The follow is ASCII text version of a few sections from the following document. The Dayside Venus Ionosphere I. Pioneer-Venus Retarding Potential Analyzer Experimental Observations KENT L. MILLER,* WILLIAM C. KNUDSEN,* AND KARL SPENNER** *Lockheed Palo Alto Research Laboratory, Department 52-12, Building 255, 3251 Hanover Street, Palo Alto California 94304, and **Institut fur Physikalische Messtechnik, D78 Freiburg, BRD, Heidenhofstrass 8, West Germany Received August 23, 1983; revised January 2, 1984 These sections have been provided in order to describe some of the problems associated with the intercalibration of the charged particle experiments aboard the PVO spacecraft. ================================================================================ ================================================================================ DISCUSSION OF RESULTS Total ion density measured by the RPA is FIG. 9. Median profiles of the particle pressure nik(T{e} + T{i}) within the 65 +/- 5 deg. and 25 + 5 deg. SZA intervals. Medians are medians of the local product and not the product of the medians. The altitude scales for the two different SZA intervals are different. 395 an independently measured quantity and is not the numerical sum of constituent densi- ties. It is derived from the saturated ion current measured by the RPA (Knudsen et al., 1979a) and the normal component of the bulk ion velocity relative to the RPA surface grid. The constituent densities are derived from differences in ion current measured over an appropriate retarding po- tential interval. By subtracting the sum of the measured constituents from N{i}, the to- tal concentration of unmeasured constitu- ents can be estimated. The RPA does not have the mass resolu- tion of an ion mass spectrometer (IMS), and does not, for example, resolve the atomic ions O+, N+, and C+ into separate peaks. We have relied on information sup- plied by the P-V ion mass spectrometer to verify that O+ is the dominant ion of the group, and also to estimate the ratio of ([C+] + [N+])/[O+] (~0.07) (Taylor et al., 1979a). Use of this ratio makes a 7% differ- ence in the value of [O+] and also improves the ion temperature derived by the RPA. Otherwise, the values reported in this study are derived independently of IMS results. We have made the assumption in fitting the theoretical values of current differences to measured differences (Knudsen et al., 1979a) that the ratio ([N+] + [C+])/[O+] is constant and has the value 0.07. Therefore, the sum ([O+] + [N+] + [C+]) measured by the RPA is a factor of 1.07 times the value of [O+] given in Figs. 6 and 7 and Tables I and II. Since at higher altitudes in Fig. 7 the ratio [O+]/N{i} is approximately 0.86, the ra- tio ([O+] + [N+] + [C+])/N{i} is ~0.93 and all other ions including H+, D+, He+, O^2+, O{2}+, NO+, N{2}+, and CO{2}+ (Taylor et al., 1979a; 1980) are approximately 7~ of the total. The RPA does not resolve the ion masses 32(O{2}+), 30(NO+), and 28(CO+ + N{2}+) into separate peaks. However, at the low alti- tude at which these ions become signifi- cant, T{i} is relatively small and the single, measured peak is clearly distorted by the presence of more than one ion mass. There- fore, we have assumed in our least-squares analysis that the peak is composed of two ions with masses 32 amu (O{2}+) and 29 amu (NO+ + N{2}+ + CO+) and have let the least squares fitting program assign separate con- centrations to these two constituents as re- quired to minimize the variance of the fit. The separate densities and their sum have been presented in Figs. 6 and 7. We esti- mate that [O{2}+ + M29+] is measured to an accuracy of approximately 5% below 200 km altitude but make no estimate for the accuracy of [O{2}+] and [M29+] separately. It is gratifying that our