Observations of shock waves in the solar system which show a pronounced difference in the plasma wave and particle environment depending on whether the shock is propagating along or perpendicular to the interplanetary magnetic field are discussed. Theories for particle acceleration developed for quasi-parallel and quasi-perpendicular shocks, when extended to the interstellar medium suggest that the relativistic electrons in radio supernova remnants are accelerated by either the Q parallel or Q perpendicular mechanisms. A model for the galactic magnetic field and published maps of supernova remnants were used to search for a dependence of structure on the angle Phi. Results show no tendency for the remnants as a whole to favor the relationship expected for either mechanism, although individual sources resemble model remnants of one or the other acceleration process

Cairns, I. H.

Leckband, J. A.

Spangler, S. R.

English

NASA Technical Reports Server

N89- 24 948
251
ARE SUPERNOVA REMNANTS QUASI-PARALLEL OR QUASI-PERPENDICULAR
ACCELERATORS?
S R Spangler, J A Leckband & I H Cairns
Dept. of Physics & Astronomy, University of Iowa, USA
ABSTRACT
Observations of shock waves in the solar system show a
pronounced difference in the plasma wave and particle en-
vironment depending on whether the shock is propagat-
ing along or perpendicular to the interplanetary magnetic
field. Quite different theories for particle acceleration have
been developed for quasi-parallel and quasi-perpendicular
shocks. Extension of these ideas to the interstellar medium
would lead one to believe that the relativistic electrons in
radio supernova remnants are accelerated by either the QII
or Q± mechanism. If this is the case, the observed morphol-
ogy of a remnant should depend on the angle ff between the
line of sight and the galactic magnetic field at the position
of the remnant. The dependence of observed structure on
this angle will be quite different for QII and Q.l_ mecha-
nisms. We have used a model for the galactic magnetic
field and published maps of supernova remnants to search
for a dependence of structure on the angle ft. We did not
find a tendency for the remnants as a whole to favor the
relationship expected for either mechanism, although in-
dividual sources resembled model remnants of one or the
other acceleration process. We will discuss a number of
possible explanations for this result.
1. INTRODUCTION
Earlier in this meeting I spoke about radioastronomical ob-
servations of relevance to diffusive shock acceleration. I
would like to return to this theme now and discuss radioas-
tronomical observations which might illuminate the process
by which supernova remnants accelerate charged particles.
Once again, we will draw on analogies with phenomena in
the solar wind.
Observations of shock waves in the solar wind show pro-
nounced differences depending on the orientation of the
shock normal to the interplanetary magnetic field. Shocks
for which the shock normal is roughly parallel to the mag-
netic field are characterized by an extensive upstream layer
of hydromagnetic turbulence termed a foreshock. Shocks
for which the shock normal is roughly perpendicular have
no such foreshock, and the plasma waves excited at the
shock are electrostatic.
For both types of shocks, particle acceleration, primarily
or exclusively of ions, is observed. The mechanisms pro-
posed for acceleration are quite different in the two cases,
being first-order Fermi acceleration for quasi-parallel shocks
(Ref. 1), while for quasi-perpendicular shocks a shock-drift
mechanism (Ref. 2) and acceleration via electrostatic waves
(Ref. 3) have been proposed.
Now let us consider the situation for supernova remnant
shocks. Radioastronomical observations show that super-
nova remnants are accelerating electrons to relativistic en-
ergies. It is also generally assumed that they are responsi-
ble for the cosmic ray ions. This being the case, they are
vastly more effective accelerators of charged particles than
are solar system shocks.
Given this, I will postulate that if these solar system mech-
anisms are operative in supernova remnants, then one or
the other will dominate. It seems to me somewhat far-
fetched, though obviously not out of the question, that
both shock drift and first-order Fermi acceleration are ac-
tive in supernova remnants. I will assume therefore that
either quasi-parallel or quasi-perpendicular acceleration is
occurring and that it would be interesting to determine the
responsible mechanism. A possible way of making such a
discrimination is indicated in Figure 1.
