M. R. Collier

XMM-Newton Observation of Solar Wind Charge Exchange Emission

We present an XMM-Newton spectrum of diffuse X-ray emission from within the solar system. The spectrum is dominated by probable C VI lines at 0.37 and 0.46 keV, an O VII line at 0.56 keV, O VIII lines at 0.65 and ~0.8 keV, Ne IX lines at ~0.92 keV, and Mg XI lines at ~1.35 keV. This spectrum is consistent with that expected from charge exchange emission between the highly ionized solar wind and either interstellar neutrals in the heliosphere or material from Earths exosphere. The emission is clearly seen as a low-energy (E < 1.5 keV) spectral enhancement in one of a series of four observations of the Hubble Deep Field-North. The X-ray enhancement is concurrent with an enhancement in the solar wind measured by Advanced Composition Explorer, Wind, and Solar and Heliospheric Observatory spacecraft. The solar wind enhancement reaches a flux level an order of magnitude more intense than typical fluxes at 1 AU and has a significantly enhanced O+7/O+6 ratio. Besides being of interest in its own right for studies of the solar system, this emission can have significant consequences for observations of cosmological objects. It can provide emission lines at zero redshift, which are of particular interest in studies of diffuse thermal emission (e.g., O VII and O VIII), and which can therefore act as contamination in the spectra of objects that cover the entire detector field of view. We propose the use of solar wind monitoring data as a diagnostic to screen for such possibilities.

Ionospheric mass ejection in response to a CME

We report observations of a direct ionospheric plasma outflow response to the incidence of an interplanetary shock and associated coronal mass ejection (CME) upon the earths magnetosphere. Data from the WIND spacecraft, 185 RE upstream, document the passage of an interplanetary shock at 23:20 UT on 24 Sept. 1998. The polar cap plasma environment sampled by the POLAR spacecraft changed abruptly at 23:45 UT, reflecting the compressional displacement of the geopause relative to the spacecraft. POLAR left the polar wind outflow region and entered the mantle flows. Descending toward the dayside cusp region, POLAR later returned from the mantle to an enhanced polar wind flux dominated by O+ plasma and eventually containing molecular ions. The enhanced and O+− dominated outflow continued as the spacecraft passed through the high altitude cleft and then the southern cleft at lower altitude. Such a direct response of the ionosphere to solar wind dynamic pressure disturbances may have important impacts on magnetospheric dynamics.

Timing accuracy for the simple planar propagation of magnetic field structures in the solar wind

Results from a correlation analysis of Wind and IMP 8 magnetometer data for a total of 543 two hour periods covering Jan.–Jul. 1995 were analyzed to determine that: (1) the timing accuracy Δτ for advecting solar wind magnetic field structures goes as (dper/dpar) · τcon where dpar is the spacecraft separation along the Sun-Earth line, dper is the transverse separation, and τcon is the predicted convection lag time, (2) “good” correlation time periods (peak correlation coefficient >0.80) are about twice as likely at solar maximum than at solar minimum, (3) geometry affects timing accuracy more than any propagation of features with respect to the solar wind, and (4) there is a “high” probability (greater than predicted by a Gaussian distribution) of “very bad” (|dparΔτ/dperτcon| >3) timing agreement due to the long tail on the probability distribution.

THE LOW-ENERGY NEUTRAL ATOM IMAGER FOR IMAGE

The ‘Imager for Magnetosphere-to-Aurora Global Exploration’ (IMAGE) will be launched early in the year 2000. It will be the first mission dedicated to imaging, with the capability to determine how the magnetosphere changes globally in response to solar storm effects in the solar wind, on time scales as short as a few minutes. The low energy neutral atom (LENA) imager uses a new atom-to-negative ion surface conversion technology to image the neutral atom flux and measure its composition (H and O) and energy distribution (10 to 750 eV). LENA uses electrostatic optics techniques for energy (per charge) discrimination and carbon foil time-of-flight techniques for mass discrimination. It has a 90° x 8° field-of-view in 12 pixels, each nominally 8° x 8°. Spacecraft spin provides a total field-of-view of 90° x 360°, comprised of 12 x 45 pixels. LENA is designed to image fast neutral atom fluxes in its energy range, emitted by auroral ionospheres or the sun, or penetrating from the interstellar medium. It will thereby determine how superthermal plasma heating is distributed in space, how and why it varies on short time scales, and how this heating is driven by solar activity as reflected in solar wind conditions.

