I. S. Kim

Delay times between geoeffective solar disturbances and geomagnetic indices

We have examined delay times between solar disturbances (X-ray flares and DSFs) and storm sudden commencements(SSC) as well as between SSC and major geomagnetic storms. To carry out cross-correlation analysis of these point series data, we have introduced a new correlation measure which is defined by the ratio of the median value of the absolute residual differences between two sets of time series data to the one determined from hypothetical target series. We have confirmed from the correlation analyses that (1) the most probable traveling time of a solar disturbance from the Sun to the Earth is estimated to be about 2 days for a disturbance associated with major (X and M class) solar flares, and about 3 days for a disturbance associated with DSFs, (2) long-duration flares are better correlated with SSCs than short-duration flares, (3) travelling times of solar disturbances strongly depend on the heliolongitude where they originate, and (4) solar disturbances associated with flares and DSFs at the western limb can hardly reach the Earth.

Near-Limb Zeeman and Hanle Diagnostics

Abstract“Weak” magnetic-field diagnostics in faint objects near the bright solar disk are discussed in terms of the level of non-object signatures, in particular, of the stray light in telescopes. Calculated dependencies of the stray light caused by diffraction at the 0.5-, 1.6-, and 4-meter entrance aperture are presented. The requirements for micro-roughness of refractive and reflective primary optics are compared. Several methods for reducing the stray light (the Lyot coronagraphic technique, multiple stages of apodizing in the focal and exit pupil planes, apodizing in the entrance aperture plane with a special mask), and reducing the random and systematic errors are noted. An acceptable level of stray light in telescopes is estimated for the V-profile recording with a signal-to-noise ratio greater than three. Prospects for the limb chromosphere magnetic measurements are indicated.

Measurements of the magnetic field in solar prominences with a spectrally scanning magnetograph

We describe observations with a new magnetograph capable of recording the whole profile of emission lines in prominences. Two recordings are used simultaneously to study the Zeeman effect in circularly polarized light. The spectral scan is produced by the action of piezo ceramics of a Perot-Fabry inter ferometer combined with a narrow band interference filter.The instrument is calibrated using 100% circularly polarized light and an emission line produced in Laboratory conditions in a simulated longitudinal magnetic field. The magnetograph was attached to the large coronagraph (Ø 53 cm) of Kislovodsk to give a series of measurements of the Hβ line of several quiescent and active prominences. The observed values of the longitudinal component of the magnetic field are between: -25 G + 13 G with a noise level at ±2 G for a corresponding resolution of 8 arc sec.Effects produced by the instrumental polarization are discussed.

Observations of neutral hydrogen in the corona

Measurements of the linear polarization degree (p) of the emission in the inner corona (<1.5 R⊙) in the green and red continuum ranges are discussed. “Excess p” (Δp = pgreen — pred), which reaches its maximum at 1.2–1.3 R⊙ (<5%) in the polar regions and 20% within large-scale streamers, is present everywhere in the corona. It has been mentioned that excess p can be explained by the presence of neutral hydrogen in the corona.

On the Fabry-Perot investigations of the solar corona: Eclipse observations of large-scale dynamics

Eclipse data on the large-scale velocity field of the green coronal line obtained by a Fabry-Perot interferometer combined with a blocking interference filter are presented. Eclipse interferograms of the solar corona of 1972 and 1981 indicate that coronal conditions are relatively quiet but not homogeneous. Doppler shifts of 95 % profiles do not exceed 20 km s−1. Large scale motions with velocities of 10 km s−1 are found for coronal regions. Effect of coronal rotation is evidently noted. Mean values of half width are are different for different coronal structures. In terms of “turbulent” velocities Vt equals 19.5, 22.5 and 27.5 in streamers, helmets and coronal holes respectively. A tendency for the decreasing of half width with distance in one helmet, possible increasing in holes and no evident dependence are found.

The coronograph-polarimeter: An algorithm for creation of solar corona polarization images

An algorithm for polarization imaging of the white light corona has been devised on the basis of the Stokes vector presentation and the IDL software applied to statistical data. The use of the algorithm for a 24-frame color negative polarization film of the corona of November 3, 1994, digitized in the “green” light allowed new data to be obtained on the distribution of the polarization degree in the “white” (summary of the blue and red spectral interval emissions) corona. On the average, the monotonic increase in the polarization degree, which is characteristic for the van de Hülst model, was not found over the range (1.2–2.0)R⊙in heliolatitudes ±45°.The polarization degree was virtually constant in the region (1.2–1.4)R⊙.The apparent “plateau” in the polarization degree distribution is obviously due to the deviation of the model with local spherical symmetry from the actual 3D structure of the inner corona.

