P. Janardhan

Travelling interplanetary disturbances detected using interplanetary scintillation at 327 MHz

Based on the advance predictions of two flare-generated shock fronts, obtained from the Space Environment Centre (SEC, NOAA, Boulder), observations of interplanetary scintillation (IPS) were carried out with the Ooty Radio Telescope (ORT) on a grid of appropriately located sources during the period 31 October to 5 November, 1992. Solar wind velocities were derived by fitting model spectra to the observed spectra and two travelling interplanetary disturbances were detected. Both disturbances were traced back to an active region on the Sun which was located close to a large coronal hole. The roles of flares and coronal holes in producing such disturbances are examined and it is shown that in the present case both the coronal hole and the active region probably played key roles in generating the two IPS disturbances.

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.

Combining visibilities from the giant meterwave radio telescope and the Nancay radio heliograph - High dynamic range snapshot images of the solar corona at 327 MHz

We report first results from an ongoing program of combining visibilities from the Giant Meterwave Radio Telescope (GMRT) and the Nancay Radio Heliograph (NRH) to produce composite snapshot images of the sun at meter wavelengths. We describe the data processing, including a specific multi-scale CLEAN algorithm. We present results of a) simulations for two models of the sun at 327 MHz, with differing complexity b) observations of a complex noise storm on the sun at 327 MHz on Aug. 27, 2002. Our results illustrate the capacity of this method to produce high dynamic range snapshot images when the solar corona has structures with scales ranging from the image resolution of 49 �� to the size of the whole sun. We emphasize that snapshot images of a complex object such as the sun, obtained by combining data from both instruments, are far better than images from either instrument alone, because their uv-coverages are very complementary.

The prelude to the deep minimum between solar cycles 23 and 24: Interplanetary scintillation signatures in the inner heliosphere

Abstract Extensive interplanetary scintillation (IPS) observations at 327 MHz obtained between 1983 and2009 clearly show a steady and significant drop in the turbule nce levels in the entire inner heliospherestarting from around ∼1995. We believe that this large-scale IPS signature, in theinner heliosphere,coupledwith thefact thatsolarpolarfields have alsobeen de cliningsince∼1995, providea consistentresult showing that the buildup to the deepest minimum in 100 years actually began more than adecade earlier. Introduction The sunspot minimum at the end of Cycle 23, has been one of the deepest we have experienced inthe past 100 years with the first spots of the new cycle 24 appea ring only in March 2010 instead ofDecember 2008 as was expected. Also, the number of spotless days experienced in 2008 and 2009was over 70%. Apart from this, Cycle 23 has shown a slower than average field reversal, a slowerrise to maximum than other odd numbered cycles, and a second maximum during the declining phasethat is unusual for odd-numbered cycles. Though these deviations from “normal” behaviour couldbe significant in understanding the evolution of magnetic fie lds on the Sun, they do not yield anydirect insights into the onset of the deep minimum experienced at the end of cycle 23. This is becausepredictions of the strength of solar cycles and the nature of their minima are strongly dictated by boththe strength of the ongoing cycle [Dikpati et al. (2006); Choudhuri et al. (2007)] and changes in theflow rates of the meridional circulation [ Nandy et al. (2011)].1

K-band Radio Observations of Comet Hale-Bopp: Detections of Ammonia and (Possibly) Water

K-band radio observations of comet Hale-Bopp (C/1995 O1) were conducted in March/April 1997 at the 100-m Telescope of the Max-Planck-Institut für Radioastronomie. Emission was firmly detected from the five lowest metastable (J = K)inversion transitions of ammonia. Assuming a thermal distribution for the metastable states of NH3, we derive a rotational temperature of 104 ± 30 K and an ammonia production rate at perihelion of6.6 ± 1.3 × 1028 s-1.The updated ammonia-to-water abundance ratio is found to be of the order of 1.0%. We also report a marginal detection of the 616–523transition line of water at λ = 1.35 cm.

Resolving the enigmatic solar wind disappearance event of 11 May 1999

[i] On 11 and 12 May 1999, the Earth was engulfed by an unusually low-density (<1 cm -3 ) and low-velocity (<350 km s -1 ) solar wind for a period of over 1 day. Extensive studies of this unusual event that occurred during Carrington rotation 1949 (CR1949), using both ground-based and space-based in situ observations, have not as yet been able to identify the cause or the solar source of this event. Using solar wind velocity measurements from the four-station IPS observatory of the Solar-Terrestrial Environment Laboratory (STEL), Toyokawa, Japan, we investigate the structure of the solar wind in May 1999 during CR1949. IPS observations from STEL were used to make tomographic velocity maps to identify and delineate the extent and morphology of the stable solar wind flows during CR1949 in the vicinity of the Earth. Combined with in situ measurements of the interplanetary magnetic field (IMF), potential field computations of the solar magnetic fields in the period, and HeI 10830A observations of coronal hole boundaries during CR1949, we have identified the source region of the unusual flows and have shown that the flow responsible for the disappearance event was a stable unipolar flow originating in the vicinity of a large midlatitude active region AR8525, located at ∼18°N and between heliographic longitudes 280° and 300°. Earlier workers have speculated that such events may be caused by the large-scale restructuring of the solar magnetic field at the maximum of each solar cycle. However, by identifying the solar source and nature of this event, we believe that at least in this particular case, the association with global, large-scale solar phenomena like the periodic 11-year solar polar field reversal is most likely to be coincidental.

