Astronomy & Celestial Mechanics
the sky, computed
Concept

Magnetic pressure — where it appears

The isotropic pressure a magnetic field exerts across its own field lines, equal to the square of the field strength over eight pi in Gaussian units. It is the only way a field enters a hydrostatic balance, and comparing it with the gas pressure gives the plasma beta.

Named by 6 essays across 3 fields — each of them below, with the objects they name alongside it.

A spot is dark because it is squeezed. Pressure against depth below the quiet photosphere's optical surface, drawn as a ratio to the pressure there. The rising curve is the surrounding gas, which grows exponentially with a 140-kilometre scale height because that is what hydrostatic equilibrium in an ideal gas produces. The flat pair of bands is the spot's own budget: a magnetic pressure of 3.58·10⁵ dyn/cm² from a 3000-gauss field, which is 25 per cent of the total, plus the gas pressure left over. Horizontal balance requires the two columns to reach the same total at the same geometric level, and the level at which they do is 348 kilometres below the quiet surface — so the spot's own optical surface sits in a hollow. Measured Wilson depressions, obtained from the foreshortening of a spot near the limb, are four to six hundred kilometres. Nothing about the darkness was assumed: a field strength read off a Zeeman splitting fixes the magnetic share, the share fixes the depression, and the depression fixes the temperature the deeper layer must have to carry the reduced flux.

The darkness a field pays for

A sunspot is not cool because something is missing. It is cool because a three-thousand-gauss field supplies part of the pressure that holds the column up, the gas therefore supplies less, and the level at which that gas becomes opaque sits some hundreds of kilometres deeper than the surface around it.

stars · Plasma beta A boundary that a sixty-fourfold gust moves by a factor of two. The standoff distance of a dipole magnetosphere against a wind, in planetary radii, against the wind's proton density, for three wind speeds. The boundary sits where magnetic pressure equals ram pressure — the only balance available, since the field exerts no force on neutral gas and the wind touches nothing. Because a dipole falls as the cube of distance its pressure falls as the sixth power, so the standoff goes as the ram pressure to the −1/6: at 450 km/s a density of 6 per cubic centimetre puts the nose at 9.3 radii — against a measured average of ten to eleven — and a sixty-fourfold compression moves it only to 4.7. That exponent is the reason a magnetosphere is a stable thing to have. It is also the reason the boundary's position is a poor measurement of the field: a factor of two in the distance is a factor of sixty-four in what caused it, so the standoff measures the wind badly and the planet's moment hardly at all.

A boundary that hardly moves

The magnetopause sits where the planet's magnetic pressure equals the solar wind's ram pressure. Because a dipole falls as the cube of distance, its pressure falls as the sixth power — so a sixty-fourfold gust in the wind moves the boundary by a factor of two, and a boundary that will not move is what makes a magnetosphere a stable thing to have.

sky · Magnetosphere A layer several times thicker than its heat can explain. The vertical density profile of the neutral gas layer, drawn twice. The narrow curve is what thermal pressure alone supports: an 8-kilometre-a-second sound speed against the vertical gravity of the stellar disc gives a scale height of 47 parsecs. The broad curve adds the three pressures that are not heat — turbulent motion, the magnetic field and the cosmic rays — which together roughly triple the total and, because the scale height goes as the square of the effective dispersion, thicken the layer by a factor of 3.1 to 145 parsecs. The observed half-thickness of the H I layer is about 150. The narrow curve is a real prediction and it is wrong, and what is missing from it is precisely the two terms that emit nothing: a field measured by Zeeman splitting and Faraday rotation, and a particle population measured by what it does to a detector on a mountain. A galaxy's gas disc is inflated by things that cannot be photographed, and the thickness is how the inflation is measured.

