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On December 08, 2018 the Twin Rocket Investigation of Cusp Electrodynamics 2 (TRICE 2) mission was successfully launched. The mission consisted of two sounding rockets, each carrying a payload capable of measuring electron and ion distributions, electric and magnetic fields, and plasma waves occurring in the northern magnetospheric cusp. This study highlights the ion and wave observations obtained by TRICE 2 in the cusp and observations from the magnetospheric multiscale (MMS) spacecraft at the low‐latitude magnetopause two hours prior to the TRICE 2 traversal of the cusp. Within the cusp, typical ion cusp features were observed, that is, energy‐latitude dispersion of injected magnetosheath plasma. However, a lower energy population was also measured near the equatorward edge of the cusp on open field lines. Pitch‐angle distributions of the low‐energy ions suggest that this population was magnetospheric in origin, and not from ionospheric upflows. Data from MMS show that counterstreaming ions were present in the outer magnetosphere and low‐latitude boundary layer at similar energies to those observed by TRICE 2 in the cusp. Correlations between the low‐energy ions within the cusp and broadband extremely low frequency waves suggest that the low‐energy magnetospheric ions that filled the flux tube may have undergone wave‐particle interactions. These interactions may cause pitch‐angle scattering of low‐energy magnetospheric ions closer to the loss cone, thereby allowing them to precipitate into the cusp and be measured by the TRICE 2 sounding rockets. Key Points: Low‐energy ions measured near the equatorward edge of the cusp by TRICE 2 are locally mirroring and propagating toward the ionosphereA similar counter‐streaming ion population is observed at the magnetopause by MMS two hours prior to the TRICE 2 cusp traversalIon and wave data in the cusp suggest wave‐particle interactions are pitch angle scattering low‐energy magnetospheric ions [ABSTRACT FROM AUTHOR]

During the Twin Rockets to Investigate Cusp Electrodynamics (TRICE‐2) High‐Flyer rocket's passage through the cusp the high frequency (HF) radio wave receiver observed three intervals of banded upper‐hybrid (UH) waves. The bands begin at the UH frequency (∼1.2–1.3 MHz), descending to as low as 1.1 MHz, with amplitudes of hundreds of mV/m. The spacing of the bands are ∼4.5–6 kHz and the number of bands ranges from 3 to 10. Simultaneously, the very low frequency (VLF) radio wave receiver observed lower‐hybrid (LH) waves with amplitudes ranging from 1 to 10 mV/m and frequencies of 4.5–6 kHz. Slight variations of the spacings of the bands in the UH waves were closely correlated with variations in the LH peak frequencies. Two possible wave‐wave interactions are explored to explain this phenomenon: decay of an UH wave into a lower frequency UH wave and a LH wave, and coalescence of independent UH waves and LH waves that spawn UH waves. Using a dispersion relation calculator with electron and ion distribution functions based off those observed by the particle instruments suggests that UH waves, and to a lesser degree LH waves, can be excited by linear instabilities. Kinematic analysis of the waves dispersion relations and the wave matching conditions show that wave‐wave interactions linking UH and LH modes are possible through either decay or coalescence. This analysis along with comparisons of the energy densities of the waves, and the ratio of their occupation numbers suggest that the decay process is more likely than coalescence. Key Points: Modulated upper‐hybrid (UH) waves coincided with enhanced power near the local lower‐hybrid (LH) frequencyThe spacings of the banded UH waves are correlated with the frequency of the peak spectral density near the LH peaksKinematic constraints and energy densities of wave modes suggest wave‐wave process is plausible, with decay more likely than coalescence [ABSTRACT FROM AUTHOR]

Auroral whistler‐mode radio emissions called saucers are of fundamental interest because they require an unusually stationary emission process in the dynamic auroral environment, and it is a mystery how that can happen in this or similar conditions elsewhere in geospace. The Cusp Alfvén and Plasma Electrodynamics Rocket (CAPER‐2), launched into the cusp, obtained the first rocket measurements of a large‐scale, multiple‐armed dayside saucer, similar to those observed by the DEMETER satellite, with the addition of particle measurements and ground‐based measurements. Analysis of saucer shapes, directional measurements using waveforms, and ground‐based data show that, accounting for estimated uncertainties, these originate at altitudes ∼4,000 km within the cusp, the eastern side of which is penetrated by the rocket ∼100 s after the saucers are encountered. On‐board particle instruments show dispersed electron bursts in the cusp, Alfvénically accelerated at altitudes at or above the saucer sources. Plain Language Summary: Electrons, precipitating down Earth's high‐latitude magnetic field lines, emit radio waves at angles to the background magnetic field, depending on frequency. When spacecrafts traverse through a region close to the source, they observe descending and ascending frequency signatures referred to as saucers. This research focused on rocket measurements of large‐scale, multiarmed saucers on the dayside. Using ray‐tracing software and hodogram analysis of the electric field waveforms combined with ground‐based measurements, we were able to determine the source location of the saucers to be in the magnetic field's cusp at altitudes near 4,000 km. Additionally, particle measurements on‐board the rocket showed time dispersed bursts of electrons typically associated with Alfvénic acceleration, which can be traced back to a source height that is equal to or above the source heights of the observed saucers. This is the first time that dayside large‐scale saucers have been associated with the cusp. Key Points: First observation of large‐scale dayside saucers from a sounding rocketThe saucers are estimated to originate at ∼4,000 km on cusp field lines to within estimated uncertaintiesDispersed electrons observed in the cusp were accelerated at altitudes at or above the saucer signal source region [ABSTRACT FROM AUTHOR]