高纬电离层气候学特征研究——EISCAT雷达观测及与IRI模式的比较

利用1988～1999年欧洲非相干散射EISCAT（European Incoherent Scatter）雷达观测数据,对不同太阳活动周相、不同季节的极光椭圆区电离层F区电子密度进行统计分析,研究其气候学特征,并与IRI-2001模式比较.EISCAT观测到的电子密度显示出显著的太阳活动高年“冬季异常”和太阳活动低年半年变化等现象.EISCAT实测电子密度随时间和高度的平均二维分布和500 km高度以下总电子含量TEC,从总体来看与IRI-2001模式预测结果符合较好.但高年在TEC达到最大值前后,IRI-2001模式预测的电子密度高度剖面与EISCAT观测结果有显著差别：F2峰以上IRI-2001模式预测的电子密度过大,造成TEC明显高于雷达观测值.另外,在太阳活动下降相,EISCAT观测显示出明显的半年周期季节变化特征,但IRI-2001模式未能预测出此下降相季节变化.     

The synthesis of travelling ionospheric disturbance (TID) signatures in HF radar observations using ray tracing

Characteristic signatures are often observed in HF radar range-time-intensity plots when travelling ionospheric disturbances (TIDs) are present. These signatures, in particular the variation of the F-region skip distance, have been synthesised using a ray tracing model. The magnitude of the skip variation is found to be a function of the peak electron density perturbation associated with the TID and radar frequency. Examination of experimental observations leads to an estimate of the peak electron density perturbation amplitude of around 25% for those TIDs observed by the CUTLASS radar system. The advantage of using the skip variation over the radar return amplitude as an indicator of density perturbation is also discussed. An example of a dual radar frequency experiment has been given. The investigation of the effect of radar frequency on the observations will aid the optimisation of future experiments.     

利用射线追踪方法研究化学物质释放对高频加热的聚焦效应

电离层化学物质释放能导致释放区域电子密度的损耗,从而产生明显的电离层空洞现象.高频电波通过电离层空洞时,由于电子密度不同,对电波产生折射效果进而形成聚焦效应.本文利用射线追踪方法,评估高频电波通过空洞形成聚焦加热的效果.结果表明,释放水分子与SF6都能对电离层产生明显的空洞,空洞半径约为25~50 km,电子密度的损耗率10%以上,在释放点附近有时能达到90%.不同摩尔数的化学物质释放能带来不同的效果.在300 km高度释放100 mol的水分子并且高频电波频率为15 MHz时聚焦效果最好.聚焦后电波能量吸收可增加10 dB左右,增幅达两倍左右.加热后的温度可以提升约20%.     

Nonlinear model of short-scale electrodynamics in the auroral ionosphere

The optical detection of auroral subarcs a few tens of m wide as well as the direct observation of shears several m/s per m over km to sub km scales by rocket instrumentation both indicate that violent and highly localized electrodynamics can occur at times in the auroral ionosphere over scales 100 m or less in width. These observations as well as the detection of unstable ion-acoustic waves observed by incoherent radars along the geomagnetic field lines has motivated us to develop a detailed time-dependent two-dimensional model of short-scale auroral electrodynamics that uses current continuity, Ohms law, and 8-moment transport equations for the ions and electrons in the presence of large ambient electric fields to describe wide auroral arcs with sharp edges in response to sharp cut-offs in precipitation (even though it may be possible to describe thin arcs and ultra-thin arcs with our model, we have left such a study for future work). We present the essential elements of this new model and illustrate the models usefulness with a sample run for which the ambient electric field is 100 mV/m away from the arc and for which electron precipitation cuts off over a region 100 m wide. The sample run demonstrates that parallel current densities of the order of several hundred A m^-2 can be triggered in these circumstances, together with shears several m/s per m in magnitude and parallel electric fields of the order of 0.1 mV/m around 130 km altitude. It also illustrates that the local ionospheric properties like densities, temperature and composition can strongly be affected by the violent localized electrodynamics and vice-versa.     

