Interplanetary Scintillation Observations Research Articles

Determining source angular sizes from interplanetary-scintillation observations in the saturated regime

Interplanetary-scintillation observations of the radio source B0531+194 (J0534+1927) obtained over a wide range of elongations at 111 MHz using the Big Scanning Antenna of the Lebedev Physical Institute are presented. Near the Sun, the temporal spectra of the scintillations have a two-component form, corresponding to the superposition of refractive and diffractive scintillations that is characteristic of the saturated regime. A method for estimating the angular size of the scintillating component based on measurement of the break frequency in the diffractive part of the scintillation spectrum is presented. The scintillating component as a fraction of the total flux can be determined using the maximum scintillation index. The angular size of the scintillating component in B0531+194 is found to be 0.24″ ± 0.05″, and the ratio of the fluxes in the core and halo to be roughly one-third. The flux density in the compact radio component is 5 Jy. The estimated parameters of the angular structure of the source are compared with observations at other frequencies.

Radial Speed Evolution of Interplanetary Coronal Mass Ejections During Solar Cycle 23

We report radial speed evolution of interplanetary coronal mass ejections (ICMEs) detected by the SOHO/LASCO coronagraph, interplanetary scintillation (IPS) at 327 MHz, and in-situ observations. In this study, we analyze solar wind disturbance factor (g-value) data derived from IPS observations during 1997-2009 covering nearly the whole period of Solar Cycle 23. By comparing observations from the SOHO/LASCO, IPS, and in situ, we then identify 39 ICMEs that could be analyzed carefully. Here, we define two speeds V_SOHO and V_bg, which are initial speed of ICME and the speed of the background solar wind, respectively. Examinations of these speeds yield the following results; i) Fast ICMEs (with V_SOHO - V_bg > 500 km/s) rapidly decelerate, moderate ICMEs (with 0 km/s < V_SOHO - V_bg < 500 km/s) show either gradually decelerating or uniform motion, and slow ICMEs (with V_SOHO - V_bg < 0 km/s) accelerate. The radial speeds converge on the speed of background solar wind during their outward propagation. We subsequently find; ii) both the acceleration and deceleration are nearly complete by 0.79 (+/- 0.04) AU, and those are ended when the ICME speed reaches a given speed. We find the value of that to be 480 (+/- 21) km/s. iii) For the fast and moderate ICMEs, a linear equation with a constant coefficient is more appropriate than a quadratic equation to describe their kinematics, because the chi-square for the linear equation satisfies the statistical significance level of 0.05, while the quadratic one is not. These results support the hypothesis that the radial motion of ICMEs is governed by a drag force due to interaction with the background solar wind. These findings also suggest that ICMEs propagating faster than background solar wind are controlled mainly by the hydrodynamic Stokes drag.

Observations of Rapid Velocity Variations in the Slow Solar Wind

The technique of interplanetary scintillation (IPS) is the observation of rapid fluctuations of the radio signal from an astronomical compact source as the signal passes through the ever-changing density of the solar wind. Cross-correlation of simultaneous observations of IPS from a single radio source, received at multiple sites of the European Incoherent SCATter (EISCAT) radio antenna network, is used to determine the velocity of the solar wind material passing over the lines of sight of the antennas. Calculated velocities reveal the slow solar wind to contain rapid velocity variations when viewed on a time-scale of several minutes. Solar TErrestrial RElations Observatory (STEREO) Heliospheric Imager (HI) observations of white-light intensity have been compared with EISCAT observations of IPS to identify common density structures that may relate to the rapid velocity variations in the slow solar wind. We have surveyed a one-year period, starting in April 2007, of the EISCAT IPS observing campaigns beginning shortly after the commencement of full science operations of the STEREO mission in a bid to identify common density structures in both EISCAT and STEREO HI datasets. We provide a detailed investigation and presentation of joint IPS/HI observations from two specific intervals on 23 April 2007 and 19 May 2007 for which the IPS P-Point (point of closest approach of the line of sight to the Sun) was between 72 and 87 solar radii out from the Sun’s centre. During the 23 April interval, a meso-scale (of the order of 105 km or larger) transient structure was observed by HI-1A to pass over the IPS ray path near the P-Point; the observations of IPS showed a micro-scale structure (of the order of 102 km) within the meso-scale transient. Observations of IPS from the second interval, on 19 May, revealed similar micro-scale velocity changes, however, no transient structures were detected by the HIs during that period. We also pose some fundamental thoughts on the slow solar wind structure itself.

