Observations of waves artificially stimulated by an electron beam inside a region with auroral precipitation

The POLAR 5 sounding rocket, launched from Andøya, Norway on 1 February, 1976 was of a “mother-daughter” configuration. An electron accelerator, mounted on the “daughter,” produced a pulsed electron beam with currents up to 130 mA and electron energies up to 10 keV. The waves, artificially stimulated by the injected electron beam, was studied using wave receivers, mounted on the “mother.” The receivers covered the frequency range from 0.1 kHz to 5 MHz.

In addition to the stimulated waves observed during beam injection, enhanced wave emissions were observed 10–20 ms after the end of beam injection. This emission seemed to be relatively independent of whether the electron beam is launched up or down along the geomagnetic field.

The high frequency emission observed after beam injection is found to be correlated with the passage through an auroral arc. In particular this emission is closely correlated with the flux of 4–5 keV auroral electrons.

The low frequency emissions observed after beam injection are concentrated in two bands below the lower hybrid frequency.

Different mechanisms for explaining the observed time delays between the beam injection and the observation of the emissions are discussed.     

Electron excitation and auroral emission parameters

Auroral luminosities of the main emission lines in the aurora have been calculated for excitation by an isotopic primary electron flux with spectra of the form J(E) = AE exp (−E/E₁) + B(E₂)E exp (−E/E₁). The variation of emissions from O and N₂⁺ with height are shown, as are the variations of column integrated intensities and pertinent intensity ratios with the characteristic energy E₂, this leading to a method of estimating the electron spectrum from ground observation.     

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.     

A rocket-borne photometric investigation of the oxygen lines at 5577 Å and 6300 Å, the sodium D-lines and the continuum at 5300 Å in the night airglow

The relative variations between 82 km and 205 km in the emission rates of nightglow radiation features at 5300 Å, 5577 Å, 5893 Å and 6300 Å have been photometrically measured from a Skylark rocket flown from Woomera, S. Australia at 2053 hours CST (1123 hours GMT) on 18 October, 1965.

Emission profiles obtained for the first three features show that these layers have their centre of intensity at, respectively, 94.0 ± 1 km, 94.5 ± 0.5, and 98.0 ± 2 km. The results further indicate that not more than 10% of the 6300 Å radiation was emitted below apogee at 205 km.

By virtue of a rather complicated vehicle motion—almost a slow tumble in the vertical plane—evidence is adduced which suggests that differences between these profiles and those of previous flights could be explained by insufficient or incorrect account being then taken of the contamination from extra-atmospheric sources such as starlight and galactic light.

Regarding the continuum, it is found that, depending on the particular region of sky background, up to 80% of the 5300 Å emission observed from the Earth may be extra-atmospheric in origin. Furthermore, of the extra-atmospheric component, again depending on the viewing direction in the sky, the emission intensity at 5577 Å may be from 10% to 50% greater than that at 5300 Å.

While it is to be expected that, before penetrating the layer, the zenithal emission intensity as registered by the photometers should remain constant, this constancy was not generally observed and the 5300 Å and 5577 Å photometers, both of which were independently duplicated, indicate an initial increasing emission intensity. Marked differences in the variation of each pair of photometers suggest that interpretation by means of aerosol absorption of the radiation in the 80 to 100-km region is incorrect and that the effect is probably instrumental in origin and of a temporary nature.     

Spatial and temporal relations between auroral emission and cosmic noise absorption

An investigation has been made of the relation between auroral emission at λ5577 Å and the cosmic noise absorption using a new technique. A photometer and the antenna of a riometer were mounted on a 60 m long rotating antenna boom which had a speed of 1 rev per 3 min. The instruments were directed at an elevation angle of 45°. From the analysis of several break-ups of the aurora it has been found that during a period of 15–20 min in the middle of a break-up there may be an increase of the absorption by a factor 2 to 4 which does not correspond to a similar increase of the auroral emission. These changes in the emission-absorption ratio has been interpreted as peaks in the energy spectrum of the incoming particles. The structures of auroral emission and auroral absorption are sometimes very similar over periods of hours and the appearance of the structure is usually in the form of an east-west oriented arc. The cross correlation coefficient may be as high as 0·9 during these events over long periods of time. However, a number of exceptional cases have appeared where little structure was found in the riometer record while the photometer showed structure and vice versa.     

