Astron. Astrophys. 316, 413–424 (1996)
ASTRONOMY
AND
ASTROPHYSICS
Kilometre-wave radio observations of solar type III bursts
by Ulysses compared with decametre-wave observations
from the Earth
C.H. Barrow1 , S. Hoang2 , R.J. MacDowall3 , and A. Lecacheux4
1
2
3
4
Max-Planck-Institut für Aeronomie, D-37189 Katlenburg-Lindau, Germany
DESPA, Observatoire de Paris-Meudon, F-92195 Meudon Cedex, France
NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
ARPEGES, Observatoire de Paris-Meudon, F-92195 Meudon Cedex, France
Received 5 February 1996 / Accepted 5 April 1996
Abstract. Observations of solar type III bursts, made at kilometric frequencies by the radio receiver of the Ulysses Unified Radio and Plasma Wave (URAP) investigation, are compared with simultaneous observations, made at decametric frequencies at the Nançay Radio Astronomy Station of the ParisMeudon Observatory, during the period following the spacecraft
encounter with Jupiter until the south solar polar pass. Of the
events suitable for study, 57 can be identified in both frequency
bands having delay times consistent with prediction based upon
geometry and upon the frequency drift between the lowest frequency observed on the ground (25 MHz) and the highest frequency observed by URAP (940 kHz). The good agreement between calculated and measured delay times suggests that the
large delay anomalies, reported by Steinberg et al. (1984) sometimes to be in excess of 8 minutes in duration, may be confined
to frequencies below the 500 kHz limit studied by these authors
and not detectable at 940 kHz with the time resolution of the
URAP receiver obtained in this study. There are cases when
bursts recorded at Ulysses are not seen at Nançay. This may
just be due to the source at 25 MHz being beyond the solar limb
and so obscured from the Earth. There are other cases, however, when bursts recorded at the Earth are not seen at Ulysses.
Such cases do not appear to be correlated with either the EarthSun-Ulysses (ESU) angle or with the heliographic latitude of the
spacecraft. As the source region at 940 kHz is large and at a considerable distance from the Sun, some part of it will usually be
visible from the spacecraft no matter what the relative positions
of the Sun, the source and Ulysses may be; this suggests either
that there may sometimes be a low frequency cutoff, inherent to
the burst itself somewhere between 940 kHz and 25 MHz, or else
that the emission has somehow been occulted. Representative
examples are presented and discussed.
Key words: Sun: radio radiation – corona
Send offprint requests to: C.H. Barrow
1. Introduction
The general characteristics of type III bursts have been reviewed
by the Solar Radio Group Utrecht (1974) and by Suzuki & Dulk
(1985). The present paper compares type III bursts observed by
the Ulysses Unified Radio and Plasma Wave (URAP) investigation at kilometric frequencies with simultaneous observations
made at decametric frequencies at the Nançay Radio Astronomy
Station of the Paris-Meudon Observatory, during the period following the spacecraft encounter with Jupiter until the south solar
polar pass (Smith et al., 1995).
Comparative studies are important because they can provide
information concerning the directivity of type III emission, extending to frequencies not accessible from the Earth, and hence
give insight to coronal and interplanetary medium conditions
affecting propagation. There have been a number of comparative studies in the past; see, for example, Steinberg et al. (1984),
Dulk et al. (1985), Sawyer & Warwick (1987), Lecacheux et al.
(1989). Hoang et al. (1994) studied 16 type III bursts recorded
by URAP during 1990 and 1991, prior to the spacecraft encounter with Jupiter. All of these previous studies, however,
were confined to observations made by receivers close to the
plane of the ecliptic. The URAP observations reported here, on
the other hand, were made after the spacecraft had encountered
Jupiter and was travelling towards south solar polar pass, thus
providing the first opportunity to make comparisons over a wide
range of heliographic latitudes. Comparisons are also of interest
because, as Dulk (1990) has pointed out, there have been relatively few observations of solar radio emission between about
20 MHz, the low frequency limit of most observations from the
Earth, and about 2 MHz where, prior to the WIND mission,
most spacecraft observations commenced. Preliminary results
have been presented briefly elsewhere (Barrow et al., 1995).
