Particle Energization

Scientific Investigation 4: Identify and advance understanding of particle injection and acceleration processes near the Sun, in the heliosphere and heliosheath. 

To discover the origins of accelerated particles near the Sun, in the heliosphere and heliosheath on IMAP will make: a) high-time-resolution observations of SW protons, suprathermal proton tails, and accelerated protons; b) energy coverage of heavy ions (He-Fe) for resolving energetic particle source populations and injection into acceleration with adequate energy resolution to resolve energy spectral features and slopes within suprathermal tails; c) angular distributions for PUI ring, shell distributions and scattering, and angle-averaged energy distributions for interstellar PUIs throughout the year and through the gravitational focusing cone for independent interstellar He flow direction derivation from PUIs, complementing; d) anisotropies for protons, He, and electrons in directions sunward, anti-sunward, and perpendicular to the magnetic field to identify transport effects in remote acceleration events; and e) ion composition for identifying seed populations and distinguishing impulsive and gradual SEPs. 

L1 Science

Our suite of instruments on IMAP is ideally suited for comprehensive L1 science and real-time SW observations. These include SW ion, heavy ion and electron instruments, time-resolved suprathermal ions and compositionally resolved pickup ions, solar energetic particle instruments, and magnetometer measurements.

These observations represent the complete in situ measurements to discover the origin of suprathermal tails from particle acceleration. The observational timescale extends into the dissipation range of turbulence and is needed to separate discrete episodic events in contrast to continuously active processes in the production of suprathermal tails.

The suprathermal observations required for IMAP must also be made in conjunction with complete SW measurements to resolve the relationship between accelerated PUIs and suprathermal particles with SW structures; fast, slow, and transitional flows; turbulence; transients; and coronal mass ejections. The necessary SW observations on IMAP include SW heavy ion composition measurements, charge-state measurements, SW ion and electron moments, and magnetic field observations to resolve shocks and turbulence. 

IMAP will continuously observe ENA energy spectra covering the energy range of particle injection into diffusive shock acceleration and stochastic particle acceleration from the outer heliosphere. By examining the variability of these ENA energy spectra as a function of location within the heliosheath (nose, flanks, tail, poles, latitude structure), IMAP provides global measurements to explore how pickup ions and suprathermal ions are injected and accelerated across the termination shock, within the heliosheath, and in the Ribbon and Belt. 

The populations observed at 1 AU including PUIs from interstellar neutrals, suprathermal ions, and energetic particles are also present in the heliosheath globally distributed flux and likely in the Ribbon and Belt, suggesting that the physical understanding of particle acceleration and suprathermal ions at 1 AU should feed directly into our understanding of these processes active in the outer heliosphere and LISM. It must be emphasized, however, that the SW at 1 AU is quite different from the regimes of the outer heliosphere. For example, PUIs become the dominant source of internal pressure in the outer heliosphere and therefore participate in the heliosheath dynamics to a much larger extent there than at 1 AU. Thus, a central opportunity with IMAP is to develop the physical understanding of suprathermal particles so that it can be applied across the broad regimes from 1 AU out through the heliosphere. 

SWAP Observations

NH/SWAP spectrum (inset) and radial trends from the literature. Observations (thick lines) from 22 to 38 AU and extrapolated radial trends (thin lines) of the SW dynamic pressure (black) and SW thermal pressure (blue) as well as the pickup H+ pressure (red). Magnetic pressure (green dashed) and another measure of SW particle pressure (blue dashed) are taken from Voyager. Additional measurements of the pickup ion pressure from the Ulysses SWICS instrument out to 5 AU and inferred values from pressure balance structures are plotted in red.

Lastly, IMAP will “connect the dots” between 1 AU in situ observations of particle acceleration and pickup ions with the populations observed in the distant SW (but still inside the termination shock). This connection is essential because these populations feed into the termination shock, emerging downstream in the inner heliosheath as the shocked, heated, and slowed populations that become pickup ions and suprathermal ENAs observed directly by IMAP. Closing the observational triad between 1 AU (IMAP in situ data), the outer heliosphere supersonic SW (from complementary New Horizons plasma data), and the heated and slowed plasma of the inner heliosheath (from IMAP ENA data) is essential to fully resolving and connecting the origin of particle acceleration with global heliospheric properties and evolution.