Variability of the ring current source population

The relationship between the strength of the storm-time ring current and the available density in the presumed source region, the plasma sheet, is examined for 23 storms with Dst * (minimum Dst corrected for magnetopause currents) ranging from -50 to -164 nT. A good correlation is found between the plasma-sheet density at geosynchronous orbit and the minimum Dst * , The minimum Dst * is also well correlated with the eastward interplanetary electric field measured upstream by the Wind spacecraft, in agreement with previous studies, but the dependence of Dst * on plasma-sheet density is unrelated to its dependence on the electric field. The best correlation is between the minimum Dst * and the product of the plasma-sheet density and the eastward interplanetary electric field. These results are consistent with a scenario in which the intensity of the storm-time ring current is determined by a combination of source strength (plasma-sheet density) and injection strength (interplanetary electric field).

The suprathermal seed population for corotating interaction region ions at 1 AU deduced from composition and spectra of H + , He ++ , and He + observed on Wind

We have measured H + , He ++ , and He + distribution functions over the solar wind through the suprathermal energy range during two corotating interaction region (CIR) events o b- served by the STICS, MASS, and STEP instruments on board the Wind spacecraft at 1 AU during April and May 1995. The major properties we find are as follows : In the suprathermal energy range (~10-500 keV/nucleon), the particle intensities peak inside the CIR itself, in the compressed and decelerated fast solar wind, in contrast to the situation at MeV energies, where the peak inte n- sities are observed outside the CIR in the fast solar wind. The distribution functions of solar wind H + and He ++ change smoothly from the core at solar wind speeds to a power law or exponential form at higher energies, with no turnover observed at intermediate energies. CIR He + is observed with an abundance ratio He + /He ++ ~ 16-17%, orders of magnitude higher than that in the bulk solar wind but nevertheless lower than that observed in CIRs at 4.5 AU. The H + , He ++ , and He + spectra have similar slopes above speeds of ~2.5-3 times the solar wind speed ( Vsw) in the space- craft frame. The ion speed at which the CIR He ++ /H + ratio changes from typical solar wind va l- ues of 4-5% to the higher (>10%) value typical of CIRs is ~1.5-1.7 Vsw, measured in the space- craft reference frame. Analyzing these observations in the context of previous global observations and simple models of CIR acceleration and transport ( Fisk and Lee, 1980), we conclude the fol- lowing: (1) Suprathermal CIR ions at 1 AU originated close (within ~0.5 AU) to the point of o b- servation, not in the outer heliosphere; (2) the injection/acceleration mechanism is not especially sensitive to charge-to-mass ratio over the range 0.25-1.0; (3) since the particles are locally accel - erated, the low-energy ion populations we observe contain the seed population; (4) the bulk solar wind itself is not the source of the energetic ions; rather, the source is in the suprathermal tail, with an injection threshold in the spacecraft frame of ~1.8-2.5 times the solar wind speed; and (5) in at least one of these CIRs, suprathermal particle acceleration is not shock associated and must therefore be associated with a statistical mechanism or co mpression in the solar wind.

