Effective Recombination Coefficient Research Articles

INCREASING POSITIVE ION NUMBER DENSITIES BELOW THE PEAK OF ION-ELECTRON PAIR PRODUCTION IN TITAN'S IONOSPHERE

We combine derived ion-electron pair formation rates with Cassini Radio Plasma Wave Science Langmuir Probe measurements of electron and positive ion number densities in Titan's sunlit ionosphere. We show that positive ion number densities in Titan's sunlit ionosphere can increase toward significantly lower altitudes than the peak of ion-electron pair formation despite that the effective ion-electron recombination coefficient increases. This is explained by the increased mixing ratios of negative ions, which are formed by electron attachment to neutrals. While such a process acts as a sink for free electrons, the positive ions become longer-lived as the rate coefficients for ion-anion neutralization reactions are smaller than those for ion-electron dissociative recombination reactions.

Influence of water vapour on the height distribution of positive ions, effective recombination coefficient and ionisation balance in the quiet lower ionosphere

Abstract. Mesospheric water vapour concentration effects on the ion composition and electron density in the lower ionosphere under quiet geophysical conditions were examined. Water vapour is an important compound in the mesosphere and the lower thermosphere that affects ion composition due to hydrogen radical production and consequently modifies the electron number density. Recent lower-ionosphere investigations have primarily concentrated on the geomagnetic disturbance periods. Meanwhile, studies on the electron density under quiet conditions are quite rare. The goal of this study is to contribute to a better understanding of the ionospheric parameter responses to water vapour variability in the quiet lower ionosphere. By applying a numerical D region ion chemistry model, we evaluated efficiencies for the channels forming hydrated cluster ions from the NO+ and O2+ primary ions (i.e. NO+.H2O and O2+.H2O, respectively), and the channel forming H+(H2O)n proton hydrates from water clusters at different altitudes using profiles with low and high water vapour concentrations. Profiles for positive ions, effective recombination coefficients and electrons were modelled for three particular cases using electron density measurements obtained during rocket campaigns. It was found that the water vapour concentration variations in the mesosphere affect the position of both the Cl2+ proton hydrate layer upper border, comprising the NO+(H2O)n and O2+(H2O)n hydrated cluster ions, and the Cl1+ hydrate cluster layer lower border, comprising the H+(H2O)n pure proton hydrates, as well as the numerical cluster densities. The water variations caused large changes in the effective recombination coefficient and electron density between altitudes of 75 and 87 km. However, the effective recombination coefficient, αeff, and electron number density did not respond even to large water vapour concentration variations occurring at other altitudes in the mesosphere. We determined the water vapour concentration upper limit at altitudes between 75 and 87 km, beyond which the water vapour concentration ceases to influence the numerical densities of Cl2+ and Cl1+, the effective recombination coefficient and the electron number density in the summer ionosphere. This water vapour concentration limit corresponds to values found in the H2O-1 profile that was observed in the summer mesosphere by the Upper Atmosphere Research Satellite (UARS). The electron density modelled using the H2O-1 profile agreed well with the electron density measured in the summer ionosphere when the measured profiles did not have sharp gradients. For sharp gradients in electron and positive ion number densities, a water profile that can reproduce the characteristic behaviour of the ionospheric parameters should have an inhomogeneous height distribution of water vapour.

Effective recombination coefficient and solar zenith angle effects on low-latitude D-region ionosphere evaluated from VLF signal amplitude and its time delay during X-ray solar flares

