Diffuse auroral precipitation in the jovian upper atmosphere and magnetospheric electron flux variability

The deposition of energetic electrons in Jupiter's upper atmosphere provides a means, via auroral observations, of monitoring electron and plasma wave activity within the magnetosphere. Not only does particle precipitation indicate a potential change in atmospheric chemistry, it allows for the study of episodic, pronounced flux enhancements in the energetic electron population. A study has been made of the effects of such electron injections into the jovian magnetosphere and of their ability to provide the source population for variations in diffuse auroral emissions. To identify the source region of precipitating auroral electrons, we have investigated the pitch-angle distributions of high-resolution Galileo Energetic Particle Detector (EPD) data that indicate strong flux levels near the loss cone. The equatorial source region of precipitating electrons has been determined from the locations of Galileo's in situ measurements by tracing magnetic field lines using the KK97 model. The primary source region for Jupiter's diffuse aurora appears to lie in the magnetic equator at 15-40 RJ, with the predominant contribution to precipitation flux (tens of ergs cm⁻² s⁻¹ sr⁻¹) stemming from <30 RJ. Variability of flux for energetic electrons in this region is also important to the irradiation of surfaces and atmospheres for the Galilean moons: Europa, Ganymede, and Callisto. The average diffuse auroral precipitation flux has been shown to vary by as much as a factor of six at a given radial location. This variability appears to be associated with electron injection events that have been identified in high-resolution Galileo EPD data. These electron flux enhancements are also associated with increased whistler-mode wave activity and magnetic field perturbations, as detected by the Galileo Plasma Wave Subsystem (PWS) and Magnetometer (MAG), respectively. Resonant interactions with the whistler-mode waves cause electron pitch-angle scattering and lead to pitch-angle isotropization and precipitation.     

Electron precipitation in a non-uniform glow aurora

Electron intensities at 5 keV >18 keV and >45 keV, were measured on a Petrel rocket flown from Kiruna, Sweden, into a non-uniform glow aurora during the recovery phase of a magnetic bay. The intensities depend on pitch-angle in a way that is consistent with the precipitation being caused by pitch-angle diffusion from reservoirs of geomagnetically-trapped electrons. The scattering process that causes pitch-angle diffusion, and leads to three regions of relatively high intensity, appears to have properties different from the scattering process that leads to two intervening regions of low intensity. A spatial structure in electron reservoir intensity, is attributed mainly to variations in the rate of erosion by pitch-angle diffusion of an initially nearly-uniform reservoir intensity. An expression is derived for the minimum lifetime of trapped electrons undergoing strong pitch-angle diffusion.     

On the origin of time delays in hard X-ray and gamma-ray emission of solar flares

The possibility of a strong pitch-angle diffusion regime as well as of turbulent propagation of energetic ions and electrons in flaring loops has been shown. The strong diffusion regime suggests that two regions with a high level of small-scale turbulence are formed in the magnetic trap. Such additional turbulent mirrors scatter energetic particles and, therefore, the flux of precipitating particles decreases and the mean lifetime of electrons and protons in a flaring loop grows. We cannot rule out that the turbulent propagation of the particles can be responsible for the energy dependence of hard X-ray delays as well as the time lag of the gamma-ray line peaks with respect to the hard X-ray peaks as the electrons and ions are accelerated simultaneously. The trap plus turbulent propagation model may also explain the lack of abundant population of 10–100 keV electrons in interplanetary space in proton-rich events and offers new possibilities for flare plasma diagnostics.     

磁重联电流片的参数变化对电子加速的影响

通过采用试验粒子的方法,研究了在有引导磁场Bz存在的磁重联电流片中,电子被super-Dreicer电场Ez加速后的运动特征.首先,考虑了引导磁场恒定且与电场有不同方向时对粒子加速的影响.在这种情况下,Bz方向的改变直接改变了电子的运动轨迹,使其沿着不同的路径离开电流片.在Bz和Ez同向时,高能电子的pitch-angle接近于180°.然而,当2者反向时,高能电子的pitch-angle接近0°.引导磁场的取向只是使电场有选择地对不同区域的电子进行加速,不会最终影响电子的能量分布,最终得到的能谱是普遍的幂率谱E-γ.在典型的日冕条件下, γ大约等于2.9.进一步的研究表明γ的大小依赖于引导磁场及磁重联电场的强弱,以及电流片的尺度.随后,也研究了包含多个X-点和O-点电流片中被加速粒子的运动特征.结果表明X-点和O-点的存在使得粒子被束缚在加速区并获得最大的加速,而且最终的能谱具有多幂率谱的特征.     

