Preferred Bartels days of high speed solar wind streams: An update

Comparison of distribution of solar wind streams as a function of Bartels rotation days for two successive epochs indicates several significant differences. These are likely to be due to the differing nature of IMF and solar wind in the two solar cycles 20 and 21.     

The February 1986 solar activity: A comparison of GIOTTO,VEGA-1, and IMP-8 solar wind measurements with MHD simulations

Large disturbances in the interplanetary medium were observed by several spacecraft during a period of enhanced solar activity in early February 1986. The locations of six solar flares and the spacecraft considered here encompassed more than 100° of heliolongitude. These flares during the minimum of cycle 21 set the stage for an extensive multi-spacecraft comparison performed with a two-dimensional, magnetohydrodynamic (MHD) numerical experiment. The plasma instruments on the European Space Agency (ESA)'s GIOTTO spacecraft, on its way to encounter Comet Halley in March 1986, made measurements of the solar wind for up to 8 hours per day during February. We compare solar wind measurements from the Johnstone Plasma Analyzer (JPA) experiment on GIOTTO with the MHD simulation of the interplanetary medium throughout these events. Using plasma data obtained by the IMP-8 satellite in addition, it appears that an extended period of high solar wind speed is required as well as the simulated flares to represent the interplanetary medium in this case. We also compare the plasma and magnetometer data from VEGA-1 with the MHD simulation. This comparison tends to support an interpretation that the major solar wind changes at both GIOTTO and VEGA-1 on 8 February, 1986 were due to a shock from a W05° solar flare on 6 February, 1986 (06:25 UT). The numerical experiment is considered, qualitatively, to resemble the observations at the former spacecraft, but it has less success at the latter one.     

Study of cosmic ray intensity in relation to the interplanetary magnetic field and geomagnetic storms for solar cycle 24

In the present study, we investigate the association of cosmic ray intensity (CRI) with various solar wind parameters (i.e. solar wind speed V, plasma proton temperature, plasma proton density), interplanetary magnetic field (IMF B), geomagnetic storms (GSs), averaged planetary A-index (Ap index) and sun spot number (SSN) for the period 2009–2016 (solar cycle 24) by using their daily mean average. To find the association of CRI with various solar wind parameters, GSs, IMF B, Ap index and SSN, we incorporate the analysis technique by superposed-epoch method. We have observed that CRI decreases with the increase in IMF B. Moreover the time-lag analysis has been performed by the method of correlation coefficient and observed a time lag of 0 to 2 day between the decrease in CRI and increase in IMF B. In addition, we show that the CRI is found to decrease in a similar pattern to disturbance storm time (Dst index) for most of the period of solar cycle 24. The high and positive correlation is found between CRI and Dst index. The CRI and Ap index are better anti-correlated to each other than CRI and IMF. CRI and SSN are positively correlated with each other. Solar wind parameters such as solar wind speed V is a CR-effective parameter while plasma proton temperature and plasma proton density are not CR-effective parameters. The indicated parameters such as Dst index, Ap index, IMF B and solar wind parameters such as solar wind speed V, plasma proton temperature, plasma proton density shows a kind of irregular variations for solar cycle 23 and 24 while CRI and SSN shows distinct behaviour for the two cycle.     

Remote sensing of the heliospheric solar wind using radio astronomy methods and numerical simulations

The ground-based radio astronomy method of interplanetary scintillations (IPS) and spacecraft observations have shown, in the past 25 years, that while coronal holes give rise to stable, reclining high speed solar wind streams during the minimum of the solar activity cycle, the slow speed wind seen more during the solar maximum activity is better associated with the closed field regions, which also give rise to solar flares and coronal mass ejections (CME’s). The latter events increase significantly, as the cycle maximum takes place. We have recently shown that in the case of energetic flares one may be able to track the associated disturbances almost on a one to one basis from a distance of 0.2 to 1 AU using IPS methods. Time dependent 3D MHD models which are constrained by IPS observations are being developed. These models are able to simulate general features of the solar-generated disturbances. Advances in this direction may lead to prediction of heliospheric propagation of these disturbances throughout the solar system.     

