Acceleration of energetic ions to approx 200 MeV and electrons to approx 2 MeV were detected by the Low Energy Charged Particle (LECP) instrument on Voyager 2 in association with a quasiperpendicular shock of theta sub Bn - 87.5 deg at 1.9 AU. The measurments, obtained at a time resolution of approx. 1.2 sec, reveal structure of the energetic particle intensity enhancements down to a scale of the order of the particle gyroradius, and suggest that acceleration takes place within a gyrodiameter of the shock. The observations are consistent with the prediction of the shock drift acceleration (SDA) mechanism. The absence of any fluctuations in the magnetic field during the shock passage suggest that turbulence is not essential to the shock acceleration process in the interplanetary medium

Krimigis, S. M.

Sarris, E. T.

English

NASA Technical Reports Server

r 170
SH-1.5-4
ACCELERATION OF IONS AND ELECTRONS TO NEAR-COSMIC RAY ENERGIES
IN A PERPENDICULAR SHOCK:THE JANUARY 6, 1978 EVENT
S. M. Krimigis
Applied Physics Laboratory, The Johns Hopkins University
Laurel, MD 20707
E. T. Sarris
The University of Thrace
Polytechnic School
Xanthi, Greece
ABSTRACT
Acceleration of energetic ions to N 200 MeV and electrons
to _ 2 MeV were detected by the Low Energy Charged Particle
(LECP) instrument on Voyager 2 in association with a quasi-
perpendicular shock of @Bn = 87"5° at 1.9 AU. The meas-
urements, obtained at a time resolution of N 1.2 sec, reveal
structure of the energetic particle intensity enhancements
down to a scale of the order of the particle gyroradius, and
suggest that acceleration takes place within a gyrodiameter
of the shock. The observations are consistent with the pre-
dictions of the shock drift acceleration (SDA) mechanism.
The absence of any fluctuations in the magnetic field during
the shock passage suggests that turbulence is not essential
to the shock acceleration process in the interplanetary
medium.
I. Introduction. It is a well documented observational fact that even
weak (MA = 1.5) quasl-perpendlcular shocks readily accele_te ions to
relatively high energies (up to N 50 MeV, Sarris et al., 1976), while
quasi-parallel shocks rarely accelerate ions to rather modest (N 200-300
keV maximum) energies, if such shocks are more or less suDercritical
(MA = 2.8, Kennel et al., 1984). Surprisingly, theoretical work on
shock acceleration of cosmic rays has concentrated on quasl-parallel
shocks (Axford et al., 1977; Blandford and Ostriker, 1978), despite the
fact that such shocks are demonstrably incapable of producing high
energy particles in the interplanetary medium. In an earlier paper
(Sarris and Krimigis, 1985) we reported the details of a high mach
number shock (MA = 3.4) observed by Voyager 2 at 1.9 AU. In this paper
we examine further the development of the energy spectrum and the time
variability of the intensity at the time of shock passage. For details
of the detector system and instrument operation see Krimlgis et al.,
1977. \\
2. Results. Figure 1 shows the count_rate profile of several channels
from the LECP instrument from January I I- 7, 1978, following a 2N flare
at 2145 UT on January I, 1978. The shoc_ wave intercepted the Voyager 2
spacecraft on January 6, 1978 at 00:01:30 UT, as is evident from the
intenslty-tlme profile in the figure. The energy extent of the enhance-
ment can be seen from the fact that the iron channel in the range _ 2-12
MeV/nuc (i.e. a total energy > 112 MeV) increased in intensity by well
170 
SH-1.5-4 
ACCELERATION OF IONS AND ELECTRONS TO NEAR-COSMIC RAY ENERGIES 
IN A PERPENDICULAR SHOCK:THE JANUARY 6, 1978 EVENT 
S. M. Krimigis 
Applied Physics Laboratory, The Johns Hopkins University 
Laurel, MD 20707 
E. T. Sarris 
The University of Thrace 
Polytechnic School 
Xanthi, Greece 
ABSTRACT 
Acceleration of energetic ions to ~ 200 MeV and electrons 
to ~ 2 MeV were detected by the Low Energy Charged Particle 
(LECP) instrument on Voyager 2 in association with a quasi-
perpendicular shock of eBn = 87.5° at 1.9 AU. The meas-
urements, obtained at a time resolution of ~ 1.2 sec, reveal 
structure of the energetic particle intensity enhancements 
down to a scale of the order of the particle gyroradius, and 
suggest that acceleration takes place within a gyrodiameter 
of the shock. The observations are consistent with the pre-
dictions of the shock drift acceleration (SDA) mechanism. 
