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Modern-Era Retrospective Analysis for Research and Applications, Version-2 (MERRA-2) total column ozone (TCO) data from 1980 to 2018 and ground based TCO data were used to investigate its responses to periodic variations concerning stratospheric dynamics such as tropopause heights and Quasi-Biennial Oscillations (QBO), and solar activity signatures of Magnesium II core-to-wing ratio (Mg II) and F10.7 cm solar radio flux (F10.7) observed over Mountain (Mt.) Waliguan (36.17°N, 100.53°E), China. Cross-correlation and wavelet analyses were used for carrying out the investigation. The MgII and F10.7 show a periodicity of around an 11-year solar cycle (SC) while the TCO trend is though periodic and it does not exhibit one to one correspondence with the 11-year SC. The highest value in TCO is 355.7 DU noted in February (1982) whereas, the lowest value of TCO is 256.02 DU in October (1988) during the entire study period. The average value in TCO is 291.17 ± 0.94 DU with an amplitude of variation of 99.68 DU (∼ 28%). The trend of TCO depicts a persistent distinct seasonal pattern with maxima in winter/spring (December through April) and minima in summer/autumn (August through November). The long-term variability shows a notable decreasing trend in TCO by ∼ 30%, 13%, and 5% per decade for SCs 21-23, where SC 24 represents an increasing trend of ∼ 1% per decade. Hence, this illustrates the significant recovery of TCO over Mt. Waliguan, China which is as a result of the reduction of the ozone depletion agents arising as a positive outcome of the implementation of the Montreal regulatory policy. From wavelet analysis, the F10.7, MgII, tropopause height, QBO and TCO show a significant dominant periodicity of 10-12 months. It is concluded that the variations in mean TCO are strongly correlated with variations of the F10.7 cm and Mg-II solar indices, and the mean TCO responds rapidly to solar activity variations.

The Square Kilometre Array Observatory (SKAO) is perhaps the most ambitious radio telescope envisaged yet. It will enable unprecedented studies of the Sun, the corona and the heliosphere and help to answer many of the outstanding questions in these areas. Its ability to make a vast previously unexplored phase space accessible, also promises a large discovery potential. The Indian solar and heliospheric physics community have been preparing for this science opportunity. A significant part of this effort has been towards playing a leading role in pursuing science with SKAO precursor instruments. This article briefly summarises the current status of the various aspects of work done as a part of this enterprise and our future goals., Comment: 34 pages, 18 figures, accepted for publication in Journal of Astronomy and Astrophysics

The present study examines the response of the terrestrial magnetosphere to the long-term steady declining trends observed in solar magnetic fields and solar wind micro-turbulence levels since mid-1990's that has been continuing beyond the mini-solar maximum of cycle 24. A detailed analysis of the response of the terrestrial magnetosphere has been carried out by studying the extent and shape of the Earth's magnetopause and bow shock over the past four solar cycles. We estimate sub-solar stand-off distance of the magnetopause and bow shock, and the shape of the magnetopause using numerical as well as empirical models. The computed magnetopause and bow shock stand-off distances have been found to be increasing steadily since around mid-1990's, consistent with the steady declining trend seen in solar magnetic fields and solar wind micro-turbulence levels. Similarly, we find an expansion in the shape of the magnetopause since 1996. The implications of the increasing trend seen in the magnetopause and bow shock stand-off distances are discussed and a forecast of the shape of the magnetopause in 2020, the minimum of cycle 24, has been made. Importantly, we also find two instances between 1968 and 1991 when the magnetopause stand-off distance dropped to values close to 6.6 earth radii, the geostationary orbit, for duration ranging from 9$-$11 hours and one event in 2005, post 1995 when the decline in photospheric fields began. Though there have been no such events since 2005, it represents a clear and present danger to our satellite systems., Accepted for publication in JGR-Space Physics. arXiv admin note: text overlap with arXiv:1709.01407

