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Modeling and Observations of the Effects of the Alfvén Velocity Profile on the Ionospheric Alfvén Resonator

Published: 25 January 2024

By Rosie Hodnett (University of Leicester)

The Ionospheric Alfvén Resonator (IAR) occurs when Alfvén waves partially reflect from boundaries in the ionosphere, towards the bottom of the ionosphere and above the F-region peak. The frequencies of the IAR are strongly controlled by the plasma mass density in the ionosphere, which is not uniform.

We have observed IAR in induction coil magnetometer data at Eskdalemuir, UK (BGS site), and extracted the harmonic frequencies for nine years of data. To model the harmonic frequencies, we used the International Reference Ionosphere and the International Geomagnetic Reference Field to model Alfvén velocity profiles. By solving a one-dimensional wave equation, we modelled the first five harmonics of the IAR for times where we had data. The wave structure of the electric field for a uniform case is shown in panel (a), and the resulting modelled harmonics for a non-uniform case is shown in panel (b). We modelled the frequencies with the lower boundary condition of the electric field of the wave being a node (shown in the figure below) and an antinode. By looking at the percentage difference between the fundamental frequency and the average separation of the harmonics (ζ) for both the node and antinode models and comparing this with the data, we find that the lower boundary is closest to being a node. ζ is presented for the node case, with UT, in panels (c) and (d), which show the data and the model respectively, binned by UT. The trend of increasing ζ towards midnight is due to changing Alfvén velocity profiles (shown in panel (e)), and suggests that the ionosphere is becoming more non-uniform. As such, measurements of IAR could be used to gain insight into the shape of the Alfvén velocity profile of the ionosphere.

Figure: (a) Shows the wave structure of the first three harmonics of the electric field for the IAR for a uniform Alfvén velocity profile, and (b) for a non-uniform Alfvén velocity profile, with a node at the lower boundary. (c) Shows the percentage offset of the fundamental frequency and the average harmonic frequency separation for the data, and (d) for the model, for the node case. For each day where there is data, the values are averaged into hourly bins of UT. Green shaded bins have a significant number of data points. (e) Shows average modelled Alfvén velocity profiles for 18:00 – 01:00 UT, normalised to their minimum value.

BGS induction coil magnetometer data, search for 'induction coil': https://webapps.bgs.ac.uk/services/ngdc/
accessions/index.html

See publication for further information:

Hodnett, R. M., Yeoman, T. K., Beggan, C. D., & Wright, D. M. (2024). Modeling and observations of the effects of the Alfvén velocity profile on the Ionospheric Alfvén Resonator. Journal of Geophysical Research: Space Physics, 129, e2023JA032308. https://doi.org/10.1029/2023JA032308

Topology of turbulence within collisionless plasma reconnection

Published: 21 December 2023

Bogdan Hnat (University of Warwick)

Collisionless magnetic reconnection [1] and plasma turbulence [2] are fundamental mechanisms that transfer energy across scales and between electromagnetic fields and particles. Stretched turbulent vortices and thin reconnection current sheets are prime sites of plasma heating and particle acceleration. Magnetic field line topology is central to both these processes.

We have classified the magnetic field topology observed as the four MMS spacecraft fly through a well resolved reconnection site. The MMS spacecraft separation defines a spatial 'yardstick', which is of order of the ion inertial range di, for sampling magnetic field topology. However, spatial variation of the topology is indirectly captured on a much finer spatial scale due to high time resolution of the magnetic field measurements, 8192 samples per second.

We find two distinct types of the magnetic field line topology near and at the electron dissipation region (EDR). At the edges of the EDR turbulent-like topology, identical to the topology of stretched vortices in hydrodynamic turbulence, is dominant. It coincides with large high-frequency electromagnetic perturbations. At the EDR the topology departs from turbulence and the structures appear to be two-dimensional, coinciding with suppression of electromagnetic fluctuations. The topology of the magnetic field line directly orders electron acceleration and heating. Suprathermal electrons are absent where turbulent-like topology dominates, but the bulk electron temperature anisotropy is enhanced. Reduced two-dimensional topology at the EDR coincides with the suprathermal electrons. The turbulent-like topology can arise in EMHD in scales smaller than electron inertial scale when vorticity dominates the dynamics. We find that vorticity is indeed dominant at all times within our interval.

