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

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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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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Oxygen abundance in coronal streamers during solar minimum

Annales Geophysicae ◽

10.5194/angeo-19-135-2001 ◽

2001 ◽

Vol 19 (2) ◽

pp. 135-145 ◽

Cited By ~ 26

Author(s):

D. Marocchi ◽

E. Antonucci ◽

S. Giordano

Keyword(s):

Solar Wind ◽

Solar Minimum ◽

Ultra Violet ◽

Solar Wind Plasma ◽

Slow Solar Wind ◽

Streamer Belt ◽

Oxygen Abundance ◽

The Core ◽

Interplanetary Physics ◽

Slow Wind

Abstract. We present a study of the oxygen abundance relative to hydrogen in the equatorial streamer belt of the solar corona during the recent period of activity minimum. The oxygen abundance is derived from the spectroscopic observations of the outer corona performed during 1996 with the Ultraviolet Coronagraph Spectrometer (SOHO) in the ultra-violet region. This study shows that the depletion of oxygen, by almost one order of magnitude with respect to the photospheric values, found in the inner part of streamers by Raymond et al. (1997a) is a common feature of the solar minimum streamer belt, which exhibits an abundance structure with the following characteristics. In the core of streamers the oxygen abundance is 1.3 × 10-4 at 1.5 R&odot;, then it drops to 0.8 × 10-4 at 1.7 R&odot;, value which remains almost constant out to 2.2 R&odot;. In the lateral bright structures that are ob-served to surround the core of streamers in the oxygen emission, the oxygen abundance drops monotonically with heliodistance, from 3.5 × 10-4 at 1.5 R&odot; to 2.2 × 10-4 at 2.2 R&odot;. The oxygen abundance structure found in the streamer belt is consistent with the model of magnetic topology of streamers proposed by Noci et al. (1997). The composition of the plasma contained in streamers is not the same as observed in the slow solar wind. Even in the lateral branches, richer in oxygen, at 2.2 R&odot; the abundance drops by a factor 2 with respect to the slow wind plasma observed with Ulysses during the declining phase of the solar cycle. Hence the slow wind does not appear to originate primarily from streamers, with the exception perhaps of the plasma flowing along the heliospheric current sheet.Key words. Interplanetary physics (solar wind plasma) – Solar physics, astrophysics and astronomy (corona and transition region; ultraviolet emissions)

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

Annales Geophysicae ◽

10.5194/angeo-20-1265-2002 ◽

2002 ◽

Vol 20 (9) ◽

pp. 1265-1277 ◽

Cited By ~ 22

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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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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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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In situ local shock speed and transit shock speed

Annales Geophysicae ◽

10.1007/s00585-998-0370-9 ◽

1998 ◽

Vol 16 (4) ◽

pp. 370-375 ◽

Cited By ~ 10

Author(s):

S. Watari ◽

T. Detman

Keyword(s):

Solar Wind ◽

Solar Wind Plasma ◽

Interplanetary Shock ◽

Propagation Model ◽

Interplanetary Shocks ◽

Shock Speed ◽

Interplanetary Physics ◽

Speed Solar Wind ◽

Transit Speed

Abstract. A useful index for estimating the transit speeds was derived by analyzing interplanetary shock observations. This index is the ratio of the in situ local shock speed and the transit speed; it is 0.6–0.9 for most observed shocks. The local shock speed and the transit speed calculated for the results of the magnetohydrodynamic simulation show good agreement with the observations. The relation expressed by the index is well explained by a simplified propagation model assuming a blast wave. For several shocks the ratio is approximately 1.2, implying that these shocks accelerated during propagation in slow-speed solar wind. This ratio is similar to that for the background solar wind acceleration.Keywords. Interplanetary physics (Flare and stream dynamics; Interplanetary shocks; Solar wind plasma)

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Perturbations of the solar wind flow by radial and latitudinal pick-up ion pressure gradients

Annales Geophysicae ◽

10.5194/angeo-22-2229-2004 ◽

2004 ◽

Vol 22 (6) ◽

pp. 2229-2238 ◽

Cited By ~ 5

Author(s):

H. J. Fahr ◽

K. Scherer

Keyword(s):

