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Quasi-periodic Oscillation of the Radio Emission

of the Solar Plasma Structures and Their Nature

G.B. Gelfreikh

Central (Pulkovo) Astronomical Observatory RAS

St,.-Petersburg, 196140, Russia

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CONTENTS

1. Introduction

2 Historical background

3. Modern Instruments used

4. Methods of analysis

5. Oscillations in different structures

6. Main types of oscillation parameters

7. Physical nature of the radio oscillations

8. Significance for physics of the sun

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1. INTRODUCTIONThe observations of the quasi-periodic pulsations of the

microwave solar radio emission are made for about 40 years. Their effectiveness as a method of study of the physics of the solar plasma became especially evident in the last decade when new large high spatial resolution instruments have been used.

The significant progress in usage of this method is due to some special features of the solar radio astronomy and radio astrophysics:

- Regular full day observations with radio heliographs- Diagnostics of the magnetic fields in the solar corona- Simultaneous coverage of the total disk of the sun

(no need for preliminary choice of the object to study)

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2. Historical background

The quasi-periodic oscillation (QPO) at microwaves were first studied by the group of Prof. Kobrin in Gorkiy (Nizhniy Novgorod) in early 60st. The ground for the program was the discovery by optical method the 5 minute oscillations in the solar atmosphere. So, they tried to find similar effects using radio observations. However, usage of small (no spatial resolution) dishes limited the results obtained.

Better, more reliable conclusions were made (Pulkovo, Siberian Institute of the Solar-terrestrial physics - Irkutsk) using small-spacing interferometers and polarization measurements. It was found that most significant QPO are due to the local radio sources connected with the solar active regions.

Next few decades the study of the QPO were based on the large antenna dishes (22 – 64 meters in diameter). In this case separate ARs were analyzed with high sensitivity. Small details responsible for a particular period could not be identified.

The last decade opened a new era in the problem due to observations with higher spatial resolution allowing to identify the position of the oscillting region.

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3. Instruments used today for observations of the radio oscillations

(1) Radioheliograph Nobeyama (=1.76cm)(2) Radioheliograph Badary (SSRT) (=5.4cm)(3) Radio Array VLA (=2-20cm)(4) Reflector r/t RATAN-600 (=1.7-32 cm)(5) Reflector dish RT-22 (=2-3.5 cm)

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4. Methods of analysisIn observations of the QPO at microwaves we deal mostly with

nonstationary processes. So, classical Fourier transformation method is not applicable. Some versions of the wavelet analysis of the oscillation spectrum and its variation with the time were used. Such an approach yields the information on the length of a timescale of monochrome oscillations and variation of their frequency.

Dynamic spectrum based on wavelet-transform Wavelet - elementary function localizing both current frequency and time-coordinate

W x a b x t dt a x tt b

adtab ( , ) ( ) ( )* / *

1 2

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Nobeyama radio-heliograph: examples of image data

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Typical non-stationary (wavelet-) spectrum of large sunspot (30.06.93: H = 2500N)

10 100

0

1000

2000

3000

4000

5000

6000

30m

50m

90m

160m

5m

3m

20070402053

1% c.l.5% c.l.>5%

Am

plit

ud

e

Period, minutes

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Short-time (3-min) oscillations

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Dominant 3-min oscillations and weak amplitude long-periodic 10-100 min variations

0

1000

2000

3000

4000

5000

6000

1 10 100

1% c.l. 5% c.l.>5% c.l.

Am

plitu

de

0 50 100 150 200 250 300 350

1

10

100

70402053

5

3

70

40

0 50 100 150 200 250 300 350

-10000

-5000

0

5000

10000

Period, m

inutesTime, minutesPeriod, minutes

20

Inte

nsity

Time, minutes

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Wavelet-entropy and typical time of reorganization of 3-min oscillation pattern

),(ln),()(),(

),(),( 2

2

tqtqtEftA

tAtq

),(tq – part of the dispersion corresponding of given frequency ),(tEf – entropy of the dispersion distribution over spectrum

1 10 100

0.00

0.05

0.10

0.15

0.20 Tmax

= 15m

Period, min502053

Am

plit

ude

1.01.21.41.61.82.0

Wa

ve

let-

en

tro

py

-4000

-2000

0

2000

40000 50 100 150 200 250 300 350

Time, min

Ite

nsity

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Time variations of intensity for sunspots 24.07.98: A) H=3000S,(23S,47E); B) H=2400N,(24S,57E); C) H=2000N,(32N,52E); D) H=2300S,(24N,01E).

