D. N. Baker

Neutral line model of substorms: Past results and present view

The near-Earth neutral line (NENL) model of magnetospheric substorms is reviewed. The observed phenomenology of substorms is discussed including the role of coupling with the solar wind and interplanetary magnetic field, the growth phase sequence, the expansion phase (and onset), and the recovery phase. New observations and modeling results are put into the context of the prior model framework. Significant issues and concerns about the shortcomings of the NENL model are addressed. Such issues as ionosphere-tail coupling, large-scale mapping, onset trigger- ing, and observational timing are discussed. It is concluded that the NENL model is evolving and being improved so as to include new observations and theoretical insights. More work is clearly required in order to incorporate fully the complete set of ionospheric, near-tail, midtail, and deep- tail features of substorms. Nonetheless, the NENL model still seems to provide the best avail- able framework for ordering the complex, global manifestations of substorms.

Wave acceleration of electrons in the Van Allen radiation belts

The Van Allen radiation belts are two regions encircling the Earth in which energetic charged particles are trapped inside the Earths magnetic field. Their properties vary according to solar activity and they represent a hazard to satellites and humans in space. An important challenge has been to explain how the charged particles within these belts are accelerated to very high energies of several million electron volts. Here we show, on the basis of the analysis of a rare event where the outer radiation belt was depleted and then re-formed closer to the Earth, that the long established theory of acceleration by radial diffusion is inadequate; the electrons are accelerated more effectively by electromagnetic waves at frequencies of a few kilohertz. Wave acceleration can increase the electron flux by more than three orders of magnitude over the observed timescale of one to two days, more than sufficient to explain the new radiation belt. Wave acceleration could also be important for Jupiter, Saturn and other astrophysical objects with magnetic fields.

Rapid local acceleration of relativistic radiation-belt electrons by magnetospheric chorus.

Recent analysis of satellite data obtained during the 9 October 2012 geomagnetic storm identified the development of peaks in electron phase space density, which are compelling evidence for local electron acceleration in the heart of the outer radiation belt, but are inconsistent with acceleration by inward radial diffusive transport. However, the precise physical mechanism responsible for the acceleration on 9 October was not identified. Previous modelling has indicated that a magnetospheric electromagnetic emission known as chorus could be a potential candidate for local electron acceleration, but a definitive resolution of the importance of chorus for radiation-belt acceleration was not possible because of limitations in the energy range and resolution of previous electron observations and the lack of a dynamic global wave model. Here we report high-resolution electron observations obtained during the 9 October storm and demonstrate, using a two-dimensional simulation performed with a recently developed time-varying data-driven model, that chorus scattering explains the temporal evolution of both the energy and angular distribution of the observed relativistic electron flux increase. Our detailed modelling demonstrates the remarkable efficiency of wave acceleration in the Earth’s outer radiation belt, and the results presented have potential application to Jupiter, Saturn and other magnetized astrophysical objects.

Electron Acceleration in the Heart of the Van Allen Radiation Belts

Local Acceleration How the electrons trapped in Earth-encircling Van Allen radiation belts get accelerated has been debated since their discovery in 1958. Reeves et al. (p. 991, published online 25 July) used data from the Van Allen Radiation Belt Storm Probes, launched by NASA on 30 August 2012, to discover that radiation belt electrons are accelerated locally by wave-particle interactions, rather than by radial transport from regions of weaker to stronger magnetic fields. Satellite observations provide evidence for local relativistic electron acceleration in Earth’s radiation belts. The Van Allen radiation belts contain ultrarelativistic electrons trapped in Earth’s magnetic field. Since their discovery in 1958, a fundamental unanswered question has been how electrons can be accelerated to such high energies. Two classes of processes have been proposed: transport and acceleration of electrons from a source population located outside the radiation belts (radial acceleration) or acceleration of lower-energy electrons to relativistic energies in situ in the heart of the radiation belts (local acceleration). We report measurements from NASA’s Van Allen Radiation Belt Storm Probes that clearly distinguish between the two types of acceleration. The observed radial profiles of phase space density are characteristic of local acceleration in the heart of the radiation belts and are inconsistent with a predominantly radial acceleration process.