It is generally agreed that type III bursts are generated when
electrons are accelerated in solar active regions and travel outward along open magnetic field lines through the solar corona
414
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
and into interplanetary space with speeds of some 0.1 to 0.5c.
As their radial distance increases, the electrons generate Langmuir waves at the local plasma frequency, fp2 = 81Ne , where
Ne is the electron density in m−3 and fp is in Hz. Some of
the Langmuir wave energy is converted into electromagnetic
radiation either at the fundamental frequency f = fp , the second harmonic f = 2fp , or both. Thus the type III bursts drift
from higher to lower frequencies as the electrons travel outwards through decreasing densities. Evidently, the source regions will be higher in the solar corona for lower frequencies.
For example, the plasma level corresponding to fp = 940 kHz
(the upper frequency limit of URAP) will be at about r ' 5R
= 0.0235 AU while for fp = 25 MHz (the effective lower frequency limit of the Nançay system for daytime observations,
because of interference) this level will be at about r ' 1.6R
= 0.0077 AU. However, Steinberg et al. (1984) found, from
ISEE-3 data, that the “apparent” source heights at the lowest
frequencies, below 500 kHz, are considerably higher (2 to 5
times) than the corresponding plasma levels while Bougeret et
al. (1984a), from Helios data, proposed that fp ∼ f /3.5. If
this relation holds above 500 kHz, it would place the 940 kHz
source at about r ' 8R , comparable with the value quoted
by Dulk (1990) of about r ' 10R at 1 MHz. Similarly, the
frequency drift-rate implied by the Alvarez & Haddock (1973)
equation, given in Sect. 3, would put the 940 kHz source at some
r ' 10R if the electron streams rise at a rate of 0.2 c.
Steinberg et al. (1984) also found anomalies of up to 500 s
in the delays between type III emission at frequencies between
40 and 500 kHz, observed simultaneously by ISEE-3 and by
Voyager. These anomalies, they conclude, are probably due to
propagation effects in the interplanetary medium. Further comparison of ISEE-3 and Voyager observations (Dulk et al., 1985)
revealed that type III bursts recorded at kilometric wavelengths
by Voyager were usually visible to the ISEE-3 receiver no matter where the source was located, either in front of or behind the
Sun. It should be noted, however, that the ISEE-3 experiment
was considerably more sensitive than the Voyager experiment
(Lecacheux et al., 1989).
2. Antennae/receivers
The URAP experiment has been described in detail by Stone et
al. (1992a). The spacecraft approached Jupiter from the plane
of the ecliptic, passing briefly into an extreme northerly jovicentric declination before entering a southerly declination close
to −38◦ after encounter and thence proceeding to south solar polar pass in June-October, 1994. Receivers covering two
bands, from 1.25 to 48.5 kHz (lo-band) and from 52 to 940 kHz
(hi-band), provided the opportunity of investigating the characteristics of solar and jovian radio emission beyond the possibilities provided by Voyager where all observations were made
from close to the ecliptic plane. Hi-band operates in 12 channels, approximately logarithmically spaced, each frequency being determined by one of twelve crystal local oscillators. The intermediate frequency (IF) amplifier frequency is 10.7 MHz, the
dynamic range about 70 db and the bandwidth 3 kHz. Lo-band
operates in 64 channels, arithmetically spaced. The IF amplifier
frequency is 432.25 kHz, the dynamic range about 70 db and the
bandwidth 750 Hz. The receivers are connected to a 72 m wire
antenna perpendicular to the spacecraft spin axis and to a 7.5 m
monopole antenna along the spin axis. The spacecraft and the
antenna system spin with a 12-s period. The inputs from the antennas can be combined to synthesize an equivalent dipole tilted
with respect to the spin axis. By combining the inputs with suitable phase differences the polarization of the incoming waves
can be obtained. The sensitivity, when used in the separation
mode (Stone et al., 1992a), is about Smin ' 10−21 W m−2 Hz−1 .
