Yam T. Chiu

Auroral plasmas in the evening sector - Satellite observations and theoretical interpretations

Observations and theoretical interpretations of auroral plasma distributions have led to a spectacular advance, in the latter part of the 1970s, in understanding the formation of auroral arcs and the role that the aurora plays in the coupling between the magnetosphere and ionosphere in the evening sector. The key to this understanding is the verification of the existence of electric field components parallel to the magnetic field. The parallel electric field accelerates electrons downwards to form the aurora. At the same time, it accelerates ionospheric ions upwards to provide the magnetosphere with a new source of hot plasma. The auroral plasma observations indicate that the hot auroral plasmas behave according to laws of adiabatic motion coexisting with a measure of plasma turbulence. Theoretical considerations of auroral arc formation are in accord with this plasma characteristic.

Imaging the plasmasphere and trough regions in the extreme-ultraviolet region

EUV images from emissions of O+ (83.4 nm) and He+ (30.4 nm) distributions in the plasmasphere and trough regions are constructed for a satellite at 9 Re (where Re is Earth radii). A diffusive equilibrium model is used to describe the density distribution along the field lines for ions in the plasmasphere, and a kinetic collisionless model is assumedto calculate ion densities in the high-latitude regions beyond the plasmapause. In our model ofthe plasmasphere, we assumethat ions move along astatic dipole field. Observational data on O+ and He+ densities in the ionosphere are used as boundaryconditionsto calculatethespatial distribution of densities along the field lines. Assuming thatthe day and night boundary conditions are asymmetric and exobase densities vary with latitude, we discuss how this would be reflected in the intensities and structure of the magnetospheric images.

Imagers for the magnetosphere, aurora, and plasmasphere

We present a small Explorer mission, Imagers for the Magnetosphere, Aurora, and Plasmasphere (IMAP), to provide the first global magnetospheric images that will allow a systematic study of major regions of the magnetosphere, their dynamics, and their interactions. The mission objective is to obtain simultaneous images of the inner magnetosphere (ring current and trapped particles), the plasmasphere, the aurora, and auroral upflowing ions. The instruments are (1) a Low Energy Neutral Particle Imager for imaging H and O atoms, separately, in the energy range of ~1 to 30 keV, in several energy passbands; (2) an Energetic Neutral Particle Imager for imaging H atoms in the energy range ~15 to 200 keV and, separately, O atoms in the energy range ~60 to 200 keV, each in several energy passbands; (3) an Extreme-Ultraviolet Imager to obtain images of the plasmasphere (the distribution of cold He + ) by means of He + (30.4 nm) emissions; and (4) a Far-Ultraviolet Imaging Monochromator to provide images of the aurora and the geocorona. All images will be obtained with time and spatial resolutions appropriate to the global and macroscale structures to be observed. IMAP promises new quantitative analyses that will provide great advances in insight and knowledge of global and macroscale magnetospheric parameters. The results expected from IMAP will provide the first large-scale visualization of the ring current, the trapped ion populations, the plasmasphere, and the upflowing auroral ion population. Such images, coupled with simultaneously obtained auroral images, will also provide the initial opportunity to globally interconnect these major magnetospheric regions. The time sequencing of IMAP images will also provide the initial large-scale visualization of magnetospheric dynamics, both in space and time.

Geophysical effects on magnetospheric images

The concept of using solar EUV line resonance scattering to image ion populations in the magnetosphere has been studied extensively in the last decade. Global magnetospheric EUV images can display the effects of scatterer density, injection, the geomagnetic field, and the changing perspective of earthshine source intensity with altitude, latitude, and local time. Successful use of these images for magnetospheric plasma diagnostics or for examining the morphology of the magnetosphere depends on properly accounting for these eftects and incorporating them into the models used to interpret such images. The importance of each of these effects is examined for varying levels of magnetospheric activity and different observer perspectives. Oxygen 834-A resonance scattering is used as a testbed. A Tsyganenko 1987 magnetospheric model is employed to study the effects of different levels of magnetospheric activity. The resonance scattering formulation employed assumes an optically thin magnetospheric medium, with full inclusion of Doppler shift effects, for 834-A radiation; Monte Carlo techniques are used for the construction of simulated magnetospheric images.

Instrumental and observational requirements for space-based imaging of magnetospheric emissions

Simulated images of extreme ultraviolet (EUV) emissions from energetic outfiowing ions have been constructed to study techniques for remotely sensing the dynamic behavior of hot plasmas in the near-Earth environment. These calculations include realistic assumptions about the energetic ion outflow from high latitudes and take into account the effects of cold plasmasphenc and ionospheric ions. The energetic ion outflow is determined from a statistical study based on five years of measurements from the Energetic Ion Composition Spectrometer on Dynamics Explorer 1. The simulated images change significantly with viewing geometry and certain spacecraft locations are clearly favorable for observing emissions from energetic ions. For example, for a near equatorial orbit, viewing locations greater than 9 Earth radii are required to observe outfiowing ions above the cold plasmaspheric background. We will discuss other important considerations for magnetospheric imaging including the sensitivity requirements of the detector. In particular, we consider the performance of multi-layer optics for EUV wavelengths.

Imaging the plasmasphere and trough regions in the extreme ultraviolet

EUV images from emissions of O+ (83.4 nm) and He+ (30.4 nm) distributions in the plasmasphere and trough regions are constructed for a satellite at 9 RE. A diffusive equilibrium model is used to describe the density distribution along field lines for ions in the plasmasphere while a kinetic collisionless model is assumed to calculate ion densities in the high latitude regions beyond the plasmapause. In our model of the plasmasphere we assumed that ions move along a static dipole field. Observational data on O+ and He+ densities in the ionosphere are used as boundary conditions to calculate spatial distribution of densities along field lines. Assuming that the day and night boundary conditions are asymmetric and exobase densities vary with latitude we will discuss how this would be reflected in the intensities and structure of the magnetosphere images.

Fabrication and test of a wide-field multilayer mirror for imaging the magnetosphere

We have developed techniques for depositing a uniform multilayer on a highly curved mirror surface. The multilayer was designed for normal incidence reflection of the emission line of He II at a wavelength of 304 angstroms. The mirror was of a design suitable for broad field imaging of the resonantly scattered solar He line from He+ in the earths plasmasphere. A spherical proto-type mirror was chosen for the tests having a radius of curvature of 9.8 cm (focal length of 4.9 cm) and a diameter of 14 cm. The sagitta for this highly curved mirror is 2.8 cm and the angle that the mirror surface makes with its axis varies from 90 deg to 45 deg, center to edge. This poses a challenge to produce a multilayer that is uniform in response over the mirrors surface. For normal incidence reflection of 304 angstroms photons, molybdenum and silicon were chosen as the multilayer materials. A vacuum sputtering process, involving planar magnetrons, was used to fabricate the multilayers. Details of the deposition techniques, results of subsequent testing and ways of further improving the uniformity will be presented.