Response of the lower equatorial ionosphere to strong tropospheric disturbances

A morphological analysis of the results of sounding the lower equatorial ionosphere (the D region) in the region of action of strong tropospheric vortex disturbances (tropical cyclones, TC) is presented in this work. Based on the rocket sounding of the lower ionosphere at Thumba rocket site (8° N, 77° E) in May–June 1985 and on the satellite monitoring of TC in the northern Indian Ocean, it is demonstrated that a sharp depletion (by a factor of 2–4) of the electron concentration at altitudes of 60–80 km could be a response of the ionosphere during the TC active phase. In this case the lower boundary of the D region rose by several kilometers (not more than 5 km), and the temperature in the region of the stratopause slightly (by 2°–3°) increases. It is assumed that internal gravity waves (IGWs) generated by TC cause the effect on the lower ion-osphere.     

地球极区电离层对行星际激波的响应

本文就地球电离层对行星际激波的动力学响应进行三维全球数值模拟研究．背景行星际磁场为螺旋场,南北分量为零;初始电离层由Ⅰ区场向电流和相应的晨昏电场所主导;行星际激波沿日地连线方向撞击地球．模拟结果表明,在激波的作用下,电离层Ⅰ区电流系统向子夜方向运动,在向阳侧相继出现与原Ⅰ区电流反向的异常场向电流对和同向的新生Ⅰ区电流对．该异常场向电流对在极盖区形成瞬间昏晨电场,尾随原Ⅰ区电流向夜侧方向漂移直至湮没．与此同时,新生的Ⅰ区电流不断增强并向夜侧和赤道方向延伸,最终取代原Ⅰ区电流,相应极盖区又恢复到原来的晨昏电场状态．这一响应过程和行星际激波强度有关：激波强度越强,新生的Ⅰ区场向电流也越强,它向赤道方向延伸的距离也越大,能到达的纬度也越低．上述结果在趋势上与观测到的输运对流涡旋和亚极光块的运动特征一致．     

On the role of collective interactions in asymmetric ring current formation

The contribution of resonant wave-particle interactions to the formation and decay of the magnetospheric ring current is analysed in the framework of a self-consistent set of equations which take into account azimuthal plasmasphere asymmetry. It is shown that the cyclotron interaction of westward drifting energetic protons with Alfven waves in the evening-side plasmaspheric bulge region leads to the formation of a ring current asymmetry located near 18:00 MLT. The time-scale of this asymmetry is determined by the proton drift time through the plasmaspheric bulge and is about 1 - 3 h. A symmetrical ring current decays mainly due to charge exchange processes. The theory is compared with known experimental data on ions and waves in the ring current and on low-latitude magnetic disturbances. New low-latitude magnetometer data on the magnetic storm of 24 - 26 July 1986 are also discussed. The model presented explains the observed localization of an asymmetrical ring current loop in the evening sector and the difference in relaxation time-scales of the asymmetry and the D_st index. It also explains measured wave turbulence levels in the evening-side plasmasphere and wave observation statistics.     

Multiple scattering effects under vertical sounding of equatorial ionosphere

This paper presents a theoretical study of the influence of electron density small-scale irregularities on radio propagation under vertical sounding of the equatorial ionosphere. The sounder is assumed to radiate in the plane perpendicular to geomagnetic field lines. An approximate analytical solution of the equation of radiation energy balance in a plane layer of randomly inhomogeneous plasma has been obtained for this case. Analysis of the results shows that multiple scattering leads to attenuation of signal power and change of the signal arrival angles in the sounder vicinity.     

