共查询到20条相似文献，搜索用时 31 毫秒
1.
Morphological features of whistlers recorded at low latitude ground station Gulmarg (geomag. lat., 24 26N) are studied to deduce information about ducts. The morphological characteristics of low latitude whistlers are discussed and compared with the characteristics of middle and high latitude whistlers. The maximum electron density (N
m
) at the height of the ionosphere obtained from whistler dispersion comes out to be higher than that of the background, which is in accordance with the characteristics of the whistler duct. The equivalent width of the whistler duct at the maximum height of its path is found to be close to the value obtained from satellite observations. The characteristics of whistler ducts in low latitude ionosphere are similar to those in middle and high latitude ionosphere. The width of ducts estimated from the diffuseness of the whistler track observed during magnetic storm is found to lie in the range of 50–200 km. 相似文献
2.
A 3-D particle simulation of excitation of whistler waves driven by an electron temperature anisotropy (T
> T
) is presented. Results show that whistler waves can have appreciable growth driven by the anisotropy. The maximum intensity of the excited whistler waves increases as a quadratic function of the anisotropy. Due to the presence of a threshold, one needs a relatively large electron temperature anisotropy above threshold to generate large-amplitude whistler waves. The average amplitude of turbulence in the context of whistler waves is up to as large as about 1% of the ambient magnetic field when T
/T
. The total energy density of the whistler turbulence is adequate for production of relativistic electrons in solar flares through stochastic acceleration. 相似文献
3.
Whistler precursors observed during day time at low latitude ground station Gulmarg (Geomag. Lat. 24 10 N) and their morphological features are reported. Transverse resonance interaction between whistler mode wave and counter streaming energetic electrons as the probable generation mechanism has been worked out. Minimum anisotropy required for wave amplification, parallel energy of resonating electrons and wave growth rate relevant to generation mechanism is studied. 相似文献
4.
A new coherent radiation mechanism, involving nonlinear interaction of whistler solitons with upperhybrid waves, excited by energetic electrons of energies of 10 keV–100 keV, is proposed for type IV solar bursts of both moving (type IV M) and stationary (type IV S) types. We show that the type IV M bursts occur when the interaction of whistler solitons and upperhybrid waves takes place in the coronal transients whereas the type IV S bursts originate provided this interaction takes place in stationary loops where density has been increased. The emitted radiation is right-hand circularly polarized with 100% polarization. Increase of brightness temperature, T
b
, at lower frequencies and also its decrease, at all frequencies, with the passage of time is predicted for type IV M bursts; this agrees fully with the observations. Furthermore, the decrease of T
b
, with time for stationary type IV component, is easily explained if the source which supplies energetic electron to the loop, becomes weaker with time. 相似文献
5.
L. Staveland 《Solar physics》1974,36(1):235-238
An analytical expression for the limb profile and the aureole is derived for a Gaussian spread function with b 2. Power series with 3 terms are used as approximation to the limb darkening given by HSRA (Gingerich et al., 1971). For smaller values of the spread parameter b the expression given by Staveland (1972) gives a more correct result. The rms difference in the intensity is 0.005 between these analytical expressions and a numerical integration (Brahde, 1972) both inside and outside the limb. Mullan's (1973) expression gives too low intensities in the range ±2b around the limb. 相似文献
6.
The large-scale photospheric magnetic field, measured by the Mt. Wilson magnetograph, has been analyzed in terms of surface harmonics (P
n
m
)()cosm and P
n
m
()sinm) for the years 1959 through 1972. Our results are as follows. The single harmonic which most often characterized the general solar magnetic field throughout the period of observation corresponds to a dipole lying in the plane of the equator (2 sectors, n = m = 1). This 2-sector harmonic was particularly dominant during the active years of solar cycles 19 and 20. The north-south dipole harmonic (n = 1, m = 0) was prominent only during quiet years and was relatively insignificant during the active years. (The derived north-south dipole includes magnetic fields from the entire solar surface and does not necessarily correlate with either the dipole-like appearance of the polar regions of the Sun or with the weak polar magnetic fields.) The 4-sector structure (n = m = 2) was prominent, and often dominant, at various times throughout the cycle. A 6-sector structure (n = m = 3) occasionally became dominant for very brief periods during the active years. Contributions to the general solar magnetic field from harmonics of principal index 4 n 9 were generally relatively small throughout this entire solar cycle with one outstanding exception. For a period of several months prior to the large August 1972 flares, the global photospheric field was dominated by an n = 5 harmonic; this harmonic returned to a low value shortly after the August 1972 flare events. Rapid changes in the global harmonics, in particular, relative and absolute changes in the contributions of harmonics of different principal index n to the global field, imply that the global solar field is not very deep or that very strong fluid flows connect the photosphere with deeper layers.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
7.
An improved X-ray polarimeter is briefly described and preliminary results of the measurements carried out on the satellite Intercosmos-7 are presented. One flare with considerable polarization (P 16%) was observed on 1972 August 4. Two other flares with rather low polarization (P 4%; P 2%) were observed on 1972 August 7 and 11. 相似文献
8.
