High-latitude Topside Ionosphere Research Articles

Multi-scale response of the high-latitude topside ionosphere to geospace forcing

We investigate the response of the topside ionosphere, auroral and polar sectors, to the forcing of the geospace during September 2017. Specifically, we aim at characterizing such a response in terms of the involved spatial scales and of their intensification during the different auroral and polar cap activity conditions experienced in the selected month, that is characterized by severe geomagnetic storm conditions. For our purposes, we leverage on and compare various in situ plasma density data products provided by the Swarm constellation of the European Space Agency (ESA). The spatio-temporal variability of the involved scales in the plasma density observation is featured through the application of the Fast Iterative Filtering (FIF) signal decomposition technique and, for the first time in the ionospheric field, of a FIF-derived dynamical spectrum called “IMFogram”. The instantaneous time-frequency representation provided through the IMFogram illustrates the time development of the multi-scale processes with spatial and temporal resolutions higher than those obtained with traditional signal processing techniques. To demonstrate this, the IMFogram is tested against Fast Fourier and Continuous Wavelet Transforms. With our fine characterization, we highlight how scale cascading and intensification processes in the plasma density observations follow the ionospheric currents activity, as depicted through the auroral activity and polar cap indices, and through the field-aligned currents data product provided by Swarm.

Looking for a proxy of the ionospheric turbulence with Swarm data

The present work focuses on the analysis of the scaling features of electron density fluctuations in the mid- and high-latitude topside ionosphere under different conditions of geomagnetic activity. The aim is to understand whether it is possible to identify a proxy that may provide information on the properties of electron density fluctuations and on the possible physical mechanisms at their origin, as for instance, turbulence phenomena. So, we selected about 4 years (April 2014–February 2018) of 1 Hz electron density measurements recorded on-board ESA Swarm A satellite. Using the Auroral Electrojet (AE) index, we identified two different geomagnetic conditions: quiet (AE < 50 nT) and active (AE > 300 nT). For both datasets, we evaluated the first- and second-order scaling exponents and an intermittency coefficient associated with the electron density fluctuations. Then, the joint probability distribution between each of these quantities and the rate of change of electron density index was also evaluated. We identified two families of plasma density fluctuations characterized by different mean values of both the scaling exponents and the considered ionospheric index, suggesting that different mechanisms (instabilities/turbulent processes) can be responsible for the observed scaling features. Furthermore, a clear different localization of the two families in the magnetic latitude—magnetic local time plane is found and its dependence on geomagnetic activity levels is analyzed. These results may well have a bearing about the capability of recognizing the turbulent character of irregularities using a typical ionospheric plasma irregularity index as a proxy.

The ionospheric source of magnetospheric plasma and the importance of solar EUV flux measurements

Ions of ionospheric origin (mostly O+ and He+) form a significant component of the plasma in the Earth's magnetosphere. These ions are extracted from the high-latitude topside ionosphere by a series of processes which heat the ions both parallel and perpendicular to the geomagnetic field. The amount of plasma outflow is correlated with geomagnetic and solar activity. However, previous studies of ion outflow in the ionosphere have relied on F10.7 as a proxy for solar activity because no routine solar EUV flux measurements were available. The simultaneous availability of solar EUV flux measurements and ion outflow data from the Fast Auroral Snapsho T Explorer (FAST) during the rising phase of the solar cycle allows us to make definitive measurements of the dependence of ion outflow at auroral latitudes on solar EUV irradiance.

Comparison of measured high latitude F-region ion composition climatological variability with models

Satellite measurements of the global F-region ion composition morphology, due to orbit-to-orbit variations in space and time, are most effectively examined statistically, taking into consideration the ranges of variability in the data. Simple averages of the measurements do not clearly delineate narrow and time varying features such as density troughs/holes, sharp ionospheric layers, or even auroral region enhancements of electron temperature/molecular ion concentrations. These in fact are the most prominent features seen along individual satellite passes and are features which have prominent climatological variations To explore how well current ionosphere models fare with reality at middle and high latitudes, the entire ensemble of available solar minimum satellite ion composition measurements (predominantly from Atmosphere Explorer C and D) are sorted into spatial, temporal and magnetic/solar activity indices bins. Ion concentrations obtained from multiple runs of the International Reference Ionosphere (IRI) and the Utah State University Time Dependent Ionosphere model (TDIM), sorted into bins similar to those used for the satellite data sets are compared to the measurements. As currently configured the IRI model does not make a good match with the middle and high latitude topside ionosphere ion composition measurements. The TDIM is a better match to the measurements but does not reproduce the deepest high latitude ionospheric troughs and auroral-cusp molecular ion enhancements.

Low-frequency magnetic turbulence in the high-latitude topside ionosphere: low-frequency waves or field-aligned currents

A detailed analysis of the ULF magnetic field fluctuations observed on board the ARCAD-Aureol-3 satellite during a crossing of the northern evening auroral zone is presented. It is shown that 0magnetic turbulence occurs at the interface between region 1 and region 2 large-scale field-aligned currents. From a minimum variance analysis, the main directions of the fluctuations are determined. The magnetic field turbulence is generally polarized in a plane perpendicular to the Earth's magnetic field. In this plane, the polarization is highly variable, alternatively linear and circular. A spectral analysis allows us to study the frequency variation of the power spectrum, wave normal direction and polarization. The main characteristics of the turbulence agree quite well with a parallel current interpretation, with a maximum intensity as large as 350 μAm −2. However, in the region where the turbulence is most intense the presence of oblique Alfvén waves, superimposed on the current sheets, may explain the observed signals.

