Friday, November 29, 2013

Where Was Comet ISON?

By now I am sure people are aware that something continues to move along the orbit of Comet ISON in the LASCO observations. It is being discussed whether it is debris from Comet ISON or a remnant. In either case we should try to understand why we didn't see that debris or remnant as it went through perihelion yesterday.

When SDO looks for a comet it does not see the dust and ion tails seen in a comet far from the Sun. We see oxygen ions, oxygen atoms with some electrons stripped off, glowing when they are hit by the hot electrons in the corona. The AIA telescopes normally look at iron ions, but the enormous amount of oxygen that comes off a comet allows that element to emit brighter for a while after the comet passes.

The images we showed yesterday were our best guesses for which AIA passbands would show bright emissions from Comet ISON as it flew past the Sun. They are the passbands that showed Comet Lovejoy in December 2011 and the earlier comet in June 2011. All four passbands would show emission from O IV, O V, or O VI, oxygen atoms with 3, 4, or 5 electrons removed. This is a chain, where O V is produced from O IV and then O VI is produced from O V.

We pointed SDO at three locations along the projected orbit. If you compare the predicted orbit with the SOHO images it looks like we had pretty good alignment. I was watching how the comet moved along its predicted orbit as we changed the SDO pointing and the comet moved through the frames we provided. Here is a snapshot of what I was watching, at 1844 UTC yesterday, about the predicted time of perihelion. You can see that the position of Comet ISON is in the upper left part of the square the size of the Sun showing where SDO is pointed. The AIA telescopes have a larger field of view, because they need to see the corona around the Sun. So the comet is well inside the AIA images at this time. There are some movies posted under "Processed Movies" that show the predicted position of Comet ISON along with the frames taken during the perihelion point. Check out the Comet ISON AIA 171 movies (.mov) or Comet ISON AIA 171 Perihelion (.mp4). The color table in these movies has been fixed to bring out dim features in the corona. You can watch the corona loops extending out farther into the corona then we would normally see.

To estimate the brightness of the comet we had to assume a composition of the stuff coming off the comet and the density of the corona it was moving through. Even though the comet is heated by the glow of the Sun to sublimate the material, it is the corona that strips off the electrons to create the oxygen ions. This means we have a couple of things to check as we try to understand the lack of any visible trail in the SDO images. Did Comet ISON have a strange composition? By strange it would have to have very little oxygen. Many molecules that are found in comets have oxygen atoms in them. Water, carbon dioxide, and most rocks have oxygen in them. Comet ISON appeared normal as it moved through the solar system. It may have been running out of ice as it moved into perihelion, but the dust particles should also have sublimated and contributed oxygen to the corona as they flew past.

Perhaps the corona was not as dense as we assumed. The reactions that cause oxygen ions need fast electrons. Each step in the ion chain takes a little time. If the density of the fast electrons is smaller each step takes a little longer. At the same time the oxygen ions are moving away from the orbit of the comet. As the new oxygen ions get further away from the comet their density goes down and they become more difficult to see. This will require us to go look more carefully at those places in the frames where the comet material has been the longest. Maybe a faint signal can be detected.

We have now seen two sungrazing comets in SDO data. The first disappeared before its perihelion and Comet Lovejoy broke up after perihelion and eventually dissipated. We have also looked for at least two others. One never appeared, and now Comet ISON, which appears to have snuck by perihelion without a trace. We will work to understand what happened as Comet ISON, either as a remnant or debris, flew by the Sun.

We are tied at 2-all for sungrazing comets that we saw and didn't see in AIA data. I hope another sungrazing comet comes by soon to break to tie.

Thursday, November 28, 2013

SDO has Repointed to the Sun

And the dust trail of Comet ISON has moved around the Sun.

Thanks for taking part in our SDO Data Event. A lot of people have help, especially Phil Scherrer, the HMI PI, Karel Schrijver, the AIA PI, Barbara Thompson, Leila Mays, and Kevin Addison. Many thanks to Judy Pepoy for keeping track of SDO during the off-points. The Google+ hangout had a lot of fun!

