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1.	Chong, Ghai Siung, et al. (författare) Dawn-Dusk Ion Flow Asymmetry in the Plasma Sheet : Interplanetary Magnetic Field B-y Versus Distance With Respect to the Neutral Sheet 2022 Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 127:4 Tidskriftsartikel (refereegranskat)abstract Previous studies have shown that the average dawn-dusk component of the perpendicular plasma flow in the plasma sheet (V-perpendicular to) can vary depending on the distance relative to the neutral sheet and the dawn-dusk component of the interplanetary magnetic field (IMF B-y). In this study, we combined 33 years of data from the Geotail, Time History of Events and Macroscale Interactions during Substorms, Cluster, and magnetospheric multiscale missions to study the slow (<200 km/s) ion flows perpendicular to the magnetic field. We find that IMF B-y has a hemispheric dependent influence on both the tail B-y and tail V-perpendicular to. Particularly, the influence is more prominent in the midnight sector (compared to both the pre- and post-midnight sectors) and at distances far from the neutral sheet (compared to the distances close to the neutral sheet). However, at distances close to the neutral sheet, there is an increased dominance of duskward flows which dominates over the systematic influence of IMF B-y on tail V-perpendicular to. Our results indicate that IMF B-y has a major influence on the magnetic flux transport in the magnetotail, mainly at distances far from the neutral sheet. The influence is weaker at distances close to the neutral sheet.
2.	Chong, Ghai Siung, et al. (författare) Ion Convection as a Function of Distance to the Neutral Sheet in Earth's Magnetotail 2021 Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 126:12 Tidskriftsartikel (refereegranskat)abstract We utilized 33 years of data obtained by the Geotail, THEMIS, Cluster and MMS missions to investigate the slow (<200 km/s) ion flows perpendicular to the magnetic field in Earth's magnetotail plasma sheet. By using plasma β as a proxy of distance to the neutral sheet, we find that the ion flow patterns vary systematically within the plasma sheet. Particularly, in regions farther from the neutral sheet, earthward (tailward) flows exhibit a strong tendency to diverge (converge) quasi-symmetrically, with respect to the midnight meridional plane. As the distance becomes closer toward the neutral sheet, this tendency to diverge and converge gradually weakens. Moreover, duskward flows become the dominant components in both the earthward and tailward flows. These variations in ion flow patterns with distance to neutral sheet are hemispherically independent. We suggest that the spatial profiles of the electric and diamagnetic drift vary with distance to the neutral sheet and are therefore responsible for the varying ion flow patterns.
3.	Chong, Ghai Siung, et al. (författare) Tailward Flows in the Vicinity of Fast Earthward Flows 2021 Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 126:4 Tidskriftsartikel (refereegranskat)abstract The occurrence of tailward flows in the magnetotail plasma sheet is closely linked to the dynamics of earthward bursty bulk flows (BBFs). Tailward flows that are observed in the vicinity of these BBFs (or TWABs – Tailward flows around BBFs) may hold unique information on its origin. In this study, we conduct a statistical survey on TWABs by using data from the Cluster mission. We find that TWABs are observed in the vicinity of ∼75% of the BBFs and their occurrence does not depend on BBF velocity magnitude. TWABs have a flow convection pattern consistent with the general tailward flows (GTWs) in the plasma sheet and they do not resemble vortical-like flows. However, TWABs have a flow velocity magnitude twice larger than the GTWs. The plasma density and temperature of TWABs are comparable with BBFs. It is more common to observe a TWAB succeeding than preceding a BBF. However, there is no distinctive difference (in flow pattern, plasma density and temperature) between preceding and succeeding TWABs. We suggest that TWABs are likely the “freshly” rebounded BBFs from the near-Earth region where the magnetic field is stronger. TWABs may represent the early stage of the evolution of tailward flows in the plasma sheet. We also discuss and argue that other mechanisms such as shear-induced vortical flows and tailward slipping of depleted flux tubes cannot be the principal causes of TWABs.
4.	De Spiegeleer, Alexandre, et al. (författare) In Which Magnetotail Hemisphere is a Satellite? Problems Using in Situ Magnetic Field Data 2021 Ingår i: Journal of Geophysical Research - Space Physics. - : Blackwell Publishing. - 2169-9380 .- 2169-9402. ; 126:2 Tidskriftsartikel (refereegranskat)abstract In Earth's magnetotail plasma sheet, the sunward-tailward Bx component of the magnetic field is often used to separate the region above and below the cross-tail current sheet. Using a three-dimensional magneto-hydrodynamic simulation, we show that high-speed flows do not only affect the north-south magnetic field component (causing dipolarization fronts), but also the sunward-tailward component via the formation of a magnetic dent. This dent is such that, in the Northern Hemisphere, the magnetic field is tailward while in the Southern Hemisphere, it is earthward. This is opposite to the expected signatures where Bx > 0 (Bx < 0) above (below) the neutral sheet. Therefore, the direction of the magnetic field cannot always be used to identify in which hemisphere an in situ spacecraft is located. In addition, the cross-tail currents associated with the dent is different from the currents in a tail without a dent. From the simulation, we suggest that the observation of a dawnward current and a tailward magnetic tension force, possibly together with an increase in the plasma beta, may indicate the presence of a magnetic dent. To exemplify, we also present data of a high-speed flow observed by the Cluster mission, and we show that the changing sign of Bx is likely due to such a dent, and not to the spacecraft moving across the neutral sheet.
5.	De Spiegeleer, Alexandre, et al. (författare) Oscillatory Flows in the Magnetotail Plasma Sheet : Cluster Observations of the Distribution Function 2019 Ingår i: Journal of Geophysical Research - Space Physics. - : AMER GEOPHYSICAL UNION. - 2169-9380 .- 2169-9402. ; 124:4, s. 2736-2754 Tidskriftsartikel (refereegranskat)abstract Plasma dynamics in Earth's magnetotail is often studied using moments of the distribution function, which results in losing information on the kinetic properties of the plasma. To better understand oscillatory flows observed in the midtail plasma sheet, we investigate two events, one in each hemisphere, in the transition region between the central plasma sheet and the lobes using the 2-D ion distribution function from the Cluster 4 spacecraft. In this case study, the oscillatory flows are a manifestation of repeated ion flux enhancements with pitch angle changing from 0 degrees to 180 degrees in the Northern Hemisphere and from 180 degrees to 0 degrees in the Southern Hemisphere. Similar pitch angle signatures are observed seven times in about 80 min for the Southern Hemisphere event and three times in about 80 min for the Northern Hemisphere event. The ion flux enhancements observed for both events are slightly shifted in time between different energy channels, indicating a possible time-of-flight effect from which we estimate that the source of particle is located similar to 5-25R(E) and similar to 40-107R(E) tailward of the spacecraft for the Southern and Northern Hemisphere event, respectively. Using a test particle simulation, we obtain similar to 21-46 R-E for the Southern Hemisphere event and tailward of X similar to - 65R(E) (outside the validity region of the model) for the Northern Hemisphere event. We discuss possible sources that could cause the enhancements of ion flux.
6.	De Spiegeleer, Alexandre, et al. (författare) Oxygen Ion Flow Reversals in Earth's Magnetotail : A Cluster Statistical Study 2019 Ingår i: Journal of Geophysical Research - Space Physics. - : Wiley-Blackwell. - 2169-9380 .- 2169-9402. ; 124:11, s. 8928-8942 Tidskriftsartikel (refereegranskat)abstract We present a statistical study of magnetotail flows that change direction from earthward to tailward using Cluster spacecraft. More precisely, we study 318 events of particle flux enhancements in the O+ data for which the pitch angle continuously changes with time, either from 0 degrees to 180 degrees or from 180 degrees to 0 degrees. These structures are called "Pitch Angle Slope Structures" (PASSes). PASSes for which the pitch angle changes from 0 degrees to 180 degrees are observed in the Northern Hemisphere while those for which the pitch angle changes from 180 degrees to 0 degrees are observed in the Southern Hemisphere. These flux enhancements result in a reversal of the flow direction from earthward to tailward regardless of the hemisphere where they are observed. Sometimes, several PASSes can be observed consecutively which can therefore result in oscillatory velocity signatures in the earth-tail direction. The PASS occurrence rate increases from 1.8% to 3.7% as the AE index increases from similar to 0 to similar to 600 nT. Also, simultaneously to PASSes, there is typically a decrease in the magnetic field magnitude due to a decrease (increase) of the sunward component of the magnetic field in the Northern (Southern) Hemisphere. Finally, based on the 115 (out of 318) PASSes that show energy-dispersed structures, the distance to the source from the spacecraft is estimated to be typically R-E along the magnetic field line. This study is important as it sheds light on one of the causes of tailward velocities in Earth's magnetotail.
7.	De Spiegeleer, Alexandre, 1990- (författare) There and back again... An Earth magneto-tale : understanding plasma flows in the magnetotail 2020 Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract On average, the Earth's magnetotail plasma sheet seems to be a calm region of the magnetosphere where the plasma moves slowly towards Earth. However, the plasma sheet actually hosts many phenomena, some of which can affect Earth. For example, high-speed flows of plasma with speed larger than 400 km/s are observed in the plasma sheet and they can lead to aurorae. Such dynamical phenomena and the impact they may have on Earth naturally makes the plasma sheet an important region of study. Even though these high-speed flows can affect Earth, they are observed less than 5% of the time, meaning that most of the time, other disturbances take place in the plasma sheet. Our aim is to investigate and better understand the plasma dynamics in the plasma sheet.The plasma above and below the cross-tail current sheet was previously thought to convect in the same direction. However, we find that under clearly non-zero Interplanetary Magnetic Field (IMF)By (dawn-dusk component), the plasma has a tendency to convect in opposite dawn-dusk direction across the current sheet near the midnight sector depending on the sign of IMF By.The high-speed plasma flows are known to be associated with an increase of the northward component of the magnetic field as they propagate toward Earth. Using simulations, we notice that the magnetic field lines are bent by the high-speed flows and dents can appear. The deformation of the magnetic field is such that it may be directed towards the tail above the cross-tail current sheet and towards the Earth below it. This is opposite to the expected orientation of the magnetic field thus making this feature important in order to properly identify the region in which a spacecraft is located.At times, the plasma can be seen to move back and forth in an oscillatory manner. We investigate statistically such oscillatory behaviour and compare them to high-speed flows and to time periods when the plasma is calm. These oscillatory flows are observed about 8% of the time in the plasma sheet. They typically have a frequency of about 1.7 mHz (~10 min period) and usually last about 41 min.Some oscillations of the plasma velocity are observed along the magnetic field. The particles measured by the satellite initially have a velocity parallel to the magnetic field and towards Earth. Gradually with time, the measured velocity of the particles turns around to first become more perpendicular to the magnetic field and then be along the magnetic field but away from Earth. These signatures are interpreted simply as being due to mirroring particles injected tailward of the satellite and move toward Earth. The particles are then reflected, and move away from Earth. We investigate the general features of such oscillations along the magnetic field and find that the source of the particles is typically located less than 25 RE (Earth's radii) tailward of the satellite.
