Photoelectron Flux Research Articles

Morphology of OI 8446Å dayglow emission

A comprehensive model is developed to study the atomic oxygen OI 8446 Å dayglow emission. The emission rate profiles and intensities are obtained using the recently developed Solar2000 EUV (Extreme Ultra Violet) flux model. These emission profiles are used to construct the morphology of the 8446 Å emission between equator and 45°N in the northern hemisphere. The longitudinal variation of 8446 Å dayglow emission is found about 5% and is not included in the presentation of morphology. A span of five years is chosen to study the effect of varying solar activity on the morphology of the OI 8446 Å dayglow emission. The morphology is studied on April 3 which lies under the equinox conditions. In year 2001 the solar F10.7 index on the chosen date was as high as 223.1 which is the case of solar maximum. From the present calculations it is found that the intensity does not vary linearly with the F10.7 solar index. The morphology shows that the region of maximum emission rate expands towards the higher latitudes as F10.7 solar index increases. The similar effects have also been found in the morphology of 7320 Å dayglow emission ( Sunil Krishna and Singh, 2009 ). The similarities in the morphology of 7320 Å and 8446 Å dayglow emissions further suggest that the photoelectron flux has strong bearing on the production of these emissions.

Predawn ionospheric heating observed by Hinotori satellite

Predawn ionospheric temperature has been known to increase with conjugate sunrise. This paper presents the onset time of predawn ionospheric heating and heating rate at around 600 km height globally using Hinotori electron temperature data for magnetically quiet (Kp < 4 and Dst > −50 nT) medium to high solar activity conditions. The analysis of the data shows that the onset of predawn ionospheric heating occurs at nearly the same solar zenith angle (SZA) of the conjugate point at low latitudes where the geomagnetic field line is shorter than about 5000 km. However, at higher latitudes with longer field lines, the conjugate SZA decreases with increasing field line length. In addition, the heating rate decreases with increasing field line length until the field line becomes about 5000 km long, and the rate remains nearly constant for longer field lines. The conjugate SZA increases with increasing solar activity (F10.7) until F10.7 reaches about 200, and the conjugate SZA remains nearly constant for higher F10.7. The observations indicate that the photoelectron flux causing predawn ionospheric heating is attenuated by scattering in the high‐altitude (>600 km) ionosphere and plasmasphere.

Photoelectrons as a tool to evaluate spectral variations in solar EUV irradiance over solar cycle timescales

There is limited information about the relative magnitude of the spectral variations in the ionizing component of solar irradiance on solar cycle timescales. We found that the Thermosphere, Ionosphere, Mesosphere, Energetics, and Dynamics (TIMED)/Solar Extreme Ultraviolet Experiment (SEE) Version 9 irradiance values predict relatively more ionospheric heating at solar minimum than those from Version 8. These changes have direct impacts on solar cycle timescale variations in ionospheric and thermospheric energy inputs derived from them. Photoelectron observations from the Fast Auroral Snapshot (FAST) satellite obtained from 2002 to 2008 are used with solar irradiance data, photoelectron flux models, and models of solar irradiance to examine the solar cycle variations of irradiance in the 4–27 nm range derived from the XPS sensor in the TIMED/SEE instrument suite. Good (±50%) agreement is found between daily photoelectron observations and model predictions. The largest differences between observed and modeled fluxes are in the 4–10 nm range, where the Fast Auroral Snapshot data show that the SEE Version 9 irradiances are systematically low. Our analysis suggests that variation on solar cycle timescales in the TIMED/SEE Version 9 and Flare Irradiance Spectral Model irradiance derived from them are systematically low in the 18–27 nm region. Because of uncertainties in the absolute value of the observed photoelectron fluxes and solar irradiances, differences between observed and modeled photoelectron fluxes are not sufficient to determine more exactly the magnitude of variation on solar cycle timescales of solar irradiance in the 4–27 nm region. These suggestions can be confirmed by higher spectral resolution observations that will be made on the Solar Dynamics Observatory mission.

Angular Correlation between Photoelectrons and Auger Electrons fromK-Shell Ionization of Neon

We have used cold target recoil ion momentum spectroscopy to study the continuum correlation between the photoelectron of core-photoionized neon and the subsequent Auger electron. We observe a strong angular correlation between the two electrons. Classical trajectory Monte Carlo calculations agree quite well with the photoelectron energy distribution that is shifted due to the potential change associated with Auger decay. However, a striking discrepancy results in the distribution of the relative angle between Auger and photoelectron. The classical model predicts a shift in photoelectron flux away from the Auger emission direction, and the data strikingly reveal that the flux is lost rather than diverted, indicating that the two-step interpretation of photoionization followed by Auger emission is insufficient to fully describe the core-photoionization process.