The absence of any fluctuations in the magnetic field during 
the shock passage suggests that turbulence is not essential 
to the shock acceleration process in the interplanetary 
medium. 
1. Introduction. It is a well documented observational fact that even 
weak (MA '" 1.5) quasi-perpendicular shocks readily accelerate ions to 
relatively high energies (up to ~ 50 MeV, Sarris et al., 1976), while 
quasi-parallel shocks rarely accelerate ions to rather modest (~ 200-300 
keV maximum) energies, if such shocks are more or less supercritical 
(MA '" 2.8, Kennel et a1., 1984). Surprisingly, theoretical work on 
shock acceleration of cosmic rays has concentrated on quasi-parallel 
shocks (Axford et al., 1977; Blandford and Ostriker, 1978), despite the 
fact that such shocks are demonstrably incapable of producing high 
energy particles in the interplanetary medium. In an earlier paper 
(Sarris and Krimigis, 1985) we reported the details of a high mach 
number shock (MA'" 3.4) observed by Voyager 2 at 1.9 AU. In this paper 
we examine further the development of the energy spectrum and the time 
variability of the intensity at the time of shock passage. For details 
of the detector system and instrument operation see Krimigis et a1., 
1977 • 
1 2. Results. Figure 1 shows the count" tate profile of several ch nnels 
from the LECP instrument from January 1 )- 7, 1978, following a 2N flare 
at 2145 UT on January 1, 1978. The shock wave intercepted the Voyager 2 
spacecraft on January 6, 1978 at 00:01 :30 UT, as is evident from the 
intensity-time profile in the figure. The energy extent of the enhance-
ment can be seen from the fact that the iron channel in the range ~ 2-12 
MeV/nuc (i.e. a total energy> 112 MeV) increased in intensity by well 
https://ntrs.nasa.gov/search.jsp?R=19850026460 2020-03-20T17:02:20+00:00Z
LECP on Voyager-2 
43<EpC80keV  Z > 1  
2 5 EFL 5 12 MeVInuc 
. ; -U 
I 
T - T  - : - 7  ,-- : - :  r i  
2 3 4 5 6 7 8  
Day of year, 1978 
Voyager-2 
1978 
Peak Jan 5, 23 18 - Jan 6, 00 18 
Arnb~ent Jan 5, 03 42 - 04 54 UT 
LE P on oyager-2 
1014.-'-'-'-'-"'-'-'-'-r-r-~,, 
l-HR AVE 
R; 195AU 
1012 
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~ 43<Epo;;;;SOkeV z;;r., 
1010 ~ .... r-"\..- ~ 
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~ 
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U 
ay of year, 1978 
Fig 1 
N 
o 
--I 
Cl-
102 
LECP 
Voyager 2 I .' 
p ': 
PL02 (pi 
43 80 keV : 
- .. ~-- .. - .---::- ... ~", 
\ 
\ 
-: 103 
102 
CH10 (pJ 
44114MeV 
---'!.-... 
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CH1 (pJ 
052145 MeV 
:J: 
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102 
101 
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100 -
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2356 0000 0006 
Day 5, 1978 Day 6 
Fig 2 
.. 
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E 
u 
oyager-2 
1978 
eak Jan 5, 23 18 - Jan 6, 00 18 
mbient Jan 5, 03 42 - 04 54 UT 
106r-----~----~-----r-----,_, 
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103 
102 
101 
\ 
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.. 
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• Ions Z ~ 1 
o Protons 
• Alphas 
-:::; 100 
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E (MeV) 
Fig. 3 
.. 
.... 
.. 
172
SH-1.5-4
Figure I Hourly averages of the responses of selected channels from the
LECP instrument on Voyager-2 following the January i, 1978
solar flare event; the shock wave (dashed line) encountered
the spacecrar[t on January 6, 1978 at 00:01:30 UT.