Solar Wind ◽

Radial Distance ◽

Solar Wind Plasma ◽

Slow Solar Wind ◽

Pressure Gradients ◽

Quantitative Calculation ◽

Symmetric Density ◽

Solar Wind Flow ◽

Loading Effects ◽

Density Perturbations

Abstract. It has been found that pick-up ions at their dynamical incorporation into the solar wind modify the original conditions of the asymptotic solar wind plasma flow. In this respect, it has meanwhile been revealed in many papers that these type of solar wind modifications, i.e. deceleration and decrease of effective Mach number, are not only due to the pick-up ion loading effects, but also to the action of pick-up ion pressure gradients. Up to now only the effects of radial pick-up ion pressure gradients were considered, however, analogously but latitudinal pressure gradients also appear to be important. Here we study the effects of radial and latitudinal pick-up ion pressure gradients, occurring especially during solar minimum conditions at mid-latitude regions where slow solar wind streams change to fast solar wind streams. First, we give estimates of the latitudinal wind components connected with these gradients, and then after revealing its importance, present a more quantitative calculation of solar wind velocity and density perturbations resulting from these pressure forces. It is shown that the relative density perturbations near and in the ecliptic increase with radial distance and thus may well explain the measured non-spherically symmetric density decrease with distance. We also show that the solar wind decelerations actually seen with Voyager-1/2 are in conciliation with interstellar hydrogen densities of nH∞≥0.1cm-3, in contrast to earlier claims for nH∞=0.05cm-3.

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Small-scale variations of helium abundance in different large-scale solar wind structures

10.5194/egusphere-egu2020-9348 ◽

2020 ◽

Author(s):

Alexander Khokhlachev ◽

Maria Riazantseva ◽

Liudmila Rakhmanova ◽

Yuri Yermolaev ◽

Irina Lodkina ◽

...

Keyword(s):

Solar Wind ◽

Large Scale ◽

Solar Wind Plasma ◽

Magnetic Clouds ◽

Slow Solar Wind ◽

Small Scale ◽

Plasma Stream ◽

Helium Abundance ◽

Russian Science ◽

Plasma Parameters

<p>The boundaries between large-scale solar wind streams are often accompanied by sharp changes in helium abundance. &#160;Wherein the high value of relative helium abundance is known as a sign of some large-scale solar wind structures ( for example magnetic clouds). Unlike the steady slow solar wind where the helium abundance is rather stable and equals ~5%, in magnetic clouds its value can grow significantly up to 20% and more, and at the same time helium component becomes more variable. &#160;In this paper we analyze the small-scale variations of solar wind plasma parameters, including the helium abundance variations in different large-scale solar wind streams, especially in magnetic clouds and Sheath regions before them. We use rather long intervals of simultaneous measurements at Spektr-R (spectrometer BMSW) and Wind (spectrometer 3DP) spacecrafts. &#160;We choose the intervals with rather high correlation &#160;level of plasma parameters as a whole to be sure that we are deal with the same plasma stream. &#160;The intervals associated with different large scale-solar wind structures are selected by using of our catalog ftp://ftp.iki.rssi.ru/pub/omni/catalog/. For selected intervals we examine cross-correlation function for Spektr-R and Wind measurements &#160;to reveal the local spatial inhomogeneities by helium abundance which can be observed only at one of spacecrafts, and we determine properties of ones. Such inhomogeneities can be generate by turbulence, which is typically getting more intense in the considered disturbed intervals in the solar wind. The work is supported by Russian Science Foundation grant 16-12-10062.</p>

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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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On the heliolatitudinal variation of the galactic cosmic-ray intensity. Comparison with Ulysses measurements

Annales Geophysicae ◽

10.5194/angeo-21-1341-2003 ◽

2003 ◽

Vol 21 (6) ◽

pp. 1341-1345 ◽

Cited By ~ 1

Author(s):

G. Exarhos ◽

X. Moussas

Keyword(s):

Solar Wind ◽

Cosmic Rays ◽

Interplanetary Space ◽

Cosmic Ray ◽

Solar Wind Plasma ◽

Simple Method ◽

Cosmic Ray Intensity ◽

Interplanetary Physics ◽

Diffusion Convection ◽

Galactic Cosmic Ray

Abstract. We study the dependence of cosmic rays with heliolatitude using a simple method and compare the results with the actual data from Ulysses and IMP spacecraft. We reproduce the galactic cosmic-ray heliographic latitudinal intensity variations, applying a semi-empirical, 2-D diffusion-convection model for the cosmic-ray transport in the interplanetary space. This model is a modification of our previous 1-D model (Exarhos and Moussas, 2001) and includes not only the radial diffusion of the cosmic-ray particles but also the latitudinal diffusion. Dividing the interplanetary region into "spherical magnetic sectors" (a small heliolatitudinal extension of a spherical magnetized solar wind plasma shell) that travel into the interplanetary space at the solar wind velocity, we calculate the cosmic-ray intensity for different heliographic latitudes as a series of successive intensity drops that all these "spherical magnetic sectors" between the Sun and the heliospheric termination shock cause the unmodulated galactic cosmic-ray intensity. Our results are compared with the Ulysses cosmic-ray measurements obtained during the first pole-to-pole passage from mid-1994 to mid-1995.Key words. Interplanetary physics (cosmic rays; interplanetray magnetic fields; solar wind plasma)

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