We present the results of the complex study of ionospheric parameter variations during two geomagnetic storms, which occurred on April 12–15, 2016. The study is based on data from a set of radiophysical and optical instruments. Both the storms with no sudden commencement were generated by high-speed streams from a coronal hole. Despite the minor intensity of the storms (Dst ≥ –55 and –59 nT), we have revealed a distinct ionospheric response to these disturbances. A negative response of electron density and F2-layer critical frequency was observed during the main phase of both the storms. The amplitude of the negative response was higher for the second storm. The period of negative electron density deviations was accompanied by an increase in the peak height, as well as by the downward plasma drift in the evening and night hours, which is not typical of quiet conditions. We have also recorded sharp peaks in the AATR (Along Arc TEC Rate) index and in total electron content noise spikes on average 2–2.5 times. This indicates an intensification of small-scale ionospheric disturbances caused by disturbed geomagnetic conditions and high substorm activity.

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The term deviation frequency (fd) denotes the boundary between the variable part of the amplitude and phase scintillation spectrum and the part of uninformative noises. We suggested the concept of the “characteristic deviation frequency” during the observation period. The characteristic deviation frequency is defined as the most probable value of the deviation frequency under current local conditions. Our case study involved GPS, GLONASS, Galileo and SBAS data under quiet and weakly disturbed geomagnetic conditions (geomagnetic storm on 16 April 2021, Kpmax = 5, SYM-Hmin = −57 nT) at the mid-latitude GNSS station. Our results demonstrated that the deviation frequency for all signal components of GPS, GLONASS and Galileo varies within 15–22 Hz. The characteristic deviation frequency was 20 Hz for the mentioned GNSS signals. The SBAS differs from other systems: deviation frequency varies within 13–20 Hz. The characteristic deviation frequency is lower and equal to 18 Hz. We suggest the characteristic deviation frequency to determine the optimal sampling rate of the GNSS carrier phase data for the ionospheric studies. In turn, the deviation frequency can be considered as a promising index to estimate the boundary of non-variability of the ionosphere.

The term deviation frequency denotes the boundary between the variable part of the amplitude and phase scintillation spectrum and the part of uninformative noises. We suggested the concept of the “characteristic deviation frequency” during the observation period which is defined as the most probable value of the deviation frequency under current local conditions. This work is a case study of the characteristic deviation frequency (fd) registered for GPS, GLONASS, Galileo and SBAS signals under quiet and weakly disturbed geomagnetic conditions in April 2021 at the mid-latitude GNSS station. Our results demonstrated that the fd value for all signal components of GPS, GLONASS and GALILEO signals varied within 15-22 Hz. The characteristic deviation frequency was 20 Hz for the mentioned GNSS signals. In difference, the deviation frequency was limited within 13-20 Hz for SBAS with the lower characteristic deviation frequency at 18 Hz. We assume that the concept of the characteristic deviation frequency can be used to determine the optimal sampling rate of the GNSS carrier phase data for the ionospheric studies. The characteristic deviation frequency can also characterize the state of the regular trans-ionospheric radio channel.

Natural hazards and geomagnetic disturbances can generate a combination of atmospheric and ionospheric waves of different scales. The carrier phase of signals of global navigation satellite system (GNSS) can provide the highest efficiency to detect and study the weak ionospheric disturbances in contrast to total electron content (TEC) and TEC-based indices. We consider the border between the informative part of the carrier phase spectrum and the uninformative noises—the deviation frequency—as the promising means to improve the GNSS-based disturbance detection algorithms. The behavior of the deviation frequency of the carrier phase spectra was studied under quiet and disturbed geomagnetic conditions. The results showed that the deviation frequency value increases under magnetic storms. This effect was revealed for all GNSS constellations and signals regardless the GNSS type, receiver type/make and data rate (50 or 100 Hz). For the 100 Hz data, the most probable values of the deviation frequency grouped within ~28–40 Hz under quiet condition and shifted to ~37–48 Hz during the weak geomagnetic storms. Additionally, the lower values of deviation frequency of ~18–25 Hz almost disappear from the distribution of the deviation frequencies as it becomes narrower during geomagnetic storms. Considering that the small-scale irregularities shift the deviation frequencies, we can use this indicator as a “red alert” for weakest small-scale irregularities when the deviation frequency reaches ~35–50 Hz.