Pulsations in solar flares:
matching observations and models

ISSI-BJ International Team led by Dr Ivan Zimovets & Prof Zongjun Ning

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• 1. Scientific Rationale of the Project and its Timeliness
  • 1.1. Introduction
  • 1.2. Motivation
  • 1.3. Current Difficulties in Understanding of Pulsations
  • 1.4. Project Timeliness
  • 1.5. Project Goals
• 2. Expected Output
• 3. Added Value from ISSI-BJ
• 4. List of Confirmed Participants
• 5. Schedule of the Project
• 6. Facilities Required
• 7. Financial Requirements

Many solar flares, if not all, are accompanied by sequences of intermittent bursts/pulses of electromagnetic emission, from radio waves up to gamma-rays, with characteristic durations ranging from several milliseconds up to several tens of minutes. Similar pulsations are found in emissions of flares on stars of different classes. Of special interest are pulsations in superflares recently found on solar-type stars with the Kepler mission. The presence of pulsations in flare emissions indicates that flaring processes of energy release are essentially non-stationary. One of the most intriguing peculiarities of these processes is that sometimes they are seen to be quasi-periodic, but sometimes are not. It is still a riddle as to why some flares display quasi-periodic pulsations (QPPs), whereas others seem to produce apparently randomly distributed bursts. Undoubtedly, reliable models of flares must be able to explain this feature. Moreover, the apparent similarity of QPPs detected in solar flares and stellar superflares may indicate the similarity of the physical mechanisms operating in them. This further stimulates the study of the solar flare pulsations, since the Sun is the nearest star, and thus can be observed in much greater details in comparison with all other stars.

Considerable progress in the study of pulsations in solar flares has been achieved in recent years. This is in large part due to spaceborne observatories such as RHESSI, Hinode and SDO which make spatially-resolving observations of the emitting sources in the X-ray and EUV energy ranges with high angular (up to a few arcseconds) and temporal (reaching a few seconds) resolutions. These observations from space are well supported by several ground-based solar radio receivers, the major advantages of which are the very high sensitivity and time resolution (up to a few milliseconds, such as the Chinese Solar Broadband Radio Spectrometer – SBRS/Huairou). Some of the solar radio instruments (e.g., NoRH, OVSA, SSRT) can also make spatially-resolved observations with good angular resolution up to 5-10 arcseconds. There are also several X-ray and gamma-ray instruments in space (e.g., XRS/GOES, Konus/Wind, GBM/Fermi, INTEGRAL), which can detect solar flare emissions with high time resolution (up to a fraction of second) but without spatial resolution. Some progress has also been achieved recently in modeling non-stationary flare energy release processes. This gives us confidence that now it is a favorable time to make breakthroughs in understanding the physical mechanisms leading to pulsations (in general) and QPPs (in particular) in solar flares.

Up to now, the problem of flare pulsations has been largely approached by individuals scattered all over the world. However, it seems that this approach is neither efficient nor effective, because revealing the nature of pulsations requires quite a broad range of specialists, in particular: in analysis of solar observational multi-wavelength multi-instrumental data sets, in advanced time-series analysis, in analytical and numerical magnetohydrodynamic (MHD) and kinetic modeling. We propose an international team of specialists in the relevant areas of solar physics, which will be specially concentrated on the problem of solar flare pulsations. The team will summarize, discuss and evaluate all currently available knowledge of this phenomenon. Based on this, the team will develop and implement the strategy of purposeful analysis of modern observational data. Both case studies of individual flares and statistical investigation of large sets of flaring events will be done. An advanced classification of different types of pulsations will be developed. Via comparing numerical MHD modeling and analytical calculations of the flare energy release processes with the results of observational data analysis the team will assess adequacy of the existing models and propose their necessary modifications.

