Ultraviolet Coronagraph Spectrometer Research Articles

Ultraviolet spectroscopy of the extended solar corona

The first observations of ultraviolet spectral line profiles and intensities from the extended solar corona (i.e., more than 1.5 solar radii from Sun-center) were obtained on 13 April 1979 when a rocket-borne ultraviolet coronagraph spectrometer of the Harvard-Smithsonian Center for Astrophysics made direct measurements of proton kinetic temperatures, and obtained upper limits on outflow velocities in a quiet coronal region and a polar coronal hole. Following those observations, ultraviolet coronagraphic spectroscopy has expanded to include observations of over 60 spectral lines in coronal holes, streamers, coronal jets, and solar flare/coronal mass ejection (CME) events. Spectroscopic diagnostic techniques have been developed to determine proton, electron and ion kinetic temperatures and velocity distributions, proton and ion bulk flow speeds and chemical abundances. The observations have been made during three sounding rocket flights, four Shuttle deployed and retrieved Spartan 201 flights, and the Solar and Heliospheric Observatory (SOHO) mission. Ultraviolet spectroscopy of the extended solar corona has led to fundamentally new views of the acceleration regions of the solar wind and CMEs. Observations with the Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO revealed surprisingly large temperatures, outflow speeds, and velocity distribution anisotropies in coronal holes, especially for minor ions. Those measurements have guided theorists to discard some candidate physical processes of solar wind acceleration and to increase and expand investigations of ion cyclotron resonance and related processes. Analyses of UVCS observations of CME plasma properties and the evolution of CMEs have provided the following: temperatures, inflow velocities and derived values of resistivity and reconnection rates in CME current sheets, compression ratios and extremely high ion temperatures behind CME shocks, and three dimensional flow velocities and magnetic field chirality in CMEs. Ultraviolet spectroscopy has been used to determine the thermal energy content of CMEs allowing the total energy budget to be known for the first time. Such spectroscopic observations are capable of providing detailed empirical descriptions of solar energetic particle (SEP) source regions that allow theoretical models of SEP acceleration to be tailored to specific events, thereby enabling in situ measurements of freshly emitted SEPs to be used for testing and guiding the evolution of SEP acceleration theory. Here we review the history of ultraviolet coronagraph spectroscopy, summarize the physics of spectral line formation in the extended corona, describe the spectroscopic diagnostic techniques, review the advances in our understanding of solar wind source regions and flare/CME events provided by ultraviolet spectroscopy and discuss the scientific potential of next generation ultraviolet coronagraph spectrometers.

Resonant Heating of Ions by Parallel Propagating Alfvén Waves in Solar Coronal Holes

Resonant heating of H, O +5 , and Mg +9 by parallel propagating ion- cyclotron Alfven waves in solar coronal holes at a heliocentric distance is studied using the heating rate derived from the quasilinear theory. It is shown that the particle-Alfven-wave interaction is a significant microscopic process. The temper- atures of the ions are rapidly increased up to the observed order in only microsec- onds, which implies that simply inserting the quasilinear heating rate into the fluid/MHD energy equation to calculate the radial dependence of ion tempera- tures may cause errors as the time scales do not match. Different species ions are heated by Alfven waves with a power law spectrum in approximately a mass order. To heat O +5 over Mg +9 as measured by the Ultraviolet Coronagraph Spectrometer (UVCS) in the solar coronal hole at a region > 1.9R⊙, the energy density of Alfven waves with a frequency close to the O +5 -cyclotron frequency must be at least dou- ble of that at the Mg +9 -cyclotron frequency. With an appropriate wave-energy spectrum, the heating of H, O +5 and Mg +9 can be consistent with the UVCS

