MIST

by Neil Rogers (Lancaster University)

Strong electrical currents in the Earth’s ionosphere and magnetosphere can produce geomagnetically induced currents (GIC) in ground-based infrastructure, such as electricity cables. For extreme conditions this can lead to instability and failure of the electricity supply. The magnitude of these currents is proportional to the rate of change of the horizontal geomagnetic field, dB_H/dt.  Climatological statistics of |dB_H/dt| may be combined with models of ground conductivity and impedances in the electricity network to evaluate the risk of GICs.     

Using 1.9 billion measurements from 125 magnetometers worldwide we fitted Generalised Pareto (GP) distributions to occurrences of dB_H/dt above the 99.97^th percentile (P_99.97). By extrapolating the GP tail distributions we predicted the magnitude of dB_H/dt expected every 200 years. This is shown in Figure 1a (with 95% confidence intervals) as a function of corrected geomagnetic (CGM) latitude. 

Figure 1. a) 200-year return levels for |dB_H/dt|. b) Occurrence probabilities of |dB_H/dt| > P_99.97 vs CGM latitude and MLT.

The sharp increase near 53° CGM latitude suggests that the largest |dB_H/dt| result from substorm expansions in a greatly expanded auroral region. Figure 1b presents the occurrence probability of |dB_H/dt| > P_99.97 vs latitude and magnetic local time (MLT). In the auroral zones this maximises in the hours before midnight due to substorm activity and in the 3-10 MLT sector due to ULF wave activity. Poleward of the dayside cusp region (~77° CGM latitude) occurrence rates increase near local noon, in summer, and under northward interplanetary magnetic field (IMF), indicating a relation to magnetospheric tail-lobe reconnection. At latitudes below 40° most occurrences were related to Sudden Commencements, the effect of shock fronts arriving in the solar wind. 

This study models extreme dB_H/dt as functions of latitude, MLT, month, and compass direction for return periods up to 500 years, and examines the effect of IMF orientation. The results demonstrate the response of the geomagnetic field to different drivers, and have significant potential in advancing modelling of GIC hazards.

For more information, please see the paper:

Rogers NC, Wild JA, Eastoe EF, Gjerloev JW & Thomson AWP. 2020. A global climatological model of extreme geomagnetic field fluctuations. J. Space Weather Space Clim. 10, 5. https://doi.org/10.1051/swsc/2020008

By Karl Bolmgren (University of Bath)

The ionosphere, the electrically charged upper atmosphere, has important effects on technologies like radio communication and satellite-based positioning. For high-accuracy positioning using Global Navigational Satellite Systems (GNSS), such as the Global Positioning System (GPS), ionospheric models are often used to estimate the ionospheric effect on satellite to ground communication. This effect is determined by the ionospheric electron content, and sudden changes or disturbances in the electron content can be challenging to include in such models.

A common type of disturbance called Travelling Ionospheric Disturbances (TIDs) are caused by gravity waves in the ionosphere, which are present all over the globe. They can be observed as wave-like fluctuations in Total Electron Content (TEC) and come in widely different spatial and temporal scales. The largest TIDs are generally caused by geomagnetic storm activity, while the more common, smaller TIDs can be caused by activity in the neutral atmosphere, like thunderstorms, perturbations from earthquakes or tsunamis, and the sudden temperature gradients associated with the solar terminator. In order to improve existing models and learn more about TIDs, we need reliable methods to study them.

Figure: Cross-section electron density of a modelled TID used to evaluate the tomographic images. For this particular simulation, a horizontal wavelength of 700 km, an initial perturbation speed of 20 m/s, and a period of 30 min was used.

Computerised ionospheric tomography is a powerful tool to image the ionosphere. Tomography is a technique used to reconstruct the 3D structure of an object from integrated measurements and is commonly used in e.g. medical imaging. In ionospheric tomography, the 3D ionospheric electron density is reconstructed from integrated measurements of TEC. We have used simulated TIDs to test how well ionospheric tomography can be used to image different scales of TIDs, and an example of a simulated TID is shown in the figure. We showed that incorporating geostationary satellites can significantly improve the imaging of TIDs. The imaging technique has significant implications for how we observe and investigate ionospheric features, such as TIDs, and presents a method to incorporate these phenomena into existing ionospheric delay correction techniques for applications like GNSS.

For more in details, please see:

Bolmgren, K., Mitchell, C., Bruno, J., & Bust, G. (2020). Tomographic imaging of traveling ionospheric disturbances using GNSS and geostationary satellite observations. Journal of Geophysical Research: Space Physics, 125, e2019JA027551. https://doi.org/10.1029/2019JA027551  

By Chris Lorch (Lancaster University)

Jupiter’s large, rapidly rotating magnetosphere is highly influenced by the presence of a global, centrifugally confined current disk comprised of co-rotating plasma from the volcanic moon Io. Azimuthal and radial currents flow through this current disk closing via magnetic field aligned currents (FACs), which are associated with Jupiter’s main auroral emission. These currents arise from dynamical processes within the magnetosphere, driven by the transport and circulation of Iogenic plasma. Characterising the structure and asymmetries in this current system is key to deciphering the dominant drivers of Jupiter’s magnetosphere-ionosphere (MI) coupled system and the behaviour of its plasma disk. 

