The type 2 radio burst was identified as a shock propagating through solar corona. Radio emission from shocks travelling through the interplanetary (IP) medium was observed. Using the drift rates of IP type II bursts the velocity characteristics of eleven shocks were investigated. It is indicated that shocks in the IP medium undergo acceleration before decelerating and that the slower shocks take longer to attain their maximum velocity

Cane, H. V.

English

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VELOCITYPROFILESOFINTERPLANETARYSHOCKS
H.V.Cane
NASA/GoddardSpace Flight Center,
Greenbelt, MD20771
ABSTRACT
The type II radio burst has long been identified as being generated by a
shock propagating through the solar corona. Only recently has it been
possible to observe radio emission from shocks travelling through the inter-
planetary (IP) medium. Using the drift rates of IP type II bursts the
velocity characteristics of eleven shocks have been investigated. The
analysis indicates that shocks in the IP mediumundergo acceleration before
decelerating. The slower shocks take longer to attain their maximumvelocity.
Inconsistencies arise when the analysis is extended to the higher frequency(>500 kHz) data.
Introduction
At the time of the first description of the type II phenomenon(Wild and
McCready, 1950) it was realized that the responsible disturbances propagated at
speeds greater than 500 km/sec (Wild, 1950). Such speeds are well in excess
of the speed of sound in the corona. The coincidence between the detection of
a sudden com_mencementgeomagnetic storm, initiated by a shock, and the observa-
tion of a type II burst at the sun a few days beforehand (Roberts, 1959) led to
the acceptance of the theory that type II bursts are generated by shocks. The
presence of two emission bands separated in frequency by about a factor of two
indicated that fundamental and first harmonic electron plasma emission was
being observed. Assuming a coronal density model the rate at which the burst
drifted through the frequency range was then related to the shock's velocity.
With the availability of 2-D spectro-heliographs it was determined that the
assumeddensity models were consistent with the observed source positions(Wild, 1970). It should be noted though, that the deduced coronal electron
densities are about a factor of i0 greater than values determined from white
light measurementsof the quiet corona (Stewart, 1976).
The propagation of shocks through the corona and IP mediumis of
considerable interest. Shocks play an important role in the energetics of the
solar corona and in particle acceleration, both at the sun and in the IP
medium. In situ measurementsof shocks have been madefor more than fifteen
years and muchprogress has been madein understanding shock structure.
However, the majority of the spacecraft used for these studies have been in
earth orbit providing information at only one heliocentric distance. Thus not
much is knownabout the evolution of shocks. Oneproperty that is known, and
was first pointed out by Gosling et ai.(1968), is that most shocks decelerate
during their transit from the sun. Shock speeds at 1AU are less than the
transit speeds. The deceleration is presumedto be as a result of expansion
of the wave and energy transfer to the ambient solar wind.
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https://ntrs.nasa.gov/search.jsp?R=19840005054 2020-03-22T07:42:45+00:00Z
The technique of 'tracking' a shock via its radio emission provides
information about the same shock over a large range of heliocentric distances.
However, until the ISEE-3 radio astronomy experiment only a few events had been
detected. Malitson et a1.(1973, 1976) reported detecting two events with an
experiment on IMP-6 and Boischot et a1.(1980) reported several bursts seen with
experiments on the Voyager spacecraft. These observations were obtained by
much less sensitive experiments and provided no information about shock
velocities. Although in the literature (e.g. Dryer, 1975) the IMP-6 data have
been used to show a constant shock velocity, in fact, this was assumed in the
original data analysis.
Data Analysis
In Figure 1 we sketch the ground-based dynamic spectrum for a type II
event observed at the Culgoora Observatory in October 1978. This event
continued to low frequencies and the 2 and i MHz intensity-time profiles from
the ISEE-3 experiment are shown. Note that ground-based observations are not
usually possible below about 20 MHz, whereas the top frequency of the ISEE-3
experiment is 2 MHz. Thus a gap of about a decade in frequency exists in the
frequency regime of interest.
1.0
g
z
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07'30
OCTOBER I,197B
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If
GROU-NDBASED OBSERVATIONS7
08'00 ' z083o 09=00
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Fig. I. This figure shows the relationship between meter wavelength
phenomena and the activity detected at low frequencies. In particular the SA
event is shown to precede the type II burst at 2 M11z. The dashed lines
indicate what we would expect to have occurred in the unobserved region of the
spectrum.
