Interplanetary origins of moderate (−100 nT < Dst ≤ −50 nT) geomagnetic storms during solar cycle 23 (1996–2008)

Abstract

[1] The interplanetary causes of 213 moderate-intensity (À100 nT < peak Dst ≤ À50 nT) geomagnetic storms that occurred in solar cycle 23 (1996-2008) are identified.Interplanetary drivers such as corotating interaction regions (CIRs), pure high-speed streams (HSSs), interplanetary coronal mass ejections (ICMEs) of two types [those with magnetic clouds (MCs) and those without (nonmagnetic cloud or ICME_nc)], sheaths (compressed and/or draped sheath fields), as well as their combined occurrence were identified as causes of the storms. The annual rate of occurrence of moderate storms had two peaks, one near solar maximum and the other in the descending phase, around 3 years later. The highest rate of moderate storm occurrence was found in the declining phase (25 storms year À1 ). The lowest occurrence rate was 5.7 storms year À1 and occurred at solar minimum. All moderate-intensity storms were associated with southward interplanetary magnetic fields, indicating that magnetic reconnection was the main mechanism for solar wind energy transfer to the magnetosphere. Most of these storms were associated with CIRs and pure HSSs (47.9%), followed by MCs and noncloud ICMEs (20.6%), pure sheath fields (10.8%), and sheath and ICME combined occurrence (9.9%). In terms of solar cycle dependence, CIRs and HSSs are the dominant drivers in the declining phase and at solar minimum. CIRs and HSSs combined have about the same level of importance as ICMEs plus their sheaths in the rising and maximum solar cycle phases. Thus, CIRs and HSSs are the main driver of moderate storms throughout a solar cycle but with variable contributions from ICMEs, their shocks (sheaths), and combined occurrence within the solar cycle. This result is significantly different than that for intense (Dst ≤ À100 nT) and superintense (Dst ≤ À250 nT) magnetic storms shown in previous studies. For superintense geomagnetic storms, 100% of the events were due to ICME events, while for intense storms, ICMEs, sheaths, and their combination caused almost 80% of the storms. CIRs caused only 13% of the intense storms. The typical interplanetary electric field (Ey) criteria for moderate magnetic storms were identified. It was found that~80.1% of the storms follow the criterion of Ey ≥ 2 mV m À1 for intervals longer than 2 h. It is concluded that southward directed interplanetary magnetic fields within CIRs/HSSs may be the main energy source for long-term averaged geomagnetic activity on Earth.

Ionospheric forecasts for the European region for space weather applications

“…In contrast, HSSs or CIRs seem to be not effectively captured by SWIF's present formulations. Such interplanetary structures usually drive the development of storm events of moderate intensity (e.g., Richardson & Cane 2012;Echer et al 2013) that are still able to produce significant ionospheric disturbances as they are described here, but also reported by others (e.g., Pokhotelov et al 2010;Tsurutani et al 2014). Such events are often ''missed'' by SWIF's ADA.…”

“…Indeed, the results obtained by Echer et al (2008) (Richardson 2013), and they were all correctly forecasted by SWIF. In contrast, Echer et al (2013) analyzed the interplanetary causes of moderate-intensity geomagnetic storms that occurred in solar cycle 23 (1996-2008) to demonstrate that most of these storms were associated with CIRs and pure HSSs (47.9%), followed by MCs and noncloud ICMEs (20.6%), pure sheath fields (10.8%), and sheath and ICME combined occurrence (9.9%). The HSSs as the dominant driver of moderate storms were also recognized by Richardson & Cane (2012).…”

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

“…The primary link between a geomagnetic storm and a CME is the out of the ecliptic component (Bz) of the interplanetary magnetic field (Gonzalez et al, 1994;Zhang et al 2007;Gopalswamy 2008;Echer et al 2008aEcher et al , 2008bEcher et al , 2013Cid et al 2012). Echer et al (2008a) conclusively showed that for all 90 major (Dst < −100 nT) storms that occurred during cycle 23, it was the Bz component that was responsible for the storms (some people have thought that it was possible that the IMF By component was also important).…”

Annual fractions of high‐speed streams from principal component analysis of local geomagnetic activity

“…Consequently, CMEs and CIRs/HSSs drive geomagnetic activity differently. Most intense (Dst < −100 nT) geomagnetic storms are produced by CMEs, while CIR-driven storms are typically limited to smaller intensities [Zhang et al, 2007;Richardson et al, 2006], and about 50% of moderate geomagnetic storms (−100 nT < Dst < −50 nT) are driven by CIRs/HSSs [Echer et al, 2013]. Due to their longer duration and the embedded…”