Solar Wind Disturbances Research Articles

Large solar wind disturbances during late May and early June 1991

Solar wind data from the Ames experiment aboard the Pioneer Venus Orbiter, coincident with a period of unprecedented solar activity that began at the end of May, 1991, within the highly active earlier portion of 1991, are summarized and discussed. Some comparison is made with corresponding data from Earth. Some particularly large, strong shocks and solar ejecta were observed at Venus. The solar longitude of Venus, relative to associated flares, varied over a wide range, for a series of flares that produced X-rays that saturated the GOES X-ray counters. Some of the disturbances at Venus must be due to CMEs with longitudinal extents up to ∼40–50 deg.

Deimos: An Obstacle to the Solar Wind

Two isolated solar wind disturbances about 5 minutes in duration were detected aboard the Russian spacecraft Phobos-2 upon its crossing the wake of the martian moon Deimos about 15,000 kilometers downstream from the moon on 1 February 1989. These plasma and magnetic events are interpreted as the inbound and outbound crossings of a Mach cone that is formed as a result of an effective interaction of the solar wind with Deimos. Possible mechanisms such as remanent magnetization, cometary type interaction caused by heavy ion or charged dust production, and unipolar induction resulting from the finite conductivity of the body are discussed. Although none of the present models is fully satisfactory, neutral gas emission through water loss by Deimos at a rate of about 10(23) molecules per second, combined with a charged dust coma, is favored.

A CME‐driven solar wind disturbance observed at both low and high heliographic latitudes

A solar wind disturbance produced by a fast coronal mass ejection, CME, that departed from the Sun on Feburary 20, 1994 was observed in the ecliptic plane at 1 AU by IMP 8 and at high heliographic latitudes at 3.53 AU by Ulysses. In the ecliptic the disturbance included a strong forward shock but no reverse shock, while at high latitudes the disturbance was bounded by a relatively weak forward‐reverse shock pair. It is clear that the disturbance in the ecliptic plane was driven primarily by the relative speed between the CME and a slower ambient solar wind ahead, whereas at higher latitudes the disturbance was driven by expansion of the CME. The combined IMP 8 and Ulysses observations thus provide a graphic illustration of how a single fast CME can produce very different types of solar wind disturbances at low and high heliographic latitudes. Simple numerical simulations help explain observed differences at the two spacecraft.

Reply [to “Comments on ‘Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field' by Yu‐Qing Lou”

Reply [to “Comments on ‘Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field' by Yu‐Qing Lou”

Comments on “Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field” by Yu‐Qing Lou

Comments on “Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field” by Yu‐Qing Lou

Flux dropouts of plasma and energetic particles at geosynchronous orbit during large geomagnetic storms: Entry into the lobes

Geosynchronous orbit spacecraft occasionally observe the magnetospheric tail lobe during disturbed geomagnetic conditions. These lobe intervals are characterized by an absence of detectable ion flux above 1 eV and an absence of electron flux above several hundred eV. This study is an extension of Thomsen et al. (1994) and examines geosynchronous orbit plasma measurements for lobe encounters during a 4‐year period involving three spacecraft, for a total of 85 spacecraft‐months of observations. During the interval surveyed, 160 lobe encounters or close encounters were observed. These events are divided into the following two classes: events observed near midnight and those observed along the flanks. Both classes of events were strongly associated with very active geomagnetic conditions as indicated by Kp (mean value of 5), Dst (mean value of −104 nT), and the Auroral Boundary Index, particularly the flank events. Fifty percent of the lobe encounters occurred within ±24 hours of a geosynchronous magnetopause crossing, which indicates that many of the lobe encounters occurred when there was severe compression and/or erosion of the magnetosphere. Most of the magnetopause‐crossing‐associated events were flank events. The flank‐lobe events were also generally associated with a large interplanetary magnetic field magnitude, with the By or Bz component being dominant. Dropouts in the energetic electron and ion fluxes were observed concurrently with the thermal ion plasma dropouts, though they often extended over a longer time interval. For most of the events a geosynchronous injection of energetic electrons and/or protons was observed following the beginning of the thermal ion dropout, often at a different satellite than the one observing the lobe. For a limited subset of the lobe events (17 events) in which a GOES satellite was nearby, the magnetic field data indicate that the field is often highly stretched or radial near the time of the lobe events. We conclude that the near‐midnight and flank‐lobe events are due to two different mechanisms as follows: the near‐midnight events are consistent with the growth phase picture of large geomagnetic substorms, while the flank events are due to a major reconfiguration of the magnetosphere during large geomagnetic storms which are triggered by the passage of a large solar wind disturbance such as a fast coronal mass ejection. These observations show that the magnetosphere can be highly distorted at synchronous orbit, even far from midnight.