Effect of solar heliospheric parameters on different components of daily variation in cosmic rays

In the present work the data of three different neutron monitoring stations, Deep River, Tokyo and Inuvik located at different geomagnetic cutoff rigidities and altitudes has been harmonically analysed for the period 1980–1993, 1980–1990 and 1981–1993 respectively to investigate for a comparative study of diurnal, semi-diurnal and tri-diurnal anisotropies in cosmic ray (CR) intensity in connection with the change in IMF Bz component and solar wind velocity on 60 quietest days. It is observed that the amplitude of first harmonic is highly anti-correlated to the solar wind velocity during the period of high-speed solar wind stream (HSSWS) epoch on quiet days for three neutron monitor stations at different geomagnetic rigidity thresholds. During quiet days the amplitude of all the three harmonics significantly deviates on the onset of HSSWS epoch, whereas the direction of the anisotropy of all the three harmonics remains time invariant at three different cut off rigidity stations. The amplitude as well as the direction of anisotropy of all the three harmonics does not have time variation characteristics associated with Bz component of IMF on geo-magnetically most quiet days.     

A Complete Catalogue of High-Speed Solar Wind Streams during Solar Cycle 23

High-speed solar wind streams (HSSWSs) are ejected from the Sun and travel into the interplanetary space. Because of their high speed, they carry out energetic particles such as protons and heavy ions, which leads to an increase in the mean interplanetary magnetic field (IMF). Since the Earth is in the path of those streams, Earth’s magnetosphere interacts with the disturbed magnetic field, leading to a significant radiation-induced degradation of technological systems. These interactions provide an enhanced energy transfer from the solar wind/IMF system into the Earth’s magnetosphere and initiate geomagnetic disturbances that may have a possible impact on human health. Solar cycle 23 was a particularly unusual cycle with many energetic phenomena during its descending phase and also had an extended minimum. We have identified and catalogued the HSSWSs of this cycle and determined their characteristics, such as their maximum velocity, beginning and ending time, duration, and possible sources. We identified 710 HSSWSs and compared them with the corresponding characteristics of the streams of previous solar cycles. For first time, we used the CME data to study the stream sources, which led to useful results for the monitoring and forecasting of space weather effects.     

The 27-Day Cosmic Ray Intensity Variations During Solar Minimum 23/24

We have studied the 27-day variations and their harmonics in Galactic cosmic ray (GCR) intensity, solar wind velocity, and interplanetary magnetic field (IMF) components during the recent prolonged solar minimum 23/24. The time evolution of the quasi-periodicity in these parameters connected with the Sun’s rotation reveals that the synodic period of these variations is ≈?26?–?27 days and is stable. This means that the changes in the solar wind speed and the IMF are related to the Sun’s near-equatorial regions in considering the differential rotation of the Sun. However, the solar wind parameters observed near the Earth’s orbit provide only the conditions in the limited local vicinity of the equatorial region in the heliosphere (within ±?7^° in latitude). We also demonstrate that the observed period of the GCR intensity connected with the Sun’s rotation increased up to ≈?33?–?36 days in 2009. This means that the process that drives the 27-day GCR intensity variations takes place not only in the limited local surroundings of the equatorial region but in the global 3-D space of the heliosphere, covering also higher latitude regions. A relatively long period (≈?34 days) found for 2009 in the GCR intensity gives possible evidence of the onset of cycle 24 due to active regions at higher latitudes and rotating slowly because of the Sun’s differential rotation. We also discuss the effect of differential rotation on the theoretical model of the 27-day GCR intensity variations.     

