Transcript Solar Wind-Magnetosphere-Ionosphere Coupling: Dynamics in
Prompt Penetration of Magnetospheric Convection to Low Latitudes: What is the Physical Mechanism?
Vytenis M. Vasyliūnas
Max-Planck Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany
Paul Song
Center for Atmospheric Research University of Massachusetts Lowell
2006 AGU Fall Meeting San Francisco, 11-15 December Paper SA44A-05
Conventional Model
Can Electric Field Drive Magnetosphere/Ionosphere?
• • • • Imposing an E-field (without flow): charge separation at boundaries in plasma oscillation period, nearly no E-field inside. Most E-field is concentrated in the sheath near the boundary Imposing a flow at the top boundary: perturbation propagates along the field (Alfven wave), E-field is created accordingly.
Finite collisions result in leakage current and small E-field inside Flow is driven by forces and not by E-field!
Evolutionary Equations (time derivative determined by present values):
B
t
E
f t a
4
r v
t
j E B
f a
r
•
f
v
a
f t a
coll m a
q a
(
E v
c
B
)
m a
g
Divergence equations: 0
E
4
c
g
4
Definition of current density:
J
a q a
v
f a
Generalized Ohm’s Law:
J
t
a
2
q n a m a
(
E
V
a c
Plasma momentum equation:
B
)
q a m a
(
κ
a
)
V
t
1
c
J B
Collision terms (ionosphere):
t
V
coll a
g
J
t
coll
J
t
t
V
coll coll
(
ei
(
m i
in en
m e m i
in
)
J
m e
en
)
n e
(
V
en e
(
en
V
n
)
in
)(
V
m e
(
in
V
n
)
en
)
J
e
Simplified overview of key equations
E
t
4
j B J
B
J
t
(
p
2
E B
/
c
B
/(
n ec e
)] ....
produces change of , on time scale ~ p -1 .
B
t
E
V
t
J B
/
c
in
(
V
V
n
) change of bulk flow produced only by stress imbalance.
•
J
Implications
is determined by the motion of all the charged particles, and there is no
a priori
(
c
/4 )
B
.
reason why it should equal • The equality of the two is established as consequence of the
E
/ t (“displacement current”) term.
• In a large-scale plasma ( primarily by changing
J
p >>1, L p /
c
>> 1), this occurs to match the existing (
c
/4 )
B
, while
E
takes the value implied by the generalized Ohm’s law (LH side = 0), both on time scale of order ~ p -1 .
•
V
is changed by stress imbalance, while B changes as consequence of changing
B
to achieve stress balance, both on time scale typically of order ~ L/V A .
Proposed Model
• Distortion of the field lines result in current • Continuity requirement produces convection cells through fast mode waves in the ionosphere and motion in closed field regions.
• Poleward motion of the feet of the flux tube propagates to equator and produces upward motion in the equator.
Conclusions
• Throughout the magnetosphere and the ionosphere, large-scale plasma flows and magnetic field deformations are determined by stress considerations. Tangential stress from the solar wind is transmitted predominantly by Alfven (shear) waves along open magnetic field lines and by fast-mode (compressional/rarefactional) waves across closed magnetic field lines. Large-scale electric fields and currents are determined as consequences of the above.
• Within the poorly conducting atmosphere below the ionosphere, electromagnetic propagation at nearly the speed of light can occur, but the resulting fields have only a minor effect on the ionosphere.
• Magnetospheric convection propagates from the polar cap to low latitudes on a time scale set by the fast-mode speed ( Alfven speed) just above the ionosphere.
References
• Vasyli ū nas, V. M.: Electric field and plasma flow: What drives what?, Geophys. Res. Lett., 28, 2177 –2180, 2001.
• Vasyli ū nas, V. M.: Time evolution of electric fields and currents and the generalized Ohm’s law, Ann. Geophys., 23, 1347 –1354, 2005.
• Vasyli ū nas, V. M.: Relation between magnetic fields and electric currents in plasmas, Ann. Geophys., 23, 2589 – 2597, 2005.
• Song, P., Gombosi, T. I., and Ridley, A. J.: Three-fluid Ohm’s law, J. Geophys. Res., 106, 8149–8156, 2001.
• Vasyli ū nas, V. M., and Song, P.: Meaning of