Conjugate Photoelectron Research Articles

Conjugate Photoelectron Energy Spectra Derived From Coincident FUV and Radio Measurements

AbstractWe present a method for estimating incident photoelectrons' energy spectra as a function of altitude by combining global scale far‐ultraviolet (FUV) and radio‐occultation (RO) measurements. This characterization provides timely insights important for accurate interpretation of ionospheric parameters inferred from the recently launched Ionospheric Connection Explorer (ICON) observations. Quantification of photoelectron impact is enabled by the fact that conjugate photoelectrons (CPEs) directly affect FUV airglow emissions but not RO measurements. We demonstrate a technique for estimation of photoelectron fluxes and their spectra by combining coincident ICON and COSMIC2 measurements and show that a significant fraction of ICON‐FUV measurements is affected by CPEs during the winter solstice. A comparison of estimated photoelectron fluxes with measured photoelectron spectra is used to gain further insights into the estimation method and reveals consistent values within 10–60 eV.

Conjugate photoelectron recapture peaks in the high resolution Auger electron spectra following near-threshold Ar 2p photoionization

We have observed formation of Rydberg series superimposed on the Ar (L3M2,3M2,3) Auger electron spectral profile with photoexcitation just above the 2p−13/2 photoionization threshold. This is due to the recapture of photoelectrons induced by post-collision interaction between the Auger electron and photoelectron: Ar+ 2p−13/2 + e− → Ar+ 3p−2ml + e−. The intensity distribution of the angular momentum state resolved Rydberg peaks revealed prominent conjugate peaks due to angular momentum exchange between photoelectrons and Auger electrons in the process. In addition, we measured the 2p−13/2nl → 3p−2ml + e− resonant Auger spectra with high resolution below the 2p−13/2 threshold. Our results showed that conjugate shake-up/down 2p−13/2nd → 3p−2mp + e− processes are stronger than the direct 2p−13/2nd → 3p−2ms, (m−1)d + e− processes in the spectator Auger processes, implying a strong angular momentum exchange effect between photoelectron and Auger electron.

Anomalous nighttime electron temperature events over Millstone Hill

This paper presents a statistical analysis of anomalous, short duration, nighttime, electron temperature enhancements (NETEs) in the ionosphere over Millstone Hill (42N, 71W). These temperature enhancements are characterized by a several hundred degree increase in the topside electron temperature with a typical duration of two to four hours. The NETEs can occur anytime at night but most frequently between eighteen and twenty hours local time. Occasionally, two NETEs may be observed in the same night. The maximum temperature reached is often comparable to the daytime temperature. NETEs are most frequent in the winter, but do occur in the spring and fall. There are rare cases of the NETE occurring in the summer. There does not appear to be a clear dependency on either solar or geomagnetic activity. The NETEs association with winter suggests a relationship with conjugate photoelectrons which, if it exists, is not simple. The observed durations are too short to be attributed solely to conjugate photoelectron heating.

Mid-latitude electron content modelling: The role of interhemispheric coupling

A modelling study of the electron content of the mid-latitude ionosphere and protonosphere has been carried out for solstice conditions using the mathematical model of Bailey (1983). In the model calculations coupled time-dependent O +, H + continuity and momentum equations and O +, H + and electron heat balance equations are solved for a magnetic shell extending over both hemispheres. The inclusion of interhemispheric flow of plasma and of heat balance has enabled us to investigate the role of interhemispheric coupling on the electron content and related shape parameters. The computed results are compared with results from slant path observations of the ATS-6 radio beacon made at Lancaster (U.K.) and Boulder, Colorado (U.S.A.). It has been found that the conjugate photoelectron heating has a major effect on the shape of the daily variation of slant slab thickness (τ) and also on the magnitude of the protonospheric content ( N p ). Some of the main features of τ are closely related to the sunrise and sunset times in the conjugate ionosphere. Also it is found that night-time increases in total electron content ( N T ) and F2 region peak electron density ( N max) in winter are natural consequences of ionization loss at low altitudes causing an enhanced downward flow of plasma from the protonosphere which is coupled to the summer hemisphere. One other important consequence of the coupled protonosphere is that the effects on N T of the neutral air wind are not much different in winter from those in summer.

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.

