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At the start of the 20th century, scientists have been puzzled by the fact that the air in electroscopes-instruments became charged or ionized no matter how well the containers were insulated.

It was thought that radioactivity from ground minerals was responsible. So if this were the case the effect should diminished with altitude.

In 1912 the puzzle was partly solved by an Austrian physicist, Victor Hess, who took a gold leaf electroscope on a balloon flight. Discovery of cosmic rays When an electroscope is charged, its leaves repel. A radioactive source can ionize air molecules, which carry away charge, ending the repulsion.

1In the late 1800s and early 1900s, physicist designed various instruments to characterize the radio activity found in certain materials.

Scientists have been puzzled by the fact that the air in electroscopes-instruments for detecting charge particles became electrically charged (ionized) no matter how well the containers were insulated.

It was thought that radioactivity from ground minerals was responsible, but if such were the case the effect should diminished with altitude.

In 1912 the puzzle was partly solved by an Austrian scientist, Victor Hess, who took a gold leaf electroscope on a balloon flight.Instead of observing less background as he got farther away from the ground, the only source of radioactivity then known, the amount of radiation increased with altitude. He concluded, "a radiation of very high penetrating power enters our atmosphere from above."

Hess lands following a historic 5,300 meter flight. August 7, 1912National Geographic photographAs his balloon ascended to 5300m, he observed the rate of charging in his electroscope increased with altitude.Initially Hess's theory about rays from space did not receive general acceptance, but additional observations after World War I supported it. The newly discovered radiation was dubbed Cosmic Rays" by Robert A. Millikan in 1925. 2As his balloon ascended to 5300m, he observed the rate of charging in his electroscope increased with altitude.Instead of observing less background as he got farther away from the ground, the only source of radioactivity then known, the amount of radiation increased with altitude. He concluded, "a radiation of very high penetrating power enters our atmosphere from above." Initially Hess's theory about rays from space did not receive general acceptance as one would expect !!There was even some panic of the idea of raining harmful radiation from outer space.. And there were some who thought he should bring a oxygen bottle with him next time.. In any event increased research after World War I supported it. The newly discovered radiation was dubbed Cosmic Rays" by Robert A. Millikan in 1925.

The bulk of cosmic rays, called Galactic Cosmic Rays come from supernova explosions within our Galaxy.

Fermis 1st order shock acceleration theory predicts a power law spectrum.

Local magnetic inhomogeneities are dragged by the shockParticle acceleration occurs across the shock (1st order Fermi Acceleration)Rest frame of a shock waveStrong shock waves are generated from Super Nova

33Roughly every 50 years a super nova occurs within our galaxy, today it is generally accepted the bulk of cosmic rays come from these events. The resulting expanding shock drives surrounding magnetic clouds. These magnetic clouds or inhomogeneities act as particle mirrors or scattering centers. Each time a charged particle crosses the shock boundary, energy from the system is transferred to the particle. Since the escape probability from these acceleration site is proportional to energy, a power law spectrum is expected. This process is referred as Fermis 1st order shock acceleration.

A power law index of 2.1 is estimated at the acceleration site, however the effects from Galactic propagation to Earth produces a softer spectrum yielding roughly 2.7 for nucleons and 3 for electrons.

6Arriving the edge of the heliosphere, cosmic rays encounter the solar wind, a low density stream of hot plasma emanating radially from the sun.Solar magnetic field is embedded in the solar wind and carried outward becoming interplanetary magnetic field (IMF).The interplanetary magnetic field lines are not perfectly smooth, but are wavy / kinky in some places. These isolated fluctuations give rise to regions that act as a particle diffuser.

The population density and amplitudes of these magnetic fluctuations vary with the solar activity cycle effecting the energy spectrum of cosmic rays at Earth

This time variation in cosmic rays is referred to as Solar Modulation which 11 year cycle

Trajectory of charged particle in a smooth magnetic field11 year cycle animation(rotation rate is scaled for viewing)

44When Cosmic Rays arrive the edge of the heliosphere, they encounter the solar wind, a low density stream of hot plasma emanating from the sun. The heliosphere is defined as the region of space dominated by the solar magnetic field or Interplanetary Magnetic Field (IMF) carried outward by the Solar Wind.

