Sunday, August 31, 2014

What’s at the Center of the Sun?

Here is an illustration of a rocket flight from the surface of Earth to the center of the Sun. A few altitudes in each atmosphere are shown. The Earth altitudes are the surface, tropopause (where airliners fly), and the Kármán line (the boundary between our atmosphere and space). The Sun shows the photosphere (the visible surface of the Sun), the bottom of the convection zone, the top of the nuclear core, and the center of the Sun. I used atmospheres (atm) as the unit because 1 atm is the pressure we are most used to.

A question about the interior of the Sun was recently asked on the Little SDO Facebook page:

"No one will ever know what is at the core of the sun. How do the scientist know what’s at the core, have they been in there? Did they actually build something that can withstand the heat of the sun to go in there and look?"

Amazing as it seems, we know quite a bit about the conditions at the center of the Sun. They are measured in two independent ways, sound waves and neutrinos. We also build models of the Sun that use what we know about the physics of plasmas that must agree with those measurements.

Let’s think about the models first. Stars live in a state of precarious balance between gravity trying to pull material inward and pressure pushing the same stuff outward. At every altitude in an atmosphere the pressure needs to be sufficient to hold up the mass of material above it.

You live at the bottom of an atmosphere. The mass of the atmosphere above you produces a pressure at the surface of (believe it or not) 1 atmosphere! It is also a pressure of 1015 mbar or hPa. How does an atmosphere work? Think of the atmosphere as having layers that start at the ground and going toward space. If you move from the lowest level (at the ground) up to the next level we find that the pressure is smaller because the higher level supports less mass. As we continue moving up the pressure continues to decrease until finally the density is too low and we say we are in outer space.

How do we relate this to the Sun? Starting at the top of the Sun we find a small pressure is necessary to hold up the small mass of the atmosphere. Moving down into the Sun we find the pressure continues to increase until we reach the core. Pressure comes from temperature and density (remember the ideal gas law). That means the temperature and density also increases as we move into the Sun

Astronomers know how to use these models to estimate the pressure, temperature and density in the core of the Sun. The simplest models give a core temperature of 20 million K, a core density of 75 gr/cm3, and a central pressure of 1.2e17 dyne/cm2. That is a pressure of 120 billion atmospheres! Even at these incredible pressures the core of the Sun is hot enough that the material remains a gas. More accurate models that follow the fusion reactions that power the Sun give the pressure as 2.4e17 dyne/cm2 (230 billion atm), a temperature of 15.7 million K, and a density of 154 gr/cm3. The illustration shows some pressure values at some altitudes in the atmospheres of the Earth and Sun as if we could fly a rocket to the center of the Sun.

But scientists are skeptical of models and those numbers should also be checked by a measurement. This led to one of the great debates of physics, the core conditions deduced from Sun’s sound waves and the core deduced from the observed number of neutrinos, known as “Solar Neutrino Problem”.

When the velocity of the solar surface is analyzed we find that it is made up of millions of waves moving with measurable frequencies. These waves also move around inside the Sun. Roughly speaking the lower the frequency of the wave the deeper the wave moves into the Sun. We can also calculate the frequencies of the waves that move in the models we built earlier. Because there are so many waves, matching the model and observed frequencies gives a very good check on our model. (This is like tuning a large bell or pipe organ.) Our current model agrees very well with the observed frequencies. That means the sound waves confirm that the temperature and pressure at the center of the Sun agrees with the frequencies from the accurate model of the Sun.

Another check on this comes from the neutrinos that are emitted by the fusion reactions in the core of the Sun. Neutrinos are neutral particles that interact very poorly with other particles. They have very small masses and come in three flavors, electron, muon, and tau. Neutrinos are very difficult to detect but would tell us whether the reactions used in the model were correct. When they were first detected, the meaasured number of neutrinos did not agree with the model that worked so well for the waves. Less than half of the number predicted by the model were observed!

