narrowband ion cyclotron waves at and near the moon in the ... · 8/18/2012 · waves suggest...
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Narrowband Ion Cyclotron Waves at and near the Moon in the Earth’s Magnetotail and the Association with the Plasma Environment
Peter J. Chi,1 C. T. Russell,1 H. Y. Wei,1 V. Angelopoulos,1 W. M. Farrell,2 and J. S. Halekas3
With special acknowledgments to: Palmer Dyal, David Williams, Greg Schmidt, and Yoshio Nakamura 1. UCLA 2. NASA GSFC 3. UC Berkeley
Lunar Science Forum NASA Ames, Moffett Field, CA
July 17-19, 2012
Lunar Portable Magne-‐tometer
Apollo Magne+c Field Experiments
Apollo 15 & 16 subsatellites
Lunar Surface Magnetometer (LSM)
Sub-‐satellite Biaxial Magnetometer (SBM)
Charles Sonnet & Palmer Dyal (NASA Ames)
Paul Coleman (UCLA)
LSM: A12, A15, A16
SBM: A15, A16 LPM: A14, A16
Restora+on of Apollo Magne+c Field Data
• For many years the Apollo magne+c field data were not accessible due to their archaic format.
• Recently a data restora+on effort has made available all magne+c field data collected from Apollo sub-‐satellites and some data from Apollo Lunar Surface Magnetometers (LSM).
Apollo LSM Observa+on of Monochroma+c Waves
A15 LSM
Apollo LSM 15, 16 Observa+ons in the Earth’s Magnetotail
A15
A16
𝑓↓𝑐𝑝 = 𝑞𝐵/2 𝑚↓𝑝
A15 LSM (B0 ~ 10 nT)
A16 LSM (B0 ~ 240 nT) Narrowband waves were frequently observed by Apollo LSMs when the Moon was in the Earth’s magnetotail.
Other Examples of 0.1-‐10 Hz Waves at the Moon Nakagawa et al. [2003]; GEOTAIL Farrell et al. [1996]; WIND
Halekas et al. [2006]; LP Nakagawa et al. [2011]; Kaguya Tsugawa et al. [2011]; Kaguya
Solar Wind Solar Wind
Solar Wind Solar Wind Solar Wind
Solar wind ni ~ 7 cm-‐3
Turbulent magnetosheath
Magnetotail ni ~ 0.3 cm-‐3 (plasma sheet) 0.01 cm-‐3 (lobes)
Narrowband waves observed by Apollo LSM
Wave Polariza+on (Apollo LSM)
• Characteris+cs of ion cyclotron waves: – (1) Excited when T⊥ > T//; (2) f ≤ ion cyclotron frequency; (3) Leg-‐hand polarized; (4) Propaga+on tends to be along magne+c field
• Apollo LSM observa+ons show consistency with all the magne+c field proper+es of ion cyclotron waves.
EllipBcity PropagaBon Angle
𝑓↓𝑐𝑝 = 𝑞𝐵/2 𝑚↓𝑝
Why Ion Cyclotron Waves: (1) In the Earth’s Magnetosphere
Causes of thermal anisotropy: • Substorm injec+on • Drig-‐shell spliing • Loss cone distribu+on
• (and more) Major purpose: Wave interacBon with parBcles for assessment of parBcle loss or energizaBon; Inference of ion composiBon.
Roux et al. [1982]
Why Ion Cyclotron Waves: (2) In the Solar System
Ion cyclotron waves associated with pickup ions have been observed in the vicinity of: • Mars exosphere • Venus exosphere • Neutral torus of Io • E-‐ring of Saturn ObservaBon of wave amplitude can be used to esBmate the amount of the pick-‐up ions and atmospheric density. (from Russell and Blanco-‐Cano, 2007)
Plasma Environment for Wave Occurrence
Ion cyclotron waves
In the Earth’s magnetotail the magne+c field is greatest in the lobes and weakens in the plasma sheet near the equatorial plane. Many wave events occurred in the plasma sheet when the field was reduced, but waves could also occur near or in the lobes.
0 3 6 9 12 15 18 21 24-10
-5
0
5
10
15
20
25
Time [hours]
B [n
T]
Apollo 15 Lunar Surface Magnetometer
BTBX SEL1972 Day 117
X
Observa+ons of ICW at Two Apollo Sites
• Differences are found in both phase and wave vector (k).
• Wave amplitude observed at Apollo 16 LSM is persistently lower.
• Wavelength is expected to be at the order of a few hundred km or less.
• These clues suggest that the remnant crustal magne+c field is strong enough to make significant changes in wave proper+es.
1346 1347 1348 1349-2-1012
dBX
A15 LSMA16 LSM
1346 1347 1348 1349-2-101
dBY
22:25 22:26 22:27 22:28 22:29 22:30-2-101
Universal Time (hh:mm)
dBZ
A15
A16
Selenographic Coordinates
[nT]
Radial field at 100 km al+tude (From
Purucker and
Nicholas, 2010)
ARTEMIS Observa+ons
ARTEMIS Observa+ons of ICW
• ARTEMIS has observed several events of ion cyclotron waves.
• All clear wave events were found close to the lunar dayside (highly loca+on-‐dependent).
• f <= fcp
-6 -4 -2 0 20
5
10
-1
0
1
YSSE [RM]
XSSE [RM]
ARTEMIS TH-B 2011 Jul 13-16
Z SSE
[RM
]
TH-‐B
Wave Proper+es (ARTEMIS)
• The wave proper+es observed by ARTEMIS are also consistent with the characteris+cs of ion cyclotron waves. – Ellip+city: -‐0.71 (leg-‐hand) – Propaga+on angle θBk = 56°
• Poppe et al. [2011] also observed upward photoelectron beam during this wave event.
Power EllipBcity
PropagaBon Angle
Ion Energy/Velocity Distribu+on during Wave Event
• Magne+c field line connected to the Moon • Ion temperature ~3 keV (plasma sheet) • Ion distribu+on func+on shows a clear shortage of par+cles
in the sunward direc+on from the Moon. • The anisotropy of ion temperature resul+ng from the “crab-‐
shaped” velocity distribu+on could lead to ion cyclotron instability.
Ion Cyclotron Waves Farther From the Moon
• ARTEMIS in SSE: (-‐2.61, -‐5.88, -‐1.13) RM
• (1) f at fc,He+; (2) Leg-‐hand polarized; (3) field-‐aligned propaga+on
• Could be associated with H+ , H2+, or He+
• Convec+on speed ~ 100 km/s à PUI?
fci (N=2)
Associated with H2+?
Possible Scenarios 1. Par+cle distribu+on, altered by the presence of the Moon, leads to ion cyclotron instability
2. Pickup ions from the lunar exosphere: ring-‐type distribu+on is no+ceable only in the tenuous magnetotail.
• Reflected ions may become neutrals, such of which with sub-‐escape speeds
F(V⊥)
V⊥
0.01*F(V⊥)
V⊥
Magnetotail Solar Wind B
Summary � Narrowband waves with f ~ fci have been
observed at/near the Moon in the Earth’s magnetotail.
� These narrowband waves can often be seen on the lunar surface and occasionally seen by orbiting satellites.
� Surface and spacecraft observations of these waves suggest plasma sheet particles (with velocity distribution altered by surface absorption) and pickup ions from lunar atmosphere could be the sources.
� These ion cyclotron waves at the Moon provide clues to the conditions of surrounding plasmas and the lunar exosphere.