Convection Electric Field Research Articles

New model fit functions of the plasmapause location determined using THEMIS observations during the ascending phase of Solar Cycle 24

AbstractIt is well known that the plasmapause is influenced by the solar wind and magnetospheric conditions. Empirical models of its location have been previously developed such as those by O'Brien and Moldwin (2003) and Larsen et al. (2006). In this study, we identified the locations of the plasmapause using the plasma density data obtained from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) satellites. We used the data for the period (2008–2012) corresponding to the ascending phase of Solar Cycle 24. Our database includes data from over a year of unusually weak solar wind conditions, correspondingly covering the plasmapause locations in a wider L range than those in previous studies. It also contains many coronal hole stream intervals during which the plasmasphere is eroded and recovers over a timescale of several days. The plasmapause was rigorously determined by requiring a density gradient by a factor of 15 within a radial distance of 0.5 L. We first determined the statistical correlation of the plasmapause locations with several solar wind parameters as well as geomagnetic indices. We found that the plasmapause locations are well correlated with the solar wind speed and the interplanetary magnetic field Bz, therefore the y component of the convective electric field, and some energy coupling functions such as the well‐known Akasofu's epsilon parameter. The plasmapause locations are also highly correlated with the geomagnetic indices, Dst, AE, and Kp, as recognized previously. Finally, we suggest new model fit functions for the plasmapause locations in terms of the solar wind parameters and geomagnetic indices. When applied to a new data interval outside the model training interval, our model fit functions work better than existing ones. The new model fit functions developed here extend the range of conditions from those used in previous works.

Momentum transfer from solar wind to interplanetary field enhancements inferred from magnetic field draping signatures

AbstractCharacterized by a cusp‐shaped enhancement in the magnetic field strength, the magnetic structure in the solar wind, called an interplanetary field enhancement (IFE), has been investigated since its discovery. To understand its three‐dimensional magnetic field geometry, we study an IFE detected by five spacecraft simultaneously. Field lines are seen draping around in the upstream region and rotating in the ambient convection electric field direction in the downstream region. Earlier studies suggest that IFEs are created when the solar wind accelerates newly formed dust clouds. Both signatures found in our study support this hypothesis: the field line draping is caused by dust‐solar wind momentum exchange, while the field line rotation is a typical signature of dusty plasma pickup. The force that exchanges the momentum is approximately 106 N. This study illustrates the nature of the interaction between two flowing plasmas of very different mass‐to‐charge ratio.

Auroral vortex street formed by the magnetosphere–ionosphere coupling instability

Abstract. By performing three-dimensional magnetohydrodynamic simulations including Alfvén eigenmode perturbations most unstable to the ionospheric feedback effects, we examined the auroral vortex street that often appears just before substorm onset. We found that an initially placed arc splits, intensifies, and rapidly deforms into a vortex street. We also found that there is a critical convection electric field for growth of the Alfvén eigenmodes. The vortex street is shown to be a consequence of coupling between the magnetospheric Alfvén waves carrying field-aligned currents and the ionospheric density waves driven by Pedersen/Hall currents.

Generation of kinetic Alfven waves in the high-latitude near-Earth magnetotail: A global hybrid simulation

In this paper, effects of a fast flow in the tail plasma sheet on the generation of kinetic Alfven waves (KAWs) in the high-latitude of the near-Earth magnetotail are investigated by performing a two-dimensional (2-D) global-scale hybrid simulation, where the plasma flow is initialized by the E×B drift near the equatorial plane due to the existence of the dawn-dusk convection electric field. It is found that firstly, the plasma sheet becomes thinned and the dipolarization of magnetic field appears around (x,z)=(−10.5RE,0.3RE), where RE is the radius of the Earth. Then, shear Alfven waves are excited in the plasma sheet, and the strong earthward flow is braked by the dipole-like magnetic field. These waves propagate along the magnetic field lines toward the polar regions later. Subsequently, KAWs with k⊥≫k∥ are generated in the high-latitude magnetotail due to the existence of the non-uniformity of the magnetic field and density in the polar regions. The ratio of the electric field to the magnetic field in these waves is found to obey the relation (δEz)/(δBy )∼ω/k∥ of KAWs. Our simulation provides a mechanism for the generation of the observed low-frequency shear Alfven waves in the plasma sheet and kinetic Alfven waves in the high-latitude near-Earth magnetotail, whose source is suggested to be the flow braking in the low-latitude plasma sheet.

