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At least 1,369 records · Page 76
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NASA NTRS1980

Distribution function representation of energy spectra of H, He, C, O and Fe in corotating particle streams

From an analysis of nine corotating events observed near 1 AU during the 1974-1976 solar minimum, the spectral parameters for H, He, C, O and Fe and relative abundances of these elements and of Ne, Mg, Si, and S-Ca were determined. The distribution functions of H, He, O and Fe are well represented by an exponential in particle speed over the energy range of the measurements from 0.3 to 5 MeV/nucleon. The composition resembles that of the solar corona, particularly in the O/C ratio which is 0.95 + or - 0.19, and the He/H and He/Ne ratios are similar to the respective ratios in the solar wind. The results are consistent with interplanetary statistical acceleration of these particles out of the high-energy tail of the high-speed solar wind.

Gloeckler, G.↗
NASA NTRS1988

Determination of auroral electrostatic potentials using high- and low-altitude particle distributions

The auroral electrostatic potential differences were determined from the particle distribution functions obtained nearly simultaneously above and below the auroral acceleration region by DE-1 at altitudes 9000-15,000 km and DE-2 at 400-800 km. Three independent techniques were used: (1) the peak energies of precipitating electrons observed by DE-2, (2) the widening of loss cones for upward traveling electrons observed by DE-1, and (3) the energies of upgoing ions observed by DE-1. The assumed parallel electrostatic potential difference calculated by the three methods was nearly the same. The results confirmed the hypothesis that parallel electrostatic fields of 1-10 kV potential drop at 1-2 earth radii altitude are an important source for auroral particle acceleration.

Reiff, P. H.↗
NASA NTRS1980

On the origin of corotating energetic particle events

The paper examines energetic corotating particle events produced by acceleration at the forward and reverse shock pair which bounds a corotating interaction region by the process of successive multiple reflections. In the steady state, the energetic particle intensity at the shock depends only on the intensity of shock particles, and measurements of energetic particles show systematic variations between intensities at the forward and reverse shocks. It is concluded that the solar wind is the original particle population that is accelerated in the stationary shocks up to several MeV.

Scholer, M.↗
NASA NTRS1979

On the connection of interplanetary shock wave parameters and energetic storm particle events

A series of energetic storm particle (ESP) events is investigated on the basis of observations of energetic solar protons by IMPs IV and V between 1967 and 1972. The total number and energy of particles associated with 20 ESP events are analyzed in connection with several plasma parameters at the shock surface; i.e., magnetic and thermal pressure, magnetic field and plasma mass flux. Correlation is established between the total energy of ESPs in the energy range of 1-80 MeV and the jump in the total pressure. It is pointed out that the amount of shock energy converted to the acceleration of ESP particles is not negligible and could, in principle, contribute to the deceleration of the shock wave.

Gombosi, T.↗
NASA NTRS2001

Correlation of Upper-Atmospheric Be-7 With Solar Energetic Particle Events

Surprisingly large concentrations of radioactive Be-7 have been found in the upper atmosphere at levels of one to three orders of magnitude greater than observed in the stratosphere. This phenomenon was originally observed on the LDEF satellite which was recovered in January 1990 following a period of extremely high solar activity in the fall of 1989. We report on follow-up measurements on the Russian COSMOS and RESURS F1 spacecraft during the period of 1996 to 1999 which was a period of minimal to moderate solar activity. The Be-7 concentrations observed on these flights were down substantially from the LDEF observations but were still one to two orders of magnitude higher than stratospheric levels. A significant correlation is observed between the Be-7 activity and the combined fluence of solar energetic protons (SEP) and galactic cosmic-ray (GCR) protons. The Be-7 activity is not correlated with overall solar activity as represented by the solar x-ray flux. We discuss possible mechanisms for the solar proton correlation. However, it is likely that the Be-7 is ionized and it is unknown how this will affect the calculations. There were several large solar flares in the fall of 1989 that produced extraordinarily intense solar particle events at the Earth and record geophysical disturbances. These may have acted to increase production of Be-7 from spallation in the stratosphere and also to enhance transport to higher altitudes from the effects of heating and expansion of the upper atmosphere. Be-7 in the upper atmosphere may also have been produced directly at the Sun. Be-7 and Li-7 are produced in solar flares when accelerated alpha-particles fuse with He-4 in the solar atmosphere. Under optimistic assumptions for Sun to Earth transport and subsequent insertion into low Earth orbit, a Be-7 density of about 10(exp -7) atom/cubic cm at 310 km is estimated.

