the total Van Rhijn correction applied. The Van Rhijn ratio does change and may be asymmetric due to asymmetric extinction and ground albedo. If it is assumed that the Van Rhijn ratios used are representative of an exceptionally clear night then it is conceivable that increased and asymmetric extinction could account for the observed lack of continuity in the data. The observed asymmetry in the data is relatively constant on all four nights. This reduces the probability that it arises from such an effect although the effect may contribute to differences from night to night. The possible effects could not be measured so an arbitrary correction to the intensity distribution on each frame was made in an attempt to derive a more probable distribution. The intensity/time plots were examined to find a region where the centre of the band of emission appeared to be in the zenith and the band extended over the whole field of view. It was assumed that at such a time the band of emission was sufficiently broad to be uniform over the entire part of the field of view analysed (above 17’ elevation). A group of three frames taken at such a time was used to derive correction factors. First the three values of emission intensity for each field point were averaged, then a mean value for the field point averages for a region near the zenith was obtained. The ratio of a field point average to the zenith mean was taken as the correction factor for that field point. Each field point intensity for other frames for the whole period of observation was then corrected by the appropriate ratio in this 16 x 16 set. The intensity/time plots so derived for the latitude interval 70.3’ to 70.7” are shown in Fig. 4. A considerable dispersion of the short curves derived from individual data frames remains but general trends are clearly apparent. 3.6
Emission from the Milky Way
The Milky Way has been shown by MONTBRIAND et al. (1965) and others to be a source of hydrogen emission comparable in intensity to aurora1 hydrogen emission; their charts of this emission were gathered from the astronomical literature. There is a region along the galactic equator generally about 10” wide emitting H, with intensities up to 60R. (The ratio of intensities H,/H, N 3.) Within this region are some much brighter areas, the largest of which is about 12” square with an emission intensity greater than 60R in H,. It is situated near New Galactic Longitude 260”. The intensities are quoted to be accurate to within a factor of 3. EATHERand JACKA (1966cc), using an instrument with a 4” field of view found emission intensities in the Milky Way up to 65R in H,. The image intensifier, when well focused, showed the Milky Way clearly, both in H0 and at the background wavelength. The brightest features could be identified with the region mentioned above at NGL 260”, and with others at NGL 290“ and 340”. They contributed no more than 15R through the titer pass band, this being an average intensity over a 12’ square area of the sky. These areas where the Milky Way could be discerned were omitted from the plots of emission zone intensity.
R. J. FRANCISand F. JACKA
330
Gap in records here Room light left on
60
30 July
c
t
16
‘64
coqlp.
17
60 3 Aug
‘64
40
20-
fi_.
eh_knd
&
2 f
I
17
8
I8
I
19
I
I
20
I
21
22
.$ 60 h s
3 Aug ‘64 (cot? 1 40
Comp.
-
\
--
23
00
01
02
03
04
60 l-
Eccentric
dipole time,
hr
Fig. 4(a). Fig. 4. Ho intensity/time plots for the invariant latitude range 70.3” to 70.7” which includes Mawson, after application of compensating factors derived from the regions marked “Camp.” 3.7 Error sources and magnitudes 3.7.1 Absolute errors. The largest source of error in the calibrationprocedureis expected to be the choice of H.,zJ line prof?le. Choosing other profiles from among those published would result in increases in the derived intensities by up to 50 per cent but confidencein the profile selected suggests an error much less than this. Other errors of smaller magnitude could arise from changes in the position of the filter bandpass with temperature, errors in the assumed reflection coefficientsof the field compressor surfaces and differencesin the sky background intensity over the two filter pass bands.
331
Studies on aurora1hydrogen emission using an image intensifier 60$ or
7 Aug ‘64 Ccont 1
more cloud
t
40-
60 r
Obrstart
6 Sept’64
6 Sept 64 jcont
1
80-
1
02 Eccentric
dipoie time,
I
03
hr
Fig. 4 (b). The overall absolute accur~y of the derived intensities is estimat.ed to be within i50% i 3R. 3.7.2 Rek&t? ewors. In this category are differencesin intensity between individual field points, between individual frames, and between individual nights of observation. Some of the factors mentioned in the previous section would have contributed also to relative errors. 1. Statistical fluctuations. !Chiswas the largest source of Merences in derived intensities between individual field points and depended on the accuraoy of measurementof densitiesand on the differencein density betweenemissionand backgroundframes. The backgroundintensity was relatively stable and deviations are better expressedabsolutely than in ratios; the observed scatter in the field point intensitieswas an RMS deviation of about 2R near the field centre and 4R near the edges where the compensating faotors were larger. 2. Van Rhijn ratios. Changes in atmospheric conditions could have led to slow changes in the derived distributionover the field of view during the period of observation. The oompensating factors would eliminate any asymmetry at the time for which the factors were derived but would not eliminate errors due to changes in conditions that occurred before or after this time.
332
R. J. FRANCISand F. JAGKA Auroroe cd KIndii
12 hrs eccentric
‘12 hrs eccentric
dipole
time
dipole tik
Fig. 5(a). Fig. 5. Isophotes of HP intensity and rough plots of aurora1activity and K indices. An estimate of the magnitude of the changes may be gained from the slope or shape of the intensity distributionsacross individual frames on the intensity/time plots. Examination shows that the errors cannot have exceeded 50 per cent at the northern or southern limits of a frame. 3. ~ompensat~g factors. The nature of the derivation of the compe~at~g factors could lead to systematic distortions of the overall distribution on each frame if the referenceframea from which the factors were derived had not in fact resulted from a uniform emission distribution. The test of this was the continuity effected when individualframes were incorporated into an intensity/time plot as shown in Fig. 4. In these plots the small scale (statistical) fluctuations have been smoothed but the larger scale systematic errors are still apparent in the lack of precise coincidenceof the short curves representingsuccessiveframes. 4. Smoothing by frame overlap. In the intensity/time plots of successiveframes the overlapping of frames allows a running mean of intensity to be made to smooth out distortions in the frame distribution introduced by errors in the data reduction. Similar smoothing cannot be made along the latitude axis and a systematic distortion in the form of an over or under estimation of intensity at the highest or lowest latitudes relative to the oentral latitude may occur.
Studies on aurora1 hydrogen emission using an image intensifier
333
4~ Isophotes 7 A..^,rn
12hrr eccentric dipole time
End
6 Sept’64 I
Obs.
____.-. , Intensi~y(
--me
*
Brightness
Royleigh)
Fig.
5
-3 index
-4
(b).
