Magnetospheric Spacecraft Research Articles

A Substorm‐Dependent Negative Limit of Non‐Eclipse Surface Charging of a Chinese Geosynchronous Satellite

AbstractSurface charging is one of the most common causes of spacecraft anomalies. When and to what potential the spacecraft is charged are two important questions in space weather. Here, for a Chinese geosynchronous navigation satellite, we infer the extreme negative surface charging potentials from the ion differential fluxes measured by a low‐energy ion spectrometer. Without the solar eclipse effect away from the midnight, the charging potentials are found to have a negative limit which is determined by the maximum SuperMAG electrojet index in the preceding 2 hr. Such an empirical relation can be reasonably explained by the dependence of 1–50 keV electron fluxes on substorm strength. Similar relations may also exist for other inner magnetospheric spacecraft in the non‐eclipse region, which would be useful for spacecraft engineering and space weather alerts.

Unusual Magnetospheric Dynamics During Intense Substorm Initiated by Strong Magnetospheric Compression

AbstractWe investigate an unusual sequence and peculiar features of magnetotail changes during a storm‐range substorm initiated by the interplanetary shock. Auroral observations and measurements at several favorably distributed magnetospheric spacecraft allowed the construction of an adaptive time‐dependent magnetospheric model to quantitatively characterize the configurational changes and mapping variations. Several passages of low‐altitude spacecraft in polar orbits near midnight help reveal the magnetic configuration of the nightside tail‐dipole transition region. In this event, an intense auroral and convection activity (accompanied by an up to 1,500 nT increase in the SuperMag AL‐index index) emerged in the highly compressed magnetosphere after the passage of interplanetary shock followed by strongly southward interplanetary magnetic field (IMF). This directly driven phase of the activity continued for an hour and resulted in the formation of a hybrid magnetic configuration with dipolarized midtail and stretched field lines in the transition region. Observations of energetic particle isotropy boundary latitudes near midnight are consistent with the modeled magnetic configuration. In concert with a downward turn of the solar wind (SW) flow, and weakening of the IMF driver and convection, an unusual stretching signature of the inner magnetosphere magnetic field was observed as close as at r ∼ 5 to 7 Re; which resulted mostly from an increasing downward tilt of the thin azimuthal current. Classic substorm breakup signatures commenced at fairly low, ∼60° magnetic latitude, deep in the closed field line region, in association with the current sheet upward motion. It was followed by strong stepwise poleward auroral expansion. We discuss how these signatures deviate from standard substorm scenarios and may be potentially imparted by the aforementioned changes in SW flow direction and pressure.

Inner Magnetospheric Response to the Interplanetary Magnetic Field By Component: Van Allen Probes and Arase Observations

AbstractWe utilize 17 years of combined Van Allen Probes and Arase data to statistically analyze the response of the inner magnetosphere to the orientation of the interplanetary magnetic field (IMF) By component. Past studies have demonstrated that the IMF By component introduces a similarly oriented By component into the magnetosphere. However, these studies have tended to focus on field lines in the magnetotail only reaching as close to the Earth as the geosynchronous orbit. By exploiting data from these inner magnetospheric spacecraft, we have been able to investigate the response at radial distances of <7RE. When subtracting the background magnetic field values, provided by the T01 and IGRF magnetic field models, we find that the IMF By component does affect the configuration of the magnetic field lines in the inner magnetosphere. This control is observed throughout the inner magnetosphere, across both hemispheres, all radial distances, and all magnetic local time sectors. The ratio of IMF By to the observed By residual, also known as the “penetration efficiency,” is found to be ∼0.33. The IMF Bz component is found to increase, or inhibit, this control depending upon its orientation.