Longitudinal (UT) effect in the onset of auroral disturbances over two solar cycles as deduced from the AE-index

Statistical study on the universal time variations in the mean hourly auroral electrojet index (AE-index) has been undertaken for a 21 y period over two solar cycles (1957–1968 and 1978–1986). The analysis, applied to isolated auroral substorm onsets (inferred from rapid variations in the AE-index) and to the bulk of the AE data, indicates that the maximum in auroral activity is largely confined to 09–18 UT, with a distinct minimum at 03–06 UT. The diurnal effect was clearly present throughout all seasons in the first cycle but was mainly limited to northern winter in the second cycle. Severe storms (AE > 1000 nT) tended to occur between 9–18 UT irrespective of the seasons whereas all larger magnetic disturbances (AE > 500 nT) tended to occur in this time interval mostly in winter. On the whole the diurnal trend was strong in winter, intermediate at equinox and weak in summer. The implication of this study is that Eastern Siberia, Japan and Australia are mostly at night, during the period of maximum auroral activity whereas Europe and Eastern America are then mostly at daytime. The minimum of auroral activity coincides with near-midnight conditions in Eastern America. It appears that the diurnal UT distribution in the AE-index reflects a diurnal change between interplanetary magnetic field orientation and the Earths magnetic dipole inclination.     

Imaging of structures in the high-latitude ionosphere: model comparisons

The tomographic reconstruction technique generates a two-dimensional latitude versus height electron density distribution from sets of slant total electron content measurements (TEC) along ray paths between beacon satellites and ground-based radio receivers. In this note, the technique is applied to TEC values obtained from data simulated by the Sheffield/UCL/SEL Coupled Thermosphere/Ionosphere/Model (CTIM). A comparison of the resulting reconstructed image with the input modelled data allows for verification of the reconstruction technique. All the features of the high-latitude ionosphere in the model data are reproduced well in the tomographic image. Reconstructed vertical TEC values follow closely the modelled values, with the F-layer maximum density (NmF2) agreeing generally within about 10%. The method has also been able successfully to reproduce underlying auroral-E ionisation over a restricted latitudinal range in part of the image. The height of the F2 peak is generally in agreement to within about the vertical image resolution (25 km).     

Combined ESR and EISCAT observations of the dayside polar cap and auroral oval during the May 15, 1997 storm

The high-latitude ionospheric response to a major magnetic storm on May 15, 1997 is studied and different responses in the polar cap and the auroral oval are highlighted. Depletion of the F₂ region electron density occurred in both the polar cap and the auroral zone, but due to different physical processes. The increased recombination rate of O⁺ ions caused by a strong electric field played a crucial role in the auroral zone. The transport effect, however, especially the strong upward ion flow was also of great importance in the dayside polar cap. During the main phase and the beginning of the recovery phase soft particle precipitation in the polar cap showed a clear relation to the dynamic pressure of the solar wind, with a maximum cross-correlation coefficient of 0.63 at a time lag of 5 min.     

Observations of thermospheric neutral winds within the polar cusp and the auroral oval using a Doppler imaging system (DIS)

Two Doppler imaging systems (DIS) or wide-field imaging Fabry-Perot interferometers (FPI), have recently been commissioned, one at the Auroral Station, Adventdalen, Longyearbyen, Svalbard, and the second at the IRF, Kiruna, Sweden. These instruments can provide wide-field (600 * 800 km) images of neutral wind flows in the upper thermosphere, by measuring the Doppler shift of the atomic oxygen forbidden near 630 nm, which is emitted from an altitude of approximately 240 km. From the instrument in Svalbard, at mid-winter, it is possible to observe the dayside polar cusp and the polar cap throughout the entire day, whereas from Kiruna, the night-time auroral oval is observable during the hours of darkness. Measurements of thermospheric dynamics from the DIS can be used in conjunction with observations of ionospheric plasma flows and thermal plasma densities by the EISCAT-Svalbard radar (ESR) and by EISCAT, along with other complementary observations by co-located instruments such as the auroral large-scale imaging system (ALIS). Such combined data sets will allow a wide range of scientific studies to be performed concerning the dynamical response of the thermosphere and ionosphere, and the important energetic and momentum exchange processes resulting from their complex interactions. These processes are particularly important in the immediate vicinity of the polar cusp and within the auroral oval. Early results from Svalbard in late 1995 will be discussed. The DIS in Kiruna observed two interesting geomagnetic disturbances in early 1997, the minor geomagnetic storm of 10, 11 January, and the disturbed period from 7–10 February. During these events, the thermospheric wind response showed some interesting departures from the average behaviour, which we attribute to the result of strong and variable Lorenz forcing (ion drag) and Joule and particle heating during these geomagnetic disturbances.     