In this article, we present the application of the weighted horizontal gradient of magnetic field (WG_M) flare prediction method to 3-dimensional (3D) extrapolated magnetic configurations of 13 flaring solar active regions (ARs). The main aim is to identify an optimal height range, if any, in the interface region between the photosphere and lower corona, where the flare onset time prediction capability of WGM is best exploited. The optimal height is where flare prediction, by means of the WG_M method, is achieved earlier than at the photospheric level. 3D magnetic structures, based on potential and non-linear force-free field extrapolations, are constructed to study a vertical range from the photosphere up to the low corona with a 45 km step size. The WG_M method is applied as a function of height to all 13 flaring AR cases that are subject to certain selection criteria. We found that applying the WGM method between 1000 and 1800 km above the solar surface would improve the prediction of the flare onset time by around 2-8 hours.Certain caveats and an outlook for future work along these lines are also discussed. ispartof: Astrophysical Journal vol:896 issue:2 status: published

The very low frequency (VLF) regime below 30 MHz in the electromagnetic spectrum has presently drawing global attentions in radio astronomical research due to its potentially significant science outcomes exploring many unknown extragalactic sources, transients, and so on. However, the non-transparency of the Earth's ionosphere, ionospheric distortion and artificial radio frequency interference (RFI) have made it difficult to detect the VLF celestial radio emission with ground-based instruments. A straightforward solution to overcome these problems is a space based VLF radio telescope, just like the VLF radio instruments onboard the Chang'E 4 spacecraft. But building such a space telescope would be inevitably costly and technically challenging. The alternative approach would be then a ground based VLF radio telescope. Particularly, in the period of post 2020 when the solar and terrestrial ionospheric activities are expected to be in a 'calm' state, it will provide us a good chance to perform VLF ground-based radio observations. Anticipating such an opportunity, we built an agile VLF radio spectrum explorer co-located with the currently operational Mingantu Spectra Radio Heliograph (MUSER). The instrument includes four antennas operating in the VLF frequency range 1-70 MHz. Along with them, we employ an eight-channel analog and digital receivers to amplify, digitize and process the radio signals received by the antennas. We present in the paper this VLF radio spectrum explorer and the instrument will be useful for celestial studies of VLF radio emissions., Comment: Accepted by Research in Astronomy and Astrophysics, 13 pages, 12 figures

We re-examined solar polar magnetic fields, using ground based synoptic photospheric magnetograms, during solar cycle 24. IThe signed polar magnetic fields showed an unusual hemispheric asymmetry in the polar field reversal process with a single unambigous reversal in the Southern hemisphere around late 2013 while the polar reversal in the Northern hemisphere started earlier around June 2012, but was completed only by the end of 2014. The examination of the unsigned polar magnetic fields in cycle 24 showed a continuing decline of fields in the Northern hemisphere whereas in the Southern hemisphere, it had partially recovered. However, the overall declining trend in solar polar fields, which began in the mid-1990’s, is still in progress. The continued decline seen in solar photospheric fields raises thequestion of whether we are heading towards a Grand or Maunder like solar minimum.

We connect the time sequence of changes in the IMF-Bz to the development of spread-F at an equatorial station Kwajalein on three different nights in November 2004, one during a geomagnetic quiet period and other two during geomagnetic disturbed periods. The chosen days show clear and smooth variations of IMF-Bz without any large fluctuations thereby enabling one to correlate changes in equatorial spread-F with corresponding changes in IMF-Bz. It is shown that a slow and continuous increase in the IMF-Bz over a duration of few hours has a similar effect on the equatorial ionosphere as of a sudden northward turning of the IMF-Bz in causing an electric field through the polar region and then to the equator. We conclude that the Spread-F at equatorial and low latitudes are due to echoes from ionization irregularities that arise due to the plasma instabilities generated by an eastward electric field on the large plasma density gradient in or below the base of the F-layer during any period of the night time along with the gravity driven Rayleigh-Taylor instability.

