Calculated photoelectron pitch angle and energy spectra

Calculations of the steady-state photoelectron energy and angular distribution in the altitude region between 120 and 1000 km are presented. The distribution is found to be isotropic at all altitudes below 250 km, while above this altitude anisotropies in both pitch angle and energy are found. The isotropy found in the angular distribution below 250 km implies that photoelectron transport below 250 km is insignificant, while the angular anisotropy found above this altitude implies a net photoelectron current in the upward direction. The energy anisotropy above 500 km arises from the selective backscattering of the low energy photoelectron population of the upward flux component by Coulomb collisions with the ambient ions. The total photoelectron flux attains its maximum value between about 40 and 70 km above the altitude at which the photoelectron production rate is maximum. The displacement of the maximum of the equilibrium flux is attributed to an increasing (with altitude) photoelectron lifetime. Photoelectrons at altitudes above that where the flux is maximum are on the average more energetic than those below that altitude. The flux of photoelectrons escaping to the protonosphere at dawn was found to be 2.6 × 10⁸ cm^?2 sec^?1, while the escaping flux at noon was found to be 1.5 × 10⁸ cm^?2 sec^?1. The corresponding escaping energy fluxes are: 4.4 × 10⁹ eV cm^?2 sec^?1 and 2.7 × 10⁹ eV cm^?2 sec^?1.     

Field-aligned suprathermal electron fluxes below 270 km in the auroral zone

Observations are reported of field aligned etectron fluxes in the energy range 50–500 eV at altitudes below 270 km from two rocket flights in the auroral zone. The regions of field aligned suprathermal electrons occurred in bursts of a few seconds duration, and in some instances the energy of the peak field aligned flux was in the range 100–500 eV. Theoretical calculations of the pitch angle distribution were made using the Monte Carlo technique for two model atmospheres having exospheric temperatures of 750 and 1500 K bracketing the expected auroral zone exospheric temperature. The calculations were made for the case of incident field aligned suprathermal fluxes with no local parallel electric field and also for the case of a local constant parallel electric field. Comparison of theoretical and experimental pitch angle distributions showed that in one case at 270 km a parallel electric field of 1–2 mV/m fitted the data whereas another burst at 210 km required a parallel electric field of about 10 mV/m to produce a field aligned distribution of 230 eV electrons as pronounced as was observed. Furthermore in this latter case the lack of strong field alignment at 500 eV pointed to localisation of the parallel electric field to an altitude range of 20–30 km about the rocket altitude.     

Measurements of the ambient photoelectron spectrum from atmosphere explorer: II. AE-E measurements from 300 to 1000 km during solar minimum conditions

The ambient photoelectron spectrum above 300 km has been measured for a sample of 500 AE-E orbits during the period 13 December 1975 to 24 February 1976 corresponding to solar minimum conditions. The 24 h average and maximum ΣK_p were 19 and 35, respectively. The photoelectron flux above 300 km was found to have an intensity and energy spectrum characteristic of the 250–300 km production region only when there was a low plasma density at the satellite altitude. Data taken at local times up to 3 h after sunrise were of this type and the escaping flux was observed to extend to altitudes above 900 km with very little modification, as predicted by several theoretical calculations. The flux at high altitudes was found to be extremely variable throughout the rest of the day, probably as a result of attenuation and energy loss to thermal plasma along the path of the escaping photoelectrons. This attenuation was most pronounced where the photoelectrons passed through regions of high plasma density associated with the equatorial anomaly. At altitudes of 600 km, the photoelectron fluxes ranged from severely attenuated to essentially unaltered—depending on the specific conditions, Photoelectron fluxes from conjugate regions were often less attenuated than those observed arriving from the high density regions immediately below. Comparison of the observed attenuations, photoelectron line broadening, and energy loss due to coulomb scattering from the thermal plasma with rough calculations based on stopping power and transmission coefficients of thermal plasma for fast electrons yielded order of magnitude agreement—satisfactory in view of the large number of assumptions necessary for the calculations. Overall, the impression of the high altitude photoelectron flux which emerges from this work is that the fluxes are extremely variable as a consequence of interactions with the thermal plasma whose density is in turn affected by electrodynamic and neutral wind processes in the underlying F region.     

