Monday, December 30, 2013

Was Comet ISON Seen in AIA 4500?

A recent blog post at The Astronomy Stackexchange asked whether an AIA image taken during the perihelion passage of Comet ISON showed the comet. It does not show the comet. The portion of the image where the bright spot is visible has a complicated pattern that is only partially removed during image processing. I will only discuss the image, not the other points made in the blog post.

The AIA 4500 image described in the blog is shown on the left. An arrow points to a bright area that is claimed to be fragments of Comet ISON breaking up near perihelion. A large format version is available from our jpeg archive on the SDO website.

This bright area is an artifact resulting from stray light entering the telescope through a pinhole leak in one of the front filters. Our science data files are available from the SDO JSOC as fits (Flexible Image Transport System) files, the astronomy way to store image data. Each CCD image moves through a series of image processing to subtract the bias field, correct for dark current, subtract the flat field, remove proton hits, and other known effects. The final science data is the processed image. You need special software to work with fits files, so we convert the images to jpeg files to make them easier for a larger audience to use.

I downloaded the fits file that leads to the AIA 4500 image at 1800 UTC, opened it in fv (a free fits viewer), and made a screenshot. Here is that original image, with an inverted color table to enhance the pattern to the left and below the image of the Sun in the upper right. This pattern is a result of a pinhole light leak in a front filter and scattering inside the telescope. While the spacecraft is moving to the offpoint, the brightness of the pattern changes, and its structure is a little smeared as well. With the solar disk out of the center of the field of view, the pattern stands out more clearly than usual; however, a similar stray light pattern is visible in all AIA 4500 images. We have decided to stop serving the AIA 4500 Å images because of this pattern (see the blog post on December 17, 2013.)

AIA 4500 images were taken once an hour during the Comet ISON perihelion passage. It was only chance that this one was taken while we were moving. The AIA 4500 image an hour later, which is much closer to perihelion and a much better exposure, is shown in the second screenshot. The pattern is now much closer to the usual pattern and the processed image at the SDO website shows no sign of the comet, but there is a little brightness in the same area as the earlier image (1289 pixels over and 1379 pixels down). This shows the difficulty in removing the pattern, especially in regions with no direct solar input.

SDO Science Team members were very disappointed to not see Comet ISON as it flew by the Sun. We are convinced that SDO data does not reveal the presence of the cometary detritus.

Monday, December 23, 2013

JSOC data flow has been restored

Images are flowing again from the SDO JSOC. Many thanks to the people who worked this weekend to repair the system.

Sunday, December 22, 2013

JSOC Disks Under Repair

Update: Images are flowing again, thanks to everyone for their patience.

The SDO HMI/AIA data distribution system is undergoing some emergency repairs, and image updates from have been suspended. SDO personnel are diagnosing the problem and it is not clear how long the outage will last. Data from SDO/EVE, including SAM images, will continue to be available during the outage.
We are sorry for any inconvenience.

Friday, December 20, 2013

It's The Solstice

Tomorrow, December 21, 2013 at 1711 UTC (12:11 pm ET) is the winter solstice in the northern hemisphere (the summer solstice in the southern hemisphere). In the north we will have the shortest day and longest night, which usually means its cold. What is really means is that the Earth's rotation axis is pointing 23° away from the Sun. The north pole of the Earth is in continuous night. Many holidays are associated with the solstices and I hope you enjoy your celebration.

The Sun just keeps on producing sunspots and I would like to comment on some of the other features (prompted by a question on the SDO Twitter feed and the Rainbow of Wavelengths video.) The question was how common are dark plasmas? When we look at the Sun in EUV wavelengths it is actually quite common to see both bright and dark regions. The triptych shows AIA 193 Å, HMI magnetic field, and AIA 304 Å, all from 1800-1830 UTC today. I identified several dark regions, a coronal hole and three filaments, which will morph into prominences when they rotate onto the edge of the Sun.

Coronal holes are dark because there is very little material in that part of the corona. No material means no glow. As solar maximum starts to wind down we will probably see many more coronal holes on the Sun. The ones that form at the poles are particularly neat, as they can last for several years as little caps.

Filaments are dark for the opposite reason. There is too much material and the light from anything beyond the filament is absorbed by the filament. More or less like holding your fingers in front of your face to keep bright light out of your eyes. It is also cooler material and doesn't glow as brightly. Filaments like to form along the lines where oppositely pointed magnetic fields meet. That is easy to see in the filaments on the right (one with an arrow, the other just the letter F.) Look for the grey area between white and dark magnetic fields in the magnetogram at the same place as the filaments in the EUV images. This line is called the "Polarity Inversion Line". Can you see the Polarity Inversion Line in the filament on the left? It will probably get easier to see as it rotates towards the center of the disk.

The coronal hole has only outwardly directed magnetic field (white in the magnetogram). The field is not as strong as near the active regions and it is usually open, or goes far out into space.

The Rainbow of Wavelengths video shows how the many wavelengths of light that SDO measures allow to see the Sun in many different ways. Bright or dark, our job is to understand all of them.

Enjoy the solstice and keep watching the Sun!

Tuesday, December 17, 2013

SDO is Almost Four Years Old and Our First Problems Arise

SDO is now approaching four years old and we are seeing things that we have to figure out how to handle. Today I'll write about the AIA 4500 Å images.

The AIA 4500 Å images have been removed from the SDO web page. Several streaks have appeared in the images (you can see some on the solar disk between about 9 and 10 o'clock.) We have decided to stop serving those images and recommend that people use the HMI continuum images.

There are four channels in the 4500 Å telescope, 4500 Å, 1600 Å, 1700 Å, and 171 Å. A series of filters is rotated in front of the CCD to measure the light in each channel. About a year ago a small pinhole appeared in the front filter of the 171 Å side of the filters. This is a thin foil filter that keeps the visible light out of the EUV images. The pinhole allows visible light into the telescope. Although the 171 Å has a second filter close to the CCD to keep out the visible light, the 4500 Å cannot as it is designed to look at the visible light.

There is a selector that allows light going through either the foil filter with the pinhole or the other front window. That selector must be rotated between those windows between exposures. You see strange streaks on the 4500 Å images because the selector and other filters are being moved while the 4500 Å is being read out. The light that comes through the pinhole is enough to create a signal on the CCD.

The 4500 Å images are still available through the JSOC.

Monday, December 2, 2013

Lunar Transit, December 2, 2013

Today from 1503 – 1551 UTC (10:03-10:51 am ET) SDO watched the moon move across our view of the Sun. The next lunar transit is closer than you think, January 30, 2014, from 1331–1556 UTC (8:31-10:56 am ET). Today's lasted 48 minutes, the next will last almost 2.5 hours!