In the summation mode the effective sensitivity is down by about
10 db.
The source direction, intensity and polarization are determined from Fourier analysis of the spin modulated signals obtained by summation, alternately in phase and in quadrature,
of the equatorial and the axial antenna outputs (Manning &
Fainberg 1980). Direction finding is possible if the source intensity varies slowly compared to the spin period.
The Nançay Decametre Array instrument (NDA) has been
described in detail by Boischot et al. (1980). The antenna array
consists of 144 broad-band conical spirals, 72 in each polarization sense, with the cone axes tilted 20◦ south in the plane
of the meridian. The measured gain is about 26.5 db in each
polarization, decreasing somewhat below 30 MHz but essentially independent of frequency above 30 MHz. By phasing the
inputs, the array can be steered within the main lobe of the spirals so that a source can be followed for some ±3.5 h around
meridian transit with very little change in gain. The effective
bandwidth is about two octaves for declinations close to the
plane of the ecliptic and can be selected anywhere between 10
and 120 MHz. During the daytime, interference is prohibitive
at the lower frequencies and solar observations are usually conducted over the band 25 to 75 MHz. Each period of observation is recorded as two separate routine dynamic spectra in leftand right-hand polarization. Other receivers may also operate
from the same antenna simultaneously. Generally, Jupiter observations take priority over solar observations and so, at certain
times of the year when Jupiter is close to conjunction, the solar
observing period may be quite short. The overall sensitivity at
25 MHz is about 10−23 W m−2 Hz−1 .
3. Observations
If a type III burst is observed at both Ulysses and at the Earth,
a delay is to be expected between the two observations which
will be the sum of the delay due to the difference in distances
of the two receivers from the source and the delay due to the
difference in frequency of the two observations. Thus, the Predicted Total Delay = Distance Delay + Frequency Delay. The
former is purely geometric and easily calculated. The latter has
been given by Alvarez & Haddock (1973), from a review of experimental observations covering a wide range of frequencies,
as:
df
= −0.01f 1.84
dt
(1)
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
415
Fig. 1. Type III event recorded on June 6, 1992, first at the Nançay Radio Astronomy Station and then 37.1 min later at Ulysses when the
spacecraft was at heliographic latitude 11.9◦ S
where df /dt is the frequency drift-rate in MHz s−1 . Thus, for a
difference in distance of 4 AU and the drift in frequency from
25 MHz down to 940 kHz, we would expect a delay of about
35 minutes. An example of this is shown in Fig. 1 where the
difference between the measured and predicted time of commencement is about one minute. Note that here and in other
figures the time scales on the Nançay and the URAP spectra
run in opposite directions. Also, the Nançay observing period is
given at the left-hand side of each spectrum. Only one polarization sense (the one most free of interference) is shown as type
III bursts are generally unpolarized.
Event onset times were measured from the dynamic spectra
with a reading accuracy of about 3 min for the URAP spectra
and, for most of the Nançay spectra, about 2 s, apart from a few
cases where the data were not archived on computer disc and
could not be expanded. The source is assumed to be at the centre
416
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
all three possibilities occurred with a few days of each other
during the second half of July, 1994. The approximate ESU
configuration during July and August, 1994, is shown in Fig. 2a.
The event shown in Fig. 1 occurred when the spacecraft was
at a fairly low southerly heliographic latitude. However, it was
also possible for bursts to be identified at both receivers when
Ulysses was at high southerly heliographic latitudes and an example of this is shown in Fig. 2b. Some general statistics for the
57 events identified in both spectral bands are shown in Fig. 3.
