Research Article

Pluto’s interaction with its space environment: Solar wind, energetic particles, and dust

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Science  18 Mar 2016:
Vol. 351, Issue 6279, aad9045
DOI: 10.1126/science.aad9045
  1. Interaction of the solar wind with Pluto’s extended atmosphere.

    Protons and electrons streaming from the Sun at ~400 km s–1 are slowed and deflected around Pluto because of a combination of ionization of Pluto’s atmosphere and electrical currents induced in Pluto’s ionosphere.

  2. Fig. 1 Geometry of the New Horizons trajectory through the solar wind interaction with Pluto’s atmosphere on DOY 195.

    Along the gray section of the trajectory between points A and B, the New Horizons spacecraft pointed the SWAP instrument’s field of view away from the solar direction. The pink circle shows the extent of Pluto’s atmosphere (2).

  3. Fig. 2 Overview of SWAP data.

    Color spectrogram of coincidence count rate (COIN) as a function of E/q and derived proton flow speed, density, and temperature values for the interval surrounding the Pluto flyby. The vertical dashed green line shows the time of closest approach. Unshaded regions indicate times during which some portion of SWAP’s very broad field of view was pointed within 5° of the Sun direction, and thus SWAP would be able to observe a radially outflowing solar wind. Moments are derived for these times when solar wind–like distributions were observed. Black data points at the start and end of this interval allow linear least-squares fits to these samples of essentially pristine solar wind (nearly horizontal lines), whereas disturbed, higher-temperature solar wind (red points) is evident in the interaction region several hundred RP behind Pluto.

  4. Fig. 3 Overview of PEPSSI data taken near Pluto.

    (A) Measurement of ions with Poisson error bars based on TOF data. Ion speeds inside the instrument correspond to 1 to 4 keV/amu. The colors of the symbols identify the viewing angles relative to the Sun direction (D). Background colors refer to time periods discussed in the text. Dashed lines denote locations where New Horizons had its closest approach to Pluto and was directly antisunward of Pluto and Charon. (B) Energy spectrogram of TOF data, assuming that ions have not been accelerated by the potential but not accounting for energy losses in the foils. (C) Angle of PEPSSI’s sector S0 to the Sun direction, with colors corresponding to those used in (A). (D) Location of New Horizons relative to Pluto and Charon. Black, radial distance to Pluto; blue, distance along the Pluto-Sun line; red, distance to the Pluto-Sun line; orange, distance to the Charon-Sun line.

  5. Fig. 4 Burst of energetic ions.

    Zooming in to the interval shaded pink in Fig. 3A, we show PEPSSI TOF observations from 11:52 to 12:01 UTC on 14 July 2015. (A) Color spectrogram of 6625 single TOF events returned during this period from all PEPSSI sectors. Raw data are acquired once every second and have been binned into 5-s average rates and logarithmically spaced bins in energy per mass. (B) Corresponding total counts per second integrated across all energies. (C) Angle between the direction to the Sun and the normal to the S0 sector. (D) Distance of New Horizons to Pluto and to the Sun-Pluto line during this period.

  6. Fig. 5 Energetic particles along the trajectory of New Horizons.

    The xP axis points away from the Sun. The vertical axis shows ρP, the distance from the xP axis in a plane containing Pluto and the trajectory. Green circles show the locations of Pluto and Charon. Open circles show times in UTC. Pink-filled circles mark the range of the ion enhancement; blue-filled circles delimit the wake. Color along the trajectory shows a PEPSSI count rate (same as in Fig. 3A).

  7. Fig. 6 Events around Pluto.

    (A) The events recorded during ±5 days of the closest approach to Pluto. Gray colored dots indicate noise events identified to be coincident dust hits or a single event coincident with thruster firings. The color code represents the boresight-to-ram angle (b2r), measured between the SDC surface normal and the velocity vector of the spacecraft. SDC’s sensitivity rapidly drops to zero for impact angles >45°; hence, events marked by red and green dots are also noise events. A single detection at a distance of ~3000 RP (11 July 2015), before the closest encounter remains, the only candidate for detecting a Pluto-system dust particle. (B) The amplitude distribution of all the noise events during this period is well fit to a Gaussian curve with an average of 1.1 × 107 e and a 1σ error = 1.5 × 106 e, indicating that our candidate impact event generated a charge that had an amplitude with a 2σ error above the average.

  8. Fig. 7 Dust in the outer solar system.

    The image shows dust density or particles with radii >1.4 μm, as measured by SDC in the outer solar system. The last data point with an error bar shows the data collected since 1 January 2015. The green dot indicates the most likely dust density of 1.2 km–3, on the basis of a single candidate dust event.

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