A characteristic of the October 1978 event, and other energetic events, is
the occurrence of fast drift elements emerging from the 'backbone' of the type
II burst observed from the ground. The 'herring-bones', as these elements are
called, have been attributed to streams of relativistic electrons accelerated
by the shock (Wild et al., 1963). In very energetic events these electrons
propagate to i AU causing intense emission at low frequencies. We presume
that the release and/or acceleration of these electrons ceases above approxim-
ately the 5 MHz plasma level because at 2 MHz the observed activity ceases
704
after about 25 minutes (see figure 1 for clarification). The intense low
frequency outburst has been called a shock accelerated (SA) event (Cane et al.,
1981). Type II bursts observed by ISEE-3 (which we call IP type II bursts)
are preceeded by SA events. Since the SA event at 2 MHz begins within a
minute or so of the type II burst observed at about 20 MHz, associations
between ground and ISEE-3 type II events can be made unambiguously despite the
lack of coverage in the regime between about 20 MHz and 2 MHz.
Figure 1 shows ISEE-3 intensities on a logarithmic scale and it is clear
that the type II emission is much weaker than the SA event. The 2 and I MHz
type II emission appears to be very weak but this is only because it is
competing with the intense background emission from our Galaxy. We have
detected type II emission at 2 and 1MHz in only about 50% of the IP type II
events. At lower frequencies the Galaxy decreases in intensity and the type
II bursts are more easily discerned - provided other solar activity is at a
minimum.
The characteristics of a type II burst at lower frequencies are shown in
Figure 2. In the figure we show intensities as a function of time for eight
frequencies for an event on May 16, 1981 and in this figure we use a linear
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I.,__ START:0825 UT \/
MAY 16, 1981 "TRANSIT=I080 km/s
.0246 8 I012 141618202223242628
TIME (HOURS)
Fig. 2, Intensity versus time profiles for eight of the ISEE-3 observing
frequencies. The type II emission is indicated by the stippling and the SA
event by hatching.
scale. The strong type III bursts (flat-topped intensifications seen on all
frequencies essentially simultaneously) have been chopped above a certain
intensity to enhance the much weaker type II emission. The SA event is
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indicated by diagonal hatching. The type II event, indicated by stippling,
is seen to drift through this frequency range (466 - 80 kHz) in about 25 hours.
For this event the transit velocity, i.e. the velocity determined from the time
interval between the start of the event at the sun and the sudden commencement,
was about i000 km/sec.
Figure 3 shows the profiles for another event whose transit velocity was
about 800 km/sec and it is seen that this event drifts through the same
frequency range more slowly. The drift rates of all the bursts studied show
this consistency with the derived transit velocities. This result substan-
tiates our identification of the flare associated with a particular sudden
commencement. Apart from a few isolated cases the transit velocities of the
shock studied are basically independent of flare longitude. This means that
to a first approximation the shocks expand isotropically.
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MAY 8,1981 i
I I I III I III I I II I I II I I III II III I I I I I II
0 2 4 6 8 I0 12 14 16 18 20 22 24 26 28 50 5254
TIME (HOURS)
Fig. 3. Intensity profiles for another event. This event has been
attributed to a slower shock than that associated with the event in fig.2 and
has a slower drift rate.
We have determined the height of a number of shocks as a function of time
by using an appropriate density model and assuming that the maximum of the
burst at each frequency is the time when the shock was at the appropriate
plasma level, i.e. coronal height. The occurrence of SA events before IP type
II bursts provides unambiguous identification of the start of the event at the
sun. The initial height of the shock at the commencement of the SA event is
essentially the height at which shocks produce meter wavelength type II
emission. This height is in the range 1.5 - 3 solar radii heliocentric
distance. The density model comprised a constant R -6 term and a variable R -2
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term. The R-6 term is negligible beyond about i0 solar radii. The amplitude
of the R-2 term was varied for different events so that a smooth height-time
curve was obtained, with the end points anchored by the start of the event at
the sun at 2 solar radii and the SC at the earth at 215 solar radii. Small
variations in the amplitude make little difference to the basic shapes of the
velocity curves, and thus for most events the same density scale was used.
Figure 4 shows data for the event of May 16 of 1981. We note that a data gap
at the beginning of the event covered the time interval when we expected to
detect the event at I and 2 MHz. The smooth curve is the spline function of
least curvature fitted to the data taking into account the accuracy to which
the maximum of the burst at each frequency can be estimated.
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MAY 16,1981
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TIME (HOURS}
Fig. 4. Deduced height versus time profile for an event. The smooth curve
is the spllne function of least curvature fitted to the data.
This technique has been applied to a number of events. Shock velocities
as a function of heliocentric distance were obtained by taking the first
derivative of the spline function fitted to the height-time data.
Results and Discussion
Figure 5 shows the velocity curves for four events. The events have been
organized as a function of transit velocity. The fastest event is at the top
left and the slowest event is bottom right. Ignoring for the moment the
profiles below about 20 solar radii one can see that for these events the
shocks accelerate over a distance ranging from about 60 to i00 solar radii.