Excess solar X-ray radiation during solar flares causes an enhancement of ionization in the ionospheric D-region and hence affects sub-ionospherically propagating VLF signal amplitude and phase. In first part of the work, using the well known LWPC technique, we simulated the flare induced excess lower ionospheric electron density by amplitude perturbation method. Unperturbed D-region electron density is also obtained from simulation and compared with IRI-model results. Using these simulation results and time delay as key parameters, we calculate the effective electron recombination coefficient ($\alpha_{eff}$) at solar flare peak region. Our results match with the same obtained by other established models. In the second part, we dealt with the solar zenith angle effect on D-region during flares. We relate this VLF data with the solar X-ray data. We find that the peak of the VLF amplitude occurs later than the time of the X-ray peak for each flare. We investigate this so-called time delay ($\bigtriangleup t$). For the C-class flares we find that there is a direct correspondence between $\bigtriangleup t$ of a solar flare and the average solar zenith angle $Z$ over the signal propagation path at flare occurrence time. Now for deeper analysis, we compute the $\bigtriangleup t$ for different local diurnal time slots $DT$. We find that while the time delay is anti-correlated with the flare peak energy flux $\phi_{max}$ independent of these time slots, the goodness of fit, as measured by $reduced$-$\chi^2$, actually worsens as the day progresses. The variation of the $Z$ dependence of $reduced$-$\chi^2$ seems to follow the variation of standard deviation of $Z$ along the $T_x$-$R_x$ propagation path. In other words, for the flares having almost constant $Z$ over the path a tighter anti-correlation between $\bigtriangleup t$ and $\phi_{max}$ was observed.

Injection and temperature dependent carrier recombination rate and diffusion length in freestanding CVD diamond

AbstractWe applied time‐resolved nonlinear optical techniques for investigation of carrier recombination and diffusion processes in a 420 μm thick CVD diamond, relatively free from structural and point defects. Injection of 1015–1017 cm−3 carrier density by two‐photon excitation and monitoring free carrier absorption decay in 80–800 K range provided lifetime of 720 ns at T ≥ 300 K. At low temperatures, a linear increase of the carrier recombination rate with the injection level was fitted by an effective bimolecular recombination coefficient B = 10−11–10−9 cm3 s−1 and attributed to the impact of excitons and biexcitons, contributing to phonon‐assisted indirect Auger recombination. At high temperatures, increase of recombination rate was governed by Coulomb enhanced direct Auger process with a 520 meV threshold. An ambipolar carrier mobility with its peak value of ∼1500 cm2 V−1 s−1 at room temperature was determined by transient grating technique at ∼2 × 1015 cm−3 excess carrier density. A strong decrease of mobility at higher injections revealed the role of carrier‐carrier scattering, exciton formation and bandgap renormalization. The simultaneous measurements of carrier lifetime and diffusivity provided carrier diffusion length, which varied in a 1–50 μm range, depending on injection and temperature.

On the thermal electron balance in Titan’s sunlit upper atmosphere

The Cassini mission has investigated Titan’s upper atmosphere in detail and found that, under solar irradiation, it has a well-developed ionosphere, which peaks between 1000 and 1200km. In this paper we focus on the T40, T41, T42 and T48 Titan flybys by the Cassini spacecraft and use in situ measurements of N2 and CH4 densities by the Ion Neutral Mass Spectrometer (INMS) as input into a solar energy deposition model to determine electron production rates. We combine these electron production rates with estimates of the effective recombination coefficient based on available laboratory data for Titan ions’ dissociative recombination rates and electron temperatures derived from the Langmuir probe (LP) to predict electron number densities in Titan’s upper atmosphere, assuming photochemical equilibrium and loss of electrons exclusively through dissociative recombination with molecular ions. We then compare these predicted electron number densities with those observed in Titan’s upper atmosphere by the LP. The assumption of photochemical equilibrium is supported by a reasonable agreement between the altitudes where the electron densities are observed to peak and where the electron production rates are calculated to peak (roughly corresponding to the unit optical depth for HeII photons at 30.38nm). We find, however, that the predicted electron number densities are nearly a factor of two higher than those observed throughout the altitude range between 1050 and 1200km (where we have made estimates of the effective recombination coefficient). There are different possible reasons for this discrepancy; one possibility is that there may be important loss processes of free electrons other than dissociative recombination in Titan’s upper atmosphere.

