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1.
Experimental results are presented for the impedance of a rocket-borne dipole antenna immersed in the ionospheric plasma. The dependence of several interesting impedance artifacts upon the antenna position relative to the Earth's magnetic field and rocket motion through the ionospheric plasma are shown. Possible evidence for plasma compressibility is provided by an impedance discontinuity occurring consistently at approximately twice the electron cyclotron frequency and a frequency shifted cyclotron-resonance impedance minimum.  相似文献   

2.
The frequency specifications of the Square Kilometre Array (SKA) call for an optimum operation of the antenna elements from 25 down to 100 MHz. The current 12 m diameter US-SKA design is specified from 500 up to 25 GHz, with an upper goal of 35 GHz. At the low frequency end of the band (i.e., 100 MHz), a 12 m reflector antenna is about four wavelengths in diameter. Then, the question is: how well can you do, at this low frequency end of the specified band of operation for the SKA, with a symmetric reflector configuration using an ultra-wide-band prime focus feed? This paper presents the analysis of the antenna performance, in terms of A eff/T A, of three symmetric configurations of the 12 m US-SKA antenna design between 100 and 200 MHz.  相似文献   

3.
An analysis is made to calculate input impedance of a loop antenna for radiation of the VLF whistler mode in the magnetosphere. The magnetosphere is assumed to be represented by a cold, uniform and collisionless magnetoplasma medium. Assuming a uniform current distribution of a circular loop, oriented at an arbitrary angle with respect to the Earth's magnetic field line, several closed-form expressions for the loop impedance have been derived. It is found that the loop input reactance is in substantial agreement with the self-inductance of a loop in free space and that the radiation resistance for a small loop can be as large as ～10 2 Ω. It is also found that a second order quasi-static theory is quite valid for determining the input impedance for small loops radiating VLF whistlers in the magnetosphere.  相似文献   

4.
We present the results from modeling the coronal mass ejection (CME) properties that have an effect on the Faraday rotation (FR) signatures that may be measured with an imaging radio antenna array such as the Murchison Widefield Array (MWA). These include the magnetic flux rope orientation, handedness, magnetic-field magnitude, velocity, radius, expansion rate, electron density, and the presence of a shock/sheath region. We find that simultaneous multiple radio source observations (FR imaging) can be used to uniquely determine the orientation of the magnetic field in a CME, increase the advance warning time on the geoeffectiveness of a CME by an order of magnitude from the warning time possible from in-situ observations at L 1, and investigate the extent and structure of the shock/sheath region at the leading edge of fast CMEs. The magnetic field of the heliosphere is largely “invisible” with only a fraction of the interplanetary magnetic-field lines convecting past the Earth; remote sensing the heliospheric magnetic field through FR imaging from the MWA will advance solar physics investigations into CME evolution and dynamics.  相似文献   

5.
Effects of plasma turbulence on the stability of electrostatic ion loss-cone waves are examined. The turbulence is assumed to be electrostatic with frequencies near 1.5 times the electron gyrofrequency and the frequencies of the generated waves are below the ion plasma frequency ωpi>. A nonlinear growth rate of the order of 10?2ωpi may be obtained, when the amplitude of the turbulence is 20 mV/m. This is comparable to previously found growth rates of the linear ion loss-cone instability, in a plasma with large pitch angle anisotropy. Bounce averaged pitch angle diffusion coefficients are also presented for different models of the ion loss-cone wave spectrum.  相似文献   

6.
The Antarctic Ross Ice Shelf Antenna Neutrino Array (ARIANNA) is a high-energy neutrino detector designed to record the Askaryan electric field signature of cosmogenic neutrino interactions in ice. To understand the inherent radio-frequency (RF) neutrino signature, the time-domain response of the ARIANNA RF receiver must be measured. ARIANNA uses Create CLP5130-2N log-periodic dipole arrays (LPDAs). The associated effective height operator converts incident electric fields to voltage waveforms at the LDPA terminals. The effective height versus time and incident angle was measured, along with the associated response of the ARIANNA RF amplifier. The results are verified by correlating to field measurements in air and ice, using oscilloscopes. Finally, theoretical models for the Askaryan electric field are combined with the detector response to predict the neutrino signature.  相似文献   

