solar active regions with peculiar spectral polarization emission and its possible diagnostics

Solar active regions with Solar active regions with peculiar spectral peculiar spectral

polarization emission polarization emission and its possible and its possible

diagnosticsdiagnostics

V.M. Bogod, T.I. Kaltman, V.M. Bogod, T.I. Kaltman, Special astrophysical observatory, Special astrophysical observatory,

RussiaRussia L.V. Yasnov.L.V. Yasnov.

St.Petersburg State UniversitySt.Petersburg State University, , RussiaRussia

Last years the various unusual Last years the various unusual spectra of solar flare productive spectra of solar flare productive active regions were discovered by active regions were discovered by radio telescope RATAN-600 in 2 – 16 radio telescope RATAN-600 in 2 – 16 GHz frequency range. GHz frequency range.

1. 1. RATAN-600 as a solar dedicated RATAN-600 as a solar dedicated telescopetelescope

high flux sensitivity (~Jy) high flux sensitivity (~Jy) High accuracy of polarization degree measurements High accuracy of polarization degree measurements

(0.02%) (0.02%) broad frequency range (2-18 GHz) broad frequency range (2-18 GHz) high spectral resolution (1% )high spectral resolution (1% )

Limitations: Limitations: Low temporal resolution (4 hours coverage near 7-11 UT Low temporal resolution (4 hours coverage near 7-11 UT

with 4 min interval )with 4 min interval ) 1D imaging (17 arcsec x 20 arcmin at 16.4 GHz)1D imaging (17 arcsec x 20 arcmin at 16.4 GHz)

Receivers

Secondary mirror

feed

antenna

South sector of the RATAN-600 in combination with the Flat mirror forms a Kraus-type periscope system. Combined with the receiver complex it provides unique possibilities for solar radio research:

http: //www.spbf.sao.ru/prognoz/

Pre-flare diagnostic Statistical study revealed an existence of special features of flare-productive active regions (Bogod and Tokhchukova, 2003, 2006). The regular and broadband RATAN-600 observations are convenient for the daily analysis and diagnostic of the pre-flare plasma. Now the web applications for the such automatic diagnostic are under development.

V. M. Bogod and S.Kh.Tokhchukova, Astronomy Letters, 2003, 29,4,p.263.V. M. Bogod and S.Kh.Tokhchukova, Cosmic Research, 2006, 44,6,p.506.

Features of the preflare plasma Features of the preflare plasma spectra spectra in the range 2.0 in the range 2.0 ссmm – – 55.0 .0 ссm m observed with RATAN-600observed with RATAN-600• I. Short-wave polarization emission brightening I. Short-wave polarization emission brightening

• II. Polarization inversion at short centimeter wavesII. Polarization inversion at short centimeter waves

• III. Appearing of the low polarization band in III. Appearing of the low polarization band in frequency spectrumfrequency spectrum

• IV. Multiple inversions of polarization signIV. Multiple inversions of polarization sign • V. Darkening effect several hours before the flareV. Darkening effect several hours before the flare

• VI. Polarization flux variations in broad frequency VI. Polarization flux variations in broad frequency band before and after a big flareband before and after a big flare

In this presentation we consider the flare In this presentation we consider the flare productive active regions spectra with a productive active regions spectra with a sufficient sufficient depression of polarized emissiondepression of polarized emission (Stokes (Stokes parameter V) parameter V) at 6 -12 GHzat 6 -12 GHz ̶̶ in the middle of in the middle of registered frequency range 2 -16 GHz , up to registered frequency range 2 -16 GHz , up to inversion of polarization sign.inversion of polarization sign.

2. Observations of solar regions with RATAN-600 (2 – 16 GHz)

2 4 6 8 10 12 14 16

-1000

0

1000

2000

3000

4000

TVa,K

f,GHz2 4 6 8 10 12 14 16

0

10000

20000

30000

40000

50000

f,GHz

TIa,K

AR 9077, 2000, Jule, 10

2 4 6 8 10 12 14 16

-2000

-1500

-1000

-500

0

500

1000 AR 9073

AR 9070

AR 9069

f,GHz

TVa,K

AR 9068

2 4 6 8 10 12 14 160

100000

200000

300000

400000

500000

TIa,K

f,GHz

()

5 10 15

0

20000

40000

f,GHz

TVa,K

АО 9077, 2000, Jule, 16

0 2 4 6 8 10 12 14 16 18

-1000

0

1000

2000

3000

4000

TVa,K

f,GHz

0 6 12 18

0

4000

8000

f,GHz

TVa,K

0 2 4 6 8 10 12 14 16 18-1000

0

1000

2000

f,GHz

Ta,V

2004.03.05 AR 10570

2004.03.06

2004.03.07

The coronal loops are an inherent part of active regions and play an important role in processes occurring in these regions and resulting in solar flares. The hot loop radiation can markedly influence on the active region properties in cm-dm range.

Research of specific spectral and polarizational features of microwave emission from coronal magnetic loops is an available way to retrieve physical conditions in coronal magnetic loops.

3. Model of hot loopodel of hot loop

R0 – radius of the loop, B0 – magnetic field at the loop axis

22

00

hy

RBB

Coronal loops usually connect opposite polarities of magnetic field. We consider 3D-model of the coronal magnetic loop as a hot half-torus. For our simulations we have chosen a simple form of magnetic field with magnetic force lines as semicircles (similar to Zlotnik et al. 2007a, 2007b; Brosius and Holman,1987). This form of magnetic field lines coincides with the levels of equal magnetic field, determining gyroresonance layers, where the cyclotron radiation at a given frequency emerges.

