Nighttime electron density enhancements at middle and low latitudes in East Asia

Nighttime enhancements in ionospheric electron density at mid- and low-latitudes are investigated by using the critical frequency of the F2-layer(fo F2) data measured from ionosonde stations at Okinawa(26.3°N, 127.8°E, Geomagnetic 15.3°N), Yamagawa(31.2°N, 130.6°E, Geomagnetic 20.4°N), Kokubunji(35.7°N, 139.5°E, Geomagnetic 25.5°N), and Wakkanai(45.4°N, 141.7°E, Geomagnetic 35.4°N) in East Asia during several solar cycles. The results show that there are obvious seasonal and solar activity dependencies of the nighttime electron density enhancements. The enhancements are termed pre-midnight enhancement and post-midnight enhancement, according to the local time when the enhancement appeared. The former has a higher occurrence probability in summer months than in winter months. In contrast, the latter has a larger occurrence probability in winter months than in summer months. Moreover, the nighttime enhancements in electron density are more likely to occur at lower solar activity. These seasonal and solar activity variations of the nighttime enhancements in electron density can be explained in terms of the combined effects of downward plasma flux from the plasmasphere and the neutral winds.     

Nitric oxide density measurements at middle latitudes

Summary The measurements of nitric oxide density were made by the photoionization method at 30–90km in several rocket flights near Volgograd. TheseNO densities are well within the range of other measurements below 60km, but become rather high above 70km. The upper mesosphericNO densities estimated by two different methods from ionospheric data in Central Europe (50 °N) are rather high, as well. The appropriateNO densities in the upper mesosphere still seem to be an open question.     

Non-trough foF2 enhancements at near-equatorial dip latitudes

Fine resolution series from three equatorial ionosondes of the IEEY network in West Africa have revealed small-scale daytime peak F2 structures, superposed on the slowly varying minimum or –trough distribution in the ±5° magnetic latitude zone. We report this new morphology, concentrating on foF2 enhancements of two types: near-equatorial crests (which travel either northwards or southwards) and magnetic field-aligned domes, whose onsets last only tens of minutes. Both types are observed to start at mid-morning or early afternoon hours. We relate their occurrence with the available variations of Vz = E × B upward drift which feeds the equatorial plasma fountain. We suggest the foF2 enhancements to be triggered by brief slow-downs of the Vz velocity near F2 peak altitude in our West African sector. Their short latitude extent differentiates them from the larger-scale tropical crest system. Further analysis of these features should lead to weather-like models of the low latitude ionosphere variations, where unstable local coupling between processes seems to be the trigger.     

Hysteresis of foF2 at European middle latitudes

The hysteresis of foF2 is studied for several European stations over the whole 24-hour diurnal interval for the equinoctial months of the years just before and just after the solar cycle minimum for solar cycles 20 and 21. Based on previous results, the hysteresis is expected to develop best just for the equinoctial months and near the solar cycle minimum. The hysteresis is generally found to be negative, i.e. higher foF2 for the rising branch compared to the falling branch of solar cycle. However, this is not the case in some individual months of some years. The noontime hysteresis represents the hysteresis at other times of the day qualitatively (as to sign) but not quantitatively. The hysteresis appears to be relatively persistent from one solar cycle to another solar cycle in spring but not in autumn. A typical value for springtime hysteresis is about 0.5 MHz. The inclusion of hysteresis into long-term ionospheric and radio wave propagation predictions remains questionable.     

Evening enhancements in F-region electron temperature at subtropical latitudes during June solstice in the Indian SROSS C2 RPA data

In-situ measurments of the topside F-region ionospheric electron and ion temperatures are very few over the low and equatorial latitudes during the last two solar cycles, particularly in the Indian sector. The SROSS C2 satellite has provided some valuable data on the thermal structure of the topside ionosphere over the Indian region. This article reports a typical evening enhancement in the topside F-region electron temperatures around 18:00 IST observed in the subtropical latitudes of 15–20°N. These enhancements that are seen during the low sunspot activity periods show a latitudinal difference with an early and sharp peak at higher latitudes (23°N). The observed features are explained on the basis of equatorial plasma dynamics associated with the Appleton ionization anomaly.     

