Ion-neutral Coupling Research Articles

Different Behavior of Density Perturbations Between Dayside and Nightside in the Martian Thermosphere and the Ionosphere Associated With Atmospheric Gravity Waves.

To investigate the excitation mechanism of ionospheric perturbations on Mars by the Neutral Gas and Ion Mass Spectrometer (NGIMS) onboard Mars Atmosphere and Volatile EvolutioN (MAVEN), we categorize ionospheric perturbations into three cases: (a) the ion-neutral coupling cases where ion and neutral perturbations are well coupled, (b) the ion-specific cases where ion perturbations move independently from neutrals, and (c) the coronal mass ejection cases associated with solar wind extreme events. A representative number of cases from total profiles are compared with a numerical model to determine the fraction that can be explained by an atmospheric gravity waves (GW). The neutral perturbations on the dayside at 170-190km altitudes are in excellent agreement with the GW. Whereas, contrary to previous thoughts, neutral perturbations are not necessarily explained by the GW especially on the nightside at 190-210km. Ion perturbations on the dayside at 170-190km also show a good agreement with the GW. The agreement becomes extremely low on the nightside at 190-210km, reaching the limit of strong ion-neutral coupling around 190km. Further investigation found that the behavior of the ion perturbations explicitly depends on the dayside and nightside. Its dominant driver potentially differs clearly between dayside and nightside. Statistics of relative perturbations demonstrate a clear effect associated with species scale height in neutrals. Whereas, the correlation between ions and neutrals breaks down at high solar zenith angle near southern dusk. We see currently unexplained behavior that cannot be fully interpreted by GW both at night and near southern dusk.

Variability of Earth’s ionospheric outflow in response to the dynamic terrestrial exosphere

The most abundant neutral constituent in the exospheric region (i.e., beyond ≈ 500 km altitude) is the atomic hydrogen (H); however, its density distributions predicted by physics-based models have been challenged by satellite-based observations of its far ultraviolet emissions. This discrepancy may impact magnetospheric ions’ densities and velocities since numerous chemistry and ion-neutral coupling interactions rely sensitively on the underlying neutral hydrogen population. The Polar Wind Outflow Model a first-principled model for relevant ion species in the high-latitude ionosphere, is employed to investigate the role of neutral H on the ionospheric outflow. Specifically, variability in the outflow of ionospheric H+, He+, N+, and O+ as a response to systematic enhancement and depletion of H number densities were simulated. The altitude-dependent ion density and energy partition profiles vary with neutral H density, solar activities, and ion species. These findings suggest that the exosphere plays a crucial role in controlling the production and loss of ions through ionospheric chemistry, as well as the energy contributions by altering ion-neutral-electron collisions and the ambipolar electric field to the high-latitude ionospheric outflow. As a result, the escape rates of the ionospheric outflow are directly associated with exospheric distributions. This work potentially helps understand the dominant mechanisms of atmospheric escape, particularly during a hydrogen-rich early Earth’s and exoplanet’s atmosphere, which is known to play a significant role in understanding the evolution of Earth’s atmosphere.

Ion-acoustic waves in weakly ionized plasmas with charge-exchange collisions

Ion-neutral charge-exchange collisions in plasmas of laboratory, space, and astrophysical origins are fundamental to understanding wave dissipation and wave generation phenomena. This paper implements a charge-exchange collision operator in the Boltzmann–Poisson system equations for a weakly ionized plasma. When considering an electric field perturbation, the governing kinetic equations provide significant results concerning the plasma conductivity and the dielectric function, appearing in simple, sensible forms. The present analysis reveals a backward wave propagation phenomenon at maximum conductivity when the wavenumber of the plasma wave is smaller than the reciprocal of the ion-neutral collisions mean free path. In addition, it is shown that ion-neutral coupling resulting from charge-exchange collisions enhances ion-acoustic waves below and beyond the ion plasma frequency and leads to the onset of a fundamental instability that overcomes Landau damping under certain circumstances. The collisionless model is recovered as a limiting case, i.e., in the asymptotic limit of a long mean free path.