Waveguide radio-wave propagation in the equatorial ionosphere according to the Interkosmos-19 data

Additional strongly remote (up to 2000 km) radio-signal reflection traces on Intercosmos-19 ionograms obtained in the equatorial ionosphere have been considered. These traces, as a rule, begin at frequencies slightly lower than the main trace cutoff frequencies, which indicates that an irregularity with a decreased plasma density exists here. The waveguide stretched along the magnetic-field line is such an inhomogeneity in the equatorial ionosphere. The ray tracing confirm that radio waves propagate in a waveguide and make it possible to determine the typical waveguide parameters: ?δN _e ≥ 10%, with a diameter of 15–20 km. Since the waveguide walls are smooth, an additional trace is always recorded distinctly even in the case in which main traces were completely eroded by strong diffusivity. Only one additional trace (of the radio signal X mode) is usually observed one more multiple trace is rarely recorded. Waveguides can be observed at all altitudes of the equatorial ionosphere at geomagnetic latitudes of ±40°. The formation of waveguides is usually related to the formation of different-scale irregularities in the nighttime equatorial ionosphere, which result in the appearance of other additional traces and spread F.     

赤道电离层R-T不稳定性发展的控制因素分析

本文从质量和电荷守恒方程出发,分析了控制电离层等离子体R-T不稳定性线性增长的各种因素,重点研究了热层风和背景电场的空间梯度对R-T不稳定性线性增长的影响.结果表明,热层风场和背景电场两者的空间梯度对R-T不稳定性线性增长有不可忽视的促进或抑制作用;对R-T不稳定性线性增长起促进作用还是起抑制作用,依赖热层风场和背景电场及其空间梯度的方向;对R-T不稳定性线性增长影响的显著程度主要依赖于热层风场和背景电场两者空间梯度的大小.数值计算结果表明,对典型的背景电离层条件,磁力线顶点高度为330 km时,对线性增长率的影响最高达到120%.     

The equatorial and low-latitude ionosphere within the context of global modeling

The thermosphere–ionosphere–mesosphere-electrodynamics coupled model TIME-GCM, coupled to NCEP lower atmosphere data, is used to simulate the noontime ionospheric peak electron density NmF2 at low latitudes for year 2002. Model output are compared with observations a three ionosonde stations: Jicamarca, Ascension Island and Darwin, stations at geomagnetic latitudes of 3°S, 10°S and 22°S, respectively. The modeled electron density at the peak of the F2-layer (NmF2) matches the general trend of the data fairly well at noon throughout the year. The shapes of the diurnal curves of NmF2 vs. local time are not well produced in the model, and particularly so at the two stations away from the geomagnetic equator. At all sites the day-to-day variability of NmF2, assessed using the percent standard deviation about the monthly mean, is about twice the modeled variability. Possible sources of this shortfall in the model may be due to the under-representation of coupling from below and/or from auroral sources.     

Response of the equatorial Pacific thermocline to climate warming

Climate models project a significant shoaling of the thermocline over the western equatorial Pacific Ocean under global warming, which has been generally regarded as a direct response to surface wind change. This study investigates the formation processes for the equatorial Pacific thermocline response to CO₂ quadrupling using the Community Earth System Model version 1 (CESM1). In particular, an overriding method is applied to isolate and quantify the wind stress effect and the direct radiative effect of CO₂ emissions. Results show that both effects of the wind stress and direct radiative forcing are equally important for shoaling the equatorial thermocline, with the former responsible for its upper portion change and the latter for its lower portion change. Further passive tracer experiments with the ocean component of the CESM1 verify the role of ocean surface warming in shoaling the equatorial thermocline and identify the ocean circulation change in response to the surface warming as its dynamic cause of formation.     

Daytime efficiency and characteristic time scale of interplanetary electric fields penetration to equatorial latitude ionosphere

We analyze the daytime efficiency of the interplanetary electric field (IEF) penetration to the equatorial ionosphere based on a correlation analysis carried out between different levels of decomposition applied to IEF intensity measured at the ACE spacecraft and ionospheric electric field intensity inferred from ground based magnetometers located in the equatorial region in Brazil. We compare the time variations of those two electric field intensities by means of a scale-by-scale decomposed time series through wavelet multi-resolution analysis. Efficiency is here defined as the fraction of the variation of the IEF intensity that has penetrated into the equatorial region, and it increases with increasing fraction. Two cases of prompt penetration electric fields (PPEF) are analyzed: one occurring on March 31, 2001, and other on April 17, 2002. Our results show that the penetration effect with time scale ranging around 1 h is maximized in relation to other scales.     