Simultaneous conjugate ground-satellite ULF pulsation measurements at the middle latitudes

The spectra of three series of simultaneous ground (Budkov Observatory) and satellite (Freja) ULF measurements have been processed. The following features were studied in the FFT spectra, power-spectra and cross-spectra of the six-minute samples, recorded during quasi-conjugate approaches of Freja to the local Budkov magnetic field line: (a) the Doppler frequency shift (about 1 mHz and less) in the predominant and some particular spectral components on Freja; (b) the amplitude relations between the ground and satellite (ratios 0.3 to 0.8); (c) phase shifts between particular signal components on the spacecraft above the ionosphere (in the azimuthal y-direction on Freja) and on the ground (in the meridional x-direction at Budkov) have been estimated (phase lags on the ground 0.9 rad) and (d) also dynamical characteristics of the fundamental wave modes in the (x,y)-plane, perpendicular to the magnetic field line, have been estimated for the predominant spectral components, considering standing oscillation along the field line. Azimuthal wave numbers were about 2.0, propagating northwest at 400km/s.     

Geomagnetic variation peculiarities at middle latitudes of the East European Platform

The results of instrumental observations of geomagnetic variations at the Mikhnevo midlatitudinal observatory of Institute of Geosphere Dynamics of the Russian Academy of Sciences (54.9595° N; 37.7664° E) are presented and discussed. The main periodicities of the local background variations of geomagnetic field are determined. Variations of ~ 27 days have been registered, as well as periodicities with periods of ~6–9, 12–14, 60 days, and a semiannual periodicity. It has been shown that the background geomagnetic variation periodicities have a sporadic and scaling character. An alternating effect of increasing and degradation periods in geomagnetic variation intensity (the intermittency effect) is found.     

Daytime F2-layer positive storm effect at middle and lower latitudes

Daytime F2-layer positive storm effects at middle and lower latitudes in the winter thermosphere are analyzed using AE-C, ESRO-4 neutral gas composition data, ground-based ionosonde observations and model calculations. Different longitudinal sectors marked by the storm onset as ‘night-time’ and ‘daytime’ demonstrate different F2-layer positive storm mechanisms. Neutral composition changes in the ‘night-time’ sector with increased [O] and [N₂] absolute concentrations, while (N₂/O)_storm/(N₂/O)_quiet\approx1 at F2-layer heights, are shown to contribute largely to the background N_mF2 increase at lower latitudes lasting during daytime hours. Storm-induced surges of the equatorward wind give rise to an additional N_mF2 increase above this background level. The mid-latitude F2-layer positive storm effect in the ‘daytime’ sector is due to the vertical plasma drift increase, resulting from the interaction of background (poleward) and storm-induced (equatorward) thermospheric winds, but not to changes of [O] and [N₂] concentrations.     

Galactic cosmic ray variation influence on baric system dynamics at middle latitudes

Variations of atmospheric pressure in the North Atlantic region during Forbush decreases of galactic cosmic rays were investigated. A noticeable pressure growth with the maximum on the 3rd and 4th days after the Forbush decrease onsets was revealed over Scandinavia and the northern region of the European part of Russia. It was shown that the observed pressure growth was caused by the formation of blocking anticyclones in the region of the climatic Arctic front, as well as by the sharp slowing of the movement of North Atlantic cyclones. It was suggested that the particles that precipitate in the regions of the climatic Arctic and Polar fronts, with the minimum energies E～20–80 MeV and ～2–3 GeV, respectively, may influence the processes of cyclone and anticyclone formation and development at extratropical latitudes.     

Low-frequency variability in atmospheric middle latitudes

A short review of research trends in the study of low-frequency mid-latitude variations is presented. Theoretical developments have been concentrated upon with the major themes of multiple equilibria, flow stability and eddy-mean flow interaction reflecting the authors main interests. A new interpretation of the role of transient eddies in maintaining atmospheric blocking is also suggested in which eddy potential vorticity fluxes are considered to mediate a downstream transition between a zonal and a steady free Rossby wave flow. This treatment avoids emphasizing local balances and fluxes by the transients which are often entirely or partially reversible. The consequences of this interpretation are explored in the barotropic model of blocking first presented by Shutts (1983). This interpretation is used to suggest conditions in which the jetstream may be unlikely to undergo transition to a blocked weather regime.     