For the goals of the proposal, we seek funding for two workshops at ISSI-BJ: autumn 2017, and summer/autumn 2018, to be attended by 14 team members each. During the first workshop, we are going to merge and extend the existing catalogues of pulsations-associated solar flares; summarize different types of solar flare pulsations; discuss current observational data sets, modern techniques of their analysis and detection of quasi-periodic signals in noisy time series; review the existing models of flare pulsations and identify the most preferred one; prepare a list of the best-observed pulsations-associated flares for further detailed analysis and modelling between the two workshops. During the second workshop, we will comprehensively discuss results of the modelling and compare them with the data analysis results. This will allow us to identify advantages and drawbacks of the models, and outline possible ways to overcome these shortcomings for improving the models in future. Another goal of the second workshop is to prepare the strategy for exploration of the solar flare pulsations jointly with the planned space- (Solar Orbiter, Solar Probe Plus, GRIS/ISS) and ground-based (ALMA, AOVSA, MUSER, SSRT) observations.

The expected outcome from our team is: several topical publications in leading research journals (such as SoPh, ApJ, A&A), online open-access catalogue of pulsations-accompanied solar flares with their main characteristics, and identification of the opportunities for joint application for future collaborations and funding on the related topics.
Solar flares are the most powerful natural phenomena in the solar system, and hence have been subject to extensive studies (e.g., Shibata & Magara, LRSP, 2011). The main practical interest in solar flares is connected with their influence (via radiation and energetic charged particles) on environments of the inner solar system planets and interplanetary space (e.g., Tsurutani et al., GRL, 2005; Baker et al., SSS of ISSI, 2006; Qian et al., GRL, 2012). This is related to the rapidly developing field of space weather and the issues of stability and safety of various space-borne and ground-based technologies. Moreover, solar flares provide a unique possibility for the study of basic physical processes in plasmas, such as charged-particle acceleration and magnetic energy conversion. This knowledge is fundamental for various astrophysical and geophysical plasma systems, and also relevant to controlled-fusion efforts. Also, the use of similarities and differences between solar flares and flares observed on other sun-like stars allows one to assess the habitable conditions on exoplanets (Armstrong et al., MNRAS, 2016). Recent discovery of the giant flares (superflares) on sun-like stars with the Kepler and ground-based telescopes poses a question of whether the Sun is able to produce such a superflare, and with which probability (Maehara et al., Nature, 2012).

A solar flare is a complex process of rapid (on a time scale of minutes/hours) convertion of magnetic energy of a parent active region into kinetic energy of charged particles and their electromagnetic radiation (Shibata & Magara, LRSP, 2011). In a flaring region, plasma is heated up to several tens of millions of kelvins and some populations of charged particles can be accelerated up to relativistic energies. Due to this, a flaring region emits (by different mechanisms) in a very broad range of wavelengths – from radio waves up to gamma-rays. Intensity time profiles of flare emissions differ from each other in different wavelength ranges. Intensity time profiles are usually not smooth. They consist of sequences of intermittent bursts (or spikes, pulses, pulsations, oscillations) of various amplitudes and durations. Pulsations are found in different types of flares, e.g. two-ribbon, (Zimovets & Struminsky, SoPh, 2009; Chen et al., CSB, 2012; Inglis & Dennis, ApJ, 2012), "single-loop" (Zimovets et al., AL, 2013; Kupriyanova, Melnikov, Nakariakov et al., SoPh, 2010), impulsive (Ning, SoPh, 2014; Tian et al., ApJL, 2016), long-duration (Tan, ApJ, 2013), etc., and in different flare stages: in pre-impulsive (Tan et al., ApJ, 2016), impulsive (Kuznetsov, Zimovets et al., SoPh, 2016; Kumar, Nakariakov et al., ApJ, 2017) and decay phases (Hayes et al., ApJ, 2016; Dennis, Tolbert, Inglis et al., ApJ, 2017). The typical "periods" (time scales) of pulsations range from a fraction of second to several minutes, and the modulation depth can reach 100% (Nakariakov & Melnikov, SSR, 2009).