It can be seen that the delays are remarkably consistent with the
prediction above. There is no obvious indication of either ESU
angle or heliographic latitude influencing the delays within the
accuracy of the measurements. The normalised delay difference
is defined, in min/AU, as:
(Measured delay − Calculated Delay)/(Ulysses to Sun
distance − 1)
Fig. 2a. Approximate Earth-Sun-Ulysses configuration during July and
August, 1994. The Sun is at the origin of the coordinate system, with
the Earth on the X-axis, the Y-axis in the plane of the ecliptic and the
Z-axis perpendicular to the plane of the ecliptic. The observation point
is at infinity, 20◦ from the X-axis (towards the positive Y-axis) and 20◦
above the plane of the ecliptic
of the Sun but this is not unreasonable as the difference in travel
times of the two frequencies, due to different source heights in
the solar corona, is of the order of seconds only.
The period studied was from April 8, 1992 through October
25, 1994. A list was compiled of the 114 type III bursts which
occurred when both receivers were operating simultaneously.
Of these, 12 were eventually discarded as being inadequate in
some respect leaving a total of 102 events. This latter list contained 57 clearly defined, isolated, bursts which could be identified unambiguously at both receivers with delays comparable
to the predicted values. An example is shown in Fig. 1. An arbitrary criterion of ±10 min was used to specify these 57 bursts
although, in fact, the differences (measured delay - calculated
delay) were found to be considerably smaller than this with an
average value of 0.7 min and standard deviation, σ, of 1.4 min.
Only three events were found to have delay differences greater
than 1.96σ.
There were other events where type III activity was only
received at one or other of the two receivers. To demonstrate
further the non-reception at Ulysses, we also included, in this
second list, one or two instances where groups of type IIIs were
observed at Nançay but not by Ulysses. Nine events have been
selected from the total of 102 as representative. These are listed
in Table 1 and are discussed in detail below. It can be seen that
As the calculated delay is largely dependent upon the distance of Ulysses from the Sun, plots 3(a) and 3(d) are very similar. The events outlined above are interesting in that the predicted
delays are so close to the measured delays even when the spacecraft is at high southerly heliographic latitudes or when the ESU
angle is large. The good agreement between the predicted and
the measured delays suggests that the delay anomalies, reported
by Steinberg et al (1984) sometimes to be in excess of 8 minutes, may be largely confined to the lowest frequencies, below
the 500 kHz limit studied by these authors. As these anomalies,
according to Lecacheux et al. (1989), are approximately proportional to f −1 , smaller delay differences may not be detectable at
940 kHz with the measuring accuracy available for the URAP
spectra.
There are a number of other cases, however, when bursts
were recorded at Ulysses but not at Nançay and vice-versa.
There is no obvious pattern to these, so far, and therefore the
present discussion will be confined to the representative examples listed in Table 1. Each event is clearly defined and all of the
Earth-based observations are confirmed by listings at adjacent
frequencies in the monthly reports of “Solar-Geophysical Data”
(SGD) and/or by the daily observations made by the Nançay
Radio Heliograph (NRH) at 164 MHz. Flare activity was also
compared and some, but not all, of the events listed could be
associated with an Hα or an X-ray flare, as shown in Table 1.
