Browsing by Author "Dimmock, Andrew P."
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Item Alfvén Ion Cyclotron Waves in Sheath Regions Driven by Interplanetary Coronal Mass Ejections(WILEY-BLACKWELL PUBLISHING, INC, 2019-06-01) Ala-Lahti, Matti; Kilpua, Emilia K.J.; Souček, Jan; Pulkkinen, Tuija I.; Dimmock, Andrew P.; Department of Electronics and Nanoengineering; University of Helsinki; Czech Academy of Sciences; Uppsala UniversityWe report on a statistical analysis of the occurrence and properties of Alfvén ion cyclotron (AIC) waves in sheath regions driven by interplanetary coronal mass ejections (ICMEs). We have developed an automated algorithm to identify AIC wave events from magnetic field data and apply it to investigate 91 ICME sheath regions recorded by the Wind spacecraft. Our analysis focuses on waves generated by the ion cyclotron instability. AIC waves are observed to be frequent structures in ICME-driven sheaths, and their occurrence is the highest in the vicinity of the shock. Together with previous studies, our results imply that the shock compression has a crucial role in generating wave activity in ICME sheaths. AIC waves tend to have their frequency below the ion cyclotron frequency, and, in general, occur in plasma that is stable with respect to the ion cyclotron instability and has lower ion β‖ than mirror modes. The results suggest that the ion beta anisotropy β⊥/β‖>1 appearing in ICME sheaths is regulated by both ion cyclotron and mirror instabilities.Item Cavitons and spontaneous hot flow anomalies in a hybrid-Vlasov global magnetospheric simulation(2018-08-08) Blanco-Cano, Xochitl; Battarbee, Markus; Turc, Lucile; Dimmock, Andrew P.; Kilpua, Emilia K.J.; Hoilijoki, Sanni; Ganse, Urs; Sibeck, David G.; Cassak, Paul A.; Fear, Robert C.; Jarvinen, Riku; Juusola, Liisa; Pfau-Kempf, Yann; Vainio, Rami; Palmroth, Minna; Department of Electronics and Nanoengineering; Universidad Nacional Autónoma de México; University of Helsinki; University of Colorado Boulder; NASA Goddard Space Flight Center; West Virginia University; University of Southampton; University of TurkuIn this paper we present the first identification of foreshock cavitons and the formation of spontaneous hot flow anomalies (SHFAs) with the Vlasiator global magnetospheric hybrid-Vlasov simulation code. In agreement with previous studies we show that cavitons evolve into SHFAs. In the presented run, this occurs very near the bow shock. We report on SHFAs surviving the shock crossing into the downstream region and show that the interaction of SHFAs with the bow shock can lead to the formation of a magnetosheath cavity, previously identified in observations and simulations. We report on the first identification of long-term local weakening and erosion of the bow shock, associated with a region of increased foreshock SHFA and caviton formation, and repeated shock crossings by them. We show that SHFAs are linked to an increase in suprathermal particle pitch-angle spreads. The realistic length scales in our simulation allow us to present a statistical study of global caviton and SHFA size distributions, and their comparable size distributions support the theory that SHFAs are formed from cavitons. Virtual spacecraft observations are shown to be in good agreement with observational studies.Item The dawn-dusk asymmetry of ion density in the dayside magnetosheath and its annual variability measured by THEMIS(2016-05-10) Dimmock, Andrew P.; Pulkkinen, Tuija I.; Osmane, Adnane; Nykyri, Katariina; Department of Radio Science and Engineering; Embry-Riddle Aeronautical UniversityThe local and global plasma properties in the magnetosheath play a fundamental role in regulating solar wind-magnetosphere coupling processes. However, the magnetosheath is a complex region to characterise as it has been shown theoretically, observationally and through simulations that plasma properties are inhomogeneous, non-isotropic and asymmetric about the Sun-Earth line. To complicate matters, dawn-dusk asymmetries are sensitive to various changes in the upstream conditions on an array of timescales. The present paper focuses exclusively on dawn-dusk asymmetries, in particularly that of ion density. We present a