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A Twenty Year Decline in Solar Photospheric Magnetic Fields: Inner-Heliospheric Signatures and Possible Implications?  

Susanta Kumar Bisoi   Submitted: 2015-06-11 20:46

We report observations of a steady 20 year decline of solar photospheric fields at latitudes ≥45o 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 (IPS) 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 micro-turbulence 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) 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).

Authors: P. Janardhan, Susanta Kumar Bisoi, S. Ananthakrishnan, M. Tokumaru, K. Fujiki, L. Jose, R. Sridharan
Projects: None

Publication Status: JGR (Space Physics)(In Press)
Last Modified: 2015-06-15 10:45
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A study of density modulation index in the inner heliospheric solar wind during solar cycle 23  

Susanta Kumar Bisoi   Submitted: 2014-08-19 19:51

The ratio of the rms electron density fluctuations to the background density in the solar wind (density modulation index, εN \equiv Δ{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 εN in the inner-heliosphere from 0.26 to 0.82 AU. The density fluctuations Δ{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 (ACE). Our analysis reveals that 0.001 ≤sssim εN ≤sssim 0.02 and does not vary appreciably with heliocentric distance. We also find that εN 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.

Authors: Susanta Kumar Bisoi, P. Janardhan, M. Ingale, P. Subramanian, S. Ananthakrishnan, M. Tokumaru, and K. Fujiki
Projects: ACE

Publication Status: ApJ (in Press)
Last Modified: 2014-08-20 09:47
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Changes in quasi-periodic variations of solar photospheric fields: precursor to the deep solar minimum in the cycle 23?  

Susanta Kumar Bisoi   Submitted: 2013-05-06 01:41

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 45 degree 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 degree to 60 degree. 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 45 degree 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 45 degree 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 on our current understanding of the solar dynamo.

Authors: Susanta Kumar Bisoi, P. Janardhan, D. Chakrabarty, S. Ananthakrishnan, and Ankur Divekar
Projects: National Solar Observatory (Sac Peak)

Publication Status: Solar Physics (accepted for publication)
Last Modified: 2013-05-06 11:08
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Abstracts by Author
A Twenty Year Decline in Solar Photospheric Magnetic Fields: Inner-Heliospheric Signatures and Possible Implications?
A study of density modulation index in the inner heliospheric solar wind during solar cycle 23
Changes in quasi-periodic variations of solar photospheric fields: precursor to the deep solar minimum in the cycle 23?

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