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.
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.