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Title: Turbulent Acceleration of Interstellar Pickup Ions at the Heliospheric Termination Shock Forms the Global ENA Spectrum

Journal Article · · The Astrophysical Journal. Letters
ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [3]; ORCiD logo [1]
  1. Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States)
  2. Southwest Research Institute, San Antonio, TX (United States); Univ. of Texas at San Antonio, TX (United States)
  3. Univ. of Waikato, Hamilton (New Zealand)
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The heliospheric energetic neutral atom spectrum observed by the Interstellar Boundary Explorer (IBEX) reveals that the heliosheath proton distribution is consistent with a power law. The origin of the spectrum is likely from interstellar pickup ions (PUIs) accelerated at the heliospheric termination shock (HTS). We present an explanation of the proton spectrum origin using a test particle simulation of PUIs accelerated at the HTS. PUIs experience preferential heating by the motional electric field in the shock foot, but do not develop a power-law tail without the presence of turbulence at wavenumbers (k) close to the PUI gyroradius scale (Rg). Voyager 2 observations of the magnetic field downstream of the HTS indicate a moderate amount of turbulence at kRg ≅ 1, (δB /B0)2 @ 0.01, which we find to be sufficient for producing a downstream suprathermal PUI tail but not at intensities observed by IBEX. Within the shock ramp, however, Voyager observed the turbulence power at much smaller scales to be nearly 100 times stronger, suggesting the possibility of strong turbulence at the PUI gyroradius scale. We show that a proton distribution can develop a power law downstream of the HTS consistent with IBEX observations if (δB /B0)2 ≳ 0.1 at kRg ≅ 1 in the shock foot. Shock drift acceleration of PUIs by the motional electric field is aided by interactions with turbulence upstream of the shock overshoot. Steepening of the IBEX proton spectrum in directions farther from the heliospheric nose suggests the HTS compression ratio and/or turbulence power weakens near the heliotail.

Research Organization:
Princeton Plasma Physics Lab. (PPPL), Princeton, NJ (United States); Southwest Research Institute, San Antonio, TX (United States); Univ. of Texas at San Antonio, TX (United States); Univ. of Waikato, Hamilton (New Zealand)
Sponsoring Organization:
National Aeronautics and Space Administration (NASA)
Grant/Contract Number:
AC02-09CH11466; 80MSSC20K0719; 80GSFC19C0027: 80NSSC20K0783
OSTI ID:
1814205
Journal Information:
The Astrophysical Journal. Letters, Vol. 916, Issue 2; ISSN 2041-8205
Publisher:
IOP PublishingCopyright Statement
Country of Publication:
United States
Language:
English

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Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
heliosheath
pickup ions
solar wind
termination shock
space plasma
interplanetary turbulence
interplanetary particle acceleration