V. A. Oleneva

What determines the magnitude of forbush decreases

Abstract Cosmic ray Forbush effects (FEs) have been studied for more than 60 years, but even today this phenomenon has no universally accepted definition, which leads to misunderstanding among researchers. There are no complete and convincing answers to the following questions: What are the distinctive features of FEs? What determines the magnitude of FEs and their variety? How are FEs associated with disturbances of the interplanetary medium, coronal mass ejections and high speed flows of the solar wind? What relation exists between FEs and geomagnetic storms? In order to answer these and other questions a data base of transient effects in cosmic rays and in the interplanetary medium is constructed and is being upgraded continuously. It combines variations in cosmic ray densities and anisotropies obtained from data of the world-wide neutron monitor (NM) network, characteristics of solar wind disturbances, solar and geomagnetic data. This data base presently contains more than 1400 events, observed during 1978–1996. The preliminary analysis allowed us to get the main characteristics of FEs, to determine what distinguishes this phenomenon among other cosmic ray variations, and propose a definition of the Forbush effect. In addition, an interplanetary disturbance parameter was derived, which is most closely associated with the magnitude of a FE. Using this large observational database, a relation between FEs and geomagnetic activity was analyzed statistically as a dependence of the FE magnitude on the maximum Kp-index measured during the associated magnetic storm.

Behavior of the cosmic-ray vector anisotropy before interplanetary shocks

The behavior of the galactic cosmic ray density and vector anisotropy during geomagnetic storms with sudden storm commencements has been investigated based on the global survey data. It is shown that the average anisotropy significantly increases directly before a sudden storm commencement; the larger the subsequent Forbush decrease, the larger the increase in anisotropy. The averaged values of the CR anisotropy and density start to be affected by the shock wave approximately 5 h before its arrival. Changes in the anisotropy direction (especially for west Forbush sources) can be observed much earlier.

Forbush effects with a sudden and gradual onset

For a comprehensive study of the Forbush effects and their relation to solar and geomagnetic activity, a database of transient phenomena in cosmic rays and the interplanetary medium has been created, which is continuously updated with data on new events. Based on these data, we study the dependence of the Forbush effects on various internal and external parameters, as well as select different groups of events. In this paper, we consider recurrent (caused by high-speed solar wind streams from coronal holes) and sporadic (associated with coronal mass ejections) events. We investigate groups of events with a sudden and gradual onset. We show that the resulting dependencies of the Forbush effects (on the parameters of interplanetary disturbances, geomagnetic activity indices, etc.) are substantially different for the above-mentioned groups. Most likely, these differences are caused by different sources of solar wind disturbances.

Main Properties of Forbush Effects Related to High-Speed Streams from Coronal Holes

The IZMIRAN database of Forbush effects and interplanetary disturbances was used to study features of the action of high-speed solar wind streams from coronal holes on cosmic rays. Three hundred and fifty Forbush effects created by coronal holes without other actions were distinguished. The mean values and distributions have been found for different characteristics of events from this group and compared with all Forbush effects and Forbush effects caused by coronal ejections. Despite the great differences in high-speed streams from coronal holes, this group turned out to be more compact and uniform as compared to events related to coronal ejections. Regression dependences and correlation relations between different parameters of events for the studied groups have been obtained. It has been shown that Forbush effects caused by coronal ejections depend considerably more strongly on the characteristics of interplanetary disturbances as compared to Forbush effects related to coronal holes. This suggests a significant difference between the modulation mechanisms of Forbush effects of different types and corroborates earlier conclusions based on indirect data.

Relationship between Forbush effect parameters and the heliolongitude of solar sources

All significant events in galactic cosmic rays for the last 55 years have been collected in a Forbush effect database created at the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation (hereinafter, IZMIRAN) based on data from the global network of neutron monitors. The solar sources of ∼800 of these events have been identified. These events were divided into five groups with respect to the heliolongitudes of the associated X-ray solar flares, and typical behavior of their characteristics such as cosmic ray density and anisotropy, was studied independently for each group. The Forbush effect characteristics, which are the most dependent on the source heliolongitude, have been identified.

Global Survey Method for the World Network of Neutron Monitors

One of the variants of the global survey method developed and used for many years at the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation of the Russian Academy of Sciences is described. Data from the world network of neutron monitors for every hour from July 1957 to the present has been processed by this method. A consistent continuous series of hourly characteristics of variation of the density and vector anisotropy of cosmic rays with a rigidity of 10 GV is obtained. A database of Forbush decreases in galactic cosmic rays caused by large-scale disturbances of the interplanetary medium for more than half a century has been created based on this series. The capabilities of the database make it possible to perform a correlation analysis of various parameters of the space environment (characteristics of the Sun, solar wind, and interplanetary magnetic field) with the parameters of cosmic rays and to study their interrelationships in the solar–terrestrial space. The features of reception coefficients for different stations are considered, which allows the transition from variations according to ground measurements to variations of primary cosmic rays. The advantages and disadvantages of this variant of the global survey method and the opportunities for its development and improvement are assessed. The developed method makes it possible to minimize the problems of the network of neutron monitors and to make significant use of its advantages.

Behavior of the cosmic ray density during the initial phase of the Forbush effect

Variations in the cosmic ray density during the initial phase of the Forbush effect during the first hours after the arrival of the interplanetary shock wave have been studied with the use of data on variations in the cosmic ray density with a rigidity of 10 GV obtained by the global survey method by the world network of neutron monitors in 1957‒2012. It is found that behavior of this parameter after the arrival of the shock wave demonstrates high variability. A small (~1/5 of total number), though distinct, group of Forbush effects, in which the density of the cosmic ray increases (not decreases) after the arrival of the shock wave, is defined. As a whole, the initial variation in cosmic ray density is correlated with the Forbush effect magnitude and the strength of the associated geomagnetic disturbance.

Modeling the behavior of the cosmic ray density in magnetic clouds

Our study focuses on the behavior of the density of cosmic ray particles at 10 GV rigidity in a magnetic cloud at Earth. It is shown that it can be mostly described by a simple parabolic dependence over distance from the centre of the cloud, when measured in gyroradii. The majority of magnetic clouds modulate cosmic rays, decreasing their density. However, there is a group of events (about 1/5 part of the total sample) during which the cosmic ray density increases within the magnetic cloud. The factors that contribute to the model description are considered, and estimates of their influence are carried out and discussed.

Density variations of galactic cosmic rays in magnetic clouds

Galactic cosmic ray behavior features in events when magnetic clouds that have been observed in interplanetary disturbances near the Earth are investigated. It is shown in most cases (but not in all) that the cosmic ray density behavior in the magnetic cloud near the Earth can be described by a simple parabolic distance dependence measured in gyroradii. Most magnetic clouds modulate cosmic rays by reducing their density, but there is a group of events (about 1/5) in which the cosmic ray density in the magnetic cloud increases. The extremum (minimum or maximum) of the cosmic ray density is often located closer to the cloud center rather than its edges. A number of factors that contribute to the model description are considered, and the contributions are estimated.

Anomalous Forbush effects from sources far from Sun center

The Forbush effects associated with far western and eastern powerful sources on the Sun that occurred on the background of unsettled and moderate interplanetary and geomagnetic disturbances have been studied by data from neutron monitor networks and relevant measurements of the solar wind parameters. These Forbush effects may be referred to a special sub-class of events, with the characteristics like the event in July 2005, and incorporated by the common conditions: absence of a significant disturbance in the Earth vicinity; absence of a strong geomagnetic storm; slow decrease of cosmic ray intensity during the main phase of the Forbush effect. General features and separate properties in behavior of density and anisotropy of 10 GV cosmic rays for this subclass are investigated.