Nandy et al. Progress in Earth and Planetary Science
https://doi.org/10.1186/s40645-021-00430-x(2021) 8:40

Progress in Earth andPlanetary Science

REVIEW

Open AccessSolar evolution and extrema: current stateof understanding of long-term solar
variability and its planetary impacts
Dibyendu Nandy1,2* , Petrus C. H. Martens3, Vladimir Obridko4, Soumyaranjan Dash1 and Katya Georgieva5

Abstract
The activity of stars such as the Sun varies over timescales ranging from the very short to the very long—stellar andplanetary evolutionary timescales. Experience from our solar system indicates that short-term, transient events suchas stellar flares and coronal mass ejections create hazardous space environmental conditions that impact Earthorbiting satellites and planetary atmospheres. Extreme events such as stellar superflares may play a role inatmospheric mass loss and create conditions unsuitable for life. Slower, long-term evolutions of the activity of Sunlike stars over millennia to billions of years result in variations in stellar wind properties, radiation flux, cosmic ray
flux, and frequency of magnetic storms. This coupled evolution of star-planet systems eventually determines
planetary and exoplanetary habitability. The Solar Evolution and Extrema (SEE) initiative of the Variability of the Sun
and Its Terrestrial Impact (VarSITI) program of the Scientific Committee on Solar-Terrestrial Physics (SCOSTEP) aimed
to facilitate and build capacity in this interdisciplinary subject of broad interest in astronomy and astrophysics. In
this review, we highlight progress in the major themes that were the focus of this interdisciplinary program,namely, reconstructing and understanding past solar activity including grand minima and maxima, facilitating
physical dynamo-model-based predictions of future solar activity, understanding the evolution of solar activity over
Earth’s history including the faint young Sun paradox, and exploring solar-stellar connections with the goal ofilluminating the extreme range of activity that our parent star—the Sun—may have displayed in the past, or may
be capable of unleashing in the future.
Keywords: Solar magnetic fields, Stellar evolution, Stellar flares, Stellar wind, Space weather, Space climate, Faint
Young Sun paradox, Star planet interactions, Planetary evolution, Habitability

1 Introduction
The Sun is a typical middle-aged G-type star whose
current age is 4.6 billion years. It is an active star, withits magnetic activity driving short-to-long-term fluctuations in its radiative, electromagnetic, and particulate
output. Occasionally, magnetic structures in the Sun
generate magnetic storms—flares and coronal mass ejections (CMEs)—that are accompanied by intense* Correspondence: dnandi@iiserkol.ac.in
1
Center of Excellence in Space Sciences India, Indian Institute of Science
Education and Research Kolkata, Mohanpur, West Bengal 741246, India
2
Department of Physical Sciences, Indian Institute of Science Education andResearch Kolkata, Mohanpur, West Bengal 741246, India
Full list of author information is available at the end of the article

radiation output, solar energetic particles (SEPs) andmagnetized plasma ejecta that perturb planetary spaceenvironments (Schwenn 2006; Pulkkinen 2007). Studiesof short-term solar variability and their influence on
space-reliant technologies and planetary atmospheres
and magnetospheres, including efforts to understandand predict solar activity are collectively termed as thescience of space weather (Schrijver et al. 2015). Slower,
long-term fluctuations in solar output over decades tomillennia are driven by long-term variations in the solar
cycle which is powered by the dynamo mechanism thatworks in the solar interior (De Jager 2005). These variations create space climate—changing radiative and© The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License,
which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give
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Nandy et al. Progress in Earth and Planetary Science(2021) 8:40

particulate environment in the heliosphere that can impact planetary atmospheres and climate ranging fromdecadal to longer timescales (Mursula et al. 2007; Nandy
and Martens 2007; Versteegh 2005). On top of this variability manifest over few generations of humans, longer term stellar evolutionary processes drive changes inthe solar convection zone (SCZ) and differential rotationthat alter the nature of the solar dynamo, and sustain
evolutionary changes in solar wind, solar radiation, andsolar storm frequency which together determine spaceenvironmental forcing of planetary atmospheres over billions of years—i.e., the evolutionary history of planets(Nandy and Martens 2007).
While ground-based observations spanning four centuries have given us a fairly good idea of decadal tocentury-scale fluctuations in the solar magnetic cycle,
and more recent space-based observations have uncovered subtle variations in solar radiation, constrained energetic processes such as flares and CMEs, andmeasured properties of the solar wind, one has to takerecourse to observations of other Sun-like stars and exoplanetary systems to appreciate the full range of plausible activity levels across the history of a Sun-like starand its consequence of planets (Nandy and Martens
2007; Brun et al. 2015). The rapidly emerging field of
Star-Planet Interactions opens up this window to explorations of the coupled evolution of star-planet systems
such as the Sun-Earth system or exoplanets around active stars (Deeg and Belmonte 2018; Das et al. 2019).
In this context, the Scientific Committee on SolarTerrestrial Physics (SCOSTEP) supported the Solar Evolution and Extrema (SEE) program to address the interdisciplinary nature of the science that seeks to uncoverand explore solar activity across a wide range of timescales so that one can fathom the impact of this activity
on planet Earth and beyond. The SEE program sought
to facilitate community research to broadly seek answers
to the following set of problems: Are we at the verge ofa new grand minimum? If not, what is the expectation
for cycle 25? For the next few decades, what can we expect in terms of extreme solar flares and storms, andalso absence of activity? What is the largest solar
eruption/flare possible? What is the expectation for periods with low activity? Does our current best understanding of the evolution of solar irradiance and massloss resolve the “Faint Young Paradox”? How does geospace response to extreme solar events such as superflare or energetic CMEs. The SEE program spawned
various collaborative efforts, supported long-term dataarchives through the SCOSTEP program, and hosted
several meetings over the course of 4 years in order tobuild capacity in this domain of research. Individual results from the SEE program are reported elsewhere(Obridko and Georgieva 2018). Here, we review the

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general state of the field that the SEE program targeted
and summarize the current state of our understandingof solar activity evolution and extrema over timescales
relevant for atmospheric forcing and planetary evolution.
For purposes of readability and with the motivation toprovide a seamless context, here we do not attempt to
explicitly segregate research results obtained during andprevious to the SEE era; rather, our focus is on providing
the relevant context and current status that would inform and motivate future research in this domain.

