Current State of Reduced Solar Activity: Intense Space Weather Events in the Inner Heliosphere

Abstract

We present a study of 21 geomagnetic storms, occurred during 2011-2017 in association with the propagation of coronal mass ejections (CMEs). These storms are selected with the minimum storm disturbance index (SYM-H) intensity of -100 nT or less and are distributed from the maximum to the minimum of the weak solar cycle 24. We identify and investigate these storm-driving CMEs (halo and partial halo CMEs) by combining EUV and white-light images in the near-Sun region, interplanetary scintillation images in between the Sun and the Earth, and in-situ measurements at the near-Earth orbit. These CMEs cover a wide range of initial speeds, ~180 to 2680 km/s. For about 50% of the CMEs, the fast initial speed at the near-Sun region does not correlate with the final speed at the near-Earth orbit. The storm indexes range between -100 and -233 nT and they are associated with minimum Bz values in the range of -12 to -38 nT. The Forbush decrease (FD) levels associated with these storms vary in the range of about -2% to -10%. A comparison of travel times of CMEs to 1 AU with the observed initial/final speeds and estimated initial speed suggests that a large fraction of fast initial speeds could possibly be due to the sudden expansion of the CME into a relatively low pressure interplanetary medium. Most of the geomagnetic storms (i.e., 19 storms) have been caused by the strong intrinsic magnetic field of the CME and only 2 storms are produced by the sheath region between the arrival times of interplanetary shock and CME. The geomagnetic storm index is compared with the possible reconnection electric field component, BzVICME. It suggests an empirical relationship for the likely lower level of storm index, SYM-H = -70 -0.003·BzVICME. (nT), in which Bz and VICME are respectively given in units of nT and km/s.

Peculiar Solar Sources and Geospace Disturbances on 20–26 August 2018

“…For example, Grechnev et al (2014) argued that a very strong magnetic field (up to 56 nT) in the magnetic cloud that caused the superstorm on 20 November 2003, was due to an unusually weak expansion of the disconnected spheromak in an enhanced-density environment constituted by the tails of the preceding ICMEs. According to Gopalswamy et al (2014) and Manoharan et al (2018), the weak Solar Cycle 24 is characterized by a reduced total pressure in the heliosphere that usually leads to the relatively large expansion of ICMEs. However, the degree of expansion is positively correlated with the radial propagation speed of CMEs/ICMEs (Gopalswamy et al, 2014).…”

Improving the Medium-Term Forecasting of Space Weather: A Big Picture Review From a Solar Observer's Perspective

“…While solar wind reached its lowest values ever measured (McComas et al, 2013), the cosmic ray background reached records values raising serious concerns on the viability of human deep space exploration during weak cycles (Schwadron et al, 2018). Although Earth experienced weaker geomagnetic storms than in SC23 (Manoharan et al, 2018) and only two Ground Level Enhancement (GLE) particle events, the CME rate was largely the same (Lamy et al, 2017). A likely reason for the absence of strong SWx events may be the lower rate of fast and/or wide CMEs in SC24 (Gopalswamy et al, 2020).…”

Regular Solar Radio Imaging at Arecibo: Space Weather Perspective of Evolution of Active Regions

“…Flares and CMEs are energetic phenomena, involving bursts of electromagnetic radiation and dynamical eruption of plasma and magnetic field from active regions, caused by the magnetic reconnection process (Priest and Forbes 2002). In the context of space weather, intense flares and their associated CMEs, particularly those directed towards the Earth, are important, because they drive the large-scale energetic space-weather storms in the interplanetary space and give rise to hazardous effects at the near-Earth environment (e.g., Xie et al 2006;Kumar et al 2011;Manoharan et al 2018). The essential need of spaceweather forecasting is to monitor the evolution of an active region, as it rotates close to the central meridian of the Sun, and assess its likelihood of releasing an Earth-directed solar flare and/or CME (e.g., Manoharan and Kundu 2005).…”