Research PaperSolving the auroral-arc-generator question by using an electron beam to unambiguously connect critical magnetospheric measurements to auroral images
Introduction
One of the outstanding questions in magnetospheric physics (Denton et al., 2016; Lanchester, 2017; Denton, 2019; Borovsky et al., 2020a) is: what magnetospheric mechanisms produce the aurora in the upper atmosphere? Of the many different types of aurora, how the magnetosphere drives auroral arcs (discrete aurora) is a particularly longstanding and important mystery (Falthammar, 1977; Atkinson, 1978; Swift, 1978; Borovsky, 1993; Paschmann et al., 2002; Haerendel, 2011, 2012): competing theories for the generator mechanisms of quiescent auroral arcs in the equatorial magnetosphere have recently been reviewed by Borovsky et al. (2020b). By generating discrete aurora, the magnetosphere transfers some of its energy to the atmosphere. Because the mechanisms acting to power the aurora arcs are not known, the type of energy extracted from the magnetosphere is not known: it could be thermal energy from ions, thermal energy from electrons, magnetic energy, flow kinetic energy, Poynting flux of waves in the solar wind, etc. And because the form of energy extraction is not known, the impact of the aurora on the magnetospheric system is not known. Beyond auroral arcs, these same issues pertain to other discrete auroral forms.
Besides the importance of knowing (1) what causes aurora and (2) how aurora impact the magnetospheric system, there has long been a third quest (3) to use auroral observations as a window to observe the global operation of the magnetosphere (Akasofu, 1965; Mende, 2016a,b). To satisfy these desires, the space science community must learn what processes in the magnetosphere create the aurora.
The main reason for the lack of knowledge about how the magnetosphere drives auroral arcs is the lack of mapping knowledge between the magnetosphere and the ionosphere: we have auroral observations and we have spacecraft measurements from the equatorial magnetosphere, but we can't unambiguously connect them together. In the mapping, the important connection between the magnetosphere and the ionosphere is the magnetic-field-line connection: magnetic-field lines guide particle orbits (Feldstein and Galperin, 1993) and magnetic-field lines act as electrical transmission lines guiding current and the Poynting-flux transport of energy (Goertz and Boswell, 1979).
An important example of our poor state of understanding of the magnetosphere-ionosphere connection is the mapping of the observed low-latitude quiescent auroral arc (a.k.a. the “growth-phase arc”) into the magnetosphere. Connecting the near-Earth acceleration region of arcs to optical auroral observations has been straightforward (cf. Ono et al., 1987; Stenbaek-Nielsen et al., 1998a, Stenbaek-Nielsen et al., 1998b; Colpitts et al., 2013), but the mapping to the equatorial region where the arc generator operates is ambiguous. One school of thought says the low-latitude arcs magnetically connect into the dipolar portion of the nightside magnetosphere (McIlwain, 1975; Meng et al., 1979; Kremser et al., 1988; Mauk and Meng, 1991; Pulkkinen et al., 1991; Lu et al., 2000; Motoba et al., 2015) while another school believes that the these arcs magnetically connect into the stretched magnetotail (Yahnin et al., 1997, 1999; Birn et al., 2004a,b, 2012; Sergeev et al., 2012; Hsieh and Otto, 2014).
Magnetic mapping between the magnetosphere and the ionosphere is usually done with the use of a magnetic-field model of the magnetosphere (e.g. Tsyganenko and Usmanov, 1982; Tsyganenko, 1989; Tsyganenko and Sitnov, 2007; Sitnov et al., 2008). In these models, as solar-wind and geomagnetic-activity conditions change, the magnetic connection from a point in the nightside ionosphere out into the magnetosphere changes dramatically. Tests of the accuracy of these magnetic-field models in connecting the magnetosphere and ionosphere indicate that the errors are substantial (Thomsen et al., 1996; Weiss et al., 1997; Ober et al., 2000; Shevchenko et al., 2010; Nishimura et al., 2011), too large for the models to be useful for connecting observed auroral arcs with measurements in the magnetosphere. Further, the magnetosphere of the Earth exhibits short-timescale dynamics of the magnetic field are not captured in the parameterized field models.
To overcome these critical mapping issues, a research effort commenced in the 1990's at Los Alamos National Laboratory to develop a technology and a methodology to accurately and unambiguously connect magnetospheric spacecraft measurements to the visible aurora. The solution involves firing an energetic electron beam from a magnetospheric spacecraft, having the Earth's magnetic field guide the beam to the atmosphere, and imaging the optical spot of the beam in the upper atmosphere (Borovsky et al., 1998a; Borovsky, 2002; NASA, 2003, 2006; Delzanno et al., 2016; Borovsky and Delzanno, 2019 Sanchez et al., 2019), while overcoming the technological risks associated with that process (National Research Council, 2012). This paper discusses auroral arcs and current theories for their generation and outlines the mission concept to solve the auroral-arc problem, discussing technological tradeoffs in the experiment design.