Phillips, G. W.↗
NASA NTRS1990

Cosmic rays in the heliosphere - Present status and future opportunities

The general theory of charged-particle transport and acceleration is reviewed and application to the heliosphere discussed in detail. The present status of understanding is briefly surveyed, with emphasis on large-scale processes. Opportunities for increasing understanding through the confrontation of theory, modeling and observations are outlined. It is concluded that the largest advances in this understanding in the next few years will come from coordinated theoretical and observations studies of large-scale phenomena requiring energetic-particle, magnetic-field, and plasma data from several spacecraft. Fundamental studies of the interactions between cosmic rays and turbulent plasma to study transport coefficients, details of interactions with shocks, etc, are also important in the longer term.

Jokipii, J. R.↗
NASA NTRS1992

Neutron energy determination with a high-purity germanium detector

Two areas that are related to planetary gamma-ray spectrometry are investigated. The first task was the investigation of gamma rays produced by high-energy charged particles and their secondaries in planetary surfaces by means of thick target bombardments. The second task was the investigation of the effects of high-energy neutrons on gamma-ray spectral features obtained with high-purity Ge-detectors. For both tasks, as a function of the funding level, the experimental work was predominantly tied to that of other researchers, whenever there was an opportunity to participate in bombardment experiments at large or small accelerators for charged particles.

Beck, Gene A.↗
NASA NTRS2012

Kinetic Model of Electric Potentials in Localized Collisionless Plasma Structures under Steady Quasi-gyrotropic Conditions

Localized plasma structures, such as thin current sheets, generally are associated with localized magnetic and electric fields. In space plasmas localized electric fields not only play an important role for particle dynamics and acceleration but may also have significant consequences on larger scales, e.g., through magnetic reconnection. Also, it has been suggested that localized electric fields generated in the magnetosphere are directly connected with quasi-steady auroral arcs. In this context, we present a two-dimensional model based on Vlasov theory that provides the electric potential for a large class of given magnetic field profiles. The model uses an expansion for small deviation from gyrotropy and besides quasineutrality it assumes that electrons and ions have the same number of particles with their generalized gyrocenter on any given magnetic field line. Specializing to one dimension, a detailed discussion concentrates on the electric potential shapes (such as "U" or "S" shapes) associated with magnetic dips, bumps, and steps. Then, it is investigated how the model responds to quasi-steady evolution of the plasma. Finally, the model proves useful in the interpretation of the electric potentials taken from two existing particle simulations.

Schindler, K.↗
NASA NTRS1996

Excitation of Plasma Waves in Aurora by Electron Beams

In this paper, we study numerically the excitation of plasma waves by electron beams, in the auroral region above 2000 km of altitude. We have solved the fully kinetic dispersion relation, using numerical method and found the real frequency and the growth rate of the plasma wave modes. We have examined the instability properties of low-frequency waves such as the Electromagnetic Ion Cyclotron (EMIC) wave as well as Lower-Hybrid (LH) wave in the range of high-frequency. In all cases, the source of free energy are electron beams propagating parallel to the geomagnetic field. We present some features of the growth rate modes, when the cold plasma parameters are changed, such as background electrons and ions species (H(+) and O(+)) temperature, density or the electron beam density and/or drift velocity. These results can be used in a test-particle simulation code, to investigate the ion acceleration and their implication in the auroral acceleration processes, by wave-particle interaction.

daSilva, C. E.↗
NASA NTRS1978

The nuclear and ionic charge distribution particle experiments on the ISEE-1 and ISEE-C spacecraft

