Wenya Li

Identifying the Driver of Pulsating Aurora

Auroral Chorus Energetic particles that arrive from near-Earth space produce photon emissions—the aurora—as they bombard the atmosphere in the polar regions. The pulsating aurora, which is characterized by temporal intensity variations, is thought to be caused by modulations in electron precipitation possibly produced by resonance with electromagnetic waves in Earths magnetosphere. Nishimura et al. (p. 81) present a detailed study of an event that showed a good correlation between the temporal changes in auroral luminosity and chorus emission—a type of electromagnetic wave occurring in Earths magnetosphere. The results points to chorus waves as the driver of the pulsating aurora. Correlations are found between aurora light intensity and a type of electromagnetic wave in Earth’s magnetosphere. Pulsating aurora, a spectacular emission that appears as blinking of the upper atmosphere in the polar regions, is known to be excited by modulated, downward-streaming electrons. Despite its distinctive feature, identifying the driver of the electron precipitation has been a long-standing problem. Using coordinated satellite and ground-based all-sky imager observations from the THEMIS mission, we provide direct evidence that a naturally occurring electromagnetic wave, lower-band chorus, can drive pulsating aurora. Because the waves at a given equatorial location in space correlate with a single pulsating auroral patch in the upper atmosphere, our findings can also be used to constrain magnetic field models with much higher accuracy than has previously been possible.

Spatial distribution of Kelvin‐Helmholtz instability at low‐latitude boundary layer under different solar wind speed conditions

Using the PPMLR-MHD global simulation model, we examined the Kelvin-Helmholtz (K-H) instability at the low-latitude boundary layer (LLBL) under northward interplanetary magnetic field (IMF) conditions with various solar wind speeds (400, 600, and 800 km/s). The spatial distribution of the K-H wave power in the equatorial plane shows two distinct power populations, referring to the two modes of K-H surface waves. The spatial evolution of K-H instability at the boundary layer is classified into four phases: quasi-stable, exponential growth, linear growth, and nonlinear phases. The boundary layer is quasi-stable near the subsolar point region. The K-H instability starts at about 30 degrees longitude, and grows exponentially with a spatial growth rate of 0.28 similar to 0.87 R-E(-1) until similar to 45 degrees longitude where the vortices fully develop. At larger longitudes, the instability grows linearly, while the vortices grow in size. From similar to 80 degrees longitude to the distant magnetotail, the K-H instability develops nonlinearly and the vortices roll up. The wave frequency, wavelength, and phase speed are given at various spatial points. Model results show that the higher solar wind speed generates K-H waves with higher frequency under the northward IMF, and the wavelengths and the phase speeds increase with the increase of the longitude. Moreover, we made a comparison of the K-H wave periods on Earths, Mercurys and Saturns magnetopauses by a proposed prediction method.

Electron jet of asymmetric reconnection

We present Magnetospheric Multiscale observations of an electron-scale current sheet and electron outflow jet for asymmetric reconnection with guide field at the subsolar magnetopause. The electron ...

Properties of Kelvin‐Helmholtz waves at the magnetopause under northward interplanetary magnetic field: Statistical study

We search the plasma and magnetic field data of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes B and C during 2008 and 2009 for observation evidences of the Kelvin-Helmholtz instability (KHI). Fourteen KHI events with rolled-up vortices are identified under the northward interplanetary magnetic field (IMF) at the low-latitude boundary layer (LLBL). We collect another 42 events reported from the observations of the Geotail, Double Star TC-1, and Cluster for a statistical study of the KH wave properties. All the 56 rolled-up KH wave events are quantitatively characterized by the dominant period, phase velocity, and the wavelength. We further explore the relationship between the KH wave period and the solar wind velocity (VSW) and the IMF clock angle. It is found that the KH period tends to be shorter under a higher VSW, and longer with a larger IMF clock angle. The spatial distribution of the KH wavelength shows a longitudinal growth with increasing distance from the subsolar point along the flank magnetopause. The statistical results provide new insights for the development of KH waves and their connection with the interplanetary conditions and deepen our understanding of the KHI at the magnetopause.

The magnetosphere under the radial interplanetary magnetic field: A numerical study

We investigate the magnetosphere under radial interplanetary magnetic fields (IMF) by using global magnetohydrodynamic simulations. The magnetosphere-ionosphere system falls into an unexpected state under this specific IMF orientation when the solar wind electric field vanishes. The most important features that characterize this state include (1) magnetic reconnections can still occur, which take place at the equatorward of the cusp in one hemisphere, the tailward of the cusp in the other hemisphere, and also in the plasma sheet; (2) significant north-south asymmetry exists in both magnetosphere and ionosphere; (3) the polar ionosphere mainly presents a weak two-cell convection pattern, with the polar cap potential valued at approximate to 30 kV; (4) the whole magnetosphere-ionosphere system stays in a very quiet state, and the AL index does not exceed -70 nT; and (5) the Kelvin-Helmholtz instability can still be excited at both flanks of the magnetosphere. These results imply the controlling role of the IMF direction between the solar wind and magnetosphere interactions and improve our understanding of the solar wind-magnetosphere-ionosphere system.

Kinetic evidence of magnetic reconnection due to Kelvin-Helmholtz waves

The Kelvin-Helmholtz (KH) instability at the Earths magnetopause is predominantly excited during northward interplanetary magnetic field (IMF). Magnetic reconnection due to KH waves has been suggested as one of the mechanisms to transfer solar wind plasma into the magnetosphere. We investigate KH waves observed at the magnetopause by the Magnetospheric Multiscale (MMS) mission; in particular, we study the trailing edges of KH waves with Alfvenic ion jets. We observe gradual mixing of magnetospheric and magnetosheath ions at the boundary layer. The magnetospheric electrons with energy up to 80 keV are observed on the magnetosheath side of the jets, which indicates that they escape into the magnetosheath through reconnected magnetic field lines. At the same time, the low-energy (below 100 eV) magnetosheath electrons enter the magnetosphere and are heated in the field-aligned direction at the high-density edge of the jets. Our observations provide unambiguous kinetic evidence for ongoing reconnection due to KH waves.

Magnetic reconnection and modification of the Hall physics due to cold ions at the magnetopause.

Observations by the four Magnetospheric Multiscale spacecraft are used to investigate the Hall physics of a magnetopause magnetic reconnection separatrix layer. Inside this layer of currents and st ...

Cold ion demagnetization near the X‐line of magnetic reconnection

Although the effects of magnetic reconnection in magnetospheres can be observed at planetary scales, reconnection is initiated at electron scales in a plasma. Surrounding the electron diffusion reg ...

Signatures of complex magnetic topologies from multiple reconnection sites induced by Kelvin-Helmholtz instability

The Magnetospheric Multiscale mission has demonstrated the frequent presence of reconnection exhausts at thin current sheets within Kelvin-Helmholtz (KH) waves at the flank magnetopause. Motivated ...

MMS observation of magnetic reconnection in the turbulent magnetosheath

In this paper we use the full armament of the MMS (Magnetospheric Multiscale) spacecraft to study magnetic reconnection in the turbulent magnetosheath downstream of a quasi-parallel bow shock. Cont ...