Transient emissions on the wavelength of helium I, 5876 Å recorded during auroral break-up

During the break-up phase of two strong auroral events, emissions of short duration on the wavelength of He I, 5876 Å have been observed. These records are accurate within 0.5 Å and intensities of up to 120 R have been measured. This high value is not consistent with the theoretical limit suggested by other authors. Simultaneous observations of H, 6563 Å show that the He I and H emissions are not closely related to each other with time, which may be one reason for explaining the discrepancy with the predicted intensity derived from observed ratios of He⁺⁺/H⁺ in the solar wind. The emission on 5876 Å has only been detected at the lowest border of very intense ray bundles towards north but not yet in auroral arcs and diffuse glow. It is suggested that two principally different helium events in aurora may be observable, one resulting in a low level He emission lasting for longer time and another in a stronger He emission of short duration. The observational difficulties caused by the presence of OH bands are discussed.     

High temporal and spatial resolution observations of low energy electrons by a mother-daughter rocket in the vicinity of two quiescent auroral arcs

Low energy precipitated electrons have been measured with high time resolution through an auroral display by a series of high geometrical factor particle counters on a ‘mother-daughter’ sounding rocket, launched during wintertime near 2100 LT from Andenes, Norway.The observations show that the 0·5–3 keV electron fluxes are anisotropically distributed, with a maximum in a direction parallel to the local geomagnetic field vector at all latitudes covered by the rocket, except within the visual auroral forms where the pitch-angle distributions are isotropic or slightly peaked in a direction normal to the geomagnetic field. The 1 and 3 keV electron fluxes are weakly anticorrelated in the vicinity of the arcs, where also the 3 keV electron flux displays a more structured variation than the 0.5 and 1 keV electron fluxes.     

Upper atmospheric temperatures from Doppler line widths—V. Auroral electron energy spectra and fluxes deduced from the 5577 and 6300 Å atomic oxygen emissions

This is a report upon further data obtained from the auroral OI 5577 Å emission with a Wide Angle Michelson Interferometer (WAMI), and upon our first observations made with it on the 6300 Å emission. The method used for converting emission intensities and temperatures to auroral electron fluxes and energy spectra is described. Data for the 5577 Å emission are presented for the (lack of) heating in auroral forms, vertical temperature profiles in aurora, electron flux and energy spectrum variations in pulsating aurora, and a ‘cold’ subvisual auroral arc. Data from the OI 6300 Å emission are presented for the diurnal variation of exospheric temperature and for the thermalization of $\text{O}\text{(}{}^1\text{D)}$ in the F-region.     

Electric field oscillations measured near an auroral arc

A sounding rocket, launched into the expansive phase of an auroral substorm, measured bursts of electric field oscillations with a typical period of one second and a magnitude exceeding 20 mV/m. The oscillations appear to be due to an MHD wave propagating along the magnetic field. The bursts were observed as the sounding rocket passed over the southern border of an auroral arc. The southern border coincided with an increase in 1–5 keV electron flux and an increase in field-aligned current.     

Interferometric phase velocity measurements in the auroral electrojet

A double-probe electric field detector and two spatially separated fixed-bias Langmuir probes were flown on a Taurus-Tomahawk sounding rocket launched from Poker Flat Research Range in March 1982. Interesting wave data have been obtained from about 10s of the downleg portion of the flight during which the rocket passed through the auroral electrojet. Here the electric field receiver and both density fluctuation (δn/n) receivers responded to a broad band of turbulence centered at 105 km altitude and at frequencies generally below 4 kHz. Closer examination of the two (δn/n) turbulent waveforms reveals that they are correlated, and from the phase difference between the two signals, the phase velocity of the waves in the rocket reference frame is inferred. The magnitude and direction of the observed phase velocity are consistent either with waves which travel at the ion sound speed (C_s) or with waves which travel at the electron drift velocity. The observed phase velocity varies by about 50% over a 5 km altitude range—an effect which probably results from shear in the zonal neutral wind, although unfortunately no simultaneous neutral wind measurements exist to confirm this.     