The other events that have been analyzed have similar profiles. If the shocks
are being driven over the distances indicated by the acceleration region then
the data suggest that the slower shocks are driven further than the fast
shocks.
707
The upper right profile shows a large peak below 20 solar radii and this
was obtained from the 1 and 2 MHz data. Note that had 2 and 1 Nttz data points
been available we expect, based on our results for other events, that the upper
left event would also have had a sharp impulsive peak. Other events show this
peak. Thus velocities obtained from the high frequency data seem to be
inconsistent with the low frequency results. However, they are not
inconsistent with analyses of ground-based events which give typical velocities
of the order of 1000 to 1500 km/sec (Maxwell and Thompson, 1962). For some
events even greater velocities have been derived (Gergely, 1982). Our
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60 120 180
Fig. 5. Velocities as a function of heliocentric distance for four events.
The top figures (L-R) are for events commencing May 16 and May 8 1981. The
bottom figures (L-R) are for events commencing December II, 1978 and August 26,
1979.
analysis assumes that the density scale derived for the low frequency (<i MHz)
data could be extended smoothly to the high frequency (i and 2 MHz) data and
that the same mode of emission was occurring. If these assumptions are
incorrect certain conditions could result in the high frequency plasma levels
being placed too high in the corona and this would lead to high velocities.
Another explanation for our results is that the type II emission profile is
being incorrectly interpreted. However, the differences in appearance of the
type II emission in the two regimes suggests an interesting alternative which
is that there are two shocks. The shock responsible for the high frequency
emission could be a blast wave which dissipates in the low corona. The
increase in velocity deduced from the low frequency data implies a driven
shock. The driver would presumably be a mass ejection detectable as a white
light coronal transient. From the preliminary analysis to date there is a
good correlation between IP type II events and fast mass ejections as observed
by the P78-I coronagraph. Whatever the answer is it is clear that there are
708
differences between the low corona and the interplanetary medium as probed by
radio emission from shocks.
In summary, interplanetary shocks produce radio bursts with frequency drift
rates dependent on the shock velocities. Analyses of these drift rates indicate
that the shocks accelerate out to some distance (of the order of 80 solar radii)
and then decelerate. Some change in the properties of the corona or the typ_ II
emission occurs below 20 solar radii which remains to be explained.
References
Boischot, A., Riddle, A.C., Pearce, J. B. and Warwick, J. W. Shock waves and
type II radiobursts in the interplanetary medium, Solar Phys., 65, 397, 1980.
Cane, H. V., Stone, R. G., Fainberg, J., Stewart, R. T., Steinberg, J. L. and
Hoang, S. Radio evidence for shock acceleration of electrons in the solar
corona, GeoDhys. Res. Lett., _, 1285, 1981.
Dryer, M. Interplanetary shock waves: recent developments, SPace Sci. Rev., 17,
277, 1975.
Gergely, T. E. On the relative velocity of coronal transients and type II
bursts, Paper presented at the SCOSTEP-STIP symposium on solar interplanetary
intervals, Maynooth, 1982.
Gosling, J. T., Asbridge, J. R., Bame, S. J., Hundhausen, A. J. and Strong, I.
B. Satellite observations of interplanetary shock waves, J. GeoDhys. Res.,
/_i, 43, 1968.
Malitson, H. H., Fainberg, J. and Stone, R. G. Observation of a type II solar
radio burst to 37R , AstroDhvs. Lett.,R14 111, 1973.Malitson, H. H., Fain_erg, J. and Stone, w_. Hectometric and kilometric solar
radio emission observed from satel]ites in August 1972, Space Sci. Rev., lq,
511, 1976.
Maxwell, A. and Thompson, A. R. Spectral observations of radio bursts. II.
Slow-draft bursts and coronal streamers, J[_, I___, 138, 1962.
Roberts, J. A. Solar radio bursts of spectral type II, Aust. J. Phys., 12, 327,
!959.
Stewart, R. T. Source heights of metre wavelength bursts of spectral types i
and III, Solar Phys., 50, 437, 1976.
Wild, J. P. Observations of the high-intensity solar radiation at metre
wavelengths. II Outbursts, Aust. J. Sci. Res., A_, 399, 1950.
Wild, J. P. Some investigations of the solar corona: The first two years of
observation with the Culgoora radioheliograph, Proc. ASA., _, 365, 1970.
Wild, J. P. and McCready, L. L. Observations of the spectrum of high-intenslty
solar radiation at metre wavelengths. I. The apparatus and spectral types of
solar burst observed, Aust. J. Sci. Res., Ae, 387, 1950.
Wild, J. P., Smerd, S. F. and Weiss, A. A. Radio bursts from the solar corona,
Ann. Rev. Astron. _d AstroDhys., !, 291, 1963.
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Velocity profiles of interplanetary shocks

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