Does magnetic reconnection have a characteristic scale in space and time?
Lead Research Organisation:
British Antarctic Survey
Department Name: Physical Sciences
Abstract
The Earth is continually enveloped by the expanding atmosphere of the Sun (known as the solar wind) that carries with it charged particles (plasma) and the Sun's magnetic field. The Earth's own magnetic field, extending into space, protects us from this solar wind, forming a cavity known as the magnetosphere. A process known as magnetic reconnection allows the Sun's magnetic field to connect to that of the Earth leading to the transfer of particles and energy from the solar wind into the magnetosphere. Some of these particles may travel along the Earth's magnetic field lines and strike the atmosphere, causing it to give off light in displays known as the aurora (commonly known as the northern lights). In fact, magnetic reconnection is the most significant transport process in the Earth's magnetosphere. There is also compelling observational evidence that magnetic reconnection processes take place in the Sun's atmosphere near its surface (the solar corona) and magnetic reconnection may be important for understanding the formation of stars, the origin of cosmic rays (high energy particles coming from space), accretion disks (found around stars, galaxies and black holes), and the generation of magnetic fields in planets, stars and galaxies. Closer to home, understanding and controlling magnetic reconnection is vital in the development of nuclear fusion reactors. Whilst there is a basic understanding of the magnetic reconnection process, exactly how, where, and when it occurs is still not understood. Recent observations of plasma flows and electric currents in the magnetosphere and of brightenings in the aurora, have provided evidence of scale-free behaviour i.e. that fluctuations in these quantities do not have a typical size and are seen across a wide range of sizes. This behaviour is typical of fractals and is often seen in nature such as in the shape of coastlines and the size of snow avalanches. These scale-free observations have, in turn, promoted the idea that magnetic reconnection is itself scale-free. However, there is no direct evidence for this and it is contrary to the traditional understanding of the magnetic reconnection process. Researchers at the British Antarctic Survey have developed a new technique to remotely sense magnetic reconnection in the magnetosphere using a combination of observations made from spacecraft and on the ground with auroral imagers and a radar network known as SuperDARN(the Super Dual Auroral Radar Network). We will use this technique to characterise statistically the structure of magnetic reconnection in both space and time over a wide range of scales. By doing this we will test the hypothesis that magnetic reconnection is scale-free and provide a constraint for new multi-scale magnetic reconnection models.
Organisations
Publications
Longden N
(2010)
Estimating the location of the open-closed magnetic field line boundary from auroral images
in Annales Geophysicae
Longden N
(2014)
Magnetic local time variation and scaling of poleward auroral boundary dynamics
in Journal of Geophysical Research: Space Physics
| Description | The main objective of the project was to investigate whether magnetic reconnection has a characteristic scale in space and time by characterising statistically the spatial and temporal structure of the footprint of reconnection as observed in the Earth's ionosphere. This would then provide constraints for multi-scale reconnection models. To a great extent the project has realised this objective, although not through using the exact methods and work plan that were outlined in the original proposal. In particular, the reconnection rate was not measured by the proposed combination of measurements of auroral boundary motion and plasma velocity but rather by inferring the reconnection character from the auroral boundary motion alone using a reconnection model. In detail, the most important achievements of the project have been: (1) The development of an improved, more general method for accurately identifying ionospheric auroral boundaries in images of the polar ionosphere taken by satellite ultra-violet imagers. In particular, the identification of the poleward auroral luminosity boundary that is a proxy for the open-closed magnetic field line boundary (the location of the ionospheric footprint of magnetic reconnection). The method was calibrated by a statistical comparison with established proxies for the boundary in particle precipitation data measured by low-altitude spacecraft which themselves sample the boundary only sparsely and infrequently. This achievement partially addresses objective 5 in the original proposal. (2) The production of a large temporal database of auroral boundary locations, including boundaries approximately every 2 minutes for about 2 years of data. This database allows the estimation of the total net reconnection rate in the magnetosphere from the rate of change of area enclosed by the open-closed field line boundary. This database and its associated metadata have been made publicly available through a dedicated website hosted by BAS and is already being used by other scientists. This achievement partially addresses objectives 1 and 3 in the original proposal. (3) The characterisation of the poleward auroral boundary motion, across the full range of magnetic local times, on timescales from 2 minutes to several hours. A structure function analysis was used to investigate the scaling of the boundary motion (and hence, of the reconnection process; see (4) below). A fractal regime of boundary motion was identified up to a time scale of 90 minutes suggesting that the reconnection process is scale-free on time scales less than the substorm time scale and that its character varies with magnetic local time. This achievement addresses objectives 2 and 4 in the original proposal. (4) The development of a theory, based on stationary solutions of the Ornstein-Uhlenbeck process, that explains the observed scaling variation with magnetic local time as well as the observed distributions of boundary motion for a full range of time scales. The theory is a mathematical and statistical development of the well-established expanding-contracting polar cap model which describes how the region of open magnetic flux in the polar cap responds to variations in both dayside and nightside magnetic reconnection. This achievement did not originally feature in the original proposal but represents a highly important result in that it links the characteristics of the boundary motion to the reconnection rate, thereby addressing objectives 2 and 4 in the original proposal by a different means. |
