Acceleration of particles in earth shock transition region and beyon

Fan, C. Y.

Gloeckler, G.

Simpson, J. A.

English

NASA Technical Reports Server

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E F I NS -65-86 
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' -- ACCELERATION OF PARTICLES IN THE EARTH'S TRANSITION REGION 
AND BEYOND 
-/. * C. Y. Fan, G. Gloeckler and J. A. Simpson 
Enrico Fermi Institute for Nuclear Studies 
University of Chicago 
Presented at  International Conference on Cosmic Rays, London 
6-17 September 1 965 
To be published in the 
Proceedings of the International Conference on Cosmic Rays, London 
by the Physical Society, London 
NASA Predoctoral Fellow 
https://ntrs.nasa.gov/search.jsp?R=19660003722 2020-03-16T21:35:43+00:00Z
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Acceleration of Particles in  the Earth's Shock Transition Region and Beyond * 
C. Y. Fan, G. Gloeckler and J .  A. Simpson 
Enrico Fermi Institute for Nuclear Studies 
/YL?// University of Chicago 
Abstract: Pulses of hish energy electrons were observed in  the vicinity 
of the bow shock of the terrestrial magnetosphere with both the Au-Si surface 
barrier solid state detector (the front detector of the cosmic-ray telescope of the 
University of Chicago) and the GM counter (University of California) on board the 
IMP-I satellite. These particle pulses were also detected inside the shock transi- 
tion region with GM counters in  earlier satellites. There i s  experimental evidence 
that these electrons are accelerated in  the shock transition region. By comparing 
the counting rates of the solid state detector and that of the GM counter on board 
IMP-I, the energy spectrum of the electrons may be estimated. If the integral 
spectrum i s  expressed as F = F E 
to + 4.5. 
, the value of 7 varies from + 2.5 -b' 
0 
Beyond the shock transition region up to the apogee of the IMP-I 
satellite (200,000 km), pulses of high energy particles were also observed with the 
Au-Si solid state detector prior to the arrival of the 27day recurrent solar plasma 
of enhanced velocity. It i s  suggested that these are particles accelerated i n  
space by multiple shock waves 
* NASA Predoctoral Fellow. 
associated with the co-rotating shock front. 
L . 
The mechanism of acceleration OF high energy particles i n  nature 
i s  of importance for understanding the origin of cosmic radiation. The purpose 
of this paper i s  to report the observation of pulses of electrons i n  the earth's shock 
transition region and beyond with the IMP-I satellite, and to present experimental 
evidence showing that they are accelerated in these regions. The acceleration by 
the Fermi mechanism i s  discussed. 
IMP-I (Explorer 18) was launched on November 27, 1963. I t s  ini t ial  
apogee was on the sunlit side of the earth a t  an angle to the sun of 32.7' and a t  
a geocentric distance of 31.9 earth radii. On board the satellite there were two 
detector systems which have energy thresholds low enough for detecting low energy 
electrons. They are the Geiger-Muller counter of the University of California 
and the gold-silicon surface barrier detector, the front detector D 
ray telescope of the University of Chicago. Their energy thresholds for the detec- 
tion of electrons are 40 kev and 30 kev respectively. 
of the cosmic 1' 
Figures 1A-C show the counting rates of D averaged over four telem- 
1' 
etry sequences (-328 sec), plotted as a function of the geocentric range of the 
satellite for the first inbound, second outbound and inbound passes. For conven- 
ience of discussion, the time scale in UT (days are numbered from Jan. 1, 1963) 
also included along with the geocentric range. The arrow with a letter M indicates 
the position of the boundary of the magnetosphere and the arrow with a letter S 
indicates the position of the bow shock as determined by Ness et al. (1964) with a 
magnetometer on board the same satellite. 
i s  
-- 
From Fig. lA, i t  i s  seen that the space may be divided into three 
, . 
I . 
-2- 
regions according to the D counting rates: 1) the magnetosphere (Region 1) where 1 
the counting rates go off scale, reaching thousands per second; 2) the interplanetary 
space (Region Ill) where the counting rates are low and only slowly varying; and 
3) the region of interaction between the solar wind and the terrestrial magnetic 
fields (Region II) where the counting rates abruptly increase for short intervals of 
time above the interplanetary value. These counting rate spikes are identified as 
electron pulses i n  the region of interaction between the solar wind and the terres- 
trial magnetic field. We shall present evidence that these electrons are locally 
accelerated i n  this region. 
The number and spatial distribution of counting rate spikes depend 
strongly upon interplanetary conditions, such as the solar wind velocity. For 
example, during the first inbound pass, the counting rates in  Region Ill, about 3 
counts/sec, were due to cosmic ray particles. These observations represent a normal, 
quiescent state in  space with a solar wind at  average velocity. However, during 
the second outbound and inbound pass (Fig. 1B and 1C) when the solar plasma of 
enhanced velocity from an active region a t  the sun swept over the earth and the 
