Interplanetary acceleration processes are shown as the most plausible explanation for the observed corotating energetic particle events. The relation between the energetic particle events and the properties of the high speed solar wind streams observed at 1 AU were investigated along with the form of the energy spectrum of the corotating energetic particle streams and its variation with respect to CIR boundaries and with radial distance. It is shown that: (1) at 1 AU a correlation exists between the j particle intensity and the solar wind velocity measured during the rising part of the event, of the form I is proportional to exp (V sub sw/V sub o); and (2) the energy spectra from .5 to 20 MeV are well represented by an exponential in momentum of the form dJ/dP = C exp (-P/P sub o). This representation is found to apply from .45 AU to beyond 5 AU. The variation of P sub o with respect to the CIR boundaries was studied using a method of superposed epoch analysis. It is shown that at 1 AU the spectrum remains constant during the first two days and then progressively flattens; between 3-4AU

Mcdonald, F. B.

Trainor, J. H.

Vanhollebeke, M. A. I.

Vonrosenvinge, T. T.

English

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Technical Memorandum 79700
Corotating Energetic Particle
and Fast Plasma Streams in
the Inner and Outer Solar
System - Radial Dependence
and Energy Spectra
I
M. A. I. Van Hollebeke, F. 3. McDonald,
J. H. Trainor and T. T. von Rosenvinge
(NASA-TM-79700) COROTATING ENERGETIC 	 N79-16976
PARTICLE AND FAST PLASMA STREAMS IN THE
INNER AND OUTER SOLAR SYSTEM: RADIAL
DEPENDENCE AND ENERGY SPECTRA (NASA) 19 P
	 Onclas
HC A02/MF A01
	 CSCL 03B G3/93 16189
JANUARY 1979
`	 National Aeronautics and
Space Administration
Goddard Space Flight Center
Greenbelt, Maryland 20771
_	 r	 ,
-2-
Introduction
Corotating energetic particle events appear to be the major source
of MeV protons in the outer heliosphere. In a recent paper, Van Hollebeke
,	 et al (1978) summarized the observations of these particle streams using
the data from Goddard cosmic ray experiments on Helios, Pioneer and IMP.
These studies covered much of the solar minimum period of cycle 20 and
rai.ged from 0.30 to 10 AU. The main characteristics of this class of
energetic particle events are:
1. The existence of a positive radial gradient of some +350% per
AU between .3 AU and 1 AU, an average gradient of +100% per AU between
1 AU and 4 AU and a negative gradient of some --40% to -100% per AU beyond
4-6 AU (e.g. Van Hollebeke et al., 1978 and references therein).
2. The close association of those events with corotating inter-
action regions (CIR's) formed between high speed and low speed solar
wind streams (McDonald et al., 1976; Barnes and Simpson, 1976; Pesses
et al., 1978; Van Hollebeke et al., 1978).
The discovery of this positive radial gradient between 1 AU and
4 AU led McDonald et al., 1976 to propose interplanetary acceleration
as the most plausible explanation for these events and to suggest the
supr.athermal distribution of the solar wind as a possible source of
particles. Detailed studies on the association between corotating
particle streams and corotating interaction regions (CIR's) which develop
forward and reverse shocks beyond 1.1.5 AU (Hundhausen and Gosling, 1976;
Smith and Wolfe 1976) have further pointed to the CIR's and their related
shocks as a possible source of particle acceleration (Barnes and Simpson,
1r
-3-
1976; Pesses et al. 1978). In support of particle acceleration near
the boundary region where shocks are observed, Barnes and Simpson 1976
have shown: 1) In many cases, the positions of the double peaks in
recurring proton intensity increases coincide with the forward and
reverse shocks; 2) The energy spectrum varies greatly at the passage of
the forward shock where the spectrum is the steepest; 3) The proton to
alpha ratio varies near the passage of the forward shock with the ratio
being the largest at the Forward shock itself. The possibility that
there may be two types of acceleration process superimposed on each other,
one associated with shock, the other associated with turbulent regions
has also been suggested (McDonald et al., 1976; Barnes and Simpson, 1976).
However, this question as well as the crucial question of the origin of
the pre-accelerated particles remains unresolved. To assist in defining
the possible types of acceleration mechanisms we propose in this report
to investigate:
A,	 The relation between the energetic particle events and the
propoerties of the high speed solar wind streams.
B. The form of the energy spectrum of the corotating energetic
particle streams and its variation with respect to CIR boundaries and
with radial distance.
A.	 Relation Between the Energetic Particle Events and the Properties
of High Speed Solar Wind
Measurements of the corotating energetic particle events obtained
