For the first time, the boundary of the heliosphere has been mapped, giving
scientists a better understanding of how solar and interstellar winds
interact.
"Physics models have theorized this boundary for years," said Dan
Reisenfeld, a scientist at Los Alamos National Laboratory and lead author on
the paper, which was published in the Astrophysical Journal today. "But this
is the first time we've actually been able to measure it and make a
three-dimensional map of it."
The heliosphere is a bubble created by the solar wind, a stream of mostly
protons, electrons, and alpha particles that extends from the Sun into
interstellar space and protects the Earth from harmful interstellar
radiation.
Reisenfeld and a team of other scientists used data from NASA's
Earth-orbiting Interstellar Boundary Explorer (IBEX) satellite, which
detects particles that come from the heliosheath, the boundary layer between
the solar system and interstellar space. The team was able to map the edge
of this zone -- a region called the heliopause. Here, the solar wind, which
pushes out toward interstellar space, collides with the interstellar wind,
which pushes in towards the Sun.
To do this measurement, they used a technique similar to how bats use sonar.
"Just as bats send out sonar pulses in every direction and use the return
signal to create a mental map of their surroundings, we used the Sun's solar
wind, which goes out in all directions, to create a map of the heliosphere,"
said Reisenfeld.
They did this by using IBEX satellite's measurement of energetic neutral
atoms (ENAs) that result from collisions between solar wind particles and
those from the interstellar wind. The intensity of that signal depends on
the intensity of the solar wind that strikes the heliosheath. When a wave
hits the sheath, the ENA count goes up and IBEX can detect it.
"The solar wind 'signal' sent out by the Sun varies in strength, forming a
unique pattern," explained Reisenfeld. "IBEX will see that same pattern in
the returning ENA signal, two to six years later, depending on ENA energy
and the direction IBEX is looking through the heliosphere. This time
difference is how we found the distance to the ENA-source region in a
particular direction."
They then applied this method to build the three-dimensional map, using data
collected over a complete solar cycle, from 2009 through 2019.
"In doing this, we are able to see the boundary of the heliosphere in the
same way a bat uses sonar to 'see' the walls of a cave," he added.
The reason it takes so long for the signal to return to IBEX is because of
the vast distances involved. Distances in the solar system are measured in
astronomical units (AU) where 1 AU is the distance from the Earth to the
Sun. Reisenfeld's map shows that the minimum distance from the Sun to the
heliopause is about 120 AU in the direction facing the interstellar wind,
and in the opposite direction, it extends at least 350 AU, which is the
distance limit of the sounding technique. For reference, the orbit of
Neptune is about 60 AU across.
Reference:
“A Three-dimensional Map of the Heliosphere from IBEX” by Daniel B.
Reisenfeld, Maciej Bzowski, Herbert O. Funsten, Jacob Heerikhuisen, Paul H.
Janzen, Marzena A. Kubiak, David J. McComas, Nathan A. Schwadron, Justyna M.
Sokół, Alex Zimorino and Eric J. Zirnstein, 10 June 2021, The Astrophysical
Journal.
Tags:
Space & Astrophysics