Auroral Particle Acceleration Research Articles

Field Line Resonances and Cavity Modes at Earth and Jupiter

Ultra-Low-Frequency (ULF) waves provide a means for the rapid propagation of energy and field-aligned current in planetary magnetospheres. At Earth, the ULF frequency range is usually defined as including waves with periods of 0.2–600 s; however, at Jupiter these waves can extend to periods of tens of minutes. In both magnetospheres, shear mode Alfvén waves can form field line resonances that exist between the ionospheres, with periods of a few minutes at Earth and a few tens of minutes at Jupiter. A major distinction between these two magnetospheres is in the density distribution. Earth has a dense ionosphere full of heavy ions, an extended, cold plasmasphere and a relatively low-density plasma sheet. In contrast, at Jupiter, the ionosphere is largely hydrogen (both in atomic form and in the H3+molecular ion), there is no appreciable plasmasphere and the plasma disk is dense and populated with heavy ions (largely sulfur and oxygen) originating at the moon Io and to some extent from other moons. As at Earth, the sharp Alfvén speed gradient above the ionosphere forms an ionospheric Alfvén resonator at Jupiter with periods of seconds. Furthermore, the high-latitude lobes at Jupiter have very low density and a resonant structure can be formed by waves bouncing between the ionosphere and the dense plasma disk. This structure leads to periods of tens of seconds. Finally, the dense Io plasma torus and plasma sheet provide conditions for compressional cavity modes to form in this region. Thus, the structure of the field line resonance modes is quite different at the two planets. Implications of these resonances on auroral particle acceleration will be discussed.

Evidence of Alfvénic Activity in Jupiter's Mid‐To‐High Latitude Magnetosphere

AbstractUsing a combination of Juno magnetometer and plasma data, we show evidence of Alfvénic turbulence within the mid‐to‐high latitude magnetosphere with sufficient conditions to trigger auroral particle acceleration. We analyze 12 events that, in agreement with theoretical results, are found to be dissipative at the electron inertial scale. Furthermore, these events contain significant Poynting flux in the range ∼0.8–20 mW/m2 at ionospheric altitudes. This is sufficient to generate auroral emissions. We confirm that such events are incompressible, confirming their Alfvénicity, occur at dissipative scales, have intermittent features present and are multifractal in nature. These results illustrate the importance of turbulence in the mid‐to‐high latitudes of Jupiter's magnetosphere as a driver of particle acceleration.

The Jovian Ionospheric Alfvén Resonator and Auroral Particle Acceleration

AbstractThe ionospheric Alfvén resonator (IAR) is a structure formed by the rapid decrease in the plasma density above a planetary ionosphere. This results in a corresponding increase in the Alfvén speed that can provide partial reflection of Alfvén waves. At Earth, the IAR on auroral field lines is associated with the broadband acceleration of auroral particles, sometimes termed the Alfvénic aurora. This arises since phase mixing in the IAR reduces the perpendicular wavelength of the Alfvén waves, which enhances the parallel electric field due to electron inertia. This parallel electric field fluctuates at frequencies of 0.1–20.0 Hz, comparable to the electron transit time through the acceleration region, leading to the broadband acceleration. The prevalence of such broadband acceleration at Jupiter suggests that a similar process can occur in the Jovian IAR. A numerical model of Alfvén wave propagation in the Jovian IAR has been developed to investigate these interactions, indicating that the IAR resonant frequencies are in the same range as those at Earth. This model describes the evolution of the electric and magnetic fields in the low‐altitude region close to Jupiter that is sampled during Juno's perijove passes. In particular, the model relates measurement of magnetic fields below the ion cyclotron frequency from the MAG and Waves instruments on Juno and electric fields from Waves to the associated parallel electric fields that can accelerate auroral particles.

The High‐Latitude Extension of Jupiter's Io Torus: Electron Densities Measured by Juno Waves

AbstractWe present Jovian electron densities measured by the Juno Waves instrument at high‐latitudes (with magnetic latitudes of on magnetic field lines threading the Io torus and plasma sheet (with M‐shells of ) using the first 29 orbits of the Juno mission. We infer electron densities from characteristic frequency cutoffs and resonances in plasma wave spectra. Often more than one of the characteristic frequencies of the plasma can be identified, whose consistency provides credibility to the plasma density estimates. The majority of density measurements are on magnetic field lines threading the Io torus and intersecting Jupiter's ionosphere between Io's M‐shell and the auroral main oval, but there is evidence of density dropouts near and poleward of the auroral oval, which may be important in auroral particle acceleration. Sharp density gradients are found near field lines mapping to the inner edge of the Io torus M‐shells. These observations demonstrate the extension of elevated Io torus densities along field lines to higher latitudes.

