Abstract

In May 2019 there was a long and sloping decreasing of cosmic ray’s intensity (up to ~4%), which was observed on neutron monitors. Despite this was a small decreasing compared to quasi-eleven-period variation, it stands out well in 24th cycle of solar activity. According to LASCO/SOHO and STEREO-A data from spectrometer in different UHF bands and from coronograph, there was a series of CMEs which affected on modulation of cosmic rays by creating a series of Forbush decrea - sing, which didn’t restore. This series was connected to two active regions on sun and began on April 30 from “reversed halo” CME. This CME didn’t reach the earth, but led to significant additional modulation of cosmic rays, mostly on east side. Later there was a series of smaller CMEs on May 1-6, which also didn’t reach the earth, but were gradually approaching to Earth. Recent CMEs on 8-9 and 12-13 created a normal Forbush decreasing. In May 2019, cosmic rays shown again, that they can collect information about distant objects of geliosphere and transmit it to Earth. The ground-level detectors sometimes can observe an interaction of interplanetary distur- bances, which didn‘t reach the earth. East CMEs are especially effective, because they closing magnetic field lines beyond the orbit of earth and can interfere the restoring of cosmic ray’s intensity.

Highlights

  • Since July 2018, the calmest period in the minimum solar activity of the 24th solar cycle has been observed

  • According to LASCO/SOHO and STEREO-A data from spectrometer in different UHF bands and from coronograph, there was a series of CMEs which affected on modulation of cosmic rays by creating a series of Forbush decreasing, which didn’t restore

  • There is some decrease in all indicators, which is replaced by another growth, which indicates the Forbush decrease

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Summary

Introduction

Since July 2018, the calmest period in the minimum solar activity of the 24th solar cycle has been observed. Small (up to 4%), but prolonged decrease in the intensity of cosmic rays measured by neutron monitors (~10 days) This decrease is considered a relatively small change in intensity in General, but rather unusual and interesting for minimal solar activity. The total duration of the event is about 25 days Such a large time interval indicates that there were significant changes in the characteristics of the interplanetary environment almost on the scale of the semi-heliosphere. Such events are rare: a similar event was observed in September 1979 (Belov et al 1983), but with a much greater decrease in the intensity of cosmic rays (more than 10%). The first two were observed at the maximum of solar activity, the last one - at the end of the maximum- at the beginning of the minimum

Solar and geomagnetic conditions (activity) during the reviewingperiod
Method of selecting Forbush effects for the May decrease
Results of the analysis of the zero harmonic of cosmic rays
Distribution of solar wind density for CMEs in April-May
The decreasing on the other planets
Findings
Discussion and conclusion
Full Text
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