Figure 2 Expanded time scale of ion and electron counting rates with a
time resolution of 1.2 sec. for channels PL02 and EB03 and
I0 sec. for Channel CHI and CHIO. The length of time corre-
sponding to the particle gyroradius as convected past the
spacecraft is marked for each channel with the exception of
EBO3. The magnetic field measurements are shown in the bottom
three panels (Courtesy of Drs. N. F. Ness and R. P. Lepping).
Figure 3 Energy spectra of ions Z > 1 at lower energies and protons and
He at higher (_ 100 MeV) energies, when shock effects were
weak (ambient) and around the shock (peak).
over 2 orders of magnitude.
The detailed time interval to within ± 5 minutes of the passage of
the shock is shown in Figure 2. The magnetic field parameters (courtesy
of N. F. Ness and R. P. Lepping) are shown in the bottom three panels.
The top curve in the upper panel shows that the enhancement for the low-
est energy ions (_ 40 keV) occurs immediately after the passage of the
shock; at an energy about a factor of I0 higher (second curve), however,
the intensity peaks at the shock passage with more of the increase up-
stream rather than downstream of the shock. At an energy one order of
magnitude higher still (third curve from the top) the increase is most
notable upstream of the shock while the intensity drops by well over an
order of magnitude downstream within a gyro-diameter of the shock. We
also note that the energetic electrons are essentially attached to the
shock itself with the intensity being higher upstream than downstream.
It is important to note that this energy separation effect is distinct
in both the general large scale structure of the energetic particle
intensity enhancements geen in Figure I, as well as in the fine scale
intensity structure which is attached to the shock. The peak inten-
sities of the fine structure are found downstream (low energies) or
upstream (high energies) at distances from the shock of the order of the
particle gyroradius. It is essential to note that measurements of the
magnetic field with a time resolution of 1.92 sec. indicate that the
magnetic field is very quiet upstream from the shock front and show no
evidence for the presence of significant magnetic turbulence in thefrequency range from 0.2 Hz to N i0-_ Hz.
Figure 3 shows the energy spectra of ions with Z _ I up to N 3
MeV, and of protons and alpha particles at energies _ 0.5 MeV where the
elements can be clearly separated (Sarris and Krimigis, 1985). The
ambient population some twenty hours prior to the passage of the shock
is shown for comparisons. It is evident that the enhancements were
largest for helium ions, especially at the higher energies (_ 10 MeV).
Although not shown here, the enhancements for the heavier elements ex-
tended to total energies ~ 200 MeV (Sarris and Krimigis, 1985).
3. Discussion. The shock acceleration event described above is remark-
172 
SH-1.5-4 
Figure 1 Hourly averages of the responses of selected channels from the 
LECP instrument on Voyager-2 following the J nuary 1, 1978 
solar flar  event; the shock wave (dashed line) encountered 
the spacecrarft on January 6, 1978 at 00:01:30 UTe 
Figure 2 Expanded time scale of ion and electron cou ting rates with a 
time resolution of 1.2 sec. for channels PL02 and EB03 and ~ 
10 sec. for Channel CHI and CHIO. The length of time corre-
spo ding to the particle gyroradius as convected past the 
spacecraft is marked for each channel with the exception of 
EB03. The magnetic field measurements are shown in the b ttom 
three panels (Courtesy of Drs. N. F. Ness and R. P. Lepping). 
Figure 3 Energy spectra of ions Z ) 1 at lower energies and protons and 
He at igher (> 100 MeV) energies, when shock effects were 
weak (ambient) ~nd around the shock (peak). 
over 2 o ders of magnitude. 
The detailed time interval to within ± 5 minutes of the passage of 
the shock is shown in Figure 2. The magnetic field parameters (courtesy 
of N. F. Ness and R. P. Lepping) are shown in the b ttom three panels. 
The top curve in the upper panel shows that the enhancement for the low-
est energy ions (~ 40 keV) occurs immediately after the p ssage of the 
shock; at an energy about a factor of 10 igher (second curve), however, 
the intensity peaks at the shock p ssage with more of the increase up-
stream rather than downstream of the shock. At an energy one order of 
magnitude igher still (third curve from the top) the increase is most 
notable upstream of the shock while the intensity drops by well over an 
order of magnitude downstream within a gyro-diameter of the shock. We 
also note that th  en rgetic electrons ar  essentially attached to the 
shock itself with the intensity being igher upstream than downstream. 