4. EXPERIMENTAL RESULTS AND DISCUSSION 4.1 ~~~e~~~~a2 ~0~~0~~ of Hs ~~te~s~t~ 4.1.1 Emtiaiolz isophotes. Figure 5 shows the isophotes of HP intensity derived from the data for the four nights analysed, interspersed with rough plots of aurora1 activity obtained from all-sky camera photogmphs, together with K,- and local K-indices. The zone of emission deduced by EATHEB md SANDFORD (1966) is shown for eomp&rison in Fig. 0. 4.1.2 I&e-n&y/time plots. While isophotes give a convenient indication of I-I, zone position and intensity, the intensity/time plots are better indioators of zone behaviour. The intensity/ time plots for the O-5 degree wide latitude band inoluding Mawson (70+30-70*7”) for the four nights snalysed are given in Fig. 4. On these are shown the smoothed intensities across individual frames. In many places where zone intensity changes occur successive frame pairs have been
R. J. FRANCISand F. JACKA
334
plotted, at other times approximately every third has been plotted. A plot of the background frame intensity is given to show its relative stability. The frame rate (and time resolution)was one pair every 4 or 5 mins.
4.2 Characteristics of the HP zone On all nineteen nights when observations were attempted H, emission was found to be present although only four nights records were suitable for detailed analysis. The isophotes (Fig. 5) for these four nights are consistent in the large scale with an emission zone fixed in space, relative to the sun and Invariant pole, underneath
,06
18
Eccentric
Dipo1e
L”~jzz SUN
Fig. 6. EATHERand SANDFORD’S (1966) zone of hydrogen emission.
which the earth rotates. The zone is of order 1000 km wide between half intensity points at 18.00E.D.T. (Eccentric Dipole Time). The differences in position of the zone from night to night are generally consistent with an expansion to lower latitudes with increasing magnetic activity. The magnitude of this expansion is of order 3’ of latitude per unit of K, or 5’ per unit of local K-index change. Examination of the intensity/time plots reveals many details that are not consistent with the fixed, stable zone concept. Some of the irregularities arise from inaccuracies in the data reduction such as inappropriate choice of compensating factors but most are real and indicate real features of zone behaviour. In addition to the mechanism producing the stable zone it appears that a separate mechanism operates at times, showing a sudden onset and giving rise to a
Studies on aurora1hydrogen emission using a image intensifier
335
poleward expansion of the zone. It is associated with Slowly Varying Ionospheric Absorption of cosmic radio noise (SVIA). This has been observed also by EATHER and JACKA(19663). 4.3 Description of events m each night In this section an attempt is made to describe the observations in terms of the ideas advanced in the previous sections. While some of the characteristics and zone movements referred to may be more clearly inferred from the isophote maps (Fig. 5), other characteristics are more readily inferred from the intensity/time plots (Fig. 4). 30 July 1964. The pattern of isophotes shown in Fig. 5 represents a relatively complex but stationary distribution of HP under which the earth rotates. In fact, as can be seen from the intensity/time plot in Fig. 4, there were discontinuities around 17.30 and 19.00. At 17.30 there was an intensification of the emission in the northern sky the detail of which cannot be accurately represented on the scale of isophotes used in Fig. 5. Similarly at 19.00 an intensification occurred near the zenith. In both these cases the features drifted out of the field of view as the earth rotated. The riometer record did not show absorption of cosmic radio noise associated with either of these features. By 21.20, rotation of the earth had carried the field of view poleward of the emission zone shortly before moonrise. Near 18.30 one frame suggested a short duration change in HP distribution but succeeding frames reverted to the general trend. 3 August 1964. The intensity/time plots in Fig. 4 show relatively little dispersion and no discontinuities. The isophotes in Fig. 5 therefore correctly represent a stable distribution of HP emission through which the field of view was carried by rotation of the earth. The maximum of the emission zone apparently lay across the path of the observing field at about 19.00-20.00 and equatorwards of it around to 04.00 when observations were terminated by approaching twilight. A few minutes after this an SVIA began, reaching 1.5 dB within 30 min and lasting for 24 hr. The development of this event was associated with intense aurora followed by weak diffuse aurora over the whole sky and K, = 6. From the work of EATHERand JACKA(1966b) one would expect strong enhancement of H, during this interval. If this did occur it must have had a sudden onset as the isophotes, which extend nearly 30 min after the SVIA began, show no sign of it. 7 August 1964. Observations began at 15.57 with intensities >30R. Shortly after 16.00 the emission region apparently retreated poleward accompanied by a decrease in K-index ; this is the cause of the discontinuities in Figs. 4 and 5. The riometer record does not show absorption although in the previous three hours it was somewhat disturbed. The zone then showed stable behaviour, the path of the observing field being polewards of the brightest part. After 23.00 observing conditions deteriorated; substantial cloud cover increased until observations were terminated at 02.15 with complete cloud cover. Intensities after 23.00 could be underestimated but enough light diffused through the cloud to show that the field of view was again moving nearer to the emission zone after 01.00.
336
R. J. FRANCIS and F. SACKA
Two frames, one at 17.00 the other at 19.20, seem inconsistent with the rest of the data. No explanation in terms of zone behaviour or experimental factors is apparent. 6 Xe@ember 1964. Observations began at 16.56 with moderate H, intensity present. The background level shows that twilight is still contributing; allowance for the Fraunhofer absorption of Ix, in this would tend to increase the derived ~tensities at the western ends of the frames affected. There was a fast reduction in intensities near 17.20due probably to a poleward shift of the emission; the detail of this is not showu in Fig. 5. The riometer record for this period is smooth. Intensities after this time are consistent with rotation under a stable zone until 00.00, the zone maximum being located equatorwards of Lawson as one would expect from the magnetic activity indices. A world wide Sudden Commencement of magnetic activity occurred at 21.37 (19.55 U.T.). No immediate reaction was evident, but near 00.00 there was a sudden increase in both aurora1 activity and HP emission. This explains the ~scontinuities in Figs. 4 and 5. At this time also the riometer records show an SVIA event which EATHER and JAGKA (1966o,b) found associated with proton precipitation and a poleward expansion of the HP emission zone. This is consistent with the present observations. The aurora1 activity preceded the rise in H, intensity and the fast increase in background level accounts for the large negative intensities derived from frames in this transition period; the background frames were exposed after the emission frames, hence the background level is overest~ated as fast increase occurs. The aurora1 activity then decreased in intensity (as did the background level) but remained widespread.