Formation of the oxygen torus in the inner magnetosphere: Van Allen Probes observations

AbstractWe study the formation process of an oxygen torus during the 12–15 November 2012 magnetic storm, using the magnetic field and plasma wave data obtained by Van Allen Probes. We estimate the local plasma mass density (ρL) and the local electron number density (neL) from the resonant frequencies of standing Alfvén waves and the upper hybrid resonance band. The average ion mass (M) can be calculated by M ∼ ρL/neL under the assumption of quasi‐neutrality of plasma. During the storm recovery phase, both Probe A and Probe B observe the oxygen torus at L = 3.0–4.0 and L = 3.7–4.5, respectively, on the morning side. The oxygen torus has M = 4.5–8 amu and extends around the plasmapause that is identified at L∼3.2–3.9. We find that during the initial phase, M is 4–7 amu throughout the plasma trough and remains at ∼1 amu in the plasmasphere, implying that ionospheric O+ ions are supplied into the inner magnetosphere already in the initial phase of the magnetic storm. Numerical calculation under a decrease of the convection electric field reveals that some of thermal O+ ions distributed throughout the plasma trough are trapped within the expanded plasmasphere, whereas some of them drift around the plasmapause on the dawnside. This creates the oxygen torus spreading near the plasmapause, which is consistent with the Van Allen Probes observations. We conclude that the oxygen torus identified in this study favors the formation scenario of supplying O+ in the inner magnetosphere during the initial phase and subsequent drift during the recovery phase.

Solar wind effect on Joule heating in the high‐latitude ionosphere

AbstractThe effect of solar wind on several electrodynamic parameters, measured simultaneously by the European Incoherent Scatter (EISCAT) radars in Tromsø (TRO, 66.6° cgmLat) and on Svalbard (ESR, 75.4° cgmLat), has been evaluated statistically. The main emphasis is on Joule heating rate QJ, which has been estimated by taking into account the neutral wind. In addition, a generally used proxy QE, which is the Pedersen conductance times the electric field squared, has been calculated. The most important findings are as follows. (i) The decrease in Joule heating in the afternoon‐evening sector due to winds reported by Aikio et al. (2012) requires southward interplanetary magnetic field (IMF) conditions and a sufficiently high solar wind electric field. The increase in the morning sector takes place for all IMF directions within a region where the upper E neutral wind has a large equatorward component and the F region plasma flow is directed eastward. (ii) At ESR, an afternoon hot spot of Joule heating centered typically at 14–15 magnetic local time (MLT) is observed during all IMF conditions. Enhanced Pedersen conductances within the hot spot region are observed only for the IMF Bz + /By− conditions, and the corresponding convection electric field values within the hot spot are smaller than during the other IMF conditions. Hence, the hot spot represents a region of persistent magnetospheric electromagnetic energy input, and the median value is about 3 mW/m2. (iii) For the southward IMF conditions, the MLT‐integrated QE for By− is twice the value for By+ at TRO. This can plausibly be explained by the higher average solar wind electric field values for By−.

Lack of relationship between geoeffectiveness and orientations of magnetic clouds with bipolar Bz and unipolar southward Bz