2 The solar activity cycle: past, present, andfutureThe Sun is a middle-aged G-type star whose current age
is 4.6 billion years. It is a moderately active star, with its
magnetic activity driving short-to-long-term fluctuationsin its activity output. The emergence of sunspots on thesolar surface varies in a periodic fashion. This is referred
to as the solar cycle, which originates in a magnetohydrodynamic (MHD) dynamo mechanism that is sustained by plasma motions in the Sun’s interior. Much of
the constraints on this dynamo mechanism come fromobservations of sunspots (Fig. 1)—dark, magnetized regions on the solar surface—whose magnetic fieldstrength can reach a few thousand gauss (G). Long-term
observations over four centuries, since the invention of
the telescope in the early 1600s, show that the numberof sunspots observed on the Sun increases and decreases
in a cyclic fashion (Schwabe 1844; Hathaway 2015),
thereby generating the solar cycle (Fig. 2). The sunspots
observed on the surface show a systematic tilt which increases with latitude. This is also known as Joy’s law(Hale et al. 1919). In reality, the variation is quasiperiodic, with cycle periods ranging between 9 and 14
years with a mean period of 11 years and strong fluctuations in the amplitude of the cycle. In the era of direct
sunspot observations, there has been a long period of absence in solar activity between 1645 and 1715—with very
few sunspots observed (see Fig. 2); this period is known
as the Maunder minimum (Eddy 1976). There are secular trends observed in the sunspot time series with episodes of aperiodic increasing or decreasing activity
which last over several cycles (scale of centuries)—with
no regularity (Usoskin 2017). The quasi-periodic natureof the solar cycle, with no apparent relation between thestrength of subsequent sunspot cycles makes predictionsa challenging task—an exercise that has to be firmly
rooted in the physics of the sunspot cycle. The subjectof MHD deals with the creation of astrophysical magnetic fields and provides the theoretical basis for understanding the genesis of solar-stellar magnetism.
Material in the solar interior exists in the plasma state,
wherein convective and large-scale motions can both
destroy and induct magnetic fields through the dynamo

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Fig. 1 A solar magnetogram shows regions of strong magnetic fields (solar active regions) on the solar surface which are typically composed ofbipolar sunspot pairs (white and black regions indicate positive and negative polarity, respectively). These bipolar sunspots appear with asystematic tilt with respect to the local parallel of latitude and their relative orientation is reversed in the two hemispheres. The tilt angle that isobserved within the solar active regions increases with latitude which is known as Joy’s law. Sunspots and associated overlying magnetic field
loops generate solar storms and modulate the radiative energy output of the Sun. They are tracers of the underlying solar dynamo mechanism.
Credit: NASA HMI/SDO

mechanism (Fig. 3). When conditions are right, inductive
processes result in generation of large-scale magnetic
fields in stellar interiors that sustain the plethora ofmagnetic activity that stars display. Stars are generallyborn with preexisting magnetic fields that exist in thestar-forming gases and rotate differentially. A preexistingpoloidal (dipolar) component of the magnetic field canbe stretched by differential rotation to form the toroidal
component (Parker 1955a) from which buoyancy instability generates sunspots (Parker 1955b). During theirbuoyant rise, toroidal flux tubes are subject to the Coriolis force which imparts tilt and thus sunspots appear as
bipolar pairs with a systematic tilt relative to the local
East-West direction. Turbulent, helical convection in rotating systems can work on the buoyantly rising toroidal
flux tubes to regenerate the poloidal field through aprocess known as the mean-field α-affect (Parker 1955a).
The decay and dispersal of the flux of tilted bipolar sunspot pairs, whose evolution is governed by near-surface

plasma flows—such as supergranular diffusion, differential rotation, and meridional circulation—can also regenerate the large-scale dipolar field of the Sun resulting in
the reversal of the old cycle polar field and imparting a
new polarity; this process is widely known as theBabcock-Leighton mechanism (Babcock 1961; Leighton1969) and is now believed to be the dominant process
driving solar cycle fluctuations (Cameron and Schüssler
2015; Bhowmik and Nandy 2018). This continuous recycling of flux between the toroidal and poloidal components of the solar magnetic field is believed to sustain
the solar cycle.
Modeling the full breadth of complex, non-linear interactions between plasma flows and magnetic fields inthe extreme regimes of stellar interiors remains a challenging task. While full MHD, direct numerical simulations are yet to achieve realistic parameter regimes ofstellar convection zones, they are beginning to display
some of the characteristics of solar plasma flows andNandy et al. Progress in Earth and Planetary Science(2021) 8:40

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Fig. 2 The yearly mean total sunspot number time series. Observations spanning four centuries show that the number of sunspots observed on
the solar surface increases and decreases in a quasi-periodic fashion with a mean period of 11 years. The plot shows the yearly mean ISN (v 2.0)
from 1700 until 2019.5. The plot 1700 (the vertical dashed line) show Group Number series (v 1.0). This is scaled by a multiplicative factor of 10 toamplify the plot until the dashed line. The amplitude of sunspot cycles varies from one cycle to another and displays secular trends that are notwell-understood. A phase of very low solar activity with very few sunspot observations—known as the Maunder minimum occurred between
1645 and 1715. Solar activity reconstructions over thousands of years indicate the presence of many more extremely low or strong activity phasesin the solar cycle (Usoskin 2017). Credit: SILSO, Royal Observatory of Belgium, Brussels

Fig. 3 A rendering of a solar dynamo simulation from Muñoz-Jaramillo et al. (2009) shows the distribution of magnetic fields within the Sun’sconvection zone at a phase near solar minimum. The left meridional cut depicts the poloidal component (in purple color) and the right
meridional cut depicts the toroidal component (red and blue colors indicate opposite polarities). The central yellow region corresponds to thebase of the convection zone. On the right, a bar plot denotes the number of sunspots on the solar surface (with yellow lines depicting different
values). Since this snapshot corresponds to a solar minimum, there are no sunspots on the surface. The full movie of the simulation can be foundat https://svs.gsfc.nasa.gov/3521. Credit: Tom Bridgman, NASA/Goddard Space Flight Center Scientific Visualization StudioNandy et al. Progress in Earth and Planetary Science(2021) 8:40

complexes of magnetic activity, including cyclic reversal(Brun et al. 2004; Ghizaru et al. 2010; Nelson et al. 2013;
Hotta et al. 2016; Karak and Miesch 2017; Käpylä et al.
2017). At the same time, physically inspired kinematic
dynamo models based on the Babcock-Leighton idea—
increasing in sophistication and parameterization of
known physics—have been developed which better reproduce the observed features of the solar cycle, such as
cyclic reversal, latitudinal distribution of sunspots, andthe observed phase relationship between the sunspot
cycle and polar fields (Durney 1995; Choudhuri et al.
1995; Dikpati and Charbonneau 1999; Nandy and
Choudhuri 2001, 2002; Nandy 2002; Guerrero andMuñoz 2004; Chatterjee et al. 2004; Guerrero et al.
2008; Muñoz-Jaramillo et al. 2009; Muñoz-Jaramillo
et al. 2010; Nandy et al. 2011; Yeates and Muñoz-Jaramillo 2013; Hazra and Nandy 2016; Kumar et al. 2019).
Figure 3 depicts the internal solar magnetic fields generated from such kinematic dynamo simulations (MuñozJaramillo et al. 2009; Nandy et al. 2011).
2.1 Predicting solar activity

