Monday, October 27, 2014

What an Active Region!

What an active region 12192 has been! Seven X-class flares in 9 days. Here is an HMI image of AR 12192 at 1615 UTC on 23-Oct-2014. You can see a couple light bridges going across the sunspots. There are some partial bridges in the left hand sunspot. But no matter how hard I squint I can't see a jack-o-lantern. It will soon be lost to view as it rotates over the limb of the Sun. Large flares often (but not always) come from large active regions. How did AR 12192 rank in size among active regions?

I downloaded the active region dataset produced by David Hathaway at Marshall SFC and looked at the largest active regions since 1874. This area data comes from the photographs taken at the Royal Greenwich Observatory until 1976 and various sources after that. I checked the list against the tables in Sunspot and Geomagnetic-Storm Data by Sir H. Spencer Jones just to make sure. I also added two regions from 2014 (AR 12192, of course, and AR 11967). Areas of active regions are measured in micro-hems (millionths of a solar hemisphere, where 1 micro-hem is about 1.54 million sq km.)

Here is the plot of maximum area of active regions in time. There are 32908 active regions in this plot. The color and symbol size changes as the area increases. AR 12192 is drawn as a black dot all the way at the right side of the plot. There is a dashed line drawn at 2750 micro-hems to help you compare this area with the others. From this we see that AR 12192 ranks 33rd in a list of active region sizes, not the biggest but in the top 0.1%. It is bracketed by AR 4497 on 08-Jan-1897 (2743 micro-hems) and AR 19109 on 08-Jan-1959 (2805 micro-hems). The area of the Earth is 83 micro-hems, meaning AR 12192 has an area of about 33 Earths.

The yearly Sunspot Number is drawn as a red line. This allows you compare how the maximum area of active regions changes with the number of active regions. It was surprising that the largest active regions are not in Solar Cycle 19, which has the largest amplitude in Sunspot Number, but in Solar Cycle 18. The top 5 active regions appeared between 1946 and 1951. Solar Cycle 19 started in 1954. The next set of large active regions have areas of 3000-4000 micro-hems and are spread across many sunspot cycles.

How big was the active region that was the site of the Carrington Flare? It was 2300 micro-hems, not even in the top 50! You can look it up on p. 102 of the table by Jones. The flare on 1-Sep-1859 has been estimated at X35, showing that large flares can come from medium-sized active regions.

So long for now to AR 12192 but perhaps we will see you next rotation!

Wednesday, October 22, 2014

It's Flare Season!

Active Region 12192 has grown to 2410 millionths of a solar hemisphere (0.2410 % of one side of the Sun), making it one of the larger active regions in Solar Cycle 24. It unleashed an X1.6 solar flare at 1400 UTC (10:00 am ET) today. Here is an HMI continuum, image with the visible active regions labeled. It had already been the site of several M flares and a half dozen C flares. This pattern of large active regions (and large flares) is common for Solar Cycles. Some of the largest flares ever recorded were in the later stages of Solar Cycle 23, and were also in the southern hemisphere of the Sun. I have been told AR 12192 is visible to a well-protected eye without a telescope. The rain in the Mid-Atlantic will prevent me from testing that, but others can check that out. (Always use appropriate eye protection when looking at the Sun! Eclipse glasses or other tested and approved filters should be used.)

This active region has caused a "Major Flare Watch" to be declared, so the two calibration maneuvers SDO had scheduled for today will be done in two weeks. Here is a summary of our projected activities for the rest of 2014.

  • 10/20/2014: Null Bias Application
  • 10/29/2014: EVE Cruciform (@1800ut)
  • 10/30/2014: Null Bias Removal
  • 11/05/2014: EVE FOV (1315-1537 UTC) and HMI/AIA Flatfield (1630-1907 UTC) Calibration Maneuvers
  • 11/12/2014: Null Bias Application
  • 11/22/2014: Lunar Transit (2229-2304 UTC)
  • 12/01/2014: Null Bias Removal

Tuesday, September 23, 2014

Fall 2014 Eclipse Season Ends; Lunar Transit is Next

Yesterday saw the end of the Fall 2014 eclipse season. If you look carefully in the upper right corner of this AIA 211 sequence you can see the limb of the Earth. The image at 0629 UTC looks dimmer and the corona is missing because of the atmosphere of the Earth absorbing the sunlight. The image is darker over most of the polar cap, although that is easier to see if you use the SDO data browser. Select the times for 22-Sept-2014 and AIA 211 to watch the dimming of the Sun caused by the atmosphere of the Earth.
The Sun appears dimmer in the upper right corner where the Earth's atmosphere is absorbing the EUV light.
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Our next lunar transit is Wednesday, 24-Sept-2014, from 0650-0720 UTC (2:50-3:20 am ET). This movie from the SDO FDS team shows that the Moon will cover less than half of the solar disk during this transit. There are a lot of bright active regions in that part of the Sun, so the sharp edge of the Moon will probably cover something interesting.
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Our last lunar transit is 22-Nov-2014, from 2225-2305 UTC (5:25-6:05 pm ET). This movie from the SDO FDS team shows that the Moon will cover only a small part of the Sun during this transit. There aren't a lot of bright active regions in that part of the Sun, but the sharp edge of the Moon often covers something interesting. This is the fourth and final lunar transit that SDO will see in 2014.
Edited 14:45 ET to correct the movie of tomorrow's transit.

Wednesday, September 10, 2014

Stationkeeping Burn #9 and an X1.6 Flare

Stationkeeping burn #9 will happen today (September 10, 2014) at 2215 UTC (6:15 p.m. ET). SDO data will be unavailable for about 30 minutes starting 2200 UTC (6:00 p.m. ET). This burn was delayed from last Wednesday when we became concerned about another satellite whose orbit would have come close to SDO after the burn.

Today the sun let loose a X1.6 flare, starting at 1721 and peaking at 1745 UTC.

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 1983 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).
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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.