Ann. Geophys., 38, 557–574, 2020 https://doi.org/10.5194/angeo-38-557-2020 © Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. Cosmic noise absorption signature of particle precipitation during interplanetary coronal mass ejection sheaths and ejecta Emilia Kilpua1, Liisa Juusola2, Maxime Grandin1, Antti Kero3, Stepan Dubyagin2, Noora Partamies4,5, Adnane Osmane1, Harriet George1, Milla Kalliokoski1, Tero Raita3, Timo Asikainen6, and Minna Palmroth1,2 1Department of Physics, University of Helsinki, Helsinki, Finland 2Finnish Meteorological Institute, Helsinki, Finland 3Sodankylä Geophysical Observatory, University of Oulu, Sodankylä, Finland 4Department of Arctic Geophysics, The University Centre in Svalbard, Longyearbyen, Norway 5Birkeland Centre for Space Science, Bergen, Norway 6ReSoLVE Center of Excellence, Space Climate Research Unit, University of Oulu, Oulu, Finland Correspondence: Emilia Kilpua (emilia.kilpua@helsinki.fi) Received: 30 October 2019 – Discussion started: 3 December 2019 Revised: 20 February 2020 – Accepted: 26 February 2020 – Published: 21 April 2020 Abstract. We study here energetic-electron (E > 30 keV) tance of substorms and magnetospheric ultra-low-frequency precipitation using cosmic noise absorption (CNA) during (ULF) waves for enhanced CNA. the sheath and ejecta structures of 61 interplanetary coro- nal mass ejections (ICMEs) observed in the near-Earth so- lar wind between 1997 and 2012. The data come from the Finnish riometer (relative ionospheric opacity meter) chain 1 Introduction from stations extending from auroral (IVA, 65.2◦ N geomag- netic latitude; MLAT) to subauroral (JYV, 59.0◦ N MLAT) Precipitation of high-energy (E > 30 keV) electrons from the latitudes. We find that sheaths and ejecta lead frequently to inner magnetosphere into the Earth’s ionosphere is an inter- enhanced CNA (> 0:5 dB) both at auroral and subauroral esting fundamental plasma process that can have significant latitudes, although the CNA magnitudes stay relatively low consequences on the atmospheric chemistry (e.g. Rodger (medians around 1 dB). Due to their longer duration, ejecta et al., 2010; Andersson et al., 2014; Seppälä et al., 2015; typically lead to more sustained enhanced CNA periods (on Turunen et al., 2016; Newnham et al., 2018). This process average 6–7 h), but the sheaths and ejecta were found to be is also of particular interest for climate models (e.g. Matthes equally effective in inducing enhanced CNA when relative- et al., 2017), but its details and external factors governing occurrence frequency and CNA magnitude were considered. the efficiency of precipitation are currently not well under- Only at the lowest-MLAT station, JYV, ejecta were more stood. The electron precipitation can result from substorm effective in causing enhanced CNA. Some clear trends of injections from the nightside plasma sheet or from the scat- magnetic local time (MLT) and differences between the tering of trapped electrons in the Van Allen radiation belts ejecta and sheaths were found. The occurrence frequency surrounding the Earth. Precipitation can occur also during and magnitude of CNA activity was lowest close to mid- pulsating aurora and exhibit modulation at similar timescales night, while it peaked for the sheaths in the morning and as the auroral emission (e.g. Grandin et al., 2017b). afternoon/evening sectors and for the ejecta in the morning Substorm injections occur predominantly from before and noon sectors. These differences may reflect differences midnight to midnight (e.g. Aminaei et al., 2006; Beharrell in typical MLT distributions of wave modes that precipitate et al., 2015) from geostationary orbit to about 9 Earth radii substorm-injected and trapped radiation belt electrons during (e.g. Spanswick et al., 2010), and electrons then drift around the sheaths and ejecta. Our study also emphasizes the impor- the Earth along the morning side (e.g. Kavanagh et al., 2007; Grandin et al., 2017a). The precipitation signatures also vary Published by Copernicus Publications on behalf of the European Geosciences Union. 