"Panels — Panels (a), (b) and (c): Time series of in situ observations of: (a) the magnetic field magnitude (black) and the electric field component E_N (blue) in the event LMN coordinates, (b) band pass filtered magnetic field components (blue) and magnetic field magnitude (black) within frequency range 64-256 Hz; the dashed red vertical lines mark the outer extent of large magnetic field fluctuations, and (c) the same quantity as panel (b) calculated for the electric field fluctuations. All traces are based on the reconnection region transit seen by MMS 3. The EDR is indicated with blue shading on all panels. Green shading indicates the time interval in which at least one spacecraft samples the EDR. Panel (d) shows the phase space of invariants of magnetic field gradient tensor. Elliptic (flux ropes) and hyperbolic (X-point) magnetic field lines are separated by the magenta line. Panel (c) shows phase space of invariants for the curl-free deformations of magnetic field lines. The red line is the boundary of possible invariants. The magenta line in panel (c) corresponds to triaxial deformations with eigenvalue ratios -3:-1:4 (Rs<0) and 3:1:-4 (Rs>0), as found in the strain tensor of a three dimensional hydrodynamic flow.

References:
[1] J. Birn, E.R. Priest, Reconnection of Magnetic Fields: Magnetohydrodynamics and Collisionless Theory and Observations (Cambridge University Press, New York, 2007).
[2] Matthaeus, W.H. and Velli, M.,Space Science Reviews, 160(1), pp.145-168 (2011).

See publication for further information:
Hnat, Bogdan, Sandra Chapman, and Nicholas Watkins. "Topology of turbulence within collisionless plasma reconnection." Scientific Reports 13.1 (2023): 18665.

Plasma vorticity in the high-latitude ionosphere

Published: 30 November 2023

By Gareth Chisham (British Antarctic Survey)

Measurements of ionospheric plasma flow vorticity can be used for studying ionospheric plasma transport processes, such as convection and turbulence, over a wide range of spatial scales. This study presents an analysis of probability density functions (PDFs) of ionospheric vorticity for selected regions of the northern hemisphere high-latitude ionosphere as measured by the Super Dual Auroral Radar Network (SuperDARN) over a 6-year interval (2000-2005 inclusive). Making certain assumptions, the observed asymmetric vorticity PDFs can be decomposed into two separate components: (1) A single-sided function that results from the large-scale vorticity inherent in the ionospheric convection pattern, driven by magnetic reconnection; (2) A symmetric double-sided function that results from meso-scale vorticity that derives from fluid processes such as turbulence, and from measurement uncertainties.

Figure with three vertical panels. This figure demonstrates the decomposition of the probability density function (PDF) of ionospheric vorticity into its large-scale and meso-scale components. The data is from 73-77 degrees AACGM latitude and 0800-1100 MLT (dawn convection cell), for IMF By positive conditions, for the years 2000-2006 inclusive. (a) PDF of all the measured vorticity measurements; (b) Separated components of the PDF with model fits; (c) Percentage contribution of each component for different values of vorticity. — This figure demonstrates the decomposition of the probability density function (PDF) of ionospheric vorticity into its large-scale and meso-scale components. The data is from 73-77 degrees AACGM latitude and 0800-1100 MLT (dawn convection cell), for IMF By positive conditions, for the years 2000-2006 inclusive. (a) PDF of all the measured vorticity measurements; (b) Separated components of the PDF with model fits; (c) Percentage contribution of each component for different values of vorticity.

Being able to model ionospheric vorticity in this way will help to improve models of ionospheric plasma flow that are often used in larger-scale system models. At the present time, these plasma flow models typically only consider the larger-scale convection flow. Our observation of a significant meso-scale flow vorticity component due to turbulence will have implications for the fidelity of these models.

See paper for further details: Chisham, G. and Freeman, M. P. (2023). Separating contributions to plasma vorticity in the high-latitude ionosphere from large-scale convection and meso-scale turbulence. Journal of Geophysical Research: Space Physics, 128, e2023JA031885, https://doi.org/10.1029/2023JA031885.

Detection of the northern infrared aurora at Uranus using the W.M. Keck II Telescope and NIRSPEC instrument

Published: 16 November 2023

By Emma Thomas (University of Leicester)

Three decades of searching for the infrared aurorae finally come to a successful conclusion as portions of the northern (IAU southern) aurorae have been confirmed at Uranus. The icy planet represents an enigma within our solar system, with the first and only visit by Voyager II in 1986, it remains one of the least documented planets in our solar system. This is exceptionally apparent with the planet’s history of auroral observations, where the UV aurorae have been observed a handful of times but no infrared (IR) counterpart has been confirmed, despite both aurorae appearing at Jupiter and Saturn. Analysis of IR aurorae at both Jupiter and Saturn have challenged what we know about magnetosphere-ionosphere coupling, highlighting a need for IR analysis at Uranus to uncover its mysteries. Since 2020 our team has meticulously analysed archived data of Uranus during 2006 from the Keck II telescope on Mauna Kea in Hawai’i. The timing of these observations was key, close to equinox, as it provided an optimal view of the predicted locations of the northern and southern aurorae. By examining the emission lines from these aurorae (the emitting ion being H3+) between 3.94 to 4.01 μm, we carried out a full spectrum best fit across 5 fundamental lines for each spatial pixel across the planet’s disk. By comparing these lines at specific locations, we were able to identify an average 88% increase in column ion densities with no significant temperature changes localised close to or at expected auroral locations for the northern aurora. With this confirmation at Uranus, we look forward to a new age of auroral investigations at both ice giant planets.

