Wave emissions at half electron gyroharmonics in the equatorial plasmasphere region: CLUSTER observations and statistics

Intense (n + 1/2) f_ce emissions are a common phenomenon observed in the terrestrial inner magnetosphere. One of their interests is their possible effect in the pitch angle scattering of plasmasheet keV-electron, leading to diffuse auroras. In this paper, we present CLUSTER’s point of view about this topic, in the equatorial region of the plasmasphere, via a statistical study using 3 years of data. Spectral characteristics of these waves, which represent an important clue concerning their generation mechanism, are obtained using WHISPER data near perigee. Details on the wave spectral signature are shown in an event study, in particular their splitting in fine frequency bands. The orbit configuration of the four spacecraft offers a complete sampling on all MLT sectors. A higher occurrence rate of the emissions in the dawn sector and their confinement to the geomagnetic equator, pointed out in previous studies, are confirmed and described with additional details. The proximity of emission sites, both to the plasmapause layer and to the geomagnetic equator surface, seems to be of great importance in the behaviour of the (n + 1/2) f_ce wave characteristics. Our study indicates for the first time, that both the intensity of (n + 1/2) f_ce emissions, and the number of harmonic bands they cover, are increasing as the observation point is located further away outside from the plasmapause layer. Moreover, a study of the wave intensity in the first harmonic band (near 3/2 f_ce) shows higher amplitude for these emissions than previous published values, these emissions can play a role in the scattering of hot electrons. Finally, geomagnetic activity influence, studied via time series of the D_st index preceding observations, indicates that (n + 1/2) f_ce emission events are observed at CLUSTER position under moderate geomagnetic activity conditions, no specific D_st time variation being required.     

Hemispheric Distribution of Lower-band Chorus Waves Observed by Van Allen Probes

Whistler mode chorus waves are important electromagnetic emissions due to their dual roles in acceleration and loss processes of Earth’s radiation belt electrons. A detailed global survey of lower-band chorus is performed using EMFISIS data from Van Allen Probes in near-equatorial orbits. In addition to the confirmation of the positive correlation of chorus wave intensities to geomagnetic activity and dayside-nightside distribution asymmetry of wave amplitude and occurrence probability, the analysis results find that in statistics lower-band chorus emissions exhibit higher wave occurrence rates and larger normalized peak wave frequencies in the magnetically northern hemisphere but somehow stronger peak wave intensities in the magnetically southern hemisphere. While overall the differences between the two magnetically hemispheric distributions tend to be not significant, it is important to establish the magnetically hemispheric distribution profiles of lowerband chorus with respect to L-shell, magnetic local time, and geomagnetic latitude for improved understanding of chorus-induced dynamics of radiation belt electrons.      

Model calculations of the dayside ionosphere of Venus

Model calculations of the dayside ionosphere of Venus are presented. The coupled continuity and momentum equations were solved for O₂⁺, O⁺, CO₂⁺, C⁺, N⁺, He⁺, and H⁺ density distributions, which are compared with measurements from the Pioneer Venus ion mass spectrometer. The agreement between the model results and the measurements is good for some species, such as O⁺, and rather poor for others, such as N⁺, indicating that our understanding of the dayside ion composition of Venus is incomplete. The coupled heat conduction equations for ions and electrons were solved and the calculated temperatures compared with Pioneer Venus measurements. It is shown that fluctuations in the magnetic field have a significant effect on the energy balance of the ionosphere.     

Relative contributions of terrestrial and solar wind ions in the plasma sheet

A major uncertainty concerning the origins of plasma sheet ions is due to the fact that terrestrial H⁺ can have similar fluxes and energies as H⁺ from the solar wind. The situation is especially ambiguous during magnetically quiet conditions (AE < 60γ) when H⁺ typically contributes more than 90% of the plasma sheet ion population. In this study we examine that problem using a large data set obtained by the ISEE-1 Plasma Composition Experiment. The data suggest that one component of the H⁺ increases in energy with increasing activity, roughly in proportion to $\text{1}\text{4}$ the energy of the He⁺⁺, whereas the other H⁺ component has about the same energy at all activity levels, as do the O⁺ and the He⁺. If we can assume that the H⁺ of solar wind origin on the average has about the same energy-per-nucleon as the He⁺⁺, which is presumably almost entirely from the solar wind, then the data imply that as much as 20–30% of the H⁺ can be of terrestrial origin even during quiet conditions.     

