Saturday, July 13, 2013

First View Of the Solar System's Tail

NASA's Interstellar Boundary Explorer (IBEX) spacecraft recently provided the first complete pictures of the solar system's downwind region, revealing a unique and unexpected structure.
High resolution still from animation of the heliotail - side view Credit: NASA
Researchers have long theorized that, 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 IBEX images released in 2009 showed an unexpected ribbon of surprisingly high energetic neutral atom (ENA) emissions circling the upwind side of the solar system.

An artist's concept of our heliosphere, which is a bubble in space created by the solar wind and solar magnetic field. 
An artist's concept of our heliosphere, which is a bubble in space created by the solar wind and solar magnetic field. Credit: NASA
Credit: Goddard

With the collection of additional ENAs over the first year of observations, a structure dominated by lower energy ENAs emerged, which was 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.

As the next two years of IBEX data filled in the observational hole in the downwind direction, researchers found a second tail region to the side of the previously identified one. The IBEX team reoriented the IBEX maps and two similar, low-energy ENA 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," says Dr. Dave McComas, IBEX principal investigator and assistant vice president of the Space Science and Engineering Division at Southwest Research Institute. "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."

The conditions necessary to make the heliosphere, namely the balance of an outward pushing stellar wind and the inward compression of surrounding interstellar gas is so common, that perhaps most stars have analogous structures, called astrospheres. Photographs of three such astrospheres are shown, as taken by various telescopes.
Credit: NASA/ESA/JPL-Caltech/GSFC/SwRI

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.

"We're seeing a heliotail that's much flatter and broader than expected, with a slight tilt," says McComas. "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."

This graph depicts the fraction of high energy cosmic rays (greater than 100 MeV) that pass through the boundary of the solar system. 100% of them are present outside of the Bow Shock. There is a small dropoff in the number that make it through to the heliopause. More than 50% are stopped between the heliopause and termination shock, which is at approximately 100 AU. This leaves a fraction less than 25% to permeate to the inner solar system.

Credit: Adler Planetarium/Chicago

The IBEX spacecraft uses two novel ENA cameras to image and map the heliosphere's global interaction, providing the first global views and new knowledge about our solar system's interaction with interstellar space.

"We often think we know what we're going to study in science, but the work sometimes takes us in unexpected directions," says McComas. "That was certainly the case with this study, which started by simply trying to better quantify the small structure incorrectly identified as an 'offset heliotail.' The heliotail we found was much bigger and very different from what we expected."

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. These clouds are identified by their motions.
Credit: NASA/Adler/U. Chicago/Wesleyan

This animation shows a charged solar particle's path leaving the sun, while following the magnetic field lines out to the heliosheath. The solar particle hits a hydrogen atom, stealing its electron and becoming neutral. We then follow it until we see it hit one of IBEX's detectors.

Credit: NASA

The paper, "The heliotail revealed by IBEX," by D.J. McComas, M.A. Dayeh, H.O. Funsten, G. Livadiotis, and N.A. Schwadron, was published today in the Astrophysical Journal.

IBEX is part of NASA's series of low-cost, rapidly developed Small Explorer space missions. Southwest Research Institute in San Antonio leads the IBEX mission with teams of national and international partners. NASA's Goddard Space Flight Center in Greenbelt, Md., manages the Explorers Program for NASA's Science Mission Directorate in Washington.


Contacts and sources: 
Maria Martinez
Southwest Research Institute

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