| DATA_SET_DESCRIPTION |
Data Set Overview
=================
This data set contains higher level data products generated by the New
Horizons Particles and Plasma science team from data taken by the Pluto
Energetic Particle Spectrometer Science Investigation (PEPSSI)
instrument team during the PLUTO mission phase.
PEPSSI is a particle telescope and a time-of-flight (TOF) spectrometer
that measures ions and electrons over a broad range of energies and
angles. Particle composition and energy spectra are measured for H to
Fe from ~ 30 keV to ~1 MeV (but not all species are uniquely separated)
and for electrons from ~30 keV to 700 keV. PEPSSI comprises a
time-of-flight (TOF) section and a solid-state detector (SSD) array
that measures particle energy. The combination of measured energy and
TOF provides unique particle identification by mass and particle energy
depending on the range: for protons from ~30 keV to ~1 MeV; for heavy
(CNO) ions from ~80 keV to ~1 MeV. Lower-energy (>3 keV) ion fluxes are
measured by TOF only, but without the SSD signal, providing velocity
spectra at these energies as well. Due to storage and bandwidth
limitations, all event data cannot be stored or telemetered to the
ground. Instead, a round-robin algorithm is used to save Energy, TOF,
and timing data for select events.
A new coordinate system is defined for this dataset, where data is
described with respect to PSH X, Y, and Z, where PSH is Plutocentric
Solar Heliographic, based on the HelioCentric Inertial (HCI) reference
frame, where HCI Z is the solar north rotational axis, and X is the
solar ascending node on the J2000 ecliptic.
The headers and contents for these higher level derived data products
are described below.
These data products are averages for various PEPSSI channels based on
calibrated PEPSSI data and contain fluxes averaged over Sectors 0-2.
The file pep_encounter_l4_triples_3h.csv includes 3 hour averages of:
B01 proton fluxes averaged over S0-S2 w/ statistical uncertainties
B02 proton fluxes averaged over S0-S2 w/ statistical uncertainties
B03 proton fluxes averaged over S0-S2 w/ statistical uncertainties
B09 helium fluxes averaged over S0-S2 w/ statistical uncertainties
B10 helium fluxes averaged over S0-S2 w/ statistical uncertainties
with 11 columns of data:
Epoch (SCET), B01, B01Unc, B02, B02Unc, B03, B03Unc, B09H, B09HUnc,
B10H, B10HUnc
with all data in units of 1/s/ster/cm^2/keV.
The lower and upper bounds for B01, B02, and B03, for the species H+,
respectively, in keV are:
Lower Bound: 25.713, 56.421, 119.63;
Upper Bound: 55.183, 118.48, 217.92.
The lower and upper bounds for B09 and B10, for the species He+,
respectively, in keV are:
Lower Bound: 13.03, 255.9;
Upper Bound: 254.63, 1199.6.
For the doubles files pep_encounter_l4_doubles_5min.csv and
pep_encounter_l4_doubles_5sec.csv:
Both of the doubles files include 5 columns:
Epoch (SCET), L09H, L09HUnc, L11H, L11HUnc
with all data in units of nuc/s/ster/cm^2/keV.
L09H and L09HUnc are the L09 Total Ion fluxes for He+ averaged over
S0-S2 w/ statistical uncertainties; and
L11H and L11HUnc are the L11 Total Ion fluxes for He+ averaged over
S0-S2 w/ statistical uncertainties.
The lower and upper bounds for L09H, in keV/nuc, for species He+, are:
Lower Bound: 11.946;
Upper Bound: 22.001.
The lower and upper bounds for L11H, in keV/nuc, for species He+, are:
Lower Bound: 6.2758;
Upper Bound: 11.858.
This dataset contains averaged data where the time tag is centered in
the time range bin. Time averages were chosen such that they were
appropriate for the statistics for the number of counts seen during
the Pluto Encounter. If the time range for the average is too large,
there is not enough time resolution, and if it is too small there
could be no counts recorded.
The overall time period of this dataset covers the 4 days which
correspond to when PEPSSI was in its highest resolution mode
(Encounter Mode). The most interesting channels were chosen, where the
L channels are indicative of the suprathermal population, and the B
channels are indicative of energetic particles.
This dataset was downselected from the full Pluto dataset, which can
be found in the lower level Pluto Encounter datasets for PEPSSI.
The dataset contains the cleanest channels with the lowest background.
The B channels are Time-Of-Flight (TOF) vs. energy which allows for
composition separation, and the L channels are TOF only. Please see
the SOC to Instrument ICD, included in this dataset, for a description
of the PEPSSI channels.
The statistical uncertainties given in this dataset are based on
Poisson statistics on counts. Poisson errors are propagated through
from counts into intensity. The uncertainties do not include any
systematic uncertainty.
