Indian Journal of Radio & Space PhysicsVo1.12. June 1983. pp.68-73

Nature of Solar Radio Burst Spectra ill

Frequency Band 0.245-35 GHzT K OAS & M K OAS GUPTA

Centre of Advanced Study in Radio Physics & Electronics, Calcutta 700 009

Received 23 August 1982

A total of 132 solar radio burst spectra in the frequency band 0.245-35 GHz have been classified into 15 distinct typesaccording to the nature of variation of emission intensity with the observed frequencies. Of these 15 types. IU-IU, IU-1. O-IU.0-1 and U-IU occur most. They generally have the most pronounced peaks at 0.245-0.606 GHz and the less pronounced peaksat around 8.8 GHz. The mean spectral indices In the ranges of frequencies 0.245-0.606.1.415·2.6'15 and 4.995-15.4 GHz are2.7. 1.3 and 1.0. respectively. In the decimetre-wave region the bursts having U-shaped spectra correlate well with type IImetre-wave bursts and proton events. and the bursts having LS-shapcd spectra with type III bursts. In the centimetre-waveregion. IF-type spectra are associated With type III bursts and proton events and the Dvtype ones with type II bursts. Thespectral shapes have been interpreted on recalling the existing knowledge of different types of emission and absorptionmechanisms known 10 take place in the solar atmosphere.

1 Introduction

The study of energy or peak flux spectra of solarradio bursts in a wide band bears importance becauseof the fact that it helps to gain an insight into thegeneration mechanisms of radio emission at differentwavelengths, which effectively depend on the actualconditions at the centre of activity. In the microwaveregion the spectra are usually classified into thefollowing groups according to the peak flux or energyexcesses: (i) increasing with frequency, (ii) decreasingwith frequency, (iii) showing an inverted-U shape witha peak in between I and 10 GHz,(iv) giving aU-shapedspectrum with a minimum roughly in between 0.5 and2 GHz and (v) having a zigzag nature of variation withfrequency. Das Gupta and Sarkari reported that outof these five different types of spectral distributions,inverted-U and complex or zigzag types were found tobe most predominant, the respective percentages being51% and 38.2%. Hachenberg/ as also Castelli et al.3,

however, examined the increasing type to be mostpredominant. Das Gupta and Sarkar ' further assertedthat most of the spectra ofinverted-U type have peaksaround 4.995 GHz. From a statistical analysis Castelliand Guidice" emphasized that: (i) most (appro-ximately 76%) of the microwave bursts have peaks inthe 5-10 GHz range and (ii) although only 5% of allmicrowave bursts exceed 500 flux unit (1 f.u. =to -22 W 1m21Hz), those that do so can reach 100 timesthis value. With the 'help of difTerent emission andabsorption mechanisms quantitative studies about themicrowave bursts have also been done during the lastfew years" -t7.

68

Fokker 18 studied the spectrum of the time ofmaximum intensity of medium-sized solar radio eventsthat cover both the metre and centimetre frequencybands. Two types of spectra can be distinguished,namely, (i) V-type of spectrum, where the straightlines that can be drawn to represent the centimetre andmetre branches, meet each other at a frequencysomewhere in the frequency range 0.65-1.8 GHz, and(ii) a jump-type of spectrum where a discontinuityoccurs somewhere in the low frequency part of thespectrum (0.3-1 GHz). There are a few radio eventswhose spectra can extend well in the millimetre-wavelength region. These events which have peaks inthe millimetre region are usually associated with themore important solar events, particularly in relation totheir terrestrial effects, and can be as intense as anyrecorded at centimetre-wavelength 19

From the above discussion it appears that theprevious studies about the radio burst spectra wereconfined to some selected bands of frequencies. Hardlyany attempt has been made for determining thespectral shape in a continuous manner from metre- tomillimetre-wave region on consideration of burst eventshaving the characteristics of simultaneous multi-frequency emission. This is because of the paucity ofsimultaneous observational data over the entirefrequency range. But recent radio observationtechniques cover a wide range of frequencies whichstarts with the decametre region and ends with the nearinfrared wavelengths. In the prevent paper an attempthas been made to ascertain the nature of spectra in themillimetre- to metre-wavelength region underdifTerentconditions with the help of the same observatory data.

