High latitude airglow observations of correlated short-term fluctuations in the hydroxyl meinel 8-3 band intensity and rotational temperature

Planet.

Space

Sci. Vol. 23, pp.

1211 to 1221.

Perpamon

Press,

1975.

Printed

in Northern

Ireland

HIGH LATITUDE AIRGLOW OBSERVATIONS OF CORRELATED SHORT-TERM FLUCTUATIONS IN THE HYDROXYL MEINEL 8-3 BAND INTENSITY AND ROTATIONAL TEMPERATURE JOHN W. MBRIWBTHBR* Photometrics, Inc., 442 Marrett Rd., Lexington, Massachusetts 02173, U.S.A. (Received 5 February 1975)

Abstract--An application of a tilting tilter photometer for the ground-based measurement of the atmospheric temperature at the menopause altitude (~85 km) is described. The technique uses selected rotational emission lines of the OH Meinel night airglow to determine a rotational temperature. A sampling rate of approximately one per minute with a precision of &5K can be achieved with a field of view (4-km transverse at the mesopause height) sufficient to detect fine structure variations in the temperature and intensity. The systematic error of these measurements is comparable with those of rocket in situ measurements by falling spheres or parachute-borne thermistors. Results obtained March 1974, at Ester Dome, Alaska, indicate the presence of systematic fluctuations in the rotational temperature and the 8-3 band intensity of period 16 rnin and amplitude 2-4 per cent. INTRODUCI’ION The night-time behavior of the OH Meinel airglow emissions is an area receiving considerable attention by ground-based observers in recent years as modeling techniques coupled with our improving understanding of the mesospheric chemical processes permit the close comparison of predicted behavior with the observed amplitude and night-time temporal pattern. For example, Dick (1972) has shown that a typical pattern of correlated intensity and rotational temperature minima near midnight occurs for a mid-latitude station as opposed to the more irregular behavior reported by Takahashi et al. (1974) and Wiens (1974) for near-equatorial stations (15-25ON). Moreover, Fiocco and Visconti (1974) have paid considerable attention to the long term behavior of band intensity ratios between transitions originating at high vibrational levels (V = 8,9) and those arising from lower levels (V = 5,6) with the implication from their analysis that the topside quenching by atomic 0 and the bottomside depletion Jf the mesospheric hydrogen concentration during the night-time plays a much more important role than had been previously appreciated. Notably, however, these studies have focused attention upon the long term behavior of the OH emissions because generally the instrumental sensitivity was insufficient to permit the sampling frequency to be greater than some 5-10 data points per hr. Inadvertently then, this limitation leads to a biased selection of the possible dynamical modes

of the atmosphere’s temperature and density (which relates to airglow intensity) that one could have observed. It is known, however, that inhomogeneities do exist in the spatial distribution of the D region airglow (Chamberlain, 1961) so an important aspect of the airglow phenomenology is being ignored by such temporal filtering. Indeed, reports of short term fluctuations in the OH temperature and intensity have been published by Krassovsky and Shagaev (1974, in which see earlier references) and by Kieffaber and Petersen (1973). In both cases the sampling frequency is on the order of 30 points per hr-considerably higher than the other works. Physical justification for such local inhomogeneities have been attributed by Krassovsky to the presence of gravity waves propagating within the mesosphere (Hines, 1972; Yeh and Liu, 1974). Evidence from pictures of noctilucent clouds (Hines, 1972) suggests strongly that such an interpretation may be valid. Hence, attention paid to the portion of the dynamical power spectrum of airglow fluctuations above lo-5 Hz would likely be of considerable interest.

DESCRIPTION OF TECHNIQUE Application of modem refinements in photometry by virtue of the availability of stable, narrow-band interference filters, inexpensive pulse counting circuitry, and photoemissive cathodes with high quantum efficiences extended to 4.9 pm makes possible a low cost technique for the measurement of mesospheric temperatures at high sampling rates with a ground-based tilting filter photometer (Eather and Arecibo Observatory, Box 995, Arecibo, l Now at: Reasoner, 1969). With minimum supervision, data Puerto Rico 00612, U.S.A. 1211

1212

J.W.

