arXiv subjects

Heather A. Elliott

Publications and source records attributed to Heather A. Elliott.

7 recordsLinked to original sources

arXiv2026

A Catalog of Automatically Identified Multi-Signature ICMEs Observed by Solar Orbiter

In this study, we present a catalog of 138 interplanetary coronal mass ejections (ICMEs) identified using multiple distance-normalized ICME criteria within sliding time windows applied to \textit{Solar Orbiter} in-situ measurements. We observe sub-adiabatic proton cooling, a near-spherical radial density decrease, a steep decline in dynamic pressure indicative of enhanced expansion in the inner heliosphere, and a magnetic-field profile shallower than Parker-spiral expectations. Approximately $55\%$ of the ICMEs are associated with a preceding shock and sheath, while $45\%$ are not. Compared with non-sheath ICMEs, sheath-associated events are $\approx27\%$ longer, $\approx68\%$ more magnetized, $\approx30\%$ faster, $\approx2.6$ times hotter, $\approx34\%$ denser, and $\approx2.8$ times more over-pressured, while expanding $\approx96\%$ faster and maintaining low plasma $β$. Expansion speed also scales strongly with ICME size and bulk speed, suggesting that sheath dynamics regulate ejecta expansion. For the 32 ICMEs with Heavy Ion Sensor (HIS) observations, $\approx30\%$ exhibit $\mathrm{O}^{7+}/\mathrm{O}^{6+}\approx1$, whereas Fe-based composition signatures ($\langle Q_{\mathrm{Fe}}\rangle$, $\mathrm{Fe}^{\ge16+}/\mathrm{Fe}$, and $\mathrm{Fe}/\mathrm{O}$) identify $\approx50$--$65\%$ of events, consistent with lower charge-state enhancements during solar minimum. Superposed epoch analysis reveals enhanced heavy-ion signatures within magnetic-cloud intervals, linking in-situ composition to coronal heating. This catalog provides a robust benchmark for multi-mission studies, machine-learning applications, CME evolution models, and space-weather investigations.

astro-ph.SR↗

arXiv2026

Polarimeter to Unify the Corona and Heliosphere (PUNCH)

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 degrees (6 R$_\odot$) to 8 degrees (32 R$_\odot$); and three separate Wide Field Imager heliospheric imagers together capture views of the entire inner solar system, at solar elongations from 3 degrees (12 R$_\odot$) to 45 degrees (180 R$_\odot$) 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 degree 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.

astro-ph.SR↗

arXiv2025

Ionization Rate of Interstellar Neutral Helium from New Horizons/SWAP Observations

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.

physics.space-ph↗

arXiv2025

High-Resolution Observations of Pickup Ion Mediated Shocks to 60 au

This study provides a detailed analysis of fourteen distant interplanetary shocks observed by the Solar Wind Around Pluto (SWAP) instrument onboard New Horizons. These shocks were observed with a pickup ion data cadence of approximately 30 minutes, covering a heliocentric distance range of ~52-60 au. All the shocks observed within this distance range are fast-forward shocks, and the shock compression ratios vary between ~1.2 and 1.9. The shock transition scales are generally narrow, and the SW density compressions are more pronounced compared to the previous study of seven shocks by McComas et al. (2022). A majority (64%) of these shocks have upstream sonic Mach numbers greater than one. In addition, all high-resolution measurements of distant interplanetary shocks analyzed here show that the shock transition scale is independent of the shock compression ratio. However, the shock transition scale is strongly anti-correlated with the shock speed in the upstream plasma frame, meaning that faster shocks generally yield sharper transitions.

physics.space-ph↗

arXiv2024

Forecasting Shock-associated Energetic Particle Intensities in the Inner Heliosphere: A Proof-of-Concept Capability for the PUNCH Mission

Solar energetic particles (SEPs) associated with shocks driven by fast coronal mass ejections (CMEs) or shocks developed by corotating interaction regions (CIRs) often extend to high energies, and are thus key elements of space weather. The PUNCH mission, set to be launched in 2025, is equipped with photometric that enables 3D tracking of solar wind structures in the interplanetary space through polarized light. Tracking techniques are used to estimate speeds and speed gradients of solar structures, including speed jumps at fast shocks. We report on a strong and a robust relation between the shock speed jump magnitude at CME and CIR shocks and the peak fluxes of associated energetic particles from the analysis of 59 CME-driven shocks and 74 CIRs observed by Wind/STEP between 1997-2023. We demonstrate that this relation, along with PUNCH anticipated observations of solar structures can be used to forecast shock-associated particle events close to the Sun; thus, advancing and providing a crucial input to forecasting of SEP fluxes in the heliosphere.

astro-ph.SR↗

arXiv2020

Cometary ions detected by the Cassini spacecraft 6.5 au downstream of Comet 153P/Ikeya-Zhang

During March-April 2002, while between the orbits of Jupiter and Saturn, the Cassini spacecraft detected a significant enhancement in pickup proton flux. The most likely explanation for this enhancement was the addition of protons to the solar wind by the ionization of neutral hydrogen in the corona of comet 153P/Ikeya-Zhang. This comet passed relatively close to the Sun-Cassini line during that period, allowing pickup ions to be carried to Cassini by the solar wind. This pickup proton flux could have been further modulated by the passage of the interplanetary counterparts of coronal mass ejections past the comet and spacecraft. The radial distance of 6.5 Astronomical Units (au) traveled by the pickup protons, and the implied total tail length of >7.5 au make this cometary ion tail the longest yet measured.

astro-ph.EP↗

arXiv2016

Long-Term Trends In The Solar Wind Proton Measurements

We examine the long-term time evolution (1965-2015) of the relationships between solar wind proton temperature (Tp) and speed (Vp) and between the proton density (np) and speed using OMNI solar wind observations taken near Earth. We find a long-term decrease in the proton temperature-speed (Tp-Vp) slope that lasted from 1972 to 2010, but has been trending upward since 2010. Since the solar wind proton density-speed (np-Vp) relationship is not linear like the Tp-Vp relationship, we perform power law fits for np-Vp. The exponent (steepness in the np-Vp relationship) is correlated with the solar cycle. This exponent has a stronger correlation with current sheet tilt angle than with sunspot number because the sunspot number maxima vary considerably from cycle to cycle and the tilt angle maxima do not. To understand this finding, we examined the average np for different speed ranges, and found that for the slow wind np is highly correlated with the sunspot number with a lag of ~4 years. The fast wind np variation was less, but in phase with the cycle. This phase difference may contribute to the np-Vp exponent correlation with the solar cycle. These long-term trends are important since empirical formulas based on fits to Tp and Vp data are commonly used to identify ICMEs, but these formulas do not include any time dependence. Changes in the solar wind density over a solar cycle will create corresponding changes in the near Earth space environment and the overall extent of the heliosphere.

astro-ph.SR↗