8.	Hamrin, Maria, 1972-, et al. (författare) Can Reconnection be Triggered as a Solar Wind Directional Discontinuity Crosses the Bow Shock? : A Case of Asymmetric Reconnection 2019 Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 124:11, s. 8507-8523 Tidskriftsartikel (refereegranskat)abstract Here we present some unique observations of reconnection at a quasi-perpendicular bow shock as an interplanetary directional discontinuity (DD) is crossing it simultaneously with the Magnetospheric Multiscale (MMS) mission. There are no burst data, but available data show indications of ongoing reconnection at the shock southward of MMS: a bifurcated current sheet with signatures of Hall magnetic and electric fields, normal magnetic fields indicating a magnetic connection between the two reconnecting regions, field-aligned currents and electric fields, E . J > 0 indicating a conversion of magnetic to kinetic energy, and subspin resolution ion energy-time spectrograms indicating ions being accelerated away from the X-line. The DD is also observed by four upstream spacecraft (ACE, WIND, Geotail, and ARTEMIS P1) and one downstream in the magnetosheath (Cluster 4), but none of them resolve signatures of ongoing reconnection. We therefore suggest that reconnection was temporarily triggered as the DD was compressed by the shock. Reconnection at the bow shock is inevitably asymmetric with both the density and magnetic field strength being higher on one side of the X-line (magnetosheath side) than on the other side where the plasma flow also is supersonic (solar wind side). This is different from the asymmetry exhibited at the more commonly studied case of asymmetric reconnection at the magnetopause. Asymmetric reconnection of the bow shock type has never been studied before, and the data discussed here present some first indications of the properties of the reconnection region for this type of reconnection.
9.	Hamrin, Maria, 1972-, et al. (författare) Space weather disturbances in non-stormy times : occurrence of dB/dt spikes during three solar cycles 2023 Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 128:10 Tidskriftsartikel (refereegranskat)abstract Spatio-temporal variations of ionospheric currents cause rapid magnetic field variations at ground level and Geomagnetically Induced Currents (GICs) that can be harmful for human infrastructure. The risk for large excursions in the magnetic field time derivative, “dB/dt spikes”, is known to be high during geomagnetic storms and substorms. However, less is known about the occurrence of spikes during non-stormy times. We use data from ground-based globally covering magnetometers (SuperMAG database) from the years 1985–2021. We investigate the spike occurrence (\|dB/dt\| > 100 nT/min) as a function of magnetic local time (MLT), magnetic latitude (Mlat), and the solar cycle phases during non-stormy times (−15 nT ≤ SYM-H < 0). We sort our data into substorm (AL < 200 nT) intervals (“SUB”) and less active intervals between consecutive substorms (“nonSUB”). We find that spikes commonly occur in both SUBs and nonSUBs during non-stormy times (3–23 spikes/day), covering 18–12 MLT and 65°–80° Mlat. This also implies a risk for infrastructure damage during non-stormy times, especially when several spikes occur nearby in space and time, possibly causing infrastructure weathering. We find that spikes are more common in the declining phase of the solar cycle, and that the occurrence of SUB spikes propagates from one midnight to one morning hotspot with ∼10 min in MLT for each minute in universal time (UTC). Finally, we discuss causes for the spikes in terms of spatio-temporal variations of ionospheric currents.
10.	Hamrin, Maria, et al. (författare) The evolution of flux pileup regions in the plasma sheet : Cluster observations 2013 Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202 .- 2169-9380 .- 2169-9402. ; 118:10, s. 6279-6290 Tidskriftsartikel (refereegranskat)abstract Bursty bulk flows (BBFs) play an important role for the mass, energy, and magnetic flux transport in the plasma sheet, and the flow pattern in and around a BBF has important consequences for the localized energy conversion between the electromagnetic and plasma mechanical energy forms. The plasma flow signature in and around BBFs is often rather complicated. Return flows and plasma vortices are expected to exist at the flanks of the main flow channel, especially near the inner plasma sheet boundary, but also farther down-tail. A dipolarization front (DF) is often observed at the leading edge of a BBF, and a flux pileup region (FPR) behind the DF. Here we present Cluster data of three FPRs associated with vortex flows observed in the midtail plasma sheet on 15 August 2001. According to the principles of Fu et al. (2011, 2012c), two of the FPRs are considered to be in an early stage of evolution (growing FPRs). The third FPR is in a later stage of evolution (decaying FPR). For the first time, the detailed energy conversion properties during various stages of the FPR evolution have been measured. We show that the later stage FPR has a more complex vortex pattern than the two earlier stage FPRs. The two early stage FPR correspond to generators, EJ<0, while the later stage FPR only shows weak generator characteristics and is instead dominated by load signatures at the DF, EJ>0. Moreover, to our knowledge, this is one of the first times BBF-related plasma vortices have been observed to propagate over the spacecraft in the midtail plasma sheet at geocentric distances of about 18R(E). Our observations are compared to recent simulation results and previous observations.
11.	Hølland, Vidar, et al. (författare) Nordic ionospheric sounding rocket seeding experiment (NISSE) 2009 Ingår i: Proc. 19th ESA Symposium on European Rocket and Balloon Programmes and Related Research, Bad Reichenhall, Germany, 7-11 June 2009. ; , s. 467-472 Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract The Nordic Ionospheric Sounding rocket Seeding Experiment (NISSE) is a student research project in the REXUS student rocket experiment program. The NISSE experiment flew onboard a sounding rocket, the REXUS 6, which was launched at the Esrange rocket range on March 12, 2009. In the NISSE experiment about 8.3 kg of water was to be released into the ionosphere at the REXUS 6 apogee altitude of about 95 km. The EISCAT UHF incoherent scatter radar system located in Northern Fennoscandia, was in action for detection and observation of the effects of the released water on the upper atmosphere. Although NISSE was only partially successful, we are motivated to present here the conceptual description of the experiment and discuss the experience gained from an educational point of view.
