ULF Parameters

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ULF Parameters Derived from Ground Magnetometer Measurements

ULF data products are derived from the 1-sec SuperMAG data. These data are not equivalent to the original data which can be obtained from the SuperMAG collaborators. If you use the ULF data products in publications and/or presentations please clearly indicate that these are SuperMAG derived ULF data.

According to MoU between Intermagnet and SuperMAG high fidelity data from Intermagnet stations cannot be downloaded from SuperMAG. These high fidelity data must be obtained from Intermagnet.


  1. Despite comprehensive and time consuming data correction and validation SuperMAG does not guarantee the quality of the data from the individual station and thus does not guarantee the quality of the ULF data either. The user is encouraged to validate the data before using them in publications.
  2. The ULF data obtained from coastal stations may be distorted by the conductivity of the nearby sea-water, known as the coastal effect.
  3. If data is higher than 1 Hz, data is resampled to 1 Hz using the median.
  4. If sampling rate of data is below 1 Hz, SuperMAG resamples the data to 1 Hz using linear interpolation. However, the lowest sampling rate included is 0.5 Hz.

Method of derivation:

Using the 1-sec SuperMAG data, we calculate the 10 parameters for each of Pc2 - Pc5 ULF bands as follows:

  1. In the derivation of the ULF parameters, the N and E components are used.
  2. 1 hour data segment is selected starting at full hours.
  3. 30 min (1800 data points) are added at both ends of above segment.
  4. Wavelet analysis (Morlet base used for optimal frequency resolution) is applied to 2 hour data segment.

Data gaps in the ULF amplitude occur at times when the wavelet power spectrum has no particular frequency peak in the Pc2-Pc5 ULF bands.

Data Products:

Period ranges:

ULF parameters:

  • Amp:
Sum of N and E amplitudes, (nT)
  • Namp:
N amplitude, (nT)
  • Eamp:
E amplitude, (nT)
  • Ipow:
Integrated Power, Log (nT2)
  • Prd:
ULF Period, (sec)
  • Elp:
ULF Ellipticity (ratio of minor axis to major axis of ULF polarization ellipse)
  • Azi:
ULF Azimuthal angle, (degrees; Positive azimuth angles means that the major axis of the polarization ellipse is directed to the north-east quadrant)
  • Qval:
Q value (an indication to check whether ULF wave is a narrow-band oscillation OR an indication of whether or not ULF oscillations are monochromatic. Generally, for Q > 2.0 the ULF could be monochromatic)
  • Slev:
Significant level of P amp (significance levels for a wavelet transform using a Chi squared test and significance level of 90%; for Slev > 1, wave power could generally be significant)
  • Slop:
Slope of power spectral density at each time