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.     

The magnetosphere–ionosphere system from the perspective of plasma circulation: A tutorial

This tutorial review examines the role of O⁺ in the dynamics of magnetosphere–ionosphere coupling. The life cycle of an O⁺ plasma element is considered as it circulates from the mid- to high-latitude ionosphere. Energization and diversion of the convecting plasma element into outflows involves Alfvénic turbulence at the low-altitude base of the cusp and plasmasheet boundary layer and in downward-current “pressure cookers.” Observational evidence indicating that O⁺ dominates the plasmasheet and ring current during extreme storm intervals is reviewed. The impacts of an O⁺-enriched plasma on solar wind–magnetosphere–ionosphere coupling are considered at both the micro and global scales. A synthesis of results from observation, theory and simulations suggests that the presence of O⁺ in the magnetosphere is both a disruptive and a moderating agent in maintaining the balance between dayside and nightside magnetic merging.     

Current state-of-the-art for the measurement of non-Maxwellian plasma parameters with the EISCAT UHF Facility

New results on the information that can be extracted from simulated non-Maxwellian incoherent radar spectra are presented. The cases of a pure ionosphere and of a composite ionosphere typical of a given altitude of the auroral F region are considered. In the case of a pure ionosphere of NO⁺ or O⁺ ions it has been shown that the electron temperature and the electron density can be derived from a Maxweilian analysis of radar spectra measured at aspect angles of 0° or 21° respectively; the ion temperature and ion temperature anisotropy can be derived from a non- constraining model such as the ID Raman fitting of a complementary measurement made at an aspect angle larger than 0° for the NO⁺ ions, or at an aspect angle larger than 21° for the O⁺ ions. Moreover with such measurements at large aspect angles, the shape of the velocity ion distribution functions can simultaneously be inferred. The case of a composite ionosphere of atomic O⁺ and molecular NO⁺ions is a difficult challenge which requires simultaneously a complementary measurement of the electron temperature to provide the ion composition and the electron density from the incoherent radar spectra at a specific aspect angle of 21°; hence, a model dependent routine is necessary to derive the ion temperatures and ion temperature anisotropies. In the case where the electron temperature is not given, a routine which depends on ion distribution models is required first: the better the ion distribution models are, the more accurately derived the plasma parameters will be. In both cases of a composite ionosphere, the 1D Raman fitting can be used to keep a check on the validity of the results provided by the ion distribution model dependent routine.     

Geomagnetic activity dependence of O in transit from the ionosphere

Energetic O⁺ ions have important dynamic effects on the ring current. Insights into the effects of O⁺ on ring current dynamics have come primarily from models, not observations. Here, we discuss observations of O⁺ populations escaping from the ionosphere and their access to the plasma sheet and ring current. We review data establishing that a significant flux of O⁺ escapes the ionosphere during geomagnetically quiet intervals. We then estimate the relative magnitude of the O⁺ population in transit between the ionosphere and ring current during quiet intervals before geomagnetic storms. Our analysis suggests that dynamic reconfigurations of the magnetosphere during geomagnetic storms significantly alter the O⁺ transport paths from the ionosphere to the ring current. During these reconfigurations some of the pre-existing, quiet time, in-transit O⁺ populations are captured on magnetic field lines leading to the ring current. The prompt appearance of this O⁺ population in the ring current could modify the evolution of the ring current in the storm growth phase. Our analysis suggests that the consequences of an activity-dependent O⁺ transport path to the ring current should be systematically investigated.     

Time-dependent simulations of the global polar wind

It has been clearly established that there is a substantial outflow of ionospheric plasma from the Earth's ionosphere in both the northern and southern polar regions. The outflow consists of both light thermal ions (H⁺ and He⁺) and an array of energized ions (NO⁺, O₂⁺, N₂⁺, O⁺, N⁺, He⁺, and H⁺). If the outflow is driven by thermal pressure gradients in the ionosphere, the outflow is called the “classical” polar wind. On the other hand, if the outflow is driven by energization processes either in the auroral oval or at high altitudes in the polar cap, the outflow is called the “generalized” polar wind. In both cases, the field-aligned outflow occurs in conjunction with magnetospheric convection, which causes the plasma to drift into and out of the sunlit hemisphere, cusp, polar cap, nocturnal auroral oval, and main trough. Because the field-aligned and horizontal motion are both important, three-dimensional (3-D) time-dependent models of the ionosphere–polar wind system are needed to properly describe the flow. Also, as the plasma executes field-aligned and horizontal motion, charge exchange reactions of H⁺ and O⁺ with the background neutrals (H and O) act to produce low-energy neutrals that flow in all directions (the neutral polar wind). This review presents recent simulations of the “global” ionosphere–polar wind system, including the classical, generalized, and neutral polar winds. The emphasis is on displaying the 3-D and dynamical character of the polar wind.     

