stp seminar august 26, 2009 masaki n. nishino * 1

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Two mechanisms of solar-wind proton entry

deep into the near-Moon wake revealed by SELENE (KAGUYA)

STP seminar

August 26, 2009

Masaki N. Nishino*1

Collaborators: Masaki Fujimoto1, Kiyoshi Maezawa1, Yoshifumi Saito1, Shoichiro Yokota1, Kazushi Asamura1, Takaaki Tanaka1, Hideo

Tsunakawa2, Hidetoshi Shibuya3, Masaki Matsushima2, Hisayoshi Shimizu4, Futoshi Takahashi2, and Toshio Terasawa2

•ISAS/JAXA, (2) TITECH, (3) Kumamoto Univ., (4) ERI, Univ of Tokyo

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Outline

• Introduction

• SELENE spacecraft & instruments

• Proton reflection at the dayside surface (Saito et al. GRL, 2008)

• Type-1 entry (Nishino et al. GRL, 2009)• Type-2 entry (Nishino et al. GRL, in

press)

• Summary

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More than 80 % of time ...

• The moon stays in the solar wind

– interaction btwn SW and the Moon

• Why important ?– Wake formation behind the

moon

– Particle/dust acceleration– Hazardous in future missions– Space plasma and planetary

surface• no thick atmosphere• no intrinsic magnetic field

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A traditional view of the lunar wake

• Electron-rich– high thermal speed of e-– generation of E field

• Gradual acceleration of SW ions

• No solar wind ions

How do ions behave in the near-Moon wake ?

E

E

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Wind and SELENE

Comparison of wake observations by Wind and SELENE

WindSELENE

(KAGUYA)

detector ions + electrons ions + electrons

altitude 11,000 km 100 km

SW proton intrusion

• gradual intrusion in the distant wake

• along IMF

?

interpretation fluid-type ?

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Outline

• Introduction


• Proton reflection at the dayside surface

• Type-1 entry• Type-2 entry

• Summary

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SELENE (Kaguya) spacecraft

• Launch– on Sept. 14, 2007

• Orbit– polar orbit– 2-hour period– 3-axis stabilized

• Plasma measurement– Ions (composition)– Electrons– Magnetic fields– Waves

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MAP (PACE+LMAG) onboard SELENE (Kaguya)Orbit• 2-h period• polar orbit• 100 km alt.

MAP-PACE• electrons x2• ions x2• each 2 str.

FOV

MAP-LMAG• magnetic field• 32 Hz

MAP measures the near-Moon plasma environment comprehensively.

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Outline

• Introduction




• Summary

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Solar-wind proton reflection at the dayside surface

protons reflected/scattered at the dayside surface

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Outline

• Introduction



• Type-1 entry– Observation– Model calculations

• Type-2 entry

• Summary

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Ion energy gain & loss at wake boundary (1)

SP: accelerationNP: deceleration

downe-

upe-

downion

upionupion

day SP wake NP day SP wake NP

gain loss gain loss

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gain loss gain loss

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gain loss gain loss

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gain loss gain loss

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gain loss gain loss

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gain loss gain loss

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Dependence on SW magnetic field By

By>0

Implication of particle dynamics

By<0

small By

SP: gainNP: loss

SP: lossNP:

gain

no energy gain nor loss

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Outline

• Introduction


• Type-1 entry– Observation– Model calculations

• Type-2 entry

• Summary

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Larmor phase filtering effect ? Wake E field ?

E

E

Vx decreases

Vx increases

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Wake E-field model

How does this simple E field change the SW proton energy ?

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Model calculations : SW proton intrusion

(a) No E field• no acceleration• cutoff due to thermal

motion

(b) with E field• energy gain & loss

no energy change

energy gain energy loss

observedions

SW•By=4 nT•Vsw=350 km/s•Vth=35 km/s

wake potential 300 eV,(width Rm/4, E ~ 0.7 mV/m)

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Model calculations : SW proton intrusion

Trajectory of SW protons

• intrusion to mid- and low-latitude region

Energy in the rest frame

• gain and loss

Energy in the SW frame• gain (as much as

wake potential)

bulk: Vsw=350 km/s, Larmor: v=70 km/s

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Summary of Type-1 entry

Solar wind protons can easily access to the lunar night side.

Before SELENEBefore SELENE Now with SELENENow with SELENE

Complicated plasma environment

We are now constructing a new model.

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Outline

• Introduction




– Observation– Model calculation

• Summary

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Ion found in the deepest wake2 kinds of wake ?(1)almost vacuum(2)plasma entry

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Proton entry into the deepest wake

Proton sneaking into the deepest wake

(from dayside ?)

Accompanied by bi-streaming e-

By-dominant IMF

SZA 168 deg

100 km height

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Obliquely-going protons are detected by IMA

Protons turn upward just near the nightside surface

E-t scatter plot along virtual spacecraft orbit

g E-t scatter plot along the virtual spacecraft orbit scatter location : every 5 degrees in the dayside region of Lon. and Lat. -70~+70 deg)scatter angle : every 2 degrees

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Validity of our model of Type-II entry

Similar patterns related to Type-II entry are reproduced.

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Formation of PGR (proton-governed region)

• Scattered protons sneak into one hemisphere on the nightside

• formation of PGR

• Generation of outward E field around PGR

• PGR absorbs ambient electrons along the IMF

• counter-streaming electrons are found in the PGR

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Wind and SELENE

Comparison of wake observations by Wind and SELENE

WindSELENE

(KAGUYA)

detector ions + electrons ions + electrons

altitude 11,000 km 100 km

SW proton intrusion

• gradual intrusion in the distant wake

• along IMF

• Two types in the near-Moon wake

• perp. to IMF

interpretation• fluid-type• electron

dominant

• particle dynamics

• formation of PGR

References

• Ogilvie et al. GRL 1996• Halekas et al. JGR 2005• Saito et al. GRL 2008• Nishino et al. GRL 2009a• Nishino et al. GRL 2009b

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stp seminar august 26, 2009 masaki n. nishino * 1

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