The four cases when an event could be identified at both
Ulysses and Nançay, were observed when Ulysses was at heliographic latitudes ranging from some 12◦ to 80◦ S. The five other
events when the emission was observed at one or other receiver
only were as follows:
1. March 6, 1993 (Fig. 4): The NDA spectra showed a type III
storm from about 10:26 to 13:18 UT while NRH reported a
noise storm at 164 MHz from 09:00 to 15:10 UT and beyond,
centred on heliographic position EW −0.48 and NS 0.09 (i.e. in
the NE solar quadrant). This same activity was reported in SGD
as intermittent type III plus continuum from 09:16 to 12:00 UT
and as continuum subsequently. Storm type III bursts, like other
type III bursts, can sometimes extend to very low frequencies
and have been detected at hectometre and kilometre wavelengths
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
417
Table 1. List of selected type III events (Ulysses 940 kHz, Nançay 25 MHz) which represents all the possibilities to appear from the present
comparative study, where: HELC DIST is heliocentric distance, HELG LAT is heliographic latitude, SOL LONG is the ESU angle projected
on the Sun’s equatorial plane with the current Earth-Sun line representing zero longitude, NRH POS are EW and NS heliographic position
coordinates. The flare data refer to the flare closest in position and time to the radio event where Hα LAT and CMD are, respectively, average
heliographic latitude and average heliographic central meridian longitude, and NOAA REGION is the serial number of the active region in
which the flare occurred
DATE
YMD
ESU
ANG
(Deg)
HELC
DIST
(AU)
HELG
LAT
(Deg)
SOL
LONG
(Deg)
ULYS
BEGIN
(UT)
DECAM
BEGIN
(UT)
920606
930306
930520
940719
940721
940722
940727
940806
940910
96.5
19.6
77.3
98.1
98.5
98.7
99.5
100.6
99.9
5.36
4.92
4.71
2.68
2.67
2.66
2.63
2.56
2.33
-11.9
-26.4
-31.0
-73.6
-73.9
-74.1
-74.8
-76.4
-80.1
263.3
356.3
299.2
257.0
255.9
255.3
253.2
249.6
253.8
1153
None
1203
1203
None
1216
0951
None
1040
1116
1026-1318M
1130
1148
1023
0858-1345N
0857-1325N
1135-1249M
1027
Nobs
None
M
N
S
X
m
t
DELAY
PRED
MEAS
(Min)
(Min)
38.1
34.4
32.7
15.8
15.8
15.7
15.4
14.8
12.9
37
33
15
13
NRH POS
EW
NS
(R )
(R )
Nobs
-0.48
Nobs
None
None
None
None
-0.23
1.15
Nobs
0.09m
Nobs
None
None
None
None
0.01m
-0.48t
BEGIN
(UT)
1113
1026
None
1151
None
None
None
1007-1311S
1025X
Hα FLARE
LAT
CMD
(Deg)
(Deg)
N10
S08
S12
N05
-
E19
E35
E60
E27
-
NOAA
REGION
7186
7440
7758
7762
7773
No observation
No event
Many bursts within the period
No activity within the period
Several flares within the period
X-ray flare
Mean heliographic position
Burst position at 1027 UT
by Fainberg & Stone (1974) and by Bougeret et al. (1984b). In
the present decametric storm, a section of which is shown in the
Figure, it can be seen that some of the bursts cut-off a little above
the NDA low frequency limit of 25 MHz while others extended
below this frequency. The type III activity may be associated
with several Hα flares close to E35, S08, NOAA Region 7440,
beginning 09:59 and continuing until 11:25 UT. None of this
activity was recorded at Ulysses, however, even though the ESU
angle (19.6◦ ) and the spacecraft heliographic latitude (26.4◦ S)
were quite low. There is no obvious reason for this unless all of
the emission cut off somewhere above the upper frequency of
the URAP hi-band, 940 kHz, and below the lower frequency of
NDA, 25 MHz. The type III recorded by Ulysses close to 09:00
SCET, predicted delay 34.4 min, would have been outside the
NDA observing period.
2. July 21, 1994 (Fig. 5): A single intense type III recorded at
Nançay at about 10:23 UT was not observed by Ulysses at heliographic latitude 73.9◦ S. No Hα or X-ray flares were reported
close to the time of the event. Direction finding data is not available for this day. Sources at low frequencies are so large and so
far from the Sun that some part of the 940 kHz source would
almost certainly have been visible from Ulysses which was then
at a distance from the Sun of 2.67 AU. The fact that this type
III was not recorded at the spacecraft indicates either that the
emission was somehow occulted or directed away from Ulysses
or else that the burst cut off at some frequency between 25 MHz
and 940 kHz.