On two‐component models of solar wind fluctuations

Matthaeus et al. first suggested a two‐component scenario for solar wind fluctuations which contains both parallel propagating Alfvén waves together with a population of quasi‐two‐dimensional turbulence. On the basis of these earlier ideas and further evidence, Tu and Marsch developed a two‐component model, in which small‐scale incompressible fluctuations are decomposed into Alfvén waves and convective structures, in an attempt to account for observed radial evolutions of energy density, normalized cross helicity σc, and Alfvén ratio rA of solar wind fluctuations. According to a recent analysis on two‐dimensional Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field, we discuss the theoretical basis of this important two‐component concept and further indicate necessary ingredients that a more self‐consistent two‐component turbulence model should contain. From the perspective of Alfvénic disturbances in the solar wind, the wave and convective components coexist in a naturally intermixed manner. Our analysis specifically describes behaviors of Alfvénic disturbances of all scales, including the intermediate ones. In a fully developed spiral magnetic field at large radii, the criterion for separating radially propagating Alfvén waves and radially convective Alfvénic structures depends on whether |ƒ + mƒS| is greater or less than the characteristic frequency ƒc ≡ (FMU³/FB²)1/2, where ƒ is the perturbation frequency, ƒS is the solar rotation frequency, m is an integer for the azimuthal wavenumber, FM is the solar wind mass flux, FB is the radial magnetic flux, and U is the asymptotic wind speed. For typical solar wind parameters, the timescale corresponding to ƒc is ≳ 15 hours.

Motion of the heliospheric termination shock at high heliographic latitude

We expect the mean distance of the heliospheric termination shock to be greater (smaller) at polar latitudes than at equatorial latitudes, depending on whether the mean dynamic pressure of the solar wind is greater or smaller at high latitudes. The heliospheric termination shock is expected to move in response to variation in upstream solar wind conditions, so that at any particular instant the termination shock will resemble a distorted asymmetric balloon with some parts moving inward and others moving outward. If the shock is a gasdynamic or magnetohydrodynamic shock the results of the analysis depend only very weakly on the nature of the upstream disturbance; typical speeds of the disturbed shock are -100 to 200 km/s. In the absence of a significant latitude gradient of the typical magnitude of solar wind disturbances typical motions of the disturbed shock at polar latitudes would be about twice as fast, due to the higher speed of the high-latitude wind. If the dynamics of the termination shock are dominated by acceleration of the anomalous component of the cosmic rays, the motion of the shock in response to a given disturbance is substantially slower than in the gasdynamic case. Conceivably, particle acceleration might be a less important effect at higher latitudes, and we envision the possibility of a termination shock that is dominated by particle acceleration at lower latitudes and is an MHD shock at high latitudes. In this event high latitude solar wind disturbances would produce substantially larger inward and outward motions of the shock in the polar regions.

Cosmic ray decreases and solar wind disturbances during late October 1989

We describe the interplanetary phenomena (energetic particles, solar wind plasma, and magnetic field) seen at IMP 8 and at ICE, located 65° west of IMP 8, during the period October 19‐31, 1989, when neutron monitors observed three ground level events originating in one active region when it was in the longitude range E09° to W57°. At least four shocks, associated with energetic particle enhancements, which can be attributed to a sequence of coronal mass ejections from the same active region, were seen at both spacecraft. An additional shock was observed only at ICE late in this period when the active region was behind the west limb. Considering all the data (which unfortunately suffer from large gaps), it appears that the ejecta associated with the shocks were detected only when the spacecraft and solar source longitude were separated by less than 50°. The shocks extended over a greater range of longitudes. The cosmic ray record at Earth is consistent with this picture such that only the first two shock‐associated cosmic ray decreases had the signature expected for intercepting ejecta material. This same time period was also examined by Bavassano et al. (1994). However, we do not agree with their conclusion that “magnetic clouds” extending at least 75° from the source longitude were present.

Solar wind interaction with small bodies: 1. Whistler wing signatures near Galileo's closest approach to Gaspra and Ida

Abstract Data from Galileo's two asteroid flybys reveal magnetic fluctuations that we interpret as perturbations of the solar wind magnetic field caused by an interaction with the nearby asteroid. The scale sizes of the bodies (∼14 km for Gaspra and ∼30 km for Ida) are intermediate between the ion and electron gyroradii, which implies that the asteroid-imposed perturbations propagate in the whistler mode. Special properties of the whistler mode include phase velocities that can exceed the solar wind speed and confinement of the disturbance to directions nearly aligned with the magnetic field. These features of the interaction impose a structure on the solar wind disturbance that differs greatly from the forms familiar for either magnetized or unmagnetized bodies of magnetohydrodynamic spatial scales. We examine both data and computer simulations of the interaction with special attention to what can be inferred about the interaction itself from analysis of the data.

