Neutral Atom Imaging of the Earths Magnetosphere

1
Neutral Atom Imaging of the Earths Magnetosphere
Ruth Skoug LASSO July, 2008
2
Why Imaging?
(After Williams et al., 1992)
3
Why Imaging?
(After Williams et al., 1992)
4
Why Imaging?
(After Williams et al., 1992)
5
The importance of neutral atom imaging
40 years of single point measurements have
provided a schematic understanding of the Earths
magnetosphere. But, the state of the
magnetosphere at any given time is complex and
uncertain. Global Imaging is required
6
ENA Imaging

• Measure energetic neutral atoms formed by charge
  exchange interaction between energetic
  magnetospheric ions and cold neutral geocorona
• Goal is to study energetic ion populations
• Requires inversion of measured ENA images
• Challenge measurements made in intense UV
  background
• UV background is 410 orders of magnitude higher
  than expected ENA fluxes
• Detectors are sensitive to both UV and particles

7
Charge Exchange
JENA ?dx nH JION ?
8
Neutral Hydrogen Geocorona
Measured from Apollo 16, April 21, 1972
9
ENA Imaging
E. Roelof, GRL, 14, 652, 1987
10
(No Transcript)
11
The IMAGE mission

• First dedicated platform for ENA imaging of the
  magnetosphere
• Launched March 25, 2000
• Satellite communication ended December 2005
• Science goal To study the global response of the
  Earths magnetosphere to changes in the solar
  wind
• Instrumentation
• Neutral atom imagers (LENA 10300 eV,
• MENA 130 keV, HENA 10200 keV)
• Ultraviolet imagers (EUV 30.4 nm, FUV 120-180
  nm)
• Radio plasma imager (RPI 3 kHz 3 MHz)

12
IMAGE Instruments
13
IMAGE Spacecraft in LMMS Shock Test Facility
IMAGE Satellite
14
(No Transcript)
15
IMAGE/MENA Flight Instrument - Ready for Cal
16
Principle of Operation

• Electrostatic collimator rejects incoming
  ions, allows ENAs to pass into the instrument
• Gold gratings block UV background while
  allowing passage of ENAs in the target energy
  range
• Passage of ENA through ultra-thin foil ejects
  secondary start electron for time-of-flight
  measurement

17
MENA Sensor Head
18
IMAGE/MENA, TWINS Imaging Technique

• Collimator Plates
• reject charged particles
• define azimuthal field-of-view

ENA

• Freestanding Transmission Gratings
• reject ambient UV
• Ultrathin carbon foil
• secondary electron (SE) emission for TOF
• and coincidence

SEs

• SE Acceleration grid
• direct position mapping of SEs on detector

• Microchannel Plate Detector
• detection of ENAs and correlated SEs

SE Anode
ENA Anode
ENA Anode

• Position-Sensitive Anode
• position measurement of ENAs and SEs

19
Transmission Gratings
ENA transmission gt 4
UV transmission lt 2 x 10-5 (1216 Å)
20
(No Transcript)
21
ENA Flux vs. Deconvolved Ions
22
Storm-time Ring Current
MENA (and HENA) observations have demonstrated
directly and conclusively the evolution of the
storm-time ring current, from main-phase
domination by an asymmetric component
(attributable to freshly-injected plasma-sheet
material flowing through the inner magnetosphere
on open drift paths) to the recovery-phase
symmetry of the fully-trapped remaining ring
current.
23
Main phase domination by tail current
MENA observations of major geomagnetic storms
show that the main phase magnetic field
perturbations can be dominantly produced by an
asymmetric ring current carried by particles from
the tail plasma sheet convected deep into the
night-side near-earth region. In the case of the
March 31, 2001, storm, it was not until the
recovery phase that ring current carriers were
able to drift in significant numbers into the
afternoon sector.
24
Sawtooth Flux Variations
Storm Oct 04-06, 2000
1
1
3
3
2
4
4
2
1
2
3
4
25
Storm-time Sawtooth Injections
Global observations of the magnetosphere enabled
by MENA and HENA energetic neutral atom imaging
have allowed us to determine that the sawtooth
energetic particle events observed by in situ
detectors on geosynchronous satellites during
moderate storm-time conditions do indeed
correspond to very large-scale substorm-like
injections of freshly energized particles, rather
than simply to the periodic motion of
trapped-particle boundaries.
26
The TWINS mission

