Imaging coherent backscatter radar studies of equatorial spread F

A new interferometric imaging technique has been used at Jicamarca to study the morphology and dynamics of plasma irregularities occurring during spread F conditions. The technique produces two-dimensional, in-beam images of the coherent scatter from these irregularities, instantaneously mapping the structure of the underlying plasma instabilities in the equatorial plane. We present sequences of images depicting backscatter from bottom-type layers, bottomside layers, and topside plumes drifting and rising through the ionospheric regions illuminated by the Jicamarca radar. The high spatial and temporal resolution of the images permits us to observe dynamical phenomena missed by conventional fixed- and steered-beam techniques. The new images show evidence of intermediate- and large-scale plasma depletions bifurcating and pinching off from the bottomside as they ascend. Analytic models are presented to describe these dynamical effects. The images also differentiate between bottom-type and bottomside spread F layers, the former being shown to have much smaller scale primary waves than the latter. Ample evidence of secondary plasma instabilities appear throughout the images. The consequences of these secondary instabilities and of shear flow are discussed.     

On the plausible linkage of thermospheric meridional winds with the equatorial spread F

Some of the characteristic features of thermospheric meridional winds during equinoctial period, associated with equatorial spread F (ESF) and their possible role in the triggering of ESF are presented through case studies of observational events under different geophysical conditions that essentially control the post-sunset F-layer height (h′F) rise. The present study reveals that the polarity and magnitude of the meridional winds become significant with the equatorward wind being present when the h′F is below a critical height for the instability to get triggered. The distinctly different characteristic features of the meridional winds during ESF and non-ESF events are presented and discussed.     

An overview and synthesis of plasma irregularities in equatorial spread F

A unified picture of plasma irregularities in equatorial spread F is developed from the analysis of satellite, sounding rocket, and coherent scatter radar observations. The coherent scatter data are analyzed using a new in-beam radar imaging technique that permits direct comparison between radar data, in situ data, and computer simulations of the irregularities. Three varieties of irregularities, all produced by ionospheric interchange instabilities, are found to occur. Thin bottom-type layers are composed of waves with primary transverse wavelengths less than about 1 km and with significant parallel wavenumbers. These exist on magnetic flux tubes controlled by the E region dynamo and drift westward in the postsunset ionosphere. A nonlocal analysis is used to calculate their linear growth rate. When the F region dynamo takes control of the flux tube, bottomside irregularities can emerge. These are more robust irregularities with longer primary wavelengths and which exhibit greater vertical development. Nonlinear analyses explain the appearance of steepened structures in rocket observations and solitary waves in satellite observations of bottomside layers. The one-dimensional spectra of these irregularities obey power laws but are anisotropic and have variable spectral indices and spectral breaks. Very strong polarization electric fields can eject large regions of deeply depleted plasma through the F peak and form topside irregularities. Theoretical calculations supported by satellite data show that ion inertia may become important for topside irregularities. The one-dimensional spectra of irregularities in the inertial regime obey a k^−5/3 power law, but strong plasma inhomogeneity implies that Kolmogorov weak turbulence is not the explanation. Topside depletions are shown to bifurcate and also to pinch off from the bottomside.     

Typical disturbances of the daytime equatorial F region observed with a high-resolution HF radar

HF radar measurements were performed near the magnetic equator in Africa (Korhogo 9°2463N–5°3738W) during the International Equatorial Electrojet Year (1993–1994). The HF radar is a high–resolution zenithal radar. It gives ionograms, Doppler spectra and echo parameters at several frequencies simultaneously. This paper presents a comparative study of the daytime ionospheric structures observed during 3 days selected as representative of different magnetic conditions, given by magnetometer measurements. Broad Doppler spectra, large echo width, and amplitude fluctuations revealed small-scale instability processes up to the F-region peak. The height variations measured at different altitudes showed gravity waves and larger-scale disturbances related to solar daytime influence and equatorial electric fields. The possibility of retrieving the ionospheric electric fields from these Doppler or height variation measurements in the presence of the other possible equatorial ionospheric disturbances is discussed.     