Mesospheric H2O and H2O2 densities inferred from in situ positive ion composition measurement

The measurements of positive ion composition in the high latitude D-region have revealed an excess of 34⁺ under distrubed conditions which has been interpreted as H₂O₂⁺. At the same altitude range near the transition height oxonium ions were measured as well. This paper presents a new model for the production and loss of oxonium ions with their production from H₂O₂⁺ + H₂O → H₃O⁺ + HO₂ and their loss by attachment of N₂ and/or CO₂. A reaction constant of 8.5×10^?28 (300/T)⁴ cm⁶s^?1 has been obtained for the three body attachment H₃O⁺ + CO₂ + M → H₃O⁺.CO₂ + M from the measured density profile of 63⁺ in flight 18.1020. Mesospheric H₂O and H₂O₂ densities are inferred from measurements of four high latitude ion compositions based on the oxonium model. The mixing ratios of hydrogen peroxide are up to two orders of magnitude higher compared to previous model calculations. In order to explain the missing production of odd hydrogen, we consider larger O(¹D) densities, surface reactions of O(³P) on particles, and cathalytic photodissociation of water vapor on aerosol particles.     

Xe+-induced ion-cyclotron harmonic waves

Porcupine sounding rockets launched from Kiruna in March 1979 carried comprehensive payloads of field, wave and particle experiments. In addition, Xenon ion sources on an ejectable package separated from the main payload during flight. The effects of the Xe⁺ beam as detected by the LF (f<16 kHz) wide-band electric field experiment are discussed. Of particular interest is the stimulation by the Xenon ions of ion-cyclotron harmonic waves covering the whole frequency range up to 16 kHz. These waves are usually linked to the local proton gyro-frequency but occasionally they are related to half that frequency. A possible generation mechanism based on the excitation of waves in the presence of a light minor ion is suggested.     

Positive ion distributions in the morning auroral zone: Local acceleration and drift effects

We report on the typical structure of the large scale ion precipitation in the morning sector of the auroral zone and associated low frequency electromagnetic waves. Data obtained during near radial passes of the AUREOL-3 satellite point to a distinction between two main precipitation regions: 1) In the poleward part of the auroral zone the latitudinal variation of the average energy (or temperature) of the precipitated ions (mainly H⁺) indicate that they are adiabatically accelerated in the outer magnetosphere. This “high energy” (? 3 to > 20 keV) precipitation is usually associated with a low energy (E < 110 eV) upward flowing 0⁺ and H⁺ component, and 2) near the boundary between discrete and diffuse electron aurorae a drastic change in the ion characteristics is observed. The flux of energetic precipitated H⁺ ions is sharply reduced, which suggests the formation of an Alfvén layer. However, intense fluxes of precipitated H⁺, O⁺, and He⁺ ions with energies < 3 keV are observed equatorward of the Alfvén layer, in coincidence with the diffuse aurora and in association with quasi-monochromatic electromagnetic waves with frequencies around the proton gyrofrequency. As the characteristic convection and bounce times of the low energy upward flowing ion component are comparable (τ > 3 hours) we suggest that the precipitation of ionospheric ions inside the diffuse aurora results from convection and corotation of the ions accelerated to suprathermal energies at higher latitudes.     

Generation mechanism of the whistler-mode waves in the plasma sheet prior to magnetic reconnection

The whistler-mode waves and electron temperature anisotropy play a key role prior to and during magnetic reconnection. On August 21, 2002, the Cluster spacecrafts encountered a quasi-collisionless magnetic reconnection event when they crossed the plasma sheet. Prior to the southward turning of magnetospheric magnetic field and high speed ion flow, the whistler-mode waves and positive electron temperature anisotropy are simultaneously observed. Theoretic analysis shows that the electrons with positive temperature anisotropy can excite the whistler-mode waves via cyclotron resonances. Using the data of particles and magnetic field, we estimated the whistler-mode wave growth rate and the ratio of whistler-mode growth rate to wave frequency. They are 0.0016f_ce (Electron cyclotron frequency) and 0.0086f_ce, respectively. Therefore the whistler-mode waves can grow quickly in the current sheet. The combined observations of energetic electron beams and waves show that after the southward turning of magnetic field, energetic electrons in the reconnection process are accelerated by the whistler-mode waves.     

