# #19.     The Magnetopause

The magnetic boundary between the Earth's field and the solar wind, named the magnetopause, has a bullet-shaped front, gradually changing into a cylinder. Its cross-section is approximately circular.

 Distances in the magnetosphere are often measured in Earth radii (RE), with one Earth radius amounting to 6371 km or 3960 miles. In these units, the distance from the Earth's center to the "nose" of the magnetosphere is about 10.5 RE and to the flanks abreast of the Earth about 15 RE, while the radius of the distant tail is 25-30 RE. By way of comparison, the moon's average distance is about 60 RE.
These, though, are just averages: the pressure of the solar wind rises and falls, and as it does, the magnetopause shrinks or expands. For instance, when the boundary is hit by a fast flow from a coronal mass ejection, the "nose" is pushed in, occasionally (a few times a year, usually) even past the synchronous orbit at 6.6 RE.

Why is the Earth's field an obstacle to the solar wind?

As noted earlier (discussion of the solar wind), field lines of the interplanetary magnetic field (IMF) are carried along by the solar wind as if they were strings and the moving ions were beads strung on them. A "bead" strung on a solar field line will thus always be on a solar field line, and unless field lines from different sources somehow manage to interconnect, it will never be on a field line connected to Earth. The two plasmas, the Earth's and the solar wind's, then form two separate families, and the magnetopause is simply the boundary separating them.

### Neutral Points and the Cusps

 The tight bond between particles and field lines can sometimes be broken, e.g. when the particles undergo collisions or when the plasma flows through a "neutral point" where the field's intensity drops to zero. On a plot of magnetic field lines, such points stand out as the ones where field lines appear to intersect each other. Intersecting field lines seem to make no sense: how can the magnetic field point in two directions at once? If our plots nevertheless show such points, the intensity of the magnetic force at them must be zero, so that the direction of that force is irrelevant.
For example, one gets such points when one plots the field line configuration inside a magnetopause which perfectly confines all of the Earth's field lines (drawing above). There will be two such points, known as the cusps of the magnetosphere, and they mark the separation between lines going sunward and those going tailward.

When spacecraft were actually sent to the cusp region - the European HEOS 1 and HEOS 2 (1968, 1972) and the University of Iowa's "Hawkeye" (1974), they seemed to observe a disordered magnetic field, weak but not zero. The absence of a strong field to hold off the solar wind means that these are "weak spots" on the magnetopause, and solar wind plasma penetrates there to fill two funnel-shaped regions. Some of the plasma particles travel all the way to the top of the atmosphere, where their collisions produce a red auroral glow.

### The Open Magnetosphere

 A feature of simple neutral points is that the magnetic fields flanking them have opposite directions (drawing). Field lines just inside the "nose" of the magnetopause point northward. On the other hand, at times when the IMF has a southward slant, as its field lines become draped against the nose (drawing) they have a generally southward orientation.

The northward flowing group would migrate to line "3" in the drawing, an "open magnetic field line" linking Earth to interplanetary space. A short time later the field line linking those particles will occupy position "4", then "5". Dungey proposed that the process was reversed at some distant neutral point (or neutral line) "6" in the far tail, as illustrated here by a figure adapted from his original article. There the interplanetary line halves were reunited and flowed away, and the ends connected to Earth were also joined up again.

Since the nose of the magnetosphere (on the average) maintains its position and is not "worn away" by the reconnection process, the magnetic field lines swept into the tail, and the plasma particles strung along them, must somehow flow back towards Earth, inside the magnetosphere. This (according to Dungey) explained the large-scale sunward flow of the tail, deduced from auroral motions and flows in the polar ionsphere, on field lines which extended into the tail.

Dungey's process of "magnetic reconnection" is expected to increase the flow of energy from the solar wind to the magnetosphere, by establishing a direct field-line link between the two. It does, however, require a southward slant of interplanetary magnetic field lines, a condition which only exists about half the time.

In 1966, soon after regular observations of the IMF began, a student of Dungey, Don Fairfield, did in fact note a strong correlation between such southward slants and the storminess of the magnetic field ("magnetic activity"). Nowadays "southward IMF" is recognized as the most important factor promoting storms and substorms in the magnetosphere, far more important than increased velocity or pressure of the solar wind, sunspot numbers, etc. This connection supports Dungey's idea of an "open magnetosphere," though direct evidence by observing interconnecting field lines is rather hard to obtain.

### Reconnection with a Northward IMF?

In the figure of the Earth's northern cusp, on the right, all magnetic field lines are directed earthward. It follows that near the magnetopause, field lines located equatorward of the cusp are pointed northward, but those more distant point southward. If magnetic reconnection requires oppositely directed field lines, this may suggest not only that when the IMF lines slant southward, they reconnect near the "nose" of the magnetosphere (as Dungey proposed), but also that when the IMF slants northward, it might reconnect poleward of the cusp, as proposed by Maezawa in Japan. It is an odd type of reconnection, affecting mainly the far tail, whose structure is still not completely clear.

 Currently (1997), the "Polar" spacecraft is orbiting in an elongated orbit rising above the northern polar cap to distances of up to 9 RE. The magnetopause is usually further away, but at times of high solar wind pressure, "Polar" has found itself in the cusp region and even outside the magnetopause. On May 29, 1996, this happened during a time of strong northward IMF, and features were observed which might have originated in this strange sort of reconnection.

Next Stop: #19H.  The Magnetopause--History

Authors and Curators:
• David P. Stern - NASA/GSFC Code 695 (u5dps@lepvax.gsfc.nasa.gov)
• Mauricio Peredo - Raytheon STX Corporation (peredo@istp1.gsfc.nasa.gov)

Last updated March 13, 1999