A&A 425, 281-285 (2004)
F. Y. Xu1,2 - H. A. Wu1,2
1 - Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China
2 - National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, PR China
Received 16 October 2002 / Accepted 31 March 2004
A rare solar radio V-shaped structure was observed on August 25, 1999 by the 4.5-7.5 GHz spectrograph of the Purple Mountain Observatory, China. It consists of a reverse drift component followed by a normal drift one, which seems to be an unusual frequency drift event because of the jump of reverse drift rate from 3.22 to 1.53 GHz/s and the fastest drift rate of -3.53 GHz/s. According to their different drift rates, the V-shaped structure can be divided into three parts. By the consideration of plasma emission of an electron beam, a three-component solar atmospheric model with large and small scale lengths , and to describe the equilibrium solar atmosphere and local inhomogeneity as well as the energy loss of the electron beam due to electron-electron collision is proposed. The local scale lengths , cm and c, c at Parts 1 and 2 of the reverse drift component are obtained. The variations of scale length of and , especially , could be responsible for the jump of drift rate. The normal drift rate would be interpreted as the increase of average energy of the electron beam at the mirror due to the decrease in electrons of lower energy via energy loss of collision. The three-component solar atmospheric model and the energy loss of the electron beam are discussed.
Key words: sun: flares - Sun: radio radiation
Earlier observations of solar radio type III bursts were made mainly in meter/decimeter wavebands, and their properties were characterized by narrow bandwidth, frequency drift and harmonic structure etc. (Suzuki & Dulk 1985). The characters of narrow bandwidth and frequency drift suggest that they may originate from a coherent emission mechanism such as from local plasma emission at its characteristic frequency, , excited by a source moving upward/downward along the field line. The exciting Langmuir wave would be converted into fundamental or harmonic escaping radiation at characteristic frequencies , by scattering or mode-mode coupling processes (Zheleznyakov 1970; Melorose 1980). The frequency of escaping radiation follows the variation of exciting local plasma frequency which decreases/increases with time as the electron beam moves upward/downward, and leads to a detectable normal/reverse drift type III burst. Such a picture is still accepted today for type III or type III-like bursts.
Since the 1980s, the surveys of solar radio type III bursts have been conducted in microwave bands, such as 0.1-3 GHz, 4-8.5 GHz and 6-8.5 GHz (Stähli & Benz 1987; Alloart et al. 1999; Bruggmann et al. 1990). The observed characters of microwave type III (III) bursts are similar to those of meter/decimeter type III bursts except that the drift rate of type III bursts is almost two orders of magnitude faster than that of meter/decimeter type III bursts, and also the dominant drift direction is a reverse drift in the microwave band but normal one in the meter and decimeter waveband. What electron density is required for a type III burst? For example, in the range of 4.5-7.5 GHz, the required electron density is about to if harmonic emission is assumed. This probably corresponds to the region between the lower corona and higher chromosphere. Because type III and type III-like structures may be the reflection of the electron beam trace or magnetic field configuration, the type III and type III-like structures in the range of 4.5-7.5 GHz may play an important role in plasma diagnosis in the transition region.
In this paper a rare type III-like structure, a V-shaped structure, observed on August 25, 1999 will be presented. The variation of reverse drift rate will be analyzed by means of the larger-scale inhomogeneity of the plasma density distribution in the solar atmosphere. The different effects of drift rate on both reverse and normal drift components as well as the parameters associated with the source region and environment are discussed.
