Observations of forced oscillations of the magnetosphere by a geostationary satellite and an extensive ground magnetometer array

We present results from the analysis of magnetometer measurements of one of the clearest observations of a double resonance Pc4 pulsation to date. The Pc4, with a period of 55 s, was measured by 18 ground magnetometers and also on board the ATS-6 satellite at geostationary orbit. Using a subsequent observation of a second harmonic guided poloidal mode pulsation at ATS-6, we have been able to estimate the plasma density at geostationary orbit. We then calculated periods of theoretical cavity mode resonances in the plasmatrough and the eigenperiods of different wave modes and harmonics at geostationary orbit. We developed a model of the variation of plasma density, and hence eigenperiods, within the magnetosphere which is consistent with these calculations and with the amplitude, phase and ellipticity observations made over the array of ground observatories. In this model we suggest that hydromagnetic field line resonances occur in the plasmatrough and in the plasmasphcre, which are the second and fundamental harmonic guided toroidal mode resonances, respectively. The model also allows us to evaluate the damping experienced by hydromagnetic standing waves in the magnetosphere. The damping is found to be slightly higher than that previously suggested for daytime conditions.     

X-ray and EUV diagnostics of active plasma structures with the RES spectroheliograph in the SPIRIT experiment onboard the CORONAS-F satellite

We provide a brief overview of the main methods and results of spectroscopic studies of several active plasma structures in the solar corona with the RES spectroheliograph in the SPIRIT experiment. This instrument has allowed ～ 150 monochromatic images of the entire Sun in extreme UV (EUV) lines in the 175-to 205-and 280-to 330-Å spectral bands and in the X-ray Mg XII 8.42-Å line to be simultaneously obtained for the first time. The RES instrument has taken ～ 300000 spectroheliograms with a high time resolution over the period of its operation since the launch of the satellite on July 31, 2001. The accumulated data were used to construct and calibrate the spectra of solar flares and compact active regions with a spectral resolution of 0.04 Å. Based on EUV spectra, we determined the temperature distributions of the electron density and differential emission measure (DEM) for several active plasma structures observed in the RES X-ray channel: active regions, flares, and spiders. The results of modeling the physical conditions in an emitting plasma were used to analyze the formation and dynamics of plasma structures detected in the monochromatic X-ray images of the entire Sun.     

The motion of a satellite in a ring potential

This investigation presents the orbital elements of a satellite moving in a circular ring potential. The ring is considered to be of infinitesimal thickness and of unit radius. The components of the perturbing accelerations due to the ring potential have been substituded into the Gauss form of Lagrange's planetary equations to yield the first-order approximations. The elements of the orbit have been expressed by means of Hansen coefficients. The results include the effects produced by the 2nd, 4th, 6th, and 8th spherical harmonics. Due to their importance we present separately the secular terms from the periodic ones. The general expressions for the orbital elements can be easily extended to include the effects produced by any other higher harmonic.List of Symbols semi-major axis - C _jK ⁿ (u, ) cosine functions ofu and - e eccentricity of the orbit - f sin² - inclination of the orbit - M mean anomaly - n mean motion - p semi-latus rectum of the orbit - R, S, andW components of the perturbing acceleration - r magnitude of position vector - S _jK ⁿ (u, ) sine functions ofu and - T time of periapse passage - u argument of latitude - U gravitational potential - V perturbing potential - G(M _r +m) (gravitational constant times the sum of the masses of ring and satellite) - _{n, k} coefficients ofR component of disturbing acceleration (functions off) - _{n, k} coefficients ofS andW components of disturbing acceleration (functions off) - mean anomaly at timet=0 - X ₀ ^{n, m} zero-order Hansen coefficients - argument of periapse - longitude of the ascending node     

