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

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

Annales Geophysicae ◽

10.1007/s00585-000-0845-9 ◽

2000 ◽

Vol 18 (8) ◽

pp. 845-851 ◽

Cited By ~ 1

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)

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Observations of interplanetary scintillation during the 1998 Whole Sun Month: a comparison between EISCAT, ORT and Nagoya data

Annales Geophysicae ◽

10.1007/s00585-000-1003-0 ◽

2000 ◽

Vol 18 (9) ◽

pp. 1003-1008 ◽

Cited By ~ 10

Author(s):

P. J. Moran ◽

S. Ananthakrishnan ◽

V. Balasubramanian ◽

A. R. Breen ◽

A. Canals ◽

...

Keyword(s):

Solar Wind ◽

Solar Wind Velocity ◽

Solar Physics ◽

Solar Wind Plasma ◽

Interplanetary Scintillation ◽

Velocity Measurements ◽

Plasma Sources ◽

Interplanetary Physics ◽

Instruments And Techniques ◽

Good Agreement

Abstract. Observations of interplanetary scintillation (IPS) allow accurate solar wind velocity measurements to be made at all heliographic latitudes and at a range of distances from the Sun. The data may be obtained with either single, double or multiple antennas, each requiring a different method of analysis. IPS data taken during the 1998 whole sun month (30th July-31st August 1998) by EISCAT, the ORT (Ooty Radio Telescope), India, and the Nagoya IPS system, Japan, allow the results of individual methods of analysis to be compared. Good agreement is found between the velocity measurements using each method, and when combined an improved understanding of the structure of the solar wind can be obtained.Key words: Interplanetary physics (solar wind plasma; sources of the solar wind) - Solar physics, astrophysics and astronomy (instruments and techniques)

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Compressive fluctuations in high-latitude solar wind

Annales Geophysicae ◽

10.5194/angeo-22-689-2004 ◽

2004 ◽

Vol 22 (2) ◽

pp. 689-696 ◽

Cited By ~ 23

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)

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Simultaneous interplanetary scintillation and optical measurements of the acceleration of the slow solar wind

Annales Geophysicae ◽

10.1007/s00585-000-0995-9 ◽

2000 ◽

Vol 18 (9) ◽

pp. 995-1002 ◽

Cited By ~ 7

Author(s):

A. R. Breen ◽

S. J. Tappin ◽

C. A. Jordan ◽

P. Thomasson ◽

P. J. Moran ◽

...

Keyword(s):

Solar Wind ◽

Solar Wind Plasma ◽

Optical Measurements ◽

Slow Solar Wind ◽

Wind Instruments ◽

Interplanetary Physics ◽

Instruments And Techniques ◽

Average Profile ◽

Soho Spacecraft ◽

Made In

Abstract. Simultaneous observations of the slow solar wind off the southeast limb of the Sun were made in May 1999 using optical measurements from the C2 and C3 LASCO coronagraphs on board the SOHO spacecraft and radio-scattering measurements from the MERLIN and EISCAT facilities. The observations show the slow solar wind accelerating outwards from 4.5 solar radii (R), reaching a final velocity of 200-300 km s-1 by 25-30 R. The acceleration profile indicated by these results is more gentle than the average profile seen in earlier LASCO observations of larger scale features, but is within the variation seen in these studies.Key words: Interplanetary physics (solar wind plasma; sources of the solar wind; instruments and techniques)

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EISCAT measurements of solar wind velocity and the associated level of interplanetary scintillation

Annales Geophysicae ◽

10.5194/angeo-20-1279-2002 ◽

2002 ◽

Vol 20 (9) ◽

pp. 1279-1289 ◽

Cited By ~ 13

Author(s):

R. A. Fallows ◽

P. J. S. Williams ◽

A. R. Breen

Keyword(s):

Solar Wind ◽

Wind Velocity ◽

Solar Wind Velocity ◽

Solar Wind Plasma ◽

Scintillation Index ◽

Line Of Sight ◽

The Sun ◽

Interplanetary Scintillation ◽

Interplanetary Physics ◽

Eiscat Measurements

Abstract. A relative scintillation index can be derived from EISCAT observations of Interplanetary Scintillation (IPS) usually used to study the solar wind velocity. This provides an ideal opportunity to compare reliable measurements of the solar wind velocity derived for a number of points along the line-of-sight with measurements of the overall level of scintillation. By selecting those occasions where either slow- or fast-stream scattering was dominant, it is shown that at distances from the Sun greater than 30 RS , in both cases the scintillation index fell with increasing distance as a simple power law, typically as R-1.7. The level of scintillation for slow-stream scattering is found to be 2.3 times the level for fast-stream scattering.Key words. Interplanetary physics (solar wind plasma)

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

Annales Geophysicae ◽

10.5194/angeo-20-1509-2002 ◽

2002 ◽

Vol 20 (10) ◽

pp. 1509-1518 ◽

Cited By ~ 7

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)

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Latitudinal extent of large-scale structures in the solar wind

Annales Geophysicae ◽

10.5194/angeo-21-1331-2003 ◽

2003 ◽

Vol 21 (6) ◽

pp. 1331-1339 ◽

Cited By ~ 2

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)

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Interplanetary Scintillation - Preliminary Observations at 103 MHz

Symposium - International Astronomical Union ◽

10.1017/s0074180900067917 ◽

1980 ◽

Vol 91 ◽

pp. 405-408

Author(s):

S. K. Alurkar ◽

R. V. Bhonsle

Keyword(s):

Solar Wind ◽

Solar Wind Plasma ◽

Scintillation Index ◽

Interplanetary Scintillation ◽

Antenna Aperture ◽

Scale Size ◽

Intensity Fluctuations

A 3-station interplanetary scintillation (IPS) observatory is being developed mainly with a view to study the solar wind plasma. The first IPS telescope operating at 103 MHz at Thaltej near Ahmedabad has been put into regular operation since April 1979. With only half the antenna aperture (≃2500 m2) presently in use, observations of 8-10 sources are being made to calculate scintillation index, temporal spectrum of intensity fluctuations and scale size of density irregularities.

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

Annales Geophysicae ◽

10.5194/angeo-21-1347-2003 ◽

2003 ◽

Vol 21 (6) ◽

pp. 1347-1357 ◽

Cited By ~ 17

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)

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Alfvénic fluctuations in "newborn"' polar solar wind

Annales Geophysicae ◽

10.5194/angeo-23-1513-2005 ◽

2005 ◽

Vol 23 (4) ◽

pp. 1513-1520 ◽

Cited By ~ 1

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)

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