Coronal and heliospheric magnetic flux circulation and its relation to open solar flux evolution

Solar cycle 24 is notable for three features that can be found in previous cycles but which have been unusually prominent: (1) sunspot activity was considerably greater in the northern/southern hemisphere during the rising/declining phase; (2) accumulation of Open Solar Flux (OSF) during the rising phase was modest, but rapid in the early declining phase; (3) the Heliospheric Current Sheet (HCS) tilt showed large fluctuations. We show these features had a major influence on the progression of the cycle. All flux emergence causes a rise then a fall in OSF, but only OSF with footpoints in opposing hemispheres progresses the solar cycle via the evolution of the polar fields. Emergence in one hemisphere, or symmetric emergence without some form of footpoint exchange across the heliographic equator, causes poleward-migrating fields of both polarities in one or both (respectively) hemispheres which temporarily enhance OSF but do not advance the polar field cycle. The heliospheric field observed near Mercury and Earth reflects the asymmetries in emergence. Using magnetograms, we find evidence that the poleward magnetic flux transport (of both polarities) is modulated by the HCS tilt, revealing an effect on OSF loss rate. The declining phase rise in OSF was caused by strong emergence in the southern hemisphere with an anomalously low HCS tilt. This implies the recent fall in the southern polar field will be sustained and that the peak OSF has limited implications for the polar field at the next sunspot minimum and hence for the amplitude of cycle 25

Transport of Mass and Energy in Mercury’s Plasma Sheet

We examined the transport of mass and energy in Mercury’s plasma sheet (PS) using MESSENGER magnetic field and plasma measurements obtained during 759 PS crossings. Regression analysis of proton density and plasma pressure shows a strong linear relationship. We calculated the polytropic index γ for Mercury’s PS to be ~0.687, indicating that the plasma in the tail PS behaves nonadiabatically as it is transported sunward. Using the average magnetic field intensity of Mercury’s tail lobe as a proxy for magnetotail activity level, we demonstrated that γ is lower during active time periods. A minimum in γ was observed at R ~ 1.4 RM, which coincides with previously observed location of Mercury’s substorm current wedge. We suggest that the nonadiabatic behavior of plasma as it is transported into Mercury’s nearâ tail region is primarily driven by particle precipitation and particle scattering due to large loss cone and particle acceleration effect, respectively.Plain Language SummaryThe transport process of mass and energy within Mercury’s magnetotail remains unexplored until now. The availability of in situ magnetic field and plasma measurements from National Aeronautics and Space Administration’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft provides us with the first opportunity to study the thermodynamic properties of particles within sunward convecting closed flux tubes in the plasma sheet. In this study, we study how mass and energy are transported in Mercury’s magnetotail by investigating the relationship between the thermal pressure and number density of the plasma in Mercury’s plasma sheet given by the equation of state in magnetohydrodynamics theory. We determined, for the first time, that the plasma behaves nonadiabatically as it is transported sunward toward Mercury. We suggest that precipitation of particles due to Mercury’s large loss cone and demagnetization of particles due to finite gyroradius effect contributes to this nonadiabatic behavior of plasma in the plasma sheet. Our results have major implications in our understanding of particle sources and sinks mechanisms in Mercury’s magnetotail.Key PointsWe calculated the value of polytropic index γ for Mercury’s plasma sheet to be ~0.687, which is smaller than 5/3 (adiabatic)Nonadiabatic plasma behavior is driven by ion precipitation and ion demagnetization due to large loss cone and finite gyroradius effectWe demonstrated that γ is lower during active time and determined a relationship between γ and the location of flow breaking regionPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/147033/1/grl58293_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/147033/2/grl58293.pd