It is important to note that this energy separation effect is distinct 
in both the general large scale structure of th  en rgetic particle 
intensity e hancements geen in Figure 1, as well as in the fine scale 
intensity structure w ich is attached to the shock. The peak inten-
sities of the fine structure are foun  downstream (low energies) or 
upstream (high energies) at distances from the shock of the order of the 
particle gyroradius. It is ssential to note that measurements of the 
magnetic field with a time resolution of 1.92 sec. indicate that the 
magnetic field is very quiet upstream from the shock front and show no 
evidence for the presence of signi~~cant magnetic turbulence in the 
frequency range from ~ 0.2 Hz to ~ 10 Hz. 
Figure 3 shows th  energy spectra of ions with Z > 1 up to ~ 3 
MeV, and of protons and alpha particles at energies> 0.5 MeV where the 
el ments can be clearly separated (Sarris and Kri;igis, 1985). The 
ambient population some twenty hours prior to the passage of the shock 
i  shown for comparisons. It is evident that th  enhancements were 
largest for helium ions, specially at the igher energies (~ 10 MeV). 
Although not shown here, th  e hancements for the heavier elements ex-
tended to total energies ~ 200 MeV (Sarris and Krimigis, 1985). 
3. Discussion. The shock acceleration event described ahove is remark-
173
SH-1.5-4
able in that not only did it accelerate nuclei and electrons to almost
cosmic ray energies, but that it did so in the absence of any discern-
ible level of turbulence in the magnetic field. Further, the data de-
monstrate that all intensity enhancements and fine structure occur
within a gyrodiameter of the shock from the lowest (_ 40 keV) to the
highest (> 20 MeV) energies. This event and many other similar ones
(Armstrong et al., 1985) demonstrate that turbulence is not a necessary
condition for particle acceleration in shocks, to the highest energies
observed in the interplanetarymedium.
Recent simulation work on quasi-perpendicular and quasl-parallel
shocks in the presence of turbulence (Decker and Vlahos, 1985) shows
that magnetic fluctuations have a tendency to cause a test particle to
encounter the shock more than once. In each of these encounters, how-
ever, the energy gain is mostly through drift acceleration along the
shock surface, rather than compression. This result helps to explain
the observation that maximum energies attained in quasl-parallel shocks
are at most _ 300 keV. The apparent maximum energy gain is likely due
to the fact that the time during which the instantaneous magnetic field-
shock normal geometry is near-perpendicular is rather small and
consequently the energy gain through shock-drlft is low. This energy
gain "saturation" effect at relatively low energies suggests that the
particle escapes the shock turbulence region after only a few encounters
with the shock.
Similarly in the astrophysical case, quasi-perpendicular shocks
would energize the ions to several hundred MeV. Such particles would
then become the seed population; further energization would obtain
through scattering in magnetic field fluctuations which may cause the
ions to undergo additional encounters with the shock for further energy
gains. At some point (relativistic energies?) compression may begin to
become an appropriate description for the process.
4. Acknowledgements. We are grateful to Dr. A. Vinas of GSFC for
providing us with the shock parameters. This work was supported by NASA
under Task I of Contract N00024-83-E-5301.
5. References. Armstrong, T. P. et al., Shock Drift Acceleration, Pro-
ceedings of the Chapman Conference on Collissionless Shocks, AGU, 1985.
Axford, W. I. et al., The acceleration of cosmic rays by shock waves,
Proceedings, International Conf. of Cosmic Rays 15th, II, 132, 1977.
Blandford, R. D. and J. P. Ostziker, Particle acceleration by
astrophysical shocks, Astrophzs. J., 221, L29, 1978.
Decker, R. B. and_ L. Vlahos, Energetic ion acceleration at
collisionless shocks, 19th ICRC Conference, SH 1.5-3, 1985.
Krimigis, S. M. et al. The Low Energy Charged Particle (LECP)
experiment on the Voyager spacecraft, Space Sci. Rev., 21, 329, 1977.
