Twin payload observations of incident and backscattered auroral electrons

Abstract

Energy spectra and pitch angle distributions of auroral electrons have been measured in a premidnight multiple arc auroral display by a Javelin rocket containing two identical payloads that separated in flight. The rocket (code-named Twins 2) was launched from Fort Churchill, Canada, at 0459 UT on March 2, 1968, and covered an altitude range up to 800 km. The electron energy spectra between 40 ev and 20 key show a 'continuum' spectrum with a superimposed energetic peak. The center energy of the peak was observed to shift from 10-12 key over the arcs to 2-3 key between the arcs. This spectral structure is shown to be similar to the inverted 'V' stYucture reported by other investigators. The in flight separation of the two payloads allowed investigation of spatial versus temporal effects in the auroral precipitation. In one interval of the flight an arc was observed to be moving northward with a velocity of 0.6 km/sec. Calculated backscattered spectra are compared with those actually measured. Good agreement was observed for electron energies above about 1.5 key, but below this energy the backscatter ratio was observed to be ~100%. Several explanations for this unusually high ratio are considered, and a likely possibility is shown to be a parallel downward-directed electric field between •150 and 500 km. Measurementsof auroral particle fluxes, their spectra, and pitch angle distributions. have been the object of numerous satellite and rocket investigations. A recent review of pertinent measurements has been given by Paulikas [1971]. The general features of auroral particle precipitation are now well known, although the details of the origins of the particles and of their interaction with the atmosphere are far from understood. Several investigators have reported 'anomalous' results in measurements of auroral particles. For example, Mozer and Bruston [1966a, b] reported energetic protons traveling upward from mirror points well within the atmosphere and anticorrelations between electron and proton fluxes. A recent study by R•me and Bosqued [1971] reported observations of anticorrelated proton and electron fluxes and proton pitch angle distributions peaked along field lines. These observations were interpreted as indicating the presence of electric fields parallel to magnetic field lines in the upper ionosphere with magnitudes ranging up • Permanent address: to a few hundred millivolts/meter. Furthermore, Mozer and Bruston [1967] reported direct experimental evidence by means of rocket-borne electric field probes of such parallel electric fields in the upper ionos.phere. Choy and Arnoldy [1971] have observed an anomalously high reflection coefficient or backscatter ratio for auroral electrons with energies below i kev. These results are indeed surprising in view of the relatively low altitude of the measurements (( 150 km, R. L. Arnoldy, private communication, 1972). Chappell [1968] has compared calculated and measured auroral electron backscatter ratios using data from the Twins I twin payload sounding rocket [Weste...

Nonlinear production of suprathermal tails in auroral electrons

“…The above predictions of secondary flux in the energy range 0.5 to •2 keV are at least a factor of 30 lower than these observations. Similar spectra have been observed by others [e.g., Reasoner and Chappell, 1973;Arnoldy and Choy, 1973].…”

“…Observation of ionization irregularities, or measurement of enhanced resistivity, will be crucial. At present we can say that our spectra and those observed by Arnoldy and Choy [1973] and Reasoner and Chappell [1973] are consistent with the theory. We have not yet found any counterexamples.…”

electron currents associated with an auroral band

“…In addition the backscatter ratio tends to be larger south of the main portion of the arc. A high backscatter ratio for lowenergy electrons has been noted previously by Reasoner and Chappell [1973], who report ratios of • 1 for electrons below 1.5 keV in a rocket flight over several auroral forms. Arnoldy and Choy [1973] report backscatter ratios for 100-eV electrons with typical values of 2-4 during much of one of their rocket flights.…”

Collisionless Effects on the Spectrum of Secondary Auroral Electrons at Low Altitudes.

“…The differential flux of primary and secondary electrons under auroral activity has been the subject of a series of experimental measurements [Feldman et al, 1972;Arnoldy and Choy, 1973;Reasoner and Chappell, 1973;Feldman and Doering, 1975;Matthews et al, 1976]. A common feature of all these rocket experiments is that between 30 and 85 eV, where the measurements overlap in energy, the electron flux data can be fitted by a power law E -• with 0.5 < a < 1.0.…”