Suzanne M. Imber

MESSENGER observations of Mercury's dayside magnetosphere under extreme solar wind conditions

CLJ and RMW acknowledge support from the Natural Sciences and Engineering Research Council of Canada, and CLJ acknowledges support from MESSENGER Participating Scientist grant NNX11AB84G. The MESSENGER project is supported by the NASA Discovery Program under contracts NASW- 00002 to the Carnegie Institution of Washington and NAS5-97271 to The Johns Hopkins University Applied Physics Laboratory.

Comment on "Jupiter: A fundamentally different magnetospheric interaction with the solar wind" by D. J. McComas and F. Bagenal

[1] McComas and Bagenal [2007] (hereinafter referred to as MB) have presented a discussion of the reconnectionmediated interaction of the Jovian magnetosphere with the interplanetary medium, which they suggest to be significantly different to that at Earth. In the latter case, it is well established that ‘open’ flux is produced at the magnetopause when the interplanetary magnetic field (IMF) is directed opposite to the equatorial planetary field, is transported to the tail by the solar wind, and returns as closed flux via plasma sheet reconnection preferentially during substorms, thus forming the Dungey cycle of flux transport [e.g., Dungey, 1961]. MB propose that the consequences of open flux production at Jupiter are different, however, due to a suggested difficulty of closed flux tube return from the tail against a substantial down-tail flow of iogenic plasma. They suggest instead that open flux is effectively removed by two-lobe reconnection when the IMF has the opposite polarity, such that the open flux in the system remains small. Two-lobe reconnection has been discussed theoretically for many years [e.g., Dungey, 1963; Cowley, 1981], though convincing evidence for its occurrence at Earth has only recently been found [e.g., Imber et al., 2006, 2007]. Here, however, we question both aspects of MB’s discussion. [2] With regard to the return of tail flux by plasma sheet reconnection, MB characterise the process as requiring closed field contraction over distances of 1500–2000 RJ at speeds of 40 km s , thus requiring 30–40 days. They suggest this to be unlikely given the surrounding fast down-tail flow of iogenic plasma. However, we regard this scenario as being unduly pessimistic, first because the estimate of the distance to the tail reconnection site is unrealistically large, and second because the closed flux tube contraction speed is unrealistically small, both contributing to unrealistically large estimates of the transport time. MB’s estimate of the distance to the tail reconnection site is essentially the length of the entire tail of open field lines, obtained by multiplying the solar wind speed by the residence time of open flux tubes in the lobe. This time is estimated to be 3–4 days on the basis that open field lines flow toward the plasma sheet at 10% of the solar wind speed ( 40 km s ) for 10% reconnection efficiency with the IMF, leading to a tail length of 1500–2000 RJ as indicated above. In fact, this significantly underestimates the length of the Jovian tail, since the lobe flow speed is slowed relative to MB’s estimate by the ratio of the lobe and IMF field strengths, i.e. factors of two to three, while an overall magnetopause reconnection efficiency of 10% seems optimistic. A more realistic residence time is 10– 20 days [Nichols et al., 2006], leading to tail lengths of 5000–10000 RJ in agreement with Lepping et al. [1983]. [3] The main point to emphasise, however, is not the inaccuracy of MB’s tail length estimate, but that such estimates provide no information about the location of the tail reconnection sites, other than an upper limit. For Earth, for example, similar estimates produce tail lengths of 1000 RE [e.g., Milan, 2004], while substorm-related reconnection is typically initiated at down-tail distances of 20–30 RE [e.g., Nagai and Machida, 1998]. While flux return from the distant tail may be unlikely as MB suggest, a reasonable conclusion is that open flux will then accumulate until reconnection occurs substorm-like sufficiently close to the planet that the closed flux is indeed able to return. The return flow speeds are then expected to be comparable to the lobe Alfven speed [e.g., Badman and Cowley, 2007], at least an order of magnitude faster than the return speeds employed in MB’s estimate. [4] Significant evidence indeed exists for sporadic reconnection in the Jovian nightside plasma sheet at distances of 100 RJ, resulting in ion jets directed both toward and away from the planet [e.g., Woch et al., 2002]. These dynamics are generally assumed to relate to pinch-off of distended closed field lines and the down-tail release of iogenic plasma occurring as part of the Vasyliunas cycle [Vasyliunas, 1983]. However, supposing that after plasmoid release the reconnection continues into the tail lobe, then closed flux is generated that will clearly flow back to the planet unencumbered by surrounding down-tail flow, whether the combined reconnection is envisaged as largescale [Cowley et al., 2003], or occurs more sporadically and multiply on smaller scales [Kivelson and Southwood, 2005]. While Dungeyand Vasyliunas-cycle tail reconnection need not be coherently related in this way, the argument is sufficient to show that open flux return from the Jovian tail by plasma sheet reconnection is not as problematic as MB suggest. [5] We now turn to MB’s second argument, that open flux can instead be effectively removed from the tail by two-lobe reconnection poleward of the cusp, such that the amount of open flux in the system remains small. This requires the open flux removal rate by two-lobe reconnection for southward-directed IMF, averaged over typical GEOPHYSICAL RESEARCH LETTERS, VOL. 35, L10101, doi:10.1029/2007GL032645, 2008

MESSENGER observations of large dayside flux transfer events: Do they drive Mercury's substorm cycle?