The solar flare (1N/M 3.6) on August 25, 1999 occurred near the
region of NOAA 8673 and 8674. The accompanying radio burst
consists of three Parts I, II and III (Fig. 1), and contains rich
fine structures. To search for fine structures, especially for
faint fine structures, the dynamic spectrogram corresponding to
Part I was enlarged interval by interval, and its chromatic scale
was also adjusted until the contrast between emissions of faint
fine structure and background was large enough. Two rare V-shaped
structures located in the interval of 01:34:44.396-01:34:44.960 UT
and frequency range of 5.57-6.54 GHz were detected, and their
spectrogram together with the partial enlargement of the 2nd
V-shaped spectrum as well as the time profile are given in Figs. 2a,
b and c. As seen from Fig. 2a, the 1st V-shaped structure is a
faint featureless structure and is due to an electron beam moving
downward along a magnetic loop then mirroring upward. The 2nd
V-shaped structure, however, is an unusual one. The 2nd one began
at 01:34:44.480 UT with a starting frequency of 5.57 GHz, then
drifted downward to the frequency 6.01 GHz with a reverse
drift rate 3.22 Ghz/s. It then drifted downward to the turnover
frequency 6.26 GHz. In this latter part, however, the reverse
drift rate was about 1.53 GHz/s and was slower than that of the
initial part. From the turn-over frequency, the normal drift
component began with the fastest drift rate of -3.53 GHz/s which
led to an increase of the instantaneous bandwidth after the change
of frequency drift. Such a feature will be discussed in Sect. 3.
The variation of drift rate is a rare phenomenon.
|Figure 1: Time profile of the solar radio event on August 25, 1999. It consists of three bursts, complex, strong impulse and weaker complex burst, denoted by I, II and III, respectively.|
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|Figure 2: Dynamic spectrogram and corresponding time profile of V-shaped structures with time resolution of 5 ms and frequency resolution of 10 MHz. a) Spectrogram of two V-shaped structures. The 1st V-shaped structure located at frequency range of 6.01-6.6 GHz is a weak and featureless structure but the 2nd is an unusual one with a jump of drift rate on the reverse drift component and a faster normal drift rate (compared with the reverse drift rate). b) The partial enlargement of the 2nd V-shaped structure, which could show the jump of drift rate clearly, Note that there is a strong diffusion in the frequency range of 5.90 to 6.26 GHz (only the higher frequency of 6.18 GHz is shown here). c) Corresponding time profile of V-shaped structures at selected frequency. The 2nd V-shaped structure could be seen still. Also note that in the frequency range of 5.9 to 6.26 GHz the peak of drift structure emission is superposed to form a continual enhancement emission, which may be caused by diffusion due to deflected electrons of lower energy.|
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Benz et al. (2001) stated that some artificial effects may produce a spurious symmetric pattern in the solar radio spectrogram. Therefore, a series of methods has been adopted to avoid the origin of spurious spectra during the design of the solar radio spectrometer at 4.5-7.5 GHz. More details about this technology have been reported by Yao et al. (2002). The V-shaped structure does not seem to be produced by an artificial effect due to simultaneous observations, because the simultaneous observations of the V-shaped structures were independently obtained in the same interval and frequency range by two spectrometers located at two observatories (PMO and NAO) separated by a distance of over one thousand km. Therefore, the V-shaped structure presented here has a solar origin and hence is reliable.
Under the assumption of an isothermal atmospheric model, the
electron density exponentially decreases with increasing height
above photosphere i.e.
drift rate of type III or type III-like burst may be written as
The electron density in the microwave source region, however, is
two orders of magnitude higher than that in the meter or decimeter
source region. Thus, energy loss of the electron beam and hence
the corresponding velocity decrease via electron-electron
collision must be taken into account. For an average electron of
energy E, the energy loss rate then may be given by
|Figure 3: Model of a three-component atmosphere. The density distribution in the magnetic loop is characterized by the scale length , while the inhomogeneity is described by two small local scale lengths and .|
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The solar atmosphere is not in equilibrium during a flare and thus an isothermal atmospheric model seems to be invalid. Based on the two-component model suggested by Aschwanden & Benz (1986), a similar three-component model described by three exponential parts with differing scale lengths (see Fig. 3) will be assumed. The first component with a large scale length represents an isothermal atmospheric model in a magnetic loop, the second and the third components with small local scale lengths and describe an inhomogeneous region superimposed on the first component.
