scholarly journals On the alignment of velocity and magnetic fields within magnetosheath jets

2020 ◽  
Vol 38 (2) ◽  
pp. 287-296
Author(s):  
Ferdinand Plaschke ◽  
Maria Jernej ◽  
Heli Hietala ◽  
Laura Vuorinen
Keyword(s):  
Magnetic Fields ◽  
Entire Range ◽  
Dynamic Pressure ◽  
Bow Shock ◽  
Superposed Epoch Analysis ◽  
Magnetospheric Multiscale ◽  
Study Results ◽  
Epoch Analysis ◽  
The Magnetic Field

Abstract. Jets in the subsolar magnetosheath are localized enhancements in dynamic pressure that are able to propagate all the way from the bow shock to the magnetopause. Due to their excess velocity with respect to their environment, they push slower ambient plasma out of their way, creating a vortical plasma motion in and around them. Simulations and case study results suggest that jets also modify the magnetic field in the magnetosheath on their passage, aligning it more with their velocity. Based on Magnetospheric Multiscale (MMS) jet observations and corresponding superposed epoch analyses of the angles ϕ between the velocity and magnetic fields, we can confirm that this suggestion is correct. However, while the alignment is more significant for faster than for slower jets, and for jets observed close to the bow shock, the overall effect is small: typically, reductions in ϕ of around 10∘ are observed at jet core regions, where the jets' velocities are largest. Furthermore, time series of ϕ pertaining to individual jets significantly deviate from the superposed epoch analysis results. They usually exhibit large variations over the entire range of ϕ: 0 to 90∘. This variability is commonly somewhat larger within jets than outside them, masking the systematic decrease in ϕ at core regions of individual jets.

scholarly journals On the alignment of velocity and magnetic fields within magnetosheath jets

2019 ◽  
Author(s):  
Ferdinand Plaschke ◽  
Maria Jernej ◽  
Heli Hietala ◽  
Laura Vuorinen
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Entire Range ◽  
Dynamic Pressure ◽  
Bow Shock ◽  
Superposed Epoch Analysis ◽  
Study Results ◽  
Epoch Analysis ◽  
The Magnetic Field

Abstract. Jets in the subsolar magnetosheath are localized enhancements in dynamic pressure that are able to propagate all the way from the bow shock to the magnetopause. Due to their excess velocity with respect to their environment, they push slower ambient plasma out of their way, creating a vortical plasma motion in and around them. Simulations and case study results suggest that jets also modify the magnetic field in the magnetosheath on their passage, aligning it more with their velocity. Based on MMS jet observations and corresponding superposed epoch analyses of the angles φ between the velocity and magnetic fields, we can confirm that this suggestion is correct. However, the effect is small: Typically, reductions in φ of only 10° are observed at jet core regions, where the jets' velocities are largest. Furthermore, time series of angles φ pertaining to individual jets significantly deviate from the superposed epoch analysis results. They usually exhibit large variations over the entire range of φ: 0° to 90°. This variability is commonly somewhat larger within jets than outside, masking the systematic decrease in φ at core regions of individual jets.

scholarly journals Scattering of field-aligned beam ions upstream of Earth's bow shock

2007 ◽  
Vol 25 (3) ◽  
pp. 785-799 ◽  
Author(s):  
A. Kis ◽  
M. Scholer ◽  
B. Klecker ◽  
H. Kucharek ◽  
E. A. Lucek ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Field Measurements ◽  
Bow Shock ◽  
Ion Distribution ◽  
Parallel Side ◽  
Earth’S Bow Shock ◽  
High Mach ◽  
The Magnetic Field

Abstract. Field-aligned beams are known to originate from the quasi-perpendicular side of the Earth's bow shock, while the diffuse ion population consists of accelerated ions at the quasi-parallel side of the bow shock. The two distinct ion populations show typical characteristics in their velocity space distributions. By using particle and magnetic field measurements from one Cluster spacecraft we present a case study when the two ion populations are observed simultaneously in the foreshock region during a high Mach number, high solar wind velocity event. We present the spatial-temporal evolution of the field-aligned beam ion distribution in front of the Earth's bow shock, focusing on the processes in the deep foreshock region, i.e. on the quasi-parallel side. Our analysis demonstrates that the scattering of field-aligned beam (FAB) ions combined with convection by the solar wind results in the presence of lower-energy, toroidal gyrating ions at positions deeper in the foreshock region which are magnetically connected to the quasi-parallel bow shock. The gyrating ions are superposed onto a higher energy diffuse ion population. It is suggested that the toroidal gyrating ion population observed deep in the foreshock region has its origins in the FAB and that its characteristics are correlated with its distance from the FAB, but is independent on distance to the bow shock along the magnetic field.