Electron density profiles in the quiet lower ionosphere based on the results of modeling and experimental data

Abstract. The theoretical PGI (Polar Geophysical Institute) model for the quiet lower ionosphere has been applied for computing the ionization rate and electron density profiles in the summer and winter D-region at solar zenith angles less than 80° and larger than 99° under steady state conditions. In order to minimize possible errors in estimation of ionization rates provided by solar electromagnetic radiation and to obtain the most exact values of electron density, each wavelength range of the solar spectrum has been divided into several intervals and the relations between the solar radiation intensity at these wavelengths and the solar activity index F10.7 have been incorporated into the model. Influence of minor neutral species (NO, H2O, O, O3) concentrations on the electron number density at different altitudes of the sunlit quiet D-region has been examined. The results demonstrate that at altitudes above 70 km, the modeled electron density is most sensitive to variations of nitric oxide concentration. Changes of water vapor concentration in the whole altitude range of the mesosphere influence the electron density only in the narrow height interval 73–85 km. The effect of the change of atomic oxygen and ozone concentration is the least significant and takes place only below 70 km. Model responses to changes of the solar zenith angle, solar activity (low–high) and season (summer–winter) have been considered. Modeled electron density profiles have been evaluated by comparison with experimental profiles available from the rocket measurements for the same conditions. It is demonstrated that the theoretical model for the quiet lower ionosphere is quite effective in describing variations in ionization rate, electron number density and effective recombination coefficient as functions of solar zenith angle, solar activity and season. The model may be used for solving inverse tasks, in particular, for estimations of nitric oxide concentration in the mesosphere.

Estimation of the effective recombination coefficient by variations in the maximum usable frequencies of one-hop high-frequency radio lines during the solar eclipses of March 29, 2006, and August 1, 2008

This paper presents the results of experimental investigations of variations in maximal usable frequencies (MUFs) during solar eclipses with the use of the method of panoramic tilt sounding by chirps in Eastern Europe and Western Siberia. The characteristics of perturbations of MUFs have been obtained experimentally. It has been established that the effective recombination coefficient increases by 24% in the ionospheric F region with an increase in the maximum eclipse phase from 0.49 to 1.0.

Carrier recombination and diffusivity in microcrystalline CVD‐grown and single‐crystalline HPHT diamonds

AbstractWe report investigation of carrier recombination and diffusivity in bulk diamonds of different crystalline structure – microcrystalline (MC) CVD‐grown and single crystalline HPHT diamonds. Presence of neutral and positively charged nitrogen and hydrogen defects was determined from NIR and UV–IR absorption spectra. Carrier injection into 1‐mm thick bulk layers was realized by two‐photon absorption at 351 nm wavelength. Carrier lifetimes of 150–330 ns in IIa type HPHT crystals correlated with N density, while the lifetimes in CVD crystal exhibited very fast (80 ps), slower one (3–8 ns), and µs‐duration thermally‐activated (∼1.5 eV) decay components. The initial two components correlated with the grain size at the front and backsides of the MC diamond. Linearly increasing with injection carrier recombination rates were observed in both CVD and HPHT samples at 800 K, and fitted with effective recombination coefficient B = 3–4 × 10−9 cm3/s. Ambipolar mobility and thermal diffusivity parameters in CVD and HPHT bulk crystals were measured by light‐induced free carrier and thermal grating techniques.

Effective electron recombination coefficient in ionospheric D-region during the relaxation regime after solar flare from February 18, 2011

In this paper, we present a model for determination of a weakly time dependent effective recombination coefficient for the perturbed terrestrial ionospheric D-region plasma. We study consequences of a class M1.0 X-ray solar flare, recorded by GOES-15 satellite on February 18, 2011 between 14:00 and 14:15UT, by analyzing the amplitude and phase real time variations of very low frequency (VLF) radio waves emitted by transmitter DHO (located in Germany) at frequency 23.4kHz and recorded by the AWESOME receiver in Belgrade (Serbia). Our analysis is limited to ionospheric perturbations localized at altitudes around 70km where the dominant electron gain and electron loss processes are the photo-ionization and recombination, respectively.