Testing Efficiency of Empirical, Adaptive, and Global MHD Magnetospheric Models to Represent the Geomagnetic Field in a Variety of Conditions

AbstractWe used data for eight magnetospheric spacecraft providing magnetic observations in various magnetospheric domains during a six‐day time period, including the June 2015 storm, and a five‐day period including the March 2015 storm. For these time intervals, containing different solar wind regimes and different activity levels, we used three types of metrics to compare predictions of all existing types of magnetospheric magnetic field models, including empirical models (TA15, TS05, and T96), a data‐adapted model (AM03), and a global magnetohydrodynamic model (Space Weather Modeling Framework/Block Adaptive Tree Solar Wind–Roe–Upwind Scheme coupled with Rice Convection Model). In total, the models are ranked in the order: AM03, TS05, TA15, Space Weather Modeling Framework, and T96. The regional scores may differ from the average ones (in particular, better scores are obtained at GEO, compared to the inner magnetosphere) and also the model effectiveness varies with activity conditions (for example, TA15 outperforms other models in quiet conditions, but TS05 leads during active times). Quite unexpectedly, although run at modest resolution, the community available global magnetohydrodynamic model closely approached the best scores of empirical statistical models during the moderate/high disturbed periods in the June event, suggesting that this kind of modeling may now compete with other type models. We also carried out the field line mapping from fixed points in the equatorial magnetosphere to the ionosphere, using different models to examine the storm time foot point excursions (which may be as large as ~10° CGLat during storm time sudden commencements) and their uncertainties, quantified by the difference between the model predictions.

Magnetotail Configuration During a Steady Convection Event as Observed by Low‐Altitude and Magnetospheric Spacecraft

AbstractMotivated by ongoing discussion regarding the magnetic configuration in the near‐Earth and midtail regions and its role in populating the inner magnetosphere during long‐duration steady magnetospheric convection (SMC) events, we analyze a rich collection of observations during ~10 hr of strong, steady solar wind driving. Auroral boundaries and regions of stretched and dipolarized magnetic field in the plasma sheet were monitored using solar electron loss cone anisotropy observed by low‐altitude spacecraft. Following a southward turning of the interplanetary magnetic field and a subsequent 3‐ to 4‐hr period of large‐scale substorm‐related reconfigurations and plasma injections, the near‐Earth magnetic configuration evolved into a nonstandard type, which lasted until the end of this SMC event (5 hr). During that time a dipolarized region with complicated Bz landscape persisted in the midtail, while the configuration was very stretched in the near tail. This was manifested as a highly depressed magnetic Bz component at geostationary orbit and as persistent nonadiabatic electron scattering at the periphery of the outer radiation belt. In addition, in situ observations suggest that a thin current sheet extended longitudinally toward the dawn terminator. In the return convection region near the terminator, observations of this azimuthal current sheet were sporadically interrupted/modulated by earthward convecting plasma structures, either remnants of reconnection‐produced plasma bubbles or flapping waves. The hybrid magnetotail configuration (dipolar in the midtail and stretched in the near tail) observed during this long‐duration SMC event poses a challenge for empirical magnetospheric modeling.

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.

Spacecraft‐Charging Mitigation of a High‐Power Electron Beam Emitted by a Magnetospheric Spacecraft: Simple Theoretical Model for the Transient of the Spacecraft Potential

AbstractA spacecraft‐charging mitigation scheme necessary for the operation of a high‐power electron beam in the low‐density magnetosphere is analyzed. The scheme is based on a plasma contactor, that is, a high‐density charge‐neutral plasma emitted prior to and during beam emission and its ability to emit high ion currents without strong space‐charge limitations. A simple theoretical model for the transient of the spacecraft potential and contactor expansion during beam emission is presented. The model focuses on the contactor ion dynamics and is valid in the limit when the ion contactor current is equal to the beam current. The model is found in very good agreement with particle‐in‐cell simulations over a large parametric study that varies the initial expansion time of the contactor, the contactor current, and the ion mass. The model highlights the physics of the spacecraft‐charging mitigation scheme, indicating that the most important part of the dynamics is the evolution of the outermost ion front, which is pushed away by the charge accumulated in the system by the beam. The model can be also used to estimate the long‐time evolution of the spacecraft potential. For a short contactor expansion (0.3‐ or 0.6‐ms helium plasma or 0.8‐ms argon plasma, both with 1‐mA current) it yields a peak spacecraft potential of the order of 1–3 kV. This implies that a 1‐mA relativistic electron beam would be easily emitted by the spacecraft.

