linkages between the atmosphere on the Earth and sun Phần 7

320 Fig. 2 Same as Fig.1, but at 109MHz. The hump between 6.1 and 6.3UT, to the left of the Stokes I deflection, is a sidelobe artifact 2.5 x 105 2 R. Ramesh et al. Sun − 109 MHz − 2006/08/11 1.5 Stokes I 1 0.5 Stokes V 0 6.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9 7 7.1 Universal Time (hrs) Fig. 3 GRH observations of 11 August 2006 at 77MHz. The peak value is Tb 2:6 108 K. The open circle at the center represents the limb of the solar photosphere, the ellipse near the bottom-right corner the GRH “beam” at 77MHz 4 N 3 Gauribidanur radioheliogram − 77 MHz E 2 1 0 −1 −2 −3 2006/08/11 − 06:30 UT −4 4 −3 −2 −1 0 1 2 3 4 Solar radii the radioheliograms, was about 4:10/day. This corresponds to r 1:1Rˇ in the solar atmosphere for the emission region (Ramesh et al. 2000). A comparison of the radioheliogramsfrom 11 and 18 August 2006 with the ones observed on 10 and 19 August 2006 indicates that the discrete source of intense emissionwas presentonlyduringtheinterval11–18August2006.We calculatedthe spectral index of the Stokes I emission observed with the polarimeter; its average value was 0:84, indicating that the observed emission is nonthermal in nature. As radio noise storms are the only long-duration (days) events in the solar atmosphere that belong to this category [McLean and Labrum 1985 and references therein], we argue that the circularly polarized emission observed with the new Gauribidanurra-dio polarimeter must be due to noise storm emission. The estimated average degree of circular polarization was V=I 0:53 at 77MHz and 0.64 at 109MHz, which supports this claim (Kai 1962). Low-Frequency Radio Observations of Coronal Magnetic Fields 321 Fig. 4 Same as Fig.3, but for 18 August 2006. The peak value is Tb 3:1 108 K. The isolated contour beyond the east limb is due to local interference 4 N 3 Gauribidanur radioheliogram − 77 MHz E 2 1 0 −1 −2 −3 2006/08/18 − 06:30 UT −4 4 −3 −2 −1 0 1 2 3 4 Solar radii Fig. 5 SoHO-MDI magnetogram obtained on 11 August 2006. The bright region located close to the east limb is AR 10904. Other than AR 10903 (located to the west of AR 10904), there are no dominant magnetic regions on the solar disk 3 Results and Discussion In general, any transient or long-duration phenomenonobserved in the solar corona should have its origin in corresponding activities at lower levels in the atmosphere. This is particularly true for radio noise storms as it is well established that these are closely associated with sunspot groups in the photosphere (e.g., Elgarøy 1977). We therefore inspected SoHO-MDI magnetogram images to identify the photospheric counterpartofthe observedcircularpolarizationat 77and109MHz. Figures5and6 showtheseimagesfor11and18August2006(CR2046).AcomparisonwithFigs.3 and 4 indicates that the bright magneticregionAR 10904(S14 E63)located close to the east/west limb of the Sun on 11/18 August 2006,respectively,must be primarily Evolution of Near-Sun Solar Wind Turbulence P.K. Manoharan Abstract This paper presents a preliminary analysis of the turbulence spectrum of the solar wind in the near-Sun region R < 50Rˇ, obtained from interplanetary scintillation measurements with the Ooty Radio Telescope at 327MHz. The results clearly showthat the scintillation is dominatedby densityirregularitiesof size about 100–500km. The scintillation at the small-scale side of the spectrum, although sig-nificantly less in magnitude, has a flatter spectrum than the larger-scale dominant part. Furthermore, the spectral power contained in the flatter portion rapidly in-creases closer to the Sun. These results on the turbulence spectrum for R < 50Rˇ quantify the evidence for radial evolution of the small-scale fluctuations (50 km) generated by Alfve´n waves. 1 Introduction The solar windis highlyvariableand inhomogeneous,andexhibits fluctuationsover a wide range of spatial and temporal scales. The properties of these fluctuations, as they move outward in the solar corona, are controlled by the presence of both waves and turbulence (e.g., Coleman 1968, Belcher & Davis 1971). However, their relative contributions to the heating and acceleration of the solar wind have yet to be assessed fully (Tu & Marsch 1995, Harmon & Coles 2005). Radio scattering and scintillation experiments measure density fluctuations, which are related to the wave field, density fluctuations, and magnetic turbulence (e.g., Higdon 1986, Montgomery et al. 1987). The density fluctuation spectrum roughly follows a Kolmogorov power law in the spatial scale range 100–1,000km, at distances well outside