Ionization sources in Titan's deep ionosphere

We analyze a multi‐instrumental data set from four Titan encounters by the Cassini spacecraft to investigate in detail the formation of the ionosphere. The data set includes observations of thermospheric and ionospheric species and suprathermal electrons. A model describing the solar and electron energy deposition is used as an organizing element of the Cassini data set. We first compare the calculated secondary electron production rates with the rates inferred from suprathermal electron intensity measurements. We then calculate an effective electron dissociative recombination coefficient, applying three different approaches to the Cassini data set. Our findings are threefold: (1) The effective recombination coefficient derived under sunlit conditions in the deep ionosphere (<1200 km) is found to be independent of solar zenith angle and flyby. Its value ranges from 6.9 × 10−7 cm3 s−1 at 1200 km to 5.9 × 10−6 cm3 s−1 at 970 km at 500 K. (2) The presence of an additional, minor source of ionization is revealed when the solar contribution is weak enough. The contribution by this non‐solar source—energetic electrons most probably of magnetospheric origin—becomes apparent for secondary electron production rates, due to solar illumination alone, close to or smaller than about 3 × 10−1 cm−3 s−1. Such a threshold is reached near the solar terminator below the main solar‐driven electron production peak (<1050 km). (3) Our ability to model the electron density in the deep ionosphere is very limited. Our findings highlight the need for more laboratory measurements of electron dissociative recombination coefficients for heavy ion species at high electron temperatures (especially near 500 K).

Charge Mobility and Recombination in a New Hole Transporting Polymer and Its Photovoltaic Blend

The charge mobility in a new hole transporting polymer, poly(2,6-bis(thiophene-2-yl)-3,5-dipentadecyldithieno[3,2-b;2',3'-d]thiophene) (PBTDTT-15), and its blend with (6,6)-phenyl-C(70)-butyric acid methyl ester (PC(70)BM) in a weight ratio of 1:3 at ambient atmosphere condition was investigated using time-of-flight (TOF) photoconductivity and photoinduced charge extraction by linearly increasing voltage (PhotoCELIV) techniques. The bulk heterojunction based photovoltaic (PV) blend (PBTDTT-15:PC(70)BM (1:3)) exhibited a promising power conversion efficiency (PCE) of 3.23% under air mass 1.5 global (AM 1.5G) illumination of 100mW/cm(2). The charge mobility and recombination properties of the best performing cells were investigated. The hole mobility in the pure PBTDTT-15 was in the range of 4 x 10(-4) cm(2)/(V s), which was reduced almost 5 times in the PBTDTT-15:PC(70)BM (1:3) blend. The PhotoCELIV transient observed for the photovoltaic (PV) blend was dominated by electrons, with the charge mobility of the order of 10(-3) cm(2)/(V s), and a weak shoulder at a long time scale due to holes. The effective bimolecular recombination coefficient (beta) obtained for the PV blend deviated significantly from the Langevin recombination coefficient (beta(L)) indicating a phase-separated morphology. The obtained results indicate that the PBTDTT-15:PC(70)BM blend can be potential for organic solar cell applications.

Transient Absorption Studies of Bimolecular Recombination Dynamics in Polythiophene/Fullerene Blend Films

Bimolecular recombination, an important loss mechanism in organic solar cells, has been investigated using transient absorption spectroscopy for poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) films and analogues of these components. Data are analyzed as a function of blend composition, postdeposition thermal annealing, and excitation density. Comparison of transient spectra for P3HT:PCBM films with analogous films employing P3HS and PC70BM allows the assignment of the photoinduced absorption features. These decay dynamics are analyzed on the nanosecond to millisecond time scales and are shown to be in excellent agreement with a bimolecular recombination model in the presence of an exponential distribution of localized (trap) states. Thermal annealing results in an acceleration of these decay dynamics, which is assigned to a reduction in the depth of the trap states and correlated with an increase in film crystallinity. The decay dynamics are analyzed to obtain an effective recombination coefficient that is charge density dependent at low polaron densities, but becomes independent of charge density at high charge carrier densities (>1018 cm−3). This transition is assigned to trap filling, with the recombination coefficient measured at high charge density (k = 3 × 10−12 cm3 s−1) corresponding to the trap-free limit. From transient spectroscopic behavior we estimate the density of intraband trap states in unannealed P3HT:PCBM blend films to be ∼7 × 1017 cm−3.

