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The aurora results from electrons that first are energized at very high altitudes by the solar wind-earth interactions and then travel down magnetic field lines until they hit the atmosphere. The basic physical processes are described that lead to the energization of these electrons and thus to the aurora.
The detection of soft X-rays of about 1 keV energy from the eclipsing pulsar PSR1957+20 is reported. This high-energy radiation should be a valuable diagnostic of the wind in this recycled pulsar system. Possible sources of the X-ray emission are the interstellar nebula driven by the pulsar wind, the interaction between the pulsar and its evaporating companion, and the pulsar itself. The small apparent size of the X-ray object argues against the first of these possibilities and suggests that the X-rays are produced within the binary.
An approach to the study of the solar wind-magnetosphere interaction by signal type, that is, by examining the effect in the magnetosphere of well defined interplanetary structures, is presented. Focus is on the response of the magnetosphere to interplanetary magnetic clouds. Among their properties are: the slow and smooth variation of the magnetic field vector, with fluctuation level well below common interplanetary values; the similarly well behaved bulk flow; the wide range of field and flow parameters; and the longevity of passage (1 to 2 days). If the magnetic cloud is oriented such that a long period of uninterruptedly northward pointing field is followed by a long interval of continuously southward pointing field, then the transition of the magnetosphere from a quiescent state (the 'ground state') to a very active state can be studied, the latter being sustained by continued forcing from the magnetic cloud. A synopsis of the main findings of a recent study in such an interaction is given, concentrating on the substorm activity attending the second part of cloud passage.
The rate of merging and the strength of the region 1 Birkeland currents increase during periods of southward interplanetary magnetic field (IMF). Fringe fields of the Birkeland currents depress dayside magnetospheric magnetic field strengths and remove magnetic flux from the dayside magnetophere, thereby allowing the dayside magnetopause to move inward and the cusp equatorward. We use previously derived fits to the magnetospause location as a function of IMF B(sub z), the condition of pressure balance at the magnetopause, and an idealized model of region 1 Birkeland currents to estimate that strong southward IMF turnings will produce approximately 13- to 26-nT depressions in the geosynchronous magnetic field strength over periods of 30-60 min. We then present three case studies of geosynchronous magnetic field strength variations during periods of nearly constant solar wind dynamic pressure and southward IMF. The dayside magnetospheric magnetic field strength was depressed approximately 10 nT during a period of strongly southward IMF (B(sub z) = -6 nT), but only approximately 5 nT during two more typical periods of slightly southward IMF (B(sub z) = -2 to -3 nT). The depressions correspond to periods of enhanced AL index, which we interpret as evidence for directly driven solar wind-magnetosphere interaction rather than the unloading of energy stored within the magnetotail. Dayside geosynchronous magnetic field strengths are weakly correlated with IMF B(sub z).
A coupled magnetospheric-ionospheric (M-I) magnetohydrodynamic (MHD) model has been used to simulate the formation of Sun-aligned polar cap arcs for a variety of interplanetary magnetic field (IMF) dependent polar cap convection fields. The formation process involves launching an Alfven shear wave from the magnetosphere to the ionosphere where the ionospheric conductance can react self-consistently to changes in the upward currents. We assume that the initial Alfven shear wave is the result of solar wind-magnetosphere interactions. The simulations show how the E region density is affected by the changes in the electron precipitation that are associated with the upward currents. These changes in conductance lead to both a modified Alfven wave reflection at the ionosphere and the generation of secondary Alfven waves in the ionosphere. The ensuing bouncing of the Alfven waves between the ionosphere and magnetosphere is followed until an asymptotic solution is obtained. At the magnetosphere the Alfven waves reflect at a fixed boundary. The coupled M-I Sun-aligned polar cap arc model of Zhu et al.(1993a) is used to carry out the simulations. This study focuses on the dependence of the polar cap arc formation on the background (global) convection pattern. Since the polar cap arcs occur for northward and strong B(sub y) IMF conditions, a variety of background convection patterns can exist when the arcs are present. The study shows that polar cap arcs can be formed for all these convection patterns; however, the arc features are dramatically different for the different patterns. For weak sunward convection a relatively confined single pair of current sheets is associated with the imposed Alfven shear wave structure. However, when the electric field exceeds a threshold, the arc structure intensifies, and the conductance increases as does the local Joule heating rate. These increases are faster than a linear dependence on the background electric field strength. Furthermore, above the threshold, the single current sheet pair splits into multiple current sheet pairs. For the fixed initial ionospheric and magnetospheric conditions used in this study, the separation distance between the current pairs was found to be almost independent of the background electric field strength. For either three-cell or distorted two-cell background convection patterns the arc formation favored the positive B(sub y) case in the northern hemisphere.
