Effects on the Ionosphere Due to Phenomena Occurring Below it

The terrestrial thermosphere and ionosphere form the most variable part of theEarth's atmosphere. Because our society depends on technological systems thatcan be affected by thermospheric and ionospheric phenomena, understanding,monitoring and ultimately forecasting the changes of the thermosphere–ionosphere system are of crucial importance to communications, navigation and the exploration of near-Earth space. The reason for the extreme variability of the thermosphere–ionosphere system isits rapid response to external forcing from various sources, i.e., thesolar ionizing flux, energetic charged particles and electric fields imposed via the interaction between the solar wind, magnetosphere and ionosphere, as well as coupling from below (meteorological influences) by the upward propagating, broad spectrum,internal atmospheric waves (planetary waves, tides, gravity waves) generated in thestratosphere and troposphere. Thunderstorms, typhoons, hurricanes, tornadoes andeven seismological events may also have observable consequences in the ionosphere.The release of trace gases due to human activity have the potential to cause changes inthe lower and the upper atmosphere.A brief overview is presented concerning the discoveries and experimentalresults that have confirmed that the ionosphere is subject to meteorologicalcontrol (especially for geomagnetic quiet conditions and for middle latitudes).D-region aeronomy, the winter anomaly of radiowave absorption, wave-liketravelling ionospheric disturbances, the non-zonality and regional peculiaritiesof lower thermospheric winds, sporadic-E occurrence and structure, spread-Fevents, the variability of ionospheric electron density profiles and Total ElectronContent, the variability of foF2, etc., should all be considered in connection withtropospheric and stratospheric processes. Ionospheric weather, as a part of spaceweather, (i.e., hour-to-hour and day-to-day variability of the ionospheric parameters)awaits explanation and prediction within the framework of the climatological, seasonal,and solar-cycle variations.     

Lidar observations and formation mechanism of the structure of stratospheric and mesospheric aerosol layers over Kamchatka

Lidar observations during 2007–2008 in Kamchatka revealed aerosol layers in the upper stratosphere at heights of 35–50 km and in the mesosphere at heights of 60–75 km. It is well known that forces of gas-kinetic nature, i.e., photophoretic forces, act on aerosol particles that absorb solar radiation and terrestrial IR radiation; these forces can counteract the gravitational force and even lead to the levitation of these particles at particular heights. The accumulation of particles at these heights may lead to the formation of aerosol layers. We calculated these forces for the conditions of lidar observations in Kamchatka. Aerosol layers were observed at heights where particle levitation can occur. Thus, the stratospheric and mesospheric aerosol layers, detected at heights of 30–50 and 60–75 km, respectively, may be due to the effect of the photophoretic force on aerosol particles.     

Variability of the Earth's Atmospheric Electric Field and Ion-Aerosols Kinetics in the Troposphere

The paper deals with the mechanism of generating a ground potential gradient electric field in regions of seismic activity and its penetration into the ionosphere. The mechanism is based on the electrode effect of charge separation under the action of the natural atmospheric electric field, A large, non-compensated, space charge is formed following a chain of ion-molecular reactions as a result of the anomalous increase of radon and aerosol emanation from the crust. This space charge leads to anomalous variations of the electric field ground level, which is supported by the experimental observations made in the seismo-active regions. In turn the variations of the strong electric field over the large earthquake areas lead to the modification of ionospheric parameters due to penetration of the anomalous electric field into the ionosphere. A theoretical model of these phenomena is proposed in this paper.     

Detection and analysis of ion-acoustic waves in the ionosphere

Summary Propagating acoustic-like disturbances generated by nuclear explosions have been observed to split in the lower ionosphere. The resulting two disturbances propagate vertically into the upper ionosphere at different speeds, the slower at ordinary sound speed in a neutral medium and the faster at almost twice the speed of sound.The faster disturbance travels at the speed of an ion-acoustic wave in the ionic fluid. Such identification is prohibited theoretically by the large damping, or coupling, of the ionic mode to the neutral particle medium through ion-neutral elastic collisions; however, if inelastic collisions are included in the theory, the principal exothermic charge-exchange and charge-transfer reactions in the ionosphere provide enough additional coherent momentum in the charged particles to offset the losses through elastic collisions. It is shown quantitatively that in some regions of the ionosphere ionacoustic wave propagation can occur almost losslessly.It is possible that under some conditions the ionosphere approaches an unstable chemical equilibrium that is relieved by the spontaneous generation of ion-acoustic wavelets. Various ionospheric observations are examined in which there is some evidence of the effects of ionospheric ionacoustic disturbances.     