Slow wind and magnetic topology in the solar minimum corona in 1996–1997

This study examines the physical conditions of the outer solar corona in order to identify the regions where the slow solar wind is accelerated and to investigate the latitudinal transition from slow to fast wind during the minimum of the solar cycle. The analysis is based on observations of six streamers obtained during the years of solar minimum, 1996 and 1997, with the Ultraviolet Coronagraph Spectrometer (UVCS) onboard the Solar and Heliospheric Observatory (SOHO). The outflow velocity of the oxygen ions and the electron density of the coronal plasma are determined in altitude ranging from 1.5 to 3.5 solar radii (). The adopted diagnostic method, based on spectroscopic analysis of the O VI 1032 and 1038 Å lines, fully accounts for the large expansion factor of the magnetic field lines expected in the regions surrounding the streamers. The analysis leads to the conclusion that the slow coronal wind is observed (i) in the region external to and running along the streamer boundary; and (ii) in the region above the streamer core beyond 2.7 , where the transition between closed and open magnetic field lines takes place and the heliospheric current sheet forms. Regions in the immediate vicinity of the streamer boundary can be identified with the edges of the large polar coronal holes that characterize solar minimum. Results point to gradual variations of the properties of a coronal hole from the streamer boundary to its polar core, most likely related to the topology of the coronal magnetic field.

Direct Observations of the Magnetic Reconnection Site of an Eruption on 2003 November 18

We report direct observations of the magnetic reconnection site during an eruptive process that occurred on 2003 November 18. The event started with a rapid expansion of a few magnetic arcades located over the east limb of the Sun and developed an energetic partial-halo coronal mass ejection (CME), a long current sheet, and a group of bright flare loops in the wake of the CME. It was observed by several instruments, both in space and on the ground, including the EUV Imaging Telescope, Ultraviolet Coronagraph Spectrometer, and Large Angle Spectrometric Coronagraph experiment on board the Solar and Heliospheric Observatory, the Reuven Ramaty High Energy Solar Spectroscopic Imager, and the Mauna Loa Solar Observatory Mark IV K-Coronameter. We combine the data from these instruments to investigate various properties of the eruptive process, including those around the current sheet. The maximum velocities of the CME leading edge and the core were 1939 and 1484 km s-1, respectively. The average reconnection inflow velocities near the current sheet over different time intervals ranged from 10.5 to 106 km s-1, and the average outflow velocities ranged from 460 to 1075 km s-1. This leads to a corresponding rate of reconnection in terms of the Alfvén Mach number MA ranging from 0.01 to 0.23. The composite of images from different instruments specifies explicitly how the different objects developed by a single eruptive process are related to one another.

UVCS Observation of Sungrazer C/2001 C2: Possible Comet Fragmentation and Plasma‐Dust Interactions

In this paper we analyze SOHO Ultraviolet Coronagraph Spectrometer (UVCS) observations of the sungrazing cometC/2001C2,amemberoftheKreutzfamily,observedon2001February7atheliocentricdistancesof4.98and 3.60 R� . This comet apparently went through sequential fragmentation events along its path: further indication of fragmentation processes is provided by UVCS observations, which show the presence of two separate tails in the 4.98 Rdata set, which we interpret as two fragments unresolved by LASCO images, one of which sublimates before reaching 3.60 R� . The cometary hydrogen Lysignal, decaying exponentially with time, has been inter- preted in terms of the H2O outgassing rate and the interactions of coronal protons with atoms created by the photodissociation of water. However, one of the fragments shows an additional Lycontribution, constant with time, which adds to the temporally decaying signal. This contribution has been ascribed to the sublimation of pyroxene dust grains, whose end products neutralize coronal protons via charge exchange processes. Hence, the twofragmentshavedifferentcomposition;differencesthroughoutthecometbodymayhavebeentheprimarycause for the comet fragmentation. Subject headingg comets: general — comets: individual (C/2001 C2) — ultraviolet: general

Modeling the Energy Budget of Solar Wind Minor Ions: Implications for Temperatures and Abundances

The outflow of oxygen and silicon ions in the solar wind has been studied using a model that extends from the chromosphere into interplanetary space, with emphasis on understanding the energy budget of the minor ions. The model solves coupled gyrotropic transport equations, which account for temperature anisotropies and heat conduction, for all charge states, and includes ionization and recombination. The minor ions are heated with a constant heating rate per particle in the corona. In the transition region the thermal force causes minor ions to flow faster than protons, with an abundance that can be less than half of the chromospheric abundance. The ions quite suddenly decouple from the proton flow in the corona, and above this point the ion flow is independent of the proton flow. For high heating rates the coronal abundance is comparable to the chromospheric abundance, the ion terminal wind speed is high, and most of the deposited energy is lost into the solar wind. Low heating rates lead to very large coronal abundances and a low terminal flow speed, and the main energy loss is then through collisions with protons and electrons in the corona. The heavy ions become much hotter than protons in the corona, even without preferential heating of the ions. However, preferential heating is necessary to prevent a large abundance enhancement in the corona and to achieve flow speeds close to the speeds observed by the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO). The abundance enhancement implies that lowering the heating rate per particle in general leads to an increase in the total energy flux absorbed by the ions.

