Modern Maximum: Solar Activity Era

• Modern Maximum is a period (1920s–mid 20th century) of anomalously high solar activity, defined by record sunspot counts and elevated solar radio flux.
• It is characterized by distinctive solar wind patterns, evolving coronal hole structures, and significant shifts in the Sun’s magnetic field configuration.
• Observational proxies such as high-latitude geomagnetic indices enable robust reconstructions of solar wind speed and heliospheric impacts throughout this era.

The Modern Maximum refers to a period in the 20th century characterized by exceptionally high solar activity, with solar indicators such as sunspot numbers, solar radio flux, and geomagnetic activity all reaching sustained maxima not observed at any other time in the direct observational record. This epoch, peaking with solar cycle 19 and decaying through subsequent cycles, is distinguished by dramatic changes in the Sun’s magnetic field configuration, coronal hole structure, solar wind properties, and terrestrial space climate impacts.

1. Definition and Historical Context

The Modern Maximum (often abbreviated as MM or Grand Modern Maximum, GMM) designates an interval from approximately the 1920s through the mid-to-late 20th century, during which solar activity indices such as the sunspot number and radio flux displayed anomalously high and persistent amplitudes. The maximum phase peaked near the maximum of solar cycle 19 (late 1950s), with the sunspot number reaching an all-time recorded high of 285, subsequently declining to 116.4 by the maximum of cycle 24, marking a cycle amplitude reduction by a factor of ~2.4 (Mursula et al., 12 Mar 2024).

This interval stands out from prior grand minima, such as the Maunder Minimum, in both total activity level and the structure of the solar and heliospheric environment.

2. Solar Wind and Coronal Hole Structure During the Modern Maximum

A key feature of the GMM is the systematic behavior of high-speed solar wind streams (HSS) and their solar sources—namely, the coronal holes and the Sun's large-scale open magnetic field.

• High-latitude geomagnetic indices, specifically the difference between magnetically disturbed and quiet days in the polar cap's vertical geomagnetic field component ($\Delta Z$), serve as robust proxies for annual HSS occurrence (Mursula et al., 2015). This method isolates the HSS signal from other drivers such as CMEs, which are more prominent in lower-latitude or global averages.
• Annually averaged solar wind speeds reconstructed from $\Delta Z$ show that HSS activity consistently peaks during the declining phase of each solar cycle (cycles 16–23), with the intensity and duration of these peaks serving as a diagnostic of the underlying coronal hole dynamics and polar field strength.
• The declining phase of cycle 18 (early 1950s) exhibited the most persistent HSS activity of the entire Modern Maximum, as evidenced by records at Godhavn (GDH) and by the maximum in annual mean solar wind speed proxies (Mursula et al., 2015, Mursula et al., 2016).
• Seasonal decomposition of geomagnetic proxies reveals centennial maxima in equinoctial wind speeds during 1952 (cycle 18 decline) and in solstitial wind speeds in 2003 (cycle 23 decline). The former is attributed to persistent extensions of polar coronal holes, while the latter signals a unique episode of isolated, low-latitude coronal holes during the demise of the GMM, a pattern not observed even during previous low-activity cycles (Mursula et al., 2016).

Year	Feature	Cause
1952	Equinox wind speed	Strong polar coronal hole extensions
2003	Solstice wind speed	Large, persistent low-latitude holes

These patterns support a view of the GMM as not only a time of high overall activity but also of distinctive solar magnetic topology evolution, shifting from polar-dominated HSS generation at the maximum to unusual low-latitude structures at its demise.

3. Solar Dynamo, Polar Fields, and HSS–Activity Coupling

The MM period offers critical support for core predictions of solar dynamo theory. Observations indicate:

• The strength and persistence of HSSs during cycle decline directly track the build-up of polar fields at sunspot minimum. The intense, sustained HSS event during cycle 18's decline signals the generation of exceptionally strong polar fields, in turn seeding the unprecedented amplitude of cycle 19's sunspot maximum (Mursula et al., 2015).
• Stochastic variations in the emergence (tilt angle and polarity orientation) of active regions, as shown in coupled surface flux transport–dynamo models, are sufficient to explain extreme cycle-to-cycle variability—e.g., the rapid transition from the maximum of cycle 19 to the much weaker cycle 20 (Pal et al., 21 May 2025).
• There is no observed need for slow deep-seated nonlinearities or exotic physical mechanisms; the stochastic "surface noise" in AR properties dominates the centennial-scale evolution, and even a single rogue, anomalously tilted AR can catalyze dramatic changes in the polar field and trigger new activity regimes ("phase switching") (Pal et al., 21 May 2025).