Pulsations of microwave emission and flare plasma diagnostics

We consider the modulation of nonthermal gyrosynchrotron emission from solar flares by the ballooning and radial oscillations of coronal loops. The damping mechanisms for fast magnetoacoustic modes are analyzed. We suggest a method for diagnosing the plasma of flare loops that allows their main parameters to be estimated from peculiarities of the microwave pulsations. Based on observational data obtained with the Nobeyama Radioheliograph (17 GHz) and using a technique developed for the event of May 8, 1998, we determined the particle density n≈3.7×10¹⁰ cm^?3, the temperature T≈4×10⁷ K, and the magnetic field strength B≈220 G in the region of flare energy release. A wavelet analysis for the solar flare of August 28, 1999, has revealed two main types of microwave oscillations with periods P₁≈7, 14 s and P₂≈2.4 s, which we attribute to the ballooning and radial oscillations of compact and extended flare loops, respectively. An analysis of the time profile for microwave emission shows evidence of coronal loop interaction. We determined flare plasma parameters for the compact (T≈5.3×10⁷ K, n≈4.8≈10¹⁰ cm^?3, B≈280 G) and extended (T≈2.1≈10⁷ K, n≈1.2≈10¹⁰ cm^?3, B≈160 G) loops. The results of the soft X-ray observations are consistent with the adopted model.     

Comparison of mm-wave and X-ray diagnostics of flare plasma

To compare mm-wave and X-ray diagnostics of solar flare plasma, five flares observed in 1980–1991 in Metsähovi at 22 and 37 GHz and with GOES, SMM, and GRO are studied. The first impulsive peak of the mm-wave bursts under investigation coincides in time with hard X-ray emission. The second gradual component in mm-wave emission coincides with the maximum of the soft X-ray emission measure. The bremsstrahlung mm-wave radiation from hot chromospheric plasma and gyrosynchrotron radiation driven by common population of superthermal electrons are calculated. It is shown that for mm-wave events with the first peak intensity 100 s.f.u., the thermal bremsstrahlung is more important than the gyrosynchrotron emission. The total energy of fast electrons deduced from the first peak of mm-wave bursts is one to two orders of magnitude less than that determined from the hard X-ray emission in the approximation of a thick-target nonthermal model. That can testify in favour of the hybrid thermal/nonthermal model proposed by Holman and Benka (1992). The emission measure and the energy of evaporated plasma using both mm-wave and soft X-ray data are also determined. For events investigated here the energy of evaporated chromospheric plasma is larger than the total energy of fast electron beams. We have concluded that, for evaporation, additional energy release in the chromosphere is needed. The possibility of such energy release in the framework of an advanced circuit model for solar flares is discussed.     

Efficient algorithms for the earth's gravitational potential and its partial derivatives for satellite prediction

In this paper, economical recursion formulae for the Earth's gravitational potential and its partial derivatives with respect to the Cartesian coordinates, and time will be established for any number and any type of the harmonic coefficients. General computational algorithms for their implementations on digital computers are also given.Now at the Department of Astronomy, King Abdul-Aziz Univ., Jeddah, Saudi Arabia.Now at the Department of Mathematics, Girls College of Education, Jeddah, Saudi Arabia.     

Nonsingular expansions of the gravity potential and its derivatives at satellite altitudes in the ellipsoidal coordinate system

The series in ellipsoidal harmonics for derivatives of the Earth’s gravity potential are used only on the reference ellipsoid enveloping the Earth due to their very complex mathematical structure. In the current study, the series in ellipsoidal harmonics are constructed for first- and second-order derivatives of the potential at satellite altitudes; their structure is similar to the series on the reference ellipsoid. The point P is chosen at a random satellite altitude; then, the ellipsoid of revolution is described, which passes through this point and is confocal to the reference ellipsoid. An object-centered coordinate system with the origin at the point P is considered. Using a sequence of transformations, the nonsingular series in ellipsoidal harmonics is constructed for first and second derivatives of the potential in the object-centered coordinate system. These series can be applied to develop a model of the Earth’s potential, based on combined use of surface gravitational force measurements, data on the satellite orbital position, its acceleration, or measurements of the gravitational force gradients of the first and second order. The technique is applicable to any other planet of the Solar System.     