Solar-cycle variation of the daily foF2 and M(3000)F2

Daily values of the ionospheric characteristics foF2 and M(3000)F2 for a given hour and month are correlated with the corresponding daily values of sunspot number using measured data collected at seven European locations. The significance of applying different-order polynomials is considered and the times are confirmed when the higher-order terms are important. Mean correlation coefficients for combined data sets over all hours, months and stations are determined, together with the standard errors of estimates. Comparisons are made with corresponding figures for monthly median values derived from the same data sets.     

Comparison of the orientation of small-scale electron density irregularities and F region plasma flow direction

Results are shown from an experimental campaign where satellite scintillation was observed at three sites at high latitudes and, simultaneously, the F region plasma flow was measured by the nearby EISCAT incoherent scatter radar. The anisotropy parameters of field-aligned irregularities are determined from amplitude scintillation using a method based on the variance of the relative logarithmic amplitude. The orientation of the anisotropy in a plane perpendicular to the geomagnetic field is compared with the direction of F region plasma flow. The results indicate that in most cases a good agreement between the two directions is obtained.     

Iterative deconvolution of teleseismic P waves from the Thessaloniki (N. Greece) earthquake of June 20, 1978

Teleseismic long-period P waves from the June 20, 1978, Thessaloniki (N. Greece) earthquake (M _s=6.4) were modeled in an attempt to extract information about asperities or barriers on the fault plane. The analysis is based on the inversion method of complex P waves developed by Kikuchi and Kanamori (1982). A far-field source time function with a rise time of 2 sec and a process time of 5 sec is inferred, corresponding to a source dimension of about 10 km when a rupture velocity of 2 km/sec is assumed.The source depth of this shock, estimated by matching synthetic seismograms to observations, is found to be 8 km. The sum of the seismic moments of the individual subevents amounts to 3.3×10²⁵ dyn-cm.     

On the usefulness of E region electron temperatures and lower F region ion temperatures for the extraction of thermospheric parameters: a case study

Using EISCAT data, we have studied the behavior of the E region electron temperature and of the lower F region ion temperature during a period that was particularly active geomagnetically. We have found that the E region electron temperatures responded quite predictably to the effective electric field. For this reason, the E region electron temperature correlated well with the lower F region ion temperature. However, there were several instances during the period under study when the magnitude of the E region electron temperature response was much larger than expected from the ion temperature observations at higher altitudes. We discovered that these instances were related to very strong neutral winds in the 110–175 km altitude region. In one instance that was scrutinized in detail using E region ion drift measurement in conjunction with the temperature observations, we uncovered that, as suspected, the wind was moving in a direction closely matching that of the ions, strongly suggesting that ion drag was at work. In this particular instance the wind reached a magnitude of the order of 350 m/s at 115 km and of at least 750 m/s at 160 km altitude. Curiously enough, there was no indication of strong upper F region neutral winds at the time; this might have been because the event was uncovered around noon, at a time when, in the F region, the E × B drift was strongly westward but the pressure gradients strongly northward in the F region. Our study indicates that both the lower F region ion temperatures and the E region electron temperatures can be used to extract useful geophysical parameters such as the neutral density (through a determination of ion-neutral collision frequencies) and Joule heating rates (through the direct connection that we have confirmed exists between temperatures and the effective electric field).     

