Convection Electric Field Research Articles

Martian magnetic storms

AbstractThe response of Mars to the major space weather events called interplanetary coronal mass ejections (ICMEs) is of interest for both general planetary solar wind interaction studies and related speculations on their evolutionary consequences—especially with respect to atmosphere escape. Various particle and field signatures of ICMEs have been observed on Phobos‐2, Mars Global Surveyor (MGS), Mars Express (MEX), and now Mars Atmosphere and Volatile EvolutioN (MAVEN). Of these, MAVEN's combined instrumentation and orbit geometry is particularly well suited to characterize both the event drivers and their consequences. However, MAVEN has detected only moderate disturbances at Mars due in large part to the general weakness of the present solar cycle. Nevertheless, the strongest event observed by MAVEN in March 2015 provides an example illustrating how further insights can be gained from available models. Here we first look more closely at what previously run BATS‐R‐US MHD simulations of the combined MAVEN observations tell us about the March 2015 event consequences. We then use analogous models to infer those same responses, including magnetic field topology changes and ionospheric consequences, to a hypothetical extreme ICME at Mars based on STEREO A measurements in July 2012. The results suggest how greatly enhanced, yet realistic, solar wind pressure, magnetic field, and convection electric field combine to produce strong magnetospheric coupling with important consequences for upper atmosphere and ionosphere energization.

The effects of spacecraft charging and outgassing on the LADEE ion measurements

AbstractAbnormal ion signals can be usually seen in the Lunar Atmosphere and Dust Environment Explorer (LADEE) mission, including a suddenly enhanced current observed by the Lunar Dust Experiment (LDEX) near the sunlight‐shadow boundary and an unexpected water ion measured by the neutral mass spectrometer (NMS), with their magnitudes insensitive to the convection electric field of solar wind but dependent on the SW density and the elapsed time of LADEE mission. By analyzing both the LDEX measurements and the NMS measurements, we find that the current enhancement can be caused by a negatively charged spacecraft in the shadow region while the significant water ions should be some artificial ions from spacecraft outgassing. The artificial water ions show a peak near 8:00 LT that may be related to a sunlight‐controlled surface outgassing. In addition, the H2O flux can be enhanced near the end of the mission when the spacecraft has a lower altitude. It is found that the H2O enhancement is actually caused by an exosphere‐contributed return flux, rather than a real water exosphere.

Role of lower hybrid waves in ion heating at dipolarization fronts

AbstractOne of the important sources of hot ions in the magnetotail is the bursty bulk flows propagating away from the reconnection region and heating the ambient plasma. Charged particles interact with nonlinear magnetic field pulses (dipolarization fronts, DFs) embedded into these flows. The convection electric fields associated with DF propagation are known to reflect and accelerate ambient ions. Moreover, a wide range of waves is observed within/near these fronts, the electric field fluctuations being dominated by the lower hybrid drift (LHD) instability. Here we investigate the potential role of these waves in the further acceleration of ambient ions. We use a LHD wave emission profile superimposed on the leading edge of a two‐dimensional model profile of a DF and a test particle approach. We show that LHD waves with realistic amplitudes can significantly increase the upper limit of energies gained by ions. Wave‐particle interaction near the front is more effective in producing superthermal ions than in increasing the flux of thermal ions. Comparison of test particle simulations and Time History of Events and Macroscale Interactions during Substorms observations show that ion acceleration by LHD waves is more important for slower DFs.

Efficient shock drift acceleration in the collision of two asymmetric pair plasma shells

The shock drift acceleration (SDA) process in the collision between two asymmetric relativistic pair plasma shells was investigated. It is found that the density ratio (nL/nR) of the two different plasma shells plays a crucial role in the SDA process and determines the efficiency of the SDA process. By increasing this parameter, the plasma bulk velocity and so as to the convection electric field in the downstream region can be much enhanced compared with the symmetric case, in which the densities in the two plasma shells are the same. As a result, the particles are efficiently accelerated by the large convection electric field and the efficiency of the SDA process is much increased as the density ratio is increased. Our particle-in-cell simulations demonstrate that the SDA efficiency can be improved by three to four times in the asymmetric case. In this way, more high energy particles could enter into the phase of the diffusive shock acceleration process.

