We use the Magnetospheric Multiscale mission to investigate electron‐scale structures at a dipolarization front. The four spacecraft are separated by electron scales and observe large differences in ...plasma and field parameters within the dipolarization front, indicating strong deviation from typically assumed plane or slightly curved front surface. We attribute this to ripples generated by the lower hybrid drift instability (LHDI) with wave number of kρe≃0.4 and maximum wave potential of ∼1 kV ∼kBTe. Power law‐like spectra of E⊥ with slope of −3 indicates the turbulent cascade of LHDI. LHDI is observed together with bursty high‐frequency parallel electric fields, suggesting coupling of LHDI to higher‐frequency electrostatic waves.
Plain Language Summary
Dipolarization fronts (DFs) are narrow boundaries with sharp enhancement of magnetic field, located at the leading part of fast plasma jets observed in Earth's magnetotail. DFs are typically assumed to be smooth boundaries at scales comparable to the ion gyroradius and below. In this study, we use the four Magnetospheric Multiscale spacecraft separated by several electron gyroradii to investigate fine structure of a DF. Surprisingly, we observe significant differences in the fields and plasma measurements between the spacecraft despite their small separation. We attribute these signatures to electron‐scale disturbances propagating along the DF surface, and thus the DF surface is not smooth as expected but rather rippled. The ripples develop as a result of a plasma instability driven by the strong inhomogeneities present at the DF. The fact that the ripples have such small scales means that they can effectively interact with plasma electrons.
Key Points
Large differences in plasma and field parameters at electron spatial scales are observed within a dipolarization front
The differences are consistent with ripples generated by lower hybrid drift instability (LHDI)
Power law‐like spectra for E and B indicate a turbulent cascade of LHDI
This paper reviews the state of knowledge concerning the source of magnetospheric plasma at Earth. Source of plasma, its acceleration and transport throughout the system, its consequences on system ...dynamics, and its loss are all discussed. Both observational and modeling advances since the last time this subject was covered in detail (Hultqvist et al., Magnetospheric Plasma Sources and Losses,
1999
) are addressed.
We argue that many studies in space physics would benefit from putting a detailed investigation into a wider perspective. Three examples of theoretical and observational studies are given. We argue ...that space physics should aim to be less of an isolated branch of science. Rather, by putting the scientific space results into a wider perspective these results will become more interesting and important than ever. We argue that diversity in a team often is favourable for work on complicated problems and helps to present the results in a wider perspective.
This is a review of the mass balance of planet Earth, intended also for scientists not usually working with space physics or geophysics. The discussion includes both outflow of ions and neutrals from ...the ionosphere and upper atmosphere, and the inflow of meteoroids and larger objects. The focus is on ions with energies less than tens of eV originating from the ionosphere. Positive low-energy ions are complicated to detect onboard sunlit spacecraft at higher altitudes, which often become positively charged to several tens of volts. We have invented a technique to observe low-energy ions based on the detection of the wake behind a charged spacecraft in a supersonic ion flow. We find that low-energy ions usually dominate the ion density and the outward flux in large volumes in the magnetosphere. The global outflow is of the order of 1026 ions s-1. This is a significant fraction of the total number outflow of particles from Earth, and changes plasma processes in near-Earth space. We compare order of magnitude estimates of the mass outflow and inflow for planet Earth and find that they are similar, at around 1 kg s−1 (30 000 ton yr−1). We briefly discuss atmospheric and ionospheric outflow from other planets and the connection to evolution of extraterrestrial life.
The SWARM satellites have both upward looking GPS receivers and Langmuir probes. The receivers repeatedly lost track of the L1 band signal in January–February 2014 at postsunset hours, when SWARM was ...at nearly 500 km altitude. This indicates that the signal was disturbed by ionospheric irregularities at this height and above. The track losses occurred right at density gradients associated with equatorial plasma bubbles and predominantly where the measured background density was highest. The signal showed strong phase scintillations rather than in amplitude, indicating that SWARM might be in the near field of an ionospheric phase screen. Density biteouts, depletions between steep gradients, were up to almost 3 orders of magnitude deep in the background of a more shallow trough centered at the magnetic equator. Comparison between satellites shows that the biteout structure strongly varied in longitude over ∼100 km and has in north‐south steep walls.