The O2+ vibrational distribution in the Venusian ionosphere

Calculations are presented of the vibrational distribution of O₂⁺ in the Venusian ionosphere for a model atmosphere based on Pioneer Venus data. At 100 km, quenching precludes the survival of vibrationally excited O₂⁺. At the exobase, near 200 km, more than half are vibrationally excited. The effects of vibrationally excited O₂⁺ on the hot oxygen corona and the airglow are discussed.     

Identification of two X-ray miniflares with Hα-subflares

Active regions show many short-lived emissions in the 3.5 – 5.5 keV range that are 100 to 1000 times weaker than “normal” X-ray flares. The hypothesis that they may well be miniflares is supported by the simultaneous occurrence of 2 H_α-subflares at the site of weak X-ray sources.     

Vertical distribution of NH3 in the upper Jovian atmosphere from IUE observations

The first unambiguous identification of ammonia in the upper atmosphere of Jupiter has been obtained from the observation of individual NH₃ bands in an IUE high resolution spectrum in the 2100–2400 Å spectral range. The variation with wavelength of the strengths of these NH₃ bands implies that the NH₃ abundance has to be strongly reduced by photolysis in the upper jovian atmosphere. Preliminary analysis by means of scattering models shows that the ammonia mixing ratio cannot be constant with altitude. The mixing ratio NH₃/H₂ ranges from 5 10^?8 to 5 10^?7 at the 250 mb pressure level, and decreases as P or P² toward higher altitudes.     

Rocket borne electron accelerator results pertaining to the beam plasma discharge

Three flights of rocket borne electron accelerators have yielded some results concerning the Beam Plasma Discharge (BPD). The first flight, E||B, from Churchill carrying an accelerator of 2 and 4 keV electrons, produced a spectrum of whistler mode waves which was identical with that produced in a large vacuum chamber, and which we know to be an indicator of BPD. The second, Echo V, launched from Poker Flat, Alaska, carrying an accelerator of 25–35 keV electrons, produced wave emissions at 3–3.5 MHz observed on the ground. Our interpretation is that BPD was not or was weakly produced. In the third flight, NB³-II launched from Churchill with an accelerator of 2, 4 and 8 keV electrons, wave emissions well above the ambient plasma frequency were observed from a separated payload, but very close to the beam, and are interpreted as demonstrating BPD.     

Quantitative determination of the outgassing water vapor concentrations surrounding space vehicles from ion mass spectrometer measurements

Outgassing from materials as well as deliberate gaseous and liquid releases create contaminant clouds around spacecraft that can degrade both instrumentation and measurements. This paper describes a new method for estimating outgassing water vapor concentrations around space vehicles. Water vapor ions measured in the course of a rocket experiment performed at Eglin AFB, Florida, on December 12, 1980 at 2311 UT are utilized to demonstrate the technique. The H₂O concentration near the payload's surface is calculated using the rate coefficient for the fast charge transfer process, O⁺ + H₂O + H₂O⁺ + O, the source of the observed water vapor ions. It is found that the measured H₂O⁺ ions were produced within 3–4 cm of the sampling plate's surface and that the average H₂O pressure over this distance was relatively constant on ascent at 8 × 10^?6 torr, within a factor two, implying a steady outgassing rate.     

Ion precipitation into the ionosphere during geomagnetic storms

This review presents numerous recent examples of interesting variations in the composition and intensity of the hot ion flux (10 eV - 15 keV/e) provided by the AUREOL-3 satellite as a function of latitude and local time during periods of magnetic activity. In particular, these results reveal that although H⁺ is the most abundant ion during magnetically quiet periods, the ion composition of hot plasma at ionospheric altitudes is quite variable, and depends strongly on magnetic activity; results obtained during main and recovery phases of several magnetic storms demonstrate clearly (below 15 keV/Q) the great importance of the low altitude ionospheric source (H⁺, O⁺, and to a lesser degree He⁺) particularly at low latitudes (L ～ 3 - 4) where the flux of O⁺ ions becomes very large and even dominates. The results of the AUREOL-3 ion spectrometers establish the fact that upflowing suprathermal ionospheric ions (E_i < 100 eV/e) appear over large regions of the auroral ionosphere, the polar caps, and the polar cusp, as well as in or at the boundary of the plasmasphere during magnetospheric substorms or magnetic storms, and may consequently contribute significantly to the plasma sheet and to the inner storm time ring current. Most of the properties of the storm time ring current found by the GEOS, SCATHA, and ISEE satellites apply to lower altitudes, although the role of the ionospheric and/or plasmaspheric source appears accentuated.     

