evidence of individual solar proton events in antarctic snow

E V I D E N C E O F I N D I V I D U A L S O L A R P R O T O N E V E N T S I N

A N T A R C T I C S N O W

GISELA A. M. D R E S C H H O F F and EDWARD J. ZELLER

Space Technology Center, University of Kansas, Lawrence, KS 66045, U.S.A.

(Received 2 May, 1989: in revised form 11 July, 1989)

Abstract. The high-resolution nitrate analyses of a snow sequence in Antarctica reveals clear evidence that the snow contains a chemical record of ionization from charged particles incident upon the upper atmosphere of the Earth. The Antarctic continent acts as a cold trap that effectively freezes out this signal and retains it in the stratigraphy of the ice shelves and the continental ice sheet. The signal that we measure results from the ionization of nitrogen and oxygen, the two primary constituents of the Earth's atmosphere, which subsequently react to form oxides of nitrogen. A large portion of the nitrogen oxides produced are ultimately oxidized to nitric acid and incorporated in snow crystals together with nitrates from tropospheric sources that also contribute to the general back~ound. The nitrate concentration in a firn core was measured in Antarctica by ultraviolet spectrophotometry under tightly controlled experimental procedures. Based on uninterrupted, high-resolution sampling, variations in nitrate concentration were found to average about 53~ (one standard deviation) of the mean concentration for the entire core. Short pulses of high nitrate concentration were found to show a variance of up to 1 l standard deviations above the mean. At the series mean, the precision of analysis is better than 2~

The firn core was drilled by hand to a depth of 21.7 m corresponding to 62 years and including more than 5 solar cycles. The time series that resulted from a total of 1393 individual analyses shows a statistically significant modulation of the background signal that is clearly tracable to solar activity. Several anomalously large concentration peaks were observed that have been dated and found to correlate with the major solar proton events of August 1972, July 1946, and the white-light flare of July 1928.

1. Introduction and Previous Work

Solar particles that are incident upon the Earth cause ionization in the polar atmosphere

(Swider and Narcisi, 1975; Swider, 1977, 1979; Crutzen, Isaksen, and Reid, 1975;

Jackman, Frederick, and Stolarski, 1980) and generate NOx, including nitrate ion

(NO~-). Zeller and Parker (1981), reported that modulat ion by solar activity caused

detectable variations in nitrate concentrat ions at South Pole and Vostok Stations on

the Antarctic ice sheet. Since the majority of solar particles are of low energy they

deposit most of their energy at high altitude within the auroral zone. Because of the time

lag related to downward transport of reaction products formed in this zone, the effects

tend to be spread over longer periods of time of up to 2 years. Comparison of the

long-term effects with a data series for carbon-14 (Eddy, 1977; Zeller and Parker, 1981)

shows a clear anticorrelation. This hypothesis was substantiated (Dreschhoff, Zeller,

and Parker, 1983) when nitrate data from the upper 200-years of a South Pole firn core

were subjected to harmonic analysis and found to show evidence of 11 and 22 year solar

activity cyclicity. During preliminary studies in 1985-1986 it was shown that the nitrate

signal also exhibits pulses that can be correlated with specific major solar flare events

(Zeller, Dreschhoff, and Laird, 1986). This conclusion was tested more fully at two

widely separated locations in Antarct ica during the 1987-1988 field season and these

Solar Physics 127: 333-346, 1990. �9 1990 Kluwer Academic Publishers. Printed Or Belgium.

334 GISELA A. M. DRESCHHOFF AND EDWARD J. ZELLER

results led to the design of the high-resolution, on-site analysis project that was completed during the 1988-1989 Antarctic field season.

The present report enlarges the scope of the initial investigation by increasing both the sample resolution and the total thickness of the snow sequence that was studied. Only through the use of extremely high-resolution sampling and analysis is it possible to obtain a signal which provides enough detail to permit the solar flare contribution to be resolved unambiguously. From the record that we obtained it is possible to show that the largest variations can be linked directly with the largest known solar flares over the last 62 years.

