Abstract

Almost 40 years ago the concept of the substorm current wedge (SCW) was developed to explain the pattern of magnetic signatures observed on the ground and in geosynchronous orbit during the substorm expansion phase. The ensuing decades saw advancements in our understanding of this system from new observations, including radar and low-altitude spacecraft, theoretical considerations, and MHD simulations, and the SCW remains a guiding paradigm on the large scale to this day. Yet recent results from new radar instrumentation, Cluster, and the coordinated ground and in situ measurements of the THEMIS mission have identified key areas in which the SCW paradigm needs to be revised and extended. Specific questions include the role of current filamentation and other small-scale processes, how the current systems of multiple onsets are related, and how other processes that contribute to substorm currents fit within the SCW model. We propose to bring together an inter-disciplinary group of experts on the ionosphere, magnetosphere, modeling and theory to produce a comprehensive review paper with two primary goals. First, we will review the work of the past 40 years covering historical development, a thorough summary of the ground, ionospheric and magnetospheric observations that underpin the model, and an overview of the necessary theoretical frameworks. Second, we will pull together the wealth of recent advancements enabled by the THEMIS and Cluster missions, with an emphasis on how recent observations have expanded our knowledge beyond the simple conceptual model, and necessitated an extension of the phenomenological picture.

Scientific Rationale

The concept of the Substorm Current Wedge (SCW) grew out of a flurry of substorm-related research performed in the late 1960’s and early 1970’s that culminated in a collection of 9 papers published in the Journal of Geophysical Research in 1973, entitled, “Satellite Studies of Magnetospheric Substorms on August 15, 1968.” The ninth paper of this collection, “Phenomenological Model for Substorms,” introduced the now ubiquitous drawing at right that shows the short-circuiting of the cross-tail current that is known to occur at substorm onset [McPherron et al., 1973]. The phenomenological picture that emerged built upon much previous research and represented a major breakthrough in organizing substorm observations.

The substorm current wedge is a simple model of the 3-dimensional current system created during the expansion phase of a magnetospheric substorm. At the end of a prolonged period of solar wind IMF Bz < 0, the magnetotail lobes contains the stored magnetic flux that is ultimately the source of energy for magnetospheric substorms. By the end of the substorm growth phase this increased lobe magnetic field strength has caused the current sheet within the plasmasheet to thin and move earthward, sometimes to the near-geosynchronous region. At substorm onset the magnetic field within an azimuthally limited patch quickly relaxes to a dipolar state (called “dipolarization”), while field lines adjacent to this region still maintain a stretched configuration. Current continuity requires that the current flowing from the east (the dawn side) still flow on the westward side of this dipolarized region. The current is diverted along field lines into the ionosphere on the eastern edge of the disrupted region, flows westward in a small ionospheric segment, then outward along the western edge. As the substorm expansion phase relaxes, the substorm current wedge dissipates and disappears by the end of the recovery phase.

The importance of the substorm current wedge lies in its ability to link ionospheric and magnetospheric processes during the dynamic collapse of the near-Earth current sheet under a single, coherent phenomenological picture. The timeliness of this proposal lies with the recent THEMIS observations which have added significant new information regarding the dynamics of the SCW, especially on smaller scales and within the first few minutes of onset. Significant questions remain, including: the role of current filamentation, waves, and other small-scale processes; how the current systems of multiple onset arcs are related; how much of the diverted current is flowing through the ionosphere; how BBF-associated currents merge into the SCW; and what other processes contribute to substorm currents, and on what spatial and temporal scales do they work. With 40 years of research and the recent advancements in understanding enabled by the THEMIS observations, it is timely and necessary to consolidate knowledge and, where appropriate, revise and extend the SCW model, to lay the groundwork for further research in the area.

