Decameter radio emission of the Sun

   Observation of radio emission of the Sun at the lowest frequencies 10-30 MHz, which can be registered on the Earth, have been carried out from 70th of the last century. Up to 2000 the registration was fulfilled at separate frequencies that complicated its analysis. Nevertheless we succeeded to analyze such components of sporadic solar radio emission as Type III bursts, their harmonic structure, stria bursts, drift pairs. There were also some chosen observations of the quiet Sun. Activation of new modern spectrometers with wide frequency band for analysis allowed to study essentially more phenomena.

Radio telescope UTR-2 and registered equipment

   Solar observations were performed with 3 sections of North-South antenna of the UTR-2 telescope. It provided the main beam width of 10 and 130 in declination and right ascension planes respectively at frequency 25MHz. Observations were carried out on each day from 6:00 to 12:00 or ±3 hours from the local noon. Digital spectrum polaremeter operated in frequency band 12MHz from 18 to 30MHz with frequency resolution 12kHz and time resolution 2, 50 and 100ms. Independently the 60-channel spectrometer was used. It had frequency range from 10 to 30MHz with resolution about 300kHz and time resolution 20 and 50ms.

 

Observations and properties of sporadic solar radio emission and quiet Sun at frequencies 10-30 MHz

   Present time we have observed and analyzed follow components of sporadic solar radio emission: Type III bursts and bursts related to them, Type II bursts, Type IV bursts, S-bursts, drift pairs, spikes, bursts in absorption. Also we obtained new results concerning to radio emission of the quiet Sun. In many cases the properties found for the first time.

Type Ш bursts and bursts related to them

Type III bursts with fine time structure

   Sometimes Type III bursts observed at decameter wavelengths reveal fine time structure in the form sub-bursts, which have duration about 1s and drift from high frequencies to low frequencies with frequency drift rates more, equal or essentially smaller than that for parent burst [1]. In some cases this fine structure is as drift pairs or S-bursts. On our mention this points on nonhomogeneity of both electron beam responsible for Type III bursts and solar corona, through which these beams move.

 

Figure. Type III bursts with different form of fine structures.

Powerful Type III bursts

   These are the bursts with fluxes more than  10^-19(W/(m²Hz)) .  In the whole frequency band 10-30 MHz the frequency drift rate dependence on frequency is linear. It show that corona, through which electron beams associated with these bursts propagate, is exponential. Their drift rates are approximately to 1 MHz/s at low frequencies and about 2.5 MHz/s at high frequencies. Durations of these bursts are in the band 4-12 s. Powerful Type III bursts do not show any definite dependence their fluxes on frequency. It can be both decreasing and increasing. 

Figure. The powerful Type III burst (10:05:50 UT on August, 17, 2002) against a background of Type III burst storm.

 

Type III - like bursts

   When an active region is placed near the central meridian, we observed remarkable number of Type III – like bursts [2]. Their drift rates are changed from 4-5 MHz/s to 30 MHz/s and even 40 MHz/s. The duration of these bursts in most is equaled to 1 s. Radio fluxes of Type III – like bursts are moderate 200 s.f.u. we think that large drift rates are connected with closeness of velocities of fast electrons, which generate these bursts, and group velocities of generated electromagnetic waves.

Figure. Type III-like burst (10:44:30 UT) against a background of Type III burst storm.

Type IIIb bursts

   We observe very frequently Type IIIb bursts with fine frequency structure in the form of stria [3]. The frequency band of stria is about 60-70 kHz and is governed with active region associated with these bursts. Stria duration is about 1.2 s mainly.

Figure. Type IIIb burst with followed type III burst.

Inverted U- и J- bursts

   We observed such bursts at so low frequencies for the first time [4]. It indicates directly on existence of high coronal magnetic arches in the solar corona. Harmonic J-J and Jb-J pairs as groups of J-bursts were registered from time to time. We concluded that in some cases groups of J-bursts were the radio emissions from the same magnetic arch and were caused with periodic injection of fast electrons in this arch.

Figure. Dynamic spectrum of U-bursts with descending branch.

Figure. F-H pairs of “Jb-J” bursts.

Figure. Splitted U – burst.

Type II bursts

   For the first time Type II bursts were observed in the frequency range 10-30 MHz [5]. At these frequencies Type II bursts lasted some minutes and had drift rates from 30 to 70 MHz/s. We found that usual Type II bursts consisted from a lot of sub-bursts whose durations were about 1s and drift rates were some MHz/s. This is evidence of electron acceleration on the shock front.

