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At least 613 records · Page 34
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NASA NTRS2002

Sources of Geomagnetic Activity during Nearly Three Solar Cycles (1972-2000)

We examine the contributions of the principal solar wind components (corotating highspeed streams, slow solar wind, and transient structures, i.e., interplanetary coronal mass ejections (CMEs), shocks, and postshock flows) to averages of the aa geomagnetic index and the interplanetary magnetic field (IMF) strength in 1972-2000 during nearly three solar cycles. A prime motivation is to understand the influence of solar cycle variations in solar wind structure on long-term (e.g., approximately annual) averages of these parameters. We show that high-speed streams account for approximately two-thirds of long-term aa averages at solar minimum, while at solar maximum, structures associated with transients make the largest contribution (approx. 50%), though contributions from streams and slow solar wind continue to be present. Similarly, high-speed streams are the principal contributor (approx. 55%) to solar minimum averages of the IMF, while transient-related structures are the leading contributor (approx. 40%) at solar maximum. These differences between solar maximum and minimum reflect the changing structure of the near-ecliptic solar wind during the solar cycle. For minimum periods, the Earth is embedded in high-speed streams approx. 55% of the time versus approx. 35% for slow solar wind and approx. 10% for CME-associated structures, while at solar maximum, typical percentages are as follows: high-speed streams approx. 35%, slow solar wind approx. 30%, and CME-associated approx. 35%. These compositions show little cycle-to-cycle variation, at least for the interval considered in this paper. Despite the change in the occurrences of different types of solar wind over the solar cycle (and less significant changes from cycle to cycle), overall, variations in the averages of the aa index and IMF closely follow those in corotating streams. Considering solar cycle averages, we show that high-speed streams account for approx. 44%, approx. 48%, and approx. 40% of the solar wind composition, aa, and the IMF strength, respectively, with corresponding figures of approx. 22%, approx. 32%, and approx. 25% for CME-related structures, and approx. 33%, approx. 19%, and approx. 33% for slow solar wind.

Richardson, I. G.↗
NASA NTRS1999

Some Peculiar Properties of Magnetic Clouds as Observed by the WIND Spacecraft

We aimed at understanding the common characteristics of magnetic clouds, relevant to solar-interplanetary connections, but exceptional ones were noted and are stressed here through a short compendium. The study is based on analyses of 28 good or better events (Out of 33 candidates) as identified in WIND magnetic field and plasma data. These cloud intervals are provided by WIND-MFI's Website under the URL (http://lepmfi.gsfc.nasa.gov/mfi/mag_cloud_publ.html#table). The period covered is from early 1995 to November 1998. A force free, cylindrically symmetric, magnetic field model has been applied to the field data in usually 1-hour averaged form for the cloud analyses. Some of the findings are: (1) one small duration event turned out to have an approximately normal size which was due to a distant almost "skimming" passage by the spacecraft; (2) One truly small event was observed, where 10 min averages had to be used in the model fitting; it had an excellent model fit and the usual properties of a magnetic cloud, except it possessed a small axial magnetic flux; (3) One cloud ha a dual axial-field-polarity, in the sense that the "core" had one polarity and the annular region around it had an opposite polarity. This event also satisfied the model and with a ve3ry good chi-squared value. Some others show a hint of this dual polarity; (4) The temporal distribution of occurrence clouds over the 4 years show a dip in 1996; (5) About 50 % of the clouds had upstream shocks; any others had upstream pressure pulses; (6) The overall average speed (390 km/s) of the best 28 events is less than the normally quoted for the average solar wind speed (420 km/s) The average of central cloud speed to the upstream solar wind speed was not much greater than one (1.08), even though many of these clouds were drivers of interplanetary shocks. Cloud expansion is partly the reason for the existence of upstream shocks; (7) The cloud axes often (about 50 % of the time) revealed reasonable attitudes with respect to the interpreted solar source, from simple geometry, but many bore no relationship, suggesting that their observations at 1 AU were probably those of the legs of the global cloud often having near-radial axes; (8) many clouds appear to have magnetic holes at or their their boundaries.

Berdichevsky, D.↗
NASA NTRS1978

Numerical simulation of MHD shock waves in the solar wind

The effects of the interplanetary magnetic field on the propagation speed of shock waves through an ambient solar wind are examined by numerical solutions of the time-dependent nonlinear equations of motion. The magnetic field always increases the velocity of strong shocks. Although the field may temporarily slow down weak shocks inside 1 AU, it eventually also causes weak shocks to travel faster than they would without the magnetic field at larger distances. Consistent with the increase in the shock velocity, the gas pressure ratio across a shock is reduced considerably in the presence of the magnetic field. The numerical method is used to simulate (starting at 0.3 AU) the large deceleration of a shock observed in the lower corona by ground-based radio instrumentation and the more gradual deceleration of the shock in the solar wind observed by the Pioneer 9 and Pioneer 10 spacecraft.

