Equatorial Plasma Sheet Research Articles

Parker Solar Probe Observations of High Plasma β Solar Wind from the Streamer Belt

In general, slow solar wind from the streamer belt forms a high plasma β equatorial plasma sheet around the heliospheric current sheet (HCS) crossing, namely, the heliospheric plasma sheet (HPS). Current Parker Solar Probe (PSP) observations show that the HCS crossings near the Sun could be full or partial current sheet (PCS) crossings, and they share some common features but also have different properties. In this work, using the PSP observations from encounters 4–10, we identify streamer belt solar wind from enhancements in plasma β, and we further use electron pitch angle distributions to separate it into HPS solar wind around the full HCS crossings and PCS solar wind in the vicinity of PCS crossings. Based on our analysis, we find that the PCS solar wind has different characteristics as compared with HPS solar wind: (a) the PCS solar wind could be non-pressure-balanced structures rather than magnetic holes, and the total pressure enhancement mainly results from the less reduced magnetic pressure; (b) some of the PCS solar wind is mirror-unstable; and (c) the PCS solar wind is dominated by very low helium abundance but varied alpha–proton differential speed. We suggest that the PCS solar wind could originate from coronal loops deep inside the streamer belt, and it is pristine solar wind that still actively interacts with ambient solar wind; thus, it is valuable for further investigations of the heating and acceleration of slow solar wind.

Evolution of Alfvén Waves in the Solar Wind. Monochromatic Driver

We use a 2.5D magnetohydrodynamic model to investigate the propagation of azimuthally driven Alfvén waves with different periods and their interaction with the solar wind. In the absence of waves, the dipole field is stretched into a helmet streamer by the solar wind. The wind speeds near the equator are lower than those in the mid and high latitudes. Magnetic reconnection in the equatorial plasma sheet regularly triggers a breakup and expulsion of a plasmoid. We next inject monochromatic Alfvén waves with a moderate amplitude of 9 km s−1 and a period of τ = 1000 s at the coronal base. A cavity showing features of forward and backward propagating modes is formed. The backward waves are able to accelerate the background plasma at mid and high latitudes through the nonlinear coupling to compressional waves. The size of the cavity increases with the period of the Alfvén waves as long as the outer boundary remains in the sub-Alfvénic wind. When τ = 4000 s, we find enhanced acceleration and heating of the solar wind plasma as well as suppression of the reconnection in the equatorial plasma sheet. The amplitudes of the backward Alfvén waves remain large inside the cavity and modify its size. The cavity ceases to exist as its outer boundary gradually moves into the super-Alfvénic wind and the large amplitude backward waves are swept away by the wind. Results suggest that Alfvén waves with moderate amplitudes can modify the dynamics and the energetics of the solar wind plasma with the embedded magnetic field.

Physical Origin of Recurrent Magnetic Dipolarization Events in Saturn’s Equatorial Plasma Sheet

AbstractWe consider the physical origin of reconnection‐related events in Saturn’s equatorial current sheet revealed by Cassini, specifically the dipolarization events discussed by Yao et al. (https://doi.org/10.1029/2018JA025837) that recur close to the ∼10.7 h planetary period oscillation (PPO) period. We argue that Yao et al.’s preferred recurrence explanation in terms of complete rotations of dipolarized field/current structures around the planet at close to the PPO period is highly implausible. All‐mission observations of ion flows at relevant radial ranges ∼20–30 Saturn radii are essentially invariably sub‐corotational, including reconnection‐related hot ion injections, thus requiring implausible structure propagation at significant speeds through the plasma. We further show their assertion that the nightside events occur at PPO phases unfavorable for reconnection is incorrect. These events instead occur under PPO conditions associated with outward radial plasma displacement and/or a thinning plasma sheet with falling colatitudinal field, shown previously to be optimal for modulated reconnection bursts leading to dipolarizations and plasmoids. We instead suggest these events are related to well‐documented Vasyliunas cycle disturbances, when two reconnection episodes happen to be triggered at similar favorable PPO phases on successive PPO cycles. Yao et al.’s dayside events do occur at phases unfavorable for recurrent reconnection, but may then simply be an effect of periodic field/plasma modulations during the regular PPO cycle combined with subcorotating small‐scale structures often present in the plasma sheet. Both dayside and nightside events can thus be understood within existing knowledge of PPO modulations of the structure and dynamics of Saturn’s equatorial current sheet.

