Organizing disparate communities to address the challenges of Solar Flare Energy Release: the SolFER model

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J. F. Drake – University of Maryland

The SolFER DRIVE Science Center: vision statement

• Our goal is a transformative advance in the understanding of explosive magnetic energy release and energetic particle production in the solar corona across a range of scales, from major eruptions that significantly impact the Earth space environment to small events that may play a role in coronal heating.

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SPHERE 2022 Workshop

7/13/2022

Some of the challenges to understanding flare energy release: the measurements

• No in situ measurements
• Cadence of EUV measurements is sometimes inadequate to directly deduce the Alfven speed
• Electron spectra reasonably well constrained but inadequate dynamic range in Bremsstrahlung measurements to separate footpoint from coronal emission – EOVSA gyrosynchrotron measurements a game changer
• Energetic ion spectra measurements limited to greater than an MeV – how much energy is really going into the ions?
• Evidence of turbulence from non-thermal line broadening no detailed spectral information
• Typically have complex magnetic geometry with no direct measurement of the coronal magnetic field
• Some direct measurements from EOVSA now and DKIST in the future

SPHERE 2022 Workshop

7/13/2022

Some of the challenges to understanding flare energy release: theory and modeling

• Enormous separation between dissipation and kinetic scales and the global system scale means that simulations are highly unrealistic
• Global particle-in-cell (PIC) simulations are infeasible since they have to resolve the Debye length (around 0.1cm)
• PIC simulations don’t reveal the extended powerlaw spectra seen in the data
• Particles become demagnetized as they gain energy so energy gain is inhibited
• MHD simulations are also highly unrealistic
• The Lundquist number, which is the ratio of the resistive to the Alfven time, is typically around 10¹⁵ while simulations are typically limited to 10⁶
• A real flaring systems is likely to be much more turbulent than in simulations
• Energetic particles in flares are dynamically significant so their absence in MHD models is a significant deficiency

SPHERE 2022 Workshop

7/13/2022

Achieving SolFER goals through a diverse scientific team

• Essential to accomplish SolFER scientific goals was to assemble a diverse scientific team
• Establish collaborations between outstanding scientists who normally don’t work together
• Observers with theorists/modelers
• Solar physicists with those doing measurements in the solar wind with Parker Solar Probe, Solar Orbiter, …
• Solar physicists with those exploring energetic events in the magnetotail (MMS)

SPHERE 2022 Workshop

7/13/2022

Fostering scientific discussion and collaboration with topical working groups

• Establish working groups on key scientific questions
• Six working groups established to facilitate interaction of scientists across the country and world
• Co-led by an observer and a theorist/modeler
• Working groups typically met bi-weekly

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SPHERE 2022 Workshop

7/13/2022

SolFER DRIVE Science Center: scientific working groups

• What mechanisms facilitate the fast release of magnetic energy in impulsive flares? (leads: L. Glesener, M. Swisdak)
• What controls the onset of flare energy release? (leads: S. Antiochos, J. Qiu)
• Why and how do flares transfer a large fraction of released magnetic energy into energetic electrons? (leads: B. Chen, F. Guo)
• What mechanism drives energization of ions and the measured abundance enhancements of some species during impulsive flares? (leads: C. Cohen, W. Daughton)
• What mechanisms control energetic particle transport in flares? (leads: D. Gary, M. Oka)
• How does reconnection heat plasma in flares and the small events (nanoflares) that may be responsible for heating the corona? (leads: T. Phan, K. Reeves)

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SPHERE 2022 Workshop

7/13/2022

Exploring reconnection-driven energetic events with in situ observations

• Explore the physics of reconnection-driven particle energy gain in areas where in situ measurements are possible
• Energetic events in the Earth’s magnetotail with MMS data (Ergun+ ‘18, ‘20)
• Energetic events in the near solar environment with Parker Solar Probe (PSP) and Solar Orbiter (SO) (Desai+ ‘22. Phan+ ‘22}

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SPHERE 2022 Workshop

7/13/2022

MMS energetic particle measurements

• MMS July 26, 2017, magnetotail reconnection event (Ergun+ 2018, 2020)
• Ions and electrons with energies above 100keV
• Strong turbulence
• The available magnetic energy per particle during reconnection is given by W₀ = m_iC_Ar²

SPHERE 2022 Workshop

7/13/2022

W₀ ~ 24keV

Characteristic electron and ion spectra

• Both electrons and protons exhibit well-defined powerlaw spectra as well as hot thermal components
• Hot thermal component a fraction of W₀
• Powerlaw spectra of both species extend down to just above W₀

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SPHERE 2022 Workshop

7/13/2022

Measuring ejecta from impulsive events in the near solar environment

• The PSP will soon be within around 10R_s at perihelion
• In situ measurements of flare ejecta will be possible, including the complete proton and alpha spectra
• A key measurement goal will be to determine the low energy limit of the proton powerlaw in flares
• Will it be as low as 10’s of keV as expected from modeling?

SPHERE 2022 Workshop

7/13/2022

The bursty solar wind from coronal holes is linked to interchange reconnection in the low corona

• Close to the sun the solar wind becomes bursty with a periodicity linked to the surface mixed polarity magnetic field
• Points to interchange reconnection as the driver of the fast wind
• Energetic protons and alphas above 100keV

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SPHERE 2022 Workshop

7/13/2022

Bale+ ‘22

Comparison of simulation spectra with SPANi and IOSIS measurements

• Powerlaw slopes from simulation and data from SPANi and ISOIS match
• Matching the lower edge of the powerlaw in SPANi protons to the simulation yields
• Full ion spectra from impulsive flare events will come!

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SPHERE 2022 Workshop

7/13/2022

Bale+ ‘22

Tools for modeling particle energization in macroscale flaring systems

• PIC model are limited to domain sizes of around 100m
• In adequate separation of the kinetic from the macroscale leads to demagnetization of particles as they gain energy
• Sharply slows particle energy gain
• Prevents the formation of the extended powerlaw spectra seen in observations
• The kglobal model mixes an MHD and particle description of the reconnection dynamics (Drake+ ‘19, Arnold+ ‘19)
• All kinetic scales are ordered out of the equations but the large-scale dynamics of growing and merging magnetic islands necessary for efficient particle acceleration is retained

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SPHERE 2022 Workshop

7/13/2022

Electron energy spectra

• Electron powerlaw spectra from kglobal -- it works!
• Powerlaws extend nearly three decades in energy
• Strong dependence on the ambient guide magnetic field
• Increased system size increases the number of nonthermal electrons but not the powerlaw index
• Good agreement with observations of the Feb 10, 2017, flare

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SPHERE 2022 Workshop

7/13/2022

Guide field

scaling

System size

scaling

Arnold+’20

Fermi drive

Including particle ions in the kglobal model

• The kglobal model has been upgraded to include particle electrons and ions
• The exploration of the full spectrum of electrons and ions is now ongoing
• The partitioning of energy between the two species can now be explored
• The plan under SolFER Phase II was to merge kglobal with the ARMS MHD model to produce KARMS
• KARMS would enable macroscale simulations of particle energization in full flare geometry

SPHERE 2022 Workshop

7/13/2022

W/m_iC_A0²