Geotail Satellite Research Articles

Investigations of the degree of asymmetry across the dayside magnetopause under southward interplanetary magnetic field using GEOTAIL observations

The magnetic reconnection environment around the dayside geomagnetopause under long-lasting southward interplanetary magnetic field (IMF) conditions is investigated using the in-situ observation by GEOTAIL satellite from 1994 to 2019. We focus on the degrees of asymmetry in the plasma density, ion temperature and the magnetic field strength between both sides of the magnetopause, that is, the ratio of the value in the magnetosphere to that in the magnetosheath in order to compute the much more realistic current sheet systems in numerical simulations. To exclude all of the interplanetary disturbance events such as Corotating Interaction Regions (CIRs) and Coronal Mass Ejections (CMEs), we investigate magnetopause crossings under long-lasting southward IMF conditions. GEOTAIL satellite sometimes repeatedly across the magnetopause during each pass due to the oscillating of the magnetopause. The degrees of asymmetry vary even during a single pass. This variation depends on the locus of the observation point, particularly the GSM Y-position, not on the time. The degrees of asymmetry in the plasma density, ion temperature and the magnetic field strength have significant variation in the data. The data points of the degree of asymmetry in the plasma density significantly spread in the duskside, while that in the magnetic field strength further widely spread in the dawnside. The degree of asymmetry in the plasma density and that in the magnetic field strength have fixed negative correlation on a log–log scale.Graphical

The impact of electron beams on the arbitrary amplitude electron–acoustic solitons in a nonthermal plasma

This study examines the nonlinear dynamics of high-frequency electron–acoustic waves (EAWs) in a collisionless, unmagnetized plasma consisting of several components, including inertial cold electrons, an inertial electron beam, and inertialess Cairns-distributed hot electrons in addition to background stationary ions. We use a nonlinear pseudopotential (Sagadeev potential) method to investigate the possibility of stationary-profile electron–acoustic solitons (EASs). In this study, the nonthermal parameter, the temperature ratio between hot and cold electrons, density ratios, and electron beam parameters are carefully examined to see how they change the features of EASs. As the nonthermality of hot electrons rises, the beam speed decreases, the density ratio of the beam to the cold electron increases, and the existence domain for EASs gets bigger. The current theoretical model shows a link between the wideband noise seen in Geotail satellites and the plasma sheet boundary layer in Earth's magnetosphere.

Equatorward Moving Auroral Arcs Associated with Impulse-Excited Field Line Resonance

The theory of equatorward moving east-west elongated auroral arcs associated with field line resonance (FLR) has been proposed for decades. However, confirming this theory requires in-situ observations of FLR within the magnetosphere and simultaneous all-sky imager observations of equatorward moving auroral arcs near satellite footpoints. In this study, we present the first observations of multiple equatorward moving auroral arcs related to impulse-excited FLR, using datasets from the WIND, Geotail satellites, and an all-sky imager at China’s Zhongshan Station (ZHS) in Antarctica. In the presented event, the ultra-low-frequency waves associated with solar wind dynamic pressure pulse was mainly toroidal mode, which is consistent with the theory that the toroidal mode waves usually related with external source. The all-sky imager located in Zhongshan station recorded several equatorward moving auroral arcs, followed by reverse propagating ones. The latitudinal width of the equatorward moving auroral arcs was on the order of 25 km and had an average equatorward propagation of ~0.37 km/s, which is very similar to the value from previous work. To better illustrate the observed evolution of auroral arcs related with the FLRs we proposed a simple model to evaluate the FACs induced by the FLRs in different latitudes. The latitudinal distribution evolution of FACs agrees well with the ground-based optical observations.