Ionosphere Feedback to Electron Scattering by Equatorial Whistler Mode Waves

AbstractPlasma sheet electron precipitation, much of it driven by whistler mode waves, is critical for magnetosphere‐ionosphere coupling. This precipitation leads to a secondary electron population at low altitudes, which moves upward along magnetic field lines to the equatorial plasma sheet. We investigate observational evidence for such electron precipitation and the ionospheric feedback provided by the secondary electron outflows. Using the near‐equatorial Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, we show that whistler mode wave bursts are accompanied by enhanced field‐aligned electrons. The properties of these electrons are not related to the whistler mode waves, as would have been expected from local wave‐particle interactions. Thus, it is unlikely that the field‐aligned electrons are formed by electron Landau resonance with whistler mode waves. This population can be the secondary electron outflows resulting from plasma sheet electron precipitation to the ionosphere. Combining THEMIS, Cluster, and Fast Auroral Snapshot Explorer (FAST) measurements, we show that this field‐aligned electron population is also observed at low altitudes, where it is associated with high‐frequency electrostatic waves. These low‐altitude waves are likely generated by the secondary electron outflows and, in turn, scatter these electrons outside of the loss cone, allowing them to be observed by the wide field‐of‐view particle instrument on the near‐equatorial THEMIS spacecraft. We discuss how this secondary electron population can subsequently alter whistler mode wave generation and propagation in the magnetosphere.

Method to Derive Ion Properties From Juno JADE Including Abundance Estimates for O+ and S2+

AbstractThe Jovian Auroral Distributions Experiment Ion sensor (JADE‐I) on Juno is a plasma instrument that measures the energy‐per‐charge (E/Q) distribution of 0.01 to 46.2 keV/q ions over a mass‐per‐charge (M/Q) range of 1– 64 amu/q. However, distinguishing O+ and S2+ from JADE‐I's measurements is a challenging task due to similarities in their M/Q (∼16 amu/q). Because of this, O+ and S2+ have not been fully resolved in the in situ measurements made by plasma instruments at Jupiter (e.g., Voyager PLS and Galileo PLS) and their relative ratios has been studied using physical chemistry models and ultraviolet remote observations. To resolve this ambiguity, a ray tracing simulation combined with carbon foil effects is developed and used to obtain instrument response functions for H+, O+, O2+, O3+, Na+, S+, S2+, and S3+. The simulation results indicate that JADE‐I can resolve the M/Q ambiguity between O+ and S2+ due to a significant difference in their charge state modification process and a presence of a large electric potential difference (∼8 kV) between its carbon foils and MCPs. A forward model based on instrument response functions and convected kappa distributions is then used to obtain ion properties at the equatorial plasma sheet (∼36 RJ) in the predawn sector of magnetosphere. The number density ratio between O+ and S2+ for the selected plasma sheet crossings ranges from 0.2 to 0.7 (0.37 ± 0.12) and the number density ratio between total oxygen ions to total sulfur ions ranges from 0.2 to 0.6 (0.41 ± 0.09).

First Evidence for Multiple‐Harmonic Standing Alfvén Waves in Jupiter's Equatorial Plasma Sheet

AbstractQuasiperiodic pulsations in the ultralow‐frequency band are ubiquitously observed in the Jovian magnetosphere, but their source and distribution have until now been a mystery. Standing Alfvén waves on magnetic field lines have been proposed to explain these pulsations and their large range in observed periods. However, in situ evidence in support of this mechanism has been scarce. Here we use magnetometer data from the Galileo spacecraft to report first evidence of a multiple‐harmonic ultralow‐frequency event in Jupiter's equatorial plasma sheet. The harmonic periods lie in the 4‐ to 22‐min range, and the nodal structure is confined to the plasma sheet. Polarization analysis reveals several elliptically polarized odd harmonics and no presence of even harmonics. The harmonic periods, their polarization, and the confinement of the wave to the plasma sheet are strong evidence supporting the standing Alfvén wave model. Multiple‐harmonic waves therefore potentially explain the full range of periods in quasiperiodic pulsations in Jupiter's magnetosphere.

