The Cusp as a VLF Saucer Source: First Rocket Observations of Long‐Duration VLF Saucers on the Dayside

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 rocket The saucers are estimated to originate at ∼4,000 km on cusp field lines to within estimated uncertainties Dispersed electrons observed in the cusp were accelerated at altitudes at or above the saucer signal source region