Steinolfson, R. S.↗
NASA NTRS2012

Solar Sources of Earth-affecting Energetic Particles

Particle radiation from the Sun is one of the most important sources of hazardous space weather in the vicinity of Earth. Detailed studies of the origin of the so-called large solar energetic particle (SEP) events became possible only during the solar cycle 23, thanks to the availability of nearly continuous observation of the solar sources ofthese events. In particular, coronal mass ejections (CMEs), which are found to be a key requirement for the occurrence of an SEP event, have been recorded continuously only starting in the 1990s. The physical connection between CMEs and SEPs is that the CMEs drive a fast-mode MHD shock, which accelerates SEPs in the corona and interplanetary medium. The earliest indication of a shock is the occurrence of a type II radio burst at frequencies anywhere from more than a hundred MHz to a few MHz. Recent investigations using STEREO observations have revealed that the shock forms very close to the Sun - a mere 100,000 km above the surface. The shock formation depends not only on the CME properties, but also on the physical conditions in the ambient medium that supports shock propagation. This paper considers extreme cases of SEP events and the associated CMEs and type II radio bursts to illustrate the variability observed in SEP event properties. Comparison will be made between the events of solar cycles 23 and 24.

Gopalswamy, Nat↗
NASA NTRS1971

Electron energy flux in the solar wind.

Description of studies of electrons between 10 eV and 9.9 keV in the solar wind. The transport of energy in the rest frame of the plasma is evaluated and shown to be parallel to the interplanetary magnetic field. The presence of electrons from solar events causes this energy-flux density to exceed the heat flow due to thermal electrons. In one such event, the observations are shown to be consistent with the solar-electron observations made at higher energies. When observations are made at a point connected to the earth's bow shock by an interplanetary-field line, a comparatively large energy flux along the field toward the sun is observed, but the heat flow remains outwardly directed during this time interval. In either situation the heat flow is found to be consistent with measurements made on Vela satellites by a different method. These values, less than .01 ergs/sq cm/sec, are sufficiently low to require modifications to the Spitzer-Harm conductivity formula for use in solar-wind theories.

Ogilvie, K. W.↗
NASA NTRS1972

Comments on the measurement of power spectra of the interplanetary magnetic field

Examination of possible noise sources in the measurement of the power spectrum of fluctuations in the interplanetary magnetic field shows that most measurements by fluxgate magnetometers are limited by digitization noise whereas the search coil magnetometer is limited by instrument noise. The folding of power about the Nyquist frequency or aliasing can be a serious problem at times for many magnetometers, but it is not serious during typical solar wind conditions except near the Nyquist frequency. Waves in the solar wind associated with the presence of the earth's bow shock can contaminate the interplanetary spectrum in the vicinity of the earth. However, at times the spectrum in this region is the same as far from the earth. Doppler shifting caused by the convection of waves by the solar wind makes the interpretation of interplanetary spectra difficult.

Russell, C. T.↗
NASA NTRS1976

Suprathermal protons in the interplanetary solar wind

Using the Mariner 5 solar wind plasma and magnetic field data, we present observations of field-aligned suprathermal proton velocity distributions having pronounced high-energy shoulders. These observations, similar to the interpenetrating stream observations of Feldman et al. (1974), are clear evidence that such proton distributions are interplanetary rather than bow shock associated phenomena. Large Alfven speed is found to be a requirement for the occurrence of suprathermal proton distribution; further, we find the proportion of particles in the shoulder to be limited by the magnitude of the Alfven speed. It is suggested that this last result could indicate that the proton thermal anisotropy is limited at times by wave-particle interactions

Goodrich, C. C.↗
NASA NTRS1978

Energetic plasma ions within the earth's magnetosheath

Observations in the dawn magnetosheath with the LASL/MPI fast plasma experiment on ISEE 1 and 2 reveal the existence of two distinct states of plasma flow within the sheath: a quiet state and a disturbed state. Energetic ions in the range of 3-40 keV are the distinguishing feature of the disturbed state. Long period (about 1 min) fluctuations in plasma density, temperature, flow speed, and flow direction occur simultaneously with the appearance of these high-energy particles. The most likely explanation of these observations is that the energetic ions are produced in the vicinity of the bow shock under certain (unspecified) interplanetary conditions, and that the fluctuations in plasma flow are produced locally by the interaction of the energetic ions with the ambient plasma.

Asbridge, J. R.↗
NASA NTRS1980

The trapped radiations of Saturn and their absorption by satellites and rings

The Pioneer 11 encounter with Saturn has revealed the existence of a fully developed magnetosphere with high-energy trapped radiation about Saturn. The present paper gives a detailed summary of the energetic charged particle measurements, including the overall characteristics of the trapped electron, proton, and helium radiation, which was found to lie inside 20 Saturn radii from the planet, and the regions extending outward to beyond the planetary bow shocks and into the interplanetary medium.

Simpson, J. A.↗
NASA NTRS1982

Pioneer 10/11 data analysis of the plasma analyzer experiment

The Pioneer 10 plasma analyzer detected the 10 plasma torus. Evidence was found of corotating ions which appear to be primarily S (++) and O (++) in the Pioneer 10 plasma data as the spacecraft moved inward from 6.9 to 5.4 R sub J. The Pioneer plasma analyzer was effective in obtaining information on the heavy ion populations in the Jovian magnetosphere. Interplanetary solar wind plasma shocks can trap energetic particles (cosmic rays) for weeks and out to distances of 17 AU. Energetic particles (o.5 MeV to 20 MeV) were confined between two plasma shocks from solar flares as the shocks propagated outward in the solar system.