Quiet, Discrete Auroral Arcs: Acceleration Mechanisms

The theory of the acceleration of auroral particles is reviewed, focusing on developments in the last 15 years. We discuss elementary plasma physics processes leading to acceleration of electrons to energies compatible with emission observed for quiet, discrete auroral arcs, defined as arcs that have time scales of minutes or more and spatial scales ranging from less than 1 km to tens of kilometers. For context, earlier observations are first described briefly. The theoretical fundamentals of auroral particle acceleration are based on the kinetic theory of plasmas, in particular the development of parallel electric fields. These parallel electric fields can either be distributed along the magnetic field lines, often associated with the mirror geometry of the geomagnetic field, or concentrated into narrow regions of charge separation known as double layers. Observations have indicated that the acceleration process depends on whether the field-aligned currents are directed away from the Earth, toward the Earth, or in mixed regions of currents often associated with the propagation of Alfven waves. Recent observations from the NASA Fast Auroral SnapshoT (FAST) satellite, the ESA satellite constellation Cluster, and the Japanese Reimei satellite have provided new insights into the auroral acceleration process and have led to further refinements to the theory of auroral particle acceleration.

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.

Universal time control of AKR: Earth is a spin‐modulated variable radio source

AbstractAuroral kilometric radiation (AKR) is known to be transient emissions generated by rapidly accelerated electrons together with sudden auroral activation in the polar magnetosphere. In contrast, the characteristics and relationship with the auroral acceleration of rather continuous AKR emissions are not well understood. We examine the emission using long‐term data and report that the continuous AKR emission frequency changes with universal time (UT) as the Earth rotates, indicating that the Earth is a spin‐modulated variable radio source. The observed UT variation of AKR frequency means that the acceleration altitude changes periodically with planetary rotation. The observations indicate that the diurnal wobble of the tilted geomagnetic field in the solar wind flow alters the magnetosphere‐ionosphere (M‐I) coupling state in the polar magnetosphere, giving rise to periodic variation of auroral particle acceleration altitude. These observations of planetary radio wave properties provide insight into the physics of planetary particle acceleration.

Magnetic fractures or reconnection of type II

AbstractThe importance of reconnection in astrophysics has been widely recognized. It is instrumental in storing and releasing magnetic energy, the latter often in a dramatic fashion. A closely related process, playing in very low beta plasmas, is much less known. It is behind the acceleration of auroral particles in the low-density environment several 1000 km above the Earth. It involves the appearance of field-parallel voltages in presence of intense field-aligned currents. The underlying physical process is the release of magnetic shear stresses and conversion of the liberated magnetic energy into kinetic energy of the particles creating auroral arcs. In this process, field lines disconnect from the field anchored in the ionosphere and reconnect to other field lines. Because of the stiffness of the magnetic field, the process resembles mechanical fractures. It is typically active in the low-density magnetosphere of planets. However, it can also lead to significant energy conversion with high-energy particle production and subsequent gamma ray emissions in stellar magnetic fields, in particular of compact objects.

On the character and distribution of lower‐frequency radio emissions at Saturn and their relationship to substorm‐like events

With the arrival of the Cassini spacecraft at Saturn in July 2004, there have been quasi‐continuous observations of Saturn kilometric radiation (SKR) emissions. Exploration of the nightside magnetosphere has revealed evidence of plasmoid‐like magnetic structures and other phenomena indicative of the Kronian equivalent of terrestrial substorms. In general, there is a good correlation between the timing of reconnection events and enhancements in the auroral SKR emission. Eight of nine reconnection events studied occur at SKR phases where the SKR power would be expected to be rising with time. Thus, while the recurrence rate of substorm‐like events at Saturn is likely much longer than the planetary rotation timescale, the events are favored to occur at a particular phase of the rotation. We show three examples in each of which the SKR spectrum extends to lower frequencies than usual. This can be interpreted as an expansion of the auroral particle acceleration region to higher altitudes along magnetic field lines as a direct consequence of an increase in the magnetosphere‐ionosphere current density driven by substorm‐like events. We then conduct a survey of such low‐frequency extensions during the equatorial orbits of 2005–2006 and place some constraints on visibility of these radio emissions.