Observation of Suprathermal Flux from SROSS-C2 Data at Low Latitude

The studies of suprathermal flux at equatorial latitude for low solar activity year of 1996 (from January to December) are reported. The retarding potential analyzer (RPA) data of Indian space research organization Stretched Rohini Satellite Series-C2 (SROSS-C2) are used. The suprathermal electron flux data are chosen in the height range of km. Seasional, latitudinal, and height variations of suprathermal flux having energy range above 30 eV along with comparisons in latitudinal variation of small energy superathermal flux of 5 eV are also reported. The data are selected during the period of local midnight to avoid contaminations from photoelectron flux and flux coming from coating of electrodes.

Evaluation of the Asymmetry in Photoelectron Distribution Around the GEOTAIL Spacecraft

We examine photoelectron distributions detected by the low-energy-particle (LEP) instrument onboard the GEOTAIL spacecraft by means of both data analysis and numerical simulations. Statistical data analysis shows asymmetries in the photoelectron distributions. For photoelectrons incident normal to the spacecraft spin axis, a higher flux is observed in the dawnward than in the duskward sector of the LEP. The distribution significantly depends on the ratio of the photoelectron energy to the spacecraft potential. Our numerical simulations reveal that the asymmetry is caused by the electrostatic potential around the thin antenna located at +18deg anticlockwise (viewed from the top) relative to the LEP. Photoelectrons in the dawnward sector are preferentially carried from the sunlit surface by this potential. For upward/downward incident photoelectrons, a higher flux of upward photoelectrons is observed in the antisunward than in the sunward sector, whereas downward photoelectrons show a weak asymmetry. Our numerical simulations demonstrate that the greater flux of upward photoelectrons is caused by the electrons emitted from the sunlit surface; they are attracted to the antisunward sector. Based on these results, the asymmetries in the photoelectron distribution measured around GEOTAIL are found to be caused by the asymmetric positioning of the thin antennas relative to the LEP.

Measured and modeled backscatter of ionospheric photoelectron fluxes

This paper examines the backscatter of ionospheric photoelectrons from the thermosphere using photoelectron measurements made on closed field lines in the magnetosphere from 1997 to 2004. The satellite made simultaneous measurements of both the precipitating photoelectrons from the sunlit conjugate hemisphere and the backscattered electrons from the dark thermosphere. The backscatter ratios of the observed fluxes are in very good agreement with photoelectron model calculations for energies ranging from a few eV to several hundred eV and for different levels of solar activity. The backscattered electron flux consists of a direct elastic backscatter component that is a weak function of energy and a much stronger cascade component that greatly increases with decreasing electron energy so that the backscatter ratio ranges from 0.2 at high energies to greater than one at low energies. Although the photoelectron fluxes can vary by up to an order of magnitude over the solar cycle, the backscatter ratios show only a small variation above 20 eV. There can be a factor of 2 or more variability in the backscatter ratios for energies below 20 eV due to thermal electron Coulomb interactions. The calculations show that 55–60% of the precipitating flux energy is backscattered from the thermosphere back along the field line to the conjugate hemisphere. The model–data comparisons show that photoelectrons are able to travel the long journey from the sunlit hemisphere to the satellite without significant degradation indicating that pitch angle scattering and trapping of photoelectrons in the magnetosphere may be small.

Temporal and spectral variations of the photoelectron flux and solar irradiance during an X class solar flare

Photoelectrons are the main energy source of airglow used to diagnose the state of the ionosphere‐thermosphere system. Because of measurement uncertainties and substantial gaps in the historical record, parameterized models of the EUV irradiance and photoelectron flux are generally used to estimate airglow intensities. This paper compares observed and modeled photoelectron spectra from an X3 class flare that occurred on July 15, 2002. The photoelectron data were obtained from the FAST satellite. Model photoelectron spectra were obtained from the Field Line Inter‐hemispheric Plasma (FLIP) model using 10 s cadence solar spectra at 1 nm resolution from the Flare Irradiance Spectral Model (FISM). The observed and modeled spectra agree well temporally and spectrally within the uncertainties of the models and data. Systematic differences found between observed and modeled photoelectron spectra suggest that the solar irradiance from FISM could be improved at wavelengths shortward of 17 nm.

