AbstractOne of the most intense geomagnetic storms of recent times occurred on 10–11 May 2024. With a peak negative excursion of Sym‐H below −500 nT, this storm is the second largest of the space era. Solar wind energy transferred through radiation and mass coupling affected the entire Geospace. Our study revealed that the dayside magnetopause was compressed below the geostationary orbit (6.6 RE) for continuously ∼6 hr due to strong Solar Wind Dynamic Pressure (SWDP). Tremendous compression pushed the bow‐shock also to below the geostationary orbit for a few minutes. Magnetohydrodynamic models suggest that the magnetopause location could be as low as 3.3RE. We show that a unique combination of high SWDP (≥15 nPa) with an intense eastward interplanetary electric field (IEFY ≥ 2.5 mV/m) within a super‐dense Interplanetary Coronal Mass Ejection lasted for 409 min–is the key factor that led to the strong ring current at much closer to the Earth causing such an intense storm. Severe electrodynamic disturbances led to a strong positive ionospheric storm with more than 100% increase in dayside ionospheric Total Electron Content (TEC), affecting GPS positioning/navigation. Further, an HF radio blackout was found to occur in the 2–12 MHz frequency band due to strong D‐ and E‐region ionization resulting from a solar flare prior to this storm.

Geomagnetic indices have been used as a proxy for studying electromagnetic ion cyclotron (EMIC) wave occurrences under different geomagnetic conditions. However, the drivers of EMIC waves are different during non‐storm, storm time and during individual storm phases. Using ∼7 years of data from the twin Van Allen Probes, we demonstrate that the occurrence probability of EMIC waves are not well captured by a specific geomagnetic activity index alone, but is rather well manifested by considering individual storm phases. We show EMIC wave occurrence statistics during different storm phases (pre‐onset, main and recovery) for geomagnetic activity indices Sym‐H, AE, and Kp and solar wind dynamic pressure P dyn , illustrating that the occurrence rates vary significantly during different storm phases even for a given geomagnetic index. We also utilize this large database to show EMIC wave occurrence distribution, and how various wave and plasma parameters behave under different geomagnetic conditions. EMIC waves occur 2.9 times more often during geomagnetic storms than during non‐storm times. The majority (72%) of storm time EMIC waves occur during the recovery phase due to long recovering time, while the highest occurrence rates are in the pre‐onset phase, followed by main and recovery phases. EMIC waves in the main phase have occurrence peaks in the dusk to pre‐midnight sectors while recovery phase events spread to more Magnetic Local Time (MLT) sectors with peaks in the morning sector. Wave amplitudes are found to be evenly distributed across different MLT sectors during all geomagnetic conditions.

Several studies have shown the importance of electromagnetic ion cyclotron (EMIC) waves to the pitch angle scattering of energetic particles in the radiation belt, especially relativistic electrons, thus contributing to their net loss from the outer radiation belt to the upper atmosphere. The huge amount of data collected thus far provides us with the opportunity to use a deep learning technique referred to as the Bag-of-Features (BoF). When applied to images of magnetic field spectrograms in the frequency range of EMIC waves, the BoF allows us to distinguish, in a semi-automated way, several patterns in these spectrograms that can be relevant to describe physical aspects of EMIC waves. Each spectrogram image provided as an input to the BoF corresponds to the windowed Fourier transform of a ∼40 minutes to 1 hour interval of Van Allen Probes' high time-resolution vector magnetic field observations. Our data set spans the 2012 September 8 to 2016 December 31 period and is at geocentric distances larger than 3 Earth radii. A total of 66,204 spectrogram images are acquired in this interval, and about 45% of them, i.e., 30,190 images, are visually inspected to validate the BoF technique. The BoF's performance in identifying spectrograms with likely EMIC wave signatures is comparable to the visual inspection method, with the enormous advantage that the BoF technique greatly expedites the analysis by accomplishing the task in just a few minutes.

Electromagnetic ion cyclotron (EMIC) waves can act as a loss process for both ring current ions and radiation belt electrons, and the spatial and temporal characteristics of these waves are important for quantifying their effects on energetic particles. Here we utilize observations from multiple spacecraft to constrain the azimuthal and radial dimensions as well as the duration of an EMIC wave event occurring on the nightside of the inner magnetosphere on 7 July 2013. These combined observations reveal waves limited to a narrow radial extent but persisting similar to 10+ hr and spanning similar to 12 hr in local time. The solar wind conditions, geomagnetic activity, and plasma environment are also examined to better understand the conditions under which persistent nightside EMIC waves can occur. Relativistic electron phase space density profiles during this event reveal local minima concurrent with the wave activity, consistent with EMIC-driven scattering and loss of radiation belt electrons. Plain Language Summary Various oscillating electric and magnetic fields, or waves, can interact with high-energy particles in near-Earth space and cause a change in the particles' energy and/or direction of motion. Where and when these waves occur can have a significant impact on how they interact with particles. Here we combine measurements from multiple spacecraft around the Earth to study one specific wave mode. While these waves are often thought to be localized and of short durations, we observe an event on 7 July 2013 where waves persist for over 10 hr and span the entire nightside of the Earth. We explore the dynamics of the Earth's magnetic field, in response to activity on the Sun, to better understand what causes these widespread, long-lasting waves. Changes in the energetic particle environment around the Earth are also presented to examine the effects of these waves. Events like these have the potential to cause significant effects in the particle populations around the Earth.

