Fifty-year Anniversary of the First Detection of Gamma rays from a Solar Flare
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|title = Fifty-year anniversary of the first detection of gamma-rays from a solar flare | |title = Fifty-year anniversary of the first detection of gamma-rays from a solar flare | ||
|number = 434 | |number = 434 | ||
| - | |first_author = Jim RYAN, | + | |first_author = Jim RYAN, Brian DENNIS, |
| - | |second_author = | + | |second_author = and Phil DUNPHY |
|publish_date = 8 August 2022 | |publish_date = 8 August 2022 | ||
|next_nugget = TBD | |next_nugget = TBD | ||
Latest revision as of 19:52, 5 August 2022
| Nugget | |
|---|---|
| Number: | 434 |
| 1st Author: | Jim RYAN, Brian DENNIS, |
| 2nd Author: | and Phil DUNPHY |
| Published: | 8 August 2022 |
| Next Nugget: | TBD |
| Previous Nugget: | Fast Prograde Flows in Solar Active Regions |
| List all | |
Contents |
Introduction
Solar flares are wonderfully complicated, and we can detect them in many ways. Fifty years ago this week, on August 4, 1972, the Gamma Ray Monitor (GRM) on the 7th Orbiting Solar Observatory (OSO-7, as shown in Figure 1) made the first-ever detection of γ-rays from a solar flare. This immediately established a connection between these flashes of ordinary visible light, with nuclear physics in a fundamental way. The GRM instrument was built by Prof. Edward L. Chupp and his team at the University of New Hampshire. Mounted in the spinning wheel section of the spacecraft, it consisted of a single 3-inch cylindrical NaI(Tl) scintillator with an active CsI(Na) anticoincidence shield. It was designed to detect and measure individual γ-ray photons with energies between 0.3 and 10 MeV. These photons have energies many millions of times greater than photons of visible light.
Solar γ rays
The observational discovery of solar γ radiation actually followed predictions, notably in Ref. [2]. These included the possibility of emission lines due to the excitation of certain nuclei to excited energy states. Other prominent emission lines would come from positron annihilation (511 keV) and from deuterium formation (2.223 MeV) (the "neutron capture" line). There could also be continua, for example, from the bremsstrahlung of relativistic electrons.
The Observations
The first solar γ-ray event, the flare SOL1972-08-04, yielded many discoveries. Both the positron annihilation and neutron-capture lines appeared, as well as hints of nuclear lines at 4.4 and 6.1 MeV, as shown in Figure 2. Most significantly, the strong line at 2.2 MeV is produced following the formation of deuterium as hydrogen nuclei in the low solar atmosphere capture free neutrons, requiring the presence of high-energy protons or ions accelerated during the flare. This was an unambiguous signature of nuclear processes taking place on the Sun. Today, we know that the positron-annihilation line is also of nuclear origin. It comes from the production of beta-emitting nuclei, where the positive beta particle (positron) annihilates with an electron in the solar atmosphere
Conclusion
This event marked a turning point in the field of high-energy solar physics. It was first conjectured by Reuven Ramaty that the abundant energetic particles in space, at the Sun and at Earth, would radiate in gamma rays as they collide with interplanetary material, but only those actually striking the Sun could produce the deuterium formation line. Today we know that flares themselves are prolific producers of protons and ions that can induce nuclear reactions, and these are frequently distinct from the energetic particles in space that are produced by shocks associated with coronal mass ejections.
The observation of this flare, and another flare on August 7, helped motivate the development of the Solar Maximum Mission, launched in 1980, and then RHESSI itself. Each of these missions would swamp the community with abundant exciting data from flares of many sizes and forms. It all started a half-century ago. These August 1972 solar events also made it into James Michener's novel Space, where the radiation from these events would doom the crew of a fictitious Apollo 18 mission. These dangers indeed exist.
References
[1] "Solar Gamma Ray Lines observed during the Solar Activity of August 2 to August 11, 1972"
[3] "Observations of Solar Gamma Ray Continuum between 360 keV and 7 MeV on August 4, 1972"
| RHESSI Nugget Date | 8 August 2022 + |
| RHESSI Nugget First Author | Jim RYAN, Brian DENNIS, + |
| RHESSI Nugget Index | 434 + |
| RHESSI Nugget Second Author | and Phil DUNPHY + |
