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A Global View of Storms and Substorms MEETING REPORT A global view of storms and substorms Downloaded from https://academic.oup.com/astrogeo/article-abstract/60/3/3.13/5497907 by Lancaster University user on 11 June 2019 Jasmine Kaur Sandhu, Maria- Theresia Walach, Hayley Allison and Clare Watt report on the RAS meeting The Global Response of the Terrestrial Magnetosphere during Storms and Substorms. xplorations into the geomagnetic field are thought to have begun around Ethe beginning of the 11th century in China, later extending to Asia and Europe (Mitchell 1932) and leading to the discovery of the global nature of the magnetic field reported by Gilbert (1600). Since then, we have identified the highly variable and dynamic nature of the global geomagnetic field in near-Earth space, specifically mag- netic storms and substorms (e.g. Graham 1724, Birkeland 1901). Ground-based data led to the proposal of key features of our space environment: the ring current (Stoer- mer 1910), the plasma-filled magnetosphere (Gold 1959, Chapman & Ferraro 1931) and the solar wind (Parker 1958). These were all 1 A schematic illustrating the large-scale structure of the magnetosphere and the key regions. The inset later confirmed with the advent of the space shows the structure of a trapped energetic particle population in the inner magnetosphere, known as age, which also brought the discovery of the Van Allen radiation belts. (Kivelson & Bagenal 2007) new features, such as our highly dynamic radiation belts (Van Allen 1958). electrical current in the inner magneto- Substorms In 1961, Jim Dungey proposed a new sphere produced by the net westward drift In contrast to storms, substorms have theory on how our magnetosphere interacts of ions, where increases in the energy and timescales of a few hours. Substorms are with the solar wind (Dungey 1961) to explain number of ions results in increases in the characterized by a storage and rapid release the observed dependence of geomagnetic ring current intensity. During magnetic of energy by the magnetotail, and are activity on solar activity (e.g. Sabine 1852). storms, large enhancements in the ring associated with clear auroral signatures Dungey (1961) proposed the idea of an “open current intensity lead to a weakening of the and intensifications (e.g. Baker et al. 1981). magnetosphere”, where coupling between local magnetic field and are Strong coupling with the IMF the geomagnetic field and the interplanetary also associated with intense “On average, storms leads to a loading of highly magnetic field (IMF) leads to a large-scale, radiation belt activity (Gonza- last several days, with stretched open field lines to global, circulatory flow of magnetic field lez 1994, Baker et al. 2004). On the storm main phase the magnetotail. Substorm lines and plasma within the magneto- average, storms last several lasting around a day” onset is accompanied by sphere. This theory was later confirmed days, with the storm main rapid magnetic reconnection observationally (Fairfield & Cahill 1966, phase lasting around one day. Geomagnetic in the magnetotail, which promptly closes Fairfield 1967), and we now know that the storms are highly variable in terms of their large amounts of flux. The stretched field coupling between the solar wind and the intensity, duration and impacts on the inner lines contract to a more dipolar configura- magnetosphere creates a dynamical and magnetosphere. A key impact of geomag- tion, a considerable amount of energy is highly variable system, and is a key driver in netic storms is concurrent radiation belt released and highly energetic plasma is generating storms and substorms. Figure 1 activity in the inner magnetosphere. The transported earthwards on the nightside. shows a simplified schematic of our current radiation belts have a complex relationship Intense field-aligned currents drive ener- understanding of the magneto sphere and with geomagnetic storms and also exhibit getic electron precipitation and result in the the key regions of interest. a high degree of variability, shaped by the intensification, broadening and expansion Storms are characterized by rapid multitude of energization and loss processes of the auroral oval (see figure 2). enhancements in the ring current, an (e.g. Elkington 2013, Reeves et al. 2003). Although substorms are known to take A&G • June 2019 • Vol. 60 • aandg.org 3.13 MEETING REPORT place when the magnetosphere is effec- 2 The northern auroral tively coupled with the IMF, they are highly oval viewed by the variable and unpredictable. Furthermore, IMAGE spacecraft. because substorms can transport energetic The thickening and plasma to the inner magnetosphere, it has contraction of the been proposed that substorms are impor- auroral oval follow- tant in generating geomagnetic storms ing substorm onset is (Daglis et al. 1999a,b). But the role of sub- apparent. (SWRI) storms in storm generation has also been debated by others (Kamide 1979, 1992); the in a given index can differ greatly in other understanding variability in storms. Fur- coupling between substorms and storms observed characteristics. For