Generic profile of a long-lived corotating interaction region and associated recurrent Forbush decrease

¹ University of Zagreb, Faculty of Geodesy, Hvar Observatory, Zagreb, Croatia
e-mail: mdumbovic@geof.hr, bvrsnak@geof.hr
² University of Graz, Institute of Physics, Graz, Austria
³ Christian-Albrechts University in Kiel, Department of Extraterrestrial Physics, Kiel, Germany

Received: 23 March 2021
Accepted: 13 January 2022

Abstract

Context. Corotating interaction regions (CIRs), formed by the interaction of slow solar wind and fast streams that originate from coronal holes (CHs), produce recurrent Forbush decreases, which are short-term depressions in the galactic cosmic ray (GCR) flux.

Aims. Our aim is to prepare a reliable set of CIR measurements to be used as a textbook for modeling efforts. For that purpose, we observe and analyse a long-lived CIR, originating from a single CH, recurring in 27 consecutive Carrington rotations 2057–2083 in the time period from June 2007–May 2009.

Methods. We studied the in situ measurements of this long-lived CIR as well as the corresponding depression in the cosmic ray (CR) count observed by SOHO/EPHIN throughout different rotations. We performed a statistical analysis, as well as the superposed epoch analysis, using relative values of the key parameters: the total magnetic field strength, B, the magnetic field fluctuations, dBrms, plasma flow speed, v, plasma density, n, plasma temperature, T, and the SOHO/EPHIN F-detector particle count, and CR count.

Results. We find that the mirrored CR count-time profile is correlated with that of the flow speed, ranging from moderate to strong correlation, depending on the rotation. In addition, we find that the CR count dip amplitude is correlated to the peak in the magnetic field and flow speed of the CIR. These results are in agreement with previous statistical studies. Finally, using the superposed epoch analysis, we obtain a generic CIR example, which reflects the in situ properties of a typical CIR well.

Conclusions. Our results are better explained based on the combined convection-diffusion approach of the CIR-related GCR modulation. Furthermore, qualitatively, our results do not differ from those based on different CHs samples. This indicates that the change of the physical properties of the recurring CIR from one rotation to another is not qualitatively different from the change of the physical properties of CIRs originating from different CHs. Finally, the obtained generic CIR example, analyzed on the basis of superposed epoch analysis, can be used as a reference for testing future models.