The video below shows a summary of cosmic rays, their history and their importance within Astrophysics.
Cosmic Rays
Cosmic Rays are not actually "rays" at all but a stream of high Energy particles and rays from outside of our Solar System.
Cosmic rays come in a variety of forms, as show in the table below :-
The Energy associated with cosmic rays can be huge, upwards of 1 x 1018 eV.
Note - As a comparison, the greatest Energy collisions capable within the LHC are only in the 1x1012eV range.
The largest Energy particle detected was the "Oh my God" Particle, detected over Utah in 1991. This Ultra High Energy Cosmic Ray ( UHECRs )had an Energy of 3 x 1020 eV and was most likely a Proton. At present, astrophysicists are unable to explain the process that would generate such a high Energy event, but at least 15 more have been detected with similar Energy since, so it is not simply a data error. All that is understood about UHECRs is that they must occur quite close ( within ~100 million light-years ) as if they originated any further away, the UHECRs would have interacted with the CMBr and decayed into Pions.
The video below gives a more detailed look at the OMG Particle event ( and UHECRs in general )
Historical Proof of Origin
In the early 20th Century, an Austrian scientist called Victor Hess was working on measuring radiation. He found that even without an obvious radioactive source, radiation was detected - Background Radiation. At this point, all background radiation was thought to be from the Earth, so in order to prove this, Hess flew detectors from weather balloons.
His assumption was that the radiation level would drop the further from the Earth the balloon reached, proving the Earth as the source of background radiation. However, as the balloon got higher, even more radiation was detected.
Hess' conclusion that the increase in radiation was caused particles from space penetrating the atmosphere won him the Nobel Prize for Physics in 1936 for the discovery of Cosmic Rays.
However, the source of these cosmic rays was still up for debate; did they originate from the Sun or from further away ?
In order to prove one way or the other, detectors were flown on weather balloons during a total Solar eclipse. If the cosmic rays were coming from the Sun, then there should be a drop in detected radiation as the Moon would absorb some of the cosmic rays. However, if there was no change in the radiation levels detected, then the cosmic rays must have come from beyond our Solar System.
The results of this experiment showed that there was no change in the radiation levels detected - the Sun is not the source of cosmic rays.
Interactions with the Upper Atmosphere
When these high Energy cosmic rays enter the Earth's atmosphere, they interact with the gases that make up the atmosphere, producing a large chain of interactions called an Air Shower.
The diagram below shows an example of an air shower caused by a cosmic ray entering the upper atmosphere :-
The exact composition of the air shower depends upon three factors :-
1. Cosmic Ray type and Energy
2. Initial Atmospheric Gas particle interaction
3. Subsequent decay stages
Detecting Cosmic Rays
Due to the effect of the cosmic ray interactions within the upper atmosphere, there are two main areas to detect cosmic rays :-
Direct detection - above the atmosphere, by high altitude balloons or satellites.
Indirect detection - At ground level, by detecting the resulting air shower particles that reach the Earth's surface.
The Sun
The nearest star to Earth is, of course, the Sun. Being "only" 150 Million km from the Earth, Scientists have been able to study the Sun in great detail, making it the most well-understood star. The Sun itself, however, is a fairly unremarkable star; placed within an H-R Diagram, the Sun appears in the middle of the Main Sequence having approximately average Brightness, Temperature and Mass.
These observations have allowed scientists to understand the Physics of stars, to understand their composition, their Chemical and Nuclear process, as well as their full life cycle. By observing the Sun, an understanding of stars in general can be derived.
Stellar Composition
Note - The following information on the Sun's composition is non-examinable knowledge, however questions can be set in exam relating to this material as applications of other sections - e.g. Prominences are an example of charged particles moving in a Magnetic Field. Examinable material begins at Solar wind .
