| BACKGROUND | MOTIVATION | GOALS | REGISTRATION & ABSTRACT | AGENDA & INVITED SPEAKERS | MEMBERS | REFERENCES |
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This international workshop seeks to analyze and interpret Fermi/LAT and other related solar data accumulated during the interval August 2008 to December 2022. One avenue of the analysis involves shocks driven by coronal mass ejections, and the shock acceleration of protons and electrons that respectively produce gamma-rays and type II radio emission. Others include theoretical studies of the development of the shock and how particles can be accelerated into space and back to the Sun. During the workshop we will build upon the recent discovery of the quantitative relation between sustained gamma-ray emission (SGRE) and type II radio bursts detected by the Wind/WAVES experiment, by investigating SGRE association of all type II bursts and solar proton events in the Fermi era. A team from eight different countries with the diverse expertise is assembled to carry out the project. The anticipated result will establish the shock source of >300 MeV protons required to produce the neutral pions responsible for SGRE and settle a long-standing issue that has persisted for >30 years. |
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Production of gamma-rays via pion decay was proposed in the astrophysical context by Morrison (1958) and in the solar context by Lingenfelter and Ramaty (1967). Such pion-decay radiation from both impulsive flare and late phase emission was identified by Forrest et al. (1985). Chupp and Ryan (2009) list 13 sustained solar gamma-ray events observed by several telescopes between 1982 and 1991. The primary characteristic of these events is that they last beyond the impulsive phase of the associated flare for minutes to hours. Of these two events lasted for hours beyond the impulsive phase of the flares. Fermi's Large Area Telescope (Fermi/LAT; Atwood et al. 2009), which became operational in August 2008, revealed that such events at energies >100 MeV are rather common (Ackermann et al. 2017; Share et al. 2018; Ajello et al. 2021). This became possible due to the unprecedented sensitivity of Fermi/LAT in the energy range 20 MeV to 300 GeV. In some cases, the Fermi/LAT events lasted for almost a day (Ajello et al. 2014; Omodei et al. 2018; Gopalswamy et al. 2018a). These events are known as long duration gamma-ray flares (LDGRF, Ryan 2000), sustained gamma-ray emission (SGRE, Share et al. 2017; Plotnikov et al. 2017; Klein et al. 2018; Gopalswamy et al. 2018a;2019a) and late-phase gamma-ray emission (LPGRE, Share et al. 2018). SGRE truly denotes the emission beyond the flare, so we use it here. Neutral and charged pions are produced by >300 MeV protons colliding with hydrogen and heavier elements in the solar atmosphere. The source of protons producing pion-decay emission in the impulsive phase of a flare is believed to be due to magnetic reconnection. The source of protons producing SGRE has several possible origins: (i) particles accelerated in the associated flare are somehow trapped in magnetic structures and diffuse slowly to the chromosphere (Ryan and Lee, 1991; Kanbach et al., 1993; Hudson 2018; Grechnev et al. 2018), and (ii) particles accelerated at the shock front that diffuse back to the Sun (Ramaty et al. 1987; Akimov et al. 1991; Vestrand and Forrest, 1993; Cliver et al. 1993; Kocharov et al. 2015; Pesce-Rollins et al. 2015a,b; Jin et al. 2018). Presence of interplanetary type II radio bursts in the decameter-hectometric (DH) wavelengths and fast (>800 km/s) coronal mass ejections (CMEs) have been recently noted to support the shock scenario (Share et al. 2018). A recent breakthrough has been the discovery of quantitative relation between SGRE and type II burst properties (Gopalswamy et al. 2018a): (1) the SGRE and DH type II burst durations are similar and linearly related, (2) the SGRE duration is larger when the DH type II bursts extend to lower frequencies, and (3) the CMEs associated with SGREs are similar to those responsible for ground level enhancements (GLEs) in solar energetic particle (SEP) events. These results represent the strongest evidence that the >300 MeV protons travel from tens of solar radii from the Sun towards the photosphere, collide with the protons, and produce the neutral pions responsible for SGRE. Figure 1 illustrates the scenario proposed by Gopalswamy et al. (2018a,b) using 19 SGRE events that last >3 hours. 
