, the termination shock
is theorised to be a boundary marking one of the outer limits of the sun's influence. It is where the bubble of solar wind
particles slows down to below supersonic
speed and heats up due to collisions with the galactic interstellar medium. It is believed to be about 100 Astronomical Units
from the Sun.
The termination shock boundary fluctuates in its distance from the sun as a result of fluctuations in solar flare activity i.e. changes in the ejections of gas and dust from the sun.
The Voyager I spacecraft is believed to have passed termination shock in February 2003.