J. B. Cao

Intermittent energy dissipation by turbulent reconnection

Magnetic reconnection—the process responsible for many explosive phenomena in both nature and laboratory—is efficient at dissipating magnetic energy into particle energy. To date, exactly how this dissipation happens remains unclear, owing to the scarcity of multipoint measurements of the “diffusion region” at the sub-ion scale. Here we report such a measurement by Cluster—four spacecraft with separation of 1/5 ion scale. We discover numerous current filaments and magnetic nulls inside the diffusion region of magnetic reconnection, with the strongest currents appearing at spiral nulls (O-lines) and the separatrices. Inside each current filament, kinetic-scale turbulence is significantly increased and the energy dissipation, E′ ⋅ j, is 100 times larger than the typical value. At the jet reversal point, where radial nulls (X-lines) are detected, the current, turbulence, and energy dissipations are surprisingly small. All these features clearly demonstrate that energy dissipation in magnetic reconnection occurs at O-lines but not X-lines.

Statistical analysis of earthward flow bursts in the inner plasma sheet during substorms

[1] In this article, we study the velocity distribution, density, duration, and energy transport of earthward flow bursts in the inner plasma sheet (IPS) during three substorm phases using the data of Cluster in 2001 and 2002. The mean peak velocity of earthward flow bursts in recovery phases (390 km/s) is smaller than those in growth and expansion phases (490 and 520 km/s). The super earthward flow bursts (V > 1000 km/s) appear more frequently in the expansion phase. The average ion density of earthward flow bursts in the recovery phase is 0.14 cm � 3 , much smaller than those in growth and expansion phases (0.28 and 0.21 cm � 3 ), indicating that lobe reconnections most likely occur in the recovery phase. The average durations of earthward flow bursts in recovery phase are 48 s, smaller than those in growth and expansion phases (99 and 103 s), suggesting that the reconnections occurring in recovery phase are rather short-lived. The earthward flow bursts in the expansion phase have largest capability of the transport of energy, about 7 times that in the recovery phase. Thus the earthward flow bursts in the expansion phase can produce largest impact effects to the inner magnetosphere.

Multiple responses of magnetotail to the enhancement and fluctuation of solar wind dynamic pressure and the southward turning of interplanetary magnetic field

During the interval from 06:15 to 07:30 UT on 24 August 2005, the Chinese Tan-Ce 1 (TC1) satellite observed the multiple responses of the near-Earth magnetotail to the combined changes in solar wind dynamic pressure and interplanetary magnetic field (IMF). The magnetotail was highly compressed by a strong interplanetary shock because of the dynamic pressure enhancement (similar to 15 nPa), and the large shrinkage of magnetotail made a northern lobe and plasma mantle move inward to the position of the inbound TC1 that was initially in the plasma sheet. Meanwhile, the dynamic pressure fluctuations (similar to 0.5-3 nPa) behind the shock drove the quasi-periodic oscillations of the magnetopause, lobe-mantle boundary, and geomagnetic field at the same frequencies: one dominant frequency was around 3 mHz and the other was around 5 mHz. The quasi-periodic oscillations of the lobe-mantle boundary caused the alternate entries of TC1 into the northern lobe and the plasma mantle. In contrast to a single squeezed or deformed magnetotail by a solar wind discontinuity moving tailward, the compressed and oscillating magnetotail can better indicate the dynamic evolution of magnetotail when solar wind dynamic pressure increases and fluctuates remarkably, and the near-Earth magnetotail is quite sensitive even to some small changes in the solar wind dynamic pressure when it is highly compressed. Furthermore, it is found that a considerable amount of oxygen ions (O(+)) appeared in the lobe after the southward turning of IMF.