The relationship between electric fields, conductances and currents in the high-latitude ionosphere: a statistical study using EISCAT data

The relationship between electric fields, height-integrated conductivities and electric currents in the high-latitude nightside electrojet region is known to be complex. The tristatic nature of the EISCAT UHF radar facility provides an excellent means of exploring this interrelationship as it enables simultaneous estimates to be made of the full electric field vector and the ionospheric Hall and Pedersen conductances, further allowing the determination of both field-perpendicular electric current components. Over 1300 h of common programme observations by the UHF radar system provide the basis of a statistical study of electric fields, conductances and currents in the high-latitude ionosphere, from which preliminary results are presented. Times at which there is significant solar contribution to the ionospheric conductances have been excluded by limiting the observations according to solar zenith angle. Initial results indicate that, in general, the times of peak conductance, identified from the entire set of EISCAT observations, do not correspond to the times of the largest electric field values; the relative contribution of ionospheric conductance and electric field to the electrojet currents therefore depends critically on local time, a conclusion which corroborates work by previous authors. Simultaneous measurements confirm a tendency for a decrease in both Hall and Pedersen conductances to be accompanied by an increase in the electric field, at least for moderate and large electric field value, a tendency which is also identified to some extent in the ratio of the conductances, which acts as an indicator of the energy of precipitating particles.     

Trends in the characteristics of the annual and semiannual variations observed in the radio wave absorption in the lower ionosphere

The continuous increase in concentration of greenhouse gases in the atmosphere is expected to cool higher levels of the atmosphere. There is some direct and indirect experimental evidence of long-term trends in temperature and other parameters in the mesosphere and lower thermosphere (MLT). Here we look for long-term trends in the annual and semiannual variations of the radio wave absorption in the lower ionosphere, which corresponds to the MLT region heights. Data from central and southeastern Europe are used. A consistent tendency to a positive trend in the amplitude of the semiannual wave appears to be observed. The reality of a similar tendency in the amplitude of the annual wave is questionable in the sense that the trend in the amplitude of the annual wave is probably induced by the trend in the yearly average values of absorption. The phases of both the annual and semiannual waves display a forward tendency, i.e. shift to an earlier time in the year. A tentative interpretation of these results in terms of changes of the seasonal variation of temperature and wind at MLT heights does not contradict the trends observed in those parameters.     

磁正午附近极光强度与沉降粒子能量关系的参数模型

极光是日地能量耦合过程中粒子沉降到极区电离层的最直观表现,对于理解地球空间环境及预测空间天气具有重要作用.本文利用2003—2009年的北极黄河站的多波段地面极光观测,结合DMSP卫星粒子沉降探测,对磁正午附近的极光强度与沉降粒子沉降能量之间的关系进行了定量研究.统计结果表明,在10—13磁地方时(MLT)630.0 nm的极光发光占主导,以低能粒子沉降为主;而在13—14MLT,630.0 nm/427.8 nm极光强度比值降低,沉降粒子能量较高.另外,利用极光强度与沉降电子的能通量以及极光强度比值与平均能量之间的函数关系,初步建立了北极黄河站磁正午附近极光强度与沉降粒子能量关系的反演参数模型,为将来空间天气的监测服务.     

The size of the auroral belt during magnetic storms

Using the auroral boundary index derived from DMSP electron precipitation data and the Dst index, changes in the size of the auroral belt during magnetic storms are studied. It is found that the equatorward boundary of the belt at midnight expands equatorward, reaching its lowest latitude about one hour before Dst peaks. This time lag depends very little on storm intensity. It is also shown that during magnetic storms, the energy of the ring current quantified with Dst increases in proportion to L_e^–3, where L_e is the L-value corresponding to the equatorward boundary of the auroral belt designated by the auroral boundary index. This means that the ring current energy is proportional to the ion energy obtained from the earthward shift of the plasma sheet under the conservation of the first adiabatic invariant. The ring current energy is also pronortional to E_mag, the total magnetic field energy contained in the spherical shell bounded by L_e and L_eq, where L_eq corresponds to the quiet-time location of the auroral precipitation boundary. The ratio of the ring current energy E_R to the dipole energy E_mag is typically 10%. The ring current leads to magnetosphere inflation as a result of an increase in the equivalent dipole moment.     

