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Register a new userMeV electrons detected by the Alice UV spectrograph during the New Horizons flyby of Jupiter
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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.
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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.
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.
As a star spins-down during the main sequence, its wind properties are affected. In this work, we investigate how the Earths magnetosphere has responded to the change in the solar wind. Earths magnetosphere is simulated using 3D magnetohydrodynamic models that incorporate the evolving local properties of the solar wind. The solar wind, on the other hand, is modelled in 1.5D for a range of rotation rates Omega from 50 to 0.8 times the present-day solar rotation (Omega_sun). Our solar wind model uses empirical values for magnetic field strengths, base temperature and density, which are derived from observations of solar-like stars. We find that for rotation rates ~10 Omega_sun, Earths magnetosphere was substantially smaller than it is today, exhibiting a strong bow shock. As the sun spins down, the magnetopause standoff distance varies with Omega^{-0.27} for higher rotation rates (early ages, > 1.4 Omega_sun), and with Omega^{-2.04} for lower rotation rates (older ages, < 1.4 Omega_sun). This break is a result of the empirical properties adopted for the solar wind evolution. We also see a linear relationship between magnetopause distance and the thickness of the shock on the subsolar line for the majority of the evolution (< 10 Omega_sun). It is possible that a young fast rotating Sun would have had rotation rates as high as 30 to 50 Omega_sun. In these speculative scenarios, at 30 Omega_sun, a weak shock would have been formed, but for 50 Omega_sun, we find that no bow shock could be present around Earths magnetosphere. This implies that with the Sun continuing to spin down, a strong shock would have developed around our planet, and remained for most of the duration of the solar main sequence.
We present results from a multi-chord Pluto stellar occultation observed on 29 June 2015 from New Zealand and Australia. This occurred only two weeks before the NASA New Horizons flyby of the Pluto system and serves as a useful comparison between ground-based and space results. We find that Plutos atmosphere is still expanding, with a significant pressure increase of 5$pm$2% since 2013 and a factor of almost three since 1988. This trend rules out, as of today, an atmospheric collapse associated with Plutos recession from the Sun. A central flash, a rare occurrence, was observed from several sites in New Zealand. The flash shape and amplitude are compatible with a spherical and transparent atmospheric layer of roughly 3~km in thickness whose base lies at about 4~km above Plutos surface, and where an average thermal gradient of about 5 K~km$^{-1}$ prevails. We discuss the possibility that small departures between the observed and modeled flash are caused by local topographic features (mountains) along Plutos limb that block the stellar light. Finally, using two possible temperature profiles, and extrapolating our pressure profile from our deepest accessible level down to the surface, we obtain a possible range of 11.9-13.7~$mu$bar for the surface pressure.
Observations made during the New Horizons flyby provide a detailed snapshot of the current state of Plutos atmosphere. While the lower atmosphere (at altitudes <200 km) is consistent with ground-based stellar occultations, the upper atmosphere is much colder and more compact than indicated by pre-encounter models. Molecular nitrogen (N$_2$) dominates the atmosphere (at altitudes <1800 km or so), while methane (CH$_4$), acetylene (C$_2$H$_2$), ethylene (C$_2$H$_4$), and ethane (C$_2$H$_6$) are abundant minor species, and likely feed the production of an extensive haze which encompasses Pluto. The cold upper atmosphere shuts off the anticipated enhanced-Jeans, hydrodynamic-like escape of Plutos atmosphere to space. It is unclear whether the current state of Plutos atmosphere is representative of its average state--over seasonal or geologic time scales.
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