An analysis of the topside ionosphere parameters based on the long-duration Irkutsk incoherent scatter radar measurements

The topside ionosphere parameters are studied based on the long-duration Irkutsk incoherent scatter radar (52.9N, 103.3E) measurements conducted in September 2005, June and December 2007. As a topside ionosphere parameter we chose the vertical scale height (VSH) related to the gradient of the electron density logarithm above the peak height. For morphological studies we used median electron density profiles. Besides the median behavior we also studied VSH disturbances (deviations from median values) during the magnetic storm of September 11th 2005. We compared the Irkutsk incoherent scatter radar data with the Millstone Hill and Arecibo incoherent scatter radar observations, the IRI-2007 prediction (using the two topside options) and VSH derived from the Irkutsk DPS-4 Digisonde bottomside measurements.     

Electric field measurements at a southern mid-latitude station obtained using an HF digital ionosonde

It is important to understand the convection of the inner magnetosphere to fully describe the response of the low- to mid-latitude thermosphere-ionosphere system to geomagnetic storms. Realistic numerical simulations of mid-latitude electric fields suffer from limited knowledge of lower thermospheric winds and ionospheric conductivity on a global scale. Even empirical models of mid-latitude electric fields suffer from the paucity of measurements made by the handful of incoherent scatter radars concentrated in the American-European sector, and the intermittent satellite measurements made in other regions. Thus it would be very useful to show the extent to which Doppler velocity measurements made with the numerous digital ionosondes deployed around the globe can be used to infer F-region electric fields. The monthly average diurnal variation of Doppler velocity measured by a recently commissioned Digisonde at Bundoora (145.1°E, 37.7°S, geographic; 49°S magnetic) is seen to resemble the average diurnal variation of ion drift measured by the incoherent scatter radar at Millstone Hill (71.5°W 42.6°N; 57°N). Moreover, the Bundoora measurements exhibit the nighttime westward perturbation drifts found in Dynamics Explorer-2 ion drift measurements.     

Electron density inversed by plasma lines induced by suprathermal electron in the ionospheric modification experiment

Incoherent scatter radar (ISR) is the most powerful ground-based measurement facility to study the ionosphere. The plasma lines are not routinely detected by the incoherent scatter radar due to the low intensity, which falls below the measured spectral noise level of the incoherent scatter radar. The plasma lines are occasionally enhanced by suprathermal electrons through the Landau damping process and detectable to the incoherent scatter radar. In this study, by using the European Incoherent Scatter Association (EISCAT) UHF incoherent scatter radar, the experiment observation presents that the enhanced plasma lines were observed. These plasma lines were considered as manifest of the suprathermal electrons generated by the high-frequency heating wave during the ionospheric modification. The electron density profile is also obtained from the enhanced plasma lines. This study can be a promising technique for obtaining the accurate electron density during ionospheric modification experiment.     

Thermospheric model calculations based on turbulence data

Thermospheric model calculations are presented which explain the seasonal compositional changes of helium and argon by the combined effect of seasonal-latitudinal variations of turbulence and global seasonal winds. The observational base of the model calculations is given by empirical data of upper thermospheric densities and by estimates of the turbopause height derived from composition measurements and incoherent scatter temperatures in the lower thermosphere. The results are compared with observations of the seasonal variability of atomic oxygen in the turbopause region.     

Incoherent-scatter measurements of E-F valley and comparisons with theoretical and empirical models

High resolution electron density measurements from Arecibo incoherent scatter (i.s.) radar are used to make a detailed study of the E-F valley. Features of the important valley parameters like height, width and depth are examined. These features are then compared with the available theoretical and empirical models. The depth of the valley obtained from the empirical models agrees with i.s. measurements for near-noon periods, but disagrees with these measurements for pre-noon and post-noon periods. Further, the i.s. measurements indicate that E-F valley is rather small during daytime as compared to models which give larger width. During the night, the valley is quite wide and deep but the presence of sporadic-E (Es) contaminates the Ne-h profiles observed with the i.s. radar. As a result the valley parameters cannot be determined unambiguously during the night.     

Comparison of measured and predicted F2 peak altitude

Different models for the F2 peak altitude hmF2 are compared with mean values determined from incoherent scatter measurements of the radar stations Jicamarca/Peru and Arecibo/Puerto Rico at all local times. Our investigation shows that the daytime hmF2 is fairly well represented by the most recent models, whereas at nighttime, higher peak altitudes are measured than predicted by either model. The observed diurnal structure is slightly misinterpreted by the models above Jicamarca and is strongly disagreeing above Arecibo.     

