A description of the intensity-time profiles of solar proton events observed at the orbit of the earth is presented. The discussion, which includes descriptive figures, presents a general overview of the subject without the detailed mathematical description of the physical processes which usually accompany most reviews

Shea, M. A.

English

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INTENSITY/TIME PROFILES OF SOLAR PARTICLE EVENTS AT ONE ASTRONOMICAL UNIT 
8 8 9 -  2 8 4 6 0  
M. A.  SHEA 
Air Force Geophysics Laboratory 
Hanscom Air Force Base 
Bedford, Massachusetts 01731 
ABSTRACT 
A description of the intensity-time profiles of solar proton events observed at 
the orbit of the earth is presented. 
figures, presents a general overview of the subject without the detailed math- 
ematical description of the physical processes which usually accompany most reviews. 
The discussion, which includes descriptive 
1. INTRODUCTION 
Major solar flares are often associated with the acceleration of energetic 
particles at the sun and their injection into the interplanetary medium where they 
can be detected by a variety of techniques. Only high energy particles could be 
detected at the earth prior to the space age since the particles had to have enough 
energy to be able to penetrate to balloon altitudes, or in rare cases, to ground- 
level detectors. Since the advent of the space era, data obtained from particle 
sensors on spacecraft throughout the heliosphere as well as improved balloon and 
ground-based instrumentation have greatly increased our understanding of solar 
particles and their propagation in the solar system. This paper presents a summary 
of the intensity/time profiles of particle events as detected at the earth's orbit. 
2. SOLAR EMISSIONS 
Solar flares are associated with electromagnetic emissions, acceleration of 
electrons and ions, and, if conditions are favorable, the injection of these parti- 
cles into space. Each solar flare is unique, and the generation of these emissions 
can differ from event to event. Figure 1 is a representation of the propagation 
time of various types of solar emissions from the sun to the earth. Solar X-rays 
and other types of electromagnetic radiation reach the earth at essentially the 
speed of light - i.e. in approximately eight minutes. To a first order approxima- 
tion, the intensity of the radio and soft X-ray emissions observable at one Astro- 
nomical Unit is independent of the location of the flare on the visible disk of the 
sun. Energetic solar particles reach the orbit of the earth from a few minutes, if 
the particles are relativistic, to hours for the lower energy particles. Both the 
measured onset time and maximum intensity of these particles are a function of the 
solar longitude of the flare with respect to the detection location. 
plasma usually propagates to the earth within one or two days and can manifest 
itself by the occurrence of aurora and geomagnetic disturbances, the magnitude of 
which are dependent upon the interplanetary plasma and field characteristics at the 
time of the arrival of the plasma at the earth. 
time of arrival of solar particle emissions at the earth. 
Angstrom soft X-ray emission is often detected prior to the recorded onset of the 
solar flare in H-alpha; this is primarily a difference in the recording 
sensitivities. 
Enhanced solar 
Figure 2 illustrates the relative 
Note that the 1-8 
PRECEDIKG PAGE &LA%# NG'T FIiMED 
7 5  
https://ntrs.nasa.gov/search.jsp?R=19890019089 2020-03-20T02:00:27+00:00Z
SOLAR EMISSIONS 
Figure 1. Pictorial representation of energetic particle propagation 
from the sun to the earth. The relative time scales are noted. 
io3 
RELATIVE 
UNITS 
IO2 
IO' 
IO0 
A I  SHOCK 
I RELATIVE TIME OF SOLAR 
I 
I PARTICLE EMISSIONS 
I (AT  1 AU) 
PROTONS 
I I  I    
I 1 
I IO 
TIME (hours) 
I DAY PDAYS I WEEK 
Figure 2. Time scales of solar particle fluxes at 1 AU. 
7 6  
START OF 
INCREASE 
SOLAR I TIMEOF 
FLARE MAXIMUM 
f RELATIVE TIME + - TIME FROM 
PROPAGATION START TO MAX 
DELAY 
AMPLITUOE OF 
MAXIMUM FLUX 
Figure 3. Characteristic solar particle intensity/time scale profile. 
Although the general shape of the intensityltime profile, as shown in Figure.3, 
will differ from event to event and also with respect to the location of the flare 
on the sun with respect to the location of the detection point in the heliosphere, 
particle events can be characterized by the following: a propagation delay between 
the onset of the solar flare in H-alpha emission and the onset of the particle 
increase; a relatively rapid rise in intensity to a maximum value; and a slow decay 
to the background level. Although actual event profiles can be complicated by 
multiple particle injections or interplanetary perturbations, this simplified 
picture is appropriate for any one isolated event. 
3 .  SOLAR PARTICLE PROPAGATION 
The concept of solar particle propagation is discussed elsewhere in these 
proceedings (Smart, 1988) and will not be discussed in detail here. 
fact is that solar particles propagate into the interplanetary medium along the 
interplanetary magnetic field lines. If a solar particle producing flare occurs 
near the "footpoint" of the interplanetary magnetic field line connecting the earth 
with the sun (which is nominally around 60' west longitude on the sun), then a 
detector located along this field line, e.g. the earth, should record the earliest 