The interplanetary magnetic field lines are not perfectly smooth, but are wavy / kinky in some places. These isolated fluctuations give rise to regions that act as a particle diffuser.

The population density and amplitudes of these magnetic fluctuations vary with the solar activity cycle effecting the energy spectrum of cosmic rays at Earth

This time variation in cosmic rays is referred to as Solar Modulation which 11 year cycle

Thule, Greenland Neutron MonitorSun Spot Number Monthly averageLong term Effect of Solar Modulation

Balloon and space measurements of proton and He ion spectraDifferent times / solar modulations different rigidity spectraPrimary spectrum at Earth pHe6Rigidity = momentum / electric charge = measure of resistance to deflection by a magnetic field6These are observations of the primary cosmic ray spectrum for both protons and Helium during different times and different levels of solar modulation resulting in a variation in spectra shapes. The kinematic parameter used is rigidity. Rigidity is defined as the momentum of a particle divided by the electric charge. It represents the resistance to deflection by a magnetic field. The key point is that the effects of solar modulation is rigidity dependent. Higher rigidity particles are less influenced by modulation effects than lower rigidity. Primary cosmic rays must pass through the Earths magnetic field to enter the atmosphere.

The geomagnetic field is more effective at shielding the atmosphere from cosmic rays near the equator than at the poles, producing a latitude effect.

Shielding effectiveness is quantified by the Geomagnetic Cutoff Rigidity. This is the minimum rigidity (momentum/charge) a particle must have to pass through the Earths magnetosphere and enter the atmosphere.

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Global contour plot of the geomagnetic cutoff rigidity7Upon reaching Earth primary cosmic rays must pass through the geomagnetic field to enter the atmosphere. The geomagnetic field is more effective at shielding the atmosphere from cosmic rays near the equator than at the poles, producing a latitude effect mainly Shielding effectiveness is quantified by the Cutoff Rigidity. This is the minimum rigidity (momentum/charge) a particle must have to pass through the Earths magnetosphere and enter the atmosphere

8Atmospheric Propagation Primary galactic cosmic rays entering the atmosphere Some of these primaries are energetic enough to produce a nuclear or high energy interaction initiating a cascade of particles through the atmosphere. As the ensemble of cascades develop the particle density and the particle type distribution varies with atmospheric depthParticle fluxes at sea-level If these particles enter the atmosphere, most are energetic enough to produce a nuclear or high energy interaction initiating a cascade of particles through the atmosphere. As the ensemble of cascades develop the particle density and the particle type distribution varies with atmospheric depth as shown in the Figure.

The passage of each particle type through atmosphere is determined by different interaction channels that are more dominate to occur for different particle types. It is interesting to note particle maximum occurs around 40,000 ft with low energy neutrons dominating the population. The progenitor to these particles are mainly protons and helium. Note that muons and neutrons are unstable particles and are produced only during interactions. On the ground at sea-level above energies of 100MeV muons are the dominate species, however when considering all energies neutrons dominate in numbers.

8By the time you finished reading this sentence roughly a dozen electrons, muons, neutrons and gamma rays just passed through your body.

In review, this is how they got here. 1) Supernova (source/acceleration site)2) Galactic Propagation (source to Heliosphere boundary)3) Heliospheric Propagation (Solar Modulation)4) Geomagnetic Propagation (Cutoff Rigidity) 5) Atmosphere Transport (TOA to ground level)

9Cosmic Ray Gantlet 9By the time you finished reading this sentence roughly a dozen electrons, muons, neutrons and gamma rays just passed through your body. Lets review how they got hereCosmic rays begin at the Supernova and then propagate through the galaxy to the helisophere. After moving upstream the solar wind for roughly 100AU, it reaches the geomagnetic field. After passing through the geomagnetic field, it enters the atmosphere producing cascades of secondaries that make it to ground level. What a journey !! If only cosmic rays could tell us their story..

