The intensities of low energy solar-interplanetary electrons and ions at 1 AU occasionally change in a square wave manner. The changes may be increases or decreases and they typically have durations of from one hour to a few hours. In some cases these channels are bounded by discontinuities in the interplanetary field and the plasma properties differ from the surrounding solar wind. In one case solar flare particles were confined to a channel of width 3 x 10 to the 6th km at Earth. At the Sun this dimension extrapolates to about 12,000 km, a size comparable to small flares

Anderson, K. A.

Dougherty, W. M.

English

NASA Technical Reports Server

N87- 24252
5
PARTICLE PROPAGATION CHANNELS IN THE SOLAR WIND
K. A. Anderson* and W. M. Dougherty
Space Sciences Laboratory,
University of California
Berkeley, California 94720
ABSTRACT
The intensities of low energy solar-interplanetary electrons and ions
at 1 AU occasionally change in a "square wave" manner. The
changes may be increases or decreases and they typically have dura-
tions of from one hour to a few hours. In some cases these chan-
nels are bounded by discontinuities in the interplanetary field and
the plasma properties differ from the surrounding solar wind. In
one case solar flare particles were confined to a channel of width
3 x 106 km at Earth. At the Sun this dimension extrapolates to
about 12000 km, a size comparable to small flares.
1. INTRODUCTION
Bartley et al. in 1966 reported that highly anisotropic fluxes of 1 to 13 MeV
protons from solar flares occasionally changed their flow direction suddenly
and by as much as several tens of degrees. McCracken and Ness [1966] showed
that the change in flow direction was due to changes in the direction of the
interplanetary magnetic field (IMF). The time for passage of these bundles
of field lines over the spacecraft implied their widths were about 3 x 106 km
at 1 AU. Domingo, Page, and Wenzel [1976] described sudden decreases in
*Also Physics Department
C-2.
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87
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solar particle intensity lasting about one hour and emphasized that such an
effect meant that little transport of particles across interplanetary field lines
was occurring.
In 1968, Jokipii and Parker used Leighton's hypothesis of random walk of
magnetic field lines associated with granules and supergranules [1964] to
develop a picture of an interplanetary medium composed of a tangle of field
lines frozen into the solar wind, but whose feet were carried about by the
random motions at the solar surface. Jokipii and Parker noted that using a
correlation length of 15 000 km--about the radius of a supergranule--the
magnetic structure would be 3 x 106 km in size at 1 AU. This is close to the
size of the filaments as determined by Bartley et al. and McCracken and Ness.
These workers did not find changes in the solar particle intensity, anisotropy
ratio or energy spectrum as the spacecraft entered the "filament". More recent-
ly the IMF has come to be regarded as containing many discontinuities, rather
than being made up of many filaments [Burlaga, 1969].
In this paper we discuss changes in the intensity of low energy solar-
interplanetary electron and ion intensities observed at distance 1 AU from
the Sun on the International Sun-Earth Explorer ISEE-3 spacecraft. These
changes are characterized by particle increases or decreases, often having a
square wave appearance, and lasting from one to a few hours.
Our measurements extend the previous ones by showing that low energy elec-
trons and ions are "channeled" and that the intensity in the narrow channels
may be higher than in the surrounding IMF. The particle intensity changes
are usually well-defined, occurring over distances _ 10,000 km. Most impor-
tantly, we show that the net particle flow may be quite different in these nar-
row channels compared to the flow just outside.
Electrons in the energy range 2 to 10 keV were measured by a swept electric
field analyzer and ions of energy in the range 45 keV to a few MeV were
measured by a pair of solid state detector telescopes, one of which was covered
by a thin foil in order to achieve separation of electrons and ions.
Although the electrons we measured are of quite low energy (2 to 40 keV),
such particles have very high speeds (9% to 35 % the speed of light), but small
t"-,'_t
88
gyroradii.In a5 nT magneticfieldat 60o pitch angle the gyroradii are in the
range 15 to 70 km. The proton gyroradii are on the order of several thousand
kilometers. The axes of the detectors lie in the ecliptic plane and are swept
over it by rotation of the spacecraft. The view angles of the electron detector
are 10 ° in azimuth and + 23 ° in elevation. Because of the restricted viewing
in azimuth, variations in the azimuth of the interplanetary magnetic field can
cause apparent changes in the count rates, especially when the particle pitch
angle distributions are highly anisotropic. We therefore have eliminated from
our analysis events contaminated by large and rapid changes in the elevation
angle of the interplanetary magnetic field.
