Auroral Ionosphere Research Articles

Features of the HF Signal Propagation on Oblique Sounding Paths During Solar and Magnetic Activity in September 2017

We present the results of studying the influence of solar and magnetic activity in September 2017 on the HF signal characteristics on subauroral and midlatitude paths. A connection between the ionospheric effects caused by the magnetic storm and the magnetic disturbance intensity is established. It is shown that the formation of a strong sporadic Es layer in the auroral ionosphere during a magnetic storm makes it possible to use the propagation mode with reflection from Es in the interests of HF communication to neutralize the effects of the negative phase of the storm and absorption growth when unfavorable conditions for the F-mode propagation with reflection from the upper ionosphere occur.

Про можливість зондування границь північного аврорального овалу за реєстраціями високочастотних сигналів на наддалеких радіолініях

Objective. Developing a technique for investigation of a dynamic of the equatorial boundary of the night side of the Northern auroral oval using data of registrations of scattered HF signals on superlong radio paths. Methods. Spectral, time, and time-frequency analysis of HF signals from time service stations, recorded at the Ukrainian Antarctic Akademik Vernadsky station (UAS). Selection of the spatial mode of signals scattered on the plasma inhomogeneities of the high latitude ionosphere of the northern hemisphere. The developed method is based on the measurement of the Doppler frequency shift and time of the group delay of the radio signal. Results. The possibility of studying the dynamics of the equatorial boundary of the night side of the Northern auroral oval using characteristics of scattered HF signals of time service stations registered at UAS is demonstrated. Conclusions. In the registrations of HF signals of the RWM station (55.75°N, 37.64°E) at UAS (65.25°S, 64.27°W) it is possible to select the mode scattered on the inhomogeneities of the high-latitude ionosphere of the northern hemisphere. The method for diagnosing the equatorial boundary of the auroral oval by observations on UAS of scattered radio signals from stations of the time service is proposed. It is convenient to use such stations as sources of the probe signal for diagnostic of the ionosphere so far as they have a number of advantages, including low relative frequency instability of master oscillators; weakly directed antennas and sufficiently powerful transmitters thanks to which their signals can be recorded in different regions of the globe; continuous work on a previously known schedule and, finally, the fact that they simultaneously emit at several frequencies, which expands the possibilities of their diagnostic use. The technique does not require additional transmitting devices and expensive antenna systems. The regular measurements using the suggested method allow carrying out the monitoring of auroral ionosphere, which will give the possibility for improving the existent models of its behavior.

Artificial Aurora Experiments and Application to Natural Aurora

A review is given of the effects observed during injections of powerful electron beams from sounding rockets into the upper atmosphere. Data come from in situ particle and wave measurements near a beam emitting rocket and ground-based optical, wideband radiowave, and radar observations. The overall data cannot be explained solely by collisional degradation of energetic electrons but require collisionless beam plasma interactions (BPI) be taken into account. The beam-plasma discharge theory describes the features of the region near a beam-emitting rocket, where the beam-excited plasma waves energize plasma electrons, which then ignite the discharge. The observations far beneath the rocket reveal a double-peak structure of artificial auroral rays, which can be understood in terms of the beam-excited strong Langmuir turbulence being affected by collisions of ionospheric electrons. This leads to the enhanced energization of ionospheric electrons in a narrow layer termed the plasma turbulence layer (PTL), which explains the upper peak. Similar double-peak structures or a sharp upper boundary in rayed auroral arcs have been observed in the auroral ionosphere by optical, radar, and rocket observations, and called Enhanced Aurora. A striking resemblance between Enhanced and Artificial Aurora altitude profiles indicates that they are created by the above BPI process which results in the PTL.

Estimated nitric oxide density in auroras from ground-based photometric data

In this paper, we numerically estimate the nitric oxide density in auroras, using photometric data on 427.8, 557.7, and 630.0 nm emission intensities. The data were obtained at midnight at observatories of the Polar Geophysical Institute. These estimates were made using a numerical modeling procedure with a time-dependent model of the auroral ionosphere [Dashkevich et al., 2017]. It is shown that the NO density in the maximum of the altitude profile is between (1÷3.3)∙10^8 cm–3. The obtained estimates indicate the absence of a correlation between the [NO]max values and 427.8 nm emission intensities.