Table 1: Parameters and drift rates for Parts 1 and 2 calculated in comparison with the observations.Such a model will apply to not only the interpretation of the change of frequency drift but also the diagnosis for plasma in the source region e.g. the diagnosis for the deviation of plasma density distribution from the equilibrium solar atmospheric model as well as for the origin of the brightened knot in the soft X-ray image etc. The density in the inhomogeneous region then may be written as
|Figure 4: An exaggerating schematic of the 2nd V-shaped structure. The relevant parameters such as frequency f as well as the different part with differing drift rate are denoted by their subscriptions 1, 2, 3 and 4.|
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The frequency drift rate of type III or type III-like structure in the same drift direction looks the same in the dynamic spectrogram because of its small time scale and narrow bandwidth. Therefore, the beam velocity v and scale length in Eq. (1) are usually considered as constant. However, the electron energy loss and hence the corresponding velocity variation, especially in the region with high electron density, must be considered for the estimation of electron lifetime corresponding to the deflection time (usually replaced by duration ). For a given E0 and n, the time interval (i.e. the life-time) can be determined by Eq. (4) i.e. , where E corresponds to v, and c has to be satisfied. To estimate the initial velocity v0 or initial energy E0 of an electron beam as well as its energy loss due to electron-electron collisions, the following approaches are adopted.
Setting in Eq. (7) then the initial velocity v0or energy E0 will be underestimated because is shortened considerably by strong f-f absorption (Benz et al. 1992) and hence . Therefore, a mean duration of 0.65 s averaged over the duration of typical type III bursts given by Isliker & Benz (1994) to determine the initial velocity v0or initial energy E0 is much more reasonable than the measurable duration. On the other hand, however, the initial electron velocity v0 in expression (1) will lead to an over-estimation of the scale length if the drift rate is measurable. Therefore, in the course of derivation of the drift rates of Parts 1 and 2, an average velocity vi (i=1,2) instead of initial velocity v0 in Eq. (1) is suitable.
Considering an inhomogeneous region with two differing scale lengths and , as well as different mean velocities and , the estimated results of drift rates agree well with the observations. According to Eq. (8) would be determined if is given. For example, and hence , and h1, h2 are about 5.5, cm and 8.3, cm, respectively. represents the density gradient in Part 1 which is steeper than that in Part 2. and will lead to a different drift rate of . It is most likely that the jump in the drift rate is caused by the variations of both scale length and velocity, especially by velocity. The ratio of to has more effect on the jump of drift rate compared to the ratio of to (see Table 1).
The V-shaped structure is formed by an electron beam moving
downward along magnetic field lines to the mirror and then upward.
Consequently, the normal drift rate
should be slower than the reverse drift rates
according to the electron velocity decrease with
time due to energy loss. The observations, however, show that the
normal drift rate
is faster than the
reverse drift rate
Either a small scale length or
an increasing energy may cause this. According to the drift rate
c given in
Sect. 2.1 and Table 1, the scale length
109 cm which is much smaller than
and is difficult to understand. Therefore, an
increasing energy of the electron beam at the mirror should be
taken into account. Considering an electron beam distribution with
then the mean energy of the electron beam may
be expressed by
Fine structures in microwave emission are still poorly known. A spectrograph with high temporal and spectral resolution is needed to study the fine structures in the microwave band. The fine structures presented here consist of a descending branch followed by an ascending one to form a V-shaped structure, which is located between 5.57 and 6.26 GHz and lastsabout 0.4 s. The significant features of the V-shaped structure are: a distinct jump of drift rate from 3.22 to 1.53 GHz/s in the reverse drift component; a fastest drift rate of -3.53 GHz/s in the normal drift component.
Type III or type III-like events refer to the plasma-emitting process of moving electron beams. The most significant feature of type III bursts is the frequency drift. The drift rate can be determined by both excited velocity covering the range of and scale length as a measure of the density gradient. As a bi-directional frequency drift event, the V-shaped structure cannot be explained by the common beam model due to different drift rates in the same drift component. The energy loss of the electron beam and the three-component solar atmospheric model with different scale lengths are proposed to explain the different drift rates. The variations of both local scale length and the mean velocity , especially , are responsible for the jump of drift rate in the reverse drift component. While the fastest drift rate in the normal drift component is interpreted by means of the increase of the average energy of the electron beam due to the decrease of electrons lower than 40 keV caused by energy loss at the mirror. The local scale length means an asymmetry of local inhomogeneity, which is probably more close to the real inhomegeneity.
The authors are grateful to the referee for helpful comments. This work was supported by the National Natural Science Foundation under grants Nos. 10333030 and 10273025 and by a key Project of the Chinese Academy of Science.