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1.
In this study over 100 Pc5 events observed on the SAMNET magnetometer array have been catalogued. The ground horizontal polarization of these waves has been investigated and it has been shown that there are large differences between the polarization azimuth distributions on either side of noon. In the morning the azimuth distributions are spread evenly across all possible orientations whereas in the afternoon they are predominantly in a North-South orientation. The difference appears to be the result of a specific polarization pattern that occurred across the SAMNET array for 31 of the Pc5 events. These 31 Pc5 events all occurred in the morning sector, close to the dawn terminator. It is suggested that this special polarization pattern is a consequence of ionospheric conductivity gradients that occur at around dawn. This theory is reinforced by the comparison of the local times of these polarization variations with the local time of sunrise. By employing ionospheric conductivity data from the EISCAT radar for two of these Pc5 events, this connection has been studied in more detail.  相似文献   

2.
Quasi-periodic fluctuations in the returned ground-scatter power from the SuperDARN HF radars have been linked to the passage of medium-scale gravity waves. We have applied a technique that extracts the first radar range returns from the F-region to study the spatial extent and characteristics of these waves in the CUTLASS field-of-view. Some ray tracing was carried out to test the applicability of this method. The EISCAT radar facility at Tromsø is well within the CUTLASS field-of-view for these waves and provides a unique opportunity to assess independently the ability of the HF radars to derive gravity wave information. Results from 1st March, 1995, where the EISCAT UHF radar was operating in its CP-1 mode, demonstrate that the radars were in good agreement, especially if one selects the electron density variations measured by EISCAT at around 235 km. CUTLASS and EISCAT gravity wave observations complement each other; the former extends the spatial field of view considerably, whilst the latter provides detailed vertical information about a range of ionospheric parameters.  相似文献   

3.
We analyse a Pc1 pearl event observed by the Finnish search-coil magnetometer network on 15 December 1984, which subsequently developed into a structured IPDP after a substorm onset. The EISCAT radar was simultaneously monitoring the mid- to high-latitude ionosphere. We have calculated the ionospheric resonator properties during the different phases of the event using EISCAT observations. Contrary to the earlier results, we find that the Pc1/IPDP (Interval of Pulsations of Diminishing Period) frequency observed on the ground corresponds to the maximum of the transmission coefficient rather than that of the reflection coefficient. This casts strong doubts on the bouncing wave packet model of Pc1 pearls. Instead, we present evidence for an alternative model of pearl formation in which long-period ULF waves modulate the Pc1 growth rate. Moreover, we propose a new model for IPDP formation, whereby the ionosphere acts as an active agent in forming the IPDP signal on the ground. The model calculations show that the ionospheric resonator properties can be modified during the event so that the resonator eigenfrequency increases according to the observed frequency increase during the IPDP phase. We suggest that the IPDP signal on the ground is a combined effect of the frequency increase in the magnetospheric wave source and the simultaneous increase of the resonator eigenfrequency. The need for such a complicated matching of the two factors explains the rarity of IPDPs on the ground despite the ubiquitous occurrence of EMIC waves in the magnetosphere and the continuous substorm cycle.  相似文献   

4.
The ionospheric electron gas can be heated artificially by a powerful radio wave. According to our modeling, the maximum effect of this heating occurs in the D-region where the electron temperature can increase by a factor of ten. Ionospheric plasma parameters such as Ne,Te and Ti are measured by EISCAT incoherent scatter radar on a routine basis. However, in the D-region the incoherent scatter echo is very weak because of the low electron density. Moreover, the incoherent scatter spectrum from the D-region is of Lorentzian shape which gives less information than the spectrum from the E- and F-regions. These make EISCAT measurements in the D-region difficult. A combined EISCAT VHF-radar and heating experiment was carried out in November 1998 with the aim to measure the electron temperature increase due to heating. In the experiment the heater was switched on/off at 5 minute intervals and the integration time of the radar was chosen synchronously with the heating cycle. A systematic difference in the measured autocorrelation functions was found between heated and unheated periods.  相似文献   