Three-dimensional exploration of the solar wind using observations of interplanetary scintillation.

The solar wind, a supersonic plasma flow continuously emanating from the Sun, governs the space environment in a vast region extending to the boundary of the heliosphere (∼100 AU). Precise understanding of the solar wind is of importance not only because it will satisfy scientific interest in an enigmatic astrophysical phenomenon, but because it has broad impacts on relevant fields. Interplanetary scintillation (IPS) of compact radio sources at meter to centimeter wavelengths serves as a useful ground-based method for investigating the solar wind. IPS measurements of the solar wind at a frequency of 327 MHz have been carried out regularly since the 1980s using the multi-station system of the Solar-Terrestrial Environment Laboratory (STEL) of Nagoya University. This paper reviews new aspects of the solar wind revealed from our IPS observations.

Interplanetary scintillations of an ensemble of radio sources during the solar activity minimum of cycles 23/24

The results of observations of interplanetary scintillations of a statistical ensemble of radio sources in the period of 2007–2011 are presented. Observation were carried out in the monitoring regime with the BSA LPI radio telescope at the frequency 111 MHz. Fluctuations of radio emission flux of all sources (a few hundred in total) were recorded 24 hours a day. Those sources were investigated, which had a scintillating flux greater than 0.2 Jy and fell within the sky band of 8° width in declination, corresponding to radio telescope’s 16-beam system. The statistical ensemble of radio sources is characterized by the mean variance of a scintillating radiation flux, which is proportional to the squared scintillation index. It follows from the obtained data that the radial dependence of a mean scintillation index during a deep solar activity minimum of 2008–2009 occurs to be weaker than one could expect in the case of spherically symmetric geometry of the solar wind. Suppression of a radial dependence of the mean scintillation index is explained by the effect of the heliospheric current sheet, which reveals itself in a high density of solar wind’s turbulent plasma in the helioequator plane. It is shown that the level of scintillations, averaged over monthly series of observations, was changing synchronously with the solar activity level.

Inclusion of Real-Time In-Situ Measurements into the UCSD Time-Dependent Tomography and Its Use as a Forecast Algorithm

The University of California, San Diego (UCSD) three-dimensional (3D) time-dependent tomography program, used for over a decade to reconstruct and forecast coronal mass ejections (CMEs), does so from observations of interplanetary scintillation (IPS) taken using the Solar-Terrestrial Environment Laboratory (STELab) radio arrays in Japan. An earlier article (Jackson et al. in Solar Phys. 265, 245, 2010) demonstrated how in-situ velocity measurements from the Advanced Composition Explorer (ACE) space-borne instrumentation can be used in addition to remote-sensing data to constrain a time-dependent tomographic velocity solution. Here we extend this in-situ inclusion to density measurements, and show how this constrains the tomographic density solution. Supplementing remote-sensing observations with in-situ measurements provides additional information to construct an iterated solar-wind parameter that is propagated outward from near the solar surface past the measurement location, and throughout the volume. As in the case of velocity when this is done, the largest changes within the volume are close to the radial directions around Earth that incorporate the in-situ measurements; the inclusion significantly reduces the uncertainty in extending these measurements to global 3D reconstructions that are distant in time and space from the spacecraft. At Earth, this analysis provides a finely tuned real-time result up to the latest time for which in-situ measurements are available, and enables more-accurate extension of these results near Earth to those remotely sensed. We show examples of this new algorithm using real-time STELab IPS data that were used in our forecasts throughout Carrington rotations 2010 through 2016, and we provide one metric prescription that we have used to determine the forecasting accuracy one, two, and three days in advance of the time data become available to analyze from STELab. We show that the accuracy is considerably better than assuming persistence of the same signal over one to two days in advance of when the data are available.

