Heather Alison Elliott

We present a method for estimating the heating rate of solar wind (SW) and pickup ion (PUI) protons from their respective energy equations. Here, we estimate the heating rate for SW protons along the New Horizons (NH), Voyager 2 (V2), and Pioneer 10 (P10) trajectories using plasma measurements, rather than the magnetic field measurements. We also compute the heating rate for PUI protons for the first time along the NH direction. We find that the SW proton heating rate in the V2 direction is higher than that along the NH direction between ∼26 and ∼52 au. Along the P10 direction, the proton heating rate exceeds that of V2 between 20 and 38 au, but is less beyond 38 au. The heating rate differs along the spacecraft trajectories in the upwind and downwind directions. The PUI proton heating rate is found to be more than 2 orders of magnitude larger than that of SW protons between ∼26 and ∼52 au. The SW/PUI heating rate correlates with SW speed and SW/PUI temperature. We present a comparison between the injection rate of turbulence energy in the energy-containing range and the intensity profile of energetic particles in the ∼50–110 keV range as a function of distance along the NH and V2 directions. It suggests that turbulence may play an important role in the modulation of energetic particles throughout the heliosphere.

The solar wind gradually slows with increasing distance from the Sun as interstellar material becomes ionized and picked up by the solar wind. When observing the solar wind in the outer heliosphere, it is difficult to differentiate temporal decreases in the speed originating at the Sun from the slowing of the solar wind with increase distance as interstellar material is picked up. In order to resolve this issue, we examine the solar wind speed in the outer heliosphere relative to the corresponding solar wind speed observed at 1 au. Additionally, we use a simulation to help discern the temporal and spatial changes in speed at New Horizons (NH) by driving the inner boundary of a heliospheric simulation with 1 au speed measurements. We conclude that the long interval of slow solar wind from 2018–2023 found at NH was caused by a combination of slow wind being directed toward NH owing to a low current sheet tilt angle keeping the streamers in the ecliptic, and the gradual slowing with increasing distance caused by additional pickup of interstellar material as the solar wind travels farther way from the Sun. When NH was between 50 and 58.5 au, the measured solar wind speed was reduced by ∼13%–15% relative to 1 au owing to pickup of interstellar material. This reduction is a further decrease from the previously reported slowing of 5%–7% in the 30–43 au region.

The large-scale dynamics of the heliosphere is driven by solar activity and variable solar wind (SW) conditions. In situ SW observations inform time-dependent heliosphere modeling efforts but are only available up until the present day, thus restricting informed predictions to the very near future. We developed and implemented a method to provide long-term forecasting of SW parameters at 1 au. Such forecasting supports modeling efforts of the time-dependent global heliosphere by providing realistic boundary conditions for heliospheric models at the upwind model-domain boundary ahead of time in order to understand the global dynamic processes in the heliosphere such as the evolution of the termination shock (TS), heliosheath, and heliopause. Such forecasting capabilities are needed to better constrain the timing of New Horizons’ TS encounter and predict energetic neutral atom flux variation for IMAP. We analyzed SW direct measurement time series for periodicities at various timescales using Lomb–Scargle periodogram analysis. Based on identified prominent periodicities, we construct quasiperiodic functions for the SW parameters. By extrapolating these functions forward in time, we obtain a prediction of the SW evolution over the next two solar cycles. Our results indicate that the next pulse in the SW dynamic pressure, which controls the global heliosphere, will occur around 2035.

The Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission is a NASA Small Explorer to determine the cross-scale processes that unify the solar corona and heliosphere. PUNCH has two science objectives: (1) understand how coronal structures become the ambient solar wind, and (2) understand the dynamic evolution of transient structures, such as coronal mass ejections, in the young solar wind. To address these objectives, PUNCH uses a constellation of four small spacecraft in Sun-synchronous low Earth orbit, to collect linearly polarized images of the K corona and young solar wind. The four spacecraft each carry one visible-light imager in a 1 + 3 configuration: a single Narrow Field Imager solar coronagraph captures images of the outer corona at all position angles, and at solar elongations from 1.5° (6 R _⊙ ) to 8° (32 R _⊙ ); and three separate Wide Field Imager heliospheric imagers together capture views of the entire inner solar system, at solar elongations from 3° (12 R _⊙ ) to 45° (180 R _⊙ ) from the Sun. PUNCH images include linear-polarization data, to enable inferring the three-dimensional structure of visible features without stereoscopy. The instruments are matched in wavelength passband, support overlapping instantaneous fields of view, and are operated synchronously, to act as a single “virtual instrument” with a 90 ^∘ wide field of view, centered on the Sun. PUNCH launched in March of 2025 and began science operations in June of 2025. PUNCH has an open data policy with no proprietary period, and PUNCH Science Team Meetings are open to all.

During the 21–22 January 2005 magnetic storm, the FAST satellite observed warm (< few keV) ions in discrete energy bands on the dayside at ∼3,000 km altitude for more than 6.5 hr. We suggest that the ionospheric energy‐banded ions represent the low‐altitude edge of the warm plasma cloak observed simultaneously by magnetospheric satellites. This is a clear example of the multi‐species ion energy bands (10 eV to several keV) observed during strong magnetic storms by the FAST satellite, stretching from the diffuse auroral region to the plasmapause with lifetimes up to 12 hr. The close association of these energy‐banded ions with magnetic storms, their broad latitudinal extent, and the presence of multiple ion species in the same energy band, rather than at the same velocity, indicate that this is a distinct phenomenon from other types of energy‐banded ions. During the 21–22 January 2005 magnetic storm, the dayside ion energy band structures, centered at 10 eV (H ⁺ ), 40 eV (H ⁺ and He ⁺ ), and 160 eV (H ⁺ , He ⁺ , and O ⁺ ), were consistent with a “time‐of‐flight and velocity filter” formation process acting on a near‐cusp, impulsive outflow of a L ∼ 1.2 in the dawn sector, significantly altering the energetics of the mid‐latitude ionosphere.

