Magnetospheric Lobes Research Articles

Causes of hemispheric differences in polar cap indices

Two recent papers by Troshichev et al. (2023) and by Lockwood (2023) have presented different physical mechanisms that cause simultaneous polar cap indices, measured close to the geomagnetic poles in the north and south hemispheres, to differ even though their overall behaviour is very similar. Several effects are described but the two main mechanisms discussed are the effect of the dawn–dusk (Y) component of the Interplanetary Magnetic Field (IMF) and inductive decoupling of electric fields caused by changes in the magnetic fields in the magnetosheath and magnetospheric lobes. This paper shows that both papers are correct and that both of the two major mechanisms are active and cause differences between the northern and southern indices. The variations in the differences with fraction of the calendar year and Universal Time are described and explained.

Over 20‐Year Global Magnetohydrodynamic Simulation of Earth's Magnetosphere

AbstractWe present our approach to modeling over 20 years of the solar wind‐magnetosphere‐ionosphere system using version 5 of the Grand Unified Magnetosphere‐Ionosphere Coupling Simulation (GUMICS‐5). As input we use 16‐s resolution magnetic field and 1‐min plasma measurements by Advanced Composition Explorer satellite from 1998 to 2020. The modeled interval is divided into 28 hr simulations including 4 hr overlap. We use maximum magnetospheric resolution of 0.5 Earth radii (RE) up to about 15 RE from Earth and decreasing resolution further away. In the ionosphere we use a maximum resolution of approximately 100 km poleward of ±58° magnetic latitude and decreasing resolution toward equator. With respect to previous version GUMICS‐4, we have parallelized the magnetosphere of GUMICS‐5 using the Message Passing Interface and have made several improvements which have for example, decreased its numerical diffusion. In total we have performed over 8,000 simulations which have produced over 10,000,000 ionospheric files and 2,000,000 magnetospheric files requiring over 100 TB of disk space. We compare these results to several empirical models and geomagnetic indices derived from ground magnetic field measurements. GUMICS‐5 reproduces observed solar cycle trends in magnetopause stand‐off distance and magnetospheric lobe field strength but consistency in plasma sheet pressure and ionospheric cross‐polar cap potential is lower. Comparisons with geomagnetic indices show better results for Kp index than for auroral electrojet index. Our extensive results can serve, for example, as a foundation for combined physics‐based and black‐box approach to real‐time prediction of near‐Earth space, or as input to other physics‐based models of the inner magnetosphere, upper and middle atmosphere, etc.

The Spacecraft Wake: Interference With Electric Field Observations and a Possibility to Detect Cold Ions

AbstractWakes behind spacecraft caused by supersonic drifting positive ions are common in plasmas and disturb in situ measurements. We review the impact of wakes on observations by the Electric Field and Wave double‐probe instruments on the Cluster satellites. In the solar wind, the equivalent spacecraft charging is small compared to the ion drift energy and the wake effects are caused by the spacecraft body and can be compensated for. We present statistics of the direction, width, and electrostatic potential of wakes, and we compare with an analytical model. In the low‐density magnetospheric lobes, the equivalent positive spacecraft charging is large compared to the ion drift energy and an enhanced wake forms. In this case observations of the geophysical electric field with the double‐probe technique becomes extremely challenging. Rather, the wake can be used to estimate the flux of cold (eV) positive ions. For an intermediate range of parameters, when the equivalent charging of the spacecraft is similar to the drift energy of the ions, also the charged wire booms of a double‐probe instrument must be taken into account. We discuss an example of these effects from the MMS spacecraft near the magnetopause. We find that many observed wake characteristics provide information that can be used for scientific studies. An important example is the enhanced wakes used to estimate the outflow of ionospheric origin in the magnetospheric lobes to about cold (eV) ions/s, constituting a large fraction of the mass outflow from planet Earth.