statistical study using THEMIS data of the dawn-dusk asymmetry of ion density in the dayside magnetosheath and its long-term variations between 2009 and 2015. Our data suggest that, in general, the dawn-side densities are higher, and the asymmetry grows from noon towards the terminator. This trend was only observed close to the magnetopause and not in the central magnetosheath. In addition, between 2009 and 2015, the largest asymmetry occurred around 2009 decreasing thereafter. We also concluded that no single parameter such as the Alfvén Mach number, plasma velocity, or the interplanetary magnetic field strength could exclusively account for the observed asymmetry. Interestingly, the dependence on Alfvén Mach number differed between data sets from different time periods. The asymmetry obtained in the THEMIS data set is consistent with previous studies, but the solar cycle dependence was opposite to an analysis based on IMP-8 data. We discuss the physical mechanisms for this asymmetry and its temporal variation. We also put the current results into context with the existing literature in order to relate THEMIS era measurements to those made during earlier solar cycles.Item Direct evidence of nonstationary collisionless shocks in space plasmas(AMER ASSOC ADVANCEMENT SCIENCE, 2019-02) Dimmock, Andrew P.; Russell, Christopher T.; Sagdeev, Roald Z.; Krasnoselskikh, Vladimir; Walker, Simon N.; Carr, Christopher; Dandouras, Iannis; Escoubet, C. Philippe; Ganushkina, Natalia; Gedalin, Michael; Khotyaintsev, Yuri; Aryan, Homayon; Pulkkinen, Tuija; Balikhin, Michael A.; Department of Electronics and Nanoengineering; University of California, Los Angeles; University of Maryland; University of California, Berkeley; University of Sheffield; Imperial College London; Université de Toulouse; European Space Agency - ESA; University of Michigan, Ann Arbor; Ben-Gurion University of the Negev; Swedish Institute of Space Physics; NASA Goddard Space Flight CenterCollisionless shocks are ubiquitous throughout the universe: around stars, supernova remnants, active galactic nuclei, binary systems, comets, and planets. Key information is carried by electromagnetic emissions from particles accelerated by high Mach number collisionless shocks. These shocks are intrinsically nonstationary, and the characteristic physical scales responsible for particle acceleration remain unknown. Quantifying these scales is crucial, as it affects the fundamental process of redistributing upstream plasma kinetic energy into other degrees of freedom-particularly electron thermalization. Direct in situ measurements of nonstationary shock dynamics have not been reported. Thus, the model that best describes this process has remained unknown. Here, we present direct evidence demonstrating that the transition to nonstationarity is associated with electron-scale field structures inside the shock ramp.Item GUMICS-4 analysis of interplanetary coronal mass ejection impact on Earth during low and typical Mach number solar winds(Copernicus Gesellschaft mbH, 2019-07-11) Lakka, Antti; Pulkkinen, Tuija I.; Dimmock, Andrew P.; Kilpua, Emilia; Ala-Lahti, Matti; Honkonen, Ilja; Palmroth, Minna; Raukunen, Osku; Department of Electronics and Nanoengineering; Tuija Pulkkinen Group; University of Helsinki; Finnish Meteorological Institute; University of Turku; Swedish Institute of Space PhysicsWe study the response of the Earth's magnetosphere to fluctuating solar wind conditions during interplanetary coronal mass ejections (ICMEs) using the Grand Unified Magnetosphere-Ionosphere Coupling Simulation (GUMICS-4). The two ICME events occurred on 15-16 July 2012 and 29-30 April 2014. During the strong 2012 event, the solar wind upstream values reached up to 35 particles cm(-3), speeds of up to 694 km s(-1), and an interplanetary magnetic field of up to 22 nT, giving a Mach number of 2.3. The 2014 event was a moderate one, with the corresponding upstream values of 30 particles cm(-3), 320 km s(-1) and 10 nT, indicating a Mach number of 5.8. We examine how the Earth's space environment dynamics evolves during both ICME events from both global and local perspectives, using well-established empirical models and in situ measurements as