Polar regionSq

Geomagnetically quiet day variations in the polar region are reviewed with respect to geomagnetic field variation, ionospheric plasma convection, electric field and current. Persistently existing field-aligned currents are the main source of the polar regionSq. Consequently, the morphology and variability of the polar regionSq largely depend upon both field-aligned currents and ionospheric conductivity. Since field-aligned currents are the major linkage between the ionosphere and the magnetosphere, the latter is controlled by solar wind state, in particular, the interplanetary magnetic field, the polar regionSq exhibits remarkable IMF dependence.     

Effects of geomagnetic activity on the mesospheric electric fields

The results of three series of rocket measurements of mesospheric electric fields carried out under different geomagnetic conditions at polar and high middle latitudes are analysed. The measurements show a clear dependence of the vertical electric fields on geomagnetic activity at polar and high middle latitudes. The vertical electric fields in the lower mesosphere increase with the increase of geomagnetic indexes K_p and K_p. The simultaneous increase of the vertical electric field strength and ion conductivity was observed in the mesosphere during geomagnetic disturbances. This striking phenomenon was displayed most clearly during the solar proton events of October, 1989 accompanied by very strong geomagnetic storm (K_p = 8+). A possible mechanism of generation of the vertical electric fields in the mesosphere caused by gravitational sedimentation of charged aerosol particles is discussed. Simultaneous existence in the mesosphere of both the negative and positive multiply charged aerosol particles of different sizes is assumed for explanation of the observed V/m vertical electric fields and their behaviour under geomagnetically disturbed conditions.Paper Presented at the Second IAGA/ICMA (IAMAS) Workshop on Solar Activity Forcing of the Middle Atmosphere, Prague, August 1997     

Nonlinear evolution of the atmosphere and ionosphere above a seismic epicenter. II. Numerical simulation

The results of numerical simulation of the nonlinear evolution of an acoustic pulse propagating above an earthquake epicenter with preset energy parameters are presented. The interaction with charged plasma particles in the lower ionosphere is taken into account. The interaction between neutral and charged components is described by the diffusion mechanism of the disturbance of the density of charged ionospheric plasma components. The simulation was carried out for three values of earthquake magnitude M = 5; 6; and 7. It is ascertained that the numerical simulation results agree with analytical estimates made during the study of main regularities of seismic events that induce weak nonlinear effects in the atmosphere.     

Dependence of the ionospheric response on the solar flare parameters based on the theoretical modeling and GPS data

The measurements of an increase in the total electron content (TEC) of the ionosphere during solar flares, obtained based on the GPS data, indicated that up to 30% of TEC increments corresponded to the ionospheric regions above 300 km altitude in some cases, and TEC increased mainly below altitudes of 300 km in other cases. The theoretical model of the ionosphere and plasmasphere was used to study the obtained effects. The altitude-time variations in the charged particle density in the ionospheric region from 100 to 1000 km were used depending on the solar flare spectrum. An analysis of the modeling results indicated that an intensification of the flare UV emission in the 55–65 and 85–95 nm spectral ranges results in a pronounced increase in the electron density in the topside ionosphere (above 300 km). The experimental dependences of the ionospheric TEC response amplitude on the localization and peak power of flares on the Sun in the X-ray range, obtained based on the GPS data, are also presented in the work.     

Variability of parameters of the <Emphasis Type="Italic">F</Emphasis>2-layer maximum in the quiet midlatitude ionosphere under low solar activity: 1. Statistical properties

Results of statistical analysis of the properties of variability of F2-layer maximum parameters (critical frequency foF2 and the height hmF2) in quiet midlatitude ionosphere under low solar activity in the daytime (1000–1500 LT) and nighttime (2200–0300 LT) hours are presented on the basis of Irkutsk station data for 2007–2008. It is found that the distribution density of δfoF2 could be presented as consisting of two distinctly different normal laws of this distribution, one of which corresponds to weak (|δfoF2| < 10%) fluctuations in foF2 and the other corresponds to strong (30% > |δfoF2| > 10%) fluctuations. Weak fluctuations in foF2 to a substantial degree are related to ionospheric variability at times less of than 1–3 h and determine the δfoF2 variability in the daytime hours. Strong fluctuations in foF2 are mainly related to day-to-day variability of the ionosphere at a fixed local time, the variability increasing by approximately a factor of 3 during the transition from day to night and determining the δfoF2 variability in the nighttime hours. The distribution density of ΔhmF2 is close to the normal distribution law. An interpretation of the different character of the distribution densities of δfoF2 and ΔhmF2 is given.     

Response of mesospheric ozone to the heating of the lower ionosphere by high-power HF radio emission

We detected a decrease in the intensity of microwave radiation at the atmospheric ozone line at a frequency of 110836.04 MHz during ionospheric modification by high-power HF radiowaves radiated by the Sura Ionospheric Heating Facility. The obtained experimental data allowed us to hypothesize that this effect was caused by the fact that mesospheric ozone was affected by internal gravity waves generated in the E region of the ionosphere during its high-power HF radiowave heating.     

