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Astronomy 305/Frontiers in Astronomy. Class web site: Office: Darwin 329A and NASA E/PO (707) 664-2655 Best way to reach me: [email protected] What is the origin of cosmic rays?. Discovery of Cosmic Rays

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Astronomy 305 frontiers in astronomy
Astronomy 305/Frontiers in Astronomy

Class web site:

Office: Darwin 329A and NASA E/PO

(707) 664-2655

Best way to reach me: [email protected]

Prof. Lynn Cominsky

What is the origin of cosmic rays
What is the origin of cosmic rays?

  • Discovery of Cosmic Rays

  • General properties of cosmic rays

  • Cosmic rays from the Sun

  • Accelerating Cosmic rays

  • Detecting cosmic rays

  • The highest energy cosmic rays

  • New cosmic ray experiments

Prof. Lynn Cominsky

Types of radiation
Types of Radiation

  • We have discussed electromagnetic radiation aka light – massless, travel at v=c

  • When scientists first started studying radiation in the early 1900s, they found 3 different types of rays

    • Alpha rays – turned out to be Helium nuclei

    • Beta rays – turned out to be electrons and positrons

    • Gamma rays – turned out to be light

  • Detectors invented to study radiation included Geiger counters, film and electroscopes

Prof. Lynn Cominsky

Discovery of cosmic rays
Discovery of Cosmic Rays

  • Viktor Hess (1912) takes electroscope on a balloon flight to 17,500 feet (without oxygen!)

  • He was trying to find the source of additional radiation seen at ground level that could not be explained by natural sources of radioactive decay

Prof. Lynn Cominsky

Hess experiment
Hess’ Experiment

  • Hess used an electroscope – detects charge on 2 thin films


  • When the cosmic rays hit the electroscope, they carried away charge

  • More cosmic rays  electroscope would discharge faster

  • Hess won the Nobel prize in 1936 for his discovery of cosmic rays

Prof. Lynn Cominsky

What are cosmic rays
What are cosmic rays?

  • Cosmic rays are charged particles such as protons, electrons and nuclei of atoms

  • They are NOT electromagnetic radiation (aka light)

  • However, sometimes cosmic rays interact with gas in our galaxy to make gamma rays

Prof. Lynn Cominsky

High energy gamma ray map
High energy Gamma-ray map

Gamma rays in the plane of the galaxy made from cosmic rays hitting gas

Prof. Lynn Cominsky

Composition of cosmic rays
Composition of cosmic rays

  • Cosmic rays are made of nuclei of different elements (and also electrons)

  • The percentage of each element of different types is called “composition”

  • All the nuclei of the elements in the periodic table are present in cosmic rays

  • The composition of cosmic rays is about the same as that of the elements in the solar system

  • Various isotopes of elements are also detected though harder to distinguish

Prof. Lynn Cominsky


  • Advanced Composition Explorer


  • Launched 8/25/97, still operational

  • Stays near L1 point in Earth-Sun Orbit

  • Studies particles in solar wind, interplanetary medium, interstellar medium and galactic matter

Prof. Lynn Cominsky

Properties of cosmic rays
Properties of cosmic rays

  • 90% of cosmic rays are hydrogen nuclei (aka protons)

  • 9% are helium nuclei

  • 1% are all the other elements

  • Thousands of low-energy cosmic rays hit every square meter of the Earth each second

  • High energy cosmic rays are rare – less than 1 per km2 per century

Prof. Lynn Cominsky

Charged particle in magnetic field
Charged particle in magnetic field


  • Magnetic fields change the direction of travel of charged particles (opposite effect for positive vs. negative particles)

  • Since the paths of cosmic rays are changed as they travel through space, it is difficult to figure out where they originated

Prof. Lynn Cominsky

Cosmic rays vs gamma rays
Cosmic rays vs. gamma rays

  • Cosmic rays (1) are deflected by magnetic fields in space

  • Gamma rays (2) travel in straight lines, unaffected by magnetic fields

Prof. Lynn Cominsky

Cosmic ray spectrum
Cosmic ray spectrum

This is a plot of how many cosmic rays are detected as a function of energy at the top of the Earth’s atmosphere

Prof. Lynn Cominsky

Solar flares make low energy crs
Solar flares make low energy CRs

  • Solar flares originate in sunspots

  • Magnetic field in sunspots stores energy than is released in solar flares

  • Sunspots often occur in pairs or groups

  • The more complex the groups, the greater probability of a resulting flare

  • A large flare has 106 times more energy than a large earthquake

Prof. Lynn Cominsky

Solar flares

Solar prominence seen by Skylab in 1973

Solar Flares

SOHO/MDI 11th magnitude earthquake on Sun following solar flare

Prof. Lynn Cominsky

Solar activity cycle



Solar Activity Cycle

  • Every 11 years, sunspots and X-rays increase

  • Increased radiation causes Earth’s atmosphere to expand

  • Solar flares cause radio interference

Prof. Lynn Cominsky

Space weather
Space Weather

  • For the latest on Space Weather, including solar flares, aurorae, blackouts, and sunspots, see

We are just past Solar Max 23!