A substantial fraction of the nitrate deposited in the snow is thought to be formed by ionization in the upper and middle atmosphere (Wada, Shibata, and Torii, 1981; Garcia et al., 1984; Kamiyama, 1989). Because of the extreme cold in Antarctica, the ice sheet accumulates and retains nitrate pulses generated by short duration events such as solar flares. Not all locations on the Antarctic ice sheet are equally capable of preserving these short duration signals, however, and specific criteria of snow accumula- tion conditions must be met if flare pulses are to be identified. The criteria are: (a) significant surface melting must not occur; (b) surface mixing must be minimal; and (c) yearly snowfall must be adequate to produce an annual snow layer thick enough to provide multiple analytical samples. The degree to which these criteria are met depends upon weather conditions existing at the specific sampling location. For example, at South Pole Station, wind erosion and mixing tend to reduce the height of nitrate pulses from individual solar events. A natural kind of signal averaging resulting from surface mixing of snow (Gow, 1965; Laird, 1986; Laird et al., 1985) prevents unambiguous detection of short-lived phenomena and may obscure the additional modulation in nitrate levels by high-energy solar proton events that cause ionization in the polar atmosphere within the south polar cap. At Vostok Station, yearly precipitation averages only about 4 cm of water and this combined with wind mixing makes monthly signal resolution doubtful in most years. At Windless Bight on the Ross Ice Shelf, all criteria

1 are fulfilled and our core includes only the upper a of the total sequence that has accumulated.

Although several glaciological studies involving nitrate measurements exist, the data they provide is inadequate for solar activity investigations (Laird, Zeller, and Dreschhoff, 1988). In most cases, the analyses are discontinuous with gaps of many meters between samples and the resolution is often very low, frequently less than one analysis per meter (Legrand and Delmas, 1986; Kirchner and Delmas, 1988). In addition, some of these studies have been conducted on old core segments that have been subjected to long distance shipment and storage periods of many years (Kirchner and Delmas, 1988) which can introduce significant variations from the original nitrate concentrations present in freshly cut cores (Grootes and Stuiver, 1987).

EVIDENCE OF INDIVIDUAL SOLAR PROTON EVENTS IN ANTARCTIC SNOW 335

2. Field Laboratory and Drilling Operations in Antarctica

All field operations were conducted in Windless Bight on the Ross Ice Shelf during

December, 1988 and January, 1989. A field laboratory was provided at Williams Airfield

approximately 7 km southwest of the drill site shown in Figure 1, and all nitrate analyses were performed in this laboratory. The laboratory consisted of a building approximately

7 x 3 m, which was heated electrically and provided with a stable source of electric power for operation of the analytical apparatus and computer. Transport to and from

Fig. 1.

1 " I I 1 0 o 166~ E 167~ E 168~ E I

.

o , / / W i n d l e s s Bight --77 45S / f i r

�9 " . . . . . . M~urdof~ ~ ROSS ICE SHELF ....-" . . . . . . . ~ ~ ' e I , 2 0 k i n ,

1~" I 1 t I I M a p showing loca t ion o f drill site in Windless Bight n e a r Ross I s l and on the Ross Ice Shelf,

A n t a r c t i c a .

the drill site was accomplished by means of a motor toboggan and when necessary, personnel and supplies were moved by attaching a Nansen sled to the motor toboggan.

Owing to the unusually severe weather conditions encountered this season with over

40 cm of snowfall during December and January, it was necessary to flag the track between the drill site and the laboratory so that transport could be accomplished safely under the frequent whiteout conditions that were encountered.

In early December, a survey of snow surface conditions in Windless Bight was made using the McMurdo Station based hovercraft. This vehicle proved to be especially useful and permitted us to make the survey for drill site selection within less than four hours. We found that the snow surface was unusually rough in many areas and we selected a site where surface relief was minimal. After site selection, a bulldozer was brought out from Williams Field and an exploratory trench was excavated to a depth of 5.6 m. This trench was used to examine the snow stratigraphy and gauge the short range horizontal variability of the individual snow layers. In addition, the excellent exposures permitted us to date the sequence in the trench wall.


All samples except those from the uppermost meter of snow were collected from drill cores obtained through the use of a hand operated PICO coring auger. The uppermost

84 cm of snow was sampled by pressing 1 inch diameter glass vials into the wall of a

shallow pit excavated upwind of the trench. Prior to use, the vials were cleaned and rinsed with double deionized water. The pit was not excavated until the end of the

season and included all of the snow that had fallen since the beginning of our field operations.