Shock Wave Driven by an Expanding System of Loops

We report on a Coronal Mass Ejection (CME) observed on June 27, 1999 by the UltraViolet Coronagraph Spectrometer (UVCS) telescope operating on board the SOHO spacecraft. The CME was also observed by the Large Angle Spectroscopic Coronagraph (LASCO). Emission of hot material has been recorded by UVCS propagating in front of an opening system of loops generated by the CME. The evolution of the UVCS structure is highly correlated with the evolution of the open- ing loop. The data reveal excess broadening of the O  doublet lines and an enhancement in the intensity of the Si  λ520.66 and λ499.37 lines due to the motion of the expanding hot gas. The hot gas emission seems to be due to a shock wave propagating in front of a very fast gas bubble traveling along the opening loop system.

Shock wave driven by an expanding system of loops

We report on a Coronal Mass Ejection (CME) observed on June 27, 1999 by the UltraViolet Coronagraph Spectrometer (UVCS) telescope operating on board the SOHO spacecraft. The CME was also observed by the Large Angle Spectroscopic Coronagraph (LASCO). Emission of hot material has been recorded by UVCS propagating in front of an opening system of loops generated by the CME. The evolution of the UVCS structure is highly correlated with the evolution of the open- ing loop. The data reveal excess broadening of the O  doublet lines and an enhancement in the intensity of the Si  λ520.66 and λ499.37 lines due to the motion of the expanding hot gas. The hot gas emission seems to be due to a shock wave propagating in front of a very fast gas bubble traveling along the opening loop system.

Magnetic Reconnection Inflow near the CME/Flare Current Sheet

This work reports direct observations of the magnetic reconnection site during an eruptive process occurring on November 18, 2003. The event started with a rapid expansion of a few magnetic arcades located over the east limb of the Sun and developed an energetic partial halo coronal mass ejection (CME), a long current sheet and a group of bright flare loops in the wake of the CME. It was observed by several instruments both in space and on ground, including the EUV Imaging Telescope, the Ultraviolet Coronagraph Spectrometer, and the Large Angle and Spectrometric Coronagraph experiment on board the Solar and Heliospheric Observatory, the Reuven Ramaty High Energy Solar Spectroscopic Imager, as well as the Mauna Loa Solar Observatory Mark IV K-coronameter. We combine the data from these instruments to investigate various properties of the eruptive process, including those around the current sheet. The composite of images from different instruments and the corresponding results specify explicitly how the different objects developed by a single eruptive process are related to one another.

Hydrogen Lyα Intensity Oscillations Observed by theSolar and Heliospheric ObservatoryUltraviolet Coronagraph Spectrometer

We report on a search for significant oscillations in different coronal structures by applying a wavelet analysis to Solar and Heliospheric Observatory UVCS observations of the hydrogen Lyα 1216 A line intensity taken between 1.5 and 2.2 R☉. Significant periodic oscillations, unlikely to be a result of instrumental effects, are shown to exist in a coronal hole, the quiet Sun, and a streamer. Observations made sequentially at different heights but at the same latitude often share similar power spectra. Neighboring pixels at the same radial distance also share similar power spectra. These results indicate both a localized structure to the periodicity and a long-range preservation of oscillation patterns in the radial expansion of the solar wind. We show that a preference for significant oscillations with periods of 7-8 minutes exists in three out of the four observations presented here. Other bands of preferred periodicity are observed at different heights.

