A Lunar Transit

Today the Moon passed between SDO and the Sun from 1331-1556 UTC (8:31-1056 am ET). At 2.5 hours in length, it was the longest lunar transit so far in the SDO mission.

You can watch the transit using the browse data option on our website. Use the times 2014-01-30 00:00:00 and 2014-01-30 17:00:00 and your favorite wavelength to enjoy the show.

Here is an image from 2014-01-30 14:29:12 in the AIA 171 passband. If you watch the movie you will see the Sun move a little bit during the transit. This transit covers a lot of the solar disk and blocks the sunlight from SDO. The fine guidance systems on AIA and HMI can't work because they need to see the whole Sun to keep the images centered from exposure to exposure. Once the transit is over the fine guidance systems started back up, giving us steady images of Sun.

And, as the Moon left SDO's field of view, an M8 flare erupted in AR 11967.

SDO Flight Dynamics Predicted Events for the Beginning of 2014

The SDO Flight Dynamics team has produced and delivered the first quarter predictions. They extend to the beginning of the Fall Eclipse Season on 2014-247. The highlights are listed below:

• 13-Jan-2014 - Maintenance Power On of IRU #1
• 15-Jan-2014 - EVE Cruciform
• 16-Jan-2014 - Power off IRU #1 - Cold Storage
• 22-Jan-2014 - Momentum Management Maneuver #17
• 28-Jan-2014 - Battery SOC to 90% - Lunar Transit Prep
• 29-Jan-2014 - AIA GT/PZT Cal
• 30-Jan-2014 - Lunar Transit (1331-1556 UTC, 8:31-10:56 ET, a long one!)
• 3-Feb-2014 - Return Battery SOC to 60%
• 5-Feb-2014 - EVE Field of View and HMI/AIA Flatfield maneuvers
• 14-Feb-2014 - Spring Handover Season Begins
• 27-Feb-2014 - Spring Eclipse Season Starts
• 3-Mar-2014 - Solar RFI (~7 minutes)
• 4-Mar-2014 - Solar RFI (~7 minutes)
• 12-Mar-2014 - Stationkeeping Maneuver #8
• 21-Mar-2014 - Spring Eclipse Season Ends
• 3-May-2014 - Spring Handover Season Ends (-Z active afterward)
• 26-Jul-2014/207 - Lunar Transit (1457z-1542z)
• 2-Aug-2014 - Fall Handover Season Begins
• 28-Aug-2014 - Fall Eclipse Season Starts

It looks like we will have three lunar transits in 2014, including one on 30-Jan that lasts almost 2.5 hours. Here is a video that shows how the long transit will look from SDO.

Welcome to 2014!

Happy Perihelion!

Today, January 4, at 1200 UTC (7 am EST), the Earth was at perihelion. This is when we were closest to the Sun in our orbit around the Sun. At perihelion, the center of the Earth was a little over 147 million km (about 91.4 million miles) from the center of the Sun. Why do we care?

The Earth orbits the Sun along an ellipse. That means sometimes we’re farther from the Sun and sometimes closer. The average distance of the Earth from the Sun is about 150 million km (about 93 million miles, one Astronomical Unit or AU). Perihelion is the starting time for that orbit. The distance of perihelion is also important. The perihelion distance of the Earth is about 1.67% closer than the average distance.

When the Earth is closer to the Sun it receives more sunlight and when it is further away it receives less sunlight. That means the timing and distance of perihelion can affect our climate. That doesn’t mean that being closer to the Sun automatically means warmer weather.

Our seasons are caused by the tilt of the earth’s rotation axis compared to our orbit around the Sun. In January the northern hemisphere of the Earth is tilted away from the Sun and the northern latitudes have winter. At the same time those living in southern latitudes experience summer. Because the tilt determines the seasons we mark our seasons by the solstices and equinoxes.

But the changing distance does affect our climate. It appears to make southern winters a little less frigid and northern summers a little milder. In the past the difference was more dramatic.

The timing of perihelion in the year changes slowly. Right now perihelion happens in northern winter, while 11,000 years ago it happened in northern summer. That changed the severity of the seasons.

If we wait even longer (several hundred thousand years) the shape of the Earth’s orbit also changes, with the eccentricity changing from nearly 0 (almost circular) to the current value of 0.0167 to 0.06. A circular orbit means the distance to the Sun does not change, while a more elliptical orbit means the amount of sunlight hitting the Earth changes even more during the year. Right now the amount of sunlight hitting the Earth at perihelion is about 6.5% greater than what hits the Earth at the furthest distance. At an eccentricity of 0.06 that would increase to 27%.

These are just two of the Milankovitch cycles that were developed to explain glaciations.

SDO instruments were built to allow for the changing distance and apparent size of the Sun during a year. But it’s nice to know that those changes do other things as well.

Have a Happy Perihelion Day!

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.

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.

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 sdo.gsfc.nasa.gov 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.

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!