(Artist concept) Far beyond the orbit of Neptune,
the solar wind and the interstellar medium interact to create a region
known as the , bounded on the inside by the termination
shock, and on the outside by the heliopause. Credits: NASA/IBEX/Adler Planetarium
This simulation shows the origin of ribbon
particles of different energies or speeds outside the heliopause
(labeled HP). The IBEX ribbon particles interact with the interstellar
magnetic field (labeled ISMF) and travel inwards toward Earth,
collectively giving the impression of a ribbon spanning across the sky.Credits: SwRI/Zirnstein
The new paper is based on one particular 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. A giant bubble, known as the
heliosphere, exists around the sun and is filled with what’s called
solar wind, the sun’s constant outflow of ionized gas, known as plasma.
When these particles reach the edges of the heliosphere, their motion
becomes more complicated.
“The theory says that some solar wind protons are sent flying back
towards the sun as neutral atoms after a complex series of charge
exchanges, creating the IBEX ribbon,” said Eric Zirnstein, a space
scientist at the Southwest Research Institute in San Antonio, Texas, and
lead author on the study. “Simulations and IBEX observations pinpoint
this process – which takes anywhere from three to six years on average –
as the most likely origin of the IBEX ribbon.”
Outside the heliosphere lies the interstellar medium, 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 – which is more than
twice as far from us as the orbit of Pluto – and on the outside by the
heliopause, the boundary between the solar wind and the comparatively
dense interstellar medium.
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 the interstellar medium, they can lose
that electron again, making them gyrate around the interstellar magnetic
field. 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 particles
carry information about all that interaction with the interstellar
magnetic field, and as they hit the detector they can give us
unprecedented insight into the characteristics of that region of space.
“Only Voyager 1 has ever made direct observations of the interstellar
magnetic field, and those are close to the heliopause, where it’s
distorted,” said Zirnstein. “But this analysis provides a nice
determination of its strength and direction farther out.”
The directions of different ribbon particles shooting back toward
Earth are determined by the characteristics of the interstellar magnetic
field. For instance, simulations show that the most energetic particles
come from a different region of space than the least energetic
particles, which gives clues as to how the interstellar magnetic field
interacts with the heliosphere.
For the recent study, such observations were used to seed simulations
of the ribbon’s origin. Not only do these simulations correctly predict
the locations of neutral ribbon particles at different energies, but
the deduced interstellar magnetic field agrees with Voyager 1
measurements, the deflection of interstellar neutral gases, and
observations of distant polarized starlight.
However, some early simulations of the interstellar magnetic field
don’t quite line up. Those pre-IBEX estimates were based largely on two
data points – the distances at which Voyagers 1 and 2 crossed the
termination shock.
“Voyager 1 crossed the termination shock at 94 astronomical units, or
AU, from the sun, and Voyager 2 at 84 AU,” said Zirnstein. One AU is
equal to about 93 million miles, the average distance between Earth and
the sun. “That difference of almost 930 million miles was mostly
explained by a strong, very tilted interstellar magnetic field pushing
on the heliosphere.”
But that difference may be accounted for by considering a stronger
influence from the solar cycle, which can lead to changes in the
strength of the solar wind and thus change the distance to the
termination shock in the directions of Voyager 1 and 2. The two Voyager
spacecraft made their measurements almost three years apart, giving
plenty of time for the variable solar wind to change the distance of the
termination shock.
“Scientists in the field are developing more sophisticated models of the time-dependent solar wind,” said Zirnstein.
The simulations generally jibe well with the Voyager data.
The IBEX ribbon is a relatively narrow strip of
particles flying in towards the sun from outside the heliosphere. A new
study corroborates the idea that particles from outside the heliosphere
that form the IBEX ribbon actually originate at the sun – and reveals
information about the distant interstellar magnetic field. Credits: SwRI
“The new findings can be used to better understand how our space
environment interacts with the interstellar environment beyond the
heliopause,” said Eric Christian, IBEX program scientist at NASA’s
Goddard Space Flight Center in Greenbelt, Maryland, who was not involved
in this study. “In turn, understanding that interaction could help
explain the mystery of what causes the IBEX ribbon once and for all.”
The Southwest Research Institute leads IBEX with teams of national
and international partners. NASA Goddard manages the Explorers Program
for the agency’s Heliophysics Division within the Science Mission
Directorate in Washington.
Related Link
- IBEX mission website
- Article: The Astrophysical Journal Letters - "Local Interstellar Magnetic Field Determined From the Interstellar Boundary Explorer Ribbon"
Source: NASA/Ibex