5. CONCLU-SIoNs 5.1 E~~~~rn~~t All sky photography, using the image intensifier and two filter method, has been shown to be an effective means of determining the temporal and spatial distribution of the aurora1 hydrogen emission. With improvements in calibration techniques and in design of optical component mountings the accuracy could be significantly improved. A more efficient photooathode material than the S9 used, such as the S20 which is now available, would improve the time resolution. Intensifiers of the type used have since been improved in respect to dark current and can be selected for uniformity of sensitivity. Intensifiers of the Cascade (phosphor/photocathode sandwich) type have also been developed yielding improved contrast and robustness. In this type of intensifier the signal induced background is very much lower. 5.2 Results It is apparent that the concept of a stable oval zone of hydrogen emission fixed in space as suggested by EATHERand SANDFORD (1966) is roughly consistent with the present observations on some nights. The H, oval shows an equatorwards expansion with increasing magnetic activity similar to that of the visible aurora1 oval (FELDSTEIN and STARKOV, 1967). ~though there is considerable overlap of the two zones, the HP oval in general lies equatorwards of the visible aurora1 oval.
Studies on aurora1 hydrogen emission using an image intensifier
337
On several occasions changes of HP intensity occurred in localized patches. These were not associated with cosmic radio noise absorption. On one night (6 September at 00.00 E.D.T.) a sudden enhancement of H, emission, extending polewards of the normal stable oval zone, was associated with an SVIA event and “breakup” of the aurora1 display characteristic of the onset of an aurora1 sub-storm (AKASOFU, 1966). This situation is not adequately represented by a poleward bulge in a stable emission zone as proposed by EATHER and SANDFORD (1966). Examination of riometer records over a long period shows that SVIA events are in general associated with moderate to large values of K,. Following ANSARI(1964) these events are presumed to be caused by energetic (30-50 kev) electrons while EATHERand JACKA(1966b) find them to be associated with enhanced H, emission as in the present investigation. It is suggested that a sudden hardening of the precipitated electron energy spectrum accompanied by enhanced precipitation of protons marks the onset of the aurora1 substorm and gives rise to the effects discussed viz. marked reduction of visible aurora1 intensity, SVIA, enhanced HP, intensified ionospheric currents and hence increased geomagnetic disturbance. Acknowledgements-The planning of this investigation and construction of the equipment was carried out in the Antarctic Division, Department of External Affairs, Melbourne. The assistance of former colleagues in that Division is gratefully rtcknowledged. The observations were made while one of us (R. J. F.) was a member of the Australian National Antarctic Research Expedition at Mawson. The assistance of D. SEEDSMAN end PT. CARDELL is gratefully acknowledged.
REFERENCES AKASOFU S-I. ANSARI Z. A. COLE K.D. EATRERR.H.~~~JACKAF. EATHERR.H.~~~JACKAF. EATHERR.H.~~~ SANDFORD B.P. EATHERR.H.~II~BURROWSK.M. EMBERSON D.L.,ToDEILLA.~~~ wILC0CK'cV.L.
1966 1964 1963 1966a 1966b 1966 1966 1962
FELDSTEINYA.I.~~~STARKOVG.V. MCILWAIN C.E. MONTBRIAND L.E.,TINsLEYB.A.~~~ VALLANCEJONESA. SIMONOW G.V. VEISSBERG O.L.
1967 1961 1965
ZWICKH.H.~~~SHEPPERD
1963
2
G.G.
Space Sci. Rev. 6, 21. J. geophys. Res. 69, 4493. Au&. Aust. Aust. Aust. Aust.
J. J. J. J. J.
Phys. Phys. Phys. Phys.
19,423. 19,241. 19,215. 19,25. Phys. 19,309.
Advances
in Electronics and Electrcn Vol. 14, p. 127, Academic Press, New York. Physics,
1963 1962
Planet. Space Sci. 15,209. J. geophys. Res. 66, 3681. Can. J. Phys. 43, 782. Geophys.
J. R. astr. Sot.
8, 258.
Aurora and Airglow 8, 36. Results of I.G.Y. U.S.S.R. Aced. Sci. J. Atmosph.
Terr.
Phys.
25, 604.
Studies on auroraI been
Epson
usingan
image
steeper
R. J. FRANCIS* and 3’. JACKA* Mawson Institute for Antarctic! Research, University of Adelaide (Received 24 June 1968)
Abs~c~-~ image intensifier and camera were used to obtain pho~~aphs of the whole sky in two wavelengths. Interference filters were used to select the wavelengths, one centred on 4859 A to pass Doppler shifted HP and one on 4879 A to assess the background emission. The equipment and data reduction methods are described in some detail. Isophotes of HP emission over a 6” wide latitude range are presented from the analysis of four nights of observations from the Australian National Antarctic Research Expeditions St&ion Mawson, Antarcticrt, in 1964. The concept of an emission zone fixed in space relative to the sun and “Invariant” poles is found to be useful but oversimplified. Usually the maximum HP intensities were about 30 Rayleigh. On one ooossion a higher intensity (approximately 70R) was observed, poleward of the usual emission zone, with sudden onset and associated with a Slowly Varying Ionospheric Absorption (SVIA) event and increased magnetic activity. This occurrence is interpreted as an aspect of the suroral sub-storm phenomenon. 1. I~T~~DuCTX~~
of spectrographic and photometric records have revealed the existence, high latitudes, of zones of emission of Doppler shifted Balmer lines of hydrogen produced by the entry of energetic protons into the upper atmosphere. EATHER and SANDFORD (1966), examining southern hemisphere data, found evidence of a poleward bulge in the zone near geomagnetic mi~ght ; this was associated with Slowly Varying Ionospheric Absorption (SVIA) of cosmic radio noise. They also found evidence of an equatorward expansion of the zone with increasing magnetic activity. The present paper reports observations made in 1964 from the Australian National Antarctic Research Expeditions station at Mawson (67*6’S, 62=9”E gg). The aim was to ~vestigate further the distribution in time and space of the HP emission. 2. INSTRUMENTATION
ANALYSES in
2.1 Image dntemijkr 2.1.1 Typo an4 parameters. The complete optical system and image intensifisr assembly is illustrated in Fig. 1. The image ~ten~er tube was & five stage tr&n~ission-secondly emission type made by “20th Century Electronics Ltd” who quoted the following basic parameters: Electron gain Photon gain (at 4500 A) Photocathode type Phosphor type Photocathode and phosphor dia. Resolution Dark emission Focussing fields
3000 106 s9 Pll 19n-Im > 15 lie p&s/mm 400 scintillations~cmS~sesec 200 gauss axial magnetic, 35 kV overall axial electric.
* Formerly with Antarctic Division, Department of External Affairs, Melbourne. 1
321
322
R. J. FRANCIS
and F. JACKA
Colllmoting lam Spherical miwor
.
9 inch f.1.
ilter and Filter
50mm. Photocathode -
-
Dynodes -
-
-
-
Phosphor
-
carriage
f/24
lens
image Intensifier
-
Solcnold
-
Camera
film
plane
Fig. 1. Sectional drawing of image intensifier assembly.
