Multi‐Instrument Observations of the Effects of a Solar Wind Pressure Pulse on the High Latitude Ionosphere: A Detailed Case Study of a Geomagnetic Sudden Impulse

Abstract

The terrestrial magnetosphere-ionosphere system is strongly coupled to the solar wind, which is highly variable on short timescales. Near-step changes in solar wind dynamic pressure, known as pressure pulses which when positive, can rapidly excite and restructure the electrodynamics within the magnetosphere-ionosphere system. When the effects of a rapid change in solar wind dynamic pressure are communicated into the magnetosphere, this is known as a geomagnetic sudden commencement (SC, Araki, 1994), indicated by a corresponding step change in the ring current index SYM-H (e.g., Hori et al., 2015;Takeuchi et al., 2002). SCs can be further subdivided into two categories: sudden impulses (SIs) and sudden storm commencements (SSCs). Although the definition of these three terms (SC, SI and SSC) is not always consistent in the literature, in this paper an SSC is defined as an SC which is rapidly (within 12 hr (Taylor et al., 1994)) followed by a geomagnetic storm, otherwise it is an SI.The majority of pressure pulses observed in the solar wind are caused by dramatic phenomena such as coronal mass ejections (CMEs) or corotating interaction regions (CIRS, e.g., Zuo et al., 2015). CMEs are violent eruptions of plasma from the Sun, resulting from magnetic reconnection (e.g., Hundhausen, 1995) and can cause SSCs (Taylor et al., 1994). CIRs, also called fast-slow stream interaction regions, have steep pressure gradients resulting from fast flowing plasma catching up with more slowly rotating plasma on more tightly wound magnetic field lines (e.g., Hundhausen, 1995). CIRs can cause Storm Gradual Commencements (SGCs, Taylor et al., 1994), which are geomagnetic storms not related to an SC. If there is no subsequent storm, a positive SI (+SI) may result from an increase in pressure, and a negative SI (−SI) may result from a decrease in pressure. An SI is generally considered to be driven by a pressure change of at least 3 nPa in under 10 min (e.g., Coco et al., 2011), although it can also be identified with SYM-H (e.g., Hori et al., 2015). Hori et al. (2015) define the SC signature in SYM-H as an increase of more than 5 nT with a gradient greater than about 15 nT per 10 s. Given the magnitude of the pressure change, SIs can have a remarkably dramatic impact on the Earth's magnetosphere, and the electrodynamics within.

Extreme Value Analysis of Ground Magnetometer Observations at Valentia Observatory, Ireland

“…This may relate to Valentia being in the region where substorm onset is most often occurs. However, $$\frac{dB}{dt}$$ changes may also be driven by step changes relating to compressional wave propagation through the magnetosphere, for example, as a result of SCs (e.g., Araki, 1994; Fogg et al., 2023; Hori et al., 2015); this may be why more $$\frac{dB}{dt}$$ extremes are seen at dayside MLTs than for B.…”

“…Any increase with a gradient of at least 3 nT min −1 is recorded as an SC. This event list was commissioned by the International Association of Geomagnetism and Aeronomy, and is part of its International Service of Geomagnetic Indices, and has been used by authors to characterise the propagation of SC effects through the magnetosphere (e.g., Fogg, 2021;Fogg et al, 2023;Gillies et al, 2012).…”

“…7 of 16 be driven by step changes relating to compressional wave propagation through the magnetosphere, for example, as a result of SCs (e.g., Araki, 1994;Fogg et al, 2023;Hori et al, 2015); this may be why more 𝐴𝐴 𝑑𝑑𝑑𝑑 𝑑𝑑𝑑𝑑 extremes are seen at dayside MLTs than for B.…”

“…The “height” phase is then the period 10 min after the end of the step change. SCs effects span the magnetosphere‐ionosphere system, and can include ULF waves (e.g., Oliveira et al., 2020) and enhancements in ionospheric convection/aurorae/FACs (e.g., Fogg et al., 2023). SCs can be further divided into events rapidly followed by a geomagnetic storm, known as SSCs, and those not followed by a storm, known as sudden impulses (SIs).…”

Why Are Some Solar Wind Pressure Pulses Followed by Geomagnetic Storms?

“…Araki (1994) suggests that the Alfvén and compressional components are observed simultaneously in magnetometer data, although Araki et al (1997);Fujita et al (2003) suggests that the Alfvén propagation follow the compressional mode. However, Fogg et al (2023) presented some evidence that the Alfvén propagation reaches the high latitude ionosphere slightly before compressional components are seen at equatorial magnetometer stations.…”