3. July 22, 1994 (Fig. 6): A single intense type III recorded by
Ulysses at 12:16 SCET from heliographic latitude 74.1◦ S was
not observed at Nançay or reported by any station in SGD. The
only flares reported, in both Hα and X-ray, occurred some twoand-a-half hours later and could hardly have been associated
with this event. For the relative positions of the Sun, the Earth
and Ulysses at the time of the type III burst, it is difficult to see
why this event was not observed at the Earth unless the source
was behind the limb of the Sun, seen from the Earth, or the burst
started at some frequency below 25 MHz.
4. July 27, 1994: A single intense type III recorded by Ulysses at
09:51 UT from heliographic latitude 74.8◦ S was not observed at
Nançay or reported by any station in SGD. An Hα flare at 06:29
to 0635 UT (E58, S03, NOAA region 7759) was the only reported optical activity. Direction finding from Ulysses suggests
a source of some 50◦ angular size, as seen from the spacecraft,
and located about 30◦ W from the spacecraft-Earth line; this
could mean that the source centre was behind the solar limb
with respect to the Earth and invisible at 25 MHz and higher
frequencies.
5. August 6, 1994 (Fig. 7): Numerous type IIIs, recorded at
Nançay from about 11:35 to 12:49 UT, were not observed by
Ulysses at heliographic latitude 76.4◦ S. SGD reported intermittent type III activity from about 11:40 until 14:20 UT. NRH
reported a noise storm at 164 MHz, centred on heliographic position EW −0.23, NS −0.01, that is on the solar equator a little
to the east of the central meridian. A number of Hα flares were
reported between 10:07 and 13:11 UT (E27, N05, NOAA region 7762) with the largest occurring at 12:45 UT. The type
IIIs recorded by NDA all extended below the 25 MHz low frequency limit of the NDA system. There was no trace of any
activity recorded by URAP, however. As in the case (3), above
(July 21, 1994), this may indicate that the emission was some-
418
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
Fig. 2b. Type III event recorded on July 19, 1994, first at the Nançay Radio Astronomy Station and then 15.0 min later at Ulysses when the
spacecraft was at heliographic latitude 73.6◦ S
how occulted or directed away from Ulysses or else that a low
frequency cutoff occurred between 25 MHz and 940 kHz.
4. Discussion
Four of the events listed in Table 1 show delays between reception at 25 MHz at the Earth and at 940 kHz at Ulysses which are
in good agreement with predicted values, based simply upon geometry and frequency drift as outlined in the previous Section.
These four events are representative of the 57 events in the list
compiled for the period preceding south solar polar pass. The
statistics of these 57 events are shown in Fig. 3 and it is evident
that the agreement between measured and predicted delays is remarkably good, even when the spacecraft is at high heliographic
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
419
Fig. 3a–d. Statistics of the 57 events identified in both spectral bands. a Measured delay against Sun-Ulysses distance. The straight
line represents the predicted delays for the
same distances. b Normalised delay difference against ESU angle. c Normalised delay difference against heliographic latitude.
d Measured delay against calculated delay.
The straight line is a least squares fit to the
data
latitude or when the ESU angle is large. Lecacheux et al. (1989)
suggest that the time delay anomalies, previously reported by
Steinberg et al. (1984), are approximately proportional to f −1
and so scale linearly with distance from the Sun. If this proportionality holds above the 500 kHz limit studied by both of
these groups then delay anomalies of about 2 min might be expected at 940 kHz. As we have seen, however, only three delay
differences were found to be greater than 1.96σ from the mean
value; these delay differences had values of 2.9, 3.9 and 4.3 min.
Thus, as far as we can tell with the present measuring accuracy, it
seems that longer anomalous delays are predominantly an effect
observed at lower frequencies, below about 500 kHz, consistent
with the f −1 relationship given by Lecacheux et al. (1989).
