Location of Magnetopause Reconnection
S M Petrinec1, S A Fuselier1, K J Trattner1, and J Berchem2
1Lockheed
Martin Advanced Technology Center, Palo Alto, CA 94304 USA
2Institute of Geophysics and Planetary Physics, University of California, Los
Angeles, CA 90095 USA
LWS Workshop, Boulder, CO
3/24/2004
Introduction
Magnetic reconnection at the magnetopause between the interplanetary
magnetic field (IMF) and the geomagnetic field is believed to be an
important, if not dominant, process for transporting plasma mass,
energy and momentum from the heliosphere into the magnetosphere.
While this concept has received strong support from observational
evidence, theory, and modeling efforts, the details of the reconnection
process remain poorly understood. In particular, it is not clear where
on the magnetopause surface reconnection occurs. This has important
consequences for understanding the rate and amount of plasma entry
into the magnetosphere. The rate and amount of plasma transferred by
reconnection strongly influence magnetospheric activity, including
ring current and radiation belt interactions. Variables such as the
magnetic shear (anti-parallel or component reconnection) and velocity
shear across the magnetopause boundary, solar wind plasma beta and
momentum flux may all play important roles in determining where the
reconnection process occurs.
Schematics of magnetopause reconnection from the literature,
illustrating the topological differences in reconnection location,
depending upon the interplanetary magnetic field direction.
Southward IMF
Adapted from Day, C., Spacecraft probes the site of magnetic
reconnection in Earth's magnetotail, Physics Today, Vol. 54,
No. 10, 16-17, 2001.
Northward IMF
Adapted from NASA Press release: Connection of Sun's and
Earth's magnetic fields provides energy for auroras, space
weather, 2000.
Types of observational evidence supporting
magnetopause reconnection:
• Enhanced plasma flows along magnetopause
• Normal component of magnetic
field across magnetopause
Sudden, bipolar signatures
appearing normal to the
magnetopause are indicative
of encountering reconnected
flux tubes. From Elphic,
AGU Monogr., 1990.
Sudden plasma flow enhancements tangential
to the nominal magnetopause surface and
much larger than neighboring flows exterior to
or interior to the magnetosphere are indicative
of reconnection. From Gosling et al., JGR,
1990.
Types of observational evidence supporting
magnetopause reconnection (cont'd):
• Magnetosheath plasma on field lines connected to ionosphere
• Ionospheric signatures
Left: Counter-streaming
solar wind alpha particles
observed simultaneously
with O+ outflowing from the
northern ionosphere.
Right: Counter-streaming
alpha particles observed
simultaneously with
counter-streaming
ionospheric O+.
Fuselier et al., JGR, 2001.
Proton FUV emissions from isolated
regions of the dayside ionosphere are
believed to be the footpoint of the
reconnected magnetic field. Phan et al.,
GRL, 2003.
Southward IMF:
Idealized sites of magnetopause reconnection (red) for two different models
Anti-parallel merging
15
Equinox
10
10
5
5
ZGSM [RE]
ZGSM [RE]
Equinox
0
-5
0
-5
IMF
-10
-15
-15
Tilted X-line
15
-10
-5
0
5
10
IMF
-10
15
YGSM [RE]
-15
-15
-10
-5
0
5
10
15
YGSM [RE]
Anti-parallel reconnection model:
Tilted X-line model:
Reconnection occurs where there is maximum
magnetic shear across the boundary separating
two disparate plasma regimes (magnetosheath
and magnetosphere). If there is a non-zero yGSMcomponent to the IMF, then the reconnecting
region is discontinuous. The regions extend from
near the equator on the magnetopause dawn-dusk
flanks up to the cusp regions at local noon.
Also referred to as component reconnection.
Occurs at the standoff location of the magnetopause (where the ambient plasma flows on either
side of the boundary are slow and where the
magnetic field shear is large), and along a line
extending in either direction from the standoff
location, at an angle which bisects the IMF and
magnetospheric magnetic field directions.
Southward IMF:
In this example, proton emissions from the
dayside ionosphere as observed by the
IMAGE FUV/SI12 look very similar to the
expected particle flux flowing into the
ionosphere predicted by the anti-parallel
reconnection model, as modeled with the
Tsyganenko 1996 magnetospheric magnetic
field model and modeled magnetosheath flow
parameters, using the solar wind as input.
There is much less agreement with the tilted
X-line model in this case.