An interpretation of the decay characteristics of solar hard X-ray bursts

A simple trap model of solar hard X-ray bursts is discussed in which nonthermal electrons trapped in a magnetic bottle precipitate into the lower chromosphere through the resonant scattering by whistlers. In such a model, the X-ray spectra produced from trapped and precipitating electrons have different spectral shape, and both of the spectra will initially soften with time, provided the precipitation dominates over collisional degradation.     

On the role of magnetic mirroring in the auroral phenomena

On the basis of field and particle observations, it is suggested that a bright auroral display is a part of a magnetosphere-ionosphere current system which is fed by a charge-separation process in the outer magnetosphere (or the solar wind). The upward magnetic-field-aligned current is flowing out of the display, carried mainly by downflowing electrons from the hot-particle populations in the outer magnetosphere (the ambient cold electrons being depleted at high altitudes). As a result of the magnetic mirroring of these downflowing current carriers, a large potential drop is set up along the magnetic field, increasing both the number flux and the kinetic energy of precipitating electrons. It is found that this simple basic model, when combined with wave-particle interactions, may be able to explain a highly diversified selection of auroral particle observations. It may thus be possible to explain both inverted-V events and auroral rays in terms of a static parallel electric field, and the electric field may be compatible with a strongly variable pitch-angle distribution of the precipitating electrons, including distributions peaked at 90° as well as 0°. This model may also provide a simple explanation of the simultaneous precipitation of electrons and collimated positive ions.     

An experimental study of low energy electron backscattering in the atmosphere

The backscatter process for low energy electrons in the upper atmosphere is studied, and it is shown how variations in the pitch-angle and energy distributions of these particles may affect the backscatter ratio and also the electron component of the upward Birkeland current. “Softening” of the electron energy spectrum combined with a field-alignment of the electron fluxes may enhance the current carried by the greater than 200 eV energy electrons significantly.     

Particle saturation of the outer zone: A nonlinear model

Properties of the steady state and transient behavior of geomagnetically trapped radiation are analyzed by means of phenomenological equations that concisely summarize the operative dynamical processes. The equations provide for a realistic coupling between electromagnetic wave energy, particle intensity, and pitch-angle anisotropy in the context of the outer zone. Applications include magnetospheric enforcement of a limit on stably trapped particle flux. the smooth transition between weak pitch-angle diffusion and strong diffusion, parasitic particle precipitation by natural and man-made radio signals, natural and artificial injections of trapped radiation, and the consequences of magnetospheric cold-plasma injection.     

Regular and chaotic dynamics in 3D reconnecting current sheets

We consider the possibility of particles being injected at the interior of a reconnecting current sheet (RCS), and study their orbits by dynamical systems methods. As an example we consider orbits in a 3D Harris type RCS. We find that, despite the presence of a strong electric field, a 'mirror' trapping effect persists, to a certain extent, for orbits with appropriate initial conditions within the sheet. The mirror effect is stronger for electrons than for protons. In summary, three types of orbits are distinguished: (i) chaotic orbits leading to escape by stochastic acceleration, (ii) regular orbits leading to escape along the field lines of the reconnecting magnetic component, and (iii) mirror-type regular orbits that are trapped in the sheet, making mirror oscillations. Dynamically, the latter orbits lie on a set of invariant KAM tori that occupy a considerable amount of the phase space of the motion of the particles. We also observe the phenomenon of 'stickiness', namely chaotic orbits that remain trapped in the sheet for a considerable time. A trapping domain, related to the boundary of mirror motions in velocity space, is calculated analytically. Analytical formulae are derived for the kinetic energy gain in regular or chaotic escaping orbits. The analytical results are compared with numerical simulations.     

Electron impact polarization of resonance lines from Lithium-like ions

The Oppenheimer-Penney theory, as developed by Percival and Seaton (1958), is applied to calculate the polarization of resonance lines from Li-like ions. Two laws for the pitch-angle distribution of electrons around the magnetic field are accounted. The degrees of polarization are averaged over the energy of non-thermal electrons generated during the initial phase of solar flares. It is found that for the full space pitch-angle distribution, as adopted by Chandra and Joshi (1984), the degrees of polarization are nearly independent of the atomic number of ion. Whereas for the forward-come distribution used by Haug (1981), they depend on the choice of the free parameterE ₀. The polarization of the resonance lines from Li-like ions is two times larger than that of the L radiations from H-like ions. Hence, under favourable conditions, it may be detected during solar flares.     