Effects from dayside magnetosphere to distant tail unleashed by a bifurcated, non-reconnecting interplanetary current sheet

Global magnetospheric effects resulting from the passage at Earth of large-scale structures have been well studied. The effects of common and short-term features, such as discontinuities and current sheets (CSs), have not been studied in the same depth. Herein we show how a seemingly unremarkable interplanetary feature can cause widespread effects in the magnetosheath-magnetosphere system. The feature was observed by Advanced Composition Explorer inside an interplanetary coronal mass ejection on 10 January 2004. It contained 1) a magnetic field dip bounded by directional discontinuities in field and flows, occurring together with 2) a density peak in what we identify as a bifurcated, non-reconnecting current sheet. Data from an array of spacecraft in key regions of the magnetosheath/magnetosphere (Geotail, Cluster, Polar, and Defense Meteorological Satellite Program) provide context for Wind’s observations of flapping of the distant (R ∼ −226 RE) magnetotail. In particular, just before the flapping began, Wind observed a hot and tenuous plasma in a magnetic field structure with enhanced field strength, with the By and Bz components rotating in a fast tailward flow burst. Closer inspection reveals a large flux rope (plasmoid) containing lobe plasma in a tail strongly deflected and twisted by interplanetary non-radial flows and magnetic field By. We try to identify the origin of this ‘precursor to flapping’ by looking at data from the various spacecraft. Working back towards the dayside, we discover a chain of effects which we argue were set in motion by the interplanetary CS and its interaction with the bow shock. These effects include 1) a compression and dilation of the magnetosphere, 2) a local deformation of the postnoon magnetopause, and, 3) at the poleward edge of the oval in an otherwise quiet polar cap flow, a strong (3 km/s) sunward flow burst in a double vortex-like structure flanked by two sets of field-aligned currents. Clearly, an intertwined set of phenomena was occurring at the same time. We learn that multi-spacecraft analysis can give us great insight into the magnetospheric response to transient changes in the solar wind.

Spatial structures in solar wind superthermal electrons and polar rain aurora

We report a special polar rain aurora case around 11:24 UT on October 27, 2003, where intense polar rain electrons produced observable polar rain auroral emission with the shape of a roughly dawn-dusk aligned bar. Associated solar wind speed and density observations during the event were around 450 km/s and 2.5 cm−3 respectively. The interplanetary magnetic field (IMF) components Bx, By, and Bz were ~5, −3, and 5 nT respectively. The negative By condition likely caused the dawnside shift and slight tilt of the polar rain aurora bar. Furthermore, although Kelvin-Helmholtz waves on the high latitude magnetopause have been previously reported to induce dawn-dusk aligned auroral bars (Zhang et al., 2007), the solar wind and IMF conditions of the event are not favorable for generating them (Zhang et al., 2013) and are therefore not a likely cause. Instead, coincident observations by the Geotail satellite show enhanced anti-sunward flux in the solar wind superthermal electrons (7 eV–42 keV) around the time of the auroral bar. The solar wind superthermal electron spatial size, when mapped into the polar ionosphere, is consistent with the width of the auroral bar, confirming a connection between the two.

Global Propagation of Magnetospheric Pc5 ULF Waves Driven by Foreshock Transients

AbstractPc5 (2–7 mHz) ultralow frequency (ULF) waves play a significant role in resonating with particles and transferring energy in the coupled magnetospheric and ionospheric system. Recent studies found that Pc5 ULF waves can be triggered by foreshock transients which can perturb the magnetopause through dynamic pressure variation. However, whether foreshock transient‐driven Pc5 ULF waves are geoeffective and can propagate globally is still poorly understood. In this study, we take advantage of the conjunction between in situ (by the THEMIS probes, Geotail satellite, GOES satellites, and Van Allen probes) and ground‐based (by the all‐sky imager at South Pole and ground‐based magnetometers) observations to simultaneously analyze the waves from the foreshock region to the dayside and nightside magnetosphere. Both of our two events show that the Pc5 ULF waves are generated by foreshock transients in the dayside magnetosphere. The in situ observations by THEMIS A and D and the 2‐D auroral signatures show that the compressional mode waves are likely broadband and coupled to the FLRs with different frequencies and different azimuthal phase speeds. This is the first report that foreshock transients can drive both low‐ and high‐m FLRs, with the azimuthal wave numbers varying from ~5 to ~23. Moreover, the Pc5 ULF waves propagated antisunward to midnight, this can potentially modulate magnetospheric and ionospheric dynamics globally.

