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Hale, Hale, Joy, Joy: a dip into the world of solar physics January 22, 2011

Posted by mareserinitatis in Uncategorized.

Until a few years ago, the most I knew about the sun is that it is huge, is the center of our solar system, provides us with warmth, and will probably flame out in a few billion years. As an undergrad, I had the opportunity to work on a research project that looked at magnetic fields on Earth caused by solar activity. I began to learn a lot more.

One of the first things I learned about was sunspot cycles, which are a consequence of the magnetic field of the sun. A good place to start with learning about magnetic fields is that, in many cases, you’ll want to think about them in terms of spheres rather than cubes. (Actually, this is true when working with antennas as well as planets.) When we do this, we can think of three directions of the magnetic field. The radial direction is the field pointing away from the center of your object.

The harder ones to describe may require the use of a donut. If we think of a donut as a short hose with the top end connected to the bottom end, it makes things a little easier. The toroidal field is the field that follows the length of the hose, which is denoted as a blue arrow in the picture above. The poloidal field goes around the hose, shown by the red arrow.

During periods when the sun is not active (i.e. there aren’t too many sunspots), the magnetic field of the sun is like a big bar magnet (or even Earth’s magnetic field). There is a north and south pole. In other words, most of the field is poloidal. [1] Sunspots, however, are actually tubes of toroidal magnetic field. They rise from a stable layer in the sun, through the convection zone, to a place where we can see them.

Sunspots are dark because they are impeding the flow of energy. This also means they are cooler than the surrounding area. The temperature of a sunspot is around 4200K, while the surrounding areas are closer to 6000K. With this much disruption in heat flow, you would think that the sun would produce less energy during a sunspot cycle. Surprisingly, it’s actually the opposite: the sun outputs more energy during a cycle. The areas around sunspots are also disrupted by the toroidal field, and so the surrounding areas actually put off more energy. These brighter areas are called faculae, which you can see in the picture below. [2]

There are two other interesting facts about sunspots which are summarized with Joy’s Law and Hale’s Law. Both of these are shown in the image below:

This image is a magnetogram. It doesn’t show the actual sunspots; it’s showing magnetic fields instead. The white is the magnetic field pointing outwards, while the black faces inward. What you’re seeing in the image is two sunspot pairs: one from one cycle and one from the next cycle.

One thing I didn’t know until recently was that sunspots tend to come in pairs. This makes sense because it is a ‘rope’ of magnetic field that is poking out. One sunspot is where the ‘rope’ is emerging while the other is where it is re-entering. Once we understand that, Hale’s Polarity Law makes a lot of sense. It states that each sunspot will have an opposite polarity: that is, one will be a ‘north pole’ with a magnetic field pointing outward while the other will be a ‘south pole’, pointing inward.

The other factoid is called Joy’s Law. I will probably explain the reason in another post, but it’s also noticeable on the above image. Joy’s law states that if we draw a line to connect a pair of sunspots, that line will be tilted. The leading spot will be closer to the equator than the one following.

Looking at the image above, we can see that the two sunspot pairs with arrows pointing to them have both a light and dark patch. The dark patches are the leading sunspots, and the light are the following. The other thing you’ll notice is that, in both sunspots, the darker patches are closer to the equator than the lighter patches.

Knowing some of these things about the sun has made me look at it in a whole new light. (Ba dum ching!) But the other fascinating thing is that sunspots come in pairs, and we all know good things that come in pairs:

[1] The heliosphere through the solar activity cycle By André Balogh, Louis J. Lanzerotti, Steven T. Suess

[2] Physics of the Sun: A First Course by Dermott J. Mullan



1. Charles J Gervasi - January 23, 2011

The skip was amazing in ’89. I remember working Australia on 28.4MHz with 25W and a half-wave vertical antenna located less than a wavelength off the ground. This was on a band that had been dead a few years earlier. And there was even skip on VHF, which I imagined was sporadic-E, but now I wonder if someone of it was tropospheric. Some of the real DX on 50 MHz had to be E layer. I had no idea what I geek I was. Sunspots made it all possible.

I’ve enjoyed the sunspot minimum this time around b/c CB is more useful for local info in the car. If I had time, I’d be doing ling some listing on the bands in the 3MHz range, which supposedly due better in low sunspots due to less D-layer absorption. I know a lot of the rare stuff has moved to the Internet, but I bet if I had a nice long antenna well off the ground and time like I had as a teenager, I could have heard some interesting stuff the past couple years during the unusually-long-lived minimum.


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