Global MHD simulation of magnetospheric response of preliminary impulse to large and sudden enhancement of the solar wind dynamic pressure
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A sudden increase in the dynamic pressure of solar wind generates a prominent and transient change in ground-based magnetometer records worldwide, which is called a sudden commencement (SC). The magnetic field variation due to an SC at high latitudes shows a bipolar change, which consists of a preliminary impulse (PI) and main impulse (MI). The largest recorded SC had an amplitude of more than 200 nT with a spiky waveform at low latitudes, and the mechanism causing this super SC is unknown. Here, we investigate the cause of the super SC using a newly developed magnetosphere-ionosphere coupling simulation, which enables us to investigate the magnetospheric response to a large increase in the solar wind dynamic pressure. To simulate SCs, the dynamic pressure of the solar wind is increased to 2, 5, 10, and 16 larger than that under the stationary condition, and two different types of dynamic pressure increase are adopted by changing the solar wind density only or the solar wind speed only. It was found that the magnetic field variations of the PI and MI are several times larger and faster for a jump in the speed than for a jump in the density. It is inferred that a solar wind velocity of more than 2500 km/s in the downstream shock, which cannot be directly simulated in this study, would be consistent with the super SC.
KeywordsSudden commencement Extreme solar wind event MHD simulation Magnetosphere
Sudden commencement (SC) is defined by a distinct variation in the horizontal component of the magnetic field on the ground, which is caused by a rapid increase in the solar wind dynamic pressure. SC shows a prominent bipolar change at auroral latitudes, in which the first variation is called the preliminary impulse (PI) and the second variation is called the main impulse (MI) (Araki 1994). On the other hand, SC shows a prominent stepwise increase at low and middle latitudes, which is called a disturbance dominant at low latitudes (DL) (Araki 1994). The PI/MI variations are caused by a field-aligned current (FAC) generated in the dayside magnetosphere (Fujita et al. 2003a, b). The DL variation is caused by compression of the magnetosphere. It is well known that the amplitude of a DL is determined by the solar wind dynamic pressure (Siscoe et al. 1968).
These rapid magnetic field variations could give rise to large power blackouts because the current induced by these magnetic field variations, which is called a geomagnetically induced current (GIC), may damage electric power stations. On 13 March 1989, for example, the power blackout of the Hydro-Quebec system in Canada was attributed to an intense geomagnetic storm (Kappenman 2001). On 30 October 2003, a power blackout of a high-voltage transmission system occurred in southern Sweden during a space storm (Pulkkinen et al. 2005). A crucial task in the study of space weather is to predict and estimate the risk from GICs.
Araki et al. (1997) reported an SC with an anomalously large amplitude on 24 March 1991. An amplitude of more than 200 nT and a rise time of 30 s were observed at Kakioka Magnetic Observatory (geomagnetic latitude = 26.6° N). A geosynchronous magnetopause crossing (GMC) event occurred at the same time, where the magnetopause entered inside the geostationary orbit. In addition, Araki (2014) investigated the other SC with an anomalously large amplitude observed at Kakioka for the period 1924 and 2013 and Colaba-Alibag for 1868 to 1967. The anomalous SC had different features from ordinary SC events. The waveform of the low-latitude ground-based magnetometer signal was not stepwise like that of a DL, but it was an unusually spiky waveform similar to the PI/MI variations typically observed at auroral latitudes. The rise time of the anomalous SC of only 30 s was several times shorter than that of ordinary SCs. Araki et al. (2004) investigated the relationship between the amplitude and rise time of a DL using nighttime data at Guam. They showed that the amplitude and rise time of a DL have a negative correlation. They assumed that the rise time is determined by the passage time of the solar wind discontinuity across an effective length in the magnetosphere, which was estimated to be ~30 Re (Re denotes earth radius). If the effective length was also 30 Re in the anomalous SC, the solar wind speed would need to be 6000 km/s to explain the short rise time of 30 s. Such a high-speed solar wind velocity has never been observed and may not be realistic. An alternative explanation would be that the anomalous SC included the magnetic field variation of the PI in addition to the DL variation, even at low latitudes, since the rise time of the PI variation is typically shorter than that of the DL variation.
To determine the mechanism controlling the amplitude of the PI and the rise time, we investigated the PI variation in detail by changing the solar wind speed and density by performing a high-resolution global MHD simulation based on magnetosphere-ionosphere coupling. Details of our simulation are described in “Simulation settings.” In “Simulation results,” we show the results for the solar wind parameter dependence of the PI amplitude and the rise time. Finally, in “Summary and discussion,” we summarize the results and discuss their application to extreme space weather events.
The numerical global MHD model developed by Tanaka (2003) self-consistently solves the solar wind-magnetosphere-ionosphere coupling process (Moriguchi et al. 2008), which is necessary for the investigation of the SC. To achieve high resolution and accurately capture discontinuities, the MHD calculation employs a finite volume (FV) total variation diminishing (TVD) scheme with an unstructured triangular grid system. The number of triangular grid points is 30,722 in the horizontal direction and 240 grids in the radial direction. The outer and inner boundaries of the simulation are set at 200 Re and 3 Re, respectively. Details of the calculation of the inner boundary are given by Tanaka (2000). In this paper, the x-axis points toward the Sun, the z-axis points north, and the y-axis is chosen to satisfy the right-handed system. To examine the magnetic field variations on the ground, only the effect from the ionospheric Hall current is calculated using the Biot-Savart law. The distribution of the Hall current is calculated as the product of the ionospheric Hall conductivity and the electric field projected along the field line from the inner boundary to the ionosphere.