P. Revathy 《Solar physics》1978,58(2):397-403
The non-linear saturation of whistler mode instability in the solar wind is considered here. The resulting heating and acceleration are calculated. It is shown that this instability heats predominantly the -particles and the ratio of heating rates of -particles to ions have been calculated. This instability is shown to be effective in accelerating -particles than ions. However, this process cannot account for V
/V
i
1. 相似文献
9.
Observation of the adiabatic behaviour of energetic particle pitch-angle distributions in the magnetosphere (Lyons, 1977, and others) in the past indicated the development of pronounced minima or drift-loss cones on the pitch-angle distributions centred at 90° in connection with storm-time changes in magnetospheric convection and magnetic field. Using a model of a drift-modified loss-cone distribution (MLCD) of the butterfly type, the linear stability of electromagnetic whistler or ion-cyclotron waves propagating parallel to the magnetic field has been investigated. The instability is shown to be quenched at high frequencies <
m
=A/(A+1), whereA is the thermal anisotropy. This quenching becomes stronger the higher are the respective parallel hot particle thermal velocityA
h and cold plasma densityn
c
. Particles around pitch-angles 90° are identified as generating electromagnetic cyclotron waves near the marginally stable frequency
m
. It is concluded that the absence of electromagnetic VLF and ELF noise during times when MLCD develops is the result of the shift of the unstable spectrum to low frequencies. 相似文献
10.
R. Tousey J. D. F. Bartoe J. D. Bohlin G. E. Brueckner J. D. Purcell V. E. Scherrer N. R. Sheeley Jr. R. J. Schumacher M. E. Vanhoosier 《Solar physics》1973,33(2):265-280
Some of the first observations obtained with the Naval Research Laboratory's Extreme Ultraviolet Spectrograph (S082A) during the first Skylab mission are presented and compared with magnetograms and other ground-based data. The instrument is a slitless objective-type grating spectrograph covering 170–630 Å and described in Solar Phys.
27, 251 (1972). Chromospheric network, loop prominences, active regions, a flare, limb brightening, XUV bright points, and coronal holes are among the phenomena shown and discussed. 相似文献
11.
Y. T. Chiu 《Solar physics》1970,13(2):420-443
We show that the observed modulation of some coronal microwave, X-ray and Type III emission into pulses of 10 sec intervals is a consequence of the stimulation of electron cyclotron waves propagated in the whistler mode in dipole-like bipolar regions of dimension 0.2 R
. Assuming that a power law spectrum of 10 keV electrons with a slope similar to solar flare protons can be trapped in a bipolar region, we show that whistlers can be generated by pitch angle instability. The resultant 10 sec bounce motion of whistler wave trains leads to enhanced, modulated emission in microwave and X-ray frequencies by pitch angle scattering of MeV electrons, and to modulated Type III emission by scattering with coherent plasma waves. A direct prediction of the theory is the existence of sympathetic pulsations at two sources a fraction of a solar radius apart. A second test of the theory is that modulated Type III emission should show strong polarization.This work was conducted under U.S. Air Force Space and Missile Systems Organization (SAMSO) Contract No. F04701-69-C-0066. 相似文献
12.
K. L. Vithal 《Astrophysics and Space Science》1977,49(2):293-316
A form of general dispersion relation for electromagnetic waves in a fully ionized anisotropic plasma with loss-cone that explicates the contribution of the loss-cone to the dispersion relation is developed. By initially ignoring effects due to anisotropy, it is shown by means of Nyquist diagram technique that an isotropic loss-cone distribution can be unstable to EM waves corresponding to the whistler mode (0<<
e
). The growth rate is then determined analytically for this distribution, assuming cyclotron resonance between the waves in the whistler mode and particles in the high energy tail of the velocity distribution. By including the effects of anisotropy, a general growth rate is obtained which is found to depend on the anisotropy, the size of the loss-cone, the softness of the energy spectrum, and the fraction of the particles which are resonant with the wave. For particular distributions the relative contributions of the anisotropy and of the loss-cone to the growth rate have been determined. It is seen that loss-cone effects, which depend on the size of the loss-cone as well as the softness of the energy spectrum, can be a significant factor in the determination of the growth rate. For the Lorentzian distribution, the half-width of unstable waves is considerably broadened and the growth rates are somewhat more severe as compared to a two-temperature Maxwellian. The threshold frequency is
which confirms the presence of unstable EM waves in the magnetospheric plasma leading to turbulence. 相似文献
13.