12.	Lindkvist, Jesper, 1986-, et al. (författare) Energy conversion in cometary atmospheres Hybrid modeling of 67P/Churyumov-Gerasimenko 2018 Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 616 Tidskriftsartikel (refereegranskat)abstract Aims. We wish to investigate the energy conversion between particles and electromagnetic fields and determine the location where it occurs in the plasma environment of comets. Methods. We used a hybrid plasma model that included photoionization, and we considered two cases of the solar extreme ultraviolet flux. Other parameters corresponded to the conditions of comet 67P/Churyumov-Gerasimenko at a heliocentric distance of 1.5 AU. Results. We find that a shock-like structure is formed upstream of the comet and acts as an electromagnetic generator, similar to the bow shock at Earth that slows down the solar wind. The Poynting flux transports electromagnetic energy toward the inner coma, where newly born cometary ions are accelerated. Upstream of the shock-like structure, we find local energy transfer from solar wind ions to cometary ions. We show that mass loading can be a local process with a direct transfer of energy, but also part of a dynamo system with electromagnetic generators and loads. Conclusions. The energization of cometary ions is governed by a dynamo system for weak ionization, but changes into a large conversion region with local transfer of energy directly from solar wind protons for high ionization.
13.	Norenius, Linus, et al. (författare) Ground-Based Magnetometer Response to Impacting Magnetosheath Jets 2021 Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 126:8 Tidskriftsartikel (refereegranskat)abstract Localized dynamic pressure pulses in the magnetosheath, or jets, have been a popular topic for discussion in recent decades. Studies show that they can propagate through the magnetosheath and impact the magnetopause, possibly showing up as geoeffective elements at ground level. However, questions still remain on how geoeffective they can be. Previous studies have been limited to case studies during few days and with only a handful of events. In this study we have found 65 cases of impacting jets using observations from the Multiscale Magnetospheric mission during 2015–2017. We examine their geoeffectiveness using ground-based magnetometers (GMAGs). From our statistics we find that GMAGs observe responses as fluctuations in the geomagnetic field with amplitudes of 34 nT, frequencies of 1.9 mHz, and damping times of 370 s. Further, the parallel length and the maximum dynamic pressure of the jet dictate the amplitude of the observed GMAG response. Longer and higher pressure jets inducing larger amplitude responses in GMAG horizontal components. The median time required for the signal to be detected by GMAGs is 190 s. We also examine if jets can be harmful for human infrastructure and cannot exclude that such events could exist.
14.	Park, Jong-Sun, et al. (författare) Auroral electrojet activity for long-duration radial interplanetary magnetic field events 2023 Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 128:3 Tidskriftsartikel (refereegranskat)abstract In this paper, we provide statistical evidence that the level of solar wind-magnetosphere-ionosphere (SW-M-I) coupling is weaker under radial (Sun-Earth component dominant) interplanetary magnetic field (IMF) conditions than non-radial IMF conditions. This is performed by analyzing auroral electrojet activity (using SuperMAG auroral electrojet indices) in the sunlit and dark ionospheres for long-duration (at least 4 hr) radial IMF events and comparing against the same for long-duration azimuthal (dusk-dawn component dominant) IMF events. We show that the north-south IMF component (IMF Bz) plays a crucial role in controlling the level of auroral electrojet activity as a negative half-wave rectifier even for both IMF orientation categories. However, it is found that the magnitudes of the auroral electrojet indices are generally lower for radial IMF than for azimuthal IMF under similar sets of solar wind (radial bulk velocity and number density) and IMF Bz conditions, regardless of whether these indices are derived in the sunlit or dark regions. Moreover, the efficiency of coupling functions is lower for radial IMF than for azimuthal IMF, implying that increased coupling strength due to the azimuthal IMF component alone cannot well explain weaker auroral electrojets during radial IMF periods. Lastly, the contribution of the radial IMF component itself to auroral electrojet activity is also lower compared to the azimuthal IMF component. Our results suggest that the level of SW-M-I coupling characterized by auroral electrojet activity can be modulated by the radial IMF component, although the effect of this component is weaker than the other two IMF components.
15.	Park, Jong-Sun, et al. (författare) Radial Interplanetary Magnetic Field-Induced North-South Asymmetry in the Solar Wind-Magnetosphere-Ionosphere Coupling : A Case Study 2022 Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 127:2 Tidskriftsartikel (refereegranskat)abstract In this paper, we present a case study of the radial interplanetary magnetic field (IMF Bx)-induced asymmetric solar wind-magnetosphere-ionosphere (SW-M-I) coupling between the northern and southern polar caps using ground-based and satellite-based data. Under prolonged conditions of strong earthward IMF on 5 March 2015, we find significant discrepancies between polar cap north (PCN) and polar cap south (PCS) magnetic indices with a negative bay-like change in the PCN and a positive bay-like change in the PCS. The difference between these indices (PCN-PCS) reaches a minimum of −1.63 mV/m, which is approximately three times higher in absolute value than the values for most of the time on this day (within ±0.5 mV/m). The high-latitude plasma convection also shows an asymmetric feature such that there exists an additional convection cell near the noon sector in the northern polar cap, but not in the southern polar cap. Meanwhile, negative bays in the north-south component of ground magnetic field perturbations (less than 50 nT) observed in the nightside auroral region of the Northern Hemisphere are accompanied with the brightening and widening of the nightside auroral oval in the Southern Hemisphere, implying a weak, but clear energy transfer to the nightside ionosphere of both hemispheres. After the hemispheric asymmetries in the polar caps disappear, a substorm onset takes place. All these observations indicate that IMF Bx-induced single lobe reconnection that occurred in the Northern Hemisphere plays an important role in hemispheric asymmetry in the energy transfer from the solar wind to the polar cap through the magnetosphere.