AbstractElectromagnetic ion cyclotron (EMIC) waves tend to occur during geomagnetic storms and solar wind pressure pulses. However, they have also been regularly observed even in the absence of these two drivers. These non‐storm time and non‐pressure pulse EMIC events are very well associated with individual nightside injections (Remya et al., 2018, https://doi.org/10.1029/2018JA025354). Nevertheless, not all substorm injections elicit wave activity. We examine the EMIC events excited during two substorm injections on 4 September 2015 and 1 October 2015. We find that injections that are associated with EMIC waves are also associated with enhanced ionospheric convection. The convective signatures occur at local times similar to those of the observed wave activity.

1Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 2Instituto Nacional de Pesquisas Espacias, Sao Jose dos Campos, SP, Brazil 3Faculty of Science, Kyoto University, Sakyo-ku, Kyoto 6068502, Japan 4Academia Sinica, Taipei, Taiwan 5Indian Institute of Geomagnetism, Navi Mumbai, India 6National Science Foundation, Wash. D.C. 7 Institute for Space Earth Environmental Research, Nagoya University, Nagoya, Japan

We present Van Allen Probe observations of electromagnetic ion cyclotron (EMIC) waves triggered solely due to individual substorm-injected ions in the absence of storms or compressions of the magnetosphere during 9 August 2015. The time at which the injected ions are observed directly corresponds to the onset of EMIC waves at the location of Van Allen Probe A (L = 5.5 and 18:06 magnetic local time). The injection was also seen at geosynchronous orbit by the Geostationary Operational Environmental Satellite and Los Alamos National Laboratory spacecraft, and the westward(eastward) drift of ions(electrons) was monitored by Los Alamos National Laboratory spacecraft at different local times. The azimuthal location of the injection was determined by tracing the injection signatures backward in time to their origin assuming a dipolar magnetic field of Earth. The center of this injection location was determined to be close to similar to 20:00 magnetic local time. Geostationary Operational Environmental Satellite and ground magnetometer responses confirm substorm onset at approximately the same local time. The observed EMIC wave onsets at Van Allen Probe were also associated with a magnetic field decrease. The arrival of anisotropic ions along with the decrease in the magnetic field favors the growth of the EMIC wave instability based on linear theory analysis.

Coherency (C) and ellipticity (c) of electromagnetic ion cyclotron (EMIC) waves are studied using Cassini data in the Earth's dayside low-latitude magnetosphere from L = 7 to 10. The results are compared with linear kinetic theory, 1-D and 2-D simulations. The EMIC waves are observed to occur in packets with multiple wave cycles. The wave cycles within a wave packet are observed to have the same general propagation angle theta(kB0) and polarization. In observations and 2-D simulations, EMIC waves have a mixture of circular and elliptical polarization for theta(kB0) < 30 degrees. This scattered ellipticity values are due to the superposition of multiple wave modes. For wave propagation angles 30 degrees < theta(kB0) < 60 degrees, the waves are highly ;elliptical (0.2 < |epsilon| < 0.7), where epsilon is the ratio of minor to major axis of the polarization ellipse. For theta(kB0) > 60 degrees, the waves are nearly linearly polarized (|epsilon| <= 0.1). This general trend is in good agreement with linear kinetic theory and 1-D simulations. Observations indicate right-hand (RH) wave packets to be interspersed with the left-hand (LH) wave packets. We show for the first time from linear theory that ion temperature anisotropies can generate RH waves at large propagation angles and for plasma beta.. beta(i) > 0.05. The observed mixture of RH and LH waves in the magnetosphere could be due to this direct generation of RH waves. Observations and simulations show that EMIC waves are coherent with 0.5 < C < 1.0 for theta(kB0) <= 50 degrees. Here C is measured as the maximum value of cross-correlation coefficient between the transverse magnetic field components of the wave.

In a recent paper, Ahmadi et al. (2016) analyze the effect of electron temperature anisotropy on proton mirror instability. They find that the electron whistler instability grows faster and consumes all the available electron free energy so that no anisotropy is left to fuel the proton mirror mode growth. In this comment we present both observational and theoretical arguments about why we think this is incorrect.

We discuss the variation of whistler mode wave electric and magnetic field polarizations as a function of propagation angle θkB0 with respect to the background magnetic field B0 using linear kinetic dispersion theory. The circular polarization of the whistler mode wave magnetic field at all propagation angles [Verkhoglyadova et al. J. Geophys. Res. 115, A00F19 (2010); P. M. Bellan, Phys. Plasmas 20, 082113 (2013)] is found to be valid only for cold plasma or low plasma beta conditions. The wave magnetic fields, on a plane orthogonal to the wave vector k, tend to become elliptically polarized with an increase in propagation angles for high beta plasma background conditions (βe≥0.1). The electric field polarization plane may not be orthogonal to wave vector k, especially for oblique propagations, and is found to be circularly polarized only at parallel propagation direction as reported by Verkhoglyadova et al. [J. Geophys. Res. 115, A00F19 (2010)] and Bellan [Phys. Plasmas 20, 082113 (2013)]. They become elliptically polarized with an increase in propagation angles. This is valid for arbitrary plasma beta conditions. The results are also analysed and compared for an inner magnetospheric plasma model with three electron species. The two major angles, Gendrin and resonance cone angles, are also discussed.