Many previous studies have shown that before the beginning of a Forbush Decrease (FD) of the cosmic ray intensity, a precursor signal can be observed. All these surveys were focused on FDs that are associated with a sudden storm com- mencement (SSC). In this work we demonstrate that precursors could also be observed in events without a SSC that is determined by an abrupt increase of the interplanetary magnetic field. The type of precursory signals and their diversity among the events are the main purpose of this study. We try to figure out similarities and differences on the signals and the associated events from both categories in the last fifty years, from 1969 to 2019, using the same selection criteria of the under investigation FDs. Simultaneously the orientation of the upcoming solar disturbances in comparison to the way they configure the increase of the interplanetary magnetic field and create these signals are discussed.

ABSTRACT The behaviour of the solar wind (SW) proton temperature and velocity and their relationship during Forbush decreases (FDs) associated with various types of solar source – coronal mass ejections (CMEs) and coronal holes (CHs) – have been studied. Analysis of cosmic ray variations, SW temperature, velocity, density, plasma beta, and magnetic field (from 1965–2019) is carried out using three databases: the OMNI database, Variations of Cosmic Rays database (IZMIRAN) and Forbush Effects & Interplanetary Disturbances database (IZMIRAN). Comparison of the observed SW temperature (T) and velocity (V) for the undisturbed SW allows us to derive a formula for the expected SW temperature (Texp, the temperature given by a T–V formula, if V is the observed SW speed). The results reveal a power-law T–V dependence with a steeper slope for low speeds (V < 425 km s−1, exponent = 3.29 ± 0.02) and flatter slope for high speeds (V > 425 km s−1, exponent = 2.25 ± 0.02). A study of changes in the T–V dependence over the last five solar cycles finds that this dependence varies with solar activity. The calculated temperature index KT = T/Texp can be used as an indicator of interplanetary and solar sources of FDs. It usually has abnormally large values in interaction regions of different-speed SW streams and abnormally low values inside magnetic clouds (MCs). The results obtained help us to identify the different kinds of interplanetary disturbance: interplanetary CMEs, sheaths, MCs, corotating interaction regions, high-speed streams from CHs, and mixed events.

An analysis is performed of a reduction in the intensity of cosmic rays during the minimum of solar cycle 24 in April–May, 2019. It is assumed that the observed slow decline and recovery of the cosmic ray flux consisted of a series of small Forbush decreases.

Analysis of the variation in galactic cosmic rays and changes in various characteristics of Forbush effects associated with the influence of interacting solar wind disturbances on the Earth (disturbances with a time interval between registration of less than 50 h) is presented based on the database of Forbush effects and interplanetary disturbances developed at the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation. Cases of the pair interaction of high-speed streams from coronal holes and coronal mass ejections in 1995–2019 are considered, and the behavior of the solar wind parameters, the interplanetary magnetic field, and the cosmic ray variations for two types of interacting solar wind disturbances (the interaction of successive coronal mass ejections and the interaction of a coronal mass ejection with a high-speed coronal hole stream) are analyzed. It is found that the average times of the onset of the Forbush effect minimum and the record of the maximum solar wind velocity and the absolute value of the interplanetary magnetic field decrease for the first of a pair of interacting events, i.e., the second events of the pair do not allow the first to develop fully. It is also found that the presence of interaction enriches the second event at the expense of the resources of the first. This increases its geomagnetic efficiency and the degree of cosmic ray modulation in comparison with isolated events.

A statistical study of interplanetary disturbances with different positions of the interplanetary magnetic field maximum that generate Forbush decreases is performed. It is shown that the most effective interplanetary disturbances are those with the position of the field maximum 6–15 h after the start of the event.

Cosmic ray cut-off rigidity tables and maps over the world concerning the epochs 2010, 2015 and the current one 2020 have been constructed. These maps display the effective cut-off rigidity in every five degrees in latitude and in longitude at the altitude of 20 km above the surface of the international reference ellipsoid. The values of the geomagnetic cut-off rigidity were calculated in every 5 degrees in latitude and in every 15 degrees in longitude applying the well-known method of particle trajectory calculations resulted from the theory of the particle motion in the Earth's magnetic field. The applied software employed the 12th Generation of the International Geomagnetic Reference Field (IGRF 12) and trajectories were calculated at 0.01 GV intervals in order to determine the vertical cut-off rigidity for each location. Beyond the use of the calculated cut-off rigidity values as a basic reference of charged particle access to different geographical locations during quiet and/or more intense geomagnetic periods, these results can be used for a long-term forecasting of the geomagnetic conditions variations. (C) 2021 COSPAR. Published by Elsevier B.V. All rights reserved.

