Declining Photospheric Fields and Solar Wind Micro-Turbulence

1. Quiet Flows the Solar Wind: Long-term Trends in Solar Photospheric Fields and Solar Wind Micro-Turbulence Janardhan, P1 and Ananthakrishnan, S.2 2.Dept. of Electronics. SP. Pune University Pune - 7 India. 1.Astronomy & Astrophysics Divn. Physical Research Laboratory Ahmedabad - 9 India. Collaborators: Susanta Kumar Bisoi. IAUS 340, Jaipur, Feb. 20, 2018

2. We all know that the Sun is exhibiting some unusual behaviour in its solar cycle activity. Each solar cycle during the past three is less active than the previous one leading to speculation of an impending Maunder minimum (no sunspots for a few decades).

3. Unusual Solar Cycle 23 Unusual Solar Cycle 23 200 Jian et al., Sol. Phys., 2011 20 21 22 23 SSN 0 1965 1975 1985 1995 2005 Top 25 – Since 1849 http://users.telenet.be/j.janssens/Spotless/Spotless.html#Year Number of Spotless Days 2007 1901 2009 1878 2008 1913

4. Sunspot - Intensities and Magnetic Field Magnetic field computed from the Zeeman splitting of the Fe I 1564.8 nm line, for umbral spectra from 1998-2005. The mean values for each year are shown. The best-fit to the original 906 data points gives a decrease in the average magnetic field strength of 52 G/yr. B Strength [Gauss] Time [year] Penn and Livingstone, ApJ.,2006

5. Sunspot - Intensities and Magnetic Field Umbral intensity normalized to the brightness of the quiet Sun. A sunspot would disappear (i.e. have zero contrast) when I reaches a value of 1.0 on this graph. Similarly, dark sunspots do not appear with a maximum magnetic field strength below 1500 G. Livingston, et al ., ApJ Lett. (2012) Caution: The intensity profile of Fe I 1564.8 nm line suffers from strong telluric blends in its red wings; hence stokes measuremt. imprecise. Since tel-blends change with solar elevation and weather conditions, it is difficult to correct for them.Rezaei et al., A&A,(2015).

6. Data Used • * Considering the above we looked at it from a different angle. • FITS format files in the from of 180 × 360 arrays: • The NSO/Kitt Peak Synoptic Magnetogram database covering 1975.13 (CR 1625) to 2003.66 (CR 2006) • (ftp://nsokp.nso.edu/kpvt/synoptic/mag/) • NSO/SOLIS facility from CR 2007 to 2014.42 (CR 2151) (ftp://solis.nso.edu/level3/vsm/merged/carr-rot/) • Data available for 526 CR’s from 1975 – 2014 - 2016 • (Data gap: 2.4 %) • Interplanetary Scintillation (IPS) measurements at 327 MHz from the Solar Terrestrial Environment Laboratory (STEL): • Data available from 1983 to 2013 - 2016 • (Data gap: 1 year in 1993)

7. Methodology Data Used Sine Latitude 1 POLAR - N Longitude is averaged to produce a 1o strip Sine Lat. 0 TORROIDAL - N TORROIDAL -S Magnetic field is averaged for selected lat. bin POLAR - S -1 “POLAR” FIELD (45-78 deg.) “TOROIDAL” FIELD (0-45 deg.)

8. Declining Photospheric Fields… Janardhan et . al., & Bisoi et. al., Sol. Phys. 2010, 2014

9. N-S Asymmetry Clearly, the toroidal fields seem to follow the solar cycle, but it is the polar fields that are declining.

10. Wavelet Spectra Polar Fields Toroidal Fields Bisoi, JP.,….et al., Sol. Phys., 2014

11. Interplanetary Scintillation (IPS)   r -4   P Sun-Earth-Line 0138+136 Line-of-sight  Offsource IPS Obs. Yield: Scintillation Index (m) m At 327 MHz: 12o  55o Heliocentric Distance (R)

12. Comparison : Photospheric Vs Heliospheric • Using the correlation between the polar field and HMF at solar minimum, the value of the HMF in 2020 was estimated to be 3.9 (±0.6) nT and a floor value of the HMF of ∼3.2 (±0.4) nT. [Janardhan, et. al., JGR, 2015 ]. • The fields will continue to decline at least until 2020, the expected minimum of the current cycle – i.e. a decline of <~25 yrs., or over one full magnetic cycle! 2031 2028 Janardhan et al., JGR., 2015

13. Polar Field Reversal – With Latitude NSO/KP Polar Field (G) Nov 2014 2010 2012 2014 2016 Year

14. More recently, using 17 GHz microwave images and high latitudes prominence activity, Gopalswamy et al. (2016) reported, that the polar reversal in the Southern hemisphere occurred around June 2014, while in the Northern hemisphere the reversal was completed only by October 2015. It is not clear why there is a difference between the microwave measurements and the photospheric field. (Different heights?)

15. Solar Wind Velocities Absence of a north polar coronal hole Both North and South polar coronal holes fully developed.