The main coring operations were conducted in three stages which provided over-

lapping core sequences that could be matched both by direct measurement of core lengths and by correlation of the nitrate concentration signal from the overlapping

sequences. The three cores were drilled approximately 6 m apart. The uppermost core included the sequence from a depth of 1 m to a depth of 6.3 m. It was set back from

the edge of the main excavation approximately 2 m and paralleled the wall of the trench. An intermediate core was drilled on the east entry ramp of the trench to a depth of 4.6 m

from the bottom of the trench excavated by the bulldozer. The total depth, excluding

overlap, covered by the three cores is 21.7 m spanning 62 years. All sampling was performed at the drill site. As soon as cores were recovered from

the drill holes they were laid out on cleaned snow surfaces in the trench. In all cases, cores were trimmed to a minimum depth of 1 cm from the core surfaces to remove any possible contamination. Similarly, core ends were trimmed to remove cuttings. After trimming, samples were cut sequentially from the core with a spacing of 1.5 cm and each sample was immediately placed in a 1 inch diameter, clean, numbered glass vial that had

been rinsed with double deionized water. Each of the bottles was then sealed with a

polyethylene cap. Cores and samples were handled only with cleaned stainless steel

implements. All samples were transported on the day they were collected, from the drill site to the field laboratory where they were kept frozen until approximately 1 hour before analysis by ultraviolet spectrophotometry. Analyses were always completed within a 24 hour period after sampling with an analytical precision of < 2% for a mean concentration of 16.28 + 0.25/~g 1 ~. Firn cores often show mass loss effects by evaporation when stored for long periods of time (Grootes and Stuiver, 1987) and errors from this source are completely avoided by measurements made on-site in Antarctica. Another

major benefit of on-site core sampling and analysis is the ability of the investigators to detect possible anomalous results and to resample immediately any portion of the

sequence that appears unusual. We took advantage of this ability on a number of occasions and were able to verify the presence of major concentration peaks.

3. Dating the Snow Sequence

Dating of the snow sequence has been accomplished by several techniques. The upper portion of the core, extending back in time to 1969, has been dated by comparing the visible snow stratigraphy with the snowfall record from nearby McMurdo Station and Scott Base. In general, the relative thickness of the individual year layers correlates closely with the precipitation record from the weather station at McMurdo. From 1969


to 1945 we were able to compare our dating with H / D ratio dating and conductivity

dating of a sequence from a core taken in Windless Bight in previous years (Palais et al.,

1983). This was of special significance because it spans a critical period of high solar

activity and several years of unusually high snowfall. F rom 1945 to the bot tom of our

core in 1927, we were forced to rely upon snow stratigraphy and the nitrate signal itself

to establish the specific year breaks. Although we recognize that dating of this portion

of the core is less certain than in the upper parts, it corresponds to year estimates based

on conductivity measurements (Palais, 1989) and we consider it generally reliable.

Maximum accuracy in dating can only be achieved by the analysis of multiple snow/tim

sequences.

4. Detection of Individual Solar Flare Events

The dated snow/tim sequence exhibits several anomalous peaks that exceed the height

of all other peaks in the sequence shown in Figure 2. These peaks tend to stand out

1972 1946 loo

& i I I [ [

, , I l , ' ' ' l t i 04

I SAMPLE NUMBER ,oo 2;0 3;0 400 . 'o 6oo' .oo' 8"~ 9'0 ,6oo' .oo' ,2'oo ,3'oo

DEPTH IN METIERS I

Fig. 2. Nitrate concentration at Windless Bight on the Ross Ice Shelf, Antarctica from analyses performed on site. The graph includes a total of 1393 individual analyses on a sequence extending from the surface to a depth of 21.7 m. The x-axis is proportional to true depth below the surface and has not been adjusted for compaction. Samples were collected continuously at an interval of 1.5 cm along the entire length of the core. The uppermost 33 samples were collected in the wall of an 84 cm deep pit with a sampling interval of 2.5 cm. Several major solar flares are visible in the record as large concentration peaks. The 1946 peak height shown in this figure corresponds to the resampled sequence shown in Figure 4. Among the most

prominent are the 1972, 1946, and 1928 flare events.

clearly and can be related to atmospheric ionization effects from specific solar proton

events. The effects o f these events are described by a number of authors: the August

1972 solar particle event (Jackman, Frederick, and Stolarski, 1980; Solomon and

Crutzen, 1981; Feynman et al., 1989, 1990); the July 1946 solar particle event and July

1928 white-light flare (Shea and Smart, 1977; Smart and Shea, 1971; Shea, 1989;

Feynman, 1989). Nitrate fallout from these events is generally very rapid and tends to

occur within the same year. I f the flare occurred after the breakdown of the polar vortex, nitrate produced by the lower energy component of the event may not reach the ground

until reestablishment of the vortex in the following year.