While efforts to understand the solar cycle have now
spanned half a century (see, e.g., the review of Charbonneau 2020), focused investigations of the physical basisof solar cycle predictions have spanned only about a decade. A scrutiny of solar cycle observations already hints
that the strength of the polar field at the minimum of acycle is a good indicator of the strength of the next cycle(Fig. 4). Indeed the more successful empirical,
precursor-based cycle predictions take advantage of theobserved correlation between the polar field at minimum
and the next cycle amplitude (Schatten et al. 1978). For
solar cycles 23 and 24, multiple different methodologies,
e.g., statistical correlations based on precursors of solar
activity, time-series analysis of solar cycle observations,
machine learning techniques, solar surface flux transportmodels, and solar dynamo models, have been utilized forpredictions often resulting in wide disagreements. It isbeyond the scope of this manuscript to review all thesepredictions and the methodologies employed; for further
details, readers are referred to the more exhaustive review of the literature by Pesnell (2008), Petrovay (2020),
Nandy (2021) and the collection of articles catalyzed by
the SEE project published in a special issue of the Journal of Atmospheric and Solar-Terrestrial Physics(Obridko and Georgieva 2018).
It is only now that we are beginning to appreciate theintricate details of how physical processes in the solar interior determine the predictability of solar cycles. We
briefly summarize here the progress in our understanding of solar cycle predictions gleaned over the last decade (and the SEE project era) which has been discussed
in details in Nandy (2021). First of all, theoretical

Page 5 of 21arguments indicate that the Babcock-Leighton dynamo
mechanism of poloidal field generation, governed by the
emergence of tilted bipolar active regions and their flux
dispersal by near-surface flux transport processes, is thedominant driver of poloidal field generation in the Sun(Cameron and Schüssler 2015). Observations of tilt angle
distribution of sunspot groups and their empirical relationship with the amplitude of sunspot cycles also support this scenario (Dasi-Espuig et al. 2010). However,
the two dynamo-based forecasts for solar cycle 24 basedon assimilating the Babcock-Leighton idea (Dikpati et al.
2006; Choudhuri et al. 2007) diverged by a wide margin
inviting controversy and uncertainty in the community.
Subsequently, Yeates et al. (2008) demonstrated that thediverging predictions resulted from different parameterizations (advection dominated versus diffusion dominated) of the SCZ in the two models and there isnothing problematic with the basic premise of theBabcock-Leighton idea itself in terms of its usage in predictive dynamo models.
Recent studies indicate that the interplay of magnetic
flux transport processes such as turbulent diffusion(Muñoz-Jaramillo et al. 2011), meridional circulation(Choudhuri et al. 1995), and turbulent pumping (Käpylä
et al. 2006a, 2006b; Guerrero et al. 2008; Hazra andNandy 2016) impact the memory and predictability ofsolar cycles with the predictive window being restricted
to only one cycle (Karak and Nandy 2012; Hazra et al.
2020). These studies suggest that very long-term prediction beyond the following solar cycle is not possible atthe previous minimum. While solar cycle amplitude variations can result from diverse physical mechanisms,
both observations ((Dasi-Espuig et al. 2010) and datadriven modeling of solar activity (Bhowmik and Nandy
2018; Jiang et al. 2018) suggest that the scatter in active
region tilt angle distribution is the dominant driver ofcycle-to-cycle amplitude variability. In this context,
anomalous active regions with tilt angles that departstrongly from Joy’s law and anti-Hale regions canstrongly perturb the polar field build-up process introducing an element of uncertainty in predictive models
relying on the Babcock-Leighton idea (Nagy et al. 2017).
However, these can be reasonably accounted for via ensemble forecasts—which imbibe reasonable uncertainty
ranges in active region input properties (tilt angle, flux,
and latitude of emergence), as has been done in recent
predictions for solar cycle 25 (Bhowmik and Nandy
2018; Jiang et al. 2018; Labonville et al. 2019).
Physical model-based predictions for solar cycle 25
have broadly followed three approaches. In one approach, surface flux transport models (which assimilate
the observed statistics of sunspot emergence) are utilized
to predict the polar field in advance of the minimum;
subsequently, the empirical relationship between polar

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Fig. 4 Constraints on the solar magnetic cycle based on observations over the past century. The top panel depicts the quasi-periodic variation ofsunspot numbers, the middle panel depicts the variation of the polar field in the northern and southern hemispheres gleaned from polar faculae
counts (Muñoz-Jaramillo et al. 2013) and polar field observations and the bottom panel depicts the sunspot butterfly diagram, i.e., the spatiotemporal evolution of latitudes where sunspots appear. Theoretical dynamo models aspire to reproduce these observational constraints, whilesolar cycle prediction efforts focus on forecasting the strength of future solar cycles(s)fields at minima and subsequent cycle amplitudes isused to get an indication of the future cycle (Cameron
et al. 2016; Iijima et al. 2017; Upton and Hathaway 2018;
Jiang et al. 2018). In another approach, data-driven solar
dynamo models (assimilating the polar field observed at
the minimum) are used to predict the upcoming solar
cycle (e.g., used for cycle 24 predictions by Dikpati et al.
2006; Choudhuri et al. 2007). In the third, recently developed and more complete physics-based approach,data-driven models of magnetic field evolution in thesolar surface and in the interior (i.e., surface flux transport and dynamo models) are coupled to generate solar

cycle predictions (Bhowmik and Nandy 2018; Labonvilleet al. 2019). The physics-based approaches for predicting
solar cycle 25 have converged and all indicate a weakmoderate cycle similar or slightly stronger than cycle 24(see Fig. 5). The timing of the peak is estimated to bearound 2024–2025. Nandy (2021) argues that this convergence in physical model-based predictions for sunspot cycle 25 is a consequence of advances in our
understanding of solar cycle predictability discussed
earlier.
Fuelled by the declining trend in solar cycle activity
over the last several decades, there was widespread

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Fig. 5 A summary of physical model-based predictions for solar cycle 25 indicates that the forecasts have converged and indicates a weak cycle,
similar or slightly stronger relative to the previous cycle 24. Adapted from Nandy (2021)

speculation of an impending Maunder-like grand minimum in solar activity prior to the minimum of cycle 24(Miyahara et al. 2010). However, as demonstrated in thevarious physical model-based predictions (Fig. 5), thereis going to be a solar cycle 25, whose early signatures are
already being observed (Nandy et al. 2020). As Nandy(2021) has argued, the current physical understanding ofsolar cycle predictability does not allow any indication of
long-term future activity beyond cycle 25 at this point intime.
2.2 Solar extrema: grand minima and maxima in activity

While direct sunspot observations exist only from theearly seventeenth century, indirect proxies of solar activity based on cosmogenic isotopes have been utilized forreconstructing past solar activity (Fig. 6). Such reconstructions indicate the presence of multiple grand maxima and minima in activity, when solar activity levels
either exceed or fall below typical activity levels over a
few cycles, the most recent and striking minimum in activity being the Maunder minimum (Usoskin et al.
2014). It is expected that during high activity levels thefrequency of solar storms would be higher, while during

activity minima, there would be hardly any solar flaresor CMEs—thereby resulting in strong and weak geospace perturbations (Gopalswamy 2016) during activity
maxima and minima, respectively. On the other hand,cosmic ray fluxes in the heliosphere are at extreme levels
during low solar activity phases. Heliospheric conditions
therefore depend on solar extrema. There is emerging(although not yet mature) understanding that very longterm solar activity levels modulate the Earth’s climate;
however, this relationship is muddled by the unprecedented anthropogenic forcing in the recent past (Haigh2007). Solar cycle variability during the maunder minimum is studied extensively using all available sunspot
observations in Usoskin et al. (2015).
While there have been some attempts at identifying
long-term periodicities such as the centennial scale
Gleissberg cycle in the sunspot record, this is more likely
a modulation of the envelope of solar activity rather than
any long-term periodic behavior (Usoskin 2017). Statistics of grand solar maxima and minima in activity—
which are widely separated intermittent occurrences—
show that there is no regularity to these events, with
their distribution being governed by a stochastic andNandy et al. Progress in Earth and Planetary Science(2021) 8:40