Equatorwards Expansion of Unperturbed, High-Latitude Fast Solar Wind

We use dual-site radio observations of interplanetary scintillation (IPS) with extremely long baselines (ELB) to examine meridional flow characteristics of the ambient fast solar wind at plane-of-sky heliocentric distances of 24-85 solar radii (R\odot). Our results demonstrate an equatorwards deviation of 3-4{\deg} in the bulk fast solar wind flow direction over both northern and southern solar hemispheres during different times in the declining phase of Solar Cycle 23.

Observations of interplanetary scintillation in China

AbstractThe Sun affects the Earth in multiple ways. In particular, the material in interplanetary space comes from coronal expansion in the form of 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. In this paper we will discuss the IPS observations in China.

Interplanetary scintillation signatures in the inner heliosphere of the deepest solar minimum in the past 100 years

AbstractWe have used interplanetary scintillation (IPS) observations at 327 MHz spanning years 1983-2009 to study microturbulence levels in the inner heliosphere. We find that the microturbulence levels show a steady and significant drop in the entire inner heliosphere starting from around 1995. The fact that the solar polar fields have also shown a similar declining trend provides a consistent result showing the buildup to the solar minimum between the solar cycles 23 and 24, the deepest in the past 100 years, actually began more than a decade earlier.

An MHD simulation model of time‐dependent co‐rotating solar wind

We present a treatment of observation‐based time‐dependent boundary conditions for the inner boundary sphere in the time‐dependent three‐dimensional MHD simulations of the global solar wind. With this boundary treatment, we obtain super‐Alfvenic MHD solutions of time‐dependent co‐rotating solar wind structures. The boundary variables on the inner boundary sphere, at 50 solar radii in this study, are assumed to change linearly from one instant to the next. A new feature is that, in order to maintain the divergence‐free condition of the magnetic field, the changes of the time‐dependent boundary magnetic field are expressed as the potential field in a thin shell volume. The solar magnetic field data from the Wilcox Solar Observatory (WSO) and the solar wind speed data from the interplanetary scintillation (IPS) observations at Nagoya University, Japan, are used as the input boundary data. The solar wind simulated with the time‐dependent boundary condition is compared with the near‐Earth and Ulysses in situ measurement data and the solar wind simulated with the fixed boundary condition over a 7‐month period in 1991. Reasonable agreements with the in situ measurements are obtained. The differences between the two simulations in the interplanetary field line paths are significant. The three‐dimensional time‐dependent MHD solution of the global solar wind will help enhance space weather models and other fields in heliophysics.

IPS observation system for the Miyun 50 m radio telescope and its commissioning observation

Ground-based observation of Interplanetary Scintillation (IPS) is an important approach for monitoring solar wind. A ground-based IPS observation system has been newly implemented on a 50 m radio telescope at Miyun station, managed by the National Astronomical Observatories, Chinese Academy of Sciences. This observation system has been constructed for the purpose of observing solar wind speed and the associated scintillation index by using the normalized cross-spectrum of a simultaneous dual-frequency IPS measurement. The system consists of a universal dual-frequency front-end and a dual-channel multi-function back-end specially designed for IPS. After careful calibration and testing, IPS observations on source 3C 273B and 3C 279 have been successfully carried out. The preliminary observation results show that this newly-developed observation system is capable of performing IPS observation. The system's sensitivity for IPS observation can reach over 0.3 Jy in terms of an IPS polarization correlator with 4 MHz bandwidth and 2 s integration time.

Long‐term evolution in the global distribution of solar wind speed and density fluctuations during 1997–2009