Vertical evolution of auroral acceleration at substorm onset

Abstract. This study describes the onset process of auroral substorms in connection with the vertical evolution of auroral particle acceleration, on the basis of auroral kilometric radiation (AKR) dynamics. We show that the auroral acceleration process at substorm onset basically consists of two stages: (1) appearance/intensification of a low-altitude acceleration region at 4000–5000 km accompanied by initial brightening and (2) breakout of high-altitude field-aligned acceleration above the pre-existing low-altitude acceleration region at 6000–12 000 km, which is followed by auroral breakup and poleward expansion. It is also revealed that this two-stage evolution of auroral acceleration corresponds to the two-step reinforcement of field-aligned current.

Propagation of kinetic Alfvén waves in the ionospheric Alfvén resonator in the presence of density cavities

Recent observations of particle distributions that are narrow in pitch angle and broad in energy have suggested that kinetic Alfvén waves are a significant contributor to auroral particle acceleration. Oscillations at frequencies near 1 Hz are a natural consequence of the propagation of Alfvén waves in the strongly varying Alfvén speed profile above the auroral ionosphere, the so‐called ionospheric Alfvén resonator. These waves often propagate in the presence of perpendicular density gradients at various spatial scales. Simulations have been performed to study the evolution of these fields including both parallel and perpendicular inhomogeneity. Phase mixing at the boundaries of the density cavity leads to small‐scale Alfvén waves, which can develop the parallel electric fields needed to accelerate the Alfvénic aurora. These simulations verify the kinetic Alfvén wave dispersion relation in the electron inertial limit, which predicts that the perpendicular phase and group velocity of these waves are in the opposite direction. In addition, the results show that narrow spatial scales are favored by high ionospheric conductance.

AKR breakup and auroral particle acceleration at substorm onset

The dynamical behavior of auroral kilometric radiation (AKR) is investigated in connection with auroral particle acceleration at substorm onsets using high‐time‐resolution wave spectrograms provided by Polar/PWI electric field observations. AKR develops explosively at altitudes above a preexisting low‐altitude AKR source at substorm onsets. This “AKR breakup” suggests an abrupt formation of a new field‐aligned acceleration region above the preexisting acceleration region. The formation of the new acceleration region is completed in a very short time (amplitude increases 10,000 times in 30 seconds), suggesting that the explosive development is confined to a localized region. AKR breakups are usually preceded (1–3 minutes) by the appearance and/or gradual enhancement of the low‐altitude AKR. This means that the explosive formation of the high‐altitude electric field takes place in the course of the growing low‐altitude acceleration. The development of the low‐altitude acceleration region is thus a necessary condition for the ignition of the high‐altitude bursty acceleration. The dH/dt component from a search‐coil magnetometer at ground shows that a few minutes prior to substorm onsets, the quasi‐DC component begins a negative excursion that is nearly synchronized with the start of the gradual enhancement of the low‐altitude AKR, indicating a precursor‐like behavior for the substorm. This negative variation of dH/dt suggests an exponentially increasing ionospheric current induced by the upward field‐aligned current. At substorm onsets, the decrease in the quasi‐DC variation of dH/dt further accelerates, indicating a sudden reinforcement of the field‐aligned current.

Large‐amplitude wave electric field in the inner magnetosphere during substorms

CRRES electric field data during substorms have been analyzed to investigate intense electric fields in the inner magnetosphere associated with dipolarizations. Substorm injections during 13:00–15:00 UT on 7 March 1991 exhibit many large‐scale electric fields and short‐duration electric field spikes. Large‐scale electric fields with durations of ∼1 min are identified as spatial structures associated with the region 1 field‐aligned currents. Amplitudes of electric fields reach 30 mV/m with a high correlation with variations of the magnetic field, and the Poynting fluxes are directed toward the ionosphere with magnitudes of more than 0.5 mW/m2. 16‐Hz high‐resolution data show intense electric field spikes with amplitudes of ∼100 mV/m with durations of ∼1 s. Most of the spikes are electromagnetic with Poynting fluxes of ∼0.1 mW/m2 directed toward the ionosphere. The electric field spikes are identified as right‐handed whistler waves with a size of ∼1000 km with frequencies just below the ion cyclotron frequency. A nearly simultaneous measurement by the DMSP‐F9 satellite shows intense plasma flows with durations of 1 s and inverted‐V electron precipitation. An estimation of the wave and particle energy fluxes shows that about half of the Poynting flux of the electric field spikes is consumed accelerating auroral particles, and 1% of the Poynting flux drives the fast plasma flows at the ionosphere. It is suggested that the electromagnetic spikes provide sufficient energy for auroral particle acceleration.