The motion of a satellite in an axi-symmetric gravitational field

The equations for the variation of the osculating elements of a satellite moving in an axi-symmetric gravitational field are integrated to yield the complete first-order perturbations for the elements of the orbit. The expressions obtained include the effects produced by the second to eighth spherical harmonics. The orbital elements are presented in the most general form of summations by means of Hansen coefficients. Due to their general forms it is a simple matter to estimate the perturbations of any higher harmonic by simply increasing the index of summation. Finally, this paper gives the respective general expressions for the secular perturbations of the orbital elements. The formulae presented should be useful for the reductions of Earth-satellite observations and geopotential studies based on them.List of Symbols semi-major axis - C _jk ⁿ (, ) cosine functions of and - e eccentricity of the orbit - f acceleration vector of perturbing force - f sin²t - i inclination of the orbit - J _n coefficients in the potential expansion - M mean anomaly - n mean motion - p semi-latus rectum of the orbit - R, S, andW components of the perturbing acceleration - r radius-vector of satellite - r magnitude ofr - S _jk ⁿ (, ) sine functions of and - T time of perigee passage - u argument of latitude - U gravitational potential - true anomaly - V perturbing potential - G(M₊+m) (gravitational constant times the sum of the masses of Earth and satellite) - _n,k coefficients ofR component of disturbing acceleration (funtions off) - _n,k coefficients ofS andW components of disturbing acceleration (functions off) - mean anomaly at timet=0 - X ₀ ^n,m zero-order Hansen coefficients - argument of perigee - right ascension of the ascending node     

The equations of motion of an artificial satellite in nonsingular variables

The equations of motion of an artificial satellite are given in nonsingular variables. Any term in the geopotential is considered as well as luni-solar perturbations up to an arbitrary power ofr/r, r being the geocentric distance of the disturbing body. Resonances with tesseral harmonics and with the Moon or Sun are also considered. By neglecting the shadow effect, the disturbing function for solar radiation is also developed in nonsingular variables for the long periodic perturbations. Formulas are developed for implementation of the theory in actual computations.     

Atomic line broadening by thermal energy fluctuations in stellar atmospheres and plasma diagnostics

A new method for finding the line widths of atomic lines produced by thermal energy fluctuations in a gaseous system is developed assuming that the atomic linear density change in the energy of the levels is equal to the linear density of the energy fluctuations per degree of freedom. A formula is derived for the atomic line widths that depends on temperature, the cubic root of the total number density of particles in the system, and on the sum of the squares of the principal quantum numbers of the states that participate in the transitions that produce the lines. The calculated widths agree well with the published experimental and theoretical values. This formula will be useful for directly diagnosing the physical state of stellar atmospheres and plasmas.     

Use of Al xii and Mg xi lines as solar plasma diagnostics

We present three sets of observations of n = 1 to n = 2 lines due to helium-like aluminium (Alxii), made during two solar flares (25 August, 1980 and 19 October, 1986), using the X-Ray Polychromator on the SMM satellite. The observed temperature-sensitive line ratio G is shown to be consistent with the close-coupling calculations of Keenan and McCann (1987), although the ratio R, which is both temperature and density-sensitive for lower-Z elements, is not sufficiently well determined from these data to say more than that the observed values of R are not inconsistent with the theoretical calculations. This region of the spectrum also includes the helium-like magnesium (Mgxi) 1¹ S - 3¹ P line, and it is shown that the ratio of this line to the Alxii resonance (1¹ S - 2¹ P) line is a more sensitive indicator of electron temperature than are the Alxii G and R ratios. We demonstrate that the three ratios may be used together in order to derive values of emission measure, electron temperature and electron density during these flares.     

The possible motions of a satellite about an oblate planet

The regions of motions of a satellite for given values of energy and angular momentum about polar axes are shown. Special attention is paid to the circular equatorial orbits which have been shown to be Hill stable. The anomalistic and the nodal period for the motions near to the circular equatorial orbits have been found.     

