Dayside tutorial: The Science of the MMS Mission
Jim Burch
GEM Workshop, Portsmouth, VA
June 17, 2014
Outline
•Fundamental Aspects of Magnetic
Reconnection
•The Magnetospheric Laboratory
•Scale Sizes and Measurement Requirements
•Magnetospheric Multiscale Mission
•Orbital Strategy and Burst-Mode Data
•Summary
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What is Magnetic Reconnection?
• Magnetic fields pointing in opposite
directions in converging plasmas tend to
annihilate each other in a diffusion
region, releasing their magnetic energy
and heating the charged particles in the
surrounding environment.
• The fast release of magnetic energy
requires that oppositely pointing
magnetic fields be torn apart and
reattached to their neighbors in a crosslinking process called magnetic
reconnection.
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A Fundamental Universal Process
(a)
(b)
(c)
Magnetic reconnection is important in the (a) Earth’s magnetosphere, (b) in
the solar corona (solar flares and CMEs) and throughout the universe (high
energy particle acceleration). Simulations (c) are used to guide experiments.
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The Magnetospheric Laboratory
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What We Know and Need to Know
Space/Lab Measurements and theory have shown that...
• All predictions about the MHD scale phenomena (e.g., ion outflow
velocity) surrounding reconnection sites are valid.
• At the ion scale the predicted quadrupolar magnetic field from Hall
MHD exists in space and in the laboratory.
• The reconnection rate is often near 0.1 VA regardless of the specific
process causing reconnection.
Need to...
• Determine the roles played by (1) electron pressure gradient and
inertial effects and (2) turbulent dissipation in driving magnetic
reconnection in the electron diffusion region.
• Determine the rate of magnetic reconnection and the parameters that
control it.
• Determine the role played by ion inertial effects in the physics of
magnetic reconnection.
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Generalized Ohm’s Law
• Electron momentum equation (or generalized Ohm’s Law):
• With no reconnection the right-hand side is zero (MHD).
• In the ion diffusion region the J x B term will be most
important (Hall MHD).
• In the electron diffusion region the first two terms will
dominate and produce reconnection because in a gyrotropic
plasma they produce mainly Epar .
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Important Scale Sizes
100,000 km
•
•
•
•
500 km
100 km
Unstable, thin current sheets have thickness < 1000 km
“Electron diffusion region” thickness is of order 10 km
Current sheet motion is typically 10 to 100 km/s
Required resolution for electron diffusion region is ~30 ms
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Measurement Requirements (1)
• Ohmic dissipation (E  J ): Relies on collisions or
turbulence to create resistivity to dissipate magnetic field
energy with inflow and outflow driven by Alfvén waves.
Reconnection
 rates are considered too slow to be
important in explosive reconnection but could be
important for the slow build-up of reconnection.
– Measurement Requirements: E-field fluctuations 0.1 to 5
kHz, current measurements from magnetometer and
electron spectrometers; scale size is the diffusion length
(20 nT/B) ~13 km.
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Measurement Requirements (2)
• Hall MHD (
): Sideways force on ions allows them to
escape from the magnetic field. Relative motion between ions
and electrons produces currents that produce a “quadrupolar”
magnetic field component perpendicular to the plane of
reconnection. Electrons responding to the magnetic field
changes generate whistler-mode waves, which then take over
the role of driving reconnection.
– Measurement requirements: B vector with 0.1 nT resolution, Eperp
(0 to 10 mV/m), full electron and ion distribution functions, ion
flow velocities (100 – 1000 km/s) with composition (H+ and O+),
electron cyclotron waves (E and B ) from 0.1 to 6 kHz, energetic
electrons and ions (w/composition) with energies up to 500 keV;
scale size is ion skin depth ~250 km.
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Measurement Requirements (3)
• Divergence of Electron Pressure Tensor (
):
Produces ambipolar electric fields and kinetic Alfvén waves.
Drives currents and magnetic field motion.
– Measurement requirements: Full electron distribution
functions, vector E field, E and B waves to several kHz;
scale size is the effective ion Larmor radius (at the electron
temperature)~20 km.
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Measurement Requirements (4)
• Electron Inertia [ E  me edv e
dt 
]: Dissipates kinetic
Alfvén waves, producing Epar and driving reconnection.

- Measurement
requirements: Full electron distribution
functions, vector E field, Jpar ; scale size is the electron skin
depth ~5-10 km.
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Need for 4 Spacecraft
• To identify reconnection events we
need to have separations up to 400 km
with spacecraft in the two inflow
regions and in the two outflow regions
(blue and red arrows).
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• To determine processes driving
reconnection we need to have
smaller separations (down to 10
km) with spacecraft within the
diffusion region (as shown).
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Magnetospheric Multiscale Mission
• The MMS Mission
science will investigate
magnetic reconnection
using the Earth’s
magnetosphere as a
laboratory. Emphasis is
on performing multipoint measurements of
plasmas and fields at the
resolutions needed to
probe the electron
diffusion region.
• Launch is scheduled for
March 11, 2015.
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http://mms.space.swri.edu
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Orbital Phases
MMS employs two mission phases with inclination of 28 deg. to
optimize encounters with both dayside and nightside
reconnection regions.
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Instrument Suite
Elements and Responsibilities
FIELDS - Electric and magnetic (E,B) measurements at <1 ms timing
resolution (DC). Roy Torbert - UNH
Fast Plasma - Image full sky at 32 energies: electrons in 30 ms, ions in 150
ms. (n, P, fe, fi). Craig Pollock - GSFC
Energetic Particles - All-sky viewing of ion and electron energetic particles
(20 – 500 keV) w/composition (fe, fi). Barry Mauk - APL
HPCA – Composition-resolved 3D ion energy distributions (fi) of H+, He++,
He+, and O+. Stephen Fuselier - SwRI
ASPOC – Maintains s/c potential to ≤ 4 V. Enables valid (fi, E, B) data.
