Brightness,polarization and electron density of the solar corona of 1980 february 16

During the eclipse of 1980 February 16 we photographed the solar corona at an effective wavelength of 6300 å. Using a quadruple camera we also obtained the coronal pictures in polarized light for four Polaroid orientations. We have used these observations to derive the coronal brightness and polarization and from these the electron densities in the corona out to a distance of about 2.5 R⊙ from the centre of the disc. The coronal brightness matches well with that of the corona of 1958 October 12.     

Ca II K line profile of the truly quiet Sun

While evaluating the chromospheric variability (solar cycle related or any other) using the Ca II K line (3933.684 Å) as an indicator, an essential prerequisite is the knowledge of the profile of a truly quiet Sun in the integrated light. Such a profile can serve as a bench mark over which enhancements can be measured, particularly when modelling variability. This paper describes how such a K-line profile has been derived for the quiet Sun using disc-integrated light.     

Duration of Polar Activity Cycles and Their Relation to Sunspot Activity

We have defined the duration of polar magnetic activity as the time interval between two successive polar reversals. The epochs of the polarity reversals of the magnetic field at the poles of the Sun have been determined (1) by the time of the final disappearance of the polar crown filaments and (2) by the time between the two neighbouring reversals of the magnetic dipole configuration (l=1) from the H synoptic charts covering the period 1870–2001. It is shown that the reversals for the magnetic dipole configuration (l=1) occur on an average 3.3±0.5 years after the sunspot minimum according to the H synoptic charts (Table I) and the Stanford magnetograms (Table III). If we set the time of the final disappearance of the polar crown filaments (determined from the latitude migration of filaments) as the criterion for deciding the epoch of the polarity reversal of the polar fields, then the reversal occurs on an average 5.8±0.6 years from sunspot minimum (last column of Table I). We consider this as the most reliable diagnostic for fixing the epoch of reversals, as the final disappearance of the polar crown filaments can be observed without ambiguity. We show that shorter the duration of the polar activity cycle (i.e., the shorter the duration between two neighbouring reversals), the more intense is the next sunspot cycle. We also notice that the duration of polar activity is always more in even solar cycles than in odd cycles whereas the maximum Wolf numbers W _\max is always higher for odd solar cycles than for even cycles. Furthermore, we assume there is a secular change in the duration of the polar cycle. It has decreased by 1.2 times during the last 120 years.     

Poleward migration of the magnetic neutral line and the reversal of the polar fields on the Sun

Poleward migration of the magnetic neutral line on the Sun has been calculated for the periods 1945–1950 and 1955–1981 using synoptic charts based on H observations. Epochs of sign reversal of the solar magnetic field at latitudes 50° to 90° have been determined for these periods. During the cycles 19 and 20 a threefold sign reversal took place in the northern hemisphere. During all the above cycles both the solar poles were of one polarity for a period ranging from 0.5 to 1 year. The poleward drift velocity of the magnetic neutral line varies from 6 to 29 m s^–1 and seems to depend on the strength of the cycle.     

Measurement of Kodaikanal white-light images

A program of digitization of the daily white-light solar images from the Kodaikanal station of the Indian Institute of Astrophysics is in progress. A similar set of white-light data from the Mount Wilson Observatory was digitized some years ago. In both cases, areas and positions of individual sunspot umbrae are measured. In this preliminary report, comparisons of these measurements from the two sites are made. It is shown that both area and position measurements are in quite good agreement. The agreement is sufficiently good that it is possible to measure motions and area changes of sunspots from one site to the next, involving time differences from about 12 hours to about 36 hours. This enables us to trace the motions of many more small sunspots than could be done from one site alone. Very small systematic differences in rotation rate between the two sites of about 0.4% are found. A portion of this discrepancy is apparently due to the difference in plate scales between the two sites. Another contributing factor in the difference is the latitude visibility of sunspots. In addition it is suggested that a small, systematic difference in the measured radii at the two sites may contribute a small amount to this discrepancy, but it has not been possible to confirm this hypothesis. It is concluded that in general, when dealing with high precision rotation results of this sort, one must be extremely careful about subtle systematic effects.Operated by the Association of Universities for Research in Astronomy, Inc., under Cooperative Agreement with the National Science Foundation.     

