Publications
106 results found
Eastwood JP, Goldman MV, Hietala H, et al., 2015, Ion reflection and acceleration near magnetotail dipolarization fronts associated with magnetic reconnection, Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 511-525, ISSN: 2169-9402
Dipolarization fronts (DFs) are often associated with the leading edge of earthward bursty bulk flows in the magnetotail plasma sheet. Here multispacecraft Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations are used to show that a spatially limited region of counterpropagating ion beams, whose existence is not evident in either the plasma moments or the electric field, is observed on the low-density side of DFs. The THEMIS magnetic field data are used to establish appropriate comparison cuts through a particle-in-cell simulation of reconnection, and very good agreement is found between the observed and simulated ion distributions on both sides of the DF. Self-consistent back tracing shows that the ion beams originate from the thermal component of the preexisting high-density plasma into which the DF is propagating; they do not originate from the inflow region in the traditional sense. Forward tracing shows that some of these ions can subsequently overtake the DF and pass back into the high-density preexisting plasma sheet with an order-of-magnitude increase in energy; this process is distinct from other ion reflection processes that occur directly at the DF. The interaction of the reconnection jet with the preexisting plasma sheet therefore occurs over a macroscopic region, rather than simply being limited to the thin DF interface. A more general consequence of this study is the conclusion that reconnection jets are not simply fed by plasma inflow across the separatrices but are also fed by plasma from the region into which the jet is propagating; the implications of this finding are discussed.
Mistry R, Eastwood JP, Hietala H, 2014, Detection of small-scale folds at a solar wind reconnection exhaust, Journal of Geophysical Research: Space Physics, Vol: 120, Pages: 30-42, ISSN: 2169-9402
Observations of reconnection in the solar wind over the last few years appear to indicate that the majority of large-scale reconnecting current sheets are roughly planar, and that reconnection itself is quasi-steady. Most studies of solar wind exhausts have used spacecraft with large separations and relatively low time cadence ion measurements. Here we present multipoint Cluster observations of a reconnection exhaust and the associated current sheet at ACE and Wind, enabling it to be studied on multiple length scales and at high time resolution. While analysis shows that on large scales the current sheet is planar, detailed measurements using the four closely spaced Cluster spacecraft show that the trailing edge of the reconnection jet is nonplanar with folds orthogonal to the reconnection plane, with length scales of approximately 230 ion inertial lengths. Our findings thus suggest that while solar wind current sheets undergoing reconnection may be planar on large scales, they may also exhibit complex smaller-scale structure. Such structure is difficult to observe and has rarely been detected because exhausts are rapidly convected past the spacecraft in a single cut; there is therefore a limited set of spacecraft trajectories through the exhaust which would allow the nonplanar features to be intercepted. We consider how such nonplanar reconnection current sheets can form and the processes which may have generated the 3-D structure that was observed.
Hietala H, Eastwood JP, Isavnin A, 2014, Sequentially released tilted flux ropes in the Earth's magnetotail, PLASMA PHYSICS AND CONTROLLED FUSION, Vol: 56, ISSN: 0741-3335
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- Citations: 17
Hietala H, Kilpua EKJ, Turner DL, et al., 2014, Depleting effects of ICME-driven sheath regions on the outer electron radiation belt, Geophysical Research Letters, Vol: 41, Pages: 2258-2265, ISSN: 0094-8276
We study the effect of interplanetary coronal mass ejection (ICME)‐driven sheath regions on relativistic outer radiation belt electron fluxes. We employ superposed epoch analysis, and, unlike previous studies, we segregate the sheath from the ejecta. We find that sheaths typically result in more than an order of magnitude decrease in the relativistic electron fluxes and that the fluxes stay below the pre‐event level for more than 2 days after the sheath passage. The electron depletions are stronger for sheaths that exhibit higher power in magnetic and dynamic pressure fluctuations in the ultralow frequency range and cause larger magnetospheric compressions. Depletions are even stronger for sheaths that encompass the entire storm main phase. Our findings suggest that sheaths are effective at depleting the electron fluxes because they increase radial diffusion under magnetospheric compression conditions, thereby enhancing magnetopause shadowing losses, particularly when the Dst effect can act in concert.
