Electron Flux Data Research Articles

Periodicities of solar electron flare occurrence: analysis of cycles 21–23

The data of energetic solar electron fluences measured in two ranges of energy values (E > 0.6 and 2 MeV) by the IMP-8 spacecraft of NASA, in the solar cycles 21, 22 and a major part of the current cycle 23, have been analysed for the first time by three different methods to detect periodicities accurately. Power spectral analysis of electron flux data shows a periodicity of ∼152 d for cycle 21, ∼330 and ∼604 d for cycle 22 and ∼152 and ∼176 d for the current cycle 23. The respective periodicities are found to be independent of the electron energy as are the other kinds of solar activity reported by earlier researchers, thus indicating the intimate relationship of electron emission with solar activity.

Sun-to-magnetosphere modeling: CISM forecast model development using linked empirical methods

The Wang–Sheeley–Arge (WSA) method is used to predict the solar wind speed (and certain other parameters) near the Earth's orbit based upon solar surface measurements. This approach gives a predicted solar wind time series with a lead time of three to four days. Such forecasted solar wind conditions can then be convolved with linear and nonlinear filters in order to provide a predicted set of geomagnetic indices or various particle flux estimates. In order to illustrate the method in a concrete way, we present here a demonstration of an end-to-end empirical forecast of relativistic electrons in the outer Van Allen radiation belt. Past work has shown that radiation belt electron fluxes are highly dependent on the speed of the solar wind striking the magnetosphere. We develop filters that predict electron fluxes using the WSA estimates of solar wind speed at L1, which allows for 3–4 days lead times. We compare the prediction efficiency (PE) provided by these filters with filters developed to use 3–4 day old values of the solar wind velocity measured at L1 and 3–4 day old values of the measured electron fluxes themselves. It is found that the WSA method provides PEs of the electron flux that are slightly lower than that provided by using old L1 or the autocorrelated electron flux data.

The Martian Electron Foreshock, Based on Mars Global Surveyor's Measurements

The Martian electron foreshock position and shape have been revealed using electron flux data obtained by the Electron Reflectometer (ER) experiment on board Mars Global Surveyor, in the time period between days 87 and 118 of 1998. From the electron flux measurements, 90 inbound and outbound electron foreshock crossings were detected. Apart from their spatial distribution, their temporal variability is also studied and investigated for possible correlation with the solar wind conditions, the solar radiation flux, and especially the interplanetary magnetic field configuration at the time. The outer boundary of the ion foreshock is identified as the inner limit of the electron foreshock. The energy distribution of the foreshock electrons is also discussed.

Mini-belt as a fine spatial structure of the outer radiation belt in quiet and disturbed conditions

Electron flux data from LANL geostationary spacecrafts were statistically treated and ordered in a special magnetic coordinate system (effective L-coordinate and MLT). The data treating procedure allowed to obtain the dynamics of quasi-trapped electrons of different energies on effective L-shells ranging from 6.6 to 7.0. It was found that in quiet conditions a stable fine spatial structure of quasi-trapped electrons exists with maximum of fluxes near L = 6.78 and MLT=12. This structure may be looked at as an asymmetrical “mini-belt”. The position of the maximum depends on electron energy and changes with magnetic activity. The dynamics of this mini-belt for both quiet and disturbed periods is illustrated and discussed. During isolated magnetic storms the mini-belt maximum shifts in a regular manner outward and inward; a diffusion wave of quasi-trapped particles propagates from outside of the geostationary orbit and serves as a source of new particles for the mini-belt. The azimuthal geometry of this diffusion wave extracted from experimental data is illustrated. The possible role of the “mini-belt” is discussed in relation with well-known “anomalous” dynamics of the inner radiation belt.

Electron and proton excitation of the FUV aurora: Simultaneous IMAGE and NOAA observations

The Far Ultraviolet (FUV) imaging system on board the IMAGE satellite provides a global view of the north auroral region in different spectral channels. The Wideband Imaging Camera (WIC) is sensitive to the N2 LBH emission and NI emissions produced by both electron and proton precipitations. The SI12 camera images the Lyman‐α emission due to incident protons only. We compare WIC and SI12 observations with model predictions based on particle measurements from the TED and the MEPED detectors on board NOAA‐TIROS spacecraft. Models of the interaction of auroral particles with the atmosphere are used together with the in situ proton and electron flux and characteristic energy data to calculate the auroral brightness at the magnetic footprint of the NOAA‐15 and NOAA‐16 orbital tracks. The MEPED experiment measures the precipitating particles with energy higher than 30 keV, so that these comparisons include all auroral energies, in contrast to previous comparisons. A satisfactory agreement in morphology and in magnitude is obtained for most satellite overflights. The observed FUV‐WIC signal is well modeled if the different spatial resolution of the two sensors is considered and the in situ measurements properly smoothed. The calculated count rate includes contributions from LBH emission, the NI 149.3 nm line, and the OI 135.6 nm line excited by electrons and protons. The proton contribution in WIC can locally dominate the electrons. The comparisons indicate that protons can significantly contribute to the FUV aurora at specific times and places and cannot be systematically neglected. The results confirm the shift of the proton auroral oval equatorward of the electron oval in the dusk sector. We also show that in some regions, especially in the dusk sector, high‐energy protons dominate the proton energy flux and account for a large fraction of the Lyman‐α and other FUV emissions.

