Giant Meterwave Radio Telescope observations of an M2.8 flare: Insights into the initiation of a flare–coronal mass ejection event

We present the first observations of a solar flare with the GMRT. An M2.8 flare observed at 1060 MHz with the GMRT on 17 November 2001 was associated with a prominence eruption observed at 17 GHz by the Nobeyama radioheliograph and the initiation of a fast partial halo CME observed with the LASCO C2 coronagraph. Towards the start of the eruption, we find evidence for reconnection above the prominence. Subsequently, we find evidence for rapid growth of a vertical current sheet below the erupting arcade, which is accompanied by the flare and prominence eruption.     

Radio Detection of a Rapid Disturbance Launched by a Solar Flare

We report the direct observation of motion associated with a solar flare at a speed of 26,000 km s-1. The motion is seen from a radio source at 0.33 GHz, which suddenly starts moving during the flare. At its peak, the radio source covers a quiet region of dimension 500&arcsec;. Emission from any given location is sporadic. The disturbance itself does not seem to radiate, but it excites coronal features that continue to radiate after it passes. The inferred velocity is larger than any previously inferred velocity of a disturbance in the solar atmosphere apart from freely streaming beams of accelerated electrons. The observed motion of the source at a fixed frequency, low polarization, and moderate bandwidth are more consistent with the typical properties of moving type IV radio bursts than with classical coronal shock-associated type II bursts, but any disturbance at such a high velocity must be highly supersonic and should drive a shock. We speculate that the disturbance is associated with the realignment of magnetic fields connecting different portions of an active region.     

Spread-F during the magnetic storm of 22 January 2004 at low latitudes: Effect of IMF-Bz in relation to local sunset time

The paper describes the results of spread-F at low latitude stations around the world during the magnetic storm starting at 0130 UT on 22 January 2004. The storm can be divided into two phases, first phase up to 1000 UT when interplanetary magnetic field IMF-Bz was highly fluctuating around a small positive value and the second phase after a sudden large southward turning of IMF-Bz at 1030 UT. The first phase produced strong spread-F at Jicamarca, Sao Luis, and Ascension Island and caused complete inhibition of spread-F at Thumba and Waltair in India. It generated weak spread-F at Ho Chi Minh City in Vietnam and strong spread-F at Hainan and Chung Li. The strong spread-F at Hainan and Chung Li were caused by the positive IMF-Bz during the first phase of the storm and not by the negative pulse of IMF-Bz at 1000 UT.     

Possible contribution of a solar transient to enhanced scintillation of a quasar

Observations of the quasar 2314 + 038 were carried out during 16–21 December, 1985 at a solar elongation () around 85 , when the plasma tail of comet Halley swept in front of it. These observations have shown a two-fold increase in scintillation index (m) as compared to the expected levels of scintillation for the source, computed using the well-known RKH model. Spacecraft data and the geomagnetic indices available during the period show that a shock-front had reached the earth on the 18 December, the day when maximum increase in scintillation was recorded. The possible contribution of such a shock-front to the enhancement has been shown to be not greater than 15 percent. Hence, the major contribution to the enhancement came from the plasma tail of a comet.     

Solar Polar Fields During Cycles 21 – 23: Correlation with Meridional Flows

We have examined polar magnetic fields for the last three solar cycles, viz. Cycles 21, 22, and 23 using NSO/Kitt Peak synoptic magnetograms. In addition, we have used SOHO/MDI magnetograms to derive the polar fields during Cycle 23. Both Kitt Peak and MDI data at high latitudes (78° – 90°) in both solar hemispheres show a significant drop in the absolute value of polar fields from the late declining phase of the Solar Cycle 22 to the maximum of the Solar Cycle 23. We find that long-term changes in the absolute value of the polar field, in Cycle 23, are well correlated with changes in meridional-flow speeds that have been reported recently. We discuss the implication of this in influencing the extremely prolonged minimum experienced at the start of the current Cycle 24 and in forecasting the behavior of future solar cycles.     

