Sudden changes in solar wind dynamic pressure can greatly influence the magnetosphere–ionosphere coupling system. It has long been known that the shape of the magnetosphere can be controlled by solar wind dynamic pressure and that the magnetosphere can be compressed by a positive dynamic pressure pulse. Furthermore, such positive pulses can generate numerous geo-effects within the magnetosphere–ionosphere coupling system. For example, observations from global magnetometer arrays indicate that a positive dynamic pressure pulse can lead to geomagnetic sudden impulses (SI+) (e.g., Araki, 1994). Positive dynamic pressure pulses have also been shown to play an important role in generating ultra-low frequency (ULF) waves (e.g., Kaufmann and Walker, 1974; Nopper et al., 1982; Wedeken et al., 1984; Cahill et al., 1990; Sarris et al., 2010; Shi et al., 2013). Zhao et al. (2019) found that such solar wind dynamic pressure pulses can directly excite field line resonances (FLRs) by using multipoint observation from THEMIS satellites, and they also provide the distribution of the FACs associated with FLRs in the same longitude but different latitudes. At lower altitudes, positive dynamic pressure pulses can induce traveling convection vortices (TCVs), as observed using incoherent scatter radars (Lühr et al., 1996; Valladares et al., 1999) and Super Dual Auroral Radar Network (SuperDARN) radars (Kataoka et al., 2003; Lyatsky et al., 1999). These TCVs are not static but propagate from the dayside to the nightside at speeds of several kilometers per second, consistent with the dynamic pressure pulse sweeping across the magnetosphere from the dayside to the magnetotail (e.g., Friis-Christensen et al., 1988; Glassmeier et al., 1989; Glassmeier et al., 1992; Kivelson and Southwood, 1991; Lühr et al., 1996; Yahnin and Moretto, 1996). At even lower altitudes, positive dynamic pressure pulses play an important role in modulating auroral phenomena, with auroral intensification typically starting near local noon and then extending toward the nightside along the dawn and dusk flanks of the auroral oval (e.g., Meurant et al., 2003; Zhou and Tsurutani, 1999; Zhou et al., 2003).

In contrast to the extensive studies on the geo-effects of positive dynamic pressure pulses, studies on the response of the magnetosphere–ionosphere coupling system to negative dynamic pressure pulses are relatively scarce (e.g., Akasofu, 1964; Araki and Nagano, 1988; Takeuchi et al., 2002). As a mirror response to the positive pulse, Araki and Nagano (1988) proposed that ground magnetic variations induced by negative dynamic pressure pulses oscillate in the opposite direction to those caused by positive pulses. Zhang et al. (2010) found that negative dynamic pressure pulses can also generate ULF waves, though the amplitudes are smaller than those associated with positive pulses. Negative pulses also play a role in generating TCVs, and Motoba et al. (2003) reported that negative pulses can induce clockwise ionospheric vortices on the duskside. Hori et al. (2012) found that evening-sector TCVs propagate poleward, consistent with negative pressure pulses sweeping the magnetosphere from the dayside to the nightside. Furthermore, Zhao et al. (2016) reported that such pulses can generate counterclockwise magnetospheric vortices on the dawnside; however, their study did not present observational or simulation results for the duskside magnetosphere.

Sibeck (1990) proposed a model illustrating the magnetospheric perturbation response to positive dynamic pressure pulses, in which magnetospheric vortices form near the dawn and dusk flanks. Shi et al. (2014) further suggested that such positive pulses can generate both clockwise and counterclockwise rotating vortices in the dawnside and duskside magnetosphere, respectively. Later, Shi et al. (2020) modified the model, proposing that negative pressure pulses can also generate vortices near the dawn and dusk flanks, but with opposite rotation directions compared to those induced by positive pulses. However, global MHD simulations by Yu and Ridley (2009) suggest that the FACs on the duskside flanks are not consistent with the Sibeck and Shi models. Specifically, the simulations show clear upward downward FACs on the duskside magnetosphere after a positive pulse. Notably, Yu and Ridley (2009) did not include any in situ observational verification, and no published studies have focused specifically on the duskside response to negative dynamic pressure pulses. Therefore, it is important to obtain in situ observations of magnetospheric vortices in the duskside magnetosphere following negative dynamic pressure pulses. Furthermore, Araki (1994) proposed that magnetospheric vortices can drive ionospheric vortices via FACs, and that these ionospheric vortices should propagate in tandem with their magnetospheric counterparts. However, no study has yet confirmed this relationship for negative dynamic pressure pulses. Thus, it remains unclear whether such pulses can generate both magnetospheric and ionospheric vortices on the duskside magnetosphere, and whether the two structures co-propagate.

