Solar cycle indices from the photosphere to the corona: measurements and underlying physics (1705.07054v1)

Abstract: A variety of indices have been proposed in order to represent the many different observables modulated by the solar cycle. Most of these indices are highly correlated with each other owing to their intrinsic link with the solar magnetism and the dominant eleven year cycle, but their variations may differ in fine details, as well as on short- and long-term trends. In this paper we present an overview of the indices that are often employed to describe the many features of the solar cycle, moving from the ones referring to direct observations of the inner solar atmosphere, the photosphere and chromosphere, to those deriving from measurements of the transition region and solar corona. For each index, we summarize existing measurements {\bf and typical use}, and for those that quantify physical observables, we describe the underlying physics.

Insights into Solar Cycle Indices from the Photosphere to the Corona

The paper "Solar cycle indices from the photosphere to the corona: measurements and underlying physics," authored by Ilaria Ermolli, Kiyoto Shibasaki, Andrey Tlatov, and Lidia van Driel-Gesztelyi, presents an extensive examination of the various indices used to describe solar cycle properties. Spanning observations from the photosphere to the corona, this paper explores the diverse manifestations of solar activity modulated by the magnetic fields of the Sun.

Summary of Solar Cycle Indices

The solar cycle, characterized by an approximately eleven-year period, produces a wide range of observable effects in the solar atmosphere. Common indices such as sunspot numbers, sunspot areas, and facular indices are strongly dependent on the magnetic dynamo processes beneath the Sun's surface. For instance, sunspot numbers, despite their historical origin dating back to the 17th century, remain a pivotal indicator of solar activity, with extensive datasets compiled by institutions such as ROB-SILSO and NOAA/NGDC. Likewise, sunspot area measurements provide key insights into magnetic flux variations, leading to more accurate models of solar irradiance changes.

Chromospheric Observations

The chromosphere's complexity is captured through indices derived from Ca II K and H-alpha line observations. These indices provide quantitative measures of non-spot magnetic activity, with cyclic and longitudinal data revealing distinct patterns in solar activity evolution not captured by sunspot indices alone. The Mg II index further complements this by serving as a proxy for UV variations, critical for understanding the solar output's interaction with Earth's atmosphere.

Coronal and Transition Region Indices

The paper discusses indices derived from the transition region and corona which illustrate the magnetic connectivity extending into the heliosphere. Notably, measurements of coronal green line emissions lead to the formulation of the coronal index, a physical representation of solar activity impacting heliospheric conditions. The F10.7 radio flux index, recognized for its stable record spanning decades, serves as an essential surrogate of the solar cycle for models predicting space weather impacts on Earth’s atmospheric conditions.

Implications and Future Research Directions

The collection and analysis of these indices have profound implications for solar and heliospheric research. Accurate, long-term datasets enable the simulation of solar irradiance models and provide crucial input into the paper of solar-terrestrial interactions, including atmospheric chemistry and climate change. Moreover, increasing the precision and calibration of these data series have the potential to improve our understanding of solar cycle predictions and its global effects. Future advancements may focus on refining these indices and extending historical records, leveraging technological progress in solar observation and data assimilation techniques to foster a deeper integration into models spanning from the Sun to the Earth’s climate system.

The research articulated in this paper underscores the significance of these indices in comprehending the solar cycle's extensive influence and offers a valuable reference point for ongoing and future explorations in astrophysical sciences.