Acceleration of charged particles by fluctuating and steady electric fields in a X-type magnetic field

Release of stored magnetic energy via particle acceleration is a characteristic feature of astrophysical plasmas. Magnetic reconnection is one of the mechanisms for releasing energy from magnetized plasmas. Collisionless magnetic reconnection could provide both the energy release mechanism and the particle accelerator in space plasmas. Here we studied particle acceleration when fluctuating (in-time) electric fields are superposed on an static X-type magnetic field in collisionless hot solar plasma. This system is chosen to mimic the reconnective dissipation of a linear MHD disturbance. Our results are compared to particle acceleration from constant electric field superposed on an X-type magnetic field. The constant electric field configuration represents the effects of steady state magnetic reconnection. Time evolution of ion and electron distributions are obtained by numerically integrating particle trajectories. The frequencies of the electric field represent a turbulent range of waves. Depending on the frequency and amplitude of the electric field, electrons and ions are accelerated to different degrees and have energy distributions of bimodal form consisting of a lower energy part and a high energy tail. For frequencies (ω in dimensioless units) in the range 0.5 ? ω ? 1.0 a substantial fraction (20%–30%) of the proton distribution is accelerated to gamma-ray producing energies. For frequencies in the range 1 ? ω ? 100.0 the bulk of the electron distribution is accelerated to hard X-ray producing energies. The acceleration mechanism is important for solar flares and solar noise storms but it could be applicable to all collisionless astrophysical plasmas.     

基于能量色散特征计算磁层顶重联位置的方法研究

磁层顶磁场重联是太阳风向磁层输入能量的主要方式.重联如何触发一直是空间物理研究的难点,其机制仍然有待深入研究.由于卫星穿越磁层顶时,很难恰好穿越重联发生的区域,因此难以观测到重联的触发条件.本文利用THEMIS卫星观测,确立了反演磁层顶重联点的方法.当重联刚开始发生时,卫星能够观测到离子的能量色散特征,可利用其计算卫星到重联发生位置的距离.沿着磁力线模型追踪该距离即可反演出磁层顶发生重联的位置.与其他方法进行了对比分析,结果显示本文方法比其他方法具有更高的精度.      

The observation of the galactic anticenter region by the balloon borne gamma-ray telescope Natalya-I

This paper presents the results of measurements of gamma radiation with energies above 5 MeV, from the galactic anticenter region. The balloon-borne gamma ray telescope “Natalya-I”, was launched on 6 November, 1980 from the Tata Institute of Fundamental Research's Balloon Facility (Hyderabad, India) and reached ceiling altitude of 35 km. The results on the accelerator calibration of the telescope, using a “tagged” gamma ray beam are also presented.     

太阳风中磁场重联离子扩散区的大尺度特征

利用ACE和WIND卫星2007年1月6日的联合探测, 在1AU附近发现了一个等离子体密度极低的Petschek-like重联喷流区. 该喷流区内部出现了非常明显的Hall双极磁场、等离子体密度下降区以及与Hall电流相符的低能段电子投掷角分布. 这些特征与重联离子扩散区的Hall效应非常吻合, 说明很可能在太阳风中观测到了一个离子扩散区. 分析表明, 与之相关的磁场重联为准稳态快速完全反向重联, 其扩散区以一对慢模波为边界, 空间尺度达到80个离子惯性长度, 表现出了大尺度重联的特征.      

Imaging of reconnection processes in hard X-rays

The Hard X-ray Spectrometer aboard the SMM detected several events of energy release late in the development of two-ribbon flares. One such event, at 21:12 UT on 21 May, 1980 (～ 20 min after the flare onset and 15 min after the peak of the impulsive phase) is studied in detail. The site of new brightening first became visible in hard X-rays (> 22 keV) and only afterwards showed up at lower energies. It was clearly located high in the corona so that one can identify it with energy release at the tops of newly formed post-flare loops. Thus, if the Kopp and Pneuman model of the loop formation is adopted, we may have imaged here a reconnection process in the solar corona. An attempt is made to estimate physical parameters at the reconnection site.     