This manuscript is organized as follows. In Section 2.1 the properties of low-latitude growth-phase auroral arcs are discussed and the properties of a “standard” auroral arc are collected and in Section 2.2 theories for the magnetospheric generation of auroral arcs are reviewed. In Section 3 the science mission concept to connect magnetospheric spacecraft measurements to aurora in the upper atmosphere is introduced. In Section 4 the tradeoffs between using a keV DC electron gun versus using an MeV radio-frequency accelerator are enumerated. Section 5 outlines the mitigation of beam-driven spacecraft charging. Section 6 studies problems associated with getting the electron beam from the spacecraft to the atmosphere: Section 6.1 deals with locating the direction of the atmospheric loss cone, Section 6.2 discusses beam stability, and Section 6.3 looks at angular scattering of the beam electrons by magnetospheric plasma waves and by field-line curvature. Section 7 overviews the optical detection and location of the beamspot by ground based cameras. Section 8 discusses orbit choices. Section 9 outlines the needed magnetospheric measurements to test the various auroral-arc-generator theories to determine the mechanisms operating to produce auroral arcs. The manuscript is summarized in Section 10. Additional science topics are enumerated in the Appendix.
Section snippets
Auroral-arc properties and auroral-arc generator theories
Auroral arcs in the auroral oval are curtain-shaped east-west aligned regions of optical emission from the upper atmosphere (Lessard et al., 2007; Karlsson et al., 2020). An auroral arc is associated with (1) accelerated energetic (keV) electrons precipitating from the magnetosphere into the atmosphere producing the optical emission, (2) an east-west aligned sheet of upward field-aligned electrical current carried by the precipitating electrons, (3) an inward-pointing electrostatic electric
Science mission concept
The concept of the mission (cf. Fig. 1) is an electron beam source mounted on a spacecraft in the equatorial region of the magnetosphere with an orbit with a 24-hr period. Through each 24-hr interval the spacecraft's magnetic footpoint in the upper atmosphere will wander across an array of ground based optical cameras in the auroral zone. The firing of the electron beam into the atmospheric loss cone will deposit energy in the atmosphere, producing an optical spot marking the location of the
A 10's-of-keV electron gun versus an MeV electron accelerator
The technology of operating high-power electron beams in space has been verified: electron guns with powers of 30 kW (O’Neil et al., 1978) and 40 kW (McNutt et al., 1995) have been operated, beams with currents of up to 18 A have been flown (Rappaport et al, 1993) and gun voltages of up to 45 kV (Winckler et al., 1975) have been flown. A 1-MeV radio-frequency proton accelerator has flown in space (Nunz, 1990; O'Shea et al., 1991; Pongratz, 2018) and a design is being prototyped for a
Spacecraft charging mitigation
Although using electron beams to trace magnetic field lines is a several-decades-old idea, it has never been realized in practice because of fear of catastrophic spacecraft charging that could be induced by a high-power electron beam in the low-density environment of the magnetosphere. A call had been made in the most-recent National Academies decadal survey to solve this problem (National Research Council, 2012). In order to get an estimate of the problem, we can consider a characteristic
Getting the beam to the atmosphere
To maximize the fraction of the electron beam entering the atmosphere, the beam must be injected into a geometrical region known as the atmospheric loss cone and must propagate far enough to reach the atmosphere. Beams with energies of up to 40 kV were propagated long distances through the magnetosphere in the Echo series of experiments (Hallinan et al., 1990; Winckler, 1992) and electron beams of 27 kV, 0.5 Amp and 15 kV, 0.5 Amp on the two ARAKS experiments were propagated 8.2 RE through the
Beamspot detection
For magnetic-field-line mapping using an electron beam, detection of the beam footpoint in the upper atmosphere is fundamental. As previous lower-energy experiments have demonstrated (e.g. Davis et al., 1980; Hallinan et al., 1990), the beam can be detected by means of its optical signature. As an example, if the velocity vectors of a 1-MeV beam of electrons are exactly aligned with the Earth's magnetic field at the top of the atmosphere, the 1-MeV electrons will deposit their energy in a
Orbit choices
Numerous orbits have been analyzed for an auroral-electron-beam mission. Focusing on the ground-based camera network situated in Canada (Spanswick et al., 2018), most of the orbits considered feature periodic revisits of the magnetic footprints over the TREx observing array. One of the most-promising orbits is the so-called “inclined geosynchronous” orbit. This orbit is a “5 × 8” elliptical 24-hour (sidereal) orbit with 12° inclination. (5 × 8 means perigee is at 5RE and apogee is at 8RE.) Note
Magnetospheric measurements needed
To determine the magnetospheric driver mechanisms of auroral arcs, spatial gradients in the magnetosphere must be measured as the illuminated spacecraft footpoint crosses the optical auroral arcs. In particular the following gradients must be measured: the ion plasma pressure, the electron plasma pressure, the ion and electron temperature anisotropy, the plasma mass density, the magnetic-field strength, and the plasma flow vector, with the gradients in the plasma flow vector v taking the forms
Summary
A mission has been outlined to provide unambiguous measurements that will bring closure to the important question: How does the magnetosphere generate auroral arcs? Answering this question will allow us to discern the impact that the aurora has on the evolution and dynamics of the magnetosphere and will allow us to look at the aurora and discern what it tells us about processes ongoing in the magnetosphere.
Decades of research went into the design of this mission. The core of the experiment is
Acknowledgements
The authors thank Matt Argall, Joachim Birn, Mick Denton, Eric Donovan, John Dorelli, Phil Fernandes, Gerhard Haerendel, Larry Kepko, Dave Knudsen, Oleksandr Koshkarov, Brian Larsen, Omar Leon, John Lewellen, Liz McDonald, Jeff Neilson, Antonius Otto, Noora Partamies, John Raitt, Geoff Reeves, Vadim Roytershteyn, Jan Sojka, Emma Spanswick, Maria Usanova, Hans Vaith, Jesse Walsh, and Kateryna Yakymenko. Work at the Space Science Institute was supported by NASA Heliophysics LWS TRT program via
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