Increase In Solar Wind Speed Research Articles

Statistical Study of Auroral/Resonant‐Scattering 427.8‐nm Emission Observed at Subauroral Latitudes Over 14 Years

AbstractAuroral emission at 427.8‐nm from N2+ ions is caused by precipitation of energetic electrons, or by resonant scattering of sunlight by auroral N2+ ions. The latter often causes impressive purple aurora at high altitudes. However, statistical characteristics of auroral 427.8‐nm emission have not been well studied. In this paper we report occurrence characteristics of high 427.8‐nm emission intensities (more than 100 R) at subauroral latitudes, based on measurements by a filter‐tilting photometer over 14 years (2005–2018) at Athabasca, Canada (magnetic latitude: ~62°). We divided the data set into solar elevation angles (θs) more than and less than −24° (shadow height of sunlight: 600 km) to separate the 427.8‐nm emissions caused by resonant scattering of sunlight and those excited by auroral electrons, respectively. The occurrence rate of 427.8‐nm emissions of more than 100 R is 10.6% and 7.65% for θs more than and less than −24°, respectively, confirming that resonant scattering of sunlight by N2+ ions is a cause of the strong 427.8‐nm emissions of more than 100 R in the sunlit ionosphere. The occurrence rate is high in the postmidnight sector and increases with increasing geomagnetic activity, solar wind speed, and density. The occurrence rate is lowest in winter. A high occurrence rate was observed in 2015–2018, during the declining phase of the 11‐year solar activity. Superposed epoch analysis indicates that the 427.8‐nm emission exceeds 100 R when solar wind speed increases and solar wind density concurrently decreases, though the standard deviation of the data is rather large.

The occurrence of upstream waves in relation with the solar wind parameters: A statistical approach to estimate the size of the foreshock region

We studied the occurrence of upstream waves in the foreshock region and their relationship with the solar wind and interplanetary magnetic field parameters. To this purpose, we developed a method for a careful identification of the upstream wave events. The results of the statistical analyses based on Cluster data (2003–2010 years) confirm that the angle between the bow shock normal direction and the interplanetary magnetic field is the key element for the wave generation; they also show the relationship between the wave occurrence and the solar wind speed and density. We focused our attention on the occurrence of wave events as a function of the distance from the bow shock. The results show that the foreshock region, where we can observe upstream waves, is characterized by an effective size that decreases with the increase of both the solar wind speed and the wave frequency. Due to the relationship between the solar wind speed and the wave frequency, we suggest that such distance is simply a function of the solar wind speed, becoming smaller when the solar wind speed increases, and then the occurring higher frequency upstream waves are confined in a more restricted region.

Changes in thermospheric temperature induced by high‐speed solar wind streams

During high‐speed stream (HSS) events the solar wind speed increases, and the cross polar cap potential increases, leading to increased Joule heating at high latitudes. The heat input at high latitudes heats the polar regions, which then conducts to lower latitudes, producing global heating. The heating occurs during the risetime of the cross polar cap potential and throughout the period of high cross polar cap potential as seen in our simulation. These simulations are performed using the Utah State University global thermosphere model driven by Joule heating rates that are consistent with electric fields observed by DMSP‐15 observations of HSS events. Cooling occurs as the cross polar cap potential decreases and continues for several days after the cross polar cap potential has returned to background values. Polar cap ionospheric observations are compared to model simulations of heating and cooling, providing evidence that the thermospheric model is capturing the HSS energy input and the post‐HSS multiday return to pre‐HSS conditions. The HSS heating can be as high as 100 K (as seen from both the model and the data) at high latitudes, with a corresponding, but lower, global increase in thermospheric temperature.

