The abrupt boundary between a magnetosphere and the surrounding plasma, the magnetopause, has long been known to support surface waves. It was proposed that impulses acting on the boundary might lead ...to a trapping of these waves on the dayside by the ionosphere, resulting in a standing wave or eigenmode of the magnetopause surface. No direct observational evidence of this has been found to date and searches for indirect evidence have proved inconclusive, leading to speculation that this mechanism might not occur. By using fortuitous multipoint spacecraft observations during a rare isolated fast plasma jet impinging on the boundary, here we show that the resulting magnetopause motion and magnetospheric ultra-low frequency waves at well-defined frequencies are in agreement with and can only be explained by the magnetopause surface eigenmode. We therefore show through direct observations that this mechanism, which should impact upon the magnetospheric system globally, does in fact occur.
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.
Key Points
First quantitative study on the connection between jets and bow shock ripples
97% of the studied high speed jets can be produced by ripples
Main ripple size found to be amplitude/wavelength ~9%, i.e., ~0.1Re/1Re
Using 2008–2011 data from the five Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in Earth's subsolar magnetosheath, we study high-speed jets identified as ...intervals when the anti-sunward component of the dynamic pressure in the subsolar magnetosheath exceeds half of its upstream solar wind value. Based on our comprehensive data set of 2859 high-speed jets, we obtain the following statistical results on jet properties and favorable conditions: high-speed jets occur predominantly downstream of the quasi-parallel bow shock, i.e., when interplanetary magnetic field cone angles are low. Apart from that, jet occurrence is only very weakly dependent (if at all) on other upstream conditions or solar wind variability. Typical durations and recurrence times of high-speed jets are on the order of tens of seconds and a few minutes, respectively. Relative to the ambient magnetosheath, high-speed jets exhibit higher speed, density and magnetic field intensity, but lower and more isotropic temperatures. They are almost always super-Alfvénic, often even super-magnetosonic, and typically feature 6.5 times as much dynamic pressure and twice as much total pressure in anti-sunward direction as the surrounding plasma does. Consequently, they are likely to have significant effects on the magnetosphere and ionosphere if they impinge on the magnetopause.
Magnetosheath high‐speed jets—localized dynamic pressure enhancements typically of ∼1 Earth radius in size—impact the dayside magnetopause several times per hour. Here we present the first in situ ...measurements suggesting that such an impact triggered magnetopause reconnection. We use observations from the five Time History of Events and Macroscale Interactions during Substorms spacecraft in a string‐of‐pearls configuration on 7 August 2007. The spacecraft recorded magnetopause in‐and‐out motion during an impact of a magnetosheath jet (VN∼−300 km/s along the magnetopause normal direction). There was no evidence for reconnection for the preimpact crossing, yet three probes observed reconnection after the impact. We infer that the jet impact compressed the originally thick (60–70 di), high magnetic shear (140–160° magnetopause until it was thin enough for reconnection to occur. Magnetosheath high‐speed jets could therefore act as a driver for bursty dayside reconnection.
Key Points
THEMIS probes (string‐of‐pearls formation) recorded magnetopause in‐and‐out motion during jet impact
No reconnection flows for the preimpact crossing; three probes observed reconnection after the impact
Compression by the high‐pressure jet initiated reconnection in an originally thick magnetopause
Abstract
Surface waves process the turbulent disturbances which drive dynamics in many space, astrophysical and laboratory plasma systems, with the outer boundary of Earth’s magnetosphere, the ...magnetopause, providing an accessible environment to study them. Like waves on water, magnetopause surface waves are thought to travel in the direction of the driving solar wind, hence a paradigm in global magnetospheric dynamics of tailward propagation has been well-established. Here we show through multi-spacecraft observations, global simulations, and analytic theory that the lowest-frequency impulsively-excited magnetopause surface waves, with standing structure along the terrestrial magnetic field, propagate against the flow outside the boundary. Across a wide local time range (09–15h) the waves’ Poynting flux exactly balances the flow’s advective effect, leading to no net energy flux and thus stationary structure across the field also. Further down the equatorial flanks, however, advection dominates hence the waves travel downtail, seeding fluctuations at the resonant frequency which subsequently grow in amplitude via the Kelvin-Helmholtz instability and couple to magnetospheric body waves. This global response, contrary to the accepted paradigm, has implications on radiation belt, ionospheric, and auroral dynamics and potential applications to other dynamical systems.
