Abstract
28 February 2017 marked 75 years since the first confident registration of solar cosmic rays (SCRs), i.e., accelerated solar particles with energies from about 106to ~1010÷ 1011eV. Modern state of the problems related to the studies of Ground Level Enhancements (GLEs) of relativistic SCRs is critically analyzed based on available direct and proxy data. We are also taking into account extremely large fluxes of non-relativistic solar energetic particles (SEPs). Both kinds of SCR events are of great astrophysical and geo-scientific (geophysical) interests. A number of the GLE properties (total statistics, occurrence rate, longitude distribution, ranking of GLEs, a number of specific GLEs – so-called “rogue” SEP events etc.) are discussed in some detail. We note also the problems of GLE identification (definition) by ground-based observations, the difficulties in the studies of weak (“hidden”, or sub-) GLEs etc. One of serious challenges to the problem of radiation hazard in space is a lack of a clear, unambiguous relation between the fluxes (fluences) of relativistic SCR and non-relativistic SEPs. Special attention is paid to the recent debate on the validity, origin and properties of the “ancient” events AD775, AD994, AD1859 (Carrington event) and BC3372. We demonstrate that, in spite of existing uncertainties in proton fluences above 30 MeV, all of them are fitted well by a unique distribution function, at least, with the present level of solar activity. Extremely large SEP events are shown to obey a probabilistic distribution on their fluences with a sharp break in the range of large fluences (or low probabilities). The studies of this kind may be extended for periods with different levels of solar activity in the past and/or in the future. Dose rates at aircraft altitudes are also demonstrated during some GLEs. Several examples of using the SCR data and GLE properties in radiation prediction schemes are considered.
Highlights
Solar energetic particles (SEPs) represent an important aspect of solar-terrestrial physics, as well as are among the three main components of space weather
Since certain periodicities found by Miroshnichenko et al (2012) are coherent for parameters g, SS, and CI, during this stage of the study, the conclusion can be made that oscillations are synchronized in different layers of the solar atmosphere – from the photosphere to the corona. This can indicate that the Solar Cosmic Rays (SCRs) (GLE) generation involves extended areas in the solar atmosphere rather than being a local process. Based on these results, interesting prospects of longterm Ground Level Enhancements (GLEs) prediction have been recently formulated by Miroshnichenko et al (2013): a) the Galactic Cosmic Rays (GCRs) oscillation distribution can be used to study a solar cycle, to predict solar proton event (SPE), and in other practical applications in the space weather problem; b) oscillations in the SS, CI, and GCRs obey the hierarchy principle (e.g., GCR intensity is modulated by solar activity SS etc.); c) oscillations in the GLE occurrence rate are apparently of absolutely different nature, with the only statistically significant oscillation with a period of ~11 years
Relativistic solar particles in their movement to the Earth, as a rule, do not undergo significant scattering, the probability of reaching the Earth obviously strongly depends on the angle of curvature of the Parker spiral of the interplanetary magnetic field (IMF)
Summary
Solar energetic particles (SEPs) represent an important aspect of solar-terrestrial physics, as well as are among the three main components of space weather (geomagnetic field disturbances; enhanced energetic particle dosages; ionospheric electron density disturbances, e.g., Song, 2001). As well established long ago, during energetic solar phenomena, high-energy particles are generated in extended energy range – from about 1 MeV to 10 Ä 100 GeV They were called Solar Cosmic Rays (SCRs) because of their close similarity to the true cosmic rays (CR) of galactic origin – Galactic Cosmic Rays (GCRs). We present an analytical (critical) review of the main features of GLEs that may be important for the Space Weather and Space Climate (SWSC) goals: 1) specific features of GLE registration; 2) general relation of GLEs to solar activity and reversals of global magnetic field (GMF) of the Sun; 3) occurrence (registration) rate of GLEs; 4) longitude distribution of the GLE sources; 5) specific ‘‘rogue’’ SEP events and GLEs; 6) small (‘‘hidden’’) events, or sub-GLEs; 7) new definition of GLE. We briefly discuss a probability of super-flares at present Sun
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