In this study, 38 magnetic clouds (MCs) that caused significant geomagnetic storms (the minimum SYM-H, SHmin, ≤−50nT) are examined, in which 17 MCs were unipolar Bz in south (S-type) and 21 MCs were bipolar Bz (north-to-south, NS-type, or south-to-north, SN-type). For S-type MC, inclination angle of the axis of the MC, |θ|, is ≥45°, while |θ|<45° for bipolar MC. This paper aims to address a question: is the intensity of a MC-driven storm correlated with the orientations of bipolar and S-type MCs? Our results demonstrate that there is no direct and significant relationship between geoeffectiveness and orientations of bipolar and S-type MCs. In other words, there is no MC preference (bipolar or S-type MC) to regulate the SHmin of the storm. On the whole, the SHmin is found to strongly correlate with southward field Bz (cc=0.96) and with the y component of the solar wind convective electric field (cc=−0.91) but to weakly correlate with solar wind speed (cc=−0.65). This result is consistent with previous studies by Wu and Lepping (2002), J. Geophys. Res. 107 (A10), 1314. doi:10.1029/2001JA000161. By separating MC-driven storms by size into moderate (−100nT<SHmin≤−50nT), intense (−200nT<SHmin≤−100nT), and super-intense (SHmin≤−200nT) categories, we find that (1) there was a high-speed stream at the trailing of NS-type MCs for moderate and intense storms, and (2) for super-intense storms driven by bipolar or S-type MCs, both solar wind speed and dynamic pressure had a significant jump at the same time before the onset of the storm, as compared to the other two categories. These jumps are the signatures of a shock. We conclude that the SHmin of a MC-driven storm is regulated by the cloud’s southward field Bz and the y component of the electric field, regardless of whether the MC is bipolar or S-type. Also, the ambient solar wind structure (e.g., shock) ahead of MC may play a role in regulating the storm’s intensity.

Estimating the vector electric field using monostatic, multibeam incoherent scatter radar measurements

AbstractAn algorithm has been developed to image the local structure in the convection electric field using multibeam incoherent scatter radar (ISR) data. The imaged region covers about 4° in magnetic latitude and 8° in magnetic longitude for the specific geometry considered (that of the Poker Flat ISR). The algorithm implements the Lagrange method of undetermined multipliers to regularize the underdetermined problem posed by the radar measurements. The error on the reconstructed image is estimated by mapping the mathematical form to a Bayesian estimate and observing that the Lagrangian method determines an effective a priori covariance matrix from a user‐defined regularization metric. There exists a unique solution when the average measurement error is smaller than the average measurement amplitude. The algorithm is tested using synthetic and real data and appears surprisingly robust at estimating the divergence of the field. Future applications include imaging the current systems surrounding auroral arcs in order to distinguish physical mechanisms.

Observations and modeling of magnetic flux tube refilling of the plasmasphere at geosynchronous orbit

AbstractThe outer plasmasphere is eroded when the strength of the convection electric field increases. Following a return of lower levels of convection, the outer plasmasphere “refills” from the ionosphere. In situ observations of the cold (~1 eV) ion number density from geosynchronous orbit indicate that within ~48 h after plasmaspheric erosion events, the number density may return to a level of ~100 cm−3, consistent with previously reported values. Current theoretical estimates of refilling rates at geosynchronous orbit are inconsistent with such rapid refilling. In order to shed light on this issue, a theoretical investigation is carried out to determine the major factors governing the refilling process. While theoretical estimates of the refilling rate reported here still fall below the observed levels by at least a factor of 2, the results of this study indicate that the morphology of the neutral atmosphere, particularly the neutral atmosphere number density, is critical in controlling the rate of refilling at geosynchronous orbit. The strength of vertical E × B drifts and horizontal neutral winds is found to play only a minor role.

On Transient Penetration of the Convection Electric Field to Low Latitudes During the Substorm Expansion Phase

The transient penetration of high-latitude electric fields into the middle and low latitude ionosphere during the substorm expansion phase is studied by using an analytical model of the magnetospheric convection. The model calculates the distribution of the electric fields of magnetospheric sources between the polar cap boundary and the geomagnetic equator by taking self-consistent account of the hot plasma belt polarization due to the electric fields both of the solar wind/magnetosphere dynamo and the corotation. It is found that a sudden strong enhancement of auroral conductivities associated with the expansive phase of severe substorm results in a significant, short-lived increase of electric field at middle and low latitudes. The electric field perturbation is essentially of transient nature stemming from the large difference between the characteristic time scales of the primary electric field driven by solar wind and of the Alfven layer’s shielding electric field. Under steady state conditions, there is no substantial penetration of the auroral electric fields to middle and low latitudes due to the enhanced auroral conductivities. The corotation driven polarization electric field can noticeably affect the electric field perturbation in the inner plasmasphere provided the ring current is rather intense.