Electron acceleration observed by the FAST satellite within the IAR during a 3 Hz modulated EISCAT heater experiment

Abstract. A quantitative analysis is presented of the FAST satellite electric field and particle flux data during an EISCAT heating experiment run on 8 October 1998. Radio frequency heating, modulated at 3 Hz, launched ULF waves from the ionosphere into the lower magnetosphere. The ULF waves were observed in FAST data and constituted the first satellite detection of artificially excited Alfvénic ULF waves. The downward electron flux data for this event contain the first observations of electrons undergoing acceleration within the Ionospheric Alfvén Resonator (IAR) due to parallel electric fields associated with an artificially stimulated Alfvén wave. The time history and spectral content of the observed down-ward electron fluxes is investigated by considering the effects of a localised parallel electric field. Furthermore, it is demonstrated that a power law electron energy distribution describes the time-variable observed fluxes better than a Maxwellian distribution.Key words. Ionosphere (active experiments; particle acceleration) – Magnetospheric physics (electric fields)

Neural network prediction of relativistic electrons at geosynchronous orbit during the storm recovery phase: effects of recurring substorms

Abstract. During the recovery phase of geomagnetic storms, the flux of relativistic (>2 MeV) electrons at geosynchronous orbits is enhanced. This enhancement reaches a level that can cause devastating damage to instruments on satellites. To predict these temporal variations, we have developed neural network models that predict the flux for the period 1–12 h ahead. The electron-flux data obtained during storms, from the Space Environment Monitor on board a Geostationary Meteorological Satellite, were used to construct the model. Various combinations of the input parameters AL, SAL, Dst and SDst were tested (where S denotes the summation from the time of the minimum Dst). It was found that the model, including SAL as one of the input parameters, can provide some measure of relativistic electron-flux prediction at geosynchronous orbit during the recovery phase. We suggest from this result that the relativistic electron-flux enhancement during the recovery phase is associated with recurring substorms after Dst minimum and their accumulation effect.Key words. Magnetospheric physics (energetic particles, trapped; magnetospheric configuration and dynamics; storms and substorms)

On the smooth surface of the northern bow shock of Mars, based on Mars Global Surveyor's measurements

A very smooth and time-invariable bow shock of Mars is revealed using Mars Global Surveyor's data. The bow-shock position has been identified using magnetic and electron flux data obtained by the Magnetometer and Electron Reflectometer (MAG/ER) experiment aboard Mars Global Surveyor, in the time period between days 87 and 255 of 1998. From the magnetic field and the electron flux measurements, 148 bow-shock crossings were detected, concentrated mostly on the northern hemisphere of the planet. With these results, a 3D configuration of the bow shock is constructed and presented. The results show that part of the observed bow shock is a surprisingly smooth surface. It is possible that the bow shock is smooth only in the northern hemisphere, since the southern surface is characterized by local magnetic anomalies. Its real shape can only be revealed in a 3D representation in the planetary centered solar ecliptic coordinate system and questions the theoretically expected variation of the bow shock.

A Windy Day on Mars

ASTRONOMY The solar wind is a plasma with interplanetary magnetic field lines. Earth's intrinsic magnetic field largely shields us from the effects of the solar wind. Mars, however, has no permanent magnetic field; when the solar wind meets Mars, it creates a bow shock, and magnetic field lines compress between the bow shock and ionopause of Mars. This thin layer of plasma is called the magnetic pile-up boundary (MPB). The Mars Global Surveyor (MGS) spacecraft passes through the MPB daily in its polar orbit. Crider et al. analyzed the electron flux data from one orbit as measured by the electron reflectometer aboard the MGS. They noticed a decrease in the electron flux in the MPB at energies between 40 to 150 electron volts, suggestive of electron impact ionization of neutral H, O, and CO2 above the ionosphere. These data were used to construct a qualitative model incorporating the density of the neutral species, the velocity of the solar wind flow around the planet, and the magnetic field geometry of the interaction. Through electron impact ionization, the incoming solar wind ionizes the neutral species and sweeps them away, thus eroding the martian atmosphere.— LR Geophys. Res. Lett. 27 , 45 (2000).

