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At least 559 records · Page 31
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

The Solar Wind Termination Shock and the Outer Heliosphere

Beginning in mid-2002, Voyager 1 observed an extended period of field-aligned flows of low energy ions and electrons that was unlike previous observations, suggesting that the spacecraft was in the vicinity of the solar wind termination shock. However, the observations differed from expectations and have resulted in differing interpretations as to whether or not Voyager 1 crossed the termination shock and entered the subsonic flow in the heliosheath for a seven month period. The puzzling aspects of the observations suggest that the shock is more complex and dynamic than generally expected and that there could be more surprises as Voyager 1 will likely encounter the shock several times before durably entering the heliosheath sometime in the next few years.

Stone, E. C.↗
NASA NTRS2011

Modeling of Coronal Mass Ejections that Caused Particularly Large Geomagnetic Storms Using ENLIL Heliosphere Cone Model

In our previous paper we reported the results of modeling of 14 selected well -observed strong halo coronal mass ejection (CME) events using the WSA -ENLIL cone model combination. Cone model input parameters were obtained from white light coronagraph images of the CME events using the analytical method developed by Xie et al. This work verified that coronagraph input gives reasonably good results for the CME arrival time prediction. In contrast to Taktakishvili et al., where we started the analysis by looking for clear CME signatures in the data and then proceeded to model the interplanetary consequences at 1 AU, in the present paper we start by generating a list of observed geomagnetic storm events and then work our way back to remote solar observations and carry out the corresponding CME modeling. The approach used in this study is addressing space weather forecasting and operational needs. We analyzed 36 particularly strong geomagnetic storms, then tried to associate them with particular CMEs using SOHO/LASCO catalogue, and finally modeled these CMEs using WSA-ENLIL cone model. Recently, Pulkkinen et al. developed a novel method for automatic determination of cone model parameters. We employed both analytical and automatic methods to determine cone model input parameters. We examined the CME arrival times and magnitude of impact at 1 AU for both techniques. The results of the simulations are compared with the ACE satellite observations. This comparison demonstrated that WSA -ENLIL model combination with coronagraph input gives reasonably good results for the CME arrival times for this set of 'geoeffective" CME events as well.

Taktakishvili, A.↗
NASA NTRS2010

Higher Energy Plasma Ions found near the Termination Shock: Analyses of Voyager 2 Data in the Heliosheath and in the Outer Heliosphere

We have found in the Voyager 2 (V2) plasma science data in the heliosheath (HS) near the termination shock (TS) high-energy ions (HEIs) in addition to the bulk plasma convective flow ions. The HEI detections temporally coincide with increased V2 plasma wave subsystem (PWS) activity in "event A"h of Gurnett and Kurth. Maxwellian fits to HEI detections indicate the HEIs are moving radially anti -Sunward with a proton speed of 600 km/s, a density of 10(exp -4) (exp -3), and a thermal speed of 10 km/s. The heliosheath bulk convective protons have a speed of 204 km/s, a density of 0.0029 cm(exp -3), and a thermal speed of 26.7 km/s. The HEI flux and ram pressure are approximately 10% and 30% of those of the bulk HS flow. Since the HEI speed is both close to twice the solar wind speed and independent of the heliosheath bulk plasma speed, the HEIs may be detections of pickup protons formed in the solar wind and convected through the TS. The HEIs also are reminiscent of the pickup protons upstream of the Mars bow shock where their energy also was independent of the bulk plasma speed and attributed to multiple reflections off the Mars bow shock. Gurnett and Kurth 's (2008) event A enhanced PWS activity may be generated by a two ]stream instability from the interaction of these HEIs with the heliosheath bulk plasma ions. We present our findings, discuss their implications, and also present alternative interpretations.

Intriligator, Devrie S.↗
NASA NTRS2018

Integrated Real-Time Modeling System for Heliospheric Space Weather Forecasting

The Sun lies at the center of space weather and is the source of its variability. The primary input to coronal and solar wind models is the activity of the magnetic field in the solar photosphere. What is needed is to develop physics-based models for the dynamics of the magnetic field from the deep convection zone of the Sun to the corona with the goal of providing robust near real-time boundary conditions at the base of space weather forecast models. NASA strategic capabilities efforts toward this goal will be summarized during the talk. These have the objective to enable characterizing and predicting the magnetic field structure and flow dynamics of the Sun by assimilating data from helioseismology and magnetic field observations into physics-based realistic magnetohydrodynamics (MHD) simulations. It is argued that an effort in integrating first-principle modeling of solar magnetism and flow dynamics with real-time observational data via advanced data assimilation methods is needed for transformative steps in space weather research and prediction.

Mansour, Nagi N.↗
NASA NTRS1995

Heliomagnetic Latitude Dependence of the Heliospheric Magnetic Field

ICE and IMP-8 data were analyzed in a heliomagnetic coordinate system defined by the orientation of the solar magnetic dipole. Observations were compared with several solar magnetic field models. The radial component of the magnetic field was found to reverse sign abruptly at the sector boundary; this is consistent with the sheet model, but inconsistent with source surface models .

ICE↗
NASA NTRS2022

Characteristics of Multi-scale Current Sheets in the Solar Wind at 1 au Associated with Magnetic Reconnection and the Case for a Heliospheric Current Sheet Avalanche

Wind spacecraft measurements are analyzed to obtain a current sheet (CS) normal width dcs-distribution of 3374 confirmed magnetic reconnection exhausts in the ecliptic plane of the solar wind at 1 AU. The dcs-distribution displays a nearly exponential decay from a peak at dcs=25 di to a median at dcs=85 di and a 95th percentile at dcs=905 di with a maximum exhaust width at dcs=8077 di. A magnetic field ϴ-rotation angle distribution increases linearly from a relatively few high-shear events toward a broad peak at 35o<ϴ<65o. The azimuthal φ-angles of the CS normal directions of 430 thick dcs≥500 di exhausts are consistent with a dominant Parker-spiral magnetic field and a CS normal along the ortho-Parker direction. The CS normal orientations of 370 kinetic-scale dcs<25 di exhausts are isotropic in contrast, and likely associated with Alfvénic solar wind turbulence. We propose that the alignment of exhaust normal directions from narrow dcs~15-25 di widths to well beyond dcs~500 di with an ortho-Parker azimuthal direction of a large-scale HCS is a consequence of CS bifurcation and turbulence within the HCS exhaust that may trigger reconnection of the adjacent pair of bifurcated CSs. The proposed HCS-31 avalanche scenario suggests that the underlying large-scale parent HCS closer to the Sun evolves with heliocentric distance to fracture into many, more or less aligned, secondary current sheets due to reconnection. A few wide exhaust-associated HCS-like CSs could represent a population of HCSs that failed to reconnect as frequently between the Sun and 1 AU as other HCSs.

Stefan Eriksson↗