In this study, we report the first direct, chain-type observation of a clockwise magnetospheric vortex and a corresponding equivalent ionospheric currents (EICs) vortex associated with a negative solar wind dynamic pressure pulse in the duskside sector. The dataset is presented in Section 2, and observations are described in Section 3. The discussion and conclusions are provided in Sections 4 and 5.

In order to correlate negative solar wind dynamic pressure pulses with magnetospheric vortices in the duskside magnetosphere, observations of solar wind and interplanetary magnetic field (IMF) parameters from the WIND spacecraft are used in this study (e.g., Farrell et al., 1995; Gloeckler et al., 1995). In addition, the solar wind dynamic pressure and SYM-H index obtained from the OMNI database are also used. The THEMIS mission provides observations of magnetospheric vortices following negative solar wind dynamic pressure pulses. The electrostatic analyzer onboard THEMIS provides magnetospheric plasma parameters (McFadden et al., 2008), while the fluxgate magnetometer measures the three components of the magnetic field (Auster et al., 2008). The time series of EICs are calculated using the spherical elementary current system (SECS) method, based on data from AUTUMN, MACCS, STEP, McMAC, CANMOS, USGS, CARISMA, and ground magnetometers operated by DTU.

In this work, we presented two individual cases demonstrating that negative solar wind dynamic pressure pulses can drive magnetospheric decompression and generate clockwise vortices in the duskside magnetosphere. The associated EICs further suggest a coupling through downward FACs, with the vortex propagating eastward in agreement with the tailward motion of the magnetospheric vortex. Moreover, the occurrence of the clockwise EIC vortex at lower latitudes, together with the satellite location, implies that the FACs linked to TCVs were generated within the nightside magnetosphere rather than at the magnetopause.

In this work, we present two individual cases showing that solar wind dynamic pressure pulses can generate clockwise magnetospheric vortices in the duskside magnetosphere. In the first case, the solar wind dynamic pressure decreased from 1.6 nPa to 1.0 nPa, while in the second case it decreased from about 9.2 nPa to 4 nPa. In both cases, the THEMIS satellites observed a clockwise magnetospheric vortex in the duskside magnetosphere. The first case involved only a small decrease of about 0.6 nPa, whereas the second case involved a much larger decrease of about 5.2 nPa. Nevertheless, both small- and large-scale negative dynamic pressure pulses were capable of generating clockwise vortices. It is well established that positive solar wind dynamic pressure pulses can induce magnetospheric vortices on both the dayside and duskside magnetosphere (e.g., Sibeck, 1990; Shi et al., 2014; Tian et al., 2016). The generation of such vortices is typically attributed to fast magnetosonic waves launched by the sudden compression of the magnetosphere (e.g., Samsonov and Sibeck., 2013; Shi et al., 2014). These waves propagate from the magnetopause toward the inner magnetosphere and then reflect at internal boundaries such as the plasmapause or ionosphere. The combined penetration and reflection of these waves subsequently form vortex-like flow patterns (e.g., Shi et al., 2014; Tian et al., 2016). For negative solar wind dynamic pressure pulses, we also adopt this explanation: the basic vortex structure can be induced by the generation and reflection of fast magnetosonic waves (e.g., Zhao et al., 2016). When a negative pressure pulse reaches the magnetosphere, it can excite a fast magnetosonic wave that propagates toward the magnetopause. Upon reaching the outer boundary, the fast mode wave is reflected at the magnetopause and then propagates earthward. Furthermore, the Ampère force at the front of the fast magnetosonic wave can accelerate plasma along the direction of wave propagation. As a result, the generation and reflection of fast mode waves form a basic plasma flow vortex, rotating in the opposite direction to that induced by positive solar wind dynamic pressure pulses (e.g., Zhao et al., 2016; Shi et al., 2020). In the first case, a solar wind dynamic pressure increase (from about 1.1 nPa to 1.6 nPa) was observed approximately 12 min before the negative solar wind dynamic pressure pulse. The negative pressure pulse occurred at around 03:13 UT, while the corresponding magnetospheric perturbations were detected by the THEMIS satellite at 03:25 UT. Furthermore, the expected sense of rotation of the magnetospheric vortex associated with a positive pressure impulse should be counterclockwise, which is opposite to the clockwise rotation of the magnetospheric vortex observed in this study. Therefore, we conclude that the magnetospheric perturbations detected by THEMIS were induced by the negative solar wind dynamic pressure pulse. Zhang et al. (2010) proposed a criterion that a solar wind dynamic pressure pulse can be identified when the variation of dynamic pressure exceeds either 1 nPa or 50%. Such pulses are considered capable of generating ultra-low-frequency waves inside the magnetosphere. In this work, the first event does not satisfy this criterion; however, the negative pulse still generated a magnetospheric vortex. We suggest that the requirement of a 50% variation may be too strict, and we plan to analyze more cases in a statistical study to determine a more appropriate threshold for whether a pulse can generate magnetospheric vortices.

