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Decker, R. B.

Publications and source records attributed to Decker, R. B..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2019

Evolution of the Suprathermal Proton Population at Interplanetary Shocks

We investigate the evolution of the suprathermal (ST) proton population as interplanetary shocks cross 1 au. The variability of the ST proton intensities and energy spectra upstream of the shocks is analyzed in terms of the shock parameters, upstream magnetic field configurations, and preexisting upstream populations. Propitious conditions for the observation of ST particles at distances far upstream from the shock occur in parallel shock configurations when particles can easily escape from the shock vicinity. In this situation, ST intensity enhancements show onsets characterized by velocity dispersion effects and energy spectra that develop into a “hump” profile peaking around ∼10 keV just before the arrival of the shock. The observation of field-aligned proton beams at low energies (5–10 keV) is possible under conditions that facilitate the scatter-free propagation of the particles streaming out of the shock. Upstream of perpendicular shocks, ST intensity enhancements are only observed in close proximity to the shock. Power-law proton spectra develop downstream of the shocks. The functional form for the downstream phase-space density proportional to v(exp -5) is observed only over a limited range of ST energies. The absence of ST populations observed far upstream of interplanetary shocks raises questions about whether ST protons contribute as a seed particle population in the processes of particle acceleration at shocks.

Shock waves↗
NASA NTRS2018

Flat Proton Spectra in Large Solar Energetic Particle Events

We present solar energetic particle events observed at 1 AU from the Sun for which the proton energy spectra at energies between ~50 keV to ~1 MeV flatten during a period of at least ~12 hours prior to the passage of the associated interplanetary shock. The flattening of the proton energy spectra occurs when the source of the particles (presumably the traveling interplanetary shock) is still downwind from the spacecraft and particle intensities are still continuously increasing. The arrival of the shock at the spacecraft is then characterized by a steepening of the spectra, where low-energy proton intensities show a more pronounced enhancement than the high-energy proton intensities. We discuss the mechanisms that may result in this flattening of the spectra in terms of current models presented in the literature.

Lario, D.↗
NASA NTRS2006

Multi-Spacecraft Observations of Interplanetary Shock Accelerated Particle Events

We use simultaneous measurements from the Wind and ACE spacecraft to determine the spatial properties of both interplanetary (IP) shocks and the shock-associated energetic particle events. We combine plasma, magnetic field and energetic particle data from ACE and Wind for 124 energetic storm particle (ESP) events from 1998 to 2003 and examine the spatial and temporal variations of these events in the Earth's vicinity. We find that even though the two spacecraft were occasionally separated by more than 400 RE, the plasma, field, and energetic particle time-intensity profiles during the events were very similar. In addition, we find that the ion composition and energy spectra in individual IP shock events are identical at the two spacecraft locations. We also use the fitted shock velocity along the normal from ACE and estimate the shock transit time to Wind location. In general, there is poor agreement between the estimated transit time and the actual measured transit time. Hence, our assumptions that a) the IP shock at 1 AU propagates radially, and/or b) the IP shock is spherically symmetric at 1 AU are not valid. In this paper, we will also study, for the first time, the anisotropy measurements of low-energy IP shock-associated ions at both ACE and Wind. We will then compare these new anisotropy analyses with locally measured shock parameters and identify possible signatures of different shock acceleration processes as predicted by the first-order Fermi and shock-drift models.

Ho, G. C.↗
NASA NTRS1994

A model of Triton's role in Neptune's magnetosphere

Escape of neutral hydrogen (H) and nitrogen (N) from Triton's maintains a large neutral cloud, called the Triton torus, in Neptune's magnetosphere. We have developed the first detailed Monte Carlo simulation model of the Triton torus that includes the collisionality, the complex geometry, the injection of two neutral species from Triton (H and N), and the combined effects of photoionization, electron impact ionization, and charge exchange. Ionization in Neptune's plasma sheet was modeled using Voyager plasma observations. Collisions cause both the H and N neutral clouds to become more radially extended, both toward Neptune and out beyond the magnetopause, as well as more extended in latitude, when compared with collisionless models. Moreover, collisions of H with the much more massive N greatly enhance the collisional ejection of H from the system and into Neptune's atmosphere. This effect decreases the probability of H ionization within the magnetosphere relative to that for N, and furthermore causes model results for two-species injection from Triton to differ significantly from those for H injection alone. For a hydrogen escape rate from Triton of 5 x 10(exp 25)/s, as given by photo-chemical models of Triton's upper atmosphere, a nitrogen escape rate of 5 x 10(exp 24)/s gives proton and N(+) sources of 5.6 x 10(exp 24)/s and 3.3 x 10(exp 24)/s, respectively, whose ratio is close to the observed ratio of protons to heavies. A nitrogen escape rate of 2 x 10(exp 25)/s, yields an N(+) source more than twice that of protons, inconsistent with the Voyager data.

