The Onset of Reconnection and Associated Turbulence in Solar Eruptions

ROSES ID: NNH19ZDA001N      Selection Year: 2019      

Program Element: Focused Science Topic

Principal Investigator: Marc Swisdak

Affiliation(s): University of Maryland, College Park

Project Member(s):
Shay, Michael A Co-I/Institutional PI University Of Delaware
Drake, James F Co-I University of Maryland, College Park
Matthaeus, William H Co-I University Of Delaware

Summary:

One of the most important outstanding problems in space science has
been to understand the mechanisms responsible for the onset of
explosive energy release during magnetic reconnection. Simulations
dating back to the GEM Challenge have demonstrated that in an
idealized case -- two-dimensional, laminar, symmetric, anti-parallel
reconnection in a collisionless electron-proton plasma -- reconnection
is mediated by the physics of whistler waves so that onset occurs when
the width of the current layer becomes thinner than the ion inertial
length. When these constraints are relaxed, however, the criteria for
onset become less clear. In the solar corona, for example, collisions
can be important during onset (i.e., the reconnection is not
super-Dreicer). In addition, turbulence can oftentimes both trigger
and disrupt reconnecting current layers to a sufficient degree
(particularly when three-dimensional dynamics are allowed) that the
two processes cannot be independently considered.

In this proposal we will focus on the onset of reconnection and
triggering of flares in the solar corona. The corona represents a
particularly challenging case because of the wide disparity in scales
-- more than ten orders of magnitude lie between kinetic length scales
and the dimensions of flare regions. Moreover, physical processes on
these scales are interconnected since, for example, the compression of
a coronal current sheet by forcing flows at large scales can lead to
turbulence and the onset of reconnection at small scales.

We propose to examine the microscale aspects of this problem through a
mixture of theory and simulation studies, but plan as well to take
advantage of the expertise offered by a Focused Science Team to
connect with the macroscales. We will examine such important
questions as: Does the onset of reconnection in coronal environments
necessarily involve turbulence? How important is the plasma
collisionality? Are there significant differences between
two-dimensional and three-dimensional systems with regards to
reconnection and the associated turbulence? Our simulations will
primarily use particle-in-cell and hybrid codes which will include
collisions self-consistently. The reconnection rate will provide a key
diagnostic to determine if onset occurs and what factors significantly
affect it. The role of turbulence will be studied by analyzing systems
with and without turbulence and noting key differences. In addition,
the statistical properties of the turbulence generated in the system
(e.g., spectra, structure functions) will be determined with an eye
for how they vary between systems and how this may affect reconnection
onset. In addition, the role of secondary reconnection sites generated
by turbulence and how they impact the global process will be
addressed.

This proposal would address a critical portion of the Focused Science
Team's effort by providing theoretical and simulation studies of
reconnection onset criteria and the associated turbulence in the solar
corona. It is anticipated that this proposal will particularly help
the Focused Science Team address the primary goal of ``Establish[ing]
an understanding of what the critical conditions are for the onset of
fast reconnection at a current sheet in the various regimes relevant
for heliophysics'', although the work will also contribute to the
other primary goals.

The PI of the proposal is Dr. Marc Swisdak, who will be responsible
for the overall direction and its integration into a Focused Science
Team. Prof. Drake will be a co-Investigator and will collaborate on
all aspects of the theoretical analysis and particle-in-cell
simulations. Prof. Shay and Prof. Matthaeus will be co-Investigators
based at the University of Delaware. They will collaborate on all
aspects of the theoretical analysis and will be primarily responsible
for the turbulence portions of the proposal.

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