Theories of diffuse shock acceleration of the cosmic rays that predict that there should be regions of intense Alfven waves both upstream and downstream of the shock front are considered. Observations of similar waves near solar system shock waves show that they produce substantial density fluctuations. Such density fluctuations might produce observable scattering of radio waves. Observations which searched for angular broadening or blurring of radio sources whose lines of sight pass close to or through supernova remnants are discussed. No definite cases of remnant-associated scattering are detected. However, the source CL 4, which is viewed through the Cygnus Loop supernova remnant, may be such an object and merits further observation

Spangler, S. R.

English

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153
SHOCK-ASSOCIATED PLASMA DENSITY FLUCTUATIONS IN THE INTERSTELLAR
MEDIUM
S R Spangler
Dept. of Physics & Astronomy, Univ. of Iowa, Iowa, USA
ABSTRACT
Theories of diffusive shock acceleration of the cosmic rays
predict that there should be regions of intense Alfv_n waves
both upstream and downstream of the shock front. Obser-
vations of similar waves near solar system shock waves show
that they produce substantial density fluctuations. Such
density fluctuations might produce observable scattering of
radio waves. We discuss observations which have searched
for angular broadening or "blurring" of radio sources whose
lines of sight pass close to or through supernova remnants.
No definite cases of remnant-associated scattering have
been detected. However, the source CL4, which is viewed
through the Cygnus Loop supernova remnant, may be such
an object and merits further observation.
I. INTRODUCTION
One of the more interesting topics in plasma astrophysics
is that of shock acceleration of charged particles. In "dif-
fusive" shock acceleration (Ref. 1), charged particles reflect
from a shock wave and generate magnetohydrodynamic
waves. These waves in turn cause pitch-angle scattering of
the particles, thus isotropizing and confining them to the
vicinity of the shock where they can be repeatedly over-
taken and accelerated. The excited hydromagnetic waves
form the key ingredient to the operation of this mecha-
nism. This theory has been given observational su!Sport
by observations of shock waves in the solar system, which
show reflected particles, the excited hydromagnetic waves,
and ions accelerated to a few hundred kilovolts.
It is widely speculated that a magnified version of this pro-
cess is responsible for production of the cosmic ray ions and
electrons. The generative shocks in this case would be su-
pernova remnants. If our speculations are correct, ahead
of a supernova remnant there should be a foreshock with
reflected ions and electrons, excited magnetohydrodynamic
waves, and particles being accelerated to relativistic ener-
gies. It would be a tremendous success for these theories if
observational evidence for this upstream wave layer, or the
equally important downstream region of magnetic fluctua-
tions, could be found.
At first, it would seem that any attempt to detect these
waves would be doomed to failure. They should have wave-
lengths comparable to and somewhat larger than the ion
inertial length, which for the interstellar medium is a few
thousand kilometers, or perhaps smaller. The detection of
objects of this size near a supernova remnant a kiloparsec
away would appear nigh-on impossible.
The purpose of this talk is to discuss a technique which, in
principal, is capable of detecting interstellar MHD waves.
To the best of my knowledge, it is the only such technique,
and relies on the measurement of radio wave scattering due
to density fluctuations in the interstellar medium. Radio
waves from objects such as pulsars and extragalactic radio
sources at low galactic latitudes are observed to be dis-
torted by a number of propagation effects. These distor-
tions are interpreted as the result of propagation through
an interstellar medium with random density fluctuations,
which engender a random refractive index. The simplest
such effect to interpret is angular broadening, or the "blur-
ring" of a radio source viewed through the turbulent inter-
stellar medium. This phenomenon is physically analogous
to the blurred appearance of objects viewed through the
turbulent exhaust of a jet engine.
How is this observational technique relevant to the ques-
tion of shock acceleration of charged particles? Spacecraft
observations of shock waves in the solar system reveal that
large-amplitude density fluctuations accompany these mag-
netohydrodynamic waves (Ref. 2). Analyses indicate that
the density fluctuations are due to (1) slightly oblique prop-
agation with wave normal angles of a few degrees, and
(2) ponderomotive effects. At any rate, the density per-
turbations are large, being about 15% or so of the mean
plasma density. By analogy, we may assume that similar
density fluctuations exist in front of supernova shock waves.
Having introduced the requisite background in space plas-
ma physics and radio astronomy, we now present the idea
of our experiment in Figure 1. We choose to observe ex-
tragalactic radio sources whose lines of sight pass close to
the edge of a supernova remnant. There are also objects
for which the line of sight passes through the post-shock
region.
If the foreshock is sufficiently extensive so that the line
of sight traverses a substantial distance through the fore-
shock, angular broadening of the source may be observable.
I should mention that there is a host of observational as-
tronomy questions which must be considered when trying
Proceedings of the Joint Varenna-Abastumani International School & Workshop on Plasma Astrophysics, hem in Varenna, Italy,
24 Aug.-3 Sept. 1988 (ESA SP-285, Vol. 1, Dec. 1988).
https://ntrs.nasa.gov/search.jsp?R=19890015562 2020-03-20T01:38:32+00:00Z
154 s R SPANGLER
to determine if the radio source is affected by interstellar
scattering. We spend a good deal of time on these deliber-
ations, but they are beyond the scope of this article.
EXTRAGALACTIC \ J/
RADIO SOURCE "_/i_"_
I
I
r"_-'-"R'_elNTERIOR OF" / [ /]_
FORESHOC_ / J
LINE OF SIGHT_ t
OBSERVER_
Figure 1. Illustration of how radioastronomical observa-
tions can be used to search for foreshocks of su-