Small-scale auroral arc distortions

Small-scale spatially periodic distortions of auroral forms have been studied utilizing low-light level television observations made at various locations in the Northern and Southern Hemispheres. The most commonly observed features were folds and vortex-like curl formations. The curls, identified here with the Kelvin-Helmholtz instability due to fluid shear, invariably had a counterclockwise rotational shape and motion when viewed in a direction anti-parallel to the Earth's magnetic field. The typical measured wavelength (5 km) and measured growth rate (4.2 sec⁻¹) were used to evaluate the Kelvin-Helmholtz dispersion relation for the apparent shear ω_s = ∂ ν_x/ ∂y (28 sec⁻¹). The apparent horizontal velocities of both folds (0–5 km/sec) and curls (0–22 km/sec) were invariably observed to be counterclockwise with respect to the multiple arc centre when viewed antiparallel to B. Consistent agreement between rotational shape and rotational motion suggests that the apparent growth rate and the apparent horizontal velocities closely approximate the actual values. If the shear results from E×B drifts in a space charge field, the calculated value for ω_s, implies an unneutralized electron density 0–1 cm⁻³ and a ΔE across the arc element 500mV/m. The velocity measurements indicate that the ΔE values for individual elements can combine to produce transient electric fields at the edges of multiple arcs as high as 1000 mV/m.     

Considerations that the source of auroral energetic particles is not a parallel electrostatic field

It is shown that electrostatic fields parallel (E₁₁) to the geomagnetic field cannot be the major mechanism that accelerates charged particles to auroral energies. Principal arguments are that electron and proton precipitation occur simultaneously, and also that precipitated electrons with energies less than 100 eV are found to accompany the electrons with energies of 1–10 keV that excite auroral luminosity. It is further shown that essentially all the ambient plasma in an entire tube of flux is required to sustain this intense low-energy precipitation, and this places a severe constraint on any replenishment process. It is found that generally the upper limit to (E₁₁) throughout the auroral regions of the ionosphere and magnetosphere is of order 10 μV/m and it may be appreciably less. Relevant measurements are reviewed briefly. It is concluded that while there may occasionally be significant E₁₁ fields, they play only a minor role-if any-in auroral phenomena.     

Altitude profiles of the photoelectron induced OD(λ6300Å) predawn enhancement by observation and theory

Emission profiles of the 6300Åline are determined from OGO 4 data in the dark ionosphere during conjugate sunrise. From Saint-Santin electron density profile measurements, it is shown that, for the two cases studied in December 1967, the recombination cannot account for the measured O¹D emission profiles. However, direct photoelectron-oxygen excitation can reproduce the data: if the photoelectron escape flux in the sunlit ionosphere, computed from standard photoelectron production, is transmitted through the field tube with an additional attenuation of 0.6 due to angular diffusion through photoelectron-electron and photoelectron-ion Coulomb collisions, the Hinteregger (1965) solar flux data must be increased by a factor 2, which agrees with previous results.     

Electron-driven excitation of O2 under night-time auroral conditions: Excited state densities and band emissions

Electron impact excitation of vibrational levels in the ground electronic state and seven excited electronic states in O₂ have been simulated for an International Brightness Coefficient-Category 2+ (IBC II+) night-time aurora, in order to predict O₂ excited state number densities and volume emission rates (VERs). These number densities and VERs are determined as a function of altitude (in the range 80-350 km) in the present study. Recent electron impact excitation cross-sections for O₂ were combined with appropriate altitude dependent IBC II+ auroral secondary electron distributions and the vibrational populations of the eight O₂ electronic states were determined under conditions of statistical equilibrium. Pre-dissociation, atmospheric chemistry involving atomic and molecular oxygen, radiative decay and quenching of excited states were included in this study. This model predicts relatively high number densities for the metastable electronic states and could represent a significant source of stored energy in O₂* for subsequent thermospheric chemical reactions. Particular attention is directed towards the emission intensities of the infrared (IR) atmospheric (1.27 μm), Atmospheric (0.76 μm) and the atomic oxygen ¹S→¹D transition (5577 Å) lines and the role of electron-driven processes in their origin. Aircraft, rocket and satellite observations have shown both the IR atmospheric and Atmospheric lines are dramatically enhanced under auroral conditions and, where possible, we compare our results to these measurements. Our calculated 5577 Å intensity is found to be in good agreement with values independently measured for a medium strength IBC II+ aurora.     

An auroral infrasonic substorm investigation using Chatanika radar and other geophysical sensors

Characteristics of the supersonic auroral arcs within the 0905 UT 2 April 1973 substorm were determined using data from (1) all-sky cameras; (2), surface magnetometers, (3) multispectral scanning photometers, (4) 30MHz riometers, (5) Chatanika incoherent-scatter radar, (6) Homer auroral radar, and (7) infrasonic microphone arrays at College and Stevens Village in Alaska. These data were analyzed to determine the properties of an auroral electrojet arc that generates auroral infrasonic waves (AIW).