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Fig. 6 Solar activity reconstruction over the past 3000 years shows a few extreme activity phases during which the sunspot cycle amplitudes
persistently exceed (grand maxima) or fall below (grand minima) the typical activity levels (denoted within the two dashed lines). Following theMaunder minimum (1645–1715) and Dalton minimum (1790–1830), a growth in general activity levels resulted in the modern maximum. Solar
cycle 24 however was very weak and cycle 25 is expected to be a below average cycle too. Further in the past, other grand maxima and minima
episodes are observed indicating extreme activity levels have occurred over the recent history with no apparent regularity. Adapted from Usoskin
et al. (2014)(or) chaotic process (Mininni et al. 2002; Usoskin et al.
2007). Theoretical arguments underscore that the grandminima phase is a special state of the solar dynamo, distinct from other normal or high activity phases (see Tripathi et al. 2021 and references therein). There are someindications from solar activity proxies that weak magnetic cycles in the large-scale solar magnetic field persisted along with few sunspot appearances during theMaunder minimum (Mursula et al. 2001; Miyahara et al.
2004). There are also hints from observations that strong
hemispheric asymmetry in solar activity (Temmer et al.
2002) is associated with grand minima episodes withsunspots predominantly erupting in one hemisphere—with consequent hemispheric decoupling in activity(Usoskin 2017). Studies indicate that the nature of thecoupling between dipolar and quadrupolar modes of thesolar dynamo mechanism (DeRosa, Brun and Hoeksema 2012) and the parity of the solar cycle may be connected to activity fluctuations (Weiss and Tobias 2016;
Beer et al. 2017; Obridko et al. 2020).
Long-term fluctuations in solar activity, including
secular trends and extreme activity levels, are theoretically understood to originate from stochastic fluctuations
in driving parameters of the solar dynamo mechanism(Passos et al. 2014), non-linear feedback mechanisms between magnetic fields, convection, and plasma flows(Tobias 2002; Wilmot-Smith et al. 2005; Muñoz-Jaramillo et al. 2011; Weiss and Tobias 2016), time-delay dynamics (Wilmot-Smith et al. 2006; Jouve et al. 2010), ora combination of these effects Hazra et al. 2014; see
Fig. 7). Long-term observations of internal plasma flow
variations such as torsional oscillations in the differential

rotation and meridional circulation—which are indicative of non-linear feedback of the magnetic field on theflows—are limited to about two solar cycles (Lekshmi
et al. 2018; Lekshmi et al. 2019). While they reveal interesting physics, they are not long enough for plasma flow
variation-driven long-term dynamo simulations. Therefore, the latter focus on a variety of theoretically expected physical mechanisms to explore very long-term
solar activity fluctuations. For more comprehensive reviews of various mechanisms that may induce long-term
modulation in solar activity, interested readers are referred elsewhere (Tobias 2002; Charbonneau (2020).
Interestingly, recent, better formulated approaches tomodel long-term solar activity variations, including entry
and exit from grand minima episodes, suggest that a
weak, mean-field α-effect is necessary in conjunction
with the large-scale Babcock-Leighton mechanism forrecovering solar-stellar activity cycles following a prolonged grand minima episode (Hazra et al. 2014; Passos
et al. 2014). While such long-term simulations are computationally expensive, 2D kinematic simulations showthat hemispheric decoupling in activity may occur during low activity phases with one hemisphere remaining
quiescent while the other hemisphere continues to show
sunspot activity (Passos et al. 2014; Fig. 8). Recent solar
dynamo models have also shown that long-term modulation and such hemispheric asymmetry may eventuallylead to parity shifts (between dipolar and quadrupolar
modes) in the solar magnetic cycle (Hazra and Nandy2019). Such studies are only now beginning to reveal
how the stochastic nature and intricate non-linear physics of MHD interactions in stellar convection zones canNandy et al. Progress in Earth and Planetary Science(2021) 8:40

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Fig. 7 Evolution of magnetic energy proxy versus time in a reduced, time-delay solar dynamo model with non-linearity and stochastic forcing(Hazra et al. 2014). These very long-term simulations recover the existence of grand maxima and minima phases which are intermittently
distributed over the course of the simulations

lead to long-term symmetry changes and activity modulation in the solar dynamo mechanism (Käpylä et al.
2016; Raynaud and Tobias 2016).
2.3 Solar forcing of the heliosphereAs discussed earlier, the activity of the Sun changes bothduring the course of a cycle and over a longer term due
to modulation of the solar dynamo mechanism over centennial to millennial timescales. This variation induces
short- and long-term variations in space environmental

conditions such as radiation, energetic particle flux, andheliospheric magnetic fields. Planets are impacted bythese variable space environmental conditions governedby solar activity, thus sharing a causal relationship with
the Sun.
Sunspots, when they first appear, tend to reduce theoverall solar radiation output due to their inhibitory effect on convective energy transfer to the surface. However, within a solar rotational timescale, sunspot decay
and their associated network regions—faculae andFig. 8 A kinematic solar dynamo simulation with stochastically fluctuating mean-field α-effect and Babcock-Leighton sources for the poloidal fieldshows long-term modulation in activity (Passos et al. 2014). The top panel depicts simulated sunspot time series in the northern (black) andsouthern (red) hemispheres and the bottom panel depicts the simulated solar butterfly diagram for the toroidal field at the base of theconvection zone. The dynamo simulation is characterized by hemispheric asymmetry in activity, parity shifts, and occasional sunspot eruptions
during the grand minimum phase observed in the simulated timeframe above

