The Ion Velocity Meter comprises two sensors that together measure the ion drift velocity vector as well as the total ion concentration, the major ion composition and the ion temperature. The ion temperature, the ion composition, and the component of the ion drift along the sensor look direction are measured by a ion retarding potential analyzer (RPA). The two mutually perpendicular components of the ion drift perpendicular to the sensor look direction are derived from ion arrival measurements made by a ion drift meter (IDM).
A retarding potential analyzer presents a circular entarnce aperture to the plasma which flows through the sensor toward a solid conducting collector. Potentials applied to the internal grids determine the behavior of the instrument.
The entrance grids insure that potentials applied internally do not escape the instrument and affect the charged particle behavior outside.
The retarding grids have positive potentials applied to them which determine the energy of ions that have access to the collector. The suppressor grid is biased negatively to prevent ambient electrons from striking the collector and to return photoelectrons that leave the collector when it is sunlit. For a satellite in orbit around the Earth in the ionosphere, the vehicle velocity is about 8 km/s. This velocity gives the ions an energy of about 1/3 eV per mass unit. Thus if the ions being detected are O+ (16 amu) these ions have about 5 volts of energy with respect to the sensor. Thus for a retarding potential of say 7 volts, no O+ ions will reach the collector, while for a retarding potential of say 3 volts, all the ions will reach the collector. The precise flux of ions reaching the collector will depend, not only on the retarding potential, but also on the temperature and the ambient drift of the ions along the sensor look direction.
The ion flux is measured as a current on the RPA collector. The precise functional form for the ion flux as a function of retarding potential is quite complex but sophisticated computer analysis can be used to fit the observations to derive the ion temperture, ion drift, ion composition and ion drift.
Curve A above shows the current versus retarding potential obtained where the ionosphere is comprised completely of O+. This so-called I-V curve can be analyzed to determine the temperature, number density, and drift of the O+. By way of comparison curve B shows the I-V curve that would result where the ionosphere is comprised of 80% H+ and 20% O+. In this case you can see the signature of each mass. The hump at low potential caused by the presence of H+ and the hump near 5 volts produced by H+. A computer analysis can, of course, "see" much smaller relative concentrations of H+.
The Ion Drift Meter presents a square entrance aperture to the plasma which flows through the sensor toward a solid conducting collector which is divided into 4 quadrants. Because the sensor is moving supersonically with respect to the plasma a beam is formed behind the aperture. Grids inside the sensor are arranged to provide a field-free drift space through which the plasma flows prior to striking the collector.
An outer grid is placed in front of the aperture to ensure that no electric fields affect the plasma. An inner grid, prior to the aperture can be biased to prevent ions with insufficient energy from entering the sensor. It is placed before the aperture so that the ion trajectories are not affected by any applied potential.
Inside the sensor a field-free region is produced between the entrance aperture and an inner shield grid. Following the field-free region a supressor grid is biased negatively to prevent ambient eletrons from reaching the collector and to return photoelectrons that leave the collector when it is sunlit.
The arrival angle of the particles entering the sensor is determined by noting that the irradiated collector area, and thus the collected current depends upon this angle as shown. We use sophisticated electronics that produce signals proportional to the logarithm of the currents. the difference between these logarithmic outputs is propotional to the ratio of the currents. The ratio of the currents can be use to derive directly the particle arrival angle.