Оценка содержания окиси азота в полярных сияниях по данным наземных фотометрических наблюдений

In this paper, we numerically estimate the nitric oxide density in auroras, using photometric data on 427.8, 557.7, and 630.0 nm emission intensities. The data were obtained at midnight at observatories of the Polar Geophysical Institute. These estimates were made using a numerical modeling procedure with a time-dependent model of the auroral ionosphere [Dashkevich et al., 2017]. It is shown that the NO density in the maximum of the altitude profile is between (1÷3.3)∙10^8 cm–3. The obtained estimates indicate the absence of a correlation between the [NO]max values and 427.8 nm emission intensities.

Resonant ULF Absorption Revealed by Auroral Doppler Radar Data

STARE Doppler data on electron drift velocities detected in the auroral ionosphere for weakly disturbed conditions are analyzed. The Doppler measurements are first averaged along each radar beam. The power spectral density (PSD) as a function of frequency is then calculated by discrete Fourier transforms of averaged signals for every radar beam. Deep stepwise drops (about 10 dB) in spectral powers of the ultra-low-frequency (ULF) range are revealed for all radar beams. These PSD drops are interpreted as manifestations of resonant ULF absorption, which occurs in the eigenfrequency continuum of standing Alfven waves excited on geomagnetic field lines. A variational analysis that models PSD decreases by stepwise profiles of the mean spectral powers is proposed. This analysis provides the least-squares fitting of model profiles to PSD decreases calculated by the data. The frequency of the mean PSD step determined for each radar beam is treated as the minimum frequency of resonant ULF absorption revealed by this beam during the given auroral event. Averaged over all beams, this frequency for the analyzed event is 4.9 ± 0.2 mHz.

Ionospheric and geomagnetic Pc5 oscillations as observed by the ionosonde and magnetometer at Sodankylä

The study is based on the data of the rapid-run ionosonde at the Sodankylä Geophysical Observatory at auroral latitude (L = 5.25) which routinely performs one-minute sounding since 2007. This dataset allows a unique opportunity for investigating possible effects of ultra-low frequency (ULF, 1–7 mHz) waves in the auroral ionosphere. Suitable observations were made during moderately disturbed geomagnetic conditions typically at recovery of the geomagnetic storms caused by solar wind high-speed streams, in the daytime between 9 and 16 MLT. The ionospheric oscillations corresponding to Pc5 geomagnetic pulsations were found in variations of the virtual height of the F layer and the power of ionosonde reflections from E and F layers. The later are most probably caused by modulation of electron precipitation, which is also manifested in weak (about 0.01–0.06 dB) variations of cosmic noise absorption. The most important and novel result is that the pulsations of power of reflection from E and F layers typically has a spectral maximum at nearly half the periodicity of the Pc5 geomagnetic pulsations, whereas such spectral peak is negligible in the geomagnetic pulsations.

The history of Andøya Rocket Range

Abstract. Just 5 years after Sputnik, on 18 August 1962, Norway launched the first sounding rocket from Andøya in northern Norway. The establishment of Andøya Rocket Range (ARR), in the Arctic and right in the middle of the night-time auroral zone, gave the scientists unique opportunities for studies of the complex processes in the auroral ionosphere and upper atmosphere. In close cooperation with the users, ARR gradually developed its technical and scientific infrastructure and is now one of the world's leading observatories in this field. ARR has also established a launch site at Svalbard, and sounding rockets from both ranges can reach far into the Arctic to study the cusp region and the daytime aurora. The ground-based instruments comprise sophisticated radars and lidars as well as passive instruments. ARR also plays an active role in space education. In 2014 Andøya Rocket Range changed its name to Andøya Space Center (ASC; https://www.andoyaspace.no, last access: 23 November 2018). This change reflects the fact that the activities now comprise much more than sounding rocket launches. ASC is an important company both nationally and in the local community of Andenes. ASC now has a staff of 95 and an annual turnover of NOK 150 million.

Collisionless relaxation of the ion ring distribution in space plasma

Energetic processes often produce transversely-heated angular distributions of the magnetized core (lowest energy) plasma. This characteristic is found in solar wind ion pickup, resulting from cometary or interstellar gas ionization, in Earths’ ionosphere, and with hot ions formed around the Space Transportation System during gas releases. We investigate the thermalization of O+ ion pickup using the 2.5D hybrid simulation method (with fluid electrons and kinetic ions) of the ion pickup (ring) distributions, formed in the auroral ionosphere, with a range of ring velocities and thermal to magnetic pressure ratios. We find that in the unstable collisonless regime the anisotropy of the non-thermal distribution produces the ion-cyclotron instability, and the nonlinear relaxation is accompanied by wave-particle scattering that results in an emitted power of EMIC waves. We conclude that ionospheric pickup thermalization is slow due to the small ring speed compared to the thermal and Alfvén speeds, while in the solar wind and other space plasmas regions with larger ion-ring velocity the collisionless relaxation and thermalization is rapid in terms of O+ ion gyro-period.

