The anisotropy of 1.3 to 2.3 MeV protons has been measured with the Caltech Electron/Isotope

Spectrometer aboard IMP-7 for periods between prompt solar particle events from 72/273 to

74/2. The diffusive anisotropy, which has been computed by subtracting the independently determined.

convective anisotropy from the observed anisotropy, is predominantly directed toward the sun with a typical radial component

of 14%

Marshall, F. E.

Stone, E. C.

It has been found that except during the onset of prompt solar particle events the diffusive flow of 1.3 to 2.3 MeV protons at 1 AU is predominantly sunward. The anisotropy of the proton intensity has been measured with the Caltech Electron/Isotope Spectrometer aboard IMP-7 for periods between prompt solar particle events from 72/273 to 74/2. The diffusive anisotropy, which has been determined by subtracting the independently determined convective anisotropy from the observed anisotropy, is predominantly directed toward the sun with a mean radial component of 14%. This sunward diffusion is typical of intensities from 0.012 to 1.2 (cm^² sec sr MeV)^(−1) for 1.3 - 2.3 MeV protons and indicates that a positive radial gradient is characteristic of these modestly enhanced fluxes. The direction of the flow is opposite to that which would be produced by previously proposed models which involve a continuous solar source for the observed particles. A steady-state propagation model which includes adiabatic energy loss with particle injection beyond 1 AU produces the average observed anisotropy for κ_rr∼4 × 10^20 cm^² sec^(−1)

