地磁场模式对低纬哨声射线追迹计算的影响

考虑到低纬地磁场与偶极子磁场偏离较大, 本文用国际参考场(n=6)进行了低纬哨声射线追迹计算.结果表明, 在约9°N和12°N处有两个很窄的哨声波非导传播出口点, 相应射线起点都在雷电活动区.哨声波非导传播出口的截止纬度约在8—9°N处.计算结果也较满意地解释了低纬哨声色散值和f₀/F₂正相关的事实.      

磁纬10°以下哨声路径纬度和所需抬升因子积分中值的估算方法

本文采用国际地磁参考场(1980IGRF,n=8)磁力线经验公式和三层模式电离层,结合哨声波沿磁力线传播所需电子浓度的横向梯度,给出了磁纬10°以下哨声路径纬度和所需抬升因子积分中值的估算方法;从而得出:(1)哨声色散值D不仅与电离层N_mF2正相关,而且与h_mF2也有明显相关性.当路径纬度ф₉₀≤10.5°(IGRF,n=8,下同)时,D与h_mF2负相关;>12时,正相关;在10.5°—12°之间时,正或负相关取决于路径顶点高度与h_m之差值.估算表明,三亚(磁纬7.04°N)收到的哨声中路径纬度ф₉₀≤10.5。的占94.5%,D与h_mF2负相关,这与观测结果一致.(2)所需抬升因子积分中值一般在7—31%之间.      

磁暴期间甚低纬哨声的导管传播

对磁暴期间甚低纬哨声导管传播的机理进行了初步的讨论，根据暴时低纬哨声的特点，认为暴时出现的扩展哨、多跳回波是沿东向扰动电场导致的场列电离增强路径传播的。对在东向电场影响下低纬Ｆ层的数值模拟表明，东向电场导致的场列电离增强结构有利于哨声的捕获和传播。     

电离层离子丰度对哨声传播经度效应的影响

Iss-b卫星探测结果表明,|电离层离子丰度呈现经度变化.本文选取较真实的电离层模型参数,对130°E和114°E两个子午面内传播的低纬哨声进行了射线跟踪.我们发现,由于[O~+],[He~+]和[H~+]决定的电子浓度垂直梯度的经度变化,导致哨声传播特性的经度效应,使得130°E比114°E内传播的低纬哨声更好地满足透射条件:|μsinδ|≤1.与电子浓度的水平梯度的影响比较,离子丰度决定的电子浓度垂直梯度随经度的变化使哨声传播产生的经度效应明显得多.武昌(地理30.5°N,114.6°E;地磁19.3°N).和鹿儿岛(31.5°N,130.8°E;20.5°N)两台站观测到的哨声出现率的经度效应证实了本文的理论预言.     

合肥地区日落哨的分析

本文根据1986年初合肥地区(地理纬度31.73°N,经度117.15°E;地磁纬度20.60°N)哨声观测资料,分析和计算了日落哨导管的抬升因子、吸收系数和导管寿命.      

中性风和垂直漂移对低纬F区电离层影响初析

利用二维低纬电离层－等离子体层时变理论模式,模拟太阳活动高年春分条件下垂直漂移和中性风强度改变对低纬F区电离层参量的影响.模式在所考察的磁子午面内求解等离子体输运方程,给出离子浓度和速度随纬度、高度、地方时的变化.模式计算结果显示,调整垂直漂移和中性风强度对低纬F区电离层电子浓度的影响与电离层所处磁纬、垂直漂移和中性风作用时段等有关,呈现出一些新特点.结果对分析不同条件下垂直漂移和中性风对低纬F区电离层影响具有一定的指导意义.      

武昌哨声的传播特征及其与地磁活动的相关

本文利用武昌(磁纬19.2°N)连续三年的哨声观测资料,研究了武昌地区哨声传播的一般特征。着重对武昌哨声的传播路径进行了分析推断,并讨论了它们与地磁活动的相关特性。发现:武昌哨声的显著特征是色散与电离层f₀F₂有明显的正相关关系;其次常出现一类色散值稳定、出现率也很高的哨声。此外,对典型事件的分析及统计分析都表明武昌哨声出现率在地磁扰动时明显增高;哨声色散在磁暴急始后不久即有一个急剧的上升,上升幅度达18%至37%。      