Key Points
Irregularities causing L1 band GPS disturbances are at 500 km height and above
Often GPS track losses at 500 km height correspond to local density gradients
The postsunset equatorial density trough can be very variable in longitude
Lower hybrid drift waves (LHDWs) are commonly observed at plasma boundaries in space and laboratory, often having the strongest measured electric fields within these regions. We use data from two of ...the Cluster satellites (C3 and C4) located in Earth's magnetotail and separated by a distance of the order of the electron gyroscale. These conditions allow us, for the first time, to make cross-spacecraft correlations of the LHDWs and to determine the phase velocity and wavelength of the LHDWs. Our results are in good agreement with the theoretical prediction. We show that the electrostatic potential of LHDWs is linearly related to fluctuations in the magnetic field magnitude, which allows us to determine the velocity vector through the relation ∫δEdt·v = ϕ(δB)(∥). The electrostatic potential fluctuations correspond to ∼10% of the electron temperature, which suggests that the waves can strongly affect the electron dynamics.
Magnetosheath jets are localized dynamic pressure enhancements in the magnetosheath. We make use of the high time resolution burst mode data of the Magnetospheric Multiscale mission for an analysis ...of waves in plasmas associated with three magnetosheath jets. We find both electromagnetic and electrostatic waves over the frequency range from 0 to 4 kHz that can be probed by the instruments on board the MMS spacecraft. At high frequencies we find electrostatic solitary waves, electron acoustic waves, and whistler waves. Electron acoustic waves and whistler waves show the typical properties expected from theory assuming approximations of a homogeneous plasma and linearity. In addition, 0.2 Hz waves in the magnetic field, 1 Hz electromagnetic waves, and lower hybrid waves are observed. For these waves the approximation of a homogeneous plasma does not hold anymore and the observed waves show properties from several different basic wave modes. In addition, we investigate how the various types of waves are generated. We show evidence that, the 1 Hz waves are connected to gradients in the density and magnetic field. The whistler waves are generated by a butterfly‐shaped pitch‐angle distribution and the electron acoustic waves by a cold electron population. The lower hybrid waves are probably generated by currents at the boundary of the jets. As for the other waves we can only speculate about the generation mechanism due to limitations of the instruments. Studying waves in jets will help to address the microphysics in jets which can help to understand the evolution of jets better.
Plain Language Summary
There is a constant plasma flow from the sun, the solar wind. The Earth's magnetic field deflects the solar wind as it flows toward Earth. As the solar wind plasma approaches Earth it gets decelerated and heated at the bow shock. Earthward of the bow shock, the magnetosheath is located where the flow diverges around Earth. In the magnetosheath plasma flows that are denser and faster than normal can sometimes be observed, so called magnetosheath jets. We investigate waves in plasmas in these magnetosheath jets and how they are generated. Studying these waves will help to understand the interaction of magnetosheath jets with their environment.
Key Points
MMS burst mode data is used to investigate waves in, and in the vicinity of, magnetosheath jets
0.2 Hz waves, 1 Hz waves, whistler waves, electron acoustic waves, lower hybrid waves, and solitary waves are observed
Waves with low frequencies cannot be explained by “basic” wave modes that are derived in homogeneous plasmas
Wakes behind spacecraft caused by supersonic drifting positive ions are common in plasmas and disturb in situ measurements. We review the impact of wakes on observations by the Electric Field and ...Wave double‐probe instruments on the Cluster satellites. In the solar wind, the equivalent spacecraft charging is small compared to the ion drift energy and the wake effects are caused by the spacecraft body and can be compensated for. We present statistics of the direction, width, and electrostatic potential of wakes, and we compare with an analytical model. In the low‐density magnetospheric lobes, the equivalent positive spacecraft charging is large compared to the ion drift energy and an enhanced wake forms. In this case observations of the geophysical electric field with the double‐probe technique becomes extremely challenging. Rather, the wake can be used to estimate the flux of cold (eV) positive ions. For an intermediate range of parameters, when the equivalent charging of the spacecraft is similar to the drift energy of the ions, also the charged wire booms of a double‐probe instrument must be taken into account. We discuss an example of these effects from the MMS spacecraft near the magnetopause. We find that many observed wake characteristics provide information that can be used for scientific studies. An important example is the enhanced wakes used to estimate the outflow of ionospheric origin in the magnetospheric lobes to about
10
26
cold (eV) ions/s, constituting a large fraction of the mass outflow from planet Earth.