The large-scale dynamic behavior of Mercurys highly compressed magnetosphere is predominantly powered by magnetic reconnection, which transfers energy and momentum from the solar wind to the magnetosphere. The contribution of flux transfer events (FTEs) at the dayside magnetopause to the redistribution of magnetic flux in Mercurys magnetosphere is assessed with magnetic field data acquired in orbit about Mercury by the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. FTEs with core fields greater than the planetary field just inside the magnetopause are prevalent at Mercury. Fifty-eight such large-amplitude FTEs were identified during February and May 2012, when MESSENGER sampled the subsolar magnetosheath. The orientation of each FTE was determined by minimum variance analysis, and the magnetic flux content of each was estimated using a force-free flux rope model. The average flux content of the FTEs was 0.06 MWb, and their durations imply a transient increase in the cross-polar cap potential of ~25 kV. For a substorm timescale of 2–3 min, as indicated by magnetotail flux loading and unloading, the FTE repetition rate (10 s) and average flux content (assumed to be 0.03 MWb) imply that FTEs contribute at least ~30% of the flux transport required to drive the Mercury substorm cycle. At Earth, in contrast, FTEs are estimated to contribute less than 2% of the substorm flux transport. This result implies that whereas at Earth, at which steady-state dayside reconnection is prevalent, multiple X-line dayside reconnection and associated FTEs at Mercury are a dominant forcing for magnetospheric dynamics.

MESSENGER observations of magnetospheric substorm activity in Mercury's near magnetotail

MErcury Surface, Space ENviroment, GEochemistry, and Ranging (MESSENGER) magnetic field and plasma measurements taken during crossings of Mercurys magnetotail from 2011 to 2014 have been examined for evidence of substorms. A total of 26 events were found during which an Earth-like growth phase was followed by clear near-tail expansion phase signatures. During the growth phase, just as at Earth, the thinning of the plasma sheet and the increase of the magnetic field intensity in the lobe are observed, but the fractional increase in field intensity could be ∼3 to 5 times that at Earth. The average timescale of the growth phase is ∼1 min. The dipolarization that marks the initiation of the substorm expansion phase is only a few seconds in duration. During the expansion phase, lasting ∼1 min, the plasma sheet is observed to thicken and engulf the spacecraft. The duration of the substorm observed in this paper is consistent with previous observations of Mercurys Dungey cycle. The reconfiguration of the magnetotail during Mercurys substorm is very similar to that at Earth despite its very compressed timescale.

Interplanetary magnetic field control of the ionospheric field‐aligned current and convection distributions

Patterns of the high-latitude ionospheric convection and field-aligned current (FAC) are a manifestation of the solar wind-magnetosphere-ionosphere coupling. By observing them we can acquire information on magnetopause reconnection, a process through which solar wind energy enters the magnetosphere. We use over 10 years of magnetic field and convection data from the CHAMP satellite and Super Dual Auroral Radar Network radars, respectively, to display combined distributions of the FACs and convection for different interplanetary magnetic field (IMF) orientations and amplitudes. During southward IMF, convection follows the established two-cell pattern with associated Region 1 and Region 2 FACs, indicating subsolar reconnection. During northward IMF, superposed on a weak two-cell pattern there is a reversed two-cell pattern with associated Region 0 and Region 1 FACs on the dayside, indicating lobe reconnection. For dominant IMF Bx, the sign of Bz determines whether lobe or subsolar reconnection signatures will be observed, but Bx will weaken the signatures compared to pure northward or southward IMF. When the IMF rotates from northward to duskward or dawnward, the distinct reversed and forward two-cell patterns start to merge into a distorted two-cell pattern. This is in agreement with the IMF By displacing the reconnection location from the open lobe field lines to closed dawn or dusk field lines, even though IMF Bz>0. As the IMF continues to rotate southward, the distorted pattern transforms smoothly to that of the symmetric two-cell pattern. While the IMF direction determines the configuration of the FACs and convection, the IMF amplitude affects their intensity.

