Research on Solar Long-term Variability and Effects

Disturbances of near-Earth space are predominantly driven by coronal mass ejections (CMEs) mostly originating from sunspots and high-speed solar wind streams (HSSs) emanating from coronal holes. The relative importance of CMEs and HSSs as well as slow solar wind in producing energetic electron precipitation was studied in detail in a recent paper by Asikainen and Ruopsa (2016) by utilizing the corrected energetic electron measurements from the MEPED instrument on board low-altitude NOAA/POES satellites from 1979 to 2013. Using solar wind observations categorized into three different flow types, it was possible to separete the contributions of these flows to annual electron precipitation and to quantify their efficiencies in producing precipitation. The study found that HSS contribution nearly always dominates over the other flows and peaks strongly in the declining solar cycle phase. CME contribution mostly follows the sunspot cycle but is typically enhanced also in the declining phase. Rather surprisingly the study found that the efficiency of both HSSs and also CMEs peaks in the declining phase. Detailed quantification of how the electron precipitation depends on solar wind southward magnetic field component, speed, and density revealed that the solar wind speed is the dominant factor affecting the precipitation in all solar wind flow types. It is well known that drag forces affecting the propagation of CMEs in in the interplanetary space act to either accelerate or decelerate CMEs towards background solar wind speed. Since HSSs enhance the average solar wind speed in the declining solar cycle phase, they also accelerate CMEs to higher speeds and thus enhance the efficiency of CMEs during these times. It thus seems that high-speed solar wind streams have thus have a double effect in enhancing energetic electron precipitation by also affecting the precipitation efficiency of CMEs embedded in high-speed streams.

Sunspot numbers provide a uniquely long index of solar activity spanning over 400 years. However they are still the subject of intense debate, with the largest uncertainty being related to the “calibration” of the visual acuity of individual observers in the past. A novel method for calibrating the visual acuity of the key solar observers in the 18th and 19th centuries to the reference data set of Royal Greenwich Observatory sunspot groups for the period 1900 – 1976, using the statistics of the active-day fraction, is presented. Observational thresholds [Ss] were defined for each observer such that the observer is assumed to miss all of the groups with an area smaller than Ss and report all the groups larger than Ss. Next, correction matrices were constructed for each observer regarding the reference data set using the Monte-Carlo method, that were applied to the original sunspot group records reported by the observers for each day (see Figure A), and finally the composite corrected series is produced for the period since 1748. The new group sunspot number series has been presented which is close to the 'classical' group sunspot number by Hoyt and Schatten (1998) but essentially lower than a recent reconstruction by Svalgaard and Schatten (2016), suggesting that the latter overestimates sunspot activity because of the used linear regression method. This result confirms the uniqueness of the grand maximum is confirmed for the last 250 years. This result is confirmed by a direct comparison with cosmogenic isotopes data of ¹⁴C in tree rings and ⁴⁴Ti in fallen meteorites.

The first direct comparison of cosmogenic ¹⁰Be record and chemical species measured in the Antarctic EPICA-Dome C ice core for the period of 38–45.5 kyr BP spanning the Laschamp geomagnetic excursion, when the geomagnetic field almost vanished, is presented. The methods of principal component analysis (PCA) and wavelet coherence (WC) were applied to group different components as a function of the main sources, transport and deposition processes affecting the atmospheric aerosol at Dome C over that period. The evident preferential association of ¹⁰Be with nitrate rather than with other chemical species was found implying for the presence of a distinct cosmogenic source of these markers. The role of calcium in driving the ¹⁰Be and nitrate relationship was shown to be insignificant, implying that the signal is mostly driven by production rather than by transport in the atmosphere. This is particularly relevant for a plateau site such as Dome C, especially in the glacial period during which the Laschamp excursion took place. The evidence that the nitrate record from the EDC ice core is able to capture the Laschamp event hints toward the possibility of using this marker for studying galactic cosmic ray flux variations and thus also major geomagnetic field excursions at pluri-centennial-millennial time scales, thus opening up new perspectives in paleoclimatic studies.