Gravity wave driven variability in the Mars ionosphere-thermosphere system

Gravity waves (GWs) have been persistently observed in neutral species in the Martian thermosphere. In this paper, we study characteristics of GWs in O2+ and O+ ions in the Mars ionosphere (∼160–200 km) and examine their relation to GWs in the thermosphere. For this purpose, we use O2+, O+, and Ar densities measured by the Neutral Gas and Ion Mass spectrometer (NGIMS) instrument aboard the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. The amplitudes and (horizontal) wavelengths of the GWs are extracted from the density profiles. The results of the present study show that GWs are ubiquitous in the Martian ionosphere-thermosphere system. We found that the amplitudes of GWs in ions are ∼2 times larger than those in neutrals. While the amplitudes in ions reach as high as 70% of the background densities, the most probable amplitude 5%. The amplitudes of the GWs in ions are more on the nightside than on the dayside; more during the 2018 global dust storm period than during nominal dust conditions; and increase with decreasing in solar activity. All these results are qualitatively similar to those observed in neutrals. In addition, amplitudes of the GWs in ions show moderate correlation with those neutrals while the wavelengths show good correlation. The amplitudes of GWs in ions and neutrals show moderate negative correlation with thermospheric temperatures. All these results strongly suggest that the GWs in ions are due to those in neutrals and ion-neutral coupling may be strong in the altitude range of interest. The correlation between the amplitudes of GWs in ions and neutral, however, is local time dependent with weak correlation on the nightside. Other factors such as recombination or particle precipitation may be playing an important role in distorting the wavy nature in ions leading to weaker correlation with neutral waves on the night side. Furthermore, the correlation between GW amplitudes in ions and neutrals is weaker in strong crustal magnetic field regions. This implies that the magnetic fields control the motion of the ions leading to distorting in wavy nature in ions.

Quantification of the role of gravity wave induced TIDs in modulating ESF day to day variability during geomagnetically disturbed periods

The control of magnetic disturbance induced seed perturbations on the daily variation in night-time ionization irregularity occurrence is studied using ionosonde data and TIMED/GUVI neutral density data at the magnetic equatorial region, Trivandrum. The study reveals that there is a requisite threshold seed amplitude for ESF to occur at a particular altitude and this requisite seed increases as the altitude decreases. This dependence of requisite seed perturbation on altitude for multiple years, which incorporates the electrodynamical effects also, is used as the basis for developing an empirical model to hind-cast ESF. Using the model, the threshold seed perturbation for any day of vernal equinox (ve) season of any year can be delineated if the solar flux index (F10.7) is known. The empirical model is also validated using the data for high and low solar activity periods. The model is able to hind-cast ESF with 92% success for low solar activity while the success rate for high solar activity is 50%. Further, the dependence of the threshold curves on neutral density is brought out, thereby confirming the role of ion-neutral coupling processes in modulating the same for magnetically disturbed periods. The geomagnetically disturbed period requisite seed amplitudes are higher than those for quiet periods at a given altitude. This study underlines the importance of disturbance induced seed perturbations and neutral density in controlling ESF occurrence.

Stability of two-fluid partially ionized slow-mode shock fronts

ABSTRACT A magnetohydrodynamic (MHD) shock front can be unstable to the corrugation instability, which causes a perturbed shock front to become increasingly corrugated with time. An ideal MHD parallel shock (where the velocity and magnetic fields are aligned) is unconditionally unstable to the corrugation instability, whereas the ideal hydrodynamic (HD) counterpart is unconditionally stable. For a partially ionized medium (for example, the solar chromosphere), both HD and MHD species coexist and the stability of the system has not been studied. In this paper, we perform numerical simulations of the corrugation instability in two-fluid partially ionized shock fronts to investigate the stability conditions, and compare the results to HD and MHD simulations. Our simulations consist of an initially steady two-dimensional parallel shock encountering a localized upstream density perturbation. In MHD, this perturbation results in an unstable shock front and the corrugation grows with time. We find that for the two-fluid simulation, the neutral species can act to stabilize the shock front. A parameter study is performed to analyse the conditions under which the shock front is stable and unstable. We find that for very weakly coupled or very strongly coupled partially ionized system the shock front is unstable, as the system tends towards MHD. However, for a finite coupling, we find that the neutrals can stabilize the shock front, and produce new features including shock channels in the neutral species. We derive an equation that relates the stable wavelength range to the ion-neutral and neutral-ion coupling frequencies and the Mach number. Applying this relation to umbral flashes gives an estimated range of stable wavelengths between 0.6 and 56 km.

