Jinbin Cao

Combined acceleration of electrons by whistler‐mode and compressional ULF turbulences near the geosynchronous orbit

[1]xa0In the quasi-linear approximation, we study electron acceleration process generated by whistler-mode and compressional ULF (fast mode waves) turbulences near the Earths synchronous orbit. The results show that the whistler-mode turbulence (0.1fce ≤ f ≤ 0.75fce) can accelerate substorm injection electrons with several hundreds of keV through wave-particle gyroresonant interaction and hence may play an important role in the electron acceleration during substorms. The compressional ULF turbulence (2–15 mHz) can accelerate both lower-energy background electrons (<30 keV) and substorm injection electrons (∼30–300 keV) through the transit-time damping mechanism. So the compressional ULF turbulence acceleration mechanism is important during both substorms and quiet times. The compressional ULF turbulence accelerates substorm injection electrons more effectively than whistler-mode turbulence. The combined electron acceleration by whistler-mode and ULF turbulences is most effective and can cause the number density of the relativistic electrons increase largely within about 8 hours. Substorms can offer both substorm injection electrons and strong turbulences, and therefore large flux enhancement events of relativistic electrons (≥1 MeV) always occur during substorm time. For magnetic storms that are composed of a series of substorms, extremely large flux enhancement events of the relativistic electrons can thus occur.

Characteristics of middle- to low-latitude Pi2 excited by bursty bulk flows

[1]xa0This paper, by using the data of Cluster, TC-1, GOES, and eight ground stations on 22 October 2004, studied the characteristics of low-latitude Pi2s generated by an earthward bursty bulk flow (BBF) in the near-Earth tail plasma sheet. The BBF excited simultaneously two distinct classes of Pi2s: one is long-period Pi2 (90–130 s) and the other is short-period Pi2 (∼50 s). The long-period Pi2 is transient response type Pi2 associated with field-aligned current produced by the braking of BBFs. The spectrum analysis show that the amplitude spectrum peak of long-period Pi2 increases with increasing latitude, indicating that the source is at higher latitudes. The time delay for the propagation of Alfven waves from Cluster to the Earth is very close to the time difference between the onset time of the BBFs at Cluster and the starting time of the long-period Pi2 on the ground. The short-period Pi2 is a global cavity mode since the Pi2s in H components at eight stations have almost the same starting time, same oscillation period, and same waveform, which are all typical characteristics of cavity mode. The amplitude spectrum peak of short-period Pi2 at NCK (N42.7) is larger than those at higher-station UPS (N56.5) and lower-station CST (N40.8). The polarization analysis at three lower-latitude stations shows that the polarization underwent two reversals. The major axis of the polarization ellipse points to approximately the north, indicating that the short-period Pi2s are not excited by nightside current system. TC-1 observed transverse mode Pi2s. Its period is almost identical with the periods of Pi2 on the ground, indicating they belong to the same wave.

Energetic ion dynamics of the inner magnetosphere revealed in coordinated Cluster-Double Star observations

[1]xa0Since early 2004 the Chinese spacecraft Tan Ce 1 (TC-1), first component of the Double Star (DSP) mission, has been on an equatorial elliptical orbit (13.4 RE apogee), allowing the study of the dynamics of the Earths magnetosphere in conjunction with the four European Cluster spacecraft (19.6 RE apogee). The Cluster and Double Star spacecraft orbits are such that the spacecraft are almost in the same meridian, allowing conjugate studies. The four Cluster spacecraft highly eccentric polar orbit at 4 RE perigee permits them to sample the ring current, the radiation belts, and the outer plasmasphere from south to north, almost following the same magnetic flux tube (latitudinal profile), whereas TC-1, with its very low-perigee equatorial orbit, gives the plasma profile across L shells. Coordinated ion measurements provided by the Cluster Ion Spectrometry and Hot Ion Analyzer instruments onboard Cluster and TC-1, respectively, obtained during quiet conditions, disturbed geomagnetic conditions, and an intense storm, are used to analyze crossings of the plasmasphere and the ring current region. Multiple narrow ion energy bands (“nose-like” structures) are simultaneously observed by both Cluster and TC-1. These observations reveal the large-scale character of these structures and pose a challenge for the simulation and modeling of the inner magnetosphere populations.

Nongyrotropy of heavy newborn ions at comet Grigg‐Skjellerup and corresponding instability

The existence of the nongyrotropic velocity distribution of newborn ions is a very important feature for comet Grigg-Skjellerup (P/G-S); this kind of distribution may be a reservoir of free energy and play a rather important role in the generation of plasma waves observed at P/G-S, especially close to the comet. In this paper we predict the distribution function of the newborn ion population, taking into account the source term for this population (implantation from neutrals) and the loss term (pickup, i.e., velocity diffusion leading to assimilation in the solar wind). We integrate the equation backward along the solar wind plasma flow line modified by the ion gyration motion using the observations of the Giotto spacecraft at P/G-S on July 10, 1992. We obtain the nongyrating zero-order nongyrotropic velocity distribution of the newborn ions which depends on the average pickup time of implanted ions and on the distance from the comet. The stability of the above equilibrium is then studied by adding linear perturbations and solving numerically the inferred dispersion equation. We show that this kind of distribution function triggers a left-handed instability which can be called “nongyrotropic ring instability” and which arises from the coupling between the two transverse electromagnetic modes and the longitudinal electrostatic mode.

