Scientists using NASA’s Interstellar Boundary Explorer (IBEX) spacecraft have revealed a unique and unexpected structure of the downwind region of the Solar system’s heliosphere.
An artist’s concept of the Solar system’s heliotail (Michael Lentz / Walt Feimer / Tom Bridgman / Genna Duberstein / Erin McKinley / Eric Christian / Karen Fox / NASA / USRA / ASI / OSU / GST / HTSI)
Previous studies have proposed, like a comet, a ‘tail’ trails the heliosphere, the giant bubble in which our Solar system resides, as the heliosphere moves through interstellar space.
The first images released in 2009 by IBEX scientists showed an unexpected ribbon of surprisingly high energetic neutral atom emissions circling the upwind side of the Solar system. The data revealed a structure preliminarily identified as the heliotail. However, it was quite small and appeared to be offset from the downwind direction, possibly because of interactions from the galaxy’s external magnetic field.
During the next two years, scientists identified a second tail region to the side of the previously identified one. The IBEX team reoriented the maps and two similar, low-energy structures became clearly visible straddling the downwind direction of the heliosphere, indicating structures that better resemble ‘lobes’ than a single unified tail.
“We chose the term ‘lobes’ very carefully. It may well be that these are separate structures bent back toward the downwind direction. However, we can’t say that for certain with the data we have today,” said Dr Dave McComas from Southwest Research Institute, who is a lead author of a paper reporting the findings in the Astrophysical Journal.
The team adopted the nautical terms port and starboard to distinguish the lobes, as the heliosphere is the ‘vessel’ that transports our Solar system throughout the galaxy.
IBEX data show the heliotail is the region where the Sun’s million mile per hour solar wind flows down and ultimately escapes the heliosphere, slowly evaporating because of charge exchange.
The slow solar wind heads down the tail in the port and starboard lobes at low- and mid-latitudes and, at least around the Sun’s minimum in solar activity, fast solar wind flows down it at high northern and southern latitudes.
The solar journey through space is carrying us through a cluster of very low density interstellar clouds. Right now the Sun is inside of a cloud that is so tenuous that the interstellar gas detected by IBEX is as sparse as a handful of air stretched over a column that is hundreds of light years long (NASA / Adler / U. Chicago / Wesleyan)
“We’re seeing a heliotail that’s much flatter and broader than expected, with a slight tilt. Imagine sitting on a beach ball. The ball gets flattened by the external forces and its cross section is oval instead of circular. That’s the effect the external magnetic field appears to be having on the heliotail,” Dr McComas said.
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Bibliographic information: McComas DJ et al. 2013. The Heliotail Revealed by the Interstellar Boundary Explorer. ApJ 771, 77; doi: 10.1088/0004-637X/771/2/77
