Variation Of Solar Wind Velocity Research Articles

Linear change and minutes variability of solar wind velocity revealed by FAST

ABSTRACT Observation of Interplanetary Scintillation (IPS) provides an important and effective way to study the solar wind and the space weather. A series of IPS observations were conducted by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The extraordinary sensitivity and the wide frequency coverage make FAST an ideal platform for IPS studies. In this paper, we present some first scientific results from FAST observations of IPS with the L-band receiver. Based on the solar wind velocity fitting values of FAST observations on 2020 September 26–28, we found that the velocity decreases with increasing frequency linearly, which has not yet been reported in literature. And we have also detected a variation of solar wind velocity on a time-scale of 3–5 min, which imply the slow change of the background solar wind, a co-existence of high- and low-speed streams, or a reflect of the quasi-periodic electron-density fluctuations.

Dependence of cosmic ray intensity on variation of solar wind velocity measured by the GRAPES-3 experiment for space weather studies

The space weather impacts propagation of galactic cosmic rays (CRs) in the heliosphere as explained by the diffusion-convection mechanism which predicts that the variation in CR intensity should be anticorrelated with changes in solar wind velocity (${\mathrm{V}}_{\mathrm{SW}}$). Several unrelated solar phenomena, including periodic ones such as 27 d solar rotation, annual, 11 yr solar activity, 22 yr solar magnetic cycle etc., and transient ones, for example, Forbush decreases (FDs), and ground level enhancements generally caused by solar flares or coronal mass ejections also affect CR intensity on Earth. These solar phenomena make a quantitative study of dependence of CR intensity on ${\mathrm{V}}_{\mathrm{SW}}$ rather difficult. Here, the high statistics muon data of six years (2000--2005) from the large GRAPES-3 muon telescope have been used to study the correlation between ${\mathrm{V}}_{\mathrm{SW}}$ and CR intensity. Data gathered during these six years were used after minimizing the contribution of various unrelated solar phenomena outlined above. We observed a strong anticorrelation between the variations in ${\mathrm{V}}_{\mathrm{SW}}$ and CR intensity at a significance of $19\ensuremath{\sigma}$.

Ground Pc3–Pc5 wave power distribution and response to solar wind velocity variations

We examine the magnetospheric wave power in the Pc3–Pc5 range in terms of its growth and decay characteristics and its distribution in L shell in response to the interplanetary plasma bulk velocity, V SW. We use linear and nonlinear (rank-order) correlation and filtering methods to quantify the effective coupling of the wave power to V SW variations. These methods are applied to measurements from 26 ground magnetometers of the IMAGE array and NOAA's GOES-10 spacecraft at geosynchronous orbit, taken over 2 years of solar-maximum activity (2002–2003). We find that the ground ULF wave power is structured in the range 3.5< L<6.4 and distributed uniformly in the range 6.4< L<15 (uncertainties in L are estimated to be ±0.5). The response of the wave power to the V SW is characterized by an increase starting 3 days before the V SW peak, intensifying several hours before the peak, and is followed by a fast decrease in the next 2 days. The rapid decay of ULF waves has two stages: one at τ=−6±2 h before the solar wind velocity reaches its peak, and one at the V SW peak, τ=0. We suggest that the first one is brought about by wave–particle interaction with inner-magnetospheric populations while the second one is a d V SW/d t effect. The correlation results are confirmed by calculating the finite-impulse response, which shows clearly the decay of the ULF waves after the V SW peak. The response of the wave power at geosynchronous orbit is remarkably similar to that of the ground wave power at comparable L shells. The above findings characterize the inner-magnetospheric response to interplanetary high-speed streams, as opposed to the more short-lived, higher-amplitude response to CMEs.

Long term measurements of radiation belts by SAMPEX and their variations

The Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX), a low‐altitude and polar‐orbiting satellite, has provided a long‐term global picture of the radiation belts since its launch on July 3, 1992. While the inner belt (L &lt; 2) protons appear to have only a solar cycle variation, the outer radiation belt (L &gt; 2) electrons vary on solar cycle, semiannual, and solar rotation time scales, and with geomagnetic storms. Recently developed models of predicting MeV electron at geostationary orbit [Li et al., 2001] and the Dst index [Temerin and Li, 2001] based on solar wind measurements are used to examine the cause of the prominent semiannual variations of outer belt electrons and the Dst index. The equinoctial effect (the angle between the Earth's dipole and the flow direction of the solar wind) contributes most to the semiannual variation of the Dst and MeV electrons deep in the inner magnetosphere (L &lt; 5). The semiannual variation of MeV electrons at geostationary orbit is attributed mostly to the semiannual variation of solar wind velocity.