Does a Local B‐Minimum Appear in the Tail Current Sheet During a Substorm Growth Phase?

AbstractMagnetic configurations with dBz/dr > 0 in the midtail current sheet are potentially unstable to various instabilities associated with the explosive substorm onset. Their existence is hard to confirm with observations of magnetospheric spacecraft. Here we use remote sensing by low‐altitude spacecraft that measured the loss cone filling rate during electron‐rich solar particle event, providing information about magnetic properties of the tail current sheet. We found a latitudinally localized anisotropic 30 keV electron loss cone region embedded inside an extended region of isotropic solar electron precipitation. It was persistently observed for more than 0.5 h during isolated growth phase event by six Polar Operational Environmental Satellites spacecraft, which crossed the premidnight auroral oval. The embedded anisotropic region was observed 1° poleward of the outer radiation belt boundary over 4–5 h wide magnetic local time sector, suggesting a persistent ridge‐type Bz2/j maximum in the equatorial plasma sheet at distances 15–20 RE. We discuss infrequent observations of such events taking into account recent results of global magnetohydrodynamic simulations.

Pulsating aurora from electron scattering by chorus waves.

Auroral substorms, dynamic phenomena that occur in the upper atmosphere at night, are caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy. These storms are characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn. Pulsating aurorae, which are quasiperiodic, blinking patches of light tens to hundreds of kilometres across, appear at altitudes of about 100 kilometres in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. This auroral pulsation, with periods of several to tens of seconds, is generated by the intermittent precipitation of energetic electrons (several to tens of kiloelectronvolts) arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere. A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves. However, no direct observational evidence of this interaction has been obtained so far. Here we report that energetic electrons are scattered by chorus waves, resulting in their precipitation. Our observations were made in March 2017 with a magnetospheric spacecraft equipped with a high-angular-resolution electron sensor and electromagnetic field instruments. The measured quasiperiodic precipitating electron flux was sufficiently intense to generate a pulsating aurora, which was indeed simultaneously observed by a ground auroral imager.

Can an electron gun solve the outstanding problem of magnetosphere‐ionosphere connectivity?

AbstractDetermining the magnetic connectivity between magnetospheric phenomena and ionospheric phenomena is an outstanding problem of magnetospheric and ionospheric physics. Accurately establishing this connectivity could answer a variety of long‐standing questions. The most viable option to solve this is by means of a high‐power electron beam fired from a magnetospheric spacecraft and spotted at its magnetic footpoint in the ionosphere. This has technical difficulties. Progress has been made on mitigating the major issue of spacecraft charging. The remaining physics issues are identified, together with the need for a synergistic effort in modeling, laboratory experiments, and, ultimately, testing in space. The goal of this commentary is to stimulate awareness and interest on the magnetosphere‐ionosphere connectivity problem and possibly accelerate progress toward its solution.