It is well known that some solar flare pulsations demonstrate quasi-periodic behavior (Nakariakov & Melnikov, SSR, 2009; Van Doorsselaere, Kupriyanova & Yuan, SoPh, 2016). In such events times ("periods") between successive pulsations are close to each other and/or frequency spectra of intensity time profiles contain narrow, statistically significant peaks. It is not clear why some flares display quasi-periodic pulsations or oscillations (QPPs or QPOs respectively), whereas others seem to produce more randomly distributed bursts. Some recent findings demonstrate that the quasi-periodicity is a common, perhaps intrinsic feature of solar flares (Kupriyanova, Melnikov, Nakariakov et al., 2010, SoPh, 2010; Simoes et al., SoPh, 2015), though there is another opinion that this may not be true (Inglis et al., ApJ, 2015; Inglis et al., ApJ, 2016). In any case, understanding of flares and of the basic physical processes operating in them is incomplete without the understanding of pulsations and QPPs.
There are at least three important reasons why solar flare pulsations should be studied with care and on a systematic basis:

• Oscillatory processes of unknown origin in natural systems are attractive to physicists a priori;
• Pulsations are a common and, perhaps, intrinsic flare phenomenon; therefore, their understanding is necessary from the standpoint of the fundamental physics of astrophysical energy releases;
• Pulsations have a significant seismological potential for the diagnostics of physical conditions in solar and stellar flaring regions and processes operating in them, because their parameters impose additional constraints on the models and interpretations (Zaitsev & Stepanov, PhyU, 2008; Nakariakov & Melnikov, SSR, 2009; Van Doorsselaere, Kupriyanova & Yuan, SoPh. 2016).

Several objective factors seriously impede understanding the nature of pulsations:

• There is a great variety of flaring phenomena called pulsations - from nearly monochromatic high-quality oscillations observed in the ranges of metric/decimetric radio or EUV emissions (Nakariakov & Melnikov, SSR, 2009) up to quite irregular sequences of noisy spikes observed in hard X-ray and microwave emissions (Tan, SoPh, 2008; Inglis et al., ApJ, 2016; Kuznetsov, Zimovets et al., ApJ, 2016). In essence, there are no yet strict criteria/definition of the quasi-periodicity and clear classification of pulsations types among solar physicists.
• Processes occurring in flaring regions are very complex, and most probably there are a number of principally distinct physical mechanisms responsible for different types of pulsations (Nakariakov & Melnikov, SSR, 2009; Afanasyev & Uralov, SoPh, 2016; Nakariakov et al., SSR, 2016; Van Doorsselaere, Kupriyanova & Yuan, SoPh, 2016). This requires a complex, synergetic approach to the problem.
• Some processes responsible for generation of pulsations, in particular magnetic reconnection and the associated acceleration of charged particles, are believed to operate simultaneously on very different time and spatial scales (Shibata & Tanuma, EPS, 2001; Nishizuka et al., ApJL, 2009; Tan & Tan, ApJ, 2012). This greatly complicates their numerical simulation and hence quantitative understanding.
• Evidently, some types of flaring pulsations, in particular, self-oscillatory mechanisms, have nonlinear and non-stationary processes in their roots. This requires a very specific mathematical apparatus, which, in essence, has not been yet fully introduced into the community of solar physicists (Nakariakov, Inglis, Zimovets et al., PPCF, 2010; Kolotkov, Nakariakov et al., MNRAS, 2015; Inglis et al., ApJ, 2016).
• Correct understanding of the nature of pulsations requires very delicate observations of their sources with sufficient time, spatial, and energy resolution, made by sensitive instruments. This is often not achieved in reality, especially in the case of stellar flares. Also, it is necessary to well exclude instrumental artifacts in the data analysis (Inglis, Zimovets et al. A&A, 2011).