Two energy release processes for CMEs: MHD catastrophe and magnetic reconnection

It remains an open question how magnetic energy is rapidly released in the solar corona so as to create solar explosions such as solar flares and coronal mass ejections (CMEs). Recent studies have confirmed that a system consisting of a flux rope embedded in a background field exhibits a catastrophic behavior, and the energy threshold at the catastrophic point may exceed the associated open field energy. The accumulated free energy in the corona is abruptly released when the catastrophe takes place, and it probably serves as the main means of energy release for CMEs at least in the initial phase. Such a release proceeds via an ideal MHD process in contrast with nonideal ones such as magnetic reconnection. The catastrophe results in a sudden formation of electric current sheets, which naturally provide proper sites for fast magnetic reconnection. The reconnection may be identified with a solar flare associated with the CME on one hand, and produces a further acceleration of the CME on the other. On this basis, several preliminary suggestions are made for future observational investigations, especially with the proposed Kuafa satellites, on the roles of the MHD catastrophe and magnetic reconnection in the magnetic energy release associated with CMEs and flares.     

The role of magnetic field shear in solar flares

We present observational results and their physical implications garnered from the deliberations of the FBS Magnetic Shear Study Group on magnetic field shear in relation to flares. The observed character of magnetic shear and its involvement in the buildup and release of flare energy are reviewed and illustrated with emphasis on recent results from the Marshall Space Flight Center vector magnetograph. It is pointed out that the magnetic field in active regions can become sheared by several processes, including shear flow in the photosphere, flux emergence, magnetic reconnection, and flux submergence. Modeling studies of the buildup of stored magnetic energy by shearing are reported which show ample energy storage for flares. Observational evidence is presented that flares are triggered when the field shear reaches a critical degree, in qualitative agreement with some theoretical analyses of sheared force-free fields. Finally, a scenario is outlined for the class of flares resulting from large-scale magnetic shear; the overall instability driving the energy release results from positive feedback between reconnection and eruption of the sheared field.     

Magnetospheric structure and high-energy photon emission of radio pulsars

Some elementary aspects of particle acceleration and flow along the polar field lines of an isolated, rotating magnetized neutron star are described. Semi-quantitaive consequences of these ideas for the emission of high energy gamma rays and their relation to gamma ray observations of radio pulsars are outlined. A global model of the magnetosphere in which these polar flows can occur is described briefly.     

Plasma flow across the cusp-magnetosheath boundary under northward IMF

In this paper, we have used two closely spaced (INTERBALL-1 and MAGION-4) satellites orbiting in the region adjacent to the magnetopause with motivation to determine the structure of the cusp-magnetosheath boundary under steady solar wind conditions and under the northward interplanetary magnetic field (IMF). Two-point observations reveal that a steady reconnection takes place tailward of the cusp and the position of the reconnection site moves in accordance with the actual IMF direction. Reconnection produces a latitudinally dispersed cusp precipitation with highest energies in the cusp poleward edge. The reconnection site is widely spread in local time and thus changes of the IMF By component only slightly modulate observed patterns. The change of the plasma flow direction suggests the presence of the vortex-like structure in the exterior cusp.     

Three-dimensional magnetic reconnection regimes: A review

The magnetic field in many astrophysical plasmas – such as the solar corona and Earth’s magnetosphere – has been shown to have a highly complex, three-dimensional structure. Recent advances in theory and computational simulations have shown that reconnection in these fields also has a three-dimensional nature, in contrast to the widely used two-dimensional (or 2.5-dimensional) models. Here we discuss the underlying theory of three-dimensional magnetic reconnection. We also review a selection of new models that illustrate the current state of the art, as well as highlighting the complexity of energy release processes mediated by reconnection in complicated three-dimensional magnetic fields.     

Hard X-ray observations of the Crab Pulsar

The Crab was observed in a balloon flight from Palestine/Texas on 9/28/81 at hard X-ray energies (20–200 keV). The light curve is significantly sharper than reported previously for this energy range. The pulse-averaged as well as the interpulse spectra show breaks in our energy-range. The variation of spectral index across the pulse has an amplitude similar to that found at lower energies by OSO-8 and larger than reported by HEAO-1 A4 at hard X-rays. For a sharp emission line at 77 keV a 99% upper limit of 1.0*10⁻³ photons/ cm² sec can be placed, a factor of 4 lower than line fluxes reported previously. Pulse-shape fits to the optical, X-ray, hard X-ray and gamma ray light-curves reveal a consistent picture of the origin of the interpulse and off-pulse emission, the breaks in the spectra and the variation of spectral index, providing arguments against a thermal component and also a polar cap emission model for NP0532.     