Nandy et al. Progress in Earth and Planetary Science(2021) 8:40

plage—result in an increase in the total solar radiative
output (Krivova et al. 2007; Yeo et al. 2020). This results
in an overall positive correlation between solar magnetic
cycle and solar radiative output over decadal and longer
timescales. This is indeed observed in satellite-based
measurements of the Total Solar Irradiance (TSI;
Fig. 9)—the solar energy incident per unit area per unit
time at 1 AU. Note that radiation fluxes in (individual)
higher frequencies (spectral irradiance) may vary much
more over the solar cycle. Given that the radiation fluxof the Sun is the primary natural energy source for heating of planetary atmospheres and driving planetary climates, there is strong interest in understanding how thepast activity of the Sun may have influenced our climate.
Efforts have been made for long-term reconstruction of
TSI variations over millennia based on physics-inspired
empirical modeling (Vieira et al. 2011; Yeo, Krivova andSolanki 2014; Wu et al. 2018). Such efforts form SunClimate relationship studies that seek to understand theimpact of long-term solar radiative variability on planetary atmospheres (Haigh 2007) and also provide inputs tothe assessment reports of the Intergovernmental Panelon Climate Change (IPCC Report 2013; https://www.
ipcc.ch/report/ar5/wg1/).
The emergence, evolution, and dispersal of solar magnetic fields on the solar surface govern the evolution ofPage 10 of 21

coronal and heliospheric magnetic fields, the latter mediated by the solar wind. The heliospheric open magnetic
flux plays an important role in determining the state of
the heliosphere. The solar open flux can be reconstructed based on established relationships between solar
surface magnetic activity and coronal fields at the source
surface—where the solar wind becomes dynamically important and carries the solar magnetic field throughout
the solar system. Ideas about solar wind and near-Earthheliospheric magnetic fieldvariations over century scale
are now available (Owens et al. 2016; Owens et al. 2017).
Based on an understanding of solar magnetic field variations, long-term reconstructions of the solar open flux
are now possible (McCracken 2007; Lockwood et al.
2014).
The heliospheric (solar) open magnetic flux is the key
contributor to variations in the cosmic ray flux (Fig. 10).
Thereby, this provides a causal connection between solar
activity and galactic cosmic ray intensity at Earth; higher
solar activity and open flux levels result in lower cosmic
ray flux. Cosmic ray interactions with the Earth’s atmosphere generate radionuclides (e.g., 10Be and 14C)—whichmake their way in to tree rings and ice sheets. Therefore,
measurements of radionuclide abundances and inversion
techniques provide an ingenious methodology to reconstruct solar activity (Usoskin 2017); it is important to

Fig. 9 Comparison of total solar irradiance (TSI) variations (top panel) with the sunspot time series (bottom panel). The TSI varies in phase withthe solar cycle indicating that during phases of very strong activity, the total radiation output—and hence radiative forcing of the heliosphere—isenhanced. The composite TSI time series data is acquired from PMOD/WRC (Fröhlich 2006)

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Fig. 10 Modulation of cosmic ray flux at Earth by the solar magnetic cycle. The top panel depicts variations in the flux of cosmic rays as
measured by neutron monitors and the bottom panel shows the sunspot time series. At times of strong (low) solar activity, the cosmic ray flux is
low (high). This modulation of cosmic ray flux is enabled by the solar magnetic fields via the solar wind—creating the heliospheric open
magnetic flux. Monthly averaged cosmic ray flux data is obtained from the World Data Center for Cosmic Rays (WDCCR). For the time period
1953–2006, the data is from CLIMAX station, from 2007 to 2017, the data is from FORT SMITH station, and beyond 2017, the data is from BAKSAN
station. This composite plot combines cosmic ray flux count from three different stations with different geomagnetic rigidity cutoffs, e.g., CLIMAX(~ 3 GV), FORT SMITH (0.3 GV, viz. polar station), BAKSAN (~ 6 GV). In order to illustrate the overall qualitative trend, the whole time series data isnormalized to the cosmic ray flux count of CLIMAX stationnote that such reconstructions are limited to about 10–
12 millennia.
Recent investigations also show other unique roles of
the heliospheric open flux; interactions mediated via
magnetic reconnection between propagating magnetic
storms from the Sun and the open flux can in fact reduce the geo-effectiveness of solar storms (Pal et al.
2020). Indeed, magnetic connectivity from the Sun andthroughout the solar system provides a causal basis for adiversity of planetary impacts of the activity of our parent star; the intricacies of these causal connections sustained over planetary lifetimes lead to other interesting
phenomena discussed in the next section.

3 Evolution of solar-stellar activity: implications
for planets
The solar system teaches us that stellar activity influences planets in profound ways. Magnetohydrodynamic
processes—similar to what is in evidence for the Sun—
operate in the interior of other solar-like stars. However,given that different stars have different rotation rates
and differential rotation—which is believed to be a function of stellar age—the exact nature of the dynamo

mechanism as well as the stellar activity is expected tovary across stars and over the lifetime of an individual
star (Nandy 2004; Brun et al. 2015; Guerrero et al. 2013,2019). Thus stellar observations and dynamo modelingof solar-like stars (or solar twins as they are known) andsolar-stellar connection studies are expected to generatebetter constraints on the solar dynamo mechanism on
the one hand and, on the other hand, provide an indication of magnetic activity evolution over the lifetime of
the Sun (Nandy and Martens 2007). In turn, the expected radiative flux, solar wind, and solar storm frequency and severity are also expected to vary in the
lifetime of the Sun. This stellar variability is expected toinfluence planetary space environments over the evolutionary lifetime of star planet systems, studies of whichhave implications for planetary habitability in the solar
system and beyond.
3.1 Star-planet interactions: impact on planetaryatmospheres

Stellar activity influences planetary environments and atmospheres in ways that may determine the habitabilityof planets (Lammer et al. 2012). For far-out planets, it isNandy et al. Progress in Earth and Planetary Science(2021) 8:40

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Fig. 11 Simulation of the interactions of solar wind with the Earth’s magnetosphere. A plane perpendicular to the solar ecliptic (xz plane) is
depicted. Earth is denoted by the white circle. Nearly vertical black contours on the left of the image indicate the solar wind magnetic field,
whereas curved black lines—one or two ends of which are rooted on Earth—indicate the Earth’s magnetospheric field following a dynamic
equilibrium with the solar wind. Magnetic reconnection and other fluid dynamical and MHD processes play critical roles in determining theimpact of stellar magnetized winds and storms on planetary magnetospheres and atmospheres. These processes can lead to atmospheric massloss or injection (via plasma flows through reconnected field lines), and over long timescales can influence planetary habitability. The above
simulation was performed using CESSI-SPIM (Das et al. 2019)

typically the activity of the star which has a one-sided
impact on planetary atmospheres and evolution (Fig. 11).
However, for close-in star-planet systems, wherein theplanet and star’s magnetospheres can dynamically interact or wherein the planet imparts sufficient tidal forces
to influence stellar flows, the influence may work eitherway (Strugarek 2016; Strugarek et al. 2014, 2019). The
environment of some planet hosting stars, e.g., Mdwarfs, would be extreme (Cohen et al. 2014), plausiblyprecluding the possibility of life. Many Jupiter-like large
planets with such extreme environments within close
proximity to host stars have been detected and are
known as Hot Jupiters. Here, we confine ourselves todiscussing Star-Planet Interactions (SPI) in the context
of Earth-like (far-out) planetary or exoplanetary systems
which are more likely to host life rather than (plausibly
uninhabitable) close-in Hot Jupiter-like planets.
Primary mechanisms of SPI include but are not limited
to radiation-driven sputtering processes that influenceplanetary atmospheric escape (Gronoff et al. 2020), radiative forcing of atmospheres (Haigh 2007), magnetic
storms, and stellar wind impacts (Das et al. 2019). It isbelieved that the existence of a magnetosphere is usefulfor the protection of planetary atmospheres against
solar-stellar wind and storm impact. Mars offers an interesting case study in this respect. It is believed that at
about 1.5 billion years age of the solar system, when the
Sun was much more magnetically active, Mars had an