The problem of flare pulsations is especially timely now. There are several reasons to encourage the initiation of our team project at this moment:

• In essence, this problem has never been studied on a systematic, coordinated basis using a large number of solar flares to look for commonalities, divergences from those commonalities, and the reasons for those commonalities. Existing knowledge has been poorly systemized, and is scattered across many different papers. However, last years this was recognized and a few attempts were done to make analysis of pulsations and QPPs in large samples of flares (Simoes et al., 290, 2015; Inglis et al., ApJ, 2016; Kuznetsov, Zimovets et al., SoPh, 2016; Cho, Cho, Nakariakov et al., ApJ, 2016). Our team (which includes the authors of the referred papers, marked by bold) aim to continue and extend this solar flare research trend.
• Advances in understanding the challenge of pulsations is highly related to the multi-wavelength spatially-resolved observation of the emitting sources in the flaring region. Currently there are a number of advanced space-based (such as RHESSI, SDO, Hinode, GOES, Fermi, IRIS) and ground-based (such as NoRH, NRH, OVSA, SSRT, and a number of optical telescopes around the world) solar observatories covering almost entire range of electromagnetic emission. They are able to perform complex state-of-the-art observations of flare regions with high temporal (up to fractions of seconds) and spatial (up to about one arcsecond and even better) resolutions. Dedicated coordinated approach is required for analysis of these complex, diverse and massive data sets to make real breakthrough in the understanding of flare pulsations.
• The problem of pulsations is highly related to MHD seismology, which is a rapidly developing branch of solar physics. Actively correlated coupling between the problems of pulsations and seismology can quickly revolutionise both these disciplines and plasma astrophysics in general (Nakariakov et al., SSR, 2016; Srivastava et al., MNRAS, 2016).
• Currently, several space- and ground-based solar and astrophysical observatories are in the phase of their preparation and close to the start of their launch/observations. In particular, the Solar Orbiter mission of the European Space Agency is planned to be launched in 2018. The Mingantu Ultrawide Spectral Radioheliograph (MUSER, China) have been established recently and is in test observations now (Yan et al., IAUS, 2016). The Atacama Large Millimeter/submillimeter Array (ALMA) just began systematic observations of the Sun. These (and some other) new observatories can definitely contribute to the problem of pulsations in solar and stellar flares. To achieve better (and quicker) output of these new observations for the problem of pulsations we must now get prepared by developing a purposeful strategy of exploitation of these forthcoming data.
• Recent finding of pulsations in optical emission of superflares at a number of solar-type stars (Balona et al., MNRAS, 2015; Pugh, Nakariakov, Broomhall, ApJ, 2015) broadens the scope of flare pulsations beyond our Sun, and makes it especially timely now, because pulsations, if well understood, represent a strong tool of diagnostics of main physical parameters (such as size, plasma density, magnetic field) of a flaring region. This, in turn, can help us better understand superflares at solar-type stars and to evaluate probability of their occurrence at the Sun.
• The proposed team is built up on the ongoing ISSI (Bern) team led by A.-M. Broomhall, which addresses the QPP detection techniques and solar-stellar analogies, while our team that mainly consists of the colleagues affiliated in the Asian-Pacific region, addresses the physical mechanisms for pulsations, their observational manifestation and modelling, and utilizes the findings of Broomhall's team.

The team will focus on the following objectives, the achievement of which should help to make significant progress in understanding pulsations and QPPs in solar flares:

Task 1 - to develop an advanced classification of different types of pulsations on the base of systematic analysis of large sample of solar flares observed by space- and ground-based instruments in the solar cycles 23-24 using the novel time-series analysis techniques;

Task 2 - to perform detailed multi-wavelength spatially-resolved analysis of the sources of pulsations in several well-observed flares of different types;

Task 3 - to assess adequacy and improve the existent models of flare pulsations; we will specifically address the repetitive and fractal regimes of spontaneous magnetic reconnection (self-oscillations), reconnection periodically induced by MHD waves, natural oscillations of magnetic X-points, sausage oscillations, and thermal over-stability.

Task 4 - to develop a strategy of exploration of pulsations in solar flares with space- and ground-based instruments in the coming years.