中性风对夜间低纬电离层参量的影响

利用一个改进的二维低纬电离层理论模式研究夜间中性风对低纬电离层多量的作用．模式计算结果表明：夜间中性风对低纬F层的影响与纬度、地方时有关．夜间低纬F层峰值电子浓度和峰高及其随时间的变化受中性风的影响与传统观点有所差异．低纬F层参量不只依赖于中性风的方向、强度,还与中性风场空间分布及时间变化率有关．中性风对低纬F层的影响值得重新认识．     

低纬电离层f0F2夜间增强的模拟研究

以太阳活动低年冬季为代表,利用二维中低纬电离层理论模式模拟低纬电离层ｆ０Ｆ２夜间增强现象,探索在赤道异常驼峰纬度附近形成该现象的可能因素或物理机制,模式在给定磁子午面内解等离子体输运方程,给出电子及各离子的浓度,速度的时空分布。在所考察条件下,低纬驼峰纬度附近ｆ０Ｆ２模式值夜间出现了明显的增强特征。     

海南岛三亚哨声观测资料的初步分析

本文主要介绍1983年初在海南岛三亚观测哨声的部分结果.记录到大量哨声,其中包括短哨、多源哨和多路哨;约有10%的哨声下截频在0.8kc/s左右.经分析发现:(1)三亚哨声几乎全部出现在f₀F₂下降的过程中,哨声出现率的峰值在f₀f₂下降最快处,即电子浓度下降变化急剧处;(2)哨声活动和磁暴的关系与中高纬有所不同,存在明显的相反情况.      

One-to-one relationship between low latitude whistlers and conjugate source lightning discharges and their propagation characteristics

One-to-one relation with its causative lightning discharges and propagation features of night-time whistlers recorded at low-latitude station, Allahabad (geomag. lat. 16.05°N, L = 1.08), India, from continuous observations made during 1–7 April, 2011 have been studied. The whistler observations were made using the Automatic Whistler Detector (AWD) system and AWESOME VLF receiver. The causative lightning strikes of whistlers were checked in data provided by World-Wide Lightning Location Network (WWLLN). A total of 32 whistlers were observed out of which 23 were correlated with their causative lightnings in and around the conjugate location (geom. lat. 9.87°S) of Allahabad. A multi-flash whistler is also observed on 1 April with dispersions 15.3, 17.5 and 13.6 s^1/2. About 70% (23 out of 32) whistlers were correlated with the WWLLN detected causative lightnings in the conjugate region which supports the ducted mode of propagation at low latitude. The multi-flash and short whistlers also propagated most likely in the ducted mode to this station.     

无碰撞电流片哨声波不稳定性

通过数值求解文献[4]中物理模型A得到的一般形式的色散关系,讨论了无碰撞电流片低频波不稳定性问题.结果表明,哨声波能被无碰撞电流片直接激发.在中性片上(z/di=0),在较宽的波数范围内,斜哨声波是可以传播的,但它基本上是稳定的.在离子惯性区内(z/di<1,电子惯性区外),斜传播的哨声波是不稳定的.在离子惯性区边缘(z/di=1),斜传播的哨声波仍然是不稳定的,增长率更大,不稳定的波频率范围更高.此外,朝向中性片方向传播(kzdi<0)的哨声波比离开中性片方向传播(kzdi>0)的哨声波有更大的增长率.     

New aspects of whistler waves driven by an electron beam studied by a 3-D electromagnetic code

We have restudied electron beam driven whistler waves with a 3-D electromagnetic particle code. In the initialisation of the beam-plasma system, “quiet start” conditions were approached by including the poloidal magnetic field due to the current carried by beam electrons streaming along a background magnetic field. The simulation results show electromagnetic whistler wave emissions and electrostatic beam modes like those observed in the Spacelab 2 electron beam experiment. It has been suggested in the past that the spatial bunching of beam electrons associated with the beam mode may directly generate whistler waves. However, the simulation results indicate several inconsistencies with this picture: (1) the parallel (to the background magnetic field) wavelength of the whistler wave is longer than that of the beam instability, (2) the parallel phase velocity of the whistler wave is smaller than that of the beam mode, and (3) whistler waves continue to be generated even after the beam mode space charge modulation looses its coherence. The complex structure of the whistler waves in the vicinity of the beam suggest that the transverse motion (gyration) of the beam and background electrons is involved in the generation of the whistler waves.     

