Mid-latitude Topside Research Articles

Evaluations of two ion temperature models in IRI-2020 based on the observations from ICON and COSMIC-2 IVM

Ion temperature is a critical parameter for understanding the ionospheric thermal processes. Its measurement is relatively sparse compared to other ionospheric parameters, and validation of existing empirical models remains an important task. This study deals with the quiet time low- and mid-latitude topside ionospheric ion temperature measured by the Ionospheric Connection Explorer (ICON) and the Constellation Observing System for Meteorology Ionosphere and Climate (COSMIC-2). We conducted statistical analysis for local time, latitudinal, seasonal and solar activity variations. The comparison of the observations with two ion temperature models (Bil-1981 and TBKST-2021) of the International Reference Ionosphere-2020 (IRI-2020) shows that TBKST-2021 performs better than Bil-1981. TBKST-2021 model captures the morning and afternoon peaks quite well although with some underestimation. The underestimation of the morning and afternoon peaks may be due to overcorrection. Further, it shows a good seasonal and latitudinal dependence, with a better performance at the equinox than in solstices. However, TBKST-2021 requires to improve the presentation of hemispheric differences in solstices.

Daytime ion and electron temperatures in the topside ionosphere at middle latitudes

AbstractIn the topside ionosphere during the daytime, the thermal electrons are directly heated by photoelectron fluxes from the local and conjugate hemispheres. The heat is lost primarily through conduction to the regions below where collisions with the ions provide the cooling. Ions are heated by collisions with both the electrons and ions and are cooled primarily by conduction to lower altitudes where the heat is lost to collisions with the neutral gas. Here we examine the effects of ion composition on the ion and electron temperatures in the midlatitude topside ionosphere during daytime using data from the Defense Meteorological Satellite Program F15 satellite in 2004–2006. The variations of electron and ion temperature with plasma density and composition indicate how the heat is exchanged between the different constituent ions and the role played by the solar zenith angle and the solar ionizing flux (F10.7). The observations show that the daytime temperature difference between the O+ and electron is smallest when the fractional contribution of H+ is the highest. In such an environment the electrons transfer heat to the H+ then to the O+ through ion‐ion collisions.

Plasma density undulations in the nighttime mid-latitude F-region as observed by CHAMP, KOMPSAT-1, and DMSP F15

We investigate plasma density undulations in the nighttime mid-latitude topside F-region. During solar maximum years the undulations are found at CHAMP, KOMPSAT-1, and DMSP F15 altitudes. The occurrence rate is higher at KOMPSAT-1 than at DMSP F15 altitude. The undulations occur infrequently during equinoxes, and the occurrence peaks are in the Asian/Oceanian (eastern Pacific/American) region during June (December) solstice. At CHAMP altitude the undulations are observed all through the night, and the occurrence rate is anti-correlated with the solar cycle. As all these results are in general agreement with known climatology of MSTIDs, we suggest that the undulations are a topside signature of MSTIDs. The undulations are often but not always accompanied by magnetic signatures indicating the presence of field-aligned current (FAC). The partial lack in correspondence might be due to the ionospheric conductivity variation. The similar distribution is, however, in support of a connection between density undulations and FACs.

Plasma temperatures in Saturn's ionosphere

We have calculated self‐consistent electron and ion temperatures in Saturn's ionosphere using a series of coupled fluid and kinetic models developed to help interpret Cassini observations and to examine the energy budget of Saturn's upper atmosphere. Electron temperatures in the midlatitude topside ionosphere during solar maximum are calculated to range between 500 and 560 K during the Saturn day, approximately 80–140 K above the neutral temperature. Ion temperatures, calculated for only the major ions H+ and H3+, are nearly equal to the neutral temperature at altitudes near and below the height of peak electron density, while they can reach 500 K during the day at the topside. Plasma scale heights of the dusk electron density profile from radio occultation measurements of the Voyager 2 flyby of Saturn have been used to estimate plasma temperature as a comparison. Such an estimate agrees well with the temperatures calculated here, although there is a topside enhancement in electron density that remains unexplained by ionospheric calculations that include photochemistry and plasma diffusion. Finally, parameterizations of the heating rate from photoelectrons and secondary electrons to thermal, ambient electrons have been developed. They may apply for other conditions at Saturn and possibly at other giant planets and exoplanets as well.