MAVEN observations of tail current sheet flapping at Mars

AbstractThe Martian magnetotail is a complex regime through which atmospheric particles are lost to space. Our current understanding of Mars' tail continues to develop with the comprehensive particle and field data collected by Mars Atmosphere and Volatile EvolutioN (MAVEN). In this work, we identify periods when MAVEN encounters multiple current sheet crossings through a single tail traversal in order to understand tail dynamics. We apply an analysis technique that has been developed and validated by using multipoint measurements in order to separate the spatial and temporal properties associated with current sheet flapping. Events are classified into periods of steady flapping, due to a global motion of the current sheet, and kink‐like flapping, resulting from localized wave propagation along the tail current sheet. Out of 106 periods during which multiple current sheet crossings were observed, 20 were due to steady flapping and 10 from kink‐like flapping. A majority of the kink‐like events resulted from waves propagating in the opposite direction of the solar wind convection electric field, regardless of their location in the tail, unlike at Earth and Venus. This finding suggests that possible magnetosphere energy sources, whereby plasma is accelerated and removed from the Martian environment, are not located in the central magnetotail; rather, these waves may be driven by a source located at the tail flank based on the direction of the solar wind electric field. Therefore, by identifying potential sources of impulsive energy release in the tail, we may better understand mechanisms that drive atmospheric loss at Mars.

A case study of EMIC waves associated with sudden geosynchronous magnetic field changes

AbstractWe present observations of electromagnetic ion cyclotron (EMIC) waves associated with a sudden commencement (SC) on 19 November 2007. In our study, we clearly showed that there was a time delay of ∼10–15 min between the SC onset and the occurrence of EMIC waves at GOES 12 and GOES 10 in the afternoon sector, while the SC‐associated EMIC wave activity observed by GOES 11 in the morning sector started almost immediately after the onset of the SC. This indicates that the EMIC wave source drifts eastward and that magnetospheric compression alone cannot generate EMIC waves without source particles. We suggest that the wave source motion drifting eastward is attributed to the SC‐associated convection electric field, which is slower than the speed of the initial SC disturbance propagating as a fast mode wave. The spectral broadening of EMIC waves for nonlinear wave growth is examined by changing a spatial gradient of the background magnetic field. We have shown that the spectral properties of the observed SC‐associated EMIC waves are in good agreement with the nonlinear theory. In addition, we observed substorm‐associated EMIC wave activity accompanied by a sudden decrease in magnetic field intensity when the satellites were near dusk. By comparing the wave onset times, we confirmed that the substorm‐associated wave source drifted westward.

The role of the convection electric field in filling the slot region between the inner and outer radiation belts

AbstractThe Van Allen Probes have reported frequent flux enhancements of 100s keV electrons in the slot region, with lower energy electrons exhibiting more dynamic behavior at lower L shells. Also, in situ electric field measurements from the Combined Release and Radiation Effects Satellite, Time History of Events and Macroscale Interactions during Substorms (THEMIS), and the Van Allen Probes have provided evidence for large‐scale electric fields at low L shells during active times. We study an event on 19 February 2014 where hundreds of keV electron fluxes were enhanced by orders of magnitude in the slot region and electric fields of 1–2 mV/m were observed below L = 3. Using a 2‐D guiding center particle tracer and a simple large‐scale convection electric field model, we demonstrate that the measured electric fields can account for energization of electrons up to at least 500 keV in the slot region through inward radial transport.