Pitch angle diffusion as seen by CRRES

Lifetimes and pitch angle distributions of radiation belt electrons injected into the slot region are determined from CRRES/MEA particle flux measurements. The dominant loss mechanism is presumed to be pitch angle scattering due to whistler waves, for which a theoretical formulation is available. The empirical lifetimes are compared to those from recent calculations, which rely on model whistler wave parameters. CRRES measurements of cyclotron-frequency electric fields are also presented and compared to the wave model.     

Simultaneous observations of some unusual whistlers and VLF hiss emission at a low L-value (L = 1.17)

The purpose of this paper is to describe some unusual whistlers (doublets and triplets) and VLF hiss emission recorded simultaneously on February 18, 1998 during nighttime at a low latitude Indian ground station Jammu (geomag. lat., 22° 26′ N; L = 1.17), and to make some discussions about their origin. The detailed structures of the observed VLF hiss emission clearly show that these emissions confined to a narrow continuous frequency band. Some times the frequency of hiss band oscillates and subsequently touches the upper edge of the first whistler component of the doublet. Detailed structures of the dynamic spectra of whistler and VLF hiss emission are briefly presented. From the dispersion analysis of the whistler doublets and triplets, it is found that the individual whistlers of the doublets and triplets simultaneously observed on the same day are one-hop whistlers having propagation path along higher and closely spaced L-values. Our result also shows that VLF hiss has been generated in the equatorial region of higher L-values. Generation and propagation mechanisms are briefly discussed.     

VLF Emissions observed at the low latitude Indian station Varanasi

VLF emissions such as hiss, chorus, pulsing hiss, triggered emissions observed at Varanasi (geom. lat. = 14° 55′ N, long. = 153° 55′ E) are reported. An attempt has been made to explain the dynamic spectra in terms of the generation mechanism and propagation through the inhomogeneous magnetoplasma. It is noted that the reported events have propagated along different L-values in the magnetosphere, although they have been recorded at the same station. The ULF waves propagating along geomagnetic field lines modulate the growth rate of VLF waves, which results in the pulsation of VLF hiss intensity. The pulsating growth rate has been computed.     

Rare observation of daytime whistlers at very low latitude (L = 1.08)

The source region and propagation mechanism of low latitude whistlers (Geomag. lat. <30°) have puzzled scientific community for last many decades. In view of recent reports, there is consensus on the source region of low latitude whistlers in the vicinity of the conjugate point. But the plausible conditions of ionospheric medium through which they travel are still uncertain. In addition to that, the whistlers in daytime are never observed at geomagnetic latitudes less than 20°. Here, for the first time, we present a rare observations of whistlers during sunlit hours from a very low-latitude station Allahabad (Geomag. Lat: 16.79°N, L = 1.08) in India on 04 February 2011. More than 90 whistlers are recorded during 1200–1300 UT during which the whole propagation path from lightning source region to whistler observation site is under sunlit. The favorable factors that facilitated the whistlers prior to the sunset are investigated in terms of source lightning characteristics, geomagnetic and background ionospheric medium conditions. The whistler activity period was found to be geomagnetically quiet. However, a significant suppression in ionospheric total electron content (TEC) compared to its quiet day average is found. This shows that background ionospheric conditions may play a key role in low latitude whistler propagation. This study reveals that whistlers can occur under sunlit hours at latitudes as low as L = 1.08 when the source lightning and ionospheric medium characteristics are optimally favorable.     

Injection mechanism for cosmic ray acceleration

The quasilinear theory of MHD waves excitation by cosmic rays accelerated at a front of supernova shock has been constructed. It is shown that the energetic particles excite the waves propagating from the shock front, the intensity and the spectrum of these waves is obtained. The role of nonlinear Landau damping in the formation of such spectrum has been analysed. The diffusive scattering length of the high energy particles in the preshock region has been calculated and it is shown with the help of these formulae that the effective Fermi acceleration at the shock front is possible upto the values of the relativistic factor = 10⁴ - 10⁵. The injection mechanism for cosmic rays acceleration has been proposed. It is based on stochastic Fermi acceleration of the thermal plasma by MHD waves excited in the preshock region. Different possibilities for wave phase velocity dispersion needed for stochastic Fermi acceleration are analysed, those are the excitation of the oblique magnetosonic waves as well as the excitation of parallel Alfven waves propagating in opposite directions. The distribution function of the suprathermal particles accelerated by MHD waves is obtained, the cosmic rays density as well as the lower boundary of their energy spectrum realised in the proposed mechanism are also calculated.