Application of the method of numerical simulation of ionospheric filtration of Pc1 signals to solving the inverse problem of ionospheric modelling

Summary The numerical method of simulating ionospheric filtration of ULF signals in the range of Pc1 frequencies has been applied to French geomagnetically conjugate observations of ULF signals made by the GEOS-1 satellite and at the observatory of Husafell (Iceland) [2]. The experimentally obtained variable values of the transmissivity of the Pc1 signal through the ionosphere [5] in the course of the micropulsation distrubance of 13.7. 1977 have been compared with the results of the numerical simulation taking into consideration the fundamental physical parameters of the high-latitude external ionosphere. This approximate form of solving the inverse problem of ionospheric modelling yielded quantitative estimates of the rapid variations of the concentration of charged particles in connection with the expected changes of their temperature. It is assumed that nonstationary states of the ionospheric plasma are caused by the very ion-cyclotron waves penetrating the ionosphere at high latitudes ( 70°) along the plasmapause.     

Charged meteoric smoke as ice nuclei in the mesosphere: Part 1—A review of basic concepts

The role of meteoric smoke as condensation nuclei for mesospheric ice has recently been challenged by model simulations on the global transport of meteoric material. At the same time a considerable fraction of smoke particles is charged in the mesosphere. This has significant effects on nucleation processes as it can remove the Kelvin barrier. We suggest that in particular nucleation on negatively charged smoke is likely to be a dominant mechanism for mesospheric ice formation. This is in contrast to nucleation on positive ion clusters as the latter is largely hampered by efficient ion/electron recombination. Surprisingly, the large potential of nucleation on charged smoke has so far not been considered in mesospheric ice models. A challenging question concerns the fraction of mesospheric smoke that is actually charged. An improved understanding of mesospheric charging and nucleation will require laboratory experiments on nuclei in the transition regime between molecular and particulate sizes.     

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.     

Revealing the classes of ionospheric disturbances on the basis of multiyear data on the critical frequency of the <Emphasis Type="Italic">F</Emphasis>2 layer

The bases of the classification method of ionospheric disturbances caused by solar-geomagnetic activity on the basis of the critical frequency of the F2 layer are developed. Data for the total solar activity cycle from 1975 to 1986 were used for studying variations in the critical frequency of the ionospheric F2 layer. The critical frequency was measured at the Moscow ionospheric observatory (55°45′N, 37°37′E) at an interval of 1 h. The gaps in the critical frequency values were filled in by the cubic interpolation method. The solar activity level was estimated using the F10.7 index. The geomagnetic disturbance was determined using the Kp · 10, Dst, and AE indices. According to the developed classification, an index of ionospheric activity is introduced. An analysis of the obtained values of the index for years of solar activity minimum and maximum shows that an increase in the absolute values of the index as a rule occurs at an increase in global geomagnetic and/or auroral disturbances. This fact indicates the sufficient information content of the developed index for characterizing ionospheric activity in any season. Moreover, using the sign of the index, one can form an opinion regarding an increase or decrease in the concentration of the ionospheric F2 layer, because the values of the considered index correspond to real oscillations in the critical frequency of the midlatitude ionosphere.     

Configuration of the upper boundary of the ionosphere

Variations of the upper boundary of the ionosphere (UBI) are investigated based on three sources of information: (i) ionosonde-derived parameters: critical frequency foF2, propagation factor M3000F2, and sub-peak thickness of the bottomside electron density profile; (ii) total electron content (TEC) observations from signals of the Global Positioning System (GPS) satellites; (iii) model electron densities of the International Reference Ionosphere (IRI^*) extended towards the plasmasphere. The ionospheric slab thickness is calculated as ratio of TEC to the F2 layer peak electron density, NmF2, representing a measure of thickness of electron density profile in the bottomside and topside ionosphere eliminating the plasmaspheric slab thickness of GPS-TEC with the IRI^* code. The ratio of slab thickness to the real thickness in the topside ionosphere is deduced making use of a similar ratio in the bottomside ionosphere with a weight Rw. Model weight Rw is represented as a superposition of the base-functions of local time, geomagnetic latitude, solar and magnetic activity. The time-space variations of domain of convergence of the ionosphere and plasmasphere differ from an average value of UBI at ∼1000 km over the earth. Analysis for quiet monthly average conditions and during the storms (September 2002, October–November 2003, November 2004) has shown shrinking UBI altitude at daytime to 400 km. The upper ionosphere height is increased by night with an ‘ionospheric tail’ which expands from 1000 km to more than 2000 km over the earth under quiet and disturbed space weather. These effects are interposed on a trend of increasing UBI height with solar activity when both the critical frequency foF2 and the peak height hmF2 are growing during the solar cycle.     