A global study of nighttime midlatitude topside spread echoes

Range spreading is frequently evident in midlatitude topside ionograms from the ISS‐B satellite. These ionograms provide a geographically unbiased perspective for the first global investigation of nighttime spread echo events that focuses solely on the midlatitude domain. The topside echo occurrence probability data show significant longitudinal variations, along with very large winter/equinox asymmetries in several geographic regions. The spread echoes are divided into two types and found to have different geophysical dependences. Small‐scale events that appear as spreading in one or two consecutive ionograms have a marked seasonal dependence on magnetic declination and no obvious correlation with auroral indices. Larger‐scale events in which the spreading is observed over spatial domains of 1000 km or more have less dependence on magnetic declination but are anticorrelated with the AE index in periods 6–8 hours prior to their appearance. The declination dependence for the small‐scale events suggests that the irregularities are most prevalent when the E region conductivity is low at both ends of the flux tube. This in turn suggests that electric fields mapping from the E region up to F region altitudes are likely involved in their generation. The correlation of the larger‐scale events with depressed AE signatures suggests that traveling ionospheric disturbances caused by auroral heating are not the most likely cause of these large features at midlatitudes.

Derivation of neutral oxygen density under charge exchange in the midlatitude topside ionosphere

We describe a new technique to derive neutral atomic oxygen density, [O], in the upper thermosphere using coincident incoherent scatter radar (ISR) and airglow emission observations from Arecibo Observatory. The technique exploits the nearly resonant charge exchange coupling between neutral and ionized hydrogen and oxygen that serves as the dominant chemical source and sink of protons near and above the F region peak. Under charge exchange production and loss of H+, the proton continuity equation can be solved for [O] using twilight H density profiles derived from measured H emission brightness at 656.3 nm together with ion density, temperature, and flux obtained simultaneously by the Arecibo ISR. We present both equilibrium and nonequilibrium solutions for [O] between 500 and 1500 km during a single quiescent nighttime interval under moderate solar activity. Comparisons with theoretical expectations and with MSIS‐model calculations of O density are used to identify the altitude and local time extent over which the technique is justified. These comparisons generally support technique validity between ∼600 and 800 km, where sufficient reactant densities are present to validate the charge exchange formulation of the continuity equation. Equilibrium solutions for [O] near 650–700 km exhibit excellent agreement with MSIS estimates before midnight, but deviations arising from ion transport become increasingly significant both above this height and as dawn approaches. Incorporation of measured proton flux gradients into the nonequilibrium solutions improves agreement between the derived and modeled estimates significantly after midnight, while the minor nonequilibrium contributions during several hours before midnight lend additional support for the presence of charge exchange equilibrium.

Altitude variations of middle-latitude topside ionospheric electron-density profiles

An approach to determine the dependence of the mid-latitude topside ionospheric electron number density ( N e) altitude distributions and ion composition changes on long-term solar-cycle variations and short-term solar-wind and magnetic disturbances has been developed. This approach focuses on N e altitude profiles from the height of the ionospheric N e maximum to ∼3000 km as deduced from ISIS (International Satellites for Ionospheric Studies) topside-sounder data. These data, obtained over a two-decade time interval, can be used to investigate secular changes in the topside N e profiles, which reflect altitude changes in plasma temperature and ion composition, over more than a solar cycle. The approach is to determine the locations of mid-latitude O +/H + transition altitudes deduced by fitting the topside N e profiles to modeled H + and O + profiles using the base electron temperatures and constant temperature gradients as variables.