Studies on aurora1hydrogen emission using an image intensifier
323
2.1.2 Operatbg p&acipZe. The image intensifier consists essentially of a photocathode, a number of electron multiplying dynodes and an output phosphor. The optical image to be intensified is focussed on the pho~~thode where it is converted into a phot~lectron image. The electrons are then accelerated and focussed on a transmissionsecondary emitting dynode in the form of a thin Clm baoked by a conducting aluminum layer and supported on a 500 A thick aluminium oxide window. The acceleratingpotentials are so chosenthat the electronshave a small probability of penetrating a dynode but a high probability of releasing a number of secondary electrons from the exit face. These secondary ebotrons are then accelerated and focussed on the second dynode where the process is repeated. After five stages of multiphcation the electrons are finally fooussedon a phosphor which transforms the electron image back to an intensifiedoptical image. The eleotxonimage is focussed between stages by combined homogeneouscoaxial magnetic and electria fields. Double loop helical fooussing was used in the photocathode and phosphor stages and single loop focussing between dynodes. 2.1.3 Reoo&w eficienc7y. EMBERSOX et al. (1962), from measurement of tube parameters and from consideration of the statistics of the secondary emission m~tip~c&tion prooess, estimated that approximately 89 per cent of the photoelectronsfrom the cathode resulted in a scintillation in the phosphor. The scintillation intensitiesare also governed by the statistics of the multiplication process and show a large spread. Using the data of EMBEIWSON et al.it is estimated that the output system used in the present investigation recorded approximately 90 per cent of the phosphor scintillations. The quantum recording efficiency of the system is thus largely limited by the quantum efficiencyof the photocathode material. The system is estimated to have a recording eBieieney, at the wavelength of maximum photocathode efficiency, of 7 per cent; this should be compared with about 0.1 per cent for direct photography using the fastest available fihn at moderate light intensities. A further advantage is achieved at the light levels involved in the present study where, using direct photography, reciprocity failure lowers film speed further. The recorded intensified image is effectively a photograph of a series of bright scintillationsand does not suffer from this effect. 2.1.4 Resolutiora. The resolution of the tube itself was quoted by the manufacturers as >I5 line pairs/mm. The input optics were designed to give 10 l.p/mm. In practice 7 l.p/mm could be resolved at high f&n densities, while at lower levels this was reduced by statistical fluctuations in the spot number density. 2.1.5 Tube mounting. 7%~ encapsulated tube as supplied was a cylinder with plastic end plates, 15 in. long and 4 in. dia. This was mounted in a sheath of black polythene for further high voltage protection. This sheath was suspendedfrom a sphericalseating inside the solenoid coil. The centre of curvature of the seating coincided with the centre of the photocathode. With this ~&ngement the geometric axis of the tube could be moved into alignment with the magnetio field without defooussingor shifting of the optical image on the photocathode. The phosphor was operated at 35 kV above Earth. For high optical aoupling efficiency lensesneeded to be brought close to the phosphor. To prevent arcingto earthed systems a transparent insulating barrier was provided. One of the lenseswas mounted inside this barrier, one outside, an arrangementwhich avoided the aberrations that would have been produced by a transparent slab between a lens and its focal surface. This arr~gement, however, slightly ineressed the vignetting in the output optics. 2.2 O$ics 2.2.1 .l%lteroptiCs. The system was designed to use narrow band pass multilayer dielectric interferencefilters to select wavelengthsof interest. The filtersused had a 6 A pass band to light at normal in&dance and the peak tra~~t~d wavelengths shifted about 1 A per degree of incidence from normal. In the system used the filters accepted a cone of light of 2-5” semiangle; the pass band wss broadened to about 8 A. Over the whole diameter of the filter the axis of the oone was parallel to the optic axis of the system ensuringthat the wavelength of pack transmission and the bandwidth were the same for all parts of the field of view. For a given image diameter the maximum effective aperture of any optical system used with
324
R. J. FRANCIS and F. JAG=
the filter is limited solely by the diameter of the filter and the permitted divergence of the beam through it; in the present system the effective aperture was f/2*8. 2.2.2 .F&& ~~~~~~0~. The filter optics had an 18” diameter field of view. To obtain an all-sky field the filter optics were preceded by a system which compressed a 180” field to an 18” field. It consisted of a convex spherical mirror surface which formed a virtual image of the whole sky in a plane half way between its top and oentre of curvature. This virtual image was reflected by a plane mirror above it into a collimating lens in the centre of the spherical mirror and positioned in the plane of the entrance pupil of the filter optics (labelled “aperture plane” in Fig. 1). 2.2.3 Output optics. A pair of flf.5 lenses mounted front to front focussed the phosphor of the image intensifier on 35 mm Ilford 5G91 film. Wiith this symmetrical system, vrtriations of the spacing of the lenses did not affect the focus; the camera position was thus not critical. 2.3 Operating programme 2.3.1 ~~~~e~ wed for ~~~roge~ ern~~~o~ o~$e~u~~o~~. For investigation of the hydrogen emission the sky was photographed through filters centred on 4859 11 and 4879 & alternately. The HP line wa.s selected although it has about one-third of the intensity of I&; the gain in photocathode efficiency at the wavelength of HP outweighs this. In addition the aurora1 spectrum is comparatively weak and flat in the HP region while the II& line may be seriously contaminated by overlapping aurora1 emissions. The integrated line intensity of HP is of comparable magnitude to the “background” emission passed by an 8 A b&nd~dth filter and ~bknowledge of the magnitude of this background is essential. The HP profile is centred on 4861 A when viewed in the magnetic horizon direction but is Doppler shifted and peaked near 4857 rf in the magnetic3zenith; the profile extends about 15 A on the long wavelength side of 4861 Bi and about 30 A on the short wavelength side. The “background” filter was selected to peak at 4879 .t%to pass little HD flux but to be as close as possible to the HP wavelengths. The Alter for XI@was centred on 4859 ,& to pass & contribution from both the horizon and zenith profiles. The relative positions and widths of the profiles and pass bands are shown in Fig. 2. After trials of various exposure times, an exposure of two minutes was adopted, this being the shortest that gave adequate film densities. Operation was attempted whenever there was no moon and no cloud. In all, observations were made on nineteen nights of which only four were suitable for detailed rtnalysis; the others were cut short by weather changes or marred by “experimental diB?oulties.” A pair of data, frames is reproduced in Fig. 3. 2.3.2 C~~~&ra~~~. The system was ~~~br&~d against a standard light source which had been calibrated at the National Standards Laboratory prior to leaving Au&r&e. This unit provided a series of eight known uniform snrfaoe intensities between 15.3 R per A and 1.76 kR per A at 4861 A. Exposures were made through two filters (6577 A and 4278 A) at each of the intensity levels. These provided s series for interpolation of any input intensity against film density. The 4859 A and 4879 .& filters were fitted into this calibration series by taking exposures through them at an intermediate intensity level. 3. DATA 3.1 F&n
ANALYSIS
scanning
Each frame was scanned with a recording microdensitometer in the direction of the magnetic meridian using a scanning slit 1 mm wide transverse to the scan direction and 0.2 mm wide (the largest dimension available) in the scan direction. Successive scans were separated by 1 mm across the frame and the scan pattern was the same for all frames, being fixed in orientation and starting point on each frame. Subsequently the density traces were further averaged over 1 mm intervals
Studies on aurora1 hydrogen emission using an image intensifier
325
in the scan direction to give a set of density readings for each frame from 1 mm square areas. The 16 x 16 set of density readings covering the sky image area for each frame was punched on computer cards for further data processing. 3.2 Theory of calibration procedure A uniform source was found to produce a non-uniform image principally because of vignetting in the output optics, non-uniformity of photocathode sensitivity and non-uniformity of filter transmission coefficient ; for this reason each field point was calibrated separately. /
Zenith profile
(Veisberg)
Arbitrary Units
Fig. 2. Shapes of assumed HP line profiles end filter passbands.
