Energetic Neutral Atom Flux Research Articles

Neutral solar wind properties: Advance warning of major geomagnetic storms

Coronal mass ejections (CMEs) have been identified as a trigger for large geomagnetic storms. A clever idea to provide advance warning of a high‐speed CME approaching the Earth was recently proposed by Hsieh et al. (1992): ejected solar matter decelerates on its way from the Sun to Earth, energetic neutral atoms (ENAs) are formed in the CME plasma due to recombination, ENAs pass the decelerating CME plasma and arrive first at the Earth. We evaluate the idea to use ENAs for advance detection of the high‐speed Earth‐approaching CMEs and consider the processes involved. Charge exchange between solar wind ions and interplanetary neutral atoms contribute effectively to ENA production. Characteristics of neutral gas within Earth's orbit are updated for both neutral atoms of interstellar origin and from outgassing from interplanetary dust. Computer simulation of CME‐produced ENAs shows that the total flux of CME‐produced ENAs is slightly smaller than the intensity of the quiescent neutral solar wind (NSW) while significantly higher ENA energies make them easily distinguishable from NSW atoms. Arrival of the CME‐produced ENAs is expected 3‐4 hours before the start of a large geomagnetic storm, which provides a basis for advance storm warning and prediction of storm magnitude. The progress in development of ENA measurement technique suggests that both the quiescent NSW and CME‐produced ENAs can be reliably measured.

Convection and diffusion of cosmic rays in the heliosheath

Diffusion and convection of both anomalous (ACR) and galactic (GCR) cosmic rays in the heliosheath and adjacent local interstellar plasma flow is calculated by solving the appropriate CR transport equation in the test particle approximation (background plasma flow unaffected). Upon creation at the termination shock the ACR are convected towards the tail region and form there a huge particle reservoir. Ion transcharging on neutral galactic gases leads to apex/antiapex heliosphere asymmetries in the fluxes of energetic neutral atoms.

Viewing the outer heliosphere in energetic neutral atoms

The outer heliosphere is inaccessible for repeated in situ investigations. One can, however, sample the charged-particle populations of that remote region from the inner solar system, including from Earth's orbit, by detecting the energetic neutral atoms produced by charge exchange between the energetic ions and the ambient neutral atoms. Fluxes of energetic neutral atoms emanating from the heliospheric interface region carry information on the thermalization of the solar-wind at the termination shock and the unmodulated anomalous cosmic rays, believed to be accelerated by the termination shock. We examine the techniques for viewing the outer heliosphere in energetic neutral atoms from within.

Anisotropy of the energetic neutral atom flux in the heliosphere

Characteristics of the energetic neutral atoms born at the heliospheric interface are considered for plasma flow structure resulting from a two-shock model of the interaction between the solar wind and the interstellar medium. The energy distributions of heliospheric energetic neutral atoms (HELENAs) are calculated and it is shown that the HELENA flux is highly anisotropic at the Earth's orbit. The characteristics of the HELENA flux are highly sensitive to the size of the heliosphere. This supports the conclusion that measurements of HELENAs from the Earth's orbit would give us an efficient tool to remotely study the heliosphere.

Properties of conductive zinc oxide films for transparent electrode applications prepared by rf magnetron sputtering

High-quality zinc oxide (ZnO) films have been deposited by rf magnetron sputtering from a zinc oxide target. The material has been obtained with resistivities ranging from about 1 Ω cm to 4.6×10−4 Ω cm, whereas the average transmission was always greater than 90% between 400 and 800 nm. A relatively high deposition rate usually leads to low resistivity of films. The films with the lowest resistivity differ from higher-resistivity films by their surface morphology, as observed by scanning electron microscopy and by their x-ray diffraction pattern. In particular, good quality films have a diffraction peak at 2Θ≂30.5°. The best films have also been characterized by Hall-effect measurements, Auger electron spectroscopy, and infrared transmission data. It is suggested that the film properties are dependent on the flux of energetic neutral oxygen atoms to the growing surface, which can be varied by changing the sputtering conditions, such as the target to substrate distance. These films have considerable potential in amorphous silicon solar cell technology as well as other photovoltaic and nonphotovoltaic applications.

Energetic neutral atom image of a storm‐time ring current

An image formed in 5 minutes by energetic neutal atoms (ENA) emitted by charge exchange with ring‐current energetic ions (∼ 50 keV) has been constructed and analyzed from ISEE 1 measurements at a radial distance of 2.6 Re during the main phase (Dst = −241 nT) of the geomagnetic storm of September 29, 1978. Comparison with simulated images calculated from a simple model of the global ring‐current ion distribution implies a strong (≳ 20:1) midnight/noon ion intensity asymmetry. Estimated ion fluxes in the midnight meridian (3.0 < r < 5.0 Re) are 2.5 × 106 (cm² s sr keV) −1 for 30 keV H+ (if H‐atoms dominated the ISEE 1 ENA fluxes) or 1.0 × 106 (cm² s sr keV)−1 for 70 keV O+ (if O‐atoms dominated). ENA fluxes (whether H or O) from the brightest areas of the image exceed 10³ (cm² s sr keV)−1.

Energetic neutral atoms (E ∼ 50 keV) from the ring current: IMP 7/8 and ISEE 1

Energetic neutral atoms (ENA), emitted from the magnetosphere with energies of ∼50 keV, have been measured with solid‐state detectors on the IMP 7/8 and ISEE 1 spacecraft. The ENA are produced when singly charged trapped ions collide with the exospheric neutral hydrogen geocorona and the energetic ions are neutralized by charge exchange. ENA observations during the recovery phase of two moderate geomagnetic storms are analyzed in detail: November 22–23, 1973, from IMP 8 at 33 RE, and December 17, 1977, from ISEE 1 at 20 RE. The three‐dimensional viewing capability of the Medium Energy Particle Instrument (MEPI) on ISEE 1 provides coarse spatial information; i.e., the ENA emission fell off significantly beyond L ≃ 5 in the midnight and morning sectors of magnetic local time. The spectral resolution of the MEPI allowed us to deduce that oxygen atoms dominated the ENA fluxes at energies of ∼100 keV because the decay time constant (7.5 hours) for the ENA fluxes was identical in the two lowest energy channels over 10 hours. Combining the spatial and spectral information from ISEE 1, we estimate an ENA unidirectional differential flux jO ≃ 30 (cm² s sr keV)−1 for 100‐keV oxygen atoms with an e‐folding energy of 20 keV at the beginning of the recovery phase. This ENA flux implies a singly charged oxygen ion flux jO+ ≃ 3.5×104 (cm² s sr keV)−1 near the inner edge of the ring current region, and we estimate a corresponding decay time for geocorona H atoms ≃ 9 days at that time and location. Using a newly derived relation between the recovery rate of the ring current disturbance (dDst/dt) and the measured ENA flux, we show self‐consistently that charge‐exchange loss of O+ ions was the dominant mechanism in the recovery of Dst in both of the storms observed in ENA by IMP 8 and ISEE 1. Even though oxygen dominated the ENA flux and the ring current energy loss rate at ∼100 keV, H+ intensities at these energies could still have been comparable to those of O+ in the ring current (because the charge‐exchange cross section for H+ is much smaller than that for O+ at 100 keV).