Enhancement of Solar Energetic Particles During a Shock – Magnetic Cloud Interacting Complex Structure

The behavior of solar energetic particles (SEPs) in a shock – magnetic cloud interacting complex structure observed by the Advanced Composition Explorer (ACE) spacecraft on 5 November 2001 is analyzed. A strong shock causing magnetic field strength and solar wind speed increases of about 41 nT and 300 km s−1, respectively, propagated within a preceding magnetic cloud (MC). It is found that an extraordinary SEP enhancement appeared at the high-energy (≥10 MeV) proton intensities and extended over and only over the entire period of the shock – MC structure passing through the spacecraft. Such SEP behavior is much different from the usual picture that the SEPs are depressed in MCs. The comparison of this event with other top SEP events of solar cycle 23 (2000 Bastille Day and 2003 Halloween events) shows that such an enhancement resulted from the effects of the shock – MC complex structure leading to the highest ≥10 MeV proton intensity of solar cycle 23. Our analysis suggests that the relatively isolated magnetic field configuration of MCs combined with an embedded strong shock could significantly enhance the SEP intensity; SEPs are accelerated by the shock and confined into the MC. Further, we find that the SEP enhancement at lower energies happened not only within the shock – MC structure but also after it, probably owing to the presence of a following MC-like structure. This is consistent with the picture that SEP fluxes could be enhanced in the magnetic topology between two MCs, which was proposed based on numerical simulations by Kallenrode and Cliver (Proc. 27th ICRC 8, 3318, 2001b).

A statistical study on the correlations between plasma sheet and solar wind based on DSP explorations

Abstract. By using the data of two spacecraft, TC-1 and ACE (Advanced Composition Explorer), a statistical study on the correlations between plasma sheet and solar wind has been carried out. The results obtained show that the plasma sheet at geocentric distances of about 9~13.4 Re has an apparent driving relationship with the solar wind. It is found that (1) there is a positive correlation between the duskward component of the interplanetary magnetic field (IMF) and the duskward component of the geomagnetic field in the plasma sheet, with a proportionality constant of about 1.09. It indicates that the duskward component of the IMF can effectively penetrate into the near-Earth plasma sheet, and can be amplified by sunward convection in the corresponding region at geocentric distances of about 9~13.4 Re; (2) the increase in the density or the dynamic pressure of the solar wind will generally lead to the increase in the density of the plasma sheet; (3) the ion thermal pressure in the near-Earth plasma sheet is significantly controlled by the dynamic pressure of solar wind; (4) under the northward IMF condition, the ion temperature and ion thermal pressure in the plasma sheet decrease as the solar wind speed increases. This feature indicates that plasmas in the near-Earth plasma sheet can come from the magnetosheath through the LLBL. Northward IMF is one important condition for the transport of the cold plasmas of the magnetosheath into the plasma sheet through the LLBL, and fast solar wind will enhance such a transport process.

Late‐phase acceleration of energetic ions in corotating interaction regions

We report on new high‐sensitivity measurements from the WIND spacecraft of the spatial distributions of 30 keV/amu to 10 MeV/amu ions from corotating interaction regions (CIRs) that extend far beyond the confines of the parent high‐speed solar‐wind stream. Not only do late‐phase MeV ions persist far into the declining solar wind, but they also show a continual gain in energy, even after sector boundary passage, until the next small increase in solar wind speed occurs. These ions are accelerated in the distant heliosphere as the reverse shock from the CIR propagates completely across the rarefaction region produced by the declining solar wind, growing in acceleration efficiency as it propagates. Energetic ions from a single CIR event are seen for a period of 17 days and ∼225° in solar longitude. The observed energy spectra can be fit to the theory of Fisk and Lee [1980] only if shock compression increases with time so that the spectra harden significantly.