5.
Experiments on the generation of artificial electromagnetic pulsations constitute an important part of investigations of the magnetosphere-ionosphere system with the use of an active action. The investigation of the generation of magnetic pulsations in the Pc1 frequency range has shown that the response of the ionosphere to heating is detected only in a few experiments. Although the primary perturbed parameter is the electron temperature, the efficiency of the generation of pulsations is determined by the perturbations of the ionospheric conductivity. The magnitude of these hertz perturbations depends complexly on the electron density profile and the parameters of a pump wave. The numerical experiment demonstrates the determining effect of the electron density in the D region on the magnitude of perturbations of the ionospheric conductivity. Under conditions of a low electron density, it is impossible to create a large perturbation of the conductivity in the Pc1 frequency range, although perturbations of the electron temperature can be large in this case. In view of a large number of electrons at altitudes of 70–90 km, which absorb a considerable fraction of the energy of a high-frequency wave, the electron temperature in the E region of the ionosphere cannot be sharply increased, but the amplitude of the variations of the ionospheric conductivity in this case is larger than that for the profiles with a low electron density. In the presence of the developed D region, the efficiency of the modification of the conductivity in the indicated frequency range can be increased by choosing the optimal frequency and polarization of the pump wave. A low efficiency of the experiments on the generation of artificial magnetic pulsations in the Pc1 frequency range is apparently explained by the fact that they were performed in winter in the absence of a well-developed D region of the ionosphere.  相似文献   

6.
2007年3月3日位于磁层昏侧THEMIS的5颗卫星、同步轨道晨侧和午前的GOES 3颗卫星和地面地磁台站同时观测到了持续近4 h的Pc5 ULF波.我们用交叉小波相关分析计算脉动的传播速度,用MVA分析求解脉动的传播方向,然后结合两者的计算结果获得了Pc5相速度矢量信息.THEMIS卫星观测到Pc5具有压缩特性,且向阳传播,速度约在6~20 km/s左右,相比于磁层中阿尔芬速度(1000 km/s)较低.这些Pc5 ULF波动可能产生于磁尾或磁层内部不稳定性.GOES 3颗卫星观测到不同情况的Pc5 ULF波,极向模占主要成分,且具有波包结构,具有阿尔芬驻波特性,可能产生于K-H(Kelvin-Helmholtz)不稳定性.地面台站观测到ULF波扰动幅度随纬度升高而增强,Pc5脉动在地理纬度60°附近达到最大值, Dumont durville台站观测到的脉动与THEMIS观测到波形有很好的相似性.  相似文献   

7.
It is well known that the ionospheric plasma response to high-power HF radio waves changes drastically as the heater frequency approaches harmonics of the electron gyrofrequency. These include changes in the spectrum of the stimulated electromagnetic emission, reduction in the anomalous absorption of low-power diagnostic waves propagating through the heated volume, and reduction in the large scale F-region heating. Theoretical models as well as previous experimental evidence point towards the absence of small-scale field-aligned plasma density irregularities at pump frequencies close to electron gyroharmonics as the main cause of these changes. Results presented in this paper are the first direct observations of the reduced striations at the 3rd gyroharmonic made by the CUTLASS radar. In addition, simultaneous EISCAT observations have revealed that the “enhanced ion-line” usually present in the EISCAT ion-line spectrum during the first few seconds after heater switch on, persisted at varying strengths while the heater was transmitting at frequencies close to the 3rd electron gyroharmonics.  相似文献   

8.
Highlights of studies of ULF waves from 1995 to early 1997 are presented. The subjects covered include (1) Pc 3–5 waves excited by sources in the solar wind, with emphasis on the role of the magnetospheric cavity in modifying the external source and establishing its own resonances, and the role of the plasmapause in magnetohydrodynamic wave propagation; (2) Pi 2 waves, with emphasis on the plasmaspheric resonances and possible alternative excitation by plasmasheet source waves; (3) the spatial structure of internally excited long-period waves, including a kinetic theory for radially confined ring current instability and groundbased multipoint observation of giant pulsations; (4) amplitude-modulated Pc 1–2 waves in the outer magnetosphere (Pc 1–2 bursts) and in the inner magnetosphere (structured Pc 1 waves or pearls); and (5) the source region of the quasi-periodic emissions. Theory and observations are compared, and controversial issues are highlighted. In addition, some future directions are suggested.  相似文献   

