Comet ISON Survives

Comet ISON, or some part of it, has survived its close encounter with the Sun, but, as noted yesterday, it seems to have fizzled as a great comet. This animation of still from SOHO shows it orbiting around the sun on 28 November. ISON appears much smaller as it moves away, but its nucleus may still be intact.ison-survives_0Video Credit: NASA

Coronal Hole

cor_hole284_1The Solar and Heliospheric Observatory, or SOHO, took this picture of a massive coronal hole above the sun’s north pole on 18 July. Coronal holes are dark regions of low density matter in the Sun’s outermost atmosphere. They are cooler and therefore, appear much darker than their surroundings.

Coronal holes appear at different places and with more frequency at different times of the Sun’s activity cycle which is currently ramping up toward a solar maximum expected this year. During this portion of the cycle, the number of coronal holes decreases. With the onset of a solar maximum, the magnetic fields on the sun reverse, and new coronal holes appear near the poles with the opposite magnetic alignment. The coronal holes then increase in size and number and extend further from the poles as the cycle moves toward solar minimum.

The holes are important to our understanding of space weather because they are the source of a high-speed wind of solar particles that streams off the Sun up to three times faster than the slower wind elsewhere. It’s unclear what causes coronal holes, but they correlate to areas on the sun where magnetic fields soar up and away into space without quickly looping back down to the Sun’s surface as they do elsewhere.

Image Credit: NASA

Planetary Transits—Mercury

As seen from the Earth, the solar system’s innermost planet Mercury spent about five hours crossing in front of the enormous solar disk in 2003. The Sun was above the horizon during the entire transit for observers in Europe, Africa, Asia, and Australia, and the horizon was no problem for the NASA’s Solar and Heliospheric Observatory (SOHO). Mercury’s tiny dot has been highlighted with circles which progress from left to right (top panel to bottom) in these four images from SOHO’s extreme UV camera. Each panel’s false-colors correspond to different wavelengths of ultraviolet which highlight regions above the Sun’s visible surface. This was the first of 14 transits of Mercury which will occur during the 21st century.

Next week, however, an event much more rare but easier to spot will occur—a transit of Venus across the Sun.

Image Credit: NASA

The Sun-Grazing Comet

Comet Lovejoy (C/2011 W3) was not expected to survive its close encounter with the Sun. Very few sungrazing comets do, but it did. This image, taken of 16 December from the  SOHO (SOlar and Heliospheric Observatory) spacecraft, shows the still inbound remnants of the tail while the brilliant head or coma emerges from the solar glare. The Sun is behind an occulting disk to block the overwhelming glare and is indicated by the white circle. Separated from its tail, Comet Lovejoy’s coma is so bright it overloads the camera’s pixels causing the horizontal streaks.

Sungrazer comets are thought to belong to the Kreutz family of comets which was created by successive break ups from a single large parent comet that passed very near the Sun in the twelfth century. Most have been discovered with SOHO’s cameras, but unlike many sungrazers, this one was first spotted by Australian astronomer Terry Lovejoy from an earth-based observatory. Comet Lovejoy is estimated to have come within 120,000 kilometers of the Sun’s surface and must have had a large cometary nucleus to have survived its intense encounter with the Sun. Remarkable videos of the encounter from NASA’s Solar Dynamics Observatory can be found here.

Image Credit: NASA

Solar Flares

The Solar and Heliospheric Observatory (SOHO) spacecraft captured this image of a solar flare as it erupted from the sun early on 28 October, 2003. Image Credit: NASA/SOHO

Solar flares are giant explosions on the sun that send energy and high speed particles into space. The number of flares varies of a cycle of roughly11 years, and the sun is currently moving towards another solar maximum expected in 2013. More flares will be coming, some small and some big enough to send their radiation all the way to Earth.