We analyze ∼40–200 keV energetic particle observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on board the New Horizons (NH) spacecraft from 2007 to 2024 and compare them with similar measurements from the Low Energy Charged Particle experiments on the Voyager 1 and Voyager 2 spacecraft when they were at comparable heliocentric distances (5–60 au). In both Voyager data sets, particle intensities generally decrease with increasing radial distance from the Sun, reaching a minimum in the outer heliosphere before rising again prior to their respective encounters with the heliospheric termination shock (TS). This radial behavior in the intensity-time profiles is described as a heliospheric valley. The NH/PEPSSI time series from 5 au (2007) to 60 au (2024) exhibits a similar decrease in particle fluxes with distance. Analysis of the radial dependence of energetic particle intensities from all three spacecraft normalized by observations at 1 au to account for the solar cycle effects reveals an approximate piece-wise composite power-law relationship, with a slope break (steeper decline) beyond ∼33 au. This break may reflect differences in the dominant transport and acceleration mechanisms operating in the two regions demarcated by this radial break. In addition, a radial scaling method is applied to Voyager observations to best match the NH data. This comparison provides an estimate for the NH TS crossing between 2027 (68 au) and 2034 (83 au).

Interstellar neutral (ISN) atoms enable studies of the physical conditions in the local interstellar medium surrounding the heliosphere. ISN helium, which is the most abundant species at 1 au, is directly observed by space missions, such as Interstellar Boundary Explorer (IBEX). However, some of these atoms are ionized by solar UV radiation before reaching 1 au, producing pickup ions (PUIs). A recent analysis of IBEX data suggests that the helium photoionization rates predicted by models are underestimated by up to 40%. The Solar Wind Around Pluto (SWAP) instrument on board New Horizons enables the study of PUIs, giving complementary insight into the other side of the ionization process. Our goal is to verify this increased helium ionization by determining the ionization rate of ISN helium in the heliosphere based on SWAP observations of helium PUIs. For this purpose, we analyze SWAP data collected between 2012 and 2022, at distances 22–54 au from the Sun. We develop a new method for fitting model distribution functions to the observational data using the maximum likelihood method. Our approach accounts for the spacecraft’s rotation and the SWAP response function, which depends on both energy and inflow direction. We estimate SWAP’s efficiency for helium relative to that for hydrogen, and determine the ISN helium ionization rate. We find that the photoionization rate obtained from the SWAP observations is ∼43% larger than the rates predicted by models, confirming the IBEX results.

Interstellar neutral (ISN) atoms enable studies of the physical conditions in the local interstellar medium surrounding the heliosphere. ISN helium, which is the most abundant species at 1 au, is directly observed by space missions, such as Interstellar Boundary Explorer (IBEX). However, some of these atoms are ionized by solar ultraviolet radiation before reaching 1 au, producing pickup ions (PUIs). A recent analysis of IBEX data suggests that the helium photoionization rates predicted by models are underestimated by up to 40%. The Solar Wind Around Pluto (SWAP) instrument on board New Horizons enables the study of PUIs giving complementary insight into the other side of the ionization process. Our goal is to verify this increased helium ionization by determining the ionization rate of ISN helium in the heliosphere based on the SWAP observations of helium PUIs. For this purpose, we analyze SWAP data collected between 2012 and 2022, at distances 22 to 54 au from the Sun. We develop a new method for fitting model distribution functions to the observational data using the maximum likelihood method. Our approach accounts for the spacecraft's rotation and the SWAP response function, which depends on both energy and inflow direction. We estimate SWAP's efficiency for helium relative to that for hydrogen and determine the ISN helium ionization rate. We find that the photoionization rate obtained from the SWAP observations is 43% larger than the rates predicted by models, confirming the IBEX results.

Currently ∼62 au from the Sun, the New Horizons spacecraft is en route to the outer heliosphere boundaries. The first boundary it will encounter is the heliospheric termination shock (HTS), where the solar wind ion (SWI) and interstellar pickup ion (PUI) plasma mixture is slowed down to subsonic speeds, compressed, and heated. Some particles, mostly PUIs, undergo preferential acceleration at the HTS due to their higher energies and thus gain the capability to reflect from the shock and undergo, e.g., shock drift acceleration. This produces a tail in the downstream PUI energy distribution, with the potential for multiple power-law breaks. In anticipation of crossing the HTS, we have constructed a test particle model with synthetic turbulence to simulate New Horizons’ Solar Wind Around Pluto (SWAP) observations downstream of the HTS. SWAP can measure the energy/charge of multiple particle populations (thermal solar wind (SW) protons, alphas, and PUIs). Here, we calculate what SWAP might observe after it crosses the HTS. Our model shows that the count rate distribution will be very different from what is observed in the supersonic SW, with a hotter SW+PUI distribution and no sharp PUI cutoff. This will require a different method to quantify the moments of the SWIs and PUIs in the heliosheath. SWAP may be able to observe part of the PUI tail downstream of the HTS, if the upstream SW speed is ≲ 300 km s⁻¹. Moreover, if the shock compression ratio is also ≳ 2, SWAP may observe multiple populations of the accelerated PUI distribution.