K. P. Raju

The Physical Conditions in a Polar Coronal Hole and Nearby Regions from Norikura and SOHO Observations

The distribution of emission-line intensities, Doppler velocities, and line widths in a polar coronal hole and nearby regions are obtained from the spectroscopic observations carried out on 1998 November 3 at the Norikura Solar Observatory, Japan. The coronal red line [Fe X] λ6374 that is prominent at coronal hole temperatures is used for the study. The coronal images in Fe IX and Fe X 171 A and Fe XII 195 A from the Extreme-Ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO) are used to get the temperature map of the corona at the time of observation. Combining both, we have obtained the nonthermal velocities in the region without the usual assumption of a uniform ion temperature. Several plume structures are identified within the coronal hole, and it is found that line widths are smaller in plumes than in the interplume regions, which is also reported from recent SOHO observations. The line-of-sight Doppler velocities in the coronal hole are larger than those in the quiet region, probably because of the excess outflow in the coronal hole. A rough negative correlation between intensity and Doppler velocity, similar to that between intensity and line width, is observed in the coronal hole. The typical nonthermal velocity in coronal holes is 24 km s-1 while that in quiet regions is 15 km s-1. The enhanced nonthermal velocity in the coronal hole is suggestive of the important role of the nonthermal broadening mechanism in the acceleration of fast solar wind. Also, the nonthermal velocities are larger (up to 27%) at the interplume regions as compared to plumes. The findings generally support the prevailing view that the interplume regions are the source regions of the fast solar wind.

The Correlation Lifetimes of Chromospheric Ca II K Network Cells

The lifetimes and spatial scales of emission network cells in the solar chromosphere were studied from a nearly continuous sequence of Caii K filtergrams, obtained from Antarctica. The temporal autocorrelation function (ACF) was calculated for spatially-aligned windows from the time sequence of filtergrams. The lifetime of network cells was found to be dependent on the activity of the region. The estimated lifetimes are 24–34 hours for quiet-region cells and 58–61 hours for active-region cells. The temporal ACF shows prominent undulations in some of the quiet-region windows. The spatial ACF reveals the periodicity of the emission network in quiet regions.

The Dependence of Chromospheric Ca II K Network Cell Sizes on Solar Latitude

Calcium K line spectroheliograms obtained during the solar minimum phases at Kodaikanal between 1913–1974 have been used to study the network cell sizes. The autocorrelation is calculated for two-dimensional strips at 5° interval up to ±50° latitude. The average size of the network cells was found to have a dependence on solar latitude with a maximum variation of about 7%. The pattern shows an apparent north–south symmetry with two minima at about 20° N and S.

ANALYSIS OF CORONAL GREEN LINE PROFILES: EVIDENCE OF EXCESS BLUESHIFTS

Coronal green line (Fe XIV 5303 A) profiles were obtained from Fabry-Perot interferometric observations of the solar corona during the total solar eclipse of 2001 June 21 from Lusaka, Zambia. The instrumental width is about 0.2 A and the spectral resolution is about 26,000. About 300 line profiles were obtained within a radial range of 1.0-1.5 R ☉ and a position angle coverage of about 240°. The line profiles were fitted with single Gaussians, and their intensities, Doppler velocities, and line widths were obtained. Also obtained were the centroids of the line profiles, which give a measure of line asymmetry. The histograms of Doppler velocity show excess blueshifts, while the centroids reveal a predominant blue wing in the line profiles. It was found that the centroids and the Doppler velocities are highly correlated. This points to the presence of multiple components in the line profiles, with an excess of blueshifted components. We then obtained the (Blue-Red) wing intensities, which clearly reveal the secondary components, the majority of which are blueshifted. This confirms that the coronal green line profiles often contain multicomponents with excess blueshifts, which also depend on the solar activity. The magnitude of the Doppler velocity of the secondary components is in the range of 20-40 km s–1 and shows an increase toward the poles. Possible explanations of the multicomponents are the type II spicules that were recently found to be important to coronal heating or the nascent solar wind flow, but the cause of the blue asymmetry in the coronal lines above the limb remains unclear.

Network and plage indices from Kodaikanal Ca-K data

The Ca II K filtergrams from Kodaikanal Solar Observatory have been used to study solar activity. The images are dominated by the chromospheric network and plages. Programs have been developed to obtain the network and plage indices from the daily images as functions of solar latitude and time. Preliminary results from the analysis are reported here. The network and plage indices were found to follow the sunspot cycle. A secondary peak is found during the period of declining activity in both the indices. A comparison of network indices from the northern and the southern hemispheres shows that the former is more active than the latter. However such an asymmetry is not clearly seen in the plage index.

Spectroscopic observation of coronal oscillations

Abstract A time sequence of coronal green line spectra was obtained with a coronagraph at the Norikura Solar Observatory. The Fourier analysis shows oscillatory powers in the Doppler velocity of the line in the 1–3 mHz range. The phase relationship between the Doppler velocity and the line intensity indicates that the oscillatory signal is due to propagating (rather than standing) sound waves.