scholarly journals Magnetic clouds' structure in the magnetosheath as observed by Cluster and Geotail: four case studies

2014 ◽  
Vol 32 (10) ◽  
pp. 1247-1261 ◽  
Author(s):  
L. Turc ◽  
D. Fontaine ◽  
P. Savoini ◽  
E. K. J. Kilpua
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Magnetic Fields ◽  
Bow Shock ◽  
Field Direction ◽  
Magnetic Clouds ◽  
Magnetic Field Direction ◽  
Field Orientation ◽  
The Impact ◽  
The Magnetic Field

Abstract. Magnetic clouds (MCs) are large-scale magnetic flux ropes ejected from the Sun into the interplanetary space. They play a central role in solar–terrestrial relations as they can efficiently drive magnetic activity in the near-Earth environment. Their impact on the Earth's magnetosphere is often attributed to the presence of southward magnetic fields inside the MC, as observed in the upstream solar wind. However, when they arrive in the vicinity of the Earth, MCs first encounter the bow shock, which is expected to modify their properties, including their magnetic field strength and direction. If these changes are significant, they can in turn affect the interaction of the MC with the magnetosphere. In this paper, we use data from the Cluster and Geotail spacecraft inside the magnetosheath and from the Advanced Composition Explorer (ACE) upstream of the Earth's environment to investigate the impact of the bow shock's crossing on the magnetic structure of MCs. Through four example MCs, we show that the evolution of the MC's structure from the solar wind to the magnetosheath differs largely from one event to another. The smooth rotation of the MC can either be preserved inside the magnetosheath, be modified, i.e. the magnetic field still rotates slowly but at different angles, or even disappear. The alteration of the magnetic field orientation across the bow shock can vary with time during the MC's passage and with the location inside the magnetosheath. We examine the conditions encountered at the bow shock from direct observations, when Cluster or Geotail cross it, or indirectly by applying a magnetosheath model. We obtain a good agreement between the observed and modelled magnetic field direction and shock configuration, which varies from quasi-perpendicular to quasi-parallel in our study. We find that the variations in the angle between the magnetic fields in the solar wind and in the magnetosheath are anti-correlated with the variations in the shock obliquity. When the shock is in a quasi-parallel regime, the magnetic field direction varies significantly from the solar wind to the magnetosheath. In such cases, the magnetic field reaching the magnetopause cannot be approximated by the upstream magnetic field. Therefore, it is important to take into account the conditions at the bow shock when estimating the impact of an MC with the Earth's environment because these conditions are crucial in determining the magnetosheath magnetic field, which then interacts with the magnetosphere.

Drifting Electron Holes Occurring During Geomagnetically Quiet Times: BD-IES Observations

2020 ◽  
Author(s):  
Zhi-Yang Liu ◽  
Qiu-Gang Zong ◽  
Hong Zou
Keyword(s):  
Quiet Time ◽  
Drift Motion ◽  
Superposed Epoch Analysis ◽  
Bursty Bulk Flow ◽  
Magnetospheric Multiscale ◽  
Substorm Activity ◽  
Ground Magnetic ◽  
Solar Wind Parameters ◽  
Epoch Analysis ◽  
Electron Holes