The SuperDARN’s role in ion-neutral coupling research

The direct collisions that thermospheric neutral particles experience with convecting ionospheric ions is the primary way in which energy from the solar wind is deposited into the Earth’s atmosphere. Because of these collisions, the neutral gas polewards of middle latitudes (∼±40∘ geomagnetic) is often set into motion due to ion-drag, and Joule heated due to friction. Transient ionospheric phenomena, such as auroral arcs and sub-auroral polarisation streams (SAPS) also modify neutral winds in their own distinctive way. Thanks to the large spatial coverage of the Super Dual Auroral Radar Network (SuperDARN) at both middle and high latitudes, studies into ionosphere–thermosphere coupling have been enabled. These range from collaborative studies with neutral wind measurements from other instruments, to the driving electrodynamic input of various global circulation models. This paper aims to provide a general overview of the ion-neutral coupling research conducted utilising the SuperDARN, and to offer some directions as to where we believe future work should be directed.

1D photochemical model of the ionosphere and the stratosphere of Neptune

Neptune remains a mysterious world that deserves further exploration and is a high-priority objective for a future planetary mission in order to better understand the formation and evolution of ice giant planets. We have developed a coupled ion-neutral 1D photochemical model of Neptune's atmosphere to study the origin and evolution of the hydrocarbons and the oxygen species. The up-to-date chemical scheme is derived from one used for Titan's atmosphere, which led to good agreements with the Cassini-CIRS observations for oxygen species and the main hydrocarbons. The main results we obtain are the following: The ion-neutral chemistry coupling produces aromatics (and benzene in particular) in the atmosphere of Neptune with relatively high abundances. Our model results are in good agreement with observations (taking model uncertainties into account). Two ionospheric peaks are present in the atmosphere located above the pressure level of 10−5 mbar and around 10−3 mbar. The influx of oxygen species in the upper atmosphere of Neptune has an effect on the concentration of many ions. We show that in situ exploration of Neptune's atmosphere would provide very interesting constraints for photochemical models concerning in particular the origin of oxygen species and the contribution of ion chemistry. A precise description of upper atmospheric chemistry is crucial for a better understanding of the internal composition and the formation processes of this planet.

On the ion-neutral coupling in cometary comae

In a cometary coma, the ion-neutral decoupling distance, sometimes referred to as the ion exobase or collisionopause, can be defined as the cometocentric distance, r(in), where ions, initially movi ...

Dependence of mesospheric Na and Fe distributions on electron density at Arecibo

We present case studies of the mesospheric alkali and non-alkali metals, Na and Fe, along with electron concentrations [Ne] obtained from measurements made at Arecibo on nights of 17–18 and 18–19 March 2004. The background mesospheric conditions as recorded by an airglow all-sky imager displayed ripple- and band-type structures on these nights. Both of the metals display detailed structures within their neutral sporadic layer but are more pronounced in Na than for Fe. A sporadic-E (Es) with electron concentrations [Ne] exceeding 3000 electrons cm−3 is accompanied by a strong Na enhancement and a weak sporadic Fe (Fes) layer around 95-km altitude. The concentration of Fe+ and Na+ is estimated to be close to 600 and 30 ions.cm−3, respectively, within the sporadic-E layer. In order to investigate ion-neutral coupling, a correlative analysis was performed in two altitude regions. Similar features are seen between neutrals and electrons in the 96–100-km altitude range, while within the altitude range of 80–90 km, an opposite behavior is seen. A comparative study between neutral layers below 90 km often referred to as the main or permanent layer and sporadic activity above 90 km reveals different characteristics for alkali and non-alkali metal. Fe concentrations in the main layer are higher than in Fes resulting in a density ratio of less than 1 determined from two layers of 3 km thickness centered at 97 and 87 km. For the case of Na, the ratio exceeds 1 during Es activity on both the nights. The case studies discussed in this work facilitate our understanding of different factors that can influence the sporadic activity in alkali and non-alkali metals. In a region dominated by ion-molecule chemistry, temperature fluctuations that can be induced by wave activity will have more impact on Na than for Fe within their layers depending on altitude.

