[Paper Review] Observations of Extreme ICME Ram Pressure Compressing Mercury's Dayside Magnetosphere to the Surface

R. M. Winslow, Noé Lugaz+7 more|University of New Hampshire Scholars Repository (University of New Hampshire at Manchester)|Mar 1, 2019

Astro and Planetary Science4 citations

TL;DR

This study presents direct observations from MESSENGER of an extreme interplanetary coronal mass ejection (ICME) compressing Mercury’s dayside magnetosphere to the planet’s surface, confirmed by a 40% increase in magnetic field strength and enhanced sodium ion flux in the exosphere. The event demonstrates that high ram pressure from ICMEs can fully collapse Mercury’s weak magnetosphere, enabling direct plasma interaction with the surface and driving atmospheric sputtering, with implications for atmospheric loss on close-in exoplanets around M dwarfs.

ABSTRACT

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.

Motivation & Objective

• To investigate the effects of extreme ICME ram pressure on Mercury’s magnetosphere during a rare, high-intensity event.
• To determine whether Mercury’s dayside magnetosphere can be fully compressed to the planetary surface under extreme solar wind conditions.
• To link magnetospheric collapse to enhanced exospheric sodium ion production via direct plasma-surface interaction.
• To assess the implications of such magnetospheric collapse for atmospheric escape on close-in exoplanets orbiting M dwarf stars.
• To validate the use of remote and in-situ measurements for detecting magnetospheric boundary collapse during extreme space weather events.

Proposed method

• Analysis of MESSENGER magnetic field and plasma data from orbit 2577, which passed within 371 km of Mercury’s surface during an ICME passage.
• Use of STEREO-A in-situ solar wind measurements to estimate ICME ram pressure at 1 AU and extrapolate to Mercury’s orbit using a proxy method.
• Comparison of MESSENGER’s magnetopause crossing timing and magnetic field strength with pre- and post-ICME conditions to identify compression effects.
• Correlation of FIPS (Fast Imaging Plasma Spectrometer) sodium ion observations with magnetospheric boundary behavior to assess sputtering-driven exospheric enhancement.
• Application of empirical models of Mercury’s magnetospheric boundaries (magnetopause and bow shock) to estimate stand-off distances under extreme ram pressure.
• Use of statistical scaling from MESSENGER’s ICME dataset (69 events) to estimate the frequency of magnetospheric collapse events during solar maximum.

Experimental results

Research questions

• RQ1 To what extent can ICME ram pressure compress Mercury’s dayside magnetosphere to the planetary surface?
• RQ2 What evidence confirms that the magnetopause was at or below Mercury’s surface during the November 2013 ICME event?
• RQ3 How does magnetospheric collapse affect the production of sodium ions in Mercury’s exosphere?
• RQ4 What are the implications of such magnetospheric collapse for atmospheric loss on close-in exoplanets around M dwarf stars?
• RQ5 How frequently might such extreme compression events occur at Mercury during solar maximum, based on MESSENGER observations?

Key findings

• MESSENGER observed a 40% increase in magnetic field strength at 371 km altitude, indicating extreme compression of Mercury’s magnetosphere to the surface.
• The magnetospheric boundary was not crossed until 1 minute before closest approach at 400 km altitude and 84°N, confirming the absence of a dayside magnetosphere during this orbit.
• The ICME ram pressure exceeded 86 nPa, the threshold estimated to collapse Mercury’s magnetosphere, and was the highest ever measured at Mercury.
• FIPS data showed no dayside equatorial Na+ during the peak compression, consistent with magnetospheric collapse, followed by increased Na+ densities in subsequent orbits due to sputtering.
• The event provides direct evidence that ICME plasma can directly impact Mercury’s surface at low latitudes, enabling a new mechanism for exospheric sodium generation.
• The study estimates that ~30% of ICME-affected MESSENGER orbits experienced magnetospheric collapse, with ~10 such events expected during BepiColombo’s nominal mission at Mercury.