Caltech Authors - Main

Vol. 4, No. 1 Geophysical Research Letters January 1977 
PERSISTENT SUNWARD FLOW OF ~ 1.6 MeV PROTONS at 1 AU 
F. E. Marshall and E. C. Stone 
California Institute of Technology, Pasadena, California 91125 
Abstract. It has been found that except during times as large as the average events seen at Earth 
the onset of prompt solar particle events the by IMP-7 during the same time period. The authors 
diffusive flow of 1.3 to 2.3 MeV protons at 1 AU is suggested interplanetary acceleration as the most 
predominantly sunward. The anisotropy of the proton likely source of the streams seen near 3 AU. 
intensity has been measured with the Caltech Elec- In this letter we report the anisotropy of 1.3 
tron/Isotope Spectrometer aboard IMP-7 for periods to 2.3 MeV protons during 317 6-hour periods from 
between prompt solar particle events from 72/273 to 72/273 to 74/2, excluding periods when prompt so•ar 
74/2. The diffusive anisotropy, which has been det- particle events are observed. It is found that •he 
ermined by subtracting the independently determined diffusive streaming is predominantly toward the sun, 
convective anisotropy from the observed anisotropy, indicating a predominantly positive radial gradient 
is predominantly directed toward the sun with a in the particle density, 
mean radial component of 14%. This sunward dif- 
fusion is typical of intensities from 0.012 to 1.2 Data Analysis (cm 2 sec sr MeV) -1 for 1.3 - 2.3 MeV protons and • 
indicates that a positive radial gradient is char- The energetic particle data Are from the Caltech 
acteristic of these modestly enhanced fluxes. The Electron/Isotope Spectrometer (EIS) on IMP-7. Ani- 
direction of the flow is opposite to that which sotropies are determined for nucleons which stop in 
would be produced by previously proposed models a 47 •m solid-state detector D2 (the instrument has 
which involve a continuous solar source for the ob- been described by Hurford et al. (1974)). The ana- 
served particles. A steady-state propagation model lyzed nucleons a•e primarily 1.3 to 2•3 MeV protons, 
which includes adiabatic energy loss with particle but heavier nuclei contribute ~ 5%. 
injection beyond 1 AU produces the average observ- 
ed anisotropy for • ~ 4 x 1020 cm 2 sec -1 . 
rr 
Introduct ion 
The flux of low energy protons at 1 AU varies 
over Several orders of magnitude even during the 
periods between prompt solar particle flares. In- 
dividual increases, or events, have been studied 
by several authors including Bryant et al. (1965), 
Each analyzed event is labeled to indicate in 
which of 8 sectors in th$ ecliptic plane it was ob- 
served. The anisotropy • is determined by fitting 
the function A {1 + • cos(•i•- •o)} to the sectored o 
events accumulated in each 6-hour period, where %i 
is the average direction of sector i. The aniso- 
tropy direction, which is the direction toward 
which the particles are flowing, is defined to be 
o 
%o + 180 , and the anisotropy amplitude •OBS is O ß 
1.08 •. The factor 1.08 corrects for the smopthmng 
Fan et al. (1968), Anderson (1969), McDonald and effect of the finite sector width and finite in- 
Desai (1971), Krimi•is et a1.(1971) and ROelof and strumental opening angle. The solar ecliptic (SE) 
Krimisis (1973). These vents are distinguished coordinate system, in which the x-axis points to- 
from the prompt flare-associated events by their 
slower rise times, more symmetric time develop- 
ment and lack of velocity dispersion. These in- 
creases have been modeled as streams of energetic 
particles cOrotating with the sun whose source is 
either continuous solar acceleration or inter- 
mittent solar acceleration with near-sun storage. 
A more comprehensive study by Kinsey (1970) extend- 
ed this model to that of the sun as a continuous, 
but variable, source of low energy protons. 
Of these studies, only Krimi•is et a1.(1971) 
(whose data were subsequently analyzed by Roelof 
and Krimi•s (1973)) reported measurements of ex- 
extended periods with continuous streaming from the 
sun to support the proposed solar source. The ob- 
servjtiOns of Krimi$is et al. (1971) are at lower 
energies (m 0.3 MeV) than those reported in this 
letter. 
Evidence of a different source for corotating 
streams at m 3 AU has been reported by McDonald 
et a____•l•. (1975), who fodnd that the average size of 
such events seen by Pioneer 11 near 3 AU is ? 10 
ward the sun and t• z-axis points toward the North 
Ecliptic Pole, has been used. 
Jokipii and Parker (1970) have shown that the ob- 
served anisotropy is given by: 
+ + + 3 c•- (•_. vu)/u • (•) 
•OBS = •CON + •DIF = w -- 3 
in which • = particle density = 4•j/w 
V = solar wind velocity 
• = diffusion tensor 
w = particle velocity 