results are qualita- tively similar to those of the IMS (Taylor et al., 1980). The reported IMS results indi- cate that above approximately 220 km alti- tude [O{2}+] and [M29+] are approximately equal in density and that below 220 km alti- tude O{2}+ clearly becomes the dominant con- stituent. At ~160 km altitude our results yield a value of approximately 5 for the ra- tio [O{2}+]/[M29+] whereas the ratio reported by Taylor et al. (1980) is more like 10- 15. The shape and magnitude of the [O{2}+] profiles presented in Figs. 6 and 7 differ from those reported by the IMS at low alti- tudes in both absolute magnitude and shape (Taylor et al., 1979a, 1980). The IMS den- sity is a factor of approximately 2 larger than that measured by the RPA and shows a peak near 175 km altitude. Similarly, the values of [CO{2}+] reported by the IMS are ap- proximately a factor of 2-3 times larger than the RPA [CO{2}+] results. The values of [O+] reported by the IMS and RPA are ap- proximately equal in the dayside iono- sphere. In the next section we discuss the accuracy of the RPA densities. 396 ACCURACY We have estimated the accuracy with which the RPA measures N{i}, [O+], [O{2}+ + M29+], [CO{2}+], and T{i} as approximately 5% and the accuracy of T{e} as approximately 10% (Knudsen et al., 1979a). These accura- cies apply when the concentrations are well within the design range of the RPA. The accuracies are estimated from accuracies with which the RPA electrometer sensitiv- ity, grid transparencies, and instrument di- mensions are measured and are not esti- mated from calibration of the RPA against a standard instrument in a laboratory plasma chamber. Because of its simple and open geometrical configuration, we consider it a FIG. 15. Solar longitude variation of normalized T{e}. 403 good standard instrument. There is also the question of how well a laboratory plasma approximates the natural conditions on or- bit, were a laboratory standard instrument more accurate than the RPA available. In Fig. 18 we have plotted the median plasma density derived from the Pioneer- Venus RPA, IMS, Langmuir probe (LP), and radio occultation (RO) experiments. The median RPA profile was constructed as explained heretofore from all RPA data re- corded in the first 780 orbits. The IMS me- dian profile was derived from the IMS data deposited in the Pioneer-Venus low-fre- quency unified abstract data system (UADS) for the first 780 orbits. Experimen- tal data are interpolated to common univer- sal times (UT) at 12-sec intervals through- out each periapsis pass by the P-V experimenters and stored in UADS. We have summed the IMS component densities at each UT to get total plasma density, col- lected all the data falling with 65 + 5 deg. SZA, ordered the data in altitude, and taken me- dians of successive groups of 11. The elec- tron density reported by the Langmuir probe experiment N{e,LP} has been treated similarly except that summing constituent densities was unnecessary. The radio oc- cultation experiment measures N{e,RO} re- motely and does not report density in UADS. The median {Ne,RO} profile was de- rived by taking all occultation profiles which fell within a small SZA interval cen- tered close to that used for the in situ exper- iments. Eight profiles fell within the inter- val 58 + 4 deg. These profiles were obtained over a period of approximately two Earth FIG. 16. Solar longitude and latitude variation of normalized particle pressure. Below 600 km altitude the normalized particle pressure exhibits an enhanced value at the dusk terminator over that at the dawn terminator reflecting the enhanced value of Ni. 404 years. A "median" profile was found from these eight profiles by assigning the log N{e,RO} of the median value midway between the log N{e} values of the middle