h'fERlWBTHER

can be obtained with f5 K precision (1 S.D.) at a sampling rate of 35/hr for a 3’ full field of view (0.052 rad). Unlike most applications of this instrument that scan in wavelength continuously by tilting (Fiocco and Visconti, 1974; Takahashi et al., 1974), the application described herein uses a discrete number of positions in the tilt scan to enhance the sampling rate. Normally 2 would be used but 3 could be employed if the backgrounds adjacent to the emission line were both measured. Consequently, sporadic variations in temperature and intensity of short duration can be detected; these rapid changes have previously been smoothed by the signal averaging required with instruments of lower throughput (for example, grating spectrometers) or the spatial averaging introduced by a large field of view (Dyson and Hopgood, 1974). Furthermore, these instruments are often used to scan broad wavelength regions such that the bulk of the instrument’s observing period is spent sampling the background rather than the features of interest. The method to be detailed herein is very similar to the one described by Schaffer and Fastie (1972) for a rocket tilting filter photometer. The application of narrow band interference filters is used to isolate rotational lines of a vibrational (Meinel) band of the OH airglow. This chemiluminescence originates principally from the reation of ozone with atomic hydrogen to produce vibrationally-excited OH in the ground electronic state and is confined to an altitude range over which the kinetic temperature is nearly constant. The tilting-titer photometer measures the column intensities of two selected components against the background produced by the airglow continuum and scattered light. The ratio of these is used to obtain the OH rotational temperature assuming thermal equilibrium. Numerous rocket measurements of the OH volume emission profile (summarized by Valiance-Jones, 1973, for mid-latitude sites) supported by photographic triangulations of OH airglow structures (Peterson and Kieffaber, 1973), indicate that the night-time hydroxyl layer can be characterized by a Gaussian function of half width between 5 and 10 km roughly centered at the mesopause altitude, where the temperature is a weak function of altitude (Cospar International Reference Atmosphere, 1972). Therefore, the temperature deduced from the hydroxyl rotational spectrum can be related to the centroid altitude of the OH layer; that is, the single value inferred from the line intensity ratio does not differ appreciably from that derived by convolving the actual atmospheric temperature profile with the volume emission profile of the OH emitting layer. We have made

model profile convolution calculations similar to those described by Sivjee et al. (1972) to quantify this concept and, as will be shown below, the “error” is comparable with the systematic error corrections required by in situ methods such as falling spheres (Faire and Champion, 1968), rocket grenades (Smith et al., 1968), and parachute-borne thermistor sensors (Webb, 1966). OH ROTATIONAL FILTER

LINES AND INTERFERENCE CHARACTERISTICS

Figure 1 presents the hydroxyl spectrum (Dick, 1972) for the airglow emission of the Meinel 8-3 band; inspection shows the P branch components of this band to be dispersed by 9 A or more. This spectrum represents a segment of a region, 1000 A in range, accumulated from successive scans over a total duration of 13.5 min. The separation of 9 A is large enough to allow each component to be isolated with a narrow band interference titer. The passband width at half-transmission points would need to be less than 6 A for adequate resolution of individual rotational lines. Such interference filters, with peak transmission ~50 per cent and aperture diameter up to 10 cm have been fabricated by commercial manufacturers. Choosing to concentrate on the careful intensity measurement of one line rather than a pair such as PI(n) and Ps(n) together avoids possible problems

(8,3)

(7,2)

I

I

7000

I

7400A

FIG.

1. SPECTRUMOF THE OH 8-3 BAND OBTAINED BY A 1-m SPECTROMETER WITH 13XklillSUhihfATION OF SCANS, AT 5A SPECTRAL RESOLUTION (PROM DICK, 1972).

Arrows point to the P,(2) and P,(5) components selected for measuring the band rotational temperature. The horizontal line indicates the sensitivity change as a function of wavelength.

1213

High latitude airglow observations on hydroxyl8-3 band of nonequilibrium between the two il states of OH and minimizes the extent of background that is also measured. An additional benefit of using two very similar filters is the cancellation of systematic errors of calibration and measurement that occur in the extraction of the rotational temperature from the intensity ratio of two components. Statistical errors would be additive, however. We selected two components of the 8-3 vibrational band, the P,(2) line at 7316A and the P,(5) line at 7402 A as this band appears to represent the best compromise among OH source intensity, the quantum efficiency of available photoemissive cathodes, and current interference filter technology. The P branch of the 7-2 band near 6800 A is unusable because it is overlaid by absorption in the lower atmosphere by Oa in its (O-l) 3Z - 1x atmospheric band; the 5-O and 6-l bands are not sufficiently dispersed for the necessary isolation of the two rotational components; and the 9-3 band is contaminated by both emissions in the atomic oxygen lD - sP doublet (6300-6364 A) and absorption in the (O-2) 0s sI; - l1: band. Hydroxyl bands at higher wavelengths, though generally more intense, do not serve because filters cannot now be fabricated with sufhciently narrow passbands to isolate a single rotational line. The application of tilting filter photometry described herein differs from standard practice (Eather and Reasoner, 1969) in that the wavelength of the filter’s peak transmission at zero tilt angle is prescribed to lie 6-8 A above the Pr component. As discussed earlier each s(n) component is separated by nearly 10 A from the P&r) component and each such pair is isolated from adjacent pairs by about 15 A. The filter’s passband if positioned at PI(n) for zero tilt angle becomes broadened by the high tilt angle (ml0 deg for a high index filter) required to shift it below the neighboring Ps component. Leakage introduced by the adjacent P1 and Ps pair would prevent an accurate measure of the background with the filter tilted. When the background spectral radiance is measured in the zero tilt position, the tilt angle required for the P,(2) line is much lower (m5 deg), so that the passband remains narrow enough (about 9 A) to prevent substantial contamination from either the P,(2) line or the P,(3) line. This latter arrangement decreases the filter’s transmission at the line about a factor 2; the sensitivity of the phototube is sut3icient to maintain a satisfactory S/N ratio (details are presented later). Another reason for adopting this mode of positioning is that the decrease of the peak transmission as a function of tilt angle at angles near 5’ is not severe; hence, the sensitivity of the instrument to modest shifts in