Effects of He+ and O+ ions on kinetic Alfvén waves: application to PSBL region

Kinetic Alfven waves (KAWs) are investigated considering existence of multi-ions (H+, He+ and O+) in plasma sheet boundary layer (PSBL) region. The dispersion relation and damping rate of wave are derived by kinetic approach. The loss-cone index (for $J=1$ and $J = 2$ ) and densities of multi-ions are varied to study the frequency and damping rate of wave over wide range of $k_{\bot} \rho_{\mathrm{H}^{+}}$ (where $k_{\bot}$ is perpendicular wave vector and $\rho_{\mathrm{H}^{+}}$ is Larmor radius of H+ ion). The presence of multi-ions in plasma is assumed for four cases: (a) H+ only, (b) H+ and He+, (c) H+ and O+, (d) H+, He+ and O+ ions. The results of the cases (b), (c) and (d) are compared with (a) to understand the effects of He+ and O+ ions on KAW. It is observed that the frequency of the wave lies in range 0.1–4 Hz for each case. He+ enhances wave frequency with increase in steepness of loss-cone indices. O+ is more effective in Maxwellian plasma resulting maximum frequency for $J=0$ . Increasing densities of He+ and O+ result in reduction of wave frequency at $k_{\bot} \rho_{\mathrm{H}^{+}} <1$ and enhancement in frequency at higher $k_{\bot} \rho_{\mathrm{H}^{+}}$ . Presence of He+ and O+ induce fluctuations in wave frequency. Reduction in damping rate due to He+ and O+ ions in loss-cone distribution signifies propagation of wave over long distances from PSBL towards auroral ionosphere. The parameters relevant to PSBL region are used in calculation of theoretical results. The results predict that the multi-ions possessing loss-cone distribution with varying densities significantly affect nature of KAW propagation.

Effect of general loss-cone distribution function on kinetic Alfvén wave in multi-ions plasma by kinetic approach

Kinetic Alfven waves with general loss-cone distribution function are investigated in multi-ions (H+, He+ and O+) plasma. Dispersion relation and damping rate for the wave are derived using Vlasov equation. Variations in frequency and damping rate versus $$ k_{ \bot } \rho_{i} $$ (where k⊥ is wave vector perpendicular to ambient magnetic field, ρi is ion Larmor radius and i denotes multi-ions) are investigated. Parameters relevant to plasma sheet boundary layer are used for graphical analysis. It is observed that wave frequency fluctuates with loss-cone distribution indices of ions. In comparison with Maxwellian plasma (J = 0), the wave frequency is enhanced for He+ and O+ and reduced for H+ with the increase in J indices and the wave existence limit shift towards lower $$ k_{ \bot } \rho_{i} $$ for lighter ions. This may be due to difference in penetration of multi-ions through the cone. H+ and He+ ions show damping at lower k⊥ρi only, whereas O+ ions exhibit damping over wide range of $$ k_{ \bot } \rho_{{O^{ + } }} $$ . The damping is reduced with the increase in loss-cone indices for all the ions which signify propagation of wave over long distances towards auroral ionosphere. The applications of these results are in understanding the effect of gyrating multi-ions in transfer of energy and in describing the generation of aurora.

Radio Emission of the High-Latitude Ionosphere as a Result of the Cutoff Mode of the Auroral Ionospheric Duct

Issues of radio emission of the auroral ionosphere are considered with the use of the classical electrodynamics approach, according to which a natural waveguide is formed in the Earth’s ionosphere owing to formation of a trough in the high-latitude ionosphere. Parameters and characteristics of the auroral ionospheric dielectric duct have been estimated on the basis of the obtained experimental data on the properties of the ionospheric trough. The hypothesis that the LF and VLF electromagnetic auroral radio emission in the high-latitude ionosphere is due to the approach of the auroral waveguide to the cutoff mode has been considered. It has been found that the radio emission bands correspond to the eigenmodes of the ionospheric duct under consideration. It has been shown that, within the proposed approach, not only the radio emission frequency band of the high-latitude ionosphere but also the polarization of the radio emission observed in the experiments can be explained.