Ion temperature crests and troughs in the morning sector of the low‐latitude and midlatitude topside ionosphere

The low‐latitude and midlatitude topside ionospheric ion temperature at 600 km altitude has been measured with the Ionospheric Plasma and Electrodynamics Instrument (IPEI) instrument on board ROCSAT‐1 satellite in the solar maximum year of 2000. Global hemispheric asymmetrical distribution of this topside ion temperature in the morning hour is presented here for the first time and is explained with the associated observation of the field‐aligned ion flow and ion concentration. Crests of ion temperature are found along the dip latitudes between 10° and 25° in the winter hemisphere during the solstice seasons. The maximum in the temperature crests is located in a longitudinal region of positive magnetic declination in the southern hemisphere during the June solstice and in the longitudinal region of negative magnetic declination in the northern hemisphere during the December solstice. Ion compression from a downward field‐aligned ion flow driven by morning neutral wind is concluded to cause the ion heating in the winter hemisphere as modeled by Bailey et al. [1975]. On the other hand, in the 0700–0800 LT sector, there are low‐temperature regions at higher latitudes in a longitude region of negative magnetic declination in the southern hemisphere during the June solstice and in the longitudinal region of positive magnetic declination in the northern hemisphere during the December solstice. A low photoionization rate together with an ion expansion cooling process from the upward field‐aligned plasma flow has resulted in such low temperature in these regions. As for temperature troughs observed near the dip equator in the morning hour, it is attributed to the convective heat loss in the summer hemisphere initially in the early morning hour and then to the rapid increase of plasma concentration at the dip equator in the late morning hour for all seasons.

Grid effects on the derived ion temperature and ram velocity from the simulated results of the retarding potential analyzer data

The ROCSAT-1 satellite circulating at 600 km altitude in the low- and mid-latitude topside ionosphere carries a retarding potential analyzer to measure the ion composition, temperature, and the plasma flow velocity in the ram direction. Based on an existing three-dimensional model, the particle's motion inside the instrument is simulated with the exact wire and mesh sizes but with a smaller aperture of the real sensor configuration. The simulation results indicate that the retarding grids could not provide a uniform retarding potential barrier to completely repel low energy particles. Some of low energy particles could pass through those grids and arrive at the collector. The leakage will cause the ram velocity to be over-estimated for by about 180 m/sec. Furthermore, the simulated O + temperature derived from the I-V curve is lower than the input temperature due to ion losses from colliding with the grids from the non-uniform potential field generated by the high retarding voltage.

Sources of Plasma Troughs in the Midlatitude and Subauroral Topside Ionosphere

The data of observations of plasma density in the large-scale ionospheric trough by the ion trap onboard the Kosmos-900 satellite on April 15, 1977, are presented. They enable one to understand both the general configuration and the fine details of the trough. There are sharp gradients of density (especially, in the subtroughs) superimposed on the less prominent variations of density in the main trough, which are characteristic of the average pattern of the trough. It was demonstrated that one of the possible mechanisms of forming the troughs(i.e., plasma cavities) is the electrostatic instability of plasma in the topside ionosphere.

Quasi-equilibrium theory of the midlatitude protonosphere

The hydrodynamic equations for H + and O + ions in the midlatitude topside ionosphere are solved analytically to study the magnetic tube depletion and refilling with a thermal plasma. Explicit expressions for the electron number density in the protonosphere and the refilling time constant of the tube are obtained. The influence of the H + ion diffusion and of charge exchange reactions is shown to be essential. It is shown that the filling of the midlatitude magnetic tubes is controlled significantly by the exospheric temperature. It is also shown that under certain conditions the depletion time may be considerably less than the refilling time. Large-scale irregularities of the electron number density can arise in the inner plasmasphere from the depletion of plasma on certain magnetic tubes.

Field-aligned flows of H + and He + in the mid-latitude topside ionosphere at solar maximum

A time-dependent mathematical model of the Earth's ionosphere and plasmasphere has been used to investigate the field-aligned flows of H + and He + in the topside ionosphere at L = 3 during solar maximum. When the flux-tube content is low there are upward flows of H + and He + during daytime in both the winter and summer topside ionospheres. During winter night-time the directions of flow are, in general, downwards for He + because of the night-time decrease in He + scale height, and upwards for H +, because of the replenishment needs of the flux tube. In the winter topside ionosphere, during the later stages of flux-tube replenishment, H + generally flows downwards during both day and night as a result of the greater plasma pressure in the summer hemisphere whilst He + flows upwards during the day and downwards at night. In the summer topside ionosphere H + flows upward to replace the H + lost from the plasmasphere to the winter topside ionosphere whilst the winter helium bulge leads to flows of He + that are in the direction winter hemisphere to summer hemisphere. When the flux-tube content is low, counterstreaming of H + and He +, with H + flowing upwards and He + downwards, occurs for most of the day above about 5000 km altitude in the summer hemisphere. There are occurrences of this type of counterstreaming in both the summer and winter hemispheres during the night. When the flux-tube content is high, counterstreaming of H + and He + occurs less frequently and over smaller regions of the flux tube. There are regions in both hemispheres where H + flows downwards whilst He + flows upwards.