The magnetic flux cancellation on the Sun plays a crucial role in determining the manner in which the net magnetic flux changes in every solar cycle, affecting the large scale evolution of the coronal magnetic fields and heliospheric environment. We investigate, in this paper, the correlation between the solar magnetic flux cancelled at the equator and the solar magnetic flux transported to the poles by comparing the net amount of magnetic flux in the latitude belt 0${^{\circ}}$-5${^{\circ}}$ and 45${^{\circ}}$-60${^{\circ}}$, estimated using synoptic magnetograms from National Solar Observatory at Kitt Peak, during Solar Cycles 21-24. We find a good correlation between the net flux in the latitude band 0${^{\circ}}$-5${^{\circ}}$ and 55${^{\circ}}$-60${^{\circ}}$ for the Northern as well as for the Southern hemispheres. However, we find a poor correlation if the net flux for the Northern and Southern hemispheres are considered together. In addition, we investigate the correlation between the net flux cancelled at the equator and the strength of solar polar field at cycle minimum, and find a good correlation between the two. We discuss the implication of the correlation of flux transported across the equator and to the poles that has an important bearing in the estimation of the residual polar cap field strength at the cycle minimum. This can be used a predictive tool for estimating the amplitude of subsequent cycles and we use this to estimate maximum smoothed sunspot numbers of 77$\pm$5 and 85$\pm$5 for the Northern and Southern hemispheres, respectively, for the upcoming Solar Cycle 25., Submitted to Solar Physics Journal (19 Pages, 7 Figures)

The extended minimum of Solar Cycle 23, the extremely quiet solar-wind conditions prevailing, and the mini-maximum of Solar Cycle 24 drew global attention and many authors have since attempted to predict the amplitude of the upcoming Solar Cycle 25, which is predicted to be the third successive weak cycle; it is a unique opportunity to probe the Sun during such quiet periods. Earlier work has established a steady decline, over two decades, in solar photospheric fields at latitudes above $45^{\circ}$ and a similar decline in solar-wind micro-turbulence levels as measured by interplanetary scintillation (IPS) observations. However, the relation between the photospheric magnetic fields and those in the low corona/solar-wind are not straightforward. Therefore, in the present article, we have used potential-field source-surface (PFSS) extrapolations to deduce global magnetic-fields using synoptic magnetograms observed with National Solar Observatory (NSO), Kitt Peak, USA (NSO/KP) and Solar Optical Long-term Investigation of the Sun (NSO/SOLIS) instruments during 1975-2018. Furthermore, we have measured the normalized scintillation index [m] using the IPS observations carried out at the Institute of Space Earth Environment Research (ISEE), Japan during 1983-2017. From these observations, we have found that, since the mid-1990s, the magnetic-field over different latitudes at 2.5 $\rm R_{\odot}$ and 10 $\rm R_{\odot}$(extrapolated using PFSS method) has decreased by $\approx 11.3-22.2 \%$. In phase with the declining magnetic-fields, the quantity m also declined by $\approx 23.6 \%$. These observations emphasize the inter-relationship between the global magnetic-field and various turbulence parameters in the solar corona and solar wind., Comment: Accepted for publication in Solar Physics, 16 pages, 5 Figures, 1 Table

The energy released during flares accelerates particles somewhere in the lower corona, which can be located and best studied using solar radio images in the decimetric frequency range. However, due to the limited imaging available at this frequency range, the exact location of electron acceleration and so the flare energy release has still remained poorly known. The low frequency radio array of Miangtu Spectra Radio Heliograph (MUSER), operating in the frequency range 400-2000 MHz with its capabilities of high temporal (25 to 200 ms), spatial (1.3 to 50 arcsec) and spectral (25 MHz) resolution is very useful in this regard. We present here high dynamic range imaging of decimetric Type III bursts using MUSER observations associated with a short-duration GOES C2.3 class solar flare that erupted on 2015 June 20. The Type III bursts identified are actually narrow band reverse Type III bursts with the brightest ones observed at the very beginning. Also, identified few seconds apart are narrow band spike-like Type III bursts coinciding with the hard X-ray (HXR) and microwave radio burst peak activities. The radio images from MUSER show that the Type III bursts are located far from the location of HXR and microwave burst locations, while each Type III bursts, as revealed by MUSER images, show different spatial locations. From the different locations of Type III bursts, we suggest the presence of different flare energy release sites, which shows that the flare energy release is indeed fragmentary in nature.