MESSENGER Observations of a Flux-Transfer-Event Shower at Mercury

Analysis of MESSENGER magnetic field observations taken in the southern lobe of Mercury's magnetotail and the adjacent magnetosheath on 11 April 2011 indicates that a total of 163 flux transfer events (FTEs) occurred within a 25 min interval. Each FTE had a duration of ∼2-3 s and was separated in time from the next by ∼8-10 s. A range of values have been reported at Earth, with mean values near ∼1-2 min and ∼8 min, respectively. We term these intervals of quasiperiodic flux transfer events "FTE showers." The northward and sunward orientation of the interplanetary magnetic field during this shower strongly suggests that the FTEs observed during this event formed just tailward of Mercury's southern magnetic cusp. The point of origin for the shower was confirmed with the Cooling model of FTE motion. Modeling of the individual FTE-type flux ropes in the magnetosheath indicates that these flux ropes had elliptical cross sections, a mean semimajor axis of 0.15RM (where RM is Mercury's radius, or 2440 km), and a mean axial magnetic flux of 1.25 MWb. The lobe magnetic field was relatively constant until the onset of the FTE shower, but thereafter the field magnitude decreased steadily until the spacecraft crossed the magnetopause. This decrease in magnetic field intensity is frequently observed during FTE showers. Such a decrease may be due to the diamagnetism of the new magnetosheath plasma being injected into the tail by the FTEs

FOREWORD

We analyzed MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) magnetic field and plasma measurements taken during 319 crossings of Mercury’s crossâ tail current sheet. We found that the measured BZ in the current sheet is higher on the dawnside than the duskside by a factor of â 3 and the asymmetry decreases with downtail distance. This result is consistent with expectations based upon MHD stress balance. The magnetic fields threading the more stretched current sheet in the duskside have a higher plasma beta than those on the dawnside, where they are less stretched. This asymmetric behavior is confirmed by mean current sheet thickness being greatest on the dawnside. We propose that heavy planetary ion (e.g., Na+) enhancements in the duskside current sheet provides the most likely explanation for the dawnâ dusk current sheet asymmetries. We also report the direct measurement of Mercury’s substorm current wedge (SCW) formation and estimate the total current due to pileup of magnetic flux to be â 11 kA. The conductance at the foot of the field lines required to close the SCW current is found to be â 1.2 S, which is similar to earlier results derived from modeling of Mercury’s Region 1 fieldâ aligned currents. Hence, Mercury’s regolith is sufficiently conductive for the current to flow radially then across the surface of Mercury’s highly conductive iron core. Mercury appears to be closely coupled to its nightside magnetosphere by mass loading of upward flowing heavy planetary ions and electrodynamically by fieldâ aligned currents that transfer momentum and energy to the nightside auroral oval crust and interior. Heavy planetary ion enhancements in Mercury’s duskside current sheet provide explanation for crossâ tail asymmetries found in this study. The total current due to the pileup of magnetic flux and conductance required to close the SCW current is found to be â 11 kA and 1.2 S. Mercury is coupled to magnetotail by mass loading of heavy ions and fieldâ aligned currents driven by reconnectionâ related fast plasma flow.Key PointsHeavy planetary ion enhancements in Mercury’s duskside current sheet provide explanation for crossâ tail asymmetries found in this studyThe total current due to the pileup of magnetic flux and conductance required to close the SCW current is found to be almost equal to 11 kA and 1.2 SMercury is coupled to magnetotail by mass loading of heavy ions and fieldâ aligned currents driven by reconnectionâ related fast plasma flowPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/138879/1/jgra53698.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/138879/2/jgra53698_am.pd

MESSENGER and Mariner 10 flyby observations of magnetotail structure and dynamics at Mercury

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94896/1/jgra21525.pd

MESSENGER Observations of Magnetotail Loading and Unloading: Implications for Substorms at Mercury