338 G I S E L A A. 5,I. D R E S C H H O F F A N D E D W A R D J. Z E L L E R

5. Direct Observation of Nitrate Variability

To judge the significance of the 'solar flare' peaks, the different levels of variability in

the nitrate concentration signal must be examined in detail. Specifically, it is necessary

to determine the extent to which these peaks differ from those that result from the annual

nitrate deposition cycle. Relatively sharp concentration maxima frequently develop

during the summer months whereas broad minima often characterize the winter months

(Laird et al., 1985). The origin of the cycle is apparently related to two factors. First,

the summer period is a time of relatively low snowfall which tends to enhance all trace impurity concentrations. Second, nitrate ion is nonvolatile and is concentrated on the

exposed surface of the snow by sublimation processes that are active during the period

of maximum insolation associated with the summer solstice.

Certain specific observations can be made from Figure 3:

(1) The snow deposited in the last two years, 1988 and 1987, is the least compressed

and displays summer highs and winter lows.

i

z i

" ' 1

80

40

0

1198811987 I A

C

B

SAMPLE NUMBER

0 60

Fig. 3. This figure shows the uppermost portion of the snow sequence. This portion has not undergone substantial compaction. Peak A, represents the nitrate concentration variation within a single snow storm.

Peaks B and C were formed in summer periods.

EVIDENCE OF INDIVIDUAl- SOLAR PROTON EVENTS IN ANTARCTIC SNOW 339

(2) The peaks A, B, and C are known to have formed in summer and they are relatively high. We know that compaction resulting from the deposition of additional snow will reduce the width and the height of these peaks.

(3) Peak A is closest to the surface and has undergone the least compaction. Its half-width and amplitude are larger when compared to other summer peaks in the sequence. Fortunately, we know the history of this specific peak because it formed during the beginning phases of a large snowstorm that lasted several days and occurred while we were on-site in Antarctica. We measured the nitrate concentration repeatedly throughout the progress of the storm and found that it initially rose sharply and then fell rapidly as the storm progressed. It is important to note that the McMurdo weather station had recorded no significant snowfall for more than 3 weeks prior to this specific storm. This effect can occur at any time during the year when snowfall is interrupted for extended periods of time. The stratigraphic characteristics of the snow sequence can provide key information in determining the time of occurrence of peaks within the year. By examining the time series in detail it may be possible to separate peaks of different origin. The shape parameters, specifically the derivatives, may provide useful information to differentiate between peaks produced by solar proton events and those derived from meteorological effects.

Figure 4, shows the reproducibility of two of the major peaks associated with solar flare events. It shows that the resampled peaks vary by 28?/o and 177/o in amplitude.

1 5 0

1946 PEAK 1972 PEAK

1 0 0

1 0 0

Z

5O t I

I,,,, / - ~ ' \ ,

, / ,,..---

O '

8O

60

40

2 0 '

/ 4 I I

I I l l / , ] 1

REPEAT SAMPLING

S A M P L E N U M B E R S A M P L E N U M B E R

I E [ I I I I I 1 0 5 5 1 0 6 5 3 7 5 3 8 5

Fig. 4. Resampling of the core through the interval of the 1946 and 1972 nitrate concentration peaks shows the level of reproducibility of the nitrate signals. Solid and broken lines indicate the nitrate concentration values obtained by repeat sampling of the core. The mean nitrate concentration and standard deviation for

the entire core is indicated on each graph.

340 G I S E L A A. M. D R E S C H H O F F A N D E D W A R D J. Z E L L E R

However, when the area under the curve is compared they vary by only 8 o/ and 7 o, /O /O "

The plots also show the mean nitrate concentration for 1393 sequential samples (horizontal bar) and the variance of + 1 standard deviation (vertical bar). Both peaks are highly significant and deviate from the mean by 11 and 7 standard deviations.

