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.     

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.     

Pitch angle and energy distributions of auroral electrons measured by the ESRO 4 satellite

Energy spectra and angular distributions of auroral electrons in the energy range 0.2–16 keV measured by the low-altitude polar orbiting satellite ESRO 4 are presented. The observations were made in the altitude range 800–1000 km near magnetic midnight. Energy-time spectrograms show inverted-V structures with peaked energy spectra. The inverted-V events are associated with anisotropic electron pitch angle distributions peaked at 0 deg. Frequently these distributions have a maximum also at 90 deg. Measurements of >43 keV electrons indicate that the acceleration probably occurs on closed field lines. It is found that many properties of the observed particle distributions can be explained by acceleration in an electric field parallel to the magnetic field lines, if trapping of particles under an increasing potential drop is included in the model.     

Auroral beam/plasma interaction observed directly

On 9 December 1981 rocket borne energetic electron spectrometers measured energy spectra over a stable auroral arc. An associated microprocessor accurately timed the electron detection pulses to calculate auto-correlation functions for each of 16 energy levels between 300 eV and 19 keV.Energy spectra measured up to 230 km altitude contained a secondary peak around 5 keV, corresponding to the auroral beam. Derived velocity distribution functions contain a plateau or table extending round from 0 to 90° pitch angle with a weak positive gradient (+ ve d?(ν)/dν) near zero pitch angle. Autocorrelation functions made at energy levels corresponding to the location of the positive gradient showed the electrons of this region of phase space to be strongly modulated (～ 30%) at a frequency of 2.65 MHz or approximately at twice the electron gyrofrequency.This observation provides the most direct measurement of the auroral beam/ionospheric plasma interaction to date. It provides hard experimental evidence to support the theories which have previously predicted that a major wave-particle interaction responsible for the evolution of the auroral distribution function occurs at heights where the upper hybrid frequency equals twice the local electron gyrofrequency.     

Rocket observation of conjugate photoelectrons in the predawn ionosphere

Photoelectron flux in the energy range 6–70 eV coming from the sunlight conjugate ionosphere has been measured directly by the rocket borne low energy electron spectrometer in the altitude region of 210–350 km. Pitch angle distribution of the measured flux is nearly isotropic, the flux decreasing slightly with pitch angle. The photoelectron fluxes measured at 350 km at the energies of 15 and 30 eV are 3 × 10⁶ and 1 × 10⁶ (cm² s str eV)^?1 respectively which decrease to 1 × 10⁶ and 1 × 10⁵ at 250 km at the same energies. These values are consistent with the vertical profile of the 630 nm airglow intensity measured simultaneously. The fluxes obtained near apogee show peaks in the range 20–30 eV which also appear in the daytime photoelectron flux, indicating reduced loss of electrons during the passage from the conjugate ionosphere through the plasmasphere at the low geomagnetic latitude where observation was made. Photoelectron fluxes observed below the apogee height are compared to the calculated fluxes to investigate the interaction of electrons with the atmospheric species during the passage in the ionosphere. Calculated fluxes obtained by using continuous slowing-down approximation and neglecting pitch angle scattering are in good agreement with the observations although there still remain disagreements in detailed comparison which may be ascribed to the assumptions inherent in the calculation and/or to the uncertainties of the input data for the calculation.     

Study of electron energy and angular distributions and calculations of X-ray,EUV line flux and rise times

Evolution of energy and angular distributions of electrons has been studied by combining small-angle analytical treatment with large-angle Monte Carlo calculations as a function of column density for initially monoenergetic and monodirectional electrons. The incident electron energies considered are 20, 30 and 60 keV at 0°, 30° and 60° angles of incidence. Using these distributions, time evolution of extreme ultraviolet (EUV) spectrum has been studied. The slopes of the curves calculated compare well with the experimentally observed curve.     

Field line topology in the dayside cusp region inferred from low altitude particle observations

Dayside low altitude satellite observations of the pitch angle and energy distribution of electrons and protons in the energy range 1 eV to 100 eV during quite geomagnetic conditions reveal that at times there is a clear latitudinal separation between the precipitating low energy (keV) electrons and protons, with the protons precipitating poleward of the electrons. The high energy (100 keV) proton precipitation overlaps both the low energy (keV) electron and proton precipitation. These observations are consistent with a model where magnetosheath particles stream in along the cusp field lines and are at the same time convected poleward by an electric field.The electrons with energies of a few keV move fast and give the “ionospheric footprint” of the distant cusp. The protons are partly convected poleward of the cusp and into the polar cap. Here the mirroring protons populate the plasma mantle. Equatorward of the cusp the pitch angle distribution of both electrons and protons with energies above a few keV is pancake shaped indicating closed geomagnetic field lines. The 1 keV electrons, penetrate, however, into this region of closed field line structure maintaining an isotropic pitch angle distribution. The intensity is, however, reduced with respect to what it was in the cusp region. It is suggested that these electrons, the lowest energies measured on the satellite, are associated with the entry layer.     