Observational study of the five-minute oscillations in the solar atmosphere

The 5-min oscillations in the photospheric velocity fields have been studied in detail from measurements on 14 absorption lines from three time sequences of spectrograms of high quality. The lines cover a range of heights in the solar atmosphere from log = + 0.2 to -1.2. Regions oscillating coherently are seen to have an average dimension of 8000 km and the oscillations in general last for 2 to 3 periods. The power spectrum analysis of high resolution enabled to determine the period of oscillation at each level very precisely. The period decreases with increase in height, being 304 s at the level log = + 0.2 and 295 s at the level log = -1.2. The low level lines possess considerable power in the low frequency range representing the convective overshoot from below. The oscillatory power increases with height, while the low frequency power decreases and the high frequency component remains substantially constant in the heights studied.The intensity fluctuations in the continuum, the line wing and core of Fe i 6358.695 have also been studied. The continuum power spectrum has practically all the power near the zero frequency range, with a very weak oscillatory component. The line wing intensity fluctuations resemble those in the continuum, whereas the line core clearly shows an oscillatory component similar to the velocity oscillations.     

IGU Commission on Changing Rural Systems Subcominission on Highlands and High-Latitude Zones

The International Geographical Union Reports

IGU Commission on Changing Rural Systems Subcominission on Highlands and High-Latitude Zones     

Does the Poleward Migration Rate of the Magnetic Fields Depend on the Strength of the Solar Cycle?

We present the pattern of the polar magnetic reversal for cycle 23 derived from H synoptic charts and have also included the reversals of the earlier cycles 18–22 for comparison. At the beginning of a new cycle (i.e., soon after the polar reversal) the zonal boundaries of unipolar magnetic regions of opposite polarities (seen as filament bands on the synoptic charts) appear close to and on either side of the equator continuing through the years of minimum indicating the onset of the cancellation of flux at these low latitudes. The cycle thus starts with cancellation of flux close to the equator and ends with the polar reversal or flux cancellation near the poles. The filament bands just below the polemost ones migrate and reach latitudes 35°–45° by the time of polar reversal and become the polemost, once the polar reversal has taken place. During the years of minimum that follow, these filament bands remain more or less stagnant at the latitudes 35°–45° except for occasional slow migration towards the equator. The migration to the poles starts at a low speed of 3 m s^–1 only when the spot activity has risen to a significant level and then it accelerates to 30 m s^–1 at the peak of the activity. It takes 3–4 years for the polemost bands to reach the poles moving at these high speeds. We quantify this possible cause and effect phenomenon by introducing the concept of the `strength of the solar cycle' and represent this by either of a set of three parameters. We show that the velocity of poleward migration is a linear function of the `strength of the solar cycle'.     

Archean Granitoids at the Contact of Eastern Ghat Granulite Belt and Singhbhum-Orissa Craton, in Bhuban-Rengali Sector, Orissa, India

The small granite plutons occurring at the contact of the Singhbhum-Orissa Iron Ore craton (IOC) to the north and the Eastern Ghat Granulite Belt (EGGB) to the south in eastern Indian shield are characterised by the presence of enclaves of the granulites of EGGB and the greenschist facies rocks of IOC. These granites also bear the imprints of later cataclastic deformation which is present at the contact of the IOC and the EGGB. In situ Pb-Pb zircon dating of these granites gives minimum age of their formation 2.80 Ga. A whole-rock three point Rb-Sr isochron age of this rock is found to be 2.90 Ga. Therefore, the true age of formation of these granites will be around 2.90–2.80 Ga. These granitic rocks also contain xenocrystic zircon components of 3.50 Ga and show a later metasomatic or metamorphic effect 2.48 Ga obtained from the analyses on overgrowths developed on 2.80 Ga old zircon cores. The presence of granulitic enclaves within these contact zone granite indicates that the granulite facies metamorphism of the EGGB is 2.80 Ga or still older in age. The cataclastic deformations observed at the contact zone of the two adjacent cratons is definitely younger than 2.80 Ga and possibly related to 2.48 Ga event observed from the overgrowths. As 2.80 Ga granite plutons of small dimensions are also observed at the western margin of the IOC; it can be concluded that a geologic event occurred 2.80 Ga over the IOC when small granite bodies evolved at the marginal part of this craton after its stabilisation at 3.09 Ga.     

An empirical relation of daytime equatorial total electron content with equatorial electrojet in the Indian zone

The variability of Total Electron Content (TEC) at Trivandrum, located within equatorial anomaly region at the dip equator, with respect to a reference level derived from the TEC measurements at Shimla, located outside the region has been studied during low solar activity period. Chapman function is assumed to hold good for regions outside the anomaly extent. It shows that the difference of total measured TEC at the equator from the derived reference is highly correlated with equatorial electrojet. The observations conform to the previous investigations and are interpreted in light of established relations. A stochastic relationship with electrojet is derived and validated.