pernova remnants. We observe (with multifre-
quency VLBI) sources whose lines of sight pass
close to supernova remnants. If the line of sight
passes through a pre-shock region of enhanced
turbulence, angular broadening or blurring of
the source may be observed.
2. OBSERVATIONAL RESULTS
Since 1984, we have been carrying out VLBI observations
of radio sources near supernova remnants, looking for en-
hanced scattering which might be attributable to MHD
waves near the shocks. It should be mentioned that from
the outset we realized that the probability of success was
not high. Extensive, wave-dominated foreshocks will occur
only at portions of the shock where the shock normal is
parallel to the galactic magnetic field. For a given radio
source, the line of sight may pass through a region where
this condition is not satisfied. Alternatively, the density in
the region into which a given shock is expanding may be
too low to host detectable perturbations. There can easily
be other agents for frustrating this experiment.
I will now proceed to discuss the results to date, which I
would describe as mixed. In other words, we have not de-
tected what I would describe as categorical or even strong
evidence for MHD wave-associated turbulence. However,
in spite of the relatively small number of lines of sight in-
vestigated, we already have a case which at the very least
encourages us to continue these types of observations.
In Spangler et al (Ref. 3), we presented three frequency
VLBI observations of five radio sources whose lines of sight
pass close to supernova remnants. In four of the remnants
the radio structure is shell-like, whereas one (the remnant
G74.9+1.2) allegedly resembles the Crab Nebula. In each
case, there is a juxtaposed radio source which can act as
a probe of the interstellar medium in the vicinity of the
remnant.
For three of the sources we were not able to detect inter-
stellar scattering; only upper limits to the intensity of tur-
bulence along the line of sight could be set. For two of
the objects, scattering was detected. Our observations of
2013+370 were sufficiently detailed to allow a measurement
of the spectrum of interstellar turbulence (Ref. 4). As we
shall see shortly, the source is not particularly scattered for
this part of the sky. The line of sight to the radio source
1849+005 is one of the most heavily scattered known;'our
measurement of a high degree of scattering is confirmed by
the discovery by Clifton and Lyne of a highly scattered pul-
sar a few arcminutes from 1849+005. At the present time
we are carrying out supplemental observations to see if the
observed degree of scattering is abnormal for this part of
the sky, and if the enhancement can be attributed to this
supernova remnant.
For the remaining remnants, the upper limits are perhaps
a little disappointing. For two of the sources (near the
remnants CTA 1 and HB 21) the "impact parameters" were
many parsecs, so the results are perhaps not surprising. For
the source near the remnant HB 9, however, the upper lim-
its were more interesting. Either the thickness of the SNR
foreshock is less than about 3-4% of the remnant radius, or
the foreshock turbulence does not have the properties we
would have expected from studies of the Earth's foreshock.
In contrast to these ambiguous or disappointing results, an
additional project has yielded an intriguing result. We have
measured the scattering along the lines of sight to eleven
sources in the constellation Cygnus (Ref. 5). The results
are presented in graphical fashion in Figure 2. Here the
0
_ O
_ O
UT
0
c_
I
Figure 2.
Scattered Angular Size,01 GI'[=
I .... i .... I .... I ' ' '
I .... I .... I , , , , I
90 80 70 60
GalacticLongitude (deg)
Graphical representation of interstellar scatter-
ing in the Cygnus region of the sky. The width
of the blur is propoz'tional to the scattering size
at 1 GHz. The heavily scattered source at lon-
gitude 75 ° and latitude -8 ° is 2048+313.
PLASMADENSITYFLUCTUATIONS 155
sizeoftheblurredimageisproportionaltotheinterstellar
scatteringsizeat1GHz.
Wecanseethat the scattering of 2013+370 (at galactic co-
ordinates gI! = 74.9, bII = 1.2) is not conspicuously large
for this part of the sky. The simplest description of this fig-
ure might be that strong scattering is restricted to within
a couple of degrees of the galactic plane. What obviously
prevents us from making such a statement is the highly scat-
tered source 2048+313 (at galactic longitude 75 °, latitude
-8°), which is the second most heavily scattered source in
our sample, despite its angular distance from the galactic
plane. The other remarkable feature of this source is that
it is viewed through the Cygnus Loop supernova remnant.
Although future observations are necessary to confirm our
notions, it is inviting to speculate that this enhanced scat-
tering is due to foreshock or post-shock turbulence associ-
ated with this remnant.
3. EPITOME
Research projects in progress should clarify the role of su-
pernova remnants in generating high levels of density turbu-
lence. Such radioastronomy experiments hold the promise
of valuable experimental input to theories of cosmic ray
acceleration by supernova shock waves. Scattering mea-
surements should be able to give the amplitude and spatial
spectrum of density fluctuations both upstream and down-
stream of the shock front. A knowledge of the relationship
between the magnetic field and the density perturbation in
an MHD wave would then permit us to deduce the wave
spectrum on both sides of the shock. Observational and
theoretical research in space plasmas are progressing to-
wards a good understanding of this relationship. Finally,
knowledge of the shock velocity (known for many remnants)
would complete the set of the principal parameters in the
theory of shock acceleration.
4. ACKNOWLEDGMENTS
This research was supported by grants NAGW-806 and
NAGW-831 from the National Aeronautics and Space
Administration.
5. REFERENCES
1. Lee M A 1982, J Geophys Res 87, 5063.
2. Spangler S R et al 1988, J Geophys Res 93, 845.
3. Spangler S R et al 1986, Astrophys J 301,312.
4. Spangler S R & Cordes J M 1988, Astrophys J 322, 346.
5. Fey A Let al 1988, Astrophys J in press.


Shock-associated plasma density fluctuations in the interstellar medium

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