In addition to analyzing the high-resolution details of the time series, information can also be gained by examining the year-to-year variation throughout the 62-year sequence. We have determined the yearly nitrate flux by multiplying the average yearly nitrate concentration by the total snowfall during that particular year corrected for density variations caused by compaction.

Because the yearly flux always includes nitrate produced by sources other than ionization from solar charged particles impacting the high altitude polar atmosphere, it provides only a relative estimate of the solar effect. If precise monthly precipitation for the area under study is known, the overestimation can be minimized. This is possible, however, only if the snow sequence is analyzed at a resolution better than one month. In this study our resolution varies in proportion to the yearly precipitation and the degree of compaction in that portion of the sequence under study. Within our sequence, yearly resolution varies from slightly less than an average of one sample per month in thin year layers that are compacted, to nearly one sample per week in some of the thicker, uncompacted year layers.

Figure 5 is a plot of yearly nitrate flux data. When this graph is compared with time series graphs of the total number of flares (NOAA, 1989) a general agreement is noted and periods of high solar activity tend to coincide with nitrate flux peaks during the last

h i

t-

3.0

2.0

1.il . -2 .0 '

1978 1968 1958 1948 1938 1928 I I I I I I

10 20 30 40 50 60

YEARS

6 i

0) >-

t'N

4 t E

Z I

2 E

Fig. 5. Nitrate yearly flux curve for the years 1988 to 1927. After 1956, the total number of flares that occurred each year during solar activity cycles 19 through 21 is indicated by the dotted line on the graph (NOAA, 1988). Both time series have been normalized. The scale on the right indicates nitrate flux per year

in #g-N per m z.


three solar cycles, cycles 19, 20, and 21. This comparison constitutes only a rough qualitative evaluation and will lead to a detailed analysis that will take into account the number and flux of solar proton events (SPEs) and the energy spectrum of significant SPEs (Shea and Smart, 1990; Feynman et al., 1989; Goswami et al., 1988). The inverse relationship between particle energies and the time required for fallout of the nitrate products implies that there might be a one- to two-year delay for nitrate produced high in the atmosphere by the low-energy component whereas the high-energy component could produce nitrates at much deeper levels resulting in fallout in a period of only a few months. The latter condition is especially likely in the case of high-energy events that occur during the polar night.

On the relatively large scale of one-year resolution, the curve of carbon-14 produced by solar cosmic rays can be visually compared with nitrate flux in Antarctic snow. The hyperfine structure of the carbon-14 curve reported by Damon, Cheng, and Linck (1988) for solar cycles 17 and 18, and our yearly flux curve show general agreement in trends with one major exception of an anomaly that occurs in 1939. In this case, 1939 shows low nitrate flux but high solar-produced carbon- 14. These two time series, both of which result from the interaction of solar particles on the Earth's atmosphere, merit further study.

6. Ionization Products from Charged Particles

Armstrong et al. (1989), pointed out that solar and interplanetary protons and e particles have essentially full and prompt access to the polar atmosphere. Ground-based riometer data from South Pole Station and satellite-derived charged particle fluxes have been correlated by calculating the vertical structure of the ionization induced by the protons and alpha particles during their stopping process. Energetic particle flux into the atmosphere is determined by the power law in kinetic energy

dN - A E - :',

d E

where dN = particle flux (number cm-2 s- 1 sr- l); dE = kinetic energy per nucleon; A = flux constant; y -- spectral constant.

These calculations, based upon IMP-8 measurements of protons from 0.29 to 440 MeV, show that for the years 1982-1985 the ionization effects of several SPEs reach into the lower stratosphere.

In model calculations of odd nitrogen (N, NO, NO2, NO 3 , HNO 3 , HOaNO2, N205, and CIONO2) produced by ionization of solar protons in 1978 and 1979, Jackman and Meade (1988) have limited their model to maximum energies of 100 MeV in assessing effects in the upper stratosphere. They conclude that "The SPE stratospheric source in 1978 is less than 1 ~ o of the N20 oxidation stratospheric source for the whole year, whereas the two sources are of comparable magnitude in the mesosphere for 1978". However, the stratospheric source will increase somewhat if particle energies in excess of 100 MeV are included. These high energy particles (100-440 MeV) cause additional


ionization as far down as the lower stratosphere (Armstrong et al., 1989; Laird, 1989). In any case, the local abundance at every point in the winter Antarctic atmosphere is a function of both local ionization and downward transport from higher altitudes.