The study of space-weather effects and more specifically Forbush decreases of the cosmic-ray intensity depends on space and ground measurements. Very often Forbush decreases and geomagnetic storms are accompanied by pre-increases and/or pre-decreases manifested in cosmic-ray behavior, known as precursory signs. These cosmic-ray intensity variations do not coincide with the shock arrival but begin well before (up to 24 hours) the onset of the main event. In this study a group of large Forbush decreases with amplitude ≥ 4% was examined for precursors. According to the helio-longitude of the solar source, the events were separated into three categories: western ( $21^{\circ} \leq $ helio-longitude ≤ 60∘), eastern ( $-60^{\circ} \leq $ helio-longitude $\leq -21^{\circ}$ ), and central ( $-20^{\circ} \leq $ helio-longitude $\leq 20^{\circ}$ ). The selected events cover 1967 – 2017. The analysis of the Forbush decreases and the plotting of the asymptotic longitudinal cosmic-ray distribution diagrams were based on the “Global Survey Method” and the “Ring of Stations” method, respectively. Data on solar flares, solar-wind speed, interplanetary magnetic field, and geomagnetic indices (Kp and Dst) were also used. The results show the clear signs of precursors in a significant number of events.

Статистическое исследование межпланетных возмущений, создающих Форбуш-понижения, с различным положением максимума межпланетного магнитного поля. Наиболее эффективны межпланетные возмущения с положением максимума поля через 6–15 ч после начала события.

In this paper, we study the effect of magnetic clouds on variations in the cosmic ray density recorded by neutron monitors. The statistical patterns and characteristic features of such events are distinguished from data on 252 Forbush effects caused by interplanetary disturbances containing magnetic clouds. The behavior of the main parameters of solar wind, cosmic rays, and geomagnetic activity during the passage of magnetic clouds past the Earth, as well as the characteristic features of the internal structure of magnetic clouds, are discussed. It is shown that the cosmic ray variations are closely related to the maximal parameters of the solar wind and the interplanetary magnetic field inside the magnetic clouds. It is established that the maximal velocity inside the magnetic cloud is most often registered at the beginning of the event, while the maximal value of the interplanetary magnetic field is registered both at the beginning and in the middle of the event by the time distribution of the solar wind maximal parameters. It is also found that there is a sufficiently close correlation of the variations of cosmic ray density in a magnetic cloud with its size expressed in the hyroradii.

The spectrum of cosmic ray variations for the range of rigidities most sensitive to ground-based neutron monitors is determined experimentally according to direct measurements by the AMS-02 magnetic spectrometer.

In this article the influence of the surrounding snow cover on the neutron monitors count rate of the World Wide Web was estimated using the method of reference stations. The applied technique also makes it possible to estimate the thickness of the snow cover at the observation point, which was done for more than two dozen stations. A comparison of the results of data correction for snow is carried out for case of automatic correction, based on the developed algorithm, and for manual one, with an error estimate.

For over 60 years, neutron monitors have been the main standard and high precision detectors for measuring cosmic rays with energy from 400 MeV to hundreds GeV. In order to obtain sufficiently complete information about the distribution of cosmic rays outside the magnetosphere, it is necessary to have a network of detectors spaced evenly enough around the globe. The ring of stations method is one of the most useful methods for studying the properties of the angular distribution of cosmic rays without expressing the cosmic ray intensity in terms of spherical harmonics. The method allows one to get the hourly longitude distribution of the cosmic ray intensity without modeling. The main objective of this work is to expand the use of the ring of stations method, as it is a convenient and useful method of studying cosmic ray variation. Using the ring of stations method, it is possible to study specific angular distributions of cosmic ray variation that are described poorly by the sum of the first spherical harmonics. The ring of stations method is primarily used to study Forbush decreases. Detailed descriptions of Forbush decrease investigation by the ring of stations method are presented in this study. The application of the method to the study of the precursors of Forbush decreases and cosmic rays behavior inside the solar wind disturbances is shown.

В работе проведен анализ специфического понижения на фоне минимума в 24 цикле солнечной активности в конце апреля -мае 2019. Предполагается, что наблюдаемое медленное понижение и восстановление потока космического излучения представляет собой серию небольших Форбуш понижений.