MIST

By Aisling Bergin (University of Warwick)

Geomagnetic indices, based on magnetic field observations at the Earth's surface, provide almost continuous monitoring of Earth’s magnetospheric and ionospheric activity. We analyze two geomagnetic index time series, AE and SMR, which track activity in the auroral region and around the Earth's equator, respectively. We show here that quantiles of the index distributions track solar cycle variation over solar cycles 21–24. The question is then how the likelihood of events varies with solar cycle activity.

In this paper, events are defined as bursts or excursions above a threshold which is either (i) a fixed value or (ii) a quantile of the distribution of the observed index values. We study the solar cycle dependence of the distributions of the burst return periods, R, and the burst durations, τ. A result from the theory of level crossings (LC) [1] constrains how , the ratio of the mean burst duration to return period, depends on the underlying empirical distribution of the observed quantity.

Our main results are as follows:

1. At fixed value burst thresholds, is peaked in the declining phase for AE annual samples and follows the sunspot number double peak for SMR.
2. Bursts are identified in samples at three distinct phases of the solar cycle. At fixed quantile thresholds the distributions of τ and R fall on single empirical curves for each of (i) the AE index at solar minimum, maximum, and declining phase and (ii) the SMR index at solar maximum. This goes beyond the constraint on average from LC theory.
3. The tail of the empirical cumulative distribution functions of the observed values of the AE and SMR indices collapse onto common functional forms specific to each index and cycle phase when normalized to the first two moments of their exceedance distributions.

Taken together, these results may combine to offer important constraints in the quantification of overall space weather activity levels.

Please see the paper for full details: Bergin, A., Chapman, S. C., Moloney, N. R., & Watkins, N. W. (2022). Variation of geomagnetic index empirical distribution and burst statistics across successive solar cycles. Journal of Geophysical Research: Space Physics, 127, e2021JA029986. https://doi.org/10.1029/2021JA029986

[1] Lawrance, A., & Kottegoda, N. (1977). Stochastic modelling of riverflow time series. Journal of the Royal Statistical Society: Series A, 140(1), 1–31. https://doi.org/10.2307/2344516

By Nahid Chowdhury (University of Leicester)

Planetary parameters at Saturn have exhibited mysterious time variabilities and periodicities for almost two decades. After the first Voyager measurements of the 1980s led to a determination of a rotation rate for the ringed planet, the advent of the Cassini mission in the 2000s showed that the measured length of a day at Saturn was subject to change. Numerous theories were proposed to explain the time variabilities seen in planetary parameters ranging from the radio emission through to the energies of neutral atoms and these fell into one of two general categories. The first was that a driving mechanism for the observed periodic behaviour would be situated externally to the planet, within the magnetosphere. The second was that a driving mechanism would be situated within the planet’s atmosphere.

Using infrared observations of Saturn’s northern polar auroral H3+ emission taken with Keck-NIRSPEC in 2017, we investigated ion flows in our search for a signature of the twin-vortex mechanism in the planet’s upper atmosphere that was purported to drive the observed periodicities. H3+ is an ion found in the ionosphere of gas giant planets and measurements of its line-of-sight velocity are indicative of the more general motion of constituents in the planet’s atmosphere.

By grouping our spectral data into quadrants of local northern planetary magnetic phase, we set up our experiment to probe the ion flows driven by the planetary period current. This led to the astonishing detection of the proposed ionospheric twin-vortices that are considered to be responsible for the periodicities witnessed throughout Saturn’s planetary and magnetospheric environments. Our observations show that local atmospheric weather effects at Saturn drive the twin-vortex flows while also generating some auroral emissions. This result has far-reaching implications for both other planets in our Solar System and exoplanetary systems.

Figure caption: Shown here is an example of the type of winds within the upper atmosphere that are driving the ionosphere to move in the way observed in this new paper. This set of two vortices rotate around the pole of the planet, driving currents within the ionosphere, which then reach out into the surrounding magnetosphere, producing the bright aurora and magnetic field changes observed by Cassini. This weather system was originally proposed by Chris Smith in a paper in Monthly Notices of the Royal Astronomical Society in 2011 (doi:10.1111/j.1365-2966.2010.17602.x) – and it is these modelled flows that we show here.