16. Decline in both Fields and Solar Wind Microturbulence An estimate of the polar field strength at minimum can be used to predict the strength of the next cycle maximum (Cliver & Ling 2011). We have now extended the data to the end of 2016. 1.8 G IPS Obs. @ 327 MHz The polar field in 2020 was estimated to be 1.8 ±0.08 G. Using this value, a maximum of 62±12 was predicted for cycle 25 ( also see Janardhan et. al., JGR, 2015).

17. Shape of the Terrestrial Magnetosphere Solar wind dynamic pressure too changed from 2.4 nPa(1974-1994) to 1.4 nPa(2009-2013). (Ingale et al. Priv. Commn. 2017).

18. Conclusions • Solar photospheric fields and solar wind micro-turbulence levels have been steadily declining for the past >22 years and will continue to decline at least until 2020. It is not yet clear whether we are headed towards a Maunder-like minimum beyond Cycle 25. • Based on the correlation between high-latitude magnetic field and the HMF at solar minima, the HMF is expected to decline to 3.9 ±0.6 nT by 2020.

19. Based on the predicted HMF we expect the Sunspot maximumof Cycle 25 to be 62 ±12, making it only a little stronger than the cycle preceding the Maunder Minimum. • There is indication that the ionospheric cut-off frequency is reducing significantly and making it the best time for very low frequency ground based radio astronomy observations!

20. THANKS FOR YOUR ATTENTION

21. Louis XIV – The Sun King (1638 – 1715) “The reign of Louis XIV appears to be a time of real anomaly in the behaviour of the Sun.”- John A Eddy – Science, 1976 Sunspot Number 0 50 100 150 200 1645 - 1715 1600 1650 1700 1750 Year Are we headed towards another Maunder like Minimum beyond cycle 25…..?

24. Estimating the Strength of Cycle 25 Cliver and Ling, Sol . Phys. (2011) The strength of the sun’s polar magnetic field at minimum is related to the next cycle’s sunspot activity (Cliver and Ling 2011). SSNmax = 63.4 × Bmin - 184.7 SSNmax (24) = 64.7 ±22 For a Bmin of 3.9 nT the sunspot maximum in cycle 25, SSNmax (25) = 62 ± 12

25. Polar Field Reversals Mar 2000 Jun 1981 Apr 1991 Jun 2012 Nov 2014 Magnetic Field (G) Jun 2000 Nov 2013 Mar 1990 May 1980 1980 1990 2000 2010 Year

27. Polar Field Reversal – Cycle 24

28. Prominence Eruptions NoRH 17 GHz • Prominence eruptions are traditionally observed in the H line. An indicator of solar max. phase is the phenomenon of rush to the poles (RTTP; Altrock 2014) of Polar Crown Prominences • Prominences contain cool material (~8000 K) and are optically thick in microwaves. • The thermal free-free emission from the prominence plasma is the dominant continuum emission in microwaves, outside the solar disk, and can be imaged with great clarity even when it is heated to higher temperatures (heated prominences will disappear in H).

29. Prominence Eruptions Gopalswamy, Yashiro, & Akiyama, ApJL, 2016 • The quiet sun brightness temp. at 17 GHz is ∼104 K. The lack of MBE in the north-polar region during 2012–2015 is apparent as B ∼0. • PEs are infrequent during minima . As activity rises, PEs occur at higher and higher latitudes, mimicking RTTP (Gopalswamy et al. 2003, 2012). • Cessation of HL PE’s roughly coincide with polarity reversals. • The reversal epoch in the north was estimated as late 2015 (Gopalswamy et al., 2016)

30. Shape of the Terrestrial Magnetosphere L10: Lin et. al., JGR, 2010 L11: Lu et. al., JGR, 2011

32. Ionosonde Observations – Over the dip equator , Trivandrum N = 1.24 104  [fo F2]2 electrons/cm3 F-region densities are controlled by production, loss and transport. Height (km) Dip Equator foF2 N (m-3) Dst Network Height , km

33. Sunspots and the Terrestrial Connection N = 1.24 104  [fo F2]2 electrons/cm3 • Sunspots govern the solar radiative energy (Kirvova & Solanki, 2007), ionospheric electron content, and radio flux. • In conjunction with the polar field, sunspots modulate the solar wind, heliospheric open flux and consequently cosmic ray flux at Earth (Solanki et al., 2000).

34. Long -term Radioisotope Records 0.4 0.6 0.8 1.0 10Be Concentration [10 4/g] 150 1.2 120 Sunspot No. 1.4 90 1.6 60 30 1400 1500 1800 1900 1600 2000 1700 Years • Using records of 14C in tree rings and 10Be in polar ice cores, • a number of grand minima have been detected in the past few • millennia (Usoskin et al. 2007; Steinhilber et al. 2012). • Usoskin et al. (2007) have reported about 27 grand minima in • the last 11,400 years using 14C data.

