Magnetotail Lobes Research Articles

The Relationship Between the Energization of Moon‐Originating Ions and Terrain Type on the Lunar Surface

AbstractWe analyze data acquired by the Kaguya satellite on 14 October 2008 when the Moon was in the terrestrial magnetotail lobe to gain new insight into the energization of ions originating from the Moon. The Moon‐originating ions were detected over a broad range of latitudes from −80° to 50° above the Moon's dayside at ∼100 km altitude. The fluxes of the Moon‐originating ions were observed at energies from ∼50 to ∼1,000 eV. Additionally, these ions exhibited a wide distribution pitch angle spanning from ∼45 to 90°. The energy levels of ions originating from the Moon show rapid changes, either increasing or decreasing by a factor of ∼10 within 8 min without the solar zenith angle dependence. Such rapid energy changes were observed over the highland regions. These observations are discussed in light of possible acceleration mechanisms of Moon‐originating ions, including temporal and spatial effects.

Plasma Observations in the Distant Magnetotail During Intervals of Northward IMF

AbstractWe examine a 6‐day traversal of the magnetotail by the ARTEMIS satellites during an interval of prolonged northward IMF. The electrostatic analyzer (ESA) onboard the ARTEMIS spacecraft measures high ion and electron fluxes at approximately 60 RE downtail in regions of the magnetotail which would normally be the magnetotail lobe, containing open flux evacuated of plasma. We interpret these observations as trapped plasma on closed magnetic flux indicating that the magnetotail is closed or partially closed but extends at least as far as ∼60 RE downtail. We find that the occurrence of plasma in the magnetotail and the closure of the magnetosphere results in distinct changes to the magnetotail structure including a reduction in the magnetic field strength and pressure as well as a narrowing of the tail by approximately 20 RE.

Statistical Analysis of Mercury's Magnetotail Lobe Field Using MESSENGER Observations

AbstractThe magnetotail lobe region at Mercury is characterized by low plasma density and low magnetic field variability compared to the nightside magnetosheath and central plasma sheet. At Mercury, as well as other planets, lobe magnetic fields play a crucial role in storing and releasing magnetic flux in response to changing upstream solar wind conditions such as interplanetary magnetic field (IMF) orientation and solar wind dynamic pressure (Pdyn). This makes the region significant for studying the magnetospheric interaction with the intense solar wind conditions at Mercury's orbit. Here, we identify and analyze magnetotail lobe observations made by the Mercury Surface, Space Environment, Geochemistry and Ranging (MESSENGER) spacecraft during its 4 years orbital phase. We empirically determined a set of criteria using magnetometer (MAG) and the Fast Imaging Plasma Spectrometer instruments onboard MESSENGER to identify lobe magnetic field intervals. From 3,332 MESSENGER orbits, we identify 1,242 lobe field intervals. We derive an expression for the average lobe magnetic field strength in nanotesla with respect to radial distance downtail: Blobe(r) = (135 ± 8) * r(−2.1±0.3) + (31 ± 8). The lobe magnetic field exhibits both small‐scale (∼3 min) and orbit‐to‐orbit (∼8–12 hr) variability in magnetic field strength compared to this averaged field strength expression. The orbit‐to‐orbit variability in lobe field strength is not significantly correlated with estimated IMF orientation, but is directly correlated with Pdyn. Thus, our findings provide evidence for the pressure balance between the inward facing Pdyn on the nightside magnetopause and the outward facing magnetic pressure supplied by the lobes.

Using machine learning to characterize solar wind driving of convection in the terrestrial magnetotail lobes

In order to quantitatively investigate the mechanism of how magnetospheric convection is driven in the region of magnetotail lobes on a global scale, we analyzed data from the ARTEMIS spacecraft in the deep tail and data from the Cluster spacecraft in the near and mid-tail regions. Our previous work revealed that, in the lobes near the Moon’s orbit, the convection can be estimated by using ARTEMIS measurements of lunar ions’ velocity. Based on that, in this paper, we applied machine learning models to these measurements to determine which upstream solar wind parameters significantly drive the lobe convection in magnetotail regions, to help us understand the mechanism that controls the dynamics of the tail lobes. The results demonstrate that the correlations between the predicted and measured convection velocities for the machine learning models (>0.75) are superior to those of the multiple linear regression model (∼0.23–0.43) in the testing dataset. The systematic analysis shows that the IMF and magnetospheric activity play an important role in influencing plasma convection in the global magnetotail lobes.

