High‐Resolution Optical Observations of Neutral Heating Associated With the Electrodynamics of an Auroral Arc

Abstract

We present results that indicate the existence of two distinct neutral heating processes associated with a discrete auroral arc over Svalbard. Within the order of seconds, the thermospheric temperature profile displays a significant response to the arc on spatial scales smaller than 10 km. It is suggested that both heating signatures are associated with the electrodynamic system responsible for the formation of the arc. Pedersen currents produce a temperature increase of approximately 100 K, observed at altitudes between 80 and 160 kilometres, directly adjacent to the arc structure and on its poleward edge only. In contrast, field-aligned currents produce a variable temperature increase, of approximately 50 K, which is observed within the arc itself and constrained to a narrow altitude range between 90 and 110 km. By utilizing a range of observations and new analysis methods we are able to measure the atmospheric neutral temperature profile, over auroral altitudes, at unprecedented temporal and spatial scales. The High Throughput Imaging Echelle Spectrograph records high-resolution emission spectra of the aurora, which are then fitted with synthetic N 2 spectra, generated with modeled N 2 volume emission rate profiles and a library of trial temperature profiles. The N 2 volume emission rate profiles are retrieved from the Southampton ionospheric model using precipitating particle energies and fluxes obtained from Auroral Structure and Kinetics and the EISCAT Svalbard Radar. The application of this technique allows us to produce a time series of neutral temperature profiles and measure the localized heating of the neutral atmosphere resulting from the electrodynamics of the arc. Key Points: • Development of a new observing technique allows for the measurement of neutral temperature profiles during auroral events at an unprecedented temporal resolution • A case study of a morning sector auroral arc over Svalbard reveals a region of significant Joule heating on its poleward edge • Evidence of a separate Ohmic heating signature embedded within the arc and associated with strong field aligned currents Supporting Information: • Supporting Information S1

Multiscale Observation of Two Polar Cap Arcs Occurring on Different Magnetic Field Topologies

“…The contributions from OH, N 2 , N + , and emissions have been fitted and removed using methods outlined in Chadney and Whiter (2018) and Price et al. (2019), leaving just the H‐ α emission at 6563 Å (indicated by a vertical dashed line). These observations (as well as the red line) will be discussed in more detail in sections 3.2 and 4.3, but for now, we note the presence of a peak in the black line at 6563 Å which indicates H‐ α emission, and that the peak appears to be broader toward shorter wavelengths (a “blue wing,” which will be clarified later), consistent with downward accelerated protons.…”

“…As before, a fit for the spectra (excluding H‐ α ) has been obtained using methods outlined in Chadney and Whiter (2018) and Price et al. (2019), which has then been taken away from the integrated spectra. Similarly to the “closed” event, H‐ α emission is present, at 6563 Å, but here the emission is much narrower in its wavelength extent.…”

In Praise of the Unexpected

“…A statistical study was made on the sunlit N 2 + emission, comparing it with steady state theory, and finding a clear change in the rotational development of the molecular bands in different conditions (Jokiaho et al., 2009). New science is still emerging from the latest HiTIES data (D. Price, 2021; D. J. Price et al., 2019) in which the neutral temperature of the auroral thermosphere is found at spatial and temporal resolutions not before achieved.…”

“…New science is still emerging from the latest HiTIES data (D. Price, 2021; D. J. Price et al., 2019) in which the neutral temperature of the auroral thermosphere is found at spatial and temporal resolutions not before achieved. The information contained within the auroral spectrum keeps on being mined by using a judicious choice of filters and with new analysis techniques.…”

Colocated Observations of the E and F Region Thermosphere During a Substorm

“…When taking into account the neutral wind, Joule heating can vary significantly in both rate of heating (Lu et al, 1995) and global morphology (Billett et al, 2018). Neutral temperatures also fluctuate expectedly in response, rising on the order of hundreds of Kelvin over periods of hours in both the E and F regions (Kurihara et al, 2006; Maeda et al, 2005; Price et al, 2019).…”