16.	Park, Jong-Sun, et al. (författare) Statistical features of polar cap North and South indices in response to interplanetary and terrestrial conditions : a revisit 2024 Ingår i: Space Weather. - : American Geophysical Union (AGU). - 1542-7390. ; 22:4 Tidskriftsartikel (refereegranskat)abstract In this study, we investigate statistical features of polar cap north (PCN) and south (PCS) indices in response to various interplanetary conditions (interplanetary magnetic field [IMF] orientation in three-dimensions) and terrestrial conditions (seasonal and magnetic local time [MLT] locations of the index stations). The concurrent PCN-PCS pairs for 1998–2002 and 2004–2018 are divided based on their sign type (positive-positive, negative-negative, negative-positive, and positive-negative PCN-PCS pairs) and time coverage (the times when both index stations are in the dawn/dusk MLT sector during northern summer/winter). Analyzing the IMF orientation dependence on the occurrence probabilities of concurrent indices and on the differences between the indices in various sign types for each time coverage reveals that the statistical features in PCN-PCS pairs obtained in the dawn MLT sector can be largely explained by the effects of the three-component IMF (related to the polar cap convection patterns) combined with season (related to the hemispheric asymmetry in solar illumination-induced ionospheric conductance). However, those obtained in the dusk MLT sector are controlled dominantly by seasonal effects rather than IMF orientation effects. Our findings indicate that PCN-PCS pair data provide local views about the solar wind-magnetosphere-ionosphere (SW-M-I) coupling system with different control efficiencies of IMF orientation and season depending on the MLT location of the stations. Therefore, introducing polar cap indices recorded simultaneously at various locations in both hemispheres and analyzing them are strongly required to infer global views of the coupled SW-M-I system in the open field regions with higher confidence.
17.	Park, Jong-Sun, et al. (författare) Transpolar Arcs During a Prolonged Radial Interplanetary Magnetic Field Interval 2021 Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 126:6 Tidskriftsartikel (refereegranskat)abstract Transpolar arcs (TPAs) are believed to predominantly occur under northward interplanetary magnetic field (IMF) conditions with their hemispheric asymmetry controlled by the Sun-Earth (radial) component of the IMF. In this study, we present observations of TPAs that appear in both the northern and southern hemispheres even during a prolonged interval of radially oriented IMF. The Defense Meteorological Satellite Program (DMSP) F16 and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellites observed TPAs on the dawnside polar cap in both hemispheres (one TPA structure in the southern hemisphere and two in the northern hemisphere) during an interval of nearly earthward-oriented IMF on October 29, 2005. The southern hemisphere TPA and one of the northern hemisphere TPAs are associated with electron and ion precipitation and mostly sunward plasma flow (with shears) relative to their surroundings. Meanwhile, the other TPA in the northern hemisphere is associated with an electron-only precipitation and antisunward flow relative to its surroundings. Our observations indicate the following: (a) the TPA formation is not limited to northward IMF conditions; (b) the TPAs can be located on both closed field lines rooted in the polar cap of both hemispheres and open field lines connected to the northward field lines draped over one hemisphere of the magnetopause. We believe that the TPAs presented here are the result of both indirect and direct processes of solar wind energy transfer to the high-latitude ionosphere.
18.	Pitkänen, Timo, 1979-, et al. (författare) Asymmetry in the Earth's magnetotail neutral sheet rotation due to IMF B-y sign? 2021 Ingår i: Geoscience Letters. - : Springer Nature. - 2196-4092. ; 8:1 Tidskriftsartikel (refereegranskat)abstract Evidence suggests that a non-zero dawn-dusk interplanetary magnetic field (IMF B-y) can cause a rotation of the cross-tail current sheet/neutral sheet around its axis aligned with the Sun-Earth line in Earth's magnetotail. We use Geotail, THEMIS and Cluster data to statistically investigate how the rotation of the neutral sheet depends on the sign and magnitude of IMF B-y. In our dataset, we find that in the tail range of -30 < XGSM < -15 R-E, the degree of the neutral sheet rotation is clearly smaller, there appears no significant rotation or even, the rotation is clearly to an unexpected direction for negative IMF B-y, compared to positive IMF B-y. Comparison to a model by Tsyganenko et al. (2015, doi:10.5194/angeo-33-1-2015) suggests that this asymmetry in the neutral sheet rotation between positive and negative IMF B-y conditions is too large to be explained only by the currently known factors. The possible cause of the asymmetry remains unclear.
19.	Pitkänen, Timo, 1979-, et al. (författare) Azimuthal velocity shear within an Earthward fast flow : further evidence for magnetotail untwisting? 2015 Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 33, s. 245-255 Tidskriftsartikel (refereegranskat)abstract It is well known that nonzero interplanetary magnetic field By conditions lead to a twisted magnetotail configuration. The plasma sheet is rotated around its axis and tail magnetic field lines are twisted, which causes an azimuthal displacementof their ionospheric footprints. According to the untwisting hypothesis, the untwisting of twisted field lines is suggested to influence the azimuthal direction of convective fast flows in the nightside geospace. However, there is a lack of in situ magnetospheric observations, which show actual signatures of the possible untwisting process. In this paper, we report detailed Cluster observations of an azimuthal flow shear across the neutral sheet associated with an Earthward fast flow on 5 September 2001. The observations show a flow shear velocity pattern with a Vperpy sign change, near the neutral sheet (Bx 0) within a fast flow during the neutral sheet flapping motion over the spacecraft. Firstly, this implies that convective fast flows may not generally be unidirectional across the neutral sheet, but may have a more complex structure. Secondly, in this event tail By and the flow shear are as expected by the untwisting hypothesis. The analysis of the flow shear reveals a linear dependence between Bx and Vperpy close to the neutral sheet and suggests that Cluster crossed the neutral sheet in the dawnward part of the fast flow channel. The magnetospheric observations are supported by the semi-empirical T96 and TF04 models. Furthermore, the ionospheric SuperDARN convection maps support the satellite observations proposing that the azimuthal component of the magnetospheric flows is enforced by a magnetic field untwisting. In summary,the observations give strong supportive evidence to the tail untwisting hypothesis. However, the T96 ionospheric mapping demonstrates the limitations of the model in mapping from a twisted tail.