Alfvénic Fluctuations Associated With Jupiter's Auroral Emissions

AbstractThe Alfvén wave mode transmits field‐aligned currents and large‐scale turbulence throughout Jupiter's magnetosphere. Magnetometer data from the Juno spacecraft have provided the first observations of Alfvénic fluctuations along the polar magnetic flux tubes connected to Jupiter's main auroral oval and the Jovian satellites. Transverse magnetic field perturbations associated with Io are observed up to ~90° away from main Io footprint, supporting the presence of extended Alfvénic wave activity throughout the Io footprint tail. Additional broadband fluctuations measured equatorward of the statistical auroral oval are composed of incompressible magnetic turbulence that maps to Jupiter's equatorial plasma sheet at radial distances within ~20 RJ. These fluctuations exhibit a k|| power spectrum consistent with strong magnetohydrodynamic turbulence. This turbulence can generate up to ~100 mW/m2 of Poynting flux to power the Jovian aurora in regions connected to the inner magnetosphere's central plasma sheet.

Wave‐Particle Interaction of Alfvén Waves in Jupiter's Magnetosphere: Auroral and Magnetospheric Particle Acceleration

AbstractWe investigate spatial and temporal scales at which wave‐particle interaction of Alfvén waves occurs in Jupiter's magnetosphere. We consider electrons, protons, and oxygen ions and study the regions along magnetic flux tubes where the plasma is the densest, that is, the equatorial plasma sheet, and where the plasma is the most dilute, that is, above the ionosphere, where auroral particle acceleration is expected to occur. We find that within a dipole L‐shell of roughly 30, the electron inertial length scale in the auroral region is the dominating scale, suggesting that electron Landau damping of kinetic Alfvén waves can play an important role in converting field energy into auroral particle acceleration. This mechanism is consistent with the broadband bidirectional electron distributions frequently observed by Juno. Due to interchange‐driven mass transport in Jupiter's magnetosphere, its magnetosphere‐ionosphere coupling is expected to be mostly not in local force balance. This might be a key reason for the dominant role of Alfvénically driven stochastic acceleration compared to the less frequently occurring, locally forced‐balanced, and thus static mono‐energetic unidirectional acceleration. Outside of approximately L = 30, the ion gyroperiod is the dominating scale suggesting that ion cyclotron damping of heavy ions plays a major role in heating magnetospheric plasma. We also present properties of the dispersion relationship and the polarization relationships of kinetic Alfvén waves including the important effects from the relativistic correction due to the displacement current in Ampère's law.

Does a Local B‐Minimum Appear in the Tail Current Sheet During a Substorm Growth Phase?

AbstractMagnetic configurations with dBz/dr > 0 in the midtail current sheet are potentially unstable to various instabilities associated with the explosive substorm onset. Their existence is hard to confirm with observations of magnetospheric spacecraft. Here we use remote sensing by low‐altitude spacecraft that measured the loss cone filling rate during electron‐rich solar particle event, providing information about magnetic properties of the tail current sheet. We found a latitudinally localized anisotropic 30 keV electron loss cone region embedded inside an extended region of isotropic solar electron precipitation. It was persistently observed for more than 0.5 h during isolated growth phase event by six Polar Operational Environmental Satellites spacecraft, which crossed the premidnight auroral oval. The embedded anisotropic region was observed 1° poleward of the outer radiation belt boundary over 4–5 h wide magnetic local time sector, suggesting a persistent ridge‐type Bz2/j maximum in the equatorial plasma sheet at distances 15–20 RE. We discuss infrequent observations of such events taking into account recent results of global magnetohydrodynamic simulations.

Periodic Emission Within Jupiter's Main Auroral Oval

AbstractWe have discovered pulsating emission within Jupiter's main auroral oval, providing evidence of the auroral signature of Jovian ULF wave processes. The form comprises a 1° × 2° spot located directly on the main emission, whose intensity oscillates with a period of ∼10 min throughout the 45 min observation. The feature appears on the duskward edge of the discontinuity, maps to ∼13–14 h LT and ∼20–50 RJ, and rotates at around a half of rigid corotation. We show that the period of the oscillation is similar to the expected Alfvén travel time between the ionosphere and the upper edge of the equatorial plasma sheet in the middle magnetosphere, and we thus suggest that the pulsating aurora is driven by a mode confined to the low‐density region outside the plasma sheet. This significant new observation shows that Jupiter's auroras present an important remote sensing window on Jovian magnetospheric wave processes.