The radio emission far away from a flaring site are usually observed in solar radio images at metric wavelengths. However, it is not commonly observed at decimetric wavelengths. Here we present observations of an unusual decimetric source emission located 500 arcsec away from a flaring site using the first high time cadence (0.5 s) Giant Meterwave Radio Telescope (GMRT) solar radio images at 610 MHz associated with a GOES C1.4 class solar flare that erupted on 2015 June 20. The decimetric radio sources don’t have any corresponding coronal or magnetic features nearby. In addition, we also present high time cadence solar images from GMRT associated with a GOES M1.0 class solar flare and a coronal mass ejection (CME) that erupted after GOES C1.4 flare on 20 June 2015, which show decimetric radio sources rather located near the flaring site. Also, observed are type-III burst activity by the Solar Broadband Radio Spectrometer at Yunnan Astronomical Observatory coinciding with the decimetric burst activity that observed by GMRT. Based on a multi-wavelength analysis and potential field source surface extrapolation, we suggest that the source electrons of the decimetric radio source and type III bursts were originated from a common electron acceleration site located near the flaring site. Also, we show that the distant decimetric radio source is actually the apparent radio source, that results due to the wave-ducting effect, wherein the ducting of radio waves produced lower in the corona leads to the displacement of true radio source emission.

The polarity of the Sun’s magnetic field reverses around the maximum of each 11-year solar cycle. This is known as polar field reversal. Here we report an investigation of a prolonged and unusual hemispheric asymmetry in the polar field reversal pattern in solar cycle 24, which was carried out using measurements of solar magnetic fields in the latitude range $55^{\circ}-90^{\circ}$ and $78^{\circ}-90^{\circ}$ for the period between February 1975 and October 2017, covering solar cycles 21–24. While this study compliments a similar study carried out using microwave brightness measurements which claimed that the field reversal process in cycle 24 was completed by the end of 2015, our results show that the field reversal in cycle 24 was completed earlier that is, in late 2014. We show that the southern solar hemisphere unambiguously reversed polarity in mid-2013 while the reversal in the field in the northern solar hemisphere started as early as June 2012, was followed by a sustained period of near-zero field strength lasting until the end of 2014, after which the field began to show a clear rise from its near-zero value. Signatures of this unusual field reversal pattern were also clearly identifiable in the solar wind using our observations of interplanetary scintillation at 327 MHz which supported our magnetic field observations and confirmed that the field reversal process was completed at the end of 2014.

We examine the temporal changes in both solar polar magnetic field (PMF) at latitudes $\ge$ $45^{\circ}$ and heliospheric magnetic field (HMF) at 1 AU during solar cycles 21--24 with emphasis on the recent activity changes after July 2015, the so called "mini solar maximum" of cycle 24. While unsigned PMF shows a solar-cycle-like variation in cycles 21 and 22, it shows an anti-solar-cycle-like variation in cycle 24. In addition, the floor level of the HMF (of 4.6 nT), i.e. the value that the HMF returns to at each solar minimum, is breached about two years prior to cycle 24 minimum, indicating a reduced HMF floor level in the upcoming cycle 24 minimum. In light of the change of unsigned PMF and the availability of a revised smoothed sunspot number (SSN) after July 2015, we have revisited the correlation of unsigned PMF and HMF at solar minimum. The correlation is used to estimate a new value of the HMF of 4.16$\pm$0.6 nT at the cycle 24 minimum and the amplitude of the upcoming cycle 25. The updated prediction is 82$\pm$8 and 133$\pm$11, on the original (V1.0) and revised (V2.0) SSN scales, respectively. These better and more reliable SSN values (due to the larger data set) imply that we will witness another mini solar maximum in the upcoming cycle 25 which will be relatively stronger than cycle 24 and a little weaker than cycle 23, even if the current solar cycle minimum occurs in 2021 instead of 2020., Comment: 16 pages, 4 figures, 2 tables