Supernova remnants will be expanding into a large-scale
galactic magnetic field. The appearance of a supernova
remnant should then depend on the angle between the line
of sight and the magnetic field at the position of the rem-
nant. If quasi-parallel acceleration is occurring, a remnant
viewed across the field will appear limb brightened, while
one viewed along the field will appear of uniform brightness.
For the case of quasi-perpendicular acceleration, a remnant
viewed across the field will appear of more uniform bright-
ness, whereas one viewed along the field will appear highly
limb brightened and annular. If, in addition, there is a
large-scale systematic magnetic field in the galaxy, there
will be a dependence of remnant appearance on position in
the sky.
The purpose of this talk is to discuss an observational search
for such an effect, which would tell us whether quasi-parallel
or quasi-perpendicular acceleration is dominant in super-
nova remnants.
2. OBSERVATIONAL INVESTIGATIONS
There are three ingredients for such an investigation. First,
we need a quantitative measure of the degree and nature
Proceedings of the Joint Varenna-Abastumani International School & Workshop on Plasma Astrophysics, held in Varenna, Italy,
24 Aug.-3 Sept. 1988 (ESA SP-285, Vol. L Dec. 1988).
https://ntrs.nasa.gov/search.jsp?R=19890015577 2020-03-20T01:39:02+00:00Z
252
k. i
S R SPANGLER &AL
. 7
of limb brightening as a function of the angle between the
magnetic field and the line of sight, hereafter referred to
as _. The dependence of limb brightening on ¢ is ob-
viously drastically different for quasi-parallel and quasi-
perpendicular acceleration. Second, we need a model for
the galactic magnetic field, so that we may obtain • given
the remnant distance and position on the sky, and third,
we need a sample of remnants with usable radio maps.
To fill the first of these needs, a model was chosen in which
the volume emissivity is a simple function of spherical co-
ordinates, specifically, an exponential decline behind the
remnant shell, and a cos 2 0 or sin 2 0 dependence on angle
with respect to the B field. We then numerically calculated
the observed brightness distribution as a function of ft. Ex-
amples of such model remnants are shown in Figure 2.
For the case of quasi-perpendicular acceleration, we see a
gradual transition from an annular structure to two arcs
parallel to the galactic magnetic field. In the case of quasi-
parallel acceleration, there is a transition from uniform sur-
face brightness to two arcs which are perpendicular to the
field.
From model brightness distributions like this, we have ex-
tracted a single measure of the degree of limb brightening,
taken to be simply the ratio of the rim-to-center bright-
ness. This parameter was chosen because it may be easily
measured from published maps of supernova remnants.
The second ingredient we need is a model of the galac-
tic magnetic field. The one we chose was from Sofue and
Fujimoto (Ref. 4), who have made a fit to measurements
of galactic Faraday rotation of extragalactic radio sources.
This model also provides a good fit to the positions of galac-
tic H II regions. Given this model, and the distance to a
supernova remnant, we can calculate the angle ff between
the line of sight and the local magnetic field.
Finally, of course, one needs data on remnants. We searched
the literature for suitable sources. The requirement was
that the remnant have a good quality radio map and an
estimate of the distance. We also excluded radiative super-
nova remnants, since it is generally thought that their radio
emission arises from compression of the existing cosmic ray
gas rather than acceleration per se.
We were disappointed in that this search yielded only
13 remnants. It is somewhat dismaying that after three
decades of what one might call modern radio astronomy, so
few good maps exist of a major type of radio source.
The analysis consisted of two phases. First, our single-
parameter representation of the limb brightening was plot-
ted as a function of ¢, and the measurements compared
with model curves of the relationship for quasi-parallel and
quasi-perpendicular acceleration. Our measurements were
highly scattered on a plot of intensity ratio versus if, and
showed no adherence to the model curve for either acceler-
ation process.
A second form of analysis was a qualitative comparison
of the observed maps with the model remnants shown in
Figure 2. Although subjective, such an approach allowed
consideration of mole features of the remnants than sim-