Measured H3+ Q(1,0-) intensity mapped across the upper atmosphere of Uranus against Uranian latitude and arbitrary longitude, (b) Total H3+ Emission calculated from the temperature and column density (explained in detailed in the Methods), (c) Estimated temperatures of the H3+ emissions from all five Q-branch lines and (d) Estimated column densities of H3+ emissions from all five Q-branch. The latitude is planetocentric whereas the longitude is arbitrary due to the loss of the Uranian Longitude System (ULS) since Voyager II. The solid black lines mark out the boundaries of E1 (on the left) and E2 (on the right). Within the boundaries, the Enhanced regions are unshaded, the Dim regions are shaded with dots, and the Intermediate regions are shaded with diagonal lines. Latitudes and/or longitudes that were not recorded during the observations have been greyed out.

References:

Thomas, E.M., Melin, H., Stallard, T.S. et al. Detection of the infrared aurora at Uranus with Keck-NIRSPEC. Nat Astron (2023). https://doi.org/10.1038/s41550-023-02096-5

A Model of High Latitude Ionospheric Convection derived from SuperDARN radar EOF Data

Published: 25 October 2023

By Mai Mai Lam (British Antarctic Survey)

Variations in space weather in the ionized region of the Earth’s atmosphere (the ionosphere) can result in expansion of the atmosphere, increasing the atmospheric drag on objects, such as satellites, in the thermosphere. We aim to significantly improve the forecasting of the effects of atmospheric drag on satellites by more accurate modelling of space weather effects on the motion of ionized particles (plasma) in the ionosphere. We have developed a model of the variation in plasma motion using a small number of solar wind variables. The model was built using a solar cycle’s worth (1997 to 2008 inclusive) of 5-minute resolution Empirical Orthogonal Function (EOF) patterns derived from Super Dual Auroral Radar Network (SuperDARN) line-of-sight observations of the plasma motion in the high-latitude northern hemisphere ionosphere (Shore et al., 2021). The model is driven by four variables: (1) the interplanetary magnetic field component By, (2) the solar wind coupling parameter epsilon, (3) a trigonometric function of the day-of-year, and (4) the monthly solar radio flux at 10.7 cm (the F10.7 index). Our model is good at reproducing the original data set - if 0 indicates that there is no reproduction and 1 indicates exact reproduction, then our model scores 0.7. Data set reproduction is best around the maximum in the solar cycle and worst at solar minimum. This is mainly due to differences in the spatiotemporal data coverage between these times but possibly also due to the model’s specification of the physical processes coupling the Sun to the Earth’s ionosphere. Our model could easily be used to forecast the ionospheric electric field about 1 hour in advance, using the real-time solar wind data available from spacecraft located upstream of the Earth.

Comparison of the new Lam 2023 model velocities with the original radar EOF velocities. 5-min snapshots of the high-latitude flow in magnetic latitude (60 – 90 ºN) and magnetic local time (12 is towards the Sun), for times when Lam model can explain the variance of the original data very well (high P) and not very well (low P). The top row shows velocities at a time when the percentage of explained variance P is high (February 2001): (a) the SuperDARN radar EOF data patterns, (b) the Lam 2023 model. The bottom row is for a time of low P (June 1999): (c) the SuperDARN radar EOF patterns, (d) the Lam 2023 model. Colour is used to indicate speed.

References:
Lam, M. M., Shore, R. M., Chisham, G., Freeman, M. P., Grocott, A., Walach, M.-T., & Orr, L. (2023). A model of high latitude ionospheric convection derived from SuperDARN EOF model data. Space Weather, 21, e2023SW003428. https://doi.org/10.1029/2023SW003428

Shore, R. M., Freeman, M., Chisham, G., Lam, M. M., & Breen, P. (2022). Dominant spatial and temporal patterns of horizontal ionospheric plasma velocity variation covering the northern polar region, from 1997.0 to 2009.0 - VERSION 2.0 (Version 2.0) [Dataset]. NERC EDS UK Polar Data Centre. https://doi.org/10.5285/2b9f0e9f-34ec-4467-9e02-abc771070cd9

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Modeling and Observations of the Effects of the Alfvén Velocity Profile on the Ionospheric Alfvén Resonator

Topology of turbulence within collisionless plasma reconnection

Plasma vorticity in the high-latitude ionosphere

Detection of the northern infrared aurora at Uranus using the W.M. Keck II Telescope and NIRSPEC instrument

A Model of High Latitude Ionospheric Convection derived from SuperDARN radar EOF Data