We apply a novel pre-flare tracking of sunspot groups towards improving the estimation of flare onset time by focusing on the evolution of the 3D magnetic field construction of AR 11429. The 3D magnetic structure is based on potential field extrapolation encompassing a vertical range from the photosphere through the chromosphere and transition region into the low corona. The basis of our proxy measure of activity prediction is the so-called weighted horizontal gradient of magnetic field (WGM) defined between spots of opposite polarities close to the polarity inversion line of an active region. The temporal variation of the distance of the barycenter of the opposite polarities is also found to possess potentially important diagnostic information about the flare onset time estimation as function of height similar to its counterpart introduced initially in an application at the photosphere only in Korsós et al. (2015). We apply the photospheric pre-flare behavioural patterns of sunspot groups to the evolution of their associated 3D-constructed AR 11429 as function of height. We found that at a certain height in the lower solar atmosphere the onset time may be estimated much earlier than at the photosphere or at any other heights. Therefore, we present a tool and recipe that may potentially identify the optimum height for flare prognostic in the solar atmosphere allowing to improve our flare prediction capability and capacity. ispartof: pages:294-297 ispartof: IAU Symposium 335 vol:13 issue:S335 pages:294-297 ispartof: IAUS 335: Space Weather of the Heliosphere: Processes and Forecasts location:University of Exeter, UK date:17 Jul - 21 Jul 2017 status: published

Aims: To investigate solar polar fields during cycle 24, using measurements of solar magnetic fields in the latitude range 55 - 90 degree and 78 - 90 degree, to report a prolonged and unusual hemispheric asymmetry in the polar field reversal pattern in solar cycle 24. Methods: This study was carried out using medium resolution line-of-sight synoptic magnetograms from the magnetic database of the National Solar Observatory at Kitt Peak (NSO/KP), USA for the period between February 1975 and October 2017, covering solar cycles 21-24 and high-resolution line-of-sight synoptic magnetograms from the Michaelson Doppler Imager instrument onboard the Solar Heliospheric Observatory. Synoptic magnetograms using radial measurements from the Heliospheric Magnetic Imager instrument onboard the Solar Dynamics Observatory, covering solar cycle 23 and 24, were also used. Results: We show that the Southern solar hemisphere unambiguously reversed polarity in mid-2013 while the reversal in the field in the Northern solar hemisphere started as early as June 2012, was followed by a sustained period of near-zero field strength lasting until the end of 2014, after which the field began to show a clear rise from its near-zero value. While this study compliments a similar study carried out using microwave brightness measurements (Gopalswamy et al. 2016) which claimed that the field reversal process in cycle 24 was completed by the end of 2015, our results show that the field reversal in cycle 24 was completed earlier i.e. in late 2014. Signatures of this unusual field reversal pattern were also clearly identifiable in the solar wind, using our observations of interplanetary scintillation at 327 MHz which supported our magnetic field observations and confirmed that the field reversal process was completed at the end of 2014., 11 pages, 7 figures, Under review in A&A

We report observations of a steady 20 year decline of solar photospheric fields at latitudes ≥45° starting from ∼1995. This prolonged and continuing decline, combined with the fact that cycle 24 is already past its peak, implies that magnetic fields are likely to continue to decline until ∼2020, the expected minimum of the ongoing solar cycle 24. In addition, interplanetary scintillation observations of the inner heliosphere for the period 1983–2013 and in the distance range 0.2–0.8 AU have also shown a similar and steady decline in solar wind microturbulence levels, in sync with the declining photospheric fields. Using the correlation between the polar field and heliospheric magnetic field (HMF) at solar minimum, we have estimated the value of the HMF in 2020 to be 3.9 (±0.6) nT and a floor value of the HMF of ∼3.2 (±0.4) nT. Given this floor value for the HMF, our analysis suggests that the estimated peak sunspot number for solar cycle 25 is likely to be 62 (±12).

