Dayside Currents Research Articles

How Do Substorms Influence Hemispheric Asymmetries in Equivalent Currents?

AbstractIonospheric dynamics exhibits a distinct hemispheric asymmetry, influenced primarily by the Interplanetary Magnetic Field (IMF) By component, dipole tilt, or a combination of both. Previous studies have indicated a reduction in these asymmetries during substorms. In this study, we conduct a superposed epoch analysis using ground magnetometer data from the northern hemisphere to examine the impact of substorms on ionospheric current asymmetry. This analysis uses the assumption of mirror symmetry between the northern and southern hemispheres when IMF By and dipole tilt are reversed. We observe a significant reduction in nightside equivalent current asymmetry indicating the IMF By and dipole tilt have minimal influence on the substorm current. On the other hand, we find that substorms exert minimal or negligible effects on dayside currents. This difference in response between nightside and dayside currents emphasizes the need to incorporate nightside dynamics into existing climatological models, which presently rely mainly on upstream parameters due to a lack of robust parameters effectively representing them. Our findings provide important insights for future modeling efforts, highlighting the distinct interactions between substorms and ionospheric currents across different hemispheric regions.

Temporal and Spatial Development of Global Birkeland Currents

AbstractThe development of large‐scale Birkeland currents is examined using the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE), which measures global Birkeland currents continuously on 10‐min time scales. The integrated current was used to identify onsets of at least 1 MA preceded by periods of quiescence lasting at least 3 hr. The Region 1 currents do not fully form without Region 2. Rather, they develop together, first on the dayside, then on the nightside, and lastly, they fill in and intensify at all local times to form the nominal statistical pattern. The onsets are closely correlated with enhancements of magnetospheric forcing as indicated by the solar wind electric field and the orientation of the interplanetary magnetic field. Nightside onsets correspond to intensifications of the auroral electrojet as reflected in the AE index; they are delayed by ~40 min relative to the increase of the dayside current and are 2.8 times more rapid than the dayside current increase. After nightside onset, Birkeland currents expand toward dawn and dusk and merge with the dayside currents while also intensifying at all local times. The dayside current pattern depends on the sign of the interplanetary magnetic field BY. The nightside current distributions are the same for positive and negative BY and display a Harang discontinuity independent of the sign of BY. The predominant development and intensification of Birkeland currents occur after nightside onset at all local times with roughly 75% of the total current, both Regions 1 and 2, appearing after nightside onset.

Seasonal and Temporal Variations of Field‐Aligned Currents and Ground Magnetic Deflections During Substorms

AbstractField‐aligned currents (FACs), also known as Birkeland currents, are the agents by which energy and momentum are transferred to the ionosphere from the magnetosphere and solar wind. This coupling is enhanced at substorm onset through the formation of the substorm current wedge. Using FAC data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and substorm expansion phase onsets identified using the Substorm Onsets and Phases from Indices of the Electrojet technique, we examine the Northern Hemisphere FACs in all local time sectors with respect to substorm onset and subdivided by season. Our results show that while there is a strong seasonal dependence on the underlying FACs, the increase in FACs following substorm onset only varies by 10% with season, with substorms increasing the hemispheric FACs by 420 kA on average. Over an hour prior to substorm onset, the dayside currents in the postnoon quadrant increase linearly, whereas the nightside currents show a linear increase starting 20–30 min before onset. After onset, the nightside Region 1, Region 2, and nonlocally closed currents and the SuperMAG AL (SML) index follow the Weimer (1994, https://doi.org/10.1029/93JA02721) model with the same time constants in each season. These results contrast earlier contradictory studies that indicate that substorms are either longer in the summer or decay faster in the summer. Our results imply that, on average, substorm FACs do not change with season but that their relative impact on the coupled magnetosphere‐ionosphere system does due to the changes in the underlying currents.