Fast‐moving diffuse auroral patches: A new aspect of daytime Pc3 auroral pulsations

AbstractAuroral pulsations are a convenient diagnostic of wave‐particle interactions in the magnetosphere. A case study of a daytime Pc3 (22–100 mHz) auroral pulsation event, measured with a ~2 Hz sampling all‐sky camera at South Pole Station (74.4°S magnetic latitude) on 17 May 2012, is presented. The daytime Pc3 auroral pulsations were most active in a closed field line region where the aurora was dominated by diffuse green‐line emissions and within ±2 h of magnetic local noon. Usually, but not always, the corresponding periodic variations were recorded with a colocated search coil magnetometer. Of particular interest is the two‐dimensional auroral signature, indicating that the temporal luminosity variations at a given point were due to repeated formation and horizontal motion of faint, nonpulsating auroral patches with scale sizes of ~100 km. The individual patches propagated equatorward with speeds of 15 km s−1 up to 20–25 km s−1 one after another along the magnetic meridian through local magnetic zenith. These properties differ considerably from typical pulsating aurorae, being periodic on‐off luminosity variations in a particular auroral patch and drifting in accordance with the convection electric field in the magnetosphere. We speculate that such repetitive patterns of the fast‐moving auroral patches, being another aspect of the daytime Pc3 auroral pulsations, may be a visible manifestation of compressional Pc3 waves which propagate earthward and cause modulation of precipitating keV electron fluxes in the dayside outer magnetosphere.

Seasonal effect for polar cap sunward plasma flows at strongly northward IMF Bz

AbstractWe use Super Dual Auroral Radar Network data to study polar cap ionospheric flow under strongly dominant positive interplanetary magnetic field Bz component. We show that the near‐noon flow along the magnetic meridian is predominantly sunward in summer. The sunward velocity increase with intensification of the external driver (the reverse convection electric field) is also faster in summer, and the rate of the increase is slightly larger for the Southern Hemisphere. The sunward flows simultaneously detected in both hemispheres are faster in the summer hemisphere. In addition, while sunward flows are aligned with the midnight‐noon line in a winter hemisphere, they are oriented toward earlier magnetic local hours in a summer hemisphere.

Typical values of the electric drift E × B/B2 in the inner radiation belt and slot region as determined from Van Allen Probe measurements

AbstractThe electric drift E × B/B2 plays a fundamental role for the description of plasma flow and particle acceleration. Yet it is not well‐known in the inner belt and slot region because of a lack of reliable in situ measurements. In this article, we present an analysis of the electric drifts measured below L ~ 3 by both Van Allen Probes A and B from September 2012 to December 2014. The objective is to determine the typical components of the equatorial electric drift in both radial and azimuthal directions. The dependences of the components on radial distance, magnetic local time, and geographic longitude are examined. The results from Van Allen Probe A agree with Van Allen Probe B. They show, among other things, a typical corotation lag of the order of 5 to 10% below L ~ 2.6, as well as a slight radial transport of the order of 20 m s−1. The magnetic local time dependence of the electric drift is consistent with that of the ionosphere wind dynamo below L ~ 2 and with that of a solar wind‐driven convection electric field above L ~ 2. A secondary longitudinal dependence of the electric field is also found. Therefore, this work also demonstrates that the instruments on board Van Allen Probes are able to perform accurate measurements of the electric drift below L ~ 3.