Impact of strong geomagnetic storms on total ozone at southern higher middle latitudes

The effects of space weather on the magnetosphere-ionosphere-atmosphere system have been broadly studied within the program CAWSES (Climate and Weather of the Sun-Earth System) and project COST724. Here we show that strong geomagnetic storms do not affect total ozone at higher middle latitudes of the Southern Hemisphere contrary to the Northern Hemisphere, where an effect is observed under specific conditions.     

Cyclic variations of the initial height of the sporadicE-layer at middle geographical latitudes

Summary The variations of the initialh E _s height are investigated in the solar cycle 1957–1968, deriving the regressive dependency:h E _s =121.4–6·10^–2 R referring to the median monthly values at a solar zenith angle =75°. The similar variations ofh E _s (R) during the day and night are interpreted as a domination of the sporadic layer formation from a redistribution of the day-time ionization and secondary participation of nightly ionizing sources. The analogous cyclich E _s andh E variations confirm this conclusion while the seasonal variations in the state of the sporadic layer show outlined dynamical effects. The comparatively not big cyclic variation in the spatial state of theE-region are considered to confirm the predominating ionizing action of the ultraviolet range (933–1038 Å) in the lower part of theE-region, while the soft X-radiation influences mainly the near maximum part of this region.     

Local enhancements in the horizontal distribution of the electron density at heights of the topside ionosphere near the dayside polar cap boundary

The structure of the electron density horizontal distribution at heights of the topside ionosphere in the Northern Hemisphere under quiet magnetic conditions in the polar peak region has been analyzed based on the measurements conducted on board the STSAT-1, DMSP F13, and DMSP F15 satellites during the period of moderate solar activity. The sector near 1000 MLT (magnetic local time) for the spring equinox season has been studied most thoroughly. It has been obtained that the polar peak consists of localized irregularities in the electron density of a specific form, inside which the electron density exceeds its background values by several tens of percent. The characteristic dimensions of the irregularities vary from a few degrees to several tens of degrees in the meridional and zonal directions, respectively. The irregularities are centered along the dayside boundary of the polar cap and coincide with the region of “soft” electron precipitation (with the average energy lower than 500 eV and with the flux density above 10⁷ electrons cm^?2 s^?1 sr^?1).     

中低纬电离层F层峰高和厚度的变化特征分析

电离层F层参数对电离层空间天气研究与电波传播应用具有重要意义,以往工作主要针对电离层f₀F₂、TEC等参数.本文利用我国中纬地区的兰州、中低纬过渡区的昆明、低纬地区的海口三个观测站的电离层垂直探测数据,分析了电离层峰高h_mF₂、F层虚高h'F和定性表征的厚度h_mF₂-h'F的周日、季节、太阳活动变化特征.研究表明：（1）兰州h_mF₂在太阳活动高年和低年的数值接近,海口在太阳活动高年白天的h_mF₂比低年白天高20~30 km.（2）在海口和昆明,h_mF₂最大值多出现在中午时段,兰州站的最大值出现在夜间.（3）海口的h_mF₂在01-3LT期间出现很强的“午夜衰落”现象,此后迅速增大.（4）利用h_mF₂-h'F来表征电离层的厚度时,其季节和周日变化特征与常用的B₀存在相似之处,但未出现清晨与午后凹陷等现象.这些结果对于提高我国电离层变化特性的认识和模式化研究水平具有重要的科学意义.

     

Longitudinal effects of ionospheric responses to substorms at middle and lower latitudes: a case study

An ionospheric model is used to simulate total electron content (TEC) disturbance events observed at middle and lower latitude sites near 75○W and 7○E longitudes. Within this longitudinal range, daytime TEC disturbances show patterns that are correlated with substrom activity seen in both auroral electrojet and ring current behavior. In modeling studies of the observed ionospheric effects, both electric field and neutral wind perturbations are examined as possible mechanisms. The morphological features of the required electric field perturbations near drawn and dusk are compared with those at other times to examine the local time characteristics of magnetospheric influence. Large-scale traveling atmospheric disturbances (TADs), an alternative candidate for the disturbance source, are also characterized and compared with known thermospheric behavior.     

