A Glasgow geomagnetic observation of a solar flare

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Nugget
Number:	446
1st Author:	Hugh HUDSON,
2nd Author:	Graham WOAN, and Chris OSBORNE
Published:	March 13, 2023
Next Nugget:	TBD
Previous Nugget:	Particle Acceleration in Two Coronal Jets
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Contents 1 Introduction 2 Ionospheric Flare Detection 3 Flouting Lord Kelvin with SOL2022-05-10T13:55 4 Complexities 5 References

Introduction

This Nugget departs a little from the usual "gee whiz" reporting of a new discovery, because solar flare effects (SFEs) on the Earth's ionosphere had been reported (as a coincidence) with Carrington's original solar flare, SOL1859-09-01 (Ref. [1]). At the time of the discovery, illustrated in Figure 1, the descriptive name for the phenomenon was "magnetic crochet" because of its hook-like appearance on the time-series records of the Earth's magnetic field. Needless to say, this was so unexpected that had little credibility at the time, even though in hindsight this event was a milestone in space weather and in multi-messenger astronomy.

Ionospheric Flare Detection

The essence of this multimessenger event in 1859 was that the flare produced ionizing radiation, which substantially altered conditions in the Earth's ionosphere around the "subsolar" point. Nowadays we know about many specific observable ionospheric effects, all known by acronyms (SID in general for Solar Ionospheric Disturbance, but then SWF, SPA, SCNA, SFD, SEA, SDA, SES...; for example SCNA stands for Sudden Cosmic Noise Absorption, and there is eve such a thing as a "slow SCNA"). For a recent example see Ref. [2]. Most of these effects relate to radio-wave propagation and only became known when radio waves became useful (see for example the "Luxemburg Effect").

The geomagnetic crochet, or SFE, is a simpler thing, but already very complicated from the theory or modeling point of view. This Nugget simply notes that any high-quality geomagnetic recording can see effects much like the one detected at the time of the Carrington flare. In particular the student observatory at the University of Glasgow has a professional-grade magnetometer and detected the SFE associated with the flare SOL2022-05-10T13:58 (M1.5). (and there are hundreds of other similar magnetometers dotted around the globe, accessible via INTERMAGNET.

Flouting Lord Kelvin with SOL2022-05-10T13:55

The River Kelvin lent its name to Lord Kelvin, the University of Glasgow's great 19th-century physicist. In astronomical circles Lord Kelvin, despite his brilliance, was famous for getting things wrong. In his list of "harrumphs!" one can count not only space weather, our interest here, but also Darwinian evolution. In his defense, though, his harrumphs were always backed up by sound scientific principles.

So it is fitting Glasgow's observatory could detect a crochet. Figure 2 shows the surprisingly clean detection of SOL2022-05-10T13:55 (the red line) and also presents a bit of a mystery: why did the magnetometer signal not align better with the GOES peak time (blue line)?

The solution to the mystery that the Glasgow time stamp drifted signficantly up to 2022. It is now fixed, but without corrections yet for the earlier data. In the meanwhile, the Armagh site of the Irish magnetometer network saw the SFE as well (the black line) and found good agreement with the soft X-ray data from GOES.

Complexities

So, what is the physics behind an SFE? It is quite complicated and contains many elements totally unknown in 1859. First, the Earth's ionosphere, when sunlit, sustains two large-scale current vortices due to the dynamo action of upper-atmospheric winds. These are always present and produce a daily magnetometer signal. When a flare happens, the ionizing UV radiation from the Sun has a new EUV/X-ray enhancement, and this enables more current to flow by changing the conductivity. The magnetometer sees this excess current via the Biot-Savart law, and from Glasgow's northerly perspective, this produces a negative deflection of the total field. The Carrington event did the same for the Kew Gardens magnetometer in 1859, although London is somewhat south of Glasgow.

One can see all kinds of uncertainties here. Geographically, how does the dynamo action actually work, and does it vary from day to day?. Geomagnetically, how disturbed is the field by magnetospheric effects? From the solar point of view, what is the time-dependent spectrum of the new ionizing radiation? At the plasma-physics level, what species in the Earth's ionosphere get ionized, and how does the ionization relax? And is this in the D-region or the E-region of the ionosphere? Finally, is the timing discrpancy of Figure 2 real, or is the time stamp of the Glasgow data incorrect?

The conclusion is that the ionosphere is a great detector of solar and even cosmologically distant events (e.g., Ref. [3]), but the detections will be difficult to interpret quantitatively until some of the problems mentioned above get solved.

References

[1] "On the Great Magnetic Disturbance Which Extended from August 28 to September 7, 1859, as Recorded by Photography at the Kew Observatory"

[2] "Solar Flare Effects on the Earth's Lower Ionosphere"

[3] "Observation of an ionospheric disturbance caused by a gamma-ray burst"