# Automatic Detection of Whistler Waves in the Top-Side Ionosphere: The WhISPER Technique
## 2 Data and Methods
### Detection Algorithm
Figure 2: Spectrograms of SCM-ELF observations (X, Y, and Z components from top to bottom in the top three panels) and IPSD\({}_{t}\) (bottom panel). Local maxima are highlighted by grey dots. Vertical dashed lines highlight the most prominent peak identified at \(t_{a}\) (blue) and a representative minor peaks identified at \(t_{b}\) (red). magnetic field data, respectively. The time resolution was approximately 6 ms both for SCM and EFD. Figure 3 illustrates waveforms (left panels) and spectrograms (right panels) for the most prominent IPSD peak, detected at \(t_{a}\) (blue dashed line in Figure 2). In the left panels, a vertical dashed line marks the detection time (\(t_{a}\)). As from left panels, the algorithm accurately identifies the onset of an oscillatory signal with a maximum amplitude of approximately 0.5 nT in all three components. The spectrograms (right panels) clearly reveal a typical whistler signature, characterized by the distinctive falling tone frequency chirped structure. The signal duration is approximately 0.5 s and the maximum frequency is \(\sim\)3.8 kHz. Figure 4 presents waveforms and spectrograms associated with a representative minor IPSD peak, detected at \(t_{b}\) (red dashed line in Figure 2). As in the previous case, the local maximum in the IPSD is temporally aligned with the onset of a whistler wave. In fact, even if the maximum signal amplitude reaches approximately 0.05 nT (Y and Z components, left panels) and both the waveforms and the corresponding spectrograms (right panels) exhibit higher noise levels than those shown in Figure 3 (here, the signal-to-noise ratio SNR is \(\sim\)10\({}^{2}\), a value notably lower than the \(\sim\)10\({}^{3}\) observed in the previous case), the characteristic falling tone structure associated with whistlers remains clearly discernible (right panels). The X component is characterized by smaller amplitude of the waveform and a noisier spectrum with respect to Y and Z, suggesting a circular polarization for the whistler wave on the YZ plane. The duration and frequency range of this event are comparable to the previous case. Electric field observations acquired during the same semiorbit are depicted in Figure 5. Spectrograms and IPSD\({}_{t}\) exhibit a high noise level in the central portion of the orbit, coinciding with the satellite's passage over low-latitude regions (usually in the range between \(-\)30\({}^{\circ}\) and 30\({}^{\circ}\), corresponding to the yellowish central part of spectrograms in Figure 5). The noise affecting EFD observations is attributed to mechanical friction induced by the vibration of boom-B, which is more pronounced at low latitudes, during summer and daylight due to increased photoionization (see [35] and references therein). Figure 3: Magnetic field waveforms (left panels) and spectrograms (right panels) within a \(\simeq\) 2 s window centered on \(t_{a}\) (blue dashed line, corresponding to the largest IPSD peak in Figure 2). The top to bottom panels correspond to the X, Y, and Z components, respectively. Particularly for diurnal semiorbits as the one illustrated in Figure 5, despite the IPSD calculation for EFD commencing from \(J_{MIN}\) = 30, the yellowish part of the spectrograms corresponds to a wide region of enhanced IPSD\({}_{\rm t}\). This broad enhancement can complicate the identification of well-defined IPSD peaks. Hence, to verify the presence of a whistler wave also in the electric field data, we examined the electric field waveforms within narrow time windows (\(\simeq\)2 s) centered around \(t_{a}\) and \(t_{b}\) (blue and red dashed lines, respectively, in Figure 2). To mitigate noise, prior to STFT, noise reduction techniques were applied to the electric field data. This preprocessing phase included detrending to remove ionospheric effects, which were not included in the data. Figure 4: Magnetic field waveforms (left panels) and spectrograms (right panels) within a \(\simeq\) 2 s window centered on \(t_{b}\) (red dashed line, corresponding to the minor representative peak in Figure 2).