Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies
We report the results of UTR-2 radio telescope observations of an absorption burst at the decameter range. This event is associated with two consecutive M-class flares and happened after the second M-flare. The latter flare was accompanied by Type II, Type IV radio sweeps and a CME. The observed ban...
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Головна астрономічна обсерваторія НАН України
2005
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| Цитувати: | Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies / A.A. Konovalenko, A.A. Stanislavsky, E.P. Abranin, V.V. Dorovsky, V.N. Mel'nik // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 78-81. — Бібліогр.: 14 назв. — англ. |
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Digital Library of Periodicals of National Academy of Sciences of Ukraine| _version_ | 1859615159587176448 |
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| author | Konovalenko, A.A. Stanislavsky, A.A. Abranin, E.P. Dorovsky, V.V. Mel'nik, V.N. |
| author_facet | Konovalenko, A.A. Stanislavsky, A.A. Abranin, E.P. Dorovsky, V.V. Mel'nik, V.N. |
| citation_txt | Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies / A.A. Konovalenko, A.A. Stanislavsky, E.P. Abranin, V.V. Dorovsky, V.N. Mel'nik // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 78-81. — Бібліогр.: 14 назв. — англ. |
| collection | DSpace DC |
| container_title | Кинематика и физика небесных тел |
| description | We report the results of UTR-2 radio telescope observations of an absorption burst at the decameter range. This event is associated with two consecutive M-class flares and happened after the second M-flare. The latter flare was accompanied by Type II, Type IV radio sweeps and a CME. The observed bandwidth of the burst is about 20 MHz. The size of the absorption region is compared with the solar radius. The frequency drift rate of the absorption burst is about −0.12 MHz/s. The features of this event are discussed.
|
| first_indexed | 2025-11-28T18:22:39Z |
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ABSORPTION BURST IN THE SOLAR SPORADIC RADIO
EMISSION AT 10–30 MHz FREQUENCIES
A. A. Konovalenko, A. A. Stanislavsky, E. P. Abranin,
V. V. Dorovsky, V. N. Mel’nik
Institute of Radio Astronomy, NAS of Ukraine
4 Chervonopraporna Str., 61002 Kharkiv, Ukraine
e-mail: alexstan@ira.kharkov.ua
We report the results of UTR-2 radio telescope observations of an absorption burst at the decameter
range. This event is associated with two consecutive M-class flares and happened after the second
M-flare. The latter flare was accompanied by Type II, Type IV radio sweeps and a CME. The ob-
served bandwidth of the burst is about 20 MHz. The size of the absorption region is compared
with the solar radius. The frequency drift rate of the absorption burst is about −0.12 MHz/s.
The features of this event are discussed.
INTRODUCTION
The radio emission of solar activity strikes by the variety of its features. Probably, this is connected with
different emission processes and physical conditions. Important effects of solar activity are eruptive flares,
coronal mass ejections (CMEs), and global coronal waves. Their in situ detection is impossible, but the radio
emissions carry information on shocks, energetic electrons and their acceleration associated with the solar
activity. As is well known, these processes play a major role in the space weather having terrestrial effects.
So, CMEs are responsible for geomagnetic storms and disrupting the auroral particle belts. In view of space
weather implications, the solar radio bursts have acquired a renewed interest, particularly in combination with
observations at various wavelengths.
Among a great number of solar bursts observed there have been reported on the detection of absorption
events [1, 4, 5, 11–14]. They are poorly stated in the scientific literature due to their rarity. Absorption
features in solar radio spectra were firstly pointed out by Boischot et al. [1] based on single-frequency record.