<p>Drifting electron holes (DEHs), manifesting as sudden but mild dropout in electron flux, are a common phenomenon seen in the Earth's magnetosphere. It manifests the change of the state of the magnetosphere. However, previous studies primarily focus on DEHs during geomagnetically active time (e.g., substorm). Not until recently have quiet time DEHs been reported. In this paper, we present a systematic study on the quiet time DEHs. BeiDa Imaging Electron Spectrometer (BD-IES) measurements from 2015 to 2017 are investigated. Twenty-two DEH events are identified. The DEHs cover the whole energy range of BD-IES (50–600 keV). Generally, the DEHs are positively dispersive with respect to energy. Time-of-flight analysis suggests the dispersion results from electron drift motion and gives the location where the DEHs originated from. Statistics reveal the DEHs primarily originated from the postmidnight magnetosphere. In addition, superposed epoch analysis applied to geomagnetic indices and solar wind parameters indicates these DEH events occurred during geomagnetically quiet time. No storm or substorm activity could be identified. However, an investigation into nightside midlatitude ground magnetic records suggests these quiet time DEHs were accompanied by Pi2 pulsations. The DEH-Pi2 connection indicates a possible DEH-bursty bulk flow (BBF) connection, since nightside midlatitude Pi2 activity is generally attributed to magnetotail BBFs. This connection is also supported by a case study of coordinated magnetotail observations from Magnetospheric Multiscale spacecraft. Therefore, we suggest the quiet time DEHs could be caused by magnetotail BBFs, similar to the substorm time DEHs.</p>

Modification of the magnetosheath due to the high-speed jets propagation.

2020 ◽  
Author(s):  
Oleksandr Goncharov ◽  
Herbert Gunell ◽  
Maria Hamrin ◽  
Linus Norenius ◽  
Olga Gutynska
Keyword(s):  
Statistical Analysis ◽  
Velocity Component ◽  
High Speed ◽  
Dynamic Pressure ◽  
Cone Angle ◽  
Bow Shock ◽  
The Earth ◽  
Magnetospheric Multiscale ◽  
Wide Range ◽  
Statistical Studies

<p>Plasmoids, defined as plasma entities with a higher anti-sunward velocity component than the surrounding plasma, have been observed in the magnetosheath in recent years. Among other denominations, plasmoids are also called “magnetosheath jets” and can be classified by transient localized enhancements in dynamic pressure. Propagating through the magnetosheath, jets do not only affect the magnetopause and magnetosphere. Jets pushed slower ambient magnetosheath plasma out of their way. As a result, plasma moves around the jets, and it is slowed down or could even be pushed in the sunward direction. Consequently, jets may create anomalous flows and be a source of additional turbulence. Using the magnetosheath measurements by the Magnetospheric Multiscale (MMS) and THEMIS spacecraft, and comparing several criteria, we have identified several thousand events in the wide range of bow shock distances. Previous statistical studies have shown that jet occurrence is almost exclusively controlled by the angle between the IMF and the Earth–Sun line (cone angle), and jets are predominantly observed when this cone angle is small. However, high-speed jets downstream of the quasi-perpendicular bow shock are very common. Our statistical analysis shows differences of jets evolution in the quasi-parallel and quasi-perpendicular magnetosheath regions. We discuss their properties, nature and relation to anomalies regions in the magnetosheath.</p>

Superposed Epoch Analysis of High Latitude Ionospheric Joule Heating during Major Geomagnetic Storms over three Solar Cycles

2018 ◽  
Vol 13 (S340) ◽  
pp. 67-68
Author(s):  
K. J. Suji ◽  
P. R. Prince
Keyword(s):  
Solar Wind ◽  
Joule Heating ◽  
High Latitude ◽  
Geomagnetic Storms ◽  
Dynamic Pressure ◽  
Basic Structure ◽  
Proton Density ◽  
Solar Cycles ◽  
Superposed Epoch Analysis ◽  
Epoch Analysis

AbstractSuperposed epoch analysis (SPEA) is commonly used to determine some basic structure in a collection of geophysical time series. The present study tries to analyze ionospheric Joule heating response at high latitudes, to the prevailing solar wind and IMF conditions on the basis of SPEA. Major geomagnetic storms (CME driven) over three consecutive solar cycles (SC 22, 23 and 24) have been selected. Ascending phase, solar maximum, and declining phase are investigated separately, for each solar cycle, to find out crucial controlling parameters for the generation of high-latitude ionospheric Joule heating. SPEA results show that, IMF parameters such as IMF By, IMF Bz, IMF clock angle and solar wind parameters such as dynamic pressure and proton density influence Joule heating production rate significantly. Meanwhile, the relentlessness of the other parameters such as IMFBt and solar wind bulk speed show that they have poor impact on Joule heating.

scholarly journals Magnetic field and dynamic pressure ULF fluctuations in coronal-mass-ejection-driven sheath regions

