Selenogenic Ion Cyclotron Waves:

ARTEMIS Observations and Implications for the Lunar Exosphere

P. J. Chi1,2, H. Y. Wei1, W. M. Farrell2, J. S. Halekas3

1. UCLA 2. NASA GSFC 3. University of Iowa

Lunar Exploration Analysis Group (LEAG) Meeting Columbia, Maryland, October 20-22, 2015

Detection of Ion Cyclotron Waves (ICW) by Apollo M

agne

tota

il

Chi et al. (2013), PSS.

1344 1345 1346 1347 1348 1349 1350 1351 13528

10

12

Bx

(a)

1344 1345 1346 1347 1348 1349 1350 1351 13520

2By

1344 1345 1346 1347 1348 1349 1350 1351 1352-4

-2Bz

22:24 22:26 22:28 22:30 22:329

11Bt

1972 Apr 26 Universal Time [hh:mm]10

-210

-110

0

10-2

100

102

Frequency [Hz]P

ower

[nT2 /H

z]

(b)

P⊥P//

A15

A16

𝑓𝑐𝑐 =𝑞𝑞

2π𝑚𝑐

A15 LSM (B0 ~ 10 nT)

Locations of Waves Observed by Apollo

-60-40-200204060

-60

-40

-20

0

20

40

60

X GSE [RE]

Y G

SE

[RE

]

1st lunation2nd lunation3rd lunation

Solar wind ni ~ 7 cm-3

Turbulent magnetosheath

Magnetotail ni ~ 0.3 cm-3 (plasma

sheet) 0.01 cm-3 (lobes)

Narrowband waves were frequently observed by Apollo LSMs when the Moon was in the Earth’s magnetotail.

Possible Mechanisms of ICW Excitation at the Moon

(a) Ion Absorption at the Moon (b) Pickup Ions

Chi et al. (2013), PSS.

Pick-up Ions (PUI): ARTEMIS Observations and Modeling AR

TEM

IS P

2 –

Nov

embe

r 11,

201

1 Poppe et al. [2012; 2013]

(In magnetotail lobes)

Excitation of ICW by PUI Hybrid simulation by Cowee and Gary (2012)

Ring distribution

Ion cyclotron wave

Ion cyclotron instability reduces free particle energy by electromagnetic fluctuations .

Fluctuations scatter pickup and background ions toward a more isotropic distribution.

Estimating PUI Density from ICW Observations (Venus, Mars, Io torus, Saturn E-ring, etc.)

𝛿𝑞2 =12𝜇0𝑚𝑖𝑁𝑖𝑉𝑖𝑖𝑖2

𝑅

1 + 𝑅2 1 2⁄ − 𝑅− 2𝑅2

Huddleston et al. (1998)

• However, not all ring particle energy will be converted to waves energy.

• The velocity distribution for lunar pickup ions is unlikely to be the same as a ring. An unstable distribution is still expected.

(Assuming all ring particle energy is converted into wave energy)

Typical parameters in magnetotail lobes:

Vinj = 50 km/s |B| = 10 nT Ntail lobe = 0.01 cm-3 R ≡ VA/Vinj [ • ] ~ ½

PUI density (conjectural): Ni ∼ 0.8 (δB[nT])2 (mi[amu])-1 cm-3

Relevance to LEAG • Ion cyclotron waves can provide a new way to infer the mass numbers

and possibly the amount of pickup ions from the lunar exosphere. • Here we use observations by ARTEMIS probes, whose orbits and

field/particle instruments are well suited for understanding the nature of ICW at the Moon.

ARTEMIS Observations of ICW: Type 1

• Several clear wave events were found close to the lunar dayside (highly location-dependent).

• f <= fcp • Left-handed, elliptically

polarized

-6 -4 -2 0 2

0

5

10

-1

0

1

YSSE [RM]

XSSE [RM]

ARTEMIS TH-B 2011 Jul 13-16

Z SSE

[RM

]

11.85 11.9 11.95 12 12.05 12.1-2

0

2

4

6

8

10

12

UT [hours]

Bx,

y,z

[nT]

ARTEMIS TH-B 2011-7-16

BX

BZ

BY

-0.6 -0.4 -0.2 0 0.2 0.4 0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

dB2 [nT]

dB3 [n

T]

B0

start

Ion Energy/Velocity Distribution: Type 1 • Magnetic field line connected to the

Moon • Ion temperature ~3 keV (plasma sheet) • “Crab-shaped” velocity distribution (a

half sphere plus high-energy particles from the other side of the Moon)

• The ICW is likely associated with the temperature anisotropy due to particle absorption at the Moon.

V2

V1

Ion

ener

gy [e

V]

B ve

ctor

[nT]

Moon

Model Prediction of Wave Excitation: Type 1 • Theoretical wave growth can be modeled by the Waves in

Homogeneous Anisotropic Magnetized Plasma (WHAMP) code. • Here we model (1) a drifted anisotropic distribution to emulate the

asymmetric distribution observed; (2) field-aligned propagation • Wave growth at frequencies below proton cyclotron frequency

Maximum growth

ARTEMIS Observations of ICW: Type 2

• Observation was made by ARTEMIS in the magnetotail lobes and several lunar radii from the Moon.

• Spacecraft and the Moon are not connected by magnetic field lines.

• Wave properties: • f ≤ fc,H2+ or fc,He+

• Left-handed polarized • Field-aligned

propagation

THB(P1)

THB(P1)

Ion Energy/Velocity Distribution: Type 2

• Ion energy at ~100 eV (i.e. not plasmasheet)

• Drift velocity at ~150 km/s (i.e. net flow present)

• The ICW is likely associated with pickup He+ (although light ions may be repelled by spacecraft potential and are more difficult to observe).

Maximum growth

WHAMP calculation with 5% PUI

Searching for ion cyclotron waves associated with heavy ions • ICWs found in ARTEMIS (and Apollo) data are associated with H+,

He+ and possibly H2+.

• ICW associated with heavy ions are not apparent in data: a. These waves have much longer periods (minutes), and are less

discernible if the background field is varying. b. Larger gyroradii associated with heavy ions are less likely to form ring

distributions within the range of ARTEMIS.

Ar+

He+ OH+ CO+

H+ (in scale for typical conditions)

Summary • Narrowband ion cyclotron waves at the Moon in the

Earth’s magnetotail can provide information about the escape of pickup ions from the Moon.

• ARTEMIS observations of ion cyclotron waves suggest that these waves are generated by (1) absorption of plasmasheet ions by the Moon, and (2) pickup ions.

• ARTEMIS observed ICWs associated with H+, He+ and possibly H2

+. The ICWs associated with heavy ions are not apparent in data.

• We are conducting a statistical survey of PUI-related ICWs observed by ARTEMIS to investigate the ion escape rates.