Team Proposal

Abstract:

Cluster has been exploring the magnetosphere for over 10 years, offering new insights into the physics and phenomenology of this highly dynamic plasma environment. Since 2008, the orbit has evolved such that the spacecraft regularly pass over the auroral zones at altitudes of 5-18,000 km; a region known as the auroral acceleration region (AAR). This is the first time in history that a closely separated multi-spacecraft mission has been able to collect data in this region. Processes in this region accelerate particles into and out of the ionosphere and magnetosphere, exciting the brightest auroral emissions, and are responsible for the generation of auroral radio emissions (auroral kilometric radiation or AKR). Following on from previously successful ISSI teams using Cluster in conjunction with ground- and space-based observatories to study dayside and nightside processes in the magnetosphere, we propose to utilize these unique multi-point measurements made in the AAR between 2008 and 2011 to study this region. At intervals during these periods, the spacecraft were maneuvered into an optimum configuration to facilitate multi-point analysis techniques. Two of the spacecraft were on the same orbit track, with one trailing the other by a few minutes, two were separated in altitude but were approximately magnetically conjugate and the fourth was tangentially separated from the co-orbiting pair. The spacecraft constellation was crossing the northern hemisphere nightside auroral zone. These multi-point observations will help us to understand the variety of acceleration processes in this region, their spatial structure and temporal evolution by allowing us to directly measure the acceleration of particles, whilst simultaneously measuring the electromagnetic emission excited by these accelerated particle populations. Taken in tandem with the THEMIS mission, which has its tail season during northern hemisphere winter, and ground-based instruments monitoring the ionosphere, this will allow us to investigate how the processes in the AAR are linked with dynamic processes in the magnetosphere-ionosphere system, providing new insights into the transport of energy within the near-Earth environment. The work of this team will utilize data from many of the instruments onboard Cluster. Data from the earlier auroral campaigns is freely available from the Cluster Active Archive. Team members who are PI’s and CoI’s on the various Cluster instruments will provide data from later campaigns. Supplementary ground and spacecraft data will be sought as required. The selected team members are experts in auroral physics and phenomenology, multi-spacecraft space plasma studies and the Cluster mission. We will use this expertise to identify and exploit the datasets that can further our understanding of auroral physics, with the resulting studies being submitted to peer-reviewed journals. The output of this team may be of interest to scientists working on proposals for future multi-spacecraft missions to the auroral zones, such as the Alfvén mission.

Research domains: Space Science (Solar-Terrestrial Sciences, Space Plasma and Magnetospheric Physics, Fundamental Physics in Space)

Scientific Rationale

The aurora at Earth are a dynamic feature of the coupled magnetosphere-ionosphere system and represent the deposition of energy from the magnetosphere into the ionosphere in concert with ionospheric mass loss. Due to conservation of the first adiabatic invariant, the majority of charged particles in the magnetosphere are trapped. A small fraction, with pitch angles within approximately 3° of the magnetic field direction travel sufficiently far along the magnetic field lines to be scattered by collisions with ionospheric atoms, exciting dim auroral emissions and being lost to the ionosphere. In order to excite the brightest aurora, particles must be accelerated along the magnetic field in violation of ideal magnetohydrodynamics. This acceleration is driven by electric fields parallel to the magnetic field from quasi-static potential structures and wave electric fields and occurs at altitudes between a few thousand kilometers altitude and several Earth radii. Whilst significant advances in our understanding of the processes that excite aurora have come about through single (e.g. Freja, FAST, Polar, DMSP) and widely separated dual (e.g. Dynamics Explorer) spacecraft missions, the spatial structure and temporal evolution of these processes remains unclear. These processes are key to understanding the coupling and energy transfer between the magnetosphere and ionosphere. The recent evolution of the Cluster orbit, now regularly bringing this four-spacecraft mission into the auroral acceleration region (AAR), affords us an unprecedented opportunity to use multi-spacecraft observations to study the processes and structure of this region. Our proposed work will utilize these new datasets from the AAR to further our understanding of this region and its coupling to large scale processes in the magnetosphere. Our work is timely in light of the new dataset available and due to the increased interest in launching multi-spacecraft missions to the auroral regions, such as the Alfvén and Ohmic proposal.