A number of sources contribute to odd nitrogen on a global scale. They are GCRs (galactic cosmic rays), nuclear explosions, lightning, SPEs, relativistic electron precipitation, meteors and downward diffusion of NO from the thermosphere. Of these sources, lightning in low- and mid-latitudes plays no significant role in polar areas due to the rapid washout from rainfall and downdrafts associated with the clouds that produce most lightning (Jackman, Frederick, and Stolarski, 1980).

In model calculations, Garcia et al. (1984) include polar night effects and showed that changes in solar activity can lead to large variations in the abundances of NOx in the thermosphere and upper mesosphere, particularly by auroral particle precipitation above 90 km altitude. NO x in the stratosphere from these sources was estimated to be possibly as high as 1016 mol cm-2 y - 1 (corresponding to about 2 mg m-2 of N O 3 -N per year). The considerable amounts of NO x that are produced in the thermosphere are transported continuously downward by diffusion and mean advection during the Antarctic winter. This source surpasses by orders of magnitude the NOx production in the stratosphere by galactic cosmic rays. However, it was believed that below 30 km, large contributions of mid-latitude air would mix into the upper stratosphere and dilute the solar signal originating in the thermosphere and mesosphere. To attribute the nitrate concentrations found in the snow to a source in the upper atmosphere, it was necessary to assume that an effective cut-off of Antarctic air from that derived from low- and mid-latitudes must occur to account for the solar signal that we found in the nitrate levels of Antarctic snow.

The Antarctic vortex developes during the austral winter (May-August) as a large air mass circulating over the continent as a closed system (Maggs, 1989). It continues to maintain its structural identity into the spring and usually breaks down in late October. The most recent observations related to studies of ozone depletion, found that the vortex can remain active as late as December (Singh, 1988). During these efforts to understand the polar atmospheric circulation in Antarctica, an additional very important finding was made. Some polar stratospheric clouds (PSC's) were discovered to contain very high levels of HNO 3 . Clouds containing nitric acid had been postulated by Toon et al.

(1986) and suggested by Garcia et al. (1984) as an alternative mechanism for NOx-rich precipitation. During the polar night, condensation of extremely small quantities of H20 in the Antarctic stratosphere can remove nitric acid almost completely from the gas phase (Wofsy et al., 1988) after which it can fall out by gravitational settling. In fact, McElroy, Salawitch, and Wofsy (1988) estimate that about one half of the initial abundance of HNO3 in the PSC's is removed by fallout of particles to the surface.

7. Signature of Solar Proton Events, Cycles 16 through 21

The nitrate time series from Windless Bight represents continuous nitrate fallout information through five complete solar cycles. The experimentally determined facts can be


summarized as follows: (1) Three major peaks are present in the nitrate concentration profile which have

been dated to occur near the end of the years 1972, 1946, and 1928. The increases above the series mean are 7, 11, and 4 standard deviations in amplitude, respectively. Among these major peaks, 1928 appears to be the smallest, but it is important to note that when compaction effects are taken into account, it increases to 6 standard deviations above the mean background of the adjacent ten years.

High-resolution sampling makes it possible to define with considerable accuracy, shape, and area under the curve of specific nitrate pulses caused by solar flare events. These measurements provide important information about the progression of nitrate fallout during the year in which the event occurred.

(2) The three largest peaks in the concentration time series occur toward the end of the corresponding year and are identified as being associated with July and August SPEs for the particular year under consideration. Generally, the relative signal strength is dependent upon the known or estimated particle fluxes and the energy spectra of the particles injected into the atmosphere at high latitudes within the polar cap (Feynman etal., 1989, 1990; Reedy, 1977; Goswami etal., 1988; Shea, 1989; Shea and Smart, 1977, 1979). Furthermore, the timing of the SPE seems to have an influence upon the nature and recognizability of its nitrate signal in the Antarctic snow.

The August event of 1972 and the July events of 1946 and the white light flare of 1928 (Shea, 1989; Feynman, 1989) occur during months of total darkness and during a time when the polar vortex has built up to produce a rapid and efficient downward transport. These events are very prominent and they provide a recognizable signature which is characteristic of major SPEs that occur at this time of year. Events that occur late in the year, after the breakdown of the strong polar vortex tend to be less well defined because the downward transport is slower and the pulse is spread out. This becomes especially apparent in a plot (Figure 6) of the yearly nitrate variability represented by the yearly standard deviation. The years 1972, 1946, and 1928 stand out most clearly as periods of high variability.