Where will efficient energy release occur in 3-D magnetic configurations?

The energy needed to power flares is thought to be stored in the coronal magnetic field. However, the energy release is efficient only at very small scales. Magnetic configurations with a complex topology, i.e. with separatrices, are the most obvious configurations where current sheets can form, and then, reconnection can efficiently occur. This has been confirmed for several flares computing the coronal field and comparing the locations of the flare loops and ribbons to the deduced 3-D magnetic topology. However, this view is too restrictive taking into account the variety of observed solar flaring configurations. Indeed, “Quasi-Separatrix Layers” (QSLs), which are regions where there is a drastic change in field-line linkage, generalize the definition of separatrices. They let us understand where reconnection occurs in a broader variety of flares than separatrices do. The strongest electric field and current are generated at, or close to where the QSLs are thinnest. This defines the region where particle acceleration can efficiently occur. A new feature of 3-D reconnection is the natural presence of fast field-line slippage along the QSLs, a process called “slip-running reconnection”. This is a plausible origin for the motions of the X-ray sources along flare ribbons.     

Reconnection and fast particle production in tokamak and solar plasmas

Detailed in situ studies of magnetic reconnection and particle acceleration, which play a crucial role in the release and redistribution of energy in solar flares, can be performed in tokamak plasmas under conditions resembling those of the flaring solar corona. Recent measurements and modelling of fast particle production during reconnection events in the Mega-Amp Spherical Tokamak (MAST) are described. Specifically, observations in this device of electron acceleration during edge localised modes, and of both ion and electron acceleration during merging-compression plasma start-up, are presented, and possible implications of these studies for particle acceleration in flares are discussed. The results from MAST lend weight to the conjecture that large numbers of ions are accelerated to sub-MeV energies in flares.     

Background-sensitive event selection in the search for gamma-ray sources

Poor angular resolution of present day gamma ray telescopes (1° at 100 MEV) together with the complicated gamma ray sky structures at compareable angular extents and poor statistics available necessitate the selection of those gammas which will increase the observable signal from a suspected source. A method for such a selection using the pre-known time signature of the source as its signal, has been developed (Özel and Mayer-Haβelwander, 1983). In an iterative procedure based on the improvement of source signal, generalized zones around the source are defined, from which gamma rays should be accepted to construct the optimum light curve of the source. These zones depend on the energy of the probed gamma rays, on the spatial structure and intensity of the background around the source, on the (estimated and/or tentative) intetnsity of the source and finally on the observing instrument's angular and energy response functions. The method, when applied to simulated and observed COS-B data demonstrated its feasibility and merits over the previonsly used circular event-acceptance cones method which was the basis of searches for gamma ray emission from radio pulsars (see for example Kniffen et al., 1974; Ögelman, et al. 1976; Kanbach, et al., 1977; and references therein). Present method does not introduce “trials” which usually reduce the significance of the results obtained, fully accounts the available knowledge on the topography of gamma ray sky, makes use of all the information about the sky, source and the instrument and therefore is the proper way of handling gamma ray data in pulsar-type searches.Presently, method is applied in the analyses of COS-B data in the context of searches for gamma ray emitting X-ray and radio pulsars. Results will be published elsewhere.     

Observations of 1–30 MeV gamma rays from the galactic center

Preliminary results are reported for gamma ray observations of the galactic center region made during a 15 hour balloon flight from Alice Springs, Australia on April 18, 1979. The observations were carried out with the UCR double-scatter gamma-ray telescope at energies of 1 to 30 MeV. The observations are compatible with a galactic source of approximately equal brightness along the region 300°<ℓII<60°. The energy distribution joins smoothly to previous spark chamber results at energies above 30 MeV and to scintillator results below 1 MeV. It appears to be a combination of nuclear gamma ray lines superimposed on a bremsstrahlung spectrum with a power law (1.3±.7) × 10⁻³ E⁻(^1.7±.2). The ¹²C* line at 4.4 MeV appears to be present with a significance of about 16σ. The flux in the line is (6±3) × 10⁻⁴photons cm⁻²s⁻¹rad⁻¹. The oxygen line at 6.1 MeV does not seem to appear significantly above background.     