Seasonal and Hemispheric Asymmetries of F Region Polar Cap Plasma Density: Swarm and CHAMP Observations

AbstractOne of the primary mechanisms of loss of Earth's atmosphere is the persistent “cold” ( 20 eV) ion outflow that has been observed in the magnetospheric lobes over large volumes with dimensions of order several Earth radii. As the main source of this cold ion outflow, the polar cap F region ionosphere and conditions within it have a disproportionate influence on these magnetospheric regions. Using 15 years of measurements of plasma density Ne made by the Swarm spacecraft constellation and the Challenging Mini Satellite Payload (CHAMP) spacecraft within the F region of the polar cap above 80° Apex magnetic latitude, we report evidence of several types of seasonal asymmetries in polar cap Ne. Among these, the transition between “winter‐like” and “summer‐like” median polar cap Ne occurs 1 week prior to local spring equinox in the Northern Hemisphere (NH) and 1 week after local spring equinox in the Southern Hemisphere (SH). Thus, the median SH polar cap Ne lags the median NH polar cap Ne by approximately 2 weeks with respect to hemispherically local spring and fall equinox. From interhemispheric comparison of statistical distributions of polar cap plasma density around each equinox and solstice, we find that distributions in the SH are often flatter (i.e., less skewed and kurtotic) than those in the NH. Perhaps of most significance to cold ion outflow, we find no evidence of an F region plasma density counterpart to a previously reported hemispheric asymmetry whereby cold plasma density is higher in the NH magnetospheric lobe than in the SH lobe.

Convection in the Magnetosphere‐Ionosphere System: A Multimission Survey of Its Response to IMF By Reversals

AbstractPast studies have demonstrated that the interplanetary magnetic field (IMF) By component introduces asymmetries in the magnetosphere‐ionosphere (M‐I) system, though the exact timings involved are still unclear with two distinct mechanisms proposed. In this study, we statistically analyze convective flows from three regions of the M‐I system: the magnetospheric lobes, the plasma sheet, and the ionosphere. We perform superposed epoch analyses on the convective flows in response to reversals in the IMF By orientation, to determine the flow response timescales of these regions. We find that the lobes respond quickly and reconfigure to the new IMF By state within 30–40 min. The plasma sheet flows, however, do not show a clear response to the IMF By reversal, at least within 4 hr postreversal. The ionospheric data, measured by the Super Dual Auroral Radar Network (SuperDARN), match their counterpart magnetospheric flows, with clear and prompt responses at ≥75° magnetic latitude (MLAT) but a less pronounced response at 60–70 MLAT. We discuss the potential implication of these results on the mechanisms for introducing the IMF By component into the M‐I system.

Nonlobe Reconnection at the Earth's Magnetopause for Northward IMF

AbstractUnder northward interplanetary magnetic field conditions, magnetic reconnection at the Earth's magnetopause is usually thought to operate through the merging of magnetosheath magnetic field lines and open magnetic field lines from the magnetospheric lobe. However, reconnection also occurs between magnetosheath field lines and closed magnetic field lines in the magnetosphere. Under certain conditions, this nonlobe field line reconnection has distinct plasma and magnetic field signatures that distinguish it from reconnection of lobe field lines. A survey of these conditions suggests that nonlobe reconnection at the Earth's magnetopause may be common even for relatively strong northward IMF.

Electron Cooling and Isotropization during Magnetotail Current Sheet Thinning: Implications for Parallel Electric Fields

AbstractMagnetotail current sheet thinning is a distinctive feature of substorm growth phase, during which magnetic energy is stored in the magnetospheric lobes. Investigation of charged particle dynamics in such thinning current sheets is believed to be important for understanding the substorm energy storage and the current sheet destabilization responsible for substorm expansion phase onset. We use Time History of Events and Macroscale Interactions during Substorms (THEMIS) B and C observations in 2008 and 2009 at ~18 − 25 RE to show that during magnetotail current sheet thinning, the electron temperature decreases (cooling), and the parallel temperature decreases faster than the perpendicular temperature, leading to a decrease of the initially strong electron temperature anisotropy (isotropization). This isotropization cannot be explained by pure adiabatic cooling or by pitch angle scattering. We use test particle simulations to explore the mechanism responsible for the cooling and isotropization. We find that during the thinning, a fast decrease of a parallel electric field (directed toward the Earth) can speed up the electron parallel cooling, causing it to exceed the rate of perpendicular cooling, and thus lead to isotropization, consistent with observation. If the parallel electric field is too small or does not change fast enough, the electron parallel cooling is slower than the perpendicular cooling, so the parallel electron anisotropy grows, contrary to observation. The same isotropization can also be accomplished by an increasing parallel electric field directed toward the equatorial plane. Our study reveals the existence of a large‐scale parallel electric field, which plays an important role in magnetotail particle dynamics during the current sheet thinning process.