onset time and the highest intensity of any detector located at the same radial 
distance but at different heliolongitudes. Figure 4 illustrates this favorable 
propagation path. If a flare occurs at any other solar longitude, the particles 
planetary field line connecting the sun with the earth whereupon they propagate 
along the field line to the earth. 
The essential 
~ which reach the earth are first transported through the solar corona to the inter- 
Figures 5 and 6 illustrate typical intensityltime profiles that would be 
recorded at one Astronomical Unit from identical flares at different locations on 
the sun. The intensity/time profile shown on the right side of Figure 5 is typical 
for a flare that occurred at the "footpoint" of the interplanetary magnetic field 
line connecting the sun with the earth. Notice the rapid rise to maximum intensity. 
The particle flux would be maximum along the favorable propagation path (shown by 
the larger dots) whereas particles that diffuse through the solar corona to other 
field lines would have a smaller flux (shown by smaller dots). 
1 ' 
77 
Figure 4 .  
sun and the earth with the favorable propagation path indicated. 
Idealized interplanetary magnetic field line between the 
0 e 
0 e e 
e 
e 
e 
e 0 
e e 
e 
0 
e 
0 
Figure 5 .  
magnetic field line from the sun to the earth from a solar flare at the "footpoint" 
of the field line. The larger the dot, the larger the flux. 
time profile for this event, as measured at the earth, is shown on the right. 
Graphic representation of particle propagation along the interplanetary 
A typical intensity/ 
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Figure 6 illustrates the particle flux in the inner heliosphere from a flare to 
the east of the earth-sun line. 
dots) would be along the interplanetary magnetic field line from the flare location 
to the hypothetical spacecraft located at one Astronomical Unit. 
from this flare are propagating along the field line to the satellite, they are also 
propagating, albeit with a reduced intensity, through the solar corona to other 
field lines. Those particles which reach the interplanetary field line connecting 
the earth with the sun have started to propagate along this field line to the earth; 
however, as seen from the lower section of this figure the particle intensity at the 
earth is still at the background level whereas at the spacecraft the maximum inten- 
sity has already been measured. The top section of Part B illustrates the particle 
intensity in the inner heliosphere a few hours later when both the spacecraft to the 
left, and a hypothetical spacecraft at the earth, would be responding to an enhanced 
solar particle flux. 
profiles which would have been recorded by both spacecraft during this event. 
spacecraft at the earth would have recorded a later onset time, slower rise time, 
smaller maximum flux, and longer decay time than the spacecraft located along the 
field line connected to the flare site. 
In Part A the maximum flux (shown by the large 
While particles 
The lower section of this panel shows the. intensity/time 
The 
1 
- 
At times major flares can populate the entire inner heliosphere with solar par- 
ticles as illustrated in Figure 7. On 8 and 9 August 1970 particle increases on the 
Pioneers 8 and 9 space probes together with the small increase on the IMP 5 satel- 
lite at the earth could not he associated with any solar activity on the visible 
hemisphere of the sun; however, Dodson-Prince et al. (1977) noted that active region 
10882, which produced particle events on 13 and 14 August 1970, was on the invisible 
hemisphere of the sun about three days before east limb passage. Since Pioneer 9 
had the largest maximum increase on 8 August, and Pioneer 8 had a smaller increase 
with maximum intensity on 9 August, a possible flare located approximately 40° 
behind the east limb was assumed to be the source of this particle event. The small 
increase observed on IMP 5 is consistent with this flare location. 
RELATIVE SOLAR PROTON 
AMPLITUDE VS. 
SPACECRAFT POSITION 
\ 
/ - -  I 
/ \ 
/ \ 
/ 8-9 AUG 1970 
/ 
/ 
/ 
I 
Figure 7 .  
9, Pioneer 8 and IMP 5 (at the earth) for the particle event of 8-9 August 1970. 
The bars representing the particle flux are placed at the location of the indicated 
spacecraft. 
behind the east limb of the sun. The Pioneer measurements were for particles above 
14 MeV; the IMP measurements were for particles above 10 MeV. 
Relative solar particle intensity (on a log scale) as measured on Pioneer 
This event has been attributed to a solar flare approximately 40' 
8 0  
4 .  SPECIFIC EXAMPLES 
Figures 8 and 9 illustrate specific examples cxf intensity/time profiles as 
measured at the earth during June 1972. 
1972 as detected on the earth-orbiting Explorer 4 3 .  This event exhibited a rapid 
rise and fast decay associated with a flare to the west of the sun-earth line and 
possibly close to the west limb. Solar flare observations did not indicate any 
flare that could be reasonably associated with this particle event. 
data from the Pioneer 6 and 10 space probes coupled with the knowledge that an 
active particle producing region had just rotated over the western limb of the sun 
led to the assignment of a flare in this region as the possible producer of this 
particle event (Shea and Smart, 1975). 
Figure 8 shows the particle event on 8 June 
Examination of 
I O  
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t 