DAS & DAS GUPTA: SOLAR RADIO BURST SPECTRA

2 Data CollectionAltogether 132 multifrequency burst events which

occurred during the period July 1969-July 1979 havebeen collected from the Solar Geophysical DataBulletins issued by the V S Department of Commerce.In each of the burst events the peak fluxes in at leasteight of the nine frequencies (0.245,0.41, 0.606, 1.415,2.695,4.995,8.8, 15.4 and 35 GHz) occur generally atthe same time and thus the spectra plotted for theseevents correspond to the real instantaneous spectra. Inorder to avoid the error in the given flux densities dueto the use of different time constants of receivers andthe differences or deficiencies of the calibrationsystems, the radio burst data of only the Sagamore HillRadio Observatory, Massachusetts, have been used.

Each of the burst events has been correlated with theH,-flares, sunspots, proton events, X-ray and metre-wave bursts. The radio bursts and H,-flares have beenregarded for association when their starting times donot differ more than ± 5 min from each other. Similartime limits have been used for the correlation of protonevents, X-ray and metre-wave bursts as well. Thesunspots have been considered when they originatedfrom the same McMath plage regions as that of theburst-related H,-flares.

3 Classification of SpectraThe peak intensity values at various frequencies for

a particular burst event have been normalized withrespect to the maximum value of peak intensityobserved for the same event. The normalized spectra,thus obtained for 132 bursts, have been separately

-------- .. - . __ .._--- ---

Table I-Spectral Classification and Their Shapes

Nomen- Number Shape of spectrum in wavelength bandc1ature of of -_. - ,,-----_ .._, '---"

spectra bursts Decimetre Centimetre(0.245-2.695 GHz) P2.695 GHz)

IU-IU 26 Inverted-U lnverted-UIU-I 10 lnverted-U IncreasingIU-D 4 Inverted-U DecreasingIU-IF 6 Inverted-U Increasing,

then OatD-IU 17 Decreasing Inverted-UD-I 10 Decreasing IncreasingD-IF 9 Decreasing Increasing,

then OatD-D 6 Decreasing DecreasingU-IU \3 U Inverted-UU-D 6 U DecreasingU-U 3 U UU-LS 3 U Lateral-SLS-IU 8 Lateral-S Inverted-ULS-I 6 Lateral-S IncreasingI-D 3 Increasing DecreasingMiscel-laneous 2

---- ------~-- .--

drawn for examining their individual shapes and alliedcharacteristics. It is seen that the spectra thus obtainedcan be classified into 15 well defined types according totheir shapes. In this context it is to be noted that theobserving frequencies 0.245, 0.41, 0.606, 1.415 and2.695 GHz fall mainly in the decimetre- and the rest inthe centimetre-wave band. Hence, in designating theoverall spectral nature the present nomenclature hasbeen adopted according to the spectral shapes in thetwo bands separately, as presented in Table I.

Some typical spectral shapes of different nature areshown in Figs I and 2. These 15 types of spectra havetheir distinctive characteristics in respect of theirvariation of peak fluxes with the observingfrequencies. It appears that inverted-U, decreasing, Vand lateral-S types in the decimetre band and inverted-U, decreasing, increasing and 'increasing and then fla t'types in the centimetre band generally predominateover other types of spectra in the respective bands.

