# The Sun and Space Weather
## 1 Introduction  
The Sun is an ordinary star from an astronomical point of view, but it is the vital source of energy that supports life on Earth. Due to its proximity to Earth, we can observe and understand Sun's variability on various timescales, from sub-second to centuries. Most of the variability is caused by solar magnetism, thought to operate in the outer shell of the Sun. Observationally, the variability manifests as the appearance and dispersal of bipolar magnetic regions (e.g., sunspot regions) and unipolar regions (coronal hole regions). The solar dynamo is sustained by the exchange between toroidal flux, represented by sunspots, and the poloidal flux, represented by polar field strength. Solar eruptions are part of the life cycle of active regions (ARs), in that photospheric motions store energy in AR magnetic fields, and the stored energy is explosively released. Coronal mass ejections (CMEs) and solar flares are two manifestations of the energy release from closed magnetic regions. Coronal holes contain field lines open to the space, and the solar plasma can readily escape into space as high-speed streams (HSS). Thus, the two types of magnetic topology on the Sun result in two types of mass emission: CMEs and HSS. As CMEs and HSS propagate into the corona and interplanetary (IP) space, they interact with the ambient solar wind forming shock sheaths ahead of CMEs and stream interaction regions (SIRs) at the leading edge of HSS. A solar flare represents a transient increase in electromagnetic emission at all wavelengths from radio to gamma rays originating from localized closed magnetic field regions on the Sun. The flare emissions are caused by nonthermal electrons (radio bursts and hard X-ray bursts) and protons (impulsive gamma rays) energized in the magnetic reconnection region in the active region corona. The accelerated particles precipitating to the chromosphere cause chromospheric evaporation, and the heated flare plasma emits insoft X-rays. Magnetic reconnection results simultaneously in a post-eruption arcade (PEA) and a magnetic flux rope (FR). Thermal emission from the PEA in soft X-rays is used as an indicator of flare strength. The FR is accelerated outwards as long as the reconnection proceeds, followed by interaction with the ambient solar wind. If a FR is fast enough, it drives a fast-mode magnetohydrodynamic (MHD) shock that can accelerate particles to GeV energies. Such particles are known as solar energetic particles (SEPs). Flares may also contribute to SEPs. SIRs also accelerate particles to lower energies, typically beyond 1 au. All these phenomena--flare electromagnetic emission, CMEs, SEPs, CIRs, and HSS--can contribute to adverse space weather in the heliosphere. Space weather effects can be felt in Earth's magnetosphere, ionosphere, atmosphere, and surface when Earth is in the path of these disturbances. Therefore, forecasting the properties of these disturbances and their arrival time at Earth is important for space weather prediction. There has been significant progress in understanding how solar eruptions result in various space weather consequences over the past two decades, as reviewed recently [1; 2; 3; 4; 5; 6; 7].  
The purpose of this paper is to summarize the observational results on solar disturbances and highlight some key results relevant to space weather. The paper is organized as follows: Section 2 provides an update on the basic properties of CMEs. Section 3 highlights the shock-driving capability of CMEs. Section 4 focuses on the CME origin of SEP events. Section 5 summarizes the CME link to geomagnetic storms via the southward magnetic field component and the speed of CMEs. Section 6 highlights those CME properties that seem to be essential for the acceleration of SEPs. Section 7 discusses the spacecraft anomalies that follow SEP events and geomagnetic storms. Section 8 summarizes the solar cycle (SC) variation of the CME rate and how it affects space weather events. Section 9 describes the extreme events of SC 23. Concluding remarks are given in Section 10.