from Pioneer 10 and Pioneer 11 at large heliocentric distances (1.5-10 AU)
have shown that most of the events observed in 1973 and 1974 exhibit a
typical double peak structure (Barnes and Simpson 1976; Pesses et al.,
1978). The first peak is closely connected with the forward shock.
-61
 1
i	 I
-4-
A
Its intensity is often lower than the second peak which is generally
associated with the reverse shock (Scholer et al., 1979b). At 1 AU
and inside 1 AU, Van Hollebeke et al., 1978 have shown that the corotating
particle events have a single peak and are contained inside the high
speed solar wind stream .just adjacent to the low and high speed stream
i
•	 interaction region. Scholer et al., 1979a have shown similar results
i
for 4 successive solar -orations in 1974. We present here a more exten-
sive study using data at 1 AU from the Goddard Space Flight Center IMP 7
low energy detector for the period November 1973 - August 1974. During
that time, two well defined recurring high speed streams originating from
North and South coronal holes were observed for some 10 solar rotations
(Hundhausen, 1977). A representative sample showi:.g a plot of the >.5 MeV
particle intensity together with the bulk velocity of the solar wind and the
interplanetary field magnitude (from the NSSDC Interplanetary Medium Data
Book, J. King, 1977) as a function of time is presented in Figure 1 for
four solar rotations. The association between corotating energetic particle
increases and the high speed solar wind stream and the magnetic field
enhancement produced by the interaction between low and high speed streams
is well evidenced in this figure. A striking feature (Figure 1) is the
similarity in the time profile of the solar wind velocity and the associated
energetic particle increase during the rising part of the event, To
show this correlation, the 1.1.5 MeV proton intensity is plotted (Fig. 2)
versus the solar wind velocity for each of the 16 corotating events
observed between Nov. 1973 and Aug. 1974. Each data point represents a
6 hour average and the association is made from the first point which
i-5-
shows a significant increase either in the solar wind speed or the
coretating particle intensity up to the maximum intensity of either
the solar wind or the particle intensity (whichever occurs last). In
order to avoid a too large dispersion and to be able to see any trend
of association for individual events as well as the average trend, the
different events have been divided into 3 groups depending on their
minimum and their maximum intensity. Panel A includes small events with
Imin < 2.5x10-3 and Imax < 6x10-2 particles/cm 2-sec-sr-MV; pa-tel B
includes medium size events with 10 -3 < Imin < 5x10-3 and Imax < 5x10-1
particles/cm 2-see-sr -MV and panel C contains events for which Imin < 5x10 3
particles/cm 2sec-sr-MV. Figure 2 shows a good correlation between particle
intensity and solar wind velocity of the form: I - eVsw/Vo where I is
the particle intensity at a given energy and Vsw is the solar wind velocity.
Vo varies from event to event from 65km/s to 135km/s with a typical
value of Vo = 80km/s during the period under consideration.
From this relation it is possible to make several remarks regarding
the distinction between shock acceleration and acceleration due to
turbulence in the solar wind. In the first hypothesis, the acceleration
is essentially produced between 2 to 4 AU where strong shock pairs are
known to develop. The particle stream observed at 1 AU would be the
result of the shock accelerated particles. However it should be pointed
out that only the decay of the particle event observed at 1 AU can possibly
be connected to the particle increase observed in front of the reverse
shock at several AU's. The onset portion of the particle increase could
then be- the result of the inward diffusion of either particles accelerated
	 '
r-6-
at the shock which then traverse the shock into the interaction region
or else the particle are accelerated in a region closer to 1 AU where
weak shocks may form. The striking correlation between solar wind speed
and the magnitude of the particle increases rules out the first alternative
and may favor the second one if a correlation can be established between
shock strength and solar wind velocity.
In ! ►:e second hypothesis where acceleration is mainly produced by
fluctuations in the solar wind, it is possible that associated with the
rise time of the observed high speed solar wind stream there are produced
wave-particle interaction which are correlated with the solar wind bulk
velocity and which might accelerated particles. A preliminary analysis
of the power spectrum of the density of the solar wind presented by
D. Intrilligator (this conference) may support such association. This
needs to be further confirmed.
B.	 Fnergy Spectra of Corotating Energetic Particle Events