the ambient temperature is much less than for tilt angles less than 2 deg. The design of the filters required for this application (manufactured by Barr Associates, Concord, MA) is based on a double Fabry-Perot cavity with two layers of blocking to ensure high out-of-band rejection. ZnS rather than cryolite was chosen as the spacer material because it has a high effective refractive index (-2.3) and does not become susceptible to splitting the transmission passband into polarization until the tilt angle reaches -15-20’. Additionally, the passband of a high-index f?lter broadens less with increasing tilt angle than that of a low refractive index filter. An added benefit is that ZnS has a lower temperature coefficient than cryolite (4 A/C”), which makes its transmission characteristics substantially less sensitive to changes in ambient temperature. In any case small changes in temperature of two similar filters have only second order effects on the line intensity ratio, and therefore on the measured mesospheric temperature. SIGNAL TO NOISE RATIOS AND PRECISION OF THE TEMPERATURE MEASUREMJZNT.

To calculate the precision of the rotational temperature measurement, we adopt a value of 20 R@ick, 1972) for the zenith intensity of the P,(2) rotational component at 7316A and a rotational temperature of 220 K. appropriate for a high latitude winter mesosphere (COSPAR, 1972). The sensitivity sI of the photometer with 5-cm aperture to the P,(2) nightglow line is s, = ALlQTJ, x 10’/4n wunts/sec Rayleigh, where& is its throughput (or etendue), 0.043 crna srfora 1%deg half anele field of view (selection of which is descri&l below); Q the quantum efficiency of EM1 9685 RM photocathode (S-20, extended red) considered optimum for this application, 9 per cent for this wavelength region; T, the filter transmission at the line (about O-18, see later calibration section); and T, the transmission of the photometer’s optics including two mirror surfaces, plexiglass observatory dome and entrance window, two plastic plates in the cooler’s moisture barrier, and two lenses (obiective and Fabrv). Assuming an effective transmission ‘bf O-9 for each ‘of these elements, T. is 0.43. With these oarameters se is calculated as 2&2&R, a number somewhat higher &an the sensitivity of 186c/sR measured in the absolute intensity calibration described later. This lower value probably results from the facts that 10 per cent of the ater area A is masked by its retaining ring, and the discriminator following the photomultiplier rejects a small fraction of the pulses originating from individual photoelectrons. The statistical signal/noise ratio for the P,(2) component is then S -= N

F, [I’, + 2(Y, + Dl”’ saRptlla = bsRo +Z(s, Rb W/T, + D)]lj=

1214

J. W. Mzruwzrrixa

where Yr and Ys are the photometer count rates in response to the airdow rotational line of intensity R, (2d Rayleighs) and the continuum background Rb (1.0 R/A); W is the ares under the filter transmission curve, in A; D is the ctark count rate, measured as 125 see-l; and t is the integration time. A similar formula holds for the second component P,(5). Substituting the sensitivities and filter parameters determined by calibration, we find that for a temperature of 220 K the signal-to-noise ratios are 12*3t1’aand 10*8N* for PI(Z) &d P,(S), respectively. For the typical integration times used in the measurements (10 set) the ratios become 38.9 and 34.1, These values imply the zenithintensity statistical fluctuation would be about 0.6 R. The S/iv can be increased by a factor of l-4 by doubling the path length through the airglow emission, that is, by making use of the van Rhijn effect near 60” zenith angle. We turn next to a calculation of the dependence of the OH rotational temperature on the line ratio. The relative intensity of a rotational component with wavelen~h A and quantum number K in a vibrational band for a Boltzmann population distribution is

where Z(r) is in photon units, A a constant that cancels out when the ratio is taken, S(K) a factor proportional to the rotational line strength, F(K) the rotational term value, and he/k has its usual spectroscopic meaning. TX, the effective rotational temperature, of course represents a weighted average of distribution of temperature over the OH volume emission rate profile, a point that we will consider separately. S(K), a and F(K) were obtsined from Mies (1974). His calculations of the OH transition probabilities result in a 10 per cent decrease in TE from that determined with the older set (Sivjee et al., 1972). Evaluating this expression for K values 2 and 5 with constants appropriate for the 8-3 band, we find !fB = 362jh.1(3*44r),

