Pioneer Venus Orbiter(PVO) Electron Temperature Probe (OETP) Data Bundle
Pioneer Venus Orbiter (PVO) Electron Temperature Probe (OETP) Solar EUV Data Description
PDS3_DATA_SET_ID = PVO-V-OETP-5-SOLAREUV-24HRAVG-V1.0
PDS3_DATA_SET_RELEASE_DATE = 1993-10-01
START_TIME = 1978-12-05T00:00:00.000
STOP_TIME = 1992-01-18T23:59:59.999
PRODUCER_FULL_NAME = DR. LARRY BRACE, DR. ROBERT F. THEIS
Collection Overview
===================
This collection contains the Solar EUV Daily Values File. 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.
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).
Confidence Level Overview
=========================
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.
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., Global Structure of Ionosphere Temperature, in Space Research X. Amsterdam,
The Netherlands: North-Holland, p.633, 1970.
Brace, L.H., 'Orbiter Electron Temperature Probe', L. Colin and D.M. Hunten, Eds. Space
Sci. Rev., Vol.20, No.4, p.454, June 1977.
Brace, L.H., R.F. Theis, and A. Dalgarno, 'The Cylindrical Electrostatic Probes for
Atmosphere Explorer -C, -D, -E', Science, Vol.8, No.4, p.341, Apr.1973.
Brace, L.H., R.F. Theis, J.P. Krehbiel, A.F. Nagy, T.M. Donahue, M.E. McElroy, and A.
Pedersen, 'Electron Temperatures and Densities in the Venus Ionosphere', Science,
Vol.203, p.763, Feb.23, 1979.
Brace, L.H., H.A. Taylor Jr., P.A. Cloutier, R.E. Daniell Jr., and A.F. Nagy, 'On the
Configuration of the Nightside Venus Ionopause', Geophys. Res. Lett., Vol.6,
p.345, 1979.
Brace, L.H., R.F. Theis, H.B. Niemann, H.G. Mayr, W.R. Hoegy, and A.F. Nagy, 'Empirical
Models of the Electron Temperature and Density in the Nightside Venus Ionosphere',
Science, Vol.205, p.102, 1979.
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.
Colin, L., Pioneer Venus Overview, IEEE Transactions on Geoscience and Remote Sensing,
Vol GE-18, No. 1, pp. 5-10, 1980.
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.
Fimmel R.O., Colin L., and Burgess E., 'Pioneering Venus: A Planet Unveiled',
NASA SP-518, 1995.
Hoegy, W.R., and L.E. Wharton, 'Current to Moving Spherical and Cylindrical Electrostatic
Probes', J. Appl. Phys., Vol.44, No.12, p.5365-5371, 1973.
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.
Langmuir, I. and H. Mott-Smith, Jr., 'Studies of the Electric Discharges in Gases at Low
Pressures', Gen. Elec. Rev., p.616, Sept.1924.
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.
Mott-Smith, H. and I. Langmuir, 'The Theory of Collectors in Gaseous Discharges', Phys.
Rev., Vol.28, pp.727-763, 1926.
Nothwang, G.T., Pioneer Venus Spacecraft Design and Operation, IEEE Transactions on
Geoscience and Remote Sensing, Vol GE-18, No. 1, pp. 5-10, January 1980.
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.
Shai, M.C., 'Formulation of Electrically Conductive Thermal-Control Coatings ', NASA Tech.
Paper 1218, Apr.1978.
Smith, D., 'The Application of Langmuir Probes to the Measurement of Very Low Electron
Temperatures', Planet. Space Sci., Vol.20, p.1721, 1972
Spencer, N.W., L.H. Brace, G.R. Carignan, D.R. Taeusch, and H.B. Niemann, 'Electron and
Molecular Nitrogen Temperature and Density in the Thermosphere', J. Geophys. Res.,
Vol.70, pp.2665-2698, 1965.
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.
Yang, L., 'Preparation and Evaluation of CVD Rhenium Thermionic Emitters', in The Third
Annual Conference on Chemical Vapor Deposition, F.A. Glaski, Ed., American Nuclear
Society, 1972.
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