Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) NASA Mission-of- Opportunity - Overview, Initial Results, and Collaborative Opportunities
TWINS - Two Wide-angle Imaging Neutral-atom Spectrometers is a NASA Explorer Mission-of-Opportunity that is stereoscopically imaging the Earth's magnetosphere for the first time. TWINS simultaneously images the magnetospheric structure in Energetic Neutral Atoms (ENAs) from 1-100 keV with high angular (∼4°x4°) and time (∼1-minute) resolution. The TWINS Ly-α monitor concurrently measures the geocoronal hydrogen density to aid in ENA analysis while environmental sensors provide contemporaneous measurements of the local charged particle environments. By imaging ENAs with identical instruments from two widely spaced, high-altitude, high-inclination spacecraft, TWINS enables three- dimensional visualization of the large-scale structures and dynamics within the magnetosphere for the first time. As of the summer of 2008, both TWINS instruments are operational and providing stereo imaging of the magnetosphere. This talk briefly summarizes the TWINS mission and instruments, summarizes some of the initial results from the mission, and provides a call for future collaborations with theory, modeling, and other in situ observations. More information about TWINS and access to these data are available at http://twins.swri.edu/. * On behalf of the entire TWINS Team
First Stereoscopic Views of the Ring Current From TWINS Energetic Neutral Atoms Imagers
The TWINS (Two Wide-Angle Imaging Neutral-atom Spectrometers) mission is the first mission dedicated to obtaining stereoscopic views of the terrestrial ring current. Two identical instruments onboard two spacecraft measure 1-100 keV ENAs with high angular and time resolution simultaneously from two different vantage points. Despite a generally low level of geomagnetic activity during year 2008, TWINS captured stereoscopic images of the ring current during moderate storms of June 15 2008 and October 11 2008. We use the O/H ratio from TWINS data for these two modest storms to estimate the O+/H+ ratio, which is used as the outer boundary condition for the Comprehensive Ring Current Model (CRCM). We model 3D H+ and O+ fluxes for these storms and reconstruct H and O ENA images. As ENA fluxes were relatively small for those periods, we use the modeled results as a guide to interpret ENA data and derive the time-dependent, 3D structure of ring current during geomagnetic storms near the minimum of solar activity cycle.
Analysis and Validation of Global Ring Current Observations by TWINS
Several moderate storms have been successfully imaged by TWINS (15 June 2008, 11 October 2008, 4 February 2009, 14 February 2009). Here we present the analyses and validations of the global ring current observations by TWINS. By using a forward simulation to retrieve the global equatorial proton distribution, we show that the simultaneous dual vantage point observations by TWINS-1 and 2 are consistent. Simultaneous in-situ measurements by THEMIS reveal that the global measurements by TWINS are both spatially and spectrally consistent.
Ion Intensities from TWINS 1 & 2 ENA Images During Weak CIR Magnetic Storms
The TWINS 1 & 2 energetic neutral atom (ENA) imagers have provided energy-dependent images for a series of weak (-30 to -50 nT minimum Dst) corotating interaction regions (CIRs) magnetic storms. The brightest pixels in the images are typically from low altitude emissions, that are due to charge exchange with neutral oxygen at altitudes of 500 - 1000 km. Mapping these pixels using the Tsyganenko magnetic field model from the ionosphere to the equator suggests that this emission is from plasma sheet particles. Using this and other features of the TWINS instruments along with techniques successfully used for ENA images from the IMAGE mission, we present ion intensities deconvolved from the neutral images. We show spatial and energy distributions of the ring current as a function of time during CIR storms in the summer of 2008.
Monitoring of the Geocoronal Hydrogen Distribution with TWINS-1/2 Lyman-alpha Observations
Two satellites, TWINS-1 and TWINS-2, are presently orbiting the Earth on highly elliptic high-apogee orbits. In addition to energetic neutral atom (ENA) imagers, both satellites carry Lyman-alpha detectors (LAD). These broadband photon-counting sensors measure the line-of-sight integrated Lyman-alpha (121.6 nm) flux of solar photons resonantly scattered by exospheric hydrogen atoms. The primary goal of the LAD measurements is characterization of the asymmetric geocorona and determining the global distribution of atomic hydrogen in the near-Earth region. Charge exchange collisions of energetic ions on these background hydrogen atoms produce ENAs detected by the TWINS ENA imagers. The hydrogen geocorona is optically thin at distances larger than 3 Earth radii, with the measured photon intensities directly related to the line-of-sight column densities of exospheric atomic hydrogen. The observational geometry limits the relative contribution of the interplanetary hydrogen glow, which can be reliably quantified and subtracted. We describe the LAD sensor in detail and present first results.
High Dynamic Pressure and Strong Northward IMF: Ingredients for a New Type of Ring Current on 21-22 January 2005
During the 21-22 January 2005 magnetic storm, a highly unusual ring current developed dominantly during northward IMF and high dynamic pressure with minimum pressure-corrected symH less than -100 nT. Early in the storm, a short (less than 1 hour) interval of strong southward IMF produced a brief depression in sym H which had already recovered when the main phase development of the ring current began under northward IMF and sustained high solar wind dynamic pressure. During the ring current development, a hot dense plasma sheet was observed at geosynchronous orbit followed by a cold dense plasma sheet. The movement of these dense plasma sheets through the inner magnetosphere during the high dynamic pressure interval in the solar wind produced the ring current, which began to decay immediately following the end of the cold dense plasma sheet interval. The cross polar cap potential reached a maximum during the interval of strongest southward IMF early in the storm. However, the equatorward edge of the auroral oval (as indicated by the MBI index) moved to lowest latitudes as the hot high-density plasma sheet stretched out the magnetotail. Precipitation of protons and electrons at ring current energies maximized during the peak of the ring current development under northward IMF conditions. A simulation of this magnetic storm event using the BATS-R-US MHD model with an inner magnetosphere module based on the Rice Convection model produces all the main features of this unusual ring current development and, in addition, shows that the solar wind-magnetosphere coupling was different during the hot compared to the cold high density plasma sheet intervals. We present a comparison between observations and model results and explore the processes responsible for this unusual ring current and its decay.
Plasma Sheet Circulation Pathways
Global simulations of Earth's magnetosphere in the solar wind compute the pathways of plasma circulation
through the plasma sheet. We address the pathways that supply and drain the plasma sheet, by coupling the
LFM global circulation model with Global Ion Kinetic simulations of the outer magnetosphere and the
Comprehensive Ring Current Model of the inner magnetosphere, including plasmaspheric plasmas. We find
that the plasma sheet is supplied with solar wind plasmas via the magnetospheric flanks, most effectively for
northward IMF. For southward IMF, the innermost plasma sheet and ring current region are directly supplied
from the flanks, with an asymmetry of single particle entry favoring the dawn flank. The central plasma sheet
(near midnight) is supplied, as expected, from the lobes and polar cusps, but the near-Earth supply consists
mainly of slowly moving ionospheric outflows for typical conditions. Work with the recently developed multi-fluid
LFM simulation shows transport via plasma "fingers" extending Earthward from the flanks, suggestive of an
interchange instability. We investigate this with solar wind ion trajectories, seeking to understand the fingering
mechanisms and effects on transport rates.