Rota t iona I

where r is the ratio Z[P,(2)]/Z[P,(S)J. We estimate the upper limit of the systematic error in the temperature measurements originating from errors in the theoretical calculations of transition probabilities and our absolute photometer calibration to be about S per cent, or &lo K. L The plot of TB against line inte&ity ratio (Fig. 2) shows a variation in r of 50 ner cent for the tvnical mesosphere temperature range iSO-2S0 K, justify&g the selection of the 4(2) and P,(S) lines. Specifically, a temperature increase of 10 per cent at 220 K results in an increase of 20 per cent in the ratio, which would require a 3a fluctuation in either component’s count rate. The statistical (la) error in the OH temperature taking into account independent fluctuations in each rotational line intensity m~~ment is

For the typical zeni~-m~urement conditions cited above this fractional variation is 0*07St-xia. Thus, for an integration time of 10 set the standard deviation of an individual mesospheric temperature determination is about &SK. The functional dependence of a on T is also shown in Fig. 2. RELATlON OF THE EFFECTIVE ROTATIONAL TEMPERATURE TO THE MESOPAUSE TEMPERATURE In addition to the aforementioned systematic and statistical temperature errors, a further small error is introduced by the finite height of the hydroxyl-emitting layer and (potentially) by offsets of its c-entroid from the mesopause minimum. We calculate here the temperature differences that can be introduced by these two effects. Average temperature profiles for winter (60°N lat.), summer (6O”N), and the mean represented by CIRA 1972 are given in Figure 3 which also shows the OH AV = 2 volume emission profiles measured in a high latitude rocket flight (Rogers et al., 1973). The “high-u” protile is

temperature

&,

K

FIG. 2. DEPENDENCE ON THE ROTATIONAL ~E~TU~OF~~TIOP~~~)~P~(~)ANDT~FRACTIONAL STATXSTICALERROR FORP*(2)~NS~ OF20RAND A 10 Set COUNTING PERIOD.

High latitude airglow observations

on hydroxyl 8-3 band

Kinetic temperature,

1215

K

7065-

%lofiVe

FIG. 3. THREE1972 CIR.4 VOLUMEEMJSION PROFILES

MODEL OF

Wkm-S

WniSSiOn

profiles

(Rogers

ef

al., 1973)

6O”N lat. KINETICTEMPERATURE PROFILES (solid lines), AND RELATIVE “HIGH u” and “row 0” OH SEQUENCES (broken lines) FROMRMSERS

et al. (1973). representative of the emission from the eighth vibrational level, which is not a&c&d by quenching by 0 on the top side of the layer as the transitions from low vibrational states may be. Other rocket proties (summarized by Valiance-Jones, 1973) show the nighttime OH layer peaking at heights between 80 km and 95 km, the measurements of higher altitudes being at mid-latitude. Weighting the a&osphere temperatrire structure with the OH emission urofile aives a close estimate of the difference between’ the r&ational temperature that is measured and the mesopause temperature. We calculated fist the effect of the widthof theemittinglayer,simulating the P,(2) component’s volume emission profile by Gaussian functions, for a fixed peak volume emission altitude of 85 km. Halfwidths ranged from a minimum of 6 km, representative of the most narrow physically reasonable distribution, to 20 km, which is about the largest that could be. expected (for example, it might apply at twilight). The calculations for a typjcal Gaussran width of 8 km showed that the temperature difference is 2 per cent (4 K) for the summer high latitude distribution and 05 per cent (1 K) for the nearly isothermal distribution of the mean winter high latitude altitude profile; at 20km halfwidth, the &erences are about 10 percent (16 K)and Zoercent (4 K). The differences for winter and’equmoxcs &IRA 1972) are much less than the systematic and statistical errors, but for summer twilight some care in interpretation is required. The difference between the OH centroid altitude and the mesopause altitude is also not a serious factor except, again, for the summer temperature profile. We calculated the effect of this offset, holding the OH Gaussian halfwidth tixed at 8 km. For this set of convolution calculations, a temperature difference of 1 per cent develops for a height separation of $zS km for the winter and CIRA mean protiles; for the worst case of SUIMI~~ profile the difference ranges from 2 to 10 per cent for height differences between zero and f5 km. Thus, as would be qualitatively expected, the measurement would be least applicable at summertime twilight, where the OH layer is broadest and the atmosphere temperature minimumismost narrow. Thesecalculations are, thus, in agreement with those reported by Sivjee et al. (1972). However, this restriction does not extend

