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Solar Spicules

July 31, 2017

Household chemicals were the first chemicals that I could access as a child, and I was joyous when I discovered the reaction between vinegar and baking soda to make carbon dioxide; viz.,

NaHCO₃ + HCH₃COO -> CH₃COONa + H₂O + CO₂

in which NaHCO₃ (baking soda, sodium bicarbonate) reacts with HCH₃COO (vinegar, acetic acid) to produce sodium acetate (CH₃COONa), water (H₂O), and carbon dioxide (CO₂).

There are many more household chemicals available today; and, thankfully, most are designed and packaged for safety. Lye (sodium hydroxide, NaOH) was the traditional treatment for clogged drains in the past, but my most recent drain opener purchase came as two safer liquids that produced an active product when mixed. The innovative package for this had two bottles joined side-by-side with a single cap.

One popular household product of my childhood was Spic 'n' Span, a mixture of sodium carbonate (also called washing soda) and trisodium phosphate, first sold in 1933 as a cleaning agent.[1] Its unusual name was a combination of archaic words that meant "sharp, neat" (spick) and "fresh" (span), and the phrase "spicke and span" was used by Samuel Pepys (1633-1703) in his oft-cited 1665 diary.[2]

Samuel Pepys Stained Glass Panel, Woolwich Town Hall, Woolwich, South East London

Samuel Pepys (1633-1703), as depicted in a stained glass panel, Woolwich Town Hall, Woolwich, South East London.

Pepys' diary contains descriptions of the Great Plague of 1665 and the Great Fire of London in 1666

(Wikimedia Commons image by Kleon3.

"Spic" has a simple etymology, coming from the Latin word spiculum, a sharp point, and spiculum was also the name of the pike weapon used by Roman soldiers in the late 3rd century. There are many needle-like biological structures called spicules, but this article is about a solar surface feature called a spicule.

Galileo discovered the Galilean moons of Jupiter in 1610, and sunspots were discovered in 1611. So, it's surprising that solar spicules, which comprise about a percent of the Sun's surface and number about a quarter of a million, weren't discovered until 1877. I wrote about the history of sunspots in an earlier article (Four Hundred Years of Sunspots, March 22, 2011).

A spicule is a chromospheric eruption of solar material about 500 km in diameter, moving outwards at about 20 kilometers/sec. Spicules have a lifetime of about 15 minutes, they're two orders of magnitude higher in density than the solar wind, and they are typically found in regions of high magnetic flux. If you do the math, multiplying lifetime and velocity, you see that spicules will extend to a height of a few thousand kilometers.

Astronomers in the distant past didn't have the ability to see the Sun at the many wavelengths that are now available for solar observation, many of these wavelengths only accessible outside Earth's atmosphere, so they can be excused for only discovering spicules two and a half centuries after the discovery of sunspots. NASA's STEREO (Solar TErrestrial RElations Observatory) mission was launched on October 25, 2006, to provide the first detailed stereoscopic images of the Sun, including coronal mass ejections.

Solar image at 171 angstrom wavelength, July 15, 2015

Solar image at 171 angstrom wavelength, acquired on July 15, 2015, by the NASA STEREO-A Extreme Ultraviolet Imager.

(NASA/STEREO image.

A more recent solar observer is NASA's Interface Region Imaging Spectrograph (IRIS), a space mission also designed to image the Sun in the extreme ultraviolet. IRIS was launched on June 28, 2013, and its telescope achieved "first light" on July 17, 2013. IRIS observed a giant solar eruption on May 30, 2014, and it reached its 10,000th orbit of the Earth on May 6, 2015.

It's thought that a greater understanding of spicules will aid an understanding of how the solar corona is heated to temperatures of millions of degrees and the properties of Alfvénic waves that assist in generating the solar wind.[6-8] In a recent study, scientists from the Bay Area Environmental Research Institute (Petaluma, California), the Lockheed Martin Solar and Astrophysics Laboratory (LMSAL, Palo Alto, California), and the University of Oslo (Oslo, Norway) have examined data from IRIS and the Swedish 1-meter Solar Telescope in the Canary Islands to develop a model of spicule formation.[6-7]

Solar spicules.

(Still image from a NASA video from NASA's Goddard Space Flight Center.

Earlier attempts at a model of spicule formation just considered the lower solar atmosphere to be a fully ionized plasma; that is, a hot gas of electrically charged particles.[7] The present model also includes neutral particles not affected by magnetic fields as are the charged particles. Adding the neutral particles greatly increased computation time, and the final model ran for roughly a year on the Pleiades supercomputer at NASA's Ames Research Center.[7] The neutral particles added buoyancy that allows the hot inner plasma to reach the chromosphere, where it decomposes into spicules that release energy.[7]

The radiation-magnetohydrodynamic model was shown to create numerous spicules with properties that match observations.[6] The simulations closely match the observations from IRIS and the Swedish Solar Telescope.[7-8] The whip-like spicule formation naturally generates Alfvén waves, strong magnetic waves named after electrical engineer and plasma physicist, Hannes Alfvén (1908-1995), that propel the solar wind.[7] Hannes Alfvén was awarded the 1970 Nobel Prize in Physics for his work on magnetohydrodynamics.

Hannes Alfvén (1908-1995) in 1942.

As an electrical engineer, Alfvén had great difficulty publishing his plasma physics papers, even after being awarded the Nobel Prize.

(Wikimedia Commons image, modified for artistic effect.

Says Juan Martínez-Sykora, lead author of the study and a solar physicist at Lockheed Martin and the Bay Area Environmental Research Institute,

"With only charged particles in the model, the magnetic fields were stuck, and couldn't rise beyond the sun's surface. When we added neutrals, the magnetic fields could move more freely."[7]

Solar spicule simulation showing the magnetic field lines. (Still image from a NASA video (Bifrost, ITA-UiO/LMSAL).

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