Observations of the F3-layer at equatorial region during 2005

The existence of the F3-layer has been observed at Brazilian equatorial stations. This paper reports on a 1-year observations at Parit Raja, Malaysia (Lat. 1°52′N, Long. 103°48′E). The greatest number of appearances of the F3-layer is around local noon while the least is after local dawn. Its occurrence is more pronounced during winter and the equinoxes. The mean height of reflection for the layer is about 600 km reaching a maximum of about 900 km during the winter of 2004/2005. It is seen that the critical frequency of the F2-layer decreases with the appearance of the F3-layer.     

Quiet-time variations of F2-layer parameters at Jicamarca and comparison with IRI-2001 during solar minimum

We analyze Jicamarca ionograms to study the quiet-condition variations in the peak electron density (NmF2), its height (hmF2), and F2-layer thickness parameter (B0) of the equatorial F2 layer during solar minimum. The sunrise peak is found in hmF2 and B0 for all months. During daytime and nighttime, the variation in the hmF2 value is mainly responsible for that in NmF2 and B0. The sunset peaks of hmF2 and B0 exist in the equinoctial months, but not in the winter months. Moreover, the observed values of hmF2, NmF2, and B0 are generally similar to the modeled values of IRI-2001.     

Disturbances at F2-region heights of equatorial anomaly during geomagnetic storms

Neutral gas composition and ionospheric measurements taken by the Dynamic Explorer 2 satellite at F2-region heights during two geomagnetic storms are used to analyze the role of some possible physical mechanisms responsible for the changes of electron density at equatorial and low geomagnetic latitudes. The storms considered occurred on October 2, 1981 (storm 1) and July 13, 1982 (storm 2). During storm 1 (weak), vertical plasma drifts and equatorward storm-time winds operated increasing of the electron density at the trough of equatorial anomaly and the decreases at the crest region. During storm 2 (intense) changes of composition (increase of molecular nitrogen and atomic oxygen) played a fundamental role for the changes of electron density observed at low latitudes in summer hemisphere. It is concluded that different physical processes seem to have varying degrees of importance depending on the intensity of the storm.     

Variability of equatorial ionospheric electron density at fixed heights below the F2 peak

A study on variability of the equatorial ionosphere was carried out at fixed heights below the F2 peak for two different levels of solar activity. The study covered height range of 100 km up to the peak of F2 layer using a real height step increase of 10 km. The variability index used is the percentage ratio of standard deviation over the average value for the month. Daytime minimum variability of between 3% and 10% was observed at height range of about 150–210 km during low solar activity and between 2% and 7% at height range of 160–220 km during high solar activity. The nighttime maximum of between 70% and 187% was observed at height range of about 210–250 km during low solar activity and between 42% and 127% at height range of 210–250 km during high solar activity. The height range at which daytime minimum was observed falls within the F1 height of the ionosphere. The result obtained is consistent with previous works carried out in the low latitude locations for American sector.     

Spectral analysis of plasma drift measurements from the AE-E satellite: evidence of an inertial subrange in equatorial spread F

Plasma drift data from the AE-E satellite are spectrally analyzed to investigate the characteristics of the flow in the topside equatorial F region ionosphere during strong spread F conditions. Plasma flow around rapidly rising depletions is thought to exhibit behavior similar to two-dimensional Kolmogorov turbulence, but only on flux tubes with sufficiently small integrated ion–neutral collision frequencies. We find that one-dimensional spectra computed from vertical plasma drift measurements made in such depletions on such flux tubes tend to display a −5/3 spectral index over scale sizes from about 1 to $\text{100}\text{km}$, suggesting the operation of an inverse energy cascade. This universal spectral form is evidence of an inertial regime of the underlying ionospheric interchange instability.     

HF radar observations of equatorial spread-F over West Africa

New experimental data depicting equatorial spread-F were taken during an HF radar sounding campaign in Korhogo (Ivory Coast, 9°24N, 5°37W, dip 4°S). Range-time-intensity maps of the radar echoes have been analyzed to identify the signatures of density depletions and bottomside spread-F. Density depletions are well known features of equatorial spread-F, and are believed to emerge after the development of a Rayleigh-Taylor instability on the bottomside F-layer. A simple model is developed and used to simulate the flow of density depletions over the radar field of view. The simulation permits an interpretation of the data that yields the zonal flow velocity as a function of local time. Comparisons with previous measurements are undertaken to assess the consistency of the computational results, and qualitative arguments are presented to identify bottomside spread-F. Using the computational results as reference, a morphological study of ionograms showing spread-F is undertaken which reveals the specific signature of bottomside spread-F on ionograms recorded just after sunset.     