2. OBSERVATIONS
Figure 1 illustrates several features of low energy ion and electron propaga-
tion channels. During this 24-hour interval two increases in electron intensity
can be seen as well as a less well defined decrease beginning near 2000 UT
(Universal Time). The ion intensity decreases in coincidence with the two elec-
tron increases but are not quite so sharply defined (the ion behavior is not
shown in Figure 1).
The clear correlation with solar wind and interplanetary magnetic field
parameters are quite striking in this example. The magnetic field changes at
the beginning and end of the two electron intensity increases have been iden-
tified as tangential discontinuities [Tsurutani, private communication, 1985].
The magnetohydrodynamic (MHD) theory of such discontinuities requires
the pressure on the two sides to be equal. In response the solar wind plasma
density increases during the times the IMF has decreased.
We have surveyed 15 months of ISEE-3 data during 1978 and 1979 and find
about 80 intensity changes, either increases or decreases, resembling those
shown in Figure 1. These features are most frequent and clearest in the 2 to
10 keV electron measurements, although ion intensity changes often accom-
pany the electron intensity changes. Only a few of the examples are so clearly
defined as the ones in Figure 1 and fewer still are accompanied by tangential
discontinuities. Our basic assumption on the nature of these characteristic
particle intensity changes is that the particles are confined to a bundle of
89
X
.g
b..
W
>
...1
w
n-
105
103
I02
,,,i,,,i,,,i,,,i, ,,i,i,
2JUNEI979 ELECTRONS
2 keV -
5 keY
6 keY
,,,I,,,I,,,I,,,1,,,I,,,
O0 08 16 24
I-- 20
=
lm-
-- 0
.m. 500E
- 400
30o
,o
C,3
O0 08 16 24
UNIVERSAL TIME, Hrs
Figure 1. Changes in the intensity of low energy electrons and ions having
a "'square wave" appearance during long-lived streams ofsolarparticles were
a fairly common occurrence over 15 months of observations in 1978 and 1979.
In the two-day period shown at least three such events occur. In some cases
the intensity changes are associated with solar wind or interplanetary magnetic
field changes. For some there is no clear signature.
90 -"
magnetic field lines and that these field lines are swept past the spacecraft
by the solar wind flow. We refer to these features in the IMF as particle prop-
agation channels. Under this assumption we have converted the temporal dura-
tion of the propagation channels to a distance D given by
D = _ V a_ [sin¢_,[ At
sw
i
where V a) is the solar wind speed averaged over time interval At, taken to
sw
be 10 seconds, and O is the angle between the solar wind flow direction and
the direction of the IMF during interval i.
We found the widths of the 80 propagation channels measured in this way
to range from 1.4 to 5.0 × 106 km with an average value of 3.7 x 10 6 km.
McDonald and Burlaga [1985] have discussed another form of channeled solar
particle propagation. They found regions of compressed interplanetary
magnetic field evidently connecting back to the solar corona since solar par-
ticles were often found in them. Such channels are much larger in size than
those we are discussing here. Their importance lies in the fact that adiabatic
energy losses are reduced and thus the particles can reach interstellar space
without large energy losses.
We next discuss an example which allows some conclusions about the origin
and nature of one particular particle propagation channel. On May 20 and
21, 1979, several small solar flares occurred in McMath plage region 16014,
located at about N16 W66. One of the flares in this region accelerated elec-
trons from < 2 keV to above 40 keV and ions <_40 keV to above 270 keV
energy (see Figure 2). The electron intensity versus time profile is characteristic
of an impulsive injection of particles at the Sun into a medium whose mean
free path for pitch angle scattering is on the order of 1 AU. The slow decay
phase seen in Figure 2 is consistent with a brief injection phase followed by
scattering in the interplanetary medium [Lin, 1974]. This view is supported
by the measured pitch angle distributions. Initially, for some tens of minutes,
the electrons are highly anisotropic, then become nearly isotropic.