Measurements of the ambient photoelectron spectrum from atmosphere explorer: I. AE-E measurements below 300 km during solar minimum conditions

The ambient photoelectron spectrum below 300 km has been studied for a sample of 500 AE-E orbits taken during the period 13 December 1975 to 24 February 1976. During this solar minimum period, the average and maximum Σ K_p were 19 and 35 respectively. The agreement between the measured spectral shape and several recent calculations is extremely good. The daytime photoelectron spectrum below 300 km from 1 to 100 eV is illustrated by a number of spectra. Detailed 0–32 eV spectra are presented at various altitudes and solar zenith angles. High resolution 10–32eV spectra show the widths of the photoelectron lines in the spectrum and the variation of the linewidth and intensity with altitude. Data from the entire 500 orbit sample are combined into plots of the average flux over a number of altitude ranges up to 300 km at various local times and solar zenith angles. The data show that the photoelectron flux below 300 km is remarkably constant (typical variation less than ±50%) over a period of several months. The photoelectron lines between 20 and 30 eV are extremely sharp when the total plasma density is low but broaden significantly at high altitudes as the plasma density builds up during the day. The N₂ vibration-rotation excitation dip at 2.3 eV is strongest at the lowest altitudes and decreases with increasing altitude and plasma density. The absolute accuracy of the experiment is discussed in detail and a correction factor for previously published AE-E fluxes is given.     

Rocket observations of the interaction of auroral electrons with the atmosphere

Electron spectra obtained during the flight of Black Brant VB-31 on August 17, 1970 through a stable aurora to a height of 268 km have been analyzed in detail to obtain the pitch angle distributions from 25 to 155° and the electron energy distributions over an energy range of 18 keV to 20 eV through the region of atmospheric interaction down to 97 km. Backscatter ratios for 140° pitch angle range from 0.065 for 18 keV electrons to 0.22 for 1 keV electrons. Backscatter of lower energy electrons decreases with atmospheric depth below 200 km. The effect of the interactions between auroral electrons and the atmosphere is such as to give a peak in electron flux which moves progressively to higher energies with penetration depth. The secondary electron flux increases monotonically with height up to 200 km. The secondary electron spectrum can be approximated by an energy power over small energy ranges but its form is somewhat dependent on height and on the primary electron spectrum.     

Observed heating effects of conjugate photoelectrons

A gridded spherical electrostatic analyzer aboard Injun 5 has been used to measure fluxes of thermal and hyperthermal electrons at subauroral latitudes in the midnight sector of the northern ionosphere between altitudes of 2500 and 850 km. Due to the offset between the geomagnetic and geographic poles hyperthermal fluxes, consisting of energetic photoelectrons that have escaped from the sunlit southern hemisphere are observed along orbits over the Atlantic Ocean and North America but not over Asia. The ambient electron temperatures (T_e) near 2500 km have their highest values at trough latitudes for all longitudes. At altitudes near 1000 km elevated electron temperatures in the trough were not a consistent feature of the data. Equatorward of the trough, in the longitude sector to which conjugate photoelectrons have access, T_e ～ 4000 K at 2500 km and ～ 3000 K at 1000 km. For regions with the conjugate point in darkness T_e ? 2300 K over the 1000–2500 km altitude range. The effective thermal characteristics of conjugate photoelectrons are studied as functions of altitude and latitude. The observations indicate that (1) at trough latitudes elevated electron temperatures in the topside ionosphere are mostly produced by sources other than conjugate photoelectrons, and (2) at subtrough latitudes, in the Alantic Ocean-North American longitude sector, conjugate photoelectrons contribute significantly to the heating of topside electrons. Much of the conjugate photoelectron energy is deposited at altitudes >2500 km then conducted along magnetic field lines into the ionosphere.     

