Mercury's Magnetopause and Bow Shock from MESSENGER Magnetometer Observations

We have established the average shape and location of Mercury's magnetopause and bow shock from orbital observations by the MESSENGER Magnetometer. We fit empirical models to midpoints of boundary crossings and probability density maps of the magnetopause and bow shock positions. The magnetopause was fit by a surface for which the position R from the planetary dipole varies as [1 + cos(theta)]-alpha, where theta is the angle between R and the dipole-Sun line, the subsolar standoff distance Rss is 1.45 RM (where RM is Mercury's radius), and the flaring parameter alpha = 0.5. The average magnetopause shape and location were determined under a mean solar wind ram pressure PRam of 14.3 nPa. The best fit bow shock shape established under an average Alfvén Mach number (MA) of 6.6 is described by a hyperboloid having Rss = 1.96 RM and an eccentricity of 1.02. These boundaries move as PRam and MA vary, but their shapes remain unchanged. The magnetopause Rss varies from 1.55 to 1.35 RM for PRam in the range of 8.8-21.6 nPa. The bow shock Rss varies from 2.29 to 1.89 RM for MA in the range of 4.12-11.8. The boundaries are well approximated by figures of revolution. Additional quantifiable effects of the interplanetary magnetic field are masked by the large dynamic variability of these boundaries. The magnetotail surface is nearly cylindrical, with a radius of ~2.7 RM at a distance of 3 RM downstream of Mercury. By comparison, Earth's magnetotail flaring continues until a downstream distance of ~10 Rss

Generic magnetic field intensity profiles of interplanetary coronal mass ejections at Mercury, Venus and Earth from superposed epoch analyses

We study interplanetary coronal mass ejections (ICMEs) measured by probes at different heliocentric distances (0.3-1 AU) to investigate the propagation of ICMEs in the inner heliosphere and determine how the generic features of ICMEs change with heliospheric distance. Using data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER), Venus Express and ACE spacecraft, we analyze with the superposed epoch technique the profiles of ICME substructures, namely, the sheath and the magnetic ejecta. We determine that the median magnetic field magnitude in the sheath correlates well with ICME speeds at 1 AU, and we use this proxy to order the ICMEs at all spacecraft. We then investigate the typical ICME profiles for three categories equivalent to slow, intermediate, and fast ICMEs. Contrary to fast ICMEs, slow ICMEs have a weaker solar wind field at the front and a more symmetric magnetic field profile. We find the asymmetry to be less pronounced at Earth than at Mercury, indicating a relaxation taking place as ICMEs propagate. We also find that the magnetic field intensities in the wake region of the ICMEs do not go back to the pre-ICME solar wind intensities, suggesting that the effects of ICMEs on the ambient solar wind last longer than the duration of the transient event. Such results provide an indication of physical processes that need to be reproduced by numerical simulations of ICME propagation. The samples studied here will be greatly improved by future missions dedicated to the exploration of the inner heliosphere, such as Parker Solar Probe and Solar Orbiter.Peer reviewe

Longitudinal conjunction between MESSENGER and STEREO A: Development of ICME complexity through stream interactions

We use data on an interplanetary coronal mass ejection (ICME) seen by MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) and STEREO A starting on 29 December 2011 in a near‐perfect longitudinal conjunction (within 3°) to illustrate changes in its structure via interaction with the solar wind in less than 0.6 AU. From force‐free field modeling we infer that the orientation of the underlying flux rope has undergone a rotation of ∼80° in latitude and ∼65° in longitude. Based on both spacecraft measurements as well as ENLIL model simulations of the steady state solar wind, we find that interaction involving magnetic reconnection with corotating structures in the solar wind dramatically alters the ICME magnetic field. In particular, we observed a highly turbulent region with distinct properties within the flux rope at STEREO A, not observed at MESSENGER, which we attribute to interaction between the ICME and a heliospheric plasma sheet/current sheet during propagation. Our case study is a concrete example of a sequence of events that can increase the complexity of ICMEs with heliocentric distance even in the inner heliosphere. The results highlight the need for large‐scale statistical studies of ICME events observed in conjunction at different heliocentric distances to determine how frequently significant changes in flux rope orientation occur during propagation. These results also have significant implications for space weather forecasting and should serve as a caution on using very distant observations to predict the geoeffectiveness of large interplanetary transients.Key PointsICME complexity increases due to interaction with corotating structures in the solar windMagnetic reconnection between ICME and HPS/HCS alters the magnetic topology of the ICME flux ropeCaution on using distant observations to predict the geoeffectiveness of interplanetary transientsPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/134123/1/jgra52739.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/134123/2/jgra52739_am.pd