'Statistical uncertainties' in the PEPSSI data are calculated by
propagating the Poisson error of the individual measurements. So, for
each accumulation period, the square root of the total number of events
observed for a given channel is taken as the absolute plus or minus
uncertainty of the measurement, this is then propagated through the
calculation of counts/sec and intensity to yield a 'statistical
uncertainty' in the intensity measurement.
The triples are averaged over 3 hours because they contain the B
channels, which are high energy channels, and general trends can be
seen by looking over a longer period of time. The 3 hour averages
over the 4 day period of this dataset showed there was no predominant
energetic particle activity as there was no large signal. The 3 hour
rate was needed to have enough counts to have reasonable statistics.
For the 5 second doubles, L09 is much lower than L11. L11 is a longer
TOF and hence a lower energy box than L09, so it is expected to have
higher intensities than L09. There is a dependence of intensity on
energy, and it is not necessarily geometry leading to the difference.
The spectral shape is important.
The quantization that can be seen in the 5 second doubles is due to
low count statistics.
PEPSSI counting rates are telemetered using a lossy logarithmic
compression scheme, so that, if rates are very high (as they are for
channels such as 'L09' and 'L11'), the resulting derived count rates and
fluxes will appear 'quantized'. Since the L09 and L11 channels have
different conversions to intensity units (e.g. their efficiencies at the
time of the Pluto encounter were a bit less than a factor of 2
different), they will appear to be 'quantized' by different sized
'steps'.
In general, it is important to look at the attitude of the spacecraft
in order to interpret the data correctly because there are large
angular dependencies due to the spacecraft attitude.
The file pep_encounter_l4_attitude_and_ephemeris.csv includes these
Attitude and Ephemeris quantities at 1s cadence (the precision of the
values are from SPICE kernels):
Epoch (in spacecraft time SCET, converted to YYYY-DOYTHH:MM:SS format)
PSH X - Plutocentric Solar Heliographic X, in Rp
PSH Y - Plutocentric Solar Heliographic Y, in Rp
PSH Z - Plutocentric Solar Heliographic Z, in Rp
PSH R - Distance from Pluto, in Rp
HCI R (distance to the Sun), in AU
PEPSSI S1 Pluto Cone Angle, in degrees
PEPSSI S1 Pluto Clock Angle, in degrees
PEPSSI S1 Sun Cone Angle, in degrees
PEPSSI S1 Sun Clock Angle, in degrees
SC-Sun-Pluto Line, in km - this value equals sqrt(PSHY^2 + PSHZ^2)
Cone and clock angle are designed to provide attitude
information by describing the orientation of a given
boresight is relationship to a specific body (more
precisely a particular body reference frame). In our case
the body is either Pluto or the Sun. Cone angle is the
angle between the s/c-to-body vector and the boresight
vector. It is zero when the boresight is looking at the
body (either the Sun or Pluto) and it can be as large as
180 degrees when looking in the opposite direction. We
define the following attitude (att) frame of reference to
determine these values.
Zatt is SC-to-body [this is the s/c-body vector
referenced above]
Xatt = -Zatt x Zbod
[Zbod is the body Z-axis;
e.g., PSH Z-axis or Sun spin axis]
Yatt = +Zatt x +Xatt
(unit vector normalization not shown)
We get the cone angle by calculating the co-latitude in
the Xatt,Yatt,Zatt reference frame. The clock angle is
the longitude in this Xatt,Yatt,Zatt frame, meaning that
when the boresight vector Vbore = (Vx,Vy,Vz) = (
Vbore.Xatt, Vbore.Yatt, Vbore.Zatt) is aligned with the
Xatt axis, the clock angle is 0 degrees and when it's aligned
with the Yatt axis it's 90 degrees, etc. Explicitly this should
correspond to:
CONEangle = arccos ( Vz/V )
CLOCKangle = arccos ( Vx/sqrt(Vx^2+Vy^2) )
. is scalar (dot) product
x is vector (cross) product
The SC-Sun-Pluto Line is important because the solar wind flows
radially, and this value tells you where you are based on the shadow of
Pluto - it goes to zero when the spacecraft is directly behind Pluto
(in Pluto's shadow during the occultation).
Rp is the radius of Pluto, defined as 1187 km in this dataset.
1 AU is an Astronomical Unit, defined as 149597871 km in this dataset.
The level of unit precision given in these files are large because
of the use of the mission SPICE kernels, and is based on the default
SPICE precision.
Version
=======
This is VERSION 1.0 of this data set.
Instrument description
----------------------
Refer to the following files for a description of this instrument.
CATALOG
PEPSSI.CAT
DOCUMENT
SOC_INST_ICD.PDF
PEPSSI_SSR.PDF
Contact Information
===================
For any questions regarding the data format of the archive,
contact
New Horizons PEPSSI Principal Investigator:
Ralph McNutt, Jr., Johns Hopkins Univ., Applied Physics Lab
Ralph McNutt, Jr.
Johns Hopkins University
Applied Physics Laboratory
Space Department
11100 Johns Hopkins Road
Room MP3-E116
Laurel, MD 20723
USA
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