Wild et al. [14] mentioned also solar radio bursts showing themselves in absorption. To study a fine structure of
the Type IV event on February 5, 1965, Malville and Aller [5] noticed an apparent absorption of the background
radiation. Kai [4] described the Type II event on August 23–24, 1968, in which an absorption U-fold burst was
visible at 80–120 MHz. Sometimes, the observed characteristics of such bursts resemble those of Type III radio
bursts. Therefore, Kai [4] called the absorption bursts by “shadow” Type III events. Nevertheless, this similarity
is enough illusory and deceptive. Slottje [13] reported sudden reductions in the intensity of a Type IV radiation
at frequencies about 250 MHz. The essential difference between the aforesaid observations and the absorption
bursts represented by Sastry et al. [12] from the observations at 34.15–34.5 MHz was the extremely low level
of underlying continuum. The observations were not confined to any particular type of bursts like Type IV,
Type III, etc. The results of Ramesh and Ebenezer [11] support the contrary point of view. There exists a close
temporal association between such absorption events and the corresponding flares accompanied by the onset
of a “halo” coronal mass ejection. The authors have concluded that coronal shock waves just give rise to
favourable conditions for beginnings of absorption bursts. Besides, it should be recognized that not any shock
wave causes the presence of absorption. Thus, the experimental features of the absorption bursts require a more
comprehensive study that would be undoubtedly useful in understanding this phenomenon, its mechanism of
initiation. Melrose [8, 9] conjectured that the absorption is the inverse of one of the familiar plasma emission
processes. However, the experimental results known for such bursts badly fall into a pattern of the plasma
theory describing the generation of Type III radio bursts from the Sun.
In this paper, we report a new observation of an absorption burst at decameter wavelengths. The observations
have detected first the absorption of sporadic solar radio emission at the upper corona. This did not succeed to
realize early. Moreover, the obtained results are really reliable. The capabilities of the UTR-2 telescope permits
to discern the quiet-Sun emission, the Galactic background radiation and sporadic features of the solar activity.
c© A. A. Konovalenko, A. A. Stanislavsky, E. P. Abranin, V. V. Dorovsky, V. N. Mel’nik, 2004
78
OBSERVATIONS
The radio data reported here were obtained on August 19, 2003 with the 60-channel spectrometer operating at
the UTR-2 radio observatory near Grakovo ([2] for details on the instrument, and [7] about the spectrometer).
The frequency range of these observations was 9–30 MHz. Recall that the plasma levels of 30 and 10 MHz
correspond to radial distances 1.78 and 2.94 of the solar radius, respectively, from the center of the Sun.
10
15
20
25
30 0
10
20
30
40
10
−20
10
−18
10
−16
Time, m
in
Frequency, M
H
z
P
ow
er
, W
/H
z
Absorption
Figure 1. Time profiles of the absorption burst on August 19, 2003. Time zero corresponds to 11:00:40 UT
On this day the solar activity was moderate with observing two M-class flares from Region 431 (S13,
L = 194) [3]. The first M-flare was impulsive (M2/1n) at 07:59 UT. The second M-flare (M2.7/2f) was at
10:06 UT with associated Type II burst (614 km/s), Type IV radio sweeps and a coronal mass ejection that
was not Earth directed. Each of these events was registered in the form of a sporadic decameter radio emission
by means of the UTR-2 telescope. Starting from 07:55:37 UT to 07:55:42 UT at 30–10 MHz the solar flux
density increases suddenly of 104 times with slow relaxing to a lower level. At 08:08:44 UT and 10:01:34 UT
the two decameter Type II bursts were observed. Next, about 10:40 UT the solar flux density begins again
slowly to increase almost at all frequencies of the observation. Finally, a sudden reduction in intensity was
started at 11:20 UT. The absorption event lasted until 11:26 UT. It characterizes by clear negative frequency
drift. Figure 1 shows the time profiles obtained with the 60-channel spectrometer in the range from 11:00:40 to
11:40:40 UT.
