Munetoshi Tokumaru

Improvements to the HAF solar wind model for space weather predictions

We have assembled and tested, in real time, a space weather modeling system that starts at the Sun and extends to the Earth through a set of coupled, modular components. We describe recent efforts to improve the Hakamada-Akasofu-Fry (HAF) solar wind model that is presently used in our geomagnetic storm prediction system. We also present some results of these improvement efforts. In a related paper, Akasofu [2001] discusses the results of the first 2 decades using this system as a research tool and for space weather predictions. One key goal of our efforts is to provide quantitative forecasts of geoeffective solar wind conditions at the L1 satellite point and at Earth. Notably, we are addressing a key problem for space weather research: the prediction of the north-south component (Bz) of the interplanetary magnetic field. This parameter is important for the transfer of energy from the solar wind to the terrestrial environment that results in space weather impacts upon society. We describe internal improvements, the incorporation of timely and accurate boundary conditions based upon solar observations, and the prediction of solar wind speed, density, magnetic field, and dynamic pressure. HAF model predictions of shock arrival time at the L1 satellite location are compared with the prediction skill of the two operational shock propagation models: the interplanetary shock propagation model (ISPM) and the shock-time-of-arrival (STOA) model. We also show model simulations of shock propagation compared with interplanetary scintillation observations. Our modeling results provide a new appreciation of the importance of accurately characterizing event drivers and for the influences of the background heliospheric plasma on propagating interplanetary disturbances.

Heliospheric tomography using interplanetary scintillation observations: 2. Latitude and heliocentric distance dependence of solar wind structure at 0.1–1 AU

Interplanetary scintillation is a useful means to measure the solar wind in regions inaccessible to in situ observation. However, interplanetary scintillation measurements involve a line-of-sight integration, which relates contributions from all locations along the line of sight to the actual observation. We have developed a computer assisted tomography (CAT) program to reduce the adverse effects of the line-of-sight integration. The program uses solar rotation and solar wind motion to provide three-dimensional perspective views of each point in space accessible to the interplanetary scintillation observations and optimizes a three-dimensional solar wind speed distribution to fit the observations. We analyzed IPS speeds observed at the Solar-Terrestrial Environment Laboratory and confirmed that (1) the solar wind during the solar minimum phase has a dominant polar high-speed solar wind region with speeds of about 800 km s−1 and within 30° of the solar equator speeds decrease to 400 km s−1 as observed by Ulysses, and (2) high-speed winds get their final speed of 750–900 km s−1 within 0.1 AU, and consequently, that acceleration of the solar wind is small above 0.1 AU.

Heliospheric tomography using interplanetary scintillation observations: 3. Correlation between speed and electron density fluctuations in the solar wind

We have examined the relationship between solar wind speed and electron density fluctuations on scale sizes around 100 km in the heliocentric distance range of 0.3 to 0.8 AU using interplanetary scintillation (IPS) data obtained at the Solar-Terrestrial Environment Laboratory. The solar wind properties derived from the IPS data are biased by line of sight integration through a three-dimensional structured solar wind. Therefore we have applied a computer-assisted tomography (CAT) method to deconvolve the line of sight integration and reconstruct the solar wind structure. The analysis was made for the solar wind speed V and electron density fluctuations δNe in the solar activity minimum phase when high-speed regions are separated from an equatorial low-speed region by a sharp velocity gradient. From results of the CAT analysis we derived the best fit power law relation of δNe ∝ V−γ with γ = 0.5 ± 0.15, indicating that fractional density fluctuations δNe/Ne in the high-speed wind are larger than those in the low-speed wind. Combining this relation with results of previous workers [Coles et al., 1995; Manoharan, 1993; Celnikier et al., 1987; Jackson et al., this issue], we suggest that the fractional density fluctuation level of the high-speed wind evolves with heliocentric distance.

THE FIRST THREE YEARS OF IBEX OBSERVATIONS AND OUR EVOLVING HELIOSPHERE

This study provides, for the first time, complete and validated observations from the first three years (2009-2011) of the Interstellar Boundary Explorer (IBEX) mission. Energetic neutral atom (ENA) fluxes are corrected for both the time-variable cosmic ray background and for orbit-by-orbit variations in their probability of surviving en route from the outer heliosