Wednesday, August 24, 2011

ARIANNA 2010 - Part 3 - In the Field

Here is part3 of Jordan's account of the 2010 ARIANNA field season:

In addition to fixing the station, Steve and I also scheduled time to make measurements of certain properties of the ice beneath the detector. Spencer Klein and Thorsten Stezelberger had already calculated the depth of the shelf beneath the prototype using data taken in 2009. They did this by pulsing a radio signal down through the ice, and measuring the time for the signal to return after reflecting. This experiment requires very sensitive equipment, since the signal strength decreases with distance, and we're working with a total path length of over a kilometer. Steve and I repeated this measurement, and got the same answer as Spencer and Thorsten. We would also go on to use that data to measure how much power a radio wave loses as it travels through the Ross Ice Shelf. The key problem in the past was that no one knew how much power the radio wave looses when it reflects off of the ocean beneath the ice shelf.

To study this separately from the effect of the ice, I moved our transmitting equipment to the end of the 1 kilometer flag line we established on the first day of the expedition. I had the help of Leah Biezums, who flew in via helo to relieve Rebekah after the first week. Now we had a path length of one and a half kilometers, and the power loss from the ice would be stronger because of the increased distance the radio signals had to travel through the ice. Presumably, the properties of the reflecting ocean were the same. Thus, by comparing the amount of power received in the signals with different path lengths, but the same reflection properties, we could isolate and study the reflecting properties alone. We've found that the ocean/ice interface acts like a smooth mirror, reflecting a large fraction of the radio signals' power. This bodes well for detecting high energy neutrinos which create radio pulses, because neutrinos propagating downward through the ice shelf will create radio waves that reflect off of the bottom and back up towards the ARIANNA detectors.

As the expedition began to draw towards its close, Steve and I began to perform tasks that would ensure the viability of the detector throughout the rest of the summer and winter. We repositioned the “heartbeat” antenna, which is designed to pulse calibration signals at the prototype detector and tells us certain information about its ability to trigger on neutrinos and determine their direction. We chose a more ideal location based on heartbeat data taken the previous season. I later used this data to show that ARIANNA has the ability to distinguish the time of arrival of a signal in two different sensor antennas to a precision of 100 picoseconds. With Leah's help, I erected the larger, more powerful wind generator and watched as it begun to spin, providing sustainable power to the station which would eventually lead to data being taken further into the Antarctic winter. We began to pack up the gear, both survival and scientific, and awaited our helo extraction.

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