Comparing the diurnal variations in the SuperMAG auroral electrojet indices SML and SMU

Diurnal variations of the SuperMAG auroral electrojet indices (SML and SMU) were examined for the period of 1980–2010, and the differences between SML and SMU were especially analyzed. The diurnal variation of SML with a maximum at around 1100 UT has a prenoon-postnoon asymmetry. At solstices, the diurnal variation of SML is much stronger than that at equinoxes. For the SMU, two maxima are recorded in the diurnal variation with the bigger one at 1700 UT and the smaller one at 0400 UT. The seasonal variations are not obvious in the UT variation characteristics of SMU although the intensity of SMU is changed remarkably season by season. For both SML and SMU, the contributing stations are located at higher geomagnetic latitude around 1600 UT and at lower geomagnetic latitude around 0400 UT. These results indicate that: (1) the SML is mostly controlled by the convection electric field. Its diurnal variation is mainly correlated with the equinoctial and R-M hypothesis; (2) the SMU is largely controlled by the ionospheric conductance. Its diurnal variation is tightly correlated with the solar radiation.

MLT and seasonal dependence of auroral electrojets: IMAGE magnetometer network observations

Total eastward and westward electrojet currents (EEJ and WEJ) and their central latitudes derived from the International Monitor for Auroral Geomagnetic Effects (IMAGE) network magnetic measurements are analyzed for the combined MLT (magnetic local time) and seasonal dependence during the period 1995–2009. EEJ shows a strong MLT variation with significant dependence on season. During summer months the maxima occur around 1600–1800 MLT, whereas during winter months the maxima occur around 1800–2000 MLT. Moreover, the summer maxima are much larger than the winter maxima and appear at higher latitudes. The summer maxima are mainly associated with the solar EUV conductivity effect, while the winter maxima are mainly due to the contribution of northward convective electric field. EEJ exhibits a dominant annual variation with maximum in summer and minimum in winter. WEJ also exhibits a strong MLT variation with significant dependence on season. The maxima occur around 0200–0400 MLT during summer months, around 0000–0200 MLT during winter months, and around 0000–0400 MLT during equinoctial months. Moreover, the equinoctial maxima are much larger than the summer and winter maxima and appear at relatively lower latitudes. The seasonal variations in WEJ are the combinations of annual variations and semiannual variations. Both annual and semiannual variations show significant dependence on MLT. These results increase our knowledge on what factors contribute to the auroral electrojets as well as their magnetic signatures and hence help us better understand the limitations of global auroral electrojet indices, such as the AE and SME indices.

Annual variations in westward auroral electrojet and substorm occurrence rate during solar cycle 23

AbstractThe International Monitor for Auroral Geomagnetic Effects network magnetic measurements during the period 1995–2009 are used to characterize the annual variations in the westward electrojet. The results suggest that the annual variations in different local time sectors are quite different due to the different sources. In the MLT sector 2200–0100, the annual variations with maxima in winter suggest they are caused by the combined effects of the convective electric field and the conductivity associated with particle precipitation. Furthermore, the conductivity seems to play a more important role in the MLT sector ∼2200–2320, while the convective electric field appears to be more important in the MLT sector ∼2320–0100. In the MLT sector 0300–0600, the annual variations with maxima in summer suggest they are caused by solar EUV conductivity effect and the equinoctial effect. The solar EUV conductivity effect works by increasing ionospheric conductivity and enhancing the westward electrojet in summer, while the equinoctial effect works by decreasing solar wind‐magnetosphere coupling efficiency and weakening the westward electrojet in winter. In the MLT sector 0100–0300, the annual variations are relatively weak and can be attributed to the combined effects of annual variations caused by all the previously mentioned effects. In addition, we find that a significant annual variation in substorm occurrence rate, mainly occurring in the premidnight region, is quite similar to that in the westward electrojet. We suggest that elevated solar wind driving during the winter months contributes to higher substorm occurrence in winter in the Northern Hemisphere.