Upper limit of electron fluxes generated by kinetic Alfvén waves in Maxwellian plasma

AbstractWe consider electron acceleration by kinetic Alfvén waves in the equatorial inner magnetosphere and plasma sheet boundary layer. The competition between the accelerating effect of the wave parallel electric field and mirror force acting on particles in an inhomogeneous background magnetic field generates an effective potential well where electrons can be trapped and accelerated. We compare energy variations of trapped and transient resonant electrons and show that these variations almost compensate each other. Thus, energy provided to waves by transient particles is transferred to trapped particles. This effect allows waves accelerate trapped electrons without being significantly damped. Using energy balance equations, we estimate the maximum flux of electrons accelerated via trapping into Landau resonance with kinetic Alfvén waves. For a wide range of system parameters (i.e., ion to electron temperature ratio, magnetic field amplitude, and wave number and wave frequency), acceleration of trapped electrons can generate fluxes with amplitude about 5–25% of the background thermal fluxes. We determine parametric regions for the most efficient acceleration.

Geomagnetic conjugate observations of plasma‐sheet electrons by the FAST and THEMIS satellites

AbstractWe investigate pitch‐angle distributions and spectral shapes of auroral electrons simultaneously observed during three conjunction events by the FAST satellite at altitudes of 2500–3500 km and by the THEMIS satellite in the plasma sheet at 7–13 RE. All three events were on the lower latitude of the auroral oval. Conjunction event 1 occurred at ~1914:45 UT on 10 April 2008. Electron spectra at energies of 0.1–3 keV are correlated well between the two satellites, while the precipitating electrons above 3 keV are missing at FAST. Event 2 occurred at 1255:26 UT on 26 April 2008. Electron spectra above 3 keV are correlated well between the two satellites. An additional broad spectral peak at energies of 0.1–0.5 keV was observed by FAST. Event 3 occurred at 0058:04 UT on 25 December 2008. Precipitating electrons of 0.5–5 keV obtained by FAST are correlated well with those of THEMIS‐C, while a monoenergetic peak at 0.1–0.2 keV was observed only by FAST. For the three conjunction events, we conclude that high‐energy precipitating auroral electrons observed by FAST directly come from the equatorial plasma sheet, while low‐energy precipitating electrons may come from middle altitudes as a result of acceleration by static potential differences. For missing high‐energy (>3 keV) electrons of event 1, we speculate that the pitch‐angle scattering by waves occurs only at a limited energy range.

Fast earthward flows, electron cyclotron harmonic waves, and diffuse auroras: Conjunctive observations and a synthesized scenario

[1] We present in this paper multi-instrumental observations and analyses of fast earthward flows, electrostatic electron cyclotron harmonic (ECH) waves, and diffuse auroras, during 8–9 UT on February 5, 2009. The event began with a series of fast earthward flows detected on mid-tail probe THEMIS-C. Subsequently, magnetic dipolarizations and strong ECH wave intensifications were observed on THEMIS-D/E probes at L ∼ 11 in the equatorial plasma sheet. Concurrently, Ground optical instruments detected diffuse auroral intensifications in the region surrounding the ionospheric footprints of the THEMIS probes. Together with the theoretical simulation performed by Ni et al. (2011e), we establish a causal conjunction between the ECH waves and diffuse auroras for the reported event. We also propose that the ECH wave and diffuse auroral intensification were likely triggered by the fast flow activity from the mid-tail and its resulting magnetic dipolarization. We discuss possible mechanisms linking the fast flow and its associated dipolarization to the intensification of ECH wave and diffuse aurora in the outer magnetosphere.