Simulation of ion heating in the topside auroral ionosphere

Energization of ions to low energies (≤10 eV) in the topside ionosphere (300 – 2000 km altitude) has been observed for a decade. These ionospheric ions have been invoked as the source population of keV ions energized in the auroral magnetosphere (≥1 RE altitude). If so, the importance of the topside ionosphere as a source of magnetospheric plasma must be assessed quantitatively. To determine the efficiencies of this topside auroral ionospheric source of magnetospheric plasma, Monte Carlo simulations of wave‐particle ion heating under realistic topside conditions have been carried out. It is shown that the source efficiency is greatly affected by (1) copious charge exchange interactions between the ions and thermospheric neutrals at an early stage of heating and (2) the observed existence of downward parallel electric potential drops. The composition of warm ions escaping the topside is predicted to be drastically different from the cold ionospheric composition. This study also predicts the existence of a hot (&lt;10 eV) population of thermospheric neutrals in the high latitude topside ionosphere.

Regions of He + dominance in the high-latitude topside ionosphere

Observations of the occurrence of He + dominance in the topside ionosphere are discussed. An earlier model of the behaviour of high-latitude H + and O + thermal plasma (Quegan et al., 1982) is extended to include He + as a major ion. Calculations using the extended model show that plasma convection is likely to play a key rôle in producing regions of He + dominance. Suggested conditions for He + dominance are listed and their applicability to observed He + behaviour is discussed.

Location and source of ionospheric high latitude troughs

One prominent feature of the high latitude topside ionosphere is the existence of sharp latitudinal depletions in the total ion (electron) concentrations within the auroral/cusp regions. These high latitude troughs, as seen by the Bennett ion mass spectrometer observations on the satellite OGO 6 at altitudes between 400 and 1100km correspond to depletions in the atomic ions which are accompanied by localized enhancements of the minor molecular ion densities. All of the high latitude troughs traversed by OGO 6 (1969–1970) were recorded and the average invariant latitude-magnetic local time (M.L.T.) distribution was determined. The troughs on the average were found at all local times to be in the vicinity of the auroral oval and to move equatorward in response to increasing magnetic activity. The average trough location was compared to the average polar cap boundary as defined by the convection electric field reversal and the electron trapping boundary as well as to the maximum horizontal magnetic disturbance associated with the large scale field aligned currents. The high latitude troughs on the average best followed the maximum magnetic disturbance distribution. It is concluded that the troughs are the result predominantly of enhanced chemical 0 + losses in regions with high convection velocities.

Helium ion outflow from the terrestrial ionosphere

Extensive calculations have been made of the behaviour of He + for situations where ion outflow occurs from the topside ionosphere. For these circumstances, steady state solutions for the He + continuity, momentum and energy equations have been obtained self-consistently, yielding density, velocity and temperature profiles of He + from 200 to 2000 km altitude. To model the high latitude topside ionosphere, a range of background H +O + ionospheres was considered with variations in the H + outflow velocity, the presence of a perpendicular electric field and different peak O + densities. In addition, the atmospheric density of neutral helium was chosen to model typical observed winter and summer densities. From our studies we have found that: (a) The outflowing He + has density profiles of similar shape to those of H +, for basically different reasons; (b) The effect of the perpendicular electric field differs considerably for H + and He +. This difference stems from the fact that an electric field acts to alter significantly the O + density at high altitudes and this, in turn, changes the H + escape flux through the O ++H charge exchange reaction. A similar situation does not occur for He + and therefore the He + escape flux exhibits a negligibly small change with electric field; (c) The fractional heating of He + due to the He +O + relative flow is not as effective in heating He + as the H +O + relative flow is in heating H +; (d) During magnetospheric disturbances when the N 2 density at the altitude of the He + peak (600 km) can increase by a factor as large as 50, the He + peak density decreases only by approximately a factor of 2; and (e) The He + escape flux over the winter pole is approximately a factor of 20 greater than the He + escape flux over the summer pole. Consequently, on high latitude closed field lines there could be an interhemispheric He + flux from winter to summer.

Effect of diffusion-thermal processes on the high-latitude topside ionosphere

We have studied the extent to which diffusion-thermal heat flow affects H + temperatures in the high-latitude topside ionosphere. Such a heat flow occurs whenever there are H +−O + relative drifts. From our study we have found that at high-latitudes, where H + flows up and out of the topside ionosphere, diffusion-thermal heat flow acts to reduce H + temperatures by 500–600 K at altitudes above about 900 km.

Plasma density irregularities in the high-latitude top side ionosphere

Thermal plasma density measurements obtained by using the spherical electrostatic analyzer probe carried by the polar orbiting satellite Isis 1 have been spectrally analyzed to provide quantitative results on the amplitude and scale size distribution of the plasma irregularities in the high-latitude top side ionosphere. The range of scale sizes examined, 200 m to over 100 km under normal conditions, is more than an order of magnitude greater than that available from ground-based measurements. Data in the altitude range 574–3523 km and from the period February 1969 to April 1972 have been analyzed. Irregularities in the high-latitude region from 40° invariant latitude up to the invariant L pole have been studied. In over 90% of the cases examined, the power spectra are found to fit a power law of the form P = P0f−β, where the index β varies within the range 1.5–2.5. The power spectral density was integrated over the scale size range 1–10 km in order to analyze quantitatively latitudinal variations of the irregularity amplitude. While the boundaries of the high-latitude irregularity zone on particular orbits examined by using this technique are in agreement with earlier morphological studies, it is also evident that the amplitude of the irregularities within the high-latitude irregularity region itself shows irregular enhancements above a generally raised amplitude level.