Modeling the thermosphere as a driven-dissipative thermodynamic system

Thermospheric density impacts satellite position and lifetime through atmospheric drag. More accurate specification of thermospheric temperature, a key input to current models such as the High Accuracy Satellite Drag Model, can decrease model density errors. This paper improves the model of Burke et al. (2009) to model thermospheric temperatures using the magnetospheric convective electric field as a driver. In better alignment with Air Force satellite tracking operations, we model the arithmetic mean temperature, T1/2, defined by the Jacchia (1977) model as the mean of the daytime maximum and nighttime minimum exospheric temperatures occurring in opposite hemispheres at a given time, instead of the exospheric temperature used by Burke et al. (2009). Two methods of treating the solar ultraviolet (UV) contribution to T1/2 are tested. Two model parameters, the coupling and relaxation constants, are optimized for 38 storms from 2002 to 2008. Observed T1/2 values are derived from densities and heights measured by the Gravity Recovery and Climate Experiment satellite. The coupling and relaxation constants were found to vary over the solar cycle and are fit as functions of F10.7a, the 162 day average of the F10.7 index. Model results show that allowing temporal UV variation decreased model T1/2 errors for storms with decreasing UV over the storm period but increased T1/2 errors for storms with increasing UV. Model accuracy was found to be improved by separating storms by type (coronal mass ejection or co-rotating interaction region). The model parameter fits established will be useful for improving satellite drag forecasts.

Test particle comparison of heavy atomic and molecular ion distributions at Mars

This study uses the Mars Test Particle simulation to create virtual detections of O+, , and in an orbital configuration in the Mars space environment. These atomic and molecular planetary pickup ions are formed when the solar wind directly interacts with the neutral atmosphere, causing the ions to be accelerated by the background convective electric field. The subsequent ion escape is the subject of great interest, specifically with respect to which species dominates ion loss from Mars. O+ is found to be the dominant escaping ion because of the large sources of transported ions in the low‐energy (&lt;10 eV) and high‐energy (&gt;1 keV) range. and are observed at these energy ranges but with much lower fluxes and are generally only found in the tail between 10 eV and 1 keV. Using individual particle traces, we reveal the origin and trajectories of the low‐energy downtail O+ populations and high‐energy polar O+ populations that contribute to the total escape. Comparing them against and reveals that the extended hot oxygen corona contributes to source regions of high‐ and low‐energy escaping ions. Additionally, we present results for solar minimum and maximum conditions with respect to ion fluxes and energies in order to robustly describe the physical processes controlling planetary ion distributions and atmospheric escape.

The role of ring current particle injections: Global simulations and Van Allen Probes observations during 17 March 2013 storm

We simulate substorm injections observed by the Van Allen Probes during the 17 March 2013 storm using a self‐consistent coupling between the ring current model RAM‐SCB and the global MHD model BATS‐R‐US. This is a significant advancement compared to previous studies that used artificially imposed electromagnetic field pulses to mimic substorm dipolarization and associated inductive electric field. Several substorm dipolarizations and injections are reproduced in the MHD model, in agreement with the timing of shape changes in the AE/AL index. The associated inductive electric field transports plasma sheet plasma to geostationary altitudes, providing the boundary plasma source to the ring current model. It is found that impulsive plasma sheet injections, together with a large‐scale convection electric field, are necessary to develop a strong ring current. Comparisons with Van Allen Probes observations show that our model reasonably well captures dispersed electron injections and the global Dst index.

Active current sheets and candidate hot flow anomalies upstream of Mercury's bow shock

Hot flow anomalies (HFAs) represent a subset of solar wind discontinuities interacting with collisionless bow shocks. They are typically formed when the normal component of the motional (convective) electric field points toward the embedded current sheet on at least one of its sides. The core region of an HFA contains hot and highly deflected ion flows and rather low and turbulent magnetic field. In this paper, we report observations of possible HFA‐like events at Mercury identified over a course of two planetary years. Using data from the orbital phase of the MESSENGER mission, we identify a representative ensemble of active current sheets magnetically connected to Mercury's bow shock. We show that some of these events exhibit magnetic and particle signatures of HFAs similar to those observed at other planets, and present their key physical characteristics. Our analysis suggests that Mercury's bow shock does not only mediate the flow of supersonic solar wind plasma but also provides conditions for local particle acceleration and heating as predicted by previous numerical simulations. Together with earlier observations of HFA activity at Earth, Venus, Mars, and Saturn, our results confirm that hot flow anomalies could be a common property of planetary bow shocks and show that the characteristic size of these events is controlled by the bow shock standoff distance and/or local solar wind conditions.