The only quantitative proxy to study the long-term solar variability over a centennial-millennial timescale is related to cosmogenic isotope proxies. While essential progress has been achieved in both measurements and modelling of the cosmogenic proxy, uncertainties still remain in the determination of the geomagnetic dipole moment evolution. A new improved reconstruction of solar activity over the past nine millennia has been presented, based on cosmogenic records of ¹⁴C in tree rings and ¹⁰Be in polar ice cores, using a multi-proxy approach using the most up-to-date models of cosmogenic isotope production and transport in Earth’s atmosphere, and available geomagnetic field reconstructions, including a new reconstruction relying on an updated archeo- and paleointensity database. The reconstruction was analyzed using the singular spectrum analysis (SSA) method to study the millennial-scale trends. The primary components of the reconstructed solar activity, as determined using the SSA method, were found different for the series that are based on ¹⁴C and ¹⁰Be. This shows that these primary components can only be ascribed to long-term changes in the terrestrial system and not to the Sun. These components have therefore been removed from the reconstructed series. In contrast, the secondary SSA components of the reconstructed solar activity are found to be dominated by a common ≈2400-year quasi-periodicity, the so-called Hallstatt cycle, in both the ¹⁴C and ¹⁰Be based series. This Hallstatt cycle thus appears to be related to solar activity. It is shown that the grand minima and maxima of solar activity occurred intermittently over the studied period, with clustering near lows and highs of the Hallstatt cycle, respectively.

Direct measurements of the solar magnetic field started in 1917 with measurements of sunspot field strength at Mount Wilson Observatory. The first magnetographic observations date back to late 1950s, with routine synoptic observations of full disk magnetograms starting in early 1970s. The full disk magnetograms provide the most complete information about polar fields, although limited information can be inferred from surface distribution of certain photospheric and chromospheric structures (e.g., polar crown filaments may outline the location of lower boundary of polar unipolar field). Correlation between bright chromospheric plages observed in Ca II K 393.37 nm spectral line and unsigned magnetic flux was noted from the very beginning of magnetographic observations in 1950s. Accordingly, the brightness of the chromospheric Ca II K spectral line available in the form of full disk images (spectroheliograms) of the Sun can be used as a proxy for solar magnetic fields.

A recent article by Pevtsov et al. is the first in a series of articles attempting to develop a new proxy for the evolution of magnetic activity in past solar cycles by combining the information from historical Ca II K line spectroheliograms and sunspot magnetic field measurements. The proxy is based on the relationship between magnetic flux and CaK line intensity (contrast) obtained from using Ca K line and sunspot magnetic field observations at Mount Wilson Observatory (MWC) and magnetograms of the National Solar Observatory at Kitt Peak. They used synoptic (Carrington) maps in 1915–1985 derived from daily Ca K line observations at Mount Wilson Observatory to identify the chromospheric plages, and to create synoptic pseudo-magnetograms. The study shows that using a combination of sunspot field measurements and the plages with renormalized intensities one can successfully construct a homogeneous long-term series of pseudo-magnetograms which highly correlate with the observed magnetic field data (correlation 0.98).

The Maunder minimum (MM) in 1645-1715 was a period when there were almost no sunspots on the Sun. This forms a paradigm for the general concept of a Grand minimum of solar activity which is important for solar/stellar dynamo theory and solar-terrestrial relations. Although the Maunder minimum was covered by the benchmark sunspot number series, uncertainties still remain on the exact level of solar activity in the 17th century. In particular, several claims were made, suggesting that it may be moderate because of the 17th century astronomers might had been affected by the dogma on the "perfectness of the Sun" which should have 'imperfect' spots.