35. Modelling Grand Minima • Fluctuations in meriodional circulation initiate grand minima (Karak and Choudhuri, 2013), with gradual changes giving rise to a gradual onset. • One or two solar cycles before grand minima onset, the cycle period becomes longer (since meridional circulation determines cycle period - Karak and Choudhuri, 2013). • There is evidence of longer cycles before the start of the Maunder minimum and Spörer minimum (Miyahara et al., 2010). 13.0 m s-1 11.5 m s-1 8.5 m s-1 Hathaway & Rightmire, Science, 2010

36. Modelling Grand Minima…(Contd.) L. Zachilas · A. Gkana – Sol. Phys. 2015 Future Predictions Past Predictions • Yearly mean SSN data (1700 – 2012), indicates that it is a low-dimensional deterministic chaotic system. • Model is able to reconstruct the Maunder Minimum period (1645 – 1715), and make long-term post-facto predictions. • Predicted: Solar activity is likely to be reduced significantly during the next decades, leading to another prolonged sunspot minimum (since the era of Maunder Minimum) lasting up to the year ≈ 2100.

37. Modelling Grand Minima…(Contd.) 10Be concentration in Greenland ice cores • Data and model of 10Be isotope (a) for the past 40 kyr provided by Finkel and Nishiizumi, 1997. (b) The model in (a), is extrapolated covering the last 135 kyr, and the Stuverik-Storm et al., 2014 data is included for comparison. • A 9500 year periodicity in solar activity. “Our model confirms a Grand minimum in the period AD2050–2200 ”. J. Sánchez-Sesma, Earth Sys. Dynamics, (2015).

38. Cooling During the Maunder Minimum The Frozen Themes (1677) Mann, et al., 2009: Science, 326, 1256-1260., Shindell et al., JGR 2006 • The temperature difference between 1680, at the center of the Maunder Minimum, and 1780, a year of normal solar activity, as calculated by a general circulation model. • Already in the midst of a colder-than-average period - the Little Ice Age, Europe and North America went into a deep freeze: alpine glaciers extended over valley farmland; sea ice crept south from the Arctic; and the famous canals in the Netherlands froze regularly—an event that is rare today.

39. Conclusions • Photospheric magnetic fields and solar wind micro- turbulence levels have been steadily declining for the past 20 years and will continue to decline at least until 2020: a period of ~25 years. • Estimate of SSNmax for Cycle 25: 62 ±12, making it only a little stronger than the cycle preceding the Maunder Minimum between 1645 and 1715. • There is some indication that the ionospheric cut- off frequency has reduced to well below 10 MHz. • Our data and modelling efforts by other researchers seem to strongly suggest a grand minimum in the offing beyond cycle 25!

40. Ionogram foF2 fxF2 h’F = The base height of the F region, and the foF2 = Max frequency Both these define the F2 layer reasonably well.

41. F-Region Densities • F-region densities show diurnal, day-day, monthly (solar rotation), seasonal, semi-annual, and annual variations. • Exhibit solar cycle variations showing good correlation with the sunspot numbers, the latter being used as a proxy to the solar EUV • Over any location, F-region densities are controlled by production, loss and transport. The role of transport is crucial and differs from location to location. • To establish the dependence of solar activity represented by sunspot number alone, one should concentrate on the background representative ionization devoid of transport effects. Importance of the Magnetic Equator • Over the magnetic equator, only electro-dynamical processes dominate in changing the number density distribution • These processes dominate only during the daytime and all the direct and indirect forcing reach a steady state well past midnight, over any location • Since sunrise effects are yet to be registered, it is perceived that the densities at ~0300 LT would be a good representation of the quiescent ionosphere that is solely controlled by solar activity.

42. N = 1.24  104  [foF2]2 electrons/cm3. • Used foF2 measurements at 0300 LT for the period 1994-2014. • A strong correlation (R= 0.96) between SSN and (foF2)2 indicates no such declining trend in (foF2)2 or ionospheric electron density. • An impending Maunder minimum is not likely to have any impact on the Earth’s ionosphere – except in reducing the cutoff Freq. • The average (foF2)2 for 2008 – 2009 was around 10 (MHz)2 , implying an ionospheric reflection cutoff at 0300 hrs LT of <3.5 MHz. • A boon for low frequency ground based radio astronomy.

43. At vertical incidence, waves with frequencies larger than the electron plasma frequency (fe) of the F-layer maximum fe = 9 (Ne)1/2 kHz (Ne in cm−3 is the electron density) can propagate through the ionosphere nearly undisturbed. Waves with frequencies smaller than fe are reflected within the ionospheric D-, E-, and F-layers. fe is of the order of 8–15 MHz during day time conditions. For oblique incidence, the critical frequency becomes larger.

44. Recent Hiatus in Global Warming! Yu Kosaka & Shang-Ping Xie, Nature, (2013). Two schools of thought exist regarding the cause of this hiatus in global warming: A slowdown in radiative forcing due to stratospheric water vapour, the rapid increase of stratospheric and tropospheric aerosols. The solar minimum around 2009.