We report the discovery of a large ( \sim 8500 km diameter ) infrared-bright storm at Neptune ’ s equator in June 2017 .
We tracked the storm over a period of 7 months with high-cadence infrared snapshot imaging , carried out on 14 nights at the 10 meter Keck II telescope and 17 nights at the Shane 120 inch reflector at Lick Observatory .
The cloud feature was larger and more persistent than any equatorial clouds seen before on Neptune , remaining intermittently active from at least 10 June to 31 December 2017 .
Our Keck and Lick observations were augmented by very high-cadence images from the amateur community , which permitted the determination of accurate drift rates for the cloud feature .
Its zonal drift speed was variable from 10 June to at least 25 July , but remained a constant 237.4 \pm 0.2 m s ^ { -1 } from 30 September until at least 15 November .
The pressure of the cloud top was determined from radiative transfer calculations to be 0.3-0.6 bar ; this value remained constant over the course of the observations .
Multiple cloud break-up events , in which a bright cloud band wrapped around Neptune ’ s equator , were observed over the course of our observations .
No “ dark spot ” vortices were seen near the equator in HST imaging on 6 and 7 October .
The size and pressure of the storm are consistent with moist convection or a planetary-scale wave as the energy source of convective upwelling , but more modeling is required to determine the driver of this equatorial disturbance as well as the triggers for and dynamics of the observed cloud break-up events .