Diagnosing Heating Mechanisms in Solar Flares
through Spectroscopic Observations of Flare Ribbons
--- an ISSI team led by Hui Tian


Diagnosing heating mechanisms in solar flares through spectroscopic observations of flare ribbons

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Team leader: Dr. Hui Tian, Peking University, China
Research domain: Space Sciences (Solar and Heliospheric Physics)
ISSI location: Beijing and Bern

Abstract:Solar flares are among the most energetic events in the solar system. Flare ribbons are one of their best observed features that can provide critical diagnostics for the fundamental flare physics not yet well understood. With a high cadence up to a few seconds and a resolution of ~0.33 arcsecond, NASA's Interface Region Imaging Spectrograph (IRIS) mission has revealed unprecedented details of ribbon dynamics in hundreds of flares since July 2013. Characteristics of emission line profiles observed at flare ribbons, e.g., Doppler shifts, line widths, asymmetries, central reversals and wing enhancements, are manifestations of various physical processes involved in solar flares. We propose a timely investigation to understand flare heating mechanisms through joint efforts of spectroscopic observations focusing on flare ribbons and advanced numerical modeling. Specifically, observers on the team will fully characterize the temporal evolution of several key emission lines, e.g., Mg II, Si IV, and Fe XXI, at ribbons in at least ten IRIS flares. Modelers on the team will perform state-of-the-art radiative-hydrodynamic simulations by experimenting various heating mechanisms to reproduce observations. Supplementary space and ground-based observations will be used to provide additional constraints. We anticipate that the proposed team work will significantly advance our understanding of flare heating mechanisms and thus well prepare us for future opportunities offered by new space missions such as ESA's upcoming Solar Orbiter.

1. Introduction and overview
Solar flares are one of the most prominent forms of solar activity that has profound impacts on the near-Earth space environment and the rest of the heliosphere. Flare ribbons are locations in the lower solar atmosphere (including the photosphere, chromosphere, and transition region) of enhanced line and continuum emissions in broad wavelengths ranging from infrared to X-rays. Such ribbons consist of footpoints of hot and dense flare loops, where most of the flare energy is deposited and dissipated. Characteristics of emission line profiles observed at flare ribbons are manifestations of various physical processes that are related to heating of the chromosphere and corona, the detailed mechanisms of which are still poorly understood. The recently-launched IRIS mission (De Pontieu et al. 2014) has obtained spectra of several key emission lines, e.g., Mg II k & h (2796.35 & 2803.52Å), C II 1334.53 & 1335.71Å, Si IV 1393.76 & 1402.77Å, and Fe XXI 1354.08Å, at ribbons of hundreds of flares with unprecedented spatial and temporal resolution. The spatial distribution and temporal evolution of these line profiles contains rich information of the transport and dissipation of energy released in flares. With an ideal mix of observers and modelers, our team aims at making significant advances in the understanding of flare heating mechanisms through coordinated efforts of spectroscopic observations of flare ribbons and advanced numerical modeling.

2. Scientific rationale
During solar flares the stressed coronal magnetic field reconfigures (via magnetic reconnection), liberating a significant amount of energy that is transported from the release site through the corona into the chromosphere. Traditionally, flare heating processes are classified into chromospheric heating and coronal heating. The bulk of the flare radiative output originates from the dense chromosphere, mostly from the ribbons. At least three mechanisms have been proposed to explain the chromospheric heating: collisions by nonthermal electrons accelerated at/near the coronal reconnection site, thermal conduction from the corona, and dissipation of downward propagating Alfvén waves. Two processes could contribute to the coronal heating during flares: in-situ heating by reconnection and evaporation of the heated chromospheric plasma.

