ULF Waves

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ULF Waves Derived from Ground Magnetometer Measurements

ULF data products are derived from the 1-sec SuperMAG data. If you use the ULF data products in publications and/or presentations please clearly indicate that these are SuperMAG derived ULF data.

Caveats:

  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.

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. Wavelet analysis is applied to geomagnetic field data for a segment of 1-h length.
  3. 30 data points (15-min segment data on each end) are added at both ends of the 1-h data segment.
  4. The data segment is then shifted forward by 1 h and wavelet analysis is repeated.
  5. If a power peak is found in Pc2 - Pc5 ULF bands, the main 4 parameters (Amplitude (nT), Period (s), Ellipticity, Azimuthal angle (degrees)) are calculated. Moreover, the Q value, significance level, and amplitude of each component are also calculated. If not found, their parameters are 999999.
  6. The integrated power and slope of PSD (power spectral density) are estimated at each time.

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, (nT)^2
  • 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