[David McComas] ISIS, the Integrated Science Investigation of the Sun, is an experiment which looks at
energetic particles over a broad range of energies, from tens of thousands of electron
volts up to about a hundred million electron volts.
The ISIS instrument is based on solid state detectors.
Those are detectors that when a particle passes through them, energy’s deposited and you
can measure that energy and you can measure that the particle has actually passed through.
So there’s the solar wind, which is this continuous flow of lower-energy particles,
and then there are much more sporadic and episodic events like solar flares that spew
out great numbers of these much more energetic particles.
in our higher-energy instrument, we have a whole set of layers of these detectors so
when a particle passes through those layers it leaves energy in each and every one of
those detectors.
Those detectors are also segmented in pieces like a pie and so when a particle comes through
from a particular direction you can tell both the direction the particle came through at
and you can tell the energy and species of that particle by looking at all the different
energy depositions.
In our lower-energy instrument, we also have the same sort of solid state detectors, but
in addition to that we use an extra trick, which is when a particle passes through a
very thin foil it ejects an electron, we’re able to detect that, and that gives us the
start timing for the particle.
One of the big issues for ISIS is to understand the difference between solar energetic particles
that just come from the Sun with a certain energy and those that are energized in interplanetary
space as they move outward.
By the time they get out to the Earth’s orbit, that’s all so messed up, mangled
up, that it’s kind of hard to tell the difference between those, but as we get in closer and
closer to the Sun, we’ll be able to differentiate those two types of sources, which will be
really important for understanding the fundamentals of energetic particles in the solar system.
By going in close and measuring the detailed distributions of these energetic particles,
their energies, directions, and their species, we’re going to learn things that we couldn’t
possibly learn by looking forever from a remote distance.
And by understanding our Sun in detail, we can understand lots of other stars in much
greater detail.