Phase-standing whistler fluctuations detected by SELENE and ARTEMIS around the Moon

Transcription

1 Symposium on Planetary Science 217, February 2 21, Sendai Phase-standing whistler fluctuations detected by SELENE and ARTEMIS around the Moon Yasunori Tsugawa 1, Y. Katoh 2, N. Terada 2, and S. Machida 1 1 Institute for Space-Earth Environmental Research, Nagoya University 2 Department of Geophysics, Tohoku University

2 Abstract Low frequency (<~.1 Hz) magnetic fluctuations around the Moon in the solar wind have been reported since 196s. They are extended upstream of the lunar wake edge along the interplanetary magnetic field lines. We analyze magnetic field data detected by SELENE and ARTEMIS to reveal generation processes of the fluctuations. Our analyses indicate that observed polarizations of the magnetic fluctuations are determined by the spacecraft velocity: right-hand polarization when S/C moves downstream, left-hand polarization when S/ C moves upstream. This fact suggests that their phase velocity in the Moon frame is smaller than the spacecraft velocity and they are R-mode in plasma frame, i.e., they are phase-standing whistlers. They are possibly generated as bow waves around the lunar crustal magnetic anomalies and/ or ballistic fluctuations carried by electrons modified through the wake.

3 Low freq. fluctuation around wake edge S IO + lll5" < INY)PEN. <.1- INYisw - O.SS 5 SOLAR WIND 2 NOISY REGIONS r-noise--i r-noise-----l 27 APRIL 1968 F 1.., '""""'r'r'' I TIME 3.,.,J., 1.!1 I 4 \...,.,/'....! J I!5 1'1 2 I 6 o.,croit v 1 [Ness and Schatten et al., 1969] +SSE 282 RAD !1!1 3. magnetic fluctuations < 5 Hz detected by Explorer 35 [Ness et al., 1967, 1968; Taylor et al., 1968] on magnetic field lines which cross the lunar wake slightly more transverse than longitudinal extend up/downstream for >1 km from the wake travelling speed solar wind speed

4 212/7/26 P1 ARTEMIS event 6 B vectors if ωobs ~ -k Vsc, λ ~ 1.5 km/s /.1 Hz = 15 km comparable to MA scale P2 3 Y SSE (R M ) ion mom ion mom P2-3 if leading edge is determined by Vg_max, P1 Vg_max VSW tan 65 = 68 km/s -6 correspond to whistler-mode waves X SSE (R M ) inbound: RH, outbound: LH opposite even at the same time & space Doppler-shift by s/c velocity from terminator to upstream of expansion region larger Vg than fast MS velocity

5 SELENE event 1 equator ~1 km altitudes in the solar wind (SW) Alfven Mach number, MA ~ 8 polarized fluctuations <~.1 Hz from dayside ~SZA6 to outer wake LH inbound to wake RH outbound from wake

6 SELENE event 2 ~1 km altitudes in SW higher Mach number, MA ~ 16 polarized components <~.1 Hz in disturbed field from terminator to inner wake LH inbound to wake RH outbound from wake

7 Statistical properties, SELENE 1 X Y LSM 6 1 X Z SCO R.6 Y LSM (km) Occurrence rate (%) Z SCO (km) 5-5 Vsc Average ellipticity RH when s/c goes downstream LH when s/c goes upstream Occurrence rate (%) -1 YLSM = sig(b XSSE)(B XSSE), ZLSM < 1 km, R = alt X LSM (km) high occurrence when large Vxsc (km/s) Vx sc Average ellipticity YSCO = sig(pos_xsse)(vsc XSSE), L YSCO < 1 km, R = alt X SCO (km) Vxsc < : RH Vxsc > : LH (km/s) Vx sc -.6 upward k vector & intrinsically RH in plasma rest frame, Doppler-shifted by s/c velocity phase-standing whistlers

8 Shock/wake waves around obstacles Table 1. Summary of features seen in hybrid simulations as a function of scale size of the obstacle. D p Upstream Plasma Waves Magnetospheric features Changes <<c/ pi None Whistler None <c/ pi Some flow deflection, n increases, and v decreases at r>d p Precursor of a plasma tail >c/ pi Pileup at r~d p Flow deflection, n, T, B increase, v decreases Reflected ions Whistler wake, fast and slow magnetosonic waves at wake edges Fast mode bow wave upstream Slow mode wake Particle acceleration at dipole (Particle trapping at belts) Tail with hot plasma Reconnection precursor >>c/ Flow modified and Bow shock Magnetopause, cusp c/ωpi in SW ~ 1 km Dp at lunar magnetic anomalies ~ 1 km 2-D grobal hybrid simulation [Russell et al., 25] phase-standing whistler wake perturbation around the Moon has been also suggested [e.g., Halekas et al., 26]

9 Ballistic effect % FIELD FLUCTUAT ] field fluctuations can be carried upstream from the wake edge by reflected thermal electrons along B SOLAR WIND no relevance to wave propagation & local instabilities ω/k ~ Vdrift Ve δb > δbǁ (k, z) d(d )-i ik f dv G( )f () - Ko, plasma waves (k, z) d(d )-i f G + fav ( )f'( ' z: o,, ),. D k, o ' ballistic effects 5B,: ' ',-o [Krall and Tidman, 1969]

10 Summary 1. We revealed that polarizations of low freq (<~.1 Hz) magnetic fluctuations observed around the Moon and upstream of the lunar wake are determined by the spacecraft velocity: Vsc,x< RH, Vsc,x> LH. 2. This fact indicates that their phase velocity in the Moon frame is smaller than the spacecraft velocity and they are R-mode in plasma frame, i.e., they are phase-standing whistlers. 3. They are possibly whistler wake perturbations generated around the magnetic anomalies and/or ballistic fluctuations carried by electrons.