the solar wind acceleration region. However, nearer to the Sun the spectrum tends to be flat (e.g., Woo & Armstrong 1979). The spectrum of the high-speed streams from coronal holes is steeper than Kolmogorov decay, which is attributed to dissipation at scales above 100km (e.g., Manoharan et al. P.K. Manoharan () Radio Astronomy Centre, National Centre for Radio Astrophysics, Tata Institute of Fundamental Research, Udhagamandalam (Ooty), India S.S. Hasan and R.J. Rutten (eds.), Magnetic Coupling between the Interior 324 and Atmosphere of the Sun, Astrophysics and Space Science Proceedings, DOI 10.1007/978-3-642-02859-5 27, Springer-Verlag Berlin Heidelberg 2010 Evolution of Near-Sun Solar Wind Turbulence 325 1994, 2000). There is considerable interest to understand the radial change of the fluctuations due to both waves and turbulence in the solar wind acceleration re-gion. In this study, spectral features are analyzed over a range of distances from the Sun using interplanetary scintillation measurements made with the Ooty Radio Telescope at 327MHz (Swarup et al. 1971). 2 Interplanetary Scintillation Interplanetary scintillation (IPS) is the variability of distant compact radio source (e.g., a quasar or a radio galaxy) caused by microturbulence in the solar wind of spatial scales 10–1000km (e.g.,Manoharanet al. 1994).Scintillationmeasurements normally refer to the instantaneous departure of intensity (ıI.t/) from the mean in-tensity of the source(hIi), i.e., ıI.t/ = I.t/hIi. As the irregularitiesare convected by the solar wind, the statistical fluctuations of ıI.t/ can be used to estimate the speed and turbulence spectrum of the solar wind, integrated along the line of sight to the radio source. However, for a given line of sight, the spectrum of scintilla-tion drops rapidly with distance from the Sun, C2.R/ R4, and the scattering is thereforeconcentratedwhere the line-of-sightis closest to the Sun. The shape of the turbulence spectrum can be inferred from the temporal IPS spectrum, obtained by taking the Fourier transformation of intensity time series. The rms intensity varia-tion ıI.t/2 1=2 is the integral of the power spectrum. The scintillation index, m, is estimated by m2 D hIi2 0fc P.f /df; (1) where fc is the cutoff frequency of the spectrum at which the scintillation equals the noise level. The systematic radial variation of C2.R/ can be obtained from the index vs. distance (m R) plots as in Fig.1. These smoothed plots represent av- erage scintillations observed over several years for two well-known radio quasars (Manoharan 2008). At given heliocentric distance, a compact source scintillates more than an ex-tended one, because Fresnel filtering plays a key role in producing the intensity fluctuations and the scintillation is heavily attenuated by a large angular size =Z, where is the wavelength of observation and Z is the distance to the scat-tering screen. The observationsreported in this study have been made with the Ooty Radio Telescope (ORT), which operates at D 0:92m. In the case of near-Sun IPS measurements,the scattering mediumis located at about1AU and thereforesources having angular size > 500milliarcsec do not scintillate. Figure 1 shows that as the Sun is approached, the scintillation increases to a peak value at a distance of R 40Rˇ, and then decreases for further closer so-lar offsets (e.g., Manoharan 1993), where 1 solar radius is Rˇ D 6:96 105 km. The peak or transition distance, R 40Rˇ, is the characteristic of IPS measure-ment at D 0:92m. It is a function of observing wavelength and moves close Evolution of Near-Sun Solar Wind Turbulence 327 Fig. 2 Sample temporal power spectra of 0138C136 on log-linear scale, showing spectral shape variations with distance from the Sun. The date and time of observation and the heliocentric dis-tance (R) are specified. These observations have been made at the eastern limb of the Sun so that the source approaches the Sun with increasing day number Fig. 3 Same as Fig.2 for radio source 0202C149 mainly limited by the ORT beam width. Figures 2 and 3 display temporal scintilla-tion spectra of radio quasars 0138C136 and 0202C149, observed at different solar offsets during April 2008. The sampling rate, 50Hz, employed in the present study in principle extends the temporal frequency range of the spectrum to 25Hz, which allows to infer the statistics of even small-scale turbulence. For example, for a typi-cal value of the solar wind speed V , the spectrum can cover spatial wavenumbers in the range 0:002 < q D .2f=V / < 0:2 km1, correspondingto scales in the range 5–500km. ... - tailieumienphi.vn