Browsing by Subject "magnetic reconnection"

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  • Akhavan-Tafti, M.; Palmroth, M.; Slavin, J. A.; Battarbee, M.; Ganse, U.; Grandin, M.; Le, G.; Gershman, D. J.; Eastwood, J. P.; Stawarz, J. E. (2020)
    The Vlasiator hybrid-Vlasov code was developed to investigate global magnetospheric dynamics at ion-kinetic scales. Here we focus on the role of magnetic reconnection in the formation and evolution of magnetic islands at the low-latitude magnetopause, under southward interplanetary magnetic field conditions. The simulation results indicate that (1) the magnetic reconnection ion kinetics, including the Earthward pointing Larmor electric field on the magnetospheric side of an X-point and anisotropic ion distributions, are well-captured by Vlasiator, thus enabling the study of reconnection-driven magnetic island evolution processes, (2) magnetic islands evolve due to continuous reconnection at adjacent X-points, "coalescence" which refers to the merging of neighboring islands to create a larger island, "erosion" during which an island loses magnetic flux due to reconnection, and "division" which involves the splitting of an island into smaller islands, and (3) continuous reconnection at adjacent X-points is the dominant source of magnetic flux and plasma to the outer layers of magnetic islands resulting in cross-sectional growth rates up to + 0.3 R-E(2)/min. The simulation results are compared to the Magnetospheric Multiscale (MMS) measurements of a chain of ion-scale flux transfer events (FTEs) sandwiched between two dominant X-lines. The MMS measurements similarly reveal (1) anisotropic ion populations and (2) normalized reconnection rate similar to 0.18, in agreement with theory and the Vlasiator predictions. Based on the simulation results and the MMS measurements, it is estimated that the observed ion-scale FTEs may grow Earth-sized within similar to 10 min, which is comparable to the average transport time for FTEs formed in the subsolar region to the high-latitude magnetopause. Future simulations shall revisit reconnection-driven island evolution processes with improved spatial resolutions.
  • Price, Daniel; Pomoell, Jens; Kilpua, Emilia (2020)
    Aims. We present a detailed examination of the magnetic evolution of AR 12473 using time-dependent, data-driven magnetofrictional modelling.Methods. We used maps of the photospheric electric field inverted from vector magnetogram observations, obtained by the Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory (SDO), to drive our fully time-dependent, data-driven magnetofrictional model. Our modelled field was directly compared to extreme ultraviolet observations from the Atmospheric Imaging Assembly, also onboard SDO. Metrics were also computed to provide a quantitative analysis of the evolution of the magnetic field.Results. The flux rope associated with the eruption on 28 December 2015 from AR 12473 was reproduced by the simulation and found to have erupted due to a torus instability.
  • Sanchez-Diaz, E.; Rouillard, A. P.; Davies, J. A.; Lavraud, B.; Sheeley, N. R.; Pinto, R. F.; Kilpua, E.; Plotnikov, I.; Genot, V. (2017)
    The origin of the slow solar wind is still a topic of much debate. The continual emergence of small transient structures from helmet streamers is thought to constitute one of the main sources of the slow wind. Determining the height at which these transients are released is an important factor in determining the conditions under which the slow solar wind forms. To this end, we have carried out a multipoint analysis of small transient structures released from a north-south tilted helmet streamer into the slow solar wind over a broad range of position angles during Carrington Rotation 2137. Combining the remote-sensing observations taken by the Solar-TErrestrial RElations Observatory (STEREO) mission with coronagraphic observations from the SOlar and Heliospheric Observatory (SOHO) spacecraft, we show that the release of such small transient structures (often called blobs), which subsequently move away from the Sun, is associated with the concomitant formation of transient structures collapsing back toward the Sun; the latter have been referred to by previous authors as "raining inflows." This is the first direct association between outflowing blobs and raining inflows, which locates the formation of blobs above the helmet streamers and gives strong support that the blobs are released by magnetic reconnection.
  • Hoilijoki, S.; Ganse, U.; Sibeck, D.G.; Cassak, P.A.; Turc, L.; Battarbee, M.; Fear, R.C.; Blanco-Cano, X.; Dimmock, A.P.; Kilpua, E.K.J.; Jarvinen, R.; Juusola, L.; Pfau-Kempf, Y.; Palmroth, M. (2019)
    This paper describes properties and behavior of magnetic reconnection and flux transfer events (FTEs) on the dayside magnetopause using the global hybrid-Vlasov code Vlasiator. We investigate two simulation runs with and without a sunward (positive)B-x component of the interplanetary magnetic field (IMF) when the IMF is southward. The runs are two-dimensional in real space in the noon-midnight meridional (polar) plane and three-dimensional in velocity space. Solar wind input parameters are identical in the two simulations with the exception that the IMF is purely southward in one but tilted 45 degrees toward the Sun in the other. In the purely southward case (i.e., without B-x) the magnitude of the magnetos heath magnetic field component tangential to the magnetopause is larger than in the run with a sunward tilt. This is because the shock normal is perpendicular to the IMF at the equatorial plane, whereas in the other run the shock configuration is oblique and a smaller fraction of the total IMF strength is compressed at the shock crossing. Hence, the measured average and maximum reconnection rate are larger in the purely southward run. The run with tilted IMF also exhibits a north-south asymmetry in the tangential magnetic field caused by the different angle between the IMF and the bow shock normal north and south of the equator. Greater north-south asymmetries are seen in the FTE occurrence rate, size, and velocity as well; FTEs moving toward the Southern Hemisphere are larger in size and observed less frequently than FTEs in the Northern Hemisphere.
