Numerical analysis of equatorial enhancement of geomagnetic sudden commencement

Equatorial behaviour of a polar-originating ionospheric current is examined by solving numerically the continuity equation on a two-dimensional spherical shell with appropriate assumptions for the ionospheric conductivity and the field-aligned source currents. The results show a clear daytime equatorial enhancement of the ionospheric currents in spite of much reduced electric field due to shielding effects of the enhanced Cowling conductivity there. The results are used for interpretation of the preliminary impulse of the geomagnetic sudden commencement.     

Extension of a polar ionospheric current to the nightside equator

A detailed analysis of rapid-run magnetograms from Guam (geomagnetic latitude = 4.2°) revealed that there are two kinds of geomagnetic sudden commencement (SC) observed in nighttime. One is the ordinary SC consisting of a main impulse only which has a smooth rise of the H-component. The other is a superposition by a small positive impulse on the very beginning part of the smooth rise of the main impulse and consequently the SC starts with a small stepwise increase of the H-component. The latter type of SC occurs between 20 and 08 h L.T. and its occurrence rate takes the maximum value of about 50% around 03 h L.T. Corresponding magnetograms from a dayside equatorial station (Huancayo, geomagnetic latitude = ?0.7°) were examined and a good correlation was found between the stepwise SC at the nightside (Guam) and SC¹ with a preliminary reverse impulse (PRI) at the dayside (Huancayo). Since PRI observed at the dayside equator may be interpreted as an extension of an ionospheric current due to an dusk-to-dawn electric field impressed on the polar ionosphere, our results show that a polar originating ionospheric current can extend to the nightside equator and produce a small but observable magnetic effect in spite of much reduced nighttime ionospheric conductivity.     

Global structure of geomagnetic sudden commencements

The geomagnetic sudden commencements (SSC's) are analyzed by the use of rapid-run magnetograms from eight American zone stations (Koror, Guam, Honolulu, Tucson, Fredericksburg, Sitka, College and Pt. Barrow). The occurrence rate of the equatorial SC¹ reaches the maximum value of 60–70% in the early afternoon. The type (SC¹ or pure SC without a preliminary reverse impulse) of an SSC at the dayside equator coincides well with that in the afternoon side high latitudes. When a pure SC is observed at Koror in the early afternoon, the other seven stations also observe pure SC's, and their onset is almost simultaneous at all the stations. The dayside equatorial SC¹ usually corresponds to SC¹'s in afternoon side high latitudes and pure SC's in middle latitudes (Honolulu and/or Tucson). In this case the onset of the pure SC's in middle latitudes is delayed by several tens of seconds from the SC¹'s which appear almost simultaneously in both the dayside equatorial and the afternoon side high latitude region. In rare cases where SC¹'s are observed at all the stations including Honolulu, their onset time is almost simultaneous. The results are interpreted as showing the polar origin of the equatorial preliminary reverse impulse and the existence of two types of the interaction between the magnetosphere and the shock or discontinuity in the solar wind during SSC. Some discussions are given for the global distribution of the onset time and the waveform of SSC, the equatorial enhancement of the preliminary reverse impulse and the main impulse, and a physical decomposition of the SSC disturbance field.     

Observational features of field line resonances excited by solar wind pressure variations on 4 September 1984

In the course of the magnetic storm of 4 September 1984, after an inverse sudden impulse (SI), geomagnetic pulsations in the Pc5-frequency range were observed at magnetometer stations in the local evening sector. They occurred at L-values of 6, and lasted for several hours, their period increasing from about 320 to 550 s. In this study, two events of enhanced activity are discussed in some detail. During the 16:00 U.T. event, a favourable position of the AMPTE/IRM spacecraft allows conjugate observations in the Northern and Southern Hemispheres and in the magnetosphere. This constellation permits a precise determination of the wave mode. During a later intensification around 18:00 U.T., the AMPTE/CCE spacecraft near local noon monitored poloidal waves, obviously driving the pulsations on the ground. Generally, the observations are consistent with the theory of field line resonance. They are interpreted as being excited by pressure variations in the solar wind. The hydromagnetic cavity mode is assumed to link the magnetopause surface motions to the field line resonances.     