Onset of the Kelvin-Helmholtz instability in partially ionized magnetic flux tubes

Context. Recent observations of solar prominences show the presence of turbulent flows that may be caused by Kelvin-Helmholtz instabilites (KHI). However, the observed flow velocities are below the classical threshold for the onset of KHI in fully ionized plasmas. Aims. We investigate the effect of partial ionization on the onset of KHI in dense and cool cylindrical magnetic flux tubes surrounded by a hotter and lighter environment. Methods. The linearized governing equations of a partially ionized two-fluid plasma are used to describe the behavior of small-amplitude perturbations superimposed on a magnetic tube with longitudinal mass flow. A normal mode analysis is performed to obtain the dispersion relation for linear incompressible waves. We focus on the appearance of unstable solutions and study the dependence of their growth rates on various physical parameters. An analytical approximation of the KHI linear growth rate for slow flows and strong ion-neutral coupling is obtained. An application to solar prominence threads is given. Results. The presence of a neutral component in a plasma may contribute to the onset of the KHI even for sub-Alfv\'enic longitudinal shear flows. Collisions between ions and neutrals reduce the growth rates of the unstable perturbations but cannot completely suppress the instability. Conclusions. Turbulent flows in solar prominences with sub-Alfv\'enic flow velocities may be interpreted as consequences of KHI in partially ionized plasmas.

MAGNETOACOUSTIC WAVES IN A PARTIALLY IONIZED TWO-FLUID PLASMA

Compressible disturbances propagate in a plasma in the form of magnetoacoustic waves driven by both gas pressure and magnetic forces. In partially ionized plasmas the dynamics of ionized and neutral species are coupled due to ion-neutral collisions. As a consequence, magnetoacoustic waves propagating through a partially ionized medium are affected by the ion-neutral coupling. The degree to which the behavior of the classic waves is modified depends on the physical properties of the various species and on the relative value of the wave frequency compared to the ion-neutral collision frequency. Here, we perform a comprehensive theoretical investigation of magnetoacoustic wave propagation in a partially ionized plasma using the two-fluid formalism. We consider an extensive range of values for the collision frequency, ionization ratio, and plasma $\beta$, so that the results are applicable to a wide variety of astrophysical plasmas. We determine the modification of the wave frequencies and study the frictional damping due to ion-neutral collisions. Approximate analytic expressions to the frequencies are given in the limit case of strongly coupled ions and neutrals, while numerically obtained dispersion diagrams are provided for arbitrary collision frequencies. In addition, we discuss the presence of cutoffs in the dispersion diagrams that constrain wave propagation for certain combinations of parameters. A specific application to propagation of compressible waves in the solar chromosphere is given.

Ion–neutral coupling in Earth's thermosphere, estimated from concurrent radar and optical observations above Alaska

During March–April 2011 a campaign of coordinated observations was undertaken between the Poker Flat Advanced Modular Incoherent Scatter Radar and the Poker Flat Scanning Doppler Imager. These instruments provide horizontally resolved maps of plasma and neutral parameters in Earth's thermosphere. We report on data collected during the campaign, and use these data to investigate two key aspects of ion–neutral coupling, namely Joule heating and the ion–neutral collision frequency. Volumetric Joule heating rates were often well correlated with measured ion temperature enhancements. The contribution of the neutral wind dynamo to the observed heating rates was positive when the absolute horizontal magnetic field perturbation (|ΔH|) was less than approximately 40nT, and negative above that level. The total momentum–transfer ion–neutral collision frequency was estimated to be 1.02−0.152+0.179s−1 at an altitude of 260km, which, for a neutral composition of 75% atomic oxygen, yielded an estimate of the O+–O collision frequency of 0.766−0.114+0.134s−1.