Proxy Index Derived From All Sky Imagers for Space Weather Impact on GPS

AbstractGlobal Positioning System (GPS) signals passing through the auroral ionosphere, which exhibits multiscreen electron density structuring, maybe scintillated causing observation and possibly positioning errors. It is advantageous to determine the magnitude of GPS signal scintillation associated with a given level of auroral brightness observed around the signal's ionospheric pierce point (IPP). Such information would enable the exploitation of auroral image observations in space weather monitoring and help in assessment of impact on infrastructure/services reliant on GNSS. Studies have observed a general positive correlation between auroral brightness and GPS phase scintillation but not a definite one‐to‐one relationship. In this study a correlation coefficient of 0.38 is observed between the phase scintillation and the level of auroral arc brightness around the GPS signal's raypath for a data set of 292 events in the Canadian sector. Alternatively, a new pseudo‐scintillation index, Rate of change of Brightness index, is introduced in this study which is derived from the changing auroral brightness around the satellite's IPP. This Rate of change of Brightness index is highly positively correlated (correlation coefficient: 0.75) with GPS phase scintillation. Spatial and spectral relationships between auroral brightness around the satellite's IPP and phase scintillation were also analyzed. It is observed that probability of GPS signals experiencing phase scintillation is high when the auroral brightness around the satellite's IPP has dominant fluctuations in the frequency band ~0.06 to 0.16 Hz. These results indicate that GPS signal scintillation is related to the dynamics of the brightness around the satellite's IPP as the satellite signal propagates through the aurora.

Impact of Precipitating Electrons and Magnetosphere‐Ionosphere Coupling Processes on Ionospheric Conductance

AbstractModeling of electrodynamic coupling between the magnetosphere and ionosphere depends on accurate specification of ionospheric conductances produced by auroral electron precipitation. Magnetospheric models determine the plasma properties on magnetic field lines connected to the auroral ionosphere, but the precipitation of energetic particles into the ionosphere is the result of a two‐step process. The first step is the initiation of electron precipitation into both magnetic conjugate points from Earth's plasma sheet via wave‐particle interactions. The second step consists of the multiple atmospheric reflections of electrons at the two magnetic conjugate points, which produces secondary superthermal electron fluxes. The steady state solution for the precipitating particle fluxes into the ionosphere differs significantly from that calculated based on the originating magnetospheric population predicted by magnetohydrodynamic and ring current kinetic models. Thus, standard techniques for calculating conductances from the mean energy and energy flux of precipitating electrons in model simulations must be modified to account for these additional processes. Here we offer simple parametric relations for calculating Pedersen and Hall height‐integrated conductances that include the contributions from superthermal electrons produced by magnetosphere‐ionosphere‐atmosphere coupling in the auroral regions.

Mapping H3+ Temperatures in Jupiter's Northern Auroral Ionosphere Using VLT‐CRIRES

AbstractWe present a detailed study of the H3+ auroral emissions at Jupiter, using data taken on 31 December 2012 with the long‐slit Echelle spectrometer CRIRES (ESO‐VLT). From this data set the rotational temperature of the H3+ ions in Jupiter's upper atmosphere was calculated using the ratio of the ν2 Q(1,0−) and ν2 Q(3,0−) fundamental emission lines. The entire northern auroral region was observed, providing a highly detailed view of ionospheric temperatures, which were mapped onto polar projections. The temperature range we derive in the northern auroral region is ~750–1000 K, which is consistent with past studies, although the temperature structure differs. We identify two broad regions which exhibit temperature changes over a short period of time (~80 minutes). We propose that the changes in temperature could be due to a local time change in particle precipitation energy, or they could be caused by dynamic temperature changes generated in the neutral thermosphere due to the magnetospheric response to a transient enhancement of solar wind dynamic pressure, as predicted by models. By comparing the H3+ temperature, column density, total emission, and line‐of‐sight velocity, we were unable to identify a single dominant mechanism responsible for the energetics in Jupiter's northern auroral region. The comparison reveals that there is complex interplay between heating by impact from particle precipitation and Joule heating, as well as cooling by the H3+ thermostat effect.