Observation and analysis of NI 520.0 nm auroral emissions

The results from the analysis of simultaneous auroral ground-based optical measurements of the N( 2D) 520.0 nm, N 2 + 1NG 470.9 nm, O( 3P) 844.6 nm and the O( 1D) 630.0 nm emission intensities are presented. The data were obtained during auroral observations at Gillam (56.35°N, 265.32°E) over an observation period of about 8 hours, from UT 2:33 hrs to UT 10:06 hrs, on 20 March 1985. The soft electron flux measurements on board the DMSP satellite for the time of the experiment have also been considered in the analysis. The N( 2D) density and the N( 2D) 520.0 nm integral emission rate I(520.0) were calculated employing a one- and two-dimensional time-dependent ion-chemistry model and the model predictions have been compared with the experimental I(520.0) nm emission rates. It was found that the model predictions of the NI I(520.0) nm intensity based on the electron energy fluxes inferred from the experimental I(844.6)/I(427.8) emission rate ratios are smaller in magnitude than the experimental values by a factor of 5–8 after allowing for horizontal transport of [N( 2D)] by neutral winds. Assuming soft electron precipitation, suggested by the OI I(630.0) nm emission measurements and the DMSP satellite electron flux data, provided good agreement between the model and experimental results. Based on the results obtained it was concluded that horizontal transport played a minor role and that the observed N( 2D) I(520.0) nm emissions were mostly produced by precipitating soft electron fluxes with energies below about 100 eV.

A study of radial diffusion of energetic electrons in the Earth's radiation belts: Data satellite “Cosmos-1686”

The distribution function of energetic electrons in the inner and outer radiation belts of the Earth have been calculated from electron spectra and flux data for energetic electrons (with energy 0.03–2 Mev) for a number of satellites (Intercosmos-19, Cosmos-1686, ets). The distribution functions show evidence of changes thought to be due to diffusion processes. The present study determins coefficients of radial diffusion on different L-shells by several methods. Coefficients of radial and pitch-angle diffusion of electrons have been determined by independent methods using the particles and electron diffusion wave measurement data. The present study examines the rols of electric and magnetic fields fluctuations for redistributing relativistic and non relativistic electrons on different L-shells. Coefficients of radial diffusion in the outer radiation belt are calculated using the diffusion wave observed for energetic electrons. The new results are compared with values obtained earlier for coefficients of radial diffusion from data of many experiments on satellites and calculations and modelling for the years 1962–1993.

On Ba+ production in the CRIT II Experiment

Analysis of particle data from the CRIT II experiment, studying Alfvén's critical ionization velocity (CIV) effect, shows that the density of newly created ions (presumably Ba+ from the shaped‐charge beam) is consistent with the increase in total plasma density measured by the independent RF plasma probe on board (Swenson et al., 1990) at the most active time period. We model this ion production using the measured electron flux data and the neutral barium model of Stenbaek‐Nielsen et al. (1990a). To identify the main source mechanisms which may contribute most to the barium ionization, a simple model for barium ion density at the payload location is developed based on Liouville's theorem. We estimate that the electron impact ionization is responsible for 90% of the barium ion production observed by CRIT II in the first release and up to 45% in the second release. By employing a “two‐state approximation” calculation (Rapp and Francis, 1962), the Ba‐O+ charge exchange cross section is found to range from about 2.0 × 10−17 cm2 at a velocity of 4 km/s to 2.0 × 10−15 cm2 at a velocity of 20 km/s. This result suggests that the Ba‐O+ charge exchange is probably dominant among all the non‐CIV ionization processes. By considering the charge exchange process in our density model, the barium ion densities are calculated for the two releases on CRIT II. The comparison between the model results and the observed data is found to be reasonably consistent if the cross sections, as calculated above, are multiplied by 0.3 for the first release and 1.0 for the second release. Our result suggests that the charge exchange process could be the most important non‐CIV ionization mechanism in the CRIT II experiment and it should be considered carefully case by case in CIV experiments.