C -- Compton-Getting Factor 
= (2-e¾)/3 
e = (T + 2mc 2 )/(T + mc2) 
¾-- •lnj/•lnT 
j -- particle intensity 
T = particle kinetic energy 
The spectral index ¾ is determined by fitting the 
energy spectrum observed by the Caltech EIS experi- 
ment. The solar wind speed has been obtained from 
the MIT plasma experiment on IMP-7 (J.D, Sullivan 
and H. S. MCidge, private communication) and is 
Copyright 1977 by the American Geophysical Union. assumed to be radial. For this preliminary study 
Paper number 6L0711. 57 
58 Marshall and Stone: Persistent Sunward Flow 
50-- 
ß 
ß ß ß ß ß 
ß ß ß 
ß 
ß ß ß ß 
ß %.: 
, .. 
-•80 ø -9• O ø 90 ø 18• 
FiEure 1. The amplitude of the observed aniso- 
tropy, •OBS,as a function of the lonEitude %SAT 
of the I•-7 satellite in GSE coordinates. Each 
dot represents one 6-hour period. A typical • 
lo error bar is indicated. Since solar wind 
data are not needed for this fiEure , the data 
set has been extended from 72/273 to 75/18 to 
increase the sample size. Only periods when 
the fl• of 1.2 to 2.4 MeV protons is between 
0.12 and 1.2 (cm 2 sec sr MeV)-i are included. 
Periods when the fl• of • 0.2 to • 1.0 MeV 
electrons exceeds 2 (cm 2 sec sr) -1 are not in- 
cluded. Periods dominated by prompt solar 
particle events have been rejected usin E the 
criteria discussed in the text. 
only periods with speeds less than 700 km sec -1 
have been used. Thus •CON can be computed, and 
GDi F thereby determined from Eq. 1. 
Selection 
Not all of the 6-hour periods from 72/273 to 
74/2 are included in this study. Periods domi- 
nated by prompt solar particle events have been 
eliminated by two methods. First, periods when 
the PLO rate (which monitors nucleons stopping 
is in the magnetotail. There is no apparent dif- 
ference between the dawn and dusk side of earth 
while the spacecraft is sunward of the earth. The 
difference in the mean anisotropy amplitude for 
the sunward quadrants is 0.9% +- 2.0%. Differences 
in the mean anisotropy direction for the two quad- 
rants are also not statistically significant. Thus 
we have no evidence of magnetospheric influence on 
anisotropies for these periods when IbfP-7 is sun- 
ward of the earth. In particular, we have seen no 
evidence that the earth is a source of protons as 
has been observed at lower energies (Krimigis et 
al. , 1975). 
Results 
Figure 2a shows GOB S for the 112 6-hour periods 
meeting the above requirements and also having a 
PLO rate between 0.03 sec -1 and 0.3 sec -l. Some- 
what more than half these periods (57%) have a 
decreasing PLO rate. The mean x and y components 
of the anisotropy are: 
<G } : (-7.2% + 1.2% -1.8% + 1.1%) OBS ' 
For each of these 112 periods, the measured spec- 
tral index y and solar+wind speed V have been used 
in Eq. 1 to determine GDiF, which has been plotted 
in Figure 2b. The mean x and y components of the 
diffusive anisotropy are: 
GDi F = (13.6% + 1.2%, -3.2% + 1.1%) 
Thus although the mean observed streaming is from 
the sun, the mean diffusive streaming is toward 
the sun. The difference in the y-components is 
due to the earth's velocity. 
A+s shown in Eq. 1, the spatial gradients implied 
by GniF depend on the form of •. However, a pre- 
domiSRStly positive radial gradient is indicated 
for either isotropic or anisotropic diffusion• For 
isotropic diffusion, GDi F is proportional to VU, 
so the fact that 88% o•-the periods shown in Fig- 
ure 2b have a positive x-component indicates that 
88% of the periods have a positive radial gradient. 
If diffusion is preferentially along the m+agneti• 
field rather than across the field, then GD• F ß B 
should be used to determine whether the particle in D2) exceeds 0.3 sec-l• (corresponding to a flux 
of 1.2 (cm 2 sec sr MeV) -•) are not included. This density U is increasing along the field toward or 
criterion also means that the measured anisotro- away from the sun. Using this technique, it has 
pies are negligibly affected by the high rate been found that 80% of the periods have a posi- 
effects discussed by Roelof (1974). Second, 25 in- tive gradient along the field away from the sun. 
dividual prompt solar particle events have been T+hus for either isotropic or anisotropic diffusion, identified by their characteristically rapid on- 
sets. The day preceding, the day of, and the two 
days following the onset are eliminated. This 
criterion eliminates small events and the be- 
ginnings of large events even if the PLO rate is 
not larger than 0.3 sec -1. 
Periods are included only when IMP-7 is sunward 
of the earth, i.e.+in interplanetary space. The 
dependence of the GOBS on the position of IMP-7 
has been examined to determine any remaining mag- 
netospheric influence. For example, any blockage 