• Two Wide-angle Imaging Neutral-atom Spectrometers
  (TWINS) will image the Earths magnetosphere in
  ENAs from two widely-spaced, high-altitude
  spacecraft
• This stereo imaging will provide the first 3-D
  images of the magnetosphere
• Instrumentation
• ENA imager based on MENA imager from IMAGE
• Lyman-a detector to measure geocorona
• TWINS-1 turned on in 2006 TWINS-2 in 2008

27
Scientific Objectives

• Primary Scientific Goal
• Establish global connectivities and causal
  relationships between processes in different
  regions of the magnetosphere
• Broad Scientific Objectives
• Ion Dynamics view global dynamics, composition,
  and energization of ions throughout the
  magnetosphere
• Plasma Origins and Destinies trace sources,
  transport, and sinks of plasma populations
• Magnetospheric Evolution observe the evolution
  of the global magnetospheric structure
• Magnetospheric Structure visualize and map the
  global configuration of the magnetosphere in
  three dimensions

28
TWINS orbits are ideal for imaging the
magnetosphere. The spacecraft spend most of
their time at high altitudes and can view out to
a distance of 12 Earth radii (RE).
29
TWINS includes instruments to measure both ENAs
(ENA Sensor Heads) and the neutral hydrogen
density (Ly-a detectors)
Both instruments are located on a rotating
actuator platform to allow 3D viewing from
three-axis stabilized spacecraft.
30
TWINS Instruments
31
TWINS Instruments
Lyman-a Detector
32
TWINS Rotational Actuator
33
Volume Sampling From One And Two Spacecraft
ONE SPACECRAFT
TWO SPACECRAFT
34
TWINS stereo imaging will enable the
identification of boundaries in the Earths
magnetosphere.
A boundary in an ENA image is formed by LOS
that are tangent to a physical boundary.
Including a priori knowledge of magnetic field
and boundary symmetries allows determination of
boundary. With one spacecraft, many possible
boundaries are consistent with an ENA image.
With two spacecraft, the problem is less
model-dependent.
TWIN 2
TWIN 1
35
Simulated ENA Image and Ion InversionViewing
from dawn, 45 latitude, 4.3 RE
Simulated ENA Image
Initial Ion Distribution
Inverted Ion Distribution
36
Ion Inversion From Two Viewpoints (-68 from
midnight and 45 from dawn)
INITIAL ION DISTRIBUTION
INVERTED ION DISTRIBUTION
37
TWINS First Images
TWINS-1
TWINS-2
Simulations from E. Roelof and P. Brandt
38
TWINS First Images

• TWINS-1 and 2 have made simultaneous observations
  of the ring current during a weak geomagnetic
  storm on 0600 UTC 15 June 2008.
• We have used a parametric ion distribution to
  reproduce the ENA images obtained from TWINS-1
  and 2.
• The same ion distribution can reproduce both ENA
  images.
• The underlying ion distribution is located in the
  ring current region of the post-midnight
  magnetosphere, which is the expected location for
  low-energy ions.

Simulations from E. Roelof and P. Brandt
39
Summary

• ENA imaging provides a global view of the Earths
  magnetosphere
• IMAGE provided the first high-resolution neutral
  atom images of the magnetosphere
• TWINS is beginning to provide the first stereo,
  3-D images of the magnetosphere
• Separate spatial and pitch angle variations
• Less dependence on models