ply a center-to-limb intensity ratio. The purpose of this
deliberation was to see if there was a tendency for the rem-
nants to more closely resemble the case of quasi-parallel
or quasi-perpendicular acceleration. Again results were in-
conclusive, although specific remnants resembled the quasi-
parallel or quasi-perpendicular model.
As discussed in Leekband et al (Ref. 5), the supernova rem-
nant of 1006AD shows a twin arc appearance, with the
arcs perpendicular to the galactic plane. With our mag-
netic field model and the distance estimate employed, we
obtain a value of ¢I, = 44 °. Such an appearance is consis-
tent with that expected for a remnant radiating by quasi-
parallel acceleration. On the other hand, Tycho's super-
nova remnant presents an almost perfect annular structure,
and we estimate ff to be about 35 °. This is in accord with a
remnant characterized by quasi-perpendicular acceleration.
However, although individual remnants might resemble the
models for one or the other acceleration process shown in
Figure 1, there is no systematic tendency.
3. CONCLUSIONS AND RUMINATIONS
The results of this project were therefore negative. We
were unable to find evidence for a galactic magnetic field
modulation of the synchrotron emission in our sample of
remnants.
We can think of four possible explanations for this negative
result. First, it is possible that the notoriously uncertain
distances to supernova remnants have produced large er-
rors in our values of ¢. Second, the magnetic field model
we have used might be incorrect, or describes a systematic
component which is small in comparison with a superposed
random component. Such an effect would also render our
value of • in error.
In both of the above explanations, it is assumed that the
galactic magnetic field does play a modulating function,
but that other influences frustrate our ability to determine
_. Alternatively, one can accept our results at face value,
and conclude that true supernova remnants do not resem-
ble either of the simple models discussed above. The third
suggestion, therefore, is that the galactic magnetic field
does not modulate the acceleration of electrons. Such a
result would seem to invalidate both of the models dis-
cussed above, but would be compatible with others, such as
second-order Fermi acceleration. The fourth and final pos-
sibility is that both quasi-parallel and quasi-perpendicular
acceleration mechanisms are occurring, but that the oper-
ative acceleration mechanism varies from one remnant to
another. Although this suggestion is at odds with a pref-
erence for simplicity in nature, it quite naturally accounts
for the fact that remnants such as SN 1006 and Tycho's
remnant so closely resemble the ezempla of quasi-parallel
and quasi-perpendicular acceleration, and yet that there is
no systematic fealty to an acceleration model.
4. ACKNOWLEDGMENTS
This paper is largely based on work carried out by James
Leckband for a Master's thesis in astronomy at the Uni-
versity of Iowa and in pres3 in Ref. 5. This research was
supported by grants NAGW-806 and NAGW-831 from the
National Aeronautics and Space Administration.
ORIGINAL PAGE
BLACK AND WHITE PHOTOGRAPH
SUPERNOVA REMNANTS 253
3)
;)
QUASI PARALLE L
GALACTIC ,_,
> °(>
MAGNETIC
>
FIELD
OBSERVE D
APPEARANCE
_i{ LIMB> BRIGHTENED
j y SUPERNOVA REMNANTS "4,
>-
UNIFORMBRIGHTNESS
>
Figure 1.
QUASI PERPENDICULAR
> O > _ UNIFORMBRIGHTNESS
© ©> : @ >_ LIMBBRIGHTENED
......
> ),-
Illustration of how the appearance of supernova remnants changes with the angle between the line
of sight and the galactic magnetic field for quasi-parallel (top) and quasi-perpendicular (bottom)
remnants. Figure from Leckband et al (Ref. 5).
Quasi-Perpendicular @=0 Quasi-Perpendicular @=45 Quasi-Perpendicular @=90
Quasi-Parallel 4_=0 Quasi-Paral]el @=45 Quasi-Parallel ¢=90
"_" R:)-,
: _L':L__
-?:.
Figure 2. Appearance of model supernova remnants as a function of (_, the angle between the line of sight
and the galactic magnetic field. The top panels are for quasi-perpendicular remnants and the
bottom ones for quasi-parallel remnants. The magnetic field is in the horizontal direction for all
panels. Figure from Leckband et al (Ref. 5).
254 S R SPANGLER &AL
5. REFERENCES
1. Lee M A 1982, J Geophys Res 87, 5063.
2. Decker R B &: Vlahos L 1986, Astrophys J 306, 710.
3. Galeev A A 1984, Soy. Phys. JETP 59, 965.
4. Sofue Y & Fujimoto M 1983, Astrophys J 265, 722.
5. Leckband J A et al 1988, Astrophys J in press.


Are supernova remnants quasi-parallel or quasi-perpendicular accelerators

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