Comparison of DMSP and SECS region-1 and region-2 ionospheric current boundary

The region-1 and region-2 boundary has traditionally been identified using data from a single spacecraft crossing the auroral region and measuring the large scale changes in the cross track magnetic field. With data from the AUTUMN, CANMOS, CARISMA, GIMA, DTU MGS, MACCS, McMAC, STEP, THEMIS, and USGS ground magnetometer arrays we applied a state-of-art technique based on spherical elementary current system (SECS) method developed by Amm and Viljanen (1999) in order to calculate maps of region-1 and region-2 current system over the North American and Greenland auroral region. Spherical elementary current (SEC) amplitude (proxy for vertical currents) maps can be inferred at 10s temporal resolution, ~1.5° geographic latitude (Glat), and 3.5° geographic longitude (Glon) spatial resolution. We compare the location of the region-1 and region-2 boundary obtained by the DMSP spacecraft with the region-1 and region-2 boundary observed in the SEC current amplitudes. We find that the boundaries typically agree within 0.2°±1.3°. These results indicate that the location of the region-1 and region-2 boundary can reasonably be determined from ground magnetometer data. The SECS maps represent a value-added product from the magnetometer database and can be used for contextual interpretation in conjunction with other missions as well as help with our understanding of magnetosphere-ionosphere coupling mechanisms using the ground arrays and the magnetospheric spacecraft data.

Future beam experiments in the magnetosphere with plasma contactors: The electron collection and ion emission routes

AbstractExperiments where a high‐voltage electron beam emitted by a spacecraft in the low‐density magnetosphere is used to probe the magnetospheric configuration could greatly enhance our understanding of the near‐Earth environment. Their challenge, however, resides in the fact that the background magnetospheric plasma cannot provide a return current that balances the electron beam current without charging the spacecraft to such high potential that in practice prevents beam emission. In order to overcome this problem, a possible solution is based on the emission of a high‐density contactor plasma by the spacecraft prior to and after the beam. We perform particle‐in‐cell simulations to investigate the conditions under which a high‐voltage electron beam can be emitted from a magnetospheric spacecraft, comparing two possible routes that rely on the high‐density contactor plasma. The first is an “electron collection” route, where the contactor has lower current than the electron beam and is used with the goal of connecting to the background plasma and collecting magnetospheric electrons over a much larger area than that allowed by the spacecraft alone. The second is an “ion emission” route, where the contactor has higher current than the electron beam. Ion emission is then enabled over the large quasi‐spherical area of the contactor cloud, thus overcoming the space charge limits typical of ion beam emission. Our results indicate that the ion emission route offers a pathway for performing beam experiments in the low‐density magnetosphere, while the electron collection route is not viable because the contactor fails to draw a large neutralizing current from the background.

Future beam experiments in the magnetosphere with plasma contactors: How do we get the charge off the spacecraft?

AbstractThe idea of using a high‐voltage electron beam with substantial current to actively probe magnetic field line connectivity in space has been discussed since the 1970s. However, its experimental realization onboard a magnetospheric spacecraft has never been accomplished because the tenuous magnetospheric plasma cannot provide the return current necessary to keep spacecraft charging under control. In this work, we perform Particle‐In‐Cell simulations to investigate the conditions under which a high‐voltage electron beam can be emitted from a spacecraft and explore solutions that can mitigate spacecraft charging. The electron beam cannot simply be compensated for by an ion beam of equal current, because the Child‐Langmuir space charge limit is violated under conditions of interest. On the other hand, releasing a high‐density neutral contactor plasma prior and during beam emission is critical in aiding beam emission. We show that after an initial transient controlled by the size of the contactor cloud where the spacecraft potential rises, the spacecraft potential can settle into conditions that allow for electron beam emission. A physical explanation of this result in terms of ion emission into spherical geometry from the surface of the plasma cloud is presented, together with scaling laws of the peak spacecraft potential varying the ion mass and beam current. These results suggest that a strategy where the contactor plasma and the electron beam operate simultaneously might offer a pathway to perform beam experiments in the magnetosphere.

