Solar modulation in surface atmospheric electricity

The solar wind modulates the flux of galactic cosmic rays impinging on Earth inversely with solar activity. Cosmic ray ionisation is the major source of air’s electrical conductivity over the oceans and well above the continents. Differential solar modulation of the cosmic ray energy spectrum modifies the cosmic ray ionisation at different latitudes,varying the total atmospheric columnar conductance. This redistributes current flow in the global atmospheric electrical circuit, including the local vertical current density and the related surface potential gradient. Surface vertical current density and potential gradient measurements made independently at Lerwick Observatory,Shetland,from 1978 to 1985 are compared with modelled changes in cosmic ray ionisation arising from solar activity changes. Both the lower troposphere atmospheric electricity quantities are significantly increased at cosmic ray maximum(solar minimum),with a proportional change greater than that of the cosmic ray change

A History of Solar Activity over Millennia

Presented here is a review of present knowledge of the long-term behavior of solar activity on a multi-millennial timescale, as reconstructed using the indirect proxy method. The concept of solar activity is discussed along with an overview of the special indices used to quantify different aspects of variable solar activity, with special emphasis upon sunspot number. Over long timescales, quantitative information about past solar activity can only be obtained using a method based upon indirect proxies, such as the cosmogenic isotopes \super{14}C and \super{10}Be in natural stratified archives (e.g., tree rings or ice cores). We give an historical overview of the development of the proxy-based method for past solar-activity reconstruction over millennia, as well as a description of the modern state. Special attention is paid to the verification and cross-calibration of reconstructions. It is argued that this method of cosmogenic isotopes makes a solid basis for studies of solar variability in the past on a long timescale (centuries to millennia) during the Holocene. A separate section is devoted to reconstructions of strong solar energetic-particle (SEP) events in the past, that suggest that the present-day average SEP flux is broadly consistent with estimates on longer timescales, and that the occurrence of extra-strong events is unlikely. Finally, the main features of the long-term evolution of solar magnetic activity, including the statistics of grand minima and maxima occurrence, are summarized and their possible implications, especially for solar/stellar dynamo theory, are discussed.Comment: A review, 91 pages, 28 figures. available online at http://solarphysics.livingreviews.org/Articles/lrsp-2013-1

Occurrence of extreme solar particle events: Assessment from historical proxy data

The probability of occurrence of extreme solar particle events (SPEs) with the fluence of (>30 MeV) protons F30>10^{10} cm^{-2} is evaluated based on data of cosmogenic isotopes 14C and 10Be in terrestrial archives centennial-millennial time scales. Four potential candidates with F30=(1-1.5)x10^{10} cm^{-2} and no events with F30>2x10^{10} cm^{-2} are identified since 1400 AD in the annually resolved 10Be data. A strong SPE related to the Carrington flare of 1859 AD is not supported by the data. For the last 11400 years, 19 SPE candidates with F30=(1-3)x10^{10} cm^{-2} are found and clearly no event with F30>5x10^{10} cm^{-2} (50-fold the SPE of 23-Feb-1956) occurring. This values serve as an observational upper limit for the strength of SPE on the time scale of tens of millennia. Two events, ca. 780 and 1460 AD, appear in different data series making them strong candidates to extreme SPEs. We built a distribution of the occurrence probability of extreme SPEs, providing a new strict observational constraint. Practical limits can be set as F30~1x, 2-3x, and 5x10^{10} cm^{-2} for the occurrence probability ~10^{-2}, 10^{-3} and 10^{-4} year^{-1}, respectively. Because of uncertainties, our results should be interpreted as a conservative upper limit of the SPE occurrence near Earth. The mean SEP flux is evaluated as ~40 (cm2 sec)^{-1} in agreement with estimates from the lunar rocks. On average, extreme SPEs contribute about 10% to the total SEP fluence.Comment: accepted to Astrophys.

A solar cycle lost in 1793--1800: Early sunspot observations resolve the old mystery

Because of the lack of reliable sunspot observation, the quality of sunspot number series is poor in the late 18th century, leading to the abnormally long solar cycle (1784--1799) before the Dalton minimum. Using the newly recovered solar drawings by the 18--19th century observers Staudacher and Hamilton, we construct the solar butterfly diagram, i.e. the latitudinal distribution of sunspots in the 1790's. The sudden, systematic occurrence of sunspots at high solar latitudes in 1793--1796 unambiguously shows that a new cycle started in 1793, which was lost in traditional Wolf's sunspot series. This finally confirms the existence of the lost cycle that has been proposed earlier, thus resolving an old mystery. This letter brings the attention of the scientific community to the need of revising the sunspot series in the 18th century. The presence of a new short, asymmetric cycle implies changes and constraints to sunspot cycle statistics, solar activity predictions, solar dynamo theories as well as for solar-terrestrial relations.Comment: Published by Astrophys. J. Let

A new model of cosmogenic production of radiocarbon 14C in the atmosphere

We present the results of full new calculation of radiocarbon 14C production in the Earth atmosphere, using a numerical Monte-Carlo model. We provide, for the first time, a tabulated 14C yield function for the energy of primary cosmic ray particles ranging from 0.1 to 1000 GeV/nucleon. We have calculated the global production rate of 14C, which is 1.64 and 1.88 atoms/cm2/s for the modern time and for the pre-industrial epoch, respectively. This is close to the values obtained from the carbon cycle reservoir inventory. We argue that earlier models overestimated the global 14C production rate because of outdated spectra of cosmic ray heavier nuclei. The mean contribution of solar energetic particles to the global 14C is calculated as about 0.25% for the modern epoch. Our model provides a new tool to calculate the 14C production in the Earth's atmosphere, which can be applied, e.g., to reconstructions of solar activity in the past.Comment: Published in EPSL, 337, 114, 201

New reconstruction of event-integrated spectra (spectral fluences) for major solar energetic particle events

Fluences of solar energetic particles (SEPs) are not easy to evaluate, especially for high-energy events (i.e. ground-level enhancements, GLEs). Earlier estimates of event-integrated SEP fluences for GLEs were based on partly outdated assumptions and data, and they required revisions. Here, we present the results of a full revision of the spectral fluences for most major SEP events (GLEs) for the period from 1956 -- 2017 using updated low-energy flux estimates along with greatly revisited high-energy flux data and applying the newly invented reconstruction method including an improved neutron-monitor yield function. Low- and high-energy parts of the SEP fluence were estimated using a revised space-borne/ionospheric data and ground-based neutron monitors, respectively. The measured data were fitted by the modified Band function spectral shape. The best-fit parameters and their uncertainties were assessed using a direct Monte Carlo method. As a result, a full reconstruction of the event-integrated spectral fluences was performed in the energy range above 30 MeV, parametrised, and tabulated for easy use along with estimates of the 68% confidence intervals. This forms a solid basis for more precise studies of the physics of solar eruptive events and the transport of energetic particles in the interplanetary medium, as well as the related applications.Comment: 19 pages, 3 figures, to be published in Astronomy and Astrophysic