Solar Cosmic Rays and Polar Nitrates? Larry Kepko Boston University Center for Space Physics and...

transcript

Solar Cosmic Rays and Polar Nitrates?

Larry KepkoBoston University Center for Space Physics

andHarlan Spence (BU), Joe McConnell (DRI), Peg Shea (AFGL) and Don Smart (AFGL)

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

Cosmic raysoCosmic rays are (broadly) very energetic particles

•Typically protons, with energies ~ 1 GeV (90% c)

oA cosmic ray will strike a particle in the upper atmosphere, producing secondaries, which produce more secondaries...

•Cosmic ray shower

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are needed to see this picture.

Atmospheric interactionoGround monitors don’t measure cosmic rays

• They measure the secondaries

• During pre-spacecraft era, we have only a record of the strongest events (GLEs)

Cosmic raysoThe 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

oSome cosmic rays are accelerated at the heliopause• Anomalous Cosmic Rays (ACR)

oDetermination of the source(s) is difficult because of the deflection of particles due to the Earth’s magnetic field.

Solar cosmic raysoForbush [1946] was the

first to observe cosmic rays associated with geomagnetic activity, and suggested a solar source:

Flare,n increase

ForbushDecrease

Flare,n increase

ForbushDecrease

“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 [...]”

Solar cosmic raysoSolar flares are most closely associated with

coronal mass ejections (CMEs), and CME shocks can accelerate particles to cosmic ray energies

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Atmospheric ChemistryoCosmic ray particles dissociate O3 and N2. The

free particles combine to form “odd nitrates”• NO, NO2, NO3, etc

oDoes this change in atmospheric chemistry make its way down to the Earth’s surface? And can it become entrained in ice?

Nitrates in Antarctic ice cores

oAs early as 1986, Zeller and Dreschhoff suggested a possible link between solar cosmic rays and impulsive nitrate spikes.

SCR Event

Year following SCR

Nitrates in Antarctic ice cores

oAs early as 1986, Zeller and Dreschhoff suggested a possible link between solar cosmic rays and impulsive nitrate spikes.

oUnfortunately:• Sea spray contributes to

nitrate deposition• Antarctic ice data are

extremely noisy• Resolution was marginally

sub-annual

oInitial results inconclusive

What makes a good ice core?

oIdeally 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)

To greener pasturesoCentral greenland easily

fulfills our criteria

oSummit has the thickest ice shelf, with minimal ice movement

The GISP-H CoreoIn 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 CoreoDreschoff 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

oResulting dataset contained ~20 samples/year and extended back to ~1577.

The GISP-H Core

400045005000550060006500700075008000Sample Number

05001000150020002500300035004000

Nitrate ConcentrationConductivity

Sample Number

oIf clearly defined, one can use the annual cycle to identify yearly intervals

oPeak occurs in the summer when the polar vortex is active, and nitrates are transported downward from the upper atmosphere

Dating Cores

3000310032003300340035003600

Sample Number

Dating CoresoBut the annual cycle is not always clearly

defined

oWe need markers that allow us to reset the annual cycle.

3000310032003300340035003600

Sample Number

Dating CoresoVolcanic eruptions are the most common and

obvious temporal markers

oVolcanos produce a conductivity enhancements (dust and metals) without a nitrate enhancement.

42004300440045004600470048004900

Sample Number

Laki 1783

The GISP-H Core

400045005000550060006500700075008000Sample Number

05001000150020002500300035004000

Sample Number

1600-1800

1800-1992

The GISP-H Core

05001000150020002500300035004000

nitrate

Sample Number

oNote the increase in background nitrate since ~1950.• Anthropogenic influence

• Decreases signal to noise/background ratio

1950 1970

The GISP-H Core

05001000150020002500300035004000

nitrate

Sample Number

oNote the occasional enhancements to well above the background. Several causes:• “Biomass burning events” (fires)

• Pollution

• Solar cosmic rays?

The GISP-H Core

05001000150020002500300035004000

nitrate

Sample Number

oTo 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

The GISP-H Core

0100200300400500600700800Sample Number

300031003200330034003500360037003800Sample Number

Clear annual cycle for the deeper layers

Less defined annual cycles in upper layers

1750-1790

1950-1990

The GISP-H Core

oThe upper meters of a core consist of loosely packed snow called ‘firn’

oData collected from the firn regions are inherently more noisy, and picking out nitrate peaks was virtually impossible.

oFor the GISP-H Core, the firn extended back to ~1950.

0100200300400500600700800Sample Number

Less defined annual cycles in upper layers 1950-1990

The GISP-H Core

It was nearly impossible to correlate nitrate enhancements and SCR during the space-age.