The role of relative ion flows on the thermal structure of the ionosphere

Calculations are presented which show that, contrary to previous suggestions, ion flow velocity differences do not have a significant effect on the thermal structure of the midlatitude topside ionosphere.

Diffusion and heat flow equations for the mid-latitude topside ionosphere

For application to the mid-latitude topside ionosphere, we have derived diffusion and heat flow equations for a gas mixture composed of two major ions, electrons and a number of minor ions. These equations were derived by expanding the velocity distribution of each constituent about its 13 lower order velocity moments. As a consequence, each constituent was allowed to have its own temperature and drift velocity. The restriction to mid-latitudes results because we have assumed that the species temperature and drift velocity differences were small. In deriving the diffusion and thermal conduction equations, we have discovered some new transport effects. For the major ions, we have found that: (1) a temperature gradient in either gas causes thermal diffusion in both gases; (2) a temperature gradient in either gas causes heat to flow in both gases; and (3) a relative drift between the major ion gases induces a heat flow in both gases. Similar transport effects have also been found for the minor ions.

Light ion and electron troughs observed in the mid-latitude topside ionosphere on two passes of Ogo 6 compared to coincident equatorial electron density deduced from whistlers

Manifestations of the plasmapause in the topside ionosphere are discussed. Light ion and electron troughs observed in the topside ionosphere by Ogo 6 on a predawn and a dusk pass are compared to equatorial electron density profiles deduced from whistlers observed at the same local magnetic time and within 1.4 hours in UT on the predawn pass and 1.1 hours in UT on the dusk pass. The plasmasphere was distended in a period of low geomagnetic activity, and the whistler profiles did not extend through the plasmapause if a significant one existed. On the predawn pass, coincidence of the NH+ and NHe+ troughs at a plasmapause just beyond the L value where the whistler-deduced equatorial profile ceased at 350 el/cm3 could have occurred if reasonable suppositions are made. However, the ‘main trough’ in Ne lay at a significantly lower L value. On the dusk pass the NH+ trough lay well beyond the limit of the whistler profile, which reached down to approximately 200 el/cm3, and NHe+ showed only a steady decline with no trough. The Ne trough was shallow and coincided with the NH+ trough.

The topside ionosphere at mid-latitudes during local sunrise

Electron-density and scale height distributions obtained from more than 1000 Alouette I ionograms have been used to study the structure and variations of the mid-latitude topside ionosphere during the sunrise period from 21 May to 4 July 1963. With the assumption that the ionosphere is in a state of diffusive equilibrium, the scale heights are interpreted in terms of combined electron and ion temperature distributions and relative ion concentrations. During the overall sunrise period, there was a strong latitudinal variation of the electron density and scale height altitude distributions. It was the latitudinal scale height distribution which determined the effects of solar zenith angle variations at specific latitudes on the temperature and ion mass distributions. For latitudes south of ~55°N geomagnetic latitude, scale height variations due to solar zenith angle changes are interpreted in terms of temperature increases and mean ion mass changes. For latitudes north of ~55°N, however, these variations are characterized by temperature increases mostly. The electron densities at fixed topside altitudes decreased with increasing illumination at all altitudes and latitudes until about ground sunrise.