Spectroscopic measurement of the plasma electron density and outflow velocity in a polar coronal hole

A new spectroscopic method, aimed to derive the plasma electron density and outflow velocity in expanding solar coronal regions, is discussed in this paper. The method is based on the analysis of a pair of coronal lines emitted via collisional and radiative excitation by the same ion, such as the O VI 1032, 1037 A doublet. The merit of this technique consists in allowing us to derive at the same time electron density and outflow velocity of the coronal plasma from nearby lines detectable with the same instrument, provided the constraint on mass flux conservation along the flow tube connecting solar corona and heliosphere is taken into account. The results obtained from the analysis of the OVI emission imply that the physical conditions of a polar coronal hole plasma, observed during minimum activity, are the following. The electron density decreases from $4\times 10^5$ cm -3 at 1.7 $R_{\odot}$ to $2{-}4\times 10^4$ cm -3 at 3.1 $R_{\odot}$, whereas the outflow velocity of the oxygen ions is monotonically increasing to reach 350–500 km s -1 at 3.1 $R_{\odot}$, depending on the assumptions on the degree of anisotropy of the velocity distribution of the ions. These results of the velocity of expansion of the fast wind confirm those obtained with Doppler dimming techniques when assuming the lowest observed density values for the coronal hole plasma. This implies that, for a rarified corona, the outflow velocity of the fast solar wind in polar holes can be traced by the motion of the O VI ions at least up to 2.4 $R_{\odot}$. The analysis also shows that the degree of anisotropy of the oxygen ions, due to the acceleration of the ions across the magnetic field in a coronal hole, exhibits a steep increase and that the geometry of the flow tube diverges very rapidly low down in the inner corona/transition region. The observations of the extended corona analysed in this paper are obtained with the Ultraviolet Coronagraph Spectrometer of the SOHO space mission.

Low-latitude coronal holes during solar maximum

The Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO has been used to observe large low-latitude coronal holes during solar maximum that produced fast solar wind streams. UVCS observations show that large low-latitude coronal holes at solar maximum, coronal holes of at least 10° in longitude, have plasma properties that seem to bridge the gap between solar minimum polar coronal holes and streamers. The ion kinetic perpendicular temperatures in equatorial coronal holes are about 2 times larger than those in a solar minimum equatorial streamer, and about a factor of 2 smaller than those in polar coronal holes above 2 R ⊙. The outflow speeds for the large equatorial coronal holes observed by UVCS are 3–4 times lower than those in polar coronal holes between 2 and 3 R ⊙. The values for high- and mid-latitude coronal holes are in between those. In all these cases, the in situ data corresponding to these coronal holes showed high-speed wind streams with asymptotic speeds of 600–750 km s −1. These wind speeds approach those observed over polar coronal holes at solar minimum, but the outflow speeds in these coronal holes between 2 and 3 R ⊙ are different. In contrast to the polar coronal holes, the bulk of the solar wind acceleration must occur above 3 R ⊙ for large low-latitude coronal holes at solar maximum. These observations provide detailed empirical constraints for theoretical models and may be key to understanding how the various types of solar wind plasma are heated and accelerated.

Origin and Acceleration of Fast and Slow Solar Wind

Before the advent of the Solar and Heliospheric Observatory (SOHO, launched in 1995), we had little information on how coronal plasma gets accelerated to the high speed measured in situ. The Ultraviolet Coronagraph Spectrometer (UVCS) on SOHO, acquiring UV data over the first few solar radii, a region unexplored in this spectral range, allowed us to build a profile of the outflow plasma speed vs. heliocentric distance, based on empirical constraints. Still, much has to be learnt about the behavior of solar wind in the extended corona. In the following, after briefly reviewing the general properties of the solar wind, I'll focus on controversial issues — like the identification of the sources of fast and slow wind, and the acceleration of fast vs. slow wind streams — and I'll illustrate recent contributions to the solution of these problems.

Coronal transients and metric type II radio bursts

In this paper we investigate the relationship between type ii and coronal transient activity in terms of emission originating from the top or the flanks of a bow/piston shock surface, extending just above the coronal mass ejection (CME) leading edge surface. For this purpose, we used ground-based metric type ii radio burst observations of twenty-nine events in conjunction with Large Angle and Spectrometric Coronagraph (LASCO/SOHO) and UltraViolet Coronagraph Spectrometer (UVCS/SOHO) observations. With the refined density diagnostic offered by the UVCS instrument, we analyzed the type ii dynamics in conjunction with the associated CME dynamics. Although we found some correlation, in all but a few cases the coronal transients appeared to lead the type ii emission locations by several minutes, in apparent disagreement with a CME-driven origin interpretation. By applying a simple model, we found however that a piston-driven origin is certainly viable for all the events under study on the hypothesis that the radio emission originates in discrete locations above the top or the flanks of bow/piston shock surfaces extending just above the transient leading edges.