Interplanetary scintillation (IPS) observations made with the 327‐MHz multistation system of the Solar‐Terrestrial Environment Laboratory (STEL) are analyzed to investigate the global distribution of solar wind speed and density fluctuations (ΔNe) and their evolution during 1997–2009. This study aims at elucidating the evolution of ΔNe distribution during the cycle 23 and subsequent extended minimum, which is useful for improving understanding of the heliospheric response to the peculiar solar activity. The computer‐assisted tomography (CAT) method is used in the present study to deconvolve the line‐of‐sight integration of STEL IPS observations. This CAT method enables retrieval of the quasi‐stationary large‐scale structure of the background solar wind. The results show that the high (low)‐latitude region is dominated by reduced (enhanced) ΔNe plasma, being closely associated with the fast (slow) solar wind. The solar wind speed data show a distinct change with solar activity, and an excellent positive (negative) correlation is revealed between the fast (slow) wind area and the polar field strength of the Sun. In contrast, the ΔNe data do not show such a solar cycle variation, but instead reveal a significant increase in the fractional area of low‐ΔNe region in 2004 preceded by a constant value with a small amount of fluctuation. This change is observed for all latitudes, distinctly after 2007 for low latitudes. Our finding is consistent with the long‐term variation of the solar wind density revealed from in situ measurements at the Earth orbit, if ΔNe ∝ Ne (where Ne is the solar wind electron density), and also consistent with the coronal hole distribution during the last solar cycle. It is found that ΔNe is inversely correlated with the solar wind speed V. We obtain the best fit power law function ΔNe ∝ V−0.36±0.14 for V &gt; 350 km/s, which is basically consistent with our earlier result. This fact suggests that the fractional density fluctuations ΔNe/Ne are greater in the fast wind than in the slow wind. There is no systematic variation in the power law index or slope during 1997–2009 except for a null slope in 2000, which may be ascribed to an insufficient resolution of the CAT analysis. Thus, the inverse relation between ΔNe and V is regarded as a general rule for solar wind turbulence. The important point to note is that a marked drop in ΔNe occurs for the slow speed wind, V &lt; 350 km/s, particularly for 2004 and 2009. This fact may be attributed to different source conditions of the very‐low‐speed solar wind.

Heliolatitude and Time Variations of Solar Wind Structure from in situ Measurements and Interplanetary Scintillation Observations

The 3D structure of the solar wind and its evolution in time are needed for heliospheric modeling and interpretation of energetic neutral atoms observations. We present a model to retrieve the solar wind structure in heliolatitude and time using all available and complementary data sources. We determine the heliolatitude structure of solar wind speed on a yearly time grid over the past 1.5 solar cycles based on remote-sensing observations of interplanetary scintillations, in situ out-of-ecliptic measurements from Ulysses, and in situ in-ecliptic measurements from the OMNI 2 database. Since in situ out-of-ecliptic information on the solar wind density structure is not available apart from the Ulysses data, we derive correlation formulae between the solar wind speed and density and use the information on the solar wind speed from interplanetary scintillation observations to retrieve the 3D structure of the solar wind density. With the variations of solar wind density and speed in time and heliolatitude available, we calculate variations in solar wind flux, dynamic pressure, and charge-exchange rate in the approximation of stationary H atoms.

The Dynamic Spectrum of Interplanetary Scintillation: First Solar Wind Observations on LOFAR

The LOw Frequency ARray (LOFAR) is a next-generation radio telescope which uses thousands of stationary dipoles to observe celestial phenomena. These dipoles are grouped in various ‘stations’ which are centred on the Netherlands with additional ‘stations’ across Europe. The telescope is designed to operate at frequencies from 10 to 240 MHz with very large fractional bandwidths (25 – 100 %). Several ‘beam-formed’ observing modes are now operational and the system is designed to output data with high time and frequency resolution, which are highly configurable. This makes LOFAR eminently suited for dynamic spectrum measurements with applications in solar and planetary physics. In this paper we describe progress in developing automated data analysis routines to compute dynamic spectra from LOFAR time–frequency data, including correction for the antenna response across the radio frequency pass-band and mitigation of terrestrial radio-frequency interference (RFI). We apply these data routines to observations of interplanetary scintillation (IPS), commonly used to infer solar wind velocity and density information, and present initial science results.

The US‐UK Space Weather Workshop

The US‐UK Space Weather Workshop

Ionospheric disturbances detected by MEXART

The radio telescope MEXART was developed to make observations of interplanetary scintillation (IPS) produced by large scale disturbances associated with solar events. In this work it is shown that on occasion there are disturbances in the ionosphere that are related with these events and which cannot only contaminate the IPS but actually be the main contribution to the observed oscillations. This was the case of the event of 15 December 2006 observed by MEXART, which presented clear scintillation. The total electron content (TEC) of the ionosphere above Mexico was calculated for the same period. It was found that the variations in TEC were associated with the scintillations detected by MEXART.