Photoelectron flux variations observed from the FAST satellite

This paper examines high resolution (Δ E/ E = 0.15) photoelectron energy spectra from 10 eV to 1 keV, created by solar irradiances between 1.2 and 120 nm. The observations were made from the FAST satellite at ∼3000 km, equatorward of the auroral oval for the July–August, 2002 solar rotation. These data are compared with the solar irradiance observed by the Solar EUV Experiment (SEE) on the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite and fluxes calculated using the Field Line Interhemispheric Plasma (FLIP) code. The 41 eV photoelectron flux, which corresponds to solar EUV fluxes near 20 nm, shows a clear solar rotation variation in very good agreement with the EUV flux measurements. This offers the possibility that the 41 eV photoelectron flux could be used as a check on measured solar EUV fluxes near 20 nm. Because of unexpected noise, the solar rotation signal is not evident in the integral photoelectron flux between 156 and 1000 eV corresponding to EUV wavelengths between 0.1 and 7 nm measured by the SEE instrument. Examination of daily averaged photoelectron fluxes at energies between 25 and 500 eV show significant changes in the photoelectron spectra in response X and M class flares. The intensity of photoelectrons produced in this energy region is primarily due to two very narrow EUV wavelength regions at 2.3 and 3 nm driving Auger photoionization in O at 500 eV and N 2 at ∼360 eV. Comparison of calculated and daily averaged electron fluxes shows that the HEUVAC model solar spectrum used in the FLIP code does not reproduce the observed variations in photoelectron intensity. In principle, the 21 discrete photoelectron energy channels could be used to improve the reliability of the solar EUV fluxes at 2.3 and 3 nm inferred from broad band observations. In practice, orbital biases in the way the data were accumulated and/or noise signals arising from natural and anthropogenic longitudinally restricted sources of ionization complicate the application of this technique.

Theory of charged dust clouds: Equilibrium

Gravitational equilibria of low density dust clouds imbedded in plasma background are studied. These are a new class of astrophysical objects, in which the electric field in quasineutral plasma balances the self-gravity of the dust. Heuristic arguments based on the total potential energy of a uniform dust ball are given for the existence of stable equilibrium and the mass limit for the maximum dust mass MAS, which can be supported against its self-gravity. The gravitational collapse is initiated by the destabilization of the fundamental radial mode as the mass of the dust cloud MD→MAS. The physics of the mass limit is similar to the Chandrasekhar’s mass limit for compact objects. One parameter family of spherically symmetric, static solutions of radius R(≈0.1AU) and the total mass of the dust cloud MD(≈1019g), for given central dust density, are constructed. A finite flux of photoelectrons is shown to decrease R and MD. Two-dimensional rotating dust disk equilibrium, characterized by a sharp and diffuse boundary, is also constructed. These dust equilibria (spherical clouds, rotating disks), which are predicted to be within the interstellar clouds, could be relevant for planet formation, as they provide locations where a significant quantity of dust can be stably trapped. The accretion and coagulation interaction between these trapped dust grains may lead to planet formation.

Aspects of Spacecraft Charging in Sunlight

This paper is an overview of spacecraft charging in sunlight. The daylight photoelectron flux emitted from spacecraft surfaces normally exceeds the ambient electron flux. As a result, charging of spacecraft surfaces to positive voltage is expected to occur in sunlight. Indeed, spacecraft are often observed to charge to low positive voltages in sunlight. However, spacecraft can charge to high-level (kiloelectronvolts) negative voltages in sunlight. Why do spacecraft charge negatively in sunlight? One chief reason concerns differential charging between the sunlit and dark sides. For a satellite with dielectric surfaces, an electric field builds up on the shaded surfaces and then wraps around to the sunlit side to form a potential barrier that suppresses the photoemission. A monopole-dipole (for zero spin) or monopole-quadrupole model (for fast spin) describes the differential charging potential distribution due to blocked photoelectrons. It is shown that these cases are similar to a more general multipole potential field in that the surface node potentials satisfy an approximate linear relation. These cases are all driven by the shade side charging so that the onset for charging is approximately the same in sunlight or eclipse if conduction currents through the spacecraft can be neglected. If conduction currents are important, potential barriers can develop on the dark side, leading to suppression of the secondary emission currents and modification of charging onset. The results were briefly compared with observations. Another important reason for negative charging concerns reflectance. Highly reflective mirrors generate substantially reduced photoemission so that current balance can be achieved without barrier formation. The onset for charging in this case depends strongly on the reflectivity. The critical temperature for charging of surface materials under space substorm conditions with different ratios of photoemission current to electron ambient current, corresponding to varying satellite surface reflectivity values, was calculated. Numerical results, which show that with substantially reduced photoemission, highly reflective surfaces charge in sunlight with the critical temperature for onset decreasing with increasing reflectivity, are presented