Statistical study of inverted-V structures in FAST data

Abstract. Since the 1970s the inverted-V structures have been observed in the satellite-borne particle data. These Λ-shaped enhancements of the differential energy flux on the high energy channels on the top of low number flux on the lower energies are traces of auroral particle acceleration by U-shaped potential drops along the field lines within the auroral zones. In this study, we show the results of a statistical study of inverted-V events in five years (1997–2001) of the Fast Auroral SnapshoT (FAST) electron spectrograms. We found 588 well-defined events in total, but the statistical results are also shown for a subset of 97 very best events. The statistical analysis shows that the inverted-V's mainly occur within the auroral oval latitudes (65°–75° invariant latitude), in the pre-midnight sector (21–23 magnetic local time) and during the Northern Hemisphere winter months (November to February). The typical maximum energies of the inverted-V's are 2–4 keV, when a threshold value of 0.3 keV is used as a selection criterion. This corresponds to an acceleration potential drop of a few kV. The typical peak energy tends to increase towards the magnetic midnight. Most of the inverted-V events occur during magnetically quiet or moderate activity, Kp 1–4. The usual width of an inverted-V varies from 20 to 40 km and is comparable to the peak value of the typical meso-scale auroral arc width of 10–20 km. Furthermore, the polar cap potential values estimated from the SuperDARN radar data are always at least three times larger than the inverted-V acceleration potential drop.

Acceleration of particles in space plasmas by nonlinear magnetosonic waves

An unresolved question of magnetospheric physics concerns the creation of convergent and divergent electric field structures responsible for the acceleration of auroral particles to several kilo-electronvolt. Similarly, an unresolved problem of solar physics concerns the acceleration of particles producing the x-ray coronal emissions. Here we show that both these problems can be explained by electric field structures produced by nonlinear magnetosonic waves. The convergent electric field structures correspond to fast magnetosonic solitons, while divergent electric field structures are produced by slow solitons. Both types of solitons can build potentials of tens of kV in the magnetosphere and hundreds of kV in the solar corona. Such solitons can be created when linear magnetosonic and Alfvén waves propagate through regions of varying Alfvén speed, which occurs on auroral field lines at altitudes of 6000 km, and also above the chromosphere in the solar corona. The initial, linear MHD waves can be produced by bursty bulk flows in the Earth's magnetotail and by chromospheric/photospheric convective flows on the Sun.

Kappa distribution functions and the main properties of auroral particle acceleration

The problem of auroral particle acceleration is analyzed taking into account the filling of the magnetosphere by accelerated particles of ionospheric origin and the trapping of particles between the magnetic and electrostatic barriers. It is taken into account that kappa distributions describe experimentally measured distribution functions better than do maxwellians. The existence of conjugate field-aligned potential drops in the north and south hemispheres is suggested. Field-aligned potential drops are formed if the value of field-aligned current is higher than the threshold determined by the free gazodynamic flow of electrons along field lines. Results of experimental observations showing the possibility of field-aligned potential drop concentration in double layers are summarized. The theory of kinetic double layers is developed taking into account the nonmaxwellian forms of distribution functions. It is shown that the kinetic treatment and the existence of nonmaxwellian distributions lead to real changes in the criteria required for double-layer formation.

How important are dispersive Alfvén waves for auroral particle acceleration?

The means by which charged particles are accelerated in space to form the aurora is still not fully understood. This acceleration produces earthward streaming electrons driving auroral luminosity and outward streaming ionospheric ions which populate space with terrestrial matter. With the advent of high resolution space borne field and particle instruments, dispersive Alfvén waves (DAWs) have been identified as drivers of auroral particle acceleration and it has been shown that the Alfvén wave energy observed is sufficient to power a significant fraction of auroral luminosity. Since previously it has been considered that auroral particle acceleration occurs in quasi‐steady field‐aligned currents, quantifying the amount of particle acceleration occurring in DAWs relative to the traditionally invoked processes is fundamental to our understanding of how the aurora works. We combine coincident satellite measurements of fields and particles to demonstrate that as functions of increasing auroral activity 25–39% of the total electron energy deposited in the ionosphere and 15–34% of total energetic ion outflow may be attributed to the action of DAWs. In fact in the vicinity of the polar cusps and pre‐midnight auroral oval, DAWs may provide the dominant means for powering electron and ion acceleration during active times.