Discrete VLF emissions (7–9 kHz) displaying unusual banded and periodic structure

Rising frequency VLF emissions having unusually high frequency and exhibiting banded structure were recorded between 14.55 and 15.30 U.T. on 28 June 1972 by the VLF goniometer receiver at Halley, Antarctica. The risers were split into two frequency groups, one with frequencies in the range 6.0–7.7 kHz and the other with frequencies between 7.8 and 9.4 kHz, the former being more numerous. The gap between the lower and upper frequency risers is superficially similar to, though at a higher frequency than, the missing bands in emissions observed by satellites. However, it is found to be unlikely that the risers received at Halley can be explained by any of the mechanisms advanced to explain the banded satellite-observed emissions. Several other explanations are considered and it is shown that the most likely is partial suppression by magnetospheric line radiation propagating in the same duct.The risers are interpreted as being generated via cyclotron resonance with counterstreaming electrons. A computer program based on Helliwell's (1967) phenomenological theory is used to determine the generation region and electron energies involved.Both frequency groups of risers display a 4 min periodicity in occurrence. It is shown that this time period is consistent with that required to replenish the flux of resonant electrons, by eastwards drift into the duct, after the emissions have been quenched due to the reduction of the flux by pitch-angle diffusion into the loss cone.     

On the Dynamics of the Jovian Ionosphere and Thermosphere: III. The Modelling of Auroral Conductivity

Recent work has been concerned with calculating the three-dimensional ion concentrations and Pedersen and Hall conductivities within the auroral region of Jupiter for varying conditions of incident electron precipitation. Using the jovian ionospheric model, we present results that show the auroral ionospheric response to changing the incoming flux of precipitating electrons (for constant initial energy) and also the response to changing the initial energy (for both constant flux and constant energy flux). The results show that, for expected energy fluxes of precipitating particles, the average auroral integrated Pedersen conductivity attains values in excess of 1 mho. In addition, it is shown that electrons with an initial energy of around 60 keV are particularly effective at generating auroral conductivity: Particles of this energy penetrate most effectively to the layer of the jovian ionosphere at which the auroral conductivity is at a maximum.     

Large-scale transport of plasma in the Earth's plasma sheet: comparative analysis for adiabatic and non-adiabatic cases

A system of multi-fluid MHD-equations is used to compare adiabatic and non-adiabatic transport of the energetic particles in the magnetospheric plasma sheet. A “slow-flow” approximation is considered to study large-scale transport of the anisotropic plasma consisting of energetic electrons and protons. Non-adiabatic transport of the energetic plasma is caused by scattering of the particles in the presence of both wave turbulence and arbitrary time-varying electric fields penetrating from the solar wind into the magnetosphere. The plasma components are devided into particle populations defined by their given initial effective values of the magnetic moment per particle. The spatial scales are also given to estimate the non-uniformity of the geomagnetic field along the chosen mean path of a particle. The latters are used to integrate approximately the system of MHD-equations along each of these paths. The behaviour of the magnetic moment mentioned above and of the parameter which characterizes the pitch-angle distribution of the particles are studied self-consistently in dependence on the intensity of non-adiabatic scattering of the particles. It is shown that, in the inner magnetosphere, this scattering influences the particles in the same manner as pitch-angle diffusion does. It reduces the pitch-angle anisotropy in the plasa. The state of the plasma may be unstable in the current sheet of the magnetotail. If the initial state of the plasma does not correspond to the equilibrium one, then, in this case, scattering influences the particles so as to remove the plasma further from the equilibrium state. The coefficient of the particle diffusion across the geomagnetic field lines is evaluated. This is done by employing the Langevin approach to take the stochastic electric forces acting on the energetic particles in the turbulent plasma into account. The behaviour of the energy density of electrostatic fluctuations in the magnetosphere is estimated.     