9.
This paper is devoted to the beautiful and mysterious Pc1 geoelectromagnetic waves that attract the attention of many researchers all over the world as an essential element of the space physics. It is concluded that in spite of the recent progress in the investigation of Pc1 waves, it is still necessary to make a careful study of some unsettled problems posed in the past. Relevant problems and disputable issues of the physics of Pc1 waves are discussed in this paper. The discussion is started with the paradoxical dependence of the Pc1 wave activity on the solar wind plasma density, which is considered as the key problem. It is argued that the solution of this paradox is of paramount importance to understand the 11-year solar cycle variation of the Pc1 occurrence rate, interplay between Pc1 and oxygen ions in the magnetosphere, impact of interplanetary magnetic field sector boundaries on the Pc1 wave activity, and other similar problems. A schematic picture showing the place of Pc1 waves in the system of solar–terrestrial relations is presented.  相似文献   

10.
The variations in the daily average energy of geomagnetic pulsations and noise in the Pc3 (20–60 mHz) and Pc4 (10–19 mHz) frequency bands in the polar cap have been studied based on the data from P5 Antarctic station (corrected geomagnetic latitude ?87°) from November 1998 to November 1999. The daily average pulsation energy has been calculated using the method for detecting the wave packets, the spectral amplitude of which is higher than the threshold level, from the dynamic spectrum. A spectral analysis of the energy of pulsations and noise in the Pc3 and Pc4 bands, performed using the maximal entropy method, has revealed periodicities of 18 days in the local winter and 26, 13, and 7–9 days during the local summer. The simultaneous and coherent variations with periods of 26, 13, and 7–9 days in the solar wind velocity and IMF orientation indicate that the variations in the Pc3–4 wave energy in the polar cap at a sunlit ionosphere are mainly controlled by the parameters of the interplanetary medium. The variations in the Pc3–4 wave energy with a period of 18 days are observed only during the local winter and are supposedly related to the variations in the ionospheric conductivity modulated by planetary waves.  相似文献   

11.
We have investigated ion outflows observed by the Akebono satellite and the EISCAT radar in the nightside auroral region on February 16, 1993. The Akebono satellite at about 7000 km altitude observed the region of suprathermal ion outflows and inverted-V type electron precipitation alternately with a horizontal separation of 70–150 km at the ionospheric level. These two regions corresponded to the upward and downward field-aligned current region, respectively, and intense ELF waves were observed in the ion outflow region. From the EISCAT VHF radar observation (Common Program 7 mode), it has been suggested that the ion outflow region and the enhanced electron temperature region were aligned along geomagnetic field lines with vertical and horizontal separations of 200–400 and 70–80 km, respectively and these two regions convected equatorward across the EISCAT radar at Tromsø site. Based on these results, we propose a model for this ion outflow as follows. In the nightside auroral region, downward FAC regions exist near the edge of the inverted-V type electron precipitation regions. ELF waves are excited probably by a plasma instability due to the upward thermal electron beam carrying the downward FACs, and these ELF waves cause transverse ion heating at the top of the ionosphere. The produced ion conics contribute significantly to ion outflow.  相似文献   

12.
During the multiband wave Pc1 event on March 7, 2001 the EISCAT UHF and VHF incoherent scatter radars operated simultaneously covering an exceptionally wide altitude range of the ionosphere ～90—2000 km. This made possible to test the ionospheric Alfvén resonator (IAR) model over a large altitude range. The three lowest IAR eigenfrequencies, where the most of the Pc1 pulsation signal bands occur, were selected for the spatial analysis of the standing wave electromagnetic fields, applying the full-wave numerical simulation method. The altitude spread of amplitude maxima and nodes together with polarization characteristics of oscillation maxima in the horizontal plane are presented. The comparison of the standing wave oscillations on the altitude profile with the signal amplitude observed on the ground is also presented.  相似文献   

