Reconnection at the Magnetopause
of Saturn: Perspectives from FTE
Occurrence and Magnetosphere Size
H.R. Lai, H.Y. Wei, C.T. Russell,
C.S. Arridge, M.K. Dougherty
Magnetospheres of the Outer Planets
1358, Friday, July 15, 2011
Boston, Massachusetts
1
Magnetopause Reconnection
• Magnetopause reconnection
plays an important role in
the Earth magnetosphere
dynamics by extracting
mechanical energy from the
solar wind flow.
• How important is
magnetopause reconnection
on Saturn where the plasma
conditions are different and
there is an internal energy
source?
2
Evidence of Reconnection: Flux
Transfer Event (FTE)
• FTEs are interconnected flux
tubes seen at the terrestrial
magnetopause that contribute to
the transfer of magnetic flux
from the dayside magnetopause
to the geomagnetic tail.
• They are one manifestation of
reconnection
• They enable us to estimate the
rate of flux transport through
reconnection.
Schematic diagram of an FTE
– We can estimate their flux content.
– We can estimate their speed.
3
FTE Observations at Earth
• Observations from ISEE 1 and 2 of the magnetic field in boundary normal
coordinates.
4
• There are no FTE events
observed at Saturn.
No FTE at Saturn
• The left figure shows the
“Best example” from
magnetic field records.
• There is no electron
structure observed in
plasma data.
• Thus, even our best
example cannot be due to
reconnection. It is a
discontinuity in the
magnetosheath.
• This suggests that the high
beta plasma of the Saturn
magnetosheath is not
conducive to FTE
formation.
5
Evidence of Reconnection: Non-zero
Magnetic Normal Component
• At the Earth, magnetic field
normal to the plane of
magnetopause is occasionally
seen for short intervals inside
magnetopause crossings.
• The magnetic field normal
component at the Saturn
magnetopause is strong and
unidirectional for several
minutes.
• This behavior signifies that
small-scale reconnection does
occur at the Saturn
magnetopause.
• Perhaps the style of
reconnection varies with the
plasma beta at the boundary.
6
Reconnection Evidence: Continued
• Energized
electrons coincide
with the finite
magnetic normal
component.
• This indicates that
small and short
scale reconnection
occurs, but not via
FTE.
• Can we determine
how important is
this reconnection?
7
Short Pulses of Normal Component
at Magnetopause
• In the period of study
there are more than 100
pulses like the one just
discussed.
• Four of these occurrences
are shown here.
• The normal components
last the order of 1 minute
and are 1 to 2 nT in a field
of about 4 nT
• The wave form is very
irregular.
8
The Dynamical Importance of
Reconnection
• We study the
magnetopause
crossings from
1000~1400 SLT in
year 2004~2007,
using crossings
identified by: H. J.
McAndrews, S. J.
Kanani, A. Masters,
and J. C. Cutler.
• At Earth,
magnetopause
moves up to 2 Re
when IMF turns
southward.
9
No Erosion of Saturn Magnetopause
During Northward IMF
• We study 71 cases
of MP crossings from
1000 to 1400 SLT
(51 N and 20 S cases).
• There is no
discernable difference
of the MP distance
between N and S IMF,
thus no erosion of MP
by N IMF.
10
Concluding Remarks
• There are no FTEs observed at the magnetopause of Saturn.
• Along magnetopause crossings, we occasionally see a strong but
short-lived magnetic field along the normal direction to the current
layers. This normal field indicates that small, short-scale
reconnection does occur at Saturn, but not exactly like an FTE.
• Perhaps the high beta magnetosheath affects the style of
reconnection.
• We compared the magnetospheric size during northward and
southward IMF. No erosion of the magnetopause by northward IMF
is evident in the time the field is in that direction.
• Long transport times in Saturn magnetosphere, long time to build
up tail flux from convection, and the fact that the IMF averages to
zero over the long term also lessen the impact of dayside
reconnection on magnetopause location.
• Thus magnetopause reconnection appears not to play a significant
role in Saturn’s magnetospheric dynamics.
11