Plasma emission and the decimetre portion of type-IV solar bursts

An attempt is made to account for the decimetre portion of the Type-IV solar radio bursts by plasma emission. Non-thermal electrons (E ～ 500 keV) trapped in a magnetic mirror (IV_dm, burst source) having loss-cone gap distribution excite plasma waves which are transformed into transverse waves through non-linear scattering by ions. A good agreement was reached between the calculated spectrum and the observed fluxes for the event of 1972 August 2. A distribution of the number of non-thermal electrons with height, and a total number of 10³², were obtained. Also it was found that the Langmuir waves can accelerate some background thermal electrons to the MeV range.     

Gyrosynchrotron Emission from Anisotropic Pitch-Angle Distribution of Electrons in 3-D Solar Flare Sources

We present calculations, made for the first time, of the gyrosynchrotron emission by mildly relativistic electrons with anisotropic pitch-angle distribution using a realistic magnetic loop model in three dimensions. We investigated the intensity, spectral index of the optically thin region of the spectrum, the spatial morphology and the dependency on the source position on the solar disk. The method to describe a three-dimensional source and the procedure to perform the calculations are presented. We have modified the Ramaty’s gyrosynchrotron code to allow the evaluation of anisotropic pitch-angle electron distributions, as described in the complete formalism. We found that anisotropic electron distributions affect the intensity of the radiation, spatial morphology and spectrum of spatially resolved sources. However, the spatially integrated spectrum of the emission seems to be insensitive to the electron pitch-angle distribution, as the magnetic field inhomogeneity smooths out the effects of the anisotropic distribution in the produced radiation, in contrast to homogeneous sources.     

Current interruption in a collisionless plasma by non-linear electrostatic waves

The existence of current-interrupting non-linear electrostatic waves in the form of negative solitons is demonstrated. Self-consistent, non-linear electrostatic potentials are constructed assuming that a current may be interrupted by trapping current-carrying electrons in such a potential. A significant fraction of the current-carrying electrons is trapped by the potential if the electron thermal velocity is much less than the electron streaming velocity. In one class of solutions, the negative solitons, the current may be reduced to a fourth of its initial value in the limit of high ion-electron temperature ratio.     

Jovian proton Aurora

The planet Jupiter possesses a magnetic field and is surrounded by a magnetosphere. The occurrence of auroral and polar cap phenomena similar to those found on earth is very likely. In this work auroral and polar cap emissions in a model Jovian atmosphere are determined for proton precipitation. The incident protons, which are characterized by representative spectra, are degraded in energy by applying the continuous slowing down approximation. All secondary and higher generation electrons are assumed to be absorbed locally and their contributions to the total emissions are included. Volume emission rates are calculated from the total direct excitation rates with corrections for cascading applied. Results show that most molecular hydrogen and helium emissions for polar cap precipitation are below the ambient dayglow values. Charge capture by precipitating protons is an important source of Lyman α and Balmer α emissions and offers a key to the detection of large fluxes of low energy protons.     

Energetic particle losses and trapping boundaries as deduced from calculations with a realistic magnetic field model

An interpretation of the stable trapping boundaries of energetic electrons and protons during quiet periods is given basing on a realistic magnetospheric magnetic field model. Particle losses are explained in terms of an ionospheric and drift loss cone filling due to a non-adiabatic pitch-angle scattering in the nightside magnetotail current sheet. The proposed mechanism is shown to provide a good agreement of the observed and calculated positions of the energetic particle trapping boundaries, as well as their energy dependence. The obtained results can be applied as a tool for investigating the magnetospheric magnetic field structure using the particle data of low-altitude satellites.     

Precipitation of charged particles by a parallel electric field

A time-dependent model of the effect of a parallel electric field on particle precipitation from a closed field-line has been constructed and the results are presented. A pattern of field-aligned pitch-angle distributions and energy peaks develops rapidly and then persists unchanged in shape while the intensity decreases for a time of the order of the bounce period of the energetic particles. It is shown that the structures in velocity space are created by the juxtaposition of particles from different source populations. Four sources are found to be sufficient to reproduce the principal features observed frequently by rockets and satellites. They are, a trapped plasma sheet distribution, a loss-cone partially filled by pitch-angle diffusion at the equator, cold ionospheric plasma which has flowed outward along the field line and particles backscattered from the precipitation into the atmosphere.The model develops density gradients and discontinuities far sharper than any observed, so that any parallel electric field actually occurring in an aurora must be accompanied by strong wave-particle interactions either as part of the accelerating mechanism or as a result of the density gradients produced by it.