The Ulysses spacecraft trajectory includes a peak southern latitude of -80.2 deg, reached during September 1994, and perihelion in the ecliptic plane at 134 AU in March 1995. The near-perihelion mission phase features a rapid scan through solar latitude, with rates approaching one degree per day. We will present observations through mid-May 1995, when the spacecraft will be near 1.5 AU and +50 deg solar latitude. At the time of this writing, observations from the solar wind plasma experiment have been examined through -40 deg solar latitude. At that latitude Ulysses was still immersed in fast solar wind from the south polar coronal hole, with wind speeds of 700 to 800 km/s and with a variety of fine structure. Expectations for near-perihelion measurements include times of slow, dense wind characteristic of the near-equatorial heliomagnetic streamer belt. A non-zero tilt of the streamer belt would produce recurrent intervals of fast coronal hole wind and corotating interaction regions (CIRs) caused by fast wind overtaking slow wind. Forward and reverse shock waves bounding the CIRs, routinely observed at low and intermediate latitudes beyond approximately 2 AU, will likely be absent during the northward transit due to proximity to the Sun we will summarize solar wind plasma results concerning meridional gradients in fluid parameters, large-scale and fine structure, and transient events such as coronal mass ejections.
Analysis of solar wind fluctuation data show that the correlation between velocity and magnetic field fluctuations decreases when going farther away from the Sun. This decorrelation can be attributed either to the time evolution of the fluctuations, carried away by the solar wind, or to the interaction between the solar wind neutral sheet and Alfven waves. To check this second hypothesis we have numerically studied the propagation of Alfven waves in the solar wind neutral sheet. The initial conditions have been set up in order to guarantee B(exp 2) = const, so that the following numerical evolution is only due to the inhomogeneity in the background magnetic field. The analysis of the results shows that compressive structures are formed, mainly in the neutral sheet where they have been identified as pressure balanced structures, i.e., tangential discontinuities. Fast perturbations, which are also produced, have a tendency to leave the simulation domain, propagating also perpendicularly to the mean magnetic field. For this reason the level of fast perturbations is always smaller with respect to the previously cited plasma balanced structures, which are slow mode perturbations. A comparison between the numerical results and some particular observational issues is also presented.
The International Solar-Terrestrial Physics (ISTP) program is an internationally coordinated multi-spacecraft mission that will study the production of the supersonic magnetized solar wind, its interaction with the Earth's magnetosphere, and the resulting transport of plasma, momentum and energy through the magnetosphere and into the ionosphere and upper atmosphere. The mission will involve l4spacecraft to be launched between 1992 and 1996, along with complementary ground-based observations and theoretical programs. A list of the spacecraft, their nominal orbits, and responsible agencies is shown.