  • Hoilijoki, Sanni; Ganse, Urs; Pfau-Kempf, Yann; Cassak, Paul A.; Walsh, Brian M.; Hietala, Heli; von Alfthan, Sebastian; Palmroth, Minna (2017)
    We present results from a first study of the local reconnection rate and reconnection site motion in a 2D-3V global magnetospheric self-consistent hybrid-Vlasov simulation with due southward interplanetary magnetic field. We observe magnetic reconnection at multiple locations at the dayside magnetopause and the existence of magnetic islands, which are the 2-D representations of flux transfer events. The reconnection locations (the X lines) propagate over significant distances along the magnetopause, and reconnection does not reach a steady state. We calculate the reconnection rate at the location of the X lines and find a good correlation with an analytical model of local 2-D asymmetric reconnection. We find that despite the solar wind conditions being constant, the reconnection rate and location of the X lines are highly variable. These variations are caused by magnetosheath fluctuations, the effects of neighboring X lines, and the motion of passing magnetic islands.
  • Palmroth, Minna; Hoilijoki, Sanni; Juusola, Liisa; Pulkkinen, Tuija I.; Hietala, Heli; Pfau-Kempf, Yann; Ganse, Urs; von Alfthan, Sebastian; Vainio, Rami; Hesse, Michael (2017)
    The key dynamics of the magnetotail have been researched for decades and have been associated with either three-dimensional (3-D) plasma instabilities and/or magnetic reconnection. We apply a global hybrid-Vlasov code, Vlasiator, to simulate reconnection self-consistently in the ion kinetic scales in the noon-midnight meridional plane, including both dayside and nightside reconnection regions within the same simulation box. Our simulation represents a numerical experiment, which turns off the 3-D instabilities but models ion-scale reconnection physically accurately in 2-D. We demonstrate that many known tail dynamics are present in the simulation without a full description of 3-D instabilities or without the detailed description of the electrons. While multiple reconnection sites can coexist in the plasma sheet, one reconnection point can start a global reconfiguration process, in which magnetic field lines become detached and a plasmoid is released. As the simulation run features temporally steady solar wind input, this global reconfiguration is not associated with sudden changes in the solar wind. Further, we show that lobe density variations originating from dayside reconnection may play an important role in stabilising tail reconnection.
  • Price, D. J.; Pomoell, J.; Lumme, E.; Kilpua, E. K. J. (2019)
    Aims. We present a detailed study of the magnetic evolution of AR 12673 using a magnetofrictional modelling approach. Methods. The fully data-driven and time-dependent model was driven with maps of the photospheric electric field, inverted from vector magnetogram observations obtained from the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). Our analysis was aided by studying the evolution of metrics such as the free magnetic energy and the current-carrying helicity budget of the domain, maps of the squashing factor and twist, and plots of the current density. These allowed us to better understand the dynamic nature of the magnetic topology. Results. Our simulation captured the time-dependent nature of the active region and the erupting flux rope associated with the X-class flares on 6 September 2017, including the largest of solar cycle 24. Additionally, our results suggest a possible threshold for eruptions in the ratio of current-carrying helicity to relative helicity. Conclusion. The flux rope was found to be a combination of two structures that partially combine during the eruption process. Our time-dependent data-driven magnetofrictional model is shown to be capable of generating magnetic fields consistent with extreme ultraviolet (EUV) observations.
  • Pal, Sanchita; Kilpua, Emilia; Good, Simon; Pomoell, Jens; Price, Daniel (2021)
    Context. Magnetic clouds (MCs) are transient structures containing large-scale magnetic flux ropes from solar eruptions. The twist of magnetic field lines around the rope axis reveals information about flux rope formation processes and geoeffectivity. During propagation MC flux ropes may erode via reconnection with the ambient solar wind. Any erosion reduces the magnetic flux and helicity of the ropes, and changes their cross-sectional twist profiles. Aims. This study relates twist profiles in MC flux ropes observed at 1 AU to the amount of erosion undergone by the MCs in interplanetary space. Methods. The twist profiles of two clearly identified MC flux ropes associated with the clear appearance of post eruption arcades in the solar corona are analyzed. To infer the amount of erosion, the magnetic flux content of the ropes in the solar atmosphere is estimated, and compared to estimates at 1 AU. Results. The first MC shows a monotonically decreasing twist from the axis to the periphery, while the second displays high twist at the axis, rising twist near the edges, and lower twist in between. The first MC displays a larger reduction in magnetic flux between the Sun and 1 AU, suggesting more erosion than that seen in the second MC. Conclusions. In the second cloud the rising twist at the rope edges may have been due to an envelope of overlying coronal field lines with relatively high twist, formed by reconnection beneath the erupting flux rope in the low corona. This high-twist envelope remained almost intact from the Sun to 1 AU due to the low erosion levels. In contrast, the high-twist envelope of the first cloud may have been entirely peeled away via erosion by the time it reaches 1 AU.