Imperial College Space and Atmospheric Physics

André Balogh's Web Pages

Since the 1960s, instruments on Space Physics missions have changed and improved in many respects, but the basic physical parameters that we need to measure have remained the same. The requirement on any Space Physics mission is still to make measurements which are as extensive as possible of all the parameters of space plasmas: the distribution functions of all the constituents in the plasma populations; the DC and AC magnetic and electric fields; and the distribution functions of energetic particles species. All these parameters and distribution functions need to be measured with high spatial, directional and temporal resolution.

On past and present missions, our research group has designed and built magnetometers, energetic particle detectors, as well as onboard data processors and power supply and power management systems for space instruments. This page provides a brief overview of the kind of instrumentation in which we have acquired considerable expertise over the years and which we intend to provide for future missions.

1. Generic space physics instrument

The generic block diagram of a space physics instrument is shown in the figure above. It consists of one or more sensors or detectors; electronics units associated with the sensors which transform the basic physical parameters to be measured (e.g. magnetic fields, particle fluxes and energies etc.) into electrical signals; an onboard control and data processing unit; interface electronics to handle the telecommands sent by the spacecraft, the data sent by the instrument to the spacecraft telemetry and the timing and auxiliary signals sent by the spacecraft; and a power converter unit which takes the standard voltage from the spacecraft (usually + 28 V) and provides the required secondary voltages to the instrument.

2. Magnetometers

The measurement of the magnetic field vector is of fundamental importance to all space physics missions. Two main types of magnetometers are used. The first, most frequently used type is the triaxial fluxgate magnetometer, the second is the vector helium magnetometer. The main characteristics of both these types are be described below.

The triaxial fluxgate magnetometer FGM consists of three sensors, each of which, together with the sensor-specific electronics, provides a voltage proportional to the value of the component of the magnetic field along its axis. The magnetic field vector is measured by an orthogonal arrangement of three sensors, as shown in the figure above.

A fluxgate magnetometer sensor consists of a magnetic core, which has a toroidal drive winding around it, and a coil former surrounding the sensor core and drive winding, around which another, sense winding is placed. The two coils are effectively orthogonal, so that there is no magnetic coupling between them. Bipolar, symmetric current pulses in the drive winding are used to drive the core material deep into saturation around the hysteresis loop, at a frequency usually about fo = 15 kHz. In the absence of an external field, the symmetry of the hysteresis loop ensures that there is no signal in the sense winding. However, in the presence of a non-zero component of an external magnetic field along the axis of the sense winding, the hysteresis loop is slightly displaced, leading to a non symmetric magnetic signal which induces an alternating voltage in the sense winding, at a frequency 2fo. This signal, of order << mV, is proportional to the component of the magnetic field along the axis of the sense winding. It is first amplified, then detected, using a synchronous detector. After some further amplification, the resultant voltage signal is fed back, through a transconductance amplifier and the sense winding, as a current counteracting the effect of the external field in the core. The voltage, representing the analogue value of the magnetic field component, is fed to an Analogue-to-Digital Converter which provides the digital output required for the data processing system.

The performance of magnetometers is measured by their sensitivity (the smallest magnetic field they can measure) which is related to their noise level; their measurement range; and their frequency response. The best magnetometers used on space missions have a noise level of order 5 - 10 pT, and it is routinely possible to achieve performances better than 0.1 nT, within a frequency range from DC to 10 Hz. In and around the magnetosphere, magnetic fields to be measured range from a few nT to several thousand nT (the Earth's magnetic field at the equator, on the Earth's surface, is about 30,000 nT). However, in interplanetary space, at large distances from the sun, the magnetic field is usually 1 nT or less. It is nevertheless possible to build magnetometers which meet all the measurement requirements on current and foreseeable space missions. The measurement range is usually quite flexible; the dynamic range (ratio of the maximum field to be measured to the resolution of the instrument) is usually determined by the magnetometer electronics and the analogue-to-digital converter (ADC). On past space missions, 12-bit ADCs were used, currently 14-16 bit ADCs are routinely used, and more recently, on the latest space missions 20-bit ADCs are either used or being planned (20 bits correspond to a maximum range of one million times the digital resolution).

Magnetometers have, in general, a small offset, which means that the output values of the three magnetic sensors are not exactly zero in a zero field. These offsets are normally determined by calibration on the ground and have values of order 0.1 to 5 nT. However, the offsets are usually not constant in time, and an in-flight determination is necessary. On spinning spacecraft, the components of the offset vector in the plane perpendicular to the spin axis can easily be determined, as, in a slowly varying magnetic field, the average measured in the plane of the spin over a complete spin period should be zero. The offset component along the spin axis is more difficult to determine, but techniques have been devised, valid in interplanetary space, which rely on the statistical properties of the magnetic field to determine the spin-aligned offset component. In the magnetosphere, this is more difficult to apply, and alternative methods are used; one such method is to rotate the spacecraft spin axis by 90o. On three- axis stabilised spacecraft, offsets are usually difficult to determine; in interplanetary space, again it is the statistical properties of the magnetic field which are used. In any case, it is important to remember, that the offsets and their long-term stability are the most critical characteristics of space-borne magnetometers.

Another type of magnetometer that has been used on some major missions is the Vector Helium magnetometer (VHM). Its use has been restricted by (a) its relatively greater complexity, mass and power, when compared to the fluxgate magnetometers, and (b) its restricted availability: only the scientific team at the Jet Propulsion Laboratory have developed and used it on space missions(Pioneer 10 and 11, ISEE 3, Ulysses). However, in principle, it out- performs the fluxgate types, by its lower noise level, hence higher sensitivity, and by its greater stability (its offsets, in particular, have proved extremely stable on such missions as Pioneer and Ulysses). The VHM is based on the response of an optically pumped metastable He population (contained in a glass cell) in the presence of external magnetic fields. Such fields are detected by measuring the transmission of infrared light through the He cell; the transmission coefficient is proportional to the component of the external magnetic field along the optical axis of the He cell. It is also possible to operate this magnetometer in the scalar mode, in which its response is made dependent, with very high accuracy and precision, on the magnitude of the external magnetic field. Combining the scalar capability of the He magnetometer with the vector capability of a tri-axial fluxgate magnetometer provides a very sensitive magnetometer system; such a system is currently flying on the Cassini Saturn Orbiter mission and is expected to provide a very accurate mapping of the internal magnetic field of Saturn, when Cassini is placed in orbit around the planet in 2004.

12 January 1999
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