active dynamo; the resulting Martian magnetosphereshielded its atmosphere from the relatively strong young
Sun. Mars is thought to have had a warm moist atmosphere at this point in time which could have been suitable for hosting life. Subsequently, however, cooling ofMar’s core resulted in the loss of the Martian dynamo
and global magnetosphere. Following sustained impact
of solar radiation-driven sputtering processes, solar magnetized storms and winds, the atmosphere of Mars likely
eroded away (Basak and Nandy 2021), impacting conditions for habitability (at least on or near the surface).
Modeling such interactions leading atmospheric masslosses at the global level is a challenging task and attempts to do this are throwing up hitherto unanticipated nuances regarding how stellar magnetized
winds interact with planetary atmospheres with or without magnetospheres or weak dipolar fields (Das et al.
2019; Sakata et al. 2020, Basak and Nandy 2021).
Studies of stellar wind environment in other astrospheres are currently being undertaken; e.g., modelingindicates that the Proxima Centauri system may have
Earth-like solar wind conditions (Alvarado-Gómez et al.
2020). Search for signatures of SPI are also beginning.
However, such studies are typically limited to hot
Jupiter-like planets close to the host star—where SPI signatures may be detectable (Poppenhaeger et al. 2011;
Strugarek et al. 2019). We anticipate that the coming
decade would be an exciting era for SPI explorations,Nandy et al. Progress in Earth and Planetary Science(2021) 8:40

primarily driven by the quest to understand habitability
in exoworlds.
3.2 Solar-stellar activity over planetary evolutionary
timescales

The magnetic activity of stars evolves with age; this evolution is driven by changes in properties of the stellar
convection zone and differential rotation rate and resultsin changes in the dynamo output, stellar surface magnetic field distribution, topology of coronal magnetic
fields, frequency and strength of stellar flares, and stellar
wind output. Angular momentum losses through magnetically driven stellar winds and stellar spin-down infact reduce the efficiency of the solar dynamo mechanism with time by increasing the Rossby number (i.e., reducing the dynamo number; Nandy 2004; Ahuir et al.
2020). The magnetic output of a solar-like star therefore
reduces with age (Nandy and Martens 2007; Vidotto
et al. 2014); see, e.g., Fig. 12. There has been suggestions
that at about stellar mid-life there is a breakdown instellar gyrochronology relations (between rotation rate
and age) in a solar-like star (Van Saders et al. 2016) andthat the Sun is much less active than a typical solar-likestar (Reinhold et al. 2020a). This has spurred a debate in
the solar-stellar community on whether the Sun’s activity is special, and whether the Sun is about to transition
to a magnetically inactive future (Metcalfe and vanSaders 2020; Reinhold et al. 2020b). A recent study indicates that a subcritical magnetic dynamo in the current

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Sun, which allows both normal activity phases and grandminima activity phases to be accessed by the dynamo
mechanism, can self-consistently explain the breakdown
of gyrochronology relations and the co-existence of normal and grand minima states (Tripathi et al. 2021).
Given that the younger Sun was much more magnetically active, understanding this early solar system environment and the changing environment of planet Earththrough the history of the solar system has the potential
to inform studies of planetary habitability in the solar
system (Guinan and Engle 2007). It is expected thatthese ideas would hold in other astrospheres; studies
also indicate that the evolution of stellar activity influences the extent and properties of habitable zones (Gallet et al. 2017) within astrospheres.
The above motivation has spurred many investigations
to understand the evolution of the activity of a solar-likestar with age. Given that the solar energy output is theprimary external source for driving planetary atmospheres and climate, reconstruction of solar irradianceover the lifetime of the Sun has been undertaken (Shapiro et al. 2020). Studies have explored how the solar
wind may have changed over the lifetime of the Sun andsolar-like stars (Ó Fionnagáin and Vidotto 2018; Ó Fionnagáin et al. 2019). The expected evolution of galactic
cosmic ray flux modulated by the evolving solar activity
has been modelled (Rodgers-Lee et al. 2020). The possible impact of a variable solar wind and consequentevolution of the Earth’s magnetosphere over the solar

Fig. 12 Evolution of stellar magnetic activity with age (reproduced from Vidotto et al. 2014). The large-scale magnetic field is measured in thissample of stars utilizing the Zeeman Doppler Imaging technique and shows a power law dependence between magnetic activity (y-axis) andage (x-axis). These observations of stronger magnetic output at young stellar ages suggest that the environment of the early, young Sun was
relatively extreme, with more energetic storms, winds, and radiative output creating an intense space environment for Earth and other solar
system planets

Nandy et al. Progress in Earth and Planetary Science(2021) 8:40

main sequence lifetime has been studied (Carolan et al.
2019). These studies are breaking new ground in deciphering the evolutionary history of the Sun-Earth system
and are a window to star planet interactions and coupled
evolution in other exoplanetary systems.
3.3 Extreme solar storms: lessons from stellar superflares

Extremely strong solar-stellar super storms can inprinciple wreak havoc on planetary atmospheres. Whatcan be quantified as an extreme solar event, whether it
be a flare or a CME? Depending on the X-ray energy
output, the flares are classified into three major categories X, M, and C with energy budget ranging from 10−2
to 10−6 W.m−2. The bolometric energy (the energy emitted across the whole electromagnetic spectrum) in an
X100 class flare is 1033 erg (see, e.g., Benz 2017). Flareswith bolometric energies exceeding 1033 erg may be considered as superflares. A study in a sample of diverse
classes of young stars indicated the possibility of superflares (Wolk et al. 2005); but how about stars of solarage? A subsequent study in a wider sample of stars(Fig. 13), including many solar-like stars, showed that
faster rotating stars are statistically more likely to produce stellar superflares with energies as high as 1036 erg
with a higher frequency (Maehara et al. 2012; Shibayama
et al. 2013; Notsu et al. 2013). This series of studies concluded that superflares with energies ~ 1034 erg are possible in solar-age stars (Notsu et al. 2019) once every
6000 years (see Okamoto et al. 2021 for the most recent
update).
The Carrington flare event (1859) is widely considered
to be the most extreme flare ever recorded by humans;
however, a solar superflare with energies close to 1036
erg has never been observed. Recently, investigations ofa meteorite sample indicated that a strong superflare
had occurred when the Sun was very young, only about
500 million years old (Mishra and Marhas 2019). Other
studies, however, assert that the chances of these super
events occurring in the current Sun are extremely low(Aulanier et al. 2013); it is also not obvious that all super
events would necessarily have an extreme geomagnetic
impact (Schmieder 2018). Nonetheless, the mere possibility of the current Sun launching a superflare—however low the frequency of such flares might be—merits
further investigations on their likelihood and possible
geospace impacts.
Flares are often associated with CMEs which are another manifestation of an energetic solar event. CMEs
carry vast amounts of magnetized plasma and when
their orientation is right can cause significant geospace
impact and atmospheric mass loss. The fastest CME(therefore with high dynamic energy) in the space era
was observed on November 10, 2004, at 02:26 UT which
has a speed of 3387 km/s; there are typically very few