20.	Pitkänen, Timo, 1979-, et al. (författare) Convection electric field and plasma convection in a twisted magnetotail : t THEMIS case study 1-2 January 2009 2018 Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 123:9, s. 7486-7497 Tidskriftsartikel (refereegranskat)abstract We investigate THEMIS satellite measurements made in a tail-aligned constellation during a time interval on 1-2 January 2009, which has previously been attributed to an interval of an interplanetary magnetic fieldB(y)-driven magnetotail twisting. We find evidence for that the orientation of the convection electric field in the tail is twist-mode dependent. For earthward flow and a negative twist (induced tail B-y < 0), the electric field is found to have northward E-z and tailward E-x components. During a positive twist (induced tail B-y > 0), the directions of E-z and E-x are reversed. The E-y component shows the expected dawn-to-dusk direction for earthward flow. The electric field components preserve their orientation across the neutral sheet, and a quasi-collinear field is observed irrespective to the tail distance. The electric field associated with the tailward flow has an opposite direction compared to the earthward flow for the negative twist. For the positive twist, the results are less clear. The corresponding plasma convection and thus the magnetic flux transport have an opposite dawn-dusk direction above and below the neutral sheet. The directions depend on the tail twist mode. The hemispherically asymmetric earthward plasma flows are suggested to be a manifestation of an asymmetric Dungey cycle in a twisted magnetotail. The role of tailward flows deserve further investigation.
21.	Pitkänen, Timo, 1979-, et al. (författare) Fast Earthward Convection in the Magnetotail and Nonzero IMF By : MMS Statistics 2023 Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 128:12 Tidskriftsartikel (refereegranskat)abstract We statistically investigate convective earthward fast flows using data measured by the Magnetospheric Multiscale mission in the tail plasma sheet during 2017–2021. We focus on “frozen in” fast flows and investigate the importance of different electric field components in the Sun-Earth (V⊥x) and dusk-dawn (V⊥y) velocity components perpendicular to the magnetic field. We find that a majority of the fast flow events (52% of 429) have the north-south electric field component (Ez) as the most relevant or dominating component whereas 26% are so-called conventional type fast flows with Ey and Ex as the relevant components. The rest of the flow events, 22%, fall into the two ’mixed’ categories, of which almost all these fast flows, 20% of 429, have Ey and Ez important for V⊥x and V⊥y, respectively. There is no Y-location preference for any type of the fast flows. The conventional fast flows are detected rather close to the neutral sheet whereas the other types can be measured farther away. Typical total speeds are highest in the mixed category. Typical perpendicular speeds are comparably high in the conventional and mixed categories. The slowest fast flows are measured in the Ez category. Most of the fast flow events are measured in the substorm recovery phase. Prevailing interplanetary magnetic field By conditions influence the V⊥y direction and the influence is most efficient for the Ez-dominated fast flows.
22.	Pitkänen, Timo, 1979-, et al. (författare) IMF By Influence on Magnetospheric Convection in Earth's Magnetotail Plasma Sheet 2019 Ingår i: Geophysical Research Letters. - : Blackwell Publishing Ltd. - 0094-8276 .- 1944-8007. ; 46:21, s. 11698-11708 Tidskriftsartikel (refereegranskat)abstract We use Geotail, Cluster, and Time History of Events and Macroscale Interactions during Substorms data over 15 years (1995–2009) to statistically investigate convective ion flows (V⊥xy<200 km/s) in the magnetotail plasma sheet under the influence of a clearly nonzero dawn-dusk interplanetary magnetic field (IMF By). We find that IMF By causes an interhemispheric asymmetry in the flows, which depends on the direction of IMF By. On the average, one magnetic hemisphere is dominated by a dawn-dusk flow component, which is oppositely directed compared to that in the other hemisphere. This asymmetry is observed for both earthward and tailward flows. A comparison to tail By reveals that the region where the asymmetry in the average flows appears agrees with the appearance of the tail By direction collinear to IMF By. The results imply that IMF By has a major influence on the direction of the magnetic flux transport in the magnetotail.
23.	Pitkänen, Timo, 1979-, et al. (författare) IMF dependence of the azimuthal direction of earthward magnetotail fast flows 2013 Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 40:21, s. 5598-5604 Tidskriftsartikel (refereegranskat)abstract Cluster magnetotail data together with ACE solar wind data from 2001 to 2009 are used to investigate the dependence of the azimuthal flow direction of earthward magnetotail fast flows on the interplanetary magnetic field (IMF). We find an indication that fast flows have favored azimuthal directions that have dependence on the IMF. Our results suggest that for positive IMF By, the favored azimuthal direction of the fast flows is dawnward in the northern plasma sheet and duskward in the southern plasma sheet. For negative IMF By, an opposite situation takes place, the favored azimuthal flow directions are then duskward and dawnward in the northern and southern plasma sheet, respectively. As a possible explanation for the results, it is suggested that the untwisting reconnected magnetic field lines may direct the fast flows in the magnetotail, the field line twist itself being dependent on the IMF.