A quantitative study of magnetospheric magnetic field line deformation by a two-loop substorm current wedge

Abstract. Substorm current wedge (SCW) formation is associated with global magnetic field reconfiguration during substorm expansion. We combine a two-loop model SCW (SCW2L) with a background magnetic field model to investigate distortion of the ionospheric footpoint pattern in response to changes of different SCW2L parameters. The SCW-related plasma sheet footprint shift results in formation of a pattern resembling an auroral bulge, the poleward expansion of which is controlled primarily by the total current in the region 1 sense current loop (I1). The magnitude of the footprint latitudinal shift may reach ∼ 10° corrected geomagnetic latitude (CGLat) during strong substorms (I1= 2 MA). A strong helical magnetic field around the field-aligned current generates a surge-like region with embedded spiral structures, associated with a westward traveling surge (WTS) at the western end of the SCW. The helical field may also contribute to rotation of the ionospheric projection of narrow plasma streams (auroral streamers). Other parameters, including the total current in the second (region 2 sense) loop, were found to be of secondary importance. Analyzing two consecutive dipolarizations on 17 March 2010, we used magnetic variation data obtained from a dense midlatitude ground network and several magnetospheric spacecraft, as well as the adaptive AM03 model, to specify SCW2L parameters, which allowed us to predict the magnitude of poleward auroral expansion. Auroral observations made during the two substorm activations demonstrate that the SCW2L combined with the AM03 model nicely describes the azimuthal progression and the observed magnitude of the auroral expansion. This finding indicates that the SCW-related distortions are responsible for much of the observed global development of bright auroras.

Radiation background and dose estimates for future X-ray observations in the Jovian magnetosphere

The Jovian magnetosphere is the most luminous system at soft X-ray wave lengths in our solar system, and hence X-ray observations can provide significant information on the Jovian magnetospheric dynamics. During X-ray observations in the Jovian magnetosphere, however, penetrating particles can hit the detector, resulting in the background noise and radiation damage. Here we consider an X-ray telescope instrument (0.3–2keV) on board a future Jovian magnetospheric spacecraft and estimate the count rates and dose rates by the high-energy particles for the design of the instrument. Based on Monte-Carlo simulations with Geant4 codes, we confirmed that good signal-to-noise ratio (0.5–500) and negligible dose rate (<0.1krad/year) can be accomplished during observations in the lobe (i.e., the region where magnetic field lines are open), which would be achieved by the high-inclination orbit of the spacecraft.

Comparison between SuperDARN flow vectors and equivalent ionospheric currents from ground magnetometer arrays

Equivalent ionospheric currents obtained with the spherical elementary current systems (SECS) method and derived from nearly 100 ground magnetometers spread over North America and Greenland are compared with ionospheric flow vectors measured by the SuperDARN radars during both the summer and winter seasons. This comparison is done over a range of spatial separations, magnetic latitudes, magnetic local times, and auroral electrojet activity to investigate under what conditions the vectors are anti-parallel to one another. Our results show that in general the equivalent ionospheric currents are anti-parallel to the flows and the best results are achieved within the auroral oval during active geomagnetic conditions in the dawn, dusk and noon sectors in the northern hemisphere summer. These results indicate the best anti-parallel alignment occurs when the currents and flows are large and well defined. Factors that may influence the alignment include ionospheric conductivity gradients and quiet time backgrounds. Our results can be used to approximate the macroscopic (∼1000 km) ionospheric convection patterns. The SECS maps represent a value-added product from the raw magnetometer database and can be used for contextual interpretation; they can help with our understanding of magnetosphere-ionosphere coupling mechanisms using ground arrays and the magnetospheric spacecraft data, and they can be used as input for other techniques.

Comparison between SuperDARN flow vectors and equivalent ionospheric currents from ground magnetometer arrays

Equivalent ionospheric currents obtained with the spherical elementary current systems (SECS) method and derived from nearly 100 ground magnetometers spread over North America and Greenland are compared with ionospheric flow vectors measured by the SuperDARN radars during both the summer and winter seasons. This comparison is done over a range of spatial separations, magnetic latitudes, magnetic local times, and auroral electrojet activity to investigate under what conditions the vectors are anti‐parallel to one another. Our results show that in general the equivalent ionospheric currents are anti‐parallel to the flows and the best results are achieved within the auroral oval during active geomagnetic conditions in the dawn, dusk and noon sectors in the northern hemisphere summer. These results indicate the best anti‐parallel alignment occurs when the currents and flows are large and well defined. Factors that may influence the alignment include ionospheric conductivity gradients and quiet time backgrounds. Our results can be used to approximate the macroscopic (∼1000 km) ionospheric convection patterns. The SECS maps represent a value‐added product from the raw magnetometer database and can be used for contextual interpretation; they can help with our understanding of magnetosphere‐ionosphere coupling mechanisms using ground arrays and the magnetospheric spacecraft data, and they can be used as input for other techniques.