Studies of the ionospheric D-region using partial reflections in spring 2004 at middle and high latitudes

Results of the observations of the ionospheric effects of two solar flares in April 2004 performed using partial reflections are presented. The studies were carried out at the measuring facilities located in different latitudinal regions: at Vasil’sursk station in the Nizhni Novgorod region and at Tumannyi station in the Murmansk region. The quantitative estimates of the electron density in the polar and midlatitude D region under quiet conditions and during solar flares were obtained. The correlation between rapid variations in electron concentration at heights of about 80 km at these stations was found and it was shown that during solar flares the electron density at heights of 60–70 km corresponds to the intensity of the X-ray flux in the range of 0.5–3 Å, which points to the action of the linear law of recombination in the ionospheric D region.     

Time-dependent convection at high latitudes

A fully time-dependent ionospheric convection model, in which electric potentials are derived by an analytic solution of Laplaces equation, is described. This model has been developed to replace the empirically derived average convection patterns currently used routinely in the Sheffield/SEL/UCL coupled thermosphere/ionosphere/plasmasphere model (CTIP) for modelling disturbed periods. Illustrative studies of such periods indicate that, for the electric field pulsation periods imposed, long-term averages of parameters such as Joule heating and plasma density have significantly different values in a time-dependent model compared to those derived under the same mean conditions in a steady-state model. These differences are indicative of the highly non-linear nature of the processes involved.     

Relativistic electron enhancements,magnetic storms,and substorm activity

The relativistic electron fluxes of the Earth's outer radiation belt are subjected to strong temporal variations. The most prominent changes are initiated by fast solar wind streams impinging upon the magnetosphere, which often also cause enhanced substorm activity and magnetic storms. Using 4 years of data from the particle detector REM aboard the UK satellite Strv-1b in a GTO, we investigated the relation between these different appearances of geomagnetic activity. A typical sequence is that there is a drop in the relativistic electron intensity during the main phase of the magnetic storm and a successive enhancement during the recovery phase which sometimes leads to much higher than pre-storm fluxes. Whereas the flux drop is well correlated with the magnetic storm intensity and is mainly due to the deceleration and loss of particles caused by the ring-current-induced magnetic field changes, there is only a bad correlation between the post-storm electron flux and Dst. As we show here, it is much more the level of substorm activity during the whole event which determines the size of the flux enhancements.     

Thermosphere–ionosphere response at middle and high latitudes during perturbed conditions: A case study

Neutral gas composition and ionospheric measurements taken by the Dynamic Explorer 2 satellite at F2-region heights (280–300 km) during an intense geomagnetic storm (peak Dst=−187 nT) were used to analyze the role of some possible physical mechanisms responsible for the changes of electron density at high and middle latitudes. The storm considered in this study occurred on 26 September 1982. The main features observed were increases of electron density during the initial stages of the storm at middle latitudes; followed by decreases of electron density at high and mid-high latitudes during the main phase of the storm and the first phase of the recovery. Delayed increases of electron density during the recovery phase have also been observed at mid-high latitudes (50–60°). Several mechanisms were discussed in explaining the features observed for the electron density variations.     

NeQuick bottomside analysis at low latitudes

NeQuick ionospheric electron density model produces the full electron density profile in the ionosphere using the F2 layer peak values (foF2 and hmF2) as anchor points. Each part of the profile is modeled using Epstein layer formalism. Simple empirical relations are used to compute the thicknesses of each semi-Epstein layer. It has been observed that when NeQuick model is used to estimate total electron content at low latitudes the modeled values tend to underestimate the observed ones. Beside the F2 peak values, the most important profile parameter is the thickness of the F2 layer bottomside. The present study focuses on NeQuick model behavior at low latitudes comparing modeled profiles parameters with the ones extracted from experimental data mostly from African and Indian sector at different levels of solar activity and different time of the day. Possible model improvements are discussed.