Ionospheric current contribution to the main impulse of a negative sudden impulse

The geomagnetic field response to a moderate-amplitude negative sudden impulse (SI−) that occurred on 14 May 2009 at 10:30 UT was examined at 97 geomagnetic observatories situated all over the globe. The response signature contains a contribution from magnetospheric as well as ionospheric currents. The main impulse (MI) is defined as the maximum depression in the observed geomagnetic field. It is observed that for low-to-high latitudes, the amplitude of the MI is larger in the afternoon to post-dusk sector than in the dawn-noon sector, indicating asymmetry in the MI amplitude. We estimated the contribution at various observatories due to the Chapman-Ferraro magnetopause currents using the Tsyganenko model (T01) and subtracted this from the observed MI amplitude to obtain the contribution due to ionospheric currents. It is found that the ionospheric currents contribute significantly to the MI amplitude of moderate SI− even at low-to-mid latitudes and that the contribution is in the same direction as that from the magnetopause currents near dusk and in the opposite direction near dawn. The equivalent current vectors reveal a clockwise (anticlockwise) ionospheric current loop in the afternoon (morning) sector during the MI of the negative pressure impulse. This evidences an ionospheric twin-cell-vortex current system (DP2) due to field-aligned currents (FACs) associated with the dusk-to-dawn convection electric field during the MI of an SI−. We also estimated the magnetic field variation due to prompt penetration electric fields, which is found to be very small at low latitudes in the present case. The studied SI− is not associated with shock, and hence no preliminary reverse impulse was evident. In addition, the summer hemisphere reveals larger MI amplitudes than the winter hemisphere, indicating once again the role of ionospheric currents.

Modeling of properties of the plasmasphere under quiet and disturbed conditions

Based on theoretical models of the ionosphere and the plasmasphere, the ion composition variations in the plasmasphere and the plasmapause structure were studied depending on the choice of the distribution model of the magnetospheric convection electric field at low and high geomagnetic activity at the equinox and the December solstice. Based on the model calculations performed, the plasmapause shape and size during an increase and decrease in geomagnetic activity were studied. It was revealed that the size of the plasmasphere mainly depends on the magnetic local time (MLT) sector and the level of geomagnetic activity, and it greatly depends on the maximum universal time during the equinox. The Earth’s plasmasphere asymmetry is manifested in the noon-midnight and morning-evening directions. The analysis results of daily and seasonal variations in the ionic composition of the Earth’s plasmasphere at a moderate solar activity level show that there is a certain increase in the ion concentrations of H+ and He+ in the winter period probably due to an increase in the exospheric density at the summer to winter transition. The data obtained are in good agreement with satellite observations which makes it possible to use the model proposed to study the plasmasphere under different geophysical conditions.

Global distributions of storm-time ionospheric currents as seen in geomagnetic field variations

To investigate temporal and spatial evolution of global geomagnetic field variations from high-latitude to the equator during geomagnetic storms, we analyzed ground geomagnetic field disturbances from high latitudes to the magnetic equator. The daytime ionospheric equivalent current during the storm main phase showed that twin-vortex ionospheric currents driven by the Region 1 field-aligned currents (R1 FACs) are intensified significantly and expand to the low-latitude region of ~30° magnetic latitude. Centers of the currents were located around 70° and 65° in the morning and afternoon, respectively. Corresponding to intensification of the R1 FACs, an enhancement of the eastward/westward equatorial electrojet occurred at the daytime/nighttime dip equator. This signature suggests that the enhanced convection electric field penetrates to both the daytime and nighttime equator. During the recovery phase, the daytime equivalent current showed that two new pairs of twin vortices, which are different from two-cell ionospheric currents driven by the R1 FACs, appear in the polar cap and mid latitude. The former led to enhanced northward B z (NBZ) FACs driven by lobe reconnection tailward of the cusps, owing to the northward interplanetary magnetic field (IMF). The latter was generated by enhanced Region 2 field-aligned currents (R2 FACs). Associated with these magnetic field variations in the mid-latitudes and polar cap, the equatorial magnetic field variation showed a strongly negative signature, produced by the westward equatorial electrojet current caused by the dusk-to-dawn electric field.