Using data from NASA’s Interstellar Boundary Explorer (IBEX), researchers have accurately determined the strength and direction of the magnetic field outside what is known as the heliosphere.
Solar wind and the interstellar medium interact to create the inner heliosheath, bounded on the inside by the termination shock, and on the outside by the heliopause. Image credit: NASA / IBEX / Adler Planetarium.
The results were published earlier this month in a paper in the Astrophysical Journal Letters.
“Such information gives us a peek into the magnetic forces that dominate the Galaxy beyond, teaching us more about our home in space,” said lead author Dr. Eric Zirnstein, of the Southwest Research Institute.
Outside the heliosphere — a vast bubble that surrounds our Solar System — lies the local interstellar medium (LISM), with plasma that has different speed, density, and temperature than solar wind plasma, as well as neutral gases.
These materials interact at the heliosphere’s edge to create a region known as the inner heliosheath, bounded on the inside by the termination shock, and on the outside by the heliopause, the boundary between the solar wind and LISM.
According to Dr. Zirnstein and co-authors, some solar wind protons that flow out from the Sun to this boundary region will gain an electron, making them neutral and allowing them to cross the heliopause.
Once in LISM, they can lose that electron again, making them gyrate around the local interstellar magnetic field (ISMF).
If those particles pick up another electron at the right place and time, they can be fired back into the heliosphere, travel all the way back toward Earth, and collide with IBEX’s detector.
The IBEX ribbon is a relatively narrow strip of particles flying in towards the Sun from outside the heliosphere. Image credit: Southwest Research Institute.
“Hydrogen energetic neutral atoms (ENAs) produced by the solar-interstellar interaction carry important information about plasma properties from the boundaries of the heliosphere, and are currently being measured by NASA’s IBEX,” the scientists said.
“The IBEX observations show the existence of a ribbon of intense ENA emission — so-called the IBEX ribbon — projecting a circle on the celestial sphere that is centered near the local interstellar magnetic field vector.”
“The new paper is based on one theory of the origin of the IBEX ribbon, in which the particles streaming in from the ribbon are actually solar material reflected back at us after a long journey to the edges of the Sun’s magnetic boundaries,” they said.
This simulation shows the origin of ribbon particles of different energies or speeds outside the heliopause (HP). The IBEX ribbon particles interact with the interstellar magnetic field (ISMF) and travel inwards toward Earth, collectively giving the impression of a ribbon spanning across the sky. Image credit: E.J. Zirnstein et al / Southwest Research Institute.
“We show that the source of the IBEX ribbon as a function of ENA energy outside the heliosphere, uniquely coupled to the draping of the ISMF around the heliopause, can be used to precisely determine the magnitude (2.93 ± 0.08 μG) and direction (227.28o ± 0.69o, 34.62o ± 0.45o in ecliptic longitude and latitude) of the pristine ISMF far (approximately 1,000 astronomical units, or 93 billion miles) from the Sun.”
“We find that the ISMF vector is offset from the ribbon center by ~8.3o toward the direction of motion of the heliosphere through the LISM, and their vectors form a plane that is consistent with the direction of deflected interstellar neutral hydrogen, thought to be controlled by the ISMF.”
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E.J. Zirnstein et al. 2016. Local Interstellar Magnetic Field Determined From The Interstellar Boundary Explorer Ribbon. ApJ 818, L18; doi: 10.3847/2041-8205/818/1/L18
