Upadhyay et al. Earth, Planets and Space (2020) 72:35 https://doi.org/10.1186/s40623-020-01157-7 FULL PAPER Open Access Occurrence characteristics of electromagnetic ion cyclotron waves at sub-auroral Antarctic station Maitri during solar cycle 24 Aditi Upadhyay1*, Bharati Kakad1, Amar Kakad1, Yoshiharu Omura2 and Ashwini Kumar Sinha1 Abstract We present a statistical study of electromagnetic ion cyclotron (EMIC) waves observed at Antarctic station (geo- graphic 70.7◦ S , 11.8◦ E , L = 5 ) on quiet and disturbed days during 2011–2017. The data span a fairly good period of both ascending and descending phases of the solar cycle 24, which has witnessed extremely low activity. We noted EMIC wave occurrence by examining wave power in diferent frequency ranges in the spectrogram. EMIC wave occurrence during ascending and descending phases of solar cycle 24, its local time, seasonal dependence and dura- tions have been examined. There are total 2367 days for which data are available. Overall, EMIC waves are observed for 3166.5 h (≈ 5.57% of total duration) which has contributions from 1263 days. We fnd a signifcantly higher EMIC wave occurrence during the descending phase (≈ 6.83%) as compared to the ascending phase (≈ 4.08%) of the solar cycle, which implies nearly a twofold increase in EMIC wave occurrence. This feature is attributed to the higher solar wind dynamic pressure during descending phase of solar activity. There is no evident diference in the percentage occurrence of EMIC waves on magnetically disturbed and quiet days. On ground, EMIC waves show marginally higher occurrence during winter as compared to summer. This seasonal tendency is attributed to lower electron densities and conductivities in the ionosphere, which can afect the propagation of EMIC waves through ionospheric ducts. In local time, the probability distribution function of EMIC wave occurrence shows enhancement during 11.7–20.7 LT (i.e., afternoon–dusk sector). Daily durations of EMIC waves are in the range of 5–1015 min and it is noted that the longer duration (240–1015 min) events are prevalent on quiet days and are mostly seen during the descending phase of solar cycle. Keywords: ULF wave, EMIC occurrence, Solar cycle, Induction coil magnetometer Introduction stop band frequency for the EMIC waves. Tese waves Te electromagnetic ion cyclotron (EMIC) waves are are transverse plasma waves, and can be observed in the generated by the cyclotron-resonant instability of aniso- Earth’s magnetosphere by various satellites, like AMPTE/ tropic medium-energy ring current ions (10–100 keV) CCE (Anderson et al. 1990), Viking (Erlandson et al. 1990), Cluster (Pickett et al. 2010), THEMIS (Usanova (Cornwall 1965; Kennel and Petschek 1966). Te ions+ responsible for the wave generation can be proton H , et al. 2012), Polar (Carson et al. 2013), GOES (Park et al. helium He+, and oxygen O+ and they determine the 2016), Arase (Shoji et al. 2018), and Van Allen Probe (Fu et al. 2018). Te Cluster and THEMIS observations have reported the EMIC-triggered coherent emissions hav- *Correspondence: [email protected] ing rising tone frequencies (Pickett et al. 2010; Naka- 1 Indian Institute of Geomagnetism, New Panvel, Navi Mumbai, India mura et al. 2015). Tese rising tone emissions consist of Full list of author information is available at the end of the article © The Author(s) 2020. This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creat iveco mmons .org/licen ses/by/4.0/. Upadhyay et al. Earth, Planets and Space (2020) 72:35 Page 2 of 16 subpacket structures (Shoji and Omura 2013; Nakamura of wave injection region and the location of magnetom- et al. 2015), whose dynamics is governed by the nonlinear eter. A recent study using ground observation has dem- wave growth. Te EMIC waves can undergo anomalous onstrated that the amplitude–frequency dependence of cyclotron resonance with relativistic electrons (Lor- the EMIC wave subpacket structures proposed by theo- entzen et al. 2000; Summers and Torne 2003) and are retical studies (Omura et al. 2010; Shoji and Omura 2013) known to play a substantial role in the loss mechanism of remain unaltered during their propagation to the ground MeV electrons in the radiation belt (Bortnik et al. 2006; (Kakad et al. 2018). Tus, the ground observations of Usanova et al. 2014; Shprits et al. 2017). Tey also inter- EMIC waves can be useful to get some information about act with ring current ions and cause them to precipitate their source