The plasmapause formation seen from meridian perspective by KAGUYA

AbstractObservations by the extreme ultraviolet (EUV) imager on board the IMAGE spacecraft revealed that the formation of a sharp plasmapause occurs in the postmidnight sector soon (<1 h) after the convection enhancement. These results cannot be explained simply by the conventional theory of the plasmapause formation that the plasmapause coincides with the last closed equipotential of the convection electric field superposed on the Earth's corotation electric field. However, due to the limitation that the EUV imager provides information on only the azimuthal distribution of the plasmapause, the formation mechanism still remains an open issue. Now global images of the plasmasphere from meridian perspective become available, thanks to the telescope of extreme ultraviolet (TEX) instrument on board the KAGUYA spacecraft. Here we studied the plasmapause formation mechanism by analyzing the sequential TEX images of an erosion event during the geomagnetic disturbance (Kp = 5) on 1–2 May 2008. The temporal evolution of the plasmapause locations at postmidnight observed by TEX agreed with those predicted by the dynamic simulations based on the interchange mechanism. Furthermore, the He+ column density in the nightside plasmasphere decreased by ~30% only at the low latitudes (<20°) during the enhanced convection period. This suggests that the plasmapause formation occurs first near the equatorial region during a geomagnetic disturbance, and it agrees with the plasmapause formation mechanism based on the interchange instability. Although we cannot conclude exclusively for the interchange mechanism, this is the first study to present the plasmapause formation viewed from the meridian perspective.

High-latitude F region large-scale ionospheric irregularities under different solar wind and zenith angle conditions

A numerical model is used to study systematically the evolution of large scale irregularities depending on the IMF Bz and By components, solar zenith angle (seasonal and daily variation), solar and geomagnetic activity. The model enables to reproduce the 3-D distribution of electron density over the high-latitude F region ionosphere in the altitude range between 130 and 640km. Since the convection electric field driven by changes in solar wind conditions has an important effect on the high-latitude ionosphere, the rotation of the IMF vector in the Y–Z plane causes a significant redistribution of the ionospheric plasma. Under the southward IMF conditions the plasma density is enhanced over a large portion of the near-pole ionosphere as a tongue of ionization, while the northward IMF leads to a considerable depletion and occurrence of the polar hole. The IMF By polarity is crucial for the shift and extension of the tongue of ionization to the dusk or dawn side. Particle precipitation also plays a role through a localized increase of the electron density mostly within the auroral oval and more pronounced auroral peak. The solar zenith angle, especially its seasonal variation, is the strongest regular factor influencing the electron density magnitude and spatial distribution. In winter, when the polar ionosphere is in darkness, large variations associated with different solar wind condition are more prominent. The daily variation of the zenith angle considerably modifies the Ne within a particular pattern. At a given time, the combined action of the IMF, solar zenith angle, level of solar and geomagnetic activity produces a complicated ionospheric response which can be considered as a superposition of different effects. Quantitative estimates of the ionospheric response to each factor are presented.

Control of the innermost electron radiation belt by large‐scale electric fields

AbstractElectron measurements from the Magnetic Electron Ion Spectrometer instruments on Van Allen Probes, for kinetic energies ∼100 to 400 keV, show characteristic dynamical features of the innermost ( ) radiation belt: rapid injections, slow decay, and structured energy spectra. There are also periods of steady or slowly increasing intensity and of fast decay following injections. Local time asymmetry, with higher intensity near dawn, is interpreted as evidence for drift shell distortion by a convection electric field of magnitude ∼0.4 mV/m during geomagnetically quiet times. Fast fluctuations in the electric field, on the drift time scale, cause inward diffusion. Assuming that they are proportional to changes in Kp, the resulting diffusion coefficient is sufficient to replenish trapped electrons lost by atmospheric scattering. Major electric field increases cause injections by inward electron transport. An injection associated with the June 2015 magnetic storm is consistent with an enhanced field magnitude ∼5 mV/m. Subsequent drift echoes cause spectral structure.

Response of the equatorial ionosphere to the geomagnetic DP 2 current system

AbstractThe response of equatorial ionosphere to the magnetospheric origin DP 2 current system fluctuations is examined using ground‐based multiinstrument observations. The interaction between the solar wind and magnetosphere generates a convection electric field that can penetrate to the ionosphere and cause the DP 2 current system. The quasiperiodic DP 2 current system, which fluctuates coherently with fluctuations of the interplanetary magnetic field (IMF) Bz, penetrates nearly instantaneously to the dayside equatorial region at all longitudes and modulates the electrodynamics that governs the equatorial density distributions. In this paper, using magnetometers at high and equatorial latitudes, we demonstrate that the quasiperiodic DP 2 current system penetrates to the equator and causes the dayside equatorial electrojet (EEJ) and the independently measured ionospheric drift velocity to fluctuate coherently with the high‐latitude DP 2 current as well as with the IMF Bz component. At the same time, radar observations show that the ionospheric density layers move up and down, causing the density to fluctuate up and down coherently with the EEJ and IMF Bz.