Numerical interpretation of high-altitude photoelectron observations

The Electron Spectrometer (ELS) instrument of the ASPERA-3 package on the Mars Express satellite has recorded photoelectron energy spectra up to apoapsis ( ∼ 10 , 000 km altitude). The characteristic photoelectron shape of the spectrum is sometimes seen well above the ionosphere in the evening sector across a wide range of near-equatorial latitudes. Two numerical models are used to analyze the characteristics of these high-altitude photoelectrons. The first is a global, multi-species MHD code that produces a 3-D representation of the magnetic field and bulk plasma parameters around Mars. It is used here to examine the possibility of magnetic connectivity between the high-altitude flanks of the martian ionosheath and the subsolar ionosphere. It is shown that some field lines in this region are draped interplanetary magnetic lines while others are open field lines (connected to both the IMF and the crustal magnetic field sources). The second model is a kinetic electron transport model that calculates the electron velocity space distribution along a selected, non-uniform, magnetic field line. It is used here to simulate the high-altitude ELS measurements. It is shown that the photoelectrons are essentially confined to the source cone, as governed by magnetic field inhomogeneity along the field line. Reasonable agreement is shown between the data and the model results, and a method is demonstrated for inferring properties of the local and photoelectron source region magnetic field from the ELS measurements. Specifically, the number of sectors in which photoelectrons are measured is a function of the magnetic field intensity ratio and the field's angle with respect to the detector plane. In addition, the sector of the photoelectron flux peak is a function of the magnetic field azimuthal angle in the detector plane.

Advances in Thick GEM-like gaseous electron multipliers—Part I: atmospheric pressure operation

Thick GEM-like (THGEM) gaseous electron multipliers are made of standard printed-circuit board perforated with sub-millimeter diameter holes, etched at their rims. Effective gas multiplication factors of 10 5 and 10 7 and fast pulses in the few nanosecond rise-time scale were reached in single- and cascaded double-THGEM elements, in atmospheric-pressure standard gas mixtures with single photoelectrons. High single-electron detection efficiency is obtained in photon detectors combining THGEMs and semitransparent UV-sensitive CsI photocathodes or reflective ones deposited on the top THGEM face; the latter benefits of a reduced sensitivity to ionizing background radiation. Stable operation was recorded with photoelectron fluxes exceeding MHz/mm 2. The properties and some potential applications of these simple and robust multipliers are discussed.

HEUVAC: A new high resolution solar EUV proxy model

This paper presents a new high-resolution version of the solar EUV irradiance model for aeronomic calculations (HEUVAC) that is designed to facilitate comparisons with measured spectra and enable more accurate calculations of ionization rates, airglow emission rates, and photoelectron calculations. The HEUVAC model bins can range from 0.1 to 100 nm and extends the EUV model below 5 nm. The new solar EUV irradiance calculations with the high resolution irradiance model show good agreement with the most recent solar EUV irradiance measurements from the solar EUV experiment (SEE) instrument on the thermosphere, ionosphere, mesosphere, energetics, and dynamics satellite. Also, photoelectron fluxes calculated from both the SEE measured and EUVAC modeled solar EUV irradiances agree well with photoelectron flux measurements by the FAST satellite. The good agreement of the EUVAC and SEE derived photoelectron fluxes with the FAST measured fluxes at solar maximum lends support to an earlier finding that the previous reference solar EUV irradiances from the Atmosphere Explorer measurements need to be adjusted upward by a factor of 2–3 below 25 nm wavelength. This result is important for remote sensing of the ionosphere and thermosphere because, as this paper shows, the airglow emission rates calculated using the SEE and HEUVAC models are 50% higher than those based on earlier solar EUV irradiance models. The calculations also show that for solar maximum conditions on 21 April 2002, most of the degradation of the escaping photoelectron flux takes place below 1000 km altitude.