The role of the magnetic barrier in the Solar wind-magnetosphere interaction

The magnetized solar wind carries a large amount of energy but only a small fraction of it enters the magnetosphere and powers its dynamics. Numerous observations show that the interplanetary magnetic field (IMF) is a key parameter regulating the solar wind-magnetosphere interaction. The main factor determining the amount of energy extracted from the solar wind flow by the magnetosphere is the plasma flow structure in the region adjacent to the sunward side of the magnetopause. While compared to the energy of the solar wind flow the IMF magnetic energy is relatively weak, it is considerably enhanced in a thin layer next to the dayside magnetopause variously called the plasma depletion layer or magnetic barrier. Important features of this barrier/layer are (i) a pile-up of the magnetic field with (ii) a concurrent decrease of density, (iii) enhancement of proton temperature anisotropy, (iv) asymmetry of plasma flow caused by magnetic field tension, and (v) characteristic wave emissions (ion cyclotron waves). Importantly, the magnetic barrier can be considered as an energy source for magnetic reconnection. While the steady-state magnetic barrier has been extensively examined, non-steady processes therein have only been addressed by a few authors. We discuss here two non-steady aspects related to variations of the magnetic barrier caused by (i) a north-to-south rotation of the IMF, and (ii) by pulses of magnetic field reconnection at the magnetopause. When the IMF rotates smoothly from north-to-south, a transition layer is shown to appear in the magnetosheath which evolves into a thin layer bounded by sharp gradients in the magnetic field and plasma quantities. For a given reconnection rate and calculated parameters of the magnetic barrier, we estimate the duration and length scale of a reconnection pulse as a function of the solar wind parameters. Considering a sudden decrease of the magnetic field near the magnetopause caused by the reconnection pulse, we study the relaxation process of the magnetic barrier. We find that the relaxation time is longer than the duration of the reconnection pulse for large Alfvén-Mach numbers.     

The space environment of Mercury at the times of the second and third MESSENGER flybys

The second and third flybys of Mercury by the MESSENGER spacecraft occurred, respectively, on 6 October 2008 and on 29 September 2009. In order to provide contextual information about the solar wind properties and the interplanetary magnetic field (IMF) near the planet at those times, we have used an empirical modeling technique combined with a numerical physics-based solar wind model. The Wang–Sheeley–Arge (WSA) method uses solar photospheric magnetic field observations (from Earth-based instruments) in order to estimate the inner heliospheric radial flow speed and radial magnetic field out to 21.5 solar radii from the Sun. This information is then used as input to the global numerical magnetohydrodynamic model, ENLIL, which calculates solar wind velocity, density, temperature, and magnetic field strength and polarity throughout the inner heliosphere. WSA-ENLIL calculations are presented for the several-week period encompassing the second and third flybys. This information, in conjunction with available MESSENGER data, aid in understanding the Mercury flyby observations and provide a basis for global magnetospheric modeling. We find that during both flybys, the solar wind conditions were very quiescent and would have provided only modest dynamic driving forces for Mercury's magnetospheric system.     

Solar Energetic Particle Events in the 23rd Solar Cycle: Interplanetary Magnetic Field Configuration and Statistical Relationship with Flares and CMEs

We study the influence of the large-scale interplanetary magnetic field configuration on the solar energetic particles (SEPs) as detected at different satellites near Earth and on the correlation of their peak intensities with the parent solar activity. We selected SEP events associated with X- and M-class flares at western longitudes, in order to ensure good magnetic connection to Earth. These events were classified into two categories according to the global interplanetary magnetic field (IMF) configuration present during the SEP propagation to 1 AU: standard solar wind or interplanetary coronal mass ejections (ICMEs). Our analysis shows that around 20 % of all particle events are detected when the spacecraft is immersed in an ICME. The correlation of the peak particle intensity with the projected speed of the SEP-associated coronal mass ejection is similar in the two IMF categories of proton and electron events, ≈?0.6. The SEP events within ICMEs show stronger correlation between the peak proton intensity and the soft X-ray flux of the associated solar flare, with correlation coefficient r=0.67±0.13, compared to the SEP events propagating in the standard solar wind, r=0.36±0.13. The difference is more pronounced for near-relativistic electrons. The main reason for the different correlation behavior seems to be the larger spread of the flare longitude in the SEP sample detected in the solar wind as compared to SEP events within ICMEs. We discuss to what extent observational bias, different physical processes (particle injection, transport, etc.), and the IMF configuration can influence the relationship between SEPs and coronal activity.     