13.
Examples of long period Pc5 magnetic field pulsations near field-aligned current (FAC) regions in the high-latitude magnetosphere, observed by INTERBALL-Auroral satellite during January 11, April 11 and June 28, 1997 are shown. Identification of corresponding magnetosphere regions and subregions is provided by electrons and protons in the energy-range of 0.01–100 keV measured simultaneously onboard the spacecraft. The examined Pc5 pulsations reveal a compressional character. A fairly good correlation is demonstrated between these ULF Pc5 waves and the consecutive injection of magnetosheath low energy protons. The ULF Pc5 wave occurrence is observed in both upward and downward FACs.  相似文献   

14.
The results of magnetometric observations and radar studies of the lower ionosphere during the superstrong magnetic storm (geospace storm) of November 7–10, 2004, are presented and analyzed. An increase in the electron density and in the amplitudes of its aperiodic and quasi-periodic variations at high level of magnetic activity is detected. In individual time intervals, the relation of Pc5-6 magnetic pulsations to short-period (5–15 min) wave disturbances in the electron density in the lower ionosphere was observed.  相似文献   

15.
Early in 1996, the latest of the European inco-herent-scatter (EISCAT) radars came into operation on the Svalbard islands. The EISCAT Svalbard Radar (ESR) has been built in order to study the ionosphere in the northern polar cap and in particular, the dayside cusp. Conditions in the upper atmosphere in the cusp region are complex, with magnetosheath plasma cascading freely into the atmosphere along open magnetic field lines as a result of magnetic reconnection at the dayside magnetopause. A model has been developed to predict the effects of pulsed reconnection and the subsequent cusp precipitation in the ionosphere. Using this model we have successfully recreated some of the major features seen in photometer and satellite data within the cusp. In this paper, the work is extended to predict the signatures of pulsed reconnection in ESR data when the radar is pointed along the magnetic field. It is expected that enhancements in both electron concentration and electron temperature will be observed. Whether these enhancements are continuous in time or occur as a series of separate events is shown to depend critically on where the open/closed field-line boundary is with respect to the radar. This is shown to be particularly true when reconnection pulses are superposed on a steady background rate.  相似文献   

16.
A Pc1/IPDP event recorded by the Finnish search coil magnetometers on 15 December 1984 was analyzed in a companion paper (Mursula et al., 2000. Non-stationary Alfvén resonator: new results on Pc1 pearls and IPDP events. J. Atmos. Solar-Terr. Phys. 62(4), 299–309) using numerical simulations of the ionospheric Alfvén resonator (IAR). EISCAT incoherent scatter radar data were used to determine the vertical profiles of ionospheric plasma parameters. In this paper, the detailed altitude profiles of several wave characteristics at the IAR eigenfrequency are computed up to 1000 km height, including, e.g., the real normalized amplitude of the magnetic wave field component, ellipticity and orientation of the polarization ellipse in the horizontal plane. We also calculate the altitude profile of the energy flux density (Poynting vector). These features illustrate in detail the ionospheric effects on the wave spectral structure in a non-stationary IAR, and their significance in the formation of the Pc1/IPDP signal on the ground.  相似文献   

17.
Pc 1 pearls have been observed on the ground for about 70 years. During this time numerous publications have been written on the various properties of Pc 1 pearl waves, the related theory, and possible applications. Pc 1 waves with a clear pearl structure are only a fraction of all Pc 1 waves observed on ground, and this fraction depends on the latitude of observations, increasing from high to low latitudes. In fact, the spatial and temporal occurrence of Pc 1 pearls is closely connected with the location and development of the plasmapause. While it has been known roughly 40 years that Pc 1 waves are electromagnetic ion cyclotron waves generated by anisotropic, energetic ions in the near-equatorial magnetosphere, the formation of pearl structure is still largely in question. In situ observations of Pc 1 waves in the Earth's magnetosphere have been made since the 1970s by various satellites in different orbits. However, satellite observations of clear Pc 1 pearls are still rather few. Here we review a few crucial satellite-based observations of Pc 1 pearls, and evaluate their contribution to the understanding of pearl formation. We show that the long-held paradigm of the bouncing wave packet model is in serious contradiction with satellite observations and therefore outdated. Instead, observations support the idea that Pc 1 wave growth rate is successively modulated at the equator by long-period ULF waves.  相似文献   

