Monday, September 30, 2013

SDO Flight Dynamics' Predicted Events for the Remainder of 2013

The SDO Flight Dynamics team has generated the timing of maneuvers and other events for the next 90 days. Here is a list with the year and day of year:
  • 2013/275 - HMI Roll (this Wednesday, October 2)
  • 2013/282 - EVE Cruciform (next Wednesday)
  • 2013/289 - EVE FOV & HMI\AIA Flatfield (October 16)
  • 2013/290 - Handover Season Ends (October 19, +Z active afterward)
  • 2013/303 - Comet ISON Offpoint Dress Rehearsal
  • 2013/332 - Comet ISON Perihelion (~1841z Thanksgiving Day)
  • 2013/336 - Lunar Transit (1503z - 1551z)

At this time, calibration maneuvers are considered essential activities and will happen as scheduled.
However, this blog will not be updated if NASA is closed due to a lapse in government funding.

Friday, September 13, 2013

How Can Today's Sunspot Number be 22?

When I look at the HMI continuum image today I see only a few sunspots, which are circled in the picture on the left. But if I go to the SIDC website, their official Sunspot Number is 22! How can that be?

Rudolf Wolf, one of the first observers of sunspots, noticed that different people saw different numbers of sunspots on the same day. He realized that the different counts came from using different equipment. If you look at the Sun through properly filtered low-power binoculars you might see a few large sunspots while if you looked at the same Sun with a high-power telescope you might see many more. If you look at the Sun with young eyes you might see smaller sunspots than someone with older eyes. To deal with these differences, in 1848 Rudolf Wolf developed the Wolf Sunspot Number by combining the number of individual sunspots with the number of groups of sunspots seen on the sun through a telescope with an 80 mm aperture and a magnification of 64x. He defined the Wolf Sunspot Number as RZ = k(10g + s). To generate the Wolf Sunspot Number you had to measure

  • s = the number of individual sunspots
  • g = the number of sunspot groups (either a set of counted spots or a fuzzy blob that may contain more than one sunspot)
  • k = the observer factor (explained below)
Today RZ is also called the International sunspot number, relative sunspot number, Wolf number or Z├╝rich number.

The observer factor (k) is a number that normalizes the sunspot number observed by any person to that of Rudolf Wolf. Each observer has their own value of k, which is usually between 0.4 and 1.7. Wolf included k so that he could combine observations from observers with eyes, telescopes, and cloudiness that were not the same as his. Assigning k’s to each observer allows us to average values of RZ from several observers to get the official sunspot number. You can also use k to account for how an observer's eyes change with time.

As a rule of thumb, if you divide the Wolf number by 15, you’ll get about number of individual sunspots visible on the solar disk. (This works better at large sunspot number and does not work today.)

I see two groups on the HMI image, the group on the left is a single spot while the group on the right has maybe three. That means my estimate of the Sunspot Number is RZ = k(10 x 2 + 1 + 3) = 24 if I set k to 1. The HMI telescope has a 140 mm aperture, which is larger than Wolf's telescope, so it sees more detail or smaller spots and gives a larger Sunspot Number.

Solar maximum is usually a time of large RZ but here we are watching solar activity fade to almost a spotless Sun. Although Solar Cycle 24 has not been creating a lot of sunspots, the Sun's magnetic field (see the HMI magnetogram at left) is still doing interesting things. You can see the magnetic field of the sunspots to the left and right of the image, while the large areas of magnetic field that support prominences and coronal holes cover the disk.

That's why we study the magnetic field!

Wednesday, September 11, 2013

Stationkeeping Burn Today

Today at 2200 UTC (6 pm ET) SDO will execute a stationkeeping burn. This 2.4 second firing of four thrusters keeps SDO within its longitude limits of its assigned geosynchronous orbit. Science data will be unavailable for about an hour to allow the spacecraft to prepare for the burn, fire the thrusters, and recover from the burn.

Friday, September 6, 2013

Coronal Holes on NPR

A story yesterday on NPR's All Things Considered talked about coronal holes. I claimed that they can look like rubber chickens and kokopellis. The rubber chicken is easy to see (here on the left in a an AIA 193 image from June 1, 2012). It was nice of a filament to help form one of the legs.
The kokopelli, a flute player from the southwest Pueblo culture, requires a bit more imagination to see (on the left, another AIA 193 image from March 13, 2013).
So here is an illustration of a kokpelli to guide your eye.

There are other coronal holes in these images. The coronal hole at the south pole of the Sun got smaller going from March to June. These polar coronal holes shrink at solar maximum and then reappear at solar minimum.

Coronal holes, in all of their shapes and sizes, will become more frequent as the Sun starts to reduce solar activity back toward another solar minimum.

Like figures in clouds, these shapes are fun to find. You can look for coronal holes at the SDO data website. They show up best in the 3-color images.

You can read more about coronal holes in another blog post.

Tuesday, September 3, 2013

Fall 2013 Eclipse Season has begun

The SDO Fall 2013 Eclipse Season has begin. Every day until September 25, 2013 some part of the Earth's shadow will pass through the SDO telescopes. Here are pictures from the first eclipse of the 2013 season. On the top is a He II 304 image from 0659 on September 2 and on the bottom is a AIA 193 image from about a minute later. The Earth's shadow appears more ragged in the 193 image because of the way the gases in that atmosphere absorb the EUV light AIA is observing and the way that light is generated by the Sun. The He II plasma tends to be cooler and be held closer to the Sun's surface while the 193 emitting plasma is hot enough to extend well off the surface. You can see that difference by looking at the glow just off the edge (or limb) of the Sun near the top of the Sun.
You use the SDO data browser to flip through these images and see the interplay between the atmospheres of the Sun and Earth.