Solar Cosmic Rays and Polar Nitrates? Larry Kepko Boston University Center for Space Physics and...
-
Upload
kerry-hines -
Category
Documents
-
view
216 -
download
0
Transcript of Solar Cosmic Rays and Polar Nitrates? Larry Kepko Boston University Center for Space Physics and...
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
QuickTime™ and aYUV420 codec decompressor
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
QuickTime™ and aMPEG-4 Video decompressor
are needed to see this picture.
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
0
100
200
300
400
500
400045005000550060006500700075008000Sample Number
0
100
200
300
400
500
05001000150020002500300035004000
Nitrate ConcentrationConductivity
Sample Number
Time
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
0
50
100
150
200
250
3000310032003300340035003600
Sample Number
Dating CoresoBut the annual cycle is not always clearly
defined
oWe need markers that allow us to reset the annual cycle.
0
50
100
150
200
250
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.
0
100
200
300
400
500
42004300440045004600470048004900
Nitrate ConcentrationConductivity
Sample Number
Laki 1783
The GISP-H Core
0
100
200
300
400
500
400045005000550060006500700075008000Sample Number
0
100
200
300
400
500
05001000150020002500300035004000
Nitrate ConcentrationConductivity
Sample Number
Time
1600-1800
1800-1992
The GISP-H Core
0
100
200
300
400
500
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
0
100
200
300
400
500
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
0
100
200
300
400
500
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
cores
The GISP-H Core
0
50
100
150
200
250
300
0100200300400500600700800Sample Number
0
50
100
150
200
250
300
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.
0
50
100
150
200
250
300
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.
0
50
100
150
200
250
300
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
Solutions to GISP-H Problems
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.
Solutions to GISP-H Problems
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
Continuous Flow Analysis
Melthead at 35.1 °F
Inner ring underpumped,
analyzed
Outer ring uverpumped,
discarded
Continuous Flow Analysis
Nitrate (NO3) is reduced to Nitrite (NO2) in a copperized Cd column
Continuous Flow Analysis
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 <
1ppb
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
QuickTime™ and aVideo decompressor
are needed to see this picture.
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
0
50
100
150
200
250
300
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.
40
60
80
100
120
140
160
180
200
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
0
50
100
150
200
28.2 28.4 28.6 28.8 29 29.2
FDepth (m)
Nit
rate
(ppb
)
BU CFA
GISP-H
BU CFA vs. GISP-HoThe higher resolution afforded by CFA is
readily apparent
0
50
100
150
200
28.2 28.4 28.6 28.8 29 29.2
FDepth (m)
Nit
rate
(ppb
)
BU CFA
GISP-H
BU CFA vs. GISP-HoThe higher resolution afforded by CFA is
readily apparent
0
50
100
150
200
28.2 28.4 28.6 28.8 29 29.2
FDepth (m)
Nit
rate
(ppb
)
BU CFA
GISP-H
Some Results!oTwo largest peaks in our record occur in
1946 and 1949
0
50
100
150
200
250
300
350
400
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
0
100
200
300
400
500
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
50
100
150
200
250
108011001120114011601180
Sample Number
1955
oTimescale of deposition is short• A matter of weeks or less
Timescale of Deposition
0
50
100
150
200
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
0
50
100
150
200
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
0
50
100
150
200
28 28.5 29 29.5
Depth (m)