to the search for rapid intensity variations. For other conditions the ground-based OH line ratio measurements serve as an accurate indicator of the temperature at the mesopause. AURORAL CONTAMINATION Field measurements at high latitudes must contend with the presence of aurora1 contamination of the photometer data which was serious when the N,+(&l) First Negative 4278 A band intensity exceeded ~100 R. This would include drizzle precipitation as well as visible aurora1 display as a serious source of contamination. The oarticular emissions of concern are the N&-3) First * Positive band (which affects both rotational comnonent measurements) and the 7319 A OH forbidden atomic emission line (Meriwether et al., 1974). The intensity of the 5-3 band is almost the same as 4278 (Valiance-Jones, 1971) so (roughly speaking) the contamination at 7316 A and at 7402A is about 1 to 2 R/A. Since this is about equal to the airglow background and since it usually fluctuates, the excess radiation can seriously perturb the line or background measurement. The existence of the 011 contaminant was evidenced b enhancement of the background measurement at 7324 1 over what was observed in quiet periods; it too was variable. Thii problem is not limited to the OH 8-3 band alone, but applies to virtually all OH emissions in the visible and the near infrared where serious blends with the NI 1 PG bands generally exist (see the aurora1 and airglow spectroscopy reviews by Vallancc-Jones; 1971,1973). THE TILTING PILTER PHOTOMETER We used a four-channel photometer belonging to the Michigan Airglow Observatory located at Ester Dome near College, Alaska. Facilities include the photometer with pulse counting electronics and a double axis mirror system with azimuth-zenith angle programmable circuitry. The heating system and thermal insulation of the observatory prevents the occurrence of ambient temperature variations greater than several degrees. The specifications of the Michigan photometer are summarized in Table 1. The availability of 4 channels with

1216

J. W. MERIurBTHER TABLE 1. SPECIFICATIONSOFIXEMICHIC+ANAIROU~WOBSERVATORY PHOTOMETER (modified January 74 with cooled, extended S-20 phototube) Number of channels Entrance aperture Field of view Objective lens, Fabry lens Wavelength scanning mode

Free spectral range Scan time

Angular scanning mode

Photomultiplier Sensitivity

Dark count Electronics

this photometer makes it possible to allocate 3 channels to OH measurements and the remaining one to an aurora1 feature such as Nt+ A4278. In the measurements describe in this paper two channels were used for rotational temperature measurement as described above and the third to measure the R branch of the OH 7-2 band to observe possible changes in the band intensity ratio between the 8-3 and the 7-2 band. These results will be reported later. In principle three channels could have been allocated to the rotational temperature measurements for the 8-3 band. The half-angular field of view is a compromise between throughput and maintenance of a narrow spectral banduass at the necessary tilt angles, with spatial resolution (S-km transverse at 100&n range) ari imnortant consideration. When a 7400 A interference hlter with bandpass FWHM 3&A (measured with a parallel beam) is illuminated in a 14 deg cone with its axis normal to the filter surface, it broadens to only 4A and its peak effective transmission decreases from 47 to 39 per cent. A narrow field also limits the broadening as the titer is tilted. We used an extended-red-sensitive EM1 9658 RM

4, automatically sequenced 5 cm nominal, circular; A = 20.3 cm* Adjustable iris +3)” half angle; Aa = 0.0436 cm* sr at It” 30.5 cm, 8.9 cm focal length, respectively Cam driven filter tilt mechanism; cams with linear, off-center, and quantized-step drive available; spectral feature optimization possible with individual tilt adjustment. Filter tilt angle monitored by potentiometerencoder For high index titers (fi = 1.8) 0-20A normally; 60 A maximum Shortest scanning interval for one channel, six seconds. For automatic 4channel cycle, six speeds selectable with total scan time from 24 to 768 set in factor-of-2 increments. Mounting plate of photometer mates with driven mirror system, manually programmable for desired azimuth and zenith distance coordinates Optics designed for 5 cm cathode EM1 9658R (installed) or 9558 N@ht sky response 15200 15577 16300 A7400

