The Energetic Particles Spectrometers (EPS) on
MESSENGER and New Horizons
S. A. Livi, R. McNutt, G. B. Andrews, E. Keath, D. Mitchell, G. Ho
Johns Hopkins University - Applied Physics Laboratory
11100 Johns Hopkins Road
Laurel, MD 20723 - USA
Abstract. In the course of this decade, two NASA deep space mission to the inner and outer heliosphere, MESSENGER
to the planet Mercury and New Horizons to the planet Pluto, will carry onboard energetic particle spectrometers. The
combination of measurements near the Sun (0.3 AU), and from the outer heliosphere (up to almost 40 AU), will ideally
complement the information available from Ulysses and from near-Earth orbiting spacecraft, yielding boundary
conditions on the processes that accelerate energetic particles. EPS is a hockey-puck-size Time-of-Flight (ToF)
spectrometer that measures ions and electrons over a broad range of energies and pitch angles. Particle composition and
energy spectra will be measured for H to Fe from 15 keV/nucleon to 3 MeV/nucleon and for electrons from 15 keV to 1
MeV. The ion section of EPS is a compact ToF telescope with two main components: a ToF section and a Solid State
Detector (SSD) array to measure separately velocity and total energy of the incoming particles. Electrons are identified
in EPS by the presence of an energy signal and by the absence of start or stop pulses, since energetic electrons have low
efficiency for production of secondary electrons when passing through thin foils. For both ions and electrons the angle of
arrival is determined by the position of the solid-state detector that collects the particle.
•
How energetic particles interact with largescale magnetic structures in the solar wind (like
interplanetary coronal mass ejections), both inside and
outside the orbit of Mercury
INTRODUCTION
Both the Sun and the dynamic heliosphere are
prodigious accelerators of energetic ions and electrons.
The evolution of the particles’ intensities, angular
distributions, energy spectra, and composition can
yield incisive diagnostics of the acceleration process,
if the particle’s propagation from the acceleration site
can be understood. EPS can reveal remarkably
different scientific results in the vicinity of Mercury
and of Pluto, because the physics of energetic particles
takes on extreme opposite characteristics in the inner
and outer heliosphere.
•
Detection at the orbit of Pluto at a mean
distance of 40 AU maximizes the effects of particle
propagation and in situ energy changes.
EPS
observations can reveal the acceleration mechanisms
of plasma/particle interactions in the outer heliosphere
because:
•
Great outbursts of solar activity can create
populations of solar energetic particles that take ~100
days to pass by on their way out of the solar system
The orbit of Mercury at a mean distance of 0.39
AU minimizes the particle propagation effects from
the Sun and transient shocks in the inner heliosphere.
Therefore observations by EPS can directly address:
•
Distances are so immense that field-aligned
propagation becomes negligible compared to
convection with magnetic field lines in the solar wind,
so solar wind structure molds the energetic particle
structures.
•
Where and when prompt solar particles are
accelerated and released from the Sun in association
with solar flares and coronal mass ejections
•
Energetic particle populations can survive
only if there is a balance between acceleration at
shocks and/or magnetic field compressions on the one
hand, and adiabatic deceleration in the expanding solar
wind on the other.
•
Under what conditions energetic particles are
accelerated by fast shocks in the inner heliosphere
CP679, Solar Wind Ten: Proceedings of the Tenth International Solar Wind Conference,
edited by M. Velli, R. Bruno, and F. Malara
© 2003 American Institute of Physics 0-7354-0148-9/03/$20.00
838
transmission toward the SSDs. The secondary
electrons released by the stop foil are steered to the
MCP and generate the stop signal. Electron trajectory
simulations show that there is less then 400ps
dispersion in the transit time of the secondary
electrons from the foil to the MCP. Sub-nanosecond
dispersion is required so as not to misidentify ion
species.
EPS DESCRIPTION
The Energetic Particle Spectrometer (EPS) is a
hockey-puck-size Time-of-Flight (ToF) spectrometer
that measures ions and electrons over a broad range of
energies and pitch angles. Particle composition and
energy spectra will be measured for H to Fe from ~15
keV/nucleon to ~3 MeV/nucleon and for electrons
from 15 keV to 1 MeV. EPS was developed on a
NASA Planetary Instrument Definition and
Development (PIDDP) grant focused on providing a
low-mass, low-power instrument1,2 that could measure
the energetic pickup ions produced in the vicinity of
Pluto3,4 in a cometary-type interaction5-8 An
engineering model has operated well at the accelerator
facility at GSFC. The instrument combines minimal
spacecraft resource requirements with versatile
measurement characteristics for exploratory missions.
