NASA's IBEX charts 11 years of change at boundary to interstellar space

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Far, far beyond the orbits of the planets lie the hazy contours of the magnetic bubble in space that we call home. This is the heliosphere, the vast bubble that is generated by the Sun's magnetic field and envelops all the planets. The borders of this cosmic bubble are not fixed. In response to the Sun's gasps and sighs, they shrink and stretch over the years. Now, for the first time, scientists have used an entire solar cycle of data from NASA's IBEX spacecraft to study how the heliosphere changes over time. Solar cycles last roughly 11 years, as the Sun swings from seasons of high to low activity, and back to high again. With IBEX's long record, scientists were eager to examine how the Sun's mood swings play out at the edge of the heliosphere. The results show the shifting outer heliosphere in great detail, deftly sketch the heliosphere's shape (a matter of debate in recent years), and hint at processes behind one of its most puzzling features. These findings, along with a newly fine-tuned data set, are published in The Astrophysical Journal Supplements on June 10, 2020. IBEX, short for the Interstellar Boundary Explorer, has been observing the boundary to interstellar space for more than 11 years, showing us where our cosmic neighborhood fits in with the rest of the galaxy. "It's this very small mission," said David McComas, the principal investigator for the mission at Princeton University in New Jersey. IBEX is just as big as a bus tire. "It's been hugely successful, lasting much longer than anybody anticipated. We're lucky now to have a whole solar cycle of observations." Mapping the solar system's edge, one particle at a time The heliosphere is filled with the solar wind, the constant flow of charged particles from the Sun. The solar wind rushes out in all directions, a million miles per hour, until it butts against the interstellar medium, winds from other stars that fill the space between them. advertisement As the Sun wades through the interstellar medium, it generates a hot, dense wave much like the wave at the front of a boat coursing through the sea. Our cosmic neighborhood is called the Local Fluff, for the cloud of superhot gases that blooms around us. Where the solar wind and Local Fluff meet forms the edge of the heliosphere, called the heliopause. Just inside that lies a turbulent region called the heliosheath. Particles called energetic neutral atoms, or ENAs, that are formed in this distant region of space are the focus of IBEX's surveys. They're created when hot, charged particles like the ones in the solar wind collide with cold neutrals like those flowing in from interstellar space. Zippy solar wind particles can snatch electrons from lumbering interstellar atoms, becoming neutral themselves. The journey of these particles begins long before IBEX detects them. Past the planets, past the asteroid belt and the Kuiper Belt, to the edge of the heliosphere, it takes about a year for a gust of solar wind to race 100 times the distance between the Sun and Earth. Along the way, the solar wind picks up ionized atoms of interstellar gases that have wriggled in to the heliosphere. The solar wind that arrives at the edge is not the same wind that left the Sun a year before. Solar wind particles might spend another six months roving the chaos of the heliosheath, the gulf between the heliosphere's two outer boundaries. Inevitably, some collide with interstellar gases and become energetic neutrals. It takes the neutral particles close to another year for the return trip, traversing the space from the edge of the heliosphere to reach IBEX -- if the particles happened to be heading in precisely the right direction. Of all the neutral particles formed, only a few actually make it to IBEX. The whole trip takes two to three years for the highest-energy particles in IBEX's observing range, and even longer at lower energies or more distant regions. IBEX takes advantage of the fact that neutral atoms like these aren't diverted by the Sun's magnetic field: Fresh ..