Multiple loss processes of relativistic electrons outside the heart of outer radiation belt during a storm sudden commencement

By examining the compression-induced changes in the electron phase space density and pitch angle distribution observed by two satellites of Van Allen Probes (RBSP-A/B), we find that the relativistic electrons (>2 MeV) outside the heart of outer radiation belt (L* ≥ 5) undergo multiple losses during a storm sudden commencement. The relativistic electron loss mainly occurs in the field-aligned direction (pitch angle α  150°), and the flux decay of the field-aligned electrons is independent of the spatial location variations of the two satellites. However, the relativistic electrons in the pitch angle range of 30°–150° increase (decrease) with the decreasing (increasing) geocentric distance (|ΔL| < 0.25) of the RBSP-B (RBSP-A) location, and the electron fluxes in the quasi-perpendicular direction display energy-dispersive oscillations in the Pc5 period range (2–10 min). The relativistic electron loss is confirmed by the decrease of electron phase space density at high-L shell after the magnetospheric compressions, and their loss is associated with the intense plasmaspheric hiss, electromagnetic ion cyclotron (EMIC) waves, relativistic electron precipitation (observed by POES/NOAA satellites at 850 km), and magnetic field fluctuations in the Pc5 band. The intense EMIC waves and whistler mode hiss jointly cause the rapidly pitch angle scattering loss of the relativistic electrons within 10 h. Moreover, the Pc5 ULF waves also lead to the slowly outward radial diffusion of the relativistic electrons in the high-L region with a negative electron phase space density gradient.

Rapid loss of the plasma sheet energetic electrons associated with the growth of whistler mode waves inside the bursty bulk flows

During the interval similar to 07:45:36-07:54:24UT on 24 August 2005, Cluster satellites (C1 and C3) observed an obvious loss of energetic electrons (similar to 3.2-95keV) associated with the growth of whistler mode waves inside some bursty bulk flows (BBFs) in the midtail plasma sheet (X-GSM similar to-17.25 R-E). However, the fluxes of the higher-energy electrons (128keV) and energetic ions (10-160keV) were relatively stable in the BBF-impacted regions. The energy-dependent electron loss inside the BBFs is mainly due to the energy-selective pitch angle scatterings by whistler mode waves within the time scales from several seconds to several minutes, and the electron scatterings in different pitch angle distributions are different in the wave growth regions. The plasma sheet energetic electrons have mainly a quasi-perpendicular pitch angle distribution (30 degrees<<150 degrees) during the expansion-to-recovery development of a substorm (AE index decreases from 1677nT to 1271nT), and their loss can occur at almost all pitch angles in the wave growth regions inside the BBFs. Unlike the energetic electrons, the low-energy electrons (similar to 0.073-2.1keV) have initially a field-aligned pitch angle distribution (0 degrees 30 degrees and 150 degrees 180 degrees) in the absence of whistler mode waves, and their loss in field-aligned directions is accompanied by their increase in quasi-perpendicular directions in the wave growth regions, but the loss of the low-energy electrons inside the BBFs is not obvious in the presence of their large background fluxes. These observations indicate that the resonant electrons in an anisotropic pitch angle distribution mainly undergo the rapid pitch angle scattering loss during the wave-particle resonances.

Compression-amplified EMIC waves and their effects on relativistic electrons

During enhancement of solar wind dynamic pressure, we observe the periodic emissions of electromagnetic ion cyclotron (EMIC) waves near the nightside geosynchronous orbit (6.6RE). In the hydrogen and helium bands, the different polarized EMIC waves have different influences on relativistic electrons (>0.8 MeV). The flux of relativistic electrons is relatively stable if there are only the linearly polarized EMIC waves, but their flux decreases if the left-hand polarized (L-mode) EMIC waves are sufficiently amplified (power spectral density (PSD) ≥ 1 nT2/Hz). The larger-amplitude L-mode waves can cause more electron losses. In contrast, the R-mode EMIC waves are very weak (PSD < 1 nT2/Hz) during the electron flux dropouts; thus, their influence may be ignored here. During the electron flux dropouts, the relativistic electron precipitation is observed by POES satellite near the foot point (∼850 km) of the wave emission region. The quasi-linear simulation of wave-particle interactions indicates that the L-mod...

Identifying magnetic reconnection events using the FOTE method

A magnetic reconnection event detected by Cluster is analyzed using three methods: Single-spacecraft Inference based on Flow-reversal Sequence (SIFS), Multispacecraft Inference based on Timing a Structure (MITS), and the First-Order Taylor Expansion (FOTE). Using the SIFS method, we find that the reconnection structure is an X line; while using the MITS and FOTE methods, we find it is a magnetic island (O line). We compare the efficiency and accuracy of these three methods and find that the most efficient and accurate approach to identify a reconnection event is FOTE. In both the guide and nonguide field reconnection regimes, the FOTE method is equally applicable. This study for the first time demonstrates the capability of FOTE in identifying magnetic reconnection events; it would be useful to the forthcoming Magnetospheric Multiscale (MMS) mission.