磁云边界层中的重联慢激波观测分析

在Petschek模型中,排空区边界处的一对慢激波是能量耗散的重要机制.已有大量行星际空间的Petschek型磁场重联排空区观测事件被报道,但是只有少量的排空区边界处观测到了慢激波.针对一例位于磁云边界层中的Petschek型磁场重联排空区观测事件,在排空区靠近磁云一侧边界处证认了一例慢激波.激波跃变层两侧的磁场和等离子体参数满足Rankine-Hugoniot关系,且激波上下游的中间马赫数均小于1,上游的慢马赫数为2.94（>1）,下游的慢马赫数为0.65（<1）,符合慢激波的观测特征.磁云内部的等离子体β值很低,局地阿尔芬速度高,同时磁云边界层中可能发生丰富的磁场重联活动,这可能是磁云前边界处慢激波形成的原因.      

Observations of Magnetic Reconnection in the Magnetotail Associated With Substorms

Magnetic reconnection is one of the most important, dynamic phenomena in the magnetotail in terms of magnetic field line configuration change and energy release. It is believed to occur in the distant magnetotail mainly during southward interplanetary magnetic field periods and in the near-Earth magnetotail in association with substorms. In the present paper, we discuss several important issues concerning magnetic reconnection in the magnetotail associated with substorms, such as reconnection signatures, location, timing, spatial scale, and behavior, from the macroscopic, observational point of view.      

Low and medium energy gamma-ray astronomy — present status and future aspects

During the last few years quite some progress has been achieved in the field of low and medium energy gamma-ray astronomy below about 30 MeV. Gamma rays from the galactic center and anti-center region have been detected, which require a high interstellar electron flux in the 100 MeV range, if they are predominantly diffuse in nature. Though the Crab pulsar and its nebula are still the only galactic gamma-ray sources which definitely have been detected, some recently determined upper limits to the gamma-ray fluxes of other radio pulsars are close to the theoretically expected values. Active galaxies seem to have a maximum of luminosity in the range between several 100 keV and a few MeV and, therefore, are of special interest. First observational results have been reported on the Seyfert galaxies NGC 4151 and MCG 8-11-11, and the radio galaxy CenA. The nature of the diffuse cosmic gamma-ray component at low gamma-ray energies is not yet solved. Unresolved active galaxies are good candidates for its origin.Considering the present status of gamma ray astronomy the study of galactic sources like radio pulsars and the unidentified high energy gamma-ray sources, the Milky Way as a whole, active galaxies and the diffuse cosmic sky seem to be the prime targets for broad band observations below 30 MeV in the GRO area. An unexplored field like that of low energy gamma-ray astronomy, however, is always open for surprises.     

Two-spacecraft observations of reconnection at the magnetopause: Model results and data comparison

We revisit an example of “quasi-steady” magnetic reconnection at the dayside magnetopause on February 11, 1998, observed by Equator-S and Geotail at the dawnside magnetopause. Phan et al. [Phan, T.D. et al., 2000. Extended magnetic reconnection at the Earth’s magnetopause from detection of bi-directional jets. Nature 404, 848–850.] reported oppositely directed jets at these spacecrafts and inferred a length of the reconnection line of about 38R_E. Pinnock et al. [Pinnock, M., Chisham, G., Coleman, I.J., Freeman, M.P., Hairston, M., Villain, J.-P., 2003. The location and rate of dayside reconnection during an interval of southward interplanetary magnetic field. Ann. Geophys. 21, 1467–1482.] used measurements from SuperDARN radars to show that the reconnection electric field was variable. Here we complement this work by obtaining snapshots of the reconnection electric field from the in situ observations. To do this, we apply a reconstruction method based on a model of compressible Petschek-type magnetic reconnection. This independent method uses magnetic field observations as input data to calculate the reconnection electric field. We obtain average values of E_rec in the range of 0.4–2.4 mV/m. Further we infer a distance perpendicular to the reconnection line of 0.4–0.6R_E. The model results are compared with the two studies mentioned above. It thus appears that while the transfer of momentum for this event is indeed large-scale, the actual rate depends on the time it is measured.