For field co-ordinates 5 and 7, the film density D( 5, 7) is a function of the product is some constant dependent on the field co4~5 rl)tJ WWV) dJ, where 45,q) ordinates and on the filter used. IS(I) is the source surface intensity per unit wavelength interval, T(l) the relative transmission coefficient of the filter and t is the exposure time. The photocathode efficiency is assumed independent of 3, over the bandwidth of a filter. For the calibration frames obtained with the standard light source, &‘(I) can also be taken as constant over the bandwidth of a filter. Then D(E, 7) = A,(& r)E‘ [ts,SW)
dA]
where the subscript YCdenotes the filter used. W, = j T,(l) dil is the equivalent bandwidth of the filter and may be obtained from plots of relative transmission versus wavelength. Then D(E, 7) = &(5> 9) JVX, Wnl. By establishing a series of values for .D([, 7) for a series of “exposures” (tS,W,J we may interpolate between them from any subsequent density found to obtain
Fig. 3. A pair of image intensifier photographs of the whole sky, showing the The R.H. photo is through the 14859 b (HP) filter, the scintillation structure. L.H. photo is through the I. 4879 A filter. An intense aurora1 band runs vertically on the left of each frame.
326
R. J. FRANCIS and F. JACKA
326
the “exposure” t j iS’(A)T(1)dil. By measuring D( [, 7) using another filter, m, and knowing t, S,, and IV,, we may calculate for it a new A,([, 7) and use the same series of calibration points in a similar way. In the investigation of the hydrogen emission the quantity we wish to measure is B, the surface intensity integrated over the line profile, i.e. B =
s
I(a, 2) dil
where I( cc,A) is the emission intensity per unit wavelength interval, viewed at a magnetic zenith angle LX,the angle between the line of sight and the magnetic field line at the emission point. In addition to the hydrogen emission there will be some “background” emission consisting of starlight and unresolved aurora1 bands and continuum. In the spectral region of interest these give an intensity per filter bandwidth of the same order as the HP line. It is assumed that this background is uniform over the region of the spectrum containing both filter pass bands, and of intensity C Rayleigh per unit wavelength. The exposure through the H, filter will give a density
and an exposure through the “background” titer will give &(E, 11)= A,([, ?;I)F [t/CT,(L)
dA] = A,(E,
IlPvc~,l
where the subscript 0 refers to the 4859 A filter and 1 refers to the 4879 L%“background” filter. By interpolation from the table of D vs. exposure for each filter, values are obtained for P = t J (1(a, A) + C)!Z’,(A)dl and Q =
tCW,;
hence s
I(a,
l)T,(A)
di2 = P -
Q;. 1
Knowing T(A) vs. il and the shape of the I( a, A) curves for various a the following integrals can be calculated:Y(a)
= a(a)fi(a,
A) 02,
X(a)
= a(a)b(a,
W’&)
dk
where a(a) is an arbitrary (normalizing) constant. Hence B = s
I(a, A) d3, = $f(P-82).
Studies on aurora1 hydrogen emission using an image intensifier
327
B can then be corrected “to the zenith” by dividing by the Van Rhijn ratio for the appropriate zenith angle. Allowance must also be made for losses in the field compressor which was not used when the calibration was performed; this is done by multiplying all the intensities obtained above by an appropriate constant. 3.3 Parametersused in analysis 3.3.1 ~~~e~o~~~ f7a?% Rbijs T&O ami fw$$t o~e~~~8~on. The difbm nature of the hydrogen emission makes height d&e rminstion by triangulation impro&icable. EATHER and JACKA (1966a), found it impracticable to determine the height from the variation of Van Rhijn ratio with zenith angle because of uncertainty in the shape of the HP line profile. Instead they matched theoretical Van Rhijn curves for reasonable emission heights against those deduced using the various published line profiles to find which profile seemed most appropriate. The best mstch was obtained assuming an emission height of about 100 km md VEISSBERU’S (1962) H@ line pro6le. EATHER and BURROWS (1966) caloulated the theoretical emission height for various combinations of bombarding proton energies and pitch angle distributions. The more probable of these combinations involved emission vers-usheight profiles which peaked sharply between 100 and 120 km altitude. Most other reliable investigations of the height profile of the hydrogen emission lead to similar estim&es of the height of emission. In the present analysis an emission height of 110 km h&s been assumed; Veissberg’s profile and Esther and Jacka’s best matching Van Rhijn curve have been used. V&s&erg published only a magnetic zenith profile for Hg. An appropriate horizon profile to match this was assumed by Esther and Jacka. Protiles for intermediate zenith angles were deduced from these by interpolation. The Doppler shift of the proiile peak is proportional to cos ccwhere 0:is the magnetic zenith angle; the intermediate HP line profiles were interpolated linearly about this peak from the zenith and horizon profiles. Figure 2 shows three of the profiles used. From these pro&s values of Y/X were calculated. They range from 2.8 in the magnetic zenith direction to 2.5 on the magnetic horizon. By oomparison, Z~ICK and SIXIGPPERD’S(1963) profiles yield vdues between 3.9 in the zenith to 2.4 on the horizon, demonstrating the sensitivity of the calibration to choice of profile shape. 3.3.2 Co-ordinates for presentation of resulta. The emission intensities derived from the film images will be displayed as s function of Eccentric Dipole time (COLE 1963, SIM~NOW 1963), and Invariant latitude R d&ned in terms of McI~w~‘s (1961) L value by L cos I = 1. Invarisnt latitude approximates closely to Eccentric Dipole latitude. In analysis of the data an emission point not on the Eeoentric Dipole meridian of Mawson has been as&bed a different emission time; longitude and time have been used interchangeably. This interchangeability is valid if the emission zone is fixed with respect to the sun and the Eccentric Dipole pole (close to Invariant pole). Its justification lies in the agreement between d&a from different frames and times processed in this way. For most of the time the assumption of a fixed zone appears valid but there are instances where t#hedata indicate a fast change in position &rid/orintensity of the emission. 3.4
Computations
A computer was used to convert film densities and positions to emission zone intensities and co-ordinates (Invariant latitude and Eccentric Dipole time). Each data frame was entered as a 16 x 16 matrix of density readings. Each reading was compared with the eight calibration levels appropriate to the field point and an intensity interpolated for it. Background frames were then subtracted from emission frames after allowance for the filter bandwidths. The differences were corrected according to the magnetic zenith angle (affecting the portion of the emission line
328
R. J. FRANCIS and F. JACKA