Echo I: An experimental analysis of local effects and conjugate return echoes from an electron beam injected into the magnetosphere by a sounding rocket

On 13 August 1970, a sounding rocket carrying a high voltage electron accelerator and several electron detectors was launched from Wallops Island, Virginia. 16 msec long, 70 mA pulses were injected into the magnetosphere at pitch angles near 90°. In each pulse the electron energy was modulated between 35 and 43 keV. The electrons were trapped in the Earth's magnetic field and bounced between the northern and southern conjugate points with a period of ～0.65 sec and drifted eastward with a gradient-curvature drift velocity of ～765 m/sec. For about 90 seconds the rocket intercepted the returning echoes. Careful study of the rocket trajectory has allowed a partial space-time picture of an echo to be constructed. The bounce time and drift velocity observations are consistent with predictions based on internal magnetic field models with no electric fields. The flux has the spatial variations predicted by atmospheric scattering models at the southern conjugate point but is about a factor of 10 too low. After some injections delayed echoes are observed, apparently 40 keV electrons whose bounce time has been increased by ～75 msec, but with no change in their bounce averaged drift velocity. Study of detector response during gun pulses revealed three unexplained features: (1) a field aligned upward moving flux after downward injections; (2) a downward moving, time dependent, flux after injections at some upward pitch angles; (3) a lack of altitude (or atmospheric density) dependence on observed count rates.     

The penetration of soft electrons into the ionosphere

Calculations are presented of energy spectra and angular and spatial distributions of electron fluxes in the ionosphere resulting from precipitation ofmonoenergetic (E = 25, 50 and 100 eV) electrons. The incident electrons are assumed to be isotropic over the downward direction. It is found that the resulting steady-state electron fluxes above ca. 300 km are highly anisotropic, and that the pitch angle distribution is energy dependent. About 15 per cent of the incident electrons are backscattered elastically to the protonosphere. A much larger number of electrons escape after they have deposited a part of their energy in the atmosphere. The mean energy of the escaping electrons is about half that of the incident electrons. About 50% of the incident energy is absorbed in the atmosphere, the remainder being returned to the protonosphere. The rate of absorption of energy is a maximum at heights between 300 and 400 km. Most of the energy is absorbed in ionization and excitation of atomic oxygen. An appreciable amount of energy is, however, absorbed as heat by the ambient electron gas. Altitude profiles are presented of the rates of ionization, excitation, and electron heating caused by soft electron precipitation.     

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.     

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 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.     

Measurements of the optical emission height profiles of medium intensity aurora

Height profiles of auroral emissions at 3914 Å, 4861 Å, and 5577 Å were obtained in two rocket flights through medium intensity stable aurora. The 3914 Å N₂⁺ integral intensity data were compared with intensity variations predicted by an auroral model for a range of primary electron energy spectra. The observed profiles for the two flights were well reproduced respectively by a 5.6 keV mono-energtic flux and by a flux with an exponential spectrum cutting off around 12 to 15 keV. The data for 5577 Å (available only above 120 km) bear a constant ratio to that for 3914 Å. The emission profiles derived for 3914 Å, peak at 115 and 107 km respectively.     

Hard X-rays from solar flares — time characteristics in correlation with energy and angular distributions of electrons

Fine time variation of hard X-rays has been explained in terms of a spread in the angle of incidence of the source electrons in non-thermal thick-target model for bremsstrahlung generation. The electron energy and angular distributions have been calculated by combining small angle scatterings using analytical treatment with a large angle collision using Monte Carlo calculations as a function of column density. The incidence angles of electrons are taken as 0, 30, and 60°. Using the Bethe-Heitler cross section and the above calculated electron distributions, the bremsstrahlung flux for different photon energies as a function of column density has been studied. The computed X-ray pulse as a function of column density has been converted into time profile. It corresponds well with the observed fine time structure. The calculated spectra of X-rays at the peak and valley are also consistent with the observations. The variation of photon flux with time has also been computed for photon energies 20, 50, and 100 keV for 90 and 180° observation angles together with the changes in spectral shapes of photon energy spectrum at different times for 90 and 180° observation angles.     