Rumi Nakamura – IWF (Austria)
SOC – Responsible for operating each IS, data management and archival
of science data. Dan Baker - LASP
Theory and Modeling – Develop models of reconnection for mission design
and data interpretation. Michael Hesse - GSFC
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Fast Plasma Instrument (FPI)
First “Video” Plasma Analyzer
DIS
DES
Objective:
Image full sky at 32 energies:
electrons in 30 ms, ions in 150
ms
DES
Design Concept:
Four ion and four electron dual
DIS
deflecting-aperture top hat
sensors for field of view and
aperture
DES Cutaway
DIS
DES
DIS
DES
Dual Electron Sensor: DES
Dual Ion Sensor: DIS
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Hot Plasma Composition Analyzer
• Solves the problem of
proton spillover, which
has prevented O+
measurements at the
magnetopause.
• Toroidal tophat analyzer
w/TOF with the
additional feature shown
in blue (RF voltage
added to DC deflection
voltage)
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HPCA Proton Attenuation
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Energetic Particle Detectors
•Multiple ion and
electron solid-state
detectors are used to
decouple all-sky view
from spacecraft spin
•2.5-second time
resolution obtained
by the two FEEPS
arrays
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MMS Instrument Suite
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Field Sensors on Spacecraft
B
5m
E
50 m
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E
12 m
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Flight Hardware
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Four Spacecraft Stacked for Test
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Orbital Strategy – Day Side
• Launch Mar. 11, 2015
• 28-deg. inclination,
apogee 12 RE on day side
• 4 mo. commissioning
• Phase 1a: day side with
separations 10 – 160 km
• Phase 1b: day side with
optimum separation
• Plots show orbits in green,
magnetopause crossings in
red (Phases 1a and 1b), and
neutral sheet crossings in
orange (Phase 2).
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Orbital Strategy – Night Side
• Launch Mar. 11, 2015
• 28-deg. inclination,
apogee 12 RE on day side
• Phase 2: Apogee 25 RE in
tail with separations 10 –
400 km
• Plots show orbits in green,
magnetopause crossings in
red (Phases 1a and 1b), and
neutral sheet crossings in
orange (Phase 2).
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MMS Burst System
Basic Plan: Obtain ~20 minutes of burst per day.
•Phase 1 orbit: ~1 day -> 20 min. burst data.
•Phase 2 orbit: ~3 days -> 1 hour of burst data.
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Orbital Strategy on Day Side
Tetrahedron configuration and burst data acquisition maintained
throughout region of interest (> 9 RE day side).
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Orbital Strategy on Night Side
Tetrahedron configuration and burst data acquisition maintained
throughout region of interest (> 15 RE night side).
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MMS Burst System
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Burst Mode Strategy
• The 96-Gbyte on-board memory stores from one to three
orbits of burst data, burst quality indices, and survey data for
downlink at least once per orbit. Each downlink is limited to 4
Gbits.
• MMS will have three ways of identifying burst data intervals.
– On-board assessment of data quality and the assignment
of burst quality indices to each burst data interval.
– Inspection of the fast survey data for identification of
additional promising burst intervals for downlink (Scientistin-the-Loop).
– Automated analysis of fast survey data.
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On-Board Triggers
• On-board triggers will be used to detect reconnection regions
based on physical signatures.
Physical Signature
Trigger Parameter
Reconnection Jets
Ion flow reversals
Magnetopause and Neutral Sheet Detection
Large B variations
Large Flows Surrounding Reconnection sites
Large E
Magnetopause and Neutral Sheet Detection
Large electron currents
Particle Acceleration Produced by
Reconnection
Electron and ion beams
Electron Diffusion Region
E parallel to B
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Scientist In The Loop
• At all times an MMS scientist will be assigned to the SOC (either
physically or electronically) to examine burst trigger indices and
fast survey data to identify promising burst data intervals that
may have been missed by the on-board system.
• The scientist will have the authority to assign priorities to data
intervals that will improve the accuracy with which burst data
selection is made.
• Data from two orbits will be available for examination, and the
priorities set by the scientist in-the-loop will affect the burst
interval selection for the next contact or cause certain data
intervals to be saved for subsequent contacts.
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Heliophysics System Observatory
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Theory and Modeling
• Key to the success of the SMART science plan is the coupling
of theory and observation.
• The SMART Theory and Modeling and IDS Teams have
developed advanced models of the reconnection process.
– These models have been used to define the MMS
measurement requirements and guide mission design.
– During the development phase, the models were refined
further, and procedures for assimilating the MMS data
into the models were defined.
– In the mission operations and data analysis phase, the
Theory and Modeling and IDS Teams will use MMS data to
refine their models.
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Summary
• The measurements made to date (at the MHD and ion scales)
have been consistent with the Petschek reconnection model
and Hall MHD.
• For further progress, the most critical region to be probed is
the electron diffusion region within which specific predictions
about the electric fields, currents, and electron dynamics need
to be tested.
• MMS will make measurements of plasmas, fields and energetic
particles at the appropriate temporal and spatial scales to
make significant progress and solve most of the outstanding
questions about magnetospheric reconnection
• Theory and modeling provides the bridge between
magnetospheric results and the laboratory and astrophysical
contexts.
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