2013 ◽  
Vol 31 (9) ◽  
pp. 1559-1567 ◽  
Author(s):  
E. K. J. Kilpua ◽  
H. Hietala ◽  
H. E. J. Koskinen ◽  
D. Fontaine ◽  
L. Turc
Keyword(s):  
Magnetic Field ◽  
Dynamic Pressure ◽  
Low Frequency ◽  
Leading Edge ◽  
Interplanetary Coronal Mass Ejections ◽  
Abrupt Decrease ◽  
Superposed Epoch Analysis ◽  
Sheath Region ◽  
Epoch Analysis ◽  
The Imf

Abstract. Compressed sheath regions form ahead of interplanetary coronal mass ejections (ICMEs) that are sufficiently faster than the preceding solar wind. The turbulent sheath regions are important drivers of magnetospheric activity, but due to their complex internal structure, relatively little is known on the distribution of the magnetic field and plasma variations in them. In this paper we investigate ultra low frequency (ULF) fluctuations in the interplanetary magnetic field (IMF) and in dynamic pressure (Pdyn) using a superposed epoch analysis of 41 sheath regions observed during solar cycle 23. We find strongest fluctuation power near the shock and in the vicinity of the ICME leading edge. The IMF and Pdyn ULF power have different profiles within the sheath; the former is enhanced in the leading part of the sheath, while the latter is increased in the trailing part of the sheath. We also find that the ICME properties affect the level and distribution of the ULF power in sheath regions. For example, sheath regions associated with strong or fast ICMEs, or those that are crossed at intermediate distances from the center, have strongest ULF power and large variation in the power throughout the sheath region. The weaker or slower ICMEs, or those that are crossed centrally, have in general considerably weaker ULF power with relatively smooth profiles. The strong and abrupt decrease of the IMF ULF power at the ICME leading edge could be used to distinguish the ICME from the preceding sheath plasma.

scholarly journals Interchange instability of a curved current layerconvecting in the magnetosheath from the bow shock towards themagnetopause

2004 ◽  
Vol 22 (3) ◽  
pp. 993-999 ◽  
Author(s):  
I. L. Arshukova ◽  
N. V. Erkaev ◽  
H. K. Biernat
Keyword(s):  
Magnetic Field ◽  
Growth Rate ◽  
Solar Wind ◽  
Layer Structure ◽  
Dynamic Pressure ◽  
Bow Shock ◽  
Curvature Radius ◽  
Current Layer ◽  
Interchange Instability ◽  
The Magnetic Field

Abstract. This paper deals with nonsteady perturbations of the magnetosheath parameters which are related to variations of the interplanetary magnetic field from north to south under a constant solar wind dynamic pressure. The magnetic field changes its direction within a thin layer which is convected with the plasma from the bow shock to the ionopause. In the course of time, this current layer is amplified during its motion towards the magnetopause. The intensity of the current is increasing, the layer thickness is decreasing, and the gradients of parameters are becoming much sharper while the layer is approaching the magnetopause. The curvature radius of this layer is decreasing while it is draping around the magnetopause. This curved layer structure with reversed magnetic field in the magnetosheath is found to be unstable with respect to the interchange instability. The growth rate of the instability is obtained for different positions of the layer. Key words. Magnetospheric physics (magnetosheath)

Asymmetric reconnection at the bow shock

2020 ◽  
Author(s):  
Herbert Gunell ◽  
Maria Hamrin ◽  
Oleksandr Goncharov ◽  
Alexandre De Spiegeleer ◽  
Stephen Fuselier ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Field Strength ◽  
Magnetic Field Strength ◽  
Plasma Flow ◽  
Bow Shock ◽  
The Other ◽  
Magnetospheric Multiscale ◽  
The Magnetic Field

<p>Can reconnection be triggered as a directional discontinuity (DD) crosses the bow shock? Here we present some unique observations of asymmetric reconnection at a quasi-perpendicular bow shock as an interplanetary DD is crossing it simultaneously with the Magnetospheric Multiscale (MMS) mission. The data show indications of ongoing reconnection at the bow shock southward of the spacecraft. The DD is also observed by several upstream spacecraft (ACE, WIND, Geotail, and THEMIS B) 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. Bow shock reconnection is inevitably asymmetric with both the density and the magnetic field strength being higher on one side of the X-line (the magneosheath side) than on the other side where the plasma flow also is supersonic (the solar wind side). Asymmetric reconnection of the bow shock type has never been studied before, and the data discussed here are hence unique.</p>

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