Measurement Techniques in Space Plasmas, Vol II, (Snare) p. 101-113Measurement Techniques in Space Plasmas, Vol I, (Heelis&Hanson) p. 61-71Measurement Techniques in Space Plasmas, Vol I, (Wüest) p. 141-155Phys 954, Solar Wind and Cosmic Rays II. Instruments for PlasmasE. MöbiusII. Plasma InstrumentsInstrumentation for space plasmas can be separated into Fields and Particle instruments.Generally, under fields we subdivide into magnetic and electric field measurements.Particle instrumentation is different for charged particles and neutrals. In this course wewill concentrate on charged particle instrumentation. Electron instruments are not muchdifferent from ion instruments, except for the sign of the charge.We will start with a brief survey of the field instruments, especially with the magne-tometers.II.1 Magnetometers, E-Field InstrumentsBrandt, p. 142 – 145, Measurement Techniques in Space Plasmas, Vol II, (Snare) p. 101-113a) Search CoilThe simplest magnetometer is a search coil, i.e. a coil which spins with the S/C. Its axis is inclined with respect to the spin axis, e.g.:We make use of magnetic induction to deduce the magnetic field. ∇ xr E = −∂r B ∂tBecause we measure the time derivative, the result needs to be integrated over thespacecraft (S/C) spin to return B. As sin*B is measured, or in other words the compo-nents  to the spin axis, the component ||B is not returned by this method. This is adrawback of the method. Because only time variations of B as seen by the coil are measured, any constant B-com-ponents on the S/C (e.g. permanent magnets, magnetized screws, etc.) do not matter.This is a substantial advantage. Only time varying fields from AC currents can contami-nate the measured signal. To minimize this remaining problem calls for a careful wiringon S/C! For example, twisted pairs of wires are used to supply current.1/14/19 47BΘSpin AxisSearch CoilPhys 954, Solar Wind and Cosmic Rays II. Instruments for PlasmasE. MöbiusA search coil sensor has several disadvantages. Let us summarize them: not an absolute instrument  must be calibrated low cadence of the measured signal (S/C spin): This means, faster frequencies inspace are hard to untangle. component ||B is missing.b) Flux Gate MagnetometerThe disadvantages have lead to the development of more advanced magnetometers, forexample, the flux gate magnetometer. It makes use of the magnetization (with or with-out saturation) of a ferromagnetic core in a coil by a high frequency current.In a magnetic field H(t) = Ho sin t that is generated with an oscillator in a coil with aferrite in the center the magnetic induction B(t) = Bo sin t follows according to the fig-ure below. The wave may be clipped because of the finite permeability, but it remainsperfectly symmetric, if there is no additional external field: H1 = B1/µo = 0.With an ambient field B1 this becomes H = H1 + Ho sint and thus: B = B1 + Bo sintThe AC field isshifted along thepermeability curve. In any case (with or without satu-ration) the signal is asymmetric about 0 in B. Satura-tion is a complication that can be eliminated with thefollowing argument: If B1 = H1 = 0: B(t) = -B(t + /),whether the signal goes into saturation in the core or1/14/19 48OscillatorHBHBH1Power SourceConsumerPhys 954, Solar Wind and Cosmic Rays II. Instruments for PlasmasE. Möbiusnot. A Fourier series of the signal contains only odd harmonics. If 2 cores with oppositewindings are used and the two signals are added in a coil across both cores, the two sig-nals compensate each other perfectly for B1 = 0 and H(t) = 0. If H1  0, this symmetry isdestroyed and a residual signal B1 is measured that is proportional to the original mag-netic field and modulated with the frequency 2. f2(2) ~ H1The time resolution of the instrument is tied to the oscillator frequency . It can be usedin a DC field. With 3 units, one gets all 3 directions. In a clever arrangement and mak-ing use of the spacecraft spin even less units are needed.It is more accurate to measure an AC signal, in particular, after all other effects are com-pensated for. Therefore, a flux gate magnetometer: is very sensitive from 0.1 nT – a few 1000 nT is self-calibrating for the signal (depends only on physical constants)However: it is also sensitive to residual S/C fields  calibration needed, but the magnetometer can be flipped mechanically tosubtract residual static fields.In principle, 1 magnetometer is sufficient on a spinning S/C to get the vector field.An orientation of 54 44 w.r.t. the plane perpendicular to the spin axisprovides the same sensitivity in all directions.However, B(t) must be slow compared to the S/C spin to get the com-plete picture. Usually two cores are sufficient in a time varying field on a spinning S/C,even for moderately high frequencies. To cover even higher frequencies the search coil is the better choice, since it directlymeasures dB/dt. Thus very often both sensors are flown on space plasma missions.c) Electric Fields1/14/19 49Oscillator54o44'Phys 954, Solar Wind and Cosmic Rays II. Instruments for PlasmasE. MöbiusElectric fields are measured with probes on long boom antennas, by determining thevoltage drop between them. The tricky part is that the potential of a probe in plasmasdepends strongly on the environment (e and i temperatures, UV radiation, energeticparticles) that charges up the probes. As noted before, generally the electron current isstronger in plasmas than ion currents. Therefore, a floating surface charges up nega-tively (more electrons hit the surface). To reach equilibrium, i.e. je = ji, part of the elec-trons must be repelled. Given a Maxwellian with Te for electrons and a probe potentialUp, the electron current varies as:je(Up) = jeo*exp(-eUp/kTe)The total current is: j = je – ji = jeo*exp(-eUp/kTe) – ji with an exponential characteristics.This is the well-known Langmuir Probe characteristics. To make use for an antenna theprobes are biased with a power supply such that they are on the steepest point of thischaracteristics. This is a controlled potential close to what the local plasma potential is.Thus measuring the voltage between two probes provides the electric field betweenthem.A complication is that UV (e.g. form the sun) leads to emission of photoelectrons fromall S/C surfaces, which then can lead to positive charging of the S/C and the probes. Inaddition, any probe