NOTES FOR USERS OF UNIFIED ABSTRACT FILE DATA FROM THE ULTRAVIOLET NITRIC OXIDE (UVNO) EXPERIMENT ON ATMOSPHERE EXPLORER C (AE-C) A. EXPERIMENT RATIONALE The purpose of the UVNO experiment is to measure the distribution of, and variations in, nitric oxide molecules in the altitude range 100-250 km. Nitric oxide is produced in this region by the reaction of N(4S) and N(2D) with O2 and is removed by reaction with N(4S) or, less importantly, by diffusion. The atomic nitrogen is produced by the dissociation of N2 following the absorption of EUV sunlight by the atmosphere. The distribution of NO depends on temperature (through the strong temperature dependence of the reaction between N(4S) and O2), on composition (through the competition between the reaction between N(2D) and O2 and the quenching of N(2D) by O), and on transport processes. B. INSTRUMENT SUMMARY The UVNO instrument is a fixed-grating Ebert-Fastie spectrometer which accepts light in a 15A bandpass centered on the (1,0) Gamma band of NO at 2149A. It measures sunlight which has been resonantly scattered in this band by ambient NO molecules. The detector is a photomultiplier with a sapphire window and a CsTe cathode; its output is fed into pulse-counting electronics. The line of sight is perpendicular to the S/C spin axis, which in turn is parallel to the orbit normal vector. The field of view is 4 x l/4 (degrees), with the long axis parallel to the S/C spin axis and therefore parallel to the viewed limb. On despun orbits the instrument looks perpendicular to the line joining the center of the earth and the S/C, in the wake direction. On spinning orbits, the instrument scans the limb of the Earth once per spin. Its sampling time is 21 msec, giving an angular smearing (at 4 rpm) of 1/2 degree. C. DATA WORDS UVNO entries in the AE-C Unified Abstract File consist of 5 words per 15 sec: #67 - H(SC) The NO scale height at the S/C altitude (km) #68 - NO(SC) The NO density at the S/C altitude (/cm**3) #69 - NO(150) The NO density at 150 km (/cm**3) #70 - NO(M) The maximum NO density observed during a given limb scan (/cm**3) #71 - Z(M) The altitude at which this maximum occurs (km) See Section I for more details. D. DATA CATALOG Attached is a catalog of those February 1974 orbits for which UVNO data is available. On most orbits measurements exist for S/C altitudes less than about 250 km. On spinning orbits, measurements of NO at 150 km may exist at S/C altitudes up to 350 km. E. CALIBRATION The instrument was calibrated before launch in the laboratory. A diffusing screen was illuminated by light from a Xenon lamp which had been passed through a monochromator. The screen was then measured by the instrument and by a field-limited standard phototube. In this way the sensitivity to an extended source was measured as a function of wavelength. In flight, the detector sensitivity was periodically monitored using an internal lamp. No direct monitoring of the reflectivity of the optics was performed, but no systematic changes in the signal levels produced by viewing the Rayleigh scattering atmosphere were observed during the period between launch (in December 1973) and February 1974. F. RELIABILITY OF DATA 1. Despun orbits. On despun orbits, the accuracy of UVNO results depends on the success of the background subtraction technique (see Section I), but more importantly on the accuracy of the S/C altitude information. On an unknown number of orbits, the instrument pointing was subject to about 2 1/2 degrees error (due to an ambiguity in the S/C ACS system); on these orbits, the "true" NO density would be about 60% larger than those entered in the UAF. A list of orbits on which this problem was known to occur is attached. (The UAF entries have not been adjusted). 