Extensive interplanetary scintillation (IPS) observations at 327 MHz obtained between 1983 and 2009 clearly show a steady and significant drop in the turbulence levels in the entire inner heliosphere starting from around ~1995. We believe that this large-scale IPS signature, in the inner heliosphere, coupled with the fact that solar polar fields have also been declining since ~1995, provide a consistent result showing that the buildup to the deepest minimum in 100 years actually began more than a decade earlier.
The sunspot minimum at the end of Cycle 23, has been one of the deepest we have experienced in the past 100 years with the first spots of the new cycle 24 appearing only in March 2010 instead of December 2008 as was expected. Also, the number of spotless days experienced in 2008 and 2009 was over 70%. Apart from this, Cycle 23 has shown a slower than average field reversal, a slower rise to maximum than other odd numbered cycles, and a second maximum during the declining phase that is unusual for odd-numbered cycles. Though these deviations from "normal" behaviour could be significant in understanding the evolution of magnetic fields on the Sun, they do not yield any direct insights into the onset of the deep minimum experienced at the end of cycle 23. This is because predictions of the strength of solar cycles and the nature of their minima are strongly dictated by both the strength of the ongoing cycle [Dikpati et al. (2006); Choudhuri et al. (2007)] and changes in the flow rates of the meridional circulation [Nandy et al. (2011)].
Ulysses, the only spacecraft to have explored the mid-and high-latitude heliosphere, in its three solar orbits, provided the earliest indications of the global changes taking place in the solar wind. A significant result from the Ulysses mission came from observations of |B r |, the radial component of the interplanetary magnetic field (IMF), as a function of the heliographic distance r which showed that the product |B r | r 2 is independent of the heliographic latitude [Smith et al. (2003); Lockwood et al. (2009)]. This result has far reaching consequences in that it essentially enables one to use in situ, single-point observations to quantify the open solar flux entering the heliosphere. The second Ulysses orbit, covering the rising to maximum phase of cycle 23, found the solar wind dynamic pressure (momentum flux) to be significantly lower in the post maximum phase of cycle 23 than during its earlier orbit, spanning the declining phase of cycle 22 [Richardson et al. (2001);McComas et al. (2003)]. Finally, the third Ulysses orbit (2004 -2008) found a global reduction of open magnetic flux and showed an ∼ 20% reduction in both solar wind mass flux and dynamic pressure in cycle 23 as compared to the earlier two cycles [McComas et al. (2008)]. In addition, a study using solar wind measurements between 1995-2009 [Jian et al. (2011)] has shown that the solar minimum in 2008-2009 has experienced the slowest solar wind with the weakest solar wind dynamic pressure and magnetic field as compared to the earlier 3 cycles. Figure 1 shows the solar magnetic field in the latitude range 45 The method adopted in computing solar magnetic fields shown in Figure 1 and details of the database used has been described in [Janardhan et al. (2010)]. It is clear from Figure 1 that there has been a continuous decline in the magnetic field starting from around 1995. Since the IMF is basically the result of photospheric magnetic fields being continuously swept out into the heliosphere one would expect to see this reflected in the solar wind and interplanetary medium. Due to the fact that the solar wind undergoes an enormous change in densities ranging from ∼10 9 cm -3 at the base of the corona [Mann et al. (2003)] to ∼10 cm -3 at 1 AU, different techniques are needed to study different regions of the solar wind. However, IPS is the only technique that can probe the entire inner-heliosphere using ground based radio telescopes operating at meter wavelengths.
IPS is a scattering phenomenon in which one observes distant extragalactic radio sources to detect random temporal variations of their signal intensity (scintillation) which are caused by the scattering suffered when plane electromagnetic radiation from the radio source passes through the turbulent and refracting solar wind [Hewish et al. (1964); Ananthakrishnan et al. (1980); Asai et al. (1998); Manoharan (2010a); Tokumaru et al. (2010)]. Though IPS measures only small-scale (∼150 km sized) fluctuations in density and not the bulk density itself, Hewish et al. (1985) showed that there was no evidence for enhanced or decreased IPS that was not associated with corresponding variations in density. They went on to derive a relation between a normalized scintillation index denoted ‘g’ and the density given by g=(Ncm -3 /9) 0.52±0.05 . Thus, whenever interplanetary disturbances, containing either enhanced or depleted rms electron density fluctuations (∆n e ) as compared to the background solar wind, cross the line-of-sight (LOS) to the observed source they exhibit themselves as changes in the levels of scintillation (m), i.e. higher or lower than expected m, where m = ∆S/ S is the ratio of the scintillating flux ∆S to the mean source flux S . In other words, whenever turbulence levels change in the solar wind, they will be reflected in IPS measurements as changes in m. The main advantage of the IPS technique is that, it can probe a very large region of
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