Disturbances in the topside ionosphere during solar flares

The results of studying the ionospheric response to solar flares, obtained from the data of the GPS signal observations and incoherent scatter radars and as a result of the model calculations, are presented. It is shown that, according to the GPS data, a flare can cause a decrease in the electron content at altitudes of the topside ionosphere (h > 300 km). Similar effects of formation of a negative disturbance in the ionospheric F region were also observed during the solar flares of May 21 and 23, 1967, with the Arecibo incoherent scatter radar. The mechanism by which negative disturbances appear in the topside ionosphere during solar flares has been studied in this work based on the theoretical model of the ionosphere-plasmasphere coupling. It has been indicated that the formation of the electron density negative disturbance in the topside ionosphere is caused by an intense removal of O⁺ ions into the overlying plasmasphere under the action of an abrupt increase in the ion production rate and thermal expansion of the ionospheric plasma.     

The role of vibrationally excited nitrogen in the formation of the mid-latitude negative ionospheric storms

Millstone Hill ionospheric storm time measurements of the electron density and temperature during the ionospheric storms (15-16 June 1965; 29–30 September 1969 and 17–18 August 1970) are compared with model results. The model of the Earth’s ionosphere and plasmasphere includes interhemispheric coupling, the H⁺, O⁺(⁴S), O⁺(²D), O⁺(²P), NO⁺, O⁺₂ and N⁺₂ ions, electrons, photoelectrons, the electron and ion temperature, vibrationally excited N₂ and the components of thermospheric wind.In order to model the electron temperature at the time of the 16 June 1965 negative storm, the heating rate of the electron gas by photoelectrons in the energy balance equation was multiplied by the factors 5–30 at he altitude above 700 km for the period 4.50-12.00 LT, 16 June 1965. The [O]/[N₂] MSIS-86 decrease and vibrationally excited N₂ effects are enough to account for the electron density depressions at Millstone Hill during the three storms. The factor of 2 (for 27–30 September 1969 magnetic storm) and the & actor 2.7 (for 16–18 August 1970 magnetic storm) reduction in the daytime peak density due to enhanced vibrationally excited N₂ is brought about by the increase in the O⁺+N₂ rate factor.     

Pulsating of the generalized ion and neutral polar winds

A three-dimensional, time-dependent model of the ion and neutral polar winds was used to study their dynamic evolution during the May 4, 1998 magnetic storm. The simulation tracked the dynamics of five species (O⁺, H⁺, H_s, O_s, and electrons) and covered a 9-h period. During the storm, D_st decreased to −210 nT, Ap reached 300, and K_p was elevated. The IMF B_z component was southward at the start of the storm and for several hours thereafter and then turned northward. However, the magnetospheric energy input to the ionosphere exhibited a 50-min oscillation, with the plasma convection and particle precipitation patterns expanding and contracting in a periodic manner. As a consequence, the ion and neutral polar winds pulsated with an approximate 50-min period. The H⁺ and O⁺ ions displayed cyclic upflows and downflows in the topside ionosphere as well as a highly structured spatial distribution that varied with time. The vertical flux of the neutral H_s atoms was upward at the top of the ionosphere, but the magnitude varied in a cyclic manner in response to the oscillating stormtime energy input. The vertical flux of neutral O_s atoms was downward at the top of the ionosphere and varied significantly with the stormtime energy input. For H⁺, O⁺, and H_s, the maximum total (integrated) vertical flux during the storm was upward at the top of the ionosphere, with values of 8–9×10²⁵ particles/s for H⁺, 2–4×10²⁶ particles/s for O⁺, and 2–3×10²⁷ particles/s for H_s. The corresponding total vertical O_s flux was predominately downward, with only localized areas with positive fluxes.     

A case study of storm commencement and recovery plasmaspheric electric fields near L=2.5 at equinox

Data from the VLF Doppler experiment at Faraday, Antarctica (65○ S, 64○ W) are used to study the penetration of the high-latitude convection electric field to lower latitudes during severely disturbed conditions. Alterations of the electric field at L-values within the range 2.0 - 2.7 are studied for two cases at equinox (10 - 12 September 1986 and 1 - 3 May 1986). The recovery of the electric field is found to be approximately an exponential function of time. Values for the equatorial meridional E×B drift velocity, inferred from the data, are used as inputs to a model of the plasmasphere and ionosphere. The model and experimental results are used to investigate the post-storm alteration of ionospheric coupling processes. The magnitude of the effect of ionosphere-plasmasphere coupling fluxes on N_mF2 values and the O⁺-H⁺ transition height is dependent on the local time of storm commencement, and on the orientation of the electric field. The coupling fluxes appear to have a maximum influence on ionospheric content during the main phase of geomagnetic activity that produces outward motion of plasmaspheric whistler ducts.     

