[Paper Review] Extended Coronal Heating and Solar Wind Acceleration Over the Solar Cycle

Steven R. Cranmer, John L. Kohl+2 more|arXiv (Cornell University)|Feb 1, 2010

Solar and Space Plasma Dynamics1 references3 citations

TL;DR

This paper investigates coronal heating and solar wind acceleration across the solar cycle using UVCS/SOHO observations and theoretical models. It demonstrates that reduced magnetic fields during the 2007–2009 solar minimum led to a 20% decrease in solar wind density and 10–14% drop in temperature at 1 AU, with models successfully predicting these changes using Alfvén wave and MHD turbulence mechanisms without free parameters.

ABSTRACT

This paper reviews our growing understanding of the physics behind coronal heating (in open-field regions) and the acceleration of the solar wind. Many new insights have come from the last solar cycle's worth of observations and theoretical work. Measurements of the plasma properties in the extended corona, where the primary solar wind acceleration occurs, have been key to discriminating between competing theories. We describe how UVCS/SOHO measurements of coronal holes and streamers over the last 14 years have provided clues about the detailed kinetic processes that energize both fast and slow wind regions. We also present a brief survey of current ideas involving the coronal source regions of fast and slow wind streams, and how these change over the solar cycle. These source regions are discussed in the context of recent theoretical models (based on Alfven waves and MHD turbulence) that have begun to successfully predict both the heating and acceleration in fast and slow wind regions with essentially no free parameters. Some new results regarding these models - including a quantitative prediction of the lower density and temperature at 1 AU seen during the present solar minimum in comparison to the prior minimum - are also shown.

Motivation & Objective

• To understand the physical mechanisms driving coronal heating and solar wind acceleration in open-field regions.
• To resolve discrepancies between theoretical models and in situ observations by leveraging extended corona measurements from UVCS/SOHO.
• To compare solar minimum conditions from 1996–1997 and 2007–2009 to test model predictions of wind properties.
• To assess the role of Alfvén waves and MHD turbulence in heating and accelerating the solar wind with minimal free parameters.
• To improve observational constraints on plasma parameters (temperature, density, velocity) across different ion species in coronal holes and streamers.

Proposed method

• Utilized 14 years of UVCS/SOHO remote-sensing measurements of coronal holes and streamers to track plasma evolution in the extended corona (1.5–10 R☉).
• Applied theoretical models based on wave turbulence dissipation (WTD) and resonant linear excitation (RLO) to simulate solar wind heating and acceleration.
• Combined in situ Ulysses data with model outputs to validate predictions of wind speed, density, and temperature changes across solar minima.
• Used semi-empirical and theoretical models of the lower solar atmosphere to constrain boundary conditions for coronal heating processes.
• Incorporated multi-species temperature and density measurements (H I Lyα, O VI) to assess electron and ion temperature decoupling.
• Quantified changes in plasma parameters between solar minima by comparing observed and modeled radial profiles of temperature and density.

Experimental results

Research questions

• RQ1 How do coronal heating and solar wind acceleration mechanisms differ between the 1996–1997 and 2007–2009 solar minima?
• RQ2 To what extent can Alfvén wave and MHD turbulence models predict observed changes in solar wind density and temperature without free parameters?
• RQ3 What is the role of coronal hole morphology and magnetic field strength in shaping the extended corona and solar wind properties?
• RQ4 How do variations in the streamer belt's latitudinal extent affect solar wind source regions and plasma outflow?
• RQ5 What observational constraints are needed to distinguish between competing heating mechanisms in collisionless coronal plasma?

Key findings

• The 2007–2009 solar minimum featured polar coronal holes that were 15% smaller in area and had 40% weaker mean photospheric magnetic fields compared to 1996–1997.
• Despite weaker coronal magnetic fields, the solar wind speed at 1 AU changed by only –3% (Ulysses data) and +1% (model output), indicating robust acceleration mechanisms.
• Solar wind density decreased by 17% (observations) and 22% (model) from 1996–1997 to 2007–2009, with temperature dropping by 14% (observed) and 8% (modeled).
• Gas pressure and dynamic pressure decreased by 20–30% across both observations and models, consistent with reduced heating and lower plasma input.
• Theoretical models successfully predicted the lower electron temperature and slightly reduced density in the low corona during the 2007–2009 minimum, matching preliminary UVCS spectral line trends.
• UVCS data suggest higher H I Lyα and lower O VI intensities in 2007–2009, consistent with reduced electron temperature and density, though full spectral modeling is needed for confirmation.