24.	Pitkänen, Timo, 1979-, et al. (författare) On IMF By-induced dawn-dusk asymmetries in earthward convective fast flows 2017. - 1 Ingår i: Dawn-dusk asymmetries in planetary plasma environments. - Hoboken, NJ, USA : American Geophysical Union (AGU). - 9781119216322 - 9781119216346 ; , s. 95-106 Bokkapitel (refereegranskat)abstract Studies of earthward plasma and magnetic field transport in the Earth’s magnetotail plasma sheet have shown that, on the average, Earthward ion flows in the premidnight and midnight sectors exhibit a duskward component while flows in the postmidnight sector are dawnward. The flow pattern is more pronounced for slower flows (<100 km/s) and alters gradually to a more symmetric one with respect to midnight for increasing flow speeds. However, recent ionospheric and magnetospheric studies have suggested that a nonzero By component in the interplanetary magnetic field (IMF) may significantly influence the earthward transport, creating previously unnoticed dawn‐dusk asymmetries between the hemispheres. In this article, we give a short overview of the present understanding of the topic, present new results, and briefly discuss the importance of the IMF By component for the Earthward transport processes in the magnetotail plasma sheet.
25.	Pitkänen, Timo, 1979-, et al. (författare) Relevance of the North-South Electric Field Component in the Propagation of Fast Convective Earthward Flows in the Magnetotail : An Event Study 2021 Ingår i: Journal of Geophysical Research - Space Physics. - : AMER GEOPHYSICAL UNION. - 2169-9380 .- 2169-9402. ; 126:7 Tidskriftsartikel (refereegranskat)abstract Fast earthward plasma flows are commonly observed in the magnetotail plasma sheet. These flows are often termed as bursty bulk flows because of their bursty nature, and they are considered to be generated by magnetic reconnection. Close to the neutral sheet (B-x similar to 0), the fast flows are considered to be associated with an enhanced dawn-to-dusk electric field (E-y > 0), which together with the northward magnetic field component (B-z > 0) protrude the plasma earthward via enhanced E x B-drift. Sometimes, reversals in the dawn-dusk velocity component perpendicular to the magnetic field (V-perpendicular to y) are measured in association with B-x sign changes in the flows. This suggests that the electric field component in the north-south direction (E-z) can play a role in determining the dawn-dusk direction of the enhanced drift. We present data measured by the Magnetospheric Multiscale, which demonstrate that E-z can have a dictating role for V-perpendicular to y of fast flows. Furthermore, it is shown that the critical contribution of E-z is not limited only to V-perpendicular to y, but it can also dominantly determine the enhanced drift of the fast flows in the X direction (V-perpendicular to x). The latter can occur also near and at the neutral sheet, which adds an alternative configuration to the conventional picture of E-y and B-z being the main players in driving the earthward fast flows. The domination of E-z in the studied events appears with potential signatures of an influence of a nonzero dawn-dusk component of the interplanetary magnetic field (IMF B-y) on the magnetotail.
26.	Pitkänen, Timo, 1979-, et al. (författare) Statistical Survey of Magnetic Forces Associated With Earthward Bursty Bulk Flows Measured by MMS 2017-2021 2023 Ingår i: Journal of Geophysical Research - Space Physics. - : American Geophysical Union (AGU). - 2169-9380 .- 2169-9402. ; 128:5 Tidskriftsartikel (refereegranskat)abstract We investigate the magnetic forces (the magnetic pressure gradient force, the curvature force, and their sum the j x B-force) associated with earthward bursty bulk flows (BBFs) using Magnetospheric Multiscale (MMS) data from five tail seasons (2017-2021). For the first time, the magnetic forces are inferred downtail of XGSM = -20 R-E and in the GSM XY and YZ planes. The results suggest that BBFs tend to be accelerated earthward by the magnetic pressure gradient force tailward of similar to 19 R-E and decelerated closer to that distance in the 2017-2018 data. The force magnitudes increase with distance. This is in accordance with earlier Cluster results. In the 2019-2021 data, the pressure gradient force magnitudes are generally smaller and no clear distance for the acceleration reversal can be determined. The curvature forces for both 2017-2018 and 2019-2021 BBFs indicate earthward acceleration independent of distance, consistent with the Cluster measurements. The sum, the j x B-force, suggests for the 2017-2018 BBFs earthward acceleration tailward of XGSM similar to 15 R-E and deceleration within that distance, also consistent with Cluster. In contrast, the 2019-2021 BBFs show general earthward acceleration by j x B independent of distance. In the GSM XY plane, the average (j x B)(xy) vectors are earthward, and in the premidnight and postmidnight dawnward for the 2017-2018 BBFs. For 2019-2021, the average (j x B)(xy) vectors have components toward the tail center. In the GSM YZ plane, the average (j x B)(yz) vectors are toward the neutral sheet.
27.	Tian, Anmin, et al. (författare) Reconstruction of Plasma Structure with Anisotropic Pressure : Application to Pc5 Compressional Wave 2020 Ingår i: Astrophysical Journal. - : Institute of Physics Publishing (IOPP). - 0004-637X .- 1538-4357. ; 889:1 Tidskriftsartikel (refereegranskat)abstract The conventional Grad-Shafranov (GS) method is designed to reconstruct a two-dimensional magnetohydrostatic structure with isotropic pressure. In this work, we developed a new GS solver (GS-like) that includes the effect of pressure anisotropy based on reduced equations from Sonnerup et al. The new GS solver is benchmarked, and the results are compared with two other GS solvers based on the conventional GS method and that from Teh. This solver is applied to reconstruct a Pc5 compressional wave event, which has mirror-like features and includes a significant pressure anisotropy (p/p(parallel to) similar to 1.5, where p and p(parallel to) are the thermal pressures perpendicular and parallel to the magnetic field), observed by the Magnetospheric Multiscale mission in the duskside outer magnetosphere on 2015 September 19. The recovered maps indicate that, within some model constraints, the wave in the selected time interval consists of two magnetic bottle-like structures, each with an azimuthal size of about 9000 km (wavenumber similar to 44) and a larger field-aligned size. The spacecraft passed through the bottles at similar to 1600 km southward of the bottle centers. Further multispacecraft measurements revealed that the Pc5 compressional wave propagates sunward along with the background plasma and retains the bottle-like structures, driven mainly by the ion diamagnetic currents. The reconstructed magnetic topology is similar to that described in previous empirical or theoretical antisymmetric standing wave models. This Pc5 compressional wave is possibly driven by drift-mirror-like instabilities.