Over Cluster’s 10-year lifetime, the spacecraft orbits have evolved from their initial 19 x 4 RE polar orbits to inclined 21.4 x 1.5 RE orbits. This change in the spacecraft orbits has meant that in 2008 the spacecraft began traversing the auroral acceleration region at all magnetic local times and at a range of altitudes (see Figure 1). The inclination of the orbit means that the spacecraft tend to pass through the auroral acceleration region at lower altitude in the north than the in the south. In December 2009, the spacecraft tetrahedron was optimized for studying the AAR with two of the spacecraft separated in altitude, two spacecraft separated along the same orbit and the remaining spacecraft laterally separated from the two co-orbiting spacecraft. The spacecraft configuration at other times has already been shown to be advantageous for more detailed studies of the AAR than offered by single spacecraft missions (e.g. Marklund et al., 2011).

Observations of the electric field associated with both upward and downward auroral currents have shown that these regions are associated with electrostatic shocks and electric fields parallel and perpendicular to the magnetic field (for a review, see Paschmann, Haaland & Treumann, 2003). A schematic view of current thinking on the structure of the upward current auroral acceleration region is given in Figure 2. Vlasov simulations (Ergun et al., 2000) of the auroral acceleration region suggest that electric double layers (Block, 1972), which provide the majority of the accelerating potential, bound the region. Observations of the low altitude (ionosphere-cavity) boundary are abundant from FAST and Polar, and show that this double layer can provide 10-50% (Mozer & Hull, 2001; Ergun et al., 2002) of the accelerating potential. There are no reported direct in-situ measurements of the high altitude double layer, such that more than 50% of the accelerating potential is unaccounted for.

A recent case study (Forsyth et al., in preparation, 2011) using data from two of the Cluster spacecraft separated in altitude along the auroral cavity has shown that 25% of the accelerating potential can be located within the auroral cavity (Figure 3) and that the particle distributions are inconsistent with acceleration due to a mid-cavity double layer (Ergun et al., 2004). This is consistent with the results presented by Mozer & Hull (2001) (shown in Figure 4), which showed that between 10 and 75% of the accelerating potential was statistically located between 2 and 3 RE altitude. However, unlike the results of Forsyth et al. (in preparation), the results of Mozer & Hull (2001) only show the distribution of the potential above and below the spacecraft. Further studies using Cluster data are key to determining the distribution of the electric potential within the auroral acceleration region.

Whilst single spacecraft measurements are capable of estimating the total acceleration (potential) and the proportion of the potential located above and below the spacecraft, more quantitative descriptions of the electric potential structure are elusive. Recently, Marklund et al. (2011) showed that when two of the Cluster spacecraft crossed the same upward current region at different heights, the revealed potential structure was quite different at the two spacecraft (Figure 5). Whilst the observations from the higher spacecraft were consistent with a U-shaped potential structure, the lower spacecraft revealed that the poleward edge of the current region was associated with an S-shaped potential structure that was invisible to the higher altitude spacecraft. Further studies during the optimized tetrahedron phase of the mission will reveal how these different potential structures evolve in time and the prevalence of U and S-shaped structures at different altitudes.

Mono-energetic acceleration, such as that described above, represents one source of acceleration of auroral particles. A further source of acceleration is kinetic Alfvén waves with short perpendicular scales. These can resonantly interact with magnetospheric electrons and, depending on the phase relationship between the waves and the electrons, accelerate them to a broad range of energies. Observations of broadband acceleration of electrons often occur at the poleward boundary of the auroral zone, suggesting that the acceleration process may be closely linked with reconnection in the magnetotail. Recent modelling studies (e.g. Watt and Rankin, 2009) have shown how Alfvén waves can persist in regions of heavy damping and accelerate magnetospheric electrons to the energies expected. This complicated, non-linear acceleration process is thought to occur in a region above the quasi-static acceleration region, which may be confirmed using simultaneous multi-point observations at different heights using Cluster.

Auroral Kilometric Radiation (AKR) is radio emission from the auroral cavity, generated due to positive gradients in the electron distribution functions perpendicular to the magnetic field. These positive gradients arise from accelerated particles undergoing magnetic mirroring within the auroral cavity. Observations by Cluster (Mutel et al., 2011, in press) have shown that both X- and Z-mode radiation can be generated in the auroral cavity (Figure 6) and that the emission mode is dependant on the electron density within the auroral cavity. In theory, the electric field and density structures should be closely related in order to conserve momentum. Combined with accurate particle densities from plasma wave measurements (Masson et al., 2010), these measurements will allow us to determine the density structure of the auroral cavity and compare this to the electric field structure.