Rocket observations of electron spectral and angular characteristics in an “inverted V” event

Energy spectra and pitch angle distributions of auroral electrons in the energy range 2.5–11 keV observed on a rocket flight launched from Andøya on 13 November 1970 are presented. Strong rapidly fluctuating fluxes during the first part of the flight were succeeded by fluxes below or close to the level of detectability. Before the rocket passed through the northern precipitation boundary two spectral events of “inverted V” character occurred. Both events were associated with field aligned pitch angle distributions. While anisotropies with the flux peaked near 0° were in general associated with the spectral peak energy, isotropy over the upper hemisphere was the dominant distribution for other energies. The observations made during these events provide strong support for the theory of a parallel potential drop close to the ionosphere as an important accelerating mechanism for auroral electrons in connection with “inverted V” events.     

Observations of energetic ions in inverted V events

Simultaneous measurements of keV ions and electrons with the ESRO 1A satellite have shown the following ion characteristics among others. Ions of about 6 keV energy are strongly field-aligned on the flanks of the inverted V events (mainly through the disappearance of the ion flux near 90° pitch angle). Field-aligned electron fluxes are often found in the same regions of the inverted V events where the ions are field-aligned. At the centre of inverted V events isotropization occurs (except in some small events). The 1 keV ion flux at large pitch angles (80°) is generally not reduced very much when the 6 keV, 80° ion flux shows strongly decreased values. The ratio of the 1 to 6 keV ion flux has a maximum near the centre of an inverted V event where the electron spectrum is hardest and the 6 keV ions are isotropic (or nearly isotropic).The observations are interpreted in terms of a model with two oppositely directed field-aligned electrostatic potential drops: one upper accelerating electrons downward and one lower, produced by the electron influx, which accelerates ions downward. Ion scattering in turbulent wave fields is proposed to be responsible for the observation that the 1 keV ion flux at large pitch angles does not decrease strongly where the 6 keV ion flux does and as an explanation of the isotropization at the centre of the event. The source problem for the ions is eliminated by the precipitating electrons ionizing continuously the thin neutral atmosphere even at altitudes of a few thousand kilometers.     

Composition and temperatures of the topside ionosphere over Arecibo

Results of analysis of about 150 autocorrelation functions are presented for the period from about 2300 hr on 5 October to about 1200 hr on 7 October 1967. A large percentage concentration of helium ions are observed. It reaches a value as high as 50 per cent with a maximum at around 800 km. Downward heat fluxes deduced from the temperature variations yield a value of about 2–2.5 × 10⁹ eV cm^?2 sec^?1 during the period 1200–1600 hr and a value of about 1.5 × 10⁸ eV cm^?2 sec^?1 during the period 0100–0400 hr at night. These agree well with other measurements. The O⁺ ions are found not to be in diffusive equilibrium, and from the O⁺ fluxes and the electron density profiles, the O⁺ drift velocity has been estimated. It is found that the speed can be as high as 1–5 × 10³ cm sec^?1 even at altitudes as high as 700 km.     

Daytime ionosphere of Venus as studied with Veneras 9 and 10 dual-frequency radio occultation experiments

We present results of the dual-frequency radio sounding of the Venusian ionosphere carried out by the Venera 9 and 10 satellites in 1975. Thirteen height profiles of electron density for different solar zenith angles varying from 10 to 87° have been obtained by analyzing the refraction bending of radiorays in the sounded ionssphere. The main maximum of electron density at a height of 140–150 km depends on the solar zenith angle and is 1.4 to 5 × 10⁵ cm^?3. The lower maximum is determined definitely to be at ～130 km high. In the main and lower maxima the electron density variations with solar zenith angle are in good agreement with the Chapman layer theory. For the first time it is found that the height of the upper boundary for the daytime ionosphere (h_i) depends regularly on the solar zenith angle. At Z_⊙ < 60°, h_i does not exceed 300 km while at Z_⊙ > 60°, it increases with Z_⊙ and comes up to ～ 600 km at Z_⊙ ～ 80°.     