Dayside and nightside magnetic field responses at 780 km altitude to dayside reconnection

AbstractDuring southward interplanetary magnetic field, dayside reconnection will drive the Dungey cycle in the magnetosphere, which is manifested as a two‐cell convection pattern in the ionosphere. We address the response of the ionospheric convection to changes in the dayside reconnection rate by examining magnetic field perturbations at 780 km altitude. The Active Magnetosphere and Planetary Electrodynamics Response Experiment data products derived from the Iridium constellation provide global maps of the magnetic field perturbations. Cluster data just upstream of the Earth's bow shock have been used to estimate the dayside reconnection rate. By using a statistical model where the magnetic field can respond on several time scales, we confirm previous reports of an almost immediate response both near noon and near midnight combined with a 10–20 min reconfiguration time of the two‐cell convection pattern. The response of the ionospheric convection has been associated with the expansion of the polar cap boundary in the Cowley‐Lockwood paradigm. In the original formulation of this paradigm the expansion spreads from noon to midnight in 15–20 min. However, also an immediate global response has been shown to be consistent with the paradigm when the previous dayside reconnection history is considered. In this paper we present a new explanation for how the immediate response can be accommodated in the Cowley‐Lockwood paradigm. The new explanation is based on how MHD waves propagate in the magnetospheric lobes when newly reconnected open flux tubes are added to the lobes, and the magnetopause flaring angle increases.

Localized field‐aligned currents in the polar cap associated with airglow patches

AbstractAirglow patches have been recently associated with channels of enhanced antisunward ionospheric flows propagating across the polar cap from the dayside to nightside auroral ovals. However, how these flows maintain their localized nature without diffusing away remains unsolved. We examine whether patches and collocated flows are associated with localized field‐aligned currents (FACs) in the polar cap by using coordinated observations of the Swarm spacecraft, a polar cap all‐sky imager, and Super Dual Auroral Radar Network (SuperDARN) radars. We commonly (66% of cases) identify substantial FAC enhancements around patches, particularly near the patches' leading edge and center, in contrast to what is seen in the otherwise quiet polar cap. These FACs have densities of 0.1–0.2 μA/m−2 and have a distribution of width peaking at ~75 km. They can be approximated as infinite current sheets that are orientated roughly parallel to patches. They usually exhibit a Region 1 sense, i.e., a downward FAC lying eastward of an upward FAC. With the addition of Resolute Bay Incoherent Scatter radar data, we find that the FACs can close through Pedersen currents in the ionosphere, consistent with the locally enhanced dawn‐dusk electric field across the patch. Our results suggest that ionospheric polar cap flow channels are imposed by structures in the magnetospheric lobe via FACs, and thus manifest mesoscale magnetosphere‐ionosphere coupling embedded in large‐scale convection.

Effects of magnetospheric lobe cell convection on dayside upper thermospheric winds at high latitudes

AbstractThis paper investigates a possible physical mechanism of the observed dayside high‐latitude upper thermospheric wind using numerical simulations from the coupled magnetosphere‐ionosphere‐thermosphere (CMIT) model. Results show that the CMIT model is capable of reproducing the unexpected afternoon equatorward winds in the upper thermosphere observed by the High altitude Interferometer WIND observation (HIWIND) balloon. Models that lack adequate coupling produce poleward winds. The modeling study suggests that ion drag driven by magnetospheric lobe cell convection is another possible mechanism for turning the climatologically expected dayside poleward winds to the observed equatorward direction. The simulation results are validated by HIWIND, European Incoherent Scatter, and Defense Meteorological Satellite Program. The results suggest a strong momentum coupling between high‐latitude ionospheric plasma circulation and thermospheric neutral winds in the summer hemisphere during positive IMF Bz periods, through the formation of magnetospheric lobe cell convection driven by persistent positive IMF By. The CMIT simulation adds important insight into the role of dayside coupling during intervals of otherwise quiet geomagnetic activity

