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Register a new userThe Search for MeV Electrons 2-45 AU from the Sun with the Alice Instrument Microchannel Plate Detector Aboard New Horizons
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The Alice UV spectrograph aboard NASAs New Horizons mission is sensitive to MeV electrons that penetrate the instruments thin aluminum housing and interact with its microchannel plate detector. We have searched for penetrating electrons at heliocentric distance of 2-45 AU, finding no evidence of discrete events outside of the Jovian magnetosphere. However, we do find a gradual long-term increase in the Alice instruments global dark count rate at a rate of ~1.5% per year, which may be caused by a heightened gamma-ray background from aging of the spacecrafts radioisotope thermoelectric generator fuel. If this hypothesis is correct, then the Alice instruments global dark count rate should flatten and then decrease over the next 5-10 years.
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In early 2007, the New Horizons spacecraft flew through the Jovian magnetosphere on the dusk side. Here, we present results from a novel means of detecting energetic electrons along New Horizons trajectory: the background count rate of the Alice ultraviolet spectrograph. Electrons with energies >1 MeV can penetrate the thin aluminum housing of Alice, interact with the microchannel plate detector, and produce a count that is indistinguishable from an FUV photon. We present Alice data, proportional to the MeV electron flux, from an 11-day period centered on the spacecrafts closest approach to Jupiter, and compare it to electron data from the PEPSSI instrument. We find that a solar wind compression event passed over the spacecraft just prior to it entering the Jovian magnetosphere. Subsequently, the magnetopause boundary was detected at a distance of 67 R_J suggesting a compressed magnetospheric configuration. Three days later, when the spacecraft was 35-90 R_J downstream of Jupiter, New Horizons observed a series of 15 current sheet crossings, all of which occurred significantly northward of model predictions implying solar wind influence over the middle and outer Jovian magnetosphere, even to radial distances as small as ~35 R_J. In addition, we find the Jovian current sheet, which had a half-thickness of at least 7.4 R_J between 1930 and 2100 LT abruptly thinned to a thickness of ~3.4 R_J around 2200 LT.
The Solar Wind Around Pluto (SWAP) instrument on New Horizons will measure the interaction between the solar wind and ions created by atmospheric loss from Pluto. These measurements provide a characterization of the total loss rate and allow us to examine the complex plasma interactions at Pluto for the first time. Constrained to fit within minimal resources, SWAP is optimized to make plasma-ion measurements at all rotation angles as the New Horizons spacecraft scans to image Pluto and Charon during the flyby. In order to meet these unique requirements, we combined a cylindrically symmetric retarding potential analyzer (RPA) with small deflectors, a top-hat analyzer, and a redundant/coincidence detection scheme. This configuration allows for highly sensitive measurements and a controllable energy passband at all scan angles of the spacecraft.
The New Horizons ALICE instrument is a lightweight (4.4 kg), low-power (4.4 Watt) imaging spectrograph aboard the New Horizons mission to Pluto/Charon and the Kuiper Belt. Its primary job is to determine the relative abundances of various species in Plutos atmosphere. ALICE will also be used to search for an atmosphere around Plutos moon, Charon, as well as the Kuiper Belt Objects (KBOs) that New Horizons hopes to fly by after Pluto-Charon, and it will make UV surface reflectivity measurements of all of these bodies as well. The instrument incorporates an off-axis telescope feeding a Rowland-circle spectrograph with a 520-1870 angstroms spectral passband, a spectral point spread function of 3-6 angstroms FWHM, and an instantaneous spatial field-of-view that is 6 degrees long. Different input apertures that feed the telescope allow for both airglow and solar occultation observations during the mission. The focal plane detector is an imaging microchannel plate (MCP) double delay-line detector with dual solar-blind opaque photocathodes (KBr and CsI) and a focal surface that matches the instruments 15-cm diameter Rowland-circle. In what follows, we describe the instrument in greater detail, including descriptions of its ground calibration and initial in flight performance.
The LOng Range Reconnaissance Imager (LORRI) is a panchromatic (360--910 nm), narrow-angle (field of view = 0.29 deg), high spatial resolution (pixel scale = 1.02 arcsec) visible light imager used on NASAs New Horizons (NH) mission for both science observations and optical navigation. Calibration observations began several months after the NH launch on 2006 January 19 and have been repeated annually throughout the course of the mission, which is ongoing. This paper describes the in-flight LORRI calibration measurements, and the results derived from our analysis of the calibration data. LORRI has been remarkably stable over time with no detectable changes (at the 1% level) in sensitivity or optical performance since launch. By employing 4 by 4 re-binning of the CCD pixels during read out, a special spacecraft tracking mode, exposure times of 30 sec, and co-addition of approximately 100 images, LORRI can detect unresolved targets down to V = 22 (SNR=5). LORRI images have an instantaneous dynamic range of 3500, which combined with exposure time control ranging from 0ms to 64,967 ms in 1ms steps supports high resolution, high sensitivity imaging of planetary targets spanning heliocentric distances from Jupiter to deep in the Kuiper belt, enabling a wide variety of scientific investigations. We describe here how to transform LORRI images from raw (engineering) units into scientific (calibrated) units for both resolved and unresolved targets. We also describe various instrumental artifacts that could affect the interpretation of LORRI images under some observing circumstances.
We evaluate the modulation of Cosmic Microwave Background (CMB) polarization using a rapidly-rotating, half-wave plate (HWP) on the Atacama B-Mode Search (ABS). After demodulating the time-ordered-data (TOD), we find a significant reduction of atmospheric fluctuations. The demodulated TOD is stable on time scales of 500-1000 seconds, corresponding to frequencies of 1-2 mHz. This facilitates recovery of cosmological information at large angular scales, which are typically available only from balloon-borne or satellite experiments. This technique also achieves a sensitive measurement of celestial polarization without differencing the TOD of paired detectors sensitive to two orthogonal linear polarizations. This is the first demonstration of the ability to remove atmospheric contamination at these levels from a ground-based platform using a rapidly-rotating HWP.
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