Measurement of magnetic field aligned potential differences using high resolution conjugate photoelectron energy spectra

Simultaneous high resolution observations of a distinctive feature in the energy spectrum of conjugate photoelectrons and spacecraft potential relative to the local ionosphere have allowed the net potential difference between magnetic conjugate points at latitudes below the region of low energy (E 〈 100 eV) auroral electron precipitation to be determined. Measurements made at 300 km, from Atmosphere Explorer C show that there is normally no net potential difference between hemispheres in this region which extended up to invariant latitudes as high as 74°. Two types of apparently related anomalous behavior were infrequently observed at high latitudes. During these periods the incident flux of conjugate photoelectrons was either decelerated by about 3 eV or was not detected.

Conjugate photoelectron fluxes observed on Atmosphere Explorer C

High resolution energy spectra of the flux of precipitating photoelectrons which arise from solar illumination of the area near the magnetic conjugate point have been measured with the Photoelectron Spectrometer Experiment on the Atmosphere Explorer C Spacecraft. Conjugate photoelectrons have been observed when the spacecraft was well away from the terminator at local solar zenith angles up to 160°. The shape of the observed electron energy spectrum is almost identical to that reported for high altitude photoelectron spectra from the Atmosphere Explorer E Spacecraft by Doering et al., 1976. Identification of the residual spectral features in the energy spectra of the conjugate photoelectrons shows that to within the experimental uncertainty of ± 1 eV, the photoelectrons from the magnetic conjugate point have not experienced any acceleration. The magnitude of the observed conjugate photoelectron flux varies considerably between passes, but in some cases the 250 km flux has been observed to be as much as half of the magnitude of the escaping photoelectron flux at 300 km near the sunlit conjugate point. Observations of the precipitating and backscattered flux provide a direct measurement of the low energy (10 eV ‐ 60 eV) electron albedo.

Twilight helium 10,830-Å calculations and observations

The 10,830-A column emission rates from metastable He(2 3S) atoms in the upper atmosphere are calculated and compared with other calculations and observations. It is shown that the calculated emission rates agree well with the observed emission rates provided that the cross sections for loss of He(2 3S) by Penning ionization on atomic oxygen is near 5 A 2 and not 44 A 2 as previously measured. It is assumed that the local and conjugate photoelectron fluxes above 300 km are about double those calculated. The dawn/dusk intensity ratio observed in New Mexico, Brazil and Peru is accounted for by changes in the ambient thermal electron concentration in Brazil and Peru and by changes in plasmasphere opacity to conjugate point electrons for New Mexico and to some extent for Brazil. The rate at which He-4 would escape from the earth if sufficient energy were imparted to the He-4 atoms during the Penning reaction was recalculated and found to be inadequate by an order of magnitude to balance the terrestrial production.

Observation of the conjugate sunrise effect on the OI 6300 Å radiation at high latitude

The paper reports on an experiment to evaluate the high latitude limit of conjugate photoelectron effects on the airglow red line intensity in Western Europe. The predawn enhancement was observed up to L = 5.1 and may extend into higher latitudes.

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.

Reaction of the oxygen emission at 6300 å during the onset of pre-dawn conjugate photoelectron precipitation

From observations of the pre-dawn enhancement of the red oxygen line at 6300 Å at the Observatory in Stara Zagora, Bulgaria and at the Abastumani Observatory in U.S.S.R., it has been established that the enhancement is usually preceded by a small minimum which we call a “valley.” This phenomenon can be used to separate the onset of photoelectron precipitation from the conjugate region from variations in the 6300 Å airglow due to other mechanisms. A theoretical model of the phenomenon is presented which indicates that it is due to a combination of the temperature dependence of the rate coefficient for dissociative recombination and the direct excitation of oxygen atoms by electrons of > 2 eV energy.

Twilight OI 6300 A airglow measurement using a high dispersion monochromator.

The OI 6300 A airglow in the morning twilight was observed with a high dispersion Ebert spectrometer. The 6300 A emission could be detected against the background continuum due to Rayleigh scattering of the solar radiation with good accuracy for the solar zenith angle (SZA) larger than 96°. The O2 photodissociation may be the main excitation process for SZA=102°-98°; a theoretical calculation based on this excitation process can reproduce the observed 6300 A intensity. The emission intensity observed shows a long-term variation in correlation with the solar activity, which may be caused by the variation of the thermospheric O2 density due to the variation of the thermopause temperature. A seasonal variation of the emission intensity, which shows large enhancement in winter, would be ascribed to the additional excitation by the local and conjugate photoelectron.