4 Spectral PeaksThe frequency at which the spectrum is peaked is an

important parameter for ascertaining the characteris-tics of the source region, especially the magnetic field.The maxima of the spectra classified above are seen tooccur at different frequencies; but for a definite class ofspectrum a particular frequency is generally preferredin respect of the occurrences of maxima, as it is evidentfrom Table 2. The spectra generally have two peaks,one below 0.606 GHz and the other at around 8.8 GHzfrequency (Table 2).

5 Spectral IndicesThe spectral index, grving the rate at which the

intensity of the burst changes with frequency is givenby the relationship:

wherefis the frequency, S the observed flux and (J. thespectral index. The values of (J. have been determinedby finding the slopes of the curves both in the high andlow frequency directions from the regions ofoccurrences of different peaks stated earlier. Thedistributions of spectral indices have been found outfor the ranges of frequencies 0.245-0.606, 1.415-2.695and 4.995-15.4 GHz within which the spectral peaksgenerally occur. The results are shown in Fig. 3.

Most frequent values of (J. at the high frequency endsof the spectra are 1-4, 0.5-2, 0-1.5 for the ranges offrequencies 0.245-0.606, 1.415-2.695 and' 4.995-15.4GHz, respectively; the respective mean values are 2.7,1.3 and 1.0. As the power spectral index (J. is related to)', the energy spectral index, by .(J. =(~,- 1)/2, therespective values of}' become 6.4, 3.6 and 3.0.

69

INDIAN J RADIO & SPACE PHYS, VOL.l2, JUNE 1983

6 Association of Radio Burst Spectra with :;unspots,Metre-wave and X-ray Bursts and Proton Events

The different types of radio burst spectra have beencorrelated with the sunspots possessing differentconfigurations and magnetic fields, with the metreband and X-ray bursts and also with the proton events,if any. As regards the association of different spectral

classes with the sunspots, it is generally observed thatthe sunspots are mostly (about 78'\,) of bipolar anddelta configurations having magnetic fields in therange 1600-3000G. The data regarding the associationof these spectra with metre-wave and X-ray bursts (0.5-20 A) and proton events are presented in Table 3. Theremarkable points to be noted in Table 3 are that the

9.7.11

""<', , ,." ''b

:~ 2',478/ <~--,10, <4 73

( ~_.-o13.6.70

//

,,

1·0

,\\\

\ ~\~\

\ ~\ ,P\ \ .:

I \'...---,:.-

I \ /IVI

\~.~ ;P'/ Y

/. YO\ / /~.--J' ( ~

/

10

100 1.0 100

!U-IU

!I. II. 74 / \

21.4.13'/ \

I/1

0·1 . \

71.7l I I~/

14.5,7110 to1.5,78

100

~~~~~~~~--~~~~~ __~~~~'6° 0·7,--~-L-LLL~~~~~~~~--~~~

100

10

II

/\d \

/ ,\

"'bI.IO.14

U-IU

1-0

()ol

"oz.s. "1

10 30.'.7 100

tOO tOO to

3.10.11

10 24.4.13 1001••• 72

22.12.'14

07,,---L-L-LLL~~~--~-U~~--~-L~5~ci0 o~,,--~-L-L~~~~--~-W~~--~~~40

FRE~UENCY (GHz)

Fig. l--Normalized spectra of solar radio bursts (The nomenclature corresponds to that in Table 1. The dates ofoccurrences of bursts are designated in the corresponding curves. The arrow at the bottom of each of the spectral

nomenclature indicates the scale used for the respective type.)

70

DAS & DAS GUPTA: SOLAR RADIO BURST SPECTRA

bursts with U-shaped spectral nature in the deqimetre-vave region have greater association with type IImetre-wave bursts and proton events, while those withLS-shaped in the same wavelength region prefer typeIII bursts. But for the centirnetre-wavelength band, IF-shaped spectra are mostly related with type III burstsand proton events, and D-shaped spectra with type IIbursts.

~00,----Q-----r--------.--------,

19. l.79

10 ;<>--- 100~ .-......, r .-

'"<t I 5.5.13.... Ia. I..J / 19. 9. 77

<t /cr P ~~tJ I•.......

/-a.V> I 10

....•... <I,..<t Iz I-~.... I;;; Iz I....,.. J~ Id

100 Ii 1. 78 1·0

~

1~·~.I--L-L~LU~IO~~~~~~~IO~~~~50·FREQUENCY (GHz)

Fig. 2 =Same as Fig. I but for spectral types LS-IU. LS-l and 1-0

7 DiscussionThe wide band spectra of solar radio bursts obtained

from different points of view enlighten the concept ofgeneration mechanisms of radio emission at variouswavelengths and throw some light on the position ofthe source region for the bursts. The existence of twopeaks, one near to metre-wave and the other in thecentimetre-wave region (see Figs I and 2), appears tofavour the argument that two separate sources withdifferent physical parameters, such as magnetic fieldand/or energy distribution of electrons. are responsiblefor these two peaks. This can be juiged from the factthat the multifrequency radio events have uniformcharacteristics with respect to the optical thick-ness20

.21 and also inferred from the observation that inthe majority of the cases the times of maximum of0.245 GHz are found to be behind those of2.695 GHz.whereas. the same for 8.8 GHz and 1-8 A X-ray burstsare ahead of those for 2.695 GHz. These resultsobviously justify the variation of effective height withfrequency? (. The possible spectral shapes both in thedecimetre- and centirnetre-wave radio band knownfrom the present study are interpreted as below.

7.1 Centimetre-wavelength Region

In this region the spectra obtained are generally ofthe following types.

Type-Lll > This shape simulates quite well with thetheory pertaining to the mechanism of gyro-synchrotron emission which is incoherent andnonthermal in nature. As the peaks of the spectra inmost cases lie around 8.8 G Hz, the magnetic field in thesource region can be evaluated from the relation+'

fmax=15x2.8B

.' ---_._-_._- ... -"----'.--'-'._.

Table 2 Occurrences of the Peaks at Various Regions of Spectra

Type of Occurrences corresponding tospectrum

Peak I at freq. (GHz) Peak II at freq. (GHz) Peak III at freq. (GHz)--_. -_._---

0.245 0.41-0.606 0.606-1415 2.695-4.995 2.695-4.995 8.8-15.4 35IU-IU 26 8 18IU-I 10 4 6lU-D 4IU-IF 6 6D-IU 17 5 120-1 10 7 30-0 6D-IF 9 3 6U-IU 13 4 9U-D 5 6U-U 3 3U-LS 3 3 3LS-IU 8 8 3 5LS-I 4 2 6 6Total 78 49 14 3 32 76 9

71

INDIAN J RADIO & SPACE PHYS, VOL.12, JUNE 1983

40

Table 3-Percentage Association with Metre-wave and X-so 4·995-15·4 GHz ray Bursts and Proton Events

Spectral Metre-wave bursts of tyoe X-ray Protonshape ._--_._- bursts events

II III IV V

Decimetre-wave'0

IU 2.2 6.5 56.5 6.5 19.6 39.0 6.50 9.5 69.0 2.4 33.3 50.0 4.8