A disc held open by what cannot be photographed

Solve hydrostatic equilibrium for the galaxy's gas layer using its temperature and the disc's gravity, and the answer is a layer several times thinner than the one that is there. What is missing from the calculation is turbulence, a magnetic field and a population of cosmic rays — three pressures of comparable size, two of which emit nothing.

galaxies · Interstellar medium Plasma beta from the corona's base outward. The ratio β of gas pressure to magnetic pressure against distance from the Sun's centre, one to ten solar radii on logarithmic scales, for a static isothermal corona in a dipole field. quiet Sun, 5 G at the base, with 10⁹ electrons per cubic centimetre at 1.5 million kelvin: β starts at 0.41 and crosses one again at 3.46 solar radii; quiet Sun, 10 G at the base, with 10⁹ electrons per cubic centimetre at 1.5 million kelvin: β starts at 0.1 and crosses one again at 5.00 solar radii; active region, 30 G at the base, with 3·10⁹ electrons per cubic centimetre at 2.5 million kelvin: β starts at 0.058 and crosses one again at 2.97 solar radii. The gas pressure falls steeply at first and then levels off, because gravity weakens with distance; the field's pressure, falling as the sixth power, never levels. Every corona therefore has a second place where the gas takes over, and it sits at two to four solar radii — where the closed loops of the corona end and the streamers' tips begin.

A corona held by its field and opened by its gas

A thousand kilometres above the Sun's surface the gas pressure has fallen so far that the magnetic field takes charge, and the corona's loops are the shape the field gives it. But the gas pressure of a hot atmosphere stops falling a few solar radii out, while the field's keeps falling as the sixth power of distance. So there is a second place where the gas takes over — and it is where the loops end, the streamers begin, and the solar wind is let out.

stars · Plasma beta A storm's depression at the equator, as an energy. The depression of the horizontal magnetic field at the Earth's surface against the total kinetic energy of the particles trapped in the ring current, on logarithmic axes. The Dessler–Parker–Sckopke theorem makes the two proportional — ΔB/B₀ = −⅔ E/Eₘ, with Eₘ = 8.4·10¹⁷ joules the energy of the Earth's dipole field above the surface — whatever the particles' energies, pitch angles or distribution in radius. The marked storms are read off the line: a moderate storm of 50 nT is 2·10¹⁵ J of trapped particles, an intense one of 100 nT 4·10¹⁵ J, the storm of March 1989 at 589 nT 2.4·10¹⁶ J, and the lowest estimate for the Carrington storm of 1859 3.4·10¹⁶ J — a few hours of the solar wind's power flowing past the magnetosphere.

A storm weighed by a compass needle

During a geomagnetic storm the horizontal field at the equator drops by tens or hundreds of nanotesla, and a theorem says the drop is proportional to the total energy of the particles trapped around the Earth — whatever their energies, their pitch angles or their distances. A ground magnetometer therefore weighs a population tens of thousands of kilometres up, in joules, and the shape of the record says what that population is made of.

sky · Magnetosphere Ions picked up from Mars and swept off on loops. The paths of a hydrogen and an oxygen ion created at rest just above the dayside of Mars and picked up by the solar wind, flowing left to right at 420 km/s through an interplanetary field of 3 nT perpendicular to the page. In the planet's frame each moves on a cycloid: it is accelerated by the wind's electric field, turns in the magnetic field, comes momentarily to rest and repeats, drifting downstream at the wind's speed and reaching twice it at the top of each loop — four times the wind's energy per unit mass. The loop height is twice the gyroradius: 2923 km for hydrogen, 46774 km for oxygen, against a planetary radius of 3390 km. The oxygen is thrown clear of the planet in a single plume to one side, which is how a planet without a magnetic field loses its heavy atmosphere to the wind.

A planet that borrows the wind's field

Venus and Mars have no magnetic field of their own, yet the solar wind does not reach their atmospheres. It is stopped by their ionospheres, whose thermal pressure balances the wind's at an altitude that moves logarithmically with every gust, and the interplanetary field piles up and drapes around the obstacle to make a magnetosphere on loan. The arrangement holds only while the ionosphere out-pushes the wind, and it leaks heavy ions on loops wider than the planet.

sky · Magnetosphere

Named alongside it

The objects these essays reach for when they reach for this one.

Solar windMagnetosphereGeomagnetic stormHydrostatic equilibriumMagnetopausePlasma betaScale heightAtmospheric escapeAuroraBow shockCharge exchangeConvection

All concepts