Ion-neutral coupling during deep solar minimum

The equatorial ionosphere under conditions of deep solar minimum exhibits structuring due to tidal forces. Data from instruments carried by the Communication/Navigation Outage Forecasting System (C/NOFS) which was launched in April 2008 have been analyzed for the first 2 years following launch. The Planar Langmuir Probe (PLP), Ion Velocity Meter (IVM) and Vector Electric Field Investigation (VEFI) all detect periodic structures during the 2008–2010 period which appear to be tides. However when the tidal features detected by these instruments are compared, there are distinctive and significant differences between the observations. Tides in neutral densities measured by the Gravity Recovery and Climate Experiment (GRACE) satellite were also observed during June 2008. In addition, Broad Plasma Decreases (BPDs) appear as a deep absolute minimum in the plasma and neutral density tidal pattern. These are co-located with regions of large downward-directed ion meridional velocities and minima in the zonal drifts, all on the nightside. The region in which BPDs occur coincides with a peak in occurrence rate of dawn depletions in plasma density observed on the Defense Meterological Satellite Program (DMSP) spacecraft, as well as a minimum in radiance detected by UV imagers on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) and IMAGE satellites.

KELVIN-HELMHOLTZ INSTABILITY IN PARTIALLY IONIZED COMPRESSIBLE PLASMAS

The Kelvin-Helmholtz Instability (KHI) has been observed in the solar atmosphere. Ion-neutral collisions may play a relevant role for the growth rate and evolution of the KHI in solar partially ionized plasmas as in, e.g., solar prominences. Here, we investigate the linear phase of the KHI at an interface between two partially ionized magnetized plasmas in the presence of a shear flow. The effects of ion-neutral collisions and compressibility are included in the analysis. We obtain the dispersion relation of the linear modes and perform parametric studies of the unstable solutions. We find that in the incompressible case the KHI is present for any velocity shear regardless the value of the collision frequency. In the compressible case, the domain of instability depends strongly on the plasma parameters, specially the collision frequency and the density contrast. For high collision frequencies and low density contrasts the KHI is present for super-Alfvenic velocity shear only. For high density contrasts the threshold velocity shear can be reduced to sub-Alfvenic values. For the particular case of turbulent plumes in prominences, we conclude that sub-Alfvenic flow velocities can trigger the KHI thanks to the ion-neutral coupling.

OBSERVING THE FINE STRUCTURE OF LOOPS THROUGH HIGH-RESOLUTION SPECTROSCOPIC OBSERVATIONS OF CORONAL RAIN WITH THE CRISP INSTRUMENT AT THE SWEDISH SOLAR TELESCOPE

We present here one of the first high resolution spectroscopic observations of coronal rain, performed with the CRISP instrument at the Swedish Solar Telescope. This work constitutes the first attempt to assess the importance of coronal rain in the understanding of the coronal magnetic field in active regions. A large statistical set is obtained in which dynamics (total velocities and accelerations), shapes (lengths and widths), trajectories (angles of fall) and thermodynamic properties (temperatures) of the condensations are derived. Specifically, we find that coronal rain is composed of small and dense chromospheric cores with average widths and lengths of 310 km and 710 km respectively, average temperatures below 7000 K, displaying a broad distribution of falling speeds with an average of 70 km/s and accelerations largely below the effective gravity along loops. Through estimates of the ion-neutral coupling in the blobs we show that coronal rain acts as a tracer of the coronal magnetic field, thus supporting the multi-strand loop scenario, and acts as a probe of the local thermodynamic conditions in loops. We further find that the cooling in neighboring strands occurs simultaneously in general suggesting a similar thermodynamic evolution among strands, which can be explained by a common footpoint heating process. Constraints for coronal heating models of loops are thus provided. Estimates of the fraction of coronal volume with coronal rain give values between 7% and 30%. Estimates of the occurrence time of the phenomenon in loops set times between 5 and 20 hours, implying that coronal rain may be a common phenomenon, in agreement with the frequent observations of cool downflows in EUV lines. The coronal mass drain rate in the form of coronal rain is estimated to be on the order of 5x10^9 g/s, a significant quantity compared to the estimate of mass flux into the corona from spicules.