Sapphirine-bearing Granulites from Abu-Balaram Area, Gujarat State: Implications for India-Madagascar Connection

P-T conditions of sapphirine bearing granulites from a new locality from the north eastern part of Gujarat range from 8.2–8.8 kbars at T of 950°C. The discovery of this UHT assemblage in the north western part of the Indian shield bordering the Aravalli-Delhi fold belt provides additional evidences for India-Madagascar connection, linking the Aravalli- Malani sequences to similar lithologies in northern Madagascar.     

Flow Sources and Formation Laws of Solar Wind Streams

The large-scale stream structure of the solar wind flow is studied in the main acceleration zone from 10 to 40 solar radii from the Sun. Three independent sets of experimental data were used: radio astronomical observations of radio wave scattering using the large radio telescopes of the Lebedev Physical Institute; dual-frequency Doppler solar wind speed measurements from the Ulysses Solar Corona Experiment during the spacecraft's two solar conjunctions in summer 1991 and winter 1995; solar magnetic field strength and configuration computed from Wilcox Solar Observatory data. Both the experimental data on the position of the transonic region of the solar wind flow and the solar wind speed estimates were used as parameters reflecting the intensity of the solar wind acceleration process. Correlation studies of these data with the magnetic field strength in the solar corona revealed several types of solar wind flow differing in their velocities and the location of their primary acceleration region.     

Changes in Quasi-periodic Variations of Solar Photospheric Fields: Precursor to the Deep Solar Minimum in Cycle 23?

Possible precursor signatures in the quasi-periodic variations of solar photospheric fields were investigated in the build-up to one of the deepest solar minima experienced in the past 100 years. This unusual and deep solar minimum occurred between Solar Cycles 23 and 24. We used both wavelet and Fourier analysis to study the changes in the quasi-periodic variations of solar photospheric fields. Photospheric fields were derived using ground-based synoptic magnetograms spanning the period 1975.14 to 2009.86 and covering Solar Cycles 21, 22, and 23. A hemispheric asymmetry in the periodicities of the photospheric fields was seen only at latitudes above ±?45^° when the data were divided into two parts based on a wavelet analysis: one prior to 1996 and the other after 1996. Furthermore, the hemispheric asymmetry was observed to be confined to the latitude range of 45^° to 60^°. This can be attributed to the variations in polar surges that primarily depend on both the emergence of surface magnetic flux and varying solar-surface flows. The observed asymmetry along with the fact that both solar fields above ±?45^° and micro-turbulence levels in the inner-heliosphere have been decreasing since the early- to mid-nineties (Janardhan et al. in Geophys. Res. Lett. 382, 20108, 2011) suggest that around this time active changes occurred in the solar dynamo that governs the underlying basic processes in the Sun. These changes in turn probably initiated the build-up to the very deep solar minimum at the end of Cycle 23. The decline in fields above ±?45^°, for well over a solar cycle, would imply that weak polar fields have been generated in the past two successive solar cycles, viz. Cycles 22 and 23. A continuation of this declining trend beyond 22 years, if it occurs, will have serious implications for our current understanding of the solar dynamo.     

The Morphology of Decimetric Emission from Solar Flares: GMRT Observations

Observations of a solar flare at 617 MHz with the Giant Meter-wave Radio Telescope (GMRT) are used to study the morphology of flare radio emission at decimetric wavelengths. There has been very little imaging in the 500 – 1000 MHz frequency range, but it is of great interest, since it corresponds to densities at which energy is believed to be released in solar flares. This event has a very distinctive morphology at 617 MHz: the radio emission is clearly resolved by the 30″ beam into arc-shaped sources seeming to lie at the tops of long loops, anchored at one end in the active region in which the flare occurs, with the other end lying some 200 000 km away in a region of quiet solar atmosphere. Microwave images show fairly conventional behaviour for the flare in the active region: it consists of two compact sources overlying regions of opposite magnetic polarity in the photosphere. The decimetric emission is confined to the period leading up to the impulsive phase of the flare, and does not extend over a wide frequency range. This fact suggests a flare mechanism in which the magnetic field at considerable height in the corona is destabilized a few minutes prior to the main energy release lower in the corona. The radio morphology also suggests that the radiating electrons are trapped near the tops of magnetic loops, and therefore may have pitch angles near 90˚.