Plain Language Summary
Wakes caused by spacecraft motion or drifting plasma are common behind spacecraft with scientific instruments and disturb in situ observations of space plasmas. We review the impact of wakes on observations by the Electric Field and Wave double‐probe instruments on the Cluster satellites. In the solar wind, the wake behind a Cluster spacecraft is caused by the spacecraft body, is narrow, and can partly be compensated for when analyzing data. In the regions above the Earth's polar regions, the wake behind a Cluster spacecraft is caused by an electrostatic structure around the positively charged spacecraft, causing an enhanced wake. The charging stops positive ions from reaching the spacecraft. Rather, this wake can be used to estimate the flux of cold (eV) positive ions escaping from the ionosphere. Above the poles the flux is about
10
26
ions/s, constituting a large fraction of the mass outflow from planet Earth. For an intermediate range of parameters, when the drift energy of the ions is comparable to the equivalent charge of the spacecraft, also the charged wire booms of a double‐probe instrument must be taken into account. We discuss an example from the MMS spacecraft near the magnetopause.
Key Points
Plasma wakes are common behind scientific spacecraft
Wakes in the solar wind can be compensated for in data analysis
Enhanced wakes in the polar lobes can be used to detect cold outflowing ions
We study Magnetospheric MultiScale observations in the outflow region of magnetotail reconnection. We estimate the power density converted via the three fundamental electron acceleration mechanisms: ...Fermi, betatron, and parallel electric fields. The dominant mechanism, both on average and the peak values, is Fermi acceleration with a peak power density of about +200 pW/m3. The magnetic field curvature during the most intense Fermi acceleration is comparable to the electron gyroradius, consistent with efficient electron scattering. The peak power densities due to the betatron acceleration are a factor of 3 lower than that for the Fermi acceleration, the average betatron acceleration is close to zero and slightly negative. The contribution from parallel electric fields is significantly smaller than those from the Fermi and betatron acceleration. However, the observational uncertainties in the parallel electric field measurement prevent further conclusions. There is a strong variation in the power density on a characteristic ion time scale.
Key Points
The power density converted via the Fermi, betatron, and parallel electric field acceleration in a tailward reconnection jet is estimated
The dominant electron acceleration mechanism is the Fermi acceleration in the current sheet center
Significant variation in the power density is observed on the characteristic time scale of ions
Observations obtained by the Freja satellite at altitudes around 1700 km in the high‐latitude magnetosphere are used to study ion energization perpendicular to the geomagnetic field. Investigations ...of ions, electrons, plasma densities, electric and magnetic wave fields, and field‐aligned currents are used to study O+ heating mechanisms. Three ion heating events are studied in detail, and 20 events are used in a detailed statistical study. More than 200 events are classified as belonging to one of four major types of ion heating and are ordered as a function of magnetic local time. The most common types of ion heating are associated with broadband low‐frequency electric wave fields occurring at all local times. These waves cover frequencies from below one up to several hundred hertz and correspond to the most intense O+ energization. Heating by these waves at frequencies of the order of the O+ gyrofrequency at 25 Hz seems to be the important energization mechanism, causing O+ ion mean energies up to hundreds of eV. The broadband waves are associated with Alfvén waves with frequencies up to at least a few hertz and with field‐aligned currents. Other types of O+ energization events are less common. During these events the ions are heated by waves near the lower hybrid frequency or near half the proton gyrofrequency. These waves are generated by auroral electrons or in a few cases by precipitating ions.
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