(counts/set-R) 120 385 110 25

125 C&c nominal at -20°C Pulse counting throughout High speed pulse detector and amplifier HP scaler-timer, model 5201 L HP D/A converter, model 580A. multiplier phototube since it offered a factor of 10 increase (to 9 per cent) in quantum efficiency at 7400 A over a 9558B S-20 phototube. Cooling the cathode from 25 to -20°C reduced the dark count rate from 50,000 to 125 sec. The noise equivalent radiance from dark count was about equal to the continuum radiance from the clear zenith night sky measured with a 4A filter at 7324A (1 R/A). In the Michigan photometer design the filter tilt scan is controlled by the figure of a cam. A quadratic surface gives a linear wavelength scan; alternatively, a cam with two or more constant radius arcs (appropriately paired toaether) can step the nassband from line to background wiThout dwelling’ on intermediate wavelengths permitting data to be acauired more raddlv. The design of a twoposition cam ‘(as described ‘earlier) positio&d the filter passband at the high wavelength side (zero tilt angle) of the line for the background measurement and obtained the line radiance at the tilted position. After the passband of a filter in the tilted position has been tuned to the optimum wavelength deter&ned by a linear scan, the linear cam is replaced with the stepper cam. The tuning of the passband was controlled by an adjustment of the

High latitude airglow observations

counts $ i

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FIG.~. EXAMPLEOFDATAACQIJBTTIONWITHTHESTJZPPER CAM. strip chart speed is identical for both a and b; the temperature computed from the ratio of intensities of the two lines in a is 210 &- 5 K.

filter’s housing in the filter wheel and was checked each night by us&the linear cam for several scans. Figure 4 shows examnles of scans obtained with both the linear cam and a 3:position stepper cam. FILTER TRANSMISSION

CALIBRATION

Accurate measurements of the filter transmission in a geometry similar to that of the field measurements are necessary to determine the intensity ratio of the OH rotational lines against the continuum background. The transmission function at each tilt angle must be measured because the passband both broadens and shifts in the off-axis, converging light beam. This calibration is best carried out with the filter in place in the actual photometer, with the field filled by a near-monochromatic light source of closely uniform surface brightness. A diffuser is necessary to remove irregularities from the monochromator’s beam. A further potential problem here is the polarized light of the slit image, to which the tllters and photometer reflecting surfaces can have different transmission than to the virtually unpolarized night airglow. Calibration facilities of the University of Michigan High Altitude Laboratory include a half-meter monocb&nator under control of a DEC PDP-lminicomputer, which also handles the outnut data. The diffuser consisted of two pieces of fro&d glass; visual and photographic inspection indicated that this source had the desired uniformity. The calibration procedure requires that the source be stable, as was the case, for the thirty minutes needed for the calibration at the chosen tilt angle. The monochromator is positioned by the computer to each of a series of programmed wavelengths. First counting rates with no Slter, representing 100 per cent transmission, are measured and stored in the computer’s memory; then the tilter was moved into position and the count rates again measured and recorded. The wavelength increment was normally chosen

to be 0.3 A for filters with FWHM - 5 A. After each step the computer calculated and printed out the filter transmission together with corrected wavelengths. In this way we obtained transmission measurements for 1” angular increments between 0 and 8” from the normal. Figure 5 shows the results for the tilter used to isolate the P,(2) OH line. The computer also calculated the integral of the transmission curve between the two limits used in the run (usually a range of 35 A; additional readings were made to check leakage). This number is the “equivalent width” W mentioned earlier needed for the calibration of the photometer system response to a calibrated continuum source. DATA REDUCTION Line intensity values (in Rayleighs) are obtained by rates for the “line” and the “background” filter positions

Here sB’ is the photometer’s continuum radiance sensitivity (counts A/RX+ determined by the calibration to be discussed shortly; W, and W, are the equivalent widths of the filter at the continuum tilt position and line position, C is the night sky’s continuum spectral radiance (R/A), and T,, Ys and Rs have their earlier meaning (we are using the P,(2) component as an example). The contribution of the continuum plus dark count is evaluated separately by centering the filter’s passband 6-8 A above the line, as explained previously. The factor m is included to correct for the small contributions to the background signal from incomplete blockage of adjacent rotational components; Fig. 5 shows that P-branch lines contribute~less than or?&part in seventy, Initially m is assumed to be unity and the correction to R, (and R‘) is then computed by an iteration procedure in which the contributions to the background signal are

1218

J. W. MERIWETHER OH spectrum obtained by a l-m spectrometer (Dick, 1972) rotational temperature decreased from 220 to 190 K during integration period (22:18-03:56 MST) Spectral slit width 5a

F II

iG x- bOu% .-z 0.8 c g 0.6 z .E

04

-

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2 .g c P

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0.30 -

0.20 : c ii

0.2 -

0.10 -

1

7300

7310

7320

Wavelength,

7330 %

FIG. 5. TRANSMISSION OF THEP,(2) FILTJJR AS A PUNWON OFTILTANGLEIN A 3” CONVERGING BEAM, WITH OH 8-3iPp1(2),P,(2), P,(3) ANDQ,(4) COMPONENTS AND l-m MONOCHROMATOR DATA FROMFIG. 1.

