[Paper Review] On Competing Models of Coronal Heating and Solar Wind Acceleration: The Debate in '08

Steven R. Cranmer|ArXiv.org|Apr 18, 2008

Solar and Space Plasma Dynamics84 references3 citations

TL;DR

This paper evaluates two competing models for solar wind acceleration: wave/turbulence-driven (WTD) and reconnection/loop-opening (RLO). WTD models attribute heating and acceleration to Alfvén waves generated by photospheric motions, while RLO models propose energy transfer via magnetic reconnection between closed loops and open flux tubes. The key finding is that both models can reproduce observed fast and slow wind streams, suggesting a combined mechanism may be necessary to fully explain solar wind properties, including charge state ratios and FIP effects.

ABSTRACT

In preparation for lively debate at the May 2008 SPD/AGU Meeting in Fort Lauderdale, this document attempts to briefly lay out my own view of the evolving controversy over how the solar wind is accelerated. It is still unknown to what extent the solar wind is fed by flux tubes that remain open (and are energized by footpoint-driven wavelike fluctuations), and to what extent much of the mass and energy is input more intermittently from closed loops into the open-field regions. It may turn out that a combination of the two ideas is needed to explain the full range of observed solar wind phenomena.

Motivation & Objective

• To evaluate the viability of wave/turbulence-driven (WTD) and reconnection/loop-opening (RLO) models in explaining solar wind acceleration and coronal heating.
• To assess whether WTD or RLO models better account for observed differences between fast and slow solar wind streams.
• To determine if both models can reproduce in situ measurements of ion charge states and FIP effects in the solar wind.
• To explore whether a combination of WTD and RLO mechanisms is required to fully explain the range of solar wind phenomena.
• To identify limitations in current models and highlight the need for more comprehensive, multi-scale simulations.

Proposed method

• Uses 2D MHD models to simulate coronal holes and streamers at solar minimum, varying magnetic field properties and expansion factors.
• Applies Alfvén wave propagation and dissipation models to simulate energy deposition in the corona and solar wind.
• Incorporates Laming’s (2004) theory of preferential wave-pressure acceleration to model elemental fractionation and FIP effects.
• Varies the coronal expansion factor and photospheric wave properties to test their impact on wind speed, density, and temperature profiles.
• Compares model outputs with in situ measurements of O⁷⁺/O⁶⁺ ratios and Fe/O abundance ratios from Ulysses and other missions.
• Analyzes feedback from higher coronal regions on lower boundary conditions to assess self-consistency across flux tubes.

Experimental results

Research questions

• RQ1 Can wave/turbulence-driven models reproduce the observed latitudinal bifurcation of fast and slow solar wind streams?
• RQ2 To what extent can reconnection/loop-opening models explain the differences in ion charge states and FIP effects between fast and slow wind?
• RQ3 Do both WTD and RLO models produce consistent results with in situ observations of wind speed, density, and temperature?
• RQ4 Can variations in coronal expansion factor and wave properties in WTD models account for the observed range of solar wind conditions?
• RQ5 Is a hybrid mechanism combining WTD and RLO processes necessary to fully explain the solar wind's physical properties?

Key findings

• The WTD model successfully reproduces the latitudinal bifurcation of fast and slow solar wind streams observed by Ulysses, using only variations in coronal expansion factor.
• A larger coronal expansion factor leads to slower, denser wind, higher base temperature, and reduced Alfvén wave intensity at 1 AU.
• The WTD model produces a factor of ~30 higher O⁷⁺/O⁶⁺ ratio in slow wind streams compared to fast streams, matching in situ observations despite lower 1 AU temperatures.
• When combined with Laming’s (2004) wave-pressure acceleration theory, the WTD model reproduces the enhanced low-FIP element abundance in slow wind streams.
• Self-consistent solutions show that feedback from higher coronal heating and expansion rates alters lower atmospheric conditions, even with identical photospheric wave inputs.
• Both WTD and RLO models can reproduce key observational trends, suggesting that a combination of both mechanisms may be required for a complete explanation of solar wind acceleration.