7.
We have examined statistically, structural asymmetries and simple relativistic beaming/source orientation in a sample of Lobe-Dominated Quasars (LDQs) using the source size D as orientation parameter; relative core strength R as beaming parameter; arm-length ratio Q, apparent flux ratio R ?, and bending angle Φ as asymmetric parameters. Our result for Q>1.5, based on the median value data is inconsistent with beaming scenario, where we expect stronger negative correlation for more asymmetric sources, between our beaming parameter R and orientation parameter D than for less asymmetric sources Q≤1.5. This observation indicates that structural asymmetries may depend more on intrinsic factors than beaming. Our kinematic asymmetric model of extra galactic radio sources suggests that larger (possibly older) sources are less asymmetric, which may be interpreted to be indicative of other factors other than beaming as responsible for the observed asymmetries in radio sources.  相似文献   

8.
Related investigations concerning the source and occurrence of Pi 2 micropulsations indicate that (1) Pi 2 peak occurrence is near 2230 LMT, before (after) which the initial disturbance vector orientation is primarily northeast (northwest) in the northern hemisphere; (2) peak occurrence is near 2330 (2030) LMT during intervals of low (high) magnetic activity; (3) also, solar wind pressure is directed at a streaming angle of 188° (165°) during low (high) magnetic activity; (4) Pi 2 rate of occurrence peaks when 1 + ? Kp ? 2?. The diurnal variation of occurrence and of the initial disturbance vector orientation may be caused by a magnetospheric Pi 2 source located near the 2230 LMT meridian. Comparison of Pi 2 peak occurrence time and solar wind variations with magnetic activity indicates possible relationships of the two. The fact that processes generating Pi 2 appear optimum, when 1 + ? Kp ? 2?, is consistent with the inverse relation of the yearly Pi 2 occurrence frequency to solar activity.  相似文献   

9.
The influence of low-frequency electrostatic turbulence on the flux of precipitating magnetospheric electrons is analyzed in the framework of the quasilinear kinetic equation. It is shown that an electron population in a turbulent region, with an electric field parallel to the ambient magnetic field, can be separated into two parts by introducing a pitch angle dependent runaway velocity vr(θ). Lower energy electrons with parallel velocity v∥ < vr are effectively scattered by plasma waves, so that they remain in the main population and are subjected to an anomalous transport equation. A distribution function f ∞ v?4 (or the particle flux vs energy J ∞ E?1) is established in this velocity range. Faster electrons with v∥ ? vr are freely accelerated by a parallel electric field, so that they contribute directly to hot electron fluxes which are observed at ionospheric altitudes. New expressions are derived for the magnetic-field aligned current and the electron energy flux implied by this model. These expressions agree well with empirical relations observed in auroral inverted-V structures.  相似文献   

10.
The relationship between the orientation of the interplanetary magnetic field (IMF), represented by the clock angle which is the angle defined by IMF-By and -Bz components, and the AL and AU indices is examined at various dipole tilt angles for the period of 1978-1988. We use the IMF data obtained from the IMP 8 satellite, AL and AU indices with corrected seasonal variations, and the dipole tilt angle, which is the dipole magnetic latitude of the subsolar point calculated as a function of the day of year and universal time. For both positive (dipole tilted to the Sun) and negative dipole tilt angles, the values of |AL| and AU decrease as the IMF clock angle moves away from 180°, becoming more northward. The indices also tend to become smaller for larger dipole tilt angle, either toward or away from the Sun. This dependence on dipole tilt angle enhances the semiannual variation of geomagnetic activity.  相似文献   