As far as the other five events are concerned, it must be
remembered that at kHz frequencies a type III source region
is so large and so far from the Sun that some part of it will
usually be directly visible from Ulysses, no matter what the relative positions of the Sun, the source and the spacecraft may be.
Steinberg et al. (1984, 1985) showed that the angular size of the
type III sources, observed from ISEE-3, increases considerably
with both elongation and decreasing frequency, ranging from
about 30◦ to 70◦ at 100 kHz as scattering effects broaden the
primary source. This is not the case for Earth-based observations at decametre-wave frequencies, however. Thus, if activity
is observed at Ulysses but not at the Earth this may be simply an
effect of geometry. If, however, the emission is observed at the
Earth but not at Ulysses we must consider other explanations.
The higher sensitivity of the NDA system must be partially responsible for this but not every case can be explained in this
way. It is very unlikely, for example, that within a period of intermittent type III activity there is no single burst with sufficient
intensity to be recorded by URAP. The event of March 6, 1993,
indicates that this is not simply an effect of large ESU angle or
of high heliographic latitude of Ulysses.
The visibility of the radio source may depend upon whether
the interplanetary (IP) type III emission (frequencies below a
few MHz) is fundamental or harmonic. This is because fp emission occurring on the far side (with respect to Ulysses) of a
volume containing this and larger plasma frequencies would
be occulted by the volume as the fp emission is, effectively,
emitted at the surface of the volume. The effect would be enhanced by refraction of the radio waves propagating into the
denser regions of the volume, which would direct them away
from the observer. 2fp emission is far more likely to travel from
one side of the occulting volume to a detector on the other side
because this emission is generated far from the surface of the
occulting volume. The URAP investigation has shown that IP
type III emissions can occur at both fp and 2fp (Reiner et al.,
420
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
Fig. 4. Numerous type IIIs recorded at Nançay, March 6, 1993, 10:26 to 13:18 UT, were not observed by Ulysses at heliographic latitude 26.4◦ S
when the ESU angle was 19.6◦ . SGD reported intermittent type III plus continuum from 09:16 to 12:00 UT
1992; Hoang et al., 1994). Furthermore, studies based upon
observations made close to the plane of the ecliptic, such as
those of MacDowall (1983), Dulk et al. (1985), Sawyer & Warwick (1987), and Lecacheux et al. (1989), have demonstrated
that a sensitive radio receiver can often detect emission from a
moderately intense IP type III burst occurring anywhere in the
interplanetary medium, even if the emission is behind the Sun
with respect to the observer. This result, explained in terms of
scattering by both small- and large-scale structures, produces
an effective directivity pattern for the bursts which is very wide
(Lecacheux et al., 1989).
It is quite possible that some type IIIs bursts display a low
frequency cutoff somewhere between 25 MHz and 940 kHz, perhaps because the decametric type III burst has no interplanetary
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
421
Fig. 5. A single type III recorded at Nançay, July 21, 1994, 10:23 UT, was not observed by Ulysses at heliographic latitude 73.9◦ S
continuation, that is the electrons do not continue out sufficiently
far into the interplanetary medium to generate the 940 kHz emission. As the WIND spacecraft can make radio observations from
20 kHz up to 14 MHz, with an instrument of similar sensitivity
and design to URAP, this should allow direct observation of the
phenomenon. It is also possible that the non-detection of events
by URAP results from some effect of enhanced directivity or a
narrowing of the effective beam pattern although this seems
rather unlikely. It should be noted, however, that McComas
et al. (1995) report both small-scale compressional and noncompressional pressure balance structures observed at higher
heliographic latitudes, 36◦ to 76◦ . The density enhancements
are less than those typically observed close to the plane of the
ecliptic and large scale-effects are less pronounced because the
422
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
Fig. 6. A single type III recorded by Ulysses, July 22, 1994, 12:16 UT, at heliographic latitude 74.1◦ S, was not observed at Nançay or by any
other reporting station in SGD. The dark band across the Nançay spectrum is due to calibration
solar wind speed is more constant than at higher latitudes. Nevertheless, the source-observer geometry may still be affected.
