High‐Resolution Poynting Flux Statistics From the Swarm Mission: How Much Is Being Underestimated at Larger Scales?. Issue 7 (17th July 2022) - Billett, D. D.
Abstract: Underestimation of the transfer of energy between the magnetosphere and ionosphere, the Poynting flux, is a persistent issue in space weather studies and the high‐latitude ionospheric models. Thought to be due to the inability to resolve small‐scale fluctuations of the ionospheric electric field, this underestimation could lead to significant further underestimations in parameters such as the thermospheric mass density and consequential satellite drag. Utilizing 16 Hz ion velocity and magnetic field measurements from the Swarm satellite mission, we examine the observed Poynting flux due to electric field fluctuations on very small spatial scales (∼1 km), and then artificially smooth the data to increase the observed scale. We quantify the decrease of integrated Poynting flux, poleward of 60/‐60° geomagnetic latitude, with increasing spatial scale. The decrease can be underestimated by as much as 15% by increasing scale from 1 km to only 8.6 km, or 16–2 Hz equivalent, with upward Poynting flux decreasing significantly faster. Our results thus point to a significant Alfvén wave driven component of the Poynting flux on kilometer scales. Additionally, we observe a northern hemisphere preference for increased Poynting flux, of which we examine its dependence on scale size and interplanetary magnetic field. Plain Language Summary: At Earth's high‐latitudes, energy from space weather enters the upper atmosphere (>100 km) and is deposited mostly as heat. This heat can have numerous knock‐on effects on the atmosphere, such as causing the air density to increase, which poses a risk to satellites orbiting at the same altitude. It is thus very important to accurately quantify the energy that enters the atmosphere from space, but it has been found that a large proportion of the energy is released on very small spatial scales (on the order of kilometers). These scales are often difficult to measure due to resolution limitations of most instruments, however, the Swarm constellation of satellites are equipped with electric field instruments that can retrieve measurements 16 times per second (16 Hz), equivalent to observing spatial scales of around 1 km. In this study, we use nearly 7 years of high‐resolution Swarm data to calculate the average space weather energy input, known as the Poynting flux, and investigate the effect of artificially smoothing out the data to simulate larger spatial scales. We find significant underestimations of Poynting flux at larger scales compared to the smaller ones, which stresses the importance of small‐scale measurements in estimating the space weather—atmosphere energy budget. Key Points: High‐resolution Swarm satellite data is used to examine Poynting flux across various spatial scales statistically Poynting flux decreases significantly with increasing spatial scale, dropping faster at scales under 10 km Area Integrated Poynting flux is 9–28% larger in the northern hemisphere than in the south