T. R. Sanderson

Tracing the topology of the October 18–20, 1995, magnetic cloud with ∼0.1–10² keV electrons

Five solar impulsive ∼1–10² keV electron events were detected while the WIND spacecraft was inside the magnetic cloud observed upstream of the Earth on October 18–20, 1995. The solar type III radio bursts produced by these electrons can be directly traced from ∼1 AU back to X-ray flares in solar active region AR 7912, implying that at least one leg of the cloud was magnetically connected to that region. Analysis of the electron arrival times shows that the lengths of magnetic field lines in that leg vary from ∼3 AU near the cloud exterior to ∼1.2 AU near the cloud center, consistent with a model force-free helical flux rope. Although the cloud magnetic field exhibits the smooth, continuous rotation signature of a helical flux rope, the ∼0.1-1 keV heat flux electrons and ∼1–10² keV energetic electrons show numerous simultaneous abrupt changes from bidirectional streaming to unidirectional streaming to complete flux dropouts. We interpret these as evidence for patchy disconnection of one end or both ends of cloud magnetic field lines from the Sun.

Observations of Energetic Ions from Comet Giacobini-Zinner

During the encounter with comet Giacobini-Zinner, the energetic particle anisotropy spectrometer on the International Cometary Explorer spacecraft observed large fluxes of energetic ions, believed to result principally from ionization of the cometary atmosphere followed by pickup and acceleration by the ambient flow of the solar wind. These heavy cometary ions were observed from approximately 1 day before closest approach to about 2� days afterward. Three regimes of differing ion characteristics have been identified. An outer region with a scale of ∼106 kilometers contains variable fluxes of antisolar-streaming pick-up ions in the undisturbed solar wind. In the middle region, of ∼105 kilometers, fluxes have less large-scale variability and broader angular and energy distributions. This region is separated from the outer zone by a sharp transition. The inner region has a scale of ∼104 kilometers and is characterized by reduced fluxes and complex angular distributions.

Wind Spacecraft Observations of Solar Impulsive Electron Events Associated with Solar Type III Radio Bursts

We present Wind spacecraft observations of solar impulsive electron events associated with locally generated Langmuir waves during solar type III radio bursts. The solar impulsive electrons had energies from ~600 eV to greater than 300 keV. Local Langmuir emissions associated with these fluxes generally coincided with the arrival of 2-12 keV electrons. A survey of 27 events over 1 yr shows that there were few occurrences of electron distributions (~96 s averaged) that were unstable to Langmuir waves and none that had a substantial growth rate (>3 × 10-2 s-1) or endured for more than 96 s. Intense solar impulsive electron events that occurred on 1995 April 2 are studied in detail. Marginally stable (plateaued) distributions occasionally coincided with a periods of local Langmuir emissions, but the electron distributions were otherwise stable. These observations suggest that kinetic processes were modifying the electron distribution but also suggest that processes other than one-dimensional quasilinear relaxation were involved. We find that solar impulsive electron distributions were often unstable to oblique waves, such as quasi-electrostatic whistler waves or electromagnetic ion cyclotron waves, suggesting that competition between Langmuir and oblique emissions may be important. There are several other features in the Wind spacecraft solar impulsive electron observations that are noteworthy. Nondispersive flux modulations were visible in many of the events (also visible in the published ISEE 3 data) in ~1-4 keV electrons, suggesting that a local hydromagnetic instability may have accompanied the lowest energy solar impulsive electron fluxes. The Wind data differ from the ISEE 3 data in the energy spectra of the electron events. ISEE 3 recorded few events with only high-energy (>10 keV) electron fluxes, whereas a survey of the Wind events shows a substantially higher ratio of high-energy events. The high-energy events were often associated with solar flares that could not have been magnetically well connected with the satellite.