Radio Observations of Rapid Acceleration in a Slow Filament Eruption/Fast Coronal Mass Ejection Event

We discuss a filament eruption/coronal mass ejection (CME) event associated with a flare of GOES class M2.8 that occurred on 2001 November 17. This event was observed by the Nobeyama Radio Heliograph (NoRH) at 17 and 34 GHz. NoRH observed the filament during its eruption both as a dark feature against the solar disk and a bright feature above the solar limb. The high cadence of the radio data allows us to follow the motion of the filament at high time resolution to a height of more than half a solar radius. The filament eruption shows a very gradual onset and then a rapid acceleration phase coincident with the launch of a fast halo CME. Soft X-ray and extreme-ultraviolet (EUV) images show heating in a long loop underneath the filament prior to the flare. The NoRH height-time plot of the filament shows a roughly constant gradual acceleration for 1 hr, followed by a very abrupt acceleration coincident with the impulsive phase of the associated flare, and then a phase of constant velocity or much slower acceleration. This pattern is identical to that recently found to occur in the motion of flare-associated CMEs, which also show a sharp acceleration phase closely tied to the impulsive phase of the flare. When the rapid acceleration occurs in this event, the flare site and the filament are separated by ~0.5 R☉, making it unlikely that a disturbance propagates from one location to the other. Models in which a disruption of the large-scale coronal magnetic field simultaneously permits the acceleration of the filament and the flare energy release seem to be a better explanation for this event.

A Study of Density Modulation Index in the Inner Heliospheric Solar Wind during Solar Cycle 23

An understanding of variations of density modulation index εN = ΔN/N, the ratio of the electron density fluctuation (ΔN) to the absolute solar wind density (N), in the inner heliosphere is of vital importance for understanding turbulent dissipation and consequent local heating of solar wind. In addition, the density modulation index plays crucial role in understanding the propagation of energetic electrons, through the heliosphere, produced by solar flares and other explosive solar surface phenomena. We have made a detailed study of εN in the inner heliosphere spanning the distance range from 0.2 to 0.8 AU, for the period 1998-2008, covering solar cycle 23. The rms electron density fluctuations (ΔN) have been deduced using ground-based interplanetary scintillation (IPS) observations at 327 MHz from the multi-station IPS observatory, at STEL, Japan. Before deriving ΔN, we have appropriately normalized scintillation measurements to remove the effect of finite source size. The absolute solar wind density (N), on the other hand, has been obtained from the space-borne Advanced Composition Explorer (ACE) mission. However, ACE density measurements are effectively at a distance of 1 AU at the Largangian point L1. Thus, for estimation of density at the location of the relevant scintillating sources, spreading over distances of 0.2-0.8 AU, the measured ACE densities at 1 AU are extrapolated in the sunward direction using an electron density model. Our analysis shows that εN does not vary with heliocentric distances r and the typical value of εN ranges from 1% to 10% which is is consistent with the earlier findings. A systematic decline in the solar wind electron density turbulence levels has been reported earlier for the period 1995 to 2008. Our investigation of the long-term temporal variations of εN over the distance range 0.2-0.8 AU have also shown a similar decline during the period 1998-2008. It therefore appears reasonable, from the linear relationship between the density fluctuations and magnetic field fluctuations, to conclude that the decrease in εN is connected to the unusual solar magnetic activity during the long and deep solar minimum at the end of the solar cycle 23.

The structure of solar radio noise storms

The Nan\c{c}ay Radioheliograph (NRH) routinely produces snapshot images of the full sun at frequencies between 150 and 450 MHz, with typical resolution 3 arcmin and time cadence 0.2 s. Combining visibilities from the NRH and from the Giant Meterwave Radio Telescope (GMRT) allows us to produce images of the sun at 236 or 327 MHz, with a large FOV, high resolution and time cadence. We seek to investigate the structure of noise storms (the most common non-thermal solar radio emission). We focus on the relation of position and altitude of noise storms with the observing frequency and on the lower limit of their sizes. We present results for noise storms on four days. The results consist of an extended halo and of one or several compact cores with relative intensity changing over a few seconds. We found that core sizes can be almost stable over one hour, with a minimum in the range 31-35 arcsec (less than previously reported) and can be stable over one hour. The heliocentric distances of noise storms are

A 20 year decline in solar photospheric magnetic fields: Inner‐heliospheric signatures and possible implications

\sim 1.20