A study of F2 region daytime vertical ionization fluxes at Millstone Hill during 1969

A study has been undertaken of the vertical fluxes of ionization in the F2 region over Millstone Hill (L = 3.2) utilizing incoherent scatter measurements of electron density, electron and ion temperatures, ion composition and vertical velocity, made over 24-hr periods twice per month during 1969. The paper presents the results for all these parameters on five representative days, and discusses the distribution of the vertical flux observed during the daytime at other times during the year.Near noon the downward flux reached a peak near 300 km with an average value of ～3 × 10⁹ el/cm²/sec in winter and ～1.6 × 10⁹ el/cm²/sec in summer. The difference is thought to be real and be caused by the higher loss rates prevailing in summer. Above 550 km there is usually a transition to upward flux, which appears to be fully established by 700 km and has an average value of the order of 5 × 10⁷ l/cm²/sec. From ion composition measurements, it appears that this flux is carried almost entirely by O⁺ ions to at least ～900 km, as the H⁺ ion concentration is small (<2% at ～775 km altitude) in this region by day. While the value of the escape flux appears in fair agreement with theoretical estimates of the limiting flux for this portion of the sunspot cycle, the extremely low H⁺ concentrations do not appear to be in accord with existing models.The diurnal variation of the upward flux through 650 km exhibits an abrupt onset close to the time of sunrise at the 200 km level (χ = 103°). A reversal to downward flux usually begins before sunset, often in the early afternoon.     

Theory of photoelectron thermalization and transport in the ionosphere

A new theoretical approach for calculating the equilibrium photoelectron flux energy and pitch angle distribution in the ionosphere is presented. Photoelectron transport, secondary electron production, and energy degradation by the excitation of the discrete energy states of the neutral atmospheric constituents and by continuous energy transfer to the ambient thermal electron gas are included in a manner consistent with the Boltzmann equation which constitutes the foundation of the theory. A difference equation, suitable for numerical solution, is given, and a numerical method for the solution of this equation is discussed in detail.     

The atmosphere of Io from Pioneer 10 radio occultation measurements

The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S-band radio occultation measurements of its atmosphere. The dayside entry measurements revealed an ionosphere having a peak density of about 6 × 10⁴ elcm^?3 at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na⁺ and 495 K if N₂⁺ is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9 × 10³ elcm^?3 at an altitude of 50 km. The topside plasma temperature is 160 K for N₂^? and 131 K for Na⁺. If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10¹¹?10¹² cm³, corresponding to a surface pressure of less than 10^?8 to 10^?9 bars. Two measurements of its radius were also obtained yielding a value of 1830 km for the entry and 192 km for the exit. The discrepancy between these values may indicate an ephemeris uncertainty of about 45 km. The two measurements yield an average radius of 1875 km, which is not in agreement with the results of the Beta Scorpii stellar occultation.     

Nighttime proton fluxes at Millstone Hill

Incoherent scatter measurements of electron density and vertical O⁺ fluxes over Millstone Hill (42.6°N, 71.5°W) previously have been used to study the exchange of plasma between the ionosphere and the magnetosphere. During the daytime there is usually an upward flux of O⁺ ions above about 450 km that can be measured readily and equated to the escaping proton flux. At night the O⁺ fluxes usually are downwards everywhere owing to the decay of the F-layer, and it becomes difficult to detect effects due an arriving proton flux. In a new study of the nighttime fluxes, appeal was made to the estimated abundance of the H⁺ ions in the upper F-region which can be extracted from the observations. From a study of the behavior on 25 days over the interval 1969–1973, we conclude that in the daytime the flux always is upwards and close to its limiting value. This situation persists throughout the night in summer at times of high sunspot activity (e.g., 1969). There is a period of downward flux prior to ionospheric sunrise on winter nights whose duration increases with decreasing sunspot number. As sunspot minimum is approached (e.g., in 1973) downward fluxes are encountered for a brief period prior to ionospheric sunrise in summer also. Thus, over most parts of sunspot cycle, it appears that the protonosphere supplies ionization to the winter night ionosphere, while being maintained from the summer hemisphere. This helps explain the smallness of the day-to-night variations reported for the electron content of magnetospheric flux tubes near L = 4 in the American sector.     