The ISEE-1 and ISEE-C instruments are designed to measure the elemental abundances, charge state composition, energy spectra, and angular distributions of energetic ions in the energy range 2 keV/charge to 80 MeV/nucleon and electrons between 75 and 1300 keV. The instrument consists of three sensor systems: (1) ULECA which is an electrostatic deflection system with rectangular solid-state detectors for the 3-560 keV/charge range; (2) ULEWATT which is a double dE/dX vs E thin-window flowthrough proportional counter/solid-state detector telescope for the 0.2-80 MeV/nucleon (Fe) range; and (3) ULEZEQ which is a combination of an electrostatic deflection analyzer and a thin-window dE/dX vs E system with thin-window proportional counter and position-sensitive solid-state detector. The applications of the instruments to solar and interplanetary phenomena and to magnetospheric acceleration and trapped particles are discussed.

Hovestadt, D.↗
NASA NTRSDate not supplied

Stellar Energetic Particle-driven Production of Biologically Relevant Molecules in Atmospheres of Young Earth-like Exoplanets

The chemistry of N2, CO2, CH4-rich atmospheres of terrestrial-type exoplanets around active G-K stars is a complex problem. The star's energetic inputs in the atmospheres take the form of ionizing radiation in the form of X-ray, Extreme UV, as well as precipitating energetic particles that were accelerated in coronal mass ejection-driven shocks. These inputs lead to the ionization and dissociation of the atmospheric species, which enable complex chemical processes resulting in the creation of molecules of interest for both astrobiology and climate (Airapetian et al., 2016; Hayworth et al. 2022). We developed a set of atmospheric models to simulate the formation of organic molecules that can be considered as atmospheric pre-biosignatures from exoplanets around active stars. They take into account the dissociations and ionizations from all the energetic inputs in the atmosphere as well as their role in the enhanced chemistry in the lower atmosphere. In addition, the evaluation of stellar energetic particle fluxes is done using the latest techniques to accurately highlight the depth at which the chemistry is enabled. These models were used to simulate the chemistry of several young terrestrial-type exoplanets resembling the Hadean Earth to highlight the creation of atmospheric pre-biosignatures and relevant greenhouse gases, in function of different scenarios for its atmospheric evolution. This work shows the importance of space weather in the chemical evolution of atmospheres of astrobiological interest.

Guillaume Gronof↗
NASA NTRS1977

Solar particle propagation from 1 to 5 AU

The radial variation of the interplanetary propagation of solar cosmic rays is investigated on a grand scale by performing a statistical analysis of charged-particle data obtained by Pioneers 10 and 11 from launch to Jovian encounter (i.e., at distances of 1 to 5 AU). A numerical propagation model is adopted which includes diffusion, convection, and adiabatic deceleration, as well as a variable power-law diffusion coefficient. The study of solar particle events is carried out by analyzing individually each of five key parameters that are uniquely defined in each solar particle event: the time to maximum flux starting from the initial release at the sun, the anisotropy at the time to maximum flux, the flux at that time, the time width of the event at half the flux at the time to maximum, and the decay time. Combined results for the proton energy ranges from 3.4 to 5.2 MeV and 24 to 30 MeV are presented, the average radial diffusion coefficients between 1 and 5 AU are determined for each energy range, and it is found that the combination of a near-impusive injection with a temporally constant spectral index produces a reasonably good fit to the high-energy data but not as good a fit to the low-energy data. Implications of the results obtained are discussed for cosmic-ray propagation theory, solar modulation studies, and interplanetary acceleration of solar particles.