180000

200000

220000

240000

260000

Inte

nsi

ty

Time, minutes

A

28000

32000

36000

40000

44000

B

0 10 20 30 40 50 60

30000

32000

34000

36000

38000

40000

42000

C

0 10 20 30 40 50 60

14000

16000

18000

20000

22000

24000

DY

Axi

s T

itle

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Dynamical (wavelet-) spectra of sunspot oscillations, 24.07.1998

0 10 20 30 40 50 600

3

6

9

12

15

18A

Time, minutes

0 10 20 30 40 50 600

3

6

9

12

15

18B

0 10 20 30 40 50 600

5

10

15

20

25

Period,

min

ute

s

C

0 10 20 30 40 50 600

3

6

9

12

15

18D

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Not only large sunspots: radio-sources over polar faculae, flocculi and small spots

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Long-time oscillations

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Active Region NOAA 9866 15.03.2002 and radio-sources No 1-4

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15.03.2002. Leading sunspot of bipolar group in AR NOAA 9866

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NOAA 9866: Dynamical spectrum of Р-spot intensity

T: 20-25m, 32-38m, 40-60m, 115-120m, ~ 200m

-1 0 1 2 3 4 5 60

1

2

3

4

5

Пе

ри

од

, ми

н

f =

10

0/T

, ми

н-1

Время, час

200

100

50

40

30

25

20

Perio

d, m

inu

tes

Time, h

f =

100

/T, m

in^

(-1)

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15.03.2002. Following sunspot of bipolar group in AR NOAA 9866

T: 20-30m, 40-65m, 90-110m, 160-180m

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NOAA 9866: No 3 (flocculus)

-1 0 1 2 3 4 5 6 7

11000

12000

13000

14000

15000

16000

17000

18000

19000

UT (hours), March 15, 2002

Z

-180000

-160000

-140000

-120000

-100000

Y (km)

-60000

-40000

-20000

0

20000

X (km)

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NOAA 9866: деталь 4 (flocculus)

20000

40000

60000

80000

100000

120000

X (km)

-160000

-140000

-120000

-100000

-80000

Y (km)

-1 0 1 2 3 4 5 6 7

11000

12000

13000

14000

15000

16000

17000

18000

UT (hours), March 15, 2002

Z

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Comparison of over-sunspot (No 1) and over-flocculus (No 4) radiosources

-1 0 1 2

40000

60000

80000

100000

120000

Time, h

Inte

nsi

ty

-220

-200

-180

-160

-140

-120-1 0 1 2

No 1

X/R

*10

00

-80

-60

-40

Y/R

*10

00

-1 0 1 2

12000

14000

16000

18000

Intensity

Time, h

-1 0 1 2

40

60

80

100

120No 4

X/R

*1000

-220

-200

-180

-160

Y/R

*1000

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Wave propagation

1.1 1.2 1.3 1.4 1.5 1.6 1.712000

14000

16000

18000

Время, час

Ин

тенси

вност

ь

70

80

90

100

110

1201.1 1.2 1.3 1.4 1.5 1.6 1.7

X/R

*10

00

-200

-190

-180

-170

-160

Y/R

*1000

Фотосфера

Слой, излучающий в 1.76 см

V = 25 km/s

Photosphere

1.76 – layer

of Solar atmosphere

I

nten

sity

Time, h

24

Flares & oscillations

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NOAA 9866: one day earlier (14.03.2002),Leading spot

-50 0 50 100 150 200 250 3004.2

4.3

4.4

Lo

g I

Time, min

-10

0

10

20

Y

-200

-180

-160

-140

-50 0 50 100 150 200 250 300

X

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NOAA 9866: following spot and flare