The MESSENGER mission to Mercury: Scientific objectives and implementation

Abstract Mercury holds answers to several critical questions regarding the formation and evolution of the terrestrial planets. These questions include the origin of Mercurys anomalously high ratio of metal to silicate and its implications for planetary accretion processes, the nature of Mercurys geological evolution and interior cooling history, the mechanism of global magnetic field generation, the state of Mercurys core, and the processes controlling volatile species in Mercurys polar deposits, exosphere, and magnetosphere. The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission has been designed to fly by and orbit Mercury to address all of these key questions. After launch by a Delta 2925H-9.5, two flybys of Venus, and two flybys of Mercury, orbit insertion is accomplished at the third Mercury encounter. The instrument payload includes a dual imaging system for wide and narrow fields-of-view, monochrome and color imaging, and stereo; X-ray and combined gamma-ray and neutron spectrometers for surface chemical mapping; a magnetometer; a laser altimeter; a combined ultraviolet–visible and visible-near-infrared spectrometer to survey both exospheric species and surface mineralogy; and an energetic particle and plasma spectrometer to sample charged species in the magnetosphere. During the flybys of Mercury, regions unexplored by Mariner 10 will be seen for the first time, and new data will be gathered on Mercurys exosphere, magnetosphere, and surface composition. During the orbital phase of the mission, one Earth year in duration, MESSENGER will complete global mapping and the detailed characterization of the exosphere, magnetosphere, surface, and interior.

The Clementine Mission to the Moon: Scientific Overview

In the course of 71 days in lunar orbit, from 19 February to 3 May 1994, the Clementine spacecraft acquired just under two million digital images of the moon at visible and infrared wavelengths. These data are enabling the global mapping of the rock types of the lunar crust and the first detailed investigation of the geology of the lunar polar regions and the lunar far side. In addition, laser-ranging measurements provided the first view of the global topographic figure of the moon. The topography of many ancient impact basins has been measured, and a global map of the thickness of the lunar crust has been derived from the topography and gravity.

On the origin of relativistic electrons in the magnetosphere associated with some geomagnetic storms

There has been considerable interest of late in the sudden appearance of large fluxes of relativistic electrons (E>1 MeV) at geostationary orbit due to the adverse effect these electrons can have on operational spacecraft. The question arises as to how these electrons are accelerated to relativistic energies in the relatively short time of a few hours. We shall show that large amplitude ULF pulsations are a unique feature of intervals of time in which the fluxes of relativistic electrons rise to high levels at radial distances beyond the normal L-shell range occupied by the outer radiation belt. We use SAMPEX polar orbiter data for the magnetic storm of November 1993 to show that the fluxes of E>400 keV electrons increase simultaneously over a broad range of L-shells and use this fact to suggest that large amplitude ULF pulsations have the potential to supply the energy necessary to create the relativistic electron fluxes.

Simulation of dispersionless injections and drift echoes of energetic electrons associated with substorms

The term “dispersionless injection” refers to a class of events which show simultaneous enhancement (injection) of electrons and ions with different energies usually seen at or near geosynchronous orbit. We show that dispersionless injections can be understood as a consequence of changes in the electric and magnetic fields by modeling an electron injection event observed early on January 10, 1997 by means of a test-particle simulation. The model background magnetic field is a basic dipole field made asymmetrical by a compressed dayside and a weakened nightside. The transient fields are modeled with only one component of the electric field which is westward and a consistent magnetic field. These fields are used to model the major features of a dipolarization process during a substorm onset. We follow the electrons using a relativistic guiding center code. Our simulation results, with an initial kappa electron energy flux spectrum, reproduce the observed electron injection and subsequent drift echoes and show that the energization of injected electrons is mainly due to betatron acceleration of the preexisting electron population at larger radial distances in the magnetotail by transient fields.

Quantitative prediction of radiation belt electrons at geostationary orbit based on solar wind measurements

Solar wind measurements are used to predict the MeV electron radiation belt flux at the position of geostationary orbit. Using a model based on the standard radial diffusion equation, a prediction efficiency of 0.81 and a linear correlation of 0.90 were achieved for the years 1995–1996 for the logarithm of average daily flux. Model parameters based on the years 1995-1996 gave a prediction efficiency and a linear correlation for the years 1995–1999 of 0.59 and 0.80, respectively. The radial diffusion equation is solved after making the diffusion coefficient a function of the solar wind velocity and interplanetary magnetic field. The solar wind velocity is the most important parameter governing relativistic electron fluxes at geostationary orbit. The model also provides a physical explanation to several long standing mysteries of the variation of the MeV electrons.

The Global Geospace Science Program and its investigations

The detailed study of the solar-terrestrial energy chain will be greatly enhanced with the launch and simultaneous operation of several spacecraft during the current decade. These programs are being coordinates in the United States under the umbrella of the International Solar Terrestrial Physics Program (ISTP) and include fundamental contributions from Japan (GEOTAIL Program) and Europe (SOHO and CLUSTER Programs). The principal United States contribution to this effort is the Global Geospace Science Program (GGS) described in this overview paper. Two spacecraft, WIND and POLAR, carrying an advanced complement of field, particle and imaging instruments, will conduct investigations of several key regions of ‘geospace’. This paper provides a general overview of the science objectives of the missions, the spacecraft orbits and the ground elements that have been developed to process and analyze the instrument observations.