Ionospheric variability over Grocka during low solar activity conditions

The variability of the critical frequency of F2 layer, foF2, over ionospheric station Grocka (44.48N, 20.31E) has been studied during the declining phase of solar cycle 23 from 2004 to 2006. The variability index was introduced to identify the daily and seasonal patterns characterizing the local mid-latitude ionosphere during quiet and disturbed geomagnetic conditions. In addition, the behaviour of the vertical total electron content values, vTEC, obtained from global positioning system (GPS) measurements in the surrounding area under these conditions is reported. The analysis shows a number of interesting features representative of the ionospheric variability relevant for ionospheric modelling as well as ionospheric propagation applications based on a single station approach.     

Atmospheric electricity coupling between earthquake regions and the ionosphere

We propose a mechanism to explain suggested links between seismic activity and ionospheric changes detected overhead. Specifically, we explain changes in the natural extremely low-frequency (ELF) radio noise recently observed in the topside ionosphere aboard the DEMETER satellite at night, before major earthquakes. Our mechanism utilises increased electrical conductivity of surface layer air before a major earthquake, which reduces the surface-ionosphere electrical resistance. This increases the vertical fair weather current, and (to maintain continuity of electron flow) lowers the ionosphere. Magnitudes of crucial parameters are estimated and found to be consistent with observations. Natural variability in ionospheric and atmospheric electrical properties is evaluated, and may be overcome using a hybrid detection approach. Suggested experiments to investigate the mechanism involve measuring the cut-off frequency of ELF “tweeks”, the amplitude and phase of very low frequency radio waves in the Earth–ionosphere waveguide, or medium frequency radar, incoherent scatter or rocket studies of the lower ionospheric electron density.     

On the 18-day quasi-periodic oscillation in the ionosphere

The presence and persistence of an 18-day quasi-periodic oscillation in the ionospheric electron density variations were studied. The data of lower ionosphere (radio-wave absorption at equivalent frequency near 1 MHz), middle and upper ionosphere (critical frequencies f₀E and f₀F2) for the period 1970–1990 have been used in the analysis. Also, solar and geomagnetic activity data (the sunspot numbers Rz and solar radio flux F10.7 cm, and a_N index respectively) were used to compare the time variations of the ionospheric with the solar and geomagnetic activity data. Periodogram, complex demodulation, auto- and cross-correlation analysis have been used. It was found that 18-day quasi-periodic oscillation exists and persists in the temporal variations of the ionospheric parameters under study with high level of correlation and mean period of 18–19 days. The time variation of the amplitude of the 18-day quasi-periodic oscillation in the ionosphere seems to be modulated by the long-term solar cycle variations. Such oscillations exist in some solar and geomagnetic parameters and in the planetary wave activity of the middle atmosphere. The high similarities in the amplitude modulation, long-term amplitude variation, period range between the oscillation of investigated parameters and the global activity of oscillation suggests a possible solar influence on the 18-day quasi-periodic oscillation in the ionosphere.     

Correlation analysis of vertical total electron content

It is necessary to model and analyze the ionospheric effects due to a direct relationship between Global Positioning System (GPS) applications and changes in the ionosphere. In order to monitor these changes, the ionosphere can be represented by the vertical total electron content (VTEC) which can be used to analyze ionospheric conditions from a variety of stations. In this study, 21 stations were used to carry out analysis and estimation of VTEC. Three days during a geomagnetic storm, namely, 7, 8, and 9 January 2005, are chosen for investigation. In addition, the de-correlation time of the VTEC was estimated to define ionospheric variations in time using autocorrelation analysis. The de-correlation time of the ionosphere is based on correlation times estimated by using autocorrelation functions. From the high-latitude stations, the mean of the correlation times decreased from 8 to 6 epochs during a storm. In this time period, it was found from the station results that the ionosphere was more affected at the high-latitude than at the mid-latitude region.     

Variations in ionospheric parameters during solar flares

Results of studying the ionospheric response to solar flares, obtained based on the incoherent scatter radar observations of the GPS signals and as a result of the model simulations, are presented. The method, based on the effect of partial “shadowing” of the atmosphere by the globe, has been used to analyze the GPS data. This method made it possible to estimate the value of a change in the electron content in the upper ionosphere during the solar flare of July 14, 2000. It has been shown that a flare can cause a decrease in the electron content at heights of the upper ionosphere (h > 300 km) according to the GPS data. Similar effects in the formation of a negative disturbance in the ionospheric F region were also observed during the solar flares of May 21 and 23, 1967, at the Arecibo incoherent scatter radar. The mechanism by which negative disturbances are formed in the upper ionosphere during solar flares has been studied based on the theoretical model of the ionosphere-plasmasphere coupling. It has been shown that an intense ejection of O⁺ ions into the above located plasmasphere under the action of a sharp increase in the ion production rate and the thermal expansion of the ionospheric plasma cause the formation of a negative disturbance in the electron concentration in the upper ionosphere.