## Counting electrons in space

28 February 2012

Space isn't really a empty. In reality it's filled with particles that can be measured by instruments on spacecraft. But there aren't that many of them so special techniques need to be used to work out the density and temperature of the particles surrounding the planets, and our spacecraft.

Spacecraft missions are constrained by the weight of the payload and so it is impractical to have separate instruments to measure all the properties of the space plasma surrounding the spacecraft. Because of this weight limitation, space plasma physicists have developed techniques to calculate the basic plasma parameters simply from counting the number of electrons over a range (spectrum) of energies.

If we counted the number of particles in a box with a side of one metre in space, we would know the number density of the particles in units of per metre cubed: simply the number of particles divided by the density. We might also count the number of particles in this box that are moving at different speeds in different directions. To a space plasma physicist, this combination is called "velocity space".

Our instruments, like the Cassini Electron Spectrometer in the picture on the right, count how many electrons there are at different energies and allow us to measure how many particles there are in different regions of velocity space - so how many particles there are in certain regions of space and what speeds they are moving at in different directions. We call this "phase space density". Using a mathematical operation called integration we can calculate the density, pressure, temperature and velocity of these electrons. These measurements are important in understanding the space environment of the planets, how they are coupled to the Sun and how important the various moons are.