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exceeding speeds of 3000 km/s (Gopalswamy 2016).
From the statistics of CME speed distribution, estimatessuggest that once in 100 years a CME event may occur
with a CME speed exceeding 3800 km/s, and once in1000 years a CME event with speed exceeding 4700 km/s
may occur (Gopalswamy 2017). These CMEs have energies ranging from 1033 erg to 1034 erg. With favorable
magnetic field orientation (southward directed zcomponent of magnetic field) and angle of impact, such
CMEs could trigger devastating ionospheric and geomagnetic storms.
The energy for driving solar flares and CME events iseventually drawn from the free magnetic energy available
in active region associated coronal magnetic structures
or magnetic reconnection events. Thus, the amount ofmagnetic field output, and the possibility of super active
regions produced from a very active dynamo and largescale flux tube instability increases the chances of solar
super events (Schmieder 2018). It is no wonder then thatyounger stars with more active magnetic dynamos havea propensity of generating superflares, extreme CMEs,
and strong coronal X-ray emission (Nandy and Martens
2007). What the impact of these was on geospace andother planetary atmospheres through Earth’s history is
an intriguing topic of future investigations.

4 The Faint Young Sun Paradox
While much (necessary) attention is being paid to thepossible influence of the Sun on global climate variationin the last millennium, there is an even more astounding
problem in the mismatch between solar luminosity andterrestrial climate in the first several billions of years of
the Earth’s existence, an issue known as the Faint YoungSun Paradox (FYSP; Kasting et al. 1988). In brief, theparadox is this: The geological and biological record
support that the Earth’s biosphere was considerably
warmer than currently during the origin of life on Earthand for several billions of years thereafter (e.g., Kasting1989). Yet, stellar evolution calculations support the Sunreaching the Zero Age Mean Sequence (ZAMS) at about
70% of its present luminosity, and linearly increasing intime up to its current level. Climate models predict a“Snowball Earth;” for such a low solar constant, unless thegreenhouse effect were much stronger than what it is now(see Feulner 2012). However, there is no geological evidence for a hugely increased presence of greenhouse gasses in the early atmosphere (Kasting 2004).
Expanding our view to the evolution of stars in general, this aspect of stellar evolution theory implies thatthe habitable zone around stars moves outward as thestar matures, due to the gradual but significant increaseof stellar luminosity over the star’s lifetime. Hence,ceteris paribus, planets that start out with conditions favorable for life, liquid water in particular, heat up asNandy et al. Progress in Earth and Planetary Science(2021) 8:40

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Fig. 13 Stellar superflares. The top panel shows flare energy and the bottom panel shows the frequency of occurrence of flares per year per starversus the period of brightness variation (a proxy for stellar rotation period). These observations show that extremely strong superflares occur
more frequently in faster rotating stars and that solar-like stars can in principle produce extremely strong, superflares. Image reproduced from
Maehara et al. (2012)

their mother star grows older and end up like Mercury
or Venus. Meanwhile, planets further out will spend thefirst couple of billion years of their existence in deepfreeze, not advantageous for the development of life, andonce they melt may not have enough time for the development of multi-cellular organisms—that took four billion years on Earth.
However, recent observations point towards theSun, and more generally the stars that host planetarysystems, for a resolution of the paradox. Most important are the results confirmed by the Mars Rovers
that show that Mars has had periods with a seeming
abundance of liquid water over its first billions ofyears (e.g., Di Achille and Hynek 2010). If both Marsand Earth both have had liquid water over their history, Occam’s razor suggests we look for a common
cause, i.e., a more luminous Sun than stellar evolution
simulations indicate. One possibility (Willson et al.
1987; Graedel et al. 1991) for a brighter young Sun
would be if the Sun had only about 5% more mass at
its origin than it has now, and consequently, has lostthe excess mass through mass loss much larger thanits present value of 3 × 10−14 Msun/year. This hypothesis is attractive because it solves the FYSP for Earthand Mars jointly and increases the likelihood of morepermanent habitable zones around other Sun-likestars, assuming that the mass loss evolution of the
Sun is not exceptional.

(2021) 8:40

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To promote this hypothesis to a credible theory, it has
to pass a large number of tests, to see if the scenario canbe made to agree with the laws of physics and whether itagrees with observations. Finally, any theory needs tomake predictions about observational tests that have notbeen carried out yet, for unbiased verification. I will describe the first two types of tests here in that order andleave the final one for future research.

its stellar wind determines the magnitude of the spindown. Simulations (Keppens and Goedbloed 2000) haveshown that that torque is equal to the stellar mass loss
rate times the lever arm, which is of the order of the Alfvén radius, i.e., the radius where the wind outflow velocity equals the Alfvén speed. Martens demonstrates
that in order for the Sun (and Sun-like stars) to spin
down from their ZAMS rotation periods, with reasonable values of the Alfvén radius of 10 to 20 solar radii, amass loss of the order of 10−11 Msun/year is needed. For
the Sun to slow down with its current mass loss rate, theAlfvén radius would have to be located beyond the orbit
of Venus, which seems unphysical. Modern stellar evolution codes, such as MESA (Paxton et al. 2011), can include the effects of mass loss and spin down and hence
various evolutionary scenarios can be simulated for comparison with observational constraints. Such simulations
also produce models of the stellar interior that can deliver detailed predictions for helioseismical observables.

Nandy et al. Progress in Earth and Planetary Science

4.1 Physical constraints

The key question to be addressed is whether we cancome up with a stellar evolution scenario that produces
enough solar luminosity to keep Mars “wet” during thefirst about two billion years of its evolution, and thatkeeps Earth warm enough for liquid water over its entire
existence. Minton and Malhotra (2007) have calculated
what initial solar mass and subsequent mass loss is required to maintain solar luminosity more or less at thepresent value from the time the Sun arrived at the
ZAMS. The result is a mass loss of the order of 10−11
Msun/year for roughly the first 2.5 billion years and thenrapidly declining to its current value that is a factor 300
less.
Simulations by Suzuki et al. (2013) have demonstrated
that when sufficient Poynting flux enters the stellar
chromosphere and corona, mass loss rates dramaticallyincrease until they plateau at a value of about 10−11
MSun/year, similar to what is observed, and to the estimate by Martens (2017) based upon rotation slow-down
considerations. Cranmer and Saar (2011) develop aphysics-based model that is remarkably successful in reproducing the observed mass-loss rates of late-type stars
over their evolution. In particular, the evolution curve
for a one solar mass star produces a mass loss rate thatis substantially higher than that in standard models, although still lower than that required in Minton andMalhotra (2007). However, the predicted solar luminosity curve is nearly flat at the current value going back intime, providing a possible resolution of the Faint YoungSun Paradox—which they forget to mention. We pointout that there are still quite a few physical assumptionsin their model that are either not yet verified, such as
magnetic filling factors, or probably incorrect, e.g., thatthe final wind velocity equals the escape velocity. Also
they neglect to address the magnetic breaking that is required for the slow-down in stellar rotation. Nevertheless, their model represents a significant contribution to
the resolution of the Faint Young Sun Paradox.
Stellar evolution theory simultaneously has to accountfor the rapid spin-down of the Sun and Sun-like stars,
from a rotation period of 4 to 5 days at the ZAMS to
about a 26-day period currently. Martens (2017) pointed
out that the problems of mass loss and spin-down are
intimately intertwined: the torque applied to the star by

