  The solar wind carves in the interstellar plasma a cavity bounded by a
surface, called the heliopause (HP), that separates the plasma and magnetic
field of solar origin from the interstellar ones. It is now generally accepted
that in August 2012 Voyager 1 (V1) crossed that boundary. Unexpectedly, the
magnetic fields on both its sides, although theoretically independent of each
other, were found to be similar in direction. This delayed the identification
of the boundary as the heliopause and led to many alternative explanations.
Here we show that the Voyager 1 observations can be readily explained and,
after the Interstellar Boundary Explorer (IBEX) discovery of the ribbon, could
even have been predicted. Our explanation relies on the fact that the Voyager 1
and the undisturbed interstellar field directions (which we assume to be given
by the IBEX ribbon center (RC)) share the same heliolatitude (~34.5 degrees)
and are not far separated in longitude (difference ~27 degrees). Our result
confirms that Voyager 1 has indeed crossed the heliopause and offers the first
independent confirmation that the IBEX ribbon center is in fact the direction
of the undisturbed interstellar magnetic field. For Voyager 2 we predict that
the difference between the inner and the outer magnetic field directions at the
heliopause will be significantly larger than the one observed by Voyager 1 (~30
degrees, instead of ~20 degrees), and that the outer field direction will be
close to the RC.