The relationship between recurring cosmic ray depressions and corotating solar wind streams at ≤1 AU: IMP 8 and Helios 1 and 2 anticoincidence guard rate observations

We examine the detailed relationship between recurrent cosmic ray depressions and corotating high‐speed streams in the inner heliosphere near the ecliptic plane using counting rates from anticoincidence guards of instruments on the IMP 8, Helios 1, and Helios 2 spacecraft. These rates indicate the density of >60 MeV/amu ions with reasonable time resolution (∼15 min) and high counting statistics. Essentially all corotating streams are accompanied by significant particle depressions. The particle decrease commences most frequently (∼63% of events) at the leading edge of the stream which is typically colocated with the stream interface inside the corotating interaction region (CIR). If the depression starts ahead of the interface, there is usually an additional abrupt decrease at the interface. In ∼61% of events the onset of the depression is closely associated with the onset of enhanced field turbulence which typically occurs near the stream leading edge. Minimum particle densities are generally found in the vicinity of the maximum solar wind speed in the high‐speed stream. The density recovers during the declining phase of the stream. The observations are most consistent with modulation of the cosmic ray density in high‐speed streams by the increase in solar wind speed. Enhanced scattering by turbulence in the CIR may also contribute near the onset of the depression, in particular in cases where the decrease commences ahead of the stream interface. The absence of a consistent relationship between the depressions and magnetic field enhancements suggests that localized particle drifts in the enhanced magnetic fields of CIRs do not produce the particle depressions. The observation of a depression is not dependent on the presence of a heliospheric current sheet crossing in the CIR, in contrast to previous reports.

Coronal disturbances and their terrestrial effects

An assessment is undertaken of recent approaches to the prediction of the interplanetary consequences of coronal disturbances, with attention to the relationships of shocks and energetic particles to coronal transients, of proton events to gamma-ray and microwave bursts, of geomagnetic storms to filament eruptions, and of solar wind increases to the flare site magnetic field direction. A discussion is given concerning the novel phenomenon of transient coronal holes, which appear astride the long decay enhancements of 2-50 A X-ray emission following H-alpha filament eruptions. These voids in the corona are similar to long-lived coronal holes, which are the sources of high speed solar wind streams. The transient coronal holes may also be associated with transient solar wind speed increases.

Energy losses of solar cosmic rays in interplanetary space

A simple model of solar cosmic ray propagation which includes diffusion, convection, and energy loss by adiabatic deceleration is studied. A Monte Carlo technique is employed to investigate the variation of mean particle energy in the interplanetary medium after the impulsive release of mono-energetic particles at the Sun. At 1 AU typical energy losses are 43% at 20 h and 64% at 60 h after particle release for a diffusion coefficient κ(r)=κ 0rβ with β=+1/2 and κ0=1.33 × 1021 cm2 s−1. When κ 0 in this model is reduced by a factor of 4, the energy loss is greater by a factor of 2 at 60 h after particle release. When β is increased, the energy losses are greater. Using the model parameters above, an increase in solar wind speed from 300 to 600 km s−-1 gives rise to energy losses that are greater again by factor of 2 at a time of 60 h. Results are compared with an observation by Murray et al. (1971) of a ‘knee’ in the energy spectrum of solar protons. It is not considered likely that the change in the energy of the knee with time requires, in addition to adiabatic deceleration, another energy change process which acts to increase the energy of particles.

Ionization troughs below the F2-layer maximum

Direction-of-arrival information is available on ionograms from Ellsworth, Antarctica because of interference effects from the Filchner Ice Shelf. This has allowed the detection of extremely large ionospheric disturbances which appear to move towards the equator with speeds of the order of 100 m/sec, in the late afternoon and early evening hours. These disturbances produce troughs in the bottomside ionosphere having widths as large as 1000 km, the electron density at the maximum of the F2-layer being reduced by an order of magnitude. Disturbance effects are also recorded in the D and E-layers. Associations with radio auroras and magnetic activity have been found. It is suggested that the troughs mark the positions of red-auroral arcs. The apparent movement of the trough as observed at Ellsworth has been explained in terms of the rotation of the Earth, the trough position being imagined as stationary with respect to the Sun. It seems likely that topside troughs are the upper F2-region extensions of bottomside troughs and that these can be identified (along the Earth's magnetic field lines) with the plasmapause, at least in the night hours after midnight. The troughs seem to define the boundary between the quiet and active regions of high latitudes. It is suggested that this boundary moves closer to the equator when the solar wind speed increases.