Electron measurements (18 keV–20 eV) in auroral events

Electron spectra measured on a rocket flight AMD-VB-34 through and over a series of auroral forms at Fort Churchill, Canada on 23 January 1974 show what can be described as inverted V events. Comparison with all-sky photographs identify clearly three of the events with three periods when the aurora was successively to the south of, underneath and to the north of the field line on which the rocket was located. In each of these events the electron spectrum changed from one resembling a Maxwellian of characteristic energy 3–4 keV on either side of the form to a nearly flat one out to 18 keV while the rocket was over the form. There was no indication of any spectral peaks in these spectra, which were confined to pitch angles of 70–90°. During descent the rocket moved slowly from over a quiet, fading arc to the equatorward side. Detailed electron observations show the spectrum returning to a Maxwellian distribution with steadily decreasing characteristic energy to 2 keV.     

Observation of a flux transfer event on the earthward side of the magnetopause

The medium energy particle spectrometer (electrons of energy > 20 keV, protons > 25 keV) on board ISEE-2 has measured very similar pitch angle distributions and intensities during “flux transfer” events in the magnetosheath and events previously designated as “inclusion” events in the magnetosphere on a single pass through the magnetopause. This is interpreted as strong evidence that magnetic field lines in the magnetosphere can connect to field lines in the magnetosheath, at least locally and for brief times, allowing the same population ofparticles to be observed on both sides of the boundary. In addition, a simple mathematical model is provided incorporating a time constant for the process re-supplying particles to the open flux tube. The observed data are satisfactorily reproduced using a time constant of 46 s, which is comparable to the half-bounce time of protons at this position.     

Electron reflectometry in the martian atmosphere

The technique of electron reflectometry, a method for remote estimation of planetary magnetic fields, is expanded from its original use of mapping crustal magnetic fields at the Moon to achieving the same purpose at Mars, where the presence of a substantial atmosphere complicates matters considerably. The motion of solar wind electrons, incident on the martian atmosphere, is considered in detail, taking account of the following effects: the electrons' helical paths around the magnetic field lines to which they are bound, the magnetic mirror force they experience due to converging field lines in the vicinity of crustal magnetic anomalies, their acceleration/deceleration by electrostatic potentials, their interactions with thermal plasma, their drifts due to magnetic field line curvature and perpendicular electric fields and their scattering off, and loss of energy through a number of different processes to, atmospheric neutrals. A theoretical framework is thus developed for modeling electron pitch angle distributions expected when a spacecraft is on a magnetic field line which is connected to both the martian crust and the interplanetary magnetic field. This framework, along with measured pitch angle distributions from the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer (MAG/ER) experiment, can be used to remotely measure crustal magnetic field magnitudes and atmospheric neutral densities at ∼180 km above the martian datum, as well as estimate average parallel electric fields between 200 and 400 km altitude. Detailed analysis and full results, concerning the crustal magnetic field and upper thermospheric density of Mars, are left to two companion papers.     

A beam/plasma interaction in the high-altitude auroral ionosphere

Ambient thermal electrons are found to be heated to temperatures as high as 10⁵ K by the passage of a field-aligned beam of suprathermal electrons through the ionosphere at altitudes over 660 km. These secondary electrons will increase the proportion of 630 nm emission, caused by the primary electron precipitation, and change the secondary electron spectrum observed at lower altitudes from that expected on the basis of atmospheric collisions alone.     

Study of hard X-ray characteristics of solar flares - support for non-thermal processes

The spatial and angular distributions and also the energy spectrum of hard X-rays from solar flares have been studied in terms of the energy and angular distributions of the accelerated electron beam. The incident electron distributions as functions of column density have been computed by combining the analytical treatment of small-angle scattering with the Monte-Carlo calculations for large angle scattering. To start with monoenergetic electrons at 0°, 30°, and 60° incidence angles have been taken. Using the Bethe-Heitler total cross section and the Sauter differential cross section along with the calculated electron distributions, the bremsstrahlung flux and its angular distribution for different photon energies > 10 keV have been studied as function of column density. The shape of the calculated curves agrees with the observations of PVO/ISEE-3 lending support to the beamed thick-target model for X-ray generation with continuous injection.Physics Department, Vishwa Bharti Institution, Rainawari, Srinagar, India.