11.
In a previous paper, we showed a method for deriving the interplanetary magnetic field (IMF) orientation from the velocity distribution of ring-like distributed ions as measured by the Ion Mass Analyser (IMA) on board Mars Express (MEX). This method has been improved so that one can derive the IMF orientation from a very limited portion of the ring distributions, i.e., only the highest energy portion of the ring distribution. This method uses the maximum variance direction L instead of the minimum variance direction N, which are derived from manually selected ring data. Because IMA's count rate for a semi-persistent ring distribution is nearly proportional to energy squire, L is most likely aligned to the tangential direction of the ring distribution at its highest energy, and this tangential direction is parallel or anti-parallel to the electric field. A vector product of L and the solar wind direction (X) gives the IMF orientation projected to the Y-Z plane. The tilt angle of IMF toward the X direction from the Y-Z plane is the same as the angle between the X direction and the ring plane, and is obtained from two methods when the initial speed of the ring ions is estimated to be much smaller than the solar wind speed: (1) angle between the velocity of ring's maximum energy portion and the solar wind vector, and (2) energy ratio between the solar wind and the maximum energy of the ring. The present method is applied to the IMA data from 3 June 2005 (0605-0640 UT) when the Mars Global Surveyor (MGS) magnetometer data are available. Using these data, we also tried to determine the sign of the IMF direction by estimating the evolution direction of the ring ions.  相似文献   

12.
The NASA DS1 spacecraft encountered Comet P/Borrelly on September 22, 2001 at a distance of ∼2171 km on the sunward side of the comet. The flyby speed was ∼16.5 km s−1. Using high temporal resolution (50 μs) absolute electric field amplitude measurements from a ∼1 m dipole antenna, new features of plasma clouds created by cometary dust impacts have been detected. The pulses have 1/e exponential decays of ∼650 μs duration, exponentially shaped overshoots with rise times of ∼2 ms, and exponential-shaped overshoot decay times of ∼10 ms. Assuming a plasma temperature of 104 K, these pulse features have been explained as plasma cloud space charge effects from the electron, proton and heavy ion portions of the clouds passing the antenna. Complex pulse shapes were also detected. These are believed to be due to either plasma cloud scattering off of the spacecraft, or to secondary impacts. Small electric pulses of duration 10-15 ms of cometary origin were detected but are presently unexplained. The electric component of the plasma wave spectra at closest approach had an f−2.4 power law shape from 10 Hz to 1 kHz. The electron cyclotron frequency was approximately 1 kHz. One possible explanation of the wave spectrum is that whistler mode waves associated with phase steepened cometary plasma waves are dispersed, leading to the broad spectrum. Finally, based on the present results, a new type of low-cost, large-area dust detector is proposed.  相似文献   

13.
We investigate the relationship between magnetic structures of coronal mass ejection (CME) source regions and geomagnetic storms, in particular, the super storms when the D st index decreases below −200 nT. By examining all full halo CMEs that erupted between 1996 and 2004, we selected 73 events associated with M-class and X-class solar flares, which have a clearly identifiable source region. By analyzing daily full-disk MDI magnetograms, we found that the horizontal gradient of the line-of-sight magnetic field is a viable parameter to identify a flaring magnetic neutral line and thus can be used to predict the possible source region of CMEs. The accuracy of this prediction is about 75%, especially for those associated with X-class flares (up to 89%). The mean orientation of the magnetic structures of source regions was derived and characterized by the orientation angle θ, which is defined to be ≤ 90∘ in the case of the southward orientation and ≥ 90∘, when the magnetic structure is northwardly oriented. The orientation angle was calculated as the median orientation angle of extrapolated field lines relative to the flaring neutral line. We report that for about 92% of super storms (12 out of 13 events) the orientation angle was found to be southward. In the case of intense and moderate storms (D st≥ −200 nT), the relationship is less pronounced (70%, 21 out of 30 events). Our findings demonstrate that the approach presented in this paper can be used to perform an automatic prediction of the occurrence of large X-class flares and super geomagnetic storms.  相似文献   