It has been previously shown (Trainor et al., 1976; Van Hollebeke
et al., 1978b)that the energy spectra from 0.5-20 MeV averages over the
24 hr period of peak intensity were well represented by an exponential
in momentum of the form dJ/dP = C exp (-P/Po) where P is the particle
,	 momentum. For these events Po was found to be generally between 12 and
16 MeV/c (Fig. 3). A similar representation was also found for the alpha
particles with Po being equal or some 10% smaller. This spectral repre-
sentation is found to apply from 0.45 to beyond 5 AU. Furthermore the
spectral index obtained at event maxima was essentially the same for a
given event between 0.45 and 3.8 AU. Extending this study to the events
t •
F •
-7-
of the Nov. 1973 - August 1974 period using data from the Goddard IMP 7
experiment at 1 AU and the Goddard-University of New Hampshire experiment
on Pioneer. 11 between n,3 and 4 AU further confirms the correctness of
the exponential in momentum representation, However, for a few very
large events at 1 AU there is a spectral flattening below some 60 MeV/c
corresponding to protons of n,2 MeV. This effect is generally seen in
association with solar wind bulk velocities greater than 750 km/sec. For
this series of events, the presence of a double peak structure in the
time-intensity profile observed by Pioneer 11 in association with the
forward and reverse shocks (which was not seen in the 1976 corotating
particle events) complicates the comparison of particle spectra at different
radial distances. Between 3 and 4 AU the spectra display large changes
at the time of shock passage while at 1 ALT only a single peak is observed
as previously noted. Furthermore, the transit time of the solar wind
plasma from 1 to 3.5 AU is on the order of 7-10 days which introduces
significant time variations. To overcome these difficulties, a method
of superposed epoch analysis is introduced in which at each radial
distance the time is defined in terms of the azimuthal distance from the
interaction region. Po(t) is normalized by Po(t o) where Po(t o) is the
spectral index at the time the edge of interaction region crosses the
spacecraft and Po(t) is obtained from 6 hour averLges. At 1 AU the
crossing time of the CIR is taken as the onset of the high speed stream.
Between 3 and 4 AU the shock crossings have been defined by the magnetic
field and plasma observations (E. J. Smith, B. Tsurutani, private communi-
cation). The results of this analysis are presented in Fig. 4. At 1 AU
Op
is:
-8-
R
the spectrum remains constant during the first two days and then
progressively flattens at a rate of 'W,5% per degree away from the
interaction region. itetween 3 and 4 AU there are striking differences
between the spectral changes observed near the forward and reverse
shocks. At the passage of the forward shock the spectrum is the
steepest as shown previously for the .54 - 1.84 MeV protons by Barnes
and Simpson, 1976, and becomes increasingly harder by some 1% per degree
away from the forward shock front. Inside the interaction region the
spectrum fluctuates but becomes progressively harder near the reverse
shock. Beyond the reverse shock, Po increases slightly.
The distribution of Po measured in the forward, reverse and inter-
action regions between 3 and 4 AU as well as the 1 AU distributions
are shown in Fig. 5. Po is determined from 6 hr averages and the number
of events for each distribution has been normalized to 100. Between
3 and 4 AU the energy spectra of the cosmic rays intensity seems to
show a wider distribution of Po in front of the reverse shock than in
front of the forward shock. The average Po is 10.5 MeV/c in front of
the reverse sh ck compared to -10 for the particles in front of the
forward shock. The particle inside the interaction regions shows a
much narrower Po distribution with an average Po of 9 MeV/c. At 1 AU,
the spectrum shows a wide distribution of Po from 8 MeV/c to ti20 MeV/c
during this period with an average value of 12 MeV/c. The dominant role
of the corotating interaction region as originally proposed by Barnes and
Simpson (1976) is clearly confirmed by this study. The spectral changes
between x•.35 and 10 AU appear to be significantly smaller than would be
l-9-
expected from diffusion processes. The observed time histories,
radial gradient and spectral distribution now provide an excellent
description of these energetic particle events from 0.5 to 10 AU.
Clearly the next step is modeling the shock acceleration process
combined with interplanetary diffusion, Such a procedure should establish
whether or not additional acceleration processes are required. The 1 AU
observations during the initial phase of the events suggest other accelera-
tion processes may be necessary.
r-10-
References
Barnes, C. W. and J. A. Simpson, Evidence for Interplanetary Acceleration
of Nucleons in Corotating Interaction Regions, Astrophys. J., 210,
	