P. H. Stoker 《Astrophysics and Space Science》1995,230(1-2):405-413
The absorption of cosmic radio noise passing through the ionosphere may be described as a function of radio wave frequencyA(f
e
) f
e
-n
, with n 2.0 for spatially uniform precipitation of electrons and n < 2.0 for spatially nonuniform precipitation. Using multifrequency riometer recordings at SANAE, the following observations are reviewed: (1) The frequency distribution of the power index, n, obtained from 4 min averaged absorptions during 1983, shows a most probable value around n 1.5, indicating that mostly energetic electrons are precipitated spatially structured onto the upper atmosphere, as in optical aurora. (2) Multifrequency riometer recordings suggest that field-aligned ionospheric irregularities have scattered additional cosmic radio waves from the central region of the Galaxy into the fields of views of the riometer antennae during an auroral absorption event in the early morning hours of 27 July, 1982. With the power reflectivity by ionospheric irregularities inversely proportional to the fourth power of radio wave frequency, as required by the Bragg condition, an estimated 70% increase in the 20 MHz radio flux at 01:22 UT, at the strong absorption peak, can explain the strongly reduced absorption observed in 20 MHz relative to 30 and 51.4 MHz. (3) Gradual increases in absorptions observed at all three riometer frequencies from onset at 11:50 UT of the largest solar proton ground level enhancement on 29 September, 1989, until 18:00 UT, suggest diffusion of the much more intense low energy protons from the polar cap to the L=4.0 geomagnetic field shell and subsequent precipitation at SANAE due to the South Atlantic Geomagnetic Anomaly. (4) The flux of electron energy deposited per second at SANAE is closely related to geomagnetic activity, but has a lower maximum during the years 1971 and 1980 of solar polar magnetic reversals than in the years 1976 and 1986/87 of minimum solar activity. (5) A significant correlation has been found between the arrival of single-hop whistlers and 30 MHz riometer absorption events, using point statistics. The maximum absorption at 30 MHz was 0.04 dB with a delay of 3 ± 2 s relative to the whistler. 相似文献
14.
We have taken a plasma model of ionosphere which is quite different from the earlier model employed by several workers to study the whistler as a diagnostic technique in the ionosphere. In contrast to the earlier model, we have considered an ionosphere that includes negative ionic species and due to which the mathematical technique loses the basic assumptions chosen earlier to derive the group travel time. We have shown that the negative ion has a significant role and without its contribution, the method to diagnose the ionospheric parameters may lead to an error. We have discussed also how to uset()=
0
h
dh/v
g
as a diagnostic tool in determining the ionospheric parameters. 相似文献
15.
Observations of whistlers during quiet times made at low-latitude ground station Nainital (geomag. lat. 19 1 N) are used to deduce plasmasphere-ionosphere coupling fluxes. The whistler data from 3 magnetically quiet days are presented that show a smooth decrease in dispersion with time. This decrease in dispersion is interpreted in terms of a corresponding decrease in electron content of tubes of ionization. The electron densities, electron tube contents (1016 el/m2-tube) and coupling fluxes (10 el m–1 s–2) are computed by means of an accurate curve fitting method developed by Tarcsai (1975) and are in good agreement with the results reported by other workers. 相似文献
16.
We have considered an ionospheric plasma model that includes the thermal effect along with the newly born ionic effect and derived a group travel time for the low-frequency whistlers with a view to employing it as a diagnostic tool in the ionosphere. The mathematical development shows that the thermal effect contribution varies with (
i
– )–7/2 whereas that of the newly born ionic effect varies with
i
– )–5/2. Both the effects are discussed separately. It is concluded that the effects are reasonably countable in the ionosphere. The investigations finally conclude that both the effects should be taken into the whistler waves, otherwise the method might cause a discrepancy in the results, which could affect their accuracy. 相似文献
17.
The elements of dielectric tensor and dispersion relation for obliquely propagating whistler waves with finite in an infinite magnetoplasma are obtained for a kappa distribution in the presence of perpendicular a.c. electric field.
Integrals and modified plasma dispersion functions are reduced in power series form. Numerical calculations have been performed
to obtain temporal growth rate and real frequencies of the plasma waves for magnetospheric plasma, using linear theory of
dispersion relation. The effect and modification introduced by the perpendicular a.c. electric field on the temporal growth
rates, real frequencies and resonance condition are discussed for kappa and Maxwellian distributions. Our results and their
interpretation are compared with known whistler observations obtained by ground-based techniques and satellite observations. 相似文献
18.
19.
K. R. Sivaraman 《Solar physics》1973,29(2):333-340
An improved method is described for the measurement of both the solar radius and the height of the chromosphere in any desired wavelength. Possible sources of uncertainty are discussed and a comparison with other methods is made. The first results from the 1972 observing period are given: R = (960.24 ±0.16) for the continuum at 5011.5 Å and R = (966.9 ±0.4) for H ± 0.5 Å. This yields a mean height of Ha emission of (4900 ± 400) km. 相似文献
20.
An observation carried out with a balloon-borne detector of an additional flux of secondary X-rays (E 30 keV) at large depths in the atmosphere is described. This excess is attributed to the emission of very hard X-rays during the solar flare of August 7, 1972. The propagation in the atmosphere of the secondary photons resulting from their electromagnetic interactions in the air is computed by utilizing the Monte Carlo method. The computations agree with the observed flux when a very hard solar X-ray spectrum is assumed. 相似文献