Because of evident hard X-ray emission located within the ribbons of many flares, nonthermal electrons have been widely believed to play a key role in heating of the chromosphere. Hydrodynamic or radiative hydrodynamic models based on nonthermal electron heating are successful in producing some of the observed features of chromospheric line profiles (e.g., Canfield & Gayley 1987; Allred et al. 2005; Rubio da Costa et al. 2015). Thermal conduction from the corona has also been suggested to play a dominant role in heating of the chromosphere in some flares, especially in flares where no hard X-ray emission is detected (e.g., Qiu et al. 2013). Thermal hydrodynamic models have been constructed and a general match was also found between the calculated and observed profiles of some chromospheric lines (e.g., Gan et al. 1992). More recently, damping of downward propagating Alfvén waves generated by reconnection have also been suggested to be an efficient way of flare heating (Russell & Fletcher 2013; Reep & Russell 2016). A direct comparison of line profiles predicted from this scenario with those observed ones has not yet been made.

Existing spectroscopic observations of flares were mostly made on the ground, using mainly the Hα line and Ca II lines. Properties of these line profiles may indicate heating mechanisms as well as the physical condition of the overlying corona (Canfield et al. 1984). The Hα line has been frequently found to exhibit a centrally reversed profile at flare ribbons. While the Ca II lines often go to full emission during flares. Non-LTE calculations have shown that these are likely consequences of electron beam bombardment (e.g., Fang et al. 1993) and that increasing beam flux leads to increasingly broad Hα line profiles (Ding & Fang 2001). These lines also reveal obvious enhancement at the blue or red wing. The red asymmetries have been attributed to the condensation downflows in the chromosphere (e.g., Ichimoto & Kurokawa 1984). While the blue asymmetries could be formed through the absorption of the red wing by downflows occurring in the upper chromosphere (e.g., Heinzel et al. 1994; Ding & Fang 1996). The profile asymmetries thus might be used to diagnose the heights of energy dissipation in the chromosphere.

The overpressure associated with chromospheric heating can drive the heated plasma upward to fill flare loops. This so-called "chromospheric evaporation" process has been proposed to explain the soft X-ray emission indicative of ~10 MK plasma in flare loops. Observational evidences of chromospheric evaporation have been frequently identified through spectroscopic observations by EUV spectrometers, including SOHO/CDS and Hinode/EIS. These observations usually reveal a blueshifted component or blue wing enhancement besides a nearly stationary component in hot emission lines from ions such as Fe XIX (formed at ~9 MK), Fe XXIII (~14 MK) and Fe XXIV (~18 MK), which suggests that the spatial resolution is insufficient to separate the evaporation flow from the ambient stationary hot plasma (e.g., Milligan & Dennis 2009; Young et al. 2013). The direction and velocity of the flow can be used to diagnose heating mechanisms and electron properties in a beam (e.g., Fisher et al. 1985; Milligan et al. 2006a, 2006b; Reep et al. 2015). For instance, a relatively low flux of nonthermal electrons or thermal conduction alone leads to evaporation upflows at several tens km/s with no associated downflows (gentle evaporation), while a high electron flux can results in both an upward expansion of the hot plasma at several hundreds km/s and a downward condensation of the cooler materials at several tens km/s (explosive evaporation). The temperature at which the Doppler shift turns from red to blue may be used to test whether continuous energy deposition is present throughout the impulsive phase (Liu et al. 2009).

Information provided through ground-based flare observations is limited by the availability of high-quality observations and the lack of spectral lines formed at higher heights/temperatures. On the other hand, the resolution of past spectroscopic observations in space was not high enough to resolve the flare kernels. This situation has changed dramatically since the launch of the IRIS mission. With a spatial resolution of ~250 km, a typical cadence of a few seconds, and the ability of performing long-duration continuous seeing-free observations, IRIS has obtained high-resolution spectra of several emission lines with different formation temperatures in ribbons of hundreds of flares.