近距离罗兰-C信号周期识别的数字化方法初探

对判断罗兰-C脉冲峰值的算法作了改进。对加有噪声的近距离罗兰- C脉冲仿真信号的采样值用改进的算法进行计算后,可识别信号周期并给出相位编码信息。对改进后的算法的有效性用MATLAB进行了验证,并用MODELSIM对改进的算法进行了仿真实现。结果表明：在一定的信噪比条件下,改进后的算法初步实现了对近距离罗兰-C信号周期识别的数字化,具有一定的实用性。     

Modeling Solar Spectral Irradiance and Total Magnetic Flux Using Sunspot Areas

We show that daily sunspot areas can be used in a simple, single parameter model to reconstruct daily variations in several other solar parameters, including solar spectral irradiance and total magnetic flux. The model assumes that changes in any given parameter can be treated mathematically as the response of the system to the emergence of a sunspot. Using cotemporal observational data, we compute the finite impulse response (FIR) function that describes that response in detail, and show that the response function has been approximately stationary over the time period for which data exist. For each parameter, the impulse response function describes the physical evolution of that part of a solar active region that is the source of the measured variability. We show that the impulse response functions are relatively narrow functions, no more than 3 years wide overall. Each exhibits a pre-active, active, and post-active region component; the active region component dominates the variability of most of the parameters studied.     

Pi-2 pulsations as a result of evolution of an Alfvén impulse originating in the ionosphere during a brightening of aurora

It is assumed that the original impulse producing Pi-2 pulsations is generated in the ionosphere at the moment of a brightening of aurora. The electric field is known to decrease in the auroral arc almost by an order of magnitude. The electric impulse that appears will be transferred along magnetic field lines and reflected from the ionosphere of the opposite hemisphere, forming the standing Alfvén wave. The electric field impulse of 100 $\text{mV}\text{m}$ is capable of causing magnetic field oscillations of order of 100 γ. Reflection of the Alfvén impulse from the ionosphere with horizontal inhomogeneities corresponding to different forms of auroras is studied. The following is found: (a) the resonance is possible only for harmonics with the rotating vector of polarization; (b) the resonance periods appear to depend essentially on the ionospheric conductivity; this may bring a significant error into determination of the magnetospheric plasma density from the pulsation periods; (c) the auroral zone exerts a screening influence on the pulsations excited at latitudes higher than the zone itself.     

Upper limits in the toroidal component of force-free magnetic fields in stellar atmospheres in the context of solar and stellar flares

Doing numerical calculations of axially symmetric force-free fields, Milsom and Wright (1976) have noticed that there seem to be no solutions if the toroidal component of the field exceeds a certain limit. In the present paper this problem is reexamined in the approximation of a plane stellar surface using a very simple analytic approximation. The results of Milsom and Wright (1976) are confirmed but, in contrast to their interpretation, it is shown that these limitations do not indicate the possibility of sudden changes of the topology of the magnetic field. This is because in a stellar atmosphere the toroidal component of the surface magnetic field is no independent quantity but is produced by shearing motions in the star which will prevent the toroidal magnetic field from exceeding its maximum value. To study the possibility of sudden changes in the magnetic field, which could cause stellar flares, the calculations are re-done prescribing the motion of the magnetic footpoints (shear in the stellar surface) instead of the toroidal component of the surface field. Using the same mathematical formalism it is found that no sudden changes can occur for configurations where all field lines connect to the stellar surface but that sudden changes may be possible for a more complicated field topology.     