The Sun is a near-perfect sphere of hot Plasma ( Surface T ~ 5,800 K ) comprising mostly of Hydrogen. The structure of the Sun is split into six separate layers, as shown in the diagram below :-
In the above diagram, three layers combine together to form the Solar Atmosphere :-
1. The Photosphere
2. The Chromosphere
3. The Corona
Within this extended region (the Corona has been shown to extend to ~20 Solar Radii), there are many complex processes occurring as heat and light Energy are transferred from the Convention Zone below.
The Photosphere
The Photosphere is classed as the effective surface of the Sun. This surface from Earth appears quite featureless. However, if the Photosphere is imaged at high resolution, this region of the Sun is massively active, with the entire Photosphere showing a roiling surface covered in granules, pockmarked by Sunspots and Prominences as well as areas of massive eruptions called Solar Flares.
The image below shows a high resolution view of part of the Photosphere :-
In the above image areas of granulation can be seen across the entire limb of the Sun. Also a Solar Prominence can clearly be seen over the surface at the center of the image.
Granulation
The "mottled" patterning on the surface of the Sun is caused by a process called Granulation. This granulation is the result of convection currents within the Convection Zone below transferring warmer gas up to the Surface, whilst cooler gas sinks back down. As the Luminosity ( L ) is proportional to the 4th power of Temperature ( T 4 ), the small temperature difference gives a large contrast in brightness between the rising and falling gas.
Note - The "cooler" gas above is still at a Temperature and Luminosity greater than that of a Thermite reaction. It is darker only in comparison to the surrounding areas. This also applies to Sunspots (see below).
Prominences and Sunspots
As stated earlier, the large structure in the center of the image is known as a Prominence. A Prominence is a huge structure ( 1000s to 100,000s of km in length ) arching upwards from the Photosphere and curving back down to it again.
A Prominence forms when a region of the Magnetic field of the Sun loops out of the Sun's surface as shown in the diagram below :-
High Temperature Plasma ( consisting of ionised gases ) follows the Magnetic field lines like Iron filings around a Bar Magnet, creating the visible Prominence. The individual particles making up the Plasma are charged, and as such move in helical motion around the Field lines, as seen in Unit 3 : motion in a Magnetic Field.
As can be seen in the diagram above, Prominences and Sunspots occur together. A Sunspot is a region on the surface of the Sun which is cooler than that of the surrounding area. The Sunspot is caused by the Magnetic field preventing the hot gas rising to the surface, giving a relatively cool patch (again hotter than Thermite). These Sunspots can be huge, covering an area greater than 150,000 km in diameter.
The diagram below shows a large Sunspot, with the Earth also shown for scale :-
Sunspots are an outward sign of the seasons on the Sun. The Sun follows a cycle of variations in activity on an 11 year cycle. When the Sun is very active, the Sunspot number increases and the number of Solar Flares increases also (See Below). When the Sun is inactive the Sunspot number can drop to zero, with very few Solar Flares.
The graph below shows Sunspot numbers for the last 400 years :-
As of 2020, the Sun is currently at the minimum of the Solar Cycle, and the Solar disk appears almost blank.
The image below shows the Solar disk imaged on March 29th 2020 :-
As can be seen, as this image was taken during a Solar Minimum, the disk appears blank - no Sunspots.
The image below shows the Solar disk as imaged on March 29th 2001 :-
As can be seen in the above image, the surface of the Sun is very active, with multiple Sunspot groups across its surface. The Sunspot group 9393 (middle of image) was the largest Sunspot group in the last 30 years, with a diameter greater than 26 Earth Radii.
The Solar Wind and the Aurora
Due to the high temperature found within the Corona, high energy charged particles stream continually outward from the Sun known as the Solar Wind. The Solar Wind moves away from the Sun in a spherical shell, travelling at a velocity of approximately 200-800 kms-1. During a Coronal Mass Ejection (see below) this velocity can increase up to ~1000 kms-1. At this velocity, the Solar Wind can reach the Earth within 1-2 days.
When the Solar Wind reaches the Earth, it interacts with the Earth's Magnetic field - the Magnetosphere.
