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Abstract. In this work, we simulated the atmospheric drag effect on two model SmallSats (small satellites) in low Earth orbit (LEO) with different ballistic coefficients during 1-month intervals of solar–geomagnetic quiet and perturbed conditions. The goal of this effort was to quantify how solar–geomagnetic activity influences atmospheric drag and perturbs satellite orbits, with particular emphasis on the Bastille Day event. Atmospheric drag compromises satellite operations due to increased ephemeris errors, attitude positional uncertainties and premature satellite re-entry. During a 1-month interval of generally quiescent solar–geomagnetic activity (July 2006), the decay in altitude (h) was a modest 0.53 km (0.66 km) for the satellite with the smaller (larger) ballistic coefficient of 2.2×10-3 m2 kg−1 (3.03×10-3 m2 kg−1). The associated orbital decay rates (ODRs) during this quiet interval ranged from 13 to 23 m per day (from 16 to 29 m per day). For the disturbed interval of July 2000 the significantly increased altitude loss and range of ODRs were 2.77 km (3.09 km) and 65 to 120 m per day (78 to 142 m per day), respectively. Within the two periods, more detailed analyses over 12 d intervals of extremely quiet and disturbed conditions revealed respective orbital decays of 0.16 km (0.20 km) and 1.14 km (1.27 km) for the satellite with the smaller (larger) ballistic coefficient. In essence, the model results show that there was a 6- to 7-fold increase in the deleterious impacts of satellite drag between the quiet and disturbed periods. We also estimated the enhanced atmospheric drag effect on the satellites' parameters caused by the July 2000 Bastille Day event (in contrast to the interval of geomagnetically quiet conditions). The additional percentage increase, due to the Bastille Day event, to the monthly mean values of h and ODR are 34.69 % and 50.13 % for Sat-A and 36.45 % and 68.95 % for Sat-B. These simulations confirmed (i) the dependence of atmospheric drag force on a satellite's ballistic coefficient, and (ii) that increased solar–geomagnetic activity substantially raises the degrading effect of satellite drag. In addition, the results indicate that the impact of short-duration geomagnetic transients (such as the Bastille Day storm) can have a further deleterious effect on normal satellite operations. Thus, this work increases the visibility and contributes to the scientific knowledge surrounding the Bastille Day event and also motivates the introduction of new indices used to describe and estimate the atmospheric drag effect when comparing regimes of varying solar–geomagnetic activity. We suggest that a model of satellite drag, when combined with a high-fidelity atmospheric specification as was done here, can lead to improved satellite ephemeris estimates.

Abstract. We performed a diagnostic study of geomagnetic storm-induced disturbances that are coupled to the lower ionosphere in mid-latitude D-region using propagation characteristics of VLF radio signals. We characterised the diurnal VLF amplitude (from two propagation paths) into five metrics, namely the mean amplitude before sunrise (MBSR), midday amplitude peak (MDP), mean amplitude after sunset (MASS), sunrise terminator (SRT) and sunset terminator (SST). We analysed and monitored the trend in variations of signal metrics for up to 20 storms, to understand deviations in the signal that are attributable to the storms; five storms (and their effects on the signals) were studied in detail, followed by statistical analysis that included 15 other events. Considering the quietient pre-day level following the storm our results showed that the MDP exhibited characteristic dipping in about 67 % and 80 % of the events for GQD-A118 and DHO-A118 propagation paths, respectively. The MBSR showed respective dipping of about 77 % and 60 %, while the MASS dipped by 58 % and 67 %. Conversely, the SRT and SST showed respective dipping of 25 % and 33 %, and 42 % and 47 %. Of the two propagation paths used in this study, the dipping of the amplitude of DHO-A118 propagation path signal is larger (as also observed in previous study). To understand the state of the ionosphere over the signal propagation paths and how it affects the VLF responses, we further analysed virtual heights (h'E, h'F1 and h'F2) and critical frequencies (foE, foF1, and foF2) of the E and F regions (from ionosonde stations near the transmitters). The results of this analysis showed a significant increase and/or fluctuations of the foF2, foF1, h'F2, h'F, h'Es and h'E near both transmitters during the geomagnetic storms. The largest increase in heights of the regions (h'F2, h'F, h'Es and h'E) occured over Juluisruh station (around the DHO transmitter) in Germany, suggesting a strong storm responses over the region leading to the large dipping of the DHO-A118 propagation path signal.

Abstract. We performed a diagnostic study of geomagnetic storm-induced disturbances that are coupled to the mid-latitude D region by quantifying the propagation characteristics of very low-frequency (VLF) radio signals from transmitters located in Cumbria, UK (call sign GQD), and Rhauderfehn, Germany (DHO), and received in southern France (A118). We characterised the diurnal VLF amplitudes from two propagation paths into five metrics, namely the mean amplitude before sunrise (MBSR), the midday amplitude peak (MDP), the mean amplitude after sunset (MASS), the sunrise terminator (SRT) and the sunset terminator (SST). We analysed and monitored trends in the variation of signal metrics for up to 20 storms to relate the deviations in the signal amplitudes that were attributable to the storms. Five storms and their effects on the signals were examined in further detail. Our results indicate that relative to pre-storm levels the storm day MDP exhibited characteristic decreases in about 80 % (67 %) of the events for the DHO-A118 (GQD-A118) propagation path. The MBSR showed decreases of about 60 % (77 %), whereas the MASS decreased by 67 % (58 %). Conversely, the SRT and SST showed amplitude decreases of 33 % (25 %) and 47 % (42 %), respectively. Of the two propagation paths, the amplitude decreases for the DHO-A118 propagation path signal were greater, as previously noted by Nwankwo et al. (2016). To better understand the state of the ionosphere over the signal propagation paths and how it might have affected the VLF amplitudes, we further analysed the virtual heights (h'E, h'F1 and h'F2) and critical frequencies (foE, foF1 and foF2) from ionosondes located near the transmitters. The results of this analysis showed significant increases and fluctuations in both the F-region critical frequencies and virtual heights during the geomagnetic storms. The largest increases in the virtual heights occurred near the DHO transmitter in Rhauderfehn (Germany), suggesting a strong storm response over the region which might account for the larger MDP decrease along the DHO-A118 propagation path.

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