Polar substorm on 7 December 2015: preonset phenomena and features of auroral breakup

Abstract. Comprehensive analysis of a moderate 600 nT substorm was performed using simultaneous optical observations inside the auroral oval and in the polar cap, combined with data from satellites, radars, and ground magnetometers. The onset took place near the poleward boundary of the auroral oval that is not typical for classical substorms. The substorm onset was preceded by two negative excursions of the interplanetary magnetic field (IMF) Bz component, with a 1 min interval between them, two enhancements of the antisunward convection in the polar cap with the same time interval, and 15 min oscillations in the geomagnetic H component in the auroral zone. The distribution of the pulsation intensity along meridian has two local maxima, namely at the equatorial and poleward boundaries of the auroral oval, where pulsations occurred in the out-of-phase mode resembling the field line resonance. At the initial stage, the auroral breakup developed as the auroral torch stretched and expanded poleward along the meridian. Later it took the form of the large-scale coiling structure that also distinguishes the considered substorm from the classical one. Magnetic, radar, and the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellite data show that, before the collapse, the coiling structure was located between two field-aligned currents, namely downward at the poleward boundary of structure and upward at the equatorial boundary. The set of GEOTAIL satellites and ground data fit to the near-tail current disruption scenario of the substorm onset. We suggest that the 15 min oscillations might play a role in the substorm initiation.

Complex Analysis of the Polar Substorm Based on Magnetic, Optical, and Radar Observations near Spitsbergen

A comprehensive analysis of the polar substorm registered by IMAGE network stations close to the poleward boundary of the auroral oval has been carried out with THEMIS, CLUSTER, and GEOTAIL satellites in favorable positions for analysis. The observation interval is characterized by a low level of geomagnetic activity. The polar substorm looked like three negative, bay-like disturbances and developed within an ordinary substorm registered at stations in the middle of the auroral zone half an hour before the polar substorm. Each bay-like disturbance of the polar substorm was accompanied by a poleward drift of aurora and westward electrojet, a Pi2 pulsation group, a surge of magnetic activity over the range of 0.1–10 Hz, and enhancement of electronic precipitations over Spitsbergen. The differences between the first activation and two subsequent ones in the aurora dynamics, ionospheric convection, and electronic precipitations are revealed. The disturbed zone longitudinal sizes and position in the magnetosphere are estimated. Possible causes of the substorm onset and disturbances associated with it are discussed. The results of the study expand the statistics on the polar substorm phenomenon and will enable insight into its nature.

Development of a near‐infrared balloon‐borne camera for dayside and sunlit auroral observations

AbstractImaging aurora in daylight is a difficult and challenging task. The brightness of the sunlit atmosphere overwhelms the auroral emissions at visible wavelengths. Modeling of atmospheric brightness suggests that the contrast between auroral brightness and sky brightness makes it possible to image the aurora at near‐infrared (NIR) wavelengths from sufficient altitudes. Preliminary experiments confirmed that the auroral N2+ Meinel emissions at about 1100 nm are bright enough to be extracted from atmospheric background brightness during daylight at about 40 km of a balloon altitude, which lead to the development of a high‐performance NIR InGaAs camera that can be flown on a high‐altitude and long‐duration balloon. Auroral observations from such a platform are highly accommodated to current space missions (such as Time History of Events and Macroscale Interactions during Substorms/Acceleration Reconnection Turbulence and Electrodynamics of Moon's Interaction with the Sun, Magnetospheric Multiscale, Cluster, Geotail, and Defense Meteorological Satellite Program) and many ground‐based measurements and will enhance the science return significantly.

Bursty bulk flows at different magnetospheric activity levels: Dependence on IMF conditions