two profiles at each altitude. The temporal variance bars for N{e,RO} extend from the second to the seventh profile sequenced by magnitude at each altitude. Three-fourths of the profiles lie within the variation range. The IMS and LP data were taken from the UADS data available as of February 1982. By common agreement among P-V experiments the UADS files may be up- dated. L. Brace (private communication) has updated his files since February 1982 which might change the N{e,LP} median curve were it to be redone using his more recent (but unavailable at the time of this study) data. We infer from the data in Fig. 18 that the accuracy of the RPA total density measure- ment is within the approximately +/- 5% claimed. All four experiments are in satis- factory agreement above 200 km altitude where O+ is approximately 90% of the total ion density (Fig. 7). Below 200 km altitude the total ion density n{i,IMS} measured by the IMS systematically departs from the other three and reaches a factor of approximately 2.5 larger than that reported by the other experiments at an altitude of ~165 km. The factor then decreases toward 160 km alti- tude, below which IMS data are not re- ported in UADS. From a comparative study of the O+, O{2}+, and CO{2}+ densities reported in UADS by the IMS and RPA for the dayside hemisphere, we have found that the IMS O+ densities are reasonably consistent with those of the RPA, but that both the IMS O{2+ and CO{2}+ densities are on the order of 2.5 times too large. The O{2+ ion is the dominant ion below an altitude of ap- FIG. 17. Solar longitude and latitude values of the solar radio flux F{10.7}. 405 proximately 180 km, as revealed by both the IMS (Taylor et al., 1980) and the RPA (this study; Knudsen et al., 1979b). In Fig. 19, contours of median values of the ratio (n{i,1} + n{i,2)/N{i are plotted as a function of altitude and solar zenith angle. The quantities n{i,1} and n{i,2} are the ion densi- ties derived from the two major ion peaks telemetered to Earth in the most frequently used RPA mode of operation (Knudsen et al., 1979a). Above 250 km altitude in the dayside ionosphere (SZA < 90 deg) only one peak was usually detected corresponding to the ion group O+ + N+ + C+. This group had a median number density equal to ap- proximately 93% of N{i} which is consistent with the composition reported by the IMS. At the lowest altitude, the major ion peak (n{i,2}) became [O{2}+ + M29+] with n{i,1} being ei- ther [O+ + N+ + C+] or [CO{2}+]. The RPA measurements indicate [O{2}+ + M29+] is ap- proximately 90% of N{i} which is close to that (~95%) indicated by the IMS (Taylor et al., 1980). We conclude that since N{i}, [O+], and [O{2}+ + M29+] are essentially independent measurements and N{i} is measured to ap- proximately 5% (Fig. 18), [O+] (=0.93 x [O+ + N+ + C+ ]) and [O{2}+ + M29+] measured FIG. 18. Vertical profiles of median plasma density measured by several Pioneer-Venus experiments in the same solar zenith angle interval. 406 by the RPA are also accurate to within ap- proximately 5% whenever they individually are a substantial fraction of N{i}. The principles of operation and the on- board peak selection criteria of the RPA are such that the RPA is limited in its ability to detect a minor ion of small fractional con- centration (Knudsen et al., 1979a). The limit of detectability depends on the ion composition, ion temperature, small-scale ion density irregularity magnitude, and in- strument noise level. Prior to the P-V launch, we conducted a very realistic nu- merical simulation of the RPA operation in an ionosphere with an irregularity level of 0.5% and temperature of 500 deg. K and in which the ratio [O+]/[O{2}+] was varied. We found that when the [O+]/[O{2}+] ratio dropped below the range 5-10%, the O+ peak was not detected consistently because of the RPA restrictive peak selection crite- ria. When the O+ peak was detected, the accuracy of the derived concentration was not significantly degraded below that ob- tained when O+ was the dominant ion. Mi- nor ions with mass larger than that of the major ion such as O{2}+ and CO{2}+ in O+ or O+ in H+ are detectable at smaller fractional concentrations. Thus, in Figs. 2 and 3, the O+ curve terminates at an altitude below which the ratio [O+]/[O{2}+ + M29+] drops beneath approximately 5%. On the other hand, CO{2}+ and O{2}+ + M29+ were detect- able at altitudes at which their fractional concentrations were as small as 0.1 and 1.0%, respectively. However, the percent- age of RPA sweeps in which the minor ions were detected decreased significantly as their respective lower limits was ap- proached. Because the detectability of a minor ion increases with increasing fractional den- sity, the possibility exists that the median densities reported in this paper for O+, O{2}+ + M29+, and CO{2}+ may not accurately rep- resent the true median in the altitude range FIG. 19. Contours of the ratio (n{i,1} + n{i,2})/N{i}, where n{i,1} and n{i,2} are the ion concentrations corresponding to the two major ion peaks measured by the RPA. 407 corresponding to their minimum detectabil- ity. That is, if the actual density of the mi- nor ion is varying temporally, the RPA would more frequently detect the ion when its density was greater than the true median value. This statement applies only near the limit of detectability. The possibility exists, therefore, that the measured medians are larger than the true medians at altitudes where a minor ion density is near the limit of its detectability. From a comparison of RPA medians with IMS medians derived from UADS data, we believe a significant bias does not exist in our medians near the limits of their detectability. Due allowance was made in the comparison for the system- atic difference in IMS sensitivity described above. Any bias that may be present is probably small in comparison with the vari- ation range indicated. ACKNOWLEDGMENTS We thank Dr. Arvydas J. Kliore for supplying the radio occultation profiles required to make the median N{e,RO} profile in Fig. 13. We also thank H. A. Taylor, Jr., and L. H. Brace for use of their UADS data in constructing the median N{e,LP} and Sigma(n{i,FMS}) profiles in the same figure. We are grateful to Tom Canty for his assistance in organizing and plotting the data. Support for K. L. Miller and W. C. Knudsen was provided by NASA Contract NAS2-9481 and Lockheed Indepen- dent Research Funds. REFERENCES BRACE, L. H., H. A. TAYLOR, JR., T. I. GOMBOSI, A. J. KLIORE, W. C. KNUDSEN, AND A. F. NAGY (1983). The ionosphere of Venus: Observations and their interpretations. In Venus (D. M. Hunten, L. Colin, T. M. Donahue, and V. 1. Moroz, Eds.). pp. 779-840. Univ. of Arizona Press, Tucson. BRACE, L. H., R. F. THEIS, W. R. HOEGY, J. H. WOLFE, J. D. MIHALOV, C. T. RUSSELL, R. C. ELPHIC, AND A. F. NAGY (1980). The dynamic be- havior of the Venus ionosphere in response to solar wind interactions. J. Geophys. Res. 85, 7663-7678. CLOUTIER, P. A., T. F. TASCIONE, R. E. DANIELL, JR., H. A. TAYLOR, JR., AND R. S. WOLFF (1983). Physics of the interaction of the solar wind with the ionosphere of Venus: Flow/field models. In Venus (D. M. Hunten, L. Colin, T. M. Donahue, and V. I. Moroz, Eds.), pp. 941-979. Univ. of Arizona Press, Tucson . COLIN, L. (1980). The Pioneer-Venus program. J. Geophys. Res. 8S, 7575-7598. CRAVENS, T. E., A. J. KLIORE, J. U. KOZYRA, AND A. F. NAGY (1981). The ionospheric peak on the Venus dayside . J. Geophys. Res . 