Tiger Loose in Antarctica, Searching for Cosmic Ray Origin

A balloon-borne experiment now flying high above Antarctica is headed for an unprecedented second loop around the South Pole in search of the origin of cosmic rays, atomic particles that travel through the galaxy at near light speeds and shower the Earth constantly.

TIGER, short for Trans Iron Galactic Element Recorder, lifted off from McMurdo Station in Antarctica at 6:30 a.m. EST December 20. At about 8:00 p.m. EST January 2, TIGER entered into its second loop around the Pole, suspended from a 29-million-cubic-foot balloon approximately 125,000 feet (above 99.6% of the atmosphere).

The extended balloon flight — one of the longest scientific flights to be attempted — will maximize the number of heavier, rare cosmic rays that TIGER can collect. TIGER has already flown two weeks and could fly two more, depending on weather conditions.

TIGER is a collaboration among Washington University in St. Louis, NASA Goddard Space Flight Center, California Institute of Technology and the University of Minnesota. Principal investigator Bob Binns of Washington University is in Antarctica for the flight. Co-investigator Eric Christian of NASA Goddard, at McMurdo Station for the launch, has recently returned to Maryland.

“TIGER has already collected some valuable data in its first 13 days of flight,” said Louis Barbier of NASA Goddard, speaking on behalf of the TIGER team. “No balloon experiment has ever made it twice around the South Pole. We hope to keep TIGER afloat as long as possible.”

TIGER collects cosmic rays heavier than iron. These heavy atomic nuclei likely come from star explosions, called supernovae. How these particles become cosmic rays (by virtue of their high speed and kinetic energy) is a major mystery in astronomy.

Scientists say that heavy cosmic rays do not race out of a supernova like shrapnel. Instead these particles are expelled by one supernova or perhaps a solar flare and sit around for millions of years before the shock wave of another supernova accelerates them to high speeds. But where are they when they are sitting around?

TIGER addresses this question by measuring the elemental composition of cosmic rays between zinc and molybdenum in the periodic table. The ratio of certain pairs of elements — such as rubidium-37/strontium-38 or germanium-32/iron-26 — reflect the environment from which they came.

The cosmic-ray source material might come from a solar flare, in which hot atomic nuclei stripped of electrons are flung into space and later accelerated. If this is the scenario, then TIGER will see more cosmic rays that are low First Ionization Potential (FIP) nuclei. Low FIP refers to the type of elements that shed their most weakly held electrons more readily than other elements do in the hot environment of a solar flare.

Or, the source material might come from a supernova sending heavy elements into the interstellar medium. These elements cool; incorporate into interstellar dust grains; get accelerated to higher speeds by a later supernova shock wave; then break off the swiftly moving dust grains to become the seeds of cosmic rays. If this is the scenario, then TIGER will see more “low-volatility” nuclei, referring to the types of elements that form interstellar dust and do not easily evaporate.

The key to deciding between the two scenarios is to find pairs of elements with similar FIP but different volatilities. Rubidium and strontium have low FIP, but rubidium is relatively volatile compared to strontium. Rubidium is less likely to be in dust grains, so finding more rubidium compared to strontium points to the “solar flare” or “hot gas” scenario. Likewise, germanium is relatively volatile compared to iron. Finding less germanium compared to iron implies that “cold” material in dust is the source of cosmic rays when a supernova blast comes and kicks them to higher speeds.

Flying a balloon from Antarctica offers two advantages. The first is long, continuous balloon flights. Constant sunshine in the Antarctic summer means there are no daytime/nighttime temperature fluctuations to cause the balloon to slowly loose altitude. Also, the Earth’s magnetic field, which shields us from most cosmic rays, allows more cosmic rays to penetrate the Earth’s atmosphere above Antarctica and the Arctic than anywhere else in the world.

TIGER is a forerunner of the ENTICE (ENergetic Trans-Iron Composition Experiment) instrument on the HNX (the Heavy Nuclei eXplorer). HNX is a satellite that is being studied for possible launch in a few years, and what TIGER learns could help in the design of HNX.

Photographs of the launch, beneath the backdrop of the lovely Mt. Erebus, are available at http://tiger.gsfc.nasa.gov. Eric Christian joins Goddard through the University Space Research Association.