Geoeffective jets impacting the magnetopause are very common Plaschke, F.; Hietala, H.; Angelopoulos, V. ...
Journal of geophysical research. Space physics,
April 2016, 2016-Apr, 2016-04-00, 20160401, Letnik:
121, Številka:
4
Journal Article
Recenzirano
Odprti dostop
The subsolar magnetosheath is penetrated by transient enhancements in dynamic pressure. These enhancements, also called high‐speed jets, can propagate to the magnetopause, causing large‐amplitude yet ...localized boundary indentations on impact. Possible downstream consequences of these impacts are, e.g., local magnetopause reconnection, impulsive penetration of magnetosheath plasma into the magnetosphere, inner magnetospheric and boundary surface waves, drop outs and other variations in radiation belt electron populations, ionospheric flow enhancements, and magnetic field variations observed on the ground. Consequently, jets can be geoeffective. The extend of their geoeffectiveness is influenced by the amount of mass, momentum, and energy they transport, i.e., by how large they are. Their overall importance in the framework of solar wind‐magnetosphere coupling is determined by how often jets of geoeffective size hit the dayside magnetopause. In this paper, we calculate such jet impact rates for the first time. From a large data set of Time History of Events and Macroscale Interactions during Substorms (THEMIS) multispacecraft jet observations, we find distributions of scale sizes perpendicular and parallel to the direction of jet propagation. They are well modeled by an exponential function with characteristic scales of 1.34RE (perpendicular) and 0.71RE (parallel direction), respectively. Using the distribution of perpendicular scale sizes, we derive an impact rate of jets with cross‐sectional diameters larger than 2RE on a reference area of about
100RE2 of the subsolar magnetopause. That rate is about 3 per hour in general, and about 9 per hour under low interplanetary magnetic field cone angle conditions (<30°), which are favorable for jet occurrence in the subsolar magnetosheath.
Key Points
First determination of magnetosheath high‐speed jet cross‐sectional scale size distribution
First determination of geoeffective jet impact rates on the subsolar magnetopause
Impact rates of large jets are much higher than occurrence rates of other dayside transients
We discuss results of a superposed epoch analysis of dipolarization fronts, rapid (δt < 30 s), high‐amplitude (δBz > 10 nT) increases in the northward magnetic field component, observed during six ...Time History of Events and Macroscale Interactions during Substorms (THEMIS) conjunction events. All six fronts propagated earthward; time delays at multiple probes were used to determine their propagation velocity. We define typical magnetic and electric field and plasma parameter variations during dipolarization front crossings and estimate their characteristic gradient scales. The study reveals (1) a rapid 50% decrease in plasma density and ion pressure, (2) a factor of 2–3 increase in high‐energy (30–200 keV) electron flux and electron temperature, and (3) transient enhancements of ∼5 mV/m in duskward and earthward electric field components. Gradient scales of magnetic field, plasma density, and particle flux were found to be comparable to the ion thermal gyroradius. Current densities associated with the Bz increase are, on average, 20 nA/m2, 5–7 times larger than the current density in the cross‐tail current sheet. Because j · E > 0, the dipolarization fronts are kinetic‐scale dissipative regions with Joule heating rates of 10% of the total bursty bulk flow energy.