Long term trends in the frequency of occurrence of the F3 layer over Fortaleza,Brazil

Recent studies using model calculation and ionospheric observations have revealed the existence of an additional layer in the topside equatorial ionosphere, the F₃ layer. The observations using bottomside ionograms from locations close to the magnetic equator in Brazilian region have shown that the occurrence of the layer is very high from December to February (local summer) and from June to August (local winter). In fact, for the year 1995 the occurrence of the F₃ layer is >75% during the months of January, February and December, and it is >65% for the period of June, July and August (Geofisica Int. 39 (2000) 57). In this work, we use 25 years of data for the months of January and August to investigate how the layer occurrence varies with the magnetic dip angle and solar activity.     

The structure of the equatorial mesosphere at Thumba

The structure of the equatorial mesosphere is being investigated at Thumba by rocket borne ultraviolet absorption photometry as well as by the meteorological M-100 rocket launching programme. Whereas the meteorological M-100 rocket luanching programme has been regular, the UV absorption studies have been few in number and sporadic in nature. In this paper an attempt is made to consolidate the results so far obtained from both these investigations.     

Long term variation in the secular love number

Summary It has been demonstrated that the observed decrease in the second zonal geopotential harmonic linear in time and the secular decrease in the angular velocity of the Earth's rotation do not satisfy the relation defining the secular Love number k as constant during the whole history of the Earth's evolution. The discrepancy disappears if it is assumed thatdk/dt=–2.4×10^–6 cy^–1 .

---

aa, m a ¶rt;a au m aauunmuaa u u a ¶rt;a u mu au u ¶rt;mm mu, n¶rt; u a a nm mu umuu u. a uam, u ¶rt;nmumdk/dt=–2,4·10 ^–6 (mmu)^–1 .

     

曲靖非相干散射雷达电离层F区日间电子温度变化特征初步分析

利用曲靖非相干散射雷达2017—2018年春夏季观测数据首次分析了电离层日间150～450km电子温度的地方时与高度变化特征及其与电子密度的相关性.发现hmF2及以上的电子温度在日出日落时具有两个峰值,在11∶00—16∶00LT之间变化较小,高度越高午后上升的时间越早;从150km开始迅速增加,在约220km达到最大值,然后开始降低,在约300～350km达到最小值,最后单调上升;200km以下电子温度与电子密度成正相关(主要由热传导控制),200～450km之间存在明显的反相关(光电离过程占主导),电子-离子温度差与电子密度对数之间存在近似线性关系,电子温度逐日变化与光电离因子的变化趋势相似,这种相关性在中午与午后更明显;以上结果与其他非相干散射雷达观测和电离层模型计算结果基本一致,但也存在一些差别,需要结合更多数据深入分析.     

Seasonal characterization of the equatorial electrojet height rise over Brazil as observed by the RESCO 50 MHz back-scatter radar

In this paper, we present the seasonal characteristics of the post-noon rise of the equatorial electrojet 3-m irregularities scattering region observed over São Luís, Brazil (2.3S, 44.2W, dip:-0.5). The study is based on a 1 year data set collected by the 50 MHz coherent back-scatter radar (also known by the acronym RESCO), that started operation in 1998. Using a method to estimate the moments from each individual back-scattered power profiles that constitute the standard range–time–intensity (RTI) maps we were able to determine the following representative parameters of the electrojet: the total power back-scattered by the electrojet irregularities (EJP), the thickness of the electrojet back-scattered power profile (EJT), the height of the center of the back-scatter region, that is, the power profile, (EJC), and the noise level corresponding to each power profile (EJN). The parameterization was applied to all selected daily RTI maps from 2002. The analysis was carried out by grouping the data according to the radar beam angle (tilted 30 westward or eastward), the magnetic disturbance indices K_p and the season, which enables us to quantify the differences in the parameters on these bases. The results are presented and discussed here focusing on the post-noon ascent of the EJC. We will also present results on the east–west asymmetry in the radar back-scattered echo power confirming our previous results, and on the appearance of a scattering region after sunset during magnetically quiet conditions around the southern summer solstice.     