Development of large-scale Birkeland currents determined from the Active Magnetosphere and Planetary Electrodynamics Response Experiment

The Active Magnetosphere and Planetary Electrodynamics Response Experiment uses magnetic field data from the Iridium constellation to derive the global Birkeland current distribution every 10 min. We examine cases in which the interplanetary magnetic field (IMF) rotated from northward to southward resulting in onsets of the Birkeland currents. Dayside Region 1/2 currents, totaling ~25% of the final current, appear within 20 min of the IMF southward turning and remain steady. Onset of nightside currents occurs 40 to 70 min after the dayside currents appear. Thereafter, the currents intensify at dawn, dusk, and on the dayside, yielding a fully formed Region 1/2 system ~30 min after the nightside onset. The results imply that the dayside Birkeland currents are driven by magnetopause reconnection, and the remainder of the system forms as magnetospheric return flows start and progress sunward, ultimately closing the Dungey convection cycle.

Seasonal variations in diffuse, monoenergetic, and broadband aurora

The only previously established seasonal auroral variation is that of intense monoenergetic aurora, corresponding to quasi‐static acceleration by geomagnetic‐field‐aligned electric fields. Here we investigate the separate seasonal dependence of both types of electron accelerated aurora (broadband, or wave, in addition to monoenergetic) and both ion and electron diffuse aurora. Dayside and nightside variations are separately considered, as are conditions of low and high solar wind driving. Across these many combinations, several clear patterns emerge. One is that the dayside tends to maximize precipitation in the summer and much more so for low solar wind driving. Nightside precipitation is higher in the winter and much more so for high solar wind driving. The dayside effects are strongest in number flux and stronger in diffuse aurora than accelerated aurora. The ease of ion entry through the summer cusp, along with the constraints of charge quasi‐neutrality, and the rise in dayside currents in the summer hemisphere adequately explain much (perhaps all) of the dayside behavior. Nightside effects are more apparent in energy flux, with the winter/summer ratio of monoenergetic aurora being the largest: 1.70 for high solar wind driving. However, both other types of electron aurora, diffuse (1.30) and broadband (1.26), also have winter/summer energy fluxes well above unity for high solar wind driving. The nightside seasonal variation of ions is much smaller, with slightly more energy flux postmidnight in the winter but with slightly higher energy fluxes premidnight in the summer. Since the increased nightside fluxes into the winter hemisphere occur primarily under strong solar wind driving, and are much more prominent in energy flux than number flux, they likely reflect increased energization in the winter ionosphere when stronger currents are being driven into the ionosphere from the magnetosphere. Equinoctial behavior tends to lie between the summer and winter hemisphere values, but typically closer to the latter. As a result, nightside electron energy flux summed over the hemispheres is higher around equinox, simply because there is no summer hemisphere.

Fast Auroral Snapshot observations of the dependence of dayside auroral field‐aligned currents on solar wind parameters and solar illumination

The solar illumination dependence of solar wind‐magnetosphere coupling has been studied by examining the dependence of auroral zone field‐aligned currents on solar wind parameters. A database of Region I currents from ∼900 crossings of the auroral zone by Fast Auroral Snapshot (FAST) was parameterized by magnetic local time, invariant latitude, and whether the foot point was illuminated. The magnitudes of dayside currents (∼200 events) were correlated with solar wind parameters using the technique of rank correlation. Very significant, strong correlation between dayside sunlit currents and solar wind parameters such as Btsin(θ/2)v2 and Btsin(θ/2)Pd1/2 was observed, consistent with a reconnection source. There was no correlation with upstream parameters for dayside Region I currents when the ionospheric foot point was in darkness. This is the first experimental evidence showing that the correlation of dayside Region I currents to upstream parameters depends on ionospheric illumination. Both the strong dependence of the correlation on solar illumination (ionospheric conductivity) and the weakness of observed correlations, compared to those obtained in previous studies of the cross polar cap potential, may be interpreted to imply that the reconnection process is a voltage, rather than a current, source. However, several theoretical studies have suggested that reconnection does not act as either a pure voltage source or a current source. In addition, because the mapping of the field‐aligned currents was not examined, the observed difference between the sunlit and dark events could be explained by current preferentially flowing in the hemisphere with the higher conductivity if the source were on closed field lines.