Please see paper for full details: Chowdhury, M.N., Stallard, T.S., Baines, K.H., Provan, G., Melin, H., Hunt, G.J., Moore, L., O’Donoghue, J., Thomas, E.M., Wang, R. and Miller, S., 2022. Saturn's Weather‐Driven Aurorae Modulate Oscillations in the Magnetic Field and Radio Emissions. Geophysical Research Letters, 49(3), p.e2021GL096492. DOI: https://doi.org/10.1029/2021GL096492.

By Tom Elsden (University of Glasgow)

Field line resonances (FLRs) are the manifestation of a magnetohydrodynamic (MHD) wave coupling process where energy is transferred from a global to local field-aligned wave. The ‘resonance’ comes from a frequency matching between these waves and being a resonant process, can result in a significant accumulation of energy on a given field line. These waves play an important role in magnetospheric wave-particle interactions, the generation of aurora and can further be used as a seismological tool to remote sense the magnetosphere from ground-based observations.

The location where FLRs occur is intrinsically linked to the current state of the magnetosphere, with the magnetic field structure, plasma density and solar wind driving conditions all being key factors. Given the drastic effect of a geomagnetic storm on the morphology of the magnetosphere, we considered how such changes impact where FLRs form between storm and non-storm times.

We used ground magnetometer data to determine how the fundamental Alfven frequency of field lines varies over the course of a storm on average. These frequencies were then used to infer a plasma density profile to be used in a numerical MHD model to investigate where the FLRs would form under broadband solar wind driving conditions.

Figure 1 shows results from these simulations, displaying the field-aligned current density as an indication of FLRs, mapped from the ionosphere to the equatorial plane to display the radial structure. The left panel is from a simulation modelling the initial phase of a storm, with the right panel modelling the main phase. We show that for an average storm, the FLR moves radially inward by ~1.7RE (compare vertical line locations). This is caused by a decrease in the fundamental Alfven frequency as well as an increase in the global (fast) wave frequency which drives the FLRs.

The important aspect of the results is the overall trend of more Earthward FLR formation during storms. Particularly if extrapolated to more severe storms, this could have an impact on storm-time wave-particle interactions in the radiation belts.

Figure 1 Caption: Colour contours of field-aligned current density from near the ionospheric end of field lines, mapped to the equatorial plane. Left: simulation modelling storm initial phase. Right: simulation modelling storm main phase. Vertical lines indicate field line resonance locations.

Please see paper for full details: Elsden, T., Yeoman, T. K., Wharton, S. J., Rae, I. J., Sandhu, J. K., Walach, M.-T., et al. (2022). Modeling the varying location of field line resonances during geomagnetic storms. Journal of Geophysical Research: Space Physics, 127, e2021JA029804. https://doi.org/10.1029/2021JA029804

 By Frankie Staples (formerly at MSSL UCL, now at UCLA)

Loss mechanisms act independently or in unison to drive rapid loss of electrons from the radiation belts. Electrons may be lost by precipitation into the Earth’s atmosphere, or through the magnetopause into interplanetary space – a process known as magnetopause shadowing. The mechanisms by which electrons are lost may be identified through changes to electron phase space density (PSD). This method considers the number of particles at given adiabatic coordinates (𝝁, K, and L*), which relate the electron energy, pitch angle, and location in the magnetic field. The characteristics of PSD evolution as a function of L* can be used to identify which loss mechanism is acting. However, the rapid nature of electron flux dropouts make it extremely difficult to resolve PSD dynamics at the necessary timescales to identify the contributions of either loss mechanism.

In this study we used a new multimission dataset of PSD observations from 36 satellites to resolve the dynamics of a magnetopause shadowing induced flux dropout in September 2017. We showed that by using Van Allen Probe data alone, the physical processes causing the dropout could be misinterpreted due to limited time and/or spatial resolution. Using multimission observations provided unprecedented time and spatial resolution necessary to correctly interpret PSD dynamics. 

The labelled Figure shows the magnetopause shadowing characteristics identified in PSD observations. Each panel shows PSD as a function of L* for fixed μ = 900 MeV/G and K = 0.1 G0.5RE at 1-hour intervals through phases of the storm. Symbol colours indicate when measurements were taken within the hour period, and dotted lines show the minimum and maximum L* of the last closed drift shell (LCDS) before the magnetopause. 

Please see paper for full details: 

Staples, F. A., Kellerman, A., Murphy, K. R., Rae, I. J., Sandhu, J. K., & Forsyth, C. (2022). Resolving magnetopause shadowing using multimission measurements of phase space density. Journal of Geophysical Research: Space Physics, 127, e2021JA029298. https://doi.org/10.1029/2021JA029298