Observation of an IMF sector effect in the Y magnetic field component at geostationary orbit

One year of magnetic field data from the geostationary spacecraft ATS 6 have been analysed for effects associated with the equatorial plane components of the interplanetary magnetic field (IMF). It is shown that perturbation fields in the Y (dawn to dusk) direction appear in association with the Y component of the IMF, in agreement with previous theoretical suggestions. On average a fraction 0.28 ± 0.02 of the IMF Y field appears at geostationary orbit, such that the average ATS 6 B_y field is 1.9 ± 0.4 nT larger when IMF B_y is positive than when it is negative. The perturbation field magnitudes are also found to depend strongly on local time, however, with largest effects appearing in the midnight and dawn quadrants, where the average perturbation fields are nearly half the simultaneous IMF B_v. field. At noon this fraction drops to one fifth, and no average effect occurs in the dusk quadrant. Both the daily mean perturbation fields and the diurnal modulation are also found to depend upon the level of magnetic disturbance as measured by K_P, or equivalently upon IMF B_z, and upon season of the year. Overall stronger daily mean perturbation fields occur when K_P is low or when IMF B_z is positive, than when K_P is high or when IMF B_z is negative. This effect is not linear, however, and there is also a trend in the data towards increasing perturbation fields with IMF B_z negative and decreasing. On dividing the data according to season, increasingly strong daily mean effects are found in the order winter, summer and equinox for both quiet and disturbed magnetospheres. Diurnal modulations of the perturbation field magnitudes for low K_P (IMF B_z > 0) take the form of large amplitude quasi-sinusoidal variations about mean values which are very marked in the equinox data, are present to a lesser degree during summer and are absent during winter conditions. When K_p is high (IMF B_z < 0) significant deviations from mean perturbation field values occur generally only during nightside hours and little seasonal dependence is evident. Finally, it is shown that the highest correlation between the IMF data and the ATS 6 perturbation fields occurs with zero time delay between the two data sets, showing that a prompt response to IMF conditions occurs at geostationary orbit within the 1 h time resolution available in this study. Although many details of the above ATS 6 response remain to be understood, these results overall demonstrate in a very direct manner the magnetically “open” nature of the Earth's magnetosphere.     

The observation of extensive air showers from an Earth-orbiting satellite

We review the main issues that are relevant for the observation of extensive air showers from an Earth-orbiting satellite. Extensive air showers are produced by the interaction of ultra-high energy cosmic particles with the atmosphere and can be observed by an orbiting telescope detecting the air scintillation light.We provide the main analytical formulas and semi-analytical results needed to optimize the design of a suitable telescope and estimate the best-expected performance and the minimal necessary requirements for the observation.While we have in mind an EUSO-like general-purpose experiment, the results presented in this paper are useful for any kind of space-based experiment.     

e-VLBI技术及其在卫星定轨中的应用概况

e-VLBI技术继承了VLBI本身具有的极高角分辨率,且其利用高速通讯网络传送观测数据,能够快速得到观测结果,这些优点对于卫星的快速测定轨和提高卫星现有的定轨精度是十分有利的。通过对e-VLBI技术特点的分析及其发展历程的回顾,并结合卫星的差分VLBI观测原理,重点阐述和讨论了e-VLBI技术在我国的应用现状、发展及要求,展望了我国未来e-VLBI的发展前景。     

Orbital variation of the synchronous satellite and its calculation

This paper investigates the orbital variation of the 24-hour synchronous satellite and gives a method for calculating the variation. The results provide a theoretical basis for the design and calculation of the orbit of the communication satellites.     

The stark broadening mechanism in an unstable plasma

We consider a beam driven unstable plasma and estimate the turbulent electric fields which may be excited by this beam. We then estimate the Stark broadening due to such fields. Also with the Department of Physics and Astronomy, University of Maryland, College Park, Maryland.     

The stability of an anisotropic plasma jet

The modified Chew-Goldenberger-Low equations have been used to study the stability of a resistive anisotropic plasma jet surrounded by a non-conducting compressible gas. The dispersion relation has been obtained and discussed in three limiting situations: (i) , the resistivity vanishingly small, (ii) and (iii) short and long wavelength perturbations. In some cases, the limiting situations 1 or 1 ( is the ratio of the density of the plasma jet to the density of the exterior gas) have been discussed. The conditions for instability have been obtained. It has been found that the resistivity introduces new modes which make the plasma jet overstable. In the limit of large wavelength disturbances, the jet with finite but high conductivity is found to be unstable. For small wavelength disturbances, the jet is found to be unstable.