An experimental study of low energy electron backscattering in the atmosphere

The backscatter process for low energy electrons in the upper atmosphere is studied, and it is shown how variations in the pitch-angle and energy distributions of these particles may affect the backscatter ratio and also the electron component of the upward Birkeland current. “Softening” of the electron energy spectrum combined with a field-alignment of the electron fluxes may enhance the current carried by the greater than 200 eV energy electrons significantly.     

Latitudinal gradients in the electron temperature of the lower F-region at mid- to low latitudes

In this paper we present the results of a study of the thermal balance of the lower F-region at mid- to low latitudes. By using a mathematical model with input data based on in situ measurements along AE-C orbits 457, 666 and 677 (26 January, 1974, 14 February, 1974 and 15 February, 1974, respectively) we demonstrate that electron heat conduction along the magnetic field lines has to be included in the model if good agreement between the calculated and observed electron temperatures is to be achieved. This gives support to the suggestion made by Hoegy and Brace (1978), that the discrepancy in the shape of the electron heating and electron cooling rate distributions reported by Brace et al. (1976) resulted mainly from neglecting heat conduction in the electron gas. In addition, our results indicate that the currently used plasma heating and plasma cooling rates and the photoelectron heating rates calculated by Brace et al. (1976) for the orbits used in this study are consistent with the AE-C in situ measurements.     

Effect of low energy electron injection on storm-time evolution of radiation belt energetic electrons: three-dimensional modeling

Using the STEERB (storm-time evolution of electron radiation belt) code, we simulate the evolution of radiation belt energetic electrons during geomagnetic storms in the case of low energy electron injection. The STEERB code is used to solve the three-dimensional Fokker–Planck diffusion equation which incorporates wave-particle interaction, Coulomb collisions and radial diffusion. Numerical simulations show that under the short time (～1 h) injection of low energy (0.1 MeV≤E _k ≤0.2 MeV) fluxes of radiation belt energetic electrons can increase during the entire storm period. During the main and recovery phases, such injection efficiently enhances chorus-driven acceleration of radiation belt energetic electrons, allowing fluxes of energetic electrons by a factor of 1–2 orders higher than those in the absence of injection. The current results indicate that substorm-induced electron injection must be incorporated to investigate the evolution of radiation belt energetic electrons.     

Thermal electron energy distribution measurements in the ionosphere

Results of electron spectrometer and cylindrical Langmuir probe measurements of ionospheric electron energy distribution in the range from about 0·2 eV to 4·0 eV are presented and discussed in this paper.     

Measurements of low energy auroral ions

This paper summarizes ion meaurements in the energy range 0.1–30keV observed during the campaigns “Substorm Phenomena” and “Porcupine”. For a clear survey of the physical processes during extraordinary events, sometimes ion meaurements of higher energies are also taken into account. Generally, the pitch angle distributions were isotropic during all flights except some remarkable events. In general the ion and electron flux intensities correlated, but sometimes revealed a spectral anti-correlation.

Acceleration of the ions by an electrostatic field aligned parallel to the magnetic field could be identified accompanied by intense electron precipitation. On the other hand deceleration of the ions was observed in other field-aligned current sheets which are indicated by the electron and magnetic field measurements. Temporal successive monoenergetic ion variations pointed to energy dispersion and to the location of the source region at 9 R_E. Furthermore, ion fluxes higher than those of the electrons were measured at pitch angles parallel to the magnetic field. Each of the examples was observed during different flights. The integral down-going number and energy flux of the ions contributed to the total particle or energy influx between 65% and less than 7% and did not clearly characterize the geophysical launch conditions or auroral activities.     

