Wolf, you and Steve asked me yesterday what I thought caused the volcano – solar correlation.

My Silly Wild ass guess, would be changes in the distribution of solar wavelengths, Although total solar insolation remains relatively stable high energy EUV fluctuates wildly, and Cosmic rays. cosmic rays,are ‘modulated’ by the solar wind.

Total Solar Insolation varies ~0.1% or less UV and EUV vary ~7%

EUV and Cosmic rays are both very energetic. CO2 active IR wavelengths penetrate the ocean a few molecules deep while UV and EUV penetrate to 300 meters. How far UV and EUV actually penetrate the earth, I have no clue.

Long-term indirect indices of solar variability

Abstract

Continuous direct records of solar variability are limited to the telescopic era covering approximately the past four centuries. For longer records one has to rely on indirect indices such as cosmogenic radionuclides. Their production rate is modulated by magnetic properties of the solar wind. Using a parameterisation of the solar activity and a Monte Carlo simulation model describing the interaction of the cosmic rays with the atmosphere, the production rate for each cosmogenic nuclide of interest can be calculated as a function of solar activity. Analysis of appropriate well-dated natural archives such as ice cores or tree rings offers the possibility to reconstruct the solar activity over many millennia….

Ilya Usoskin uses cosmic rays to estimate solar activity since records back thousands of years are not available.

History of Solar Activity over Millennia. by Ilya G. Usoskin

…It is noteworthy that geomagnetic indices, in particular low-latitude aurorae (Silverman, 2006), are associated with coronal/interplanetary activity (high-speed solar-wind streams, interplanetary transients, etc.) that may not be directly related to the sunspot-cycle phase and amplitude, and therefore serve only as an approximate index of solar activity. One of the earliest instrumental geomagnetic indices is related to the daily magnetic declination range, the range of diurnal variation of magnetic needle readings at a fixed location, and is available from the 1780s (Nevanlinna, 1995). However, this data exists as several fragmentary sets, which are difficult to combine into a homogeneous data series.

Heliospheric indices are related to features of the solar wind or the interplanetary magnetic field measured (or estimated) in the interplanetary space. For example, the time evolution of the total (or open) solar magnetic flux is extensively debated (e.g., Lockwood et al., 1999; Wang et al., 2005; Krivova et al., 2007).

A special case of heliospheric indices is related to the galactic cosmic-ray intensity recorded in natural terrestrial archives. Since this indirect proxy is based on data recorded naturally through- out the ages and revealed now, it makes possible the reconstruction of solar activity changes on long timescales, as discussed in Section 3. …

For fun here is Usoskin lambasting Leif Svalgaard @ WUWT. LINK

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Now that I have put down my SWAG, I am going to go look and see if there is anything in the literature.

2022 Komitov and Kaftan, Danjon Effect”, Solar-Triggered Volcanic Activity,
and Relation to Climate Change — Russian Journal of Earth Sciences. 😄

…Trigger mechanisms of solar and geomagnetic activity over volcanic events, as well as their relation to climate change (in interaction with galactic cosmic rays (GCR) and/or solar energetic particles (SEP)), are discussed….

…The  existence  of  two  main  peaks  (near  to sunspot maximum and minimum), as well as a third one,  which is placed on 3–4 years after the sunspot maximum, indicates not one but two or three solar-modulated trigger mechanisms over the volcanic activity

The first peak, from Φ from 0.9 to 1 and after that from 0 to 0.2, corresponds to the end of one and the beginning of the next Schwabe-Wolf’s sunspot cycle. The sunspot cycles minima relate to galactic cosmic rays (GCR) flux maxima. The absolute maxima of the last one occur usually∼1 year after the sunspot cycle minima, i.e., at phase 0.1≤Φ≤0.2.  It relates to the minimal parameters of Sun’s heliosphere, which helps the GCR flux penetration in the inner part of the Solar system and the Earth’s atmosphere in particular [Usoskin,2013]. The anti-correlation between the GCR flux penetrating into the Earth’s atmosphere and the sunspot cycle phase is often labeled as one of the so-called “Forbush effects” [Forbush, 1954, 1958].As it has been shown by many authors [Svensmark and Friis-Christensen, 1997;Tinsley, 2000;Yu,2002], GCR flux changes reflect through many terrestrial processes:  aerosol and cloud formation,terrestrial current systems, atmospheric circulation, etc.  The possible physical mechanisms of GCR influences over volcanic activity and optical parameters of the Earth’s atmosphere will be dis-cussed in the next section…

The problem for possible solar-generated trigger mechanisms of volcanic events is of high importance in interdisciplinary scientific standpointat least for two reasons:  

1 – It is interesting for a better understanding of conditions when strong volcanic eruptions could occur with higher probability;  

2 – The important role of volcanic activity over the Earth’s climate change is out of doubt [Briffa et al., 1998; Brönnimann and Krämer, 2016;Cole-Dai et al., 2009;Kasatkina et al., 2018;Robock,2000]. 

Consequently, it is quite possible that an essential part of the “Sun-climate“ relation is caused not by solar electromagnetic radiation changes but rather by triggering solar activity processes in the lithosphere and particularly volcanism…

As was mentioned in section 3, an indication that the solar trigger forcing volcanic activity is realized by physical natural processes, miscellaneous but close and/or similar between themselves, is the fact of the existence of two or three peaks in the F-Φ histograms. 

  The galactic cosmic rays and solar energetic particles (SEP, pro-tons of≥10 MeV) are able to overcome the Earth’s magnetic field and penetrate in the middle and lower parts of the atmosphere.  The SEP events very often accompany strong solar flares (power class M5 and higher), big coronal mass ejections,and related strong geomagnetic storms. Both solar-modulated physical agents are of a corpuscular nature, but their maxima are in different phases of the sunspot cycle – GCR during the near sunspot cycle minima,  while the peaks of solar flares, CMEs, SEP, and strong geomagnetic storms – near sunspot cycle maxima. If the main physical factor of solar forcing over the volcanic activity is pre-dominantly closely connected with the sunspots and faculae (for example, the electromagnetic in-dexes such as the solar radio flux at 2800 MHz(F10.7), the total solar irradiance (TSI) index) in F-Φ histograms should be only one, but not two or three peaks observed. 

  

  Thus, in our opinion, the trigger mechanisms of the “Sun-volcanism” relationship is mainly realized due to cosmic electric charged particle interactions with the Earth’s magnetic field, atmosphere, and different terrestrial current systems. A serious support for this is the third peak in F-Φ histograms (mainly for VEI = 5) on the downward branch of the sunspot cycle at 0.7 ≤Φ≤ 0.8. As was already mentioned, it corresponds to high geomagnetic activity, whose primary sources are both strong solar flares and accompanying events (CMEs and SPEs) and high speed solar wind streams, caused by coronal holes. 

  

  The two- or three-peak structure of histograms, the fact that the third (CH) peak relates not to all but to a narrow-magnitude range of volcanic eruptions (VEI = 5) points that there is not a single solar triggering mechanism.  Most likely, there is a wide collection of triggering phenomena, which act under a great many of circumstances, depending on the current magma chamber conditions,the geological structure, rock and mineral composition, and current seismic conditions of the region, and its tectonic history (recent volcanic eruptions and/or earthquakes), as well as the current space weather, space climate conditions, and related phenomena and parameters (GCR flux, solar flare, and CME activity, solar proton events (SPE),geomagnetic and atmospheric conditions).