-4 -2 0 4 6 U 4.0 28 48.0 4.0 20.0 52.0 12.050 LS 86.0 7.1 28.6 35.7 7.1

1·415-2·695 GHz Centimetre-wave40

IU 3.1 10.9 54.7 3.1 25.0 34.4 3.1

30 0 21.1 57.9 21.1 58.0 15.8I 3.8 73.0 7.7 23.1 50.0 3.8IF 6.7 87.0 6.7 33.3 60.0 20.0

-6

'"IIJVZIIJa:a::)oV0~0

IIJ<:)4I-ZIIJVa:IIJQ.

-6 2-2 030

4

0·245-0'606 GHz

-6 -2 0 2

SPECTRAL INDICES

Fig. 3·· -Histograms showing the percentage occurrences of events indifferent ranges of spectral indices for the specified ranges offrequencies (The left and the right hand sides represent thedistribution of spectral indices in the low and the high frequency

directions. rcspectively.)

where B is the magnetic field. The magnetic field isfound to be about 200 G which is consistent with theobservations of Zirin et al.23 and also with the valuededuced by Takakura" with the help of his theoreticalmodel.

Type-I- This type is possibly due to the thermalfree-free emission, because in case of thermal emissionthe intensity increases with increasing frequency forany temperature distribution. This type of thermalemission is produced from the coronal condensationarising out of the plasma compression after thecatastrophic explosion of a flare which enhances theelectron density and the temperature of the sourceregion with respect to the ambient medium?".

Type-IF-- The flat spectrum results from free-freeabsorption and self-absorption of nonthermal gyro-synchrotron emission such as that described byRamaty and Petrosian 10. Takakura" conjectured thatthis flatness arises due to the inhomogeneity in themagnetic field of the source region.

72

Type-D- The decreasing type of spectrum in thisfrequency band can be linked with the suppression ofradiation by free-free absorption. It may also be due tothe effects of an anisotropic pitch angle distribution ofelectrons with energies in excess of me". For suchdistributions, if the direction of observation differssignificantly from the direction of maximumanisotropy, the radiation at high frequencies isstrongly suppressed",

7.2 Decimetre-wavelength Region

In this least studied region (0.245-2.695 GHz), thespectra which occur most are of following types.

Type-Ill - This type of spectral shape can beexplained by the same gyro-synchrotron radiation asthat discussed earlier for the centimetre-wavelengthregion. The magnetic field in the source region may beeither 5-15 G as deduced from Holt and Ramaty'sexpression, or 25-60 G as obtained from Takakura'srelations.

Type-D-In the decimetre-wavelength region thesuppression of radiation can be attributed to the self-absorption of gyro-synchrotron radiation, the Razineffect, the absorption below plasma frequency and thegyro-resonance absorption.

Type-V-In the present study, it has been examinedthat type II metre-wave bursts and proton events aremostly associated with this type compared to othertypes of spectra in the decimetre-wavelength region.Castelli and Aarons= propounded that the protonevents can be identified with a high degree ofconfidence by the occurrence of an intense centimetre-wave burst accompanied by U-shaped spectrum.Again, according to the present state of knowledge, thetype II metre-wave bursts are generated by the shockwave passing upwards into the corona from achromospheric flare almost at right angles to the

DAS & DAS GUPTA: SOLAR RADIO BURST SPECTRA

magnetic field. The violent explosion of flare whichproduces shock wave probably causes some of the fieldlines to be blown open, giving access temporarily to theenergetic particles to the high corona and theinterplanetary space. The relativistic electrons beingaccelerated by the shock wave are promptly releasedinto a much larger volume that contains a muchsmaller magnetic field, and emit optically thinsynchrotron radiation at lower frequencies and thuscause an upturn in the observed microwave spectrum.The same shock front possibly accelerates the protonsas well 26,27, thus explaining the association betweenthe proton emission and Ll-shaped spectra.