Evidence for a 17-year cycle in the IMF directions at 1 AU, in solar coronal hole variations, and in planetary magnetospheric modulations

A dominant 16–17 yr cycle was observed in the net exposure times of the Earth to Toward and Away field directions of the interplanetary magnetic field (IMF). A cycle of the same frequency and phase was observed in the polarity of the long-term hemispheric differences in coronal hole distributions. This was determined from north/south differences in average Fe xiv green line ‘quiet’ regions at high- and mid-latitudes. It is argued that the 17-yr cycle is a fundamental oscillation of coronal hole topology, which is transferred to Earth via variations in the neutral sheet. A comparison of the 17-yr cycle to the 22-yr Hale cycle indicated that they are not identical, but rather, can mix to form a 75-yr cycle plus a 9-yr cycle. Evidence for the 75-yr cycle existed in the Earth's net exposure times to fields from the solar North and South, and in the long-term imbalance of solar quiet regions between the northern and southern hemispheres. The 9-yr cycle was manifested in the mid- to low- latitude Fe xiv modulations and in solar wind velocity variations in the ecliptic. At Earth, evidence for a similar 17-yr cycle was observed in the horizontal magnetic field observations in a multitude of surface magnetic recording stations. In addition, the detection of a 17-yr cycle in the Huancayo neutron monitor cosmic ray series suggests that the effects of this cycle extend to the heliospheric boundaries. It is concluded that sufficient preliminary evidence exists to consider the hypothesis that the Sun contains a magnetic moment with an oscillatory cycle of 17 years.

Latitudinal solar wind velocity variations from polar coronal holes: A self‐consistent MHD model

Coronal holes are regions of the solar corona magnetically open to the interplanetary medium where the plasma flows outward to form the solar wind. An axisymmetric magnetohydrodynamic model has been developed that describes both the magnetic structure of the solar corona and the outward flow of coronal plasma in a self‐consistent manner, considering a dipolar field at the solar surface. Equilibrium solutions were found for a field at the solar pole of 10 G and a variety of plasma boundary conditions at 1 AU in order to explore the latitudinal dependence of the solar wind velocity from polar coronal holes. As expected, two large polar coronal holes were found, extending down to 32° latitude, separated by a belt of closed coronal helmets. The coronal helmet belt extended to 1.7 Rs, and above this height all flux lines were open and connected to one of the polar coronal holes, from which all solar wind flowed. Three cases of latitudinal velocity variation at 1 AU were considered: (1) a sharp transition between a slow wind at low latitudes and a fast wind at high latitude (in accordance with Ulysses' observations), (2) a linear latitudinal increase of velocity from slow wind at the equator to fast wind over the pole, and (3) a constant velocity irrespective of latitude. In all cases a fast wind flowed from the poles, but there always existed a low‐latitude band of slow solar wind close to the Sun. This slow wind flowed out from the boundary of both coronal holes and formed a band of slow wind on either side of the heliospheric current sheet. The transition between slow and fast wind close to the Sun was sharply defined in all cases.

The connection between solar wind velocity variations and the physical and empirical models for electron fluxes in near-Earth space

Long duration predictions of energetic electron fluxes in near-Earth space is obtained using calculated models. However the existing calculated models, constructed on the basis of experimental data, usually do not sufficiently take into account the variations of the solar wind velocity. Many experiments on satellites such as “Molnia”, “Prognoz”, “Intercosmos” and “Cosmos” show that the energetic electron flux intensity in magnetosheath, magnetopause and geosynchronous orbits depends on the solar wind velocity. Specifically, a quantitative relation has been established between energetic electron fluxes in the outer radiation belt ( L = 3.5−5) and the solar wind velocity value, and the ratios determining time of energetic electron transfer from the magnetospheric boundaries to ionospheric altitudes. A method for determining the dependence observed in physical and empirical models for energetic electron fluxes in near-Earth cosmic space is suggested.

The global solar magnetic field as a “controller” for the origin and acceleration model of solar wind streams

The 3-D MHD analytical solution as a model of the supersonic transalfvenic solar wind for the low and middle heliolatitudes at a distance of (5–20) solar radii is constructed. The solution shows that typical high-speed stream velocities observed in interplanetary space could be achieved after the flow passes through the transalfvenic region. Relatively small (5%) latitudinal or/and longitudinal magnetic variations arising in the subalfvenic region can cause great (100%) latitudinal or/and longitudinal solar wind velocity variations in the superalfvenic region and play a role of a “function of control” for the process of the high-speed stream acceleration. The conditions are indicated under which the areas of local deceleration inside of the flow could exist.