Relativistic electron flux anisotropies in the duskside Jovian magnetosphere: a test for source location and escape mechanism

Results of anisotropy analyses of >∼ 3 MeV electron flux data obtained by the high-energy telescope of the COSPIN experiment on Ulysses are presented for the period from day 40 to day 45 of 1992, when the spacecraft was outbound from Jupiter in the duskside high-latitude magnetosphere and magnetosheath. At small L-shells or minimum magnetic latitudes near the planet, the electron pitch-angle distributions are peaked at α = 90° and have net inward flows to the South Pole of the magnetic dipole. A majority of the high-latitude magnetosphere regions are dominated by electrons with field-aligned bidirectional distributions. The electron flows are essentially directed outward from the southern polar region, but inward electron flows which come from the northern polar region have also been detected, particularly near the magnetospheric boundary. The pattern of electron angular distributions in the magnetosphere is compatible with Nishida's model for recirculation of magnetospheric energetic particles by diffusion at low altitudes. Mechanisms for electron acceleration in the magnetosphere are discussed on the basis of our new observations. In the magnetosheath, alternating field-aligned electron flows are present. The observations suggest that electrons escape from the magnetosphere to the interplanetary space through diffusion across closed magnetic field lines on the boundary. It is found that at least a significant fraction of the interplanetary Jovian electrons (>∼ 10 26s −1) may escape the magnetosphere in this way.

Electrodynamics of very high latitude arcs in the morning sector auroral zone

In November 1985 an experimental campaign was conducted at Sondre Stromfjord, Greenland, to study the electrodynamics of auroral arcs at very high latitudes. The experiments involved coordinated observations by the Sondrestrom radar and two polar orbiting spacecraft. For several hours around local dawn on November 17 a system of auroral arcs was observed over Sondrestrom. These arcs varied in intensity and width but were all extended generally in the magnetic east‐west direction. A Defense Meteorological Satellite Program photograph obtained during the observations showed that two types of arcs were present. In the south the arcs were localized enhancements or striations within a broad diffuse aurora. Poleward of the diffuse aurora was a system of arc segments that extended over about 5° of latitude. The radar measured enhanced ionospheric electron densities associated with each of these arcs, although the altitudes of the enhancements were different for the two types. In the diffuse aurora the ionization peaked at altitudes near 115 km, whereas in the arc segments the ionization peaked at altitudes above 180 km. Examination of electric field data from the radar indicated that the boundary in ionization associated with the two types of aurora occurred where the southward electric field maximized. The HILAT satellite passed in the vicinity of Sondrestrom during the experiment and measured precipitating electron fluxes and field‐aligned currents. The electron flux data showed that the low‐altitude ionization observed by the radar was produced by electrons with energies near 5 keV. The higher‐altitude ionization was produced by precipitation associated with enhancements in the fluxes of low‐energy electrons. The magnetometer data indicated that the boundary between the region 1 (downward) and region 2 (upward) current sheets in the morning sector is just poleward of the boundary separating the hard and soft precipitation regions. The latitudinal variation in field‐aligned current is consistent with the measured variations in electric field and conductivity.

Characteristics and sources of ionization in the continuous aurora

The continuous aurora is defined as E region ionization that is steady in time and uniformly distributed over a large portion of the auroral oval. We have examined the characteristics and sources of ionization in the continuous aurora using data from the Chatanika incoherent scatter radar and several polar orbiting satellites. The altitude profile of ionization in the continuous aurora usually has an E region maximum of 0.5 to 5×105 cm−3 at altitudes between 110 km and 140 km. The profile is very similar to that expected from precipitating electrons with a Maxwellian energy distribution of 0.5–4 keV characteristic energy and an energy flux of 0.2 to 16 erg/cm2 s. We have used simultaneous electron flux data from the DMSP satellites to assess the extent to which precipitating electrons are responsible for the continuous aurora. We show two cases in which the simultaneous incoherent scatter radar data revealed substantial ionization equatorward of the region of electron precipitation. We attribute this ionization to precipitating protons. The presence of significant ionization produced by proton precipitation is confirmed by a third data set in which measurements of precipitating proton fluxes made by the NOAA 6 satellite over the diffuse aurora near Chatanika indicated a proton energy flux of about 0.5 ergs/cm2 s. The energy flux carried by precipitating electrons was more than an order of magnitude less. Ionization produced by proton precipitation can have an altitude distribution very similar to electron aurora. In addition, the proton aurora has the same latitudinal profile as the electron aurora, so that distinction between the two cannot be made on the basis of electron density measurements alone.

