Project description

Problem statement

Space weather is the incessant forcing exerted by solar magnetic activity on the space environment of Earth and other bodies in the solar system. This forcing perturbs the Earth’s magnetic sphere of influence (geospace) on timescales of hours and days. These perturbations can be detrimental, and potentially catastrophic, to terrestrial infrastructure and human assets in orbit, beyond the atmosphere [1]. Space climate, in turn, is due to the Sun’s variability over timescales of decades, centuries and millennia, influencing the long-term occurrence frequencies of such space weather events [2,3]. An ability to predict space climate variations is clearly important for the planning of future space missions, as well as for the incorporation of realistic solar forcing variations in models of future terrestrial climate change.

The immediate cause of space climate variations is the semi-regular nature of the 11-year cyclic variations in solar activity: the amplitude and length of successive solar cycles show quite large variations. The second half of the 20th century was characterized by a series of strong solar cycles known as the Modern Maximum. This era of strong solar activity has abruptly ended in the first decade of the 21st century. The ongoing Solar Cycle 24 which started in 2008, peaked in 2014 with a significantly lower peak amplitude, somewhat below the long-term average and similar to the solar cycles seen in the early 20th century.

This marked change has prompted increased interest in the origin of cycle-to-cycle variations in solar activity and in possibilities of predicting the amplitude of upcoming solar cycles, most notably Cycle 25. Experience has shown that the best candidate for a physical precursor of the amplitude of an upcoming cycle is the peak strength of the solar polar magnetic fields (or alternatively, the solar dipole moment), reached typically around the time of solar minimum [4].

The critical issue still open is how, in turn, the amplitude of the dipole field is determined by solar activity in the previous cycles. Observations clearly indicate that the polar fields are built up by the poleward transport of trailing polarity magnetic flux from bipolar active regions. (“Trailing” and “leading” refer to the direction of solar rotation. Trailing polarity is normally positioned at slightly higher latitudes than leading polarity, and it is the same polarity for ARs on the same hemisphere in the same solar cycle.) This transport is mainly due to a meridional flow, so variations in the meridional flow have been invoked as a factor in intercycle activity variations [5-7].

An alternative possibility has been highlighted by Cameron et al. [8] who stressed the importance of the tilt angle of bipolar active regions relative to the east-west direction. Clearly, for zero tilt, leading and trailing polarity flux would be transported towards the poles in equal rates, resulting in no net change in the polar flux. An increasing tilt angle will then lead to an increasing polar field strength. This opens two intriguing possibilities. On the one hand, some studies of the observational record indicate that tilt angles are anticorrelated to cycle amplitude [9]. The origin of this anticorrelation may be related to the dynamics of the emerging flux loop or to the meridional inflows towards the active latitude zone associated with the torsional oscillation pattern, the amplitude of which is determined by the level of solar activity. This tilt quenching is a potentially important nonlinear feedback effect of solar activity level on the tilt angles, and thereby on the buildup of polar fields that will serve as seed fields for the next cycle.

On the other hand, a random scatter of tilt angles around the mean value determined by the above process will introduce a significant degree of stochasticity in the process. As the total magnetic flux in the polar cap is comparable to the magnetic flux in a single large active region, in some cases even one AR can have a major distorting effect of polar flux buildup. Large exceptional or rogue active regions disobeying Joy’s law or Hale’s polarity rules [10,11] can potentially play havoc with the buildup of polar fields, especially if they emerge near the equator as in this case a higher fraction of one polarity can diffuse across the equator, avoiding cancellation with its opposite polarity counterpart. It has been shown in a dynamo model that in extreme cases such freak events may even trigger longer episodes of unusually low or unusually high activity, i.e. grand minima or grand maxima [12].

In view of the above developments, for improving solar activity forecasts on a decadal scale (i.e. for space climate forecasting) we need to

  • improve our theoretical understanding of the respective roles played by various nonlinear feedback effects on the dynamo and by stochastic fluctuations represented by individual AR
  • assimilate actual observations of individual solar active regions into the models and calibrate the models to solar observations
  • clarify the role, character and origin of meridional flow variations as a further potentially important mechanism in intercycle variations

Objectives

The goal of our collaborative project is to advance our understanding of the mechanisms leading to major changes in space climate by making progress in the modelling of the effects of large and complex active regions. We will combine the existing expertise of team members in order to systematically study the impact of large and complex solar active regions on space climate. We will make concerted efforts towards some or all of the following general objectives:

  • investigate what are those characteristics of active regions (other than sheer size) that contribute most to their effectivity in impacting space climate. In particular we will study how dynamo effectivity depends on the emergence latitude of ARs and, in view of some recent results [13], how the deviations of real AR from simple bipoles (e.g. timing of emergence and decay, and complex, extended spatial structure) may be best represented in the theoretical models. These studies will rely on surface flux transport (SFT) and dynamo models. In this respect we also need to
  • improve the calibration of SFT and dynamo models to observations to best represent the large-scale solar magnetic field, building on recent results [14]. This work may also involve the incorporation of hitherto neglected effects into the models, a case in point being pumping effects in dynamo models [15]. In parallel with this, we need to
  • clarify the influence of parameters and other choices in surface flux transport and dynamo models on the general characteristics of the solar magnetic cycle, on nonlinear feedback effects and on the dynamo effectivity of individual ARs. The incorporation of actual solar observations into such models should then open the way towards better forecasting of solar cycle variations. To this end, we will
  • collect, select and process observational data, analyzing a set of ARs carefully selected from the archives to test and validate our theoretical inferences. We may further collect data for and model large and complex historical (pre-1950s) active regions to assess their potential role in the space climate of the past. In addition we will select and analyze any candidate regions from solar patrol observations that may have significant effects on space climate. Finally, building on these results it will be possible to
  • attempt to explain and reproduce past intercycle variations in solar activity by modelling the effect of the rogue AR candidates selected

While not losing the clear focus of this research project, we also plan to extend its scope by considering

– hemispheric asymmetries: these may be introduced by rogue AR either directly (by the AR affecting primarily the hemisphere where it is located) or indirectly, by exciting a quadrupolar dynamo mode, as suggested recently [16]. It should be noted that the call from the NOAA/NASA cycle 25 prediction panel mentions explicitly hemispheric asymmetry as a target prediction [17].

– variations in the meridional flow/torsional oscillation pattern, as another potential factor in intercycle activity variations [5-7]

– the possibility to compute probability distributions for two-cycle predictions, if the effectivity of large active regions on upcoming cycles is accurately characterized.

Methodology

In order to make advance towards these goals the team will

– discuss the factors influencing dynamo effectivity

– summarize the available information about the issue from previous research

– conduct its own exploratory studies using existing surface flux transport and dynamo codes at the participants’ disposal (particularly the 2X2D dynamo code, the 3D STABLE dynamo code and the AFT and MPS SFT codes).

– make benchmark tests and comparisons of the aforementioned codes, and perform calibration to observations

– comb through archive data from space observatories (e.g. SoHO, SDO, Hinode) and historical observational databases to identify some rogue AR candidates, collecting available information.

References

[1] National Research Council: Severe Space Weather Events. Nat. Academies Press (2009)

[2] Petrovay K, Living Rev. Sol. Phys. 7, 6 (2010)

[3] Usoskin IG, Living Rev. Sol. Phys. 14, 3 (2017)

[4] Muñoz-Jaramillo A, Balmaceda LA, DeLuca EE: Phys. Rev. Lett. 111(4), 041106 (2013)

[5] Jiang J, Işik E, Cameron RH, Schmitt D, Schüssler M: Astrophys. J. 717, 597 (2010),

[6] Hathaway DH, Upton L: J. Geophys. Res. A 119, 3316 (2014)

[7] Upton L, Hathaway DH: Astrophys. J. 792, 142 (2014)

[8] Cameron RH, Jiang J, Schmitt D., Schűssler, M: Astrophys. J. 719, 264 (2010)

[9] Dasi-Espuig M, Solanki SK, Krivova NA, Cameron R, Peñuela T.: Astron. Astrophys. 518, A7

(2010)

[10] McClintock BH, Norton AA: Astrophys. J. 797, 130 (2014)

[11] Jiang J, Cameron RH, Schüssler M: Astrophys. J. 791, 5 (2014)

[12] Nagy M. Lemerle A, Labonville F, Petrovay K, Charbonneau P: Sol. Phys. 292, 167 (2017)

[13] Jiang J, Song Q, Wang J-X, Baranyi T: Astrophys. J. 871, 16 (2019)

[14] Virtanen I & Mursula K: Astron. Astrophys. 604, 7 (2017)

[15] Karak BB & Cameron RH: Astrophys. J 832, 94 (2016)

[16] Schüssler M, Cameron RH: Astron. Astrophys. 618, 89 (2018)

[17] Upton L: Solar Cycle 25 Call for Predictions. Solar News 2019/1