Ionospheric Plasma Structuring in Relation to Auroral Particle Precipitation

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The aurora can be seen as the signature of direct coupling of the ionosphere and magnetosphere. 26 During high geomagnetic activity, energetic particle precipitation leads to higher intensity of 27 the aurora resulting in different auroral forms. Dynamical processes in the E-and F-regions 28 of the ionosphere are often associated with instabilities and turbulence which result in plasma 29 structuring and irregularities at various scales. Such irregularities in ionospheric plasma density 30 have impact on the propagation of radio waves (e.g., Keskinen and Ossakow, 1983; Huba et al., 31 To study whether plasma structuring is driven by particle precipitation, we investigate the relative 142 with c being the speed of light, ρ the ray path (e.g., Kintner et al., 2005Kintner et al., , 2007. 143 The phase ϕ is connected to time delay ∆t and therefore to electron density variations along the (4) 160 (e.g., Briggs and Parkin, 1963; Yeh and Liu, 1982). The S 4 index describes and is effected by 161 irregularities in a range of hundreds of meters to meters (at and below the Fresnel radius) (e.g., 162 Basu et al., 1998; Kintner et al., 2007).

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In this study, we use the calibrated 60-seconds reduced data (Oksavik, 2020a)  a,d,g) and negative drops of the local B x component (shown in Figure 2 panels a,e,i). For compa-203 rability of the cases events with available data between 18-24 UT (21h-03h MLT nightside) were 204 selected, this means the data set has been reduced to find times at which all three ASC recorded  material. This indicates that the signal is impacted right at the altitude of green auroral emission.

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This behavior has not been detected in the same extent with regards to red auroral emissions.

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The emission altitude of green auroral emissions is used as the piercing point altitude for the 256 study along with ASC images of the green auroral emissions. The phase scintillation data is referred to as slightly elevated σ ϕ above 0.2 rad, moderately elevated 262 σ ϕ above 0.3 rad, strongly elevated σ ϕ above 0.5 rad and very strongly elevated σ ϕ above 0.7 rad.

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Not all of the measured data during auroral activity of that day is shown: very faint aurora, forms of the aurora may be influenced by the way the camera and brightness is calibrated and calculated.

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They are however a good measure for comparison between and relation to the σ ϕ indices.

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For the first band elevated σ ϕ values are observed also at the west-ward boundary and within the 415 form, see Figure 5b. When the auroral bands fade out, elevated σ ϕ values are still observed over 1 to 7 minutes (first and second example respectively), see Figure 5(c,d & g,h).

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In summary, we observed: (1) Elevated phase scintillation indices correspond consistently well 418 to the spatial and temporal evolution of auroral forms in the green emissions (oxygen, 557.7 nm) 419 altitudes, which means particle precipitation into the ionospheric E-region is a driver for plasma 420 structuring.
(2) There may be a time delay between the temporal evolution of aurora (f.e. com-421 mencement and fading of auroral activity) and elevated phase scintillation index measurements. been studied and referred to as a blobs type II by Jin et al. (2016). They also state that soft F-region 438 particle precipitation does not contribute much to plasma structuring processes in the nightside au-439 roral region. Our findings are in agreement with this, but we however show that more energetic 440 particle precipitation penetrating down to the E-region may be a main source and is found co-441 located with intense elevated σ ϕ . The link between elevated σ ϕ or phase scintillations and E-region

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The energy source for driving instabilities in the E-region ionosphere can be manifold, such from 502 as flow shears, from gradients or directly by kinetic energy. Instabilities associated with particle 503 precipitation are e.g. kinetic instabilities, two-stream instabilities. The flow of particles in field-504 aligned currents can also produce current-driven instabilities (e.g., Kropotkin, 2016).

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One instability that can be directly produced by a velocity shear (by particle precipitation/ elec-  In this study, the relation between auroral particle precipitation and plasma structuring was inves-536 tigated. In summary, the temporal and spatial evolution of auroral forms and phase scintillation in-537 dices were studied. For this, three event days with similar background conditions (substorm events,

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-(2) There may be a time delay between the temporal evolution of aurora (e.g. commencement 548 and fading of auroral activity) and elevated phase scintillation index measurements. Particle pre-549 cipitation enhances the plasma density. When the precipitation declines or moves, it will still 550 take some time for the plasma structures to diffuse. Until then, instabilities will further cause 551 redistribution of energy and irregularity dissipation.  The irregularities and instabilities causing the elevated phase scintillation indices especially in 556 the E-region may be due to instabilities which are driven by energy at the boundary of auroral 557 forms, such as the Kelvin-Helmholtz instability (directly produced by a velocity shear such as 558 from particle precipitation) or Farley-Buneman instability (through fast flows at the boundaries).

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Plasma structuring may predominately be observed on pole-ward boundary as the gradient in 560 plasma density is larger than it is on the equator-ward boundary.

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The question on why plasma structuring processes in the E-region are observed specifically at 562 the edges of auroral forms, such as spirals and bands, and at polew-ard boundaries for auroral arcs 563 remains open. Further case studies with even higher spatial and temporal resolution and bigger sta-564 tistical studies investigating time-delay statistics are needed to understand the structuring process.

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In the future we also need to investigate further, which instabilities are caused during the plasma 566 structuring processes and how they effect trans-ionospheric radio waves.