Previous work by Khurana [2001] examined the solar wind influence on Jupiter’s magnetosphere using equatorial maps of Jupiter’s current disk and the results demonstrated clear azimuthal asymmetries fixed with local time (LT). However, the analysis was limited the lack of spacecraft coverage in the dusk – dayside magnetosphere provided by the Galileo spacecraft.  

Figure 1: Equatorial maps detailing the structure of Jupiter’s current disk with Jupiter located at (0,0). Dashed circles and radial lines represent 20R_J and 1 hr local time boundaries respectively. A compressed bow shock and magnetopause (Joy et al [2002]) are shown as black solid lines. a) The radial height-integrated current density, warmer (cooler) colours are indicative of current flowing radially outwards (inwards). b) The azimuthal height integrated colour density, flowing in the direction of corotation. c) The divergence of the perpendicular height-integrated current density, indicating the location of upward and downward FACs. Warmer (cooler) colours indicate the presence of upward (downward) FACs.

Lorch et al. [2020] determines the structure of the current disk at all LTs by considering magnetometer data at Jupiter from every available spacecraft, including Juno, up to 28 July 2018.  We apply an automated identification tool to magnetometer data from Jupiter’s current disk in conjunction with updated models of Jupiter’s intrinsic magnetic field and current disk geometry. In total, we identify 7382 lobe traversals, calculating the associated height-integrated current density for each crossing.  Additional coverage provided by the later half of the Galileo mission and the recent Juno mission allowed us to map all LTs. Asymmetries exist in both the radial (Figure 1a) and azimuthal (Figure 1b) currents into 20 R_J. Furthermore, we quantify the structure of upward and downward FACs in previously unmapped regions of Jupiter’s magnetosphere (Figure 1c). We find a positive net current density of 1.87 MA / R_J², suggesting unmapped currents must close either down-tail or along the magnetopause. Our results demonstrate important asymmetries in Jupiter’s current systems that play a crucial role in the MI coupled system. Amalgamating these results into future MI coupling models has the potential to remove discrepancies between model predictions and observations.

For more information, please see the paper:

Lorch, C. T. S., Ray, L. C., Arridge, C. S., Khurana, K. K., Martin, C. J., & Bader, A. (2020). Local time asymmetries in Jupiter's magnetodisc currents. Journal of Geophysical Research: Space Physics, 125, e2019JA027455. https://doi.org/10.1029/2019JA027455 

By David Stansby (MSSL, UCL) 

The solar wind is a continuous flow of plasma from the surface of the Sun, flowing out into interplanetary space. Faster solar wind is known to originate in large coronal holes, but slower solar wind has a wide range of different sources. A subset of slow solar wind is filled with pure Alfvén waves, much like the fast solar wind, suggesting it has a similar origin in coronal holes.

In our study we tested this theory of coronal hole origin, using Helios in-situ measurements of the solar wind at 0.35 AU. Figure 1 shows a wide range of plasma properties in typical fast solar wind (black), highly Alfvénic slow solar wind (blue), and non-Alfvénic slow wind (red).

Figure 1: In-situ solar wind properties for three intervals of solar wind, measured at 0.35 AU. The three types of solar wind are Fast (black), Slow Alfvénic (blue), and Slow non-Alfvénic (red).

The fast and Alfvénic slow wind had similar

• Alpha particle abundances
• Alpha particle drift speeds
• Alpha particle temperature anisotropies
• Alpha to proton temperature ratios (in both parallel and perpendicular directions) 

These similarities imply that the heating and acceleration mechanisms of fast and slow Alfvénic solar wind are qualitatively similar, acting in the same way on protons and alpha particles. This agrees with the theory that slow Alfvénic wind originates in coronal holes, like fast solar wind.

In contrast, the fast and Alfvénic slow wind had different

• Electron, proton, and alpha particle temperatures
• Mass fluxes

These differences can be explained by different magnetic field geometries in the low corona: slower wind is released on magnetic field lines which undergo more expansion in the corona, increasing the mass flux and reducing the temperatures. This implies that whilst slow Alfvénic wind originates in coronal holes, it most probably originates in small coronal holes.

For more information, please see:

The origin of slow Alfvénic solar wind at solar minimum, Monthly Notices of the Royal Astronomical Society 492, 39–44 (2020), D Stansby, L Matteini, T S Horbury, D Perrone, R D’Amicis, L Berčič, https://doi.org/10.1093/mnras/stz3422