The rest of the presentation will focus on how the Neutron Monitor responds to these particles

10Measurements from the ground provide a large aperture cosmic rays making it capable to study very high energy events, however a significant amount of information is lost through the atmosphere

Measurements from the ground provide a large aperture cosmic rays making it capable to study very high energy events, however a significant amount of information is lost after CR pass through the atmosphere to ground level. 10

A significant amount of information about the structure of the Galaxy, Sources and Heliosphere can be inferred from Cosmic Ray Observations above the atmosphere

A significant amount of information about the structure of the Galaxy, Sources and Heliosphere can be inferred from a CRs, such as relative spectra shape and composition. Providing a means to send instruments above the atmosphere is critical for our research. 1112

If the solar system composition is a template of source particle composition, one could infer the differences are the result of propagation through galaxy. If the solar system composition is a template of source particle composition, one could infer the differences are caused by propagation through galaxy 12NASA Balloon Program

The primary objective of the NASA Balloon Program is to provide high altitude platforms for scientific and technological investigations.

These investigations include fundamental scientific discoveries that contribute to our understanding of the Earth, the solar system, and the universe.

The platforms also provide demonstration opportunities of potential new instrument and spacecraft technologies

As with Hess, 100 years ago, one possible means is a high altitude balloon.

The primary objective of the NASA Balloon Program is to provide high altitude platforms for scientific and technological investigations.

These investigations include fundamental scientific discoveries that contribute to our understanding of the Earth, the solar system, and the universe.

The platforms also provide demonstration opportunities of potential new instrument and spacecraft technologies

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ANITA Antarctic Impulsive Transient Antenna

AESOPAnti-Electron Sub-Orbital PayloadLEELow EnergyElectronsUniversity of DelawareDepartment of Physics and Astronomy Balloon ProgramActive Projects

AstrophysicsUltra High Energy NeutrinosSpace Physics and AstrophysicsPositrons and ElectronsGeomagnetic and Space PhysicsElectrons and X-raysHigh altitude ballooning has been an integral part of cosmic ray research at the University of Delaware for over the past 25 years. The research includes a range of investigations such as the solar wind influence on the galactic cosmic ray propagation through the solar system, spectra of Jovian electrons and extra-Galactic neutrinos.

The ANITA payload provides a means to detect of the highest-energy sources and processes in the universe. The instrument designed to detect UHE neutrinos by recording broadband radio pulses from particle showers produced by neutrino interactions in Antarctic ice.

The LEE instrument has flown 22 times since 1968 and consequently providing long time series of cosmic ray electron spectra up to roughly 5 GeV while AESOP, utilizing an magnetic spectrometer, separates positrons from electrons over a similar energy range. The LEE and AESOP instruments were designed to be low weight systems primarily to maximize balloon altitudes and reducing background while maintaining the ability to achieve adequate number of events during a typical flight.

AESOP and LEE are maintained at the University of Delaware while ANITA is maintained primarily at the University of Hawaii with Delaware as a collaborating instition. 14The LEE Payload

LEE detects electrons withPlastic scintillators T1, T3 and G (anticoincidence)Gas Cherenkov detector T2. It measures the electron energy with Cesium iodide (T4) calorimeter Lead glass (T5) calorimeterScintillator T6 assists in particle identification and energy determination by counting the number of particles that escape the calorimeter. 2515LEE

Balloon observations of electrons with the LEE begun in 1968 at the University of Chicago and has continued at the UD-Bartol Research Institute since 1984. The data from these balloon flights have been used to study solar modulation of electrons with energies up to ~ 20GeV.

Flight Log for LEE

16balloon observations of electrons with the LEE begun in 1968 at the University of Chicago and has continued at the Bartol Research Institute since 1984. The data from these balloon flights have been used to study solar modulation of electrons with energies up to ~ 20GeV. The above chart illustrates the time span of exposures which is needed to study modulation.

Outward flowing solar wind and solar rotation produce a spiral geometry of the interplanetary magnetic field lines. A+ is shorthand for the case where the dipole has a positive projection on the solar rotation axis is positive whereas the opposite projection is termed A-. Reversals of the solar magnetic field occur every 11 years.q>0q