satellite, the electron spikes beyond the magnetosphere greatly increased in  number 
and intensity, and the distribution of the spikes extended far out to distances 
greater than 200,000 km. The arrival of  the high velocity plasma was detected 
by plasma probes on board IMP-I at  2111 Dec. 2, 1963 UT (Bridge, e t  al. 1964; 
Wolfe, e t  al. 1965). It i s  interesting to note that many of the spikes at  large 
distances appeared many hours prior to that time. We suggest that these spikes are 
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particles accelerated i n  the interplanetary space by multiple shock waves. Details 
w i l l  be discussed later in this paper. 
In Figure 1C the background counting rate gradually rose from i ts 
normal value to approximately 30 counts per second. This background i s  due to the 
arrival of a 27-day recurring stream of protons with energies below 30 MeV. The 
results are discussed in another paper presented in this.conference (Fan, Gloeckler 
and Simpson). 
The mica window GM counter of the University of California was 
designed for the detection of electrons only (Anderson, e t  al. 1965). The comparison --
1 of the electron spikes observed by the GM electron detector and that by the D 
solid state detector in Region II for the second inbound pass i s  shown i n  Fig. 2. The 
one to one correspondence i s  apparent. 
The important difference between the GM counter and the solid state 
detector i n  counting electrons i s  that the response of the former i s  a linear function 
of the flux whereas the response of the latter i s  a non-linear function, since the 
solid state detector counts electrons individually a t  high energies and by pile-up 
effect a t  low energies. Consequently, it becomes possible to estimate the spectrum 
by comparing the counting rates of the two detectors. A detailed analysis w i l l  be 
published elsewhere (Fan, Gloeckler and Simpson, 1965). 
Fig. 3 shows the correlation between the peak counting rates of the -
electron spikes as observed with the GM counter and with the D solid state 1 
detector. If we assume that the electrons in the spikes observed in  the shock transi- 
t ion region and beyond have an inverse power integral spectrum: 
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F(E) = F E electrons/cm sec, ster 
- d  2 
0 
where Foand 2/ 
characteristics of the detector systems, the correlation function between the D, 
counting rate C and the GM counter counting rate R can be readily calculated, 
This function for = 2.5, 3, 3.5, and 4.5 i s  dotted i n  Fig. 3. The observed 
varied from 2.5 to 4.5. Montgomery et al. (1965) obtained values of 3.2, 
are two positive constants and E i s  i n  kev, by knowing the 
d’ 
f -- 
3.4 and 4.2 for the electron pulses a t  large svn-earth-probe angles mostly inside 
the shock transition region. The present values are i n  remarkable agreement with 
their results although they were measured at  different times and locations. 
In an earlier publication (Fan, e t  al, 1964) i t  was pointed out that --
the electrons observed as spikes in the shock transition region are most l ikely 
locally accelerated near the bow shock. On the other hand, Anderson, e t  al. (1965) 
suggested that these e!ect:cns =re of n;agne:osphei-ic origin. A strong argument 
favoring local acceleration comes from the examination of the variation of the 
--
. .  
energy spectra of the electrons i n  the spikes shown in  Figure 3 and the positions 
of the spikes shown i n  Fig. 2. The exponent for the spectrum i s  remarkably narrow 
in  range of values with a tendency of hardening with the distance of the spike from 
the magnetosphere. If these were the electrons escaping from the magnetosphere, 
the more distant electron spikes would have softer energy spectra than the spikes 
near the magnetosphere, Since this i s  not the case, we believe this i s  evidence 
against electrons escaping from the magnetosphere. Independent evidence for 
acceleration of particles i n  this region is given by the dawn-evening asymmetry 
b 
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of the electron spike distributions observed by Montogomery et al. (1965). -- 
Arguments in favor of various mechanisms of acceleration of the 
electrons which appear in the counting rate spikes depend upon the position of the 
spikes relative to the magnetosphere and the earth bow shock, From Fig. 1A-C, i t  i s  
seen that the electron spikes are found in four regions: i n  the transition region, a t  the 
shock front, within a few earth radii beyond the bow shock, and a t  geocentric range 
approximately 20 earth radii and beyond. Those spikes which coincide wi th  the 
shock front could well represent a quasi-stationary spatial structure. On the other 
hand, i t  was pointed out by Jokipii and Davis (1964) that the electron spikes a 
few earth radii outside the shock front cannot be a spatial structure since i n  this 
region the magnetic lines of force are flowing with the solar wind and thus the 
electrons would be immediately swept away with the magnetic field. Consequently 
they suggested that the electrons are accelerated by the Fermi mechanism (Fermi, 
1954) between the bow shock and the approaching irregularities carried by the 
solar wind. As soon as the "jaws" close the electrons are swept into the transition 
