The CARISMA Induction Coil Magnetometers

To complement the measurements obtained by the fluxgate magnetometers, induction coil magnetometers are co-located at 8 CARISMA station locations (DAWS, FSMI, FCHU, RABB, ISLL, MSTK, PINA, THRF). Induction coil magnetometers are vector sensors, measuring the field in the direction of their winding axis (the long axis). Two coils are located at each site, aligned orthogonally with magnetic North and East and are housed in buried enclosures on a gravel bed for drainage and stability. The magnetometers are connected to a 24-bit data acquisition system with GPS timing, and are sampled at 100 samples/second.

Induction coil, or search coil, magnetometers measure changes in magnetic field. Faraday's law of inductance describes the relationship of induced voltage to a changing magnetic field, and is the operating principle of this design of magnetometer. Essentially, the magnetometer consists of a coil with a very large number of turns of wire wrapped around a high permeability core. The relationship between the measured signal (V), the number of turns (n), the area of the coil (A), the relative permeability of the core material (µ_r) and the changing magnetic field (B) can be expressed as:

The core acts as a magnetic field concentrator, increasing the density of the magnetic field inside the coil, and so increasing the sensitivity. A feedback winding is also wound around the core to flatten the frequency response of the magnetometer.

The induction coils deployed in the CARISMA array are custom made sensors based on the LEMI-30, produced by Lviv Centre of Institute for Space Research, and are designed for measurements on the range 0.001Hz to 30Hz. The frequency sensitivity is shaped by Faraday's law for frequencies below 1Hz (proportional to frequency) and by the active filtering for frequencies above this (flat response).  Typical phenomena seen with this type of instrument include the Schumann resonances, the signature of the ionospheric Alfvén resonator and evidence of EMIC waves in the magnetosphere.

Specifications of the CARISMA induction Coils