Acceleration of interstellar pickup ions in the disturbed solar wind observed on Ulysses

Acceleration of interstellar pickup H+ and He+ as well as of solar wind protons and alpha particles has been observed on Ulysses during the passage of a corotating interaction region (CIR) at ∼4.5 AU. Injection efficiencies for both the high thermal speed interstellar pickup ions (H+ and He+) and the low thermal speed solar wind ions (H+ and He++) are derived using velocity distribution functions of protons, pickup He+ and alpha particles from < 1 to 60 keV/e and of ions (principally protons) above ∼60 keV. The observed spatial variations of the few keV and the few hundred keV accelerated pickup protons across the forward shock of the CIR indicate a two stage acceleration mechanism. Thermal ions are first accelerated to speeds of 3 to 4 times the solar wind speed inside the CIR, presumably by some statistical mechanism, before reaching higher energies by a shock acceleration process. Our results also indicate that (1) the injection efficiencies for pickup ions are almost 100 times higher than they are for solar wind ions, (2) pickup H+ and He+ are the two most abundant suprathermal ion species and they carry a large fraction of the particle thermal pressure, (3) the injection efficiency is highest for protons, lowest for He+, and intermediate for alpha particles, (4) both H+ and He+ have identical spectral shapes above the cutoff speed for pickup ions, and (5) the solar wind frame velocity distribution function of protons has the form F(w) = F0w−4 for 1 < w < ∼5, where w is the ion speed divided by the solar wind speed. Above w ∼ 5‐10 the proton spectrum becomes steeper. These results have important implications concerning acceleration of ions by shocks and CIRs, acceleration of anomalous cosmic rays, and particle dynamics in the outer heliosphere.

Alfvénic disturbances in the equatorial solar wind with a spiral magnetic field

We study a two‐dimensional propagation of Alfvénic perturbations within the solar equatorial plane (SEP) in the solar wind with a spiral magnetic field. Analytical solutions within the entire frequency range are derived at large radial distance r. Numerical solutions which smoothly pass through the Alfvén critical point rA are also obtained. Since the spiral magnetic field caused by the solar rotation scales as ∼r−1 at large r, the velocity and magnetic field perturbations, which are perpendicular to the SEP, remain small relative to the wind parameters in the entire radial range. The frequency criterion for the non‐WKB radial scalings of the perturbation variables to appear is (f + mf⊙)² < fc², where f is the perturbation frequency, f⊙ is the solar rotation frequency, m is the azimuthal wavenumber, and fc is the well‐known characteristic frequency determined by the wind parameters. We demonstrate that the process of continuous reflection in a spiral magnetic field and the effect of superposing Alfvénic perturbations with various f and m can lead to variations of the relative magnetic field fluctuation, the normalized cross helicity σc, the Alfvén ratio ra, the magnetic perturbation energy density Eb, and the kinetic perturbation energy density Eυ, which differ from the predictions of the conventional WKB theory. In particular, the non‐WKB radial scalings of the perturbation variables can appear at any timescales with appropriate values of m. The relevance of these results to data analyses on interplanetary fluctuations is discussed, and we emphasize the importance of obtaining fluctuation spectrum with respect to both f and m in the outer heliosphere.

The solar causes of major geomagnetic storms

Abstract. In this paper we reanalyse the set of ten major geomagnetic storms which occurred between August 1978 and December 1979. We relate them to the characteristics of the solar wind disturbances which caused them and the solar sources of such disturbances as tracked by means of interplanetary scintillation. It seems to us that the shock causing the sudden commencement and the plasma behind it with an important long-lasting Bz south component (Bz.

Large-scale structure of solar wind and its relationship with solar corona: Prognoz 7 observations

On the basis of selective measurements of proton and α-particle bulk parameters and magnetic field on board the Prognoz 7 satellite, the structure of solar wind is studied and compared with the known structure of solar corona. For that purpose the magnetic field value, B, proton temperature, T p, ratio of thermal and magnetic pressures, β = nkT p ( B 2 8π ) , and relative helium abundance, n alpha n p , ar as distributions on the n- V p (solar wind ion density-proton velocity) plane. The values and different behaviour of these parameters on the n- V p plane allow us to identify five regions (types of solar wind streams), which may be connected with well known solar corona structure and events: (1) heliospheric current sheet; (2) streams from closed coronal magnetic field regions (streamers); (3) streams from open magnetic field regions (coronal holes); (4) disturbed solar wind streams at large angles relative to the solar meridians of non-stationary events in the corona; and (5) disturbed streams at small angles relative to the meridians of non-stationary events, including the matter of coronal mass ejections. Distributions of velocity difference, V α − V p, and temperature ratio, T α T p , of α-particles and protons on the n- V p plane preferential acceleration ( V α > V p) and heating ( T α T p ⩾ 4 ) of α-particles are generally observed in from coronal holes.