Comparison of various empirical models of electron temperature with experimental measurements during low solar activity

Comparisons of various available empirical models of electron temperature are made with actual measurements from incoherent scatter radar and rocket and satellite probes, during low solar activity period. The models compared are those of Pandey $\text{et al.}$ (1983), Brace and Theis (1978), IRI (1979) and Bilitza (1983). It is found that our model and the Brace and Theis model are closer to actual measurements than the IRI and Bilitza models.     

Case studies of height structure of TID propagation characteristics using cross-correlation analysis of incoherent scatter radar and DPS-4 ionosonde data

The height structure of TID characteristics is studied on the base of the electron density profiles measured by two beams of the incoherent scatter radar and DPS-4 ionosonde. The height profiles of the TID propagation characteristics are obtained by means of cross-correlation and spectrum analysis of the radar and ionosonde data. The noticeable height variability of the TID parameters is observed. The variability is explained by interference of several TIDs. The obtained TID propagation characteristics are compared with known results of the TID studies.     

A steady-state model for the D- to F-region of the polar cap

For obvious reasons the ionosphere of the polar cap, surrounded by the auroral zone, is only poorly investigated. Even ionosonde data are very scant from geomagnetic latitudes beyond 70°. Since 1997 the European incoherent scatter radar facility EISCAT has an additional installation on Svalbard and has been providing electron density data nearly continuously ever since. These measurements which mainly cover the E- and F-regions are supplemented by rocket data from Heiss Island at a comparable magnetic latitude; these data are more sporadic, but cover lower altitudes and densities. A provisional, steady-state, neural network-based model is presented which uses the data of both sites.     

Studies of polar mesosphere summer echoes with the EISCAT VHF and UHF radars: Information contained in the spectral shape

The nature of PMSE in the VHF and UHF frequency range is considered taking into account the shape of corresponding Doppler spectra. Assuming a turbulence-based model of PMSE it is argued that for cases where a VHF radar detects strong PMSE, the UHF radar could either detect enhanced coherent scattering caused by the same physical process as in the VHF (i.e., turbulence with large charged ice particles), there could be incoherent scattering modified by the charged ice particles, or there could be a mixture of both. In order to distinguish these cases a simple but robust method is introduced to characterize the shape of the Doppler spectra derived from observations at both frequencies. Spectral shapes are quantified with one simple fitting parameter of a generalized fit to the autocorrelation function (=Fourier transform of the Doppler spectrum). This parameter takes a value of 1 for a Lorentzian spectrum indicative of pure incoherent scatter from the D-region, a value of 2 for coherent scatter owing to turbulence, and a value of less than 1 for incoherent scatter modified by the presence of charged aerosol particles. This method is applicable to observations at altitudes between ∼70 and ∼90 km. Simultaneous observations with the EISCAT VHF and UHF radar are presented in which all three cases mentioned above are identified. For the case of incoherent scatter modified by the presence of charged aerosol particles we quantify the radius of the involved ice particles to exceed ∼5 nm. Most importantly, however, for the case where the UHF-signal exceeded the incoherent scatter signal significantly, the spectrum revealed a clear Gaussian shape indicative of a coherent scattering process with identical spectral width as for the VHF-observations. This finding gives strong support that both echoes are created by the same turbulence-based mechanism and not by different mechanisms as speculated by several previous authors.     

Validation of FORMOSAT-3/COSMIC radio occultation electron density profiles by incoherent scatter radar data

Low Earth Orbiting satellites carrying a dual frequency GPS receiver onboard offer a unique opportunity to remote sensing of the global ionosphere on a continuous basis. No other profiling technique unifies profiling through the entire F2-layer with global coverage. The FORMOSAT-3/COSMIC data can make a positive impact on the global ionosphere study providing essential information about the height electron density distribution and particularly over regions that are not accessible with ground-based measuring instruments such as ionosondes and GPS dual frequency receivers. Therefore, it is important to verify occultation profiles with other techniques and to obtain experience in the reliability of their derivation. In the given study we present results of comparison of the electron density profiles derived from radio occultation measurements on-board FS-3/COSMIC and from the Kharkov incoherent scatter radar sounding.     

