1. Solar Cosmic Rays and Polar Nitrates? Larry Kepko Boston University Center for Space Physics and Harlan Spence (BU), Joe McConnell (DRI), Peg Shea (AFGL) and Don Smart (AFGL)

2. Background Motivation: 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

3. Cosmic rays  Cosmic rays are (broadly) very energetic particles Typically protons, with energies ~ 1 GeV (90% c)  A cosmic ray will strike a particle in the upper atmosphere, producing secondaries, which produce more secondaries... Cosmic ray shower

4. Atmospheric interaction  Ground monitors don’t measure cosmic rays They measure the secondaries During pre-spacecraft era, we have only a record of the strongest events (GLEs)

5. Cosmic rays  The majority of cosmic rays are galactic (GCR), and are accelerated outside our solar system, but inside the galaxy. Star formation Acceleration from supernova shock wave  Some cosmic rays are accelerated at the heliopause Anomalous Cosmic Rays (ACR)  Determination of the source(s) is difficult because of the deflection of particles due to the Earth’s magnetic field.

6. Solar cosmic rays  Forbush [1946] was the first to observe cosmic rays associated with geomagnetic activity, and suggested a solar source:  Flare, n increase Forbush Decrease Flare, n increase Forbush Decrease “These 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 [...]”

7. Solar cosmic rays  Solar flares are most closely associated with coronal mass ejections (CMEs), and CME shocks can accelerate particles to cosmic ray energies

8. Atmospheric Chemistry  Cosmic ray particles dissociate O 3 and N 2. The free particles combine to form “odd nitrates” NO, NO 2, NO 3, etc  Does this change in atmospheric chemistry make its way down to the Earth’s surface? And can it become entrained in ice?

9. Nitrates in Antarctic ice cores  As early as 1986, Zeller and Dreschhoff suggested a possible link between solar cosmic rays and impulsive nitrate spikes. SCR Event Year following SCR

10. Nitrates in Antarctic ice cores  As early as 1986, Zeller and Dreschhoff suggested a possible link between solar cosmic rays and impulsive nitrate spikes.  Unfortunately: Sea spray contributes to nitrate deposition Antarctic ice data are extremely noisy Resolution was marginally sub-annual  Initial results inconclusive

11. What makes a good ice core?  Ideally we would like to take our ice cores from a region that has: High snowfall rates Low noise (away from the ocean) Clearly defined annual cycle Many markers that can be used for dating (volcanoes)

12. To greener pastures  Central greenland easily fulfills our criteria  Summit has the thickest ice shelf, with minimal ice movement

13. The GISP-H Core  In 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.

14. The GISP-H Core  Dreschoff and Zeller (U. Kansas) analyzed the core for nitrate and conductivity. Core was sliced into 1.5 cm segments Samples 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.

15. The GISP-H Core Time

16.  If clearly defined, one can use the annual cycle to identify yearly intervals  Peak occurs in the summer when the polar vortex is active, and nitrates are transported downward from the upper atmosphere Dating Cores

17.  But the annual cycle is not always clearly defined  We need markers that allow us to reset the annual cycle.

18. Dating Cores  Volcanic eruptions are the most common and obvious temporal markers  Volcanos produce a conductivity enhancements (dust and metals) without a nitrate enhancement. Laki 1783

19. The GISP-H Core Time 1600-1800 1800-1992

20. The GISP-H Core  Note the increase in background nitrate since ~1950. Anthropogenic influence Decreases signal to noise/background ratio 1950 1970

21. The GISP-H Core  Note the occasional enhancements to well above the background. Several causes: “Biomass burning events” (fires) Pollution Solar cosmic rays?

22. The GISP-H Core  To test the solar cosmic ray hypothesis, we need to correlate nitrate events with SCR events (obviously) For that we need very accurate dating on the ice cores

23. The GISP-H Core Clear annual cycle for the deeper layers Less defined annual cycles in upper layers 1750-1790 1950-1990

24. The GISP-H Core  The upper meters of a core consist of loosely packed snow called ‘firn’  Data collected from the firn regions are inherently more noisy, and picking out nitrate peaks was virtually impossible.  For the GISP-H Core, the firn extended back to ~1950. Less defined annual cycles in upper layers 1950-1990

25. The GISP-H Core  It was nearly impossible to correlate nitrate enhancements and SCR during the space-age. Less defined annual cycles in upper layers

26. What we learned from GISP-H  Impulsive nitrate spikes are possibly associated with: Historical records of mid-latitude aurora Early GLEs Large geomagnetic storms A few spacecraft era events  Time from SCR event to ground level nitrate enhancement is short Weeks to months

27. Problems with GISP-H  Dating Accurate dating relies on volcanic markers and identification of the annual cycle. Nitrate does not give the strongest annual variation.  Firn Noise Because of the noise inherent in firn ice, few comparisons were made to space-age solar cosmic ray events.  Resolution At the time, the GISP-H core was the best resolution available, but it only provided ~20 samples/year.