Factor	Polar field effect	Next cycle impact
Large tilt scatter	Stronger build-up	Stronger cycle
Small (quenched) tilt	Weaker build-up	Weaker cycle
Many anti-Hale ARs	Weaker/diluted field	Weaker cycle
Rogue ARs	Rapid loss	Grand minimum

4. Long-term Trends and Transition to a Weaker Sun

The decay phase following the MM (cycles 20–24) marks significant structural changes in the solar atmosphere and its magnetic field:

• All radio-derived activity proxies (F30, F15, F10.7, F8, F3.2 cm fluxes) increased relative to the sunspot number from the 1970s to the 2010s, with the largest increases at longer wavelengths (F30: +19.9%, F10.7: +9.0%) (Mursula et al., 12 Mar 2024).
• The widely reported "1980 jump" in the F10.7–sunspot relationship is in fact only the first in a series of 1–2 year "humps" at each solar maximum, indicative of physical reorganization in the solar atmosphere, not data inhomogeneity (Mursula et al., 12 Mar 2024).
• Chromospheric/coronal indices (MgII index, number of active regions) decay less rapidly than the sunspot number, indicating a more rapid fading of strong field elements (sunspots) than moderate field components.
• These patterns are interpreted as a general shrinking and restructuring ("deflation") of the Sun’s magnetized atmosphere, compatible with a geometrically lowered, more canopy-like field configuration, and changing the spectral profile of radio emission.

Such differential restructuring modifies the solar wind environment, heliospheric fields, and has practical implications for modeling ionospheric and planetary responses to solar flux.

5. Observational Proxies and Methodological Innovation

The paper of the MM period has been enabled by methodological advances in long-baseline solar and geomagnetic proxy reconstructions:

• High-latitude geomagnetic proxies (e.g., $\Delta Z$ at polar cap observatories) provide HSS indices with low sensitivity to CMEs or secular drifts, enabling annual-scale reconstructions back to the early 20th century with strong empirical correlation to measured solar wind speeds ($r = 0.78$, $p = 5.6 \times 10^{-5}$; 43 annual values) (Mursula et al., 2015).
• Seasonal disaggregation of geomagnetic data (e.g., equinox vs. solstice solar wind speeds) reveals distinct coronal hole evolution patterns, offering insight into the large-scale solar magnetism not visible in the sunspot number alone (Mursula et al., 2016).
• These reconstructions highlight the necessity of multi-parameter monitoring. For instance, the aa index is less reliable for HSS tracking due to conflation with CME-driven activity.

Proxy	Sensitivity to HSS	Sensitivity to CME	Temporal coverage
Polar cap $\Delta Z$	High	Low	1920s–present
aa index	Moderate	High	1868–present
$\Delta H$ (SOD)	High (subauroral)	Higher (CME)	1914–present

6. Modern Maximum in the Broader Astrophysical and Space Climate Context

The GMM is not only of solar-pysical interest but also central to understanding terrestrial space climate and planetary impacts. Increased solar wind speed and associated geomagnetic activity altered the structure and coupling of the Earth's magnetosphere and ionosphere, with downstream consequences for atmospheric chemistry, radio propagation, and climate coupling mechanisms.

The observed transition toward a weaker Sun since the MM also serves as a natural analog for earlier grand minima and offers a benchmark for dynamo theory predictions, the robustness of solar–terrestrial relationships, and the potential for future extreme fluctuation events.

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In summary, the Modern Maximum denotes a uniquely high and structurally dynamic phase in solar activity, controlled by stochastic emergence and transport properties of active regions, with marked consequences for solar wind, heliospheric structure, and terrestrial space weather. Its end, marked by both long-term and abrupt shifts in key physical indicators, signals ongoing large-scale reorganization of solar magnetism and atmospheric structure. Modern proxy methodologies have allowed quantitative tracking and physically grounded interpretation of these changes, establishing a paradigm for future grand maxima and minima analyses in stellar dynamo systems.