Cluster observation ofelectron accelerationby ULF Alfvén waves

• Above the auroral region at altitude of 5-6Re, periodic upward (sometimes downward) electron beams are often observed by CLUSTER.

• They are coincide with ULF Alfvén waves (E- and B- field disturbance) of few minutes.

• The coexistence of the beams and the ULF waves implies their close relationship.

OUTLINE• CLUSTER observation above the nightside auroral region

– High latitude (~70˚-80˚)– Altitudes: 5-6 RE

– Periodic upward electron flux structures– Electric/Magnetic field disturbances at similar period– DMSP conjunction observation

• Temporal/Spatial signatures by four S/C observations– E/B ratio: Signatures of Alfvén waves– Downward Poynting flux comparable to electron’s energy– Electron acceleration signatures shows time viabilities.– Wave forms do not look like structure. – Vortex structure.

CLUSTER-II 2002-04-15 summary

B-field

E-field

dB

Poynting Flux

∑S

electronsUp

Perp.

Down

~300 km

Conjunction betweenDMSP-CLUSTER

DM

SP

SS

M

CL

US

TE

RFG

M

E/B ratio : 106 [m/s] (0.8 mho)• ≈ Alfvén speed : ≈106 [m/s]

• ≤ Pedersen conductivity (~few mho is expected.)

Electron signatures21:53:30 21:54:15

S/C

-1S

/C-2

S/C

-3S

/C-4

21:55:30

Up

Down

EF

WF

GM

S

∑S

Beast

Epole

?

BE?

BE?

BE?

summaryAbout 2 minutes oscillation of electron loss c

orn/upward electron beam and E-field, B-field have been observed at the plasmasheet boundary above the auroral region.They are coincide with …

Upward field aligned current regionUpward ion beam region.

E-, B- field disturbances are likely to be Alfvén waves, and the electrons are accelerated directly by the Alfvén waves.

• E/B ratio: Signatures of Alfvén waves• Downward Poynting flux comparable to electron’s energ

y• Electron acceleration signatures shows time viabilities.• Wave forms do not look like structure. • Several hundreds km of perpendicular wave length.• Vortex structure.