IRIS has already provided new insights into flare heating. For instance, the Mg II k and h lines often show no central reversal and many absorption lines become emission features at flare ribbons (Kerr et al. 2015; Tian et al. 2015; Liu et al. 2015). The hot Fe XXI 1354.08Å line is entirely blueshifted by up to ~300 km/s, indicating that the flare kernel is resolved by IRIS (e.g., Tian et al. 2014, 2015; Young et al. 2015; Polito et al. 2015; Li et al. 2015; Brosius & Daw 2015; Graham & Cauzzi 2015). In some flares a correlation between the time histories of the flow velocity and hard X-ray flux has been found, in agreement with models of nonthermal electron heating (Tian et al. 2015; Li et al. 2015). However, Battaglia et al. (2015) found that the Fe XXI blue shift shows no spatial and temporal correlation with the hard X-ray source in one flare, suggesting that electron beams only play a minor role in driving the evaporation in this flare. For cooler lines such as Si IV, Si II, Fe II, C I and Mg II subordinate line, IRIS has detected their red asymmetries or distinct redshifted components in flare kernels (Graham & Cauzzi 2015; Tian et al. 2015; Graham et al. 2016, in prep.). Preliminary investigation by the team members suggests that these lines may be extremely useful in probing the energy deposition height in the atmosphere. Heinzel & Kleint (2014) discovered the hydrogen Balmer-continuum emission in the near UV (NUV) IRIS channel which is the signature of hydrogen recombination in solar flares (see also Kleint et al. 2016). This provides a new, direct constraint on beam energy deposition and chromospheric heating.

Some progress has been made in modeling flare emission in general and the above-mentioned characteristics of IRIS observations in specific. Currently two radiative hydrodynamic codes are widely used to study the interaction of nonthermal electrons with the lower solar atmosphere and associated emissions: i.e., the RADYN code (Carlsson & Stein 1997; Allred et al. 2005, 2015) and the Flarix code (Kasparova et al. 2009, Heinzel et al. 2016). Recent extensive tests have revealed that both codes can provide consistent temporal evolution of the flaring chromosphere subject to electron beam heating. For example, Rubio da Costa et al. (2016) use electron beam inferred from RHESSI hard X-ray data as the input to the RADYN code to reproduce the observed Mg II line profiles. Rubio da Costa et al. (2015) also incorporated the Stanford Fokker-Planck code in their RADYN simulation to self-consistently model particle acceleration and transport as well as the hydrodynamic response of the atmosphere to electron heating. Liu et al. (2015) synthesized the Mg II k line using their non-LTE code with the Multilevel Accelerated Lambda Iteration (MALI) code, and found the sensitivity of Mg II line intensities to various electron-beam parameters. Polito et al. (2016) has run a flare loop simulation undergoing electron beam heating using the HYDRAD code (Bradshaw & Cargill 2013) and the reproduced trend of Doppler shift of hot lines is consistent with the IRIS observation in an X2.0 flare. More recently, Alfvén wave dissipation as a flare heating mechanism has also been added to the RADYN code (Kerr et al. 2016, in prep).

So far the IRIS data of only a few flares has been analyzed by the community. Clearly, a better understanding of the flare heating mechanisms requires detailed analysis of more flares observed by IRIS. Modeling of these flares involving different heating processes should also be performed, with the aim of reproducing the signatures in the IRIS lines formed at different temperatures/heights.

3. Goals
Our over-arching goal is to understand the detailed heating processes in different flares through combined efforts of spectroscopic observations of flare ribbons and advanced numerical modeling. Specific goals to achieve this objective are:

(1) Perform a survey of the IRIS data to select tens of flare observations with well defined ribbons which are suitable for studying flare heating.
(2) Characterize the temporal evolution of several key emission lines, e.g., Mg II, C II, Si IV and Fe XXI lines, at different locations of the ribbons in the selected flares.
(3) Use complementary multi-wavelength observations, including X-rays from RHESSI or Fermi/GBM and Hinode/XRT, EUV images from SDO/AIA, and EUV spectra from Hinode/EIS to provide physical constraints on relevant processes.
(4) Run radiative-hydrodynamic simulations focusing on different heating processes to reproduce signatures (e.g., Doppler shift, line width, asymmetry, central reversal) in the observed IRIS line profiles.