Anomalous electron density events in the quiet summer ionosphere at solar minimum over Millstone Hill

This study compares the observed behavior of the F region ionosphere over Millstone Hill with calculations from the IZMIRAN model for solar minimum for the geomagnetically quiet period 23–25 June 1986, when anomalously low values of hmF2(<200 km) were observed. We found that these low values of hmF2 (seen as a G condition on ionograms) exist in the ionosphere due to a decrease of production rates of oxygen ions resulting from low values of atomic oxygen density. Results show that determination of a G condition using incoherent scatter radar data is sensitive both to the true concentration of O⁺ relative to the molecular ions, and to the ion composition model assumed in the data reduction process. The increase in the O⁺ + N₂ loss rate due to vibrationally excited N₂ produces a reduction in NmF2 of typically 5–10%, but as large as 15%, bringing the model and data into better agreement. The effect of vibrationally excited NO⁺ ions on electron densities is negligible.     

Numerical simulation of the <Emphasis Type="Italic">F</Emphasis>2-layer stratification and appearance of the <Emphasis Type="Italic">F</Emphasis>3 and <Emphasis Type="Italic">G</Emphasis> layers in the equatorial ionosphere: The morphology of the phenomena

This work is devoted to a numerical simulation of the equatorial ionosphere, performed using the GSM TIP model completed with a new block for calculating the electric field. It has been indicated that the usage of the wind system calculated according to the MSIS-90 model makes it possible to reproduce the electromagnetic drift velocities at the equator, the effect of the F2-layer stratification, and the appearance of the F3 layer in the equatorial ionosphere. The calculations performed using the modified GSM TIP model made it possible to detect a maximum in the electron density vertical profile at an altitude of ∼1000 km, formed by H⁺ ions, which we called the G layer. If this layer actually exists, it can be observed during sounding the low-latitude ionosphere from satellites during dark time of day.     

Some features of nighttime enhancements in the electron concentration in the F2-layer maximum of the midlatitude ionosphere

Using the data of vertical sounding of the ionosphere in Alma-Ata (76°55′ E, 43°15′ N) conducted in 2002–2012, the reaction of parameters of the ionospheric F2 layer to various types of nighttime enhancements in the electron concentration in the maximum of the layer (NmF2) was studied, including the height of the maximum and bottom of the layer, its semithickness, and electron concentration at some fixed heights. Examples of recordings of a combination of the enhancements caused by different mechanisms are presented. The similarity of the reaction of the F2-layer parameters to the nighttime enhancements caused by the rise of the layer and plasma flux from the protonosphere and passage of large-scale travelling ionospheric disturbances was found. Difficulties in identifying these two events in the case of their equal duration are noted. The difference in the reaction of the F2-layer parameters to the enhancements caused by the rise of the layer and plasma fluxes from the protonosphere and occurrence of the summer midlatitude ionospheric anomaly is shown.     

A numerical model of the ionosphere,including the E-region above EISCAT

It has been previously demonstrated that a two-ion (O⁺ and H⁺) 8-moment time-dependent fluid model was able to reproduce correctly the ionospheric structure in the altitude range probed by the EISCAT-VHF radar. In the present study, the model is extended down to the E-region where molecular ion chemistry (NO⁺ and O⁺₂, essentially) prevails over transport; EISCAT-UHF observations confirmed previous theoretical predictions that during events of intense E×B induced convection drifts, molecular ions (mainly NO⁺) predominate over O⁺ ions up to altitudes of 300 km. In addition to this extension of the model down to the E-region, the ionization and heating resulting from both solar insolation and particle precipitation is now taken into account in a consistent manner through a complete kinetic transport code. The effects of E×B induced convection drifts on the E- and F-region are presented: the balance between O⁺ and NO⁺ ions is drastically affected; the electric field acts to deplete the O⁺ ion concentration. The [NO⁺]/[O⁺] transition altitude varies from 190 km to 320 km as the perpendicular electric field increases from 0 to 100 mV m⁻¹. An interesting additional by-product of the model is that it also predicts the presence of a noticeable fraction of N⁺ ions in the topside ionosphere in good agreement with Retarding Ion Mass Spectrometer measurements onboard Dynamic Explorer.     

