J. M. Quinn

Effect of wave-particle interactions on ring current evolution for January 10-11, 1997: Initial results

We simulate the ring current evolution during the magnetic storm caused by Earth passage of the January 1997 magnetic cloud. Compared to previous studies, we include for the first time energy diffusion caused by wave-particle interactions. The modeled Dst index agrees reasonably well with the measured one, corrected for magnetopause currents and currents induced in the solid Earth. We compare H+distributions calculated from our model with those measured by the HYDRA instrument on the POLAR spacecraft and find that: a) the agreement between theory and data at large Lshells (L>5.5) is very good; b) although the agreement at low Lshells is improved when scattering by EMIC waves is included, the result is not entirely satisfactory, suggesting that either transport in a more realistic magnetospheric electric field or additional loss processes should be considered.

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.

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.

Low energy neutral atoms in the magnetosphere

We report observations of low energy neutral atoms (LENA) from the solar wind and the ionosphere, obtained by the LENA Imager on the IMAGE spacecraft. The LENA Imager detects and images LENAs arriving at the spacecraft from within a 90° field of view (8° × 8° pixels), swept through 360° every two minutes by spacecraft spin. Neutral atoms arriving at the sensor are converted to negative ions by a conversion surface. The resulting negative ions are separated in energy (3 bins, 10–250 eV) and arrival direction (±45°). They are then accelerated, detected, and time-of-flight mass analyzed. The solar wind and the ionosphere both emit measurable neutral atom fluxes, the latter responding rapidly to to variations of the former.

Geoeffectiveness of three Wind magnetic clouds: A comparative study

We compare the large-scale goemagnetic response to the three magnetic clouds observed by Wind in October 1995 (OCT95), May 1996 (MAY96), and January 1997 (JAN97), studying specifically storm and substorm activity, and other global effects due to untypically large and variable solar wind dynamic pressures. Since the temporal profiles of the interplanetary parameters of the three clouds resemble one another closely, the comparison is meaningful. Using the integrated Poynting flux into the magnetosphere as a rough measure of energy input into the magnetosphere, we find relative energy inputs to be OCT95: JAN97:MAY96 = 22:11:4, with most of the accumulation in the 3-day periods occurring during passage of the Bz 0 cloud phase and the leading edge of the fast stream; and (2) a weakening of the control of the cloud field on magnetosheath flow during the Bz > 0 cloud phase. In summary we find that under most of the aspects considered, OCT95 is the most geoeffective. The buffetting of the magnetospheric cavity by dynamic pressure changes was, however, strongest on JAN97. The profound differences in the magnetospheric response elicited by the clouds is found to be due to the amplitude, duration and rapidity of change of the relevant interplanetary parameters. At present, interplanetary monitors are indispensable for understanding the geomagnetic response to interplanetary structures.

Field and flow perturbations in the October 18–19, 1995, magnetic cloud

We examine magnetic field and plasma perturbations in the October 18–19, 1995, magnetic cloud. Besides the front boundary, the 3-s-averaged magnetic field measurements made by the Magnetic Field Investigation on the Global Geospace Mission spacecraft Wind reveal a further 15 clear magnetic field directional discontinuities (DDs) with field rotations >15°, each lasting for ∼1 min. A number of these DDs are clustered near the trailing edge of the cloud. Using 3-s resolution proton data from the Three-Dimensional Plasma and Energetic Particle Experiment on Wind, we find that these DDs are accompanied by perturbations in the flow. We find that except for the front boundary, which is a clear tangential discontinuity, all the others are rotational. Across the DDs the bulk flow speed is sometimes enhanced and sometimes depressed. Changes in proton temperature across the DDs suggest a more elaborate structure, for example, a reconnection layer. In a search for large-scale regularities, we apply minimum variance analysis to determine the normals to contiguous 1-hour-long stretches of the cloud data and find that there are large-scale structures ordering the field and the flow for the first 21 hour of cloud data. Thus we identify three coherent segments of several hours duration each with a well-defined normal, but the normals to the individual segments are very different from each other. In particular, one segment in the cloud Bz 0 phase. The normals to the DDs within a given segment are closely aligned with the normal to that segment. For the last 6 hours of cloud data, no coherent structure was found since no reliable normals could be determined. Studying flow anisotropies of electrons in the energy range 0.1 – 100 keV and changes in intensity in the >1 keV electrons, Larson et al. [1997] inferred several instances of disconnection of cloud field lines from the Sun, which were attributed to reconnection between adjacent bundles of cloud field lines. Our results are supportive of this interpretation. Furthermore, our results suggest the presence of detailed substructure in, and/or large distortions of, the magnetic cloud which reached 1 AU on October 18–19, 1995.

Dust in the wind: The dust geometric cross section at 1 AU based on neutral solar wind observations

We report observations of the neutral component of the solar wind from the Low Energy Neutral Atom (LENA) imager on the NASA IMAGE spacecraft from year 2001. There is a pronounced annual modulation of the neutral solar wind, and the flux outside of the upstream region is used to place an upper limit on the dust geometric cross section in the sunward direction at 1 AU of Γ1AU < 6×10−19 cm−1. This value agrees with inferences made from the zodiacal light.

Energetic auroral electron distributions derived from global X-ray measurements and comparison with in-situ particle measurements

On May, 27, 1996, the Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board NASAs POLAR spacecraft was imaging the southern auroral oval during an auroral substorm. Near simultaneous particle measurements by the DMSP F12 and F13 and POLAR satellites allow us to compare measured energetic electron distributions with distributions derived from the x-ray measurements; agreement is achieved where the assumed electron distribution used in the x-ray derivations is a reasonable approximation to the measured distribution. The PIXIE data show an energy dispersion in the precipitating electrons in the morning sector such that energy increases with increasing MLT, the result of the dependence of the electron drift speed on energy and its dominance over the loss rate due to precipitation. Strong pitch angle diffusion in the morning sector depletes the source of injected electrons creating the absence of significant electron fluxes, and thus x-ray fluxes, above 1.5 keV in the afternoon sector.

NICE: an instrument for direct mass spectrometric measurement of interstellar neutral gas

The direct measurement of the neutral interstellar gas requires a very sensitive neutral particle imaging instrument in the energy range of 10 eV– 1000 eV. For successful detection and identification, the neutral particles have to be ionized first, which will be accomplished via surface ionization. This method is successfully employed in the Low Energy Neutral Atom imager (LENA) instrument on the IMAGE spacecraft launched on 25 March 2000, which still operates well. We present the laboratory prototype of the Neutral Interstellar Composition Experiment (NICE), a neutral particle mass spectrometer dedicated to the measurement of interstellar gas, and will discuss its instrumental characteristics. Performance is evaluated with emphasis on the neutral to negative ion conversion for hydrogen and oxygen and the collection of these ions by the mass spectrometer. Measurements of the detection efficiency of the prototype for primary neutral hydrogen and oxygen atoms are presented. Several conversion surfaces, conductive and insulating, were investigated and all are potential candidates for a next generation neutral particle imaging instrument.