A Disconnection Event in the Tail of Comet Leonard

Dec. 27, 2021: Something unusual is happening to the tail of Comet Leonard. It’s being disconnected. To see the break, scroll down this picture taken by Gerald Rhemann on Christmas Day:

About halfway down the picture, right here, a rupture appears. This is a disconnection event: A piece of Comet Leonard’s tail has been pinched off and is being carried away by the solar wind.

Blame space weather. CMEs hitting comets can cause magnetic reconnection in comet tails, sometimes ripping them off entirely. NASA’s STEREO-A spacecraft watched this happen to Comet Encke in April 2007: movie. Comet Leonard has not experienced a known CME impact, but solar wind streams can do the same thing. At least two high-speed streams have hit the comet in recent weeks–enough to explain the turmoil.

This photo taken by Jan Hattenbach on Dec. 26th shows Comet Leonard’s tail whipsawing across the sky over La Palma in the Canary islands:

“The tail is at least 36 degrees long!” marvels Hattenbach. “Of those, six degrees could be seen in 10×50 binoculars, including the bright ‘knot’ associated with the disconnection event. Under the dark skies of La Palma, the comet was easily seen as a 3.9mag elongated cloud with the naked eye.”

Not every knot in Leonard’s tail is a disconnection event. Comet Leonard has flared in brightness 3 times since Dec. 15th–a sign of possible fragmentation in the comet’s core. Much of the tail is doubtless an imprint of Comet Leonard’s rapid-fire instabilities.

Whatever is happening, get ready for more. Comet Leonard is approaching the sun for a 0.61 AU close approach on Jan. 3rd. Increasing heat and proximity to solar storms could spark new outbursts and ruptures. Astronomers in the southern hemisphere will have the best view as the comet glides through the constellation Microscopium. Here’s where to find it.

Forbush Decrease: A Sudden Drop in Atmospheric Radiation

Dec. 3, 2021: Last month, a “Cannibal CME” hit Earth, sparking a strong G3-class geomagnetic storm and auroras as far south as California and New Mexico. You might think such a storm would boost radiation in Earth’s atmosphere. Think again. High-altitude balloons hurriedly launched by Earth to Sky Calculus during the storm on Nov. 3rd and 4th found just the opposite. Radiation in the stratosphere plummeted:

This is called a “Forbush decrease,” named after American physicist Scott Forbush who studied cosmic rays in the early 20th century.  It happens when a coronal mass ejection (CME) sweeps past Earth and pushes galactic cosmic rays away from our planet. Radiation from deep space that would normally pepper Earth’s upper atmosphere is briefly wiped out.

We have measured Forbush decreases before. For example, here’s one from Sept. 2014. The Forbush decrease of Nov. 3-4, 2021, was the deepest in the history of our 7-year atmospheric monitoring program. Radiation levels in the stratosphere over California dropped nearly 20%, more than doubling the previous record from our entire dataset.

Above: This is the CME that caused the Forbush decrease

This event illustrates the yin-yang relationship between solar activity and cosmic rays. As solar activity increases, more and more CMEs billow away from the sun, pushing cosmic rays out of the inner solar system. This is one reason we expect cosmic radiation to subside in the years ahead as young Solar Cycle 25 intensifies. Click here to learn more.