Studies of the arrival directions of energetic solar particles during ground level enhancements (CLE's) observed by neutron monitors have shown that, in general, in the first hour of the event most of the particles arrive with a distribution of pitch angles peaked about the garden hose field direction in the vicinity of Earth. During the first hour some of the particles arrive from the antisolar direction, while in later stages of the event the intensity becomes more nearly isotropic as a result of scattering of particles in interplanetary space. An attempt is made to determine the arrival directions of the particles during the early stages of the GLE of 16 February 1984 using the data currently available from high latitude neutron monitors near sea level where the cut off is essentially atmospheric (approx. LGV)

Fenton, A. G.

Fenton, K. B.

Humble, J. E.

English

NASA Technical Reports Server

339
SH 3.2-2
ENERGETICSOLARPARTICLEEVENTS
K.B.Fenton, A.G.Fenton and J.E.Humble
Physics Department, University of Tasmania
Box 252C, G.P.O., Hobart, Tasmania, Australia, 7001
I. Introduction. Studies of the arrival directions of energetic solar
particles during ground level enhancements (GLE's) observed by neutron
monitors have shown that, in general, in the first hour of the event most
of the particles arrive with a distribution of pitch angles peaked about
the garden-hose field direction in the vicinity of Earth (e.g., McCracken,
1962, Maurer et a1.,1973, Duggal,1979). During the first hour some of the
particles arrive from the anti-solar direction, while in later stages of
the event the intensity becomes more nearly isotropic as a result of
scattering of particles in interplanetary space.
In this paper an attempt is made to determine the arrival directions of
the particles during the early stages of the GLE of 16 February 1984
using the data currently available to us from high latitude neutron mon-
itors near sea level where the cut-off is essentially atmospheric (_IGV).
2. Data. In this event the first particles arrived at Earth during the
interval 0905-0910 UT and the data we have used for this interval are
listed in Table I together with the asymptotic viewing directions assumed
for the neutron monitors for 2 GV particles (Shea and Smart, 1975). We
also list the percentage increases observed in the next five minutes.
These figures are expressed as the percentage above the level prevailing
in the hour 0800-0900 UT.
TABLE i DATA FOR GLE OF 16 FEBRUARY 1984
ASYMPTOTIC DIRECTION AT 2GV PERCENTAGE INCREASE
STATION LATITUDE LONGITUDE,0E 0905-0910 0910-0915
Goose Bay -8 0 42.4 94.2
Deep River -22 i0 39.5 71.9
Mawson -13 56 36.6 49.3
Kerguelen 26 152 5.5 Ii.0
Thule 68 324 0 8.4
Terre Adelie -53 170 0 5.0
Inuvik 16 233 0 1.5
Alert 83 8 0 0
McMurdo -67 275 0 0
3. Analysis. It was expected that during the first five minutes
virtually all the particles would arrive from the garden-hose direction
or from small angles relative to that direction, since the time would not
be available for receiving scattered particles, except those scattered
very close to Earth.
39 
E GETIC S AR P ICLE E TS 
K.B.Fenton, A.G.Fenton and J. . b1e 
Physics epartment, niversity of Tasmania 
Box 252C, G.P.O., Hobart, Tasmania, Australia, 7001 
SH 3.2-2 
1. I tr duction. Studies of t e arri al irecti ns of energetic solar 
particles during ground level enhancements ( LE's) observed by neutron 
onitors have sho n t at, i  general, i  t e first hour f t e event ost 
of the particles arrive ith a distribution of pitch angles peaked about 
the garden-hose field direction in the vicinity of arth (e. ., cCracken, 
962, aurer t 1., 73, u ga1,1979). uring t  first our s e f t  
particles arrive fro  the anti-solar direction, hile in later stages of 
t e e ent t e i tensity beco es ore early is tr pic as a r sult of 
scattering of particles in interplanetary space. 
In this paper an atte pt is ade to deter ine the arrival directions of 
the particles during the early stages of the GLE of 16 February 1984 
using the data currently available to us from high latitude neutron mon-
i rs ear  l el here  t- ff  sentia ly t ospheric ~l  
. ata.  t i  ent  t arti l s r  t arth ri   
i t r al 5- 10   t  at  e a e s  f r t i  i t r al r  
liste  in able 1 together ith the asy ptotic vie ing directions assu ed 
f r t  tr  onitors f r   arti l s ( ea a  art, 5). e 
also list the percentage increases observed in the next five inutes. 
These figures are expressed as the percentage above the level prevailing 