ESR and EISCAT observations of the response of the cusp and cleft to IMF orientation changes

We report observations of the cusp/cleft ionosphere made on December 16th 1998 by the EISCAT (European incoherent scatter) VHF radar at Troms and the EISCAT Svalbard radar (ESR). We compare them with observations of the dayside auroral luminosity, as seen by meridian scanning photometers at Ny Ålesund and of HF radar backscatter, as observed by the CUTLASS radar. We study the response to an interval of about one hour when the interplanetary magnetic field (IMF), monitored by the WIND and ACE spacecraft, was southward. The cusp/cleft aurora is shown to correspond to a spatially extended region of elevated electron temperatures in the VHF radar data. Initial conditions were characterised by a northward-directed IMF and cusp/cleft aurora poleward of the ESR. A strong southward turning then occurred, causing an equatorward motion of the cusp/cleft aurora. Within the equatorward expanding, southward-IMF cusp/cleft, the ESR observed structured and elevated plasma densities and ion and electron temperatures. Cleft ion fountain upflows were seen in association with elevated ion temperatures and rapid eastward convection, consistent with the magnetic curvature force on newly opened field lines for the observed negative IMF B_y. Subsequently, the ESR beam remained immediately poleward of the main cusp/cleft and a sequence of poleward-moving auroral transients passed over it. After the last of these, the ESR was in the polar cap and the radar observations were characterised by extremely low ionospheric densities and downward field-aligned flows. The IMF then turned northward again and the auroral oval contracted such that the ESR moved back into the cusp/cleft region. For the poleward-retreating, northward-IMF cusp/cleft, the convection flows were slower, upflows were weaker and the electron density and temperature enhancements were less structured. Following the northward turning, the bands of high electron temperature and cusp/cleft aurora bifurcated, consistent with both subsolar and lobe reconnection taking place simultaneously. The present paper describes the large-scale behaviour of the ionosphere during this interval, as observed by a powerful combination of instruments. Two companion papers, by Lockwood et al. (2000) and Thorolfsson et al. (2000), both in this issue, describe the detailed behaviour of the poleward-moving transients observed during the interval of southward B_z, and explain their morphology in the context of previous theoretical work.     

Combined CUTLASS, EISCAT and ESR observations of ionospheric plasma flows at the onset of an isolated substorm

On August 21st 1998, a sharp southward turning of the IMF, following on from a 20 h period of northward directed magnetic field, resulted in an isolated substorm over northern Scandinavia and Svalbard. A combination of high time resolution and large spatial scale measurements from an array of coherent scatter and incoherent scatter ionospheric radars, ground magnetometers and the Polar UVI imager has allowed the electrodynamics of the impulsive substorm electrojet region during its first few minutes of evolution at the expansion phase onset to be studied in great detail. At the expansion phase onset the substorm onset region is characterised by a strong enhancement of the electron temperature and UV aurora. This poleward expanding auroral structure moves initially at 0.9 km s^-1 poleward, finally reaching a latitude of 72.5°. The optical signature expands rapidly westwards at ~6 km s^-1, whilst the eastward edge also expands eastward at ~0.6 km s^-1. Typical flows of 600 m s^-1 and conductances of 2 S were measured before the auroral activation, which rapidly changed to ~100 m s^-1 and 10–20 S respectively at activation. The initial flow response to the substorm expansion phase onset is a flow suppression, observed up to some 300 km poleward of the initial region of auroral luminosity, imposed over a time scale of less than 10 s. The high conductivity region of the electrojet acts as an obstacle to the flow, resulting in a region of low-electric field, but also low conductivity poleward of the high-conductivity region. Rapid flows are observed at the edge of the high-conductivity region, and subsequently the high flow region develops, flowing around the expanding auroral feature in a direction determined by the flow pattern prevailing before the substorm intensification. The enhanced electron temperatures associated with the substorm-disturbed region extended some 2° further poleward than the UV auroral signature associated with it.     

The generation of non aspect sensitive plasma density irregularities by field aligned drifts in the lower ionosphere

A theory of the generation of plasma density irregularities with virtually no aspect sensitivity, in the lower ionosphere at high latitudes, by electron drifts aligned with the geomagnetic field, is presented. The theory is developed through fluid equations in which the destabilising mechanism involves positive feedback from electron collisional heating. When field aligned electron drift speeds exceed a few km s^–1, this effect destabilises waves with wavelengths in excess of a few tens of metres in the lower E-region, where collisional effects are sufficiently large. Furthermore, the threshold conditions are almost independent of the wave propagation direction and the unstable waves propagate at speeds well below the ion acoustic speed. The role that this new instability may play in recent radar backscatter observations of short scale irregularities propagating in directions close to that of the geomagnetic field, in the lower E-region is also considered.