Instrument Overview
    ===================
      The objective of this experiment is to measure the
      three-dimensional velocity distributions of the
      solar-wind ions and electrons in the energy range
      from 30 eV up to 16 keV within +/- 30deg of the
      ecliptic plane.  During the cometary encounter, the
      interaction of the solar wind with the cometary
      ionosphere will be investigated, paying particular
      attention to the mass loading of the solar wind by
      cometary ions, to the existence, location and
      strength of the upstream shock transition and of
      (possibly) backscattered particles from the shock
      and other,unexpected, plasma populations.  Data
      will be collected on such solar-wind phenomena as
      the halo and core distributions of electrons,
      variations in the flow velocity, and the He++/He+
      ratio in relation to solar rotation and longitude.
 
 
      The instrument consists of one sensor for electrons
      and one for positive ions.  As shown in Figure 3,
      each sensor consists of a fan-shaped collimator, a
      270deg spherical electrostatic analyzer, and a
      Micro-Channel Plate (MCP) detector with five
      discrete anodes.  The collimator's field of view is
      5deg in azimuth and 60deg in polar angle
      respectively, the latter being centered
      perpendicular to the spacecraft spin axis and thus
      in the ecliptic plane.  The 270deg spherical
      analyzer has a better-defined response in
      energy-angle space than a quadrispherical analyzer.
 
 
      After leaving the analyzer, particles are
      post-accelerated by a potential of 250 V for
      electron and 2500 - 3500 V for ion analysis.  A
      Z-type three-stacked MCP detector is used to obtain
      a saturated pulse-height distribution over a wide
      dynamic range of counting rates.  The anodes of
      each MCP are split into five sections in order to
      provide a five-point angular distribution with a
      12deg resolution in the polar direction.  The
      output pulses from each MCP anode are amplified,
      shaped and accumulated in 19-bit counters.  The
      contents of the counter are compressed into 8 bits
      and stored temporarily in the PAM buffer memory.
      The instrument's angular distribution in the
      azimuthal direction, obtained by the spacecraft
      spin, is 5.625deg within +/- 22.5deg of the solar
      direction and 22.5deg in the remaining spin phase.
 
 
      The instrument has four modes of energy scanning
      (Table below).  In each mode the energy is scanned
      in steps as E(n)=30 exp (0.0066n), where E(n) is
      the energy of the n-th step and n=n(1), . . . .
      .,n(2).  Details of the energy scanning, which is
      synchronized to the spin phase, are shown for the
      E1-mode in Figure 5.  During each spacecraft spin
      four energy steps are scanned periodically in every
      22.5deg interval, i.e.  the step is changed every
      1.40625deg around the solar direction and 5.625deg
      over the remaining azimuthal directions.
      Consequently, it takes 24 spacecraft spins in the
      E1-mode to cover the complete energy range in 96
      steps.
 
      At high bit rate (2048 bit/s), three-dimensional
      velocity distributions will be obtained for both
      electrons and ions.  These data, produced during
      one spin, will be transmitted in 10 PCM frames (5
      s) during the next spin interval.  At low bit rate
      (64 bit/s), two-dimensional distributions will be
      obtained for electrons and ions by summing the
      counts from each anode.  The energy scanning will
      be carried out over 16 spin periods once every 512
      s, required to transmit these data.
 
      The ESP instrument will be operated during the
      Halley-encounter phase as well as during the cruise
      and post-encounter phases, but only while Planet-A
      is in its high-spin-rate mode (nominally 6.3 rpm).
 
-----------------------------------------------------------------------
Table 2. Energy scanning modes of Planet-A Solar-Wind Plasma Experiment
 
                                                            Time for one
                             Total no.                      complete
Mode  n(1)    n(2)  delta n  of steps  Energy range (eV)    sequence* (s)
-----------------------------------------------------------------------
E1       0      95     1       96         30 ~ 15800         228.6
 
E2       O      63     1       64         30 ~  1920         152.4
 
E3      32      95     1       64        250 ~ 15800         152.4
 
E4       1      95     2       48         34 ~ 15800         114.3
-----------------------------------------------------------------------
n(1)  =  lowest step number   delta n  =  step interval
n(2)  =  highest step number      *  =  times refer to high bit rate;
                                        always 512 s at low bit rate