-50 0 50 100 150 200 250 300 350

4.5

5.0

5.5

6.0

Time, min

Lo

g I

-180-160-140-120-100

-80

Y

-420

-400

-380

-360

-50 0 50 100 150 200 250 300 350

X

V=30 km/c

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Following-spot (flare): Leading-spot:

-50 0 50 100 150 200 250 300

10

100

Time, min

200

50

20

5

2

Per

iod,

min

-50 0 50 100 150 200 250 300

10

100

Time, min

200

50

20

5

2

Per

iod

, min

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5. Some conclusions based on observationsPractically all plasma structures of the solar atmosphere

demonstrate in its radio emission some periodic oscillations. The oscillations may be registered in brightness, polarization, position of brightest point and QT-propagation inversion region.

The periods of oscillations are found in the limits of fraction of a minute to hundreds of minutes. Even in one AR different though of similar structures show difference in periods.

Shortest periods of about 3 minutes is a typical feature of sunspot-associated sources. However, longer periods up to hundreds are also significant and even dominating in some particular cases. (40 -60 minute oscillations are typical).

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6. Main features of the QPOThe five minute oscillations could be registered in most

features but only as temporary non stationary effect.Most of QPO are clearly of non stationary nature though

some few oscillation are very stable both in amplitude and periods.

One can summarize that most oscillations belong to the following ranges of periods ~3, ~5, ~10, 20 - 25, 40 - 60, 90 -120, ~200 minutes.

Typical time of stationary appearance of the 3 minute oscillations is about 15 min.

Some variations of oscillation parameters were found in connections with the flare activity and development of the AR structure.

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7. Physical Nature of QPO Observed radio oscillations are obviously the result of

modulation of physical parameters of the region producing the radio emission. The presence of periodicity in oscillations submit the existence of a resonance structure for some kinds of MHD waves in the plasma of the solar atmosphere. However, the position of the resonator as well as its size may be quite different from those of the microwaves feature of the solar disk under investigation.

We may propose 3 main types of the above situation:(1) The resonator coincides with the emitting region(2) The resonator is outside but close to the radio emitter(3) The resonator of global solar nature (e.g. 5 min

oscillations)

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8. Sunspot-associated sources– This type of the radio emission is generated in a very narrow

geometrically region. Its geometry and position depending on a particular wavelength. For shorter part of the spectrum it originate in the CCTR with very high gradient of temperature and the size of the source depends strongly on the magnetic field strength or the wavelength. As a result, the radio method has extremely high sensitivity to oscillation effects (e.g. a few G in the field of 2000G typical for Nobeyma data).

– Most prominent feature of the spectra are reasonably expected 3-minute oscillations due to some resonance process MHD waves below the radio emission – process widely studied by theoreticians, based on early known optical data. Radio observations presented new information related to high CCTR.

– Oscillations in the range of 10 – 30 minutes are probably due to the effect of oscillations of coronal loops beginning in the strong magnetic tube of sunspot.

– Besides strong, often the strongest oscillations are registered with longer periods, say 40, 80 minutes and longer. Such periods were found by a number of authors in sunspots earlier from optical observations but did not find proper theoretical study respond. Of special interest are some long period (more than an hour) with very stationary period parameters. Possibly they are really connected with global oscillations of the sun.

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9. Discussion The application of the modern high spatial resolution radio

telescopes to study the QPO at microwaves opened a new era in understanding its nature and significance for diagnostics of some plasma processes essential for physics of the sun.

At the same time, we are very far from understanding all the phenomena we do observe. So, one may expect that in some future, if proper efforts will be made, our usage of the radio methods based on observations of the PQO will be much wider and include reasonable solutions of some problems concerning the nature and forecasting the flare and CME activity of the sun, processes leading to heating the corona, the helioseismology.

Success in the developing the above methodology depends essentially both on the wider usage of the present day observations and further progress in constructing new instruments and methods of analysis, developing the solar physics as well.

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Acknowledgments

The work was supported by the State Research Programs “Astronomy”, the “Integration” (I0208.1173), the Scientific School grant 477.2003.2, INTAS 00-0543, grant of OFN-16 and grants of RFBR 02-02-16548, 03-02-17357, 03-02-17528.