A train of likely Kelvin–Helmholtz (K–H) vortices with plasma
transport across the magnetopause has been observed by the Time History of
Events and Macroscale Interactions during Substorms (THEMIS) at the duskside
of the magnetopause. This unique event occurs when the interplanetary magnetic
field (IMF) abruptly turns northward, which is the immediate change to
facilitate the K–H instability. Two THEMIS spacecraft, TH-A and TH-E,
separated by 3
Kelvin–Helmholtz (K–H) instability can be activated at the interface between
different plasma regimes with different velocities, and the perturbations
propagate along the direction of the velocity shear as a form of surface
wave developing into nonlinear vortices. As shown by Hasegawa (1975), the
high density and the magnetic field perpendicular to the velocity shear on
either side of the interface facilitate the unstable condition. The fastest
K–H instability occurs when the wave vector
The orbits and positions of TH-A (green) and TH-E (black) during the interval of interest 22:20–22:54 UT. The position data are expressed in GSM coordinates.
In addition to magnetic reconnections at the low-latitude (Dungey, 1961) and high-latitude magnetopause (Song and Russell, 1992), whose nature is a popular research topic (e.g., Dai, 2009, 2018; Dai et al., 2017), the K–H instability is an important way to transport solar wind into the magnetosphere when reconnections are inactive at the magnetopause. A statistical study of double star observations implies the entry of cold ions into the flank magnetopause caused by the K–H vortices that is enhanced by solar wind speed (Yan et al., 2005). However, it is noted that the K–H instability itself cannot lead to plasma transport across the magnetopause (Hasegawa et al., 2004); therefore, certain secondary processes (e.g., Nakamura et al., 2004; Matsumoto and Hoshino, 2004; Chaston et al., 2007) are necessarily coupled with the K–H instability for plasma transport into the magnetosphere via the low-latitude boundary layer (LLBL). The reconnection of the twisted magnetic field lines inside the K–H vortex was first found in a simulation (Otto and Fairfield, 2000) and has since been identified in observations (Nykyri et al., 2006; Hasegawa et al., 2009; Li, et al., 2016). The plasma transport into the magnetosphere via such a process in K–H vortices has been quantitatively investigated in a simulation (Nykyri and Otto, 2001). Most recently, energy transport from a K–H wave into a magnetosonic wave was estimated by conserving energy in the cross-scale process, and three possible ways were discussed to transfer energy involving shell-like ion distributions, kinetic Alfvén waves, and magnetic reconnection (Moore et al., 2016). Up to now, there have only been a handful of reports of direct observations of plasma transport in the K–H vortices (e.g., Sckopke et al., 1981; Fujimoto et al., 1998; Hasegawa et al., 2004). Moreover, the microphysical processes for the plasma transport remain unclear, indicating more observations of such a transport process are needed to help us understand the physics. In this work, we present the THEMIS observations of likely K–H vortices activated when the IMF abruptly turns northward. We show a solar wind transport into the magnetosphere occurs and evolves within the vortices.
The THEMIS mission (Angelopoulos, 2008) consists of five identical
spacecraft originally orbiting the Earth similarly to a string-of-pearls
configuration. In August 2009, TH-B and TH-C were pushed to the vicinity of
the lunar orbit, while the other three stayed in the near-Earth orbit with
an apogee of approximately 13
During the interval 22:20–22:54 UT on 28 March 2016, TH-A and TH-E were
located near the magnetopause (Fig. 1), while TH-D was located in the
inner magnetosphere, far from the magnetopause. TH-B, near the lunar orbit,
was immersed in the solar wind at the dawnside downstream of the other two
spacecraft. As shown in panel (a) of Fig. 3, TH-B observed an abrupt turning
of the IMF from duskward to northward at 22:32 UT, corresponding to
22:22 UT, with a time lag of 10 min ((
Fluctuations in the plasma parameters and the ion and electron
energy–time spectra. Panel
The observed plasma rotations and perturbations of the magnetic field
because of the formation of K–H vortices. Panel
In this event, the IMF is strongly northward, and the observed magnetic
field does not change much, so it could be difficult to identify the
magnetopause. We selected the four intervals of 22:24:00–22:24:40,
22:32:40–22:33:10, 22:35:50–22:36:10, and 22:28:50–22:39:20 UT, marked by the
black arrows, when the TH-A ion spectrum showed the magnetosheath feature.
