1

Dynamic Motion of

Bow Shock and MagnetopauseObserved by THEMIS

Hui Zhang (BU)

Q.-G. Zong (Umass Lowell)

D. G. Sibeck (GSFC)

T. A. Fritz (BU)

2

Magnetopause Bow shock

?

13.5

3Magnetopause Bow shock

4

Normal directions of boundaries

UT boundary Normal direction (GSE)

Method

05:50 Magnetopause (0.980,-0.103,-0.169) MVA

07:52 Magnetopause (0.996,-0. 080,-0.044) MVA

07:54 Magnetopause (0.985,-0. 077,-0.153) MVA

07:59 Bow shock (0.813,-0. 292,-0.504) Coplanarity

08:01 Bow shock (0.292,-0. 550,-0.782)* Coplanarity

11:06 Bow shock (0.956, -0.142, 0.256) CoplanarityShock normal from coplanarity method: n=(BuxBd)x(Bu-Bd)/|(BuxBd)x(Bu-Bd)|

*question: why the shock normal at 08:01 is different from others?

5

Magnetopause Discontinuity

Timing analysis shows:Both the magnetopause and the discontinuity move anti-sunward but at different speed.Magnetopause: ~30 km/s; discontinuity: ~ 90km/s.

6

Vx ~ 90 km/s Vx ~ 30 km/s

Discontinuity

Magnetopause

7

• Magetospheric population peaks at ~ 6 keV, and this peak is not present in

the magnetosheath from 0755 to 0757, suggesting these ions are not from

magnetosphere.

Where are the energetic ions (~10 keV) from?

8

• Not from the sun

9

Theta Bn:

Angle between the

IMF direction and the

shock normal

where Shock normal

is (0.956, -0.142,

0.256)GSE

(from THEMIS B

observation at

~1106UT)

Time difference is

2 hours!However…

Shock acceleration?

Quasi-Para Shock Quasi-Perp Shock

WIND data

10

Summary• The magnetopause was found moving anti-sunward at ~30 km/s.

• Two subregions have been observed by all 5 THEMIS spacecraft

in the magnetosheath associated with a discontinuity. This

discontinuity traveled from the bow shock to the magnetopause at

Vx ~ 90 km/s.

• Both low and higher energy ions are observed in the first region

and only low energy ions are observed in the other subregion.

• What caused the shock, magnetopause moving anti-sunward?

• Where are the energetic ions (~10keV) from?

– Not from the solar wind

– May not from the magnetosphere

– Quasi-Parallel shock acceleration? Has problem to correlate the IMF

with the discontinuity marked by red arrows on slide 5.