Reply

Turner et al. [2000] analyzed the contribution of cross‐tail currents to the Dst index. In order to estimate this contribution we used modified versions of the Tsyganenko models which had been adjusted to match spacecraft data in the tail, and we isolated a tail region and calculated its influence. We concluded that the tail currents were responsible for around 25% of the Dst response during moderately disturbed times. Maltsev and Ostapenko [2002] conclude that our estimate was low by a factor of 2, owing to that fact that we neglected dayside currents and that the model we used systematically underestimates the cross‐tail current system. We appreciate their insightful analysis of our work, but we disagree with their conclusions. The models we used were modified to match spacecraft data in the tail, so we do not feel they underestimate the tail currents, and we consider the tail currents to be primarily located in the magnetotail, so we feel our decision to neglect dayside currents was justified. Additionally, we feel that some of the discrepancies between our results and theirs are due to different definitions of tail and ring currents and our decisions on whether to include the induced ground current contribution in our estimates of the tail current contribution to Dst. Here we respond briefly to their arguments and conclude that we still find the approximate magnitude of the tail current contribution to Dst to be around 25%. Additionally, Maltsev and Ostapenko include their own analysis of the tail current contribution to Dst, but we will limit our response to those comments which directly relate to our work.

Large-scale distributions of ionospheric horizontal and field-aligned currents inferred from EISCAT

Abstract. Statistical models for large-scale convection and for ionospheric conductances were previously derived from observations of the incoherent-scatter radar EISCAT. We complete this large-scale description with statistical models of the horizontal and field-aligned currents achieved from the same data base and for the same ranges of the magnetic activity index Kp. Except for the high-latitude dayside currents generally located poleward of the radar field of view, a large part of the whole current system can be probed with EISCAT. Globally consistent with previously published models, our results also exhibit some differences, such as the asymmetry in the local-time extension of the current sheets, concentrated to a few hours around 18 MLT in the evening sector, while widely spread from premidnight to prenoon magnetic local times on the morningside. This statistical description of the current system above EISCAT allowed us to examine several aspects of the large-scale auroral electrodynamics, namely the relationships between convection, conductances, and currents, in particular in the vicinity of the Harang discontinuity, and the features of the global current circuit.

Freja's contribution to the ISTP event of October 27, 1992. A distorted magnetosphere

A unique arrangement of satellites and ground‐based magnetometers has provided the opportunity to study the distortion of the magnetosphere by conditions of elevated solar wind dynamic pressure and high geomagnetic activity on October 27, 1992. Data were acquired by the near‐Earth satellites Freja, UARS, DMSP‐F8 and DMSP‐F11, and by IMP‐8. This study will focus on patterns of large‐scale Birkeland currents deduced from the APL magnetic field experiments on Freja and UARS. Because of the relatively low inclinations of their orbits (63° for Freja and 57° for UARS) these satellites skim the current systems over a wide range of local time and provide a kind of “global image” of the Birkeland currents. Energetic particle data from the DMSP satellites are used in an attempt to complete the image of the auroral region. The principal findings of this study include the following. The dayside Birkeland current system is displaced 6° to 9° equatorward of the statistical pattern determined by Iijima and Potemra [1978] for undisturbed conditions (|AL|< 100 nT) and 3° equatorward of their disturbed statistical pattern (|AL|≥ 100 nT). The densities of these daytime Birkeland currents are estimated to be 1 to 2 µA/m², comparable to, but not larger than the statistical values determined by Iijima and Potemra for disturbed conditions. The nightside Birkeland currents are located close to the Iijima and Potemra disturbed statistical pattern and they are less intense than the dayside currents.