DATA_SET_ID                    = PVO-V-OETP-3-HIRESELECTRONS-V1.0
  DATA_SET_NAME                  = PVO VENUS ELECT TEMP PROBE CALIB
                                      HIGH RES ELECTRONS VER 1.0
  PRODUCER_FULL_NAME             = DR. ROBERT F. THEIS
  PRODUCER_INSTITUTION_NAME      = GODDARD SPACE FLIGHT CENTER
  Description:

 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.

The High Resolution file provides measurements of Ne (or Ni) within 30
minutes either side of periapsis at somewhat higher resolution than is
possible from the voltampere curves.  However, these measurements are
less accurate when the spacecraft is outside the ionosphere, where Ne is
typically well below 100 cm-3.  In sunlight, spacecraft photoelectron
densities at the radial probe location are of the order of 30-50 cm-3.  In
darkness, the Ne measurements can be made down to densities of about 2
cm-3 because the pe background is absent.  However, the measurements
made in the Venus umbra are often degraded at low densities because of
the presence of hot electrons that charge the spacecraft to potentials that
lie beyond the range of the OETP sweep voltage.  This makes it impossible
to drive the probe positive with respect to the plasma potential.  In
addition, deBye shielding causes the probe to become enveloped in the ion
sheath of the spacecraft at very low densities, further reducing its access
to the ambient ionospheric plasma.  Empirically derived corrections for
this effect have been applied to the high resolution data in order to
provide at least a lower limit of Ne, but the errors could exceed a factor of
2 at densities below 10 cm-3.  This correction does not allow Ne to be less
than 2 cm-3.  When the electron current at maximum positive voltage is
less than a certain very low value an Ne value of 2 cm-3 is entered in the
High Resolution file simply to serve as an upper limit on Ne, and to show
that data were actually being taken.
 
In summary, the high resolution Ne measurements provide about a factor
of 8 higher resolution than the UADS file whose resolution is limited by
the recovery rate of raw voltampere curves.  Therefore the high resolution
data better resolve such small scale features as the ionopause and the
plasma clouds often found above the ionopause.  Also, the UADS densities
often stop somewhere within the ionopause density gradient, so this
feature can best be resolved using the High Resolution data.  However,
certain artifacts have not been removed from the data, so one must be
careful not of over-interpret them. 

In general, the high resolution Ne measurements have a lower absolute
accuracy than the UADS (voltampere curve) measurements because
factors such as the spacecraft potential and Te are not available to
calculate Ne more precisely.  To reduce such errors in the high resolution
data, they are normalized to the voltampere curve measurements.  This
normalization is entirely different from the Ne-Ni normalization employed
in deriving the UADS data.
 
Another source of error in the High Resolution Ne measurements is the
jump discontinuities that occur when the spacecraft passes from sunlight
to 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 because
they are not based on voltampere curves but measurements at a fixed
positive potential. Therefore a discontinuity may occur in Ne at the
sunlight-shadow boundary if Ne is sufficiently low that spacecraft
photoelectron emission affects the spacecraft potential.
 
The precision of the high resolution data is probably somewhat better than
that of the UADS data because the latter may suffer from the effects of
volt ampere curve distortion due to small scale density variations and spin
effects which do not show up in the single point samples used in the high
resolution measurements.  This feature makes the high resolution data
more valuable in resolving small scale, and small amplitude plasma
structure.