Interplanetary scintillations of strong radio sources in the descending phase near the cycle 23 minimum

Observations of interplanetary scintillations of the 3C 298 and 3C 48 radio sources during low solar activity, performed with a BSA FIAN radio telescope at a frequency of 111 MHz, are presented. The radial dependences of the scintillation indices, where the effect of a low-latitude heliospheric current sheet is observed, have been obtained. Based on the scintillation temporal spectra, the solar wind velocity values have been obtained, and it has been indicated that these values are in good agreement with those found using the spaced measurements method.

Two-Station Interplanetary Scintillation Measurements of Solar Wind Speed near the Sun Using the X-band Radio Signal of the Nozomi Spacecraft

Interplanetary scintillation (IPS) measurements of the solar wind speed for the distance range between 13 and 37 R S were carried out during the solar conjunction of the Nozomi spacecraft in 2000 – 2001 using the X-band radio signal. Two large-aperture antennas were employed in this study, and the baseline between the two antennas was several times longer than the Fresnel scale for the X-band. We successfully detected a positive correlation of IPS from the cross-correlation analysis of received signal data during ingress, and estimated the solar wind speed from the time lag corresponding to the maximum correlation by assuming that the solar wind flows radially. The speed estimates range between 200 and 540 km s−1 with the majority below 400 km s−1. We examined the radial variation in the solar wind speed along the same streamline by comparing the Nozomi data with data obtained at larger distances. Here, we used solar wind speed data taken from 327 MHz IPS observations of the Solar-Terrestrial Environment Laboratory (STEL), Nagoya University, and in situ measurements by the Advanced Composition Explorer (ACE) for the comparison, and we considered the effect of the line-of-sight integration inherent to IPS observations for the comparison. As a result, Nozomi speed data were proven to belong to the slow component of the solar wind. Speed estimates within 30 R S were found to be systematically slower by 10 – 15 % than the terminal speeds, suggesting that the slow solar wind is accelerated between 13 and 30 R S.

Shrinking lakes of the Arctic: Spatial relationships and trajectory of change

Extensive interplanetary scintillation (IPS) observations at 327 MHz obtained between 1983 and 2009 clearly show a steady and significant drop in the turbulence levels in the entire inner heliosphere starting from around ~1995. We believe that this large-scale IPS signature, in the inner heliosphere, coupled with the fact that solar polar fields have also been declining since ~1995, provide a consistent result showing that the buildup to the deepest minimum in 100 years actually began more than a decade earlier.

The prelude to the deep minimum between solar cycles 23 and 24: Interplanetary scintillation signatures in the inner heliosphere

Abstract Extensive interplanetary scintillation (IPS) observations at 327 MHz obtained between 1983 and2009 clearly show a steady and significant drop in the turbule nce levels in the entire inner heliospherestarting from around ∼1995. We believe that this large-scale IPS signature, in theinner heliosphere,coupledwith thefact thatsolarpolarfields have alsobeen de cliningsince∼1995, providea consistentresult showing that the buildup to the deepest minimum in 100 years actually began more than adecade earlier. Introduction The sunspot minimum at the end of Cycle 23, has been one of the deepest we have experienced inthe past 100 years with the first spots of the new cycle 24 appea ring only in March 2010 instead ofDecember 2008 as was expected. Also, the number of spotless days experienced in 2008 and 2009was over 70%. Apart from this, Cycle 23 has shown a slower than average field reversal, a slowerrise to maximum than other odd numbered cycles, and a second maximum during the declining phasethat is unusual for odd-numbered cycles. Though these deviations from “normal” behaviour couldbe significant in understanding the evolution of magnetic fie lds on the Sun, they do not yield anydirect insights into the onset of the deep minimum experienced at the end of cycle 23. This is becausepredictions of the strength of solar cycles and the nature of their minima are strongly dictated by boththe strength of the ongoing cycle [Dikpati et al. (2006); Choudhuri et al. (2007)] and changes in theflow rates of the meridional circulation [ Nandy et al. (2011)].1