Short-term variability of the Sun-Earth system: an overview of progress made during the CAWSES-II period

Nat Gopalswamy,Bruce Tsurutani,Yihua Yan

doi:10.1186/s40645-015-0043-8

Abstract

This paper presents an overview of results obtained during the CAWSES II period on the short term variability of the Sun and how it affects the near Earth space environment. CAWSES II was planned to examine the behavior of the solar terrestrial system as the solar activity climbed to its maximum phase in solar cycle 24. After a deep minimum following cycle 23, the Sun climbed to a very weak maximum in terms of the sunspot number in cycle 24 (MiniMax24), so many of the results presented here refer to this weak activity in comparison with cycle 23. The short term variability that has immediate consequence to Earth and geospace manifests as solar eruptions from closed field regions and high speed streams from coronal holes. Both electromagnetic (flares) and mass emissions (coronal mass ejections, CMEs) are involved in solar eruptions, while coronal holes result in high speed streams that collide with slow wind forming the so called corotating interaction regions (CIRs). Fast CMEs affect Earth via leading shocks accelerating energetic particles and creating large geomagnetic storms. CIRs and their trailing high speed streams (HSSs), on the other hand, are responsible for recurrent small geomagnetic storms and extended (days) of auroral zone activity, respectively. The latter lead to the acceleration of relativistic magnetospheric killer electrons. One of the major consequences of the weak solar activity is the altered physical state of the heliosphere that has serious implications for the shock-driving and storm causing properties of CMEs. Finally, a discussion is presented on extreme space weather events prompted by the 2012 July 23 super storm event that occurred on the backside of the Sun. Many of these studies were enabled by the simultaneous availability of remote-sensing and in situ observations from multiple vantage points with respect to the Sun Earth line.

Highlights

The second phase of the Climate and Weather of the Sun-Earth System (CAWSES-II) was organized into task groups (TGs)
Given the observation that coronal mass ejection (CME)-driven shocks form at a heliocentric distance of 1.5 to 5 Rs (Tsurutani et al 2003a; Gopalswamy et al 2013b), these findings indicate that observations close to the Sun are most important for large solar energetic particles (SEPs) events
The paucity of Ground-level enhancement (GLE) events in cycle 24 cannot be explained by the 22% reduction in the number of fast and wide CMEs originating in the traditional GLE longitudes (W20 to W90)

Summary

Introduction

The second phase of the Climate and Weather of the Sun-Earth System (CAWSES-II) was organized into task groups (TGs). The fast component is of particular interest because it can compress the upstream slow solar wind forming a corotating interaction region (CIR). Perhaps even more important than the CIR is the high-speed stream proper It carries large nonlinear Alfvén waves, whose southward components cause reconnection at the magnetopause resulting in continuous sporadic plasmasheet injections into the nightside magnetosphere. These injections of anisotropic approximately 10- to 100-keV electrons cause the growth of an electromagnetic wave called ‘chorus’ and the chorus interacts with approximately 100-keV electrons accelerating them to MeV energies (Tsurutani et al 2006, 2010; Thorne et al 2013). The variability manifested as solar flares, CMEs, SEPs, and high-speed solar wind streams directly affects space weather on short time scales. All these phenomena are coupled near the Sun but throughout the inner heliosphere, including geospace and Earth’s ionosphere and atmosphere where the impact can be felt (Verkhoglyadova et al 2014; Mannucci et al 2014; Tsurutani et al 2014)

Methods

Findings

Conclusions