~ Space Weather Update~ ANOTHER CME IS ON THE WAY:

ANOTHER CME IS ON THE WAY: As Earth passes through the wake of one CME, which did little to stir geomagnetic activity on Aug. 20th, another CME is on the way. NOAA forecasters expect a coronal mass ejection hurled into space yesterday by an erupting magnetic filament to deliver a glancing blow to Earth's magnetic field on Aug. 23rd. High-latitude sky watchers should be alert for auroras. Aurora alerts: text, voice.

SUNDIVING COMET AND FULL-HALO CME: A small comet plunged into the sun on August 20th. Just before it arrived, the sun expelled a magnificent full-halo CME. Click to view a movie from the Solar and Heliospheric Observatory (SOHO):

In the final frames of the movie, the comet can be seen furiously vaporizing. Indeed, those were the comet's final frames. It did not emerge again from its flyby of the hot sun. "With a diameter of perhaps a few tens of meters, this comet was clearly far too small to survive the intense bombardment of solar radiation," comments Karl Battams of the Naval Research Lab, who studies sungrazing comets.

The CME (coronal mass ejection) came from an explosion on the farside of the sun. Although the CME and the comet appear to intersect, there was probably no interaction between the two. The comet is in the foreground and the farside CME is behind it.

Occasionally, readers ask if sundiving comets can trigger solar explosions. There's no known mechanism for comets to spark solar flares. Comets are thought to be too small and fragile to destabilize the sun's magnetic field. Plus, this comet was still millions of kilometers from the sun when the explosion unfolded.

The comet, R.I.P., was a member of the Kreutz family. Kreutz sungrazers are fragments from the breakup of a single giant comet many centuries ago. They get their name from 19th century German astronomer Heinrich Kreutz, who studied them in detail. Several Kreutz fragments pass by the sun and disintegrate every day. Most, measuring less than a few meters across, are too small to see, but occasionally a bigger fragment like this one attracts attention.

TRUE BLUE MOON? Was last night's full Moon a "Blue Moon?" Some observers say "yes," but not because the Moon turned blue. Behold this picture of last night's moonrise over Volterraio Castle on the Island of Elba, Italy, then scroll down for further discussion:

"The Full Moon of Aug. 20-21 is a 'seasonal Blue Moon,'" explains photographer Stefano De Rosa, "because it is the third of four full moons in a single season."

But wait--isn't a Blue Moon the second full Moon in a calendar month? That would be the modern definition, which became popular in the late 20th century. De Rosa's definition is an older and, some would say, truer definition of "Blue Moon."

Which definition is correct? Both and neither. It's all folklore! The only true-blue Moon is a Moon that actually turns blue. And, yes, that can happen. Under certain circumstances volcanic dust and ash from forest fires can scatter the reds out of moonlight, leaving only a blue-cratered disk behind.

Switch to: Europe, USA, New Zealand, Antarctica
Credit: NOAA/POES

Planetary K-index
Now: Kp= 4 unsettled
24-hr max: Kp= 4 unsettled
explanation | more data

Interplanetary Mag. Field
B_total: 8.2 nT
B_z: 1.1 nT south
explanation | more data
Updated: Today at 2227 UT

Coronal Holes: 21 Aug 13

Solar wind flowing from the indicated coronal hole should reach Earth on Aug. 23-24. Credit: SDO/AIA.