Solar wind plasma entry observed by cluster in the high‐latitude magnetospheric lobes

AbstractUsing the Cluster data during the period from January to April between 2001 and 2006, we find an observation of solar wind entry due to magnetic reconnection occurred in the terrestrial high‐latitude magnetospheric lobes, tailward of the cusps under northward interplanetary magnetic field (IMF). Occurrence rate of solar wind entry events in this study is of the same order as that for the Cluster orbital interval from August to October between the years of 2002 and 2004 as reported by Shi et al. (2013). In this paper, we further study the role of the IMF Bx and By components in the control of solar wind plasma entry based on the investigations of different magnetic dipole tilt variations between our database and Shi et al. (2013). This study shows that the asymmetry distribution of solar wind entry events in the northern and southern lobes could be caused by the variation of magnetic dipole tilt, which could influence the locations of the reconnection site on the high‐latitude lobe magnetopause. On the other hand, IMF Bx can also affect the solar wind plasma entry rate, which is also consistent with previous results. Therefore, we conclude that the “north‐south asymmetry” of solar wind entry events in the lobes could be the combined result of magnetic dipole tilt and IMF Bx. In addition, the IMF By component can influence the entry events in conjunction with the variation of IMF Bx component, which is in line with the Parker Spiral of the IMF.

Effects of Saturn's magnetospheric dynamics on Titan's ionosphere

AbstractWe use the Cassini Radio and Plasma Wave Science/Langmuir probe measurements of the electron density from the first 110 flybys of Titan to study how Saturn's magnetosphere influences Titan's ionosphere. The data is first corrected for biased sampling due to varying solar zenith angle and solar energy flux (solar cycle effects). We then present results showing that the electron density in Titan's ionosphere, in the altitude range 1600–2400 km, is increased by about a factor of 2.5 when Titan is located on the nightside of Saturn (Saturn local time (SLT) 21–03 h) compared to when on the dayside (SLT 09‐15 h). For lower altitudes (1100–1600 km) the main dividing factor for the ionospheric density is the ambient magnetospheric conditions. When Titan is located in the magnetospheric current sheet, the electron density in Titan's ionosphere is about a factor of 1.4 higher compared to when Titan is located in the magnetospheric lobes. The factor of 1.4 increase in between sheet and lobe flybys is interpreted as an effect of increased particle impact ionization from ∼200 eV sheet electrons. The factor of 2.5 increase in electron density between flybys on Saturn's nightside and dayside is suggested to be an effect of the pressure balance between thermal plus magnetic pressure in Titan's ionosphere against the dynamic pressure and energetic particle pressure in Saturn's magnetosphere.

Transpolar arc observation after solar wind entry into the high‐latitude magnetosphere

AbstractRecently, Cluster observations have revealed the presence of new regions of solar wind plasma entry at the high‐latitude magnetospheric lobes tailward of the cusp region, mostly during periods of northward interplanetary magnetic field. In this study, observations from the Global Ultraviolet Imager (GUVI) experiment on board the TIMED spacecraft and Wideband Imaging Camera imager on board the IMAGE satellite are used to investigate a possible link between solar wind entry and the formation of transpolar arcs in the polar cap. We focus on a case when transpolar arc formation was observed twice right after the two solar wind entry events were detected by the Cluster spacecraft. In addition, GUVI and IMAGE observations show a simultaneous occurrence of auroral activity at low and high latitudes after the second entry event, possibly indicating a two‐part structure of the continuous band of the transpolar arc.

