engleski

Forbush decreases associated to Stealth CME

Interplanetary coronal mass ejections (ICMEs) are structures in the solar wind that are the counterparts of coronal mass ejections (CMEs) at the Sun. It is commonly believed that enhanced magnetic elds in interplanetary shocks and solar ejecta as well as the increased solar wind speed are the cause of Forbush decreases (FDs) that are intensity decreases of galactic cosmic rays (GCRs) intensities. FDs have been extensively investigated by means of neutron monitor measurements. However, using neutron monitor measurements one has to keep in mind that its measurements not only re ect the GCR intensity variation in interplanetary space but also the variation of the geomagnetic eld as well as the conditions in the Earth atmosphere. Albeit the very high counting statistic allowing to determine intensity variation of less than a percent the interplanetary signal may be masked by e.g. the daily variation or the change of the geomagnetic eld. In contrast single detector measurements aboard a spacecraft in interplanetary space do not su er from such variations. It has been shown previously that the counting rate statistic allows the determination of FDs with less than a few per mille. However, in contrast to neutron monitors such detectors are sensitive to a few tenth of MeV/n ions and therefore the investigation of FDs becomes impossible if the event is accompanied by accelerated particles. Here we will present observations by both Electron Proton Helium INstruments (EPHIN) aboard SOHO and Chandra as well as neutron monitor measurement that were associated to Stealth CMEs. We found for each of these events an associated FD. The amplitude in space varied between 0.6% and 3.5%. Only one of the events was identi ed in the neutron monitor data unambiguously.

coronal mass ejection ; Forbush decreases

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