AbstractBased on concurrent observations of the ACE and Geotail satellites from 1998 to 2005, we statistically analyzed and compared the earthward bursty bulk flows (BBFs) with local positive Bz under different interplanetary magnetic field (IMF) conditions. Four different magnetospheric activity levels (MALs), including quiet times and substorm growth/expansion/recovery phases, are considered. The properties of the BBFs, including their ion temperature (T), Vx component, x component of the energy flux density (Qx), and the solar wind dawn‐dusk electric field Ey (observed at ~1 AU), are analyzed. Main observations include the following: (1) BBF tends to have less penetration distance for northward IMF (NW‐IMF) than for southward IMF (SW‐IMF). Inward of 15 RE the BBFs for SW‐IMF are dominant. Few BBFs for NW‐IMF occur within 15 RE. (2) The occurrence probabilities of the BBFs at each MAL depend highly on the orientations of the IMF. During quiet times, the BBFs for NW‐IMF are dominant. Reversely, during the growth and expansion phases of a substorm, the BBFs for SW‐IMF are dominant. (3) The strengths of the BBF have significant evolution with substorm development. For SW‐IMF condition, the strengths of the BBFs are the lowest for quiet times. The strength of the BBFs tends to increase during the growth phase and reaches to the strongest value during the expansion phase, then, decays during the recovery phase. For NW‐IMF condition, the strengths of the BBFs evolve with the substorm development in a similar way as for SW‐IMF condition. (4) For SW‐IMF, the solar wind Ey evolves with the substorm development in a similar way to the strength of the BBFs. However, no clear evolution is found for NW‐IMF. (5) The strengths of the BBF Qx and solar wind Ey are closely related. Both tend to be stronger for growth phase than for quite time, reach the strongest for expansion phase, then decay for recovery phase. It appears that to trigger a substorm, the strength of the BBFs should achieve energy thresholds with values different for NW‐IMF and SW‐IMF.

Temporal evolutions of the solar wind conditions at 1 AU prior to the near‐Earth X lines in the tail: Superposed epoch analysis

AbstractUtilizing conjunction observations of the Geotail and ACE satellites from 1998 to 2005, we investigated the temporal evolutions of the solar wind conditions prior to the formation of X lines in the near‐Earth magnetotail. We first show the statistical properties of Bz, By, density, and velocity of the solar wind related to the 374 tail X line events. A superposed epoch analysis is performed to study the temporal evolutions of the solar wind conditions 5 h prior to the tail X lines. The solar wind conditions for tail X lines during southward interplanetary magnetic field (IMF) (SW‐IMF) and northward IMF (NW‐IMF) are analyzed. The main results are as follows: (1) For events classified as SW‐IMF, near‐Earth X line observations in the magnetosphere are preceded by ~2 h intervals of southward IMF; (2) for events classified as NW‐IMF, the northward IMF orientation preceding near‐Earth X line observations lasts ~ 40 min.

Long-term variations in the plasma sheet ion composition and substorm occurrence over 23 years

The Geotail satellite has been operating for almost two solar cycles (~23 years) since its launch in July 1992. The satellite carries the energetic particle and ion composition (EPIC) instrument that measures the energetic ion flux (9.4–212 keV/e) and enables the investigation of long-term variations of the ion composition in the plasma sheet for solar cycles 22–24. From the statistical analysis of the EPIC data, we find that (1) the plasma ion mass (M) is approximately 1.1 amu during the solar minimum, whereas it increases to 1.5–2.7 amu during the solar maximum; (2) the increases in M seem to have two components: a raising of the baseline levels (~1.5 amu) and a large transient enhancement (~1.8–2.7 amu); (3) the baseline level change of M correlates well with the Mg II index, which is a good proxy for the solar extreme ultraviolet (EUV) or far ultraviolet (FUV) irradiance; and (4) the large transient enhancement of M is caused by strong magnetic storms. We also study the long-term variations of substorm occurrences in 1992–2015 that are evaluated with the number of Pi2 pulsations detected at the Kakioka observatory. The results suggest no clear correlation between the substorm occurrence and the Mg II index. Instead, when the substorms are classified into externally triggered events and non-triggered events, the number of the non-triggered events and the Mg II index are negatively correlated. We interpret these results that the increase in the solar EUV/FUV radiation enhances the supply of ionospheric ions (He+ and O+ ions) into the plasma sheet to increase M, and the large M may suppress spontaneous plasma instabilities initiating substorms and decrease the number of the non-triggered substorms. The present analysis using the unprecedentedly long-term dataset covering ~23 years provides additional observational evidence that heavy ions work to prevent the occurrence of substorms.