transmitted by the filter) and zenith angle (Van Rhijn effect). Those points near the field edge with zenith angles greater than 73’ were rejected since the Van Rhijn correction becomes highly inaccurate beyond this angle. The resultant set of integrated emission line intensities were printed out together with the Invariant latitude and Eccentric Dipole time ascribed to each field point. The latter co-ordinates were derived from the latitude of the observation point and time of observation after calculating the azimuth and distance (assuming an emission height of 110 km) of the field emission point. The field points correspond to the centres of each of the 1 mm square areas over which the film density was integrated. The field points for each frame were separated into the Invariant latitude groups half a degree wide in the range 67.3"to 73.2"and the individual points were plotted on intensity/Eccentric Dipole time grids. A set of background intensities for each observation period was also obtained to check that variations of emission intensity were not caused by a change in gain of the recording system. These showed that the gain was quite stable. From these intensity/time plots the isolines of constant intensity were derived and plotted on a latitude/Eccentric Dipole time grid. This was considered the most lucid way of displaying the data. 3.5 Continuity and correction factors Examination of successive sets of zone intensities derived from pairs of data frames by the above computations accepted at their face value leads to the conclusion that the emission zone within the field of view was most intense in a region of the sky to the north west and that the intensity always decreased near the horizon by up to a factor of two. That the emission should show preferential brightening in a particular region of the sky which is small in relation to the whole extent of the emission, and that the emission should at all times decrease toward the horizon around what is essentially an arbitrary observing point, leads to the suspicion that these are characteristics not of the emission zone but of the data reduction procedure. To accept the conclusion of this examination one would be obliged to abandon the concept of a zone fixed in space and its consequence that successive frames plotted on an intensity/time grid should show continuity. Factors that would cause a lack of continuity are examined below. 1. Vignetting in the jilter optics. This source must be included in the possibilities since the small collimator lens in the field compressor must be imaged accurately on the aperture of the 50 mm lens above the photocathode to avoid vignetting. Any misalignment would result in asymmetric vignetting in addition to that already observed on the calibration frames which were obtained with the field compressor removed and a relatively large area uniform source placed above the filter optics entrance pupil. 2. Vignetting changes in the output optics. This would arise if the camera mount was not replaced in exactly the same position whenever it was shifted, causing misalignment of the output optics and a consequent asymmetry in the vignetting. A shift of 2 or 3 mm between the time of the observations were made and the calibration performed is required to explain the observed asymmetry; this is physically possible but unlikely to have occurred.
Studies on aurora1 hydrogen emission using an image intensifier
329
3. Overestimation of Van Rhijn ratio. The effect observed is of the same order of magnitude as the total Van Rhijn correction applied. The Van Rhijn ratio does change and may be asymmetric due to asymmetric extinction and ground albedo. If it is assumed that the Van Rhijn ratios used are representative of an exceptionally clear night then it is conceivable that increased and asymmetric extinction could account for the observed lack of continuity in the data. The observed asymmetry in the data is relatively constant on all four nights. This reduces the probability that it arises from such an effect although the effect may contribute to differences from night to night. The possible effects could not be measured so an arbitrary correction to the intensity distribution on each frame was made in an attempt to derive a more probable distribution. The intensity/time plots were examined to find a region where the centre of the band of emission appeared to be in the zenith and the band extended over the whole field of view. It was assumed that at such a time the band of emission was sufficiently broad to be uniform over the entire part of the field of view analysed (above 17’ elevation). A group of three frames taken at such a time was used to derive correction factors. First the three values of emission intensity for each field point were averaged, then a mean value for the field point averages for a region near the zenith was obtained. The ratio of a field point average to the zenith mean was taken as the correction factor for that field point. Each field point intensity for other frames for the whole period of observation was then corrected by the appropriate ratio in this 16 x 16 set. The intensity/time plots so derived for the latitude interval 70.3’ to 70.7” are shown in Fig. 4. A considerable dispersion of the short curves derived from individual data frames remains but general trends are clearly apparent. 3.6
Emission from the Milky Way
The Milky Way has been shown by MONTBRIAND et al. (1965) and others to be a source of hydrogen emission comparable in intensity to aurora1 hydrogen emission; their charts of this emission were gathered from the astronomical literature. There is a region along the galactic equator generally about 10” wide emitting H, with intensities up to 60R. (The ratio of intensities H,/H, N 3.) Within this region are some much brighter areas, the largest of which is about 12” square with an emission intensity greater than 60R in H,. It is situated near New Galactic Longitude 260”. The intensities are quoted to be accurate to within a factor of 3. EATHERand JACKA (1966cc), using an instrument with a 4” field of view found emission intensities in the Milky Way up to 65R in H,. The image intensifier, when well focused, showed the Milky Way clearly, both in H0 and at the background wavelength. The brightest features could be identified with the region mentioned above at NGL 260”, and with others at NGL 290“ and 340”. They contributed no more than 15R through the titer pass band, this being an average intensity over a 12’ square area of the sky. These areas where the Milky Way could be discerned were omitted from the plots of emission zone intensity.
R. J. FRANCISand F. JACKA
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Gap in records here Room light left on
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Fig. 4(a). Fig. 4. Ho intensity/time plots for the invariant latitude range 70.3” to 70.7” which includes Mawson, after application of compensating factors derived from the regions marked “Camp.” 3.7 Error sources and magnitudes 3.7.1 Absolute errors. The largest source of error in the calibrationprocedureis expected to be the choice of H.,zJ line prof?le. Choosing other profiles from among those published would result in increases in the derived intensities by up to 50 per cent but confidencein the profile selected suggests an error much less than this. Other errors of smaller magnitude could arise from changes in the position of the filter bandpass with temperature, errors in the assumed reflection coefficientsof the field compressor surfaces and differencesin the sky background intensity over the two filter pass bands.