The role of atomic oxygen concentration in the ionization balance of the lower ionosphere during solar proton events

Abstract. The influence of atomic oxygen concentration on the height distribution of the main positive and negative ions and on electron density in the mesosphere is studied for the conditions prevailing during the solar proton event on 17 January 2005. It is shown by numerical modeling that the electron and ion density profiles are strongly dependent on the choice of the atomic oxygen profile. Experimental measurements of the electron density are used as the criterion for choosing the atomic oxygen profile in the mesosphere. With the help of modeling, the atomic oxygen profile in the daytime in the winter mesosphere is found to lead to a model electron density profile best matching the electron density profile obtained experimentally. As a result, with the help of modeling, we find the atomic oxygen profiles at various solar zenith angles in the winter mesosphere which lead to model electron density profiles matching the electron density profiles obtained experimentally. Alteration of the atomic oxygen concentration leads to a redistribution of the abundance of both positive and negative ion constituents, with changes in their total concentrations and transition heights. In consequence this results in changes of the electron density and effective recombination coefficient. For conditions of low concentration of atomic oxygen (during a solar proton event), the formation of cluster ions is the key process determining electron and ion densities at altitudes up to 77 km. The complex negative CO3− ion is formed up to about 74 km and the final NO3− ion, which is stable in relation to the atomic oxygen, is the dominant negative ion up to 74 km. As a result the transition heights between cluster ions and molecular ions and between negative ions and electron density are located at 77 km and 66 km, respectively.

Modeling of the D-region behavior in the polar ionosphere during the solar flare of April 5, 2004

On the basis of the ion chemistry theoretical model, the impact of a powerful solar flare on variations in the ion composition and electron density in the D region of the polar ionosphere is considered. Good agreemnt between the model profiles of the electron density Ne(h) and the experimental data obtained during the flare by the partial reflection method is found. It is shown that the decrease in the effective recombination coefficient observed during disturbances is explained by the depletion of the relative content of the rapidly recombining complex ion clusters.

D-region electron density evaluated from VLF amplitude time delay during X-ray solar flares

VLF amplitude enhancements of the NAA/24.0 kHz signal (Maine, USA), registered by the Belgrade AbsPAL facility during solar flares, have been related to solar X-ray fluxes measured by the GOES-12 satellite. The observed regular appearance of the VLF amplitude/phase peak behind the peak of the corresponding X-ray flux has been characterized by the time delay assigned to each of the 97 out of 114 flare-induced distinct amplitude/phase enhancement events analysed. The measurements reported pertain to the period May–August of the years 2004 and 2005 and have been used to model the electron density enhancements of the D-region during solar flares. On the grounds of the continuity equation for the electron component and the amplitude time delay, the effective recombination coefficient has been estimated to range from 10 - 13 m 3 / s to 10 - 11 m 3 / s , depending on the X-ray maximum flux. By integrating the continuity equation, the time variation of the electron density during solar flare occurrence has been restored. Enhancements up to two orders of magnitude, for X-class flares, with respect to undisturbed conditions were predicted. The results obtained have been found to be in good agreement with measurements by different techniques, as well as with the independent estimates of β and H ′ . Errors introduced by the applied approach have been critically examined to place the uncertainty of the results arrived at between 10% and 20%.

On the relations between proton influx and D-region electron densities during the polar-cap absorption event of 28-29 October 2003

Abstract. Observations by incoherent-scatter radar have been applied to explore relationships between the fluxes of incident protons and the resulting D-region electron densities during a polar-cap radio-absorption event. Using proton flux data from a GOES geosynchronous satellite, the energy band having the greatest influence at a selected height is estimated by a process of trial and error, and empirical relationships are defined. The height profiles of the effective recombination coefficient are determined for day and night, and the transition over the evening twilight is investigated for the height range 60-70 km. The results show that the day-night change is confined to heights below 80 km, night-time values at the lower levels being consistent with a balance between negative ions and electrons controlled by 3-body attachment and collisional detachment. The daytime results confirm that, contrary to the prediction of some chemical models, a square-law continuity equation may be strictly applied. It is confirmed that, as previously reported, the timing of the sunset change varies with altitude.

