MESSENGER observations of the dayside low‐latitude boundary layer in Mercury’s magnetosphere

Observations from MErcury Surface Space ENvironment GEochemistry, and Ranging (MESSENGER)’s Magnetometer and Fast Imaging Plasma Spectrometer instruments during the first orbital year have resulted in the identification of 25 magnetopause crossings in Mercury’s magnetosphere with significant low‐latitude boundary layers (LLBLs). Of these crossings 72% are observed dawnside and 65% for northward interplanetary magnetic field. The estimated LLBL thickness is 450 ± 56 km and increases with distance to noon. The Na+ group ion is sporadically present in 14 of the boundary layers, with an observed average number density of 22 ± 11% of the proton density. Furthermore, the average Na+ group gyroradii in the layers is 220 ± 34 km, the same order of magnitude as the LLBL thickness. Magnetic shear, plasma β and reconnection rates have been estimated for the LLBL crossings and compared to those of a control group (non‐LLBL) of 61 distinct magnetopause crossings which show signs of nearly no plasma inside the magnetopause. The results indicate that reconnection is significantly slower, or even suppressed, for the LLBL crossings compared to the non‐LLBL cases. Possible processes that form or impact the LLBL are discussed. Protons injected through the cusp or flank may be important for the formation of the LLBL. Furthermore, the opposite asymmetry in the Kelvin‐Helmholtz instability (KHI) as compared to the LLBL rules out the KHI as a dominant formation mechanism. However, the KHI and LLBL could be related to each other, either by the impact of sodium ions gyrating across the magnetopause or by the LLBL preventing the growth of KH waves on the dawnside.Key PointsInvestigation, characterization, and observation of the low‐latitude boundary layer of MercuryIs there a relation between the Kelvin‐Helmholtz instability and the low‐latitude boundary layerInvestigate for what surrounding conditions the low‐latitude boundary layer occursPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/136336/1/jgra52122_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/136336/2/jgra52122.pd

Aurora in the Polar Cap: A Review

This paper reviews our current understanding of auroral features that appear poleward of the main auroral oval within the polar cap, especially those that are known as Sun-aligned arcs, transpolar arcs, or theta auroras. They tend to appear predominantly during periods of quiet geomagnetic activity or northwards directed interplanetary magnetic field (IMF). We also introduce polar rain aurora which has been considered as a phenomenon on open field lines. We describe the morphology of such auroras, their development and dynamics in response to solar wind-magnetosphere coupling processes, and the models that have been developed to explain them

Polar auroral arcs

NR 2014080

Importance of the dusk-dawn interplanetary magnetic field component (IMF By) to magnetospheric convection in Earth's magnetotail plasma sheet

The solar wind and its embedded magnetic field, the interplanetary magnetic field (IMF) together with magnetic reconnection power the large-scale plasma and magnetic flux circulation in the Earth's magnetosphere-ionosphere system. This circulation is termed as convection and its strength is controlled by the north-south IMF component (IMF Bz). In recent years, an interest has arisen to investigate the lesser-known role of the dusk-dawn component (IMF By) in convection. It has been previously known though that prevailing nonzero IMF By can cause plasma flow asymmetries in the high-latitude ionosphere, but how the magnetospheric flows, for instance, in the magnetotail plasma sheet are affected, remains to be investigated. In this article, we introduce the recent progress and the latest achievements in the research of the influence of IMF By on tail plasma sheet convection. The research progress has been rapid and it has revealed that both fast and slow convection are affected in a manner that is in accordance with the asymmetries observed in the ionospheric convection. The results indicate the significance of the IMF By component on magnetospheric convection and they represent a major advance in the field of solar wind-magnetosphere coupling

The question of transpolar arc conjugacy:new results from comparing solar wind data and dipole tilt distribution of five different datasets

Abstract In this work, we investigate the interhemispheric transpolar arc (TPA) conjugacy using four previously published datasets based on Polar UV images, DMSP particle data, and IMAGE images, and a new TPA list based on DMSP SSUSI images during 1 Sep to 15 Oct 2015. IMF Bₓ and the Earth’s dipole tilt have often been suggested to influence the TPA conjugacy, as both induce a north-south asymmetry on the magnetosphere. However, by comparing these parameters at TPA formation with the background distribution for each dataset, we find that neither the dipole tilt nor Bₓ plays a major role for the TPA conjugacy in four of the five datasets. The well-known correlation between initial TPA location and IMF By appears in all datasets with information about the TPA formation. In addition, we find that a minority of dawnside TPAs form during the “wrong” By sign. In the northern (southern) hemisphere, dawn TPAs appear also during weakly duskward (dawnward) IMF. Due to the polar orbit of DMSP satellites, TPA conjugacy and location can be examined on a case-by-case basis with the new dataset. The results show that at least 73% of TPAs appear in both hemispheres simultaneously. IMF Bₓ and dipole tilt values for conjugate TPAs do not differ from those for non-conjugate TPAs. Most conjugate (isolated) TPAs appear on opposite oval sides in each hemisphere (57%). Interestingly, in case northern and southern hemisphere TPAs form on the same oval side, they appear typically at dawn during weak IMF By