Response of the Martian thermosphere/ionosphere to enhanced fluxes of solar soft X rays

We have investigated the response of the thermosphere and ionosphere of Mars to enhanced fluxes of solar soft X rays, such as those that have been detected by the SNOE satellite (e.g., Bailey et al., 2000). We have constructed standard models by adopting the SC#21REFW and F79050N solar fluxes from H. E. Hinteregger (private communication) (see also Torr et al., 1979) for the low and high solar activity models, respectively. We then constructed enhanced soft X‐ray models by multiplying the solar photon fluxes for wavelengths below 200 Å by a factor of 3 at low solar activity and by a factor of 6 at high solar activity. We discuss the effects of the larger fluxes of soft X rays on the ion and electron density profiles and the minor neutral density profiles. We find that increasing the Hinteregger soft X‐ray fluxes by a uniform factor below 200 Å cannot reproduce the distinct lower peaks that have been observed in some of the electron density profiles measured by the radio science experiment on the Mars Global Surveyor (MGS), and that were modeled previously by Fox et al. (1995) using the SERF2 solar fluxes of Tobiska (1991). In our model computations, the lower peak appears only as a shoulder, although the magnitude of the electron density at the shoulder is comparable to those exhibited by the MGS radio occultation profiles. This is in contrast to models of the terrestrial ionosphere, where increasing the solar fluxes by a uniform factor of 2–4 for wavelengths less than 250 Å has been found to bring the model electron and NO density profiles, and the shape of the photoelectron flux spectra, into agreement with the measured values.

Stochastic Models of Hot Planetary and Satellite Coronas: Suprathermal Nitrogen in Titan's Upper Atmosphere

The processes of dissociation and dissociative ionization of molecular nitrogen by solar UV radiation and by the accompanying flux of photoelectrons, as well as sputtering of the atmosphere by fluxes of magnetospheric ions and pick-up ions, are the main sources of translationally excited (hot, or suprathermal) nitrogen atoms and molecules in Titan's upper atmosphere. Since Titan does not possess an intrinsic magnetic field, ions from Saturn's magnetosphere can penetrate into the outer layers of Titan's atmosphere and sputter atoms and molecules from the atmosphere in momentum-transfer and charge exchange collisions. Atmospheric sputtering by corotating nitrogen ions and carbon-containing pick-up ions, as well as photodissociation-related losses, was considered previously by Lammer and Bauer (1993) and Shematovich et al. (2001, 2003). In this paper we investigate the processes of the formation and evolution of the fraction of suprathermal nitrogen atoms and molecules in the transition region of Titan's upper atmosphere using the previously developed Monte Carlo model for hot satellite and planetary coronas (Shematovich, 1999, 2004). It is established that the suprathermal nitrogen fraction in the transition region of Titan's upper atmosphere includes nitrogen atoms and molecules but the suprathermal nitrogen concentration is relatively small owing to high rates of escape from the atmosphere and to the efficient thermalization of suprathermal nitrogen at the altitudes of the relatively dense lower thermosphere. However, the scale height for suprathermal nitrogen in the transition region is much higher than that for the ambient atmospheric gas. Therefore, suprathermal nitrogen becomes one of the dominant components in the outer exosphere.

Effect of the 14 July 2000 solar flare on Earth's FUV emissions

An X‐class solar flare occurring on 14 July 2000 (Bastille Day) marked the initiation of events which led to a powerful geomagnetic storm on 15 July. At the time of the flare the NASA‐IMAGE spacecraft was near apogee and observed a global‐scale transient increase in far ultraviolet (FUV) emissions of the thermosphere in both the N2 Lyman‐Birge‐Hopfield (LBH) band and the O I 135.6‐nm line and also in geocoronal emissions of H I at 121.6‐nm. Comparison with the solar x‐ray flux measured by GOES 10 shows the delayed response of the FUV emissions compared to the peak in x‐rays. This lag is attributed to a later peak in EUV emissions, which drive the photoelectron flux in the energy range which is, in turn, most effective in producing N2 LBH and O I 135.6‐nm emissions. Measurements of solar EUV input by Solar and Heliospheric Observatory (SOHO) Charge, Element, and Isotope Analysis System (CELIAS)/Solar Extreme‐Ultra‐Violet Monitor (SEM) confirm the close correlation between solar EUV and terrestrial FUV emissions. A comparison of the flare‐time variation in the signal in each channel of the instrument allows one to (1) determine the variation in resonantly scattered solar emissions and (2) differentiate between emissions created through photodissociation by solar EUV and those driven by terrestrial photoelectron impact. The latter provides a means for positively identifying the dayglow emissions observed in the SI‐12 instrument.