Kennel, C. F., et al., Structure of the November 12, 1978 quasi-
parallel interplanetary shock, J. Geophys. Res., 89, 5435, 1984.
Sarris, E. T. and S. M. Krimigis, Quasi-perpendicular shock accelera-
tion of ions to _ 200 MeV and electrons to _ 2 MeV observed by Voyager
2, Astrophzs. J., , (in press) November 15, 1985.
Sarris, E. T. et al., Observations of a high-energy ion shock spike in
interplanetary space, Geophys. Res. Lett. 3, 133, 1976.
3 
- .5-4 
a le i  t at not only did it accelerate clei and electrons t  al ost 
cos ic ray energies, but that it did so in the absence of any discern-
   t b lence  t  agneti  d. urt , t  t  
onstrate t t ll i t sity enhance ents and fine stru ture cc r 
ithin a gyrodia eter of the shock from the lo est ('" 40 ke ) to the 
highest (~ 20 MeV) energies. This event and many other si ilar ones 
( r strong t l., ) onstr te t t t rbulence i  t a ssary 
c i tion f r arti le l r tion i  s c s, t  t e i est e er ies 
served i  t e i terplanetary ediu . 
ecent i ulati  or   uasi- erpendicular  uasi-parallel 
s c s i  t  rese ce f t r l  ( ec er a  la os, ) s s 
that agnetic fluctuations have a tendency to cause a test particle to 
counter t   or  t  .    t  counters, -
er, t  r  ai  i  ostl  t r  ri t celerati n l  t  
 r e, t r t  pre sion. hi  lt l  t  plai  
t  servati n t t a i  er i s ttain  i  asi-parallel s s 
  ost'"  .  ar t a i   i   ik   
t  t e f t t t t e time ri  hich t e i sta ta e s agnetic fi l -
s  r al etr  i  ear- erpendicular i  r t r s all  
consequently the energy gain through shock-drift is lo . his energy 
i  "s t r ti " ff t t r l ti l  lo  er i s s ests t t t  
arti le escapes the shock t r le ce region ft r only a fe  encounters 
it   . 
i ilarl  i  t e astr ysical case, asi- erpendicular shocks 
ould energize the ions to several hundred e . Such articles ould 
t  e t  s  pulati n; f rt r r i ati  oul  t i  
t r  tt ri  i  agnetic fi l  fl t ti s hi  a  s  t  
ions to undergo additional encounters ith the shock for further energy 
ai s. t so e point (r l ti isti  e er ies?) co pression ay begin t  
beco e an appropriate description for the process. 
4. Acknowledgements. We are grateful to Dr. A. Vinas of GSFC for 
providing us ith the shock para eters. This work was supported by NASA 
er as    ontract 00024-83-E-5301. 
5. eferences. r strong, . P. et l., Shock rift cceleration, ro-
ceedings of the Chapman Conference on Collissionless Shocks, AGU, 9~ 
xford, . I. t 1.,  elerati n f s i  r s  s  aves, 
Proceedings, International Conf. of Cosmic Rays 15th,~, 132, 1977. 
landford, . . and J. . stri er, arti l  accelerati n by 
astrophysical shocks, Astrophys. J., 221, L29, 1978. 
ecker, R. B. and' L. Vlahos~nergetic ion acceleration at 
c 11isi nless s c s, t  IC  onference, S  . - , 1985. 
ri igis, S. M. et al. The Low Energy Charged Particle (LECP) 
experi ent on the oyager spacecraft, Space Sci. ev., 21, 329, 1977. 
ennel, . ., t t., t t r  f t  ovember1 ,  asi-
parallel interplanetary shock, J. Geophys. es.,!2., 5435, 1984. 
arris, E. T. and S. . ri igis, uasi-perpendicular shock accelera-
tio   io  to '"  e   l t s t  '"     oyager 
2, Astrophys. J., __ , (in press) November 15, 1985. 
arris, E. T. et a1., bservations of a high-energy ion shock spike in 
interplanetary space, Geophys. Res. Lett. ~, 133, 1976. 


Acceleration of ions and electrons to near-cosmic ray energies in a perpendicular shock: The January 6, 1978 event

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