Fifty-year Anniversary of the First Detection of Gamma rays from a Solar Flare

{{Infobox Nugget
|name = Nugget
|title = Fifty-year anniversary of the first detection of gamma-rays from a solar flare  
|number = 434
|first_author = Jim RYAN, Brian DENNIS, 
|second_author = and Phil DUNPHY
|publish_date = 8 August 2022
|next_nugget = TBD
|previous_nugget = {{#ask: [[Category:Nugget]] [[RHESSI Nugget Index::433]]}}
}}

== 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 
([https://en.wikipedia.org/wiki/OSO_7 OSO-7], as shown in Figure 1)
made the first-ever detection of &gamma;-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 
[https://eos.org/articles/edward-l-chupp-1927-2017 Prof. Edward L. Chupp] 
and his team at the 
[https://www.unh.edu 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 &gamma;-ray photons with energies between 0.3 and 10 MeV.
These photons have energies many millions of times greater than photons
of visible light.

[[File:434f1.png|700px|thumb|center|<b>Figure 1:</b>
<i>
Left, the OSO-7 spacecraft was stabilized by the spinning lower wheel section that allowed the upper "sail" at the top to continuously point at the Sun.
Right, details of the pioneering &gamma;-ray instrument (Ref. [1]) that was mounted in the lower wheel structure so that it continuously scanned over the Sun each rotation.
</i>
]]

== Solar &gamma; rays ==

The observational discovery of solar &gamma; 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 
[https://en.wikipedia.org/wiki/Antimatter positron] annihilation (511 keV)
and from 
[https://en.wikipedia.org/wiki/Deuterium deuterium] formation (2.223 MeV)
(the "neutron capture" line).
There could also be continua, for example, from the
[https://en.wikipedia.org/wiki/Bremsstrahlung bremsstrahlung] 
of relativistic electrons.

== The Observations ==

The first solar &gamma;-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

[[File:434f2.png|600px|thumb|center|<b>Figure 1:</b>
<i>
The spectrum from the pioneering OSO-7 observation, as adapted from Ref. [3].
The darker histogram is from the solar direction, and the lighter one is
from the anti-solar direction. The <sup>60</sup>Co lines are in both 
sectors because they come from the on-board 
[https://www.iaea.org/topics/nuclear-science/isotopes/radioisotopes radioisotope] 
calibration source.
</i>
]]

== Conclusion ==

This event marked a turning point in the field of high-energy
solar physics.  
It was first conjectured by 
[https://en.wikipedia.org/wiki/Reuven_Ramaty 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
[https://heasarc.gsfc.nasa.gov/docs/heasarc/missions/solarmax.html 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 
[https://en.wikipedia.org/wiki/Space_(Michener_novel) Space], where the radiation from these
events would doom the crew of a fictitious Apollo 18 mission.
These dangers indeed exist.

== References ==

[1] [https://ui.adsabs.harvard.edu/abs/1973Natur.241..333C "Solar Gamma Ray Lines observed during the Solar Activity of August 2 to August 11, 1972"]

[2] [http://adsabs.harvard.edu/abs/1958NCim....7..858M "On gamma-ray astronomy"]

[3] [https://ui.adsabs.harvard.edu/abs/1975SoPh...43..415S "Observations of Solar Gamma Ray Continuum between 360 keV and 7 MeV on August 4, 1972"]