Intriligator, D. S.↗
NASA NTRS1985

Propagation of injected cosmic rays under distributed reacceleration

The cosmic ray acceleration model in which the particles gain continuously energy from stochastic processes in the interstellar space by encountering shocks leads to a logarithmic increase in the secondary to primary ratios with energy, which is inconsistent with observations. The observed decrease of the secondary to primary ratios may therefore suggest that a considerable part of the acceleration has taken place rapidly at the sources. Astronomical observations on the other hand indicate that shocks from supernovae do exist in the galaxy and observations in the interplanetary space show clear evidence that shock acceleration works.

Simon, M.↗
NASA NTRS1986

Interplanetary field control of the location of the Venus bow shock - Evidence for comet-like ion pickup

The location of the Venus bow shock is found to be sensitive to both the angle of the IMF to the solar wind flow and the phase of the solar cycle. The former dependence is attributed to the effect of planetary ion pickup by the magnetosheath convection electric field analogous to cometary ion pickup by the solar wind electric field. The latter dependence is attributed to the solar-cycle variation in the density of exospheric neutrals in the magnetosheath analogous to the cometary response to distance from the sun. Both a north-south and a pole-equator asymmetry in the location of the bow shock are also found, controlled by the direction of the IMF in the plane perpendicular to the solar wind flow.

Alexander, C. J.↗
NASA NTRS1987

The cause of high-intensity long-duration continuous AE activity (HILDCAAS) - Interplanetary Alfven wave trains

It is shown that high intensity (AE of greater than 1,000 nT), long duration (T of greater than 2d) continuous auroral activity events are caused by outward (from the sun) propagating interplanetary Alfven wave trains. The Alfven waves are often (but not always) detected several days after major interplanetary events, such as shocks and solar wind density enhancements. Presumably, magnetic reconnection between the southward components of the Alfven wave magnetic fields and magnetospheric fields is the mechanism for transfer of solar wind energy to the magnetosphere.

Tsurutani, Bruce T.↗
NASA NTRS1974

Study of the relation between Pc 3 micropulsations and magnetosheath fluctuations and for the multisatellite multimeasurement investigation of earths bow shock

Hourly averages of HEOS A interplanetary field and plasma parameters are compared with micropulsation spectrograms taken by auroral zone stations. Visual evaluation of tungsten induction coil records and a statistical summary indicate a class of pulsations sometimes in the Pc 3, sometimes in the Pc 4 range, whose appearance correlates with solar wind field flow alignment. It is concluded that there is a pulsation phenomenon of variable period strongly associated with certain interplanetary field directions.

Greenstadt, E. W.↗
NASA NTRS1983

Backstreaming ions from oblique earth bow shocks

One-dimensional, hybrid (particle ions, fluid electrons) numerical simulations of oblique shock structure are used to analyze the mechanism by which solar wind ions can be reflected off the earth's bow shock front and escape upstream. It is shown that no backstreaming ions are produced in the simulation over 50-90 degrees of theta(nB), the angle between the interplanetary magnetic field and the shock normal. Backstreaming ions are emitted for theta(nB) of 45 degrees or less, with characteristics in reasonable agreement with those of observed reflected ion streams. The simulated shock structure becomes rather turbulent as theta(nB) decreases from 45 to 30 degrees. Analysis of the backstreaming ion trajectories shows that all backstreaming ions are originally specularly reflected. The oblique shock structure and effects such as whistler formation and ion heating are discussed.

Leroy, M. M.↗
NASA NTRS1983

The effect of a coronal shock wave on the solar wind ionization state

In connection with studies of solar wind ionization state freezing, it is shown that, by using a Lagrangian approach of following individual fluid parcels, the techniques used previously for calculating ionization state variations in a steady state case can be extended straightforwardly to time-varying flows. The specific ionization state calculations presented are for a relatively simple picture of time-dependent coronal flow, based on a well-known model of a self-similar shock wave propagating through the corona. Time-dependent ionization effects for the sudden transition between two otherwise steady flows are likely to be limited to a narrow range of gas parcels which, having been shocked within the coronal freezing-in radius, pass a fixed interplanetary observer in an interval of a few tens of minutes. The amplitude of any rise in interplanetary ionization temperature associated with the coronal shock is likely to be considerably smaller than the jump in electron temperature that actually occurs in the corona.

Owocki, S. P.↗
NASA NTRS1979

Initial ISEE observations of the bow shock

ISEE 1 and 2 magnetic-field profiles across three terrestrial bow shock crossings are shown to illustrate the control of the bow shock structure and upstream waves by solar-wind conditions, especially by the direction of the interplanetary field. The quasi-perpendicular shocks examined have thicknesses of the order of a ion inertial length. Upstream waves observed for field directions of about 45 deg to the shock normal are observed to be carried back towards the bow shock by the solar wind, while propagating upstream at what appears to be the magnetosonic velocity. These studies are continuing.

Russell, C. T.↗