Two-dimensional drift velocities from the IMAGE EUV plasmaspheric imager

The Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) Mission extreme ultraviolet (EUV) imager observes He⁺ plasmaspheric ions throughout the inner magnetosphere. Limited by ionizing radiation and viewing close to the sun, images of the He⁺ distribution are available every 10 min for many hours as the spacecraft passes through apogee in its highly elliptical orbit. As a consistent constituent at about 15%, He⁺ is an excellent surrogate for monitoring all of the processes that control the dynamics of plasmaspheric plasma. In particular, the motion of He⁺ transverse to the ambient magnetic field is a direct indication of convective electric fields. The analysis of boundary motions has already achieved new insights into the electrodynamic coupling processes taking place between energetic magnetospheric plasmas and the ionosphere. Yet to be fulfilled, however, is the original promise that global EUV images of the plasmasphere might yield two-dimensional pictures of mesoscale to macroscale electric fields in the inner magnetosphere. This work details the technique and initial application of an IMAGE EUV analysis that appears capable of following thermal plasma motion on a global basis.     

Contributions to the Ring Current Energy During Geomagnetic Storms

The purpose of this research was to study the complexity of the energization of the ring current during a geomagnetic storm, produced during southern Bz(IMF) by the injection of plasma sheet ions, accelerated by enhanced convective electric fields. This model assumes that the plasma sheet is continuously populated by H⁺ from the sun and the ionosphere, and sporadically by ionospheric O⁺, making the ring current a coupled system whose energy can hardly be expressed analytically. When Bz(IMF) turns north, the ring current becomes uncoupled, and the energy decays exponentially if the storm is weak, or can be expressed as a combination of exponentials during strong storms representing the quick decay of O⁺ and the slower decay of H⁺, as has been shown.     

Model calculations of the total electron content for the system of GPS satellites

The numerical global self-consistent model of the Earth’s thermosphere, ionosphere and protonosphere (GSM TIP), which makes it possible to calculate all the main parameters of the near-Earth plasma, is used to calculate the total electron content (TEC). Calculations have been performed along the radiosignal propagation trajectory between a surface receiving point and a GPS satellite. The TEC value calculated from the satellite data have been compared with such a “true model” TEC value for magnetically quiet conditions of the spring equinox and moderate solar activity. The relative errors in determining the satellite data-based TEC for two European (Troms have been calculated. It has been indicated that an increase in the number of satellites not always results in an increase in accuracy of the TEC value measured on satellites.     

Localized structure in the cusp and high-latitude ionosphere: a modelling study

The ionospheric signature of a flux transfer event (FTE) seen in EISCAT radar data has been used as the basis for a modelling study using a new numerical model of the high-latitude ionosphere developed at the University of Sheffield, UK. The evolution of structure in the high-latitude ionosphere is investigated and examined with respect to the current views of polar patch formation and development. A localized velocity enhancement, of the type associated with FTEs, is added to the plasma as it passes through the cusp. This is found to produce a region of greatly enhanced ion temperature. The new model can provide greater detail during this event as it includes anisotropic temperature calculations for the O⁺ ions. This illustrates the uneven partitioning of the energy during an event of this type. O⁺ ion temperatures are found to become increasingly anisotropic, with the perpendicular temperature being substantially larger than the parallel component during the velocity enhancement. The enhanced temperatures lead to an increase in the recombination rate, which results in an alteration of the ion concentrations. A region of decreased O⁺ and increased molecular ion concentration develops in the cusp. The electron temperature is less enhanced than the ions. As the new model has an upper boundary of 10 000 km the topside can also be studied in great detail. Large upward fluxes are seen to transport plasma to higher altitudes, contributing to the alteration of the ion densities. Plasma is stored in the topside ionosphere and released several hours after the FTE has finished as the flux tube convects across the polar cap. This mechanism illustrates how concentration patches can be created on the dayside and be maintained into the nightside polar cap.     

Ion-neutral coupling in the high-latitude F-layer from incoherent scatter and Fabry-Perot interferometer measurements

Since the auroral ionosphere provides an important energy sink for the magnetosphere, ionosphere-thermosphere coupling must be investigated when considering the energy budget of the ionosphere-magnetosphere coupling. We present the first Scandinavian ground-based study of high-latitude F-region ion-neutral frictional heating where ion velocity and temperature are measured by the EISCAT incoherent scatter radar as well as neutral wind and temperature being measured simultaneously by a Fabry-Perot interferometer. A geomagnetically active period (K_p = 7^- - 5^-) and quiet period (K_p = 0⁺ - 0) were studied. Neglecting the neutral wind can result in errors of frictonal heating estimates of 60% or more in the F-layer. About 96% of the local ion temperature enhancement over the neutral temperature is accounted for by ion-neutral frictional heating.     

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.