Geo-effectiveness of solar wind extremes

Examples of extreme events of solar wind and their effect on geomagnetic conditions are discussed here. It is found that there are two regimes of high speed solar wind streams with a threshold of ∼ 850 km s^-1. Geomagnetic activity enhancement rate (GAER) is defined as an average increase in Ap value per unit average increase in the peak solar wind velocity (Vp) during the stream. GAER was found to be different in the two regimes of high speed streams with +ve and-ve IMF. GAER is 0.73 and 0.53 for solar wind streams with +ve and -ve IMF respectively for the extremely high speed streams (< 850 km s^-1). This indicates that streams above the threshold speed with +ve IMF are 1.4 times more effective in enhancing geomagnetic activity than those with -ve IMF. However, the high speed streams below the threshold with -ve IMF are 1.1 times more effective in enhancing geomagnetic activity than those with +ve IMF. The violent solar activity period (October–November 2003) of cycle 23 presents a very special case during which many severe and strong effects were seen in the environment of the Earth and other planets; however, the z-component of IMF (Bz) is mostly positive during this period. The most severe geomagnetic storm of this cycle occurred when Bz was positive.     

A New Prediction Method for the Arrival Time of Interplanetary Shocks

Solar transient activities such as solar flares, disappearing filaments, and coronal mass ejections (CMEs) are solar manifestations of interplanetary (IP) disturbances. Forecasting the arrival time at the near Earth space of the associated interplanetary shocks following these solar disturbances is an important aspect in space weather forecasting because the shock arrival usually marks the geomagnetic storm sudden commencement (SSC) when the IMF Bz component is appropriately southward and/or the solar wind dynamic pressure behind the shock is sufficiently large. Combining the analytical study for the propagation of the blast wave from a point source in a moving, steady-state, medium with variable density (wei, 1982; wei and dryer 1991) with the energy estimation method in the ISPM model (smith and dryer 1990, 1995), we present a new shock propagation model (called SPM below) for predicting the arrival time of interplanetary shocks at Earth. The duration of the X-ray flare, the initial shock speed and the total energy of the transient event are used for predicting the arrival of the associated shocks in our model. Especially, the background speed, i.e., the convection effect of the solar wind is considered in this model. Applying this model to 165 solar events during the periods of January 1979 to October 1989 and February 1997 to August 2002, we found that our model could be practically equivalent to the prevalent models of STOA, ISPM and HAFv.2 in forecasting the shock arrival time. The absolute error in the transit time in our model is not larger than those of the other three models for the same sample events. Also, the prediction test shows that the relative error of our model is ≤10% for 27.88% of all events, ≤30% for 71.52%, and ≤50% for 85.46%, which is comparable to the relative errors of the other models. These results might demonstrate a potential capability of our model in terms of real-time forecasting.     

Photospheric and heliospheric magnetic fields

The magnetic field in the heliosphere evolves in response to the photospheric field at its base. This evolution, together with the rotation of the Sun, drives space weather through the continually changing conditions of the solar wind and the magnetic field embedded within it. We combine observations and simulations to investigate the sources of the heliospheric field from 1996 to 2001. Our algorithms assimilate SOHO/MDI magnetograms into a flux-dispersal model, showing the evolving field on the full sphere with an unprecedented duration of 5.5 yr and temporal resolution of 6 hr. We demonstrate that acoustic far-side imaging can be successfully used to estimate the location and magnitude of large active regions well before they become visible on the solar disk. The results from our assimilation model, complemented with a potential-field source-surface model for the coronal and inner-heliospheric magnetic fields, match Yohkoh/SXT and KPNO/He?10830 Å coronal hole boundaries quite well. Even subject to the simplification of a uniform, steady solar wind from the source surface outward, our model matches the polarity of the interplanetary magnetic field (IMF) at Earth ～3% of the time during the period 1997–2001 (independent of whether far-side acoustic data are incorporated into the simulation). We find that around cycle maximum, the IMF originates typically in a dozen disjoint regions. Whereas active regions are often ignored as a source for the IMF, the fraction of the IMF that connects to magnetic plage with absolute flux densities exceeding 50 Mx cm^?2 increases from ?10% at cycle minimum up to 30–50% at cycle maximum, with even direct connections between sunspots and the heliosphere. For the overall heliospheric field, these fractions are ?1% to 20–30%, respectively. Two case studies based on high-resolution TRACE observations support the direct connection of the IMF to magnetic plage, and even to sunspots. Parallel to the data assimilation, we run a pure simulation in which active regions are injected based on random selection from parent distribution functions derived from solar data. The global properties inferred for the photospheric and heliospheric fields for these two models are in remarkable agreement, confirming earlier studies that no subtle flux-emergence patterns or field-dispersal properties are required of the solar dynamo beyond those that are included in the model in order to understand the large-scale solar and heliospheric fields.