Thus far we have considered primarily the effects in the Earth's atmosphere. It is also necessary to consider the effects of interplanetary transport of charged particles between their point of origin on the sun and their access to the Earth's atmosphere. It is well known that major anisotropies of solar cosmic ray intensities exist between stations in Antarctica (Pomerantz, 1984; Pomerantz et al., 1980) and also between the northern and southern hemispheres (Pomerantz and Duggal, 1979). The north-south anisotorpy has been found to be strongly dependant on the direction of the spiral interplanetary magnetic field (IMF) (Pomerantz, 1984; Bieber and Pomerantz, 1986). For individual solar particle events, conditions in interplanetary space may be such that unimpeded transport occurs of relativistic solar particles (greater than one gigavolt) to the Earth along the spiral IMF field lines connecting the Sun to the Earth (Pomerantz and Bieber, 1985). However, during the course of each solar cycle the well-ordered IMF sector structure undergoes varying degrees of disturbance depending on the number and intensity of solar flares. This process and its effect on the Earth's magnetic field has been

344 G I S E L A A. M . D R E S C H H O F F A N D E D W A R D J. Z E L L E R

Fig. 6.

I---

z

h i >

_A tad e r

1'5 t" 1972

1"0' "

0.5'

~

1946

1928 I

I I I I I I I I I I 10 20 30 40 50 60

YEARS Plot of the variation of the magnitude of one standard deviation determined from the nitrate

concentration variation within each individual year from 1988 through 1927.

incorporated in a geomagnetic activity Recurrence Index (Sargent, 1979). This index is derived from a combination of 110 years of geomagnetic index and the so called Bartels Rotations, the 27-day solar rotations. The Bartels records begin with rotation No. 1 on February 8, 1832 (Kertz, 1971). The 27-day Recurrence Index is highest at sunspot minimum when the highest correlations exist between successive rotation periods and it is generally lowest during sunspot maximum. However, the lengths of periods of high correlation vary between cycles and are longest in duration in alternate cycles (every 22 years). The signal from the Windless Bight core shows a variation in response to the frequency and intensity of major solar flares especially when they occur during the polar night. Preliminary examination indicates that this variation in response follows a pattern related to the 27 day Recurrence Index.

The largest and most distinct pulses, 1928, cycle 16; 1946, cycle 18; and 1972, cycle 20; occur in the transition period between the short duration and the subsequent long duration maxima of the Recurrence Index. Such major flares carry shockwaves outward in the heliosphere and distort the IMF structure for relatively short periods of time. On the other hand, 'When whole series of flares occur, the situation in the interplanetary medium can only be described as chaotic...' (Sargent, 1979). For example, in the case of solar cycle 19 (1954-1964), the Recurrence Index is consistently low for a period of more than 7 years, and the distortions in the interplanetary magnetic field appear to blur out the nitrate response. During that period of time a distinct rise in background level results and a series of groups of pulses appearing as well defined nitrate peaks occur. However, none of the sharply defined peaks reach maximum intensities, e.g., the July

E\,IDENCE OF INDIVIDUAL SOLAR PROFON EVENTS IN ANTARCTIC SNOW 345

1959 SPE which occurred during the polar night, that are comparab le to the nitrate

signal of the isolated, large 1972 event.

Detai led signal analysis must be appl ied to determine the nature of the nitrate

response to these kinds of anomalous ly large solar flares and flare groups, part icularly

with respect to solar pro ton fluxes which are based on interplanetary proton flux models

(Shea and Smart , 1990; F e y n m a n et al., 1989, 1990; G o s w a m i et al., 1988). Finally, an

a t tempt will be made to determine not only the frequency of major solar part icle events

recorded in the snow sequences but also such parameters as the part icle energies and

fluences in each individual event. The limits within which this detai led cal ibrat ion of

major nitrate pulses can be achieved will require an even higher resolut ion time series

that extends as far back as geomagnet ic observat ions have been recorded.

Acknowledgements

This work was part ial ly funded by the Nat iona l Science Founda t ion grant N S F / D P P

8715543 and by U.S. Air Force Cont rac t A F O S R - 8 8 - 0 0 6 5 .

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evidence of individual solar proton events in antarctic snow

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