Auroral morphology is highly dynamic and is linked to activity in the magnetosphere. During a substorm, the aurora develops from a thin arc, which rapidly brightens, expanding polewards and westwards, before breaking down into so-called omega bands and pulsating auroral patches during the recovery phase. Newell et al. (2010) showed that the power being dumped into the ionosphere by various types of aurora (diffuse, broadband, inverted-V) varies through a substorm, with broadband auroral power increasing by 182% in a short period after onset, compared with only 79% for inverted-V aurora. Furthermore, the increase in broadband auroral power is relatively short lived. This raises questions about the evolution of aurora precipitation during a substorm such as: are broadband and inverted-V populations concurrent, or does the broadband aurora evolve into the inverted-V aurora? Hull et al. (2010) used data from a high altitude (~3.5 RE) auroral crossing by Cluster in a “cigar” formation to examine the temporal evolution of the AAR at substorm onset and concluded that quasi-static potentials form out of broadband, Alfvénic regions. With the four Cluster spacecraft passing through the auroral region at closely separate times and separated along the magnetic field, we can further investigate the spatial evolution of the auroral cavity.

It has already been demonstrated that multi-point measurements in the auroral zones can greatly increase our understanding of the processes within this region (e.g. Marklund et al., 2001, 2011; Hull et al., 2010; Wild et al., 2011). This new dataset, which provides a wealth of multi-point data in the auroral acceleration region, is ideal to address the spatial and temporal variability of the auroral zone processes.

Expected Output

Our primary aim is to publish papers in refereed literature. We note that the topic of auroral processes has been re-invigorated in the last few years, with dedicated conferences (e.g. AGU Chapman Conference on the Relationship between magnetospheric processes and auroral morphology held in March 2011) and new proposals for multi-spacecraft missions to study the aurora. As such, we believe our topic will be of great interest to the scientific community, as well as the wider public. The proposed team consists of scientists who are experts in a range of auroral processes as well as in multi-spacecraft analysis. We will produce papers that highlight those findings which can primarily only be achieved through the multi-point, multi-spacecraft analysis afforded by Cluster, together with supporting data from other spacecraft on the input to the auroral zones and ground stations which regularly monitor auroral activity.

As in many areas of space science, previous publications have tended to focus on relatively narrow areas of research. One of the aims of this team is to produce a single, comprehensive study of as many aspects of the auroral acceleration region processes as possible; quantitatively relating the acceleration of particles with the depletion of the auroral cavity and the associated radio emissions. This will complement previous, narrower studies and provide a reference for future studies.

Added value from ISSI

The hosting of this international team by ISSI offers the opportunity to bring together a wide range of expertise to examine the most exciting new dataset in auroral physics for many years. Some work has already been done on theses datasets within the Cluster community, although this is currently uncoordinated with individual research groups concentrating on specific aspects of auroral physics. By providing funding and facilities, ISSI will allow these groups to come together and work with scientists with a background studying auroral data from earlier missions, along with modellers and theoretical physicists, providing a greater insight into the processes that drive the aurora.

Schedule

An initial five-day meeting will establish our current understanding of the auroral processes and select a number of events suitable for further in depth study, collate datasets and establish the parameters for complementary model runs. A second four-day meeting will enable us to review work done off-line and collate results for preparation to be published. A final 3-day meeting will be used to finalise the publications prior to submission or address any issues raised during the refereeing process.

Funding

The individual team members will arrange funding for travel to ISSI. We request funding to cover facilities, accommodation and subsistence of the team whilst at ISSI and travel support for the team leader. One of our team is an ESA employee. An estimate of costs is given below.

Travel for team leader

800 EUR

Subsistence for 11 team members for three meetings

11 members x 12 days = 132 per diems

6 600 EUR (50 EUR per diem)

Hotel for 11 team members for three meetings

11 members x 12 days = 132 nights

13 200 EUR (100 EUR per night)

TOTAL

20 600 EUR