Manifestations of electric currents observed in the K-corona

The 2D distribution of tangential velocities of the coronal plasma electron component (K-corona) was obtained and interpreted. Coronal continuum linear polarization films in the green spectral range obtained during the total solar eclipse of March 29, 2006, are used. The developed method of high-precision linear polarimetry made it possible to obtain the first 2D distribution in the K-corona linear polarimetry history for the polarization angle sign at distances smaller than 1.5 Rsun. For clarity, we accepted that clockwise deviations of the polarization direction from tangential to the solar limb have positive polarity, whereas counterclockwise deviations have negative polarity. The distribution differs from the anticipated pattern for scattering by resting electrons and reveals a correlation with the coronal structure and the presence of diffuse and structural components and largeand small-scale regions of opposite polarities. The interpretation in the scope of scattering by moving electrons indicates that free electron tangential velocities (tangential electric currents) are strongly fragmented in the inner corona.

The plane of polarization of the solar coronal emission on August 11, 1999

A two-dimensional polarization image of the inner regions of the solar corona (R≤1.5Rȩ) during the total solar eclipse of August 11, 1999 is presented. This image clearly exhibits both small-and large-scale structure in the distribution of deviations of the plane of polarization from its theoretical direction for coronal emission in the near infrared (570–800 nm). An accuracy for the deviation angles of ≤1° was achieved by reducing the instrumental scattered light in the telescope, installing a continuously rotating polaroid near the image plane of the entrance pupil (i.e., the Lyot stop plane), and developing a special algorithm for constructing the polarization images based on the IDL software, in which the properties of the light are described in terms of the Stokes parameters. This algorithm was used to process 24 digitized polarization images of the corona, corresponding to one complete rotation of the polaroid. Analysis of the polarization image for angles of 0°–5° indicates the existence of significant deviations in the inner corona. The polar and equatorial coronal regions are characterized by diffuse and almost uniform structure of the deviation angles, 0.5° ± 0.5°, corresponding to Thomson scattering of the photospheric radiation by free electrons. Four large-scale structures over the NE, SE, NW, and SW limbs covering about 60° in position angle have deviations of 1°–3°. Numerous small-scale structures with dimensions up to 30″ and deviation angles of 3°–5° tracing strongly curved coronal streamers were detected in active coronal regions, especially over the NE limb. Interpretation of these deviations in terms of flows of moving electrons implies tangential velocities of up to 2.5×104 km/s, i.e., electron energies of up to 2×103 eV.

Observing the Solar Magnetic Field

Problems of “weak” magnetic fields diagnostics in the upper solar atmosphere are discussed, where the Zeeman splitting is 3-4 orders of magnitude less than the line width. This corresponds to magnetic observations of chromosphere, prominences and the corona requiring more advanced technique as compared with the photosphere. Direct and indirect methods of magnetic researches in the upper solar atmosphere are briefly reviewed. The “signal-to-noise” ratio of magnetic records is shown to strongly depend on the stray light in the feeding telescope. Concordance of performances of a feeding telescope and Stokes polarimeter are analysed to carry out reliable magnetic measurements. Eclipse coronagraphic polarimetric observations are concluded to be powerful for low-resolution magnetic diagnostics based on analysis of Hanle effect in prominences and the emission line corona. Employment of a new generation large-aperture reflecting coronagraph allows to expect advances in high-resolution magnetic observations of prominence and coronal structures.

Recurrent solar wind streams

A new approach to investigating the statistical relationship between certain solar features and recurrent wind streams is presented. This approach is based, on a comparative analysis of the distributions of lifetimes of a set of solar features, recurrent geomagnetic disturbances, and geomagnetic “calms.” Correlation coefficients of 0.81, 0.85, 0.79, and 0.77 are found for the distributions of several solar features—filaments, large-scale magnetic fields, coronal features, and coronal holes, respectively—and recurrent geomagnetic disturbances. A correlation factor of 0.97 between the distributions of geomagnetic “calms” and active regions is found. The combined evidence indicates that no specific type of solar feature is responsible for the recurrent stream activity. Rather, the configuration of the large-scale magnetic field of the Sun appears to control the permanently existing corpuscular activity. Since prominences trace polarity division lines of the large-scale magnetic field structure of the Sun, they have been checked as a possible general predictor of recurrent corpuscular activity; their parameters could present the most reliable indices that relate closely with trends in geomagnetic disturbances. A comparative analysis of cyclic variations of sunspot numbers, the total number of prominences, the relative number of low-height (<-20″) prominences, and recurrent geomagnetic storms is made for solar cycle N16. The relative number of low-height prominences is found to correlate broadly (0.83) with recurrent wind streams.