4.2 Observational constraints

The key question here is what observations can be madeto verify or discard the existence of a massive solar wind
through much of the Sun’s history. Below we list the observations we know about that are relevant to thisquestion:
1. The current solar mass loss rate is 3 × 10−14 Msun/
year.
2. The current value of the Afven radius is about 12
solar radii (Velli et al. 2016).
3. Sun-like stars at very young ages have a mass lossof ~ 10−11 Msun/year. For example Gaidos et al.
(2000) find, from deep observations at radio (3.6cm) wavelengths, upper limits for the mass loss rate
above that value for a number of young nearby
solar-type stars. Wood et al. (2005) find mass loss
rates of that order for a number of young solar-typestars from the observed astrospheric absorption in
Lyman-alpha.
4. Sun-like stars near the ZAMS have a rotation
period of a few days at most (Barnes 2009).
5. The Sun and same aged Sun-like stars have a rotation period of 25–30 days (Barnes 2009).
6. Analysis of moon rocks that have been exposed tothe solar wind for as long as they have been in
existence at the lunar surface, indicate a solar massloss about an order of magnitude higher than thecurrent value over the last two or three billion years(Geiss and Bochsler 1991).
7. According to Sackmann and Boothroyd (2003):“Older, solar-flare irradiated grains from meteorites
imply an early solar flare activity about 103 times
that of the present Sun (Caffee et al. 1987); the(2021) 8:40

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associated solar wind may have been enhanced by asimilar factor.”
8. There is a surprising dearth of observations of massloss rates for Sun-like stars between one and threebillions years of age. There are indications ofmassive stellar winds sustained for ~ 3 Gyr (Melendez et al. 2017) for Sun-like stars.
9. Helioseismic observations measure the position of
the base of the Sun’s convective envelope and thesurface helium abundance, which slightly differ
between a Sun that starts out at 1.05 Msun andconstant luminosity, and one that starts out at one
solar mass and an initial luminosity of 0.7 Lsun,
increasing linearly with time. One obvious effect isthat more helium is produced in the core in theformer case, which leaves fingerprints throughout
the Sun (Sackmann and Boothroyd 2003). Oldermodels for helioseismic observables for solar
evolution with mass loss rates of the order of 10−11
Msun/year produce slightly better agreement withactual helioseismic observations than those with noor current mass loss rates (Sackmann and
Boothroyd 2003). However, the difference is within
the error margins of those models, so no conclusion
can be drawn. We have found no recent updates tothese results.

planet Earth. In their individual scope, each of these
topics is vast, with a diversity of literature. This review
cannot do justice to these individual sub-topics but has
rather focused on the interconnected themes and progress therein.
It is our hope that this will serve as a useful source ofideas, information, and references which would instigate
future investigations of solar-stellar activity and attempts
to understand their impact on planets and exoplanets.

Nandy et al. Progress in Earth and Planetary Science

More observational constraints on the mass loss history of the Sun and Sun-like stars may exist. In general,
one has to be vigilant to distinguish pure observational
constraints from those derived from models. For example, Cranmer (2017) concludes that “the cumulative
CME mass lost by the young Sun may have been as
much as 1% of a solar mass,” which is relevant in thiscontext. However, this result is based on a correlation
between current CME mass loss and magnetic filling
factor on the stellar surface, and hence is implied. This
does not diminish this work by Cranmer, but one has tobe careful in considering this an observational
constraint.
We conclude that progress may be expected over the
next decade in solving the FYSP: helioseismic models
will be improved, fresh moon rocks will be retrieved, as
well as samples for asteroids, more observations of theparameters of Sun-like stars will become available, andstellar evolution simulations with mass loss will be further explored.

5 Conclusions
Inspired by the VarSITI Solar Evolution Program of
SCOSTEP, this review summarizes the state of the fieldand current understanding of solar magnetic activity
fluctuations, solar extreme events, solar activity evolution, and their impacts on our space environment andAbbreviations
AR: Active regions; CESSI-SPIM: Center of Excellence in Space Sciences India
Star-Planet Interaction Model; CME: Coronal mass ejections; HMI/
SDO: Helioseismic Magnetic Imager/Solar Dynamics Observatory;
MHD: Magnetohydrodynamics; SCOSTEP: Scientific Committee on SolarTerrestrial Physics; SEE: Solar Evolution and Extrema; SEP: Solar energetic
particles; TSI: Total solar irradiance; VarSITI: Variability of the sun and its
terrestrial impact
Acknowledgements
We acknowledge scientific discussions facilitated by the SCOSTEP VarSITI
program and the International Astronomical Union (IAU) Working Group on
Impact of Magnetic Activity on Solar and Stellar Environments. We arethankful to Ilya Usoskin, Aline Vidotto, and Kazunari Shibata for providing
Fig. 6, Fig. 12, and Fig. 13, respectively. CESSI is funded by the Ministry of
Education of the Government of India under the Frontier Areas of Scienceand Technology (FAST) scheme.
Authors’ contributions
D.N. led the writing of this review with contributions from P.M. Some of the
figures were generated by S.D. Planning of the contents of this review weredone by D.N., P.M., V.O., and K.G. All authors have read and approved thefinal manuscript.
Authors’ information
D.N., P.M., and V.O. co-lead the Solar Evolution and Extrema program of SCOSTEP VarSITI.
Funding
The Center of Excellence in Space Sciences India (CESSI) is funded by the
Ministry of Education of the Government of India.
Availability of data and materials
Data which feature in plots highlighted in this manuscript are available fromthe sources cited at the relevant sections. Furthermore, specific data sources
used for some of the plots are available through the following online datarepositories.
Sunspot Time Series Data:
http://www.sidc.be/silso/datafiles
Total Solar Irradiance Data:
ftp://ftp.pmodwrc.ch/pub/data/irradiance/composite/DataPlots/
composite_42_65_1709.dat
Cosmic Ray Data:
http://cidas.isee.nagoya-u.ac.jp/WDCCR/

Declarations
Competing interests
The authors declare that they have no competing interests.
Author details
1
Center of Excellence in Space Sciences India, Indian Institute of Science
Education and Research Kolkata, Mohanpur, West Bengal 741246, India.
2
Department of Physical Sciences, Indian Institute of Science Education andResearch Kolkata, Mohanpur, West Bengal 741246, India. 3Department of
Physics & Astronomy, Georgia State University, 25 Park Place, 6th Floor,
Atlanta, GA 30303, USA. 4Pushkov Institute of Solar-Terrestrial Physics,
Ionosphere, and Radio Wave Propagation, Kaluzhskoe shosse 4, Troitsk,

(2021) 8:40

Page 18 of 21

Moscow 108840, Russia. 5Space Research and Technology Institute, Bulgarian
Academy of Sciences, Sofia, Bulgaria.

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