Key Points
Superposed epoch analysis of THEMIS dipolarization front events
Common pattern in field and particle variations during front crossings
Particle energization and energetic plasma transport
We investigate the accuracy with which the reconnection electric field EM can be determined from in situ plasma data. We study the magnetotail electron diffusion region observed by National ...Aeronautics and Space Administration's Magnetospheric Multiscale (MMS) on 11 July 2017 at 22:34 UT and focus on the very large errors in EM that result from errors in an LMN boundary normal coordinate system. We determine several LMN coordinates for this MMS event using several different methods. We use these M axes to estimate EM. We find some consensus that the reconnection rate was roughly EM = 3.2 ± 0.6 mV/m, which corresponds to a normalized reconnection rate of 0.18 ± 0.035. Minimum variance analysis of the electron velocity (MVA‐ve), MVA of E, minimization of Faraday residue, and an adjusted version of the maximum directional derivative of the magnetic field (MDD‐B) technique all produce reasonably similar coordinate axes. We use virtual MMS data from a particle‐in‐cell simulation of this event to estimate the errors in the coordinate axes and reconnection rate associated with MVA‐ve and MDD‐B. The L and M directions are most reliably determined by MVA‐ve when the spacecraft observes a clear electron jet reversal. When the magnetic field data have errors as small as 0.5% of the background field strength, the M direction obtained by MDD‐B technique may be off by as much as 35°. The normal direction is most accurately obtained by MDD‐B. Overall, we find that these techniques were able to identify EM from the virtual data within error bars ≥20%.
Key Points
The reconnection rate EM is estimated for one event using several techniques to find an M direction
The error bars in EM and the LMN coordinate directions are estimated from virtual data
The reconnection rate is likely EM = 3.2 mV/m ± 0.6 mV/m, which corresponds to a normalized rate of 0.18 ± 0.035
The Magnetospheric Multiscale Magnetometers Russell, C. T.; Anderson, B. J.; Baumjohann, W. ...
Space Science Reviews,
03/2016, Letnik:
199, Številka:
1-4
Journal Article, Book Review
Recenzirano
Odprti dostop
The success of the Magnetospheric Multiscale mission depends on the accurate measurement of the magnetic field on all four spacecraft. To ensure this success, two independently designed and built ...fluxgate magnetometers were developed, avoiding single-point failures. The magnetometers were dubbed the digital fluxgate (DFG), which uses an ASIC implementation and was supplied by the Space Research Institute of the Austrian Academy of Sciences and the analogue magnetometer (AFG) with a more traditional circuit board design supplied by the University of California, Los Angeles. A stringent magnetic cleanliness program was executed under the supervision of the Johns Hopkins University’s Applied Physics Laboratory. To achieve mission objectives, the calibration determined on the ground will be refined in space to ensure all eight magnetometers are precisely inter-calibrated. Near real-time data plays a key role in the transmission of high-resolution observations stored on board so rapid processing of the low-resolution data is required. This article describes these instruments, the magnetic cleanliness program, and the instrument pre-launch calibrations, the planned in-flight calibration program, and the information flow that provides the data on the rapid time scale needed for mission success.
We present a comprehensive statistical study of magnetic holes, defined as localized decreases of the magnetic field strength of at least 50%, in the solar wind near Mercury, using MESSENGER orbital ...data. We investigate the distributions of several properties of the magnetic holes, such as scale size, depth, and associated magnetic field rotation. We show that the distributions are very similar for linear magnetic holes (with a magnetic field rotation across the magnetic holes of less than 25°) and rotational holes (rotations >25°), except for magnetic holes with very large rotations (≳140°). Solar wind magnetic hole scale sizes follow a log‐normal distribution, which we discuss in terms of multiplicative growth. We also investigate the background magnetic field strength of the solar wind surrounding the magnetic holes, and conclude that it is lower than the average solar wind magnetic field strength. This is consistent with finding solar wind magnetic holes in high‐β regions, as expected if magnetic holes have a connection to magnetic mirror mode structures. We also present, for the first time, comprehensive statistics of isolated magnetic holes in a planetary magnetosheath. The properties of the magnetosheath magnetic holes are very similar to the solar wind counterparts, and we argue that the most likely interpretation is that the magnetosheath magnetic holes have a solar wind origin, rather than being generated locally in the magnetosheath.
Key Points
We present a comprehensive statistical study of magnetic holes in the near‐Mercury solar wind and magnetosheath
We find that magnetic hole scale sizes are log‐normally distributed
We suggest that a majority of the magnetosheath magnetic holes have a solar wind origin
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