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.