of particle flow by the magnetosphere should be 
more pronounced when IMP-7 is on the dawn side of 
earth than on the dusk side of earth. Figure 1 
shows the observed anisotropy amplitude as a func- 
tion of the Geocentric Solar Ecliptic (GSE) longi- 
tude of IMP-7. The outstanding feature of this 
figure is the small anisotropies found when the 
longitude of IMP-7 is near 180 ø , i.e. when IMP-7 
•DIF indicates that U is typically larger outside 
1 AU than inside. 
This diffusive flow back toward the sun was 
found to be typical of •1! •flux levels used in 
this study (0.012 to 1.2(cm-sec sr MeV) -1) and 
of both increasing and dacraasing intensities. 
Lower intensities were not included because of 
large statistical uncertainties in the measured 
anisotropy when determined over a 6-hour period. 
Jokipii and Parker (1970) have shown that the 
anisotropy following individual particles, that is 
particles with the same T in the rest frame of the 
solar wind, is 
+ 3!+ GPart -- • i V - •'•U/U j ß (2) 
The average radial component of • . is 5% + 1% 
toward the sun. Thus the individua• particles are 
Marshall and Stone: Persistent Sunward Flow 59 
a) 
+60% 
XSE 
Figure 2. a)+Six-hour averages of the observed 
time development of prompt solar particle events. 
The solution is of the form 
Vr/• U(r)=UK(-2C + 2;2;-V---- r)e rr 
• rr' 
in which U K is a solution to Kummer's equation. 
U increases approximately exponentially away from 
r -- O with a scale length of ~ K /V. This solu- 
rr 
tion was included in a discussion of the modulation 
of galactic cosmic rays by Fisk and Axford (1969). 
The model predicts an increasing radial com- 
ponent of kDIF with increasing solar wind speed. 
The observed dependence is consistent with the 
model calculations as shown in Figure 3. The data 
points are averages for 4 sets of data grouped by 
solar wind speed. 
Usin• the mean observed y of -3.15 and V of 440 
km sec -•, the model produces diffusive anisotroDies 
ra9ging from 10% to 17% for •rr from 1020 to 1021 
cm z sec-1. The mean observed radial diffusive ani- 
sotropy is 14% + 1%. A •rr of 1021 cm 2 sec-lpro - 
duces an intensity ratio of 13 between 1 and 3 AU, 
comparable to that inferred from Pioneer 11 data, 
and a ratio of 3 between 0.3 and 1 AU. Smaller 
values of •rr produce larger gradients. However, 
if particles are injected over a range of radii 
around 3 AU or if •rr increases with radius, smaller 
values of • at 1 AU would also produce a ratio 
rr 
of • 10 between 1 and 3 AU. 
Conclusion 
The diffusive streaming of low energy protons 
has been found to be predominantly toward the sun 
during periods between prompt solar particle 
events. The long-term average radial component 
anisotropy, GOB S . A typical +- lo error bar is (14%) of this anisotropy and its dependence on the 
shown. The means (p) of the x and y components solar wind speed are consistent with the anisotropy 
of the anisotropy are indicated by arrows. Only calculated using a steady-state propagation model 
periods from 72/273 to 74/2 when IMP-7 is sun- including adiabatic energy loss for K ~ 4 x 1020 
ward of Earth and the flux of 1.2 to 2.4 MeV cm 2 sec -1. More sophisticated modelsrrwill be need- 
protons is between 0.12 and 1.2 (cm 2 sec sr ed to account for the detailed time variations in 
MeV) -1 are included. Periods dominated by the anisotropy and to investigate the possibility 
prompt solar particle events have been rejec- that the particle sink inside i AU results from 
ted. propagation to and eventual outflow from higher 
b)+Six-hour averages of the diffusive solar latitudes. 
anisotropy, •DIF' Previous observations at i AU during periods 
between prompt events have been interpreted as 
diffusing toward the sun faster than they are con- indicating the sun to be a continuous source of 
vected away from the sun. Such a sunward flow in- MeV protons. The new observations of predominant- 
dicates a sink for 1.3 to 2.3 MeV particles inside ly sunward diffusive flow show that such a direct 
i AU. solar source is rare during 1973 and suggest the 
The earlier observations established the recur- possibility that some of the previously observed 
rent character of the intensity increases. This increases may not be due to such a source. 
long-term coherence suggests consideration of a At the present stage of analysis it is not 
simple steady-state propagation model in which ad- possible to specify the source mechanism(s) for 
iabatic energy loss is the sink of 1.6 MeV protons these particles. Although no direct association 
inside 1 AU. The steady-state propagation equa- has been established, the observations are con- 
tion: sistent with the increases near 3 AU reported by 
McDonald et al. (1975). Thus, interplanetary ac- 
