Dec. 20, 2005: Four rare transits of the Sun by aircraft have been captured by the Improved Solar Optical Observing Network (ISOON) telescope at the National Solar Observatory atop Sacramento Peak at Sunspot, New Mexico.

ISOON is a semi-autonomous, remotely commandable patrol telescope designed for studies of solar activity in support of space weather specifications and forecasts. It observes the Sun in hydrogen-alpha light, which reveals ribbons of relatively cool gas standing above the hotter chromosphere, the Sun's middle atmosphere. Normally, ribbons (also called filaments) snake between regions of opposite magnetic polarity.

So when ISOON captured a straight ribbon on Dec. 14, 2004, at 2342 UT (4:42 p.m. MST), it bore closer inspection. The ribbon actually was the wake of a jetliner about 177 km (110 mi) away, probably departing the El Paso (Texas) International Airport. The estimate is based on an average length of almost 40 m (130 ft.) for a Boeing 737, the apparent model in the image.

While such passages are not uncommon in urban areas near airports, they become increasingly rare with greater distance between aircraft and telescope (i.e., the alignment is less likely). Only ten or so aircraft transits a year are expected at remote Sacramento Peak. Even fewer coincide with ISOON's once-a-minute, 1/200th of a second (50 ms) exposures.

Surprisingly, another three transits were captured on Nov. 14, 15, 16, 2005, late in the afternoon. These only caught only the wake, not the aircraft, and were seen in continuum (a red off-band color used as a stand-in for white light) and were not as fine as the H-alpha image from 2004.

The 2004 image is available at the National Solar Observatory's web site where the first image is an edited holiday version showing an eight-reindeer sled making a pre-delivery check ride: http://www.nso.edu/press/holiday05/ISOON_Santa.html

Unedited copies are linked from the page.

NSO is operated by the Association of Universities for Research in Astronomy under a cooperative agreement with the National Science Foundation. ISOON is a project of the U.S. Air Force Research Laboratory. Its data are extensively used by scientists at NSO and around the world.

ISOON images are available at http://nsosp.nso.edu/isoon/