Low-latitude dusk flank magnetosheath, magnetopause, and boundary layer for low magnetic shear: Wind observations

We have studied in detail a Wind spacecraft crossing of the low-latitude dusk flank magnetosheath, magnetopause (MP), and the low-latitude boundary layer (LLBL) when the local magnetic shear across the MP was low (<30°) and the interplanetary magnetic field (IMF) was northward. We find that the magnetosheath flow tangential to the MP slows down initially as one moves from the bow shock toward the MP. However, close to the MP this flow speeds up as the MP is approached. The source of flow acceleration is likely to be the magnetic force associated with draping of the field lines around the MP. Magnetic flux pile-up and a plasma depletion layer are also observed next to the flank MP indicating that the level of magnetic flux transfer across the entire dayside low-latitude MP via reconnection is low. The MP is characterized by changes in the plasma properties. The electron parallel temperature is enhanced across the MP and continues to increase across the LLBL, while the perpendicular temperature is constant across the MP. This constancy of the perpendicular temperature suggests that the transfer of plasma takes place across the local MP. In the LLBL, the ion and electron temperatures are well correlated with the density. In addition, the flow direction in a substantial portion of the LLBL is nearly aligned with that in the magnetosheath, and the flow speed tangential to the MP decreases gradually with decreasing LLBL density. The behavior of the particle distributions suggests that the entire LLBL was on closed field lines. In essence, our findings on the topology and on the LLBL plasma characteristics suggest that even in the absence of reconnection at the local low-shear MP, the LLBL is locally coupled to the adjacent magnetosheath. The smooth variations of the plasma parameters with the density are consistent with the LLBL spatial profiles being gradual. This may suggest that diffusion processes play a role in the formation and dynamics of the LLBL. Finally, the magnetic field and the state of the plasma in the plasma sheet adjacent to the flank MP/LLBL appear to be functions of the IMF direction. Thus the IMF may control both the external (magnetosheath) and the internal (plasma sheet) boundary conditions for the flank MP processes.

The Low Energy Proton Experiment on ISEE-C

There are still many outstanding problems concerning the origin and propagation of sub-MeV protons in the heliosphere. Proton fluxes from solar flares are modulated by coronal propagation, as well as by large and small scale structures in the solar wind. Acceleration processes in interplanetary space, as well as at the sun, complicate the picture, as does the presence, in interplanetary space close to the earth of protons of magnetospheric origin. Experiment DFH was designed to investigate the problems of low energy proton behavior in the energy range 35-1600 keV. The orbit of ISEE-C is free from magnetospheric effects but permits a high data rate and DFH provides three-dimensional proton flux distribution measurements with good energy resolution at a time resolution of 16 s. Thus it provides 180 independent points on the proton phase space distribution function every 16 s. The instrument was designed and built by the Space Research Laboratory, Utrecht, The Netherlands, the Space Science Department of ESA, and the Cosmic Rays and Space Physics Group of Imperial College, London, England. Section I of this paper reviews briefly the scientific objectives of the experiment. Details of the instrument are given in Section II.

Observation of an impulsive solar electron event extending down to ∼0.5 keV energy

We present the first observation of a solar impulsive electron event spanning the entire solar wind-suprathermal particle energy range (few eV to hundreds of keV), obtained with the 3-D Plasma and Energetic Particle experiment on the WIND spacecraft. The electron energy spectrum fits to a power-law ∼ E−3 from ∼40 keV down to a peak at ≲ 1 keV, with significant flux detected down to ∼0.5 keV. Since the range of such low energy electrons in ionized hydrogen is much less than the column density of the corona, they must be accelerated high, ∼1 R⊙ (solar radius) above the photosphere, for typical active coronal density models.

Moon-solar wind interactions: First results from the WIND/3DP Experiment

Launched on November 1, 1994, the WIND spacecraft passed behind the moon on December 27, 1994 and remained in its optical shadow for ∼41 minutes. At the time the moon was immersed in the interplanetary medium and a large amount of new data has been collected by the WIND/3DP experiment. The nature of the flow near the moon is described by presenting detailed electron and ion plasma parameters in various regions; a structured lunar plasma umbra is clearly observed downstream of the moon at XSSE ∼ −6 RM, characterized by a gradual decrease of electron and ion densities to values of <0.5 cm−3 near the center of the optical umbra. The plasma umbra is surrounded by a penumbra region in which plasma densities and magnetic field magnitude decrease and plasma flow undergoes small but noticeable variations. Analysis of magnetic shielding effects show that medium energy electrons follow the interplanetary magnetic field lines and are partially occulted by the moon. These first WIND results on the detailed properties of the downstream flow and the cavity extent are compared with fluid descriptions of the global moon-solar wind interaction.