Intensification of the Cowling current in the global MHD simulation model

[1] We examine the effects of the ionospheric conductance on the intensification of the westward electrojet current in the ionosphere based on the piecewise parabolic method with a Lagrangian remap (PPMLR) global MHD simulation model. The ionospheric conductance is empirically linked to the plasma pressure in the plasma sheet. The simulation results are consistent with observations: When the Pedersen and Hall conductances are small, the ionospheric current shows a two-cell pattern; when the conductances increase and the ratio ΣH/ΣP ≥ 2, an intense westward electrojet appears in the midnight sector. This intense westward electrojet is the Cowling current driven by the induced southward electric field due to the blockage of the northward Hall current from closure in the equatorial plasma sheet. The simulation shows the development of the Cowling electrojet is essential to the intensification of the westward electrojet in the ionosphere.

H+ and O+ content of the plasma sheet at 15–19 Re as a function of geomagnetic and solar activity

We have used the ion composition data from the CIS/CODIF instrument on the Cluster spacecraft to determine how the H+ and O+ contribution to the plasma sheet density changes as a function of geomagnetic conditions and solar activity. The Cluster spacecraft are in a polar orbit that cut through the equatorial plasma sheet at ∼19 Re downtail for the first 5 years of the mission. We have restricted the data set to apogee time periods, from 15 to 19 Re, in order to give the composition at a clear position, which can then be used, for example, as a boundary condition for models. The geomagnetic conditions are described using the Kp index, while the solar activity is represented by the use of F10.7 index. Functional forms for these dependencies are provided. The statistical study covers the years from 2001 to 2005, which covers solar maximum, and the declining stage of the solar cycle. We find, as expected, that the O+ density in this region depends strongly on both solar EUV and geomagnetic activity. In addition, we find that there is a gradient in the O+/H+ density ratio, from the 15 to 19 Re plasma sheet to the 6.6 Re plasma sheet, indicating that there is significant additional entry of O+ inside of 15 Re.

Plasma sheet pressure evolution related to substorms

We have investigated the inner equatorial plasma sheet plasma pressure evolution associated with substorm onsets using the five THEMIS spacecraft data during the 2008 tail season. The central plasma sheet thermal pressure is estimated from the thermal and magnetic pressure based on one dimensional pressure balance. The results show that for a majority of substorm cases, the plasma pressure enhances after onset earthward of ∼12 RE radial distance. On the other hand, the plasma pressure beyond ∼16 RE shows a remarkable decrease near onset. During the recovery phase, the pressure gradually returns to its original level. These results indicate that during the substorm expansion phase, the current wedge usually expands to the region tailward of ∼16 RE with the inner edge earthward of this radial distance. The pressure increase earthward of ∼12 RE could be associated with an increase of cross‐tail current, though this is not necessarily so due to the nontail‐like magnetic field geometry after dipolarization. For a minority of cases, the pressure fluctuates without net change or reduces at ∼11 RE. During these events, the ground magnetic perturbations and the estimated center of the westward electrojet extend further equatorward than for the pressure increase cases. There is also evidence that aurora breakup occurs further equatorward when there is a pressure reduction, indicating a more earthward penetration of the plasma sheet. During these cases, the current wedge represented by pressure reduction is clearly observed earthward to ∼11 RE. The relationship between the auroral break up arc, which is believed to map to ∼10 RE, and may be associated with a precursor pressure enhancement, and the inner plasma sheet pressure and current evolution near onset warrants more careful analysis.

Ion temperature drop and quasi‐electrostatic electric field at the current sheet boundary minutes prior to the local current disruption

From a survey of the cross‐tail current disruption (CD) events in the near‐Earth plasma sheet collected from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, we identify a highly repeatable class of event occurring at the current sheet boundary in a few minutes before the local CD onset. Salient features of this class of event include (1) a precipitous drop of the ion temperature, (2) concurrent growth of a neutral sheet‐pointing electric field, and (3) ULF wave activations at Pi1/Pi2 bands. We interpret the ion temperature drop as a manifestation of the extremely thinning of the local current sheet prior to its disruption. This thinning process is inferred as nonadiabatic by nature. Particularly, when the current sheet thickness is down to ion kinetic scales, the ions are demagnetized, and a quasi‐electrostatic neutral sheet‐pointing electric field emerges owing to the charge separation. The ULF fluctuations of electric/magnetic field have a two‐band structure. The lower‐frequency band with a period of 50 to 80 s is interpreted as an Alfvenic mode coupled from other preonset wave modes excited at the equatorial plasma sheet such as the ballooning. The higher‐frequency wave at 10‐ to 20‐s periods is attributed to some instability mode, directly leading to the disruption of the thin current sheet (TCS). We suggest that an extremely thinned non‐Harris TCS and the emergence of quasi‐electrostatic field constitute the conducive conditions for a local CD to occur. A companion paper by Liu et al. (2009) covers some theoretical aspects of this study.

