Radio Sounding of the Near-Sun Plasma Using Polarized Pulsar Pulses

I.V.Chashei, T.V.Smirnova, V.I.Shishov

Pushchino Radio Astronomy Obsertvatory, Astrospace Center, Lebedev Physical Institute

ABSTRACT

The results are presented of radio sounding observations probing the inner solar wind near the minimum of solar activity cycle using polarized pulsar pulses from PRS B0525+21 and PSR B0531+21 received when the lines of sight toward these pulsars was close to the Sun. The observations were obtained in June 2005 and June 2007 on the Large Phased Array of Lebedev Physical Institute at 111 MHz. An upper limit for the scattering of giant pulses from PSR B0531+21 due to their passage through the turbulent solar wind plasma is determined. The arrival-time delays for pulses from PSR B0531+21 are used to derive the radial dependence of the mean density of the circumsolar plasma. The resulting density distribution indicates that the acceleration of fast high latitude solar wind outflows continues to heliocentric distances of 5-10 RS, where RS is the solar radius. The mean plasma density at heliocentric distances of about 5 RS is 1.4

104 cm-3 , substantially lower than at solar activity maximum. This is associated with the presence of polar coronal holes. The Faraday rotation measure at heliocentric distances of 6-7 RS is estimated. Deviation of the

spatial distribution of the magnetic field from spherical symmetry are comparatively modest in the studied range of heliocentric distances.

Propagation effects of pulsed signals in the solar wind

• Dispersion delay of pulses

• Scattering of pulses on electron density fluctuations

• Faraday rotation of polarization plane

Dispersion delay of pulses

t = A 2 DM , A = const

• DM = N dz

Scattering of pulses on electron density fluctuations

• Structure function of radio wave phase fluctuations

DS() =2(re)2 <Nl2(R0)> (/l)2 lReff

l – turbulence inner scale

• Wave field coherence scale 0

DS(0) = 1

• Scattering angle sc

sc = / 2 0

• Pulse broadening by scattering

sc = z sc2 / 2 c

Scattering of pulses on electron density fluctuations

t

• I(t) = I0 exp ( - t2 / 02 ) exp [( t - t) / sc ] dt

-• I(t) = I0 exp (0

2/4 sc2 – t/ sc) [ 1 - (0/2 sc – t/ 0 ) ]

• Max. at t 0

Faraday rotation of polarization plane

= C 2 RM , C = const

• RM = N Bz dz

Observations

• Radio telescope BSA LPI : geometric area 70 000 м2,

operation frequency 111 МHz

• Receiver: 96 channels 20 кГц, whole bandwidth 1.92 МHz, integration time 2.68 мs

• Pulsar В 0525+21: period 3,74 s, two components, polarization degree 40-50 %; yy 2005, 2007.

• Pulsar in the Crab nebula В 0531+21: period 33 мs, giant pulses, polarization degree10 % ; y 2007 .

Antenna BSA LPI

Pulsars coronal occultation

Example of pulsar В 0525 record

Dispersion delay for pulsar PSR В 0525, 2005, 2007

Pulse delay and ambient plasma density

• Dispersion delay upper estimate :

t < 2 ms DM < 2 1016 cm-2

N (5.5 R) < 2 104 cm-3 (2005)

t < 4 ms DM < 4 1016 cm-2

N (5.5 R) < 4 104 cm-3 (2007)

• Counselman & Rankin, 1969-1970 observations , 111-430 Мгц :

t 5 ms, N (10 R) 7 103 cm-3, N (5.5 R) 6 104 cm-3

Dispersion delay for PRS B 0531, 2007

Density dependence on the distance from the Sun, PSR B 0531, 2007

Giant pulses of PSR В 0531

PSR В 0531scattering

Scattering and density fluctuations

• Upper estimate of pulse broadening (B 0531):

sc < 2 ms scattering angle sc < 3 10-3 rad 10

• Counselman & Rankin, 1969-1970 observations, 111-430 МHz :

sc 6 ms

PSR В 0525 Faraday rotation

Rotation measure radial profile В 0525

Rotation measure

• Rotation measure upper estimate at R = 7 R :

RM < 6 rad/m2 (2005 y.) RM < 3 rad/m2 (2007 y.)

• Bird et al., observations 1978-1979 y.: RM < 10 rad/m2

Conclusions

• Our estimates of t, sc show, that in the years 2005-2007 mean density, as well density fluctuations were several times (2-3) lower, than in observations 1969-1970 г. (Counselman & Rankin).

• During period of closest approach solar offset point of radio path was located at high heliolatitudes, about 80о. Possible explanation: polar coronal hole with low plasma density at solar activity minimum.

• Our estimates of rotation measure are in agreement with the results of Bird et al., (1978-1979г.). The deviations in spatial magnetic field distribution from spherically symmetric are modest: Bns < 610-3 G, Br> 210-2 G .

• Radial density profile ( B 0531, 2007) show that the acceleration of fast solar wind continues up to the distances about 5 – 10 RS .