Our study of solar high-latitude (≥45°) photospheric magnetic fields and interplanetary scintillation observations, at 327 MHz, of solar wind micro-turbulence levels showed a steady decline for the past 20 years. Also, the fact that cycle 24 has already past its peak, implies that high-latitude fields are likely to decline until ∼2020. Based on a correlation between the solar high-latitude fields and the heliospheric magnetic fields (HMF), we estimated the HMF strength in 2020 and the floor value of the HMF, which were found to be 3.9 (±0.6) nT and 3.2 (±0.4) nT, respectively. Using estimated value of the HMF in 2020, the peak sunspot number for solar Cycle 25 was estimated to be 62 (±12). These results and the fact that solar magnetic fields continues to decline at present raise the question of a very low sunspot activity in near-future or another grand minimum similar to the Maunder minimum. So what is the possible impact of such a likely grand minimum on terrestrial ionospheric current systems? Our study night time F-region maximum electron density (a measure of terrestrial ionospheric currents) reveals that the impending minimum is not likely to have any adverse impact rather the period post cycle 25 will be useful for undertaking systematic ground based low-frequency radio astronomy as the night time ionospheric cutoff-frequency could be well below 10 MHz.

We present a study of decimetric radio activity, using the first high time cadence (0.5 s) images from the ${\textit{Giant Meterwave Radio Telescope}}$ (${\textit{GMRT}}$) at 610 MHz, associated with ${\textit{GOES}}$ C1.4 and M1.0 class solar flares, and a coronal mass ejection (CME) onset that occurred on 20 June 2015. The high spatial resolution images from ${\textit{GMRT}}$ show a strong radio source during the C1.4 flare, located ~500$''$ away from the flaring site with no corresponding bright footpoints or coronal features nearby. In contrast, however, strong radio sources are found near the flaring site during the M1.0 flare and around the CME onset time. Weak radio sources, located near the flaring site, are also found during the maximum of the C1.4 flare activity, which show a temporal association with metric type III bursts identified by the Solar Broadband Radio Spectrometer at Yunnan Astronomical Observatory. Based on a multi-wavelength analysis and magnetic potential field source surface extrapolations, we suggest that the source electrons of ${\textit{GMRT}}$ radio sources and metric type III bursts were originated from a common electron acceleration site. We also show that the strong ${\textit{GMRT}}$ radio source is generated by a coherent emission process and its apparent location far from the flaring site is possibly due to the wave-ducting effect., Comment: 24 Pages, 11 figures, accepted for publication in ApJ

Studies in recent times have reported that solar high latitude fields have been steadily declining since around 1995. The previous solar cycle 23 has also been unusual in that, it experienced one of the deepest solar minima in the past 100 years with over 70% of the days in 2007 and 2008 being entirely spotless. We have made a detailed study of the effect of such a steady and prolonged decline in solar photospheric magnetic fields on the solar wind using interplanetary scintillation (IPS) observations of solar wind micro-turbulence levels. Our study, covering solar cycles 21, 22 and 23, has also shown a steady decline in solar wind micro-turbulence levels, in the distance range 0.2 to 0.8 AU, in sync with the declining solar photospheric fields. These results beg the question as to whether we are headed towards a long period of little or no sunspot activity, in a manner similar to the period between 1645 and 1715 known as the Maunder minimum, when the sun was completely devoid of sunspots. We have estimated the expected sunspot number around the maximum of solar cycle 25 and find that cycle 25 will be much weaker than the current cycle 24 which had a peak sunspot number in November 2013 of around 75. We have also looked for signatures of the declining solar wind micro-turbulence levels in the Earth's ionosphere and our study indicates that the night time ionospheric cut-off could reduce to well below 5 MHz if the decline in the solar magnetic fields continues beyond 2020.