successively improved with the aid of the OH rotational temperature calculated from the previous estimate. These corrections for P-branch lines resulted in less than 1 K changes in the final temperature. More signihcaut is the correction of the P,(2) line intensity for the Q,(4) line at 7311 A, which is indicated in the synthetic spectrum of Figure 7 but cannot be detected in the l-m spectrometer data due to its relative weakness. The increase in the temperature from this correction is about 6 K. s,’ was determined with three different calibrated low brightness sources, two derived from radioactive phosphorescence, and the other from a tungsten halogen bulb described by Dandekar and Davis (1973). Measurements of the photometer response to the three continuum sources agreed to within 15 per cent. The continuum radiance of the tungsten source, which resulted in the median value, was adopted for the absolute calibration. DATA

ACQUISITION

AND

ANALYSIS

Measurements of the OH rotational temperature following the technique described here were made

from the Michigan Airglow Observatory located at Ester Dome near College, Alaska (64.8 N lat., 148-O W long.) in the winter months from 12 February to 20 March 1974. The occurrence of aurora1 activity (monitored with a Na+ 4278 A filter) generally prevented continual data acquisition throughout the night, although in most cases measurements were possible for a duration of several hours after evening twilight. One night, 15 March, was geomagnetically quiet (BK, = 6) without aurora1 activity and data were obtained from 06 : 55 to 13 : 50 UT (20: 55-03 : 50 AST) with two gaps of 15 and 45 mm at 08 : 00 and 11: 30 UT. The measurements began and ended at a solar depression angle of -10 deg. The night sky on 15 March was very clear with no Moon so a zenith angle of 60’ (toward geomagnetic

1219

High latitude airglow observations on hydroxyl8-3 band

average

-Smoothed

I 2001

I 0x00

08:cO

15 March

temperature

11:oo

1o:oo

09:oo UT.

Smoothed

noise

I4:' 00

13:oo

12:oo

1974

FIG. 6. OH DATA FROM THE TILTING PHOTOMETER FOR 15 MARCH 1974. Shown are the rotational temperature and the intensity of the OH 8-3 band. The heavy line represents five point moving averages for both.

south) increased the column pathlength permitting a sampling rate of 35/hr with $5 K error. The background radiance at 17324 increased from 1.1 R/A at the zenith to 1.7 R/A, and the OH line column emission rate for P,(2) increased almost a factor of 2 to an average of 37 R. Aurora1 activity in the north was weak and did not move south to Ester Dome; the drizzle zone had virtually disappeared and the Ns+ band at 4278 A remained less than 4 R during the entire night. Between 1l:OO and 11:45 UT, measurements with the linear cam were made to check on the positioning of the filter passbands with the two position stepper cam. Figure 6 presents the 15 March data for both the OH rotational temperature and the OH 8-3 band intensity calculated from a model spectrum with the temperature and the P,(2) intensity as input values. The full band intensity permits a better comparison of the OH column emission and rotational temperature, as the weak dependence of the P,(2) individual component intensity on the rotational temperature is removed. The P,(2) intensity can be recovered if desired from the given band intensity, I(8-3), by evaluation of the expression Z[8-31 = (OX00011127”a

major and minor structure; large excursions at 09 : 00 and 11: 00 UT are particularly noticeable with -20 per cent enhancements in the 8-3 band intensity. The first maximum appears about 1) hr after astronomical twilight (18’ SDA). Numerous minor intensity variations were present ; in particular,those at 07:07,07:35 and lo:45 UT. A very striking train of small wavelike fluctuations between 12:05and 13:30UTfollowed thesecondmajor intensity maximum between 11 :OOand 12:OOUT. The periods between the five minor peaks in this sequence lie between 16 and 20 min, which is also typical of the other minor intensity enhancements. The amplitudes of these variations are between 2 and 4 per cent, Detrended

0

u

20

data

40

of 15 March,

60

60

Time,

min

11:45-3:45

UT

100

120

+ 0.04451 TR - 0.2876)113?,(2)] R. The statistical fluctuations in both variables are smoothed by five point running averages; this reduces the standard temperature deviation to about f 1 per cent (f2 K) at the expense of time resolution. The temperature and intensity data show both

FIG. 7. OH ROTATIONAL T~MPERAT~JRE AND BAND INTENSITY RJ2.WXJAI.SAFTER REMOVAL OF MEANS BY DETRENDINGTREATMENT.

