ULF Waves Derived from Ground Magnetometer Measurements
ULF data products are derived from the 1-min magnetometer data provided by 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.
- 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.
- The ULF data obtained from coastal stations may be distorted by the conductivity of the nearby sea-water, known as the coastal effect.
- Indices are revised as new stations are added to the data holdings.
- Finally, the indices are based on the SuperMAG version of the data and thus have undergone error correction, temporal resampling, rotation, and the baseline has been subtracted. The data used are thus not equivalent to the original data that can be obtained from the collaborators.
Method of derivation:
Using the 1-min SuperMAG data, we calculate the 10 parameters of the ULF data products as follows:
- In the derivation of the ULF parameters, the N and E components are used.
- Wavelet analysis is applied to geomagnetic field data for a segment of 1-h length.
- 30 data points (15-min segment data on each end) are added at both ends of the 1-h data segment.
- The data segment is then shifted forward by 1 h and wavelet analysis is repeated.
- If a power peak is found in Pc5 ULF band (150Ð600 s), 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.
- The integrated power and slope of PSD (power spectral density) are estimated at each time.
- Ellipticity: ratio of minor axis to major axis of ULF polarization ellipse.
- Azimuthal angle: positive azimuth angles means that the major axis of the polarization ellipse is directed to the north-east quadrant.
- Q value: an indication of whether or not ULF oscillations are monochromatic. Generally, for Q > 2.0 the ULF could be monochromatic.
- Significance level: significance levels for a wavelet transform using a Chi squared test and significance level of 0.90 (90%). For > 1., wave power could generally be significant.