How to interpret a spectrum with FFT mirroring?
Your field data is real (as in &quot;not complex&quot;) and hence your spectrum is conjugate symmetric.
fftshift() rotates the spectrum so that 0 Hz is in the middle of the vector. Your x-axis on the plot should be fs*[-nFFT/2:nFFT/2-1]/nFFT; where $nFFT$ is the FFT length and $fs$ is the sampling rate.
Remember that time discrete signals are periodic in frequency the second half of the spectrum represents the negative frequencies.