scholarly journals Compressive fluctuations in high-latitude solar wind

2004 ◽  
Vol 22 (2) ◽  
pp. 689-696 ◽  
Author(s):  
B. Bavassano ◽  
E. Pietropaolo ◽  
R. Bruno
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
High Latitude ◽  
Correlation Coefficients ◽  
Field Measurements ◽  
Solar Wind Plasma ◽  
Ecliptic Plane ◽  
Mhd Waves ◽  
Polar Wind ◽  
Interplanetary Physics

Abstract. Solar wind compressive fluctuations at MHD scales have been extensively studied in the past using data from spacecraft on the ecliptic plane. In the present study, based on plasma and magnetic field measurements by Ulysses, a statistical analysis of the compressive fluctuations observed in the high-latitude solar wind is performed. Data are from the first out-of-ecliptic orbit of Ulysses, when the Sun's activity is low and the high-latitude heliosphere is characterized by the presence of a fast and relatively steady solar wind, the polar wind. Our analysis is based on the computation of hourly-scale correlation coefficients for several pairs of solar wind parameters such as velocity, density, temperature, magnetic field magnitude, and plasma pressures (thermal, magnetic, and total). The behaviour of the fluctuations in terms of their amplitude has been examined, too, and comparisons with the predictions of existing models have been performed. The results support the view that the compressive fluctuations in the polar solar wind are mainly a superposition of MHD compressive modes and of pressure-balanced structures. Nearly-incompressible effects do not seem to play a relevant role. In conclusion, our results about compressive fluctuations in the polar wind do not appear as a break with respect to previous low-latitude observations. However, our study clearly indicates that in a homogeneous environment, as the polar wind, the pressure-balanced fluctuations tend to play a major role. Key words. Interplanetary physics (MHD waves and turbulence; solar wind plasma) – Space plasma physics (turbulence)

scholarly journals Multi-scale analysis of compressible fluctuations in the solar wind

2018 ◽  
Vol 36 (1) ◽  
pp. 47-52 ◽  
Author(s):  
Owen W. Roberts ◽  
Yasuhito Narita ◽  
C.-Philippe Escoubet
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Electron Density ◽  
Field Measurements ◽  
Slow Waves ◽  
Alfven Wave ◽  
Magnetic Helicity ◽  
Mhd Waves ◽  
Multi Scale ◽  
Interplanetary Physics

Abstract. Compressible plasma turbulence is investigated in the fast solar wind at proton kinetic scales by the combined use of electron density and magnetic field measurements. Both the scale-dependent cross-correlation (CC) and the reduced magnetic helicity (σm) are used in tandem to determine the properties of the compressible fluctuations at proton kinetic scales. At inertial scales the turbulence is hypothesised to contain a mixture of Alfvénic and slow waves, characterised by weak magnetic helicity and anti-correlation between magnetic field strength B and electron density ne. At proton kinetic scales the observations suggest that the fluctuations have stronger positive magnetic helicities as well as strong anti-correlations within the frequency range studied. These results are interpreted as being characteristic of either counter-propagating kinetic Alfvén wave packets or a mixture of anti-sunward kinetic Alfvén waves along with a component of kinetic slow waves. Keywords. Interplanetary physics (MHD waves and turbulence)

scholarly journals Latitudinal extent of large-scale structures in the solar wind

2003 ◽  
Vol 21 (6) ◽  
pp. 1331-1339 ◽  
Author(s):  
H. A. Elliott ◽  
D. J. McComas ◽  
P. Riley
Keyword(s):  
Solar Wind ◽  
Large Scale ◽  
Spatial Scales ◽  
Coronal Holes ◽  
Solar Wind Plasma ◽  
Ecliptic Plane ◽  
Plasma Parameters ◽  
Field Polarity ◽  
Interplanetary Physics ◽  
Hydrodynamic Code