Thanks for joining us on Thanksgiving Day 2013 for Comet ISON's perihelion breakdown!

SDO is at Departure Pointing

SDO has moved to point at our Departure Pointing box. Let's keep watching!

A Pretty Dim Comet!

We are not seeing anything in 171 or 193.

Now we are hearing that the comet evaporated well before perihelion. I would think that the rocky bits would still move along the orbit and evaporate, but maybe that isn't true.

The latest SOHO LASCO C2 movies show that Comet ISON appears to have run out of steam.

SDO is Moving to the Perihelion Point Position

The Approach Pointing images will always be available for viewing.

It takes a while for the images to move through the system, about 10-15 minutes right now.

SDO is in the Approach Pointing Position

SDO has pointed toward the Approach Pointing box to look for Comet ISON. The images are moving through the data processing and should be viewable in a couple of minutes.

We are seeing images but so far no signal in 171 or 193.

Still looking here, but no comet sightings. If you look at these images long enough you can see anything!

SOHO LASCO Movies Available from our website

You can download the SOHO LASCO C2 and C3 movies of Comet ISON at and selecting "Processed Movies".

Comet ISON Looking Good for Perihelion Passage, 30 minutes and we start looking!

Here is the latest image from the SOHO LASCO C3 Coronagraph. It was taken at 1254 UTC (8:54 am ET) this morning. Comet ISON is looking like a bright sungrazing comet. You can see it moving toward the occulting disk in the center that blocks the Sun from the CCD.

Comet ISON has just entered the field of view of LASCO C2. At 1300 UTC Comet ISON was 7.5 R_sun from the center of the Sun, or 6.5 R_sun from the surface. It is nearing the point where the solar heating can evaporate the small particles, converting them to molecules and atoms, and causing the dust tail to disappear.

Still looking good. Moving toward perihelion this afternoon. It is 6.5 R_sun from the the center of the Sun (5.5 R_sun above the surface.) This is only 3.8 million km (2.4 million miles) above the surface of the Sun. For those keeping track, Comet ISON moved 700,000 km (430,000 miles) closer to the Sun in a little over an hour.

Wednesday, November 27, 2013

Comet ISON Moving into Position for Perihelion

This image from the LASCO C3 Coronagraph on the SOHO satellite shows Comet ISON moving toward its encounter with the Sun. On Thursday Comet ISON will go through perihelion (the closest point of its orbit to the Sun) at about 1:45 pm ET (1845 UTC). During the next two days the comet will go very close to the Sun and make a 140° turn, almost reversing its direction and heading out into interstellar space. After several million years of heading in almost a straight line, it will change course in 2 days!

Join us for the SDO Comet ISON Perihelion Data Event tomorrow from 1 to 3:30 pm ET (1800-2030 UTC). There is also a Google+ hangout running during the perihelion passage.

Come out and watch a comet!

Tuesday, November 26, 2013

Data Flow is back to Normal

The SDO data pipeline has been repaired and data is flowing normally.

We are ready for Comet ISON's perihelion passage on Thursday. Watch for the comet at our special Comet ISON website.

Monday, November 25, 2013

JSOC is being repaired

The SDO JSOC has been upgraded and some issues are keeping data from being available. While the data is being stored safely, they are not bring served to our website. The problems are being worked and should be fixed soon.

Wednesday, November 20, 2013

SDO's Comet ISON Perihelion Event Website

SDO Comet ISON viewing page
Next Thursday, November 28, SDO will watch Comet C/2012 S1 (ISON) fly by the Sun at 1844 UT (1:44 pm ET). We have designed a website to share the SDO images with the public. Here are some tips on the SDO Comet ISON Perihelion Page.
The top bar has a couple of links and drop-down menus. You can view a movie which shows the different areas SDO will point (off-pointing- when SDO is not point directly at the sun) and explore the orbits of Comets ISON and Lovejoy (from 2011, not the current one). The 3-D interactive works with many browsers (but not Safari). It allows you to examine two sungrazing comet orbits and see the views of different satellites. Under resources are pointers to websites that discuss Comet ISON.

