Quasi-parallel electron beams and their possible application in inferring the auroral arc's root in the magnetosphere

Liang, J.; Jiang, Feifei; Donovan, E.; Spanswick, E.; Angelopoulos, V.; Strangeway, R. J.

doi:10.5194/angeo-31-1077-2013

Cited by 5 publications

(8 citation statements)

References 57 publications

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“…To date, however, there has been no observational link between a visible auroral arc and its ultimate source in the magnetosphere, the so‐called arc generator. This limitation is due in part to the fact that even a single arc maps magnetically to a vast region of the magnetosphere, the structure of which is exceeding difficult if not impossible to resolve with a single spacecraft, or even a fleet of several spacecraft (Jiang et al, ; Liang et al, ). However, an understanding of the generator is essential for a self‐consistent theory of auroral arcs.…”

Section: Discussionmentioning

confidence: 99%

Swarm Observation of Field‐Aligned Currents Associated With Multiple Auroral Arc Systems

Knudsen

Gillies

et al. 2017

JGR Space Physics

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Auroral arcs occur in regions of upward field‐aligned currents (FACs); however, the relation is not one to one, since kinetic energy of the current‐carrying electrons is also important in the production of auroral luminosity. Multiple auroral arc systems provide an opportunity to study the relation between FACs and auroral brightness in detail. In this study, we have identified two types of FAC configurations in multiple parallel arc systems using ground‐based optical data from the Time History of Events and Macroscale Interactions during Substorms all‐sky imagers, magnetometers and electric field instruments on board the Swarm satellites. In “unipolar FAC” events, each arc is an intensification within a broad, unipolar current sheet and downward return currents occur outside of this broad sheet. In “multipolar FAC” events, multiple arc systems represent a collection of multiple up/down current pairs. By collecting 17 events with unipolar FAC and 12 events with multipolar FACs, we find that (1) unipolar FAC events occur most frequently between 20 and 21 magnetic local time and multipolar FAC events tend to occur around local midnight and within 1 h after substorm onset. (2) Arcs in unipolar FAC systems have a typical width of 10–20 km and a spacing of 25–50 km. Arcs in multipolar FAC systems are wider and more separated. (3) Upward currents with more arcs embedded have larger intensities and widths. (4) Electric fields are strong and highly structured on the edges of multiple arc system with unipolar FAC. The fact that arcs with unipolar FAC are much more highly structured than the associated currents suggests that arc multiplicity is indicative not of a structured generator deep in the magnetosphere, but rather of the magnetosphere‐ionosphere coupling process.

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Section: Discussionmentioning

confidence: 99%

Swarm Observation of Field‐Aligned Currents Associated With Multiple Auroral Arc Systems

Knudsen

Gillies

et al. 2017

JGR Space Physics

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“…[54] In this paper, we present our recently developed techniques to calculate the RCMFL in the equatorial magnetosphere using multiprobe measurements under different probe geometries and exemplify the techniques in three THEMIS events occurring during a substorm expansion phase and early recovery phase (Event 1), a substorm recovery phase (Event 2), and a quasi steady interval (Event 3), respectively. Our developed technique of in situ RCMFL calculation has already found its application in a number of researches led by the authors and their colleagues recently [e.g., Donovan et al, 2012;Liang et al, 2013;Ge et al, 2012]. The in situ RCMFL may also have other potential uses in, e.g., the study of the ballooning instability which is by nature RCMFL dependent.…”

Section: Summary and Discussionmentioning

confidence: 99%

“…The estimation of ∂ B x /∂ z and in turn the RCMFL can be performed with two‐probe observations [ Donovan et al ., ; Liang et al ., ]. The two probes are required to be very close to each other in x distance yet moderately separated in z direction ( L z > Δz > > Δx in which L z denotes the thickness of the current sheet).…”

Section: Methodsmentioning

confidence: 99%

“…This geometry, as we shall explain in the next section, is favorable for an in situ calculation of the RCMFL in the equatorial magnetosphere. In earlier studies, the authors of this paper have already attempted to use THEMIS multiprobe observations to estimate the RCMFL in a couple of events to help infer the b2i boundary in the tail [ Donovan et al ., ] or the transition region from a quasi‐dipolar magnetosphere to a highly stretched current sheet [ Liang et al ., ]. In this paper, we shall make a comprehensive and systematic introduction to our recently developed techniques to calculate the RCFML in the equatorial magnetosphere under different satellite geometries.…”