Zwickl, R. D.↗
NASA NTRS2005

MESSENGER: Exploring Mercury's Magnetosphere

The MESSENGER mission to Mercury offers our first opportunity to explore this planet s miniature magnetosphere since the brief flybys of Mariner 10. Mercury s magnetosphere is unique in many respects. The magnetosphere of Mercury is among the smallest in the solar system; its magnetic field typically stands off the solar wind only - 1000 to 2000 km above the surface. For this reason there are no closed drift paths for energetic particles and, hence, no radiation belts. The characteristic time scales for wave propagation and convective transport are short and kinetic and fluid modes may be coupled. Magnetic reconnection at the dayside magnetopause may erode the subsolar magnetosphere allowing solar wind ions to impact directly the regolith. Inductive currents in Mercury s interior may act to modify the solar wind interaction by resisting changes due to solar wind pressure variations. Indeed, observations of these induction effects may be an important source of information on the state of Mercury s interior. In addition, Mercury s magnetosphere is the only one with its defining magnetic flux tubes rooted in a planetary regolith as opposed to an atmosphere with a conductive ionospheric layer. This lack of an ionosphere is probably the underlying reason for the brevity of the very intense, but short-lived, - 1-2 min, substorm-like energetic particle events observed by Mariner 10 during its first traversal of Mercury s magnetic tail. Because of Mercury s proximity to the sun, 0.3 - 0.5 AU, this magnetosphere experiences the most extreme driving forces in the solar system. All of these factors are expected to produce complicated interactions involving the exchange and re-cycling of neutrals and ions between the solar wind, magnetosphere, and regolith. The electrodynamics of Mercury s magnetosphere are expected to be equally complex, with strong forcing by the solar wind, magnetic reconnection at the magnetopause and in the tail, and the pick-up of planetary ions all driving field-aligned electric currents. However, these field-aligned currents do not close in an ionosphere, but in some other manner. In addition to the insights- into magnetospheric physics offered by study of the solar wind - Mercury system, quantitative specification of the "external" magnetic field generated by magnetospheric currents is necessary for accurate determination of the strength and multi-polar decomposition of Mercury s intrinsic magnetic field. MESSENGER S highly capable instrumentation and broad orbital coverage will greatly advance our understanding of both the origin of Mercury s magnetic field and the acceleration of charged particles in small magnetospheres. In. this article, we review what is known about Mercury s magnetosphere and describe the MESSENGER science team s strategy for obtaining answers to the outstanding science questions surrounding the interaction of the solar wind with Mercury and its small, but dynamic, magnetosphere.

Slavin, James A.↗
NASA NTRS1985

Energetic ion acceleration at collisionless shocks

An example is presented from a test particle simulation designed to study ion acceleration at oblique turbulent shocks. For conditions appropriate at interplanetary shocks near 1 AU, it is found that a shock with theta sub B n = 60 deg is capable of producing an energy spectrum extending from 10 keV to approx. 1 MeV in approx 1 hour. In this case total energy gains result primarily from several separate episodes of shock drift acceleration, each of which occurs when particles are scattered back to the shock by magnetic fluctuations in the shock vicinity.

Decker, R. B.↗
NASA NTRS1986

Modeling of ion acceleration through drift and diffusion at interplanetary shocks

A test particle simulation designed to model ion acceleration through drift and diffusion at interplanetary shocks is described. The technique consists of integrating along exact particle orbits in a system where the angle between the shock normal and mean upstream magnetic field, the level of magnetic fluctuations, and the energy of injected particles can assume a range of values. The technique makes it possible to study time-dependent shock acceleration under conditions not amenable to analytical techniques. To illustrate the capability of the numerical model, proton acceleration was considered under conditions appropriate for interplanetary shocks at 1 AU, including large-amplitude transverse magnetic fluctuations derived from power spectra of both ambient and shock-associated MHD waves.

Decker, R. B.↗
NASA NTRS1962

ELECTRIC PROPULSION- A NEW TECHNOLOGY

Within the past 3 years, electric thrust devices have passed from the first exploratory feasibility experiments to the flight-engineered hardware phase. This rapid surge of development has been punctuated by equally rapid improvements in the performance figures of electrical thrust devices. The most spectacular advances were made in ion engine technology. Total energy expenditure per ion pair has gone from 35,000 ev in 1958 to about 600 ev at the present time leading to power utilization efficiencies of up to 80 per cent, depending upon the specific impulse. The percentage of the particles intercepting the accelerating and focusing electrodes has been reduced from more than 50 per cent in 1958 to less than 0.01 per cent at the present time. Great improvements in ion current per unit area have also been realized, leading to greater thrust "pressures."

PLASMA ENGINE↗