Excitation of Plasma Irregularities in the F Region of the Ionosphere by Powerful HF Radio Waves of X‐Polarization

AbstractWe discuss theoretically the excitation of artificial plasma irregularities in the auroral ionosphere by high‐frequency X‐mode radio wave. This is done via a two‐step process. As a first step we adopt the thermal self‐focusing instability excited in the F region of the ionosphere under the action of a strong high‐frequency (HF) radio wave. This instability causes the formation of perturbations of the electron temperature and plasma concentration across the magnetic field. In addition, the plasma becomes depleted in the regions of the electron temperature enhancements and vice versa, since the gradients of plasma concentration and the electron temperature have opposite signs. In such conditions the temperature gradient instability comes into play. As a second step we consider plasma and electron temperature inhomogeneities that appear due to this instability to be responsible for the generation of irregularities with transverse sizes smaller than the typical scales of the self‐focusing instability. Alternative mechanisms such as excitation of the gradient‐drift and the current‐convective instabilities, which are often attributed to the generation of plasma irregularities in the F region and can contribute to the formation of artificial irregularities in the case of X‐mode heating, are also discussed.

Instability Caused by an Inhomogeneous Energy Density Distribution as a Possible Source of Electrostatic Broadband Noise

Plasma instability caused by an inhomogeneous energy density distribution is considered. It is shown that this instability can lead to the excitation of electrostatic ion-cyclotron and oblique ion-acoustic waves, generated in the presence of an inhomogeneous transverse electric field and a shear in the parallel drift velocity of the plasma particles. The considered physical mechanisms of the instability generation in plasma can serve as possible sources of broadband electrostatic turbulence in the auroral ionosphere.

Jupiter's Aurora Observed With HST During Juno Orbits 3 to 7

AbstractA large set of observations of Jupiter's ultraviolet aurora was collected with the Hubble Space Telescope concurrently with the NASA‐Juno mission, during an eight‐month period, from 30 November 2016 to 18 July 2017. These Hubble observations cover Juno orbits 3 to 7 during which Juno in situ and remote sensing instruments, as well as other observatories, obtained a wealth of unprecedented information on Jupiter's magnetosphere and the connection with its auroral ionosphere. Jupiter's ultraviolet aurora is known to vary rapidly, with timescales ranging from seconds to one Jovian rotation. The main objective of the present study is to provide a simplified description of the global ultraviolet auroral morphology that can be used for comparison with other quantities, such as those obtained with Juno. This represents an entirely new approach from which logical connections between different morphologies may be inferred. For that purpose, we define three auroral subregions in which we evaluate the auroral emitted power as a function of time. In parallel, we define six auroral morphology families that allow us to quantify the variations of the spatial distribution of the auroral emission. These variations are associated with changes in the state of the Jovian magnetosphere, possibly influenced by Io and the Io plasma torus and by the conditions prevailing in the upstream interplanetary medium. This study shows that the auroral morphology evolved differently during the five ~2 week periods bracketing the times of Juno perijove (PJ03 to PJ07), suggesting that during these periods, the Jovian magnetosphere adopted various states.

Coupled SKR Emissions in Saturn's Northern and Southern Ionospheres

AbstractKilometric radiation (SKR) emitted above Saturn's auroral ionosphere is modulated in intensity at periods close to the planetary rotation period; SKR periods differ slightly for sources in the north and in the south. Although there is good evidence that the signals are generated independently in the two hemispheres, it is also well established that during southern summer power emitted from the northern hemisphere is modulated in intensity not only at the northern period but also at the southern period, an observation that requires an explanation. We examine the idea that the signal in the north at the southern period is a secondary effect of the strong field‐aligned current system centered at 270° that drives the southern signal. Basing our analysis on studies of field‐aligned current systems in the terrestrial and Jovian magnetospheres, we argue that the parallel electric fields that drive electrons into the southern auroral ionosphere and generate SKR are, at least in part, bidirectional and thus capable of accelerating electrons toward the opposite hemisphere where the secondary signal is detected with intensity lower than that of the locally generated signal. This interpretation implies that the atmospheric process that modulates the periodic responses can operate independently in each hemisphere.

Possible Mechanism for Damping of Electrostatic Instability Related to Inhomogeneous Distribution of Energy Density in the Auroral Ionosphere

Satellite observations show that the electrostatic instability, which is expected to occur in most cases due to an inhomogeneous energy density caused by a strongly inhomogeneous transverse electric field (shear of plasma convection velocity), occasionally does not develop inside nonlinear plasma structures in the auroral ionosphere, even though the velocity shear is sufficient for its excitation. In this paper, it is shown that the instability damping can be caused by out-of-phase variations of the electric field and field-aligned current acting in these structures. Therefore, the mismatch of sources of free energy required for the wave generation nearly nullifies their common effect.