Lidar observations of atmospheric sodium at an equatorial location

Lidar measurements of upper atmospheric sodium were made at Alcântara (2°20′ S, 44°30′ W), Brazil, on 8 days during the month of November 1996. These are the first sodium measurements to be reported from an almost equatorial location. The average sodium distribution at Alcântara was found to be very similar to the annual mean for São José dos Campos (23° S), but about 1 km higher than the November mean value of 92 km for the latter site. Only one, weak, sporadic sodium layer was observed during more than 26 h of observations, an occurrence frequency much less than that seen at low latitude sites in the northern hemisphere. A survey of the observations of sporadic layers at various locations suggests that both magnetic dip and declination angles might influence their rate of occurrence.     

Variability of the F region of the equatorial ionosphere during quiet geomagnetic conditions prior to strong earthquakes

The deviations in the hourly values of the F-region critical frequency from the monthly median for the ionospheric Huancayo station located near the magnetic equator during periods of quiet geomagnetic conditions are analyzed for 1957–1987. Ionospheric data for five days prior, one day after, and directly for the days of 33 strong (with a magnitude M ≥ 5.5) earthquakes with epicenters in the American longitudinal sector were used. It is revealed that, in 24 cases 1–5 days prior to the considered earthquakes, a decrease in the critical frequencies by more than 20% with a duration from one to six hours was observed mainly in the nighttime. One can assume that these effects (at least, part of them) are related to the processes of earthquake preparation. Disturbances were mainly detected in cases when the radius of the earthquake preparation zone exceeded the distance between the epicenter and the observation station. The need for a further study of the characteristics of different kinds of coupling “from below” on electrodynamical processes in the low-latitude ionosphere for successful recognition of disturbances of the seismogenic nature is noted.     

Data modeling and assimilation studies with the MU radar

We report initial results of data modeling and assimilation studies for several MU radar experiments. Various inputs to a one-dimensional ionospheric model are adjusted to provide agreement with observation and also to learn the sensitivity of the model to their variations. Certain observations are also used directly in the model to anchor or constrain its behavior. In particular, studies of the electron density from 100 to 500 km altitude in the ionosphere are carried out with the help of a theoretical model of O⁺, NO⁺, O⁺₂ and N⁺₂ densities and MU radar observations of the power, ion-drift and plasma-temperature profiles. Four typical cases are selected to study quantitatively the effects of the (A) perpendicular-north component of the plasma drift (15 December 1986), (B) atmospheric composition (7 October 1986), (C) solar EUV flux (2 August 1989) and (D) upper-boundary O⁺ density (5 October 1989) on the model N_mF2, h_mF2 and N_e profile, as well as on the neutral wind calculation from h_mF2 and drift data. It is found that the measured vertical ion drift explains quantitatively well the measured h_mF2 (particularly at low solar activity) while the model gives a better match with the measured N_e when it uses the h_mF2-based wind rather than the measured plasma drift. Different model values of the atmospheric O/N₂ ratio and EUV flux and different values of the upper-bound O⁺ density may modify not only N_mF2 markedly but also h_mF2: a lower O/N₂ ratio results in higher h_mF2; the EUVAC model gives higher h_mF2 at high solar activity than does the EUV91 model; with a smaller upper-bound O⁺ density, h_mF2 is lower by day but little changed by night. We specifically note that the meridional wind needed by the model to reproduce the observed h_mF2 differed according to how well the model reproduced the observed N_mF2. The uncertainties in the MSIS86 and EUV model predictions are also discussed. It is found that if the MSIS and EUV91 models are used together, the model gives an N_mF2 higher than that measured at high solar activity. Thus the O/N₂ ratio needs to be reduced from the MSIS value if EUV91 is used. If EUVAC is used, no large modification is required. At equinox for low solar activity, modeling with either EUV model produces N_mF2 values lower than those measured, and so the true O/N₂ ratio may be higher than that given by MSIS model.