     

Recurrency and the origin of the vertical component of the interplanetary magnetic field

The autocorrelation functions of the solar wind velocity and of the IMF components as well as of the geomagnetic activity indices are studied for two periods: August–December, 1965 and January–May, 1974. The vertical component of the IMF is shown to exhibit a rather definite recurrency relatively independent of the recurrency of the solar wind velocity.The daily mean values of the Z-component of the IMF are shown to correlate ( = -0.5) with the intensity of the meridional component of the large scale solar magnetic field with time delay of about 5 days with respect to the latter. This result is interpreted as an evidence for the Z- component of the IMF to be carried away by the solar wind from the Sun.     

A statistical study of the interdependence of solar wind parameters

Correlation analysis of solar wind parameters, namely solar wind velocity, pro- ton density, proton temperature and mean interplanetary magnetic field (IMF) from the ACE spacecraft data near Earth, was done. To our best knowledge, this study is a novel one since we consider here only the parameters inside the solar wind, including the mean IMF and, hence, the solar wind is a self consistent system. We have proposed a Multiple Linear Regression (MLR) model for the prediction of the response variable (solar wind velocity) using the parameters proton density, proton temperature and mean IMF mea- sured as dally averages. About 60% of the observed value can be predicted using this model. It is shown that, in general, the correlation between solar wind parameters is sig- nificant. A deviation from the prediction at the solar maximum is interpreted. These results are verified by a graphical method.     

Multiwavelength Study on Solar and Interplanetary Origins of the Strongest Geomagnetic Storm of Solar Cycle 23

We study the solar sources of an intense geomagnetic storm of solar cycle 23 that occurred on 20 November 2003, based on ground- and space-based multiwavelength observations. The coronal mass ejections (CMEs) responsible for the above geomagnetic storm originated from the super-active region NOAA 10501. We investigate the H?? observations of the flare events made with a 15 cm solar tower telescope at ARIES, Nainital, India. The propagation characteristics of the CMEs have been derived from the three-dimensional images of the solar wind (i.e., density and speed) obtained from the interplanetary scintillation data, supplemented with other ground- and space-based measurements. The TRACE, SXI and H?? observations revealed two successive ejections (of speeds ???350 and ???100 km?s^?1), originating from the same filament channel, which were associated with two high speed CMEs (???1223 and ???1660 km?s^?1, respectively). These two ejections generated propagating fast shock waves (i.e., fast-drifting type II radio bursts) in the corona. The interaction of these CMEs along the Sun?CEarth line has led to the severity of the storm. According to our investigation, the interplanetary medium consisted of two merging magnetic clouds (MCs) that preserved their identity during their propagation. These magnetic clouds made the interplanetary magnetic field (IMF) southward for a long time, which reconnected with the geomagnetic field, resulting the super-storm (Dst _peak=?472 nT) on the Earth.     

Investigation on spectral behavior of Solar transients and their interrelationship

