arXiv subjects

Catherine L. Johnson

Publications and source records attributed to Catherine L. Johnson.

4 recordsLinked to original sources

arXiv2025

Impact Plasma Amplification of the Ancient Mercury Magnetic Field

Spacecraft measurements of Mercury indicate it has a core dynamo with a surface field of 200-800 nT. These data also indicate that the crust contains remanent magnetization likely produced by an ancient magnetic field. The inferred magnetization intensity is consistent with a wide range of paleofield strengths (0.2-50 uT), possibly indicating that Mercury once had a dynamo field much stronger than today. Recent modeling of ancient lunar impacts has demonstrated that plasma generated during basin-formation can transiently amplify a planetary dynamo field near the surface. Simultaneous impact-induced pressure waves can then record these fields in the form of crustal shock remanent magnetization (SRM). Here, we present impact hydrocode and magnetohydrodynamic simulations of a Caloris-size basin (~1,550 km diameter) formation event. Our results demonstrate that the ancient magnetospheric field (~0.5-0.9 uT) created by the interaction of the ancient interplanetary magnetic field (IMF) and Mercury's dynamo field can be amplified by the plasma up to ~13 uT and, via impact pressure waves, be recorded as SRM in the basin antipode. Such magnetization could produce ~5 nT crustal fields at 20-km altitude antipodal to Caloris detectable by future spacecraft like BepiColombo. Furthermore, impacts in the southern hemisphere that formed ~1,000 km diameter basins (e.g., Andal-Coleridge, Matisse-Repin, Eitkou-Milton, and Sadi-Scopus) could impart crustal magnetization in the northern hemisphere, contributing to the overall remanent field measured by MESSENGER. Overall, the impact plasma amplification process can contribute to crustal magnetization on airless bodies and should be considered when reconstructing dynamo history from crustal anomaly measurements.

astro-ph.EP↗

arXiv2021

The Lunar Geophysical Network Landing Sites Science Rationale

The Lunar Geophysical Network (LGN) mission is proposed to land on the Moon in 2030 and deploy packages at four locations to enable geophysical measurements for 6-10 years. Returning to the lunar surface with a long-lived geophysical network is a key next step to advance lunar and planetary science. LGN will greatly expand our primarily Apollo-based knowledge of the deep lunar interior by identifying and characterizing mantle melt layers, as well as core size and state. To meet the mission objectives, the instrument suite provides complementary seismic, geodetic, heat flow, and electromagnetic observations. We discuss the network landing site requirements and provide example sites that meet these requirements. Landing site selection will continue to be optimized throughout the formulation of this mission. Possible sites include the P-5 region within the Procellarum KREEP Terrane (PKT; (lat:$15^{\circ}$; long:$-35^{\circ}$), Schickard Basin (lat:$-44.3^{\circ}$; long:$-55.1^{\circ}$), Crisium Basin (lat:$18.5^{\circ}$; long:$61.8^{\circ}$), and the farside Korolev Basin (lat:$-2.4^{\circ}$; long:$-159.3^{\circ}$). Network optimization considers the best locations to observe seismic core phases, e.g., ScS and PKP. Ray path density and proximity to young fault scarps are also analyzed to provide increased opportunities for seismic observations. Geodetic constraints require the network to have at least three nearside stations at maximum limb distances. Heat flow and electromagnetic measurements should be obtained away from terrane boundaries and from magnetic anomalies at locations representative of global trends. An in-depth case study is provided for Crisium. In addition, we discuss the consequences for scientific return of less than optimal locations or number of stations.

astro-ph.EP↗

arXiv2019

Observations of Extreme ICME Ram Pressure Compressing Mercury's Dayside Magnetosphere to the Surface

Mercury's magnetosphere is known to be affected by enhanced ram pressures and magnetic fields inside interplanetary coronal mass ejections (ICMEs). Here we report detailed observations of an ICME compressing Mercury's dayside magnetosphere to the surface. A fast CME launched from the Sun on November 29 2013 impacted first MESSENGER, which was orbiting Mercury, on November 30 and later STEREO-A near 1 AU on December 1. Following the ICME impact, MESSENGER remained in the solar wind as the spacecraft traveled inwards and northwards towards Mercury's surface until it reached and passed its closest approach to the planet (at 371 km altitude) without crossing into the magnetosphere. The magnetospheric crossing finally occurred 1 minute before reaching the planet's nightside at 400 km altitude and 84$^\circ$N latitude, indicating the lack of dayside magnetosphere on this orbit. In addition, the peak magnetic field measured by MESSENGER at this time was 40% above the values measured in the orbits just prior to and after the ICME, a consequence of the magnetospheric compression. Using both a proxy method at Mercury and measurements at STEREO-A, we show that the extremely high ram pressure associated with this ICME was more than high enough to collapse Mercury's weak magnetosphere. As a consequence, the ICME plasma likely interacted with Mercury's surface, evidenced by enhanced sodium ions in the exosphere. The collapse of Mercury's dayside magnetosphere has important implications for the habitability of close-in exoplanets around M dwarf stars, as such events may significantly contribute to planetary atmospheric loss in these systems.

physics.space-ph↗

arXiv2019

The Global Surface Roughness of 25143 Itokawa

Surface roughness is an important metric in understanding how the geologic history of an asteroid affects its small-scale topography and it provides an additional means to quantitatively compare one asteroid with another. In this study, we report the first detailed global surface roughness maps of 25143 Itokawa at horizontal scales from 8--32~m. Comparison of the spatial distribution of the surface roughness of Itokawa with 433 Eros, the other asteroid for which this kind of analysis has been possible, indicates that the two asteroids are dominated by different geologic processes. On Itokawa, the surface roughness reflects the results of down-slope activity that moves fine grained material into geopotential lows and leaves large blocks in geopotential highs. On 433 Eros, the surface roughness is controlled by geologically-recent large impact craters. In addition, large longitudinal spatial variations of surface roughness could impact the role of YORP on Itokawa.

astro-ph.EP↗