Type II bursts with herring borne structure

   Such bursts are also observed at the decameter wavelengths [5]. There were cases when back of Type II burst had wavelike form with average drift rate equaled to zero. We consider that in these cases corresponding shock propagate parallel to solar surface and intersect coronal structures. It gives an opportunity to evaluate sizes and densities of coronal beam walls directly.

Type II bursts with the second and third harmonics

At various times observed Type II bursts had two and even three harmonics. In all cases the second harmonic was the most bright [6]. The presence of three harmonics confirms the point of view that emission mechanism of Type II bursts is plasma, which based on generation of Langmuir waves and their conversion into electromagnetic waves through different processes of scattering on ions and confluence of Langmuir waves.

Figure. Type IV burst followed Type II burst with three harmonics.

Type IV bursts

   These bursts were observed at frequencies 10-30 MHz for the first time also [6]. Their durations are changed from 1.5 hours to 5-6 hours and even more. Radio fluxes of these bursts exceed 10-100 times quiet Sun fluxes at these frequencies. All Type IV bursts have fine structure in the form of fiber-bursts in emission and absorption [6]. Once a zebra structure was observed during Type IV burst [6].

Figure. Low frequency part of Type IV burst with attendant Type III bursts and Type II burst.

Fiber- bursts in emission and absorption

   These bursts look like usual Type III bursts. Drift rates of burst in emission are in the range 1-3.5 MHz/s and can be both positive and negative. It says in favor of propagation of electron beams toward and outward the Sun during Type IV bursts. The duration of these bursts does not depend of frequency and equal to 6 s. Fiber-bursts in absorption is seldom phenomenon. Their drift rates are 3-4 MHz/s and durations are 20-22 s. Bursts in absorption seem to connect with fast electrons too but their velocity distributions are such that electrons absorb background emission of Type IV bursts effectively.

Figure. A fragment of Type IV bursts. The concequence of fiber bursts in emission (white), which succeeded with fibers in absorption (black).

Zebra structure

   Groups of zebra patterns were observed during Type IV burst registered on Juy, 22, 2004. Sometimes up to 40 stripes were in a group. The period was different and was changed from 1.2 s to 2 s. Zebra structure was observed at frequencies 14-30 MHz primary. The single zebra pattern can have both positive and negative drift rates.

 

Figure. Three groups of zebra patterns.

 

Figure. Succession of stripes in zebra structure in emission and in absorption.

S-bursts

    These bursts are shortest phenomenon at frequencies 10-30 MHz [7]. Their durations are from 0.3 s to 0.6 s. Drift rates of S-bursts is increased with frequency as those of usual Type III bursts, but the dependence is not so steep. Single S-bursts are observed in 4-14 MHz frequency band. Their instantaneous frequency band is increased according to linear law. The value of magnetic field in the place of S-burst generation can be found on slope of this line.

Figure. S-bursts against the background of decameter activity of the Sun.

Drift pairs

   These bursts consist of two components with 1-2 s delay, which have constant drift rates [8, 9]. There are approximately equal number of drift pairs with positive and negative drift rates. In spite of their similarity these bursts have some different properties. They have different average drift rates, different drift rate dependences on frequency and so on.

Figure. Fragment of DPs storm of July, 15 2002.

Spikes

    To these bursts we belong chaotically placed on dynamic spectrum bursts with durations about 1.2 s and frequency bands about 70 kHz [10]. These properties are very close to stria bursts.

Figure. Storm of decameter spikes against storm of type III bursts.

Long duration bursts in absorption

   Such bursts are observed frequently before or during Type IV bursts [6, 11]. Their durations are as high as 2-3 minutes. Sometimes they have drift rates, which can be more than 100 MHz/s that corresponds to linear velocity more than 2000km/s. Such shadow regions can be thick formations, which absorb background emission, but it is not understood their large velocities.

Radio emission of the quiet Sun

Our observations show that radio emission of quiet Sun is changed from day to day and from month to month. In years of the minimum of solar activity average values of fluxes decreased from 712 Jy at 20 MHz and 868 Jy at 25 MHz in 2008 to 600 Jy at 20 MHz and 834 Jy at 25 MHz in 2009.

Figure. The temporal variations of flux density of the quite Sun radio emission in 2008-2009.

References

   1. V.N. Melnik, A.A. Konovalenko, E.P. Abranin, V.V. Dorovskyy, A.A. Stanislavsky, H.O. Rucker, A. Lecacheux Astronomical and Astrophysical Transactions Vol. 24, No. 5, October, 391–401, 2005.
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