 t  ur 0- 00 . 
 1        
PTOTI  I    E N  
 I  GI DE,OE -  -  
oose ay -8 0 42.4 94.2 
Deep River -22 10 39.5 71.9 
a s  -   .  49.3 
Kerguelen 26 152 5.5 11.0 
l     .  
r  deli  -5  17    
i     1.5 
l rt    0 
ur  -6  275 0 0 
3. Analysis. It was expected that during the first five minutes 
virtually all the particles would arrive from the garden-hose direction 
or from small angles relative to that direction, since the time would not 
be a il l  for rec ivin  s tter  p rti l s, exce t those sc ttere  
very close to Earth. 
https://ntrs.nasa.gov/search.jsp?R=19850026505 2020-03-20T17:01:07+00:00Z
340
SH 3.2-2
Following earlier authors, we use the symbol _ to represent the angle
between the asymptotic mean direction of arrival of the particles and the
asymptotic viewing direction of a neutron monitor, calculated in most
instances for 2 GV particles.
In spite of the apparent simplicity of this event, we have so far been
unable fully to understand the pattern of arrival directions. At 0910 UT
on 16 February 1984, the Sun was overhead at (-120,42.50), implying0that
if the solar wind speed were such that the garden-hose angle was 45 W of
the Earth-Sun llne, the mean arrival direction of the particles should
have been (-120,357.50). However, the fact that the increases observed
in the first five minutes at Goose Bay, Deep River and Mawson were so
nearly the same would appear to require a near e_uality of the 6 values
for these stations instead of the 50, 160 and 57v calculated for this
case. By trial, we have found that if, instead, the mean arrival direc-
tion were (50,250) the 6 values for Goose Bay, Deep River and Mawson
would have been 280 , 310 and 360 respectively, values consistent with the
progression towards slightly smaller intensity increases for these
stations. This arrival direction, however, implies a garden-hose angle of
less than 200W of the Earth-Sun llne which would appear to be unacceptably
small.
There is the further problem that for a mean arrival direction of (50,250 )
Kerguelen has a 6 value for 2 GV particles of 1200 and would not therefore
be expected to receive prompt particles. However, a 5.5% increase occurr-
ed in the first five minutes, and the profile has the appearance charac-
teristic of prompt particles. If it is assumed that, say, 4 GV particles
were responsible for the increase at Kerguelen, the _ value would be 740.
Although less than 900 , this value seems to be still too large for prompt
particles. In any case Thule and Alert have _ values of 740 and 780 res-
pectively but registered no increase in the first five minutes. At this
stage, we have not found a satisfactory solution to these problems.
A similar analysis of the next five minute interval, during which scatter-
ed particles were received at several monitors, has also failed so far to
yield an orderly progression of percentage increase versus 6. It is inter-
esting to note that Pomerantz et al. (1984) found by analysing the hourly
average data for this event that a smooth function of the percentage in-
crease versus cos 6 could be obtained. Whether this is due to the effect
of smoothing fine structure is a matter to be investigated.
4. Comments. As with previous analyses which we have made of five min-
ute data, e.g., the event of 7,8 December 1982 (Fenton et al., 1983), we
believe that a more detailed treatment may be necessary in which the act-
ual asymptotic directions at the time of the event are used and in which
the over-simplificatlon is eliminated of assuming that a single asymptotic
direction can be used for each station. In a more detailed analysis, the
responses of high altitude, as well as non-polar, monitors should also be
taken into account. It should be possible to extend the analysis to one-
minute data, at least for large GLE's for which accurately timed data are
available, and thus obtain additional information on the scattering
environment in the nearby interplanetary space.
340 
SH 3.2-2 
Following earlier authors, we use the symbol 0 to represent the angle between the asymptotic mean direction of arrival of the particles and the asymptotic viewing direction of a neutron monitor, calculated in most instances for 2 GV particles. 
