Global Ionospheric Response to a Periodic Sequence of HSS/CIR Events During the 2007–2008 Solar Minimum

Abstract

The thermosphere-ionosphere system constitutes the upper-most layer of the Earth's atmosphere and is characterized by high variability and a strong dependence on external drivers. The two main categories of forcings are solar radiation and geomagnetic activity at high latitudes. Under normal conditions, solar radiation is the dominant source of energy and momentum, generating the standard structure, composition, and circulation of the thermosphere-ionosphere (Fuller-Rowell, 2014;Rishbeth & Garriott, 1969). During geomagnetic storms, the energy input at high latitudes can be significant, or even dominant. During such events, there is an increase of particle precipitation and Joule heating (Codrescu et al., 1995) at high latitudes, together with penetration electric fields at mid-and low-latitudes. The resulting plasma drift and associated ion-drag will heat up the neutral atmosphere, inducing pressure gradients that can alter the global neutral circulation from a dayside-nightside pattern of winds to an equatorward pattern. Additionally, since the Joule heating maximum occurs around 100-120 km altitude, the heavier molecular species dominant here (molecular oxygen and molecular nitrogen) will be displaced toward higher altitudes, while the atomic oxygen density will decrease due to equatorward winds (Fuller-Rowell et al., 1994). The resulting modified chemical composition will in turn lead to a higher plasma loss rate through charge exchange and recombination, as well as a decrease in plasma production due to the loss of atomic oxygen.The occurrence of geomagnetic storms is linked to solar activity and transient events in the solar wind. During the descending and minimum phases of a solar cycle, the near-Earth solar wind is dominated by highspeed streams (HSSs) and their associated stream interaction regions (SIRs), characterized by increased plasma density and magnetic field intensity. These structures are formed when the fast-solar wind emanating from a solar coronal hole (CH) "collides" with the slower solar wind ahead of it. Coronal holes lasting for several solar rotations will generate recurring SIRs, also known as corotating interaction regions (CIRs). The ionospheric impact of geomagnetic storms and of HSS/CIR sequences is an active area of research, with multiple recent studies employing a variety of data types and approaches: TEC (

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“…This is probably due to the recurrent interaction with the high-speed streams and the corotating interaction regions observed in 2008 and which are characterized by a more "ordered" structure of the interplanetary magnetic field. In a recent study, Munteanu et al (2019) showed that two systems of CIRs and HSS whose origin stayed stable on the Sun (same group of coronal holes) for several months in 2008, impacted the Earth, including for the time period investigated in this study (see, also, Negrea et al, 2021). While in 2007 the Earth was also impacted by HSS and CIRs their origin was more dynamic and their properties were therefore much more variable compared to 2008.…”