| Exploitation Route | (1) Use of data analysis methodology to identify the open-closed magnetic field line boundary in auroral imager data. (2) Use of pre-derived boundary data sets for studies of magnetic reconnection. (3) Measurements of the multi-scale nature of reconnection for increasing understanding of the reconnection process within the solar terrestrial environment. |
| Sectors | Other |
| URL | http://www.antarctica.ac.uk/bas_research/our_research/az/magnetic_reconnection/auroral_boundary_data.html |
| Description | The data analysis methods developed in this project are now in use within the wider scientific community. The auroral boundary data sets derived in this project are freely available on the project website and have been used in a number of subsequent studies. |
| First Year Of Impact | 2014 |
| Sector | Other |
| Title | IMAGE Auroral Boundary database |
| Description | Estimations of the location of the poleward boundary of the auroral oval as measured by the IMAGE satellite covering the epoch 2000-2002. These boundaries are determined using the method of Longden et al. (2010). The boundaries are used for a wide range of studies of the dynamics of solar wind-magnetosphere-ionosphere coupling. |
| Type Of Material | Database/Collection of data |
| Year Produced | 2011 |
| Provided To Others? | Yes |
| Impact | Enabled further development of model boundary locations for use in magnetosphere-ionosphere coupling studies. |
| URL | https://www.bas.ac.uk/project/image-auroral-boundary-data/ |
| Title | Ionospheric boundaries derived from IMAGE satellite mission data (May 2000 - October 2002) - VERSION 2.0 |
| Description | Ionospheric boundary locations derived from IMAGE (Imager for Magnetopause-to-Aurora Global Exploration) satellite FUV (Far Ultra Violet) imager data covering the period from May 2000 until October 2002. These include poleward and equatorward auroral boundary data derived directly from the three imagers, WIC (Wideband Imaging Camera), SI12 (Spectrographic Imager 121.8 nm), and SI13 (Spectrographic Imager 135.6 nm). These also include the OCB (open-closed magnetic field line boundary) and EPB (equatorward precipitation boundary) derived indirectly from the auroral boundaries. The data set also includes model fitted circles for all the boundary data sets for all measurement times. Chisham et al. (2022) also describe that the v2 data set also includes estimates of the OCB at each time, derived from a combination of the poleward auroral boundary measurements in combination with modelled statistical offsets between the auroral boundary and the OCB as measured by the DMSP spacecraft. The v2 data set also includes estimates of the EPB at each time, derived from a combination of the equatorward auroral boundary measurements in combination with modelled statistical offsets between the auroral boundary and the EPB as measured by the DMSP spacecraft. The v2 data set also includes model circle fit boundaries for all times for all eight raw data sets. These model circle fits were estimated using the methods outlined in Chisham (2017) and Chisham et al. (2022), which involves fitting circles to the spatial variation of the boundaries at any one time. The raw auroral boundaries were derived as outlined in Longden et al. (2010) (the original v1 data set) with the application of the additional selection criteria outlined in Chisham et al. (2022). For the creation of the original v1 data set, for each image, the position of each pixel in AACGM (Altitude Adjusted Corrected Geomagnetic) coordinates was established. Each image was then divided into 24 segments covering 1 hour of magnetic local time (MLT). For each MLT segment, an intensity profile was constructed by finding the average intensity across bins of 1 degree magnetic latitude in the range of 50 to 90 degrees (AACGM). Two functions were fit to each intensity profile: a function with one Gaussian component and a quadratic background, and a function with two Gaussian components and a quadratic background. The function with a single Gaussian component should provide a reasonable model when the auroral emission forms in a continuous oval. When the oval shows bifurcation, the function with two Gaussian components may provide a better model of the auroral emission. Of the two functions fit to each intensity profile, the one with the lower reduced chi-square goodness-of-fit statistic was deemed to be the better model for that profile. The auroral boundaries were then determined to be the position of the peak of the poleward Gaussian curve, plus its FWHM (full-width half-maximum) value of the Gaussian, to the peak of the equatorward Gaussian, minus its FWHM. In the case of the single Gaussian fit, the same curve is used for both boundaries. A number of criteria were applied to discard poorly located auroral boundaries arising from either poor fitting or incomplete data. Following Chisham et al. (2022), additional criteria were used to refine the data for the v2 auroral boundary data sets. These included dealing with anomalous data at the edges of the image fields of view, and dealing with anomalous mapping issues. Funding was provided by: STFC grant PP/E002110/1 - Does magnetic reconnection have a characteristic scale in space and time? NERC directed grant NE/V002732/1 - Space Weather Instrumentation, Measurement, Modelling and Risk - Thermosphere (SWIMMR-T). NERC BAS National Capability - Polar Science for Planet Earth. |
| Type Of Material | Database/Collection of data |
| Year Produced | 2022 |
| Provided To Others? | Yes |
| URL | https://data.bas.ac.uk/full-record.php?id=GB/NERC/BAS/PDC/01631 |