Ion acceleration in Mars’ twisted magnetotail

ABSTRACT Mars lacks a global magnetic field and instead has a unique, ‘hybrid’ magnetosphere composed of crustal magnetic fields, which are stronger in the Southern hemisphere than the Northern hemisphere. These crustal fields are believed to drive a twist in the magnetotail at Mars. This is important because heavy ions escape from this induced magnetotail, which is a major source of atmospheric erosion. We present analysis of heavy ions produced in Mars’ upper atmosphere that are transported downtail along the twisted current sheet and observed by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. We identified O+, O$_2\, ^+$, and CO$_2\, ^+$ being accelerated to different energies at a range of distances downtail and found that heavy ions in the 10–50 eV energy population have the highest flux. We present tail slices of ion fluxes for different interplanetary magnetic field (IMF) configurations based on the BY component of the IMF and find that the different configurations affect the degree of tail twist and the transport of heavy ions in the induced magnetotail lobes. We find that the total escape rate for O+, O$_2\, ^+$, and CO$_2\, ^+$ is 5.5e24 s-1, with 50% escaping between 2–200 eV.

Evidence of Alfvén Waves Generated by Mode Coupling in the Magnetotail Lobe

AbstractMode coupling is an important mechanism to excite Alfvén waves and has been extensively investigated based on numerical simulations. However, no direct observational evidence has been found regarding to this mechanism in the magnetotail lobe. Here, we report an in‐situ observation that an Alfvén wave with a period of several minutes is clearly relevant to a fast mode wave in the magnetotail lobe based on observations of the Cluster spacecraft. These waves occur at the region where the Alfvén speed has a gradient during an enhancement of the solar wind dynamic pressure. This suggests that the Alfvén wave is generated by the mode conversion at the region where the Alfvén speed has a gradient, and the fast mode wave might be driven by the solar wind. Our observations provide an important insight into the wave generation and energy conversion in the magnetosphere.

Observational Properties of 15MLT‐PCA in the Southern Hemisphere and the Switching Effects of IMF By on 15MLT‐PCA Occurrence

AbstractBased on Special Sensor Ultraviolet Spectrographic Imager on Defense Meteorological Satellite Program spacecraft (DMSP/SSUSI) observations in the Northern Hemisphere, a particular polar cap arc observed at ∼1,500 magnetic local time (MLT) sector was identified and was named 15MLT‐PCA. A 15MLT‐PCA model proposed for the Northern Hemisphere suggests that the 15MLT‐PCA is caused by a lobe reconnection related to the tilt angle and lobe field line twisting due to a long‐lasting positive interplanetary magnetic field (IMF) By condition. However, it is unclear if the 15MLT‐PCA can be observed in the Southern Hemisphere and how the IMF By influences its occurrence and location. Using the long‐term DMSP/SSUSI observations in the Southern Hemisphere, we confirmed that the 15MLT‐PCA could be observed in the Southern Hemisphere in summer under long‐lasting negative IMF By and predominantly positive IMF Bx. The ion energy dispersion and some other statistical properties are also investigated. Further, we showed that the IMF By has a switching effect on the occurrence of the 15MLT‐PCA using multiple cases. We found that the 15MLT‐PCA disappear as soon as the IMF By changed to an unfavorable condition, but it seems that the favorable conditions need to be maintained for ∼1 hr for the occurrence of 15MLT‐PCA. We also found that the IMF By magnitude is well correlated to the MLT location of the 15MLT‐PCA. We suggest that all the results support the 15MLT‐PCA model and imply that a magnetotail lobe reconnection should be common in the summer hemisphere because the well‐known Parker Spiral favors meeting the model's IMF requirements.

The Magnitude of IMF By Influences the Magnetotail Response to Solar Wind Forcing

AbstractThe dynamics of substorms are known to be dominated by the North‐South () component of the Interplanetary Magnetic Field (IMF), which is the most important driver of the dayside reconnection. Even though the dawn‐dusk () component is also known to play a role in substorm dynamics, its effects are not yet fully understood. In this paper we study how IMF modulates the onset latitude, strength and occurrence frequency of substorms as well as the isotropic boundary (IB) latitude of energetic protons. We show that the substorm onset latitude and the IB latitude are about one degree lower for large magnitude ( nT) than for small . In contrast, the substorm occurrence frequency is larger for small . We suggest that the magnetotail is more stable during large , requiring the magnetotail lobes (and hence the polar cap) to contain more flux to initiate a substorm compared to the situation when is small.