2. Spinning orbits. No attitude errors exist because the UVNO detection of the limbs of the earth provided pointing information to a very high accuracy. However, the curve-fitting algorithm used to analyze spinning-orbit data (section 1) strongly weights the low-altitude, strong-signal region at the expense of the spacecraft-altitude, weak-signal region; so the reported NO density at S/C altitude, and the NO scale height (words 68 and 67) are not reliable. G. EVENTS AFFECTING DATA Except as noted in (F) above, the UVNO entries in the UAF have already been screened for spurious or misleading results. H. ANOMALIES IN DATA See (G) above. I. DATA REDUCTION 1. Noise removal. The cosmic-ray and energetic particle environment of the experiment in space produced obvious spikes in about 20% of the samples. These were removed by rejecting samples lying above a 2*sigma limit calculated from 5 neighboring samples. The remainder of the spurious counts were assumed to be random, and were removed (along with the genuine "dark counts") by background subtraction techniques. 2. Temperature and optical thickness correction. The response of the instrument to a slant column containing N mols/cm**2 at some temperature T is C = INTEGRAL(S*g dN) counts/21msec N=0 to N where g is the number of solar photons scattered per molecule per second, and S is the instrument sensitivity. A theoretical study of the scattering and detection processes, using the measured instrument slit-function, was performed. The results can be approximated by S*g = P/(Q+N) where P = 10*(70+T) Q = P*5.7E12*(1+T**2/3.8E6) Thus C = P*ln(1+N/Q) or, rearranging, N = Q*[exp(C/P)-1] (1) 3. Despun orbits. For despun orbits, the data was averaged in 15 second periods after noise removal. The background count level was obtained from measuruments at high altitudes on downleg and upleg, and subtracted; the remaining NO signal was rejected if it was less than half the subtracted background. NO was assumed to be distributed according to NO(Z) = NOsc * exp{ -(Z-Zsc)/Hsc } where Hsc was chosen to be 33 km (representative of actual values). Then N = NOsc * Hsc * Ch(Re+Zsc,H,THETA) (2) in which Ch(R,H,THETA) is the Chapman function and THETA (the look angle measured from the zenith) is obtained from S/C attitude data. NOsc is then obtained from (1) and (2). T was obtained from the formula T = 900 - 550*exp{ -(Zsc-120)/50 } (3) The error introduced by assuming this value of T rather than a measured value is small compared to other uncertainties. The parameters NO150, NOm and Zm were not obtained on despun orbits. 4. Spinning orbits. On spinning orbits the background count level was measured over a 30 degree arc near the zenith, and subtracted. At altitudes below Zsc, NO was assumed to form a layer given by NO(Z) = 2*NOm*[exp{-(Z-Zm)/Hsc} - exp{-2*(Z-Zm)/Hsc}/2] = 2*NOm*[b*x - (b*x)**2/2] (4) in which: NOm, Hsc, and Zm are to be determined; x = exp{-(Z-Zo)/Hsc} where Zo= 90 km b = exp{(Zm-Zo)/H) = 1/xm The observed slant column density is then N(Zk) = 2*NOm * sqrt{2*pi*RH} * [b*x - (b*x)**2 / 2*sqrt(2)] (5) where Zk is now the look altitude (the lowest altitude reached by the instrument line-of-sight). From (1) and (5) we have exp(C/P) = 1+ 1/Q * 2*NOm * sqrt{2*pi*RH} * [b*x - (b*x)**2 / 2*sqrt(2)] (6) The instrument counts C at each look altitude Zk sampled during the limb-scan were fittted to the expression exp(C/P) = 1 + A*x - B*x**2 by a least squares process in which Hsc was varied. Then the parameters NOm and Zm were readily obtained from A and B, using (6). The exponential expression (3) for T was used (with Zsc replaced by Zk) above 120 km, and replaced by T = 175 + 175 * [(Zk-85)/35]**2 for values of Zk less than 120 km. The actual curve-fitting process was more complex than implied above. It assumed that NO = 0 wherever expression (4) is negative, and it also included an iterative correction for Rayleigh scattering contamination of the signal below 100 km. The parameters NOsc and NO150 were obtained from expression (4) using the derived values of NOm, Hsc and Zm. At altitudes so high (>200 km) that the above fitting process failed due to poor altitude resolution, it was still possible to find NO150 by assuming an exponential distribution of NO and analyzing only the data for which Zk lay between 120 and 180 km. This procedure generally failed above about 350 km. USEFUL REFERENCES A. Dalgarno, W.B. Hanson, N.W. Spencer, and E.M. Schmerling, "The Atmosphere Explorer Mission," Radio Sci., 8, 263-266, 1973 . C.A. Barth, D.W. Ruseh and A. I. Stewart, "The UVNO Experiment for Atmosphere Explorer," Radio Sci., 8, 279-386, 1973.