91
X_..I
I..d
._.1
Ld
or"
f,,;o,,_,,,,,,,,,,.....,,,,,,,,,,,,,,,,,,,,,,,,- I MAY 979
106 ELECTRONS
__-
,k
io_2- 40-280 key --_
O0 08 16 O0 08 16 O0
UNIVERSAL TIME
Figure 2. An importance 1N flare began at 1107 UT on May 20, 1979 and
injected electrons from 2 to 100 ke V and 1ow energy ions in to in terplanetary
space. The spacecraft entered a particle propagation channel at 2030 UT. This
channel was 2.5 × 10 6 km in width at 1 A U.
The most remarkable feature during these two days is the sudden increase
in the electron and ion intensities at about 2000 UT on May 20, followed about
six hours later by a sudden return to the earlier slow decay. The increase is
simultaneous at all particle energies. This is also the case for the decrease.
During this six-hour period the pitch angle distributions became highly
anisotropic showing strong flow of electrons away from the Sun (Figure 3).
92
II ILSIILIILLLJIll LIL LLlill ill JJ IL I ' lli'J ii _LL _l i
IoeL- ,_ A B -_
5F I _ 2 keY ELECTRONS
_.1
,, ,o4_...... ,....... ,....... T.... ,,,, ....... ,...... i
oo o8 ,6 I oo I 08 t6 oo
i,i>
_ !--i' ..... i_i ..... izi ...... i_/I Tv I t
,,J ._
I A
O.1 - _
Ig
22 uu v: 04
Figure 3. About nine hours after the solar flare that began at 1107 UT, highly
anisotropic fluxes of electrons appeared for several hours (,4 to B). Before
and after this time the electron fluxes were nearly isotropic. The vertical lines
through the pitch angle distributions at the bottom of the figure lie along the
interplanetary magnetic field. The direction of the Sun lies somewhere in the
upper half of these diagrams. The pitch angle distributions are indexed to
the middle panel by means of Roman numerals.
93
Evidentlythe interplanetaryfield lineshaveconnectedto asourceregionin
thesolaratmosphereableto continuallyinjectelectronsandionsinto thein-
terplanetarymediumfor at leastsix hours.We supposethat this injection
alsooccurredbeforeandafterthissix-hourinterval.Fromtheobservedslow
decayof particleintensityduring this time, thesourcecouldhavesupplied
particlesfor intervalsas long as20hoursor more.
Fromthesolarwindspeedanddirectionof theinterplanetarymagneticfield
wefind thewidth of the particlepropagationchannelto be2.5 x 10 6 km.
Linear extrapolation of this width to the solar surface gives a size of 12 000
km. The magnitude and topology of the magnetic field near the Sun no doubt
will affect the actual size, but we appear to be dealing with a dimension which
is larger than the size of a small flare but smaller than the plage region. Both
at the Sun and in interplanetary space this structure is relatively small. One
consequence of this is that if such structures are associated with most active
regions they would be a common feature at 1 AU, but would not often be
observed because of their small size.
While the spacecraft is located in the particle propagation channel three im-
pulsive, flare-like injections of particles occur (Figure 3). They are identified
by their impulsive appearance and their velocity dispersion, consistent with
about a 1.3 AU travel distance. One of these electron events can be firmly
identified with an optical flare in region 16014. No optical association could
be found for the other two particle injections. This is not surprising since we
have found from a study of many impulsive low energy electron events that
those having very low intensity often have no reported Ha association [Pot-
ter, Lin, and Anderson, 1980]. However, it is unusual that three low energy
solar electrons events occur in a six-hour interval. This leads us to believe
that during the interval the spacecraft is situated on field lines connected to
the site of flaring on the Sun. If this is the case the field lines guiding the
electrons and ions away from the Sun from 2030 UT on May 20 to 0230 on
May 21 have their origin in or near McMath plage region 16014, located at
N16 W66 at this time. Since Earth is at a heliographic latitude of 2 ° S at this
time the field lines would have been deflected 18 ° southward. This result is
consistent with the suggestions of Schulz [1973] and by Svalgaard et al. [1975]
and Svalgaard and Wilcox [1976], that the current sheet is related to the equator
94
of a solarmagneticdipole.In suchaviewfield linesdrawnoutby thesolar
wind neartheequatorcouldtracebackto latitudeswellremovedfrom the
equatorin themannershownin Figure7of Smith,Tsurutani,andRosenberg
[1978].