Abstract. Comparison of solar wind observations from the ACE spacecraft, in the ecliptic plane at ~ 1 AU, and the Ulysses spacecraft as it orbits over the Sun’s poles, provides valuable information about the latitudinal extent and variation of solar wind structures in the heliosphere. While qualitative comparisons can be made using average properties observed at these two locations, the comparison of specific, individual structures requires a procedure to determine if a given structure has been observed by both spacecraft. We use a 1-D hydrodynamic code to propagate ACE plasma measurements out to the distance of Ulysses and adjust for the differing longitudes of the ACE and Ulysses spacecraft. In addition to comparing the plasma parameters and their characteristic profiles, we examine suprathermal electron measurements and magnetic field polarity to help determine if the same features are encountered at both ACE and Ulysses. The He I l 1083 nm coronal hole maps are examined to understand the global structure of the Sun during the time of our heliospheric measurements. We find that the same features are frequently observed when both spacecraft are near the ecliptic plane. Stream structures derived from smaller coronal holes during the rising phase of solar cycle 23 persists over 20°–30° in heliolatitude, consistent with their spatial scales back at the Sun.Key words. Interplanetary physics (solar wind plasma)

Statistical relations between in-situ measured Bz component and thermospheric density variations

2021 ◽  
Author(s):  
Sofia Kroisz ◽  
Lukas Drescher ◽  
Manuela Temmer ◽  
Sandro Krauss ◽  
Barbara Süsser-Rechberger ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Field Measurements ◽  
Satellite Orbit ◽  
Solar Wind Plasma ◽  
Statistical Investigation ◽  
Neutral Density ◽  
Special Focus ◽  
Short Term

<p>Through advanced statistical investigation and evaluation of solar wind plasma and magnetic field data, we investigate the statistical relation between the magnetic field B<sub>z</sub> component, measured at L1, and Earth’s thermospheric neutral density. We will present preliminary results of the time series analyzes using in-situ plasma and magnetic field measurements from different spacecraft in near Earth space (e.g., ACE, Wind, DSCOVR) and relate those to derived thermospheric densities from various satellites (e.g., GRACE, CHAMP). The long and short term variations and dependencies in the solar wind data are related to variations in the neutral density of the thermosphere and geomagnetic indices. Special focus is put on the specific signatures that stem from coronal mass ejections and stream or corotating interaction regions.  The results are used to develop a novel short-term forecasting model called SODA (Satellite Orbit DecAy). This is a joint study between TU Graz and University of Graz funded by the FFG Austria (project “SWEETS”).</p>

scholarly journals Dynamical evolution of the inner heliosphere approaching solar activity maximum: interpreting Ulysses observations using a global MHD model

2003 ◽  
Vol 21 (6) ◽  
pp. 1347-1357 ◽  
Author(s):  
P. Riley ◽  
Z. Mikić ◽  
J. A. Linker
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Solar Maximum ◽  
Dynamical Evolution ◽  
Solar Wind Plasma ◽  
Polar Regions ◽  
Activity Maximum ◽  
Interplanetary Magnetic Fields ◽  
Interplanetary Physics ◽  
Interaction Regions

Abstract. In this study we describe a series of MHD simulations covering the time period from 12 January 1999 to 19 September 2001 (Carrington Rotation 1945 to 1980). This interval coincided with: (1) the Sun’s approach toward solar maximum; and (2) Ulysses’ second descent to the southern polar regions, rapid latitude scan, and arrival into the northern polar regions. We focus on the evolution of several key parameters during this time, including the photospheric magnetic field, the computed coronal hole boundaries, the computed velocity profile near the Sun, and the plasma and magnetic field parameters at the location of Ulysses. The model results provide a global context for interpreting the often complex in situ measurements. We also present a heuristic explanation of stream dynamics to describe the morphology of interaction regions at solar maximum and contrast it with the picture that resulted from Ulysses’ first orbit, which occurred during more quiescent solar conditions. The simulation results described here are available at: http://sun.saic.com.Key words. Interplanetary physics (Interplanetary magnetic fields; solar wind plasma; sources of the solar wind)

scholarly journals Alfvénic fluctuations in "newborn"' polar solar wind

2005 ◽  
Vol 23 (4) ◽  
pp. 1513-1520 ◽  
Author(s):  
B. Bavassano ◽  
E. Pietropaolo ◽  
R. Bruno
Keyword(s):  
Solar Wind ◽  
Solar Activity ◽  
Solar Maximum ◽  
Activity Cycle ◽  
Residual Energy ◽  
Mhd Waves ◽  
Polar Wind ◽  
Interplanetary Physics ◽  
Low Latitudes ◽  
Comparative Statistical Analysis