Doubly ionized carbon observed in the plasma tail of comet Kudo-Fujikawa.

Comet C/2002 X5 (Kudo-Fujikawa) was observed near its perihelion of 0.19 astronomical unit by the Ultraviolet Coronagraph Spectrometer aboard the Solar and Heliospheric Observatory spacecraft. Images of the comet reconstructed from high-resolution spectra reveal a quasi-spherical cloud of neutral hydrogen and a variable tail of C+ and C2+ that disconnects from the comet and subsequently regenerates. The high abundance of C2+ and C+, at least 24% relative to water, cannot be explained by photodissociation of carbon monoxide and is instead attributed to the evaporation and subsequent photoionization of atomic carbon from organic refractory compounds present in the cometary dust grains. This result serves to strengthen the connection between comets and the material from which the Solar System formed.

Empirically Determined Anisotropic Velocity Distributions and Outflows of O5+Ions in a Coronal Streamer at Solar Minimum

Empirical constraints on the O5+ velocity distributions and outflow speeds in a solar minimum equatorial streamer between 2.6 and 5.1 R☉ are determined using a spectral synthesis code that includes O VI Doppler dimming. These constraints follow directly from UV spectra taken on 1996 October 12 with the Ultraviolet Coronagraph Spectrometer (UVCS) on the Solar and Heliospheric Observatory (SOHO) satellite and three-dimensional electron densities derived from tomography applied to a time series of polarized white-light images taken with the Large Angle and Spectrometric Coronagraph (LASCO) also on SOHO. Four conclusions result from this work: (1) our analysis shows O5+ velocity distribution anisotropy in the streamer legs and stalk and gives strong evidence that the microscopic velocity distribution (which excludes wave motions that equally affect all charged particles) is anisotropic, where the most probable speed perpendicular to the magnetic field direction exceeds that in the parallel direction; (2) there is preferential heating of the O5+ ions over the protons in the streamer stalk and legs; (3) there is no evidence for preferential O5+ heating in the core; and (4) the outflow velocity of the O5+ ions is determined at heights above 4.6 R☉. All results have a confidence level of at least 70%.

Far‐Ultraviolet Spectra of Fast Coronal Mass Ejections Associated with X‐Class Flares

The Ultraviolet Coronagraph Spectrometer (UVCS) on board the Solar and Heliospheric Observatory satellite has observed very fast coronal mass ejections (CMEs) associated with X-class flares. These events show spectral signatures different from those seen in most other CMEs in terms of very rapid disruption of the pre-CME streamer, very high Doppler shifts, and high-temperature plasma visible in the [Fe XVIII] emission line. This paper describes three very similar events on 2002 April 21, July 23, and August 24 associated with X-class flares. We determine the physical parameters of the pre-CME streamers and discuss the geometric and physical nature of the streamer blowouts. In the April 21 event, the hot plasma seen as [Fe XVIII] is not related to the structure seen in [Fe XXI] by the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) instrument at lower heights. It has the form of a rapidly expanding fan, quite likely a current sheet. In the August event, on the other hand, the [Fe XVIII] is probably a bubble of hot plasma formed by reconnection in the wake of the CME. C III emission from the July 23 flare is detected as stray light in the UVCS aperture. It precedes the hard X-ray brightening by about 2 minutes.

Physical Parameters of the 2000 February 11 Coronal Mass Ejection: Ultraviolet Spectra versus White‐Light Images

We present spectra of a three-part coronal mass ejection (CME) observed by the Ultraviolet Coronagraph Spectrometer aboard SOHO on 2000 February 11. Images of the CME in different spectral lines show how the morphology depends on the temperature, density, and outflow speed of the ejected plasma. The H I Ly? is the line that best resembles the white-light data, although it can be rather different where the outflow speed severely dims its radiative component. We estimate the ranges of temperature and density in the front, prominence core, and void. We also estimate the outflow speed that is the true speed of the ejecta as obtained from the Doppler dimming technique, its component projected on the plane of the sky, and the line-of-sight speed for the three components of the CME. The plasma in the front was denser, cooler, and more depleted in O and Si than the ambient coronal streamer. These characteristics indicate that it originated in the closed field core of the pre-CME streamer. The leading edge was not the projection of a simple spherical shell onto the plane of the sky. The line profiles suggest a wide looplike structure, although a more complete shell that was brighter in some areas could also match the data. The prominence has a structure in temperature and density with the hotter top end emitting in the Mg X and Si XII lines while the bottom end was much cooler and visible only in the H I Lyman lines. Emission in the void was rather faint. The outflow speed obtained from Doppler dimming of the radiative lines, the line-of-sight speed measured from the Doppler shift of the lines, and the plane-of-the-sky speed estimated from the comparison of the images taken at 2.3 and 2.6 R? give speeds much lower than those estimated at greater heights (>4 R?) from LASCO and indicate a stronger acceleration at lower heights.