The subsolar magnetosheath and magnetopause for high solar wind ram pressure: WIND observations

On a rapid inward pass through the subsolar magnetosheath (MSH) and magnetopause (MP), the WIND spacecraft initially encountered a moderately-compressed low-magnetic shear MP (at a radial distance of 8.6 RE), followed by multiple crossings of a high-shear MP (at 8.2 RE). The large shear resulted from a southward turning of the external MSH field. Strong magnetic field pile-up, a plasma depletion layer (PDL), and plasma flow acceleration and rotation to become more perpendicular to the local magnetic field were observed in the MSH on approach to the low-shear MP. At the high-shear MP, magnetic reconnection flows were detected, and there are some indications that plasma depletion effects were weak or absent in the adjacent MSH. We attribute the changes in the MP and MSH properties to the sudden rotation of the MSH field direction. In essence, the structure of the MP regions under the unusually high solar wind ram pressure condition in this case does not seem to be qualitatively different from that observed under more typical (less compressed) conditions. Also similar to previous observations, the mirror mode is marginally unstable in the MSH proper, but is stable in the PDL. In this region, the proton temperature anisotropy is inversely correlated with βp∥. Finally, the electron distributions are observed to be anisotropic (Te⟂/Te∥ ∼1.3) throughout the entire MSH.

The Ulysses south polar pass: Energetic ion observations

We present here a preliminary analysis of observations of energetic ions during the first polar pass of the Ulysses spacecraft, concentrating mainly on the region where the spacecraft was continually immersed in high speed flow from the polar coronal hole. During the ascent to high latitudes, a single recurrent peak was observed once per solar rotation. From 70°S during ascent to 43°S during descent no major peaks were observed. Thereafter, two peaks per solar rotation were observed.

Observations of energetic water-group ions at comet giacobini-zinner: Implications for ion acceleration processes

Abstract Observations of energetic water-group pick-up ions made by the EPAS instrument during the ICE fly-by of comet P/Giacobini-Zinner are investigated for evidence concerning the processes which accelerate the ions from initial pick-up energies of around 10 keV up to energies of a few hundred kilo-electronvolts. The form of the ion spectrum in the ion rest frame is first investigated and compared with theoretical suggestions that exponential energy distributions might be produced by either first or second order Fermi acceleration in the cometary environment. It is shown that such distributions do not fit the data at all well, but that rather (over the EPAS ion bulk rest frame energy range of ∼30 to 50 keV) data also fit a power law distribution very well, but at lower energies the data tend to show a flattening below the power law form. It is also shown that the observed spectra are much softer than those calculated by Ip and Axford (1986, Planet. Space Sci. 34, 1061) for the G-Z ion pick-up region upstream of the bow shock, indicating that if these distributions are indeed due to second order Fermi acceleration as they propose, then the ion mean free path must be rather larger than used in their calculations (i.e. rather larger than 5 water-group ion gyroradii). Overall, it is concluded that at the present stage of theoretical development it is premature to draw firm conclusions about acceleration mechanisms from studies of the ion spectrum alone. However, from the general spectral analysis it is also possible to investigate the variations of ion intensity and spectral hardness which take place during the comet encounter, and to gain an indication of the degree of isotropy of the ion distribution in the rest frame of the flow. We find that a ∼.5 × 104km-wide region (∼ 35 min along the spacecraft trajectory) of rapid ion intensification and spectral hardening occurs immediately upstream from the turbulent mass-loaded region, suggestive of a first order Fermi process in which ions are successively reflected between the outer layer of the slowed, turbulent region and waves in the faster upstream flow. It is shown that within the turbulent region the ion distribution becomes rapidly isotropized in its own bulk rest frame, indicative of a sudden reduction in ion mean free path. Additional processes (possibly second order Fermi acceleration) must also occur in order to account for the further modest intensifications of the energetic ion fluxes observed within the turbulent mass-loaded region, as well as the presence of ions in the outer pick-up region which have energies well in excess of the local pick-up energy.