A new method of studying the relation between ionization rates and radio-wave absorption in polar-cap absorption events

Abstract. During polar-cap absorption events, which are caused by the incidence of energetic solar protons, the high-latitude ionospheric D region is extended down to relatively low altitudes. While the incoming proton fluxes may be monitored by satellite-borne detectors, and the resulting radio-wave absorption with a ground-based riometer, the enhancement of electron density at a given altitude is less easily determined. Direct measurements by incoherent-scatter radar are infrequent and they tend to lack the necessary sensitivity at the lower levels. Computations of the electron density from the observed particle fluxes are handicapped by uncertainties in the height profile of the effective recombination coefficient. This paper describes a new approach based on finding the best-fit solution to an over-determined set of equations. The D region is treated as a set of slabs, each contributing to the total radio absorption, and the method relies on the fact that the proton spectrum varies during the event. The analysis produces a set of coefficients relating the absorption increment in the slab to the square root of the production rate, as a function of height. Values of effective recombination coefficient are also deduced over a range of heights, and these agree with previous estimates (Gledhill, 1986) to within a factor of 2. However, whereas the latter do not generally go below 60km altitude the new determination extends the values down to 40km. The new method provides a measurement of the height profile of the absorption in PCA events. It is shown that the slabs centred from 45 to 65km typically account for 80% of the total daytime absorption, and that less than 1% of the total arises above 80km or below 30km. At night most of the absorption comes from the slabs at 75 and 80km, with no significant contribution from slabs below 75 or above 85km. These results would not differ significantly from estimates based on the Gledhill profiles if extrapolated downward. Predictions based on the coefficients generated by the procedure are compared with the polar-cap absorption observed during some recent events. Typical electron-density values are derived, and the study provides an independent confirmation that the electron density and the production rate are related by a square-root law.

Quantitative description of electron precipitation during auroral absorption events in the morning/noon local-time sector

Flux-energy spectra of precipitating electrons are derived from electron density profiles measured by the EISCAT radar during auroral absorption events in the morning/noon local-time sector. The inversion technique uses effective recombination coefficient profiles computed on the basis of a previously validated theoretical model of the lower ionosphere. It is shown that flux-energy spectra for the energy range 30– 200 keV are in reasonable agreement with those derived for the same events using trapped flux-energy spectra from geosynchronous satellite data and a model for diffusion of trapped electrons into the loss cone by scattering on whistler waves. During individual events, strongly varying precipitating fluxes are found to be due primarily to varying pitch-angle diffusion.

Ionospheric electrical conductances produced by auroral proton precipitation

Prom incoherent scatter radar observations and space‐borne particle detector data, it appears that energetic proton precipitation can sometimes, for some locations, be a major source of ionization in the auroral ionosphere and contribute significantly to the electrical conductances. Here we propose a simple parameterization for the Pedersen and Hall conductances produced by proton precipitation. The derivation is based on a proton transport code for computing the electron production rate and on an effective recombination coefficient for deducing the electron density. The atmospheric neutral densities and temperatures and the geomagnetic‐field strength are obtained from standard models. The incident protons are assumed to have a Maxwellian distribution in energy with a mean energy 〈E〉 in the 2–40 keV range and an energy flux Q0. The parameterized Pedersen and Hall conductances are functions of 〈E〉 and Q0, as well as of the geomagnetic‐field strength. The dependence on these quantities is compared with those obtained for electron precipitation and for solar EUV radiation. To add the contribution of proton precipitation to the total conductances for electrodynamic studies in auroral regions, the conductances produced by electron and proton precipitations can be combined by applying a root‐sum‐square approximation.

The recombination of H 3+ ions with electrons: dependence on partial pressure of H 2

We observed an increase in the effective recombination coefficient ( α eff) of the recombination of H 3 + ions with electrons with a number density of H 2, n( H 2) . With increasing n(H 2) from 1×10 11 to 2×10 12 cm −3 the α eff increased from 1.3×10 −8 to 1.5×10 −7 cm 3 s −1 . The dependence of α eff on n(H 2) indicates that the recombination of H 3 + ions in an afterglow plasma is a complex process in which collisions with H 2 take place. It is stressed that at conditions corresponding to plasma in interstellar space the dissociative recombination of H 3 + ions is very slow with a rate coefficient α⩽1.3×10 −8 cm 3 s −1 .