New Theory Of ENAs At Edge Of Solar System Checks Out With Data
February 6, 2013

Understanding The Edge Of Our Solar System

John P. Millis, Ph.D. for — Your Universe Online

While our solar system contains many fascinating objects — planets, moons and the Sun, to name just a few — one of the most interesting and perplexing of these objects lies at its very edge.

Our Sun sweeps charged particles through the solar system, encasing us in a cocoon that shields us from the interstellar environment. It is at the boundary of the solar system, where the solar wind slows and the influence of our star´s magnetic field wanes, where the flow of particles comes to a standstill. Or so we thought.

In 2008 NASA launched the Interstellar Boundary Explorer (IBEX) that was tasked with imaging that region of our cosmic neighborhood. What they found wasn´t a standing shock of particles but rather ribbons of neutral atoms — elements that contain equal numbers of protons and electrons — streaming into our solar system.

This unexpected result immediately gave rise to a host of theories that sought to explain how these ribbons formed. Until recently each of these models has failed to completely account for the complexities of this phenomenon.

However, a new theory published in the February 4 issue of the Astrophysics Journal, describes a so-called Retention Model that may help explain the unusual IBEX data.

Researchers David McComas and Nathan Schwadron realized that the energy distributions of the ribbons pouring into the solar system reflected that of the solar wind pushing out. They concluded that the ions within the solar wind may actually be capturing electrons, giving rise to energetic neutral atoms (ENAs) as they crossed the interstellar boundary.

However, they also theorized that some of these ENAs would interact with the interstellar medium on the other side. While most of the ENAs will scatter off into the vastness that lies beyond, never to return, some will collide with atoms in the interstellar medium and re-ionize, allowing them to interact with the interstellar magnetic field. Eventually these ions would 'pile up' and begin to stream back toward the interstellar boundary. These ions will then recombine with free electrons, thus becoming what the authors have dubbed 'secondary ENAs.'

"The secondary ENAs coming into the solar system after having been temporarily trapped in a region just outside the solar system do the same thing. As they pile up and get trapped or retained, they produce higher fluxes of ENAs from this region and form the bright ribbon seen by IBEX," explained McComas, who is also the principal investigator on IBEX.

Dr. Schwadron adds: “Think of the ribbon as a harbor and the solar wind particles it contains as boats. The boats can be trapped in the harbor if the ocean waves outside it are powerful enough. This is the nature of the new ribbon model. The ribbon is a region where particles, originally from the solar wind, become trapped or 'retained' due to intense waves and vibrations in the magnetic field."

To test their theory the researchers created simulations based on solar wind models and found that the resulting ribbons closely resembled those found in the IBEX data. According to Schwadron, the retention theory "checks all the boxes, agrees with all the available observations, and the mathematical modeling results look remarkably like what the ribbon actually looks like."

"This substantially raises the bar for models that attempt to explain the ribbon," he concluded.