Incoherent Scatter Radar Studies of Daytime Plasma Lines

First results from wideband (electron phase energies of 5–51 eV), high-resolution (0.1 eV) spectral measurements of photoelectron–enhanced plasma lines made with the 430 MHz radar at Arecibo Observatory are presented. In the F region, photoelectrons produced by solar EUV line emissions (He II and Mg IX) give rise to plasma line spectral peaks/valleys. These and other structures occur within an enhancement zone extending from electron phase energies of 14–27 eV in both the bottomside and topside ionosphere. However, photoelectron–thermal electron Coulomb energy losses can lead to a broadened spectral structure with no resolved peaks in the topside ionosphere. The plasma line energy spectra obtained in the enhancement zone exhibit a unique relation in that phase energy is dependent on pitch angle; this relation does not exist in any other part of the energy spectrum. Moreover, large fluctuations in the difference frequency between the upshifted and downshifted plasma lines are evident in the 14–27 eV energy interval. At high phase energies near 51 eV the absolute intensities of photoelectron-excited Langmuir waves are much larger than those predicted by existing theory. The new measurements call for a revision/improvement of plasma line theory in several key areas.     

Observations of the gamma-ray flux from the galaxy Mk 501

We present two-year-long observations of the flux of very-high-energy (～10¹² eV) gamma rays from the active galactic nucleus Mk 501 performed with a Cherenkov detector at the Crimean Astrophysical Observatory. A gamma-ray flux from the object was shown to exist at confidence levels of 11 and 7 standard deviations for 1997 and 1998, respectively. The flux varied over a wide range. The mean flux at energies >10¹² eV, as inferred from the 1997 and 1998 data, is (5.0±0.6)×10^?11 and (3.7±0.6)×10^?11 cm^?2 s^?1, respectively. The errors are the sum of statistical observational and modeling errors. The mean power released in the form of gamma rays is ～2×10⁴³ erg s^?1 sr^?1.     

Thermal protons in the morning magnetosphere: Filling and heating near the equatorial plasmapause

Thermal H⁺ distributions have been measured as the European Space Agency GEOS-1 satellite passed through the late morning equatorial magnetosphere, plasmapause and plasmasphere. The unique capabilities of the on-board Supralhermal Plasma Analysers (SPA) have been used to overcome the retarding floating potential of the satellite and measure the velocity distribution of the cold protons. In the magnetosphere an enhanced source cone of such ions with a temperature of ~ 0.5 eV is a signature of the filling process occurring outside the plasmapause where flux tubes are relatively empty. In the plasmasphere the thermal H⁺ is essentially isotropic with a temperature less than 0.5 eV but the motion of the satellite introduces apparent drift.These measurements of cold proton velocity distribution now permit a reappraisal of the definition of the “plasmapause”. It becomes inappropriate to use an arbitrary empirical density, e.g. the conventional 10 cm ^?3, in order to establish a boundary. It is now possible to identify a plasmapause interaction region where the two cold proton populations co-exist. This region generally lies Earthward of the 10 cm ^?3 density level, has a width which is strongly dependent on magnetic activity and the temperature is typically between 0.5 and 1.5 eV. The change from “filled” to “unfilled” flux tubes relates to the physical processes which are occurring and the controlling electric field configuration; in particular, the last closed equipotential. Throughout this region, in going from the plasmasphere to the magnetosphere, the plasma drift motion is expected to change from corotation to a convection which is controlled by E ×B, and is predominantly Sunward due to the dawn-dusk electric field. Crossing the plasmapause on the morning side, little change in drift direction should occur but subtle variations in the ionic velocity distribution do reflect the change in the degree of flux tube density equilibrium.Our first direct measurement of the magnetospheric E × B drift has been reported previously but here measurements from a selected six day period show how the plasma in the plasmapause region responds to changing magnetospheric activity. The drift velocities cannot he derived with high accuracy but the analysis shows that the technique can provide a valid mapping of the magnelospheric electric field. In addition, since the magnetospheric cold plasma distribution is observed after it has come from the ionosphere, a distance of many Earth radii, the scattering and accelerating mechanisms along the flux tube can be studied. For this particular data-set taken in the late morning, the maximum potential drops along the flux tubes were less than a volt. The ionospheric proton source cone is observed to be broad, pitch angle scattering persists up to 40 or even 70°.Although these results throw new light on the plasmaspheric filling process one must recognise that, however the plasmapause is defined, it is not a simple matter to map this boundary from the equatorial plane down to low altitudes and the mid-latitude trough.     