Sunspots or dark regions of strong magnetic fields on the solar photosphere are generated via magnetohydrodynamic processes in the solar interior that involve the cyclic generation of toroidal fields (sunspot fields) from pre-existing poloidal fields and the eventual regeneration of the poloidal fields. This cyclic process, referred to as the solar dynamo, leads to the well known solar activity cycle of waxing and waning sunspot numbers with a period of 11 years. The sunspot minimum at the end of solar cycle 23, however, was one of the deepest recorded in the past 100 years, with cycle 24 starting about 16 months later than expected. A detailed study of solar high latitude (≥ 45∘) or polar magnetic fields using ground based magnetograms has clearly shown a steady decline in polar field strength since mid-1990's [1, 2, 3] which has continued to the present, i.e. until the end of February 2013. Since sunspot fields are generated from polar fields, this long term decline in polar field strength would eventually effect the sunspot field strength in the subsequent solar cycles. A continued decline in the polar fields in the manner indicated by our analysis would imply that the polar field strength will approach zero by ∼2031. In addition, a detailed analysis of solar wind micro-turbulence in the inner heliosphere has also shown a steady decline in sync with the declining solar photospheric magnetic fields. The fact that both solar polar fields and inner-heliospheric micro-turbulence levels show a similar decline raises the question as to whether we are headed towards an extended period of very little or no sunspot activity in a manner similar to what was seen in the Maunder minimum?

An understanding of variations of density modulation index e N = 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 e N 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 e N does not vary with heliocentric distances r and the typical value of e N 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 e N 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 e N is connected to the unusual solar magnetic activity during the long and deep solar minimum at the end of the solar cycle 23.

Using both wavelet and Fourier analysis, a study has been undertaken of the changes in the quasi-periodic variations in solar photospheric fields 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. The study, carried out using ground based synoptic magnetograms spanning the period 1975.14 to 2009.86, covered solar cycles 21, 22 and 23. A hemispheric asymmetry in periodicities of the photospheric fields was seen only at latitudes above $\pm45{^{\circ}}$ when the data was divided, based on a wavelet analysis, into two parts: one prior to 1996 and the other after 1996. Furthermore, the hemispheric asymmetry was observed to be confined to the latitude range 45${^{\circ}}$ to 60${^{\circ}}$. 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 when coupled with the fact that both solar fields above $\pm45{^{\circ}}$ and micro-turbulence levels in the inner-heliosphere have been decreasing since the early to mid nineties (Janardhan et al., 2011) suggests 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 the cycle 23. The decline in fields above $\pm45{^{\circ}}$ for well over a solar cycle, would imply that weak polar fields have been generated in the past two successive solar cycles \textit{viz.} cycles 22 and 23. A continuation of this declining trend beyond 22 years, if it occurs, will have serious implications on our current understanding of the solar dynamo., 25 pages, 8 Fiures, Accepted for publication in Solar Physics Journal

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

The paper describes the occurrence of a pair of oppositely directed sudden impulses (SI), in the geomagnetic field ($\Delta$X), at ground stations, called SI${^{+}}$ -- SI${^{-}}$ pairs, that occurred between 1835 UT and 2300 UT on 23 April 1998. The SI${^{+}}$ -- SI${^{-}}$ pair, was closely correlated with corresponding variations in the solar wind density, while solar wind velocity and the southward component of the interplanetary magnetic field (Bz) did not show any correspondence. Further, this event had no source on the visible solar disk. However, a rear-side partial halo coronal mass ejection (CME) and an M1.4 class solar flare behind the west limb, took place on 20 April 1998, the date corresponding to the traceback location of the solar wind flows. This event presents empirical evidence, which to our knowledge, is the best convincing evidence for the association of specific solar events to the observations of an SI${^{+}}$ -- SI${^{-}}$ pair. In addition, it shows that it is possible for a rear side solar flare to propagate a shock towards the earth.

