Micromachined deformable mirrors for adaptive optics

A micromachined deformable mirror (µ-DM) for optical wavefront corrections described. Design and manufacturing approaches for µ-DMs are detailed. The µ-DM employs a flexible silicon membrane supported by mechanical attachments to an array of electrostatic parallel plate actuators. Devices are fabricated through surface micromachining using polycrystalline silicon thin films. µ-DM membranes measuring 2 mm x 2 mm x 2 µm, supported by 100 actuators are described. Figures of merit include stroke of 2 µm, resolution of 10 nm, and frequency bandwidth DC - 7 kHz. The devices are compact,inexpensive to fabricate, exhibit no hysteresis, and use only a small fraction of the power required for conventional DMs. Performance of an adaptive optics system using a µ-DM was characterized in a closed-loop control experiment. Significant reduction in quasi-static wavefront phase error was achieved. Advantages and limitations of µ-DMs are described, in relation to conventional adaptive optics systems and to emerging applications of adaptive optics, such as high resolution correction, small aperture systems, and optical communication.

Transient Dayside Precipitation Structures

Auroral arcs that emanate from the poleward edge of the auroral oval and drift poleward are usually considered to be part of ionospheric signature of dayside reconnection. These arcs appear at quasi-regular 5 minute-intervals, and are associated with a double vortex convection pattern and a pair of oppositely directed field-aligned currents. For the most part, these poleward-moving auroral forms are observed when the Interplanetary Magnetic Field (IMF) Bz component is negative. In this talk, we ask what is the signature of reconnection when Bz >0, and By 0. Observations from the Sondrestrom incoherent scatter radar, the Goose Bay HF Radar, the Akebono satellite and DMSP reveal that, in the afternoon sector, E-W elongated arcs are seen to emanate from the auroral poleward boundary and appear to move equatorward. This direction-opposite to the "expected" poleward direction-is interpreted in terms of a westward (i.e. sunward) motion of elongated arcs that make a small angle with respect to the L shell. The arcs are associated with increased poleward velocity and a field-aligned current pair. We conjecture that the arcs are the Northern-Hemisphere footprints of reconnection events occurring in the Southern Hemisphere, where a clear cusp signature is apparent.

The Physics of Collisionless Magnetic Reconnection

Dr. James Drake
Date: 10/7/99
Time and Place: 3:45 in CAS Room 500
Affiliation: University of Maryland

Magnetic reconnection plays a critical role in the dynamics of the magnetosphere, solar corona and laboratory plasma by providing a mechanism for the fast release of stored magnetic energy. Typically magnetic reconnection is a collisionless process, either because collisions are essentially absent (the Earth's magnetosphere) or because the intense electric fields which are generated during reconnection exceed the Dreicer runaway field, rendering collisions irrelevant (solar corona). The rate of magnetic reconnection is controlled by the geometry of the dissipation region, where the ideal MHD description breaks down and the frozen-in condition is violated. Recent Hall MHD, hybrid and full particle simulations have revealed that the reconnection rate, as measured by the inflow rate into the magnetic x-line, is a universal constant ~ 0.1 CA, independent of the mechanism which breaks the frozen-in condition and independent of the macroscopic system scale length L. Thus, the Sweet-Parker theory of reconnection does not apply to collisionless reconnection. The underlying physics for this result has been carefully explored. The dissipation region develops a distinct two-scale structure associated with the decoupling of electron and ion motion at small scales: an outer layer of width C/(pi controlled by the ions and an inner scale length of width C/(pe which is controlled by the electrons. The plasma dynamics at sub-C/(pi scale lengths is controlled by whistler waves. Analytic arguments are presented which demonstrate that it is the quadratic dispersion character of the whistler waves which is the essential ingredient leading both to the insensitivity of the reconnection rate to the mechanism which breaks the frozen-in condition and the voids the traditional Sweet-Parker theory of slow reconnection.

Electron Dynamics at the Dayside Magnetopause

Dr. Georgios Vetoulis
Date: 10/7/99
Time and Place: 3:45 in CAS Room 500
Affiliation: CSP Boston University.

There are strong indications that at the dayside magnetopause there exists wave activity (whistlers) with characteristic length scales on the order of the electron skin depth (c/\omega_{p}). That means that the electron mass is important in this region.  Because of the low collisionality of the plasma in this area, the electron inertia is the only viable mechanism for breaking the "frozen-in" constraint and thus permitting reconnection and
transport processes through the bow shock. We have a reduced model of electron dynamics valid at length scales on the order of a few electron skin depths so that ions may be ignored.  Using numerical simulation, we studied the transport of magnetic flux and current from the solar wind to the magnetosphere and calculated spectra which we compared to published observations.  We describe some of the insights which this model gave us
regarding the dayside magnetopause.

Active Auroras and Plasma Sheet Dynamics

Dr. George Parks
Date: 10/14/99
Time and Place: 3:45 in CAS Room 500
Affiliation: CSP Boston University.