We examine the possibilities of sustaining an argon atmosphere by diffusion from the upper 10 km of crust, and alternatively by effusion from a molten or previously molten area at great depth . Ar-40 in the atmospheres of the planets is a measure of potassium abundance in the interiors since Ar-40 is a product of radiogenic decay of K-40 by electron capture with the subsequent emission of a 1.46 eV gamma-ray. Although the Ar-40 in the earth's atmosphere is expected to have accumulated since the late bombardment, Ar-40 in surface-bounded exospheres is eroded quickly by photoionization and electron impact ionization. Thus, the argon content in the exospheres of the Moon, Mercury and probably Europa is representative of current effusion rather than accumulation over the lifetime of the body. Argon content will be a function of K content, temperature, grain size distribution, connected pore volume and possible seismic activity. Although Mercury and the Moon differ in many details, we can train the solutions to diffusion equations to predict the average lunar atmosphere. Then these parameters can be varied for Hermean conditions. Assuming a lunar crustal potassium abundance of 300 ppm, the observed argon atmosphere requires equilibrium between the argon production in the upper 9 Km of the moon (1.135 x 10(exp -3) cm(exp -3) s(exp -1)) and its loss. Hodges et al. conclude that this loss rate and the observed time variability requires argon release through seismic activity, tapping a deep argon source. An important observation is that the extreme surface of the Moon is enhanced in argon rather than depleted, as one would expect from outgassing of radiogenic argon. Manka and Michel concluded that ion implantation explains the surface enhancement of Ar-40. About half of the argon ions produced in the lunar atmosphere would return to the surface, where they would become embedded in the rocks. Similarly, at Mercury we expect the surface rocks to be enhanced in Ar-40 wherever the magnetosphere has been open over time. Thus the measurement of surface composition will reveal the long-term effects of solar wind-magnetosphere interaction. Additional information is contained in the original extended abstract.
In the NUV spectrum of Eta Car, we have resolved many narrow absorption lines of neutral and singly-ionized elements with the Space Telescope Imaging Spectrograph. We report for the first time the detection of interstellar vanadium in absorption, and many highly-excited absorption lines of Fe, Cr, Ti, Ni, Co, Mn, and Mg. These elements, normally tied up in dust grains in the ISM, are located within wall of the Homunculus within 20,000 A.U. of Eta Car. Stellar radiation and stellar wind are interacting with the wall. Dust is likely being modified and/or destroyed. Previous Homunculus studies have demonstrated that nitrogen is overabundant and that carbon and oxygen emission lines are weak, or non-existent. Are the large column densities of these heavy elements due to abundance effects, excitation mechanisms, or modified grains? We may gain insight as Eta Car goes through its spectroscopic minimum in the summer of 2003.
One of the great achievements of Parker was the prediction that the solar magnetic field would be drawn into Archimedian spirals as it is carried away from the Sun by the solar wind. This prediction has been amply confirmed by many in situ measurements in the intervening four decades. But, Parker made his prediction for a solar wind that expands into infinite space while we now know that the local interstellar medium (LISM) is far from empty and, in fact, confines the solar wind to a finite volume, known as the heliosphere, that extends to approximately 100 AU in the upstream direction (the solar system is moving through the LISM). Voyagers 1/2, presently at -80 AU, are approaching the upstream boundaries of the heliosphere and returning data on the properties of the magnetic field. This is important for understanding how galactic cosmic rays (GCRs) reach the Earth. Voyagers show that the IMF at 10-80 AU behaves much as Parker predicted - with two important exceptions. This is not surprising since the field is essentially passively advected by the solar wind out to 80 AU. But, new models say that nearer the heliosphere boundaries the field plays a major role in the solar wind-LISM interaction. However, of the many physical ingredients that constitute the outer heliosphere, the magnetic field poses some of the most interesting and difficult numerical modeling problems. Presently, only a few results have been published and much remains to be done. Here I will summarize the expected and measured behavior of the magnetic field at 80 AU. Then I will describe modeling predictions beyond 80 AU: magnetic "tornadoes", polarity envelopes, the Axford-Cranfill effect, inner and outer magnetic walls and more. I will also list what I believe to be important new modeling objectives. Finally, I will speculate on what is happening with the magnetic field near the nose of the heliosphere. My conclusion is that models of GCR modulation rarely incorporate even crudely realistic magnetic fields so it is a wonder that they are as successful as they are and no surprise that there are still important discrepancies between GCR modulation observations and the models.