I '*	 L91-L96, 1976.
Hundhausen, A. J., An Interplanetary View of Coronal Holes, preprint
HAO, NCAR, 1977.
Hundhausen, A. J., and J. T. Gosling, Solar Wind Structure at Large
0
Heliocentric Distances: An Interpretation of Pioneer 10 Observations,
J. Geophys. Res. 81, 1436, 1976.
Intrilligator, D. S., Solar Wind 4 Conference, Burghausen, 1978.
King, J. H., Interplanetary Medium Data Book NSSDC/WI)C-A-R&S 77-04a.
McDonald, F. B., B. J. Teegar.de:, J. H. Trainor and T. T. von Rosenvinge,
The Interplanetary Acceleration of Energetic Nucleons, Astrophys.
J. 203, L149, 1976.
Pesses, M. E., J. A. Van Allen and C. K. Goertz, Energetic Protons
Associated with Interplanetary Active Regions, 1 to 5 AU From the
Sun, Geophys. Res. 83, 553, 1978.
Sr_holer, M., D. Hovestadt and B. Klecker and G. Gloeckler, The Composition
of Energetic Particles in Corotating Events, Astrophys. J. 227, 19/9.
	
•	 Scholer, M., G. Morfill, M. A. I. Van Hollebeke, On the Origin of
Corotating Energetic Particle Streams, to be published in J. Geophys.
Res. 1979.
Smith, E. J., J. H, Wolfe, Observations of Interaction Regions and Corotating
Shocks Between 1 and 5 AU: Pioneer 10 and 11, J. Geophys. Res. Lett.
3, 137, 1976.
s
I
I
r	 •
-11-
Trainor, J. H., M. A. I. Van Hollebeke, T. von Rosenvinge, SC 11,
EOS Trans. Am. Geophys. U. 57, 12, 1976.
Van Hollebeke, M. A. I., F. B. McDonald, J. H. Trainor, T. T. von. Rosenvinge,
The Radial Variation of Corotating Energetic Particle Streams in
the Inner and Outer Solar System, J. Geophys, Res., 83, 4723, 1978
Van Hollebeke, M. A. I., F. B. McDonald, J. H. Trainor, T. T. von Rosenvinge,
Energy Spectrum of the Corotating Energetic Particle Stream in the
s
Inner and Outer Solar System, Bull. Amer. Phys. Soc. 23, 509, 1978b.
j
1	 4
A -.:&
1
II
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Figure Captions
Figure 1	 Association between energetic particle streams observed
at 1 AU and the high speed solar wind stream shown for 4
solar rotations. The hatched area are flare accelerated
particle events.
Figure 2	 Correlation between energetic particle intensity and the
solar wind bulk velocity during the onset of each of 16
corotating streams observed from Nov. 1973 A lig. 1974.
Figure 3	 Energy spectra of a corotating energetic particle stream
observed for 3 consecurive solar rotations in 1976.
Figure 4	 Variation of Po with respect to the irteraction region.
Figure 5	 Distribution of Po measured at different radial distances,
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Corotating energetic particle and fast plasma streams in the inner and outer solar system: Radial dependence and energy spectra

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