Employing wavelet and Fourier methods, we investigate temporal variations of periodicities in the photospheric fields obtained from synoptic magnetograms of the National Solar Observatory at Kitt Peak (NSO/KP) spanning the years 1975-2009. A north-south asymmetry is noticed in the periodicities of photospheric fields in the latitude range, 45°-78°, when the data is grouped into fields prior to and after 1996. This asymmetry when coupled with the fact that both solar fields in the latitude range, 45°-78°, and the micro-turbulence levels in the inner heliosphere began declining ~ 1995-1996 suggests that active changes occurred in the underlying basic solar processes which eventually initiated, at the end of solar cycle 23, the build-up of the deepest solar minimum, in the past 100 years.

We report a pair of oppositely directed sudden impulses (SI), in the geomagnetic field (ΔX), at ground stations, called SI+ – SI− pairs, that occurred between 1835 UT and 2300 UT on 23 April 1998. The SI+ – SI− pair, was well correlated with corresponding variations in the solar wind density, while solar wind velocity and the southward component of the interplanetary magnetic field (Bz) did not show any correspondence. This event had no source on the visible solar disk but was associated with a rear-side fast partial halo coronal mass ejection (CME) and an optically occulted M1.4 class solar flare behind the west limb. This event was unique in that one could clearly identify variations in ΔX at ground stations with solar wind parameters.

We have examined solar polar fields during the solar cycles 21–23 using both ground and space based synoptic magnetograms. It has been shown that the unsigned polar fields in the latitude range (78°–90°) show a steady decline starting ~1995–1996 till the maximum of cycle 23. We also find that the long term changes in the unsigned polar fields at higher latitudes are well correlated with changes in the surface meridional flow speed that have been reported for the cycle 23.

We have used interplanetary scintillation (IPS) observations at 327 MHz spanning years 1983-2009 to study microturbulence levels in the inner heliosphere. We find that the microturbulence levels show a steady and significant drop in the entire inner heliosphere starting from around 1995. The fact that the solar polar fields have also shown a similar declining trend provides a consistent result showing the buildup to the solar minimum between the solar cycles 23 and 24, the deepest in the past 100 years, actually began more than a decade earlier.

High red-shift radio galaxies are best searched at low radio frequencies, due to its steep radio spectra. Here we present preliminary results from our programme to search for high red-shift radio galaxies to ~ 10 to 100 times fainter than the known population till date. We have extracted ultra-steep spectrum (USS) samples from deep 150 MHz Giant Meter-wave Radio Telescope (GMRT) observations from one of the three well-studied DEEP2 fields to this affect. From correlating these radio sources w.r.t to the high-frequency catalogues such as VLA, FIRST, and NVSS at 1.4 GHz, we find ~ 100 steep spectrum (spectral index, $\alpha$ $>$ 1) radio sources, which are good candidates for high red-shift radio galaxies., Comment: 3 pages, 1 figures, Revised version under review in the Journal of Astrophysics and Astronomy

The ratio of the rms electron density fluctuations to the background density in the solar wind (density modulation index, $\epsilon_{N} \equiv \Delta{N}/N$) is of vital importance in understanding several problems in heliospheric physics related to solar wind turbulence. In this paper, we have investigated the behavior of $\epsilon_{N}$ in the inner-heliosphere from 0.26 to 0.82 AU. The density fluctuations $\Delta{N}$ have been deduced using extensive ground-based observations of interplanetary scintillation (IPS) at 327 MHz, which probe spatial scales of a few hundred km. The background densities ($N$) have been derived using near-Earth observations from the Advanced Composition Explorer ($\it{ACE}$). Our analysis reveals that $0.001 \lesssim \epsilon_{N} \lesssim 0.02$ and does not vary appreciably with heliocentric distance. We also find that $\epsilon_{N}$ declines by 8% from 1998 to 2008. We discuss the impact of these findings on problems ranging from our understanding of Forbush decreases to the behavior of the solar wind dynamic pressure over the recent peculiar solar minimum at the end of cycle 23.., Comment: 13 Pages, 8 Figures, Accepted for publication in ApJ