28.  Dating Accurate dating relies on volcanic markers and identification of the annual cycle. Nitrate does not give the strongest annual variation. Solutions to GISP-H Problems Solution Appeal to a higher authority. Joe McConnell of Desert Research Institute has the most accurately dated cores available. Will provide > 10 high-resolution cores

29. Solutions to GISP-H Problems  Firn Noise Because of the noise inherent in firn ice, few comparisons were made to space-age solar cosmic ray events. Solution We use multiple runs of the same core to reduce the noise level through averaging. In addition, our cores were obtained 10 years after GISP-H – taking us into the space age.

30. Solutions to GISP-H Problems  Resolution At the time, the GISP-H core was the best resolution available, but it only provided ~20 samples/year. Solution Continuous Flow Analysis (CFA) provides resolutions perhaps 100x higher than the GISP-H analysis.

31. Continuous Flow Analysis  In the late 90’s glaciologists moved away from labor-intensive hand analysis of cores.  Instead, they moved to a closed, continuous system. Much faster analysis Less chance for contamination Allowed for easy analysis of multiple species Provides spatial (temporal) resolution an order of magnitude better than previously available.

32. Continuous Flow Analysis commercial freezer at - 20 °F

33. Continuous Flow Analysis Melthead at 35.1 °F Inner ring underpumped, analyzed Outer ring uverpumped, discarded

34. Continuous Flow Analysis Nitrate (NO 3 ) is reduced to Nitrite (NO 2 ) in a copperized Cd column

35. Continuous Flow Analysis Spectrophotometer measures absorption at 540 nm, which is proportional to nitrate+nitrite concentration Calibration curves are produced by passing NO 3 standards through the system before and after core runs Capable of measuring < 1ppb

36. BU Cores  Last summer we were fortunate to obtain 2 30-m cores from Summit, Greenland Our 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 2003 Cores were bagged, tubed, boxed, and then transported from Greenland to Scotia, NY via a LC- 130 Hercules USAF transport plane.

37. BU Cores Ice sheet accumulation 1971-90 (Bales et al., GRL, 28:2967-70, 2001) Ice Stored at BU Medical Campus in -30 °C deep freeze

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

39. BU CFA Analysis  We had first melt the week before AGU, and currently have data from Toby core segments 19- 30 and Meg cores 25-28 16-m x 4 segments each = 64 m analyzed  1-m takes ~2 hours to analyze. Unless the core gets stuck Or the lines freeze Or a tube pops out of its connector etc.

40.  Dating is rather difficult at this point, but we believe we start at ~1937 BU Results

41.  Each core segment provides 4 independent runs  After the runs, depths are hand-adjusted (mm’s) to align peaks, then averaged to produce a single curve.

42. BU CFA vs. GISP-H  The higher resolution afforded by CFA is readily apparent Depth (m) Nitrate (ppb) BU CFA GISP-H

43. BU CFA vs. GISP-H  The higher resolution afforded by CFA is readily apparent Depth (m) Nitrate (ppb) BU CFA GISP-H

44. BU CFA vs. GISP-H  The higher resolution afforded by CFA is readily apparent Depth (m) Nitrate (ppb) BU CFA GISP-H

45. Some Results!  Two largest peaks in our record occur in 1946 and 1949 GLE #3 25-Jul-1946 GLE #4 19-Nov-1949

46. Where we’re at now  Currently melting ~ 1-m/day Within a few weeks, will have a long enough record in the space age to begin comparison with space-based cosmic ray records  Joe McConnell (DRI) is assembling data from multiple Greenland cores (possibly 12 or more) These will be folded in to provide multi-point (geographic) measurements of the enhancements  Event studies are probably useless, especially as we enter the space age Instead, will rely on statistical analysis of coincidence.

47. Conclusions  The correlation between impulsive nitrate events in polar ice and SCR is still open to question There are problems with dating of the GISP-H core Resolution was poor (relative to today) Could not reliably use space-age measurements  Our current project utilizes multiple core runs and will statistically analyze the association to a degree that should definitely answer the question. These are the highest quality data available today

48. Conclusions  If the correlation persists, we will have a method of pushing the SCR record back hundreds of years.  If the correlation disappears...

49.  One of the largest nitrate enhancements occurred late in 1859 The 1859 Carrington Event 1859186118631857

50. The 1859 Carrington Event  Carrington observed a flare so bright, that it was visible with the naked eye

51. Timescale of Deposition  McCracken et al. [2001] analyzed the time of nitrate enhancement relative to the SCR onset  Claimed the February 23, 1956 ground level cosmic ray increase appears in the nitrate record 1957195619541955

52.  Timescale of deposition is short A matter of weeks or less Timescale of Deposition

53.  Timescale of deposition is short A matter of weeks or less  Time from SCR event to deposition is also short A few weeks to months SCR

54. Timescale of Deposition  Timescale of deposition is short A matter of weeks or less  Time from SCR event to deposition is also short A few weeks to months  Suggested that gravitational sedimentation through snowfall is the only mechanism that can explain such rapid transport