Astronomy Magazine
Glossary · The Sun & Space Weather
Heliosphere and heliopause
The bubble the solar wind blows in interstellar space, and the wall where it stops.
The Sun's wind pushes outward until it can no longer hold back the pressure of the interstellar medium. That boundary is the heliopause, and everything inside it is the heliosphere — a vast, teardrop-shaped bubble containing all the planets. Voyager 1 crossed it in 2012 at about 121 astronomical units and Voyager 2 in 2018, both confirming the crossing by a sudden rise in cosmic rays and a change in the pitch of plasma waves. They are the only two objects to have made the measurement.
Where it has come up lately
NASA Science
Related terms
Coronal mass ejection
A billion tonnes of solar atmosphere thrown into space, and the cause of most serious aurorae.
Solar flare classes
A, B, C, M, X — a logarithmic scale for how hard the Sun just flashed in X-rays.
Kp index
A 0–9 scale of how disturbed Earth's magnetic field is right now.
The solar cycle
An eleven-year rhythm in sunspots, flares and the Sun's entire magnetic field.
Questions
What is Heliosphere and heliopause?
The bubble the solar wind blows in interstellar space, and the wall where it stops.
Why does it matter?
Voyager 1 crossed it in 2012 at about 121 astronomical units and Voyager 2 in 2018, both confirming the crossing by a sudden rise in cosmic rays and a change in the pitch of plasma waves. They are the only two objects to have made the measurement.