High-Latitude Cold Ion Outflow Inferred From the Cluster Wake Observations in the Magnetotail Lobes and the Polar Cap Region

Cold ions with low (a few eV) thermal energies and also often low bulk drift energies, dominate the ion population in the Earth’s magnetosphere. These ions mainly originate from the ionosphere. Here we concentrate on cold ions in the high latitude polar regions, where magnetic field lines are open and connected to the magnetotail. Outflow from the ionosphere can modify the dynamics of the magnetosphere. In-situ observations of low energy ions are challenging. In the low-density polar regions the equivalent spacecraft potential is often large compared to cold ion energies and the ions cannot reach the spacecraft. Rather, a supersonic ion flow creates an enhanced wake. The local electric field associated with this wake can be used to detect the drifting cold ions, and this wake technique can be used for statistical studies. In this paper, we review some of the key results obtained from this technique. These results help us to understand how cold ionospheric outflow varies with various conditions of solar activities and the Earth’s intrinsic magnetic field.

Ion Outflow and Escape in the Terrestrial Magnetosphere: Cluster Advances

AbstractCluster was the first mission in the terrestrial magnetosphere to involve four spacecraft in a tetrahedral configuration, providing three‐dimensional measurements of the space plasma parameters. Cluster was also equipped with a very comprehensive instrumentation, allowing the measurement of the ion populations outflowing from the ionosphere, their circulation in the magnetosphere, and their eventual escape to outer space. The observations of the outflowing and escaping ion populations performed by Cluster are reviewed and the most prominent results highlighted. These show the dominance in the magnetotail lobes of cold plasma outflows originating from the polar caps. For the energetic heavy ion outflow, the cusps constitute the main source. Their transport and acceleration through the polar cap into the lobes and then into the plasma sheet has been characterized. The dependence of the polar outflow on the solar wind parameters and on the geomagnetic activity has been evaluated for both cold ion populations and heavy energetic ions. For the latter, outflow has been observed during all periods but an increase by two orders of magnitude has been shown during extreme space weather conditions. This outflow is adequate to change the composition of the atmosphere over geological timescales. At lower latitudes, the existence of a plasmaspheric wind, providing a continuous leak from the plasmasphere, has been demonstrated. The general scheme of the outflowing ion circulation in the magnetosphere or escape, and its dependence on the IMF conditions, has been outlined. However, several questions remain open, waiting for a future space mission to address them.

Hot Plasma in the Magnetotail Lobes Shows Characteristics Consistent With Closed Field Lines Trapped in the Lobes

AbstractWe examine the magnetotail using data from the Hot Ion Analyzer on Cluster 1 during 2001–2009. We develop and utilize an algorithm in order to identify times during which Cluster 1 is in the magnetotail lobe but observes plasma, which is hotter than our expectations of the lobe. We analyze the prevailing Interplanetary Magnetic Field (IMF) conditions for our algorithm and a reference algorithm (with no particle energy criteria) and find that the periods we select are, on average, nT more toward northward IMF. Examining the temperature in the magnetotail for our periods shows that the morphology of the average temperature is consistent with the Milan et al. (2005, https://doi.org/10.1029/2004JA010835) model of a magnetotail structure during northward IMF, in which closed field lines are prevented from convecting to the dayside, causing them and the plasma trapped on them to protrude into the magnetotail lobes. We also find evidence that of our identified periods may be driven by direct entry into the magnetosphere from the solar wind.

ARTEMIS Observations of Lunar Nightside Surface Potentials in the Magnetotail Lobes: Evidence for Micrometeoroid Impact Charging

AbstractThe Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun mission observes anomalously low lunar nightside surface potentials while in the terrestrial magnetotail lobes. Observed potential magnitudes are between 15% and 40% that expected from ambient tail‐plasma charging, are highly concentrated on the lunar dawn hemisphere, and are correlated with low ambient plasma densities. These characteristics suggest an additional, highly asymmetric source of cold current to the lunar surface. Given these characteristics, we identify micrometeoroid impact‐generated plasmas as the likely source of this additional current. Using laboratory measurements of impact charge yields and models of the micrometeoroid flux to the Moon constrained by in situ measurements, we show that currents due to micrometeoroid impact plasmas have the necessary magnitude and spatial distribution to explain the observed surface potential measurements. Micrometeoroid impact‐generated currents may contribute to surface charging at airless bodies with intense micrometeoroid bombardment and/or low ambient plasma densities.