An arc that was show to be the source of an AIW was found to have the following characteristics: (1) a velocity of 500 m/sec traveling from an azimuth of 350°; (2) an intensity in 4278 A of 26 Kr, (3) a maximum electron density of 2.8 × 10⁶ el/cm⁶ at 100km height, (4) an equivalent westward line current of 2.8 × 10⁶ A, (5) orientation of ΔH parallel to the AIW direction of travel and perpendicular to the arc's long axis, (6) a characteristic energy of the primary auroral electron spectrum of 3.0keV, and (7) an energy deposition rate for the auroral pdarticles of 100 erg/cm² sec.     

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.     

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.     

Boundary layer plasmas as a source for high-latitude, early afternoon, auroral arcs

Simultaneous measurements of hot boundary layer plasma from PROGNOZ-7 and particle precipitation from the TIROS/NOAA satellite in nearly magnetically conjugate regions have been used to study the dynamo process responsible for the formation of high latitude, early afternoon, auroral arcs.

Characteristic for the PROGNOZ-7 observations in the dayside boundary layer at high latitudes is the frequent occurrence of regions with injected magnetosheath plasma embedded in a “halo” of antisunward flowing magnetosphere plasma. The injected magnetosheath plasma have several features which indicate that it also acts as a local source of EMF in the boundary layer. The process resembles that of a local MHD dynamo driven by the excess drift velocity of the injected magnetosheath plasma relative to the background magnetospheric plasma.

The dynamo region is capable of driving field-aligned currents that couple to the ionosphere, where the upward current is associated with the high latitude auroral arcs.

We demonstrate that the large-scale morphology as well as the detailed data intercomparison between PROGNOZ-7 and TIROS-N both agree well with a local injection of magnetosheath plasma into the dayside boundary layer as the main dynamo process powering the high-latitude, early afternoon auroral arcs.     

Convection and field-aligned currents, related to polar cap arcs, during strongly northward IMF (11 January 1983)

Electric and magnetic fields and auroral emissions have been measured by the Intercosmos-Bulgaria-1300 satellite on 10–11 January 1983. The measured distributions of the plasma drift velocity show that viscous convection is diminished in the evening sector under IMF B_y < 0 and in the morning sector if IMF B_y > 0. A number of sun-aligned polar cap arcs were observed at the beginning of the period of strongly northward IMF and after a few hours a θ-aurora appeared. The intensity of ionized oxygen emission [O⁺(²P), 7320 Å] increased significantly reaching up to several kilo-Rayleighs in the polar cap arc. A complicated pattern of convection and field-aligned currents existed in the nightside polar cap which differed from the four-cell model of convection and NBZ field-aligned current system. This pattern was observed during 12 h and could be interpreted as six large scale field-aligned current sheets and three convective vortices inside the polar cap. Sun-aligned polar cap arcs may be located in regions both of sunward and anti-sunward convection. Structures of smaller spatial scale correspond to the boundaries of hot plasma regions related to polar cap arcs. Obviously these structures are due to S-shaped distributions of electric potential. Parallel electric fields in these S-structures provide electron acceleration up to 1 keV at the boundaries of polar cap arcs. The pairs of field-aligned currents correspond to those S-structures: a downward current at the external side of the boundary and an upward current at the internal side of it.     

Resonance scattering by N2

A study is made of the intensity distribution among the bands of the Meinel and first negative system of N₂⁺ due to resonance scattering of sunlight. Absolute transition probabilities are used to calculate the relative populations among the ion states under resonance scattering conditions; the mean lifetime for deactivation is the parameter which determines the amount of resonance scattering. Photon scattering rates are calculated for most of the ion bands and it is suggested that an appropriate value for the 3914 Å band would be 0·050 photons/ sec per ion. Observations of the Δυ = −1 sequence of the first negative system in the twilight spectrum are reported. Extended vibrational development is detected which indicates that only about 80 per cent of the emission is resonance scattered. Sunlit auroral spectra of N₂⁺, however, which have been generally considered to be due predominantly to resonance scattering, indicates only about 40 per cent of the emission is due to resonance scattering. Measurable effects resulting from a charge-transfer ion source (O⁺(²D)) are predicted.