The Making of a Magnetogram Part I: Lost in a Magnetic Field

Like the magnetic field of a bar magnet (shown here with iron filings), a sunspot group (almost always) has a north and south magnetic pole. Images courtesy of Newton Henry Black and NASA.

Magnetograms are wondrous things, that when I stop to think about what they are, I can scarcely believe that they even exist! A magnetogram is an image, where color represents magnetic field strength. The idea that you can take a picture of a magnetic field completely goes against my intuition of how magnetic fields work. Prior to studying physics, I only knew magnetic fields as funny forces one feels when pushing/pulling two refrigerator magnets together/apart. How could one possible take a picture of an invisible force!? Well, I’m not going to lie- its not a simple process at all. But I’ll give the explanation my best shot..

I will space out this explanation over a few posts, so I can go into a bit of detail about the whole process- starting from the ‘basic’ physics concepts. …I’ve never met a physics that I thought was basic 🙂

While reading this, if you have any corrections (I’ve never read or written a physics discussion with out at least one mistake) or questions (no matter how ‘basic’ you think they are), please post them in the comments section or on Sunspotter Talk!

Magnetic fields put the ‘magnet’ in Magneto

Magneto using his magnetic powers. Courtesy of James Burns.

I could try and describe magnetic fields in some arcane abstract way, but I will start with something everyone has at least heard of: light… A magnetic field is one component of a light wave; the other component is an electric field. If you remember nothing else from what follows, remember these three rules:

1. a changing electric field generates a magnetic field; (formally Ampere’s Law)
   …coiling an electrified wire around a magnet will cause it to spin = an electric motor
2. a changing magnetic field generates an electric field; (formally Faraday’s Law)
   …passing a magnet through a coil of wire will generate electricity = an electrical generator
3. an accelerating (or decelerating) electric field generates light. (e.g., synchrotron and Bremsstrahlung radiation)

An electro-magnetic (light) wave, showing the magnetic and electric field components. Courtesy of the National High Magnetic Field Laboratory.

And what is light? Incredibly, light is an oscillating electric field and an oscillating magnetic field that perpetuate each other, as long as they travel at the speed of light, 300,000 km/s. This is a constant conversion of electric energy into magnetic energy and magnetic energy back into electric energy. The energy of light is determined by how fast the electric and magnetic field is oscillating. However, in quantum mechanics, light is represented as photons, which are  light-wave ‘packets’ of energy, that behave like particles (e.g., they can bounce off of another particle, like an electron). But in classical mechanics, light behaves as a wave (e.g., two light waves can ‘interfere’ and cancel out or build in strength, just like water waves). The fact that light can simultaneously behave as both a particle and wave is known as ‘wave-particle duality‘. To make a magnetogram, you sometimes have to consider light as a particle, and sometimes as a wave.

Light as a wave. This diagram shows the spectrum of different light wavelengths.

Like a packet of crisps (chips), a photon packet of light is VERY calorie dense. Each photon has a specific amount of energy stored inside. If the conditions are right, a photon can deliver all of its energy to an atom (e.g., a hydrogen atom) by smashing into it. When it does this, the orbit of the electron going around the proton (in a hydrogen atom, for example) will become more energetic, and the photon will be completely absorbed by the atom. This is called photon absorption. On the other hand, an energetic electron in an atom can suddenly (sometimes randomly) become less energetic, and in doing so, release a photon. This is called photon emission, and releases photons at a very narrow range of energies that correspond to the change in energy of the electron’s orbit.

Light as a particle (photon). An diagram of an atom: a nucleus surrounded by orbiting electrons, where one of the electrons has decreased in energy, moving to a lower orbit, and released a photon. This is photon emission.

There is another very different way that light can be released. When anything glows (like a red-hot iron or an old fashioned light bulb) a lot of photons are being released, but in a different way than with ‘emission‘. When something glows because it is hot (like the surface of the Sun), it is usually releasing thermal radiation. When matter has a high temperature, it means that the particles it is made of are moving around really fast, and banging into each other. Remember Rule #3? Every time charged particles in the matter bang into one another, they undergoing deceleration or acceleration, like the balls bouncing around on a pool table, and they release light! Not surprisingly,  the hotter the matter is, the more thermal radiation is being released. It turns out that when something glows at a certain temperature it will release a predictable fraction of its photons at each energy; we can use Planck’s Law to predict this. Thermal radiation releases photons at many different energies (a broad spectrum).

A diagram of intensity (how much radiation is released) at each wavelength for a series of objects at different temperatures. These are blackbody spectra. The peak in wavelength for hotter objects is at a shorter wavelength (more energetic) at has a larger intensity (more light is emitted). The colors are in the reverse order, though: red is actually less energetic than green!

The main thing to remember is this: photon emission releases light in a narrow range of energies, while thermal radiation releases light waves in a broad range of energies.

The theoretical black body spectrum for the Sun (gray) and the actual light spectrum for the Sun (orange). The jagged lines are emission and absorption spectral lines. Courtesy of Charles Chandler.

Proton says to a Neutron, ‘I’ve lost my Electron!‘

The Neutron says, ‘Are you sure?‘
The Proton replies, ‘I’m positive!‘

The surface of the Sun is like an ocean, but instead of being made of water molecules, it is made of (mostly) hydrogen and some helium. Another difference between the solar surface and an ocean is that instead of a refreshing ~290 Kelvin the surface of the Sun is a blazing 5,700 Kelvin (…so hot that it glows yellow!). For this reason, it is a plasma (and not just a ‘gas’). It is a plasma because a significant fraction of the Sun’s hydrogen has been stripped of its electron, leaving just a lonely ionised proton (a positively charged particle), wandering around looking for an electron. This gives plasma some very interesting properties, which will be covered in a future blog post when we discuss the physics of sunspots.

The visible-light solar spectrum; the range of wavelengths of light from the Sun that you can see with your eyes. The dark lines are from photon absorption within the solar atmosphere. Courtesy of Aura.

The Sun isn’t just hydrogen, it also has a small amount of a lot of other elements. These other elements can also undergo photon emission, which produces photons of certain energies. We observe this light as emission lines. By studying emission lines, Helium was first discovered because we saw one of its lines when we looked at the light from the Sun (the name Helium comes from Helios, the Greek Sun god). Atoms or partially ionised atoms (that still have at least one electron) undergo photon emission and absorption all the time because of all the other particles and photons smashing into them.

When atoms or ions undergo photon emission in the presence of a magnetic field, something strange happens. And because of this we are able to measure magnetic fields on the Sun- but you’ll have to wait for The Making of a Magnetogram Part II to hear all about it!

As always if you have any questions/ comments /suggestions, we are eager to hear them!

Thanks for listening.