ANALYSIS AND RESULTS
The drift rate value of the absorption burst equals to –119±8 kHz/s at the frequency range 14–30 MHz. It is four
times more than the frequency drift rate associated with Type II burst for which it was about –0.03 MHz/s. This
shows that probably the absorber moved quicker than the shock wave generating the Type II burst. Moreover,
it follows from the spectrogram that the absorber ran to catch up with the shock wave. It should be noticed
that early the drift rate of an absorption burst was mentioned only in [4] (∼ –10 MHz/s at the single frequency
80 MHz). Let us estimate the radial velocity of the absorber traveled into the solar corona. Assume that
the bursts were conditioned by the plasma mechanism, i.e., the radio radiation is emitted/absorbed near the local
electron plasma frequency. As a density model of the corona, we use a special Parker solution [10]. Mann et al.
have shown its good agreement with observations for corona heights up to 5 AU [6]. Then the electron number
density Ne as a function of the radial distance r is expressed in the form
Ne(r) = Ns exp[As(Rs/r − 1)], (1)
As = 13.83, Ns = 5.14 · 109 cm−3, Rs is the solar radius. The frequency drift rate Df of a burst on a spectrogram
is connected with the radial velocity Vr of the corresponding source by the relation
79
Df =
df
dt
=
f
2
1
Ne
dNe
dr
Vr, (2)
where f is the frequency. Substituting the density model Eq. (1) into Eq. (2), the radial shock speed ∼ 640 km/s
is obtained. In this assumptions the absorber advanced with the velocity about 2 550 km/s. What object,
absorbing radio emission, can moved in the solar corona with such high velocity? It is not clear.
The maximum of absorption on the time profiles (Fig. 1) corresponds to the most reductions of emission
in intensity. In this connection the first crucial question to answer is just connected with the maximum of
absorption. How much is the solar radio emission absorbed? For that aim we compare the minimum power of
the absorption burst with the emission power of the quiescent Sun. It should be remembered that the quiet-
Sun flux varies considerably within a solar cycle. This is understandable, since the eruptive flares and CMEs
warm the corona in all times, and their intensity depends on time. According to the solar data, available
from NOAA [3], it has been known that on August 18, 2003 the solar activity was at low level. Therefore, for
our aims it was chosen the initial period of the observation records on August 19 before the first M-flare at
7:59 UT. We hope that by this time the corona had settled down more or less. The Sun moves 2.5′′ relative
to the background sky per minute; the UTR-2 array tracked the Sun, and the array beam covered almost
fully the solar corona at the corresponding plasma frequency. Strictly speaking, the emission contains not only
the proper emission of the quiescent Sun, but also the Galactic background emission. The two parts were not
separated specially. Nevertheless, their ratio may be estimated.
0 1000 2000 3000 4000 5000 6000 7000 8000 9000
10
1
10
2
10
3
10
4
10
5
Time in timer pulse counts
Lo
g−
sc
al
e
A
D
C
r
ea
di
ng
0 dB
42 dB
Noise
generator
level
Figure 2. Curve for calibrating a power scale in the radio observational data on August 19, 2003 at 25 MHz. The dashed
line corresponds to the absorption burst emission in minimum, the dotted line shows the sum level of the quiet-Sun
emission and the background emission, the dash-dotted line notices only the background emission level. The black heavy
line is the largest value of sporadic radio emission at 11:16:15 UT. ADC means an analog-to-digital converter
As a concrete example, we consider the observations of the absorption burst at 25 MHz. In this case
the absorption burst minimum in spectral power is 3.6 · 10−20 W/Hz, the galactic background plus the quiescent-
Sun level is 2.7 · 10−20 W/Hz, the proper galactic background level is 2.0 · 10−20 W/Hz. These power levels are
represented in Fig. 2, where for the convenience they are plotted on the calibration curve. In particular, the quiet-
Sun flux of 103 Jy is assumed as typical for the solar emission at 25 MHz, then the galactic background has
the temperature 26 400 K. In this figure it is clearly seen that all these levels lie within the limits of the calibration
curve, and thus their accuracy of measurement is beyond doubt. The calibration scale covers 42 dB with the step
of 4 dB from 0 dB to 20 dB, and next with the 2 dB step up to 42 dB. The minimum of the absorption burst
corresponds to 36.9 dB according to this scale, whereas the galactic background together with the quiescent-
Sun emission and the only galactic background had 38.1 dB and 39.6 dB, respectively. The similar situation
takes place for other observation frequencies. It should be emphasized that formerly such detailed analysis of
the emission components received by radio astronomical instruments for absorption events was not carried out.