Prof. Lynn Cominsky

Coronal mass ejections
Coronal Mass Ejections

  • CMEs are the cause of major geomagnetic storms on Earth

  • CMEs are NOT caused by solar flares, although they may both be signatures of rapid changes in the magnetic field

  • 1015 - 1016 g of material is ejected into space at speeds from 50 to >1200 km/s

  • Can only be observed with coronagraphs

Prof. Lynn Cominsky

Coronal mass ejections1

Solar Maximum Mission CME in 1989

Coronal Mass Ejections

Prof. Lynn Cominsky

Solar flares affect the earth
Solar flares affect the Earth

  • Light in solar flares travels at the speed of light (8.5 minutes to reach Earth)

  • Relativistic particles travel at near light speed – arrive in 20 minutes to hours

  • Bulk material ejected from Sun travels at 400-1000 km/hour – takes ~1 day to reach Earth

  • Charged particles that hit the Earth create aurorae

Prof. Lynn Cominsky


  • Best observed near the magnetic poles

  • Colors are due to different molecules at different heights in the Earth’s atmosphere – mostly oxygen and nitrogen

Recent auroral location

Prof. Lynn Cominsky

South atlantic anomaly and crs
South Atlantic Anomaly and CRs

  • Region where the Earth’s magnetic field dips that allows CRs to reach lower into the atmosphere

Prof. Lynn Cominsky

Medium energy cosmic rays
Medium-energy Cosmic Rays

  • 1012 – 1015 eV

  • Composition at Earth’s atmosphere

    • 50% protons

    • ~25% alpha particles

    • ~13% C/N/O nuclei

    • <1% electrons

  • Believed to originate outside of solar system but inside of Milky Way galaxy

Prof. Lynn Cominsky

Possible sources of galactic crs
Possible Sources of Galactic CRs

  • Energetic places in the Galaxy

    • Black Holes

    • Neutron stars

    • Pulsars

    • Supernovae

Red = Xrays

Blue = UV

Green = ionized H

30 Doradus star forming region

Prof. Lynn Cominsky

Accelerating cosmic rays
Accelerating cosmic rays

  • Medium energy cosmic rays must be accelerated by shock waves in our galaxy

  • Much research is going on to conclusively prove that supernovae can accelerate cosmic rays to medium energies

  • Supernovae are believed to be able to accelerate CRs up to the energy of the “knee” 3 x 1015 eV

  • How do we prove that supernovae are really the acceleration sites for CRs?

Prof. Lynn Cominsky

Asca x ray astronomy satellite
ASCA X-ray Astronomy satellite

  • ASCA = Advanced Satellite for Cosmology and Astrophysics aka Asuka or flying bird

  • Japanese X-ray astronomy satellite that observed 1993-2001

Prof. Lynn Cominsky

Asca and sn1006
ASCA and SN1006

Prof. Lynn Cominsky

Asca and sn10061
ASCA and SN1006

  • First direct evidence that supernovae can accelerate cosmic rays

  • Non-thermal synchrotron spectrum at the edges of the supernova where the shocks should occur

  • Thermal spectrum in the center of the supernova due to hot gas from explosion

  • Magnetic field in SN1006 exactly the right strength to accelerate CRs up to the “knee”

Prof. Lynn Cominsky

Detecting cosmic rays
Detecting cosmic rays

  • Cosmic rays are further classified into primaries and secondaries

  • Primaries are the particles which hit the Earth’s atmosphere

  • Secondaries are created by interactions between the primaries and the air molecules

Prof. Lynn Cominsky

Air showers of secondary crs
Air showers of secondary CRs

  • Secondaries are primarily “pions” –elementary particles with charge + - or 0

  • Charged pions hit other air molecules

  • Neutral pions decay into 2 gamma rays which then create positron/electron pairs

  • Cascade includes UV fluorescent emission, more charged particles and Cerenkov radiation – blue light caused by very fast particles moving through the atmosphere at faster than the local speed of light

Prof. Lynn Cominsky

Air showers of secondary crs1
Air showers of secondary CRs

Prof. Lynn Cominsky

Shower maximum
Shower maximum

  • Cascade continues until average particle in the shower is not energetic enough to create new particles  “shower maximum”

  • After shower maximum, particles are absorbed by atmospheric molecules and shower intensity decreases

  • Shower maximum: for each 1 GeV energy in primary cosmic ray, shower has 1-1.6 particles

  • For primaries > 1015 eV, enough particles reach ground to be detected in detector array

