Ionospheric Electron Heating Associated With Pulsating Auroras: Joint Optical and PFISR Observations

Abstract

In a recent study, Liang et al. (2017, https://doi.org/10.1002/2017JA024127) repeatedly identified strong electron temperature (T e ) enhancements when Swarm satellites traversed pulsating auroral patches. In this study, we use joint optical and Poker Flat Incoherent Scatter Radar (PFISR) observations to further investigate the F region plasma signatures related to pulsating auroras. On 19 March 2015 night, which contained multiple intervals of pulsating auroral activities, we identify a statistical trend, albeit not a one-to-one correspondence, of strong T e enhancements (~500-1000 K) in the upper F region ionosphere during the passages of pulsating auroras over PFISR. On the other hand, there is no discernible and repeatable density enhancement in the upper F region during pulsating auroral intervals. Collocated optical and NOAA satellite observations suggest that the pulsating auroras are composed of energetic electron precipitation with characteristic energy >10 keV, which is inefficient in electron heating in the upper F region. Based upon PFISR observations and simulations from Liang et al. (2017) model, we propose that thermal conduction from the topside ionosphere, which is heated by precipitating low-energy electrons, offers the most likely explanation for the observed electron heating in the upper F region associated with pulsating auroras. Such a heating mechanism is similar to that underlying the "stable auroral red arcs" in the subauroral ionosphere. Our proposal conforms to the notion on the coexistence of an enhanced cold plasma population and the energetic electron precipitation, in magnetospheric flux tubes threading the pulsating auroral patch. In addition, we find a trend of enhanced ion upflows during pulsating auroral intervals.On the other hand, if the downgoing magnetospheric electrons include a component of substantially "colder" electrons, for example, on order of~10 eV or less, electron heating in the upper ionosphere may operate in a fundamentally different way. The "cold" magnetospheric electrons cannot penetrate deeply into the ionosphere, but can strongly heat electrons near the topside boundary of the ionosphere. The heat will be conducted toward lower altitudes into the F region ionosphere. In this scenario, the T e enhancement seen in the F region ionosphere is a remote effect, meaning that the low-energy magnetospheric electrons do not actually penetrate to F region altitudes, so no significant ionization or red-line auroral excitation is LIANG ET AL. 4430

A New Technique for Estimating the Lifetime of Bursts of Electron Precipitation From Sounding Rocket Measurements

“…The lack of observed excess redline emission suggests redline temperatures at higher altitudes are at or below what ERPA measured. We do note that the magnitude of the expected temperature enhancements below 300 km is in the range of those recently reported by Liang et al () from ground‐based studies from Poker Flat, Alaska.…”

Diffuse and Pulsating Aurora

“…These authors further suggested that ion outflow could provide the requisite cold plasma, though no observations of upflow were presented in that paper. Evidence of PFISR observations of ion upflow (not outflow) over pulsating aurora are presented by Liang et al (2018), which they attribute to the deposition of heat flux from higher altitudes that lead to the formation of an ambipolar field and subsequent ion upflow. Kenward et al (2019) also used PFISR observations, in conjunction with allsky imagers filtered at 557.7 and 630.0 nm and were able to show that the soft precipitation directly heated the electron population in the F-region.…”

“…While it is still unknown how such cold plasma regions form and regulate auroral pulsation, satellite observations have shown modulating low-energy plasma fluxes that suggest to originate as plasma outflows from the ionosphere and to modulate wave growth rates Liang et al, 2015). Substantial ionization and heating in the ionosphere are associated with pulsating aurora (Hosokawa and Ogawa, 2015;Liang et al, 2018), and such processes may be the source of plasma outflow. Waves going to the southern (northern) hemisphere interact with electrons moving to the northern (southern) hemisphere.…”

“…As a result, with appropriate assumptions they can estimate the ion velocities in the F-region, and neutral-wind velocities in the E-region. Using electron density profiles from ISRs it is possible to estimate Hall and Pedersen conductivities in the ionosphere (Hosokawa et al, 2010;Kirkwood et al, 1988), energy spectra of precipitating electrons (Fang et al, 2010;Semeter and Kamalabadi, 2005;Sivadas et al, 2017), ionospheric electron heating (Liang et al, 2018), and Hall and Pedersen currents. Atmospheric radars such as PANSY in Antarctica (Sato et al, 2014) can be used to identify deep mesospheric ionization during diffuse or pulsating aurora (Kataoka et al, 2019).…”

Potential Association Between the Low-Energy Plasma Structure and the Patchy Pulsating Aurora

“…Liang et al (2017; investigated the electron temperature (T e ) enhancement in the upper/topside ionosphere associated with PPAs. Based upon Swarm satellite measurements (Liang et al, 2017) and Poker Flat Incoherent Scatter Radar (PFISR) measurements (Liang et al, 2018), a strong T e enhancement in the region of PPAs is found. The pulsating auroral precipitation itself that produces the optical luminosity usually consists of ∼10 keV or even higherenergy electrons, which is ineffective in heating electrons in the upper/topside ionosphere.…”