Observations of neutral atoms from the solar wind

We report observations of neutral atoms from the solar wind in the Earths vicinity with the low-energy neutral atom (LENA) imager on the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) spacecraft. This instrument was designed to be capable of looking at and in the direction of the Sun. Enhancements in the hydrogen count rate in the solar direction are not correlated with either solar ultraviolet emission or suprathermal ions and are deduced to be due to neutral particles from the solar wind. LENA observes these particles from the direction closest to that of the Sun even when the Sun is not directly in LENAs 90° field of view. Simulations show that these neutrals are the result of solar wind ions charge exchanging with exospheric neutral hydrogen atoms in the postshock flow of the solar wind in the magnetosheath. Their energy is inferred to exceed 300 eV, consistent with solar wind energies, based on simulation results and on the observation of oxygen ions, sputtered from the conversion surface in the time-of-flight spectra. In addition, the sputtered oxygen abundance tracks the solar wind speed, even when IMAGE is deep inside the magnetosphere. These results show that low-energy neutral atom imaging provides the capability to directly monitor the solar wind flow in the magnetosheath from inside the magnetosphere because there is a continuous and significant flux of neutral atoms originating from the solar wind that permeates the magnetosphere.

The Interstellar Boundary Explorer (IBEX)

The Interstellar Boundary Explorer (IBEX) mission is exploring the frontiers of the heliosphere where energetic neutral atoms (ENAs) are formed from charge exchange between interstellar neutral hydrogen atoms and solar wind ions and pickup ions. The geography of this frontier is dominated by an unexpected nearly complete arc of ENA emission, now known as the IBEX ‘Ribbon’. While there is no consensus agreement on the Ribbon formation mechanism, it seems certain this feature is seen for sightlines that are perpendicular to the interstellar magnetic field as it drapes over the heliosphere. At the lowest energies, IBEX also measures the flow of interstellar H, He, and O atoms through the inner heliosphere. The asymmetric oxygen profile suggests that a secondary flow of oxygen is present, such as would be expected if some fraction of oxygen is lost through charge exchange in the heliosheath regions. The detailed spectra characterized by the ENAs provide time-tagged samples of the energy distributions of the underlying ion distributions, and provide a wealth of information about the outer heliosphere regions, and beyond.

Ion outflow observed by IMAGE: Implications for source regions and heating mechanisms

Images of the Earths proton aurora from the IMAGE spacecraft on 8 June 2000 indicate a temporally and spatially isolated ionospheric response to a shock that impinged on the Earths magnetopause. Sometime after this ionospheric response, the Low Energy Neutral Atom imager on IMAGE detected enhanced ionospheric outflow. The time delay between the ionospheric response and the enhanced outflow is consistent with the travel time of ∼ 30 eV neutral Oxygen (created by charge exchange of outflowing O + with the exosphere) from the low altitude ionosphere to the spacecraft. The prompt ionospheric outflow implies that the shock deposited sufficient energy in the topside ionosphere near or above the O + exobase to initiate the outflow.

Multispacecraft observations of sudden impulses in the magnetotail caused by solar wind pressure discontinuities: Wind and IMP 8

Two upstream solar wind pressure discontinuities that were associated with storm sudden commencements have been examined to determine their effect on the geomagnetic tail lobe field. During the two events, occurring on March 9, 1995, and August 17, 1995, the Wind spacecraft was located in the upstream region monitoring the solar wind, and the IMP 8 spacecraft was in the geomagnetic tail lobe observing the tail response. The two events occurred during periods with northward or weak southward interplanetary magnetic field. In each case, the data suggest that the magnetic field in the tail lobe increased in magnitude directly in response to the external solar wind pressure increase. It is shown that a simple model in which a uniform magnetic field is compressed by a step function constriction accurately predicts characteristic timescales, which are of the order of a couple minutes, and the magnetic field profiles. The inferred flaring angles are consistent with model predictions, and the changes in the flaring angle across the discontinuities correspond to expectations based on changes in the subsolar magnetopause position and tail width. Overall, the results of this study indicate that the magnetotail maintains an approximate MHD equilibrium even as it responds rapidly to interplanetary pressure discontinuities.