A reassessment of the level of solar activity during the MM was made, using all the existing, both direct and indirect, datasets and evidence has found to show that the activity was very low, significantly lower than during any other period over the last centuries including the current weak solar cycle #24. As an example, the incut Figure shows occurrence of auroral reports for 1700-1900. The green line is the number of auroral nights at geomagnetic latitudes below 56° from a combination of several catalogues. The points show the geomagnetic latitude and time of auroral sightings from the catalogue of Vázquez et al. (2014). Once can see that during the MM, auroral reports were not only significantly less frequent, but they were also high-latitude, limited to the (sub)auroral zone, indicating the absence of moderate-strong geomagnetic storms.

The careful revision of all the presently available datasets for the Maunder minimum proves that solar activity was indeed at an exceptionally low level during that period, corresponding to a special grand minimum mode of solar dynamo.

Recent study finds that cosmogenic ⁷Be isotopes produced in the lower stratosphere were measured in near-ground air at Rio de Janeiro, Brazil, after the southern hemispheric Sudden Stratospheric Warming (SSW) of 2002. The study was based on a comparison of ⁷Be data measured around Angra Nuclear Power Station (23 S 44 W) during the last three decades and a model estimate of the near-ground air ⁷Be concentration using the CRAC:⁷Be model of cosmogenic production together with a simplified model for atmospheric ⁷Be deposition that assimilates the regional precipitation data. The results of the study indicate that an anomalous stratosphere-troposphere coupling associated to the unique SSW of 2002 allowed stratospheric aerosols carrying ⁷Be to reach the ground level very quickly. This methodology points to an important use of ⁷Be as a quantitative tracer for stratospheric influence on near-ground air patterns.

Several recent studies have found variability in the Northern Hemisphere winter climate related to different parameters of solar activity. While these results consistently indicate some kind of solar modulation of tropospheric and stratospheric circulation and surface temperature, opinions on the exact mechanism and the solar driver differ. Proposed drivers include, e.g., total solar irradiance (TSI), solar UV radiation, galactic cosmic rays, and magnetospheric energetic particles. While some of these drivers are difficult to distinguish because of their closely similar variation over the solar cycle, other suggested drivers have clear differences in their solar cycle evolution. For example, geomagnetic activity and magnetospheric particle fluxes peak in the declining phase of the sunspot cycle, in difference to TSI and UV radiation which more closely follow sunspots. Using 13 solar cycles (1869-2009), we study winter surface temperatures and North Atlantic oscillation (NAO) during four different phases of the sunspot cycle: minimum, ascending, maximum, and declining phase. We find significant differences in the temperature patterns between the four cycle phases, which indicates a solar cycle modulation of winter surface temperatures. However, the clearest pattern of the temperature anomalies is not found during sunspot maximum or minimum, but during the declining phase, when the temperature pattern closely resembles the pattern found during positive NAO. Moreover, we find the same pattern during the low sunspot activity cycles of 100 years ago, suggesting that the pattern is largely independent of the overall level of solar activity.

In the declining phase of the solar cycle (SC), when the new-polarity fields of the solar poles are strengthened by the transport of same-signed magnetic flux from lower latitudes, the polar coronal holes expand and form non-axisymmetric extensions toward the solar equator. These extensions enhance the occurrence of high-speed solar wind (SW) streams (HSS) and related co-rotating interaction regions in the low-latitude heliosphere, and cause moderate, recurrent geomagnetic activity (GA) in the near-Earth space. Here, using a novel definition of GA at high (polar cap) latitudes and the longest record of magnetic observations at a polar cap station, we calculate the annually averaged SW speeds as proxies for the effective annual occurrence of HSS over the whole Grand Modern Maximum (GMM) from 1920s onward.

We find that a period of high annual speeds (frequent occurrence of HSS) occurs in the declining phase of each of SCs 16-23. For most cycles the HSS activity clearly reaches a maximum in one year, suggesting that typically only one strong activation leading to a coronal hole extension is responsible for the HSS maximum. We find that the most persistent HSS activity occurred in the declining phase of SC 18. This suggests that cycle 19, which marks the sunspot maximum period of the GMM, was preceded by exceptionally strong polar fields during the previous sunspot minimum. This gives interesting support for the validity of solar dynamo theory during this dramatic period of solar magnetism.