region, especially a t  large sun-earth-probe angles, 
The counting rate spikes a t  great distances which were observed 
during the second outbound and inbound pass (Fige 1B and 1C) were again observed 
approximately 27 days later and i n  the following solar rotation when high velocity 
solar wind from the same active region of the sun swept over the satellite and the 
earth. The evidence for later sequences i s  unclear because the satellite gradually 
entered into the tai l  region of the magnetosphere. A detailed study revealed that 
they appeared prior to the gradual increase in the general background which leads 
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1, 
into the onset of the recurring solar proton stream. This time sequence i s  illustrated 
i n  Fig. 4 where the D counting rates and interplanetary magnetic f ie ld strength 1 
F, f ield vector angles 6’ and , the variances of the three components 
of the field, Xset 
of Ness et al. (1964) are platted as a function of time (UT). About four hours 
after several electron spikes had been detected (beyond the range of 20 earth radii), 
as measured with the onbwrd magnetometer Y se’ ‘set 
-- 
the D, counting rate increased gradually from i t s  normal value of below 3 counts/ 
sec to above 3 counts/sec. The increase seems to coincide with an abrupt increase 
in the f ield strength F and a rapid change in  the vector angles 
as indicated by an arrow in Fig. 4. The fluctuations i n  the counting rates prior 
to the sudden commencement seem to correlate to that of the magnetic field. Al- 
though the physical implications of these correlations i s  not clear at the present 
moment, we would l ike to take these as evidence of shock waves produced by the 
recurrent solar streams and the electron spikes (maybe even some of the background) 
as particles accelerated by the co-rotating shock front. 
8 and 43 
We wish to emphasize that i f  the acceleration of the electron spikes 
observed a t  geocentric distances to 200,000 km, a t  the bow shock, and inside 
the transition region are to be explained by a single process, then the Fermi mech- 
anism i s  l ikely to be the type. The assumption that the electrons are accelerated 
a t  the shock and beyond and then carried into the shock transition region and 
the magnetospheric tai l  by convection appears to be the most persuasive. 
r 
Acknowledgments 
We deeply appreciate the assistance given us i n  the design, fabrication 
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and data reduction on the IMP-I satellite by the staff of our Laboratory for 
Astrophysics and Space Research. 
We thank the staff of the Goddard Space f l i gh t  Center, NASA, for 
preparation of our experiments for flight and for data processing. 
The research was supported in  part by NASA contracts NAS-5-2990, 
NAS-5-2133 and by the Ai r  Force Cambridge Research Laboratories, Contract 
AF 19 (628)-2473. 
a 
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References 
Anderson, K. A., Harris, H. K., and Paoli, R. J., 1965, J. Geophys. Res. 70, -
1039- 1050. 
Research V (to be published). 
Fan, C. Y., Gloeckler, G., and Simpson, J. A., 1964, Phys. Rev. Letters 13, 149-153. -
Fan, C. Y., Gloeckler, G., and Simpson, J. A,, 1965 (submitted for publication 
in J. Geophys. Res.). 
Fermi, E., 1954,Astrophys. J. 119, 1-6. -
Jokipii, J. R., and Davis, L., 1964, Phys. Rev. Letters 13, 739-741. -
Montgomery, M. D., Singer, S., Conner, J. P. and Stogsdill, E. E., 1965, Phys. Rev. 
Letters 14, 209-213. -
Ness, N. F., Scearce, C. S., and Seek, J. B., 1964, J. Geophys. Res. 69, 3531-3569. -
WoIfe, J. H., Silva, R. W., and Myers, M. A. (to be published 1965). 
'I 
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Captions 
Fig. 1A-C 
3 
The counting rate of D, vs. the geocentric distances in 10 km of the 
satellite IMP-I. Solar plasma of enhanced velocity was detected by 
an onboard plasma probe a t  336-21 17 UT. The arrow with a letter M 
indicates the position of the boundary of the magnetosphere and the 
arrow with a letter S indicates the position of the bow shock as 
determined by Ness et al. (1964). 
The comparison of the counting rates of the GM counter (University 
-- 
Fig. 2. 
of California) and that of the Au-Si solid state detector (University 
of Chicago), 
The correlation of the counting rates of the electron pulses in the Fig. 3. 
shock transition regia and beyond, measured wi th  the Au-Si detector 
with that measured with the GM counter. 
Fig. 4. The correlation of the electron spikes and gradual increase in  the D, 
counting rates in interplanetary space with the variances of the 
magnetic field. F i s  the f ield strength, 8 and c#' are two 
Zse, are variances 
se' 
polar vector angles, and Xse, Y 
of the three components of the magnetic field. 
IMP I First inbound 
Apogee time 3 3 3  0118 UT 
(Nov 2 9 ,  1963) 
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60 70 10 90 100 110 120 130 140 150 160 170 180 190 200 
100 I I I I I I  I I I I I I I I -  
f 40 \ t. IMP I Second outbound I Perigee time 3 3 5  0001 - (Dec.1, 1963) 
Fig. I 8 
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Acceleration of particles in the earth's shock transition region and beyond

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