We present the first statistical study of loading and unloading of magnetic flux in Mercury's magnetotail. These events describe the global circulation of magnetic flux through the magnetosphere and provide strong evidence that terrestrial-type substorms take place at Mercury. 438 events were identified over the 4 years of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission by a gradual, short-lived increase in the magnetotail lobe magnetic field strength, coincident with an outward flaring of the magnetotail. Substorm duration ranged from tens of seconds to several minutes, with a median of 195 s and a mean of 212 s. The median amplitude of lobe magnetic field increase was ~11.5 nT, which represents an increase of 23.4% on the background lobe field strength, compared with ~10% for terrestrial substorms. The magnetotail lobes were found to contain ~2–3 MWb of magnetic flux based on 1031 tail passes, with a mean of 2.52 MWb and a standard deviation of 0.48 MWb. An estimate of the change in open flux content during the loading phase of each substorm ranged from 0.08 to 3.7 MWb with a mean value of 0.69 MWb and a standard deviation of 0.38 MWb. These changes in open flux content are an underestimate as the change in magnetotail radius during the events was not accounted for. The maximum lobe flux content during each substorm (~3 MWb) represented ~40% of the total available magnetic flux in the system (~7.5 MWb). During terrestrial substorms, the maximum lobe magnetic flux content is ~10–12% of the total flux from the dipole. A typical substorm at Mercury therefore cycles through a significantly larger fraction of the available magnetic flux than all but the largest substorms at the Earth

The Influence of IMF Clock Angle on Dayside Flux Transfer Events at Mercury

Analysis of MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) data has shown for the first time that the orientation of the interplanetary magnetic field (IMF) in the magnetosheath of Mercury plays a crucial role in the formation of flux transfer events (FTEs) at the dayside magnetopause. During the first 4 Hermean years of MESSENGER's orbit around Mercury, we have identified 805 FTEs using magnetometer data. Under conditions of near‐southward IMF, at least one FTE was detected on nearly 70% of passes through the magnetopause but the observation rate during northward IMF was less than 20%. FTEs were also observed preferentially in the prenoon sector

Field Line Resonance in the Hermean Magnetosphere: Structure and Implications for Plasma Distribution

The first statistical survey of field line resonance (FLR) events is presented using magnetometer data from the entire MErcury Surface, Space ENvironment, GEochemistry and Ranging mission. Ultralow-frequency waves are an important tool for the magnetoseismology of the Hermean magnetosphere; this study provides a completely new window onto the resonance structures and plasma density distribution in the Hermean magnetosphere. Here we assess resonance events from two categories—toroidal resonances characteristic of the classical picture of FLRs in the terrestrial magnetosphere driven by the Kelvin-Helmholtz instability and a more comprehensive approach including all observed transverse resonances with more relaxed polarization criteria. Two hundred twenty-three toroidal FLRs with characteristics consistent with Kelvin-Helmholtz-driven FLRs are found in the dayside Hermean magnetosphere. The fundamental frequencies of these waves are used to provide estimates of plasma mass density in the range of ∼ 1–650 amu/cm3. A further 343 transverse resonances are found which provide very similar density estimates to the Earth-like FLR population. Fundamental and harmonic frequencies from all 566 events are used to fit a power law to plasma mass density along the field lines. The equatorial plasma mass density is predicted to vary approximately with R−7.5. The offset of the Hermean dipole into the northern hemisphere causes significant asymmetries in the standing wave structure. Due to the extreme warping (away from a dipolar configuration) of Mercury's magnetosphere by the solar wind, the fundamental toroidal mode is predicted to oscillate with a notably lower frequency than the fundamental poloidal mode, contrary to relative toroidal and poloidal frequencies modeled for Earth's magnetosphere

What controls the local time extent of flux transfer events?

Flux transfer events (FTEs) are the manifestation of bursty and/or patchy magnetic reconnection at the magnetopause. We compare two sequences of the ionospheric signatures of flux transfer events observed in global auroral imagery and coherent ionospheric radar measurements. Both sequences were observed during very similar seasonal and interplanetary magnetic field (IMF) conditions, though with differing solar wind speed. A key observation is that the signatures differed considerably in their local time extent. The two periods are 26 August 1998, when the IMF had components BZ≈−10 nT and BY≈9 nT and the solar wind speed was VX≈650 km s−1, and 31 August 2005, IMF BZ≈−7 nT, BY≈17 nT, and VX≈380 km s−1. In the first case, the reconnection rate was estimated to be near 160 kV, and the FTE signatures extended across at least 7 h of magnetic local time (MLT) of the dayside polar cap boundary. In the second, a reconnection rate close to 80 kV was estimated, and the FTEs had a MLT extent of roughly 2 h. We discuss the ramifications of these differences for solar wind-magnetosphere coupling