# # Program information file # PROGRAM_ID 2024B034 PROGRAM_TITLE Observations of Uranus' Near-IR H2 Emission Approaching the Solar Cycle 25 Maximum PROGRAM_INV1 Laurence Trafton PROGRAM_INV2 Henrik Melin PROGRAM_INV3 PROGRAM_INV4 PROGRAM_INV5 PROGRAM_SCICAT major planets / satellites PROGRAM_ABSTRACT_BEG We propose to use iSHELL with the Kgas setting at the IRTF to observe Uranus' whole-disk, K-band, quadrupole H2 S- and Q-branch emission line spectrum through the 4 arcsec slit at effective R ~8000 for the 2024 apparition. The spectrum will be analyzed to extract the disk-averaged rotational temperature of H2, which has been in a long-term decline that has extended well into the southern spring season. These observations are needed to understand the energy balance of Uranus' upper atmosphere, particularly the source of its excess heating, which cannot be explained by solar EUV insolation alone. These observations will extend the time series documenting Uranus' seasonal response to solar forcing in anticipation of a potentially imminent seasonal temperature trend reversal. Establishing the epoch of this reversal would reveal the dissipative time constant for the response of the thermosphere to seasonal forcing, which should help to constrain the unknown heating process. The extracted vibrationally excited H2 column abundance will also be useful for establishing the cooling rate of Uranus' thermosphere -- further constraining the unknown heating source there. Finally, the 2025 Solar Cycle 25 is stronger than forecast and its elevated EUV/FUV and charged particle output during 2024B may provide an opportunity to assess the importance of solar-activity in driving Uranus' excessively high thermospheric temperature through auroral heating. We therefore also propose to search the nightly H2 images of Uranus' whole disk for any localized auroral enhancement and estimate the relative contribution of aurorae to the emission intensity. PROGRAM_ABSTRACT_END