region in the magnetosphere. into the mid-latitude ionosphere, resulting in the isolated Mursula et al. (1991) studied the long-term behav- proton auroras (Nomura et al. 2012; Søraas et al. 2013; ior of Pc1 pulsations and reported that annual EMIC Ozaki et al. 2018). A statistical study of EMIC waves con- wave occurrence varies with solar cycle. Later, Mursula ducted by Meredith et al. (2003) suggests that the energy et al. (1996) have studied the occurrence of Pc1 waves range for electrons that can resonate with the wave spans using simultaneous satellite (AMPTE/CCE) and ground a wide range and may go up to 50 MeV. Generally, MeV observations for two periods of contrasting solar activ- electrons can penetrate deeper in the Earth’s atmosphere ity levels (i.e., low and high). Tey found that the total and afect the neutral dynamics at lower altitudes (Tsuru- occurrence rate of Pc1 waves in space decreased by a fac- tani et al. 2016). Terefore, studies of the EMIC waves are tor 1.6 from low to high sunspot times, and it is in good important for better understanding of their overall role in agreement with a simultaneous decrease in occurrence the particle dynamics in the Earth’s magnetosphere–ion- observed on the ground. Kangas et al. (1998) have pro- osphere–atmosphere coupled system. vided a review of the occurrence of Pc1 waves and its link Te EMIC waves generated in the equatorial magne- with the geomagnetic storm, solar cycle, Sun’s and Earth’s tospheric region (Pickett et al. 2010; Omura et al. 2010), rotation and orbital motion. A recent study carried out propagate towards the mid-latitude ionosphere, follow- using EMIC observations at Halley station in Antarctica, ing geomagnetic feld lines. When these waves reach the indicates that the frequency of EMIC wave is also infu- ionosphere, horizontal structuring in the F-region acts as enced by the solar activity (Lessard et al. 2015). Tere is a duct. Some of EMIC wave packets can be trapped into observational evidence that supports the solar activity such ionospheric duct, which causes the EMIC wave to infuence on the EMIC wave occurrence. However, the propagate horizontally to lower or higher latitudes. Tis mechanism responsible for such dependence is not well way, the wave information can propagate thousands of understood. Te solar activity infuence on EMIC waves kilometers (Johnson and Cheng 1999; Kim et al. 2011). might be linked through the variations in interplanetary Some of EMIC wave energy can leak to the atmosphere solar wind parameters like density, velocity, and dynamic from the duct wall while propagating, and their signa- pressure. A study by Guglielmi et al. (2005) indicates tures can be manifested as geomagnetic pulsations in that the occurrence of Pc1 waves at Sodankyla, Finland = the Pc1–Pc2 frequency range (0.1–5 Hz). Te EMIC ( L 5.1 ) is controlled by the higher solar wind density wave propagation to the ground can be much complex and low values of solar wind velocity. Also, the role of because its characteristics such as polarization, wave solar wind dynamic pressure in exciting EMIC waves has power and wave normal angle change during their propa- been established through satellite observations (Usanova gation (Allen et al. 2015; Kim and Johnson 2016). Satel- et al. 2012). In this context, to get a better scenario, com- lite observations, e.g., CRESS (Loto’aniu et al. 2005) and prehensive statistical studies of EMIC waves from the GOES (Clausen et al. 2011), showed that these waves are ground as well as satellite observations are needed. left-handed polarized in the generation region. Using In this paper, we present a statistical study of occur- magnetic feld data from an array of ◦fve ground-based◦ rence and characteristics of EMIC waves using the 7 magnetometers situated between ( 62 and 87 S ), Kim years (2011–2017) of ground-based observations from et al. (2010) suggested that after the entry of EMIC waves sub-auroral station Maitri. Te observation period covers in the ionospheric duct, they tend to move poleward 40 months in the ascending phase and 44 months in the and their sense of polarization changes from left hand descending phase of the solar cycle 24. Te study of cur- to right hand. A study by Kakad et al. (2018) reported rent solar cycle would be interesting because of extremely that EMIC waves observed at Maitri are mainly associ- low solar activity during its progression. Such a decrease ated with right-handed elliptical polarization.