Prof. Lynn Cominsky

Extensive air shower arrays
Extensive air shower arrays

  • “Footprint” of shower extends several hundred square meters

  • Particles are traveling at speeds near c

  • By comparing arrival times at different detectors, direction of origin can be determined within 1o

Prof. Lynn Cominsky

Air cerenkov telescopes
Air Cerenkov telescopes

  • Cerenkov light is imaged onto segmented optical light telescopes

  • Showers initiated by gamma rays with E>TeV can be distinguished from CR showers by analyzing the shape of the shower profile

Prof. Lynn Cominsky

Ultra high energy cosmic rays
Ultra-high energy cosmic rays

  • Believed to originate outside of our Galaxy but perhaps in the local group

  • For CRs above the “knee” (>3 x 1015 eV) some other acceleration process must occur

  • Jets from active galaxies are often theorized to be the accelerators

  • What are they?

  • Where do they come from?

  • How did they get so much energy?

Prof. Lynn Cominsky

Air fluorescent detectors
Air “Fluorescent” Detectors

  • UV light flashes emitted from (mostly) Nitrogen molecules are focused and imaged with detectors on telescopes

NOTE: UV light is really scintillation not fluorescence – which is remission at visible light of UV light

Prof. Lynn Cominsky

Fly s eye 1981 1993
Fly’s Eye: 1981-1993

  • Pixels on sky from telescope array are hexagonal tiles like a fly’s eye – eventually a second array was built for stereo vision


Prof. Lynn Cominsky

Akeno giant air shower array
Akeno Giant air shower array

  • AGASA is in Japan

  • 111 surface detectors and 27 muon detectors under ground in 100 km2 separated by 1 km

  • Combining muon and surface detectors yields composition of primary cosmic ray

Prof. Lynn Cominsky


  • Large pieces of material (usually inorganic salts or organic plastics) that emit visible light when hit by CRs

  • Often used for gamma rays as well

AGASA Scintillator

Prof. Lynn Cominsky

Muon detectors
Muon Detectors

  • Many of the secondaries are muons – negatively charged particles that are cousins to electrons but 186 times more massive

Prof. Lynn Cominsky

Water cerenkov detectors
Water Cerenkov Detectors

  • Tanks of water surrounded with photo-multipliers to detect the blue Cerenkov light emitted in the water

AGASA Water Cerenkov Detector

Prof. Lynn Cominsky

Agasa highest energy event
AGASA highest energy event

  • 3 x 1020 eV – second highest energy cosmic ray ever detected

  • Shower spread over 6 x 6 km2

  • Billions of particles in shower

  • Primary probably an oxygen nucleus or similar element

Prof. Lynn Cominsky

Agasa anisotropy
AGASA anisotropy

  • CRs greater than 1019 eV seen in 11 years of observations with AGASA

  • Red are > 1020 eV, green are 4-10 x 1019 eV

  • Circles are clusters of events within 2.5o

Prof. Lynn Cominsky

Agasa data ankle to gzk cutoff
AGASA data – “ankle” to GZK cutoff

  • >1020 eV energy CRs from > 150 million light years away should not reach the Earth due to collisions with the photons in the microwave background  “GZK cutoff”

Prof. Lynn Cominsky

Pierre auger observatory being built
Pierre Auger Observatory (being built)

  • 2 water Cerenkov arrays to detect the highest energy cosmic rays – one each in the northern and southern hemispheres

  • Each location occupies 3000 km2 and has 1600 detectors



Prof. Lynn Cominsky

Pierre auger observatory argentina
Pierre Auger Observatory (Argentina)

  • 30 detectors are now operational (out of 1600 planned)

  • 2 fluorescence detectors are working (out of 24 planned)

A building housing a fluorescence detector

Prof. Lynn Cominsky

Why should we care about crs
Why should we care about CRs?

  • We are constantly exposed to background radiation from secondary CRs

  • Exposure is greater in airplanes, mountains

  • CRs produce C14 used for carbon dating

  • CRs produce single-event-upsets (mistakes) in space-based computer chips

  • We want to understand how nature can accelerate particles to near light speed

  • Highest energy CRs could signify new physics

Prof. Lynn Cominsky

Aspire lab on cosmic rays
ASPIRE lab on cosmic rays

  • Go to

  • Try at least the Hess’ balloon ride (Activity 1). Be sure to integrate counts for at least 20 seconds. What do you conclude about the origin of cosmic rays?

  • Also try activities 2, 3 and 4 if you have time.

Prof. Lynn Cominsky

Web resources
Web Resources

  • Imagine the Universe

  • Java demo

  • Cosmic and Heliospheric Learning Center

  • Astronomy Picture of the Day

Prof. Lynn Cominsky

Web resources1
Web Resources

  • History of cosmic rays

  • Pierre Auger Observatory

  • Adelaide Astrophysics Group



Prof. Lynn Cominsky