Our team will solve the tasks formulated above by comprehensive discussions during two planned workshops and by the purposeful and coordinated (by the team leader and co-leader) work between the workshops.

Workshop 1

First, by implementing the novel time-series analysis techniques, currently discussed and tested by the ongoing ISSI team led by A.-M. Broomhall (e.g., Inglis et al., ApJ, 2016; Kolotkov, Anfinogentov & Nakariakov, A&A, 2016), to the observations by space- (RHESSI, GOES, TRACE, SDO, IRIS, Fermi, INTEGRAL, Konus/Wind) and ground-based (NoRH, NoRP, SBRS) instruments of solar flares in the cycles 23-24 revealing evidences of pulsations and QPPs, we will develop an advanced classification of the main types of pulsations in various wavelength ranges in different types of solar flares (two-ribbon, circle-ribbon, "single-loop", etc.). Our starting point for the collaborative data analysis are the catalogues of the solar flare pulsations detected with GOES (AFINO, Inglis et al., ApJ, 2016) and RHESSI (Cho, Cho, Nakariakov et al., ApJ, 2016; Kuznnetsov, Zimovets et al., SoPh, 2016). Our further plan is to extend these catalogues significantly. We expect that our final list will consist of several hundred events, and will be in open access. Based on this, we will catalog, summarize and review major types of solar flare pulsations (Task 1).

Second, during the workshop, we will discuss the current status of the solar flare pulsations modelling (Tan & Tan, ApJ, 2012; Li et al., A&A; 2014; Nishizuka et al., ApJ, 2015; Afanasyev & Uralov, SoPh, 2016; Nakariakov et al., SSR, 2016; Van Doorsselaere, Kupriyanova & Yuan, SoPh, 2016). We will identify the most preferable models for each major identified type of pulsations. Also, we will prepare a list of the best-observed pulsation flares of the major types for further detailed analysis and modelling during the time between the two workshops (Task 2).

Workshop 2

This workshop will focus on comprehensive comparison between modelling predictions and data analysis of the pre-selected best-observed flares with pulsations and QPPs. This will allow us to identify advantages and disadvantages of the performed modellings, to discuss which necessary adjustments and corrections of the models should be made in near future, which types of flare pulsations and QPPs can be modelled under the current state of analytical and numerical capabilities, outline possible ways to overcome the identified problems and shortcomings (Task 3).

The second aim of the workshop is to discuss the strategy and become ready for exploration of pulsations and QPPs in solar flares with the planned space- (such as Solar Orbiter, Solar Probe Plus, GRIS/ISS) and ground-based (such as ALMA, AOVSA, SSRT) missions and instruments in the coming years (Task 4). One workshop day is planned to be dedicated specifically to discussing the possibilities of using the Mingantu Ultrawide Spectral Radioheliograph (MUSER, China), led by Prof Yihua Yan (NOAC, Beijing), and the Decimetric and metric digital solar radio spectrometers of the Yunnan Astronomical Observatories, China (Gao et al., NewA, 2014), to study the solar flare radio pulsations.
The scientific output of the project is intended to be published in several (4-6) focused papers (e.g. in A&A, ApJ, SoPh, PASJ, RAA) jointly co-authored by the team members. The publication record of the participants makes our aim realistic. It is the responsibility of the team leader and team co-leader to monitor and oversee that most of the proposed aspects are to be covered. Scientific results achieved in frames of the project will be further distributed by team-members over broad solar and astrophysical communities by presenting them at national and international science meetings (such as COSPAR, NAM, AGU, ESPM, etc.). Besides, we will prepare an open-access online catalogue of solar flares accompanied by pulsations and QPPs of different types in different wavebands, which will contain flare emission light-curves and brief descriptions of the main characteristics of the flares and pulsations; special software and scripts for their analysis. The catalogue will be hosted on the IKI (Moscow) server and mirrored on the ISSI-BJ site. Another output will be training of local students (which we will invite from CAS to attend both our workshops) that contribute to generation of skilled human resources. Also, we will identify the opportunities for joint application for future collaborations and funding on the related topics (e.g., joint grants between RFBR, NSFC, NRF, RSL, etc.).
We are sure that the recently opened ISSI-BJ facility has the same stimulating research environment and excellent research facilities as provided by ISSI-Bern. Together with its convenient geographical location, it makes this site ideal for the proposed activity, because the majority of our team members are from the Asia-Pacific region. As an additional advantage, we expect that our Team meetings will be attended by the colleagues from the Solar Physics Division of NOAC, Beijing and Beijing University: Prof Yihua Yan, Drs Jun Zhang, Dr Ting Li, et al., who are active researchers in the field, at no cost for the programme. For all team members, the site is easily accessible by direct flights to Beijing. Several participants are already familiar with the ISSI-BJ arrangements. The ISSI programme is ideal for promoting international collaboration and interdisciplinary research. Forming an ISSI team is also an excellent forum to advance both the current research of each individual participant and the overall visibility of the team activity in the space science community. Last but not the least, the team leader (Dr Ivan Zimovets) has a half-time postdoctoral researcher position in NSSC/CAS and ISSI-BJ (the contract is currently till October 31, 2017). Initiation and managing of a qualified international research team in ISSI-BJ is one of his responsibilities expected by NSSC/CAS and ISSI-BJ.
The Project Team consists of 14 approved participants, whose summary extensive expertise covering essentially entire field of the Project. The Team is balanced, the numbers of theoretical and data analysis members are almost equal.