The fact that energetic particles stream away from the Sun for periods of time
on the order of days has been known for some time. [See, for example, Simnett,
1971 and Anderson, Lin, and Potter, 1982.] There are two general views on
the mechanisms behind long-lived emission of solar particles. The first of these
is storage of flare accelerated particles in the corona and their subsequent
escape. The other hypothesis is continuous acceleration. Present observational
evidence is not sufficient to resolve this issue, and there are conceptual dif-
ficulties with each hypothesis. Radio wave observations give the best evidence
in favor of coronal storage, although the configurations of the magnetic fields
have not been made clear in this way. The difficulty with coronal storage is
the rapid rate of energy loss of the fast particles to background electrons in
the coronal plasma. Only at very high coronal altitudes is the rate of energy
loss low enough to permit storage over periods of days [Krimigis and Ver-
zariu, 1971 ]. The problem with the continuous acceleration hypothesis is that
no physical mechanism for it has been identified, and the best understood
mechanisms involve shock waves and are therefore impulsive in character.
Impulsive acceleration is noisy in the sense of copious X-ray and radio wave
emission whereas these emissions are largely absent during periods of long-
lived streaming. The present observations provide some additional informa-
tion on the problem of long-lived emission of solar particles. In the first place
only flares of small size are involved here whereas in the past the process has
been generally associated with large flares. In the case at hand the largest flare
was importance 1N and it accelerated only low intensities of ions in addition
to the rather large fluxes of electrons. However, this flare occurred in an ac-
tive region above which type III bursts frequently appeared, indicating that
beams of fast electrons were present in the corona over a period of one or
two days.
Secondly, the streaming of solar particles on May 20 to 21, 1979 is restricted
to a spatial region whose dimension at the Sun is on the order of 104 km.
The near perfect confinement of the streaming particles must be associated
95
with discontinuities or major changes in the topology of magnetic fields in
the solar atmosphere. However, no magnetic feature could be identified with
the particle channel on this occasion. In particular, nothing suggesting a neutral
sheet appeared.
The third point we would make using data from the May 20 to 21, 1979 inter-
val concerns the energy spectrum of the electrons. We noted above that the
intensity of the 2 to 10 KeV electrons changed very little over a six-hour period.
If the small change in intensity is interpreted as due to energy loss of these
electrons to background electrons we can set a lower limit to the altitude at
which the electrons must reside if indeed they are trapped in magnetic struc-
tures. A 10 to 20% energy loss in six hours at the lowest energies requires
that the average electron density not exceed 105 cm -3. This corresponds to
a heliocentric distance of 4 solar radii for the quiet Sun and 19 solar radii
if the coronal region is a streamer with enhanced density [Fainberg and Stone,
1971].
We have also compared the energy spectrum of the electrons in the impulsive
injection from the 1N flare that began at 1107 UT on May 20 with the spec-
trum of electrons streaming in the propagation channel. The results are given
in Table I. We have fitted the spectral data to a power law for 30 one-hour
data samples and obtained the power law exponent for each sample. Ten
samples are taken during times preceding entry into the particle channel, 10
samples in the channel, and 10 samples following exit from the channel. The
average value for each set of ten samples and the standard deviations are given
in Table I. We find that there is no difference in the low energy electron spec-
tra between particles in the propagation channel and those on field lines out-
side the channel. There can be little doubt that the electrons outside the channel
were accelerated by the 1N flare. We take the similarity of the energy spectra
to be significant but not conclusive evidence for storage and subsequent escape
over a period of many hours of electrons accelerated over a brief period of
time by the importance 1N flare which began at 1107 UT on May 20, 1979.
We conclude that some of the structure found in the interplanetary magnetic
field is established by spatial features on the order of 104 km near the solar
surface, and that these features persist for at least several days.
96
TableI.
Time
20-21May 1979 Numberof Intervals SpectralIndex,
14:00-19:00 10 3.77 _+ 0.08
21:00-02:00 10 3.39 _+ 0.29
03:00-08:00 20 3.49 + 0.07
Comparison of the energy spectra of 2 to 10 ke V electrons inside the particle
propagation (2100 to 0200 UT) with spectra calculated before and after entry
into the channel. The electrons outside the channel are certain to have been
accelerated by an importance 1N flare, although at this time they are no longer
streaming from the Sun. The close similarity of spectral indices suggests that
the particles streaming away from the Sun in the propagation channel were
accelerated by the same flare but then were trapped and released over a period
of many hours.
ACKNOWLEDGMENTS
This work is the result of research supported by NASA grant NAG5-376.
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Particle propagation channels in the solar wind

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