During the four intervals, TH-A observed magnetosheath cold ions without
magnetospheric hot ions (green regions at the top of panel e, Fig. 2). The
absence of hot ions indicated that the spacecraft had crossed the
magnetopause into the magnetosheath, where the outbound and inbound
crossings of the magnetopause can be identified in the ion spectrum. At each
pair of traversals, the local magnetopause coordinates LMN were calculated
by using MVA (Sonnerup and Cahill, 1967, 1968). The details and results of
MVA calculations are listed in Table 1. In the calculations of MVA, relatively
large ratios of the second to third eigenvalues
The magnetopause distortions formed by the K–H vortices deduced by the
MVA. The average magnetopause (dashed lines), approximated to the spacecraft
trajectory, was calculated from the magnetopause model (Shue et al., 1998).
Traversal pair at 22:24 UT in panel
Results of MVA analysis at the four magnetosheath encounters of TH-A.
The ratio of the second to third eigenvalues
The high-speed and low-density feature is one of the fundamental
characteristics of rolled-up vortices (Nakamura et al., 2004; Takagi et al.,
2006) and has been used to identify vortices in single spacecraft
measurements (e.g., Hasegawa et al., 2006; Hwang et al., 2011; Grygorov et
al., 2016). We estimated the magnetosheath velocity by averaging the TH-A
measurements during the four magnetosheath intervals mentioned above, with
the magnetosheath velocity of about 134 km s
The observed velocity along the tailward direction versus the ion density. Green dots are from TH-A observations and black dots from TH-E observations. The blue lines mark the high-speed and low-density region possibly caused by the acceleration of the rotation.
Before and after the 22:22–22:52 UT interval, the magnetospheric hot ions
dominated in panel (e) of Fig. 2, mainly in the 3–25 keV range with an
energy flux of 10
The coexistence of hot and cold ions is one direct feature of the solar wind transport into the magnetosphere, as clearly displayed in Geotail observations by Fujimoto et al. (1998) and in Cluster observations by Hasegawa et al. (2004). In this event, the coexistence of hot and cold ions was firstly noted near the periodically oscillating magnetopause. Furthermore, we used the enhancement of hot electron flux as an indicator of the magnetosphere and set up the more critical criteria to diagnose the coexistence and hence to display the transport regions, as marked by the green bars at the bottom of panel (f) and the black bars at the bottom of panel (j) in Fig. 2. By comparing the green bars and the black bars, it can be found that the transport regions in TH-A observations appear more periodic and those in TH-E observations more dispersed. The difference between the features of transport regions at upstream TH-A and downstream TH-E implies the plasma transport significantly occurred and evolved during the tailward propagation, along with the collapse of the vortices, leading to a kind of turbulence state, as illustrated in previous simulations (Nakamura et al., 2004; Matsumoto and Hoshino, 2004).
Typical portraits of the energy–time spectra of plasmas in different
regions. Panel
Intuitively, TH-E might be located further inward in the LLBL than TH-A and observed more dispersive oscillations. TH-A observed very clearly periodic motions of the magnetopause during the 34 min except 22:46–22:50 UT and TH-E observed a relatively much more dispersed spectrum during the interval, but five clear oscillations appeared again during 22:40–22:48 UT. However, it seems true that, on the whole, the spectrum observed at TH-E is much more turbulent than the periodic spectrum at TH-A. Such an evolution implies the collapse of the vortices and the evolution leading to a turbulence state. In previous simulations (Nakamura et al., 2004; Matsumoto and Hoshino, 2004), the vortices collapse and cause transport of the solar wind into the magnetosphere; after that, new vortices may be generated at the recovered magnetopause. The five oscillations during 22:40–22:48 UT at downstream TH-E can by explained as newly formed vortices. As mentioned above, the first K–H wave, as well as the transport regions, arrived at the upstream TH-A as soon as the IMF abruptly turned northward. The K–H vortices were evidently activated as a response to the abrupt northward turning of the IMF, which was the direct change to facilitate the K–H instability immediately.