331
Studies on aurora1hydrogen emission using an image intensifier 60$ or
7 Aug ‘64 Ccont 1
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Fig. 4 (b). The overall absolute accur~y of the derived intensities is estimat.ed to be within i50% i 3R. 3.7.2 Rek&t? ewors. In this category are differencesin intensity between individual field points, between individual frames, and between individual nights of observation. Some of the factors mentioned in the previous section would have contributed also to relative errors. 1. Statistical fluctuations. !Chiswas the largest source of Merences in derived intensities between individual field points and depended on the accuraoy of measurementof densitiesand on the differencein density betweenemissionand backgroundframes. The backgroundintensity was relatively stable and deviations are better expressedabsolutely than in ratios; the observed scatter in the field point intensitieswas an RMS deviation of about 2R near the field centre and 4R near the edges where the compensating faotors were larger. 2. Van Rhijn ratios. Changes in atmospheric conditions could have led to slow changes in the derived distributionover the field of view during the period of observation. The oompensating factors would eliminate any asymmetry at the time for which the factors were derived but would not eliminate errors due to changes in conditions that occurred before or after this time.
332
R. J. FRANCISand F. JAGKA Auroroe cd KIndii
12 hrs eccentric
‘12 hrs eccentric
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Fig. 5(a). Fig. 5. Isophotes of HP intensity and rough plots of aurora1activity and K indices. An estimate of the magnitude of the changes may be gained from the slope or shape of the intensity distributionsacross individual frames on the intensity/time plots. Examination shows that the errors cannot have exceeded 50 per cent at the northern or southern limits of a frame. 3. ~ompensat~g factors. The nature of the derivation of the compe~at~g factors could lead to systematic distortions of the overall distribution on each frame if the referenceframea from which the factors were derived had not in fact resulted from a uniform emission distribution. The test of this was the continuity effected when individualframes were incorporated into an intensity/time plot as shown in Fig. 4. In these plots the small scale (statistical) fluctuations have been smoothed but the larger scale systematic errors are still apparent in the lack of precise coincidenceof the short curves representingsuccessiveframes. 4. Smoothing by frame overlap. In the intensity/time plots of successiveframes the overlapping of frames allows a running mean of intensity to be made to smooth out distortions in the frame distribution introduced by errors in the data reduction. Similar smoothing cannot be made along the latitude axis and a systematic distortion in the form of an over or under estimation of intensity at the highest or lowest latitudes relative to the oentral latitude may occur.
Studies on aurora1 hydrogen emission using an image intensifier
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6 Sept’64 I
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4. EXPERIMENTAL RESULTS AND DISCUSSION 4.1 ~~~e~~~~a2 ~0~~0~~ of Hs ~~te~s~t~ 4.1.1 Emtiaiolz isophotes. Figure 5 shows the isophotes of HP intensity derived from the data for the four nights analysed, interspersed with rough plots of aurora1 activity obtained from all-sky camera photogmphs, together with K,- and local K-indices. The zone of emission deduced by EATHEB md SANDFORD (1966) is shown for eomp&rison in Fig. 0. 4.1.2 I&e-n&y/time plots. While isophotes give a convenient indication of I-I, zone position and intensity, the intensity/time plots are better indioators of zone behaviour. The intensity/ time plots for the O-5 degree wide latitude band inoluding Mawson (70+30-70*7”) for the four nights snalysed are given in Fig. 4. On these are shown the smoothed intensities across individual frames. In many places where zone intensity changes occur successive frame pairs have been
R. J. FRANCISand F. JACKA
334
plotted, at other times approximately every third has been plotted. A plot of the background frame intensity is given to show its relative stability. The frame rate (and time resolution)was one pair every 4 or 5 mins.
4.2 Characteristics of the HP zone On all nineteen nights when observations were attempted H, emission was found to be present although only four nights records were suitable for detailed analysis. The isophotes (Fig. 5) for these four nights are consistent in the large scale with an emission zone fixed in space, relative to the sun and Invariant pole, underneath
,06
18
Eccentric
Dipo1e
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Fig. 6. EATHERand SANDFORD’S (1966) zone of hydrogen emission.
which the earth rotates. The zone is of order 1000 km wide between half intensity points at 18.00E.D.T. (Eccentric Dipole Time). The differences in position of the zone from night to night are generally consistent with an expansion to lower latitudes with increasing magnetic activity. The magnitude of this expansion is of order 3’ of latitude per unit of K, or 5’ per unit of local K-index change. Examination of the intensity/time plots reveals many details that are not consistent with the fixed, stable zone concept. Some of the irregularities arise from inaccuracies in the data reduction such as inappropriate choice of compensating factors but most are real and indicate real features of zone behaviour. In addition to the mechanism producing the stable zone it appears that a separate mechanism operates at times, showing a sudden onset and giving rise to a
Studies on aurora1hydrogen emission using a image intensifier
335
poleward expansion of the zone. It is associated with Slowly Varying Ionospheric Absorption of cosmic radio noise (SVIA). This has been observed also by EATHER and JACKA(19663). 4.3 Description of events m each night In this section an attempt is made to describe the observations in terms of the ideas advanced in the previous sections. While some of the characteristics and zone movements referred to may be more clearly inferred from the isophote maps (Fig. 5), other characteristics are more readily inferred from the intensity/time plots (Fig. 4). 30 July 1964. The pattern of isophotes shown in Fig. 5 represents a relatively complex but stationary distribution of HP under which the earth rotates. In fact, as can be seen from the intensity/time plot in Fig. 4, there were discontinuities around 17.30 and 19.00. At 17.30 there was an intensification of the emission in the northern sky the detail of which cannot be accurately represented on the scale of isophotes used in Fig. 5. Similarly at 19.00 an intensification occurred near the zenith. In both these cases the features drifted out of the field of view as the earth rotated. The riometer record did not show absorption of cosmic radio noise associated with either of these features. By 21.20, rotation of the earth had carried the field of view poleward of the emission zone shortly before moonrise. Near 18.30 one frame suggested a short duration change in HP distribution but succeeding frames reverted to the general trend. 3 August 1964. The intensity/time plots in Fig. 4 show relatively little dispersion and no discontinuities. The isophotes in Fig. 5 therefore correctly represent a stable distribution of HP emission through which the field of view was carried by rotation of the earth. The maximum of the emission zone apparently lay across the path of the observing field at about 19.00-20.00 and equatorwards of it around to 04.00 when observations were terminated by approaching twilight. A few minutes after this an SVIA began, reaching 1.5 dB within 30 min and lasting for 24 hr. The development of this event was associated with intense aurora followed by weak diffuse aurora over the whole sky and K, = 6. From the work of EATHERand JACKA(1966b) one would expect strong enhancement of H, during this interval. If this did occur it must have had a sudden onset as the isophotes, which extend nearly 30 min after the SVIA began, show no sign of it. 7 August 1964. Observations began at 15.57 with intensities >30R. Shortly after 16.00 the emission region apparently retreated poleward accompanied by a decrease in K-index ; this is the cause of the discontinuities in Figs. 4 and 5. The riometer record does not show absorption although in the previous three hours it was somewhat disturbed. The zone then showed stable behaviour, the path of the observing field being polewards of the brightest part. After 23.00 observing conditions deteriorated; substantial cloud cover increased until observations were terminated at 02.15 with complete cloud cover. Intensities after 23.00 could be underestimated but enough light diffused through the cloud to show that the field of view was again moving nearer to the emission zone after 01.00.