Conjugate photoelectron excitation of O I 4368 airglow emission

Conjugate photoelectron fluxes are shown to be mainly responsible for the excitation of O I 4368 radiation observed with a grille spectrometer located at Langmuir Laboratory, New Mexico (L equals 1.7). On the basis of observations of O I 7774 and 4368 tropical emissions at Agulhas, Negras, Brazil (L equals 1.1), the contribution of radiative recombination to the New Mexico intensity is shown to be small. Conjugate photoelectron excitation at the Brazilian site has not been observed.

New Dynamic Aspects of Intertropical F2 Ionization

Diurnal changes in F2 ionization distribution as measured during June and July 1966 over the Tamanrasset meridian reveal two types of asymmetric phenomena distorting the crest‐and‐trough system (formerly known as the equatorial F2 ionization anomaly). The first type corresponds to a general time variation of the latitude asymmetry. Our observations yield four independent parameters of this asymmetry; for 80% of the time, the variation can be attributed to ion drag due to a time‐varying meridian air wind. Magnetic activity is correlated with the amplitude of this wind, and the low‐altitude thermospheric circulation is discussed. Day‐to‐day fluctuations of the diurnal evolution pattern suggest a possible long‐term control of the equatorial electric field by the general dynamo current system. A second type of asymmetry shows more intense, local, and short‐lived phases superimposed on the fountain and neutral wind fluctuations. It has the character of a transitory forced‐diffusion process. We believe that the factor responsible is electron temperature gradients produced by conjugate photoelectron heating.

Observations and computations of twilight helium 10,830-Angstrom emission

Observations of helium 10,830-A radiation with a grille spectrometer at Socorro, New Mexico, from 1967 to 1970 are presented. The data are discussed in light of new computations of the excitation and emission profiles. The observational data show an intensity increase of a factor of 3 during this period that is closely related to a corresponding change in the solar EUV radiation (λ<380 A). A large seasonal variation is discussed that is caused in part by a 5∶1 variation in the neutral helium abundance and that is augmented by seasonal variation in the conjugate photoelectron flux. In addition, a diurnal variation of the helium abundance of about 30% with a maximum in the morning is suggested.

Conjugate photoelectron impact ionization

The exchange of photoelectrons between ionospheres in a matter of minutes rather than at the slow ambipolar speed is discussed. It is shown that the electron density may be affected by secondary processes resulting from the conjugate photoelectron flux but not by the flux itself. The flux spectrum of conjugate photoelectrons throughout the day at the solstices for minimum solar activity is calculated for 55 N° geographic latitude over Europe, using a method previously employed by Nisbet. Summer escaping flux values range up to 9 × 10 12 electrons m −2 sec −1 and winter values to 5 × 10 12 electrons m −2 sec −1. Compared at specific solar zenith angles the computed values are in good agreement with recent satellite measurements. Approximately half of this flux is lost by Coulomb collisions along the field line path. The resulting flux arriving at the local ionosphere produces ionization by inelastic collisions in the atmosphere. This additional ionization is about 3 per cent of the ionization from local processes at summer noon and 48 per cent at winter noon. During winter nighttime this conjugate photoelectron ionization can be significant for several hours. Although small in magnitude, this additional ionization should systematically modify the summer total electron content depending on geographic location. The large seasonal differences in the relative impact ionization may explain in part the F-layer seasonal anomaly. This source may be important for maintaining and causing enhancements in the winter nighttime ionosphere.

Most Recent Studies of Low Latitude Effects Due to Conjugate Location Heating

The present experimental evidence for the existence of a streaming of a photoelectron flux between magnetically conjugate hemispheres is reviewed. During local night but sunlit conjugate conditions, significant effects are observed on electron temperature, ionic composition, electron density distribution, the intensity of a weak plasma resonance line, and 6300‐Å airglow emissions, due to the conjugate photoelectron flux. The flux is viewed as important not only in its ionospheric and magnetospheric effects, but also as a tool to probe and study related phenomena.