Abstract. The 3-D structure of the solar wind is strongly dependent upon the Sun's activity cycle. At low solar activity a bimodal structure is dominant, with a fast and uniform flow at the high latitudes, and slow and variable flows at low latitudes. Around solar maximum, in sharp contrast, variable flows are observed at all latitudes. This last kind of pattern, however, is a relatively short-lived feature, and quite soon after solar maximum the polar wind tends to regain its role. The plasma parameter distributions for these newborn polar flows appear very similar to those typically observed in polar wind at low solar activity. The point addressed here is about polar wind fluctuations. As is well known, the low-solar-activity polar wind is characterized by a strong flow of Alfvénic fluctuations. Does this hold for the new polar flows too? An answer to this question is given here through a comparative statistical analysis on parameters such as total energy, cross helicity, and residual energy, that are of general use to describe the Alfvénic character of fluctuations. Our results indicate that the main features of the Alfvénic fluctuations observed in low-solar-activity polar wind have been quickly recovered in the new polar flows developed shortly after solar maximum. Keywords. Interplanetary physics (MHD waves and turbulence; Sources of the solar wind) – Space plasma physics (Turbulence)

scholarly journals Are there current-sheet-like structures in the Earth's magnetotail as in the solar wind – results and implications from high time resolution magnetic field measurements by Cluster

2008 ◽  
Vol 26 (7) ◽  
pp. 1889-1895 ◽  
Author(s):  
G. Li ◽  
E. Lee ◽  
G. Parks
Keyword(s):  
Magnetic Field ◽  
Solar Wind ◽  
Current Sheet ◽  
Time Resolution ◽  
Field Measurements ◽  
Solar Wind Plasma ◽  
High Time ◽  
High Time Resolution ◽  
Mhd Turbulence ◽  
Flux Tubes

Abstract. Recent studies of solar wind MHD turbulence show that current-sheet-like structures are common in the solar wind and they are a significant source of solar wind MHD turbulence intermittency. While numerical simulations have suggested that such structures can arise from non-linear interactions of MHD turbulence, a recent study by Borovsky (2006), upon analyzing one year worth of ACE data, suggests that these structures may represent the magnetic walls of flux tubes that separate solar wind plasma into distinct bundles and these flux tubes are relic structures originating from boundaries of supergranules on the surface of the Sun. In this work, we examine whether there are such structures in the Earth's magnetotail, an environment vastly different from the solar wind. We use high time resolution magnetic field data of the FGM instrument onboard Cluster C1 spacecraft. The orbits of Cluster traverse through both the solar wind and the Earth's magnetosheath and magnetotail. This makes its dataset ideal for studying differences between solar wind MHD turbulence and that inside the Earth's magnetosphere. For comparison, we also perform the same analysis when Cluster C1 is in the solar wind. Using a data analysis procedure first introduced in Li (2007, 2008), we find that current-sheet-like structures can be clearly identified in the solar wind. However, similar structures do not exist inside the Earth's magnetotail. This result can be naturally explained if these structures have a solar origin as proposed by Borovsky (2006). With such a scenario, current analysis of solar wind MHD turbulence needs to be improved to include the effects due to these curent-sheet-like structures.

scholarly journals A study of phase-steepened Alfvén waves in a high-speed stream at 0.29 AU

2000 ◽  
Vol 18 (8) ◽  
pp. 845-851 ◽  
Author(s):  
P. Alexander
Keyword(s):  
Solar Wind ◽  
High Speed ◽  
A Priori ◽  
Solar Wind Plasma ◽  
Alfven Waves ◽  
Mhd Waves ◽  
Alfvén Waves ◽  
Interplanetary Physics ◽  
Speed Solar Wind ◽  
High Speed Solar Wind

Abstract. This work performs a search of phase-steepened Alfvén waves under a priori ideal conditions: a high-speed solar wind stream observed in one of the closest approaches to the Sun by any spacecraft (Helios 2). Five potential candidates were initially found following procedures established in earlier work. The observed cases exhibited arc-like or elliptical polarizations, and the rotational discontinuities that formed the abrupt wave edges were found at either the leading or the trailing part. The consideration of some additional specific parameters (mainly related to the relative orientation between mean magnetic field, wave and discontinuity) has been suggested here for an ultimate and proper identification of this kind of phenomenon. After the inclusion of these calculations in our analysis, even fewer cases than the five originals remain. It is suggested that optimum conditions for the detection rather than just for the existence of these events have to be reconsidered.Key words: Interplanetary physics (discontinuities; MHD waves and turbulence; solar wind plasma)

scholarly journals Solar wind heating by an embedded quasi-isothermal pick-up ion fluid

