Secondary electrons created within the instrument can cause inaccurate measurements as they generate an undesirable background signal. As a result, the probability of them being created in the first place must be reduced and they must also be accounted for. There are two sources of these particles, both of which populate the 1eV to 10eV energy range. The first originate from incident electrons with an energy higher than the instrument is set up to measure. They have too much energy and therefore are not deflected sufficiently by the electric field along the instrument track. Consequently, they impact with the outer hemispherical plate near the entrance aperture, creating the low energy secondary electrons. The second source is due to the influx of solar ultraviolet radiation, generating secondary photoelectrons, however, they can only be created when the Sun is within the analyser’s field of view.
The effect of sunlight on the sensor must be reduced in order to minimise any con- tamination which may occur. In order to do this, baffles are incorporated in the input collimator, along with highly absorbent, diffusely reflecting surfaces within. This method reduces the possibility in which the detector can be stimulated via internal reflections or direct illumination via the action of ultraviolet light. In addition, the background signal created by the internally generated secondaries and other penetrating radiation must be quantified in order to be accounted for. This can done by biassing a grid placed in front of the microchannel plates by -8V, repelling incident electrons and setting the plate voltage to the minimum of 0.59eV. No electrons can reach the detector through the instrument’s track once these settings are in place and therefore any signals detected
must be generated by penetrating radiation or scattered secondary particles (Johnstone et al.,1997). This phenomenon is often called internal spacecraft charging.
Charge can also accumulate on the surface of a spacecraft. The main contributors to this effect are: plasma interactions, incident charged particles, solar radiation and the Earth’s magnetic field.
Plasma affects the spacecraft by inducing charges on its surfaces due to the flux of elec- trons and positive ions. The motion of the spacecraft through the plasma environment can also lead to the creation of a local environment (spacecraft wake) which can also create additional spacecraft charging. The presence of charged particles in the local vicinity of the spacecraft plays a large part in its charging. Generally electrons are more mobile than ions due to their smaller mass, giving them a higher thermal speed, hence electrons flux tends to dominate. This means that overall, negative electron current is larger than positive ion current. From this, a spacecraft’s surface tends to charge negatively to a potential of the order of the electron temperature. However, when the spacecraft if illuminated by the Sun, Solar radiation acts to oppose this charging effect. As photons emitted from the Sun impact with the spacecraft, they interact via the pho- toelectric effect and as a result the spacecraft emits photoelectrons. These constitute as a current flowing into the spacecraft and therefore can reduce the effect of negative sur- face charging. When this occurs for an extended period, it is possible for the spacecraft to become positively charged as the rate of photo-emission can dominate.
Finally the Earth’s magnetic field determines the regions of space where spacecraft charg- ing can occur as well as affecting the escape of photoelectrons from the craft (Mikaelian, 2009).
When the spacecraft becomes charged, it interacts with the ambient charged particles surrounding it, altering the incoming and outgoing particle flux, giving rise to the mag- nitude of the spacecraft potential. Its value varies in different regions; for example, in areas of high density such as within the tail plasma sheet, the spacecraft experiences a much lower potential, of the order of tens of volts, whereas in low density regions, such as tail lobes, the potential can reach up to 100V. Cluster has the Active Spacecraft POtential Control (ASPOC) system installed which emits a beam of positive ions away from the craft, helping to reduce the spacecraft potential and maintain it below 2V. This minimises the number of electrons returning back to the spacecraft (Szita et al.,2001). The acceleration of ambient charged particles due to spacecraft charging changes the distribution in which they would ordinarily be observed. It adjusts their energy and direction of motion and therefore a measurement of the spacecraft potential must be taken so it can be accounted for in the results. This is particularly important while
measuring low-energy electrons as the energy band in which they sit can often be very close to that of the spacecraft potential. Photoelectrons are also detected by the PEACE instrument and thus the measured particle distribution can differ from the natural dis- tribution. The contribution of both photoelectron and ambient-electron contamination is mitigated from both the ground-based and onboard moments.
Onboard moment calculations follow the assumption that raw data with energies below 10 eV must be contaminated and thus all raw data in this energy range are removed before any moment calculations are made. The ground moments are somewhat different as they use the measured spacecraft potential, and in addition, the user can also add a further offset if they have grounds to believe that the measured spacecraft potential (which is the potential difference between the spacecraft and the ends of the booms) is not representative of the real spacecraft potential (between the spacecraft and infinity). This is another reason as to why the latter are more accurate.