Solar wind parameters for the density jump and speed jump conditions used in this study
(V = 372 [km/s])
(N = 5 [/cc])
The right panel of Fig. 4 shows the relationship between the amplitude and the rise time of the PI. The vertical axis indicates the magnetic field amplitude of the PI. The horizontal axis indicates the rise time of the PI. The rise times in the speed jump simulations are shorter than those in the density jump simulations. The rise time is determined by the time in which the solar wind discontinuity passes through the effective length of the magnetosphere. The solar wind velocity in the speed jump simulations is higher than that in the density jump simulations. Therefore, the rise times in the speed jump simulations are shorter than those in the density jump simulations. In both the speed jump and density jump simulations, the PI amplitude increases as the PI rise time decreases. This tendency is the same as the relation between the rise time and the amplitude of the DL investigated by Araki et al. (2004). On the other hand, a difference is that the rise time of the PI is shorter than that of the DL. The rise time of the PI is at most 90 s, while the rise time of the DL is at least 1.5–9 min. We can estimate the effective length of the PI from the product of the solar wind velocity and the rise time in our simulation results, which is ~3.5 Re from the speed jump simulations. This indicates that the effective region of the PI is more localized than that of the DL, which has an effective length of 30 Re. The reason for the short PI rise time is that the dynamo generating the FAC is localized in the dayside magnetosphere. We next discuss the mechanism by which the dynamo generates the magnetic field disturbance on the ground via the magnetosphere-ionosphere current system.
Summary and discussion
The amplitude of the PI has different values for a density jump and speed jump even though the solar wind dynamic pressure is the same.
The amplitude of the PI depends on the solar wind density and the speed. This is different from the DL response, which only depends on the solar wind dynamic pressure.
A high-speed solar wind is needed to create a large-amplitude PI because the dynamo that generates the FAC associated with the PI magnetic field variation is more effective at an enhanced speed than at an enhanced density.
The amplitude of the PI is larger when the rise time is shorter. This is consistent with the relationship between the rise time and amplitude of the DL, and the rise time of the PI is shorter than that of the DL because the effective length of the PI, i.e., the dynamo region, is shorter than that of the DL. Also, the rise time of the PI is shorter at an enhanced speed than at an enhanced density. It is therefore suggested that a high-speed solar wind is needed to create a rapid magnetic variation.
It is also found that a magnetic field variation similar to the so-called Psc appears after the PI/MI only under the speed jump condition as shown in the bottom right panel of Fig. 3. The oscillation has a period of a few minutes, corresponding to that of the vortices in the dayside equatorial plane in the magnetosphere, as shown in the bottom right panel of Fig. 1. When the high-speed solar wind impinges on the magnetosphere, vortices are repeatedly formed at the equatorial magnetopause, which is probably due to the K-H instability. It seems that the high pressure of the vortices plays an essential role as a current generator to drive the FAC and the magnetic field oscillation. The mechanisms of magnetic field oscillation driven by the dynamic pressure enhancement under the velocity jump condition will be discussed in detail in a separate paper.
We investigated the PI magnetic field variation at auroral latitudes based on the results of our simulation and that at low latitudes based on the results at auroral latitudes, because the validity of our simulation code is limited to the boundary of the ionosphere, corresponding to the inner boundary of the magnetosphere. To overcome this problem directly, we have to modify our simulation code to one with an inner boundary of less than 3 Re. However, considerable time is required to simulate the inner region because the Alfven speed in the model is greater than that in our model. In the future, we will modify our code and investigate the PI dependence on latitude.
Note that the parameters of the speed jump condition do not correspond to those of realistic solar wind because the shock conditions are not satisfied at the discontinuity between upstream and downstream. However, to investigate the effect of a speed jump, it was necessary to distinguish the effect of a density jump from that of a dynamic pressure jump to clarify which parameter should be varied to investigate PI magnetic variation. Here, we discuss a case in which the shock condition is satisfied. The solar wind discontinuity in the simulation must satisfy the shock condition to compare observations. Our simulation code is limited to high solar wind dynamic pressures of at least 16 times the solar wind dynamic pressure because the effect of a high solar wind pressure reaches the inner boundary in the simulation, which is at 3 Re. Therefore, we carried out the simulation while satisfying the shock condition by assuming 16 times the solar wind dynamic pressure, although the solar wind velocity is less than that predicted for the extreme event reported by Araki et al. (1997). The solar wind velocity in the simulation is 744 km/s and the density is 23.2/cc. According to the simulation, the amplitude of the PI is 350 nT. This amplitude is almost the same as that for the speed jump condition with a solar wind velocity of 744 km/s, which does not satisfy the shock condition. This suggests that it is not important for the amplitude and rise time of the PI to satisfy the shock condition. Therefore, the results in this paper may be reliable even though the shock condition is not satisfied.
All numerical simulations reported in this paper were performed on a HITACHI SR16000 supercomputer at NICT. This study was carried out using the resources of Science Cloud at NICT. We used a 3-D visualization system called Virtual Aurora developed on AVS/Express Developer.
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