Previously, both electron and ion distributions were used to diagnose the region of observation (Chen et al., 1993). While diagnosing the transport regions in this event, the typical plasma features in different regions were selected for comparisons (Fig. 6), as illustrated by the energy flux distributions of both ions (blue line) and electrons (red line). In panel (a), both the ion and electron fluxes show single peaks at low energy, indicating the components of a cold and dense magnetosheath plasma. In panel (b), the ion flux shows a double peak, which means the coexistence of the magnetosheath cold ions and magnetospheric hot ions. The relatively smaller peak/enhancement in the electron flux shows that the magnetospheric hot electrons are detected, but the cold electrons dominate, implying the spacecraft is located in the magnetosheath but very close to the magnetopause, a coexistence region. In panel (c), both the ion and electron fluxes show a double peak. The double peak of the ion flux indicates the coexistence of the magnetosheath cold ions and magnetospheric hot ions. For the electron flux, the peak at the high energy indicates that more magnetospheric hot electrons are detected, implying that the spacecraft is located in the magnetosphere, another example of a coexistence region. In panel (d), both ion and electron fluxes show single peaks at high energy, indicating the components of hot and tenuous magnetospheric plasma. It should be noted that the ion flux plots (blue lines in each panel) should be lower in the tail, but show no such decrease tails in part because the data were absent at the high-energy channels. The typical regions shown correspond to the magnetosheath, the energetic particle streaming layer, the LLBL, and the magnetosphere (Sibeck, 1991).
We analyzed observations from TH-A and TH-E that periodically encountered the magnetopause and the LLBL. Although they could be possibly caused by surface waves, the periodical encounters, characterized by the rotation features in the bulk velocity, magnetic field deviations, the high-speed low-density features and the distortions of the magnetopause deduced by MVA showed the likely generation of K–H vortices. The K–H vortices started, or else, the surface waves were amplified by the K–H instability as soon as the IMF turned northward abruptly, which is the direct change to facilitate the instability immediately. By considering the enhancement of the hot electrons as an indicator of the magnetosphere region, typical plasma features were observed in different regions such as the energetic particle streaming layer, the LLBL, and the magnetosphere. The evolution between periodic and dispersed magnetopause observations from TH-A to TH-E implied the possible plasma transport, which is consistent with the different features of the coexisting regions of cold and hot plasmas between TH-A and TH-E. These new observations can complement existing observations and enhance our understanding of the plasma transport processes in K–H vortices.
The data for this paper are available at the Coordinated Data Analysis Web
of NASA's Goddard Flight Center
(
GQY designed the idea, carried out the investigations, and prepared the manuscript with contributions from all the co-authors. GKP, CLC, and TC offered the valuable scientific discussions and helped to improve the manuscript. JPM ensured the data and gave valuable suggestions. YR prepared some of the figures.
The authors declare that they have no conflict of interest.
The authors are grateful to NASA's Goddard Flight Center and the associated instrument teams for supplying the data. The authors thank Chi Wang and Lei Dai for valuable scientific discussions. Part of the work was done during Guang Qing Yan's visit at UC Berkeley, who cordially appreciates the assistance from Forrest S. Mozer.
This research has been supported by the Strategic Pioneer Program on Space Science, the Chinese Academy of Sciences (grant nos. XDA15052500, XDA15350201, and XDA17010301), the National Natural Science Foundation of China (grant nos. 41574161, 41731070, 41574159, and 41004074), the National Space Science Center CAS-NSSC-135 project (grant no. Y92111BA8S), and the Specialized Research Fund for State Key Laboratories.
This paper was edited by Anna Milillo and reviewed by two anonymous referees.