In the opposite situation, where IP type III bursts are observed from sites that should be visible from the Earth, but
where no decametric type III bursts are observed, it may be that
the decametric emission is occulted by some overdense region.
This could take place at a sector boundary where (Schwenn,
1990 and references therein) density enhancements of an order
of magnitude or more above the background level close to the
plane of the ecliptic can occur. Similar phenomena are observed
for certain IP type III bursts, which may only become visible
at frequencies below a few hundred kHz. Such structures might
prevent decametric radio emission from propagating in a certain direction although, in this case, higher frequency (metric)
emission might still be expected sometimes at the Earth and this
has not been observed in any of the events reported here. It is
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
423
Fig. 7. Numerous type IIIs recorded at Nançay, August 6, 1994, 11:35 to 11:59 UT, were not observed by Ulysses at heliographic latitude
76.4◦ S. SGD reported intermittent type III activity from 11:40 UT onwards. The dark band across the Nançay spectrum is due to calibration
also possible, of course, that the type III burst started at some
frequency between 25 MHz and 940 kHz and so could not be
observed at Nançay.
5. Conclusion
Type III bursts observed by URAP at kilometric frequencies
have been compared with simultaneous decametre-wave observations made at the Nançay Radio Astronomy Station of the
Paris-Meudon Observatory, during the period following Ulysses
encounter with Jupiter until the south solar polar pass. Of the
102 events studied, 57 isolated bursts can be identified in both
424
C.H. Barrow et al.: Kilometre-wave radio observations of solar type III bursts by Ulysses
frequency bands having delay times consistent with prediction
based upon geometric and frequency drift considerations. Calculated and measured delay values are in good agreement. This
suggests that longer anomalous delays are predominantly observed below 500 kHz, as implied by the f −1 relationship suggested by Lecacheux et al. (1989).
There are 11 cases when bursts recorded at Ulysses were not
seen at Nançay. This may simply be a geometrical effect, with
the 25 MHz source behind the solar limb as seen from the Earth,
it may be due to occultation by some density enhancement or
the burst may have started at some frequency between 25 MHz
and 940 kHz.
There are 34 cases when emission was recorded at Nançay
but not at Ulysses. Some of these must be due to the greater sensitivity of the NDA, of course, but they cannot all be explained
in this way. Some part of the lower frequency 940 kHz source
will usually be visible from Ulysses no matter what the relative
positions of the Sun, the source and the spacecraft may be. As
such effects do not seem to be dependent upon either ESU angle or heliographic latitude we can only speculate that the kHz
emission has been occulted by a dense region, as outlined in the
previous section, or else that a low frequency cutoff, inherent
to the burst itself, has occurred between 25 MHz and 940 kHz.
Certainly, this region of the spectrum has hardly been explored,
generally being too low in frequency for Earth-based observation but above the frequencies studied by most spacecraft in the
past. The WIND mission should provide interesting information on this point. The observations by Dulk et al. (1985) show
that geometry is unlikely to be responsible. The other possible
explanation, that the type III emission at 940 kHz may occasionally be rather more directive than has previously been suspected,
also seems unlikely.
Acknowledgements. We thank P. Zarka and L. Denis for generous help
with the survey of the decametre-wave observations made at the Nançay
Radio Astronomy Station; also M. Pick and L. Klein for access and an
introduction to the catalogued solar observations made at metre wavelengths by the Nançay Radio Heliograph Group and G. Mann for a
list of comparative solar radio observations made at the Observatorium für Solare Radioastronomie, Tremsdorf. URAP is the collaborative effort of four institutions, NASA Goddard Space Flight Center,
Observatoire de Paris-Meudon, Centre de Recherches en Physique de
l’Environnement Terrestre et Planétaire and the University of Minnesota. The Principal Investigators are R. G. Stone and R. J. MacDowall.
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