558 E. Kilpua et al.: Cosmic noise absorption signature of particle precipitation depending on the substorm phase. The growth phase is char- and dawn sectors (e.g. Malaspina et al., 2016) with the acterized by a relatively weak but intensifying precipita- strongest amplitude recorded at dusk (Kim and Shprits, tion region which extends in longitude (in particular west- 2019). They are believed to result from nonlinear growth of ward) and moves to lower latitudes (e.g. Hargreaves et al., chorus waves when they transfer into the plasmasphere (e.g. 1975; Ranta et al., 1981). Short duration spikes are iden- Bortnik et al., 2008; Hartley et al., 2018). The EMIC waves tified during the onset associated with rapid particle accel- in turn occur predominantly in the duskside of the magneto- eration into the ionosphere (e.g. Hargreaves et al., 2001), sphere close to the plasmapause, as they are generated by while in the growth phase precipitation proceeds to lower anisotropic ring current proton distributions (Zhang et al., latitudes (e.g. Pytte et al., 1976). For both the freshly in- 2016; Saikin et al., 2016). jected substorm electrons and the electrons trapped in the Several statistical studies have highlighted differences in Van Allen belts, the precipitation is considered to be primar- geomagnetic and radiation belt responses and in precipita- ily facilitated through pitch-angle scattering following inter- tion signatures during different large-scale solar-wind drivers action with very-low-frequency (VLF) waves – in particu- (e.g. Huttunen et al., 2002; Borovsky and Denton, 2006; lar, due to cyclotron resonance with the lower-band chorus Potapov, 2013; Kilpua et al., 2015; Asikainen and Ruopsa, whistler waves. The precipitation via whistler waves is con- 2016; Kilpua et al., 2017b; Shen et al., 2017; Grandin et al., sidered most efficient for a few tens of kiloelectronvolts of 2017a; Benacquista et al., 2018), namely interplanetary coro- energy electrons, while they mostly accelerate higher-energy nal mass ejections (ICMEs; e.g. Kilpua et al., 2017a) and (> 500 keV) electrons (e.g. Bortnik and Thorne, 2007; Lam slow–fast stream interaction regions (SIRs; e.g. Richardson, et al., 2010). A high-amplitude chorus can, however, cause 2018). ICMEs are generally known to drive the strongest ge- microburst precipitation of megaelectronvolt trapped elec- omagnetic storms, while SIRs and the following fast streams trons (e.g. Thorne et al., 2005; Osmane et al., 2016; Douma cause more moderate, but sustained, geomagnetic activity et al., 2019). Ultra-low-frequency (ULF) Pc5 waves also (e.g. Borovsky and Denton, 2006; Grandin et al., 2019). At play an important role. The Coroniti and Kennel(1970) the- the geostationary orbit (L =∼ 6:6), SIR-/fast-stream-driven ory predicts that ULF waves may enhance energetic-electron storms more efficiently enhance megaelectronvolt radiation precipitation by periodically increasing the growth rate of belt electrons than ICME-driven storms that cause deeper whistler waves. Observational evidence supporting this the- and longer depletions. Shen et al.(2017), however, showed ory was provided by Motoba et al.(2013) during a conjunc- that at the heart of the outer belt (L =∼ 4–5) and low L shells tion between the Cluster satellites and a ground-based riome- ICMEs enhance more effectively both megaelectronvolt and ter (relative-ionospheric-opacity-meter) station. lower-energy electron fluxes. A comparison of energetic- Another wave mode that is invoked to precipitate electrons particle precipitation during ICMEs and SIRs shows that the through gyroresonance are electromagnetic-ion-cyclotron former tend to produce a higher riometer precipitation sig- (EMIC) waves. They have been shown to be able to pre- nal during the first hours of the geomagnetic storm (Long- cipitate both megaelectronvolt (e.g. Usanova et al., 2014) den et al., 2008), whereas the latter may lead to energetic- and sub-megaelectronvolt electrons (e.g. Blum et al., 2019; electron precipitation for up to 4 d following the storm on- Hendry et al., 2019), although the efficiency of sub- set (Grandin et al., 2017a). A case study of an ICME event megaelectronvolt electron precipitation is still debated. Both by Longden et al.(2007) found that most of the precipitation chorus and EMIC waves occur outside the high-density plas- observed during the related geomagnetic storm was produced masphere, whose outer boundary, the plasmapause, varies by substorm-injected particles on the nightside. On the day- significantly in location with geomagnetic activity and also side, precipitation events occurred in good correlation with with the magnetic local time (MLT) (O’Brien and Moldwin, solar-wind dynamic-pressure pulses. Kavanagh et al.(2012) 2003). Inside the plasmasphere, precipitation occurs primar- performed an extensive epoch analysis of electron precipi- ily due to plasmaspheric hiss (Li et al., 2015). Hiss waves tation during high-speed streams using riometer data. The can scatter electrons over a wide range of energies, but the study showed that precipitation enhances at the arrival of the scattering times increase significantly with increasing elec- SIR, peaks about half a day after its arrival and typically tron energy and decreasing hiss wave power (e.g. Selesnick continues for several days during the following high-speed et al., 2003; Summers et al., 2008; Kavanagh et al., 2018). stream, in agreement