Decker, R. B.↗
NASA NTRS1993

The role of magnetic loops in particle acceleration at nearly perpendicular shocks

The acceleration of superthermal ions is investigated when a planar shock that is on average nearly perpendicular propagates through a plasma in which the magnetic field is the superposition of a constant uniform component plus a random field of transverse hydromagnetic fluctuations. The importance of the broadband nature of the transverse magnetic fluctuations in mediating ion acceleration at nearly perpendicular shocks is pointed out. Specifically, the fluctuations are composed of short-wavelength components which scatter ions in pitch angle and long-wavelength components which are responsible for a spatial meandering of field lines about the mean field. At nearly perpendicular shocks the field line meandering produces a distribution of transient loops along the shock. As an application of this model, the acceleration of a superthermal monoenergetic population of seed protons at a perpendicular shock is investigated by integrating along the exact phase-space orbits.

Decker, R. B.↗
NASA NTRS1992

Shock drift acceleration

Basic aspects of shock drift acceleration at fast-mode collisionless shocks are reviewed. Recent modeling efforts that incorporate such effects as shock structure, shock curvature magnetic loops, rippled shocks, and magnetic turbulence are described. Theoretical and observational questions prompted by the models described are posed.

Decker, R. B.↗
NASA NTRS1990

Particle acceleration at shocks with surface ripples

The present treatment of superthermal-ion acceleration on the surface of a fast-mode hydromagnetic shock gives attention to (1) small-amplitude surface ripples characterized by width L and amplitude A that are large relative to the energetic-ion gyroradius, and (2) shocks which are on average quasi-perpendicular. An investigation is made of the effects of the confinement, evolving geometry, and finite shock curvature associated with the ripple, by integrating along the orbits of the proton test particles. As an upstream magnetic field line convects through the surface ripple, it intersects the shock at two points, thereby forming a temporary magnetic trap. Flux-line profiles and angular distributions in a given ripple differ substantially, depending on the path it takes through the ripple and its distance from the shock.

Decker, R. B.↗
NASA NTRS1988

Interplanetary protons (Ep of about 1 MeV) 1973-1986 and out to 22.4 AU

This paper uses annual mean counting rate data from detectors on two long-lived spacecraft, Pioneer 11 and IMP 8, to study the temporal and heliocentric radial distance variations of the intensity of interplanetary protons (Ep of about 1 MeV) over solar activity cycle 21. The Pioneer 11 data cover the time period April 1973 through 1986 and the heliocentric radial distance range r of between 1.0 and 2.4 AU. IMP 8, in an approximately circular geocentric orbit of semimajor axis 35 earth radii, provides comparable data at 1 AU over the time period 1974-1986. The combination of the two bodies of data shows that the annual mean intensity of such protons varies as the inverse square of the distance from the sun, irrespective of solar activity as measured by the annual mean sunspot number S. Also it is found that the annual intensity at 1 AU is approximately proportional to S, except for anomalously low values in 1979 and 1980, and that the product of the annual mean intensity at Pioneer 11 by r-squared is also approximately proportional to S, except for anomalously low values in 1979, 1980 (in particular), and 1981. The common 1980 'anomaly' is attributed to gross changes in interplanetary conditions associated with the reversal of the polarity of the sun's polar magnetic field.

Van Allen, J. A.↗
NASA NTRS1988

The latitude and radial dependence of shock acceleration in the heliosphere

Voyager 1 and 2 observations of ions accelerated at corotating shocks within about 13-28 AU are discussed. The ion spectra extend smoothly from at least 30 keV to an energy above about 3 MeV. However, these spectra are falling steeply at the other end. The event-avaraged energy spectra during 1984 are similar for both Voyagers, and the event-averaged spectra at Voyager 2 are of similar form both before and after the flux decrease in early 1985. The event-averaged intensities of about 1 MeV protons exhibit an about -3 percent per degree gradient before and after the early 1985 intensity decrease. The significance of these observations for interpreting the shock acceleration is addressed.

Gold, R. E.↗
NASA NTRS1988

The role of drifts in diffusive shock acceleration

The role played by shock-associated drifts during the diffusive acceleration of charged particles at collisionless MHD shocks is evaluated. In the rest frame of the shock, the total energy gained by a particle is shown to result from two coupled acceleration mechanisms, the usual first-order Fermi mechanism and the drift mechanism. When averaged over a distribution of particles, the ratio of the drift-associated energy gain to the total energy is found to be independent of the total energy at a given theta1 (the angle between the shock normal and the unperturbed upstream magnetic field) in agreement with theoretical predictions. No evidence is found for drift-associated deceleration, suggesting that drifts always augment acceleration.