Plasma observations from the different plasma sheet regions during active auroras will be discussed. We will focus on dynamic plasma features and relate them to the different phases of the auroral substorm. We show the plasma distributions in the plasma sheet in general consist of several distinct components, the usual hot isotropic keV component, a low energy(<100 eV) component and counter streaming and unidirectional beams (few keV) streaming parallel and antiparallel to the magnetic field direction. The electron distributions at the same time often show bidirecitonal anisotropic distributions, including weak field-aligned beams that may be accelerated in the current sheet. These features are associated with rapid magnetic field variations, with increases of the Bz component, bursty bulk flows and injection and acceleration of electrons to ionospheric and local plasma sheet sources, behave in a systematic way and they characterize both pseudo and normal auroral expansion events. Our observations paint a very different picture of the plasma sheet dynamics than the picture derived from the MHD fluid physics.

How the Sun controls Nitric Oxide in the Thermosphere

Dr. Charles Barth
Date: 10/21/99
Time and Place: 3:45 in CAS Room 500
Affiliation: Laboratory for Atmospheric and Space Science, University of Colorado

Nitric Oxide in the Earth's thermosphere is produced by the interaction of solar extreme ultraviolet and x-radiation with the neutral constituents of the thermosphere, molecular nitrogen, atomic oxygen, and molecular oxygen. Since the irradiance of solar x-rays is highly variable, the nitric oxide density in the lower thermosphere (110 km) also varies. The variability of thermospheric nitric oxide at low latitudes, particularly in the tropics, is controlled by the variability of the solar soft x-rays (2-10 nm). Nitric oxide is produced at high latitudes by the precipitation of electrons with energies in the range of 1-10 keV. These are the same electrons that produce the visible and ultraviolet aurora near 65 degrees north and south geomagnetic latitudes. The magnitude of the flux of precipitating electrons is controlled through the interaction of the solar wind with the Earth's magnetosphere. The density of nitric oxide in auroral zone reflects the complexity of this interaction by showing high variability. Global images of the nitric oxide density distribution in the lower thermosphere may be used to show the energy input into the lower thermosphere from solar x-rays and from auroral electron precipitation.

Jupiter's Rings: Signs of the Source

Professor Joseph Burns
Date: 10/25/99
Time and Place: 3:45 in CAS Room 500
Affiliation: Cornell University

New Views of Jupiter's Rings: Jupiter's rings are the archetype for ethereal planetary rings, very-low optical-depth bands containing micron-sized "dust" that encircle all the giant planets. From observations by the Galileo spacecraft and Keck 10-m telescope, we now understand this ring system: its sources and sinks, as well as the processes that cause its odd shape. The Jovian rings have three components: a vertically thick toroidal halo (1.4-1.7 RJ), a thin main ring (1.7-1.8 RJ) with the tiny moonlet Adrastea at its edge, and a pair of exterior gossamer rings (1.8-3.5RJ). Each of the gossamer rings is bounded closely by the orbit of a small satellite and has a half-thickness that closely matches the maximum vertical excursion of the satellite off Jupiter's equatorial plane. Abrupt drop-offs in brightness are seen where the gossamer rings end in both forward-scattered (Galileo) and back-scattered (Keck) light; the bands' top & bottom edges have extra brightness. Because particle lifetimes are brief, the rings must be continually regenerated by meteoroid impacts into sources. The gossamer rings are collisional ejecta derived from the ring-moons Amalthea and Thebe, then driven inward by Poynting-Robertson drag. The main ring is probably debris from Adrastea and Metis, which orbit in the equatorial plane. The halo particles are driven vertically by resonances with Jupiter's rotating magnetic field. When halo orbits become highly distorted, particles are lost into Jupiter.

Equatorial Spread-F: Cause and Effects

Dr. Santimay Basu
Date: 11/2/99
Time and Place: 3:45 in CAS Room 500
Affiliation: Naval Research Laboratory

The equatorial spread-F (ESF) or the turbulent state of the equatorial F-region is discussed. It is shown that ESF is typically a post-sunset phenomenon and it evolves in the presence of a complex set of stabilizing and destabilizing forces. The horizontal scale lengths of electron density irregularities in equatorial spread-F spans more than five decades, typically between tens of centimeters to several hundred kilometers. The thermosphere and the ionosphere internally control the generation of the irregularities and its forcing by solar transients is an additional modulating factor. The spread-F irregularities scatter radio waves to cause amplitude and phase scintillation and affect satellite communication and navigation systems. The development of global specification and forecast systems for scintillation is needed in view of our increasing reliance on space based communication and navigation systems that are vulnerable to ionospheric scintillation. Such systems are discussed in the context of the international space weather program.

Turbulent Boundary Layer and Outer Cusp: Particle Acceleration and Diffusion

Dr. Sergei Savin
Date: 11/18/99
Time and Place: 3:45 in CAS Room 500
Affiliation: Space Research Institute, Moscow, Russia

Upper Atmospheric Physics and the Meteoric Metal Layers

Prof. Chester S. Gardner
Date: 9/23/99
Time and Place: 3:45 in CAS Room 500
Affiliation: Universtiy of Illinois at Urbana-Champaign

Thermospheric Limb Imaging with WINDII: the Wind Imaging Interferometer

Prof. Gordon G. Shepherd
Date: 12/2/99
Time and Place: 3:45 in CAS Room 500
Affiliation: Centre for Research in Earth and Space Science York University, Toronto, Canada