National Aeronautics and Space Administration

Living With A Star

Targeted Research and Technology

Heating of Ions in the Low-beta Compressible Solar Wind

ROSES ID: NNH18ZDA001N      Selection Year: 2018      

Program Element: Focused Science Topic

Principal Investigator: Xiangrong Fu

Affiliation(s): NMC, Inc.

Project Member(s):
Guo, Fan Co-I Los Alamos National Laboratory
Matthaeus, William H Co-I University of Delaware
Li, Hui Co-I Los Alamos National Laboratory

Summary:

The solar wind is the high speed plasma flow originated from the Sun, carrying magnetic field and energetic particles and propagating throughout the heliosphere. In-situ measurements have shown that solar wind is turbulent and ions are heated, though the heating mechanisms for solar wind ions are still under debate and a subject of active research.

We propose to study the solar wind ion heating in the regime when the turbulent Mach number is high (between 0.1 and 1) and the plasma beta is low (< 0.1), i.e. the low-beta compressible turbulence (LBCT) regime. This regime is particularly relevant in the near-Sun region where the solar wind originates and the magnetic energy density is large, though such conditions can exist throughout the heliosphere. In particular, we will study the critical role of the parametric decay instability (PDI), which converts a large-amplitude forward Alfven wave into a backward Alfven wave and a slow mode. The backward Alfven waves can further interact with forward Alfven waves and produce a plasma turbulence mixed with compressible and incompressible components. Our recent MHD and kinetic simulations show that, in this regime minor ion species undergo efficient heating, with distinctive signatures in both parallel and perpendicular directions. Specifically, we will address three key science questions (SQ):

1) Can PDI explain enhanced density fluctuations at the center of the preferential ion heating zone (10-20 solar radii)?

2) What are the heating rates of compressible and incompressible turbulences on protons and minor ions in the low-beta compressible solar wind?

3) How does the inclusion of ion heating from compressible turbulence improve global modeling of the solar wind?

Because LBCT have been observed in the heliosphere from tens of solar radii to a few AUs, with an increasing occurrence when approaching the Sun, our proposed investigation is very timely to address the long-standing ion heating problem in the solar wind because in-situ measurements in the close-to-Sun region will soon be made available by the Parker Solar Probe (PSP).

We propose to perform local 3D MHD and hybrid simulations to address the problem of solar wind ion heating in the low-beta compressible regime, using turbulence and plasma quantities provided by global MHD simulations. The local MHD simulations will enable us to examine the similarities and differences of properties between compressible turbulence and the well-studied nearly incompressible turbulence. Hybrid simulations will allow us to directly examine the detailed ion heating processes in the low-beta compressible regime. Specifically, we will use the background plasma conditions including magnetic field, plasma density, ion temperature and the solar wind speed in several typical regions: 1 AU, 0.3 AU and 10s R_s (solar radius) and <2R_s, from observations and global MHD simulations. We anticipate to carry out a large set of hybrid simulations to study PDI and its contribution to density fluctuations. With kinetic ions properly modeled, their heating rates by compressible waves and turbulence can be quantified, and empirical models of heat functions will be derived. These studies will enable us to test competing solar wind heating mechanisms which could have different ion heating signatures and provide critical microphysics inputs for the global solar wind evolution models.
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