The Earth's Magnetic field up to this point has been described as the Field around a huge Bar Magnet, as shown in the diagram below :-
However, this is too simple of a description. In the real system, the Solar Wind deforms the Magnetosphere into a teardrop shape, as can be seen in the diagram below :-
The Earth's Magnetosphere acts like a shield around the Earth, deflecting the Solar Wind from impacting the Earth directly.
The incoming charged particles in the Solar Wind are deflected by the Magnetosphere, with the particles following a range of paths depending on the angle at which they interact with the field (see Electromagnetism for more information).
If the charged particles reach the Magnetosphere at a certain angle, the particles can follow a helical path, orbiting the Magnetic field lines and entering the atmosphere around the poles. The diagram below shows this process occurring :-
When these charged particles enter the Earth's atmosphere, they interact with molecules of gas within the atmosphere. The gas molecules enter an excited state, causing Electrons to enter higher energy levels. When these Electrons return to the ground state, their energy is released in the form of a Photon, causing the air molecules to give out light. This light is known as the Aurora. The diagram below shows a summary of this process :-
The video below shows a summary of the formation of Aurora
Coronal Mass Ejections and Solar Flares
In the section above, an explanation of the formation of a Prominence was given. These large structures of ionised Plasma normally fall back down to the Sun's surface, but sometimes this does not happen. If the Magnetic field becomes twisted, it can jump to a new location. As the Magnetic field shifts, it releases the energy stored within it, releasing a burst of Electromagnetic Radiation away from the sun known as a Solar Flare. The shift in the field can also release a large amount of Plasma (~ 2x1012 kg) from the surface, known as a Coronal Mass Ejection (CME). These explosions of material from the surface of the Sun carry high energy waves and particles away from the Sun.
A Coronal Mass Ejection can have a large effect on the Earth, if the CME is Earth-directed, ie the burst of Plasma reaches the Earth. The CME, however, doesn't actually impact the Earth itself, it is blocked by the Earth's Magnetosphere. When the large Mass of Plasma comes into contact with the Magnetosphere, the Magnetosphere becomes compressed by the impact. This causes changes in the position and strength of the Field across the Earth. This changing Magnetic field causes induced EMFs across long wires on the Earth, as well as in the ground itself. The CME also greatly increases the number of charged particles entering the atmosphere at the Poles, causing intense Aurora during the impact. The stream of charged particles can also damage Electronics in satellites, as well as exposing any astronauts or high altitude aircraft to increased levels of radiation.
On April 4th 2001, Sunspot group 9393 released the largest Coronal Mass Ejection ever imaged. The CME was not directed towards the Earth, but if it had been, would have caused major disruption to Power systems and communications across the Earth as well as posing significant risk to the astronauts aboard the International Space Station.
The animation below shows a Solar Flare on the 4th of April 2001, the largest ever recorded.
Note - The disk of the Sun in blocked by a shield within the SOHO telescope optics, without this shield the Corona could not be imaged as shown.
The Carrington Event
The CME in 2001 was the largest CME imaged, but it is not the largest known. The Largest CME known that struck the Earth is known as the Carrington Event of 1859.
On September the 1st 1859, British astronomer Richard Carrington observed a huge Solar Flare erupting from the Sun's surface. The next day, Aurora could be seen as far south as the Caribbean and Mexico, with the Aurora so bright over California that miners in the Californian Gold rush woke and started to prepare breakfast, thinking it was dawn. In the northeastern US, it was possible to read newspapers solely by the light of the Aurora.
The colossal stream of charged particles entering the atmosphere caused huge ground currents with disrupted the telegraph systems across Europe and the US. Some telegraph operators were electrocuted and the telegraph lines themselves threw off sparks for several hours.
It has been estimated that if a Carrington-class CME struck the Earth today, the disruption would be huge, due to society's dependence on electronics, all of which would be affected by the impact. It is estimated that it would cost the US alone ~ 2 Trillion Dollars in damage to infrastructure, communications, data loss and physical damage.
The video below shows a summary of the effect of a Carrington-class CME impact