Coronal holes as sources of large‐scale solar wind disturbances and geomagnetic perturbations

In this paper we present theoretical and observational evidence that low‐latitude unstable coronal holes may be, in some circumstances, the source of widespread interplanetary shocks and of the consequent geomagnetic perturbations. We also reanalyze here the shock events tracked by interplanetary scintillation in the study of Hewish and Bravo (1986), which produced the sudden commencement of a geomagnetic storm, in terms of an alternative flare, disappearing filament, or coronal hole eruption origin. The main conclusion of this study is that there are good reasons to consider coronal holes as sources of shock type interplanetary disturbances and that more effort should be made to monitor coronal holes continuously in search of evidence of large short‐time changes in their structure and physical conditions.

Large-scale structure of solar wind and solar corona: Prognoz 7 observations

Abstract On the basis of selective measurements of proton and α-particle bulk parameters and magnetic field on board the Prognoz 7 satellite the structure of the solar wind is studied and is compared with a possible structure of the solar corona. For that purpose the magnetic field value, proton temperature, ratio of thermal and magnetic pressures and relative helium abundance are represented as distributions on the n - V p (density - proton velocity) plane. Values and different behaviour of these parameters on this plane allow to identify 5 types of the solar wind streams which may be connected with the solar corona structure: (1) the heliospheric current sheets, (2) streams from closed coronal magnetic field regions (streamers), (3) streams from open magnetic field regions (coronal holes), (4) disturbed solar wind streams at large angles relative to the solar meridians of non-stationary events in the corona, and (5) disturbed streams at small angles relative to meridians of non-stationary events including coronal mass ejection matter.

The magnetospheric disturbance ring current as a source for probing the deep earth electrical conductivity

Two current rings have been observed in the equatorial plane of the earth at times of high geomagnetic activity. An eastward current exists between about 2 and 3.5 earth radii (Re) distant, and a larger, more variable companion current exists between about 4 and 9 Re. These current regions are loaded during geomagnetic substorms. They decay, almost exponentially, after the cessation of the particle influx that attends the solar wind disturbance. This review focuses upon characteristics needed for intelligent use of the ring current as a source for induction probing of the earth’s mantle. Considerable difficulties are found with the assumption that Dst is a ring-current index.

Suprathermal ions observed upstream of the Venus bow shock

Suprathermal ions with arrival directions quite distinct from those of the solar wind have been detected upstream of the Venus bow shock. We examine the possibility that these events could be caused by instrumental or spacecraft effects or that they could be either solar wind disturbances, planetary pickup ions, or Suprathermal ions upstream of the Venus bow shock and conclude that they are consistent with upstream suprathermal ions associated with the bow shock that are observed down‐stream from the points of intersection of their extrapolated trajectories with the shock.

The solar wind: Advances in our knowledge through two solar cycles

As the Pioneer and Voyager spacecraft have moved outward they have gradually unfolded a view of distant regions of the heliosphere. Information on the solar wind velocity, density and temperature as a function of distance out to more than 40 AU has been gathered. Meanwhile our description of the solar wind has evolved. Long-standing questions on the sources of the wind causing geomagnetic activity were clarified by the discovery of coronal holes and coronal mass ejections. The propagation of the resultant solar wind disturbances through the heliosphere has been studied using both observations and models. Plasma physical processes have been studied. This review focuses on the development of the concepts that have been used to describe the solar wind in the 3 dimensional heliosphere over the last two solar cycles. Collisionless shocks, transient disturbances in space, disturbance propagation and the distant solar wind are discussed.

Simulation of January 1–7, 1978, events

The solar wind disturbances during January 1–7, 1978, are reconstructed by a modeling method. First of all, the interplanetary magnetic field background pattern, including a corotating shock, is reproduced using the Stanford source surface map. Then, two solar flares with their onset times on January 1, 0717 at 17°S, 10°E and 2147 UT at 17°S, 32°E, respectively, are selected to generate two interplanetary transient shocks. It is shown that these two shocks interacted with the corotating shock, resulting in a series of interplanetary events observed by four spacecraft, Helios 1 and 2, IMP 8, and Voyager 2. In particular, our simulation results show that these three shock waves interact and coalesce in interplanetary space such that Helios 2 and Voyager 2 observed only one shock and Helios 1 and IMP 8 observed two shocks. All shocks observed by the four spacecraft, except the corotating shock at Helios 1, are either a transient shock or a shock which is formed from coalescing of the transient shocks with the corotating shock. Our method is useful in reconstructing a very complicated chain of interplanetary events observed by a number of spacecraft.