MLT dependence in the relationship between plasmapause, solar wind, and geomagnetic activity based on CRRES: 1990–1991

AbstractUsing the database of CRRES in situ observations of the plasmapause crossings, we develop linear and more complex plasmapause models parametrized by (a) solar wind parameters V (solar wind velocity), BV (where B is the magnitude of the interplanetary magnetic field (IMF)), and dΦmp/dt (which combines different physical mechanisms which run magnetospheric activity), and (b) geomagnetic indices Dst, Ap, and AE. The complex models are built by including a first harmonic in magnetic local time (MLT). Our method based on the cross‐correlation analyses provides not only the plasmapause shape for different levels of geomagnetic activity but additionally yields the information of the delays in the MLT response of the plasmapause. All models based on both solar wind parameters and geomagnetic indices indicate the maximal plasmapause extension in the postdusk side at high geomagnetic activity. The decrease in the convection electric field places the bulge toward midnight. These results are compared and discussed in regard to past works. Our study shows that the time delays in the plasmapause response are a function of MLT and suggests that the plasmapause is formed by the mechanism of interchange instability motion. We observed that any change quickly propagates across dawn to noon, and then at lower rate toward midnight. The results further indicate that the instability may propagate much faster during solar maximum than around solar minimum. This study contributes to the determination of the MLT dependence of the plasmapause and to constrain physical mechanism by which the plasmapause is formed.

Equatorward evolution of auroras from the poleward auroral boundary

An all-sky imager installed at the midnight sector in Dawson City (66.0° in geomagnetic latitude) recorded the equatorward evolution of auroras from the auroral poleward boundary. The auroras evolved as shear layers expanding southeastward with velocities of 1–4km/s, referred to as N-S auroras, and occurred during the transient intensification of the convection electric fields in the nighttime magnetosphere, as inferred from an electron spectrogram at geosynchronous altitudes. A continuous increase in the inclination angle of the field lines and magnetic field perturbations associated with propagating ionospheric loop currents were observed in the auroral zone during the N-S auroras. Simultaneously, Pc4 pulsations were observed at low latitudes from night to day sectors. We conclude the following: (1) the N-S auroras are an auroral manifestation of the earthward drift of plasma sheet electrons in the equatorial plane associated with transient and localized convection electric fields; (2) the Pc4 pulsations are produced in the magnetosphere by plasma sheet ions in the plasmasphere. The localized convection fields produce a vortical motion of plasmas in the equatorial plane, which may initiate the N-S auroras and ionospheric loop currents in the auroral zone.

Solar wind‐generated current in the Lunar Dust Experiment

AbstractMeasurements from the Lunar Dust Experiment (LDEX) show that the current associated with lofted lunar dust lacks an altitude dependence, implying that the current may come from other sources. Here we present some evidences for solar wind (SW)‐generated current. Direct SW influx on the nightside can cause a large current, and the backscattered energetic neutral atoms (ENAs) on the dayside can bring a good correlation between the current and SW density. It is found that the current favors a lower SW speed and a smaller SW incident angle, but the dependences are also affected by the solar zenith angle (SZA) and the scattering function of ENAs. Picked‐up ions can enhance the current when the angle between the convection electric field and LDEX's normal is larger than 90°. But when the angle is smaller than 90°, the enhancement is negligible.