references. We show that on the large scale, and during moderate driving, the GUMICS-4 results are in good agreement with the reference values. However, the local values, especially during high driving, show more variation: such extreme conditions do not reproduce local measurements made deep inside the magnetosphere. The same appeared to be true when the event was run with another global simulation. The cross-polar cap potential (CPCP) saturation is shown to depend on the Alfven-Mach number of the upstream solar wind. However, care must be taken in interpreting these results, as the CPCP is also sensitive to the simulation resolution.Item Jensen-Shannon Complexity and Permutation Entropy Analysis of Geomagnetic Auroral Currents(WILEY-BLACKWELL PUBLISHING, INC, 2019-04) Osmane, Adnane; Dimmock, Andrew P.; Pulkkinen, Tuija I.; University of Helsinki; Uppsala University; Department of Electronics and NanoengineeringIn this study we determine whether auroral westward currents can be characterized by low-dimensional chaotic attractors through the use of the complexity-entropy methodology developed by Rosso et al. (2007, https://doi.org/10.1103/PhysRevLett.99.154102) and based on the permutation entropy developed by Bandt and Pompe (2002, https://doi.org/10.1103/PhysRevLett.88.174102). Our results indicate that geomagnetic auroral indices are indistinguishable from stochastic processes from time scales ranging from a few minutes to 10 hr and for embedded dimensions d < 8. Our results are inconsistent with earlier studies of Baker et al. (1990, https://doi.org/10.1029/GL017i001p00041), Pavlos et al. (1992), D. Roberts et al. (1991, https://doi.org/10.1029/91GL00021), D. A. Roberts (1991, https://doi.org/10.1029/91JA01088), and Vassiliadis et al. (1990, https://doi.org/10.1029/GL017i011p01841, 1991, https://doi.org/10.1029/91GL01378) indicating that auroral geomagnetic indices could be reduced to low-dimensional systems withchaotic dynamics.Item Solar Orbiter Data-Model Comparison in Venus' Induced Magnetotail(WILEY-BLACKWELL, 2023-02) Stergiopoulou, Katerina; Jarvinen, Riku; Andrews, David J.; Edberg, Niklas J.T.; Dimmock, Andrew P.; Kallio, Esa; Persson, Moa; Khotyaintsev, Yuri V.; Department of Electronics and Nanoengineering; Esa Kallio Group; Swedish Institute of Space Physics; Centre national d'études spatialesWe investigate the structure of the Venusian magnetotail utilizing magnetic field and electron density measurements that cover a wide range of distances from the planet, from the first two Solar Orbiter Venus flybys. We examine the magnetic field components along the spacecraft trajectory up to 80 Venus radii down the tail. Even though the magnetic field behavior differs considerably between the two cases, we see extended electron density enhancements covering distances greater than ∼20 RV in both flybys. We compare the magnetic field measurements with a global hybrid model of the induced magnetosphere and magnetotail of Venus, to examine to what degree the observations can be understood with the simulation. The model upstream conditions are stationary and the solution encloses a large volume of 83 RV × 60 RV × 60 RV in which we look for spatial magnetic field and plasma variations. We rotate the simulation solution to describe different stationary upstream IMF clock angle cases with a 10° step and find the clock angle for which the agreement between observations and model is maximized along Solar Orbiter's trajectory in 1-min steps. We find that in both flybys there is better agreement with the observations when we rotate the model for some intervals, while there are parts that cannot be well reproduced by the model irrespective of how we vary the IMF clock angle, suggesting the presence of non-stationary features in the Venus-solar wind interaction not accounted for in the hybrid model.Item A statistical study of magnetic field fluctuations in the dayside magnetosheath and their dependence on upstream solar wind conditions(American Geophysical Union. Wiley, 2014) Dimmock, Andrew P.; Nykyri, Katariina; Pulkkinen, Tuija I.; Sähkötekniikan korkeakoulu; School of Electrical EngineeringThe magnetosheath functions as a natural interface connecting the interplanetary and