Team Member	Country	Project Duties
AFANASYEV Andrei	Russia	Theory and modelling of MHD waves in the solar atmosphere
CHEN Peng-Fei	China	Theory and numerical MHD modelling
GAO Guannan	China	Interpretation of radio pulsations in decimetric and metric bands
INGLIS Andrew	USA	Time-series analysis of flare light curves
KUMAR Pankaj	South Korea	Multi-wavelength solar flare observations
LI Bo	China	Analytical and numerical modelling of MHD oscillations
NAKARIAKOV Valery	UK	MHD seismology and theoretical bridge to flare pulsations
NING Zongjun	China	Multi-wavelength multi-instrumental solar flare analysis
NISHIZUKA Naoto	Japan	Analysis/interpretation of multiwavelength solar data sets
SRIVASTAVA Abhishek	India	Observations and modeling of waves in the Sun's atmosphere
TAN Baolin	China	Observations and theoretical interpretation of radio pulsations
HUI Tian	China	Analyze UV and FUV data during flares
YUAN Ding	China	MHD modelling and solar data analysis
ZIMOVETS Ivan	Russia, China	Team Leader; multi-instrumental solar flare observations

We propose to run two five-days meetings at ISSI-BJ: the first one to be held in the autumn 2017, and the second one in the summer-autumn of 2018. The timing of the workshops will be adjusted to minimize the conflict with other commitments of the participants (especially teaching and major international conferences) and with the stays of the Team Leader (Dr Ivan Zimovets) at the ISSI-BJ and NSSC/CAS.
We do not have special requirements besides the usual ISSI-BJ workshop facilities: one meeting room with projection facilities, white board, and Internet access for all team members.
We seek the standard financial support for the participants: 7 days (5 meeting days plus arrival and departure days) x 11 participants + 7 days x 12 participants = 161 per diems. Two our team members (Dr Baolin Tan & Dr Hui Tian) are affiliated in Beijing and do not required financial support from ISSI-BJ. Since the Team Leader (Dr Ivan Zimovets) is in the ISSI-BJ staff till October 31, 2017, he does not need any financial support for the workshop 1. The travel expenses for the workshop 1 could be transferred to the Team Co-Leader. If the team will be selected, we will make use of ISSI's "young scientist" funding scheme to support two PhD students or earlier carrier researcher.

Contact us Space Research Institute of the Russian Academy of Sciences (IKI), 2017