Type-LS-Two peaks, obtained below 0.245 GHzand in the region 0.606-1.415 GHz in this type ofspectra, probably originate from two separate sources.Again, this spectral nature has been seen to beassociated mostly with type III metre-wave bursts.Hence, it may be presumed that the peak around 0.606GHz is due to the gyro-synchrotron and that below0.245 GHz due to the plasma oscillations.

8 ConclusionsAs most of the radio bursts considered here are

associated with bipolar or delta types of sunspots. weconsider an asymmetrical bipolar magnetic field wherethe two poles have dissimilar magnetic fields. Iff.m andf2m be the frequencies corresponding to the twospectral peaks as obtained in most of the spectra, thenit may be written that

f:=~where B1 and B2 are the magnetic fields at the twopoles. Asf1m < f2m, it is obvious that B. < B2• Again.the intensity at spectral peak (S"J is related to thenumber of electrons (N) responsible for the gyro-synchrotron emission and the magnetic field (B)through the expression

Sm=NB

Therefore, ~=%I~'~2m 2 2

But in most of the spectra it is observed that theintensity, S.m. at the metre- to decimetre-wavetransition region is greater than that (S2"J at around8.8 GHz. Hence. one can infer the followingpossibilities: (i) the number of electrons gyrating

around the weaker pole of the sunspot is very muchgreater than that around the stronger pole, (ii) thenumber of electrons being the same, more energeticelectrons are present above the weaker pole, (iii) someother mechanism (possibly plasma oscillation)contributes to the emission intensity observed at 0.245-0.606 GHz region in addition to that due to gyro-synchrotron mechanism.

ReferencesI Das Gupta M K & Sarkar S K. J Roy Astron SOl' tCanudai, 65

(1971) 152.2 Hachenberg O. Solar System Radio Astronomy (Plenum Press.

New York). 1965.3 Castelli J P. Aarons J. Michael G A. Jones C & Ko H C. Solar

Flares and Space Research (North Holland Pub Co Ltd.Amsterdam). 1969.

4 Castelli J P &. Guidice D A. AFCRL environmental researchpaper. No 381 (Hanscom Field. Bedford. USA). 1972.

5 Takakura T. Sol Phys (Netherlands). 1 (1967) 304.6 Takakura T. Sol Phys (Netherlands). 26 (1972) 151.7 Takakura T. in High Energy Phenomena on the Sun. edited by

R Ramaty and P G Stone (Goddard Space Flight Centre.USA) 1973. 179.

8 Ramaty R. AstrophysJ(USA), 158(1969) 753.9 Ramaty R. in High Energy Phenomena on the Sun, edited by

R Ramaty and P G StonetGoddard Space Flight Centre)1973, 188.

10 Ramaty R & Pelrosian V. Astrophys.J(USA). 178(1972) 241.II Kruger A. Physics of Solar Continuum Radio Bursts (Akadernie

Verlag. Berlin. GDR) 1972.12 Kovalev V A & Korolev 0 S. Sov Astron (USA). 20 (1976) 69.13 Matzler C. Sol Phys (Netherlands). 49 (1976) 117.14 Bohme A. Furstenberg F. Hildebrandt J. Saal O. Kruger A,

Hoyng P & Stevens G A. Sol Phys (Netherlands). 53 (1977)139.

15 Schoechlin W & Magun A, Sol Physt Netherlandss, 64(1979) 349.16 Roy J. Sol Phys (Netherlands). 64 (1979) 143.17 Melrose D B. Space Sci Rev (Netherlands), 26 (1980) 3.18 Fokker A D. Sol Phys (Netherlands), 8 (1969) 376.19 Croom D L. Sol Phys (Netherlands), 19 (1970) 414.20 Kakinurna T. Yamashita T & Enome S, Proc Res Inst Atmos

Nagoya Uniu i.lapans, 16(1969) 127.21 Das Gupta M K. Das T K & Sarkar S K, Sol Physt Netherlandss,

67 (1980) 109.22 Holt S S & Ramaty R. Sol Phys (Netherlands). 8 (1969) 119.23 Zirin A. Pruss G & Vorpahl 1. Sol Phys (Netherlands). 19 (1971)

463.24 Zheleznyakov V V. Radio Emission of the Sun and Planets

(Pergamon Press. London), 1970.25 Castelli J P & Aarons J, Proceedings of the fifteenth electrical

wave propagation meet of AGARD, C P No 49, 1970.26 Gubchenko V M & Zaitsev V V, Sol Physi Netherlandst, 63(1 979}

337.27 Lin R & Hudson H S. Sol Phys i Netherlandsi, 50(1976) 153.

73