Dynamical and Physical Properties of a Post–Coronal Mass Ejection Current Sheet

In the eruptive process of the Kopp-Pneuman type, the closed magnetic field is stretched by the eruption so much that it is usually believed to be open to infinity. Formation of the current sheet in such a configuration makes it possible for the energy in the coronal magnetic field to quickly convert into thermal and kinetic energies and cause significant observational consequences, such as growing postflare/CME loop system in the corona, separating bright flare ribbons in the chromosphere, and fast ejections of the plasma and the magnetic flux. An eruption on 2002 January 8 provides us a good opportunity to look into these observational signatures of and place constraints on the theories of eruptions. The event started with the expansion of a magnetic arcade over an active region, developed into a coronal mass ejection (CME), and left some thin streamer-like structures with successively growing loop systems beneath them. The plasma outflow and the highly ionized states of the plasma inside these streamer-like structures, as well as the growing loops beneath them, lead us to conclude that these structures are associated with a magnetic reconnection site, namely, the current sheet, of this eruptive process. We combine the data from the Ultraviolet Coronagraph Spectrometer, Large Angle and Spectrometric Coronagraph Experiment, EUV Imaging Telescope, and Coronal Diagnostic Spectrometer on board the Solar and Heliospheric Observatory, as well is from the Mauna Loa Solar Observatory Mark IV K-coronameter, to investigate the morphological and dynamical properties of this event, as well as the physical properties of the current sheet. The velocity and acceleration of the CME reached up to 1800 km/s and 1 km/sq s, respectively. The acceleration is found to occur mainly at the lower corona (<2.76 Solar Radius). The post-CME loop systems showed behaviors of both postflare loops (upward motion with decreasing speed) and soft X-ray giant arches (upward motion with constant speed, or acceleration) according to the definition of Svestka. In the current sheet, the presence of highly ionized ions, such as Fe(+17) and Ca(+13), suggests temperature as high as (3-4) x 10(exp 6) K, and the plasma outflows have speeds ranging from 300 to 650 km/s. Absolute elemental abundances in the current sheet show a strong first ionization potential effect and have values similar to those found in the active region streamers. The magnetic field strength in the vicinity of the current sheet is found to be of the order of 1 G.

Alfvenic Turbulence in the Extended Solar Corona: Kinetic Effects and Proton Heating

We present a model of magnetohydrodynamic (MHD) turbulence in the extended solar corona that contains the effects of collisionless dissipation and anisotropic particle heating. Recent observations have shown that preferential heating and acceleration of positive ions occur in the first few solar radii of the high-speed solar wind. Measurements made by the Ultraviolet Coronagraph Spectrometer aboard SOHO have revived interest in the idea that ions are energized by the dissipation of ion cyclotron resonant waves, but such high-frequency (i.e., small-wavelength) fluctuations have not been observed. A turbulent cascade is one possible way of generating small-scale fluctuations from a preexisting population of low-frequency MHD waves. We model this cascade as a combination of advection and diffusion in wavenumber space. The dominant spectral transfer occurs in the direction perpendicular to the background magnetic field. As expected from earlier models, this leads to a highly anisotropic fluctuation spectrum with a rapidly decaying tail in the parallel wavenumber direction. The wave power that decays to high enough frequencies to become ion cyclotron resonant depends on the relative strengths of advection and diffusion in the cascade. For the most realistic values of these parameters, however, there is insufficient power to heat protons and heavy ions. The dominant oblique fluctuations (with dispersion properties of kinetic Alfven waves) undergo Landau damping, which implies strong parallel electron heating. We discuss the probable nonlinear evolution of the electron velocity distributions into parallel beams and discrete phase-space holes (similar to those seen in the terrestrial magnetosphere), which can possibly heat protons via stochastic interactions.