Characteristics and Frequency of Weak Stellar Impulses of the Oort Cloud

We have developed a model of the response of the outer Oort cloud of comets to simultaneous tidal perturbations of the adiabatic galactic force and a stellar impulse. The six-dimensional phase space of near-parabolic comet orbital elements has been subdivided into cells. A mapping of the evolution of these elements from beyond the loss cylinder boundary into the inner planetary region over the course of a single orbit is possible. This is done by treating each perturbation separately, and in combination, during a time interval of 5 Myr. We then obtain the time dependence of a wide range of observable comet flux characteristics, which provides a fingerprint of the dynamics. These include the flux distributions of energy, perihelion distance, major axis orientation, and angular momentum orientation. Correlations between these variables are also determined. We show that substantive errors occur if one superposes the separately obtained flux results of the galactic tide and the stellar impulse rather than superposing the tidal and impulsive perturbations in a single analysis. Detailed illustrations are given for an example case where the stellar mass and relative velocity have the ratio M∗/V_rel=0.043 M⊙/km s⁻¹ and the solar impact parameter is 45,000 AU. This case has features similar to the impending Gliese 710 impulse with the impact parameter selected to be close to the low end of the predicted range. We find that the peak in the observable comet flux exceeds that due to the galactic tide alone by ≈41%. We also present results for the time dependence of the flux enhancements and for the mean encounter frequency of weak stellar impulse events as functions of M∗/V_rel and solar impact parameter.     

Impulse response analysis of 5577Å emissions

A method of time series analysis termed impulse response estimation is described. The method is applied to simultaneous ground based photometry measurements of the N₂⁺ ING band at 4278Å and the 5577Å line emission in order to estimate the response in 5577Å to a discrete impulse in excitation rate.From these impulse response estimates the contribution from indirect excitation processes to the O(¹S) state is estimated to be of the order of 80% and the lifetime of the associated intermediate species to be approximately 0.1 s. Effective lifetimes for the O(¹S) excited state are obtained in the range 0.49–0.86 s with a distribution showing a sharp cut-off at 0.8 s and a mean of 0.71 s.     

彗木相撞的不可压缩分层流体模型

提出了一个彗木相撞的线性模型．在这个模型中，将木星大气看作是一个分层的、不可压缩和无粘滞的流体层，而与撞击相联的引力波则由一个初始冲力所造成．采用初始冲压P(r，z，0)代替初始表面形变作为初始条件．发现这种做法使得问题的数学处理更加方便，特别是在计算流体粒子的垂直位移和摄动势能方面，后者可以表为P(r，z，0)的有限傅立叶正弦和余弦变换．导出了一个联系彗星碰撞参数、木星大气参数和所设初始冲力之间的关系式．还对两种P(r，z，0)的假设形式分析地导出表面波波高．     

Optimal deflection of NEOs en route of collision with the Earth

Recently, a method for the n-body analysis of the velocity change required to deflect a hazardous near-Earth object (NEO) was presented by Carusi et al. [Carusi, A., Valsecchi, G.B., D'Abramo, G., Boattini A., 2002. Icarus 159, 417-422]. We extent this method in order to optimize the velocity change vector instead of its along-track magnitude. From an application of both methods to a fictitious NEO we find Carusi's parallel approach to be reasonable for phases of unperturbed two-body motion. But, for orbit phases inhering third-body perturbations, i.e., for planetary close approaches or prior to a collision, the results obtained from the new method show the radial component of deflection impulse to play a major role. We show that a fivefold greater efficiency can be achieved by a deflection impulse being non-parallel to orbital velocity. The new method is applied to two possible 99942 Apophis impact trajectories in order to provide constraints for future Apophis deflection mission analysis.     

Interplanetary shock waves and magnetospheric substorms

It is shown that substorm activity after a storm sudden commencement (S.S.C.) depends on whether or not an interplanetary shock wave is accompanied by a large increase of the solar wind-magnetosphere energy coupling function ε. It has long been thought that substorm activity associated with an S.S.C. results from sudden conversion of magnetic energy stored in the magnetotail and that this conversion is triggered by the shock wave. However, the present result implies that the magnetospheric substorm is not a sudden conversion of stored magnetic energy, but is a direct consequence of increased efficiency of the solar wind-magnetosphere dynamo.     