Observations of OI 7774 emission excited by conjugate photoelectrons

Observations and computer calculations of OI 7774 airglow emissions excited by conjugate photoelectrons have been carried out. The observations were made at McDonald Observatory, Texas using a 2m grille spectrometer from December 1972 to June 1973. The zenithal emission intensity during conjugate photoelectron precipitation was fairly constant at 2–4 R until conjugate sunset, after which it diminished steadily and ceased near a conjugate solar zenith angle (χ_c) of 105 ± 3°. A predawn enhancement in both OI 7774 and [OI] 6300 was observed to commence near χ_c ～ 102°.The computations utilize the two-stream technique of Nagy and Banks (1970) to obtain the escaping photoelectron flux and the local excitation rates of the oxygen emissions. Good agreement with the observations is obtained for the dependence of the emission rate on conjugate solar zenith angle. A lack of agreement in absolute intensity may not be due entirely to uncertainties in the excitation cross section. The discrepancy may indicate significant magnetospheric scattering of photoelectrons with energy greater than 15 eV.     

Electron precipitation observations from a rocket flight through the dayside auroral oval

Energy spectra of electrons between 30 eV and 18 keV were obtained with a spectrometer on a Black Brant rocket launched from Cape Parry, N.W.T. (Λ = 75.2°) on December 6, 1974 to study the dayside magnetospheric cleft. The rocket flew to an apogee of 236 km and travelled poleward to 80° invariant latitude. The cleft was observed to extend from 76.9 to 78.4° invariant latitude. Equatorward of this electrons of a few keV energy were observed with a total energy flux of up to 2 erg/cm² sec ster. Variable fluxes of electrons with a spectrum fitted by a Maxwellian distribution of 150 eV characteristic energy were observed through most of the cleft. One inverted V structure was crossed. In that region, the electron energy increased to 650 eV and a total energy flux of 8 erg/cm² sec ster was measured. The event was a temporal one and only a few km in width, as deduced from optical data. Fluxes of about 10⁻² erg/cm² sec ster were recorded poleward of the cleft.     

Transterminator ion flow in the Martian ionosphere

The upper ionospheres of Mars and Venus are permeated by the magnetic fields induced by the solar wind. It is a long-standing question whether these fields can put the dense ionospheric plasma into motion. If so, the transterminator flow of the upper ionosphere could explain a significant part of the ion escape from the planets atmospheres. But it has been technically very challenging to measure the ion flow at energies below 20 eV. The only such measurements have been made by the ORPA instrument of the Pioneer Venus Orbiter reporting speeds of 1-5 km/s for O⁺ ions at Venus above 300 km altitude at the terminator ( [Knudsen et al., 1980] and [Knudsen et al., 1982]). At Venus the transterminator flow is sufficient to sustain a permanent nightside ionosphere, at Mars a nightside ionosphere is observed only sporadically. We here report on new measurements of the transterminator ion flow at Mars by the ASPERA-3 experiment on board Mars Express with support from the MARSIS radar experiment for some orbits with fortunate observation geometry. We observe a transterminator flow of O⁺ and O₂⁺ ions with a super-sonic velocity of around 5 km/s and fluxes of 0.8×10⁹/cm² s. If we assume a symmetric flux around the terminator this corresponds to an ion flow of 3.1±0.5×10²⁵/s half of which is expected to escape from the planet. This escape flux is significantly higher than previously observed on the tailside of Mars. A possible mechanism to generate this flux can be the ionospheric pressure gradient between dayside and nightside or momentum transfer from the solar wind via the induced magnetic field since the flow velocity is in the Alfvénic regime. We discuss the implication of these new observations for ion escape and possible extensions of the analysis to dayside observations which may allow us to infer the flow structure imposed by the induced magnetic field.