14.
In this paper, we use the distributions of projected linear size (D), core- (P C ) and extended- (P E ) radio luminosities, to investigate a consequence of relativistic beaming and radio source orientation scenario for low-luminosity extragalactic radio sources. In this scenario, BL Lacertae objects (BL Lacs) are believed to be Fanaroff-Riley type I (FR I) radio galaxies, but with radio axes aligned close to the line of sight. At this orientation, the core emission is greatly enhanced by relativistic Doppler boosting and linear size foreshortened due to geometrical projection. A simple outcome of this scenario is that the extended luminosity is expected to be orientation invariant, but a D–P C correlation is envisaged. Results show that both the relative core dominance (R) and linear size are strongly correlated with extended luminosity (r≥ 0.7). Using the R-distribution and R–P E anti-correlation, we show that the difference in radio core-dominance between FR I radio galaxies and X-ray selected BL Lacs can be accounted for by a bulk Lorentz factor γ～5–13 and viewing angle ?～5–15°, which can be understood in terms of the scenario, with relativistic beaming persisting at largest scales.  相似文献   

15.
We present results of the dual-frequency radio sounding of the Venusian ionosphere carried out by the Venera 9 and 10 satellites in 1975. Thirteen height profiles of electron density for different solar zenith angles varying from 10 to 87° have been obtained by analyzing the refraction bending of radiorays in the sounded ionssphere. The main maximum of electron density at a height of 140–150 km depends on the solar zenith angle and is 1.4 to 5 × 105 cm?3. The lower maximum is determined definitely to be at ～130 km high. In the main and lower maxima the electron density variations with solar zenith angle are in good agreement with the Chapman layer theory. For the first time it is found that the height of the upper boundary for the daytime ionosphere (hi) depends regularly on the solar zenith angle. At Z⊙ < 60°, hi does not exceed 300 km while at Z⊙ > 60°, it increases with Z⊙ and comes up to ～ 600 km at Z⊙ ～ 80°.  相似文献   

16.
By using the method of separating rapid and slow subsystem, we obtain an analytical solution for a stable three-dimensional motion of a circumbinary planet around a binary star. We show that the motion of the planet is more complicated than it was obtained for this situation analytically by Farago and Laskar (2010). Namely, in addition to the precession of the orbital plane of the planet around the angular momentum of the binary (found by Farago and Laskar (2010)), there is simultaneously the precession of the orbital plane of the planet within the orbital plane. We show that the frequency of this additional precession is different from the frequency of the precession of the orbital plane around the angular momentum of the binary. We demonstrate that this problem is mathematically equivalent both to the problem of the motion of a satellite around an oblate planet and to the problem of a hydrogen Rydberg atom in the field of a high-frequency linearly-polarized laser radiation, thus discovering yet another connection between astrophysics and atomic physics. We point out that all of the above physical systems have a higher than geometrical symmetry, which is a counterintuitive result. In particular, it is manifested by the fact that, while the elliptical orbit of the circumbinary planet (around a binary star) or of the satellite (around an oblate planet) or of the Rydberg electron (in the laser field) undergoes simultaneously two types of the precession, the shape of the orbit does not change. The fact that a system, consisting of a circumbinary planet around a binary star, possesses the hidden symmetry should be of a general physical interest. Our analytical results could be used for benchmarking future simulations.  相似文献   

17.
The paper aims at removing the restriction x ? 1 (where x is the cosine of the equatorial pitch angle) in the theory of pitch angle diffusion by bounce resonance. When the fourth order anharmonicity term is included in the expansion of the magnetic field around the equator, the parallel displacement of a particle becomes a superposition of the first and third harmonics of the fundamental frequency. The diffusion coefficient for pitch angle scattering by bounce resonance has been evaluated by taking into consideration the anharmonicity effects, and this expression can be expected to be valid for particles which mirror at higher latitudes also.  相似文献   