Hietala H, Plaschke F, 2013, On the generation of magnetosheath high-speed jets by bow shock ripples, Journal of Geophysical Research: Space Physics, Vol: 118, Pages: 7237-7245, ISSN: 2169-9380
[1] The terrestrial magnetosheath is embedded with coherent high‒speed jets of about 1RE in scale, predominantly during quasi‒radial interplanetary magnetic field (IMF). When these high dynamic pressure (Pdyn) jets hit the magnetopause, they cause large indentations and further magnetospheric effects. The source of these jets has remained controversial. One of the proposed mechanisms is based on ripples of the quasi‒parallel bow shock. In this paper, we combine for the first time, 4 years of subsolar magnetosheath observations from the Time History of Events and Macroscale Interactions during Substorms mission and corresponding NASA/OMNI solar wind conditions with model calculations of a rippled bow shock. Concentrating on the magnetosheath close to the shock during intervals when the angle between the IMF and the Sun‒Earth line was small, we find that (1) 97% of the observed jets can be produced by local ripples of the shock under the observed upstream conditions; (2) the coherent jets form a significant fraction of the high Pdyn tail of the magnetosheath flow distribution; (3) the magnetosheath Pdyn distribution matches the flow from a bow shock with ripples that have a dominant amplitude to wavelength ratio of about 9% (∼0.1RE/1RE) and are present ∼12% of the time at any given location.
Battarbee M, Vainio R, Laitinen T, et al., 2013, Injection of thermal and suprathermal seed particles into coronal shocks of varying obliquity, ASTRONOMY & ASTROPHYSICS, Vol: 558, ISSN: 0004-6361
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- Citations: 20
Turc L, Fontaine D, Savoini P, et al., 2013, A comparison of bow shock models with Cluster observations during low Alfven Mach number magnetic clouds, ANNALES GEOPHYSICAE, Vol: 31, Pages: 1011-1019, ISSN: 0992-7689
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- Citations: 9
Plaschke F, Hietala H, Angelopoulos V, 2013, Anti-sunward high-speed jets in the subsolar magnetosheath, ANNALES GEOPHYSICAE, Vol: 31, Pages: 1877-1889, ISSN: 0992-7689
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- Citations: 95
Kilpua EKJ, Hietala H, Koskinen HEJ, et al., 2013, Magnetic field and dynamic pressure ULF fluctuations in coronal-mass-ejection-driven sheath regions, ANNALES GEOPHYSICAE, Vol: 31, Pages: 1559-1567, ISSN: 0992-7689
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- Citations: 37
Andreeova K, Kilpua EKJ, Hietala H, et al., 2013, Analysis of the substructure within a complex magnetic cloud on 3-4 September 2008, ANNALES GEOPHYSICAE, Vol: 31, Pages: 555-562, ISSN: 0992-7689
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- Citations: 2
Hietala H, Sandroos A, Vainio R, 2012, PARTICLE ACCELERATION IN SHOCK-SHOCK INTERACTION: MODEL TO DATA COMPARISON, ASTROPHYSICAL JOURNAL LETTERS, Vol: 751, ISSN: 2041-8205
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- Citations: 10
Hietala H, Partamies N, Laitinen TV, et al., 2012, Supermagnetosonic subsolar magnetosheath jets and their effects: from the solar wind to the ionospheric convection, ANNALES GEOPHYSICAE, Vol: 30, Pages: 33-48, ISSN: 0992-7689
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- Citations: 88
Hietala H, Agueda N, Andreeova K, et al., 2011, In situ observations of particle acceleration in shock-shock interaction, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 116, ISSN: 2169-9380
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- Citations: 15
Sator N, Hietala H, 2010, Damage in impact fragmentation, INTERNATIONAL JOURNAL OF FRACTURE, Vol: 163, Pages: 101-108, ISSN: 0376-9429
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- Citations: 20
Hietala H, Laitinen TV, Andreeova K, et al., 2009, Supermagnetosonic Jets behind a Collisionless Quasiparallel Shock, PHYSICAL REVIEW LETTERS, Vol: 103, ISSN: 0031-9007
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- Citations: 114
Sator N, Hietala H, 2009, Damage in impact fragmentation, IUTAM Symposium on Dynamic Fracture and Fragmentation, Publisher: Springer Netherlands, Pages: 355-362, ISBN: 9789048197590
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