example, two thermore, Heather McCreadie (University remains unclear. substorms may be associated with the same of Warwick) demonstrated how the vari- Downloaded from https://academic.oup.com/astrogeo/article-abstract/60/3/3.13/5497907 by Lancaster University user on 11 June 2019 The induced currents and magnetic field AL index minimum, but the duration of the ations in the Dst index during any storm perturbations mean that the existence of a bay, the auroral signatures of the substorm, can be characterized using an autonomous storm or substorm can be identified from and the impacts on the inner magneto- curve-fitting technique. McCreadie’s magnetic field observations at ground level. sphere can vary significantly between the approach in quantifying the Dst index Magnetic field data can be condensed into two events. Using a single index at a single variations during storm suggests impor- simple indices that exhibit relatively clear time to represent the globally averaged tant applications in being able to explore signatures during storms and substorms, magnetic field response cannot capture variability in the Dst response from storm and are highly useful in identifying and the large degree of variability in other to storm. exploring events (see box “The importance aspects of the magnetosphere (e.g. solar of geomagnetic indices” for further details). wind driving and plasma properties). How do we define storms and substorms? Studies of the magnetosphere, specifi- Conversely, for events that seem to have The discussion on how events character- cally the storm and substorm phenomena, the same level of solar wind coupling and ized by the same level of magnetic indices are strongly motivated by the implica- internal conditions, the response of the led to discussion of what information on tions of these processes for our everyday magnetic indices and the magnetospheric physical processes the indices provide. lives. Although very intense storms and system is wide ranging, in terms of the Is this the information that we need? If substorms are rare, when they do occur occurrence and intensity of storms and not, what is required? And how does the the ramifications for society are significant substorms, as well as steady magneto- information we have and what we need (Lanzerotti 2013). The events drive large spheric convection. influence how we define these events? ground-induced currents (GICs), which can A fundamental question is: As discussed above, disrupt ground power networks. Changes why do we observe so much “The promising storms and substorms to the ionosphere can lead to radio-wave variability? And what physi- potential for exhibit a high degree of absorption and thus communication black- cal magnetospheric processes predicting super- variability associated with outs. Additionally, geomagnetic storms drive it? Sarah Bentley (Uni- storms was explored” different features of the event can have devastating effects on satellites: versity of Reading) stressed (e.g. solar wind coupling, extreme intensifications of the radiation that it is important to review how we con- magnetic responses, inner magnetospheric belt are highly damaging to satellites, and sider the magnetospheric system. Taking response etc). In order to identify how we the increased altitude of the ionospheric a deterministic approach, there must be a define the events, we have to choose what boundary during storms increases satellite process in the magnetosphere or a charac- is the defining feature of interest. The drag for low-orbiting satellites. We need to teristic of the solar wind coupling that we choice of important feature of a storm or understand the physical processes associ- haven’t identified. Alternatively, is it just the substorm is highly dependent on the “end ated with storms and substorms, determine chaotic nature of the system that introduces user”, as pointed out by Chapman in the why they occur and identify how they this variability (e.g. Prabin Devi 2013)? This discussion. For example, Richard Horne affect our magnetosphere. There are many highlights the question of whether we can (British Antarctic Survey) discussed how outstanding questions and much remains predict when and how these events occur storms driven by coronal mass ejections to be investigated. This was our motivation and identify the source of their variations. (CMEs) are associated with a much larger for holding the RAS Specialist Discus- In contrast to the seemingly unpre- ring current enhancement and magneto- sion Meeting “The Global Response of the dictable qualities of the magnetosphere, spheric compressions than storms driven Terrestrial Magnetosphere during Storms work presented by Sandra C Chapman by corotating interaction regions (CIRs); and Substorms” (held at Burlington House (University of Warwick) demonstrated this means that the former can generate on Friday 8 February 2019), where work on clear reproducible trends in the distribu- intense GICs. In contrast, the CME-driven understanding storms and substorms was tion tails of magnetic indices (including storms are associated with a significant presented and discussed.
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