After much research, such as changing the model near the core to reduce the fusion reactions and the calculated number of neutrinos the model would emit, a new theory of neutrinos was tried. The Mikheyev-Smirnov-Wolfenstein (MSW) effect causes neutrinos to change flavor, from electron neutrino to muon or tau neutrino, as they move through matter. Could this be the reason for the “Solar Neutrino Problem”?

The first neutrino flux measurements in the Homestead Mine were of only very high energy neutrinos. Lower energy fluxes from the Sudbury Neutrino Observatory showed that most of the neutrinos produced by the fusion reactions in the core of the Sun, all of which are electron neutrinos, are changed into muon and tau neutrinos by the time they reach the Earth. Once this particle physics concept was confirmed, the central temperature of the Sun from the sound waves is confirmed.

People have thought about the center of the Sun for over 100 years. The models of the Sun are based on the 1968 Nobel Prize winning work by S. Chandrasekhar and W. A. Fowler. The 2002 Nobel Prize in Physics was awarded to R. Davis and M. Koshiba for the first detection of cosmic neutrinos. T. Duvall, a scientist who works with SDO data, recently won the AAS/SPD Hale Prize for his work using the sound waves to understand the inside of the Sun.

That’s how we know the conditions at the core of the Sun.

Wednesday, August 27, 2014

Solar Dynamics Observatory Captures Images of a Late Summer Flare


On Aug. 24, 2014, the sun emitted a mid-level solar flare, peaking at 8:16 a.m. EDT. NASA's Solar Dynamics Observatory captured images of the flare, which erupted on the left side of the sun. Solar flares are powerful bursts of radiation. Harmful radiation from a flare cannot pass through Earth's atmosphere to physically affect humans on the ground, however -- when intense enough -- they can disturb the atmosphere in the layer where GPS and communications signals travel. This flare is classified as an M5 flare. M-class flares are ten times less powerful than the most intense flares, called X-class flares.

Image Credit: NASA/SDO

Upcoming Maneuvers and Activities

On Friday August 29, 2014 the Fall 2014 eclipse season begins. Each day until September 21 the Earth will pass through the SDO telescopes' fields of view. These are opportunities to see the absorption of the solar extreme ultraviolet by the Earth's atmosphere, showing how high the atmosphere goes.
Stationkeeping burn #9 will happen next Wednesday (September 3, 2014) at 2245 UTC (6:45 p.m. ET). SDO data will be unavailable for about 30 minutes starting 2240 UTC (6:40 p.m. ET).
video
Just after eclipse seasons ends we have the second of three lunar transits in 2014. It will happen on September 24, 2014 from 0650-0720 UTC (2:50-3:20 a.m. ET). Here is the animation of the transit from the SDO Flight Dynamics Team.

Wednesday, August 20, 2014

Why NASA Studies the Ultraviolet Sun

You cannot look at the sun without special filters, and the naked eye cannot perceive certain wavelengths of sunlight. Solar physicists must consequently rely on spacecraft that can observe this invisible light before the atmosphere absorbs it.
“Certain wavelengths either do not make it through Earth’s atmosphere or cannot be seen by our eyes, so we cannot use normal optical telescopes to look at the spectrum,” said Dean Pesnell, the project scientist for the Solar Dynamics Observatory, or SDO, at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

read the rest of the article here...

Momentum Management Maneuver #20 is Today

SDO will execute Momentum Management Maneuver #20 today from about 1915-1945 UTC (3:15-3:45 p.m. ET). This Delta-H burn is used to keep the reaction wheels within their speed limits. During the maneuver the science data is usually not valid as the spacecraft moves around.

We are looking forward to eclipse season starting August 29, 2014, giving us another opportunity to see the Earth's limb against the Sun. Eclipse season ends September 21, just in time for a lunar transit on September 24 from 0650-0720 UTC (2:50-3:20 a.m. ET).A station-keeping maneuver will happen on September 3, 2014, at 2245 UTC (6:45 p.pm. ET).

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.