# Satellite Measured Ionospheric Magnetic Field Variations over Natural Hazards Sites
## 4 Data Processing Method  
For a source event with specific date and geographical coordinate a virtual spatial box of usually a few degrees in lat/long is selected. If line of sight projected footprints from satellites are within this area, the magnetic field values from these orbit segments (transits of the area) are considered and selected for further processing. In general, these segments are crossings before and after the event, the extend of the time interval is selectable (usually a few days). All field components for each segment are individually detrended, this corresponds to a high-pass filtering with low corner frequency of \(f_{c}=0.02\times f_{Ny}\), where \(f_{Ny}\) is the Nyquist frequency. A wavelet transform (Morse wavelet, symmetry \(\gamma=3\), time-bandwidth product \(P^{2}=60\)) converts the components (time series) into a time-scale decomposition. With the absolute values of this representation a binning into frequency bands (scales) is conducted. For _Swarm_ the statistics of the fluctuations are computed over the time dimension of the scalogram for each crossing, i.e., over the time shift for each scale of the wavelet transform (not over different crossings). In case of CSES HPM the segments before the event are merged to a single entity, a second entity are the merged segments after the event. This is necessary to get enough measurements for the further processing, i.e., a higher sample rate is preferable. For CSES HPM the step size is in log-scale according to \(\ln(f_{n})=-0.7651-0.069315\times\)n, where n is the number of steps shown in the ranges n = {1 - 50, 1 - 60, 1 - 63}, for _Swarm_ A/B/C VFM the step size is \(\ln(f_{n})=3.1469-0.069315\times\) n, shown are two ranges n = {1 - 40, 1 - 61}. The significance of the values are higher at the lower end of the range (the higher frequencies), because the fluctuations are usually weak for the upper range but depending also on the strength of the source event. The statistics of the segment bands are compared. For the evaluation and graphical representation (box plots) median values and the 25/75 percentiles are used, outliers are individually drawn, and the boxes/points are colour and shape coded. Segments before (after) the event are in blue (red) colour with fading intensity for larger time differences to the event time, i.e., focus is on crossings in the temporal vicinity of the event, notches indicate the medians. The widths of the boxes are decreasing with time, the furthest crossing in the past (future) has maximum (minimum) elongation. The first crossing after the event is visualised with a red bar, the length marks the percentiles range, and a circle symbol for the median is used. A Mann-Whitney U test with significance level 5 percent for the earthquakes investigated with CSES FGM1 is carried out, flags (yellow stars) at the top of the panels indicate the shift of the medians.  
The method is designed to be sensitive to detect magnetic field variations at LEO altitudes which potentially originate from natural hazards. The measured signals in the ionosphere shall carry the imprints of (and close to) lithospheric processes. The conjecture is that the transport of energy and momentum from bottom to top takes place through atmospheric waves.