Potential barrier in the electrostatic sheath around a magnetospheric spacecraft

Plasma and field measurements on board magnetospheric spacecraft can be influenced by the spacecraft potential and emitted particles such as photoelectrons and secondary electrons. There exist contradictory requirements for, on one hand, minimizing the spacecraft potential and, on the other hand, minimizing perturbations in the sheath and contamination by spacecraft‐generated electrons. Assessment and mitigation of such effects therefore require improved quantitative modeling of the spacecraft electrostatic sheath. In this paper a fully self‐consistent model of the plasma around an electron‐emitting central body in a spherically symmetric geometry is used to analyze the electrostatic sheath around an idealized magnetospheric spacecraft. Although the model is too simplistic to allow detailed comparisons with observations, it helps to analyze the phenomenon of potential barrier occurrence and some global characteristics, which can be relevant to conductive magnetospheric spacecraft like Geotail or Cluster. It is shown that nonmonotonic potential with negative potential barrier can exist all around a positively charged spacecraft even in the case of realistic illumination pattern. The magnitude of the potential barrier in regions of the magnetosphere with large Debye‐length plasma is found to be as large as a few volts negative for small positive spacecraft potential but that it quickly vanishes when the potential is increased by a few volts. Furthermore, the location of the potential barrier is found to be much more variable than previously predicted.

Constructing the magnetospheric model including pressure measurements

We modeled the magnetotail configuration during a highly disturbed period on 19 April 1985 based on data from three magnetospheric spacecraft (Active Magnetospheric Tracer Explorers (AMPTE)/IRM, AMPTE/CCE, and ISEE 1) and proton precipitation measured by DMSP spacecraft. The goal was to test the procedure of including the plasma pressure as input data when constructing the magnetotail magnetic field model. On the basis of AMPTE/IRM data we confirm statistically a high degree of pressure isotropy at 8–17 RE distances in the tail plasma sheet, which allows computation of the plasma pressure from magnetospheric magnetic field model and easy mapping of the pressure along the corresponding flux tubes. The usage of pressure data not only increases the amount of data for event‐oriented modeling but also imposes the nonlocal constraints on the configuration and mapping between magnetosphere and ionosphere to stabilize the solution of inverse problem and get it to be more realistic. Observational and modeling results indicate large systematic deviations of observed configuration from predictions of standard models (T89 or T96) [Tsyganenko, 1989, 1995] and reveal highly variable configurations, justifying a need for the development of event‐oriented modeling. Indications of localized B minimum and current density maximum at around 12 RE have been obtained in the model with best data coverage, but precipitation from this region showed no signatures of discrete structures. The structured precipitation has been mostly seen in poleward half of the auroral zone precipitation, which is mapped to rather distant parts (&gt;30 RE) of the plasma sheet.

Multiple‐spacecraft observation of a narrow transient plasma jet in the Earth's plasma sheet

We use observations from five magnetospheric spacecraft in a fortuitous constellation to show that narrow transient plasma flow jets of considerable length formed in the tail can intrude into the inner magnetosphere and provide considerable contribution to the total plasma transport. A specific auroral structure, the auroral streamer, accompanied the development of this narrow plasma jet. These observations support the ‘boiling’ plasma sheet model consisting of localized underpopulated plasma tubes (bubbles) moving Earthward at high speeds as a realistic way to resolve the ‘convection crisis’ and to close the global magnetospheric circulation pattern.