This Technical Publication (TP) addresses some climatic elements with emphasis on atmospheric composition, including gas radiative characteristics. Solar radiation is discussed with considerable information on the mathematical and statistical formulae. On a worldwide basis, temperature and precipitation for the globe are discussed along with interaction in drought. Also included is the simultaneous interaction with winds, humidity, and solar radiation. Volcanology gets minimum treatment. The oceans and seas are treated in chart form along with the interrelationship of oceanic currents and El Nino and La Nina, and ENSO phenomena. Upper air circulations are discussed. Various cloud formations up to 85-95 km altitude are described. Information on tornadoes and hurricanes is also included. One section is devoted to the climate physical-chemical elements. A short discussion is given on the importance for the quality of data and/or information in descriptions of the climate. This TP presents only an overview or survey of these and other various climatic elements.
Interaction between wind regimes and topography can give rise to complex suites of aeolian landforms. This paper considers aeolian sediment associated wit11 troughs on Mars and identifies a wider range of deposit types than has previously been documented. These include wind streaks, falling dunes, "lateral" dunes, barchan dunes, linear dunes, transverse ridges, sand ramps, climbing dunes, sand streamers, and sand patches. The sediment incorporated into these deposits is supplied by wind streaks and ambient Planitia sources as well as originating within the trough itself, notably from the trough walls and floor. There is also transmission of sediment between d~~neTsh. e flow dynamics which account for the distribution of aeolian sediment have been modeled using two-dimensional computational fluid dynamics. The model predicts flow separation on the upwind side of the trough followed by reattachment and acceleration at the downwind margin. The inferred patterns of sediment transport compare well with the distribution of aeolian forms. Model data indicate an increase of wind velocity by approx. 30 % at the downwind trough margin. This suggests that the threshold wind speed necessary for sand mobilization on Mars will be more freq~~entmlye t in these inclined locations.
Since the Apollo era the electric potential of the Moon has been a subject of interest and debate. Deployed by three Apollo missions, Apollo 12, Apollo 14 and Apollo 15, the Suprathermal Ion Detector Experiment (SIDE) determined the sunlit lunar surface potential to be about +10 Volts using the energy spectra of lunar ionospheric thermal ions accelerated toward the Moon. More recently, the Lunar Prospector (LP) Electron Reflectometer used electron distributions to infer negative lunar surface potentials, primarily in shadow. We will present initial results from a study to combine lunar surface potential measurements from both SIDE and the LP/Electron Reflectometer to calibrate an advanced model of lunar surface charging which includes effects from the plasma environment, photoemission, secondaries ejected by ion impact onto the lunar surface, and the lunar wake created downstream by the solar wind-lunar interaction.
Space Technology 5 (ST-5) is a three micro-satellite constellation deployed into a 300 x 4500 km, dawn-dusk, sun-synchronous polar orbit from March 22 to June 21, 2006, for technology validations. In this study, we use the in-situ magnetic field observations from Space Technology 5 mission to quantify the imbalance of Region 1 (R1) and Region 2 (R2) currents. During the three-month duration of the ST5 mission, geomagnetic conditions range from quiet to moderately active. We find that the R1 current intensity is consistently stronger than the R2 current intensity both for the dawnside and the duskside large-scale field-aligned current system. The net currents flowing into (out of) the ionosphere in the dawnside (duskside) are in the order of 5% of the total RI currents. We also find that the net currents flowing into or out of the ionosphere are controlled by the solar wind-magnetosphere interaction in the same way as the field-aligned currents themselves are. Since the net currents due to the imbalance of the R1 and R2 currents require that their closure currents flow across the polar cap from dawn to dusk as Pedersen currents, our results indicate that the total amount of the cross-polar cap Pedersen currents is in the order of approx. 0.1 MA. This study, although with a very limited dataset, is one of the first attempts to quantify the cross-polar cap Pedersen currents. Given the importance of the Joule heating due to Pedersen currents to the high-latitude ionospheric electrodynamics, quantifying the cross-polar cap Pedersen currents and associated Joule heating is needed for developing models of the magnetosphere-ionosphere coupling.