Topside models: Status and future improvements

This paper reviews the data base and empirical models that are available for the global representation of electron density in the topside ionosphere. Topside sounder and incoherent scatter measurements are the prime data sources. We assess their data volume and compatibility. Several empirical models are discussed (IRI, Bent, SLIM, and FAIM) and their specific characteristics and differences are pointed out. Global and temporal trends as predicted by these different models are compared and contrasted with measured results. Most models use vertical height rather than a field-aligned height coordinate, although it is well known that topside electrons are confined to moving along magnetic field lines. We examine several magnetic coordinate systems and evaluate their merits for empirical modelling.     

Relationship between electron density and electron temperature as a function of solar activity

This paper attempts to examine the control of electron density and solar activity on the F-region electron temperature. This is achieved by obtaining coefficients relating electron temperature with electron density and solar activity by using incoherent scatter radar measurements at Arecibo for the period August 1966 to May 1977. These coefficients are then used to construct an empirical model of F-region electron temperature. The model values are compared with measurements at other locations and show reasonable agreement.     

匹配滤波方法在非相干散射雷达测量空间碎片中的应用

利用2010年3月25日欧洲非相干散射雷达的常规电离层探测数据, 分析了实测数据中接收功率突变的现象. 高度发生在500 km以上的功率突变一般不是由于地球物理影响产生的, 而要归因于卫星或空间碎片. 采用匹配滤波方法实现了对空间碎片的探测和提取, 利用相干积累的方法提高了信噪比, 共获得363个碎片, 得到了碎片随高度和时间的分布. 结果表明, 碎片高度分布主要集中在800 km左右, 与欧洲非相干散射组织现有的碎片数据具有一致性.      

Time constants in the ionosphere from neural network models

Neural network (NN) models for the low latitude and the polar ionosphere from the D- to the F-region were developed which are based on incoherent scatter radar data from Arecibo and EISCAT Svalbard, respectively. The various geophysical input parameters defining the NN are not only the ones that represent the time one wants to predict, but also the geophysical conditions prior to the time of the prediction. The optimum length of these preceding periods are derived for the two models are different, but a period of 60 days is a compromise acceptable for both latitudes. Furthermore from the Arecibo data time constants of electron density decay after sundown are derived which – arguably – are also relevant elsewhere, including the polar latitudes. Whereas at all altitudes the electron densities decay exponentially after sundown, below 300 km there is an additional variation with solar zenith angle.     

Millstone地区垂直等效风场的气候学变化

基于1976---2006年美国Millstone非相干散射雷达的电离层观测数据, 分析了美国Millstone地区不同太阳活动条件下, 包括中性风场和电场漂移共同贡献的垂直等效风场的变化特征. 结果表明, Millstone地区的垂直等效风场表现出比较明显的周日、太阳活动和季节变化特征. 晚间垂直向上的等效风较强, 白天等效风较弱, 甚至接近于零. 在不同太阳活动和季节变化条件下, Millstone地区的等效风场都表现出类似的周日变化特征, 低太阳活动条件下, 晚间表现出较大的向上漂移. 这种周日变化和太阳活动变化特征与Millstone地区受到极区热源驱动大气循环的调制以及离子曳力的增减有关. 春季和秋季有相似的幅度和相位变化趋势, 表现出分点对称性; 冬季晚间向上漂移比夏季弱, 且随着太阳活动增强, 差异更加明显, 这再次体现了极区热源驱动大气循环的影响.      

Measurement of electric fields in the ionosphere by incoherent scatter radar techniques

Various methods for the extraction of electric fields from incoherent scatter radar observations are discussed. The random errors in electric field determinations are derived and it is shown that under typical conditions observatories such as EISCAT should measure electric fields with an r.m.s. error of about 1 mv/m with one minute integration time. Some preliminary EISCAT results are shown and compared with other observations.     

Neural network development for the forecasting of upper atmosphere parameter distributions

This paper presents a neural network modeling approach to forecast electron concentration distributions in the 150–600 km altitude range above Arecibo, Puerto Rico. The neural network was trained using incoherent scatter radar data collected at the Arecibo Observatory during the past two decades, as well as the Kp geomagnetic index provided by the National Space Science Data Center. The data set covered nearly two solar cycles, allowing the neural network to model daily, seasonal, and solar cycle variations of upper atmospheric parameter distributions. Two types of neural network architectures, feedforward and Elman recurrent, are used in this study. Topics discussed include the network design, training strategy, data analysis, as well as preliminary testing results of the networks on electron concentration distributions.