336
R. J. FRANCIS and F. SACKA
Two frames, one at 17.00 the other at 19.20, seem inconsistent with the rest of the data. No explanation in terms of zone behaviour or experimental factors is apparent. 6 Xe@ember 1964. Observations began at 16.56 with moderate H, intensity present. The background level shows that twilight is still contributing; allowance for the Fraunhofer absorption of Ix, in this would tend to increase the derived ~tensities at the western ends of the frames affected. There was a fast reduction in intensities near 17.20due probably to a poleward shift of the emission; the detail of this is not showu in Fig. 5. The riometer record for this period is smooth. Intensities after this time are consistent with rotation under a stable zone until 00.00, the zone maximum being located equatorwards of Lawson as one would expect from the magnetic activity indices. A world wide Sudden Commencement of magnetic activity occurred at 21.37 (19.55 U.T.). No immediate reaction was evident, but near 00.00 there was a sudden increase in both aurora1 activity and HP emission. This explains the ~scontinuities in Figs. 4 and 5. At this time also the riometer records show an SVIA event which EATHER and JAGKA (1966o,b) found associated with proton precipitation and a poleward expansion of the HP emission zone. This is consistent with the present observations. The aurora1 activity preceded the rise in H, intensity and the fast increase in background level accounts for the large negative intensities derived from frames in this transition period; the background frames were exposed after the emission frames, hence the background level is overest~ated as fast increase occurs. The aurora1 activity then decreased in intensity (as did the background level) but remained widespread.
5. CONCLU-SIoNs 5.1 E~~~~rn~~t All sky photography, using the image intensifier and two filter method, has been shown to be an effective means of determining the temporal and spatial distribution of the aurora1 hydrogen emission. With improvements in calibration techniques and in design of optical component mountings the accuracy could be significantly improved. A more efficient photooathode material than the S9 used, such as the S20 which is now available, would improve the time resolution. Intensifiers of the type used have since been improved in respect to dark current and can be selected for uniformity of sensitivity. Intensifiers of the Cascade (phosphor/photocathode sandwich) type have also been developed yielding improved contrast and robustness. In this type of intensifier the signal induced background is very much lower. 5.2 Results It is apparent that the concept of a stable oval zone of hydrogen emission fixed in space as suggested by EATHERand SANDFORD (1966) is roughly consistent with the present observations on some nights. The H, oval shows an equatorwards expansion with increasing magnetic activity similar to that of the visible aurora1 oval (FELDSTEIN and STARKOV, 1967). ~though there is considerable overlap of the two zones, the HP oval in general lies equatorwards of the visible aurora1 oval.
Studies on aurora1 hydrogen emission using an image intensifier
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On several occasions changes of HP intensity occurred in localized patches. These were not associated with cosmic radio noise absorption. On one night (6 September at 00.00 E.D.T.) a sudden enhancement of H, emission, extending polewards of the normal stable oval zone, was associated with an SVIA event and “breakup” of the aurora1 display characteristic of the onset of an aurora1 sub-storm (AKASOFU, 1966). This situation is not adequately represented by a poleward bulge in a stable emission zone as proposed by EATHER and SANDFORD (1966). Examination of riometer records over a long period shows that SVIA events are in general associated with moderate to large values of K,. Following ANSARI(1964) these events are presumed to be caused by energetic (30-50 kev) electrons while EATHERand JACKA(1966b) find them to be associated with enhanced H, emission as in the present investigation. It is suggested that a sudden hardening of the precipitated electron energy spectrum accompanied by enhanced precipitation of protons marks the onset of the aurora1 substorm and gives rise to the effects discussed viz. marked reduction of visible aurora1 intensity, SVIA, enhanced HP, intensified ionospheric currents and hence increased geomagnetic disturbance. Acknowledgements-The planning of this investigation and construction of the equipment was carried out in the Antarctic Division, Department of External Affairs, Melbourne. The assistance of former colleagues in that Division is gratefully rtcknowledged. The observations were made while one of us (R. J. F.) was a member of the Australian National Antarctic Research Expedition at Mawson. The assistance of D. SEEDSMAN end PT. CARDELL is gratefully acknowledged.
REFERENCES AKASOFU S-I. ANSARI Z. A. COLE K.D. EATRERR.H.~~~JACKAF. EATHERR.H.~~~JACKAF. EATHERR.H.~~~ SANDFORD B.P. EATHERR.H.~II~BURROWSK.M. EMBERSON D.L.,ToDEILLA.~~~ wILC0CK'cV.L.
1966 1964 1963 1966a 1966b 1966 1966 1962
FELDSTEINYA.I.~~~STARKOVG.V. MCILWAIN C.E. MONTBRIAND L.E.,TINsLEYB.A.~~~ VALLANCEJONESA. SIMONOW G.V. VEISSBERG O.L.
1967 1961 1965
ZWICKH.H.~~~SHEPPERD
1963
2
G.G.
Space Sci. Rev. 6, 21. J. geophys. Res. 69, 4493. Au&. Aust. Aust. Aust. Aust.
J. J. J. J. J.
Phys. Phys. Phys. Phys.
19,423. 19,241. 19,215. 19,25. Phys. 19,309.
Advances
in Electronics and Electrcn Vol. 14, p. 127, Academic Press, New York. Physics,
1963 1962
Planet. Space Sci. 15,209. J. geophys. Res. 66, 3681. Can. J. Phys. 43, 782. Geophys.
J. R. astr. Sot.
8, 258.
Aurora and Airglow 8, 36. Results of I.G.Y. U.S.S.R. Aced. Sci. J. Atmosph.
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25, 604.