Mercury's cross-tail current sheet: Structure, X-line location and stress balance

The structure, X-line location, and magnetohydrodynamic (MHD) stress balance of Mercurys magnetotail were examined between −2.6 < XMSM < −1.4 RM using MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) measurements from 319 central plasma sheet (CPS) crossings. The mean plasma β in the CPS calculated from MESSENGER data is ~ 6. The CPS magnetic field was southward (i.e., tailward of X-line) ~ 2–18% of the time. Extrapolation of downtail variations in BZ indicates an average X-line location at −3 RM. Modeling of magnetic field measurements produced a cross-tail current sheet (CS) thickness, current density, and inner CS edge location of 0.39 RM, 92 nA/m2 and −1.22 RM, respectively. Application of MHD stress balance suggests that heavy planetary ions may be important in maintaining stress balance within Mercurys CPS. Qualitative similarities between Mercurys and Earths magnetotail are remarkable given the differences in upstream conditions, internal plasma composition, finite gyro-radius scaling, and Mercurys lack of ionosphere.

MESSENGER observations of cusp plasma filaments at Mercury

The MESSENGER spacecraft while in orbit about Mercury observed highly localized, ~3-s-long reductions in the dayside magnetospheric magnetic field, with amplitudes up to 90% of the ambient intensity. These magnetic field depressions are termed cusp filaments because they were observed from just poleward of the magnetospheric cusp to mid-latitudes, i.e., ~55° to 85° N. We analyzed 345 high- and low-altitude cusp filaments identified from MESSENGER magnetic field data to determine their physical properties. Minimum variance analysis indicates that most filaments resemble cylindrical flux tubes within which the magnetic field intensity decreases toward its central axis. If the filaments move over the spacecraft at an estimated magnetospheric convection speed of ~35 km/s, then they have a typical diameter of ~105 km or ~7 gyro-radii for 1 keV H+ ions in a 300 nT magnetic field. During these events, MESSENGERs Fast Imaging Plasma Spectrometer observed H+ ions with magnetosheath-like energies. MESSENGER observations during the spacecrafts final low-altitude campaign revealed that these cusp filaments likely extend down to Mercurys surface. We calculated an occurrence-rate-normalized integrated particle precipitation rate onto the surface from all filaments of (2.70 ± 0.09) × 1025 s-1. This precipitation rate is comparable to published estimates of the total precipitation rate in the larger-scale cusp. Overall, the MESSENGER observations analyzed here suggest that cusp filaments are the magnetospheric extensions of the flux transfer events that form at the magnetopause as a result of localized magnetic reconnection.

Coupling between Mercury and its nightside magnetosphere: Cross‐tail current sheet asymmetry and substorm current wedge formation

We analyzed MESSENGER magnetic field and plasma measurements taken during 319 crossings of Mercurys cross-tail current sheet. We found that the measured BZ in the current sheet is higher on the dawn-side than the dusk-side by a factor of ≈ 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 dusk-side have a higher plasma beta than those on the dawn-side, where they are less stretched. This asymmetric behavior is confirmed by mean current sheet thickness being greatest on the dawn-side. We propose that heavy planetary ion (e.g. Na+) enhancements in the dusk-side current sheet provides the most likely explanation for the dawn-dusk current sheet asymmetries. We also report the direct measurement of Mercurys substorm current wedge (SCW) formation and estimate the total current due to pileup of magnetic flux to be ≈ 11 kA. The conductance at the foot of the field-lines required to close the SCW current is found to be ≈ 1.2 S, which is similar to earlier results derived from modelling of Mercurys Region 1 field-aligned currents. Hence, Mercurys regolith is sufficiently conductive for the current to flow radially, then across the surface of Mercurys highly conductive iron core. Mercury appears to be closely coupled to its night-side magnetosphere by mass loading of upward flowing heavy planetary ions, and electrodynamically by field-aligned currents that transfer momentum and energy to the night-side auroral oval crust and interior.

Mars plasma system response to solar wind disturbances during solar minimum

This paper is a phenomenological description of the ionospheric plasma and induced magnetospheric boundary (IMB) response to two different types of upstream solar wind events impacting Mars in March 2008, at the solar minimum. A total of 16 Mars Express orbits corresponding to five consecutive days is evaluated. Solar TErrestrial RElations Observatory-B (STEREO-B) at 1 AU and Mars Express and Mars Odyssey at 1.644 AU detected the arrival of a small transient interplanetary coronal mass ejection (ICME-like) on the 6 and 7 of March, respectively. This is the first time that this kind of small solar structure is reported at Marss distance. In both cases, it was followed by a large increase in solar wind velocity that lasted for ~10 days. This scenario is simulated with the Wang-Sheeley-Arge (WSA) - ENLIL + Cone solar solar wind model. At Mars, the ICME-like event caused a strong compression of the magnetosheath and ionosphere, and the recovery lasted for ~3 orbits (~20 h). After that, the fast stream affected the upper ionosphere and the IMB, which radial and tangential motions in regions close to the subsolar point are analyzed. Moreover, a compression in the Martian plasma system is also observed, although weaker than after the ICME-like impact, and several magnetosheath plasma blobs in the upper ionosphere are detected by Mars Express. We conclude that, during solar minimum and at aphelion, small solar wind structures can create larger perturbations than previously expected in the Martian system.

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

Abstract 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 that 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 foot points 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 foot point 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.