86, 1 1323- 11329. KEATING, G. M., J. Y. NICHOLSON 111, AND L. R. LAKE (1980). Venus upper atmosphere structure. J. Geophys. Res. 85, 7941-7956. KNUDSEN, W. C., J. C. BAKKE, K. SPENNER, AND V. NOVAK (1979a). Retarding potential analyzer for the Pioneer-Venus orbiter mission. Space Sci. Instrum. 4, 351-372. KNUDSEN, W. C., K. SPENNER, R. C. WHITTEN, J. R. SPREITER, K. L. MILLER, AND V. NOVAK (1979b). Thermal structure and major ion composition of the Venus ionosphere: First RPA results from Venus orbiter. Science 203, 757-759. KNUDSEN, W. C., K. SPENNER, K. L. MILLER, AND V. NOVAK (1980a). Transport of ionospheric O+ ions across the Venus terminator. J. Geophys. Res. 85, 7803-7810. KNUDSEN, W. C., K. SPENNER, R. C. WHITTEN, AND K. L. MILLER (1980b). Ion energetics in the Venus nightside ionosphere. Geophys. Res. Lett. 7, 1045- 1048. KNUDSEN, W. C., K. L. MILLER, AND K. SPENNER (1982). Improved Venus ionopause altitude calcula- 408 tion and comparison with measurement. J. Geophys. Res. 87, 2246-2254. LUHMANN, J. G., R. C. ELPHIC, AND C. T. RUSSELL (1980). Observations of large scale steady magnetic fields in the dayside Venus ionosphere. Geophys. Res. Lett. 7, 917-920. MAYR, H. G., L HARRIS, H. B. NIEMANN, H. C. BRINTON, N. W. SPENCER, H. A. TAYLOR, JR., R. E. HARTLE, W. R. HOEGY, AND D. M. HUNTEN (1980). Dynamic properties of the thermosphere in- ferred from Pioneer-Venus mass spectrometer mea- surements. J. Geophys. Res. 85, 7481-7487. MIHALOV, J. D., J. H. WOLFE, AND D. S. INTRILIGA TOR (1980). Pioneer-Venus observations of the solar wind Venus interaction. J. Geophys. Res. 85, 7613- 7624. MILLER, K. L., AND W. C. KNUDSEN (1982a). Super- rotation of the Venus ionosphere. EOS 63, 369. MILLER, K. L., AND W. C. KNUDSEN (1982b). Further evidence for an ionospheric superrotation velocity component on Venus. EOS 63, 1017. MILLER, K. L., W. C. KNUDSEN, K. SPENNER, R. C. WHITTEN, AND V. NOVAK (1980). Solar zenith angle dependence of ionosphere ion and electron tempera- ture and densities on Venus. J. Geophys. Res. 85, 7759-7764. NIEMANN, H. P., W T. KASPRZAK, A. E. HEDIN, D. M. HUNTEN, AND N W. SPENCER (1980). Mass spectrometric measurements of the neutral gas com- position of the thermosphere and exosphere of Ve- nus. J. Geophys. Res. 85, 7817-7828. SCHUBERT, G., C. COVEY, A. DEL GENIO, L. S. ELSON, G. KEATING, A. SEIFF, R. E. YOUNG, J. APT, C. C. COUNSELMAN, III, A. J. KLIORE, S. S. LIMAYE, H. E. REVERCOMB, L. A. SROMOVSKY, V. E. SUOMI, F. TAYLOR, R. WOO, AND U. VON ZAHN (1980). Structure and circulation of the Venus atmo- sphere. J. Geophys. Res. 85, 8007-8025. SHIMAZAKI, T., R. C. WHITTEN, H. T. WOODWARD, W. C. KNUDSEN, AND K. L. MILLER (1984). The steady state dayside Venus ionosphere. II. Numeri- cal model interpretation and implications for the neutral atmosphere. Submitted for publication. SPREITER, J. R., AND S. S. STAHARA (1980). Solar wind flow past Venus: Theory and comparisons. J. Geophys. Res. 85, 7715-7738. TAYLOR, H. A., JR., H. C. BRINTON, S. J. BAUER, R. E. HARTLE, T. M. DONAHUE, P. A. CLOUTIER, F. C. MICHEL, R. E. DANIELL, JR., AND B. H. BLACK- WELL (1979a). Ionosphere of Venus: First observa- tions of the dayside ion composition near dawn and dusk. Science 203, 752-203. TAYLOR, H. A., JR., H. C. BRINTON, S. J. BAUER, R. E. HARTLE, P. A. CLOUTIER, R. E. DANIELL, IR., AND THOMAS M. DONAHUE (1979b). Ionosphere of Venus: First observations of day-night variations of the ion composition. Science 205, 96-99. TAYLOR, H. A., H. C. BRINTON, S. J. BAUER, R. E. HARTLE, P. A. CLOUTIER, AND R. E. DANIEI L (1980). Global observations of the composition and dynamics of the ionosphere of Venus: Implications for the solar wind interaction. J. Geophys. Res. 85, 7765-7777. WHITTEN, R. C., A. SEIFF, B. BALDWIN, W. C. KNUDSEN, AND K. L. MILLER (1984). Asymmetries in the dayside atmosphere and ionosphere of Venus. Submitted for publication. 409