2

222

02

0 exp(1a

hyRxTTT a

0,5 1,0 1,5

0,01

0,1

1

h, 109 cm

T, 106 K

Kinetic temperature is assumed to be enhanced in torus along magnetic field line:

cc

cchchc

cchchc

chch

hhT

hhhhh

hhThhT

hhT

hT

,

,)()(

,

)(0

hch = 2.0×108

cmhc = 3.0×108

cm

Tch = 104 K

Tc = 106 K

ac

a

ch

a

ach

q

hhT

hh

T

hN

hhT

hN

hN

,105

exp105

exp

,105

exp

)(

330

30

2сch

a

hhh

0,0 0,5 1,0 1,5 2,0

1E9

1E10

1E11

Nq, 10-3 cm

h, 109 cm

The electron density decreases with the height according to barometric law from an initial value:

It is necessary to note that we do not contemplate to construct a model of the physically realistic coronal loop which would be able to provide the observed properties of a certain radio source. Our purpose is to consider a possible influence of coronal loop on the observed properties and to explain some specific sources of centimeter and decimeter radiation from active regions.

According to well known relations for optical depth and brightness temperature for thermal cyclotron radiation (Zheleznyakov,1970, Zlotnik,1968) we calculate emission spectra of ordinary and extraordinary radiation from the modeled source.

We take into account the dependence of optical thickness of gyroresonance layers on plasma temperature and electron density, as well as on the angle between magnetic field and the line-of-sight.

4. Model calculations

4 6 8 10 12 140,0

2,0x105

4,0x105

6,0x105

8,0x105

B0=500 G

f, GHz

Tv, K

6 8 10 12 14 160,00

2,50x105

5,00x105

f,GHz

TV, K

B0=700 G

300-500 G 10 GHz - 700 G. y=0.2×109, N0=2×1011 см-3.

6 8 10 12 14 160,0

2,0x105

4,0x105

6,0x105

8,0x105

1,0x106

f, GHz

Tv, K

a=2 ×108 cm; B0=700 Gs

4 6 8 10 12 140,00

0,01

0,02FV, sfu

f, GHz

TV(x,y), TI(x,y) y =[ 0 , 0.5 Ro]

5. Influence of magnetic field distribution on calculated TV and TI spectra.

322 /1),(

dyh

ByhB th

3/1300

20

3/2300

23/1300

200

200

))((

)()(

RBBBB

RBBRBBRBRBBd

thth

ththth

4 6 8 10 12 140,00

2,50x105

5,00x105

TV, K

f, GHz

y=0, R0=9×108 cm, a=1×108 cm, B0=500 G

Takakura, T.: 1972, Solar Phys. 26, 151

6 8 10 120

1x106

2x106

3x106

4x106

f, GHz

TV,K

y=1

Dipole approximation of magnetic field:

ARfmax,

109GHz

fleft,

109GHz

fright,

109GHz

B0, Gs

90772000.07.10

4.9 4.0 5.7 440 0.35

90772000.07.16

4.0 3.6 5.5 360 0.48

105702004.03.05

5.0 3.7 6.1 450 0.48

105702004.03.06

4.6 4.2 5.4 410 0.26

105702004.03.07

5.0 3.7 6.6 450 0.58

104882003.10.29

4.9 3.5 6.6 440 0.63

a magnetic field strength of the hot loop a magnetic field strength of the hot loop a product of the relative magnetic field gradient by the loop a product of the relative magnetic field gradient by the loop thicknessthickness

hB

B

The unusual spectra of solar flare productive active The unusual spectra of solar flare productive active regions were discovered by radio telescope RATAN-600 in 2 – 16 regions were discovered by radio telescope RATAN-600 in 2 – 16 GHz frequency range with a sufficient depression (at 6-12 GHz) of GHz frequency range with a sufficient depression (at 6-12 GHz) of polarized emission (Stokes parameter V).polarized emission (Stokes parameter V).

Some simple models of hot loop (as a torus and as a Some simple models of hot loop (as a torus and as a dipole approximation) were used for calculations of frequency dipole approximation) were used for calculations of frequency structure of microwave source emission with mentioned peculiarities. structure of microwave source emission with mentioned peculiarities.

These simulations have confirmed the possible These simulations have confirmed the possible interpretation of these polarization peculiarities by a presence of hot interpretation of these polarization peculiarities by a presence of hot loops in the solar corona. loops in the solar corona.

The observed parameters of polarization emission The observed parameters of polarization emission allow to estimate a magnetic field strength of the hot loop allow to estimate a magnetic field strength of the hot loop BB0=360-450 0=360-450 Gs and a product of the relative magnetic field gradient by the loop Gs and a product of the relative magnetic field gradient by the loop thickness thickness 0.26 до 0.630.26 до 0.63 . .

6. Conclusions

• These characteristics are not strongly affected by magnetic field model due to small thickness of loop;

• In frames of torus model these characteristics are not nether affected by limited space resolution of RATAN-600 as the measured spectra are approximately equals at different positions along the active region. BUT:

• For more complicated magnetic field configurations the integral characteristics of hot loop radio emission can be rather different in comparison to torus hot loop model.