Solar Flare Evolution of Temperature, Emission Measure, and Coronal Abundances

BENNET SCHWAB

TOM WOODS

MAY 27, 2022

�Key Science Question

• Which heating mechanisms are dominant in making the solar corona more than 100 times hotter than the photosphere?

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• There are two main theories that likely both play a part in coronal heating. During quiescent times, elemental abundances should remain near the currently accepted Feldman values.

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• Evidence for the dominance of one over the other can be provided by looking at the abundance of elements. During flaring times a change in these elemental abundances would support one method of coronal heating.

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For three separate flares, the abundances of Mg, Si, and Fe are analyzed and show change

Flare Phases

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• In the top plot are the GOES 1–8 SXR band and the histogram of RHESSI HXR, normalized to their maxima.

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• In the bottom plot, the flare phases can be seen along with an approximations of their time ranges.
• The onset – where fluctuations occur, and photon flux increases before the flare event
• Impulsive phase – the rapid release of energy accompanied by a quick spikes in HXR
• Gradual phase – initially a large rise in SXR that decays towards pre-flare conditions
• Extended phase – HXR emission from long-term storage of particles in coronal magnetic fields

Source: Hudson 2011

Impulsive Phase HXR

Gradual Phase SXR

Extended Phase HXR

*Tarek 2018

*Dennis 1989

*Dennis 1989

*Hudson 2011

Standard Flare Model

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• Magnetic reconnection occurs in the solar corona
• Sends non-thermal electrons and accelerated particles downwards along the magnetic loop

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• These particles collide with the underlying chromosphere at the footpoints of the loop
• Bursts of HXR and γ rays are emitted in the impulsive phase

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• This also causes immense heating and chromospheric evaporation occurs
• SXR radiation comes from the cooling chromospheric plasma in the gradual phase

This figure is from the archive of Professor Kenneth R. Lang, Tufts University

*Nakariakov 2009

*Hudson 2011

*Aulanier 2013

Other Analyses of Flare Abundance Changes

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Source: Narendranath 2020, https://arxiv.org/pdf/2011.08584.pdf

• Low FIP elements such as Ca, Fe, and Si are usually 4 times more abundant in the corona than the chromosphere and S is usually 2 times more abundant

* Feldman 1992, Landi 2002

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• During a flare this FIP bias changes and abundances shift towards chromospheric

* Narendranath 2020, Mondal 2021

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MinXSS-1 CubeSat

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• Obtained on-orbit solar SXR spectra between 0.5 – 30 keV

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• Data spans from 2016 June to 2017 April, the tail end of solar minimum

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• Captures the continuum as well as several emission line features

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• Both quiescent sun conditions as well as solar flare data are observed

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Credit: Tom Woods

Credit: James Mason

MinXSS-1 Flares Analyzed

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• A single T, single EM model with separate AF for Mg, Si, and Fe was used

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• Many questions to investigate:
• Is the decrease in AF consistent for all types of flares?
• Is the maximum T reached dependent on flare class?
• When is the maximum T observed relative to the flare peak?
• When is the minimum AF observed relative to flare peak?

Flare 1 – Compact Flare

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Flare 2 – Long-Decay Flare

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Flare 3 – Quasi-Periodic Pulsation (QPP) Flare

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Note: Relative Time is difference from peak flare intensity

Conclusions from Flare Analysis

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• Is the decrease in AF consistent for all types of flares?
• Yes, the decrease in AF was found for the three different flares analyzed that differed in magnitude as well as type

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• Is the maximum T reached dependent on flare class?
• Yes, for higher magnitude flares the maximum temperature reached by the SXR emitting plasma increased as well

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• When is the maximum T observed relative to the flare peak?
• The temperature peak for the flare occurs about 3 minutes prior to the flare peak in intensity

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• When is the minimum AF observed relative to flare peak?
• The minimum AF reached also occurs about 3 minutes prior to the flare peak in intensity.

How Do These Results Help Answer the Key Science Question?

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• The decrease in AF during the start of the gradual phase is strong supportive evidence of chromospheric evaporation caused by magnetic reconnection heating of the corona.

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• Increases in maximum temperature reached by the SXR emitting plasma for higher magnitude flares implies that higher energy flares provide more heating to the corona than lower energy flares.

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• The temperature peak indicates that the chromospheric plasma is hottest 3 minutes before the majority of the energy is emitted through SXR radiation.

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• Minimum AF values reached 3 minutes prior to the flare peak shows that the SXR radiation is coming from plasma that underwent chromospheric evaporation.

Which heating mechanisms are dominant in making the solar corona more than 100 times hotter than the photosphere?

Thank you!

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Questions?