Lunar Photoemission Yields Inferred From ARTEMIS Measurements

AbstractPhotoemission yield (the number of emitted electrons per incoming photon) is one of the fundamental properties of solid materials but is not yet well constrained for the lunar surface for photon energies >∼20 eV. In this study, we constrain this yield for incident photons with energies of ∼10–500 eV with data from the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission along with solar irradiance spectra from Version 2 of the Flare Irradiance Spectral Model. We also report the first oxygen Auger electron observations at the Moon by the ARTEMIS spacecraft, which provides a unique feature to identify photoelectrons emitted from the lunar surface. With lunar photoelectron observations identified in both Earth's magnetotail lobes and the solar wind for four selected days, we infer a lower bound of 10−3 in yield for photon energies >∼20 eV. However, our investigation also reveals an uncertainty over ∼4 orders of magnitude in derived yields with a sensitivity study, owing to a poorly constrained photoelectron energy probability function. This uncertainty motivates future experiments on lunar samples to better characterize the lunar surface charging environment.

Investigating the Moon's Interaction With the Terrestrial Magnetotail Lobe Plasma

AbstractWe present observations of the Moon's plasma interaction in Earth's magnetotail lobes by the ARTEMIS mission, and compare these to hybrid model results to constrain the global properties of the lunar electromagnetic environment. We identify, for the first time in the magnetotail lobes, a low‐energy wake extending multiple lunar radii downstream of the Moon along the ambient flow direction. This wake is tilted out of the lunar optical shadow, allowing for detection of the otherwise unobservable cold ambient magnetospheric plasma. Similar eclipse encounters may provide additional opportunities to measure this low‐energy plasma potentially originating from the terrestrial ionosphere. We find lunar ionospheric outflow extending multiple Moon radii downstream that generates asymmetries in the Moon's plasma environment and shares characteristics with the plasma interactions of Rhea, Tethys, and Dione. The extensive ARTEMIS data set may therefore provide additional insights into the plasma environments near moons of the outer solar system.

Radar Observations of Flows Leading to Longitudinal Expansion of Substorm Onset Over Alaska

AbstractLongitudinal expansion of brightening of the substorm onset arc likely represents longitudinal expansion of the substorm onset instability. We used Poker Flat radar campaign auroral observations to identify seven substorm onsets seen east or west of the radar that expanded into the radar field‐of‐view. The radar observations were used to show flows relative to the expanding onset aurora. Flow enhancements were found equatorward (poleward) of the westward (eastward) longitudinal expansion of the onset arc brightening that agree with plasma sheet bubble modeling. This lends significant additional supporting evidence that an incoming reduced entropy flow channel leads to onset. As a flow channel enters the auroral oval (plasma sheet) from the polar cap (magnetotail lobes) and starts to move equatorward (earthward in the plasma sheet) as a bubble, it simultaneously expands azimuthally due to the combination of electric drift along dawnward tilted equipotentials and westward magnetic drift of more energetic ions. As the azimuthally expanding bubble reaches near the equatorward boundary of the electron auroral oval, it leads to the onset instability indicated by the auroral beading and associated flows if conditions are appropriate. The instability grows in the initial onset region, as it simultaneously extends longitudinally with the continued azimuthal expansion of the bubble within the plasma sheet. Additionally, after onset, the auroral bulge expands poleward and can lead to a westward traveling surge. We observe strong flows heading toward the front of the surge that appear to bring low entropy plasma, perhaps from the polar cap, into the surge.

Plasma Convection in the Terrestrial Magnetotail Lobes Measured Near the Moon's Orbit

AbstractIn order to study the plasma convection in the deep magnetotail lobes near lunar orbit, we investigated ions originating from the tenuous exosphere and surface of the Moon, which are measured by the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft. Directly measuring the plasma convection in the tail lobes is difficult, due to the typically large positive spacecraft potential. In this work we show that in the terrestrial magnetotail near the Moon, the convection velocity can be estimated by measuring the velocity of lunar ions. Determining what factors control the lobe convection is important in understanding the linkage between the upstream conditions and the dynamics of the tail lobes. Based on systematic analysis of multiple ARTEMIS observations and OMNI data, we find that the interplanetary magnetic field (IMF) and magnetospheric activity plays an important role in controlling plasma convection in the near‐Moon lobes.

