High-Resolution Spectral and Anisotropy Characteristics of Solar Protons During the GLE N(circle)73 on 28 October 2021 Derived with Neutron-Monitor Data Analysis

The first ground-level enhancement of the current Solar Cycle 25 occurred on 28 October 2021. It was observed by several space-borne and ground-based instruments, specifically neutron monitors. A moderate count-rate increase over the background was observed by high-altitude polar stations on the South Pole and Dome C stations at the Antarctic plateau. Most of the neutron monitors registered only marginal count-rate increases. Using detrended records and employing a method verified by direct space-borne measurements, we derive the rigidity spectra and angular distributions of the incoming solar protons in the vicinity of Earth. For the analysis, we employed a newly computed and parameterized neutron-monitor yield function. The rigidity spectra and anisotropy of solar protons were obtained in their time evolution throughout the event. A comparison with the Solar and Heliospheric Observatory/Energetic and Relativistic Nuclei and Electron (SOHO/ENRE) experiment data is also performed. We briefly discuss the results derived from our analysis

Using global neutron monitor network data for GLE analysis:recent results

Abstract The solar cycle 23 provided several strong ground level enhancements (GLEs). The first strong GLE event of the cycle was observed on 14 July 2000 (the Bastille day event), while the last was observed on 13 December 2006. In addition, the period of late October – early November 2003 was characterized by strong cosmic ray variability and sequence of three GLEs. Cycle 23 produced also the second largest event in the observational history — 20 January 2005 event. Here we perform a precise analysis of spectral and angular characteristics of solar energetic particle (SEP) events on the basis of neutron monitor (NM) data. We model particle propagation in the Earth’s magnetosphere and atmosphere using a newly computed NM yield function at several altitudes above the sea level. The method includes several consecutive steps: detailed computation of asymptotic cones and rigidity cut-off of each NMstation used in the analysis, making an initial guess of the inverse problem by assuming the apparent source position location in a convenient way, application of the NM yield function for detector response modelling and subsequent optimization procedure in order to derive spectral and angular characteristics of SEPs. Here, we present results from analysis of several GLEs. The SEP spectra and pitch angle distributions were obtained in their dynamical development throughout the events. We briefly demonstrate the capability of the method to assess SEP features of sub-GLE events, the details reported in this volum. Several case examples are discussed

Halloween GLEs on October–November 2003, spectra and angular distribution:revised results

Abstract A precise study of solar energetic particles provides an important basis to understand their acceleration and propagation in the interplanetary space. A specific interest is paid to solar protons possessing energy high enough, so that they can induce an atmospheric cascade in the Earth’s atmosphere, whose secondary particles reach the ground, eventually being registered by ground-based detectors e.g. neutron monitors. This particular class of events is called ground-level enhancements (GLEs). The solar cycle 23 provided several strong GLEs. The first strong GLE event of the cycle was observed on 14 July 2000 (the Bastille day event), while the last was observed on 13 December 2006. In addition, the period of late October — early November 2003 was characterized by strong cosmic ray variability and a sequence of three GLEs (the so-called Halloween GLEs) was registered, which is the focus of this study. Here, we performed a precise analysis of neutron monitor records and derived the spectral and angular characteristics of the solar energetic particles during the Halloween GLEs. We modeled the particle propagation in the Earth’s magnetosphere and atmosphere using a verified NM yield function computed at several altitudes above the sea level. The solar protons spectra and pitch angle distributions were obtained in their dynamical development throughout the events. We briefly discuss the revealed features of the Halloween events

GLE # 67 event on 2 November 2003:an analysis of the spectral and anisotropy characteristics using verified yield function and detrended neutron monitor data

Abstract During Solar Cycle 23 16 ground-level enhancement events were registered by the global neutron monitor network. In this work we focus on the period with increased solar activity during late October – early November 2003 producing a sequence of three events, specifically on ground-level enhancement GLE 67 on 2 November 2003. On the basis of an analysis of neutron monitor and space-borne data we derived the spectra and pitch-angle distribution of high-energy solar particles with their dynamical evolution throughout the event. According to our analysis, the best fit of the spectral and angular properties of solar particles was obtained by a modified power-law rigidity spectrum and a double Gaussian, respectively. The derived angular distribution is consistent with the observations where an early count rate increase at Oulu neutron monitor with asymptotic viewing direction in the anti-Sun direction was registered. The quality of the fit and model constraints were assessed by a forward modeling. The event integrated particle fluence was derived using two different methods. The derived results are briefly discussed

New method of assessment of the integral fluence of solar energetic (> 1 GV rigidity) particles from neutron monitor data

Abstract A new method to reconstruct the high-rigidity part (≥ 1 GV) of the spectral fluence of solar energetic particles (SEP) for GLE events, based on the world-wide neutron monitor (NM) network data, is presented. The method is based on the effective rigidity Reff and scaling factor Keff. In contrast to many other methods based on derivation of the best-fit parameters of a prescribed spectral shape, it provides a true non-parametric (viz. free of a priori assumptions on the exact spectrum) estimate of fluence. We reconstructed the SEP fluences for two recent GLE events, #69 (20 Jan. 2005) and #71 (17 May 2012), using four NM yield functions: (CD00 — Clem and Dorman in Space Sci. Rev.93, 335, 2000), (CM12 — Caballero-Lopez and Moraal in J. Geophys. Res.117, A12103, 2012), (Mi13 — Mishev, Usoskin, and Kovaltsov in J. Geophys. Res.118, 2783, 2013), and (Ma16 — Mangeard et al. in J. Geophys. Res.121, 7435, 2016b). The results were compared with full reconstructions and direct measurements by the PAMELA instrument. While reconstructions based on Mi13 and CM12 yield functions are consistent with the measurements, those based on CD00 and Ma16 ones underestimate the fluence by a factor of 2 – 3. It is also shown that the often used power-law approximation of the high-energy tail of SEP spectrum does not properly describe the GLE spectrum in the NM-energy range. Therefore, the earlier estimates of GLE integral fluences need to be revised

