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 USA2Institute 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 Northward IMF

Adapted from NASA Press release: Connection of Sun's and Earth's magnetic fields provides energy for auroras, space weather, 2000.

Adapted from Day, C., Spacecraft probes the site of magnetic reconnection in Earth's magnetotail, Physics Today, Vol. 54, No. 10, 16-17, 2001.

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.

-15

-10

-5

0

5

10

15

ZG

SM

[R

E]

-15 -10 -5 0 5 10 15

YGSM [RE]

Equinox

IMF

Anti-parallel merging

-15

-10

-5

0

5

10

15

ZG

SM

[R

E]

-15 -10 -5 0 5 10 15

YGSM [RE]

Equinox

IMF

Tilted X-line

Southward IMF: Idealized sites of magnetopause reconnection (red) for two different models

Anti-parallel reconnection 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 yGSM-component 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.

Tilted X-line model:Also referred to as component reconnection. Occurs at the standoff location of the magneto-pause (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 y-component 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 anti-parallel 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).

14

12

10

8

6

4

2

0

MA

2520151050

Distance of reconnection site from Bx reversal [RE]

De

ple

tion

Fa

ctor (k)

5.8

5.2

4.6

4.0

3.3

2.5

1.6

11

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).

Constraints on reconnection location: Magnetosheath Alfvén Mach number (cont'd)

Summary:

 · 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.

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.