J. W. MEW

1220

which exceeds the la standard deviation of 1 per cent for the Spoint running averages. In the period before dawn between 11: 45 and 13 : 45 UT, there is a good correlation between the intensity and temperature fluctuations. Figure 7 presents this data in the form of deviations from the temperature or intensity mean computed for a given data point from a linear least square fit to the set of 70 points in this period. Oscillations with a period of 16 min are strongly evident in the temperature data but not so obvious in the band intensity measurements. Cross correlation analysis between the intensity and temperature series indicates the presence of a 4 min lag with the intensity maxima leading the temperature maxima. SUMMARY

The use of a multichannel tilting filter photometer for the acquisition of mesospheric temperatures is described in detail, and OH temperature and the OH 8-3 band intensity data for 15 March 1974 are analyzed. Two features characterized this application of the tilting filter photometer: (1) the use of a stepping cam to shift the filter passband rapidly between background and line positions increasing the rate of data acquisition; (2) the sacrifice of throughput in the choice of the passband positioning to ensure the accurate measurement of the background intensity and to decrease the instrument’s sensitivity to ambient temperature shifts. The increase in the sampling rate by the more ethcient mode of data acquisition balances the decrease in the throughput by the lower filter transmission at the line position and by the small field of view necessary to avoid the spatial filtering of possible intensity variations by a large field of view. This latter feature explains why a grating spectrometer would not be an ideal instrument for this application bebecause a large field of view is required for the filling of the grating area to utilize the maximum throughput available. Measurements obtained indicate the presence of what might be called micro-fluctuations in the intensity and temperature measurements of the OH Meinel night airglow. The physical origin of these is most likely to be density modulations originating from internal gravity waves present in the high latitude winter mesosphere (Hines, 1972; Yeh and Liu, 1974). The lack of a substantial body of similar data to reinforce the findings of one night of observations confIned to a narrow altitude range in the mesosphere makes these results more suggestive rather than conclusive. Additional effort in observing the microscale fluctuations present in OH airglow emission with high temporal resolution

combined with observations collected on other airglow emission features, such as Na, 01(5577), or O,(lC), characteristic of the D region are definitively indicated. Furthermore, the results given here of the long term fluctuations, albeit for one night only, suggest that mesospheric processes in the winter high latitude atmosphere are more complicated than that of a more southern location and may be related to circulation effects induced by Joule heating in the aurora1 belt (Hays et aZ., 1973). Acknowledgements-1 would like to thank P. B. Hays of the University of Michigan for the loan of the tilting filter photometer (under NSF Grant 28690X2), permission to use both the Michigan Airglow Observatory at Ester Dome, Alaska, and the MAO filter calibration facility, and other support. I also thank I. L. Kofsky for his critical review of the manuscript and Ken Dick for his permission to use Fig. 1 from the Ann. Geophys. article and for his various suggestions regarding the technique. I thank the Geophysical Institute of the University of Alaska for permission to use the facilities of Ester Dome. This work was partially supported by Contract DNAOOl73-0027. REFERENCES

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High latitude airglow observations vibrational transition Mies, F. (1974). Calculated probabilities of OH(X%), J. Molec. Spectrosc. 53, 150. Peterson, A. W. and Kieffaber, L. M. (1973). Infrared photography of OH airglow structures. Nature, Land. 242, 321. Rogers, J. W., Murphy, R. E., Stair, A. T., Ulwick, J. C., Baker, K. D. and Jensen, L. L. (1973). Rocket-borne radiometric measurements of OH in the aurora1 zone. J. geophys. Res. 78,7023. Schaeffer, R. C. and Fastie, W. G. (1972). Tilting-filter measurements in dayglow rocket photometry. Appl. Opt. 11,2289. Sivjee, G. G., Dick, K. A. and Feldman, P. D. (1972). Temporal variations in nighttime hydroxyl rotational temperature. Planet. Space Sci. 20,261.

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Smith, W. S., Katchim, L. B. and Theon, J. S. (1968). Meteor. Monographs 9, 170. Takahashi, H., Clemesha, B. R. and Sahai, Y. (1974). Niahtalow OH (8, 3) band intensities and rotational tempeiature at 23%. -Planet. Space Sci. 22,1323. Valiance Jones, A. (1973). The infrared spectrum of the airglow. Space Sci. Rev. 15, 355. Valiance Jones, A. (1971). Aurora1 spectroscopy. Space Sci. Rev. 11.776. Webb, W. L..(1966). Structure of the stratosDhere and mesosphere, p. 42 Academic P&s, New Yolk. Wiens, R. H. (1974). Diurnal variation of the (8-3)/ (5-O) intensity ratio of nightglow OH at Adi Ugri, Planet. Space Sci. 22, 1059. Yeh, K. C. and Liu, C. H. (1974). Acoustic-gravity waves in the upper atmosphere. Rev. geophys. Space Phys. 12,193.