Three views for the perihelion of Comet ISON

The home page will have images from four of the AIA wavelengths during each the views (off points), Approach, Perihelion, and Departure. These wavelengths were chosen because in December 2011, when Comet Lovejoy passed perihelion, it was very bright in these wavelengths,  and we believe they are the best choice for Comet ISON. We will not see a normal comet tail. The glowing material is forced to move along the Sun's magnetic field. It will look like a cloud moving along and away from the orbit of the comet.
You can watch for Comet ISON using either the kiosk mode or mp4 movies. If you select "View kiosk" a new page will open and a series of images at that wavelength will be shown. As data becomes available your kiosk display will automatically add the new images. At the end of each 1-hour phase you will be looking at about 300 images. The movie will flip through all of the data and you can watch for the comet to appear. You can run/pause the display, step through the frames, and force an update. If you pause the display you can use the slider to flip back and forth in the images. New images are available in sets of 5 every minute.
If you select "Download mp4" an mp4 movie that has all of the images currently available will be sent to your machine and you can use the movie controls on your webpage to view the images. You have to re-load the mp4 every 5 minutes to get the latest data. The mp4 movies will only be updated every 5 minutes!
Join SDO and watch for Comet ISON on November 28! A great way to spend a few hours of the American Thanksgiving Day.

Tuesday, November 19, 2013

AR 11893 Waves Goodbye with an X-1 Flare

As AR 11893 rotates over the limb of the Sun it erupted with a X-1 flare at 1030 UTC (5:30 am ET) this morning. Here is a detail from the AIA 1700 Å channel. This image was taken at 5:32:55 am ET this morning. It shows a graceful arch where the flare is occurring. The AIA 1700 Å channel is sensitive mostly to ultraviolet continuum light, but during a flare some hot lines are seen. You can see details about the flare without the blooming seen in the hotter channels.

Thursday, November 14, 2013

The Perihelion Passage of Comet ISON

Comet C/2012 S1 (ISON) will be at its perihelion (the closest it gets to the Sun) on November 28, 2013. It will then move back out into the solar system, becoming a night-sky comet and then fading from view. But watch now because Comet ISON will never return! It came from the Oort cloud and it will fly out of the solar system in a few million years.

When they are close to the Sun, sungrazing comets can be seen in the EUV channels of SDO and other solar satellites as their debris lights up the Sun's magnetic field. As Comet ISON moves through perihelion, SDO will point at three places away from the Sun to track the comet. This figure describes what we will do.

About 100 minutes before perihelion we will point SDO at the “Approach” position. It takes about 5 minutes for SDO to point to a new place in space and allow the fuel to stop moving (“Move, settle”). We allow for that in the timing of the moves. SDO will then observe that part of space for one hour. The Sun will appear in the upper right corner of the images and Comet ISON will move through the images from the lower right to the upper left. Images should be available on the SDO Comet ISON website after 12:30 pm ET.

At 1:09 pm ET, SDO will be pointed to the “Perihelion” position and observe that part of space for an hour. This location includes the perihelion, which happens at about 1:44 pm ET. At 2:14 pm ET, SDO will point at the “Departure” and observe for another hour. We will see Comet ISON moving away from the Sun in this box. At 3:19 pm ET SDO will point back at the Sun and we will resume our solar studies.

You can also see this in a movie.

The Oort cloud is many, many comets orbiting very far from the Sun. Kind of the paint splatter from making the Solar System. We think it starts about 0.1 lightyears from the Sun and extends to 0.8 lightyears — about halfway to the next star. Comet ISON is the first Oort-cloud, sungrazing comet in quite a while. It has fallen thousands of AU to just barely miss the Sun and make its escape. Join us at the SDO Comet ISON website from 1700 to 2024 UTC (noon to 3:24 pm ET) on November 28, 2013 (Thanksgiving Day in the USA) to watch this unique event.