The light-ion trough, the main trough, and the plasmapause

Extensive observations of midlatitude depletions in electron and total ion density by both direct and indirect techniques have prompted numerous studies of the possible association between these troughs, observed both in the F region and in the topside ionosphere, and the plasmapause. Within this work, one basic problem arises in that, although the plasmapause has been detected as a global phenomenon by both VLF and ion composition measurements, the electron and ion density troughs have been identified primarily as nightside features. This problem, as well as the difficulty in explaining various inconsistencies in relating the position of the plasmapause and the ionization trough, is explained by a close examination of the ion composition, generally unavailable in previous trough studies. In particular, ion composition results from the polar-orbiting Ogo satellites identify the persistence of a pronounced light-ion trough (LIT) in H+ and He+, often identified by order-of-magnitude decreases in the light-ion concentrations, that occurs within a few degrees of latitude and reaches residual concentration levels of 10²–10³ ions/cm³ near 60° dipole latitude. The LIT, observed both within the thermosphere and at high latitudes in the magnetosphere, has been correlated directly with the VLF whistler identification of the plasmapause. On the nightside near 1000 km a complex relationship is observed between the dominant ions O+ and H+. The rapidly changing mean ion mass observed at the plasmapause typically produces a resultant total ion density distribution of complexity sufficient to confuse correlative studies with the plasmapause, even though the LIT is very sharply defined within the ion composition. On the dayside, where O+ is typically the predominant ion near 1000 km, LIT structure is often hidden beneath the O+ layer, which may exhibit little or no depletion even though H+ and He+ show trough structure at the plasmapause. This analysis of the midlatitude topside ion composition indicates the need for caution both in studies of the plasmapause per se and in phenomenological studies such as the relationship between the subauroral red arc and the plasmapause, which are most likely dependent on the detailed information available within the ion composition.

Dynamical behavior of thermal protons in the mid-latitude ionosphere and magnetosphere

Satellite and other observations have shown that H + densities in the mid-latitude topside ionosphere are greatly reduced during magnetic storms when the plasmapause and magnetic field convection move to relatively low L-values. In the recovery phase of the magnetic storm the convection region moves to higher L-values and replenishment of H + in the empty magnetospheric field tubes begins. The upwards flow of H +, which arises from O +—H charge exchange, is initially supersonic. However, as the field tubes fill with plasma, a shock front moves downwards towards the ionosphere, eventually converting the upwards flow to subsonic speeds. The duration of this supersonic recovery depends strongly on the volume of the field tube; for example calculations indicate that for L = 5 the time is approximately 22 hours. The subsonic flow continues until diffusive equilibrium is reached or a new magnetic storm begins. Calculations of the density and flux profiles expected during the subsonic phase of the recovery show that diffusive equilibrium is still not reached after an elapsed time of 10 days and correspondingly there is still a net loss of plasma from the ionosphere to the magnetosphere at that time. This slow recovery of the H + density and flux patterns, following magnetic storms, indicates that the mid-latitude topside ionosphere may be in a continual dynamic state if the storms occur sufficiently often.

Topside ionosphere at sunrise

The sunrise behavior of the midlatitude topside ionosphere at low sunspot numbers was examined by using Alouette 1 electron density and plasma scale height profiles obtained at Stanford University during May and June 1963. Under the assumption that observed variations result from solar control, the data were averaged to form height profiles of plasma scale height and electron density from 450 to 750 km altitude, which vary smoothly with solar zenith angle χ. The first manifestation of sunrise is a sudden and rapid decrease in plasma scale height, beginning between χ=105° and 102° and ending between χ=98° and 96°. Electron densities increase rapidly between χ=98° and 95°. Our interpretation of the observations is that the sudden decrease in plasma scale heights is due to an increase in percentage concentration of O+ ions at all altitudes, due primarily to a thermal expansion of the ionosphere that causes an upward movement of the topside ionization. The velocity of the thermal motion is estimated, and we attempt to resolve conflicting evidence from other observations that the topside ionosphere is controlled by either geomagnetic latitude or solar zenith angle.

Electron Densities and Scale Heights in the Mid-latitude Topside Ionosphere

The diurnal and latitudinal variations of electron density and plasma scale height in the topside ionosphere during summer and winter have been calculated from Alouette I ionograms recorded at Woomera. The electron density behaviour is anomalous in that the winter night-time values are generally as large or larger than those occurring during the day. At heights near 1000 km the winter night-time values are greater than those for night-time summer. The behaviour of the scale height is very similar to that reported by others for the mid-latitude region of the northern hemisphere and implies that at night-time the transition level from 0+ to lighter ions occurs at heights of about 550 km in summer and 500 km in winter.