Ion Velocity Meter (IVM)
The IVM consists of two planar thermal ion sensors, a retarding potential analyzer (RPA) and an ion drift meter (IDM) that will provide measurements of the ion drift velocity in the spacecraft reference frame, the ion temperature and the total ion number density at the location of the spacecraft. Using a magnetic field model the drift velocity vector can be converted to a local electric field perpendicular to the magnetic field and expressed in a geographic and/or geomagnetic coordinate system. Above 250 km altitude the direction parallel to the Earth’s magnetic field is an electric equipotential for horizontal scales greater than 25 km. Thus the electric field at the spacecraft location can be translated to any other location along the magnetic field through the spacecraft.
IVM – Heritage
IVM is derived with little change from the instrument that is presently operating successfully as part of the CINDI project on the C/NOFS satellite. The IVM sensors and sensor electronics are constructed as a single package and are mounted to view approximately along the spacecraft velocity vector through a common aperture plane that is isolated from the spacecraft ground. Figure 1 shows the sensor configuration that is flying on the CINDI mission and and a schematic illustration of the ICON configuration.
The aperture plane serves as the reference ground for the sensors and ensures that all external fields are uniform and normal to the entrance aperture. A portion of the aperture plane, the SenPot reference surface, is isolated from the reference ground. The SenPot amplifier generates a potential with respect to spacecraft ground to maintain the IVM reference ground at the floating potential with respect to the plasma. The operation and data analysis techniques have extensive heritage from many previous missions starting with Atmosphere Explorer in the 1970’s and subsequently the Defense Meteorological Satellite Program and ROCSAT-1 before the CINDI mission in 2008.
Figure 1. CINDI IVM shown schematically as it will appear on ICON.
IVM – How it Works
The Retarding Potential Analyzer is a planar sensor that presents a circular aperture to the incoming plasma stream that is intersected by a number of planar semi-transparent conducting grids and a large solid collector. Two schematic views of the RPA sensor are shown in figure 2.
Figure 2. Schematic cross-section and front view of the retarding potential analyzer.
The sensor is mounted to view approximately along the satellite velocity through a large aperture plane that provides a uniform planar electrical ground reference potential. Grounded grids cover the entrance aperture to ensure that no internally applied potentials influence the ion beam trajectories prior to entering the sensor. Inside the sensor the ion beam traverses a series of semi-transparent grids before impacting the collector. The retarding grids are biased at potentials between 0 and 25 volts and thus control the minimum energy that the ions must posses to reach the collector. A suppressor grid prior to the collector is biased at -12 volts to reject ambient electrons and suppress photoelectrons ejected from the collector.
A current-voltage characteristic is obtained by measuring the ion current while the retarding voltage moves over a series of predetermined discrete levels. Retarding voltage sequences are chosen to optimize the sensor performance for the changing conditions expected in the ionosphere over a substantial part of a solar cycle. Figure 3 shows a simulated data curve obtained assuming a total ion concentration or 105 cm-3 with 20% H+ and 80% O+. This current-voltage characteristic has a well-known functional form that can be fitted to retrieve the ion drift component along the sensor look direction, termed , the ion temperature and the major constituent ions. The current at zero retarding voltage is used to derive the total ion number density.
Figure 3. RPA current-voltage characteristic typical of topside ionosphere.
The Ion Drift Meter is a planar sensor that presents a square aperture to the incoming plasma stream that is intersected by a number of planar semi-transparent conducting grids and a solid segmented collector. The collector segments are arranged such that the cuts lie approximately along the satellite nadir and the orbit normal, which define the local vertical and horizontal directions perpendicular to the satellite motion. Two schematic views of the IDM sensor are shown in figure 4.
Figure 4. Schematic cross-section and front view of the ion drift meter.
The sensor is mounted to view approximately along the satellite velocity through a larger aperture plane that provides a uniform planar electrical ground reference potential. Two planar grids are placed prior to the entrance aperture to prevent the passage of light ion species. To ensure that no internally applied potentials influence the ambient external plasma the outermost grid, which is coplanar with the aperture plane, is always tied to reference ground, as is the aperture plane. This suppressor grid is biased at -12 volts to prevent access of ambient thermal electrons to the collector and to suppress electrons liberated from the collector by solar euv radiation. Figure 5 shows a schematic cross-section and the O+ beam trajectory, in the plane of the satellite motion and the local vertical. Also shown is the electrical configuration indicating the current measured by logarithmic electrometers that provide the inputs to a linear difference amplifier.
Figure 5. Ion beam trajectories and electronic configuration for the IDM.
It can first be easily seen that a simple trigonometrical relationship exists between the component of the ion velocity normally incident on the sensor aperture and the transverse component of the ion velocity.
The primary task is to determine the arrival angle a, of the plasma from knowledge of the asymmetry in the collector currents and . It can be seen that
Thus from a measurement of the current ratio and knowledge of the sensor dimensions the tangent of the ion beam arrival angle can be determined and thus the transverse component of the ion drift can be derived.
IVM – Performance
The RPA utilizes an automatic ranging linear electrometer to measure the ion current for discrete values of the retarding potential that may specified by ground command and stepped at a 32 Hz rate. A nominal sweep sequence will comprise 16 points, recorded with 14-bit accuracy and executed in 0.5 seconds. The IDM utilizes two logarithmic electrometers that may be engaged to measure currents from collector segments that are alternately horizontal and vertical in the transverse direction. These signals provide the inputs to a linear difference amplifier that directly provides a signal proportional to the tangent of the ion arrival angle. The difference amplifier is sampled at 32 Hz with 14-bit accuracy, providing alternating horizontal and vertical arrival angles that provide a measure of the dynamic stability of the ionosphere and may be averaged to provide the transverse ion drift components that accompany the ram drift component derived by the RPA. The instrument performance capabilities are shown in Table 1.
IVM – References
Heelis, R.A. and W. B. Hanson (1998), Measurements of Thermal Ion Drift Velocity and Temperature Using Planar Sensors, Measurement Techniques in Space Plasmas: Particles, Geophys. Monogr. Ser., 102, AGU, 61.