18.
Excess solar X-ray radiation during solar flares causes an enhancement of ionization in the ionospheric D-region and hence affects sub-ionospherically propagating VLF signal amplitude and phase. VLF signal amplitude perturbation (ΔA) and amplitude time delay (Δt) (vis-á-vis corresponding X-ray light curve as measured by GOES-15) of NWC/19.8 kHz signal have been computed for solar flares which is detected by us during Jan–Sep 2011. The signal is recorded by SoftPAL facility of IERC/ICSP, Sitapur (22° 27′N, 87° 45′E), West Bengal, India. In first part of the work, using the well known LWPC technique, we simulated the flare induced excess lower ionospheric electron density by amplitude perturbation method. Unperturbed D-region electron density is also obtained from simulation and compared with IRI-model results. Using these simulation results and time delay as key parameters, we calculate the effective electron recombination coefficient (α eff ) at solar flare peak region. Our results match with the same obtained by other established models. In the second part, we dealt with the solar zenith angle effect on D-region during flares. We relate this VLF data with the solar X-ray data. We find that the peak of the VLF amplitude occurs later than the time of the X-ray peak for each flare. We investigate this so-called time delay (Δt). For the C-class flares we find that there is a direct correspondence between Δt of a solar flare and the average solar zenith angle Z over the signal propagation path at flare occurrence time. Now for deeper analysis, we compute the Δt for different local diurnal time slots DT. We find that while the time delay is anti-correlated with the flare peak energy flux ? max independent of these time slots, the goodness of fit, as measured by reduced-χ 2, actually worsens as the day progresses. The variation of the Z dependence of reduced-χ 2 seems to follow the variation of standard deviation of Z along the T x -R x propagation path. In other words, for the flares having almost constant Z over the path a tighter anti-correlation between Δt and ? max was observed.  相似文献   

19.
《New Astronomy Reviews》2002,46(2-7):231-233
We present the results of the study of the orientation of the accretion disk relative to the host galaxy disk in Seyfert galaxies. We used a sample selected by a mostly isotropic property, the flux at 60 μm, with radio and optical data homogeneously observed and analyzed, to avoid selection effects. We found that the observed i and δ values, galaxy inclination and difference between the position angle of the jet and the galaxy major axis, respectively, are consistent with a random β-distribution, the angle between the jet and the galaxy plane axis. We also found that the previously suggested Zone of Avoidance disappears and was probably due to a selection effect. We suggest several explanations for the misalignment of the accretion disk relative to the galaxy disk.  相似文献   

20.
On the basis of issues raised by observations of BL Lac objects and the qualitative jet model proposed by Bakeret al. in 1988, we have been led to consider the quantitative role of coherent, stimulated emission in jets and construct a new jet model of blazars in which a relativistic electron beam with an axial symmetric, power-law distribution is injected from the central engine into the jet plasma. We study quantitatively the synchrotron emission of the relativistic electron beams. Using the weak turbulent theory of plasma, we discuss the interaction between relativistic electron beams and jet plasma, and the roles of stimulated emission. The main results are:
  1. The synchrotron emission increases sensitively with the increase of the angle between the direction of the beam and the magnetic field. When the direction of the beam is vertical to the magnetic field, the synchrotron emission reaches its maximum, i.e. the emitted waves are beamed in the direction of the jet axis. We suggest that radio selected BL Lac objects belong to this extreme classification.
  2. The synchrotron emission of the relativistic beam increases rapidly with the increase of the Lorentz factor of the relativistic electron,γ, whenγ ≤ 22.5, then decreases rapidly with increase ofγ.
  3. The stimulated emission also increases with increasing Lorentz factorγ of the relativistic electrons whenγ ≤ 35 and then decreases with the increasingγ. The maximum stimulated emission and the maximum synchrotron emission occur at different frequencies. Stimulated emission is probably very important and reasonable flare mechanism in blazars.
  4. The rapid polarization position angle (PA) swings may arise from the interaction between the relativistic electron beam and the turbulent plasma.
  相似文献   

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