Photoelectron flux and nightglow emissions of 5577 and 6300 Å due to solar wind electron precipitation in Martian atmosphere

The detailed model calculations of photoelectron flux and nightglow emissions of 5577 and 6300 Å lines are made using analytical yield spectrum approach based on Monte Carlo method. The calculated photoelectron spectra are compared with electron reflectometer (ER) measurements made by Mars Global Surveyor (MGS) at the energy range 10–1000 eV. This calculation suggests that X‐ray ionization is an important process in the dayside ionosphere of Mars at energy greater than 90 eV, and 10‐ to 90‐eV electron population is controlled by photoionization and photoelectron impact ionization. The secondary electron flux, excitation rates, emission rates, and limb intensities of 5577 and 6300 Å lines are calculated due to precipitation of solar wind electrons observed by ER experiment at the energy range 10–1000 eV in the nighttime ionosphere of Mars. In the vicinity of the peak altitudes, it is found that excitation/emission of atomic oxygen after the electron impact dissociation of CO2 is the main source of 5577 Å emissions, while dissociative recombination of molecular oxygen ion is the major source of 6300 Å emissions after the electron impact excitation of atomic oxygen. The calculated secondary electron spectra are also compared with ER experiment measurements between energies of 10 and 1000 eV. The nightglow limb intensities are compared with the upper limit set by Mars 5 spectroscopic observations.

Solar EUV and Electron-Proton-Hydrogen Atom-Produced Ionosphere on Mars: Comparative Studies of Particle Fluxes and Ion Production Rates Due to Different Processes

The total photoelectron and secondary electron fluxes are calculated at different times and altitudes along the trajectory of Mars Global Surveyor passing through the nightside and dayside martian ionosphere. These results are compared with the electron reflectometer experiment on board Mars Global Surveyor. The calculated electron spectra are in good agreement with this measurement. However, the combined fluxes of proton and hydrogen atom as calculated by E. Kallio and P. Janhunen (2001, J. Geophys. Res. 106, 5617–5634) were found to be 1–2 orders of magnitude smaller than the measured spectra. We have also calculated ionization rates and ion and electron densities due to solar EUV, X-ray, and electron–proton–hydrogen atom impacting with atmospheric gases of Mars at solar zenith angles of 75°, 105°, and 127°. In the vicinity of the dayside ionization peak, it is found that the ion production rate caused by the precipitation of proton–hydrogen atom is larger than the X-ray impact ionization rate while at all altitudes, the photoionization rate is always greater than either of the two. Moreover, X-rays contribute greatly to the photoelectron impact ionization rate as compared to the photoion production rate. The calculated electron densities are compared with radio occultation measurements made by Mars Global Surveyor, Viking 1, and Mars 5 spacecraft at these solar zenith angles. The dayside ionosphere produced by proton–hydrogen atom is smaller by an order of magnitude than that produced by solar EUV radiation. X-rays play a significant role in the dayside ionosphere of Mars at the altitude range 100–120 km. Solar wind electrons and protons provide a substantial source for the nightside ionosphere. These calculations are carried out for a solar minimum period using solar wind electron flux, photon flux, neutral densities, and temperatures under nearly the same areophysical conditions as the measurements.

Variation of the spacecraft potential in the magnetosphere

This paper deals with the surface charging of the Polar satellite, using the spacecraft potential measurements of the electric field instrument (EFI) gathered during 48 months in 1996–1999. For most of the time, the satellite floats at a positive potential V sc in the magnetosphere because of a high flux of photoelectrons emitted from the satellite surface; typically V sc varies between 0 and 50 V so that it is 0– 1 V in the plasmasphere, at the plasmapause it exhibits a steep increase by 3– 5 V , and outside the plasmasphere V sc is usually more than 5 V . On the dayside magnetosphere V sc is 7– 8 V in average, whereas on the nightside it lies in the 10– 20 V range. In the cusp, however, V sc is only about 3 V , similar to spacecraft potentials observed in the magnetosheath. Highest V sc's occur in the high-altitude (>4 R E) polar cap, where V sc is usually between 20 and 30 V , and on auroral field lines where it is frequently in the 30– 50 V range and occasionally, approximately 1.5% of time, above 50 V .