Global Longitudinal Dependence Observation of the Neutral Wind and Ionospheric Density Distribution

The statistical global view of the low-latitude ionospheric density stimulates further interest in studying the strong longitudinal variability of the ionospheric density structures in low-to-equatorial latitudes. However, we are not completely certain how the electrodynamics and ion-neutral coupling proceeds at low latitudes; in particular, the longitudinal difference in the dynamics of plasma structures in the low-to-mid latitude ionosphere is not yet fully understood. Numerical studies of latent heat release in the troposphere have indicated that the lower atmosphere can indeed introduce a longitudinal dependence and variability of the low-latitude ionosphere during quiet conditions. For the first time, we present simultaneous observations of the tidally modulated global wind structure, using TIDI observations, in the E-region and the ionospheric density distribution using ground (global GPS receivers) and space-based (C/NOFS in situ density and GPS TEC on CHAMP) instruments. Our results show that the longitudinally structured zonal wind component could be responsible for the formation of wave number four pattern of the equatorial anomaly.

Extreme changes in the dayside ionosphere during a Carrington-type magnetic storm

It is shown that during the 30 October 2003 superstorm, dayside O+ ions were uplifted to DMSP altitudes (~850 km). Peak densities were ~9 × 105 cm−3 during the magnetic storm main phase (peak Dst = −390 nT). By comparison the 1–2 September 1859 Carrington magnetic storm (peak Dst estimated at −1760 nT) was considerably stronger. We investigate the impact of this storm on the low- to mid-latitude ionosphere using a modified version of the NRL SAMI2 ionospheric code. It is found that the equatorial region (LAT = 0° ± 15°) is swept free of plasma within 15 min (or less) of storm onset. The plasma is swept to higher altitudes and higher latitudes due to E × B convection associated with the prompt penetration electric field. Equatorial Ionization Anomaly (EIA) O+ density enhancements are found to be located within the broad range of latitudes ~ ± (25°–40°) at ~500–900 km altitudes. Densities within these peaks are ~6 × 106 oxygen ions-cm−3 at ~700 km altitude, approximately +600% quiet time values. The oxygen ions at the top portions (850–1000 km) of uplifted EIAs will cause strong low-altitude satellite drag. Calculations are currently being performed on possible uplift of oxygen neutrals by ion-neutral coupling to understand if there might be further significant satellite drag forces present.

Equatorial and Low Latitude Ionospheric Effects During Sudden Stratospheric Warming Events

The sources of ionospheric forcing are from above (solar and magnetospheric forces), internal (chemistry, local neutral dynamics), and from below (lower atmosphere). In this work we review the observed ion-neutral coupling effects at equatorial and low latitudes during large meteorological events called sudden stratospheric warming (SSW). Research in this direction has been accelerated in recent years mainly due to: (1) extensive observing campaigns, and (2) solar minimum conditions. The former has been instrumental to catch the events before, during, and after the peak SSW temperatures. The latter has permitted a reduced forcing contribution from above and internal. The main ionospheric effects are clearly observed in the zonal electric fields (or vertical E×B drifts), total electron content, and peak electron densities. We include results from different ground- and satellite-based observations, covering different longitudes and years. We also present and discuss the modeling efforts that support most of the observations. Given that SSW can be forecast with a few days in advanced, there is potential for using the connection with the ionosphere for forecasting the occurrence and evolution of electrodynamic perturbations at low latitudes, and sometimes mid latitudes, during arctic winter warmings.

Matter infall in collapsing molecular cloud cores with an axial magnetic field

The magnetic fields affect collapse of molecular cloud cores. Here, we consider a collapsing core with an axial magnetic field and investigate its effect on infall of matter and formation of accretion disk. For this purpose, the equations of motion of ions and neutral infalling particles are numerically solved to obtain the streamlines of trajectories. The results show that in non-steady state of ionization and ion-neutral coupling, which is not unexpected in the case of infall, the radius of accretion disk will be larger as a consequence of axial magnetic field.