A search for the origin of very low electron temperatures

VLET's (Very Low Electron Temperature) are regions in the solar wind (lasting 12–30 h) in which the electron temperature is abnormally low. Because it is generally believed that the thermal conductivity parallel to the interplanetary magnetic field lines is high for electrons, i.e. the contact with the Sun should be ideal, VLET's are surprising observations. In this work a statistical analysis of many of these events is made with respect to the dependence of this phenomenon on interplanetary plasma and field parameters. In contrast with earlier work it was found that VLET's exist not only after a shock front. The statistical analysis showed further that a VLET is always associated with a VLPT (Very Low Proton Temperature) event and that, whilst on average the temperature of the electrons and protons outside the low temperature regions is about the same, the mean proton temperature inside is three times lower than that of the electrons.A particular model for VLET's is investigated in detail; the closed magnetic loop or “blob” model. By assuming: (a) pure adiabatic expansion in a radially streaming solar wind without pressure equalization across the “blob” boundary, and (b) rapid pressure equalization across the boundary, an attempt was made to quantitatively investigate the feasibility of the “blob” model in the light of plasma and field data. It can be shown that the closed magnetic loop model is unlikely to be the major cause of VLET's.     

Cosmic flux of low energy gamma rays

Measurements have been made on the cosmic gamma rays of energy between 0.25 and 4.2 MeV from a balloon experiment made near the geomagnetic equator using a collimated 7.6 cm×7.6 cm NaI(T1) crystal assembly. The depth-intensity curves obtained were used to estimate the contribution due to the diffuse cosmic gamma rays in the above energy interval; an unfolding of the counting rates was then performed to obtain the energy spectrum. It is found that a power law fitted to the present data points has a spectral index of –1.8±0.2. A critical examination is then made of all the observational data between 1 keV and 100 MeV to deduce information on the spectral shape in this energy region. Upper limits on low energy gamma ray fluxes from Sco X-1 and the Galactic centre region are also reported.     

The relationship of auroral absorption to the electron energy distribution of the plasma

An auroral absorption event in the D-region of the atmosphere has been studied by simultaneous measurements of electron temperature, electron density and hyperthermal electrons with a Langmuir probe, and of radio absorption coefficient by 30 MHz riometers. The absorption of the radio waves cannot be explained only by the enhancement of the electron density but requires that the electron collision frequency v be increased above its normal value by the presence of a high energy tail in the electron distribution function. A model is used to determine the characteristics of the hyperthermal electrons in order to evaluate their contribution to the collision frequency and to the absorption coefficient. Good agreement is found between theoretical and experimental values.     

Measurement of electron density in low density plasmas from the electron accelerating region characteristics of cylindrical Langmuir probes

Measurements carried out using a cylindrical Langmuir probe operated in the electron accelerating region of the current-voltage characteristics under orbital limited conditions in low density plasmas, show the response of the probe to be in good agreement with Langmuir theory. By making observations in three different plasmas, namely a steady state plasma, an afterglow plasma and the ionospheric plasma it is confirmed that the form of the orbital limited characteristics of the probe is independent of the energy distribution of the electrons in the plasma. Comparative measurements of ionospheric electron densities made between a rocket borne cylindrical probe and a ground based ionosonde show good agreement to exist and thus demonstrate that the probe operated in this mode not only overcomes the significant problems associated with retarding region probe measurements but affords an accurate determination of electron density. This underlines the usefulness of this kind of probe for electron density measurements in plasmas where the energy distribution of the electrons is unknown.     

Type III solar radioburst profiles and the associated electron energy spectra

An attempt is made to explain the observed frequency-time profiles of type III solar radiobursts in terms of a rapid plasma wave decay rate combined with the exciter model recently proposed by the author. The decay rate is assumed to be sufficiently rapid for the plasma wave energy density profile to be similar to the excitor power density time profile; this is consistent with the exciter model, the rapid decay being caused by Landau damping on the electrons of the modified high energy tail of the ambient plasma electron velocity distribution. The model is compared with radio observations by making simple assumptions about the dependence of the radio intensity upon the plasma wave energy. A comparison is made with simultaneous radio and electron observations by further assuming a simple power-law velocity distribution for the electrons at their point of ejection from the Sun.     

Hard electron energy distribution in the relativistic shocks of gamma-ray burst afterglows

Particle acceleration in relativistic shocks is not a very well understood subject. Owing to that difficulty, radiation spectra from relativistic shocks, such as those in gamma-ray burst (GRB) afterglows, have been often modelled by making assumptions about the underlying electron distribution. One such assumption is a relatively soft distribution of the particle energy, which need not be true always, as is obvious from observations of several GRB afterglows. In this paper, we describe modifications to the afterglow standard model to accommodate energy spectra which are 'hard'. We calculate the overall evolution of the synchrotron and Compton flux arising from such a distribution. We also model two afterglows, GRB010222 and GRB020813, under this assumption and estimate the physical parameters.     