2002 ◽  
Vol 20 (10) ◽  
pp. 1509-1518 ◽  
Author(s):  
H. J. Fahr
Keyword(s):  
Solar Wind ◽  
Solar Wind Plasma ◽  
Mhd Waves ◽  
Polytropic Index ◽  
Wind Flow ◽  
Interstellar Gas ◽  
Secondary Ions ◽  
Interplanetary Physics ◽  
Solar Wind Flow ◽  
Inner Heliosphere

Abstract. It is well known that the solar wind plasma consists of primary ions of solar coronal origin and secondary ions of interstellar origin. Interstellar H-atoms penetrate into the inner heliosphere and when ionized there are converted into secondary ions. These are implanted into the magnetized solar wind flow and are essentially enforced to co-move with this flow. By nonlinear interactions with wind-entrained Alfvén waves the latter are processed in the co-moving velocity space. This pick-up process, however, also causes actions back upon the original solar wind flow, leading to a deceleration, as well as a heating of the solar wind plasma. The resulting deceleration is not only due to the loading effect, but also due to the action of the pressure gradient. To calculate the latter, it is important to take into account the stochastic acceleration that suffers at their convection out of the inner heliosphere by the quasi-linear interactions with MHD turbulences. Only then can the presently reported VOYAGER observations of solar wind decelerations and heatings in the outer heliosphere be understood in terms of the current, most likely values of interstellar gas parameters. In a consistent view of the thermodynamics of the solar wind plasma, which is composed of secondary ions and solar wind protons, we also derive that the latter are globally heated at their motion to larger solar distances. The arising heat transfer is due to the action of suprathermal ions which drive MHD waves that are partially absorbed by solar wind protons and thereby establish their observed quasi-polytropy. We obtain a quantitative expression for the solar wind proton pressure as a function of solar distance. This expression clearly shows the change from an adiabatic to a quasi-polytropic behaviour with a decreasing polytropic index at increasing distances, as has been observed by the VOYAGERS. This also allows one to calculate the average percentage of the intitial energy fed into the thermal proton energy. In a first-order evaluation of this expression we can estimate that under stationary flow conditions about 10% of the initial injection energy is eventually transfered to solar wind protons, independent of the actual injection rate. Key words. Interplanetary physics (energetic particles; interstellar gas; solar wind plasma)

scholarly journals Estimating random transverse velocities in the fast solar wind from EISCAT Interplanetary Scintillation measurements

2002 ◽  
Vol 20 (9) ◽  
pp. 1265-1277 ◽  
Author(s):  
A. Canals ◽  
A. R. Breen ◽  
L. Ofman ◽  
P. J Moran ◽  
R. A Fallows
Keyword(s):  
Solar Wind ◽  
Transverse Velocity ◽  
A Priori ◽  
Solar Wind Plasma ◽  
Flow Speed ◽  
Mhd Waves ◽  
Interplanetary Scintillation ◽  
Fast Solar Wind ◽  
Interplanetary Physics ◽  
Made In

Abstract. Interplanetary scintillation measurements can yield estimates of a large number of solar wind parameters, including bulk flow speed, variation in bulk velocity along the observing path through the solar wind and random variation in transverse velocity. This last parameter is of particular interest, as it can indicate the flux of low-frequency Alfvén waves, and the dissipation of these waves has been proposed as an acceleration mechanism for the fast solar wind. Analysis of IPS data is, however, a significantly unresolved problem and a variety of a priori assumptions must be made in interpreting the data. Furthermore, the results may be affected by the physical structure of the radio source and by variations in the solar wind along the scintillation ray path. We have used observations of simple point-like radio sources made with EISCAT between 1994 and 1998 to obtain estimates of random transverse velocity in the fast solar wind. The results obtained with various a priori assumptions made in the analysis are compared, and we hope thereby to be able to provide some indication of the reliability of our estimates of random transverse velocity and the variation of this parameter with distance from the Sun.Key words. Interplanetary physics (MHD waves and turbulence; solar wind plasma; instruments and techniques)

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