Solar Cosmic Rays and Polar Nitrates?

Extreme Solar Proton Events and Polar Nitrates?Harlan Spence(University of New Hampshire)

Larry Kepko (GSFC), Joe McConnell (DRI), Peg Shea and Don Smart

BackgroundMotivation: Some recent work (McCracken et al. [2001, and others) has suggested that impulsive nitrate events in polar ice are results of large solar proton events.

Carrington white light of 1859 observed in Greenland ice cores?

Solar cosmic raysForbush [1946] was the first to observe cosmic rays associated with geomagnetic activity, and suggested a solar source:

Flare,n increaseForbushDecreaseFlare,n increaseForbushDecreaseThese considerations suggest the rather striking possibility that the three unusual increases in cosmic-ray intensity may have been caused by charged particles actually being emitted by the Sun [...]Atmospheric ChemistryCosmic ray particles dissociate O3 and N2. They combine to form odd nitratesNO, NO2, NO3, etc

Does this change in atmospheric chemistry make its way down to the Earths surface? And can it become entrained in ice?

What makes a good ice core?Ideally we would like to take our ice cores from a region that has:High snowfall ratesLow noise (away from the ocean)Clearly defined annual cycleMany markers that can be used for dating (volcanoes)To greener pastures

Central Geasily fulfills our criteria

Summit has the thickest ice shelf, with minimal ice movementThe GISP-H CoreIn June, 1992, a 122-m core was collected at Summit as part of the Greenland Ice Core Science Project 2 (GISP-2)

The GISP2 drilling dome on the ice surface. The dome is about 105 feet (32.5 m) in diameter and encloses the lower part of the drilling tower. The dome is connected to nearby surface and buried workshops and living quarters. The GISP-H CoreDreschoff and Zeller (U. Kansas) analyzed the core for nitrate and conductivity.Core was sliced into 1.5 cm segmentsSamples were melted, and 2.5 ml injected by hand into a UV absorption cell to analyze nitrate, followed by a conductivity measurement

Resulting dataset contained ~20 samples/year and extended back to ~1577.Nitrates in Antarctic ice coresAs early as 1986, Zeller and Dreschhoff suggested a possible link between solar proton events and impulsive nitrate spikes.

SPEYear following SPENitrates in Antarctic ice coresAs early as 1986, Zeller and Dreschhoff suggested a possible link between solar cosmic rays and impulsive nitrate spikes.Unfortunately:Sea spray contributes to nitrate depositionAntarctic ice data are extremely noisyResolution was marginallysub-annualInitial results inconclusive

BU CoresIn 2006 we were fortunate to obtain 2 30-m cores from Summit, GreenlandOur cores resulted from a project needing only bore holes at Summit, Greenland (special thanks to Sarah Das, WHOI, Joe McConnell, DRI, and Jane Dione, NSF for their help in getting these cores for our project).

Summit

Jay Kyne drilling an ice core on another expedition to Greenland in summer 2003Cores were bagged, tubed, boxed, and then transported from Greenland to Scotia, NY via a LC-130 Hercules USAF transport plane.

Continuous Flow AnalysisIn the late 90s glaciologists moved away from labor-intensive hand analysis of cores.

Instead, they moved to a closed, continuous system.Much faster analysisLess chance for contaminationAllowed for easy analysis of multiple speciesProvides spatial (temporal) resolution an order of magnitude better than previously available.Continuous Flow Analysis

commercial freezer at -20 F

Continuous Flow Analysis

Melthead at 35.1 FInner ring underpumped, analyzedOuter ring uverpumped, discardedContinuous Flow Analysis

Nitrate (NO3) is reduced to Nitrite (NO2) in a copperized Cd columnContinuous Flow Analysis

Spectrophotometer measures absorption at 540 nm, which is proportional to nitrate+nitrite concentrationCalibration curves are produced by passing NO3 standards through the system before and after core runs

Capable of measuring < 1ppb

BU Cores

Ice Stored at BU Medical Campus in -30 C deep freezeBU Cores

Cores cut into 4 quarters with a bandsaw...... And analyzed at BU

BU ResultsEach core segment provided 4 independent runsAfter the runs, depths are hand-adjusted (mms) to align peaks, then averaged to produce a single curve.

BU CFA vs. GISP-HThe higher resolution afforded by CFA is readily apparent

Depth (m)Nitrate (ppb)BU CFAGISP-HBU CFA vs. GISP-HThe higher resolution afforded by CFA is readily apparent

Depth (m)Nitrate (ppb)BU CFAGISP-HBU CFA vs. GISP-HThe higher resolution afforded by CFA is readily apparent

Depth (m)Nitrate (ppb)BU CFAGISP-HResultsTwo largest peaks in our record occur in 1946 and 1949

GLE #325-Jul-1946GLE #419-Nov-1949