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Foreshock and planetary size: A Venus-Mars comparison

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Title: Reflected solar wind in the foreshock region: a Venus-Mars comparison by VEX and MEX Author: Yamau Last modified by: Yamau Created Date: 9/23/2010 1:04:06 PM – PowerPoint PPT presentation

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Title: Foreshock and planetary size: A Venus-Mars comparison


1
Foreshock and planetary size A Venus-Mars
comparison
M. Yamauchi, Y. Futaana, R. Lundin, S. Barabash,
M. Holmstrom (IRF, Kiruna, Sweden) A. Fedorov,
J.-A. Sauvaud, C. Mazelle (CESR, Toulouse,
France) R.A. Frahm, J.D. Winningham (SwRI, San
Antonio, USA) E. Dubinin, M. Fraenz (MPS,
Katlenburg-Lindau, Germany) T.L. Zhang, W.
Baumjohann (IWF, Graz, Austria) A.J. Coates
(UCL/MSSL, Surrey, UK)
2
Outline
1. Introduction Earth's knowledge 2. Venus
(similar to Earth) 3. Mars (Different from
Venus/Earth)
FAB field-aligned beam FAB FS foreshock
3
// beam Vsw
4
Venus (VEX)
connected to BS
5
Same as the Earth
SW
1. Field-aligned H. 2. Gyrating H with large
V//.
?
SW
6
cf. Earth
V?1 - V// (projection at V?20)
V?1 - V?2 (cut at V//-1000 km/s)
Venus foreshock Earth foreshock How about Mars?
7
Quite different from Venus
BS
(1) only "ring" distribution (2) no "foreshock"
signature (examined 500 traversals)
8
Venus-Mars difference (1) parameters
(1) Alfvén Mach number (MA) x ? (2) Gyroradius
(rg) / Bow-shock radius (RS) (3) Inertia length
(c/?pi) / Bow-shock size (RS)
SW parameter RS (BS radius) MA (?n1/2V/B) c/?pi (? n-1/2) c/?piRS rg (? V/B) rg/RS
Venus 1 1 1 1 1 1
Mars 0.5 1.4 3 5 4 8
For Mars RS 5000 km for Martian Subsolar 2 keV
H under 6 nT ? rg 1000 km 5/cm3 H ? c/?pi
100 km
9
1. solar wind, 23. bow-shock cold ion, 4.
sneak out
?V// ltlt V// (yes), ?V// ltlt V// (yes),
?V// V// (no),
??
Due to the finite curvature, some ions do not
re-enter
??
10
Venus-Mars difference (2) cold H
(1) Gravity Venus gt Mars ? (2) Exosphere Venus
lt Mars ? (3) newly born H Venus ltlt Mars (This
is clear from the difference in ring
distribution) ? (4) cold H at Bow shock Venus
ltlt Mars (High density cold H is observed only
for Mars)
11
Venus - Mars difference (summary)
(1) Alfvén Mach number (MA) (2) Gyroradius
(rg) / Bow-shock radius (RS) (3) Inertia length
(c/?pi) / Bow-shock size (RS) (4) Cold ion
inside Bow-shock
parameter RS MA c/?piRS rg/RS cold H at BS
Venus 1 1 1 1 very little
Mars 0.5 1.4 5 8 a lot
? ?
? ?
12
Examine close to the Bow Shock
MEX
c/?pi
rg
We sometimes observed multiple-ring structure.
13
Three types of accelerated ions
beyond rg pickup ions ?obtain B direction
beyond c/?pi within rg reflected ions
within c/?pi foot ions
14
3rd // acc 2nd // acc main // acc pre-acc heating
15
Multiple acceleration
green foot blue primary ring red 1st
branch purple 2nd branch brown 3rd branch
Gyro-phase bunching
red half gyro purple one third gyro
16
within BS
escape
?
(Since VdHT gt VSW, dHT frame is erroneous)
17
SW Reflection ? convert V? to V// in SW frame
??
The observed multiple ring structure is well
explained by multiple specular reflection. But,
why is it observed outside the foot region?
? no Finite bow shock size compared to
rg. yes Cold ion in the bow shock ? This may
explain non-specular reflection at subsolar.
18
Special features for Mars
  • Energy is stepping (due to reflection?)
  • Gyro-bunching effect (due to short distance?)
    with gradual ? acceleration (why?)
  • Two different scale length
  • No specular reflection near the bow shock (need
    to confirm)

19
Venus Earth
No internal magnetic field. Planet is the same
size as the Earth ? Smaller bow shock size than
the Earth, yet MHD regime. Effect of cod ions in
the bow shock can be ignored.
Mars ? Earth
No internal magnetic field. Planet is smaller
than the Earth. ? The bow shock size is too
small to treat with MHD. Effect of cod ions in
the bow shock cannot be ignored.
20
Ending (add Earth)
(1) Alfvén Mach number (MA) (2) Gyroradius
(rg) / Bow-shock radius (RS) (3) Inertia length
(c/?pi) / Bow-shock size (RS) (4) Cold ion
inside Bow-shock
parameter RS MA c/?piRS rg/RS cold H at BS
Earth 5 1.2 0.3 0.4 no
Venus 1 1 1 1 very little
Mars 0.5 1.4 5 8 a lot
? ?
? ?
21
End
22
(No Transcript)
23
IMA looking direction
VEX
24
(No Transcript)
25
Multiple-Reflection
x
S E
S toward BS from left
E toward BS from right
S E
SE toward BS from left
S E
S VHT along BS E along BS
x
S E
S along BS E toward BS
(0.6, -0.8, 0)XYZ
26
Time Spatial variation
3rd // acc 2nd // acc main // acc pre-acc heating
27
Classifying counts in // and ? directions
B (N-direction) is estimated from minimum
variance method applied to the ring distribution
Time Spatial variation
28
Three configurations (on-going work)
Done
2005-7-29 2005-8-3
2005-7-12
2005-8-5
29
Summary
Venus Express / ASPERA-4 often observes
back-streaming H in the foreshock region of
Venus, in a similar ways as the Terrestrial
foreshock, i.e., field-aligned component, and
intermediate (gyrating) component Mars Express
/ ASPERA-3 (same instrumentation as VEX) did not
observe similar ions in the Martian foreshock
region beyond the foot region. Instead, it shows
different type of acceleration in the foot
region, indicating the ion trajectory (history)
during its gyromotion. The finite gyroradius
effect makes Mars a perfect laboratory to study
acceleration processes.
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