Section: Introductionmentioning

confidence: 99%

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Multiprobe estimation of field line curvature radius in the equatorial magnetosphere and the use of proton precipitations in magnetosphere‐ionosphere mapping

et al. 2013

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[1] Nonadiabatic, chaotic motion of ions under a curved geomagnetic field geometry is well recognized as a key mechanism that leads to the pitch angle scattering of protons in the equatorial magnetosphere. The efficiency of such proton pitch angle scattering process is energy dependent and controlled by the radius of curvature of the magnetic field line (RCMFL). The protons scattered into the loss cone will precipitate into the ionosphere and excite proton auroras there. In this study, we propose techniques to calculate the RCMFL based upon multiprobe measurements under different probe geometries. We also demonstrate how to use the obtained RCMFL to evaluate the proton precipitation fluxes, and further use the proton precipitation features to help estimate the magnetosphereionosphere mapping. Our procedures are summarized as follows: (1) We calculate the RCMFL using Time History of Events and Macroscale Interactions during Substorms (THEMIS) multiprobe measurements. (2) With the obtained RCMFL, we evaluate the scattering efficiency and in turn the loss cone fluxes of the magnetospheric protons at different energy levels. (3) The above evaluation of the in situ loss cone fluxes is compared to the ionospheric measurements of proton precipitation fluxes in various energy ranges and/ or the optical proton auroras to estimate the possible footprint of THEMIS probes in the ionosphere.Citation: Liang, J., E. Donovan, E. Spanswick, and V. Angelopoulos (2013), Multiprobe estimation of field line curvature radius in the equatorial magnetosphere and the use of proton precipitations in magnetosphere-ionosphere mapping,

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“…That the free energy source for wave generation is this parallel current rules out the possibility of the instability being a lower hybrid instability, which is driven by diamagnetic currents in perpendicular direction [ Daughton , ; Gary , ]. The field‐aligned currents in the vicinity of the dipolarization fronts could potentially be related to the substructure parallel electron beam distribution observed near the fronts [ Liang et al ., ; Yao et al ., ]. Such instabilities, which relax by reducing the parallel current, have the potential to contribute to global energy dissipation.…”

Section: Introductionmentioning

confidence: 99%

Off‐equatorial current‐driven instabilities ahead of approaching dipolarization fronts

et al. 2017

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Recent kinetic simulations have revealed that electromagnetic instabilities near the ion gyrofrequency and slightly away from the equatorial plane can be driven by a current parallel to the magnetic field prior to the arrival of dipolarization fronts. Such instabilities are important because of their potential contribution to global electromagnetic energy conversion near dipolarization fronts. Of the several instabilities that may be consistent with such waves, the most notable are the current‐driven electromagnetic ion cyclotron instability and the current‐driven kink‐like instability. To confirm the existence and characteristics of these instabilities, we used observations by two Time History of Events and Macroscale Interactions during Substorms satellites, one near the neutral sheet observing dipolarization fronts and the other at the boundary layer observing precursor waves and currents. We found that such instabilities with monochromatic signatures are rare, but one of the few cases was selected for further study. Two different instabilities, one at about 0.3 Hz and the other at a much lower frequency, 0.02 Hz, were seen in the data from the off‐equatorial spacecraft. A parallel current attributed to an electron beam coexisted with the waves. Our instability analysis attributes the higher‐frequency instability to a current‐driven ion cyclotron instability and the lower frequency instability to a kink‐like instability. The current‐driven kink‐like instability we observed is consistent with the instabilities observed in the simulation. We suggest that the currents needed to excite these low‐frequency instabilities are so intense that the associated electron beams are easily thermalized and hence difficult to observe.

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Quasi-parallel electron beams and their possible application in inferring the auroral arc's root in the magnetosphere

Cited by 5 publications

References 57 publications

Swarm Observation of Field‐Aligned Currents Associated With Multiple Auroral Arc Systems

Swarm Observation of Field‐Aligned Currents Associated With Multiple Auroral Arc Systems

Multiprobe estimation of field line curvature radius in the equatorial magnetosphere and the use of proton precipitations in magnetosphere‐ionosphere mapping

Off‐equatorial current‐driven instabilities ahead of approaching dipolarization fronts

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