Exploring the physics of hot onset flare precursor events (HOPE)

Haotian Da, J. F. Drake, M. Swisdak and Z. Yin

University of Maryland

H. Hudson

SSL and University of Glasgow

Solar flare hot onset observations

• The onset of hard x-rays in flares is often preceded by a period of increasing soft x-rays corresponding to an increasing volume of plasma with a temperature in the range of 10-15MK (Hudson+ ‘21, Battaglia+ ‘23)
• Nearly constant elevated temperature
• Increasing emission measure suggesting continuous heating

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SPHERE 2025

08/12/2025

Battaglia+ ‘23

Does the evolution of the guide field during reconnection in flares produce the hot onset?

• Flare two-ribbon observations (Qiu+ ‘23) and MHD flare modeling (Dahlin+ ‘22) suggest that the guide field continuously decreases during flare evolution
• Reconnection theory and modeling results
• Significant electron and proton energization only takes place when the guide field is weak
• Strong guide field reconnection drives plasma heating but no energization
• The hot onset precursor might be a consequence of reconnection with a strong guide field at early time
• The reduction of the guide field at late time triggers electron energization and the production of hard x-rays

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SPHERE 2025

08/12/2025

Basic mechanisms for electron energy gain during reconnection

• In the guiding center limit

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• Curvature drift
• Slingshot term (Fermi reflection) increases the parallel energy

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• Grad B drift
• Betatron acceleration increases perpendicular energy – μ conservation

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SPHERE 2025

08/12/2025

Drake+ ‘06, Dahlin+ ‘14

Acceleration mechanism for highest energy electrons

• Fermi reflection dominates energy gain for highest energy electrons
• Because V_c ~ v_||²

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• E_|| a sink for highest energy electrons
• Data from a large 3D, strongly turbulent PIC simulation of reconnection

SPHERE 2025

08/12/2025

Li+ ‘19

Electron energy spectra

• Electron powerlaw spectra from kglobal -- a hybrid MHD/particle description to model macroscale systems
• Non-thermal electrons driven by multi-island formation and merging
• Powerlaws extend nearly three decades in energy
• Strong dependence on the ambient guide magnetic field
• A strong guide field suppresses electron energization

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SPHERE 2025

08/12/2025

Guide field

scaling

System size

scaling

Arnold+’21

Fermi drive

Impact of a guide field on particle energy gain

SPHERE 2025

08/12/2025

Guide field evolution in two-ribbon flares

• Measurement of the tilt angle of hot loops during measurements of two ribbon flares reveal the decrease of the guide field (shear angle) during flare evolution

SPHERE 2025

08/12/2025

Guide field evolution from MHD flare simulation

• MHD 3D flare simulation using the ARMS code (Dahlin+ ‘22)

SPHERE 2025

08/12/2025

B_𝜙 is the guide field

Reconnection simulation to explore flare heating and energization

• Carry out a 2D reconnection simulation with the kglobal model, including a variable guide field to mimic flare magnetic field profiles
• kglobal brings together an MHD and particle description to model heating and energization in macrosystems
• Energy normalization in units of m_iC_A² ~ 15keV for flares

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SPHERE 2025

08/12/2025

Drake+’19

Arnold+’19

Arnold+ ’21

Yin+ ‘24

Initial magnetic

field profiles

Electron energy and guide field evolution

• Formation of multiple magnetic islands leads to electron heating
• No significant energization until late time when the weak guide field reaches the current layer

SPHERE 2025

08/12/2025

Spectra at four times

Temperature, emission measure and nonthermal fraction

• Nearly constant temperature increment during reconnection with a strong guide field (T ~ 0.05m_iC_A² ~ 1keV ~ 10MK)
• Steady increase in emission measure due to increase in volume of heated plasma
• Sharp increase in the non-thermal fraction at late time

SPHERE 2025

08/12/2025

Conclusions

• The onset of hard x-rays in flares is often preceded by a period of increasing soft x-rays corresponding to an increasing volume of plasma with a temperature in the range of 10-15MK (Hudson+ ‘21, Battaglia+ ‘23)
• Theory and simulations suggest that electron energization only takes place during reconnection with a weak guide field (Drake+ ‘06, Dahlin+ ‘14, ‘17, Arnold+ ‘21, Yin+ ‘24)
• An increase in thermal energy takes place during strong guide field reconnection
• Two ribbon flare observations and MHD flare simulations suggest that the guide field decreases during flare evolution
• kglobal simulations with a variable guide field reveal particle heating in the range of 10MK during strong guide field reconnection with the onset of non-thermal electrons once weak guide field commences
• More to come: simulations in larger domains are needed produce the extended electron powerlaw distributions expected at late time

SPHERE 2025

08/12/2025