magnetospheric plasma. Since the magnetosheath houses the shocked solar wind, it is populated with abundant magnetic field turbulence which are generated both locally and externally. Although the steady state magnetosheath is to date relatively well understood, the same cannot be said of transient magnetic perturbations due to their kinetic nature and often complex and numerous generation mechanisms. The current manuscript presents a statistical study of magnetic field fluctuations in the dayside magnetosheath as a function of upstream solar wind conditions. We concentrate on the ambient higher-frequency fluctuations in the range of 0.1 Hz -> 2 Hz. We show evidence that the dawn (quasi-parallel) flank is visibly prone to higher-amplitude magnetic perturbations compared to the dusk (quasi-perpendicular) region. Our statistical data also suggest that the magnitude of turbulence can be visibly enhanced close to the magnetopause during periods of southward interplanetary magnetic field orientations. Faster (> 400 km s−1) solar wind velocities also appear to drive higher-amplitude perturbations compared to slower speeds. The spatial distribution also suggests some dependence on the magnetic pileup region at the subsolar magnetopause.Item Subcritical Growth of Electron Phase-space Holes in Planetary Radiation Belts(2017-09-01) Osmane, Adnane; Turner, Drew L.; Wilson, Lynn B.; Dimmock, Andrew P.; Pulkkinen, Tuija I.; Department of Electronics and Nanoengineering; The Aerospace Corporation; NASA Goddard Space Flight CenterThe discovery of long-lived electrostatic coherent structures with large-amplitude electric fields (1 ≤ E ≤ 500 mV/m) by the Van Allen Probes has revealed alternative routes through which planetary radiation belts' acceleration can take place. Following previous reports showing that small phase-space holes, with q φ/Tc e ≃ 10-2 10-3, could result from electron interaction with large-amplitude whistlers, we demonstrate one possible mechanism through which holes can grow nonlinearly (i.e., γ ∝ √ φ) and subcritically as a result of momentum exchange between hot and cold electron populations. Our results provide an explanation for the common occurrence and fast growth of large-amplitude electron phase-space holes in the Earths radiation belts.Item Transmission of an ICME Sheath Into the Earth's Magnetosheath and the Occurrence of Traveling Foreshocks(WILEY-BLACKWELL PUBLISHING, INC, 2021-12) Ala-Lahti, Matti; Dimmock, Andrew P.; Pulkkinen, Tuija I.; Good, Simon W.; Yordanova, Emilya; Turc, Lucile; Kilpua, Emilia K.J.; University of Helsinki; Swedish Institute of Space Physics; Department of Electronics and NanoengineeringThe transmission of a sheath region driven by an interplanetary coronal mass ejection into the Earth's magnetosheath is studied by investigating in situ magnetic field measurements upstream and downstream of the bow shock during an ICME sheath passage on 15 May 2005. We observe three distinct intervals in the immediate upstream region that included a southward magnetic field component and are traveling foreshocks. These traveling foreshocks were observed in the quasi-parallel bow shock that hosted backstreaming ions and magnetic fluctuations at ultralow frequencies. The intervals constituting traveling foreshocks in the upstream survive transmission to the Earth's magnetosheath, where their magnetic field, and particularly the southward component, was significantly amplified. Our results further suggest that the magnetic field fluctuations embedded in an ICME sheath may survive the transmission if their frequency is below ∼0.01 Hz. Although one of the identified intervals was coherent, extending across the ICME sheath and being long-lived, predicting ICME sheath magnetic fields that may transmit to the Earth's magnetosheath from the upstream at L1 observations has ambiguity. This can result from the strong spatial variability of the ICME sheath fields in the longitudinal direction, or alternatively from the ICME sheath fields developing substantially within the short time it takes the plasma to propagate from L1 to the bow shock. This study demonstrates the complex interplay ICME sheaths have with the Earth's magnetosphere when passing by the planet.