In this study, we use the in-situ magnetic field observations from Space Technology 5 mission to quantify the imbalance of Region 1 (R1) and Region 2 (R2) currents. During the three-month duration of the ST5 mission, geomagnetic conditions range from quiet to moderately active. We find that the R1 current intensity is consistently stronger than the R2 current intensity both for the dawnside and the duskside large-scale field-aligned current system. The net currents flowing into (out of) the ionosphere in the dawnside (duskside) are in the order of 5% of the total R1 currents. We also find that the net currents flowing into or out of the ionosphere are controlled by the solar wind-magnetosphere interaction in the same way as the field-aligned currents themselves are. Since the net currents due to the imbalance of the R1 and R2 currents require that their closure currents flow across the polar cap from dawn to dusk as Pedersen currents, our results indicate that the total amount of the cross-polar cap Pedersen currents is in the order of ~ 0.1 MA. This study, although with a very limited dataset, is one of the first attempts to quantify the cross-polar cap Pedersen currents. Given the importance of the Joule heating due to Pedersen currents to the high-latitude ionospheric electrodynamics, quantifying the cross-polar cap Pedersen currents and associated Joule heating is needed for developing models of the magnetosphere-ionosphere coupling.
Reusing components from a rocket launch can be cost saving. NASA's space shuttle system has reusable components that return to the Earth and impact the ocean. A primary example is the Space Shuttle Solid Rocket Booster (SRB) that descends on parachutes to the Earth after separation and impacts the ocean. Water impact generates significant structural loads that can damage the booster, so it is important to study this event in detail in the design of the recovery system. Some recent examples of damage due to water impact include the Ares I-X First Stage deformation as seen in Figure 1 and the loss of the SpaceX Falcon 9 First Stage.To ensure that a component can be recovered or that the design of the recovery system is adequate, an adequate set of structural loads is necessary for use in failure assessments. However, this task is difficult since there are many conditions that affect how a component impacts the water and the resulting structural loading that a component sees. These conditions include the angle of impact with respect to the water, the horizontal and vertical velocities, the rotation rate, the wave height and speed, and many others. There have been attempts to simulate water impact. One approach is to analyze water impact using explicit finite element techniques such as those employed by the LS-Dyna tool [1]. Though very detailed, this approach is time consuming and would not be suitable for running Monte Carlo or optimization analyses. The purpose of this paper is to describe a multi-body simulation tool that runs quickly and that captures the environments a component might see. The simulation incorporates the air and water interaction with the component, the component dynamics (i.e. modes and mode shapes), any applicable parachutes and lines, the interaction of winds and gusts, and the wave height and speed. It is capable of quickly conducting Monte Carlo studies to better capture the environments and genetic algorithm optimizations to reproduce a flight.
We use a nonstationary generalization of the higher-order structure function technique to investigate statistical properties of the magnetic field fluctuations recorded by MESSENGER spacecraft during its first flyby (01/14/2008) through the near-Mercury space environment, with the emphasis on key boundary regions participating in the solar wind - magnetosphere interaction. Our analysis shows, for the first time, that kinetic-scale fluctuations play a significant role in the Mercury's magnetosphere up to the largest resolvable timescale (approx.20 s) imposed by the signal nonstationariry, suggesting that turbulence at this plane I is largely controlled by finite Larmor radius effects. In particular, we report the presence of a highly turbulent and extended foreshock system filled with packets of ULF oscillations, broad-band intermittent fluctuations in the magnetosheath, ion-kinetic turbulence in the central plasma sheet of Mercury's magnetotail, and kinetic-scale fluctuations in the inner current sheet encountered at the outbound (dawn-side) magnetopause. Overall, our measurements indicate that the Hermean magnetosphere, as well as the surrounding region, are strongly affected by non-MHD effects introduced by finite sizes of cyclotron orbits of the constituting ion species. Physical mechanisms of these effects and their potentially critical impact on the structure and dynamics of Mercury's magnetic field remain to be understood.