Data Set Information
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| DATA_SET_NAME |
PVO VENUS ELECT TEMP PROBE CALIB HIGH RES ELECTRONS VER 1.0
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| DATA_SET_ID |
PVO-V-OETP-3-HIRESELECTRONS-V1.0
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| NSSDC_DATA_SET_ID |
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| DATA_SET_TERSE_DESCRIPTION |
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| DATA_SET_DESCRIPTION |
The High Resolution Ne File. These data are based on measurements ofthe electron saturation current or the ion saturation current taken from asmany voltampere curves as the telemetry data rate permitted. Since Ne isassumed equal to Ni everywhere in the ionosphere, either can be used as ameasure of Ne. The ion current is used at high densities (Ni > 4x104cm-3)and the electron current is used at lower densities. Typically, 4 to 8 highresolution density samples are obtained in the interval between recoveredvoltampere curves, although this ratio is bit rate dependent. This providesNe and Ni measurements at much smaller intervals than is possible fromthe voltampere curves themselves. High resolution measurements aretypically available at 2 to 8 second intervals depending upon the telemetryrate available to the OETP at the time.The High Resolution file provides measurements of Ne (or Ni) within 30minutes either side of periapsis at somewhat higher resolution than ispossible from the voltampere curves. However, these measurements areless accurate when the spacecraft is outside the ionosphere, where Ne istypically well below 100 cm-3. In sunlight, spacecraft photoelectrondensities at the radial probe location are of the order of 30-50 cm-3. Indarkness, the Ne measurements can be made down to densities of about 2cm-3 because the pe background is absent. However, the measurementsmade in the Venus umbra are often degraded at low densities because ofthe presence of hot electrons that charge the spacecraft to potentials thatlie beyond the range of the OETP sweep voltage. This makes it impossibleto drive the probe positive with respect to the plasma potential. Inaddition, deBye shielding causes the probe to become enveloped in the ionsheath of the spacecraft at very low densities, further reducing its accessto the ambient ionospheric plasma. Empirically derived corrections forthis effect have been applied to the high resolution data in order toprovide at least a lower limit of Ne, but the errors could exceed a factor of2 at densities below 10 cm-3. This correction does not allow Ne to be lessthan 2 cm-3. When the electron current at maximum positive voltage isless than a certain very low value an Ne value of 2 cm-3 is entered in theHigh Resolution file simply to serve as an upper limit on Ne, and to showthat data were actually being taken.In summary, the high resolution Ne measurements provide about a factorof 8 higher resolution than the UADS file whose resolution is limited bythe recovery rate of raw voltampere curves. Therefore the high resolutiondata better resolve such small scale features as the ionopause and theplasma clouds often found above the ionopause. Also, the UADS densitiesoften stop somewhere within the ionopause density gradient, so thisfeature can best be resolved using the High Resolution data. However,certain artifacts have not been removed from the data, so one must becareful not of over-interpret them. (See section on accuracy)
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| DATA_SET_RELEASE_DATE |
1993-10-01T00:00:00.000Z
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| START_TIME |
1978-12-05T01:31:29.898Z
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| STOP_TIME |
1992-10-07T08:11:28.102Z
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| MISSION_NAME |
PIONEER VENUS
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| MISSION_START_DATE |
1968-06-01T12:00:00.000Z
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| MISSION_STOP_DATE |
1992-10-07T12:00:00.000Z
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| TARGET_NAME |
VENUS
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| TARGET_TYPE |
PLANET
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| INSTRUMENT_HOST_ID |
PVO
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| INSTRUMENT_NAME |
ORBITER NEUTRAL MASS SPECTROMETER
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| INSTRUMENT_ID |
ONMS
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| INSTRUMENT_TYPE |
QUADRUPOLE MASS SPECTROMETER
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| NODE_NAME |
planetary plasma interactions
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| ARCHIVE_STATUS |
ARCHIVED
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| CONFIDENCE_LEVEL_NOTE |
In general, the high resolution Ne measurements have a lower absoluteaccuracy than the UADS (voltampere curve) measurements becausefactors such as the spacecraft potential and Te are not available tocalculate Ne more precisely. To reduce such errors in the high resolutiondata, they are normalized to the voltampere curve measurements. Thisnormalization is entirely different from the Ne-Ni normalization employedin deriving the UADS data.Another source of error in the High Resolution Ne measurements is thejump discontinuities that occur when the spacecraft passes from sunlightto shadow. An abrupt change in spacecraft potential occurs at that point,and this changes the probe voltage which is referenced to the spacecraft.The Ne measurements cannot easily be corrected for this change becausethey are not based on voltampere curves but measurements at a fixedpositive potential. Therefore a discontinuity may occur in Ne at thesunlight-shadow boundary if Ne is sufficiently low that spacecraftphotoelectron emission affects the spacecraft potential.The precision of the high resolution data is probably somewhat better thanthat of the UADS data because the latter may suffer from the effects ofvolt ampere curve distortion due to small scale density variations and spineffects which do not show up in the single point samples used in the highresolution measurements. This feature makes the high resolution datamore valuable in resolving small scale, and small amplitude plasmastructure.
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| CITATION_DESCRIPTION |
Theis, R. F., PVO-V-OETP-3-HIRESELECTRONS-V1.0, PVO VENUS ELECT TEMP PROBE CALIB HIGH RES ELECTRONS VER 1.0, NASA Planetary Data System, 1993.
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| ABSTRACT_TEXT |
The High Resolution Ne File. These data are based on measurements of the electron saturation current or the ion saturation current taken from as many voltampere curves as the telemetry data rate permitted. Since Ne is assumed equal to Ni everywhere in the ionosphere, either can be used as a measure of Ne. The ion current is used at high densities (Ni > 4x104cm-3) and the electron current is used at lower densities. Typically, 4 to 8 high resolution density samples are obtained in the interval between recovered voltampere curves, although this ratio is bit rate dependent. This provides Ne and Ni measurements at much smaller intervals than is possible from the voltampere curves themselves. High resolution measurements are typically available at 2 to 8 second intervals depending upon the telemetry rate available to the OETP at the time.
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| PRODUCER_FULL_NAME |
DR. ROBERT F. THEIS
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| SEARCH/ACCESS DATA |
Planetary Plasma Interactions Website
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