ALOMAR

Arctic Lidar Observatory for Middle Atmosphere Research

 
Pulsation magnetometer

Built at Andøya Rocket Range with the pulsating eurora in mind....


 
USOC - puls.mag. amplifier



Photo: Jan Olav Andersen
Basic principle:

The searchcoil (induction) magnetometer is complementing the fluxgate magnetometer, and it's primary goal is to measure the very small/rapid variations in the earths magnetic field. If we define the fluxgate's sensitivity specifications down to about 0.5 nT and it's frequency resolution from DC to 1 Hz, then the searchcoil will continue down to about 10-20 pT with a frequency resolution of 20-30 Hz for the best instruments.

The main principle is pure induction where the magnetic field is moving and not the coil. Earlier the sensor was made of a coil without any core, containing thousands of revelations and with diameters up to about 2 meters. These days one normally uses high permeability cores, and the one used by ALOMAR-ARS consists of a core with a cross section of 1 sqare centimeter with a lenght of 1 meter (ur = 30000). The core is surrounded by a coil containing 40000 revelations and a small calibration coil containing 30 revs.

The coil output is directly coupled to a 120dB amplifier giving an output signal equal to 1.2 V at 100 pT at 2 Hz. The frquency range for the ALOMAR-ARS searchcoil magnetometer is 0.001 Hz to 3 Hz. The sensors are placed in the ground beside the USOC building.

Technical details:
  • Two channel induction magnetometer recording H and D comp.
  • Two sensors, high permeability cores (50 000) with wound coils of 40 000 turns
  • Small secondary windings for calibration
  • Signal from a main coil is amplified by a diff. Amp with switchable gain of 106, 112 or 118 dB.
  • Sensitivity: 300 T/V output with a gain of 112 dB
  • Frequency response: Flat from DC to 3 Hz
  • Input noise: 0,05 V p-p
  • Filter amplifier: 0,1 - 3 Hz (3 dB) gain 14 dB


Detecting rapid variations in the earths magnetic field
H and D component