Wednesday, November 13, 2013

Sunrise from the Dark Side of a Sungrazing Comet

Can you ride a sungrazing comet around the Sun by staying on the dark side? Maybe not! When you get close enough sunlight reaches around the comet and lights up the dark side. How does that work?

This reminds me of bicycle chains and gears. How long does a chain have to be to reach around two gears? How many teeth fit into the chain as it wraps around?

Here is a picture with gears the same size and a chain wrapped around them. This is the same as gears being very far apart. The chain leaves both gears at the top and bottom of each gear. The same number of teeth are used on both gears. We can think of the Sun heating the comet in a similar way when the comet is far from the Sun. Sunlight warms only one hemisphere. But what happens if you get much closer to the Sun?
If the left gear is made much larger than the right gear we get this picture. Now the chain leaves the large gear at the arrow, far from the top and meets the small gear away from the top. More teeth are used on the large gear, but fewer on the small. This shows that light from the Sun reaches past the one hemisphere and lights up the dark side of the comet. As comet gets really close to the Sun even someone standing on the dark side will see the sunrise.

But that sunrise will not be a little Sun coming up in the East. It will look more like a total solar eclipse fading away. A bright ring will surround the edge of the comet and slowly fill your sky to the horizon as you get closer. You would be able to see the corona first, followed by the chromosphere, and finally the bright photosphere. This effect, also called the two-pulley problem, means that comets absorb sunlight over a larger area close to the Sun than they do far away. This helps them sublimate more rapidly.

Life on a sungrazing comet would sure be different!

Check out our 3-D rendering of the orbits of sungrazing Comets ISON and Lovejoy. It shows both orbits near perihelion. You can explore why different satellites see comets differently.

Wednesday, November 6, 2013

Comet ISON Off-Point Testing Today

We are testing the Comet ISON off-point sequence today. We will move to each of the three positions and wait 10 minutes, taking data to see how the detectors respond. It will take about an hour.

Here is an example of what the Sun will look like as Comet ISON goes through perihelion on November 28, 2013. Somewhere in the inky darkness the comet's light will be seen!

Update at 2:19 pm: Test is over and we are pointing at the Sun again.

Monday, November 4, 2013

November is Comet ISON Month for SDO

This lovely image of Comet ISON was taken on October 27, 2013 by astrophotographer Damian Peach. [Image credit: Damian Peach]

Now that November has arrived it's time to get ready for Comet C/2012 S1 (ISON). On November 28 (yes, Thanksgiving Day) at 1:45 pm ET, Comet ISON will fly only 730,000 miles above the surface of the Sun. Even though it is moving as fast as 375 km/s (840,000 mph), the Sun will heat Comet ISON, causing it to sublimate and leave a lot of water and grit behind. Comet ISON is about 2 km across, much bigger than Comet Lovejoy in December 2011, and it will leave a lot of itself behind.

Once the stuff sublimates we will watch for it to light up the corona in the AIA bandpasses. We saw Comet Lovejoy in 7 of the 10 bandpasses in December 2011. This time we will point SDO toward three different places where the comet is predicted to be. One will be centered about the time of perihelion for a half hour on each side of perihelion. The other two will point toward the paths the comet will use to approach and leave the Sun and take data for an hour at each place. The offpoints are much larger than the one we did for Comet Lovejoy in 2011. We expect that the comet debris will look a lot smoother for Comet ISON because the magnetic field that far from the surface is also smoother.

The near-realtime SDO images will be available as self-updating movies at a dedicated website that is being tested and will soon be ready. Until then, there is an ISON campaign website with a lot of good pictures and information about Comet ISON, written by people who study comets. You can also look at the Know Your Night Sky blog for observing hints, especially after perihelion as Comet ISON climbs away from the Sun towards the North Star.

During November we will talk about how much stuff we should see coming off Comet ISON, why the signatures of a sungrazing comet could be like how supernovae work, and how a belt going around two pulleys can help us to understand sungrazing comets.

Make November a month for Sun and comet watching!