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Explosive energy release in space plasmas


Project Description

The substorm is a repeatable space weather disturbance, which, like earthquakes, apparently unpredictably releases a considerable and variable amount of energy. This makes the substorm one of the greatest sources of uncertainty in predicting the impact of space weather on electricity supply networks, satellites, and associated services.

This project aims to improve our understanding of what determines when a substorm will occur by:

(i) comparing and synthesising diverse catalogues of thousands of substorm onsets from different observational sources and identification methods to reveal their common waiting time distribution and source/method departures from it,

(ii) examining the common distribution, or departures from it, for evidence of sub-hour recurrence of substorm onsets or of substorm intensifications in so-called compound substorms,

(iii) testing the common waiting time distribution against that predicted by the Minimal Substorm Model (MSM), and revising the model accordingly,

(iv) developing the MSM or alternative model to account for any sub-hour recurrence property.

In the standard model, the substorm evolves through 3 phases over a typical duration of ~3 hours. In the growth phase, lasting ~1 hour, energy from the solar wind is transferred into the Earth’s outer magnetic field environment known as the magnetosphere where it is stored as magnetic energy in the anti-sunward-stretched magnetic lines of force of the magnetotail. Eventually this storage of energy modifies the plasma in the magnetosphere such that a plasma instability occurs at so-called substorm onset. The instability rapidly reconfigures the geometry and topology of the magnetotail magnetic field over an expansion phase lasting ~20-30 min. The reconfiguration releases some or all of the stored magnetic energy into various forms of energy, which are transported and dissipated into the upper atmosphere, the ring current, and into Space in the expansion and subsequent recovery phase lasting 1-2 hrs.

Consequently, the minimum time between substorms would be expected to be about 3 hours and this can be lengthened by a period when the solar wind input is switched off that effectively interrupts or suspends the growth phase123. This model appears consistent with the statistical distribution of ~1000 substorm waiting times identified from energetic particle data at geostationary orbit4. The distribution has a mode at 2-3 hours, consistent with periodic substorms under continuous driving, and a larger average of 5-6 hours, resulting from a long tail extending beyond 24 h due to the interruptions of solar wind driving. This interpretation has been confirmed by the successful reproduction of the empirical waiting time distribution by the so-called Minimal Substorm Model (MSM)5. This simple mathematical model is of an integrate-and-fire process in which energy from the solar wind accumulates in the magnetotail until substorm onset occurs at some fixed energy level and then a variable amount of energy is released that is proportional to the solar wind power input at onset. (The MSM also approximately explains the statistical distribution of estimated substorm energy loss into the upper atmosphere6.

However, substorms are morphologically complex and thus the identification of onset may depend on observing instrument, location, and identification criteria. Consequently, different substorm catalogues yield generally different substorm waiting time distributions7. Whilst several appear similar to that of the standard model above, some indicate a plethora of waiting times at sub-hour time scales. Observationally, this can be understood as intensifications seen within the standard substorm, creating the so-called compound substorm8. However, it is unclear whether it is appropriate to view these as intensifications within a substorm or as superposed substorms, which would require revision of the standard model. The distinction is not merely semantic but is important to understanding the physics of the substorm instability, magnitude, and dynamics. It is also vital for operational forecasting.

In this project, the student will compare and contrast the various substorm catalogues and devise statistical and/or machine learning methods to synthesise the catalogues to yield a robust substorm list and waiting time distribution that is common to all, taking into account observational uncertainties and methodological differences. From this, the student will identify the non-standard component (i.e., sub-hour waiting times) in either the synthesised distribution or more likely in departures from it of individual catalogues. Using this information, the student shall test whether the MSM, or revisions of it, can explain the synthesised distribution and whether and what additional mathematical model is needed to account for the non-standard component. Depending on the outcome and time available, the student will attempt to relate the mathematical model(s) to our understanding of substorm physics with a view to deriving the model mathematics from the MHD equations and approximations of the kinetic instabilities.

Desired Knowledge and Skills

• Undergraduate degree in Physics, with a strong interest in solar or space plasma physics
• Strong computational skills in a relevant programming language (IDL, Matlab, Python)
• Good statistical and mathematical skills

Applications

Applications submitted by 31st January 2020 will be given full consideration. We will continue accepting applications until all places are filled. After we receive your application, we will select candidates for interviews. If you are selected, you will be invited for an interview at MSSL. You will have the opportunity to see the laboratory, students' flats and talk to current students. The studentships are for the advertised projects only. In your application, please specify which project you want to apply for.

To apply, please visit the Online Application page, select department of "Space & Climate Physics" and programme type of "Postgraduate Research". After pushing "Search Now" button, select "RRDSPSSING01: Research Degree: Space and Climate Physics" for Full-time or Part-time mode.
Our Online Applications page can be found here: https://www.ucl.ac.uk/adminsys/search/

Entry requirements

An upper second-class Bachelor’s degree, or a second-class Bachelor’s degree together with a Master's degree from a UK university in a relevant subject, or an equivalent overseas qualification.

Students from the UK or those from the EU who meet the residency requirements (3 years' full-time residency in the UK) are potentially eligible for a Science and Technology Facilities Council (STFC) studentship.

Funding Notes

These pay UK/EU tuition fees and a maintenance allowance for 3.5 years (subject to the PhD upgrade review).

EU students who do not meet eligibility requirements still qualify for the UK/EU fees rate, but not the STFC maintenance allowance.

References

1. Freeman and Farrugia, 1995
2. Freeman et al., 1999
3. Morley et al., 2009
4. Borovsky et al., 1993
5. Freeman and Morley, 2004
6. Morley et al., 2007
7. Forsyth et al., 2015
8. Sandhu et al., 2019

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