Objectives

The basic picture of the SCW has stood the test of time, and remains a unifying concept around which studies of substorms are organized. Yet important questions remain, and recent coordinated ground (magnetometer, imaging and radar) and in situ data have yielded important new insight into complex details of the simple phenomenological model. Our goal with an ISSI team is to bring together leading experts across a diverse spectrum of disciplines to review the historical development of the substorm current wedge, from both theoretical and data analysis perspectives, summarize and consolidate recent advancements and understanding, and lay the groundwork for future research. Our output would be a thorough review paper, roughly balanced between recent advancements and historical background, covering the following aspects:

Historical Development

We intend to first review the history of the theoretical concepts and observational knowledge that lead to the development of the substorm current wedge concept. Important ideas go back to at least Boström [1964], who linked three-dimensional current systems with the auroral electrojects. Further papers by Obayashi and Nishida [1968] and others established the DP-1 (polar substorm) and DP-2 (two-cell convection) descriptions of geomagnetic disturbances, and proposed equivalent current systems for their generation. Bonnevier et al. [1971] proposed a three-dimensional  current system similar to the SCW, which was used to model ground magnetic perturbations, but did not consider magnetospheric measurements. Using a combination of ground and geosynchronous magnetic field observations McPherron et al. [1973] synthesized the observational constraints of both ground and importantly magnetospheric observations and theoretical considerations into a single framework, which they dubbed the substorm current wedge, to explain the DP-1 geomagnetic disturbance.

 Observations

A tremendous body of research has been devoted to studying the influences of the substorm current wedge on different regions of geospace, and these observations provide the observational foundation for the SCW model. Perhaps the most obvious effect is on the ground. The ionospheric portion of the SCW is an azimuthally and latitudinally limited westward electrojet, and drives large negative H (north-south) perturbations (the DP-1 system) at high latitudes, as shown in the top panel of Figure 2. From Figure 1 it is clear that both sections of the field-aligned currents of the SCW produce a northward magnetic perturbation inside the wedge at midlatitudes. This northward perturbation is called a “positive bay” for historical reasons (see bottom traces in Figure 2).

The effects of the substorm current wedge are manifested in variety of locations and measurements, and we intend to provide a thorough review of these observations. Specific topics include: midlatitude magnetic variations, auroral zone magnetic variations, synchronous observations, radar observations and changes in convection patterns, statistical association with Pi 2 pulsations, and statistical relation to magnetospheric bulk flows. In addition, the innermost THEMIS spacecraft have provided new insight into the underlying physics of the SCW. We have included in our team experts from the THEMIS community to ensure capture of recent advancements in this area.

Extensions, complications, and recent results

We plan to spend at least half our effort focusing on the importance of recent results, and how they complicate and extend the simple phenomenological picture of the SCW. Although the basic SCW paradigm reasonably explains the large-scale magnetic perturbations during a substorm, a number of important questions still remain, especially in light of new observations. For example, over the past decade it has been recognized that an earthward traveling BBF imposes a current system on the ionosphere of similar geometry to the SCW. It is unclear how this current systems relates to or merges with the SCW, but recent observations, primarily from coordinated THEMIS observations, have yielded new insight. As a unified picture has yet to emerge from the new data, our team will focus on consolidating observations and knowledge related to and, where possible, answering, questions such as the following: How much of the ionospheric currents that we observe during a substorm can be explained by this paradigm? What other local processes contribute to substorm currents, on what spatial and temporal scale are they working, and what is their relative importance? On what scales can the east-west substorm currents be described as continuous, and where and when does filamentation exist? What is the structure of currents during multiple activations? The result our work at ISSI will be an identification of where the SCW model needs to be revised and/or extended, and we anticipate spending roughly half our time focusing on these new insights.

Initiation

The sudden switching-on of the substorm current wedge at substorm onset leads to a complicated interplay between the ionosphere and magnetosphere as the two regions attempt to reach an equilibrium state. Transient Pi2 pulsations (T = 40-150 s) are one result of this transitional response (see e.g., Baumjohann and Glassmeier, [1984]). There has been much theoretical and observational research into these so-called transient response Pi2 (often called ‘mid latitude Pi2’), which we intend to review. Note that these are distinct from Pi2 at low-latitude, a topic which we do not plan to include, as they are not part of the substorm current wedge system. The THEMIS mission and concomitant high-resolution ground all-sky imaging has enabled an unprecedented examination of the timing and spatial characteristics of the aurora, transient response Pi2, and the formation of the substorm current wedge within the first few minutes of onset (see e.g. Kepko et al., [2009]). We will review and include recent results from this mission, and identify key open questions for future research.

