Multi-Scale Irregularities Product: a data product utilizing the high-resolution Swarm plasma density data for space weather applications

We use the high-resolution Swarm faceplate plasma density data at 16 Hz to develop a set of parameters that can characterize multi-scale ionospheric structures and irregularities along the Swarm orbit. We present the methods for calculating density gradients over different window sizes, rate of change of density index, power spectral density and the spectral slope at both low and high latitudes. The faceplate plasma data are not continuously available through the years. However, about 8 years of data from Swarm A are processed from late 2014 to the end of 2025. Some statistical results from Swarm A are presented. The variations of plasma structures and irregularities are dependent on solar activity, season, local time and geomagnetic activities, and the variations show different patterns between low and high latitudes. For example, the high-latitude ionosphere is characterized by persistent ionospheric structures and irregularities poleward of 60 magnetic latitude, while the low-latitude ionospheric irregularities are only dominant during 19-01 local time near the magnetic equator. The occurrence of steep spectral slope at high latitudes shows clear seasonal variations, i.e., it maximizes during local summer and minimizes during local winter in both hemispheres. However, the occurrence of steep spectral slope at low latitudes is only sensible when significant plasma structures and irregularities are present. We further calculate the histogram of spectral slopes at low latitudes when the rate of change of density index is enhanced. The histogram resembles a Gaussian distribution with an expected value of 1.97. The processed data are available to the wider community. Given the high resolution, this new data product will be useful for the scientific communities that are interested in the magnetosphere-ionosphere-thermosphere coupling and near-Earth space environment.

💡 Research Summary

The paper presents a new Swarm‑derived data product, the Multi‑Scale Irregularities Product (MUSIC), which exploits the 16 Hz face‑plate (FP) plasma density measurements from the Swarm constellation to quantify ionospheric structures and irregularities across a wide range of spatial scales. The authors begin by highlighting the limitation of the traditional Langmuir Probe (LP) data, which samples at 2 Hz and therefore can only resolve structures larger than ~7.5 km. In contrast, the FP current, converted to plasma density under the assumption of quasi‑neutrality, provides a sampling interval of 0.0625 s, corresponding to a spatial resolution of roughly 470 m for a satellite speed of 7.5 km s⁻¹.

Using eight years of Swarm‑A data (late 2014–2025), the authors develop three families of quantitative descriptors: (1) density gradients (∇Ne) computed via linear regression over running windows that correspond to horizontal scales of 5, 10, 20, 50 and 100 km; (2) the rate of change of density (ROD) and its standard‑deviation index (RODI) evaluated over 1 s, 5 s, 10 s and 20 s intervals, which capture rapid fluctuations and are especially sensitive to equatorial plasma bubbles; (3) power spectral density (PSD) and the associated spectral slope (p) derived from Fourier transforms over windows ranging from 8 s to 60 s. After testing several window lengths, the authors adopt a 10 s window with a 1 s time step (i.e., down‑sampling to 1 Hz) as the optimal compromise between statistical robustness and sensitivity to sub‑kilometer structures.

Statistical analyses are performed by binning the processed parameters into 1‑day and 2° quasi‑dipole latitude grids. The long‑term behavior of electron density, ∇Ne5 km, RODI1 s, and the occurrence frequency of steep spectral slopes (p > 2) is examined in relation to solar flux (F10.7), geomagnetic activity (Kp), season, local time (LT), and latitude. Key findings include:

• High‑latitude (|QD latitude| > 60°) irregularities are persistent throughout the year, with the occurrence of steep spectral slopes peaking during local summer and diminishing in winter. This seasonal modulation is evident in both hemispheres.
• Low‑latitude irregularities are confined to the magnetic equatorial region and are most pronounced between 19:00–01:00 LT. The density gradient ∇Ne5 km is enhanced during the daytime equatorial ionization anomaly (EIA), while RODI1 s spikes after sunset, coinciding with the appearance of plasma bubbles that reach the Swarm altitude.
• When the RODI index is elevated at low latitudes, the distribution of spectral slopes approximates a Gaussian with a mean of 1.97 and a modest standard deviation, indicating that the PSD follows a well‑defined power‑law under active irregularity conditions.
• Data availability is governed by the operational schedule of the Thermal Ion Imager (TII); the FP density is recorded only when TII bias is below –2.5 V. Consequently, the dataset shows a gap in continuous coverage from late 2019 to mid‑2023, with improved availability after mid‑2023.

The MUSIC product extends the capabilities of the earlier IPIR (Ionopsheric Plasma Irregularities) dataset, which was limited to 2 Hz sampling, by providing sub‑kilometer resolution and a richer suite of diagnostics. The authors argue that this higher‑resolution product will benefit a broad community interested in magnetosphere‑ionosphere‑thermosphere coupling, space weather forecasting, and the mitigation of GNSS scintillation effects. All processed data are made publicly available through the ESA Swarm data portal, ensuring that the scientific community can readily incorporate these new metrics into models and observational studies.