Decker, R. B.↗
NASA NTRS1987

Change in interplanetary shock acceleration preceding STIP Interval 17

The intensity and frequency of shock acceleration events in the interplanetary medium decreased dramatically in early 1985. Low energy ions were observed by IMP 8 at 1 AU and Voyagers 1 and 2 at 22 and 16 AU, respectively. Voyager 1 was at 25 deg heliographic latitude while IMP 8 and Voyager 2 were near the solar equatorial plane. The decrease in low energy shock events led to a drop in the average ion flux by a factor of 20 to 50. It started about day 10 of 1985 in the approximately .5 MeV channel on IMP8 and took approximately 75 days to reach the new, lower, background level. The decrease at the Voyagers started approximately 50 days later. The time delay between the start of the decrease at IMP and at Voyager 2 implies that decrease was convected outward with a velocity of approximately 535 km/sec. The intensity and frequency of interplanetary shock events remained at the lower level for at least 1.5 years.

Gold, R. E.↗
NASA NTRS1987

Latitudinal gradient of energetic particles in the outer hemisphere during 1985-1986

A measurement of a sustained latitudinal gradient of 70-MeV galactic cosmic ray protons is reported using data from the interplanetary probes Voyager 1 and 2 and the earth-orbiting satellite IMP 8 during a 1-year period from mid-1985 to mid-1986. Starting in early 1985, the intensity of cosmic rays at Voyager 2 began increasing faster than that at Voyager 1. By mid-1985, the intensity at Voyager 2 exceeded and remained higher than that at Voyager 1 for at least 14 solar rotations. Using the Voyager 2-IMP 8 data to correct for the radial gradient, an average latitudinal gradient during this period of about -53 percent/deg or about -38 percent/deg was determined. In addition, Voyager data at very low ion energies which are associated with acceleration at corotating shocks are presented.

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 NTRS1986

On ion acceleration in interplanetary quasi-perpendicular shock waves

The kinematic and pitch angle scattering constraints for multiple shock encounters to occur are investigated. The results suggest that (1) large particle anisotropies and a large number of shock encounters are not mutually exclusive and (2) solar wind ions are not directly injected into the nearly perpendicular interplanetary shock acceleration process. Also, the average number of shock encounters required to accelerate an ion between two energies is calculated. The results are consistent with the shock geometry dependence observed in interplanetary shock spike and corotating interaction region associated energetic ion events.

Pesses, M. E.↗
NASA NTRS1986

Measurement of radial and latitudinal gradients of cosmic ray intensity during the decreasing phase of sunspot cycle 21

The cosmic ray radial and latitudinal gradients during the 1981-1984 decreasing phase of sunspot cycle 21 are investigated based on data from Voyagers 1 and 2, with a detector threshold of not less than 70 MeV/nuc, and IMP-8, with a detector threshold of not less than 35 MeV/nuc. During the interval, the heliolongitudinal separation between the Voyager spacecraft changed from about 4 to 26 deg, and comparison of the 26-day means of the cosmic ray intensities obtained show that the data is consistent on the average with a long-term zero latitudinal gradient. Comparison at 1 AU of Voyager and IMP-8 data illustrate that the radial gradient decreased over this period at the rate of about 0.4 percent per AU per year, reaching a value of about 2.0 percent/AU between 16 and 22 AU, and a value of about 0.6 percent/AU between 16 and 22 AU, a pattern which would locate both Voyagers during 1977 outside the principal cosmic ray modulation at solar minimum within 22 AU.

Venkatesan, D.↗
NASA NTRS1985

Prompt acceleration of ions by oblique turbulent shocks in solar flares

Solar flares often accelerate ions and electrons to relativistic energies. The details of the acceleration process are not well understood, but until recently the main trend was to divide the acceleration process into two phases. During the first phase elctrons and ions are heated and accelerated up to several hundreds of keV simultaneously with the energy release. These mildly relativistic electrons interact with the ambient plasma and magnetic fields and generate hard X-ray and radio radiation. The second phase, usually delayed from the first by several minutes, is responsible for accelerating ions and electrons to relativistic energies. Relativistic electrons and ions interact with the solar atmosphere or escape from the Sun and generate gamma ray continuum, gamma ray line emission, neutron emission or are detected in space by spacecraft. In several flares the second phase is coincident with the start of a type 2 radio burst that is believed to be the signature of a shock wave. Observations from the Solar Maximum Mission spacecraft have shown, for the first time, that several flares accelerate particles to all energies nearly simultaneously. These results posed a new theoretical problem: How fast are shocks and magnetohydrodynamic turbulence formed and how quickly can they accelerate ions to 50 MeV in the lower corona? This problem is discussed.

Decker, R. B.↗
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.↗