Lower thermospheric wind variations in auroral patches during the substorm recovery phase

AbstractMeasurements of the lower thermospheric wind with a Fabry‐Perot interferometer (FPI) at Tromsø, Norway, found the largest wind variations in a night during the appearance of auroral patches at the substorm recovery phase. Taking into account magnetospheric substorm evolution of plasma energy accumulation and release, the largest wind amplitude at the recovery phase is a fascinating result. The results are the first detailed investigation of the magnetosphere‐ionosphere‐thermosphere coupled system at the substorm recovery phase using comprehensive data sets of solar wind, geomagnetic field, auroral pattern, and FPI‐derived wind. This study used three events in November 2010 and January 2012, particularly focusing on the wind signatures associated with the auroral morphology, and found three specific features: (1) wind fluctuations that were isolated at the edge and/or in the darker area of an auroral patch with the largest vertical amplitude up to about 20 m/s and with the longest oscillation period about 10 min, (2) when the convection electric field was smaller than 15 mV/m, and (3) wind fluctuations that were accompanied by pulsating aurora. This approach suggests that the energy dissipation to produce the wind fluctuations is localized in the auroral pattern. Effects of the altitudinal variation in the volume emission rate were investigated to evaluate the instrumental artifact due to vertical wind shear. The small electric field values suggest weak contributions of the Joule heating and Lorentz force processes in wind fluctuations. Other unknown mechanisms may play a principal role at the recovery phase.

Universal time effect in the response of the thermosphere to electric field changes

AbstractUnderstanding the dynamics of the thermospheric mass density is of paramount importance for predicting drag on low‐altitude satellites, particularly during geomagnetic storms. Transient enhancements in ion velocities, which frequently occur as a result of storm‐driven solar wind electric field fluctuations, cause increases in neutral density and temperature. Since the Earth's quasi‐dipolar magnetic field is tilted and offset from the center of the planet, it is hypothesized that hemispheric asymmetries arise, altering the thermospheric response to energy input based upon the time of day. This study used the Global Ionosphere‐Thermosphere Model (GITM) to investigate this phenomenon via a series of 22 idealized simulations, where the convective electric field was enhanced for 1 h of the day. Two configurations of the Earth's magnetic field were considered, the International Geomagnetic Reference Field (IGRF) and a centered dipole. These runs were conducted at March equinox when the amount of sunlight falling on the two hemispheres was the same. Two additional sets of runs were conducted at the June and December solstices for comparison. It was found that the most geoeffective times were those times when the geomagnetic poles were pointed toward the Sun. This orientation maximizes the photoionization colocated with the high‐latitude potential pattern, leading to more in Joule heating.

Middle‐ and low‐latitude ionosphere response to 2015 St. Patrick's Day geomagnetic storm

AbstractThis paper presents a study of the St Patrick's Day storm of 2015, with its ionospheric response at middle and low latitudes. The effects of the storm in each longitudinal sector (Asian, African, American, and Pacific) are characterized using global and regional electron content. At the beginning of the storm, one or two ionospheric positive storm effects are observed depending on the longitudinal zones. After the main phase of the storm, a strong decrease in ionization is observed at all longitudes, lasting several days. The American region exhibits the most remarkable increase in vertical total electron content (vTEC), while in the Asian sector, the largest decrease in vTEC is observed. At low latitudes, using spectral analysis, we were able to separate the effects of the prompt penetration of the magnetospheric convection electric field (PPEF) and of the disturbance dynamo electric field (DDEF) on the basis of ground magnetic data. Concerning the PPEF, Earth's magnetic field oscillations occur simultaneously in the Asian, African, and American sectors, during southward magnetization of the Bz component of the interplanetary magnetic field. Concerning the DDEF, diurnal magnetic oscillations in the horizontal component H of the Earth's magnetic field exhibit a behavior that is opposed to the regular one. These diurnal oscillations are recognized to last several days in all longitudinal sectors. The observational data obtained by all sensors used in the present paper can be interpreted on the basis of existing theoretical models.