Importance of the dusk-dawn interplanetary magnetic field component (IMF By) to magnetospheric convection in Earth’s magnetotail plasma sheet

The solar wind and its embedded magnetic field, the interplanetary magnetic field (IMF) together with magnetic reconnection power the large-scale plasma and magnetic flux circulation in the Earth’s magnetosphere-ionosphere system. This circulation is termed as convection and its strength is controlled by the north-south IMF component (IMF Bz). In recent years, an interest has arisen to investigate the lesser-known role of the dusk-dawn component (IMF By) in convection. It has been previously known though that prevailing nonzero IMF By can cause plasma flow asymmetries in the high-latitude ionosphere, but how the magnetospheric flows, for instance, in the magnetotail plasma sheet are affected, remains to be investigated. In this article, we introduce the recent progress and the latest achievements in the research of the influence of IMF By on tail plasma sheet convection. The research progress has been rapid and it has revealed that both fast and slow convection are affected in a manner that is in accordance with the asymmetries observed in the ionospheric convection. The results indicate the significance of the IMF By component on magnetospheric convection and they represent a major advance in the field of solar wind-magnetosphere coupling

Magnetic forces associated with bursty bulk flows in Earth's magnetotail

We present the first direct measurements of magnetic forces acting on bursty bulk flow plasma in the magnetotail. The magnetic forces are determined using Cluster multispacecraft measurements. We analyze 67 bursty bulk flow (BBF) events and show that the curvature part of the magnetic force is consistently positive, acting to accelerate the plasma toward Earth between approximately 10 and 20 R-E geocentrical distances, while the magnetic field pressure gradient increasingly brakes the plasma as it moves toward Earth. The net result is that the magnetic force accelerates the plasma at distances greater than approximately 14 R-E, while it acts to decelerate it within that distance. The magnetic force, together with the thermal pressure gradient force, will determine the dynamics of the BBFs as they propagate toward the near-Earth tail region. The determination of the former provides an important clue to the ultimate fate of BBFs in the inner magnetosphere

Aurora in the polar cap: a review

This paper reviews our current understanding of auroral features that appear poleward of the main auroral oval within the polar cap, especially those that are known as Sun-aligned arcs, transpolar arcs, or theta auroras. They tend to appear predominantly during periods of quiet geomagnetic activity or northwards directed interplanetary magnetic field (IMF). We also introduce polar rain aurora which has been considered as a phenomenon on open field lines. We describe the morphology of such auroras, their development and dynamics in response to solar wind-magnetosphere coupling processes, and the models that have been developed to explain them.</p

Multi-instrument observations of multiple auroral arcs in the duskside polar cap region

International audiencePolar cap auroral arcs (PCAs) are one of the outstanding phenomena in the polar cap region during periods of northward interplanetary magnetic field (IMF). Smaller scale PCAs tend to occur either in the duskside or dawnside of the polar cap and are known to drift in the dawn-dusk direction depending on the sign of the IMF By. Studies of PCAs are of particular importance because they represent dynamical characteristics of their source plasma in the magnetosphere, for example in the interaction region between the solar wind and magnetosphere or in the boundary between the plasma sheet and tail lobe. To date, however, very little has been known about the spatial structure and/or temporal evolution of the magnetospheric counterpart of PCAs. In order to gain more comprehensive understanding of the origin of PCAs, we have investigated an event of PCAs on November 10, 2005, during which multiple PCAs were detected by a ground-based all-sky camera at Resolute Bay, Canada. During this interval, several PCAs were detached from the duskside oval and moved poleward. The large-scale structure of these arcs was visualized by space-based imagers of TIMED/GUVI and DMSP/SSUSI. The images from these instruments indicate that the arcs were pointing towards the dayside cusp. In addition to these optical observations, we employ the Cluster satellites to reveal the particle signature corresponding to the small-scale PCAs. The ionospheric footprints of the 4 Cluster satellites encountered the PCAs sequentially and observed well correlated enhancements of electron fluxes at weak energies (< 1 keV). The Cluster satellites also detected signatures of upflowing ion beams exactly at the times of the satellite crossing of the PCAs. This implies that the ions were accelerated upward by a quasi-stationary electric field existing above the PCAs. Ionospheric convection measurement from one of the SuperDARN radars shows an existence of velocity shear across one of the PCAs. This signature is consistent with converging electric field structure in the vicinity of the arc. In the presentation, we will show the results of detailed comparison between the ground-based radio and optical signatures of the PCAs and those obtained by the Cluster spacecraft at magnetospheric altitudes