From MariachiWiki

Contents 1 Cosmic Rays 2 The Mystery of UHECR and MARIACHI 3 Cosmic Ray and Radar 4 Links for Further Reading

Cosmic Rays

Cosmic rays are particles (maily consisted of protons) that are produced in cosmological events. They were first discovered by Victor Hess in 1912 during his balloon flights over Vienna, Austria. Ever since their discovery the nature and origin of these cosmic rays have been a mystery. By the 1930's, the subject of cosmic rays was intensively debated in public as shown in this 1932 article from the New York Times featuring Robert Millikan and Arthur C. Compton. Later, in 1938, Pierre Auger, discovered extensive showers by simultaneously detecting particles at two detectors located at some distance from one another. The most comprehensive and recent review of the scientific literature on cosmic ray research is an article written by Gaisser and Stanev for the Particle Data Group.

Thanks to the development of numerous techniques the cosmic ray energy spectra has been measured to very high energies.

The Mystery of UHECR and MARIACHI

Ultra High Energy Cosmic Rays (UHECR) with energies in excess of 10²⁰eV (100 EeV) have been detected by several experiments, albeit with low statistics. They present a conundrum whose solution may provide insight into the origins and evolution of the universe. There are no known sources within our galaxy or those close to us that could accelerate particles to these almost macroscopic energies, and yet the turn-on of pion production through the interactions of high energy charged particles with the 2.7K microwave background provides a strong limit (Greisen, Zatsepin, Kuzmin (GZK) cutoff) for propagation from greater distances. The associated neutrino flux, which does not suffer such a cutoff, has not yet been detected, and may provide an even wider window and better directionality on ultra high energy processes in the universe.

The detection of UHECR to date has been accomplished either by detection of the particles from the extensive air showers (EAS) by ground arrays (scintillator or water Cerenkov detectors); or by means of detection of the light produced by the EAS in the atmosphere from Cerenkov radiation or fluorescence of atmospheric nitrogen. MARIACHI (Mixed Apparatus for Radar Investigation of Cosmic-rays of High Ionization), as shown in Figure 1, is an innovative concept that will explore the detection of UHECR by observing radio signals originating from commercial VHF transmitters being reflected (or absorbed) by the high ionization densities produced in the EAS. If successful, the MARIACHI technique of Radio Cosmic Ray Scatter (RCRS) will allow for detection of UHECR economically over much larger areas than currently possible, and might provide for detection of the associated ultra high energy neutrino (UHEN) flux. MARIACHI is also innovative in that ground array detectors that will initially confirm the radio signals are scintillator arrays to be built and operated by high school students and teachers. The data gathered by MARIACHI will include other sporadic ionized atmospheric phenomena such as meteors and lightning. Thus MARIACHI is unique not only for being on the forefront of cosmic ray detection techniques, but also for its incorporation of a diverse and interdisciplinary group of scientists, educators, and students in a large collaboration for the support of cutting-edge research.

Cosmic Ray and Radar

The first experimental attempt to detect UHECR using radar was performed by Bernard Lovell and Patrick M.S. Blackett in the early 1950's. In the 70's, Suga performed similar experiments but was unsuccessful. In Lovell's experiment, the observed signals were later interpreted as reflection from meteor trails. Indeed, ionization trails left by meteors when they vaporize in the upper atmosphere are now routinely detected by a technique known as Radio Meteor Scatter (RMS).

In recent times Peter Gorham revisited the idea of cosmic ray detection via radar ranging techniques using dedicated transmitters and receivers. He pointed out that the ionization trails left by UHECR should be comparable to those of micrometeors. He explored, in detail, about the reflection of signals from the ionization densities to be expected in several regimes, and calculated the expected return power for normal and oblique angles of incidence. He also noted the applicability of this technique to horizontal showers indicative of neutrino-induced events.

Radar technology has evolved tremendously since the first attempts to detect cosmic ray showers. For example, a technique known as "Radar imaging" is now widely used. Meyer and Sahr have implemented a interferometric radar for the observation of ionospheric irregularities at VHF frequencies, and observed irregularities in the ionosphere. They achieved resolutions as fine as 2 km at a range of 1000 km and formed two-dimensional spatial images. In the MARIACHI experiment, receivers will continuously listen to fixed frequency transmitters below the horizon which are normally not detectable. If a reflective object such as lightning, a meteor trail, or an extreme energy cosmic ray intervenes, then for a brief time the receiver can pick up a reflected signal from one or several transmitters. This is depedent upon the location, altitude, and lifetime of the ionization cloud.

The detection of meteors via Radio Meteor Scatter is possible because the ionization density left behind by the ionization trail is very large. For a given frequency, the reflection of electromagnetic waves transitions from partially to completely above a critical density defined by the plasma frequency, f_p:

where n_e is the ionization density in electrons/m³, me and e are the electron mass and charge respectively. The ionization densities created by meteors are in the range of 10¹² to 10¹⁴ electrons/m³ and can reflect radio waves from distant sources such as TV and FM radio stations with signals lasting from 0.1s up to 50-60s depending on the size of the meteor. The vast majority of meteors move at speeds of 10-20km/s and are no larger than a grain of sand. They leave a cylindrical ionization trail a few kilometers in length and with a 1m radius, typically at altitudes above 80km. The reflective lifetime is determined initially by the high temperature in the hot gases of the meteor trail, as well as the diffusion and subsequent cooling of the trail plasma. RMS can detect meteors that are up to 1,000 kilometers away from the receiving station.

Extended air showers that ionize the atmosphere follow a dissimilar mechanism, where the numerous charged particles produced in the shower ionize air molecules. Shower simulations show that ionization densities comparable to meteors (10¹² to 10¹⁴ electrons/m³) can be produced by UHECR. It is striking to see that the ionization cloud can be 15 km long and up to 50m in radius. For energies on the order of 50 EeV, the core ionization in the simulations can reach 10¹⁸ electrons/m³. These shower profiles differ depending on whether the primary particle was a proton or a heavy nucleus. The goal of MARIACHI will be to use reflected images of these profiles to extract the energy and direction of the primary UHECR.

Links for Further Reading

1. Comprehensive list of Scientific References on Cosmic Rays, and to past, present, and future Experiments check these links.
2. General introduction to cosmic rays start with the SLAC Cosmic Ray page. This site also has a cosmic ray array that can be used interactively to make measurements of the flux.
3. Good introductory material at NASA Educational page on cosmic rays.
   
4. Nice introductions to Ultra High Energy Cosmic rays (UHECR)- the goal of the MARIACHI experiment- can be found at the University of Adelaide site, and the University of Leeds web site, which operated the Haverah Park Ground Array for many years.
5. See also our Introductory Materials for our WSE 187 course.
6. Introduction to UHECR from the Pierre Auger Observatory.