4. Timeliness
The proposed team work is extremely important and timely mainly because of the unprecedented high-quality and high-resolution spectroscopic data provided by IRIS, which were very scarce in the past but hold the key to uncover fundamental heating mechanisms in flares. In just 2.5 years since its launch, IRIS has already observed hundreds of flares in various lines formed in a wide range of temperatures and heights. Reports mentioned above have already demonstrated the diagnostic power of such observations. Recent progress in both observations and models and more promising ongoing research carried out by team members has laid a solid foundation for the proposed project. As such, this ISSI team is currently well-positioned to undertake the challenging tasks proposed here.

In addition, use of complementary X-ray and EUV observations from RHESSI, Fermi/GBM, SDO, and Hinode by this team will help the solar flare community prepare for ESA's upcoming Solar Orbiter mission (2018), which will carry an X-ray telescope called STIX, an EUV imager (EUI), and an EUV spectral imager (SPICE). Several team members, e.g., Kleint and Heinzel, are already involved in hardware and/or software development for these instruments.

5. Expected outcome
Major outcome expected from the proposed team work includes:
(1) A list of flares observed with IRIS, including detailed description of the characteristics of key emission lines, will be posted on the team website.
(2) Analysis of the spatial distribution and temporal evolution of the key IRIS lines and continua in at least ten flares will lead to refereed publications.
(3) Through a combination of ribbon observations and modeling, new tools of diagnosing flare heating processes will be developed. This will also lead to refereed publications.
(4) A major review summarizing the outcome of the proposed project will be published, e.g., in Space Science Reviews.

6. Team
We have assembled a strong team, as listed below, with a diverse age, gender, and geographic distribution. More importantly, the combination of broad and complementing expertise is key to the proposed project. More than 2/3 of the members are young researchers actively working on the forefront of flare research. Specifically, members Reeves, Liu, Tian and Kleint are or were on the IRIS science team and have obtained a wealth of valuable flare data when serving as IRIS planners. Heinzel is the IRIS Associate Scientist. In particular, Reeves and Tian have been maintaining an exhaustive list of IRIS-observed flares to date. Members Young, Graham, Li, Kerr and Tripathi have rich experience in analyzing IRIS flares. Members Rubio da Costa, Heinzel, Kasparova, Kerr, and Cheng are experts on modeling flare dynamics and emission. Members Liu, Qiu, Rubio da Costa, and Simões have extensive experience analyzing EUV/X-ray data and/or modeling particle kinetics.

   Table 1: List of team members

7. Project schedule
We plan to have one five-day meeting in Beijing and the other one in Bern. We anticipate the first meeting taking place in the first half of 2017. Before the first meeting, observers on our team will analyze the IRIS spectra at ribbons of about ten flares. In the first meeting, we will discuss various spectral signatures and possible underlying heating processes, and identify key challenges for the modeling efforts. After this meeting the observers will continue to analyze more flare data acquired by IRIS. At the same time, modelers on our team will construct flare models and perform numerical simulations to reproduce the spectral signatures. The second meeting will be held one year later, and both observational and numerical results will be presented and discussed. After that we will publish a review paper summarizing the new insights into flare heating processes revealed by the proposed work.

8. Added value provided by ISSI
The proposed ISSI international team will serve as an ideal platform for a small group of researchers to work intensively on flare heating. Our goal is to achieve a better understanding of flare heating processes through combined efforts of IRIS observations and advanced modeling. Such a goal is relevant to both solar and stellar physics and thus matches the goals of ISSI. Considering the diverse geographic locations of the team members, we plan to have one meeting in Beijing and the other in Bern, which will serve to maximize the scientific return and expand the influence of this project in both Europe and East Asia.

9. Facilities required
For each meeting we request a room for up to ~18 people. Internet access and projector should be available in the room.

10. Financial support
Financial support is requested for 12 of the team members (see the table above). The team leader Hui Tian will transfer the travel refund to another team member.

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