The Acceleration of Lunar Ions by Magnetic Forces in the Terrestrial Magnetotail Lobes

AbstractIn order to study the acceleration of ions originating from the tenuous exosphere and surface of the Moon, we analyzed data from the ElectroStatic Analyzer (ESA) and Flux Gate Magnetometer (FGM) carried by the Acceleration, Reconnection, Turbulence, and Electrodynamics of Moon's Interaction with the Sun (ARTEMIS) spacecraft. Previous investigations have modeled the acceleration of lunar ions by the motional electric field of the surrounding plasma. However, in the terrestrial magnetotail, where the lunar ion density can equal or even exceed the ambient plasma density, other forces may play an important role in the tenuous plasma environment. Determining what forces govern lunar ion motion is important in understanding their interaction with the ambient plasma in the unique environment of the magnetotail. Based on a detailed analysis of two individual ARTEMIS observations, we find that magnetic pressure and magnetic tension forces may play an important role in accelerating the lunar ions.

Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity

AbstractIn this study we use high‐quality convection data from the Electron Drift Instrument on board Cluster to investigate how near‐Earth tail activity affects the average convection pattern in the magnetotail lobes when the interplanetary magnetic field has a dominating east‐west (By) component. Two different proxies have been used to represent different levels of reconnection in the near‐Earth tail: The value of the AL index and the substorm phases identified by the Substorm Onsets and Phases from Indices of the Electrojet algorithm. We find that the convection changes from a dominantly YGSM direction, but opposite in the two hemispheres, to a flow oriented more toward the plasma sheet, as the north‐south component of the convection increases when reconnection enhances in the near Earth tail. This result is consistent with recent observations of the convection in the ionosphere, which suggest that the nightside convection pattern becomes more north‐south symmetric when tail reconnection increases. This is also supported by simultaneous auroral observations from the two hemispheres, which shows that conjugate auroral features become more symmetric during substorm expansion phase. The reduced asymmetry implies that the asymmetric pressure balance in the lobes is altered during periods with strong reconnection in the near‐Earth tail.

Asymmetric Transport of the Earth’s Polar Outflows by the Interplanetary Magnetic Field

Abstract The polar outflows, as an important plasma source of the Earth’s magnetosphere, usually exhibit significant north–south asymmetries, which can strongly affect the plasma distributions in the magnetotail lobe and perhaps contribute to the substorm triggering. But the mechanism of the asymmetric transport of these outflows is still unclear. In this Letter, 3D global magnetohydrodynamic (MHD) simulations are performed to investigate the development of the polar outflows after their escapes from the inner boundary under influences of the interplanetary magnetic field (IMF) B x . It is found that the velocity of northern polar outflows is much stronger than the south. We suggest that the IMF B x causes the north–south asymmetries in the magnetospheric configuration, and subsequently, great differences of the force and mass distributions appear between the two hemispheres, which lead to the significant north–south asymmetries in the transport of the polar outflows. We also discuss the differences in the acceleration mechanisms of the polar outflows between the northward and southward IMF cases.

Response of Earth's Neutral Sheet to Reversals in the IMF By Component

AbstractThe By component of the interplanetary magnetic field (IMF) plays an integral part in the coupling of the solar wind‐magnetosphere‐ionosphere system. In this study, we use a suite of magnetospheric spacecraft, with data spanning nearly 25 years, to statistically investigate the control of the IMF By component on the neutral sheet. Past studies have identified that this thin layer of the magnetotail, which separates the oppositely directed magnetotail lobes, responds to reversals in the IMF By component. The proposed method for driving this response is through unequal flux loading in the magnetotail lobes, and we present lobe flow data from the Cluster mission, which suggests that this is indeed the case. We find that the neutral sheet responds in a statistical fashion, with a clear twist or rotation whose orientation depends upon the polarity of the IMF By component. Additionally, under individual analyses, we find the response time of the neutral sheet to IMF By reversals is much shorter than possible through the classical dayside reconnection driven picture. We find an average response time of between 10 and 20 min with little to no dependence on distance downtail. These results suggest that the neutral sheet responds to the IMF By component on multiple timescales.