The difference in time between start and stop (1ns
to 200ns), together with the distance between the two
foils, give a measure of the particle’s velocity. The
total charge released in the SSD is proportional to the
energy of the parent particle.
Start Foil
Stop foil
SSD
-2900 V
Ion and Electrons
Electrons
Electrons
-2900 V
-2600 V
-2100 V
Cover closed
-2600 V
-2100 V
Collimator
Gnd
Annular MCP
The EPS was chosen for and will fly on the
MESSENGER (NASA Discovery Program) mission to
Mercury that launches in March 20049. Only minor
changes will be made to this instrument for the Pluto
mission: (1) more open collimator to increase the
geometric factor, (2) slower microprocessor clock to
decrease secondary power by 375 mW, (3) low-current
microchannel plates (MCPs) to decrease secondary
power another 250 mW, and (4) a thinner front foil
(6.6 µg C (300 Å)) to lower the energy threshold in the
much lower UV background present.
Start Anode
Gnd
Stop Anode
Gnd
60 mm
SSD
Start Foil
Start
Anode
(1 of 6)
Stop
Anode
Collimator
(conceptual)
Stop Foil
Ion Measurement
EPS is a compact ToF telescope with two main
components: a ToF section and a Solid State Detectors
(SSD) array. The SSD comprises six ion implanted
planar silicon detectors, each with four pixel, two
dedicated to ion measurements and two to electron
measurements.
FIGURE 1. Principle of operation of EPS.
Electron Measurement
Energetic electrons have low efficiency for
production of secondary electrons when passing
through thin foils. A zero ToF technique for measuring
energetic electrons would suffer from low foregroundto-background ratio, in a radiation environment like
the Earth radiation belt. A thin layer (flashing) of 1mm
of aluminum covers half of the SSD in EPS. This dead
layer stops protons with energy less then 200 keV; on
the other hand electrons lose less then 10 keV energy
by the interaction with this dead layer. Electrons are
identified in EPS by the presence of an energy signal
and by the absence of start and stop pulses. Under such
conditions, a signal of 50keV in the detector can be
generated either by a 250keV proton that did not
generate either a start or a stop pulse (a low probability
event), or by a 60 keV electron. The ToF spectra
Particles enter the system through a mechanical
collimator that delimits the look direction and hit a
thin (4 mg/cm2) polyimide foil. A layer of 500 Å of
Aluminum covers the entrance foil, to enhance Lyα rejection properties. A deployable cover protects the
EPS front foil during launch.
Electrons are released from the inner surface of the
start foil and focused to a well-defined region on a
microchannelplate (MCP) to generate the start signal
in a dedicated anode. The incident ions are not
affected by the electric fields of the focusing optics.
After a 6-cm flight path ions traverse the stop foil,
which is a 4 mg/cm2 polyimide foil, covered by 500 Å
of palladium to reduce light (both visible and UV)
839
collected in the adjacent SSD (without flashing) will
be used during ground data analysis for checking and
correcting for the proton contamination.
these spacecraft are three-axis stabilized during data
taking, so the azimuthal angle of arrival is set by the
instantaneous orientation of the spacecraft..
For both ions and electrons the elevation angle of
arrival is determined by the position of the solid-state
detector that collected the particle. This type of
instrument is best suited to a spinning spacecraft, but
The main instrument parameters of EPS are
summarized in Table 1.
Table 1. EPD Sensor Characteristics
Geometric Factor (G-total instrument) =
0.03 cm2 sr
Angular Coverage (Instantaneous) =
160° x 12°
Angular Pixels (elevation x azimuth) =
13.5° x 22.5°
Energy Coverage
See Table Below
Energy Resolution (Spectral Mode) =
8 keV
∆E/E Capability at 50 keV =
16%
∆E/E Capability at >100 keV =
<8%
Mass Species Separation
See Table Below
Species
Energy Range
Logic
Ions
> 5 keV
ToF only
Protons
15 keV - 3.0 MeV
ToF x E
Alphas
25 keV - 3.0 MeV
ToF x E
CNO
60 keV - 30 MeV
ToF x E
Electron Spectra
15 keV - 1.0 MeV
Singles/Foil Technique
Relative Electron Rates
1.0 to 2.0 MeV
Singles
CONCLUSIONS
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