18.
Backscatter from E-region irregularities was observed at aspect angles close to 90° (almost parallel to the direction of the magnetic field) using the ALOMAR SOUSY radar at Andoya/Norway. Strong electric fields and increased E-region electron temperatures simultaneously measured with the incoherent scatter facility EISCAT proved that the Farley-Buneman plasma instability was excited. In addition, strong particle precipitation was present as inferred from EISCAT electron densities indicating that the gradient drift instability may have been active, too. Backscatter at such large aspect angles was not expected and has not been observed before. The characteristics of the observed echoes, however, are in many aspects completely different from usual auroral radar results: the Doppler velocities are only of the order of 10 m/s, the half-width of the spectra is around 5 m/s, the echoes originate at altitudes well below 100 km, and they seem to be not aspect-sensitive with respect to the magnetic field direction. We, therefore, conclude that the corresponding irregularities are not caused by the mentioned instabilities and that other mechanism have to be invoked.  相似文献   

19.
Ionospheric heating experiments were done by the EISCAT Heater in Tromsø on 15–19 November, 1993. A low-light TV camera was installed at the VLF receiving station at Porojärvi about 100 km to the south-east of Tromsø. The spectral analysis of the auroral luminosity variations showed that the brightness of the aurora varied at the modulation frequency of the heating wave. The results of this analysis and the numerical simulations of the auroral luminosity variations caused by the HF heating are shown. The variations of the optical emission intensity at the heating frequency occur during the auroral ionosphere modification. The observed intensity variation of the auroral green line during the interval of enhanced electron temperature is explained by a decreasing rate of the O2+ ion dissociative recombination when the electron temperature increases. The brightness variation depends on the characteristic energy and the intensity of the auroral electron flux and the heating wave parameters. The artificial luminosity pulsations caused by HF heating are estimated.  相似文献   

20.
本文首先从电子密度及电子温度的输运方程和考虑自作用时的电磁波波动方程出发,利用简正模展开的方法推导出泵波在反射区域激发出热自聚焦不稳定性(thermal self-focusing instabilities,TSFI)所需电场阈值以及其增长率的完整数学表达式,并估算了TSFI激发阈值及所对应的有效辐射功率(ERP)的量级.随后利用三维垂直加热的理论模型,结合国际参考电离层(IRI-2012)和中性大气模型(MSIS-E-00)给出的背景参数,数值模拟了大功率高频泵波加热电离层时泵波反射区域电子密度及电子温度因TSFI而产生的变化及发展的过程,并对比分析了不同背景参数对较热效果的影响.结果表明:当高频泵波的加热阈值达到或超过百毫伏每米的量级时,即可激发TSFI,发展出大尺度电子密度及温度不均匀体,这些不均匀体内的密度耗空约为4%~10%,而电子温度剧烈增长,到达背景温度值的1.6~2.1倍;且在相当的加热条件下,背景电子温度越低、电子密度越小,加热效果越显著;电子密度及电子温度的扰动幅度随着加热时间的推移而逐渐减小,即扰动逐渐趋于饱和,且电子温度要快于电子密度达到饱和状态.本文还对泵波反射高度处的电子密度及电子温度变化率进行采样并求得其功率谱密度,分析结果表明:TSFI发展出的大尺度不均匀体满足幂律谱结构,谱指数随着加热的进行逐渐趋于稳定,白天与夜间的幂律谱指数区别不大,但电子密度与电子温度的幂律谱有所区别.  相似文献   

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