80
For the sake of simplicity hereinafter the sum of the galactic background and the quiescent-Sun emission will be
called simply by the background emission. The results of the frequency comparison of the minimum power of
the absorption burst with the power of the background emission derived from the Sun clearly shows that the most
reductions of emission reached background levels on the corresponding frequencies, but they did not fall down
lower. The correlation coefficient between the background emission levels and the minima of the absorption
burst tends to 0.95, and their spectral index comes to –1.93±0.1. The latter is found in a good agreement with
the solar measurements fulfilled early by means of the UTR-2 telescope.
All this allows us to define the absorbing region size. As the almost full absorption was observed, the cross-
section of the region must be certainly more than the source size of the Type II burst at 11:20 UT. Though
the solar radiogeliograph did not take part in our observations, it has been known that, for example, at 80 MHz
the source size of Type II bursts comes to 12′×7′ [4]. At decameter wavelengths it will be still more. The si-
multaneous frequency band of the absorption burst exceeds 20 MHz that speaks about the longitudinal size of
the absorber. It was more than one solar radius. The absorbing region must be optically thick for the radiation
before absorption to be absorbed significantly.
SUMMARY
We observed a transient reduction of solar sporadic radio emission at decameter wavelengths on August 19,
2003. The observations have been stated that during the absorption the sporadic radio emission decreased
almost to a background emission level. The drift rate value of the absorption burst is more than the drift rate of
Type II solar bursts, but considerably less than one for Type III bursts at the same frequencies. The geometric
size of the absorber is comparable with the solar size. A more detailed analysis of this event will be carried out
elsewhere.
Acknowledgements. The work was supported by the INTAS call 2003.
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|
| id | nasplib_isofts_kiev_ua-123456789-79608 |
| institution | Digital Library of Periodicals of National Academy of Sciences of Ukraine |
| issn | 0233-7665 |
| language | English |
| last_indexed | 2025-11-28T18:22:39Z |
| publishDate | 2005 |
| publisher | Головна астрономічна обсерваторія НАН України |
| record_format | dspace |
| spelling | Konovalenko, A.A. Stanislavsky, A.A. Abranin, E.P. Dorovsky, V.V. Mel'nik, V.N. 2015-04-03T15:16:53Z 2015-04-03T15:16:53Z 2005 Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies / A.A. Konovalenko, A.A. Stanislavsky, E.P. Abranin, V.V. Dorovsky, V.N. Mel'nik // Кинематика и физика небесных тел. — 2005. — Т. 21, № 5-додаток. — С. 78-81. — Бібліогр.: 14 назв. — англ. 0233-7665 https://nasplib.isofts.kiev.ua/handle/123456789/79608 We report the results of UTR-2 radio telescope observations of an absorption burst at the decameter range. This event is associated with two consecutive M-class flares and happened after the second M-flare. The latter flare was accompanied by Type II, Type IV radio sweeps and a CME. The observed bandwidth of the burst is about 20 MHz. The size of the absorption region is compared with the solar radius. The frequency drift rate of the absorption burst is about −0.12 MHz/s. The features of this event are discussed. The work was supported by the INTAS call 2003. en Головна астрономічна обсерваторія НАН України Кинематика и физика небесных тел MS1: Decameter Radioastronomy Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies Article published earlier |
| spellingShingle | Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies Konovalenko, A.A. Stanislavsky, A.A. Abranin, E.P. Dorovsky, V.V. Mel'nik, V.N. MS1: Decameter Radioastronomy |
| title | Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies |
| title_full | Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies |
| title_fullStr | Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies |
| title_full_unstemmed | Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies |
| title_short | Absorption burst in the solar sporadic radio emission at 10−30 MHz frequencies |
| title_sort | absorption burst in the solar sporadic radio emission at 10−30 mhz frequencies |
| topic | MS1: Decameter Radioastronomy |
| topic_facet | MS1: Decameter Radioastronomy |
| url | https://nasplib.isofts.kiev.ua/handle/123456789/79608 |
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