Southward IMF:
In this example, proton emissions from the
dayside ionosphere as observed by the
IMAGE FUV/SI12 look different from the
anti-parallel model results, but agree more
closely with the tilted X-line reconnection
model. The more relevant reconnection model
for a given interval likely depends upon other
factors, such as solar wind plasma momentum
flux, or plasma beta.
Northward IMF:
In this example, proton emissions from the dayside ionosphere are observed by the
IMAGE FUV/SI12 during an interval of northward IMF and positive IMF By. The
pre-noon emissions are believed to be due to precipitating protons from a
reconnection site tailward of the northern cusp (post-noon emissions are a different
phenomenon associated with the plasmasphere). The location of the precipitation is
well-predicted with the Tsyganenko 1996 magnetospheric magnetic field model and
magnetosheath flow parameters, using the solar wind as input. The third panel
illustrates the magnetospheric field lines on the dayside only, along with the
reconnection regions. An outstanding question regards how far down the tail the
reconnection regions actually extend.
Northward IMF:
Same event as on the previous page, but a few minutes later, when the ycomponent of the IMF changed sign. It can be seen that the dayside
proton emissions have now moved to the post-noon sector. The location
of the precipitation is again well-predicted with the Tsyganenko 1996
magnetospheric magnetic field model and magnetosheath flow
parameters, using the solar wind as input. The difference between antiparallel and component reconnection is much less pronounced; expressed
primarily as the width of the reconnection regions at the magnetopause.
Constraints on reconnection location: Magnetosheath Alfvén Mach number
View of the magnetopause from the Sun. Large black circle represents the magnetopause at XGSM=0.
Yellow circles represent the cusp locations at equinox; blue and red squares are the cusp locations at
winter and summer solstices, respectively. With analytic formulations of the variation of density,
velocity, and magnetic field in the magnetosheath along the magnetopause boundary, the magnetosheath
Alfvén Mach number can be determined everywhere. Where the Alfvén Mach number is greater than
unity, reconnection is unstable (most of the magnetosheath plasma undergoing reconnection can reach
the ionosphere, but the reconnected flux tube moves tailward). Where the Alfvén Mach number is
greater than two, most of the magnetosheath plasma having undergone reconnection cannot reach the
ionosphere. (Petrinec et al., JGR, 2003).
Constraints on reconnection location: Magnetosheath Alfvén Mach number (cont'd)
Same as previous slide, but includes a plasma depletion layer. The plasma depletion layer has been
observed by many spacecraft, primarily during periods of northward IMF. The decrease in
magnetosheath plasma density and increase in magnetosheath magnetic field intensity results in a
lowering of the Alfvén Mach number, such that steady reconnection can occur over the dayside
magnetopause, and into the nightside as well (from Petrinec et al., JGR, 2003).
Constraints on reconnection location: Magnetosheath Alfvén Mach number (cont'd)
12
5.2
10
4.6
8
4.0
6
3.3
4
2.5
2
1.6
1
MA
5.8
1
0
0
5
10
15
20
Depletion Factor (k)
14
25
Distance of reconnection site from Bx reversal [RE]
Using in situ particle observations of reconnecting plasma from the TIMAS (Toroidal Imaging
Mass Angle Spectrometer) instrument on board the Polar spacecraft near the cusp during times
of northward IMF, and considering time-of-flight arguments, it was determined for several
passes how far downstream was the reconnection site. In almost all cases, the estimated Alfvén
Mach number at the reconnection region was greater than two. This is very unlikely to be true,
since the reconnection plasma was observed. Therefore, a depletion layer at the magnetopause
almost certainly existed during these intervals (from Petrinec et al., JGR, 2003).
Summary:
There are several lines of evidence which support the concept of magnetic reconnection at
the Earth's magnetopause as a mechanism for transporting plasma into the magnetosphere.
· For southward IMF, there are two different models for determining the reconnection
regions at the dayside magnetopause: Anti-parallel reconnection and the X-line
(component merging) model of reconnection. Observations sometimes support one model,
and sometimes support the other. Other factors such as solar wind momentum flux or
solar wind plasma beta are thought to play important roles, but need to be systematically
tested with observations and numerical models.
· For northward IMF, ionospheric emissions of the footpoint of reconnection agree very
well with the predicted location of reconnection. Anti-parallel and component merging
locations are very similar; differing only in the extent in local time.
· For northward IMF, it is also important to understand how far downstream (tailward of
the cusp) the magnetopause reconnection site lies. This depends upon the particular
properties of the magnetosheath plasma (such as the variation of Alfvén Mach number
and the existence of a plasma depletion layer), as well as on dipole tilt angle.