The solar system: a laboratory for the study of the physics of particle acceleration

AbstractA remarkable variety of particle acceleration occurs in the solar system, from lightning-related acceleration of electrons to tens of MeV energy in less than a millisecond in planetary atmospheres; to acceleration of auroral and radiation belt particles in planetary magnetospheres; to acceleration at planetary bow shocks, co-rotating interplanetary region shocks, shocks driven by fast coronal mass ejections, and possibly at the heliospheric termination shock; to acceleration in magnetic reconnection regions in solar flares and at planetary magnetopause and magnetotail current sheets. These acceleration processes often occur in conjunction with transient energy releases, and some are very efficient. Unlike acceleration processes outside the solar system, the accelerated particles and the physical conditions in the acceleration region can be studied through direct in situ measurements, and/or through detailed imaging and spectroscopy. Here I review recent observations of tens of MeV electron acceleration in the Earth's atmosphere and in the Earth's radiation belts, electron and ion acceleration related to magnetic reconnection in solar flares, electron acceleration to ≥ 300 keV in magnetic reconnection regions in the Earth's deep magnetotail, and acceleration of solar energetic particles (SEPs) by shocks driven by fast coronal mass ejections (CMEs).

Anomalous resistivity of current-driven isothermal plasmas due to phase space structuring

The anomalous electric resistivity of collisionless plasmas is an important issue in the physics of hot plasmas, e.g., in the context of auroral particle acceleration and of reconnection in the solar corona. The linear stability theory of isothermal current driven space plasmas predicts an ion-acoustic instability if the relative drift velocity of the current carrying particles exceeds a certain threshold, which, generally, depends on the plasma parameters. The spectrum of waves, excited by a marginal instability, is very narrow. Hence, the wave power at saturation and the corresponding electric resistivity due to wave-particle interaction cannot be obtained by means of a quasilinear, weak turbulence approach and the nonlinear single mode theory provides too small saturation amplitudes. To solve the nonlinear problem a newly developed unsplit conservative Eulerian Vlasov code is applied to simulate a strongly magnetized current driven plasma, which can be considered in 1D1V (one spatial, one velocity space direction). Instead of periodic boundary conditions, usually used as they are simpler to treat, open boundaries are implemented which allow to maintain a constant current flow. Simulated is a typical almost isothermal (Te=2Ti) hot (κTi=1keV) space plasma for the real mass ratio mi∕me=1836. The initial spontaneous instability is followed by a three-stage nonlinear evolution: First electron trapping leads to the formation of electron phase space holes. Due to a steepening of the leading edges of the potential wells the electron phase space holes gradually become asymmetric, they grow in size and deepen. The phase space holes accelerate until they move much faster than the initial ion-acoustic waves. The interaction of the current carriers with the asymmetric potential wells and causes a nonvanishing net momentum transfer between the particles and the self-generated electric field. After a few ion plasma periods ion trapping starts until, finally, an electrostatic double layer arises. It is found that the nonlinear saturated state of the system is dominated by the particle interaction with coherent phase space structures. The corresponding anomalous resistivity is slightly modulated with an oscillation period τ≈ωpi−1). For a macroscopic description its major part can be parameterized by means of an effective collision rate νeff of the order of 10−2ωpe≈0.5ωpi, where ωpe is the electron and ωpi the ion plasma frequency.

Displacement Current and the Generation of Parallel Electric Fields

We show for the first time the dynamical relationship between the generation of magnetic field-aligned electric field (E||) and the temporal changes and spatial gradients of magnetic and velocity shears, and the plasma density in Earth's magnetosphere. We predict that the signatures of reconnection and auroral particle acceleration should have a correlation with low plasma density, and a localized voltage drop (V||) should often be associated with a localized magnetic stress concentration. Previous interpretations of the E|| generation are mostly based on the generalized Ohm's law, causing serious confusion in understanding the nature of reconnection and auroral acceleration.