Roles of whistler mode waves and magnetosonic waves in changing the outer radiation belt and the slot region

Using the Van Allen Probe long-term (2013 – 2015) observations and quasi-linear simulations of wave-particle interactions, we examine the combined or competing effects of whistler-mode waves (chorus or hiss) and magnetosonic (MS) waves on energetic ( 0.5 MeV) electrons inside and outside the plasmasphere. Although whistler-mode chorus waves and MS waves can singly or jointly accelerate electrons from the hundreds of keV energy to the MeV energy in the low-density trough, most of the relativistic electron enhancement events are best correlated with the chorus wave emissions outside the plasmapause. Inside the plasmasphere, intense plasmaspheric hiss can cause the net loss of relativistic electrons via persistent pitch angle scattering, regardless of whether MS waves were present or not. The intense hiss waves not only create the energy-dependent electron slot region, but also remove a lot of the outer radiation belt electrons when the expanding dayside plasmasphere frequently covers the outer zone. Since whistler-mode waves (chorus or hiss) can resonate with more electrons than MS waves, they play dominant roles in changing the outer radiation belt and the slot region. However, MS waves can accelerate the energetic electrons below 400 keV and weaken their loss inside the plasmapause. Thus, MS waves and plasmaspheric hiss generate different competing effects on energetic and relativistic electrons in the high-density plasmasphere.

Suprathermal electron acceleration in the near-Earth flow rebounce region

Flux pileup regions (FPRs) are traditionally referred to the strong-Bz bundles behind dipolarization fronts (DFs) in the Earths magnetotail and can appear both inside earthward and tailward bursty bulk flows. It has been widely reported that suprathermal electrons (40–200 keV) can be efficiently accelerated inside earthward FPRs, leaving the electron acceleration inside tailward FPRs as an open question. In this study, we focus on the electron acceleration inside a tailward FPR that is formed due to the flow rebounce in the near-Earth region (XGSM ≈ −12 RE) and compare it quantitatively with the acceleration inside an earthward FPR. By examining the Cluster data in 2008, we sequentially observe an earthward FPR and a tailward FPR in the near-Earth region, with the earthward one belonging to decaying type and the tailward one belonging to growing type. Inside the earthward FPR, Fermi acceleration and betatron cooling of suprathermal electrons are found, while inside the tailward FPR, Fermi and betatron acceleration occur. Whistler-mode waves are observed inside the tailward FPR; their generation process may still be at the early stage. We notice that the suprathermal electron fluxes inside the tailward FPR are about twice as large as those inside the earthward FPR, suggesting that the acceleration of suprathermal electrons is more efficient in the flow rebounce region. These acceleration processes have been successfully reproduced using an analytical model; they emphasize the role of flow rebounce in accelerating suprathermal electrons and further reveal how the MHD-scale flow modulates the kinetic-scale electron dynamics in the near-Earth magnetotail.

Propagation characteristics of plasmaspheric hiss: Van Allen Probe observations and global empirical models

Based on the Van Allen Probe A observations from 1 October 2012 to 31 December 2014, we develop two empirical models to respectively describe the hiss wave normal angle (WNA) and amplitude variations in the Earths plasmasphere for different substorm activities. The long-term observations indicate that the plamsaspheric hiss amplitudes on the dayside increase when substorm activity is enhanced (AE index increases), and the dayside hiss amplitudes are greater than the nightside. However, the propagation angles (WNAs) of hiss waves in most regions do not depend strongly on substorm activity, except for the intense substorm-induced increase in WNAs in the nightside low-L region. The propagation angles of plasmaspheric hiss increase with increasing magnetic latitude (MLAT) or decreasing radial distance (L value). The global hiss WNAs (the power-weighted averages in each grid) and amplitudes (medians) can be well reproduced by our empirical models.