The thermal structure of Titan’s upper atmosphere, I: Temperature profiles from Cassini INMS observations

We derive vertical temperature profiles from Ion Neutral Mass Spectrometer (INMS) N2 density measurements from 32 Cassini passes. We find that the average temperature of Titan’s thermosphere varies significantly from pass-to-pass between 112 and 175K. The temperatures from individual temperature profiles also varies considerably, with many passes exhibiting wave-like temperature perturbations and large temperature gradients. Wave-like temperature perturbations have wavelengths between 150 and 420km and amplitudes between 3% and 22% and vertical wave power spectra of the INMS data and HASI data have a slope between −2 and −3, which is consistent with vertically propagating atmospheric waves. The lack of a strong correlation between temperature and latitude, longitude, solar zenith angle, or local solar time indicates that the thermal structure of Titan’s thermosphere is not primarily determined by the absorption of solar EUV flux. At N2 densities greater than 108cm−3, Titan’s thermosphere is colder when Titan is observed in Saturn’s magnetospheric lobes compared to Saturn’s plasma sheet as proposed by Westlake et al. (Westlake, J.H. et al. [2011]. J. Geophys. Res. 116, A03318. http://dx.doi.org/10.1029/2010JA016251). This apparent correlation suggests that magnetospheric particle precipitation causes the temperature variability in Titan’s thermosphere; however, at densities smaller than 108cm−3 the lobe passes are hotter than the plasma sheet passes and we find no correlation between the temperature of Titan’s thermosphere and ionospheric signatures of enhanced particle precipitation, which suggests that the correlation is not indicative of a physical connection. The temperature of Titan’s thermosphere also may have decreased by ∼10K around mid-2007. Finally, we classify the vertical temperature profiles to show which passes are hot and cold and which passes have the largest temperature variations. In a companion paper (Part II), we estimate the strength of energy sources and sinks in Titan’s thermosphere.

Estimating the capture and loss of cold plasma from ionospheric outflow

An important source of magnetospheric plasma is cold plasma from the terrestrial ionosphere. Low energy ions travel along the magnetic field lines and enter the magnetospheric lobes where they are convected toward the tail plasma sheet. Recent observations indicate that the field aligned ion outflow velocity is sometimes much higher than the convection toward the central plasma sheet. A substantial amount of plasma therefore escapes downtail without ever reaching the central plasma sheet. In this work, we use Cluster measurements of cold plasma outflow and lobe convection velocities combined with models of the magnetic field in an attempt to determine the fate of the outflowing ions and to quantify the amount of plasma lost downtail. The results show that both the circulation of plasma and the direct tailward escape of ions varies significantly with magnetospheric conditions. For strong solar wind driving with a southward interplanetary magnetic field, also typically associated with high geomagnetic activity, most of the outflowing plasma is convected to the plasma sheet and recirculated. For periods with northward interplanetary magnetic field, the convection is nearly stagnant, whereas the outflow, although limited, still persists. The dominant part of the outflowing ions escape downtail and are directly lost into the solar wind under such conditions.

On the effect of IMF turning on ion dynamics at Mercury

Abstract. We investigate the effect of a rotation of the Interplanetary Magnetic Field (IMF) on the transport of magnetospheric ion populations at Mercury. We focus on ions of planetary origin and investigate their large-scale circulation using three-dimensional single-particle simulations. We show that a nonzero BX component of the IMF leads to a pronounced asymmetry in the overall circulation pattern. In particular, we demonstrate that the centrifugal acceleration due to curvature of the E × B drift paths is more pronounced in one hemisphere than the other, leading to filling of the magnetospheric lobes and plasma sheet with more or less energetic material depending upon the hemisphere of origin. Using a time-varying electric and magnetic field model, we investigate the response of ions to rapid (a few tens of seconds) re-orientation of the IMF. We show that, for ions with gyroperiods comparable to the field variation time scale, the inductive electric field should lead to significant nonadiabatic energization, up to several hundreds of eVs or a few keVs. It thus appears that IMF turning at Mercury should lead to localized loading of the magnetosphere with energetic material of planetary origin (e.g., Na+).

Modelling of spacecraft spin period during eclipse

Abstract. The majority of scientific satellites investigating the Earth magnetosphere are spin stabilized. The attitude information comes usually from a sun sensor and is missing in the umbra; hence, the accurate experimental determination of vector quantities is not possible during eclipses. The spin period of the spacecraft is generally not constant during these times because the moment of inertia changes due to heat dissipation. The temperature dependence of the moment of inertia for each spacecraft has a specific signature determined by its design and distribution of mass. We developed an "eclipse-spin" model for the spacecraft spin period behaviour using magnetic field vector measurements close to the Earth, where the magnetic field is dominated by the dipole field, and in the magnetospheric lobes, where the magnetic field direction is mostly constant. The modelled spin periods give us extraordinarily good results with accumulated phase deviations over one hour of less than 10 degrees. Using the eclipse spin model satellite experiments depending on correct spin phase information can deliver science data even during eclipses. Two applications for THEMIS B, one in the lobe and the other in the lunar wake, are presented.

3-D mesoscale MHD simulations of a cusp-like magnetic configuration: method and first results

Abstract. We present a local mesoscale model of the magnetospheric cusp region with high resolution (up to 300 km). We discuss the construction and implementation of the initial configuration and give a detailed description of the numerical simulation. An overview of simulation results for the case of strongly northward interplanetary magnetic field (IMF) is then presented and compared with data from Cluster 2 spacecraft from 14 February 2003. Results show a cusp diamagnetic cavity (CDC) with depth normal to the magnetospheric boundary on the order of 1–2 RE and a much larger extent of ~5–9 RE tangential to the boundary, bounded by a gradual inner boundary with the magnetospheric lobe and a more distinct exterior boundary with the magnetosheath. These results are qualitatively consistent with observational data.

Titan's thermospheric response to various plasma environments

[1] TheCassini‐HuygensmissionhasbeenobservingTitansinceOctober2004,resultingin over 70 targeted flybys. Titan’s thermosphere is sampled by the Ion and Neutral Mass Spectrometer (INMS) during several of these flybys. The measured upper atmospheric density varies significantly from pass to pass. In order to quantify the processes controlling this variability, we calculate the nitrogen scale height for a variety of parameters related to the solar and plasma environments and, from these, we infer an effective upper atmospheric temperature. In particular, we investigate how these calculated scale heights and temperatures correlate with the plasma environment. Measured densities and inferred temperatures are found to be reduced when INMS samples Titan within Saturn’s magnetospheric lobe regions, while they are enhanced when INMS samples Titan in Saturn’s plasma sheet. Finally the data analysis is supplemented with Navier‐Stokes model calculations using the Titan Global Ionosphere Thermosphere Model. Our analysis indicates that, during the solar minimum conditions prevailing during the Cassini tour, the plasma interaction plays a significant role in determining the thermal structure of the upper atmosphere and, in certain cases, may override the expected solar‐driven diurnal variation in temperatures in the upper atmosphere. Citation: Westlake, J. H., J. M. Bell, J. H. Waite Jr., R. E. Johnson, J. G. Luhmann, K. E. Mandt, B. A. Magee, and A. M. Rymer (2011), Titan’s thermospheric response to various plasma environments, J. Geophys. Res., 116, A03318,

Ion dynamics during compression of Mercury's magnetosphere

Abstract. Because of the small planetary magnetic field as well as proximity to the Sun that leads to enhanced solar wind pressure as compared to Earth, the magnetosphere of Mercury is very dynamical and at times subjected to prominent compression. We investigate the dynamics of magnetospheric ions during such compression events. Using three-dimensional single-particle simulations, we show that the electric field induced by the time varying magnetic field can lead to significant ion energization, up to several hundreds of eVs or a few keVs. This energization occurs in a nonadiabatic manner, being characterized by large enhancements of the ion magnetic moment and bunching in gyration phase. It is obtained when the ion cyclotron period is comparable to the field variation time scale. This condition for nonadiabatic heating is realized in distinct regions of space for ions with different mass-to-charge ratios. During compression of Mercury's magnetosphere, heavy ions originating from the planetary exosphere may be subjected to such an abrupt energization, leading to loading of the magnetospheric lobes with energetic material.