Solar EUV Dataset Description FILE_NAME = SOLAREUV.TAB RECORD_TYPE = FIXED_LENGTH FILE_RECORDS = 250 DATA_SET_ID = PVO-V-OETP-4-SOLAREUV-24HRAVG-V1.0 SPACECRAFT_NAME = PIONEER VENUS ORBITER INSTRUMENT_NAME = ELECTRON TEMPERATURE PROBE TARGET_NAME = VENUS START_TIME = 1978-12-05T00:00:00.000Z STOP_TIME = 1985-10-09T23:59:59.999Z START_ORBIT = 1 STOP_ORBIT = 2500 Solar EUV Daily Values This file gives the magnitude of the photoemission current from the radial probe, Ipe, (in units of 10-9 amps). Ipe dominates the ion current measurements outside the Venusian ionosphere, making possible the serendipitous measurement of the total solar EUV flux. The latter is an important parameter because solar EUV is the main source of ionization and heating for the Venusian thermosphere and ionosphere. The method is discussed by Brace et al., (1988). The pe current measurements are taken just before PVO leaves the solar wind and enters the magnetosheath (usually an hour or two before periapsis). This approach provides the solar EUV flux that the Venus thermosphere received just before the periapsis measurements. The maximum value of the spin modulated Ipe is taken because it corresponds to a probe orientation perpendicular to the Sun when the maximum area of the probe is exposed to the Sun. Ipe is proportional to the intensity of the ionizing component of solar radiation, so it is possible to derive the total solar EUV (and far UV) flux. Ly alpha contributes approximately half of the Ipe while nearly all of the rest is produced by radiation between 200 A and 1200 A which ionizes, excites and dissociates thermospheric neutrals. This file contains the daily average value of the photoelectron emission current, Ipe, from the radial probe, usually measured about 1 hr before periapsis. The Ipe values are given in units of 10-9 amperes. The data cover the interval from 1979 through early 1992 when periapsis got low enough to cause photoelectric yield changes that have not been fully resolved and corrected for appropriately. The data provided cover orbits 1 to 4800. After orbit 4800, when PVO began to enter the atmosphere, the Langmuir probe could no longer be kept clean, and as a result the yield changed For further details, contact Walt Hoegy at GSFC code 914, (301) 286-3837 or email hoegy@mite.gsfc.nasa.gov. The daily Ipe measurements can be converted into the total solar EUV flux (VEUV) using the following the equation given by Brace et al., (1988), VEUV = 1.53 x 1011 Ipe (photons/cm2/s) VEUV represents the total solar flux, weighted by the known wavelength- dependent yield of the collector. A standard Hinteregger solar EUV/UV spectrum is assumed to derive the coefficient, but the measurement is relatively insensitive to this assumption over the typical range of variations in the solar spectrum. The VEUV data have been useful in the study of solar EUV effects on the ion production and electron heating rates in the Venus ionosphere. VEUV variations have been correlated with changes in the density and temperature of the ionosphere (Elphic et al., 1984), the height of the bow shock (Alexander et al., 1985, Russell et al., 1988), and changes in the density and temperature of the thermosphere (Mahajan et al., 1990). DATA QUALITY/ACCURACY The Ipe measurements themselves are made with an absolute accuracy of 1 to 2%, depending upon where the current falls within the decade range of the ranging electrometer. The absolute accuracy of the measurements is also limited by our knowledge of the photoelectric yield of the radial probe collector, and our assumption that a Hinteregger standard EUV/UV spectrum is correct. We estimate a 10% absolute accuracy in the total EUV flux and a 1 to 2% relative accuracy or precision provided by the accuracy of the current measurements themselves. See Brace et al.(1988) for details of the method. The data file contains Venus Solar Flux information in the form of Ipe values in units of 10-9 amperes.. This file is formatted for printing and is self explanatory. The total solar EUV flux (VEUV)is derived by multiplying by the factor given in the equation presented earlier. Dates 78339-78348 Orbits 1- 10 ipe 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Dates 78349-78358 Orbits 11- 20 ipe 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Dates 78359-79003 Orbits 21- 30 ipe 0.00 0.00 0.00 0.00 10.30 0.00 0.00 0.00 0.00 10.30 Dates 79004-79013 Orbits 31- 40 ipe 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Dates 79014-79023 Orbits 41- 50 ipe 0.00 10.30 0.00 0.00 0.00 10.30 10.02 10.02 0.00 10.02 Dates 79024-79033 Orbits 51- 60 ipe 0.00 0.00 10.02 10.11 0.00 0.00 0.00 0.00 0.00 0.00 Dates 79034-79043 Orbits 61- 70 ipe 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 10.98 11.19 Dates 79044-79053 Orbits 71- 80 ipe 11.03 11.19 10.34 9.74 10.39 11.40 11.61 0.00 11.40 10.78 Dates 79054-79063 Orbits 81- 90 ipe 9.52 0.00 9.70 10.11 9.97 0.00 9.97 9.65 0.00 9.52 Dates 79064-79073 Orbits 91- 100 ipe 9.70 10.39 10.54 10.59 10.78 10.83 10.98 10.88 10.83 0.00 REFERENCES Alexander, C. J., C. T. Russell, Solar cycle dependence of the location of the Venus bow shock, Geophys. Res. Lett., 12, 369, 1985. Brace, L. H., W. T. Kasprzak, H. A. Taylor, Jr., R. F. Theis, C. T. Russell, A. Barnes, J. D. Mihalov, and D. M. Hunten, The ionotail of Venus: Its configuration and evidence for ion escape, J. Geophys. Res., 92, 15, 1987. Brace, L. H., W. R. Hoegy, and R. F. Theis, Solar EUV measurements at Venus based on photoelectron emission from the Pioneer Venus Langmuir probe, J. Geophys. Res., 93, 7282, 1988. Brace, L. H., R. F. Theis, and J. D. Mihalov, The Response of the Venus Nightside Ionosphere and Ionotail to Solar EUV and Solar Wind Dynamic Pressure, J. Geophys. Res., 95, 4075, 1990. Elphic, R.C., L. H. Brace, R. F. Theis, and C. T. Russell, Venus Dayside Ionosphere Conditions: Effects of magnetic field and solar EUV flux, Geophys. Res. Lett., 11, 124, 1984. Krehbiel, J. P., L. H. Brace, J. R. Cutler, W. H. Pinkus, and R. B. Kaplan, Pioneer Venus Orbiter Electron Temperature Probe, IEEE Transactions on Geoscience and Remote Sensing, GE-18, 49, 1980. Mahajan, K. K, W. T. Kasprzak, L. H. Brace, H. B. Niemann, and W. R. Hoegy, Response of the Venus Exospheric Temperature Measured by Neutral Mass Spectrometer to the Solar EUV Measured by Langmuir Probe on the Pioneer Venus Orbiter, J. Geophys. Res., 95, 1091, 1990. Russell, C. T., E. Chou, J. G. Luhmann, P. Gazis, L. H. Brace, and W. R. Hoegy, Solar and interplanetary control of the location of the Venus bow shock, J. Geophysic. Res., 93, 5461, 1988. Theis, R. F., L. H. Brace, K. H. Schatten, C. T. Russell, J. A. Slavin, J. A. Wolf, The Venus ionosphere as an obstacle to the solar wind, Advances in Space Research, 1, 47, 1980. Theis, R. F., L. H. Brace, R. C. Elphic, and H. G. Mayr, New empirical models of the electron temperature and density of the Venus ionosphere, with applications to transterminator flow, J. Geophys. Res., 89, 1477, 1984.