Oblique ring instability driven by nongyrotropic ions: Application to observations at comet Grigg-Skjellerup

We investigate the oblique behavior of low-frequency left-handed electromagnetic waves driven by a ring of nongyrotropic ions. We use the unperturbed orbit integral technique to obtain for the first time the dispersion equation of oblique modes for a nongyrotropic particle distribution in a magnetoplasma. When the oblique propagation angle θ with respect to the magnetic field increases, the polarization of the unstable wave mode rapidly loses the left hand circular nature of the parallel mode and tends to be linear. Electron density compression and magnetic field compression appear and both maximize at θ ∼22°. The difference between gyrotropic ring instability and nongyrotropic ring instability diminishes with the increase of obliquity. The nongyrotropy of the ring ions decreases the growth rate for large wave numbers and to a lesser extent at its maximum. In addition, the nongyrotropy increases slightly the electron density and magnetic field compressions. The growth rate of the mode with θ >0 (k• Vp 0); this effect does not exist in the gyrotropic case. The plasma parameters used in these theoretical calculations are those observed during the Giotto encounter with comet Grigg-Skjellerup, when nongyrotropic newborn heavy ion ring distributions as well as associated low frequency waves have been reported. The results on the electron density compression are in good quantitative agreement with the observations upstream of the cometary shock.

Density holes in the upstream solar wind

Larmor size transient structures with depletions as large as 99% of ambient solar wind density levels occur commonly upstream of Earth’s collisionless bow shock. These “density holes” have a mean duration of ∼17.9 ± 10.4s but holes as short as 4s have been observed. The average fractional density depletion (δn/n) inside the holes is ∼0.68 ± 0.14. The density of the upstream edge moving in the sunward direction can be enhanced by five or more times the solar wind density. Particle distributions show the steepened edge can behave like a shock, and measured local field geometries and Mach number support this view. Similarly shaped magnetic holes accompany the density holes indicating strong coupling between fields and particles. The density holes are only observed with upstream particles, suggesting that back‐streaming particles interacting with the solar wind are important.

Electron acceleration by whistler-mode waves around the magnetic null during 3D reconnection

The magnetic field configuration around a magnetic null pair and its associated electron behavior during 3D magnetic reconnection have recently been reported from in situ observations. Electrons are suggested to be temporarily trapped in the central reconnection region as indicated by an electron density peak observed near the magnetic null (He J-S et al 2008 Geophys. Res. Lett. 35 L14104). It is highly interesting that energetic electron beams of a few kiloelectronvolts are found to be related to the magnetic null structure. However, the acceleration mechanism is still not fully understood. In this paper, we show that strong whistler-mode electromagnetic waves are indeed found around the magnetic null. Further we propose a new electron acceleration scenario of trapped electrons near the magnetic null points driven by the whistler-mode waves, which is confirmed by numerical results. It is demonstrated that whistler waves can enhance the phase space density (PSD) of electrons for energies of ~2?keV by a factor of 100 at lower pitch angles very rapidly, typically within 2?s. The accelerated electrons may escape from the loss cone of the magnetic cusp mirrors around the magnetic null, leading to the observed energetic beams.

Alfven waves in regions of magnetic reconnection and the acceleration of newborn ions

Abstract We use a 2-D hybrid numerical simulation to study velocity-driven magnetic reconnection in low-beta plasmas. Our results show that Alfven waves can be produced in the process. Under their action newborn ions undergo pitch-angle scattering and assume a spherical shell distribution. A part of the ions are accelerated to a maximum energy of about 4(miV2A0/2). The acceleration lasts over a time scale of 100/Ωi, and is extremely rapid. The energy spectrum of the accelerated ions is a double power-law spectrum.

Particle Simulations for Electron Beam-Plasma Interactions

The computer simulations of high-frequency instabilities excited by the high density electron beam and their nonlinear effect are presented. One-dimensional electromagnetic particle simulations are performed with different values of the electron beam-to-plasma density ratio. The results show that for the high electron beam-to-background plasma density ratio, all the Langmuir waves and two electromagnetic waves with left-hand and right-hand circular polarizations (i.e., the L-O mode and the R-X mode) propagating parallel to the magnetic field can be generated and the maximum values of wave electric fields are nearly the same. The electron beam and background plasma are diffused and a part of energetic background electrons are obviously accelerated by the wave-particle interactions. The heating of the beam and background plasma is mainly due to the electrostatic (Langmuir) wave-particle interactions, but the accelerations of a part of energetic background electrons may be mainly due to the electromagnetic wave-particle interactions.