Mercury's Surface Magnetic Field Determined from Proton-Reflection Magnetometry

Solar wind protons observed by the MESSENGER spacecraft in orbit about Mercury exhibit signatures of precipitation loss to Mercury's surface. We apply proton-reflection magnetometry to sense Mercury's surface magnetic field intensity in the planet's northern and southern hemispheres. The results are consistent with a dipole field offset to the north and show that the technique may be used to resolve regional-scale fields at the surface. The proton loss cones indicate persistent ion precipitation to the surface in the northern magnetospheric cusp region and in the southern hemisphere at low nightside latitudes. The latter observation implies that most of the surface in Mercury's southern hemisphere is continuously bombarded by plasma, in contrast with the premise that the global magnetic field largely protects the planetary surface from the solar wind

Plasma pressure in Mercury's equatorial magnetosphere derived from MESSENGER Magnetometer observations

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95264/1/grl28621-sup-0002-txts01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/95264/2/grl28621.pd

Low‐degree structure in Mercury's planetary magnetic field

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96330/1/jgre3134.pd

Spatial Distribution and Spectral Characteristics of Energetic Electrons in Mercury's Magnetosphere

The Energetic Particle Spectrometer (EPS) on the MESSENGER spacecraft, in orbit about Mercury since March 2011, has detected bursts of low- and moderate-energy (tens to hundreds of keV) electrons during portions of most orbits. There have been periods when such bursts were observed regularly on every orbit over a span of several weeks, and other periods when electrons were not observed for several days at a time. We have systematically characterized these energetic events on the basis of particle intensity over the 12-month period since MESSENGER began orbital operations. Now that MESSENGER has sampled most Mercury longitudes and local times, it is evident that the largest burst events were either at high northern latitudes or near local midnight. Lower-energy events were also seen near the equator but were mostly absent in both the dawn and dusk local time sectors. The high-latitude and nightside events are similar in particle intensity, spectra, and pitch angle and are interpreted to be the result of acceleration by the same mechanism. Another group of events occurred upstream of Mercury's bow shock. For two examples of this group of upstream events with good pitch angle coverage, the particles were field-aligned and traveling away from the bow shock. This group of events is interpreted to be similar to upstream events found at Earth during which particles are accelerated at the bow shock and subsequently travel upstream into the solar wind

Comprehensive survey of energetic electron events in Mercury\u27s magnetosphere with data from the MESSENGER Gamma-Ray and Neutron Spectrometer

Data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Gamma-Ray and Neutron Spectrometer have been used to detect and characterize energetic electron (EE) events in Mercury\u27s magnetosphere. This instrument detects EE events indirectly via bremsstrahlung photons that are emitted when instrument and spacecraft materials stop electrons having energies of tens to hundreds of keV. From Neutron Spectrometer data taken between 18 March 2011 and 31 December 2013 we have identified 2711 EE events. EE event amplitudes versus energy are distributed as a power law and have a dynamic range of a factor of 400. The duration of the EE events ranges from tens of seconds to nearly 20 min. EE events may be classified as bursty (large variation with time over an event) or smooth (small variation). Almost all EE events are detected inside Mercury\u27s magnetosphere on closed field lines. The precise occurrence times of EE events are stochastic, but the events are located in well-defined regions with clear boundaries that persist in time and form what we call “quasi-permanent structures.” Bursty events occur closer to dawn and at higher latitudes than smooth events, which are seen near noon-to-dusk local times at lower latitudes. A subset of EE events shows strong periodicities that range from hundreds of seconds to tens of milliseconds. The few-minute periodicities are consistent with the Dungey cycle timescale for the magnetosphere and the occurrence of substorm events in Mercury\u27s magnetotail region. Shorter periods may be related to phenomena such as north-south bounce processes for the energetic electrons

Characteristics of the plasma distribution in Mercury's equatorial magnetosphere derived from MESSENGER Magnetometer observations

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/96228/1/jgra22273.pd