Energetic ion acceleration during magnetic reconnection in the Earth’s magnetotail

In this paper, we present a comprehensive study of the energetic ion acceleration during magnetic reconnection in the Earth’s magnetosphere using the Geotail data. A clear example of the energetic ion acceleration up to 1 MeV around an X-type neutral line is shown. We find that the energetic ions are localized at far downstream of reconnection outflow. The time variation of energetic ion and electron is almost the same. We observe ∼100 keV ions over the entire observation period. We study ten events in which the Geotail satellite observed in the vicinity of diffusion region in order to understand the reconnection characteristics that determine the energetic ion acceleration efficiency. We find that the reconnection electric field, total amount of reduced magnetic energy, reconnection rate, satellite location in the Earth’s magnetosphere (both X GSM and Y GSM) show high correlation with energetic ion acceleration efficiency. Also, ion temperature, electron temperature, ion/electron temperature ratio, current sheet thickness, and electric field normal to the neutral sheet show low correlation. We do not find any correlation with absolute value of outflow velocity and current density parallel to magnetic field. The energetic ion acceleration efficiency is well correlated with large-scale parameters (e.g., total amount of reduced magnetic energy and satellite location), whereas the energetic electron acceleration efficiency is correlated with small-scale parameters (e.g., current sheet thickness and electric field normal to the neutral sheet). We conclude that the spatial size of magnetic reconnection is important for energetic ion acceleration in the Earth’s magnetotail.

X lines in the magnetotail for southward and northward IMF conditions

AbstractUtilizing associated observations of Geotail and ACE satellites from the year of 1998 to 2005, we investigated the X lines in the near‐Earth tail under different interplanetary magnetic field (IMF) conditions. The X lines are recognized by the tailward fast flows with negative Bz. Statistically, the X lines in the tail can be observed for southward as well as northward IMF, but more frequently observed for southward IMF. A typical case on 26 April 2005 showed clear evidence that the X line can occur for northward IMF while the geomagnetic activity is particularly quiet. Further analysis showed that the X line‐related solar wind has stronger Ey and Bz components for southward than northward IMF. In addition, the X line‐related geomagnetic activities are stronger for southward than northward IMF.

Probabilities of magnetic reconnection encounter at different activity levels in the Earth’s magnetotail

With the upcoming Magnetospheric Multiscale mission by NASA to investigate magnetic reconnection (MR) in detail, it is imperative to identify more precisely the occurrence probabilities of observing MR signatures in different magnetospheric activity levels. An extensive investigation is conducted on the probabilities of observing MR feature at different magnetospheric activity levels with data from Geotail satellite. A newly developed method is used to categorize the state of magnetosphere in five different activity levels. The result shows quantitatively the probabilities in encountering MR features for these five different activity levels, providing valuable guidance to satellite operations to enhance encounter of MR in the Earth’s magnetotail.

Further evidence for a connection between auroral kilometric radiation and ground‐level signals measured in Antarctica

AbstractInspired by recent observations of three cases of coincident ground‐level auroral kilometric radiation (AKR)‐like signals and outgoing AKR measured with the Geotail satellite, investigation of 2008 data from four Antarctic observatories yields >30 additional examples. The occurrence rate peaks near 22 MLT similar to that of AKR. Polarization measurements of one event show it to be right‐hand polarized. Correlation analysis of ground‐level and satellite data suggests an imperfect correlation with 2–3 sigma significance, although occasionally much better. (Perfect correlation is not expected, for at least two reasons: distant satellites detect AKR from many sources over a significant part of the oval, and many effects can hinder transmission of AKR from those sources to either distant satellites or ground stations.) Statistical analysis of the existing quantity of data is therefore suggestive but does not prove a connection between the ground‐level AKR‐like signals and outgoing AKR. However, two other methods provide evidence favoring such a connection. The first full‐resolution measurements of ground‐level AKR‐like signals show that their fine structure strongly resembles that of AKR as known from high‐resolution spacecraft receivers. Two case studies show that the location of active aurora, presumed connected with the AKR sources, controls whether or not the ground station detects the AKR, or which of several ground stations detects it most strongly. These investigations strengthen the hypothesis that through some mechanism, sources of outgoing AKR detected by distant satellites occasionally generate signals detectable as whistler mode waves at low altitude or right‐hand polarized signals at ground level.

Internally and externally induced deformations of the magnetospheric equatorial current as inferred from spacecraft data

Abstract. Based on a data pool of 79 yearly files of space magnetometer data by Polar, Cluster, Geotail, and THEMIS satellites between 1995 and 2013, we developed a new quantitative model of the global shape of the magnetospheric equatorial current sheet as a function of the Earth's dipole tilt angle, solar wind ram pressure, and interplanetary magnetic field (IMF). This work upgrades and generalizes an earlier model of Tsyganenko and Fairfield (2004) by extending the modeling region to all local times, including the dayside sector. In particular, an essential feature of the new model is the bowl-shaped tilt-related deformation of the equatorial surface of minimum magnetic field, similar to that observed at Saturn, whose existence in the Earth's magnetosphere has been demonstrated in our recent work (Tsyganenko and Andreeva, 2014).