Phase relation of oscillations near the planetary period of Saturn's auroral oval and the equatorial magnetospheric magnetic field

Previous analyses of Hubble Space Telescope (HST) images of Saturn's southern auroras obtained during two campaigns, in January 2007 and February 2008, have revealed that the auroral oval oscillates at a period close to the planetary rotation period, with its center describing an elongated ellipse of semimajor axis ∼2° colatitude aligned along the prenoon to premidnight direction. Previous analyses of Cassini magnetic field data from Saturn's near‐equatorial quasi‐dipolar magnetosphere have also established the presence of a rotating pattern of magnetic field perturbations near the planetary period, a phase model which has been derived from data over the interval from mid‐2004 to the end of 2007, whose extrapolation is verified here for use during the February 2008 HST campaign. In this paper we compare the phases of these oscillatory phenomena and show that the southern oval displacement was directed approximately opposite to the rotating equatorial perturbation field during both HST campaign intervals. We also examine the relation of the southern oval oscillations to the periodic power modulations in Saturn kilometric radiation (SKR) and show that the southern oval was displaced sunward at SKR maxima. It is suggested that the oval displacements are related to magnetospheric field line distortions associated with the rotating magnetic field perturbations, this picture also being consistent with recently reported periodic tilting of the equatorial plasma sheet. We note, however, that this picture provides no immediate explanation for the significantly elliptical nature of the observed oval motion.

Implication of multiple dipolarization event near 9 R E for the physics of substorms

In 2001, 2002 and 2003, the Polar spacecraft probed the near equatorial plasma sheet at 9 R E near local midnight. Using the magnetic field observations, the signatures at substorm onsets are studied. Close to the flux pile-up region, the Polar spacecraft readily detects the dipolarization front, especially for pseudo onsets. An event with two distinct onsets has been examined. The signatures are found to be consistent with the multiple-onset model suggested by Russell [Russell, C.T. How northward turnings of the IMF can lead to substorm expansion onsets. Geophys. Res. Lett. 27, 3257–3259, 2000] which is a modified Near-Earth Neutral Line (NENL) model. Another similar event is also examined showing the effects of different Interplanetary Magnetic Field (IMF) conditions upon substorms. Moreover, ground effects can be very weak compared to in situ observations, especially for pseudo onsets, because these signatures appear to be localized and not global.

Configuration and dynamics of the Jovian magnetosphere

A massive rotating equatorial plasma sheet dominates Jupiter's magnetosphere and the solar wind and the interplanetary magnetic field (IMF) are not thought to be as important as at Earth. However, in a recent simulation study we found that for a purely northward IMF the Jovian magnetosphere reached an unstable state in which a nearly periodic series of magnetic X and O lines were launched tailward. In this study we have carried out three‐dimensional global magnetohydrodynamic simulations to investigate the causes of this dynamic behavior. First, we examined the effects of dynamic pressure on the magnetospheric configuration in the absence of an IMF. We examined a series of northward IMF simulations in which we varied both the IMF magnitude and dynamic pressure. If the outer edge of the rotating plasma sheet is far from the X‐line (for large pressure and small IMF), the reconnected flow reaches the dawn magnetopause and exits down the tail. If the neutral line forms closer to the rotation boundary (small pressure and IMF), then the reconnected flux tubes can convect all of the way around Jupiter. When they reach the nightside, they become stretched tailward and can reconnect again. This leads to the periodic behavior. If the neutral line forms very close to the rotation boundary (large pressure and IMF), the Jupiterward flow compresses the rotating plasma sheet. In this case the flow goes around Jupiter but the flux tubes return along the dusk magnetopause and do not participate in reconnection a second time.