Effect of electron trapping and background nonextensivity on the ion-acoustic soliton energy

Weak ion-acoustic (IA) solitary wave propagation is investigated in the presence of electron trapping and background nonextensivity. A physically meaningful distribution is outlined and a Schamel-like equation is derived. The role a background electron nonextensivity may play on the energy carried by the IA soliton is then examined. It is found that nonextensivity may cause a soliton energy depletion. An increase of the amount of electron trapping leads to a net shift towards higher values of the soliton energy.     

Convective transport of low energy cosmic rays in the interplanetary region

A complete solution has been obtained of the steady-state transport equations, including energy losses, for cosmic-rays in the interplanetary region for conditions in which diffusive transport is negligible and convective effects dominate. The region of validity of the solution will in general be a shell between heliocentric radiiR ₁ andR ₂ (R ₂ may be infinite). The precise range of kinetic energyT and heliocentric radiusr in which the solution is valid is not known but it appears to be applicable in the vicinity of Earth to protons withT≤1 MeV. ForT～0.5 MeV near Earth,R ₁ may be ～0.5 AU andR ₁ will decrease asT, observed near Earth, decreases. The solution is simple in form but quite general; it predicts the differential number densityU (r, T) in terms of that observed at radius a (near Earth, say). Thus it may be quite useful in interpreting and co-ordinating steady-state cosmicray observations atT～1 MeV. The differential and integral intensities, differential anisotropy and differential radial-gradient at (r, T) also are determined. A simple interpretation of the solution is given in terms of energy losses due to adiabatic deceleration of the particles as they are being convected outward from the Sun. This leads to the useful notion of following a particle in (r, T) as it increasesr and decreasesT. Particles convected from the outer corona to Earth decrease their kinetic energy by factor ～500.Following a particle the Compton-Getting factor remains constant. Particles observed at (a, T) in convective transport have come from nearer the Sun; they may be of solar origin but may also be of galactic origin having penetrated tor<R ₁   

Ionization states and the origin of low energy cosmic ray nuclei

The origin of the new component of cosmic ray nuclei in 1–30 MeV amu^–1 recently detected through space vehicles in interplanetary space is investigated in detail. It is assumed that these particles may originate from nearby sources, e.g., from novae type explosions, which have peculiar C, N and O compositions. These particles are further assumed to be accelerated and modulated within the heliosphere. The charged states of these ions in the interstellar space have been calculated in detail and it is shown that the same charged states are preserved in the heliosphere when they are accelerated to energies of the order of 10⁷eV amu^–1 from energies of 10⁵ ev amu^–1. Modulation of these ions are calculated and it is found that because of low charged states of the ions these have high rigidities and are modulated in such a way as to enhance the O-ion abundances as compared to C-ions. A comparison is made of the demodulated composition of C to Si-ions with available abundance data of some novae.     

Analytic approach to auroral electron transport and energy degradation

The interaction of a beam of auroral electrons with the atmosphere is described by the linear transport equation, encompassing discrete energy loss, multiple scattering and secondary electrons. A solution to the transport equation provides the electron intensity as a function of altitude, pitch angle (with respect to the geomagnetic field) and energy. A multi-stream (discrete ordinate) approximation to the transport equation is developed. An analytic solution is obtained in this approximation. The computational scheme obtained by combining the present transport code with the energy degradation method of Swartz (1979) conserves energy identically. The theory provides a framework within which angular distributions can be easily calculated and interpreted. Thus, a detailed study of the angular distributions of “non-absorbed” electrons (i.e., electrons that have lost just a small fraction of their incident energy) reveals a systematic variation with incident angle and energy, and with penetration depth. The present approach also gives simple yet accurate solutions in low order multi-stream approximations. The accuracy of the four-stream approximation is generally within a few per cent, whereas two-stream results for backscattered mean intensities and fluxes are accurate to within 10–15%.     