We probe the spectral hardening of solar flares emission in view of associated solar proton events (SEPs) at earth and coronal mass ejection (CME) acceleration as a consequence. In this investigation we undertake 60 SEPs of the Solar Cycle 23 along with associated Solar Flares and CMEs. We employ the X-ray emission in Solar flares observed by Reuven Ramaty Higly Energy Solar Spectroscopic Imager (RHESSI) in order to estimate flare plasma parameters. Further, we employ the observations from Geo-stationary Operational Environmental Satellites (GOES) and Large Angle and Spectrometric Coronagraph (LASCO), for SEPs and CMEs parameter estimation respectively. We report a good association of soft-hard-harder (SHH) spectral behavior of Flares with occurrence of Solar Proton Events for 16 Events (observed by RHESSI associated with protons). In addition, we have found a good correlation (R=0.71) in SEPs spectral hardening and CME velocity. We conclude that the Protons as well as CMEs gets accelerated at the Flare site and travel all the way in interplanetary space and then by re-acceleration in interplanetary space CMEs produce Geomagnetic Storms in geospace. This seems to be a statistically significant mechanism of the SEPs and initial CME acceleration in addition to the standard scenario of SEP acceleration at the shock front of CMEs.     

The solar wind during the Maunder minimum

It is presently believed that the high speed solar wind originates almost entirely in coronal holes. Theory suggests that the origin of the high speeds is extended energy deposition in proportion to the magnetic field intensity in the holes and at 1.5–3.0 solar radii heliocentric distance. Evidence from the time of the Maunder Minimum, together with the above results, allows a hypothesis to be made for the state of the solar wind at that time. Firstly, carbon-14 data indicate an enhanced cosmic ray intensity, with the conclusion that the interplanetary magnetic field (IMF) was smooth and perhaps of low intensity. Secondly, the apparent absence of a corona during eclipses requires low coronal density, suggesting an absence of closed magnetic loops. Thirdly, the absence of sunspots eliminates the possibility of a solar maximum type of corona of low emission intensity and implies a low large-scale photospheric field intensity. Finally, the absence of mid-latitude aurorae implies either that the solar wind speed or the IMF intensity or both, were low and not irregular.A resulting self-consistent hypothesis is that the solar wind was of the simplest variety, analogous to that described in models of the so-called “quiet solar wind”. All closed coronal field regions would have been absent and extended energy deposition in the corona would have been far less important than today. At 1 a.u., the density and speed would have been less than 5 cm^?3 and 300 km^?1s, respectively. At the same time, there would have been a very low level of fluctuation all the way from the microscale up to the contrast between high and low speed solar wind streams. Also, if the IMF is the source of the 22 yr and magnetic sector associated modulations in the present terrestrial climate, these modulations may have been suppressed during the Maunder Minimum. Recently, it has been discovered that the 22 yr modulation in fact was suppressed during the Maunder Minimum (C. Stockton and M. Mitchell, personal communication), in support of the above suggestion.     

Short-term variations of cosmic-ray intensity and flare related data in 1981–1983

A statistical analysis of the cosmic-ray intensity (CR) daily means, registered at three Neutron Monitor stations with different cut-off rigidities (Deep River, Climax and Alma-Ata), as well as, of the solar hard X-ray flares fluence recorded by Venera-13, -14 space-probes, has been performed for the time interval 1981–1983. Various methods of time series spectrum analysis, such as Fast Fourier Analysis (FFT) and Maximum Entropy (MESA), accompanied by appropriate statistical tests, have been employed to detect periodicities, while the method of Successive Approximations (SA) is used independently in order to define the amplitude and the phase of each fluctuation. New short-term periodicities of 100, 70, 50 and 32 days, in addition to the known ones of 152, 27 and 14 days, appeared in cosmic ray data. During this particular time interval, similar spectral behaviour has been reported in the solar hard X-ray flares data. The influence of the solar hard X-ray flares variability in the energy range 50–500 keV, expressed by their fluence values, upon the cosmic-ray modulation, is discussed.     

Observations of interplanetary scintillation with a single-station mode at Urumqi

The Sun affects physical phenomena on Earth in multiple ways.In particular,the material in interplanetary space comes from coronal expansion in the form of inhomogeneous plasma flow (solar wind),which is the primary source of the interplanetary medium.Ground-based Interplanetary Scintillation (IPS) observations are an important and effective method for measuring solar wind speed and the structures of small diameter radio sources.We discuss one mode of ground-based single-station observations: Single-Station Single-Frequency (SSSF) mode.To study the SSSF mode,a new system has been established at Urumqi Astronomical Observatory (UAO),China,and a series of experimental observations were successfully carried out from May to December,2008.