Response of the ionosphere to natural and man-made acoustic sources

A review is presented of the effects influencing the ionosphere which are caused by acoustic emission from different sources (chemical and nuclear explosions, bolides, meteorites, earthquakes, volcanic eruptions, hurricanes, launches of spacecrafts and flights of supersonic jets). A terse statement is given of the basic theoretical principles and simplified theoretical models underlying the physics of propagation of infrasonic pulses and gravity waves in the upper atmosphere. The observations of “quick” response by the ionosphere are pointed out. The problem of magnetic disturbances and magnetohydrodynamic (MHD) wave generation in the ionosphere is investigated. In particular, the supersonic propagation of ionospheric disturbances, and the conversion of the acoustic energy into the so-called gyrotropic waves in the ionospheric E-layer are considered.     

Observations of Pc5 micropulsation-related electric field oscillations in the equatorial ionosphere

A 54.95-MHz coherent backscatter radar, an ionosonde and the magnetometer located at Trivandrum in India (8.5○N, 77○E, 0.5○N dip angle) recorded large-amplitude ionospheric fluctuations and magnetic field fluctuations associated with a Pc5 micropulsation event, which occurred during an intense magnetic storm on 24 March 1991 (A_p=161). Simultaneous 100-nT-level fluctuations are also observed in the H-component at Brorfelde, Denmark (55.6○N gm) and at Narsarsuaq, Greenland (70.6○N gm). Our study of the above observations shows that the E-W electric field fluctuations in the E- and F-regions and the magnetic field fluctuations at Thumba are dominated by a near-sinusoidal oscillation of 10 min during 1730–1900 IST (1200-1330 UT), the amplitude of the electric field oscillation in the equatorial electrojet (EEJ) is 0.1-0.25 mV m⁻¹ and it increases with height, while it is about 1.0 mV m⁻¹ in the F-region, the ground-level H-component oscillation can be accounted for by the ionospheric current oscillation generated by the observed electric field oscillation in the EEJ and the H-component oscillations at Trivandrum and Brorfelde are in phase with each other. The observations are interpreted in terms of a compressional cavity mode resonance in the inner magnetosphere and the associated ionospheric electric field penetrating from high latitudes to the magnetic equator.     

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.     

Numerical simulation of the electric field and zonal current in the Earth’s ionosphere: The dynamo field and equatorial electrojet

The work is devoted to the numerical simulation of the dynamo electric field and its effects in the Earth’s ionosphere within the scope of the thermosphere-ionosphere-protonosphere global self-consistent model developed at WD IZMIRAN. The new electric field calculation block, which was used to obtain results of the self-consistent calculations of the electric field potential generated by the dynamo effect of the thermospheric winds (the dynamo field) and the equatorial electrojet for March 22, 1987, is briefly described in this work. A comparison of the obtained results with the experimental data showed a satisfactory agreement. Moreover, the proposed model was used to calculate the diurnal variations in the ionospheric parameters for Jicamarca equatorial station under the same conditions with the help of the new block of the electric field. The results of these calculations are also presented and discussed in this work. It has been indicated that the new model satisfactorily describes the specific features of electric field distribution at the geomagnetic equator and the well-known phenomenon of equatorial electrojet.     

Response of the outer ionosphere to a magnetic storm at equinox

Summary The observations of the ACTIVE Project satellites in the interval of March 17–23, 1990 were analyzed for the purpose of studying the response of the outer ionosphere to the magnetic storm with SSC on March 20 at 22.43 UT. In particular, measurements of thermal plasma parameters were used, but VLF broadband measurements and data on energetic ion and electron fluxes in the range of 17.7–272 keV were also available. The results of this case study show that the observations in the morning sector at altitudes around 2000 km reflect well the state of the plasmasphere during enhanced activity, namely the depth to which the plasmasphere has been affected by enhanced magnetospheric convection. They also provide the possibility of monitoring the initial phase of recovery. The early evening observations of the plasma parameters in the outer ionosphere at altitudes of 500–1000 km indicate a distinct trough in electron concentration. In the dusk sector, the equatorward edge of this trough can be assumed to be the projection of the equatorial plasmapause. This, combined with the occurrence of electron temperature peaks and with the morning plasmapause position, enables one to judge whether the plasmaspheric bulge has formed and whether an inner plasmapause has occurred.     

