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!

Space Weather on Brewster Rockit

Check out the July 7 Brewster Rockit, a comic at GoComics.com. A nice little illustration about space weather effects in the heliosphere. A bit of a preview is at left.

What is space weather and what research is SDO doing with space weather?

Space weather is (1) the study of eruptive solar activity (solar storms) such as solar flares, coronal mass ejections, solar energetic particles, (2) the effect these storms have on Earth, the rest of the solar system (for example the other planets) and (3) the effect on our technology in space and on Earth.

SDO has provided incredibly detailed observations of the sun and the areas that produce space weather activity. This includes observing the growth of sunspot regions (main flare and major CME producers) as well as the birth of CMEs and the evolution of solar prominences, which often produce large CMEs.  Link to larger version of the picture http://sdo.gsfc.nasa.gov/gallery/main/item/150