Saturday, July 19, 2014

This Week in SDO

On July 9 SDO did the EVE FOV and HMI/AIA Flatfield maneuvers. You may have seen the images from AIA wiggle and blur during these maneuvers. This is a normal part of running a solar observatory in a geosynchronous orbit.

This week we have the EVE cruciform maneuver on Wednesday, July 23. On Saturday, July 26 we will have a lunar transit. Here is a short movie showing how the Moon moves through the SDO field of view. The transit lasts from 1457–1542 UTC (10:57-11:42 a.m. ET).

video
It isn't a very long transit, nor does the Moon cover a lot of the Sun. SDO will watch two more lunar transits this here, so stay tuned.

The Sun had a spotless day on July 17! According to the SIDC there were no sunspots on the Sun that day. The average sunspot number for 2014 remains at 90, even though the daily value has been as high as 150. This was the first spotless day since 2011. Unlike the spotless days in 2009-2011, several small sunspots appeared that day. They weren't seen at the earlier time used by the official observer. Here is an HMI with the sunspot circled.

They were pretty small sunspots!

Thursday, July 10, 2014

What Is a Magnetic Field?

Recently I was working at the City Skies Solar Workshop at the Franklin Institute. This program brings middle school teachers and community center leaders together to provide STEM activities at the centers with the teachers ensuring the science content. It’s a really nice program that SDO has worked with for two years.

During the June workshop one of the center leaders asked, “What is a magnetic field?” Now that should be an easy question to answer. SDO/HMI measures the magnetic field of the Sun (see today's solar magnetic field map on the left); we use compasses to find directions using the Earth’s magnetic field. So, what is a magnetic field? It wasn’t as easy to answer as I thought.

A magnetic field is one of the fields used to track forces. It gives us a recipe to describe magnetic forces around the source of the field.

Gravitational fields are used to describe the orbits of planets around the Sun. Electric fields describe how currents flow in the power grid and how radios work. A magnetic field is how we keep track of the magnetic force created by many moving particles (or electrical currents). There are two nuclear fields as well. Fields are not simple concepts. They do not follow our usual experiences. A concept like Newton’s law of motion, that force is equal to mass times acceleration, can be seen every day. But even though they are invisible, fields are as real as the forces they allow us to calculate.

The magnetic force doesn’t work in the way the electric and gravitational force work. Both of those draw particles together (or make them move apart for like electrical charges). Only moving charges can feel the force from a magnetic field. As soon as a particle feels the effect of a magnetic field it starts to move in a circle. The speed of the particle doesn’t change but the direction of the velocity does. This makes a magnetic field a good deflector of particles. There are only the holes at the poles that allow particles in.

Magnetic fields come from electrical currents, whether in the Sun or a magnet. The strength of magnetic forces can be greater than the force of gravity (that’s why magnets work) but it is weaker than the electric forces between charges. It also works when the source of the magnetic field is electrically neutral!

Magnetic forces from those fields push around moving charged particles. Those moving charged particles also produce a magnetic field. Interesting things, like the solar dynamo, happen where the strengths of the magnetic fields from the two sets of moving charged particles are about the same. By concentrating on what a magnetic field does to the charged particles moving through it we are acting like the scientists who first tried to understand why compasses were affected by electrical currents in nearby wires. They also mapped the shape of magnetic fields on Earth with iron filings (and more complicated instruments). On the Sun we can trace the magnetic field in the corona using EUV images (such as from SDO/AIA). Newer instruments can measure the actual coronal magnetic field, which can be compared with how the plasma moves in the EUV images. We use the Zeeman effect on iron atoms by measure the magnetic field near the surface of the Sun. Those scientists also began thinking about fields and started us down the road to modern physics.

What is a magnetic field? A magnetic field is the region of space near a body where magnetic forces due to the body can be detected. It’s the reaction of the particles that counts, not the region.

Edited 08/05/2014 to fix the bar magnet picture.