In spite of the apparent simplicity of this event, we have so far been unable fully to understand the pattern of arrival directions. At 0910 UT on 16 February 1984, the Sun was overhead at (-12°,42.5°), implying that if the solar wind speed were such that the garden-hose angle was 45 0W of the Earth-Sun line, the mean arrival direction of the particles should have been (-12°,357.5°). However, the fact that the increases observed in the first five minutes at Goose Bay, Deep River and Mawson were so nearly the same would appear to require a near equality of the 0 values for these stations instead of the 5°, 16° and 57° calculated for this case. By trial, we have found that if, instead, the mean arrival direc-tion were (5°,25°) the 0 values for Goose Bay, Deep River and Mawson would have been 28°, 31° and 36° respectively, values consistent with the progression towards slightly smaller intensity increases for these stations. This arrival direction, however, implies a garden-hose angle of less than 200W of the Earth-Sun line which would appear to be unacceptably small. 
Th re is the further problem that for a mean arrival direction of (5°,25°) Kergu len has a 0 value for 2 GV particles of 120° and would not therefore be expected to receive prompt particles. However, a 5.5% increase occurr-ed in the first five minutes, and the profile has the appearance charac-teristic of prompt particles. If it is assumed that, say, 4 GV particles w re responsible for the increase at Kerguelen, the 0 value would be 74°. Although le s than 90 0 , this value seems to be still too large for grompt particles. In any case Thule and Alert have 0 values of 74° and 78 res-pectively but registered no increase in the first five minutes. At this stage, we have not found a satisfactory solution to these problems. 
A similar analysis of the next five minute interval, during which scatter-ed particles were received at several monitors, has also failed so far to yield an orderly progression of percentage increase versus o. It is inter-esting to note that Pomerantz et al. (1984) found by analysing the hourly average data for this event that a smooth function of the percentage in-crease versus cos 0 could be obtained. Whether this is due to the effect of smoothing fi e structure is a matter to be investigated. 
4. Co ments. As with previous analyses which we have made of five min-ute data, e.g., the event of 7,8 December 1982 (Fenton et al., 1983), we believe that a more detailed treatment may be necessary in which the act-ual asymp otic directions at the time of the event are used and in which the over-simplification is eli inated of assuming that a single asymptotic direction can be used for each station. In a more detailed analysis, the responses of high altitude, as well as non-polar, monitors should also be taken i t  account. It should be possible to extend the analysis to one-minute data, t least for large GLE's for which accurately timed data are available, and thus obtain additional information on the scattering envir ent in the nearby interplanetary space. 
341
SH 3.2-2
5. Acknowledgements. We wish to thank the people responsible for the
operation of the neutron monitors listed in Table i and their organiza-
tions for supplying us with their data. The project is supported in part
by the Australian Research Grants Scheme.
References.
Duggal,S.P.,(1979), Rev. Geophys. and Space Phys., 17, 1021.
Fenton,K.B., Fenton,A.G. and Humble,J.E.,(1983), Proc.18th ICRC, 4, 189.
McCracken,K.G.,(1962), J. Geophys. Res., 67, 435.
Maurer,R.H., Duggal,S.P. and Pomerantz,M.A.,(1973), J. Geophys. Res.,78,29.
Pomerantz,M.A., Bieber,J.W. and Evenson,P.A.,(1984), Proc. Int. Symp.
Cosmic Ray Modulation in the Hellosphere, Morioka, Japan, 437.
Shea,M.A. and Smart,D.F.,(1975), Air Force Cambridge Research Laboratories
Environmental Research Papers, No. 502, AFCRL-TR-75-0177.
341 
SH 3.2-2 
5. Acknowledgements. We wish to thank the people responsible for the 
operation of the neutron monitors listed in Table 1 and their organiza-
tions for supplying us with their data. The project is supported in part 
by the Australian Research Grants Scheme. 
References. 
Dugga1,S.P.,(1979), Rev. Geophys. and Space Phys., 17, 1021. 
Fenton,K.B., Fenton,A.G. and Humb1e,J.E.,(1983), Proc.18th ICRC, 4, 189. 
McCracken,K.G.,(1962), J. Geophys. Res., 67, 435. 
Maurer,R.H., Dugga1,S.P. and Pomerantz,M.A.,(1973), J. Geophys. Res.,78,29. 
Pomerantz,M.A., Bieber,J.W. and Evenson,P.A.,(1984), Proc. Int. Symp. 
Cosmic Ray Modulation in the He1iosphere, Morioka, Japan, 437. 
Shea,M.A. and Smart,D.F.,(1975), Air Force Cambridge Research Laboratories 
Environmental Research Papers, No. 502, AFCRL-TR-75-0177. 


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