0100200300400500600700800Sample Number

Less defined annual cycles in upper layers

What we learned from GISP-H

oImpulsive nitrate spikes are possibly associated with:• Historical records of mid-latitude aurora

• Early GLEs

• Large geomagnetic storms

• A few spacecraft era events

oTime from SCR event to ground level nitrate enhancement is short• Weeks to months

Problems with GISP-HoDating

• Accurate dating relies on volcanic markers and identification of the annual cycle. Nitrate does not give the strongest annual variation.

oFirn Noise• Because of the noise inherent in firn ice, few

comparisons were made to space-age solar cosmic ray events.

oResolution• At the time, the GISP-H core was the best resolution

available, but it only provided ~20 samples/year.

oDating• 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

oFirn 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.

oResolution• 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.

Continuous Flow AnalysisoIn the late 90’s glaciologists moved away

from labor-intensive hand analysis of cores.

oInstead, 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.

Continuous Flow Analysis

commercial freezer at -20 °F

Melthead at 35.1 °F

Inner ring underpumped,

analyzed

Outer ring uverpumped,

discarded

Nitrate (NO3) is reduced to Nitrite (NO2) in a copperized Cd column

Spectrophotometer measures absorption at

540 nm, which is proportional to nitrate+nitrite concentration

Calibration curves are produced by passing NO3

standards through the system before and after

core runsNitrate Calibration

-0.0400

-0.0300

-0.0200

-0.0100

0.0000

0.0100

0.0200

0.0300

0 50 100 150 200 250 300 350 400 450

Concentration (ppb)

Absorbance

Capable of measuring <

BU CoresoLast summer 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 2003

Cores were bagged, tubed, boxed, and then transported from Greenland to Scotia, NY via a LC-

130 Hercules USAF transport plane.

BU CoresIce Stored at BU

Medical Campus in -30 °C deep freeze

BU CoresCores cut into 4 quarters with a bandsaw...

... And analyzed at BU

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BU CFA AnalysisoWe 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

o1-m takes ~2 hours to analyze.• Unless the core gets stuck

• Or the lines freeze

• Or a tube pops out of its connector

• etc.

oDating is rather difficult at this point, but we believe we start at ~1937

BU Results

1940194419481952195619601964

Date (Still a little rough)

BU ResultsoEach core segment provides 4 independent runs

oAfter the runs, depths are hand-adjusted (mm’s) to align peaks, then averaged to produce a single curve.

27.9 28 28.1 28.2 28.3 28.4 28.5

Core 29a

Depth (m)

BU CFA vs. GISP-HoThe higher resolution afforded by CFA is

readily apparent

28.2 28.4 28.6 28.8 29 29.2

FDepth (m)

BU CFA

GISP-H

readily apparent

28.2 28.4 28.6 28.8 29 29.2

FDepth (m)

BU CFA

GISP-H

readily apparent

28.2 28.4 28.6 28.8 29 29.2

FDepth (m)

BU CFA

GISP-H

Some Results!oTwo largest peaks in our record occur in

1946 and 1949

19451946194719481949195019511952

Date (Still a little rough)

GLE #325-Jul-1946

GLE #419-Nov-1949

Where we’re at nowoCurrently 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

oJoe 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

oEvent studies are probably useless, especially as we enter the space age• Instead, will rely on statistical analysis of coincidence.

ConclusionsoThe 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

oOur 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

ConclusionsoIf the correlation persists, we will have a

method of pushing the SCR record back hundreds of years.

oIf the correlation disappears...

oOne of the largest nitrate enhancements occurred late in 1859

The 1859 Carrington Event

3000305031003150320032503300

Nitrate Concentration

Conductivity

Sample Number

1859 1861 18631857

The 1859 Carrington EventoCarrington observed a flare so bright, that it

was visible with the naked eye

Timescale of DepositionoMcCracken et al. [2001] analyzed the time of

nitrate enhancement relative to the SCR onset

oClaimed the February 23, 1956 ground level cosmic ray increase appears in the nitrate record

1957195619540

108011001120114011601180

Sample Number

oTimescale of deposition is short• A matter of weeks or less

Timescale of Deposition

28 28.5 29 29.5

Depth (m)

Timescale of DepositionoTimescale of deposition is short

• A matter of weeks or less

oTime from SCR event to deposition is also short• A few weeks to months

28 28.5 29 29.5

Depth (m)

SCR SCR

Timescale of DepositionoTimescale of deposition is short

• A matter of weeks or less

oTime from SCR event to deposition is also short• A few weeks to months

oSuggested that gravitational sedimentation through snowfall is the only mechanism that can explain such rapid transport

28 28.5 29 29.5

Depth (m)

Solar Cosmic Rays and Polar Nitrates? Larry Kepko Boston University Center for Space Physics and...

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