The symmetric baroclinic instability of an equatorial current

Abstract

The stability of a baroclinic zonal current to symmetric perturbations on an equatorial β-plane is considered. The fluid is assumed to be Boussinesq, inviseid, adiabatic, hydrostatic, and stably stratified. The solutions exhibit the same stability properties as those on an f-plane: instability occurs whenever Ri < 1/(1 + d), where Ri is the Richardson number and d is a measure of the horizontal shear of the current; the most unstable motions tend to parallel the isotherms of potential temperature; and they have infinitely small scales of variation perpendicular to the isotherms. The variation of Coriolis parameter leads to one important difference in the structure of the eigenfunctions: the rapidly growing modes are concentrated in high latitudes, and the slowly growing ones in low latitudes.

The suggestion that the symmetric cloud bands observed at low latitudes in Jupiter's atmosphere are caused by symmetric instabilities is re-examined in the light of these results. These cloud bands would have to be associated with the slowly-growing, low-latitude modes. These modes consist of small scale motions parallel to the isotherms, with the magnitude of the motions having a large scale modulation as a function of latitude. The time scales of these modes and the latitude scales of their modulation agree qualitatively with the observations of Jupiter's cloud bands, so long as Ri is not very close to zero or to its critical value.     

Response of the topside ionosphere over Arecibo to a moderate geomagnetic storm

We analyze the data obtained using the Arecibo incoherent scatter radar to examine the response of the topside ionosphere to a moderate geomagnetic storm that occurred during the period March 7–11, 2008. During this time period a magnetic storm with a non-monotonic main phase decrease in the Dst index occurred. The recovery phase also exhibited a secondary Dst decrease. During the initial phase of the storm, Te and Ti increased coincident with the arrival of the solar wind. The main phase registered an increase in proton concentration proportional to Ne while temperatures reached the lowest values. Variations in O⁺ concentration were not significant but a reduction in helium fraction was observed. Soon after the peak of the storm, the transition height between the topside ionosphere and the protonosphere, where H⁺ ions dominate composition, was lower than would be expected during quiet conditions and this behavior lasted for approximately 12 h.     

Response of the ionosphere to the Baltic Sea earthquake of 21 September 2004

GPS observations of the European permanent network were used to identify seismo-ionospheric precursors of Baltic Sea earthquake of 21 September 2004. It is a very rare event for this region of Europe (magnitude of about 5.0). This value is the threshold for the occurrence of seismic effects in the ionosphere. In total electron content (TEC) data over the region of the earthquake, a specific ionospheric anomaly appeared one day before the earthquake was detected. The ionospheric variability had a positive sign with an enhancement of about 4–5 TECU (1 TECU = 10¹⁶ electrons/m²) relative to the non-disturbed state of the ionosphere. The anomaly had a duration of 4–5 hours in the day time. The special size of this anomaly was about 1000 km. The characteristic parameters of the anomaly show that it can be associated with ionospheric precursors of an earthquake.     

Response of the nighttime midlatitude ionosphere to the passage of an atmospheric gravity wave

Using the data of the ionospheric vertical sounding in Almaty, the response of various parameters of the nighttime F layer to the passage of an atmospheric gravity wave, generated during the large magnetic storm on July 24–25, 2004, is studied. The analysis of the phase relations between the variations in the electron density at the F layer maximum (NmF), the layer maximum height (hmF), and the layer half-thickness showed that they are determined by the slope of the wave phase front. It is shown that the half-thickness of the layer changes in anti-phase with the variations in NmF2. The known fact that the amplitudes of variations in the critical frequencies of the F ₂ layer are smaller than the amplitudes of electron density variations at fixed heights is explained.