Quenching of auroral hydrogen line emission by collisional ionization

With the aid of the classical impulse approximation a simple formula is derived for the cross section for electron loss from fast excited hydrogen atoms passing through a neutral gas. Detailed computations relevant to the quenching of hydrogen line emission in auroras are performed. It is found, incidentally, that the population distribution amongst the states of a level is unlikely to be brought into statistical equilibrium by the collisions suffered by the hydrogen atoms.     

Momentum Distribution in Solar Flare Processes

We discuss the consequences of momentum conservation in processes related to solar flares and coronal mass ejections (CMEs), in particular describing the relative importance of vertical impulses that could contribute to the excitation of seismic waves (“sunquakes”). The initial impulse associated with the primary flare energy transport in the impulsive phase contains sufficient momentum, as do the impulses associated with the acceleration of the evaporation flow (the chromospheric shock) or the CME itself. We note that the deceleration of the evaporative flow, as coronal closed fields arrest it, will tend to produce an opposite impulse, reducing the energy coupling into the interior. The actual mechanism of the coupling remains unclear at present.     

An incompressible stratified fluid model ofComet Shoemaker-Levy 9''s collision with Jupiter

A linear model of Comet Shoemaker-Levy 9's collision with Jupiter ispresented, in which the Jovian atmosphere is taken to be a stratified, incompressible and inviscid fluid layer and the gravity wave associated with the impacts is caused by an initial impulse. Instead of adopting an initial surface deformation, we take an initial impulsive pressure P(r, z, 0) as the initial condition. It is found that this approach is more convenient for the mathematical formulation of the problem, especially for the computation of the vertical displacement of the fluid particles and the perturbation potential energy which can then be expressed by finite Fourier sine and cosine transform of P(r, z, 0).

A relationship among parameters of the impact and the Jovian atmosphereand the assumed initial impulse is deduced. We also derive the wave height of the surface wave analytically for two assumed forms of P(r, z, 0).     

低频时码接收机的FIR滤波器设计及其DSP实现

将FIR(有限冲击响应)数字滤波器应用于低频时码接收机中,并利用DSP(digital signal proccssor)芯片TMS320C5402实现了低频时码信号的FIR滤波。实验表明,FIR滤波可以使信号中混杂的较为严重的干扰得到很好的抑制,并且失真较小,有利于实现高性能的定时接收机。     

On mini-halo encounters with stars

We study, analytically and numerically, the energy input into dark matter mini-haloes by interactions with stars. We find that the fractional energy input in simulations of Plummer spheres agrees well with the impulse approximation for small and large impact parameters, with a rapid transition between these two regimes. Using the impulse approximation, the fractional energy input at large impact parameters is fairly independent of the mass and density profiles of the mini-halo; however, low-mass mini-haloes experience a greater fractional energy input in close encounters. We formulate a fitting function which encodes these results and use it to estimate the disruption time-scales of mini-haloes, taking into account the stellar velocity dispersion and mass distribution. For mini-haloes with mass     on typical orbits which pass through the disc, we find that the estimated disruption time-scales are independent of mini-halo mass, and are of the order of the age of the Milky Way. For more massive mini-haloes, the estimated disruption time-scales increase rapidly with increasing mass.     

Disappearence of equatorial Es, associated with magnetic field depressions

The phenomenon of the sudden disappearance of equatorial sporadic E at two stations, namely, Trivandrum and Kodaikanal is studied. It is established that whenever there is a sudden disappearance of E_s, there is a depression in the horizontal magnetic field (H) range. Electron velocities during the presence and absence of sporadic E have been estimated. These results show that the irregularities responsible for sporadic E are present even when the electron velocities are less than the ion thermal velocity.