+ +  
++ * E>30 \ 
+ MeV 
I EXPLORER 43 I 
eV 
+A 
a 9 
JUNE 1972 
IO  
Figure 8. Intensity/time profiles f o r  three proton channels on the 
Explorer 4 3  spacecraft for the event with onset on 8 June 1972. 
a i  
Another particle event occurred later in the same month as shown in Figure 9. 
There were two possibilities for the solar activity associated with this event - a 
series of flares near the cenlpal meridian of the sun and eastern limb activity 
which might have been associated with a flare behind the east limb. Because of the 
large variations in the intensity/time profiles as measured on different spacecraft 
it has not been possible to associate a specific flare to this event except to 
assume that more than one solar event on the eastern hemisphere may have contributed 
to the total flux at the earth. 
indicative of a source region (or regions) to the east of central meridian. 
The particle flux profile shown in Figure 9 is 
20 
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14 15 16 17 18 19 
JUNE 1972 
Figure 9. 
Explorer 4 3  spacecraft for the event with onset on 16 June 1972. 
Intensity/time profiles for two proton channels on the 
82 
5. DISPELLING POPULAR MYTHS 
As in many scientific fields, a certain amount of folklore has been generated 
which may, or may not, have a factual basis. For example, it is often mentioned 
that relativistic solar particle events (i.e. the so-called ground-level events) 
occur on the rising and falling portion of the solar cycle, but not during solar 
maximum or solar minimum. 
solar particle events between 1956 and 1984, two occurred in 1968 and one in 1979 - 
both years of sunspot number maximum, and one occurred in 1976 within three months 
of solar minimum. 
portions of the solar cycle, a statement that they never occur at solar maximum or 
minimum is based on mythology. 
Inspection of Figure 10 shows that of the 35 relativistic 
While most of these events do occur on the rising and falling 
Another myth which has been dispelled since the era of precise satellite 
measurements is that solar particle events are always associated with solar flares. 
First, one should define the term solar particle event which is usually defined as 
an increase in particle intensity associated with a solar flare. 
flare could not be located on the sun in reasonable time association with a particle 
increase as measured by satellites, a hypothetical flare was often identified as 
occurring during times of no flare patrol or on the invisible solar disk. With 
improved resolution of satellite measurements, particle increases have now been 
associated with disappearing filaments, interplanetary magnetic sector boundary 
crossings, coronal mass ejections, and acceleration by interplanetary shocks. 
Although the energies and flux associated with these events are g e n e r a l l y  r e l a t i v e l y  
small, as shown in Figure 11, there have been exceptions such as the Fermi 
acceleration of protons to relativistic energies during the August 1972 solar- 
terrestrial events (Pomerantz and Duggal, 1974). 
When a specific 
RELATIVISTC SOLAR PRON EVEMS 
I 
TO EARTH 
Figure 10. 
three solar cycles as shown by the smoothed sunspot number. 
on the sun of the flares associated with relativistic solar proton events. 
Left Side: Frequency of relativistic solar proton events throughout 
Right Side: Location 
8 3  
1215 UT 1315 UT 1457 UT 
X 
3 1  
i 
1 ' ) < * I  30-45 MEV 
$1 PROTONS 
I /  1 
10-5 1 1  ' .  
5 6 7 a 
DECEMBER 1981 
Figure 11. 
filament on 5 December 1981. (From Kahler, et al., 1986) 
Solar particle increase associated with a disappearing 
6 .  CONCLUDING REMARKS 
Solar particle events can occur at any time in the solar cycle. They come in 
various sizes and with different intensity/time profiles usually dependent upon the 
location of the flare with respect to the detection site and the characteristics of 
the interplanetary medium at the time of the event. There is no guarantee that 
specific events will or will not occur during a projected mission time frame. 
only guidelines that can be given will be generated from statistical studies of 
ground-based and spacecraft measurements conducted over the past three solar cycles. 
The 
REFERENCES 
Dodson-Prince, H.W., E.R. Hedeman, and O.D. Mohler, Survey and Comparison of Solar 
Activity and Energetic Particle Emission in 1970, 
Laboratory Technical Report, AFGL-TR-77-0222, 1977. 
Air Force Geophysics 
Kahler, S.W., E.W. Cliver, H.V. Cane, R.E. McGuire, R.G. Stone, and N.R. Sheeley, 
Jr., Solar filament eruptions and energetic particle events, Astrophys. J., my 
504, 1986. 
~ 
Pomerantz, M.A., and S.P. Duggal, Interplanetary acceleration of solar cosmic rays 
to relativistic energy, J. Geophys. Res., 79, 913, 1974. 
Shea, M.A., and D.F. Smart, An analysis of the solar particle data obtained during 
CINOF, in Results Obtained During the Campaign for Integrated Observations of 
Solar Flares (CINOF), edited by M.A. Shea and D.F. Smart, Air Force Cambridge 
Research Laboratories Technical Report No. AFCRL-TR-75-0437, 1975. 
Smart, D.F., Predicting the arrival times of solar particles, Proceedings of the JPL 
Workshop in the Interplanetary Charged Particle Environment (this publication) , 
1988. f . 8  ; v n r f i i  
"YGF 
v. -,?-?;;";7 t.i , ' *- : - 8 4 b. . : 2, :<, _I 


Intensity/time profiles of solar particle events at one astronomical unit

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