28.	Tian, Anmin, et al. (författare) Structure of Pc 5 Compressional Waves Observed in the Duskside Outer Magnetosphere: MMS Observations 2022 Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 127:3 Tidskriftsartikel (refereegranskat)abstract The geometrical structure of the Pc 5 compressional wave is important in judging its generation mechanism and the wave-particle interaction process. In this work, 117 magnetic troughs (where magnetic field strength transiently decreases) identified from 50 Pc 5 compressional wave events in the duskside (15.5–18.5 local time) magnetosphere are studied based on the Magnetospheric Multiscale (MMS) data. We derived the three dimensional geometry of the magnetic trough by including the normal and velocity information at its boundaries from the multi-spacecraft analysis method. The magnetic trough has a magnetic bottle shape along the magnetic field line with the most probable center (with weakest magnetic field) located at θ = (Formula presented.), while the widest part of the magnetic bottle located around θ (Formula presented.) (θ denotes the angle between spacecraft position vector and the ambient magnetic field). The cross section of the magnetic trough is eccentric and has a “wedge-like” shape whose average open angle is ∼23° toward radial outward. It is found that the radial component of the current density is the dominant one at the boundaries, and the value is generally proportional to the depth of the magnetic trough. The generation of these Pc 5 compressional waves can be attributed to the drift Alfvén ballooning mirror instability. This work reveals the possible changes of magnetic field configuration caused by the Pc 5 compressional wave in the magnetosphere and may bring new ideas to the interaction way between wave field and ring current particles.
29.	Vaverka, Jakub, et al. (författare) Detection of EMPs generated by meteoroid impacts on the MMS spacecraft and problems with signal interpretation 2017 Ingår i: 2017 XXXIInd General Assembly and Scientific Symposium of the International Union of Radio Science (URSI GASS). - : IEEE. - 9789082598704 Konferensbidrag (refereegranskat)abstract Signatures of hypervelocity dust impacts detected by electric field instruments are still not completely understood. We have used the electric field instrument onboard one of the MMS spacecraft orbiting the Earth since 2015 to study various pulses in the measured electric field detected simultaneously by multiple antennas. This unique instrument allows a detailed investigation of registered waveforms. The preliminary results shown that the solitary waves can generate similar pulses as dust impacts and detected pulses can easily by misinterpreted when only one antenna is used.
30.	Wang, Mengmeng, et al. (författare) Statistical analysis of whistler precursors upstream of foreshock transient shocks : MMS observations 2024 Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 51:8 Tidskriftsartikel (refereegranskat)abstract Using the high-time-resolution data from the Magnetospheric Multiscale mission, precursor waves upstream of foreshock transient (FT) shocks are statistically investigated using the four-spacecraft timing method. The wave frequencies and wave vectors determined in the plasma rest frame (PRF) are shown to follow the cold plasma dispersion relation for whistler waves. Combining with the feature of the right-hand polarization in the PRF, the precursors are identified as whistler-mode waves around the lower hybrid frequency. The occurrence of whistler precursors is independent of the Alfvén Mach number and the FT shock normal angle. More importantly, all the whistler precursors have group velocities pointing upstream in the shock frame, suggesting the dispersive radiation to be a possible generation mechanism. The study improves the understanding of not only the whistler precursors but also the overall FT shock dynamics.
31.	Xiao, Y.C., et al. (författare) Statistical properties of the distribution and generation of kinetic-scale flux ropes in the terrestrial dayside magnetosheath 2023 Ingår i: Geophysical Research Letters. - : American Geophysical Union (AGU). - 0094-8276 .- 1944-8007. ; 50:23 Tidskriftsartikel (refereegranskat)abstract The generation of kinetic-scale flux ropes (KSFRs) is closely related to magnetic reconnection. Both flux ropes and reconnection sites are detected in the magnetosheath and can impact the dynamics upstream of the magnetopause. In this study, using the Magnetospheric Multiscale satellite, 12,623 KSFRs with a scale <20 RCi are statistically studied in the Earth's dayside magnetosheath. It is found that they are mostly generated near the bow shock (BS), and propagate downstream in the magnetosheath. Their quantity significantly increases as the scale decreases, consistent with a flux rope coalescence model. Moreover, the solar wind parameters can control the occurrence rate of KSFRs. They are more easily generated at high Mach number, large proton density, and weak magnetic field strength of the solar wind, similar to the conditions that favor BS reconnection. Our study shows a close connection between KSFR generation and BS reconnection.
32.	Yao, S.T., et al. (författare) Ion-Vortex Magnetic Hole With Reversed Field Direction in Earth's Magnetosheath 2023 Ingår i: Journal of Geophysical Research - Space Physics. - : John Wiley & Sons. - 2169-9380 .- 2169-9402. ; 128:7 Tidskriftsartikel (refereegranskat)abstract Plasma vortices are ubiquitous in space and play important roles in the transmission of energy and mass at various scales. For small-scale plasma vortices on the order of ion gyroradius, however, their properties and characteristics remain unclear. Here, we provide unique findings of an ion-scale vortex observed in the Earth's magnetosheath. The vortex is generated by the ion diamagnetic drift associated with an isolated magnetic hole (MH). The magnetic field in the axial direction is reversed in the vortex center, which is consistent with ring-shaped currents carried by the ions. The field strength becomes very weak (<1 nT) at the field reversal region, although the ion distributions vary rather continuously across the entire structure. A kinetic equilibrium model is then applied to reconstruct the above features. These findings can help us understand the plasma vortex and MH from magnetohydrodynamics to kinetic scales.

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