New Horizons Solar Wind Around Pluto (SWAP) Observations of the Solar Wind From 11-33 AU

TL;DR

This study analyzes solar wind data collected by the SWAP instrument on the New Horizons mission from 11 to 33 AU, revealing how solar wind properties evolve and interact in the outer heliosphere.

Contribution

We developed a forward model for SWAP count rates and validated it against observations, providing new insights into solar wind behavior beyond 20 AU.

Findings

Solar wind structures diminish with distance due to interactions.

The T-V relationship becomes steeper and less correlated beyond 20 AU.

Longer periodicities in wind parameters strengthen with distance.

Abstract

The Solar Wind Around Pluto (SWAP) instrument on NASA's New Horizon Pluto mission has collected solar wind observations en route from Earth to Pluto, and these observations continue beyond Pluto. Few missions have explored the solar wind in the outer heliosphere making this dataset a critical addition to the field. We created a forward model of SWAP count rates, which includes a comprehensive instrument response function based on laboratory and flight calibrations. By fitting the count rates with this model, the proton density (n), speed (V), and temperature (T) parameters are determined. Comparisons between SWAP parameters and both propagated 1 AU observations and prior Voyager 2 observations indicate consistency in both the range and mean wind values. These comparisons as well as our additional findings confirm that small and midsized solar wind structures are worn down with increasing distance due to dynamic interaction of parcels of wind with different speed. For instance, the T-V relationship steepens, as the range in V is limited more than the range in T with distance. At times the T-V correlation clearly breaks down beyond 20 AU, which may indicate wind currently expanding and cooling may have an elevated T reflecting prior heating and compression in the inner heliosphere. The power of wind parameters at shorter periodicities decreases with distance as the longer periodicities strengthen. The solar rotation periodicity is present in temperature beyond 20 AU indicating the observed parcel temperature may reflect not only current heating or cooling, but also heating occurring closer to the Sun.