V-(•_-VU) - V-(W) +] V •-•(e(T) TU)= 0 (3) celeration mechanisms uch as transit-time damp- 
- ing (Fisk, 1976) which have been suggested to pro- 
duce the increases seen by McDonald et al. (1975) 
has been simplified by assuming a power-law energy may be relevant to the new observations at 1 AU. 
spectrum, azimuthal•symmetry, Krr independent of However, Fermi acceleration throughout the helio- 
radius and energy, V radial and independent of r, sphere, which would result in ~ 1% diffusive ani- 
U finite at r = 0, and a source of particles at sotropy (Fisk, 1976), is inconsistent with the 
some r > 1 AU. The same assumptions about K__, V, large diffusive anisotropy observed at i AU. Other 
and U have been used by Lupton and Stone (1973) possibilities may include stream-stream interac- 
in a time-dependent model to fit successfully the tions, out-of-the-ecliptic propagation, or stor- 
60 Marshall and Stone: Persistent Sunward Flow 
30% - 
20% 
I0% 
x: -3.15 Krr = 10 21 crn2/sec 
x: -3.15 Krr =I0 20 cm2/sec 
/ 
/ 
, , I, I 
0 500 I000 
VSW (km/sec) 
Figure 3. The average x-component of •DIF as a 
function of the solar wind speed. Only periods 
from 72/273 to 74/2 when IMP-7 is sunward of 
Earth and the flux of 1.2 to 2.4 MeV protons is 
from 0.04 and 1.2 (cm 2 sec sr MeV)-lare included. 
Periods dominated by prompt solar particle events 
have been rejected. 
age beyond 1 AU of particles from solar flares not 
observed as prompt events at Earth. 
Acknowledgments. We are grateful to H. S. 
Bridge and J. D. Sullivan for providing IMP-7 
plasma data prior to publication. We are also 
grateful to R. E. Vogt who has been intimately 
involved throughout the IMP-7 program. Signifi- 
cant contributions to this project have also been 
made by W. E. Althouse, G. J.Hurford, and R. A. 
Mewaldt. This work was supported in part by the 
National Aeronautics and Space Administration 
under Contract NAS5-11066 and Grant NGR 05-002- 
160. F. E. Marshall has received valuable sup- 
port from an NDEA Fellowship. 
References 
Anderson, K.A., Electrons and Protons in Long- 
Lived Streams of Energetic Solar Particles, 
Solar Physics, •, 111, 1969. 
Bryant, D.A., T.L. Cline, U.D. Desai, and F.B. 
McDonald, Continual Acceleration of Solar 
Protons in the MeV Range, Phys. Re•. Lett., 
1__4, 481, 1965. 
Fan, C.Y., M. Pick, R. Pyle, J.A. Simpson, and 
D.R. Smith, Protons Associated with Centers of 
Solar Activity and Their Propagation in Inter- 
planetary Magnetic Field Regions Corotating with 
the Sun, J. Geophys. Res., 73, 1555, 1968. 
Fisk, L.A., On the Acceleration of Energetic Par- 
ticles in the Interplanetary Medium, GSFC Pre- 
print X-660-76-25, 1976. 
Fisk, L.A. and W.I. Axford, Solar Modulation of 
Galactic Cosmic Rays, 1, J.. Geophys.Res., 74, 
4973, 1969. 
Hurford, G.J., R.A. Mewaldt, E.C. Stone, and 
R.E. Vogt, The Energy Spectrum of 0.16 to 
2 MeV Electrons During Solar Quiet Times, 
Astrophys. J., 192, 541, 1974. 
Jokipii, J.R. and E.N. Parker, On the Convection, 
Diffusion, and Adiabatic Deceleration of 
Cosmic Rays in the Solar Wind, Astrophys. J., 
160, 735, 1970. 
Kinsey, J.H., Identification of a Highly Vari- 
able Component in Low-Energy Cosmic Rays at 
1 AU, Phys. Rev. Lett., 24, 246, 1970. 
Krimigis, S.M., E.C. Roelof, T.P. Armstrong, and 
J.A. Van Allen, Low-Energy (> 0.3 MeV) Solar 
Particle Observations at Widely Separated Points 
(> 0.1 AU) during 1967, J. Geophys. Res., 76, 
5921, 1971. 
Krimigis, S.M., J.W. Kohl, and T.P. Armstrong, 
The Magnetospheric Contribution to the Quiet- 
Time Low Energy Nucleon Spectrum in the Vici- 
nity of Earth, Geophys. Res. Lett., 2, 457,1975. 
Lupton, J.E. and E.C. Stone, Solar Flare Particle 
Propagation -- Comparison of a New Analytic So- 
lution with Spacecraft Measurements, J. Geophys. 
Res., 78, 1007, 1973. 
McDonald, F.B. and U.D. Desai, Recurrent Solar 
Cosmic Ray Events and Solar M Regions, J. Geo- 
phys. Res., 7__6, 808, 1971. 
McDonald, F.B., B.J. Teegarden, J.H. Trainor, 
and T.T. von Rosenvinge, The Interplanetary 
Acceleration of Energetic Nucleons, Astrophys. 
J., 203, L149, 1975. 
Roelof, E.C., On the Measurement of Energetic 
Particle Flux Anisotropies with a Class of Spin- 
ning Detectors, J. Geophys. Res., 7__9, 1535, 1974. 
Roelof, E.C. and S.M. Krimigis, Analysis and Syn- 
thesis of Coronal and Interplanetary Energetic 
Particle, Plasma, and Magnetic Field Observations 
over Three Solar Rotations, J. Geophys. Res., 7__8, 
5375, 1973. 
(Received October 18, 1976; 
accepted November 11, 1976.) 


Persistent Sunward Flow of ~1.6 MeV Protons at 1 AU

Persistent Sunward Flow of ~1.6 MeV Protons at 1 Au

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Persistent Sunward Flow of ~1.6 MeV Protons at 1 Au

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