Azimuthal structure of the solar wind velocity andK-corona brightness in the heliospheric current sheet

The solar wind velocity near Earth shows systematic structure in and around the heliospheric current sheet. The solar wind velocity measurements at IMF sector boundary crossings at 1 AU during 1972–1977 have been used to infer the azimuthal structure of the solar wind velocity in the current sheet. We found that the solar wind velocity in the in-ecliptic portion of the current sheet varies from longitude to longitude, where it originates from the corona. Also, the yearly average value of solar wind velocity in the HCS is found to vary with the phase of the solar cycle; with a maximum value around 1974. TheK-corona brightness on the source surface corresponding to the IMF sector boundary crossings during the period of study also shows a similar but opposite pattern of variation when the data are averaged over a long period. However, this relation is not observed when we considered them individually. So, we conclude that there exists a longitudinal variation of solar wind velocity in the heliospheric current sheet.

IPS measurements and azimuthal variation of solar-wind velocity on the heliospheric current sheet

The solar-wind velocity tends to be a minimum along the heliospheric current sheet and increases with heliomagnetic latitude. In addition, an azimuthal variation of solar-wind velocity is found to exist on the current sheet. In this study of the solar-wind velocity variations on the current sheet, we have made use of interplanetary scintillation (IPS) measurements. This method provides solar-wind data over a larger range of heliolatitudes: ± 80° or greater. We found that there exists a clear longitudinal variation of solar-wind velocity on the current sheet during 1973–1985. It is also noticeable that the pattern of variation changes with the phase of the solar cycle. On average, the solar-wind velocity varied from 300 to 550 km s −1 on the current sheet during the period of study. It is also found that the longitudinal variation of the solar-wind velocity on the current sheet obtained using near-Earth satellite data agrees well with that obtained using IPS data.

Observations of the interactions of heavy ions from Comet P/Giacobini-Zinner with the solar wind

During the encounter between the ICE spacecraft and Comet Giacobini-Zinner, intense fluxes of energetic heavy ions were observed at distances up to 4 × 10 6 km from the comet. These ions were observed with steep energy spectra and highly anisotropic angular distributions, and are consistent with a composition comprising mainly ions from the water group. The flux versus time profiles have a general fall-off with increasing distance from the comet, but are modulated by both changes in the magnetic field direction and the solar wind velocity, the magnetic field variations being mainly responsible for variations on a time scale of minutes, and the solar wind velocity variations being responsible for much larger time-scale modulations, such as the inbound/outbound asymmetry of the intensity profile. In this paper we present correlated observations of heavy ions, the solar wind velocity and the magnetic field direction, and compare the observations of the ions with the theoretical predictions for their variations with distance from the comet, with the solar wind velocity and with the magnetic field direction.

Response of earth and venus ionospheres to corotating solar wind stream of 3 July 1979

Corotating solar wind streams emanating from stable coronal structures provide an unique opportunity to compare the response of planetary ionospheres to the energy conveyed in the streams. For recurrent solar conditions the “signal” propagating outward along spiral paths in interplanetary space can at times exhibit rather similar content at quite different downstream locations in the ecliptic plane. Using solar wind measurements from plasma detectors on ISEE-3, Pioneer Venus Orbiter (PVO) and Helios-A, as well as in-situ ion composition measurements from Bennett Ion Mass Spectrometers on the Atmosphere Explorer-E and PVO spacecraft, corotating stream interactions are examined at Earth and Venus. During May–July 1979 a sequence of distinct, recurrent coronal regions developed at the Sun. Analysis of these regions and the associated solar wind characteristics indicates a corrresponding sequence of corotating streams, identifiable over wide distances. The time series of solar wind velocity variations observed at Earth, Venus, and the Helios-A positions during June–July attests to intervals of corotating stream propagation. The characteristics of the stream which passed Earth on July 3, are observed at Helios-A and at Venus (PVO) about 8 days later, consistent with the spiral path propagation delay times between the locations in the ecliptic plane. On July 3, Earth and Venus have a wide azimuthal separation of about 142 ‡. Although the planetary environments are distinctly different, pronounced and somewhat analagous ionospheric responses to the stream passage are observed at both Earth and Venus. The response to the intercepted stream is consistent with independent investigations which have shown that the variability of the solar wind momentum flux is an important factor in the solar wind-ionosphere interaction at both planets.

Acceleration of the solar wind by whistler waves from coronal holes

We investigate the possibility of an additional acceleration of the high speed solar wind by whistler waves propagating outward from a coronal hole. We consider a stationary, spherically symmetric model and assume a radial wind flow as well as a radial magnetic field. The energy equation consists of (a) energy transfer of the electron beam which excites the whistler waves, and (b) energy transfer of the whistler waves described by conservation of wave action density. The momentum conservation equation includes the momentum transfer of two gases (a thermal gas and an electron beam). The variation of the temperature is described by a polytropic law. The variation of solar wind velocity with the radial distance is calculated for different values of energy density of the whistler waves. It is shown that the acceleration of high speed solar wind in the coronal hole due to the whistler waves is very important. We have calculated that the solar wind velocity at the earth's orbit is about equal to 670 km/sec (for wave energy density about 10 −4 erg cm −3 at 1.1R⊙). It is in approximate agreement with the observed values.