Application of verified neutron monitor yield function for GLE analysis

Abstract Systematic study of solar energetic particles provides an important basis to understand their acceleration and propagation in the interplanetary space. After solar eruptive processes, such as solar flares and/or coronal mass ejections, solar ions are accelerated to high energy. In the majority of cases, the maximum energy of the accelerated solar ions is several tens of MeV/nucleon, but in some cases, it exceeds 100 MeV/nucleon or even reaches the GeV/nucleon range. In this case, the energy is high enough, so that solar ions generate an atmospheric cascade in the Earth’s atmosphere, whose secondary particles reach the ground, being eventually registered by ground-based detectors, specifically neutron monitors. This particular class of events is known as ground-level enhancements (GLEs). Several methods for analysis of GLEs, using neutron monitor data were developed over the years. Here, we present a method for assessment of the spectral and angular characteristics of the GLEs using data from the world-wide NM network, namely by modeling the global neutron monitor network response with a new verified yield function. The method is based on consecutive steps, specifically detailed computation of asymptotic cones and rigidity cut-off of each station used in the analysis and optimization of the global neutron monitor response over experimental and modeled count rate increases. The method is compared with other methods, including in-situ measurements. A very good agreement between our method and space-borne measurements with PAMELA space probe, specifically the derived fluence of solar protons during GLE 71 was achieved, therefore verification of the method is performed

Can we properly model the neutron monitor count rate?

Abstract Neutron monitors provide continuous measurements of secondary nucleonic particles produced in the atmosphere by the primary cosmic rays and form the main tool to study the heliospheric modulation of cosmic rays. In order to study cosmic rays using the world network of neutron monitor and needs to be able to model the neutron monitor count rate. Earlier it was difficult because of the poorly known yield function, which has been essentially revisited recently. We have presented a verification of the new yield function of the standard neutron monitor (NM) using a recently released data on the direct in situ measurements of the galactic cosmic rays energy spectrum during 2006-2009 (the period of the record high cosmic ray flux) by Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics spaceborne spectrometer, and on NM latitude surveys performed during the period of 1994-2007, including periods of high solar activity. We found a very good agreement between the measured count rates of sea level NMs and the modeled ones in very different conditions: from low to high solar activity and from polar to tropical regions. This implies that the count rate of a sea level neutron monitor can be properly modeled in all conditions, using the new yield function

Updated neutron‐monitor yield function:bridging between in situ and ground‐based cosmic ray measurements

Abstract An updated yield function for a standard NM64 neutron monitor (NM) is computed and extended to different atmospheric depths from sea level to 500 g/cm² (∼5.7 km altitude) and is presented as lookup tables and a full parametrization. The yield function was validated using the cosmic ray spectra directly measured in space by the AMS‐02 experiment during the period May 2011 through May 2017 and confronted with count rates of all NM64‐type NMs being in operation during this period. Using this approach, stability of all the selected NMs was analyzed for the period 2011–2017. Most of NMs appear very stable and suitable for studies of long‐term solar modulation of cosmic rays. However, some NMs suffer from instabilities like trends, apparent jumps, or strong seasonal waves in the count rates

Heliospheric modulation of cosmic rays during the neutron monitor era:calibration using PAMELA data for 2006–2010

Abstract A new reconstruction of the heliospheric modulation potential for galactic cosmic rays is presented for the neutron monitor era, since 1951. The new reconstruction is based on an updated methodology in comparison to previous reconstructions: (1) the use of the new-generation neutron monitor yield function; (2) the use of the new model of the local interstellar spectrum, employing in particular direct data from the distant missions; and (3) the calibration of the neutron monitor responses to direct measurements of the cosmic ray spectrum performed by the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) spaceborne spectrometer over 47 time intervals during 2006–2010. The reconstruction is based on data from six standard NM64-type neutron monitors (Apatity, Inuvik, Kergulen, Moscow, Newark, and Oulu) since 1965 and two International Geophysical Year-type ground-based detectors (Climax and Mount Washington) for 1951–1964. The new reconstruction, along with the estimated uncertainties is tabulated in the paper. The presented series forms a benchmark record of the cosmic ray variability (in the energy range between 1 and 30 GeV) for the last 60 years and can be used in long-term studies in the fields of solar, heliospheric, and solar-terrestrial physics.Plain Language Summary Solar magnetic activity modulates the flux of energetic galactic cosmic rays bombarding Earth. A newly revised method for a quantitative assessment of the cosmic ray modulation parameter using data from the ground-based network of neutron monitors is presented. The new reconstruction employs a calibration of the ground-based data to the spaceborne direct measurements during a period of 2006–2010. The presented reconstructions form a benchmark record of the cosmic ray variability (in the energy range between 1 and 30 GeV) for the last 60 years and can be used in long-term studies in the field of solar, heliospheric, and solar-terrestrial physics