Theory and modeling

The SCW is an equivalent current system that approximately reproduces the magnetic variations seen at midlatitudes during substorms. In the simplest models the current is portrayed as a line (or sheet) current connecting the tail to the ionosphere, down in the morning sector, across the auroral bulge in the ionosphere, and upward in the premidnight sector (e.g., Kisabeth and Rostoker, [1977]). Parameters in these models include the L shell on which the current flows, the strength of the current, and the local times of the downward and upward currents. The current strength provides a quantitative measure of the current linking the magnetosphere and the ionosphere. Model parameters can be determined by inversion of midlatitude ground magnetometer data augmented by knowledge of the location of the center of the substorm electrojet. The primary application of this analysis is in the study of phenomena in the tail where it is important to locate satellites relative to the auroral bulge, and has played an especially important role during the analysis of THEMIS substorms.

On the theoretical side, signatures of the SCW were first identified in 3D MHD simulations of magnetotail reconnection [Birn and Hesse, 1991; Scholer and Otto, 1991]. These simulations demonstrated the important role of flow shear and diversion in distorting the magnetic field and building up the wedge currents. The role of flow braking was further established by Hesse and Birn [1991] and Shiokawa et al. [1998], which was an important step in linking the dipolarization of the SCW to magnetospheric flows, generated by reconnection. Birn and Hesse [1991] and Birn et al. [1999]) further demonstrated that the current system in the simulations was associated with pressure gradients (rather than inertia effects),  suggesting that off-equatorial pressure gradients in the z (vertical) direction are the primary drivers. Our review paper will cover the analytical work, starting with simple current sheet models and magnetogram inversion, and proceed further to summarizing simulation results while also providing the mathematical background of the physics responsible for the SCW.

Future work

The final section of the review paper will identify the key open questions related to the substorm current wedge. Many of these questions have evolved from the THEMIS data analysis and recent theoretical and modeling results. For example, the relation of the SCW to high-latitude features, such as the onset arc, is an active and still controversial area of research, but the THEMIS results are adding important observational constraints. There are additional questions about how the current is generated in the near-geosynchronous region, and what fraction of the current closes locally as opposed to passing through the ionosphere.

Expected outputs

Our goal is to publish a comprehensive review paper in Space Science Reviews or similar journal that provides a historical perspective on the development of the substorm current wedge, summarizes previous observational and theoretical work, identifies key open questions. Further, we will consolidate recent observations and theoretical considerations that indicate the SCW is more complicated than the simple paradigm, and we will revise and extend the model as appropriate. We expect the paper to become a primary reference for researchers in the area.

Schedule

Late 2010: Review the current knowledge within the categories identified in the proposal. Team members are organized into respective areas of expertise and present information to the group and team leader who synthesizes the input into a coherent narrative. Of primary importance is consolidating recent advancements and understanding, and identifying areas in which the SCW paradigm needs to be revised and/or extended.

Mid 2011: Identify areas that need additional work and finalize a draft of the review paper. Submittal of paper by late 2011.

References

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Birn, J., and M. Hesse, The substorm current wedge and field-aligned currents in MHD simulations of magnetotail reconnection, J. Geophys. Res., 96, 1611, 1991.

Birn, J., M. Hesse, G. Haerendel, W. Baumjohann, and K. Shiokawa, Flow braking and the substorm current wedge. J. Geophys. Res., 104, 1999.

Bonnevier, B., R. Boström, and G. Rostoker, A three-dimensional model current system for polar magnetic substorms. J. Geophys. Res., 75, 1970.

Boström, R, A Model of the Auroral Electrojets, J. Geophys. Res., 69, 1964.

Hesse, M., and J. Birn, On dipolarization and its relation to the substorm current wedge, J. Geophys. Res., 96, 19,417, 1991.

Kepko, L., E. Spanswick, V. Angelopoulos, E. Donovan, J. McFadden, K.-H. Glaßmeier, J. Raeder, and H. J. Singer, Equatorward moving auroral signatures of a flow burst observed prior to auroral onset, Geophys. Res. Lett., 36, 2009.

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McPherron, R. L., Russell, C. T., and M. P. Aubry, Satellite studies of magnetospheric substorms on August 15, 1968. 9. Phenomenological model for substorms, J. Geophys. Res., 78, 1973.

Sergeev, V., et al., Simultaneous THEMIS observations in the near-tail portion of the inner and outer plasma sheet flux tubes at substorm onset, J. Geophys. Res., 113, 2008.

Scholer, M., and A. Otto, Magnetotail reconnection: Current diversion and field-aligned currents, Geophys. Res. Lett., 18, 733, 1991.

Shiokawa et al, High-speed ion flow, substorm current wedge, and multiple Pi 2 pulsations. J. Geophys. Res., 103, 1998.