To Understand Solar Activity And Its Terrestrial Impact, One Has to Know The Past

  • Date: 30/09/17
  • Alexei A. Pevtsov and Frédéric Clette, EOS

Solar activity waxes and wanes in 10- to 11-year cycles; this is now general public knowledge. However, we know this only because of existing long-term records. Thanks to these histories, we also know that properties of solar cycles vary on timescales of 100 years and even longer. Thus, some of the most important processes on the Sun may take decades if not centuries to reveal themselves [Owens, 2013].

This long timescale means that some issues are not resolved, or even identified, at the time when data are acquired. Synoptic observations of solar activity, programs that span many years, feed future research to solve these issues.

However, present-day research funding schemes tend to focus on providing effective funding for rapidly changing research goals. Funding agencies and the National Academies emphasize short grants, lasting 3–5 years, as the prime vehicle for funding scientific research, a duration that is too short to ensure the survival of synoptic programs.

How do we change this focus so that the synoptic studies so critical to our understanding get sustainable funding?

Insights Gained from Taking the Long View

Historical data show the presence of grand minima and grand maxima, when the Sun was either inactive or extremely active for an extended period. Those major changes in solar activity seem to have created significant changes in the past Earth climate, making long-term records essential to solve critical issues of the 21st century.

Even records of past solar activity that come from isotopic sources such as ice cores and tree rings rely on establishing a relationship between radiocarbon measurements and the direct observations of solar activity. Because the natural circulation of carbon in Earth’s atmosphere was affected by the explosive increase in the use of fossil fuel at the beginning of the Industrial Revolution, only historical observations of solar activity can be used for calibrating radiocarbon data. The absolute radiocarbon standard is based on 1890 wood.

Sometimes, historical records of direct observations of solar activity themselves may require critical analysis. Recently recalibrated records of sunspot numbers [Clette et al., 2016] indicate, for instance, that solar cycle amplitudes may have been more uniform in the past 3 centuries than assumed until recently. If this result can be fully confirmed, it weakens the evidence for a solar cause of global warming.

A series of images of the Sun taken 7–17 March 1989 shows the evolution of a large sunspot group (NOAA 5395) as it moves around the Sun. The top sequence, taken in white light, shows the sunspot as it appears in the photosphere (solar visible surface). The bottom images are magnetograms of the same sunspot region, showing variations in magnetic polarity. During its disk passage, this active region produced more than 100 X-ray flares, including 11 flares in the most powerful X class. This eruptive activity was the cause of the “great geomagnetic storm” of 13–14 March, which affected radio communications and satellite operations and caused the famous Quebec blackout on 13 March 1989. See Allen et al. [1989] for a detailed description of solar and geomagnetic activity associated with this active region. Credit: NOAO/AURA/NSF

We also have historical time series of direct measurements of sunspot magnetic field strengths (e.g., see Figure 1) and ultraviolet observations of the Sun going back more than a century. In combination with modern dynamo models, these historical data allow us to explore the possible changes in properties of solar plasma in the convection zone, where these magnetic fields are generated, and form a better understanding of future cycles.

Paraphrasing Carl Sagan, “You have to know the past to understand the future.”

Shortsighted Funding Strategies

Unfortunately, despite the importance of long-term time series, we are witnessing an alarming decline in funding, and even cancellation, of long-term programs. For example, last year brought us the disbanding of a solar group at Debrecen Heliophysical Observatory in Hungary, thus interrupting the recording of a historical time series of sunspot group areas that spans more than a century. This project had started at Greenwich Royal Observatory in May of 1874 and transferred to Debrecen at the end of 1978.

At Mount Wilson Observatory in California, scientists continue direct measurements of sunspot field strength that began in 1917. Funding for this project has been discontinued, but the effort lives on because of heroic efforts of remaining observing personnel. Similar cuts to sunspot measuring programs threaten research around the world.

Orchestrating Change

The success of long-term synoptic observations requires long-term sustainable funding. The short-duration project funding schemes that have prevailed over recent years are unsuitable for long-term data collection and continuous monitoring. Indeed, long-term continuity is a key requirement for producing meaningful and usable data sets.

This does not mean that nothing changes over the term of a time series. Not all historical time series need to be continued, and instruments inevitably change over the lifetime of long-term time series. However, change must be carefully planned and orchestrated to maintain the uniformity of a time series, including cross calibration of new and old instruments.

Although some efforts are being made to develop replacements for aging synoptic facilities, there is an overall lack of long-term planning for such programs. This lack of planning may lead to the creation of ad hoc “networks” of nonuniform instrumentation and unnecessary duplication.

This area of research also benefits from close international collaboration. One strategy for using funding efficiently would be to establish a list of observables that the international community considers worthy to continue for an extended period of time. Then the funding agencies and the National Academies could be approached to establish a mechanism for shared funding for such time series. In this funding model, even the countries with limited research capabilities may contribute to the overall success.

To ensure the survival of historical time series, this work needs to be done now.

Fig. 1. This sunspot drawing shows one of the largest sunspot groups observed over the past 100 years. Observations were taken on 7 April 1947 at Mount Wilson Observatory in California. Solar north is toward the top of the drawing, and solar west is to the left. Markings on the drawings indicate the positions and magnetic fields of all sunspots measured that day. Credit: Carnegie Observatories

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