A new approach to pitch angle scattering in the magnetosphere

We develop a theory to describe the spatial variation of the particle pitch angle distribution function along a geomagnetic field line. Because spatial variation is most significant inside the loss cone, our theory is tailored to that part of velocity space. A boundary layer equation for the loss cone is derived and then solved. The description of the pitch angle distribution is completed by asymptotically matching the boundary‐layer solution to the distribution outside the loss cone, which can be obtained by using the bounce‐averaged diffusion equation. Calculated results for the particle distribution function in representative cases are presented. We apply our theory to ISIS 2 electron flux data for the continuous aurora to deduce the magnitude and the energy dependence of the pitch angle diffusion coefficient. We find that the diffusion coefficient is peaked at the invariant latitude at which the total energy flux into the atmosphere is largest, and that its energy dependence can be fitted with a power law E−n, with ½ ≲ n ≲ 1.

Winter nighttime ion temperatures and energetic electrons from OGO 6 plasma measurements

This paper presents and discusses ion temperature and suprathermal electron flux data acquired with the retarding potential analyzer on board the OGO 6 satellite when it was in solar eclipse. Attention is directed to measurements in the 400‐ to 800‐km height interval between midnight and predawn in the northern winter nonpolar ionosphere. Statistical analysis of data recorded during a 1‐month time span permits a decoupling of horizontal and altitude effects. A distinct longitudinal variation is observed for ion temperature above 500 km, with a significant relative enhancement over the western North Atlantic. Altitude distributions of ion temperature are compatible with Millstone Hill profiles within the common region of this enhancement. Large fluxes of energetic electrons are observed and extend to much lower geomagnetic latitudes in the same longitude sector. Both a direct correlation in magnitude and a strong similarity in spatial extent are demonstrated for these ion temperature and electron flux data. The location of the limiting low‐latitude boundary for observation of the electron fluxes is variable, dependent on local time and season as well as longitude. Variations in this boundary are found to be consistent with a calculated conjugate solar zenith angle of 99°±2° describing photoproduction of energetic electrons in the southern hemisphere. The OGO 6 data are considered to be indicative of an energy source originating in the sunlit summer hemisphere and providing heat via transport of photoelectrons to a broad but preferential segment of the winter nighttime mid‐latitude ionosphere. Ions at other longitudes are without access to this energy source and cool to near the neutral temperature at heights to above 800 km in the predawn hours.

Direct measurement of conjugate photoelectrons and predawn 630 nm airglow

A sounding rocket was flown during the predawn on 17 January, 1976 from Uchinoura, Japan, to measure directly the behaviour of the conjugate photoelectrons at magnetically low latitudes. On board the rocket were an electron energy analyzer, 630 nm airglow photometer, and plasma probes to measure electron density and temperature. The incoming flux of the photoelectrons was measured in the altitude range between 210 and 340 km. The differential flux at the top of the atmosphere was determined to be F = (1.3 ± 0.4) × 10 11 exp[− E( eV) 12 ] electron · m −2 · sr −1 · s −1 in the energy range 10 ⩽ E ⩽ 50 eV. The emission rate of the 630 nm airglow was observed in the altitude range between 90 and 360 km. The apparent emission rate observed at 80 km was 32 ± 5 R. From a theoretical calculation of the optical excitation rate using the observed electron flux data along with a model distribution of atomic oxygen, it was estimated that more than 65% of the emission could be produced by direct impact of the photoelectrons with atomic oxygen in the thermosphere between 200 and 360 km. Using the observed electron density and the model distribution of oxygen molecules the residual of the emission was ascribed to the excitation of O( 1D) through dissociative recombination, O 2 ++ e→ O ∗ + O ∗∗ . The direct collisional excitation by ambient electrons is estimated to be negligibly small at the level of observed electron temperature.

Auroral electrons and the optical emissions of nitrogen

The differential flux of auroral primary and secondary electrons and the emission of the 1 PG and 2 PG (first and second positive) systems of N2 and the 1 NG (first negative) system of N2+ were measured in a rocket experiment during a steady IBC II+ aurora. The electron data indicate that the incident auroral electrons were nearly monoenergetic with energy of about 25 keV. A single power law of the form F = F0Eα cannot accurately describe the secondary electron spectrum in the energy range of 3–85 eV. However, the spectrum may be approximated by using α = −3.2 between 7 and 20 eV and α = −0.9 between 25 and 85 eV. The measured intensity and altitude profile of the N2 1 PG (5, 2) band at 6685 Å agree well with the values calculated by using the measured electron flux and laboratory cross-section data. About 40% of the N2+ 1 NG (0, 0) band emission at 3914 Å is excited by electrons with energy below 85 eV.

Auroral electron spectra

Discussion of the suitability of available differential electron flux data for auroral analyses. The differential energy spectra of auroral electrons at several altitudes computed by Berger et al. (1973) and Rees and Jones (1973) for initial electron energies of 10, 5, and 2 keV are reviewed, and it is concluded that they are suitable for auroral analyses.