Evidences of a distorted electron energy distribution in ionospheric plasma

The energy distribution of thermal electrons in the ionospheric plasma was measured by means of a glass-sealed Langmuir probe. Second derivatives of the v-i curves were obtained electrically by using the second harmonic method. The height of the measurement was from 103 to 360 km.Above 130 km the energy distribution of thermal electrons were Maxwellian enough to evaluate electron temperature. Below 130 km the electrons appeared to consist of two groups of electrons of different temperatures. Because of the bi-Maxwellian energy distribution, the apparent electron temperature obtained from the above method differed from that of an electron temperature probe.     

Relation between hard X-ray spectra and electron energy spectra

The relationship between hard X-ray spectra and energetic electron spectra in solar X-ray bursts is investigated, and a simplified cross-section for bremsstrahlung which is applicable to the region of mildly relativistic energies is proposed. Using the proposed cross-section, we solve an integral equation to obtain the electron energy spectrum. The validity of the proposed cross-section is checked by comparing the spectrum calculated by the exact Bethe-Heitler formula. A good agreement between two calculated spectra is obtained up to 10 MeV energy with an accuracy of 20 %.     

Detection of ferric iron in an exsolved lunar pyroxene using electron energy loss spectroscopy (EELS): Implications for space weathering and redox conditions on the Moon

To shed light on the mechanism of formation of nanophase iron particles (npFe) in space-weathered materials from airless bodies, we analyzed exsolved and unexsolved space-weathered lunar pyroxenes from Apollo 17 sample 71501. The exsolved pyroxene allowed for the observation of the effects of space weathering on similar mineral phases with variable composition. Using coordinated scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy (EELS), we determined that two coexisting pyroxenes in the exsolved grain showed systematic variations in response to space weathering, despite equivalent exposure conditions. The npFe in the space-weathered rim of augite lamellae were smaller and fewer than the npFe in the rim of pigeonite lamellae. EELS spectrum imaging revealed the presence and heterogeneous distribution of Fe⁰, Fe²⁺, and Fe³⁺ in the exsolved pyroxene. Metallic iron occurred in the npFe, a mixture of Fe²⁺ and Fe³⁺ occurred in the pigeonite lamellae, and the augite lamellae contained virtually all Fe³⁺. Approximately 50% of the total Fe measured in the exsolved pyroxene grain was ferric. Partitioning of Fe²⁺ and Fe³⁺ among the lamellae is invoked to explain the difference in npFe development in pigeonite and augite. The results of this study, the first to identify Fe³⁺ in a crystalline lunar ferromagnesian silicate, have implications for our understanding of how space weathering might proceed in oxidized phases. Furthermore, the discovery of an Fe³⁺-rich pyroxene also supports attribution of the 0.7 μm absorption feature observed in Galileo Solid State Imager data to oxidized Fe in clinopyroxenes.     

Positrons and low energy cosmic rays from supernovae

Burgeret al. (1970) calculated the positron flux from the decay of⁵⁶Co⁵⁶Fe from cosmic rays injected from supernovae. The plasma properties of the ejected matter are determined in the present calculation in order to include the ionization loss of the positrons as the matter expands. It is found that using the matter velocity distribution of previous supernova model calculations that roughly 10% of the positrons escape. The average lifetime in the galaxy due to ionization loss is found to be relatively small, 1.5×10⁵ yr, and with the above injection results in ×3, the observed flux. The same matter velocity distribution is subjected to ionization loss in the galaxy and a steady state low energy, 10E200 MeV, differential flux spectrum is found,J(E)E ^–1.2. This removes the difficulty of the high galactic energy density resulting from a steeper spectrum.     

The formation of high energy electron fluxes in the inner radiation belt of earth

Measurements on board the low altitude polar orbiting satellite Intercosmos-17 /nearly circular orbit h = 500 km, i = 83.5°/ have shown relatively high fluxes of high energy electrons /E_e > 100 MeV and E_e > 300 MeV respeetively/ at minimum-B-equator. Computation of the electron production spectra assuming the interaction of high energy protons of the inner radiation belt with residual atmosphere is made. The considered mechanism can explain the enhanced flux of high energy electrons registered in the Brazil magnetic anomaly.