Through the different amplitudes of solar wind dynamic pressure decreases, both cases in this work generate magnetospheric vortices, but the corresponding magnetospheric and ionospheric responses are very different. In the first case, the solar wind dynamic pressure decreased by only about 0.6 nPa, whereas in the second case it decreased by about 5.2 nPa. The amplitude of the magnetospheric plasma flow velocity perturbation is about 50 km/s in the first case, and about 100 km/s in the second case. Keiling et al. (2009) proposed that the magnitude of FACs is proportional to the speed of the rotating plasma flow vortex. This suggests that the FACs in the second case should be approximately twice those in the first case, assuming other parameters remain similar. In this study, we used the spherical elementary currents systems method to calculate EICs. If we assume a zero gradient of ionospheric conductance in the direction perpendicular to the electric field, the Hall-to-Pedersen conductance ratio becomes proportional to the FACs intensity (e.g., Amm and Kauristie, 2002; Weygand et al., 2011). Using this approach, we calculated the FACs induced by the negative solar wind dynamic pressure pulses. The downward FACs induced by the clockwise magnetospheric vortex are about −13,053 A for the first case, and about −25,984 A for the second case. These results are consistent with the theoretical framework proposed by Keiling et al. (2009), confirming that stronger plasma flow vortices produce stronger FACs. The FACs results show that while solar wind dynamic pressure pulses of different amplitudes can both generate magnetospheric vortices, but the resulting FAC intensities are very different. Furthermore, in both cases the SYM-H index lags behind the negative solar wind dynamic pressure pulse by several minutes. In the first case, the delay is about 5 min, while in the second case it is about 3 min. When the solar wind dynamic pressure decreases, the magnetopause expands outward, leading to a global reconfiguration of magnetospheric current systems. This process involves several stages. First, the pressure disturbance must be transmitted from the solar wind into the magnetosheath and then into the magnetosphere. Although fast-mode and Alfvén waves can propagate rapidly, the perturbation still requires a finite time (typically a few minutes) to reach the inner magnetosphere. Second, while the magnetopause and tail currents are modified almost immediately, the ring current and partial ring current respond more gradually, as their variations depend on the drift and redistribution of energetic ions in the equatorial plane. This inertia of the current systems delays the manifestation of the disturbance in ground-based indices. Third, the SYM-H index is derived from an average of multiple low-latitude magnetometer stations. Therefore, even if local perturbations appear promptly at some stations, the index only reflects the global change once the disturbance has developed coherently across a broad longitudinal extent. Taken together, the propagation of the pressure pulse, the dynamical adjustment of magnetospheric currents, and the averaging nature of SYM-H account for the observed several-minute delay.

According to Sibeck (1990), magnetospheric vortices driven by dynamic pressure variations propagate from the dayside toward the nightside along the magnetopause, implying that they are generated near the magnetopause. Araki (1994) further proposed that positive dynamic pressure pulses can simultaneously excite paired vortices in both the dawnside and duskside magnetosphere, each linked to corresponding ionospheric vortices via FACs. This model suggests a mirrored relationship between magnetospheric and ionospheric vortices, implying that ionospheric vortices should propagate in concert with their magnetospheric counterparts. Thus, if a magnetospheric vortex moves from the dayside to the nightside, the associated ionospheric vortex should exhibit a similar motion. In the second case, we present multi-satellite THEMIS observations of a magnetospheric vortex. A clear time lag is observed in the magnetospheric plasma flow velocity (Figure 7), allowing us to use multi-point measurements to calculate the vortex’s propagation speed. The estimated speed is about 297 km/s, which is smaller than the solar wind velocity of 460 km/s. The corresponding EICs vortex propagation speed is about 7.8 km/s, which corresponds to roughly 290 km/s at the magnetopause, similar to the magnetospheric vortex propagation speed. Furthermore, the propagation speed of the EICs vortex closely matches the shock aurora propagation speed of 6–11 km/s (e.g., Zhou et al., 2009). This similarity suggests a strong relationship between shock auroras and magnetospheric vortices following solar wind dynamic pressure pulses. Although no corresponding auroral observations are available for these cases, we plan to address this question in future studies by analyzing additional events.

In this work, we present a causal chain demonstrating that a negative solar wind dynamic pressure pulse can generate a clockwise magnetospheric vortex in the duskside magnetosphere. A corresponding clockwise EICs vortex can also be generated via downward FACs. The time series of the EICs results indicates that the EICs vortex propagated eastward, consistent with a tailward-moving magnetospheric vortex. Furthermore, the FACs associated with TCVs are located inside the magnetosphere rather than on the magnetopause in the nightside sector. In future studies, we will investigate more cases of magnetospheric and corresponding ionospheric vortices using in situ observations and global MHD simulations.