Rotational constants of linear and/or bent Cn+1H and CnN(n = 1–6): A DFT study

The geometries, dipole moments, and rotational constants for the linear and/or bent cations, C_n+1H⁺ and C_nN⁺(n = 1–6), were studied by the B3LYP method with the modest basis sets. For C_nH⁺(n = odd; 3, 5, 7) and C_nN⁺(n = even; 2, 4, 6), the theoretical rotational constants (B_es) of closed-shell singlet C₃H⁺, C₅H⁺, C₇H⁺, CCN⁺, C₄N⁺, and C₆N⁺ were calculated to be about 11,244, 2420, 885.2, 11,970, 2439, and 880.8 MHz, respectively. By contrast, the triplets are stable than the corresponding singlets for C_nH⁺(n = odd; 2, 4, 6) and C_nN⁺(n = even; 3, 5) except CN⁺.     

Charging of dust grains by anisotropic solar wind multi-component plasma

In this paper we study the charging process of small grain particles by anisotropic multi-component solar wind plasmas (electrons, protons and heavy ions), versus two-component (electron/proton) plasmas. We are focusing attention on the important characteristics of the charging process, namely the charging time, floating potential and current content as functions of plasma parameters such as He⁺⁺/H⁺ (α/p) number density and T_α/T_p temperature ratios of alpha particles to protons, as well as plasma streaming velocity v₀. Measured statistical properties of solar wind plasma parameters at 1 AU show considerable variations in α/p-temperature ratios from 1 to 10, in α/p-number density ratio from 0.01 to 0.35, as well as in values of streaming velocity v₀ from 200 km/s to 1000 km/s and more. Periods of these variations could last for several days each, leading to significant variability in the charging process, according to newly derived general analytical expressions. Numerical calculations performed for protons/alphas plasmas showed large disparity in the charging characteristics. For example, in anisotropic plasma, grain charging time varies up to 90% depending on α/p-particles temperature and number density ratios, whereas changes in floating potential are up to 40%. In contrast, in isotropic plasma, charging characteristic for grains do not change very much for the same plasma parameters variations, with charging time varying about 12% and floating potential only varying about 4%. It is also shown that in highly anisotropic plasma, with all ballistic electrons and ions, dust grains could not hold their charges, and characteristic discharged time is calculated. We note that the analysis is equally applicable to any sized body immersed in solar wind plasma.     

Positive ion composition and electron density in a combined auroral and NLC event

The positive ion composition and electron density were measured in the lower ionosphere above Kiruna in salvo A of CAMP (Cold Arctic Mesopause Project). The CAMP/P (S37/P) payload carrying a magnetic ion spectrometer, positive ion and electron probes, and propagation experiments was launched on 3 August 1982 2332 UT during extended Noctilucent Clouds (NLC) and auroral activities over Kiruna. The measured electron density was 5×10³cm^?3 at 80 km and 2.5×10⁵cm^?3 at 90 km. The increase of ion and electron densities in the D- and E-region during twilight was caused by precipitating auroral particles. The height distribution of the positive ions measured by the mass spectrometer in the mass range 19–280 amu is different from a winter flight with similar auroral conditions. Below 85.5 km proton hydrates H⁺(H₂O)₃ ? H⁺(H₂O)₈ were the dominant ions. The heaviest proton hydrates H⁺(H₂O)₇ and H⁺(H₂O)₈ were most abundant at 82–85.5 km, the altitude of visible NLC. Above 85.5 km O₂⁺ and NO⁺ became dominant. A small metal ion layer was observed between 90.5–93 km with a maximum ion density of 10% of the total positive ion density at 91 km altitude. The metal ion density disappeared within about a km below 90.5 km.