Figure 1. (a) SGRE duration (Y) is anti-correlated with the type II ending frequency (X) indicating that the longer the SGRE duration, the farther the shock travels from the Sun. (b) SGRE duration (Y) is linearly related to the type II duration (X). In (a) and (b) the 95% and 99% confidence intervals are shown by the blue and yellow shaded regions. The blue data points are from the 2014 September 1 backside event not included in the correlation but fits well. (c) A cartoon showing the accelerated protons (p) and electrons (e-) at the CME-driven shock. Protons arriving at the Sun produce SGRE (γ) via neutral pions; those going into the heliosphere are detected as SEPs if the observer is well connected. The electrons produce a type II radio burst in the shock upstream via the plasma emission. Higher energy electrons propagating to the Sun may contribute to the bremsstrahlung continuum [from Gopalswamy et al. 2019a]. |
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Although the results presented in Fig. 1 are quite appealing and are on solid grounds, there are several unanswered questions. For example, Bazilevskaya (2017) wonders why only about half of the SEP events are associated with SGRE events. On the other hand, there are SEP events with no indication of high-energy particles, yet they are associated with intense SGRE (Winter et al. 2018) events. The problem seems to be worse for DH type II bursts: there were about 176 DH type II bursts since the launch of Fermi launch until the end of 2017, yet only about 40 gamma-ray events have been detected (Gopalswamy et al. 2019b). The 2012 March 13 SEP event was intense with >100 MeV protons detected at Earth, yet there was no SGRE event (Share et al. 2018). One must also take into account the limited duty cycle of the LAT observations, however. From the modeling point of view, the current model that high-energy particles are accelerated in a narrow ray near the shock nose (Kocharov et al. 2015) needs to be revised because this model ignores the presence of the shock driver immediately behind the shock. A realistic scenario is shown in Fig. 1(c), which also has some important implications to the spatial distribution of gamma-ray emission. In some cases, weak DH type II bursts and CMEs with less-than average speed are associated with SGRE (Gopalswamy et al. 2019b). When SGRE is caused by shock particles, one expects a correlation between the number of high energy protons inferred from the gamma-ray flux and that from the SEPs detected in space (de Nolfo et al. 2019; Gopalswamy et al. 2021). Large uncertainties have been found in these numbers, which need to be minimized to Clarifying these issues will greatly enhance understanding the origin of >300 MeV protons that produce SGRE via pions. |
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The scientific goal of this workshop is to understand the origin of SGRE from the Sun that last for hours, and sometimes almost a day after the end of the impulsive phase of the associated flare. The scientific objectives of this workshop are: (i) to determine why all large solar energetic particle events are not associated with SGRE, (ii) to determine why all type II radio bursts in the decameter-hectometric wavelengths are not associated with SGRE; (iii) to check if the spatial distribution of SGRE with respect to the source active region is compatible with the interplanetary shock source for the required high-energy protons; (iv) to come up with a realistic model of particle acceleration and transport to the Sun that accounts for the physical conditions in the ambient medium (turbulence, seed particles, magnetic mirroring). These objectives can be achieved by carrying out a set of tasks that involve analyzing Fermi/LAT data ( https://umbra.nascom.nasa.gov/fermi/lat/) in conjunction with data on hard X-rays from RHESSI, and low-energy gamma rays from Fermi Gamma-ray Burst Monitor (GBM), DH type II radio bursts from Wind/WAVES and STEREO/WAVES ( https://cdaw.gsfc.nasa.gov/CME_list/radio/waves_type2.html), CME data from SOHO/LASCO, and STEREO/SECCHI ( https://cdaw.gsfc.nasa.gov/CME_list/), and GOES SEP data ( https://cdaw.gsfc.nasa.gov/CME_list/sepe/). Type II bursts at higher frequencies (metric) will be used to identify shock formation closer to the Sun. All these data sets are available in the public domain and overlap with the Fermi/LAT data since its launch. The modeling experts (theory and numerical simulation) will work with the team to develop a realistic model noted above. A lot of work has already gone into identifying the flares and CMEs with all the relevant parameters, so we can hit the road running for the proposed project. |
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Research Institute Building 2 (tentative) Institute for Space-Earth Environmental Research (ISEE) Nagoya University Chikusa Ward, Nagoya, Aichi 464-8601, Japan Click here to download the campus map in pdf. In the campus map, the metro station is in the C3 box and the Research Insitute Building 2 (F3-8) is the F3 box. |
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Team Members |
Local Organizing Committee |
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N. Gopalswamy (Team Leader) |
S. Masuda (Team Co-leader) |
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S. Gunaseelan
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M. Jin
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P. Makela
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S. Masuda |
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N. V. Nitta
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M. Pesce-Rollins
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J. Ryan
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S. Yashiro |
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A. Shih
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R. Vainio
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A. Warmuth
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A. Asai
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K. Iwai
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T. Minoshima
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Y. Muraki
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H. Tajima
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K. Watanabe
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A. Afanasiev
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A. Mohan
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