Responses of different ion species to fast plasma flows and local dipolarization in the plasma sheet

AbstractWe investigate the responses of different ion species (H+, He+, He++, and O+) to fast plasma flows and local dipolarization in the plasma sheet in terms of energy density. We use energetic (9–210 keV) ion composition measurements made by the Geotail satellite at r = 10~31 RE. The results are summarized as follows: (1) whereas the O+‐to‐H+ ratio decreases with earthward flow velocity, it increases with tailward flow velocity with steeper Vx dependence for perpendicular flows than for parallel flows; (2) for fast earthward flows, the energy density of each ion species increases without any clear preference for heavy ions; (3) for fast tailward flows, the ion energy density initially increases, then it decreases to below the preceding levels except for O+; (4) the O+‐to‐H+ ratio does not increase through local dipolarization irrespective of dipolarization amplitude, background Bz, X distance, and Vx; (5) in general, the H+ and He++ ions behave similarly. Result (1) can be attributed to radial transport in the presence of the earthward gradient of the background O+‐to‐H+ ratio. Results (2) and (4) suggest that ion energization at local dipolarization is not mass dependent in the energy range of our interest because the ions are not magnetized irrespective of species. Result (3) can be attributed to the thinning of the plasma sheet and the preferable field‐aligned escape of the H+ ions on the tailward side of the reconnection site. Result (5) suggests that the solar wind is the primary source of the high‐energy H+ ions.

Magnetospheric signatures of auroral disturbances during the passage of the solar wind’s CIR and sheath regions

Based on Polar satellite data, the authors have studied the auroral disturbances that arose during the passage by the Earth of compressed plasma regions formed in front of high-speed solar wind streams (the CIR region) and in front of magnetic clouds (the Sheath region). The aurorae observed by the Polar satellite possessed basic signatures of a substorm: a localized onset and expansion toward the pole and westward and eastward. However, in these cases they had a very large size in longitude and latitude and occupied a very large area. All disturbances observed by the Polar satellite during the Sheath and CIR regions of the solar wind in December of 1996, in 1997–1998, and in 2000 were analyzed. Eight events during disturbance development in the ionosphere, when the Geotail satellite was located in the plasma sheet of the magnetospheric tail, were selected. It is shown that in all selected cases some typical signatures of substorm development in the magnetospheric tail were observed, namely: (1) fast plasma flows (flow reversal, i.e., from tailwards to Earthwards) and (2) a sharp decrease of the total pressure, which followed an interval of total pressure increase. One can draw the conclusion that in the CIR and Sheath regions with a high solar wind density, substorm disturbances of a specific type are observed, with large latitudinal and longitudinal size (sometimes occupying the entire polar cap).

IMF effect on the polar cap contraction and expansion during a period of substorms

Abstract. The polar cap boundary (PCB) location and motion in the nightside ionosphere has been studied by using measurements from the EISCAT radars and the MIRACLE magnetometers during a period of four substorms on 18 February 2004. The OMNI database has been used for observations of the solar wind and the Geotail satellite for magnetospheric measurements. In addition, the event was modelled by the GUMICS-4 MHD simulation. The simulation of the PCB location was in a rather good agreement with the experimental estimates at the EISCAT longitude. During the first three substorm expansion phases, neither the local observations nor the global simulation showed any poleward motions of the PCB, even though the electrojets intensified. Rapid poleward motions of the PCB took place only in the early recovery phases of the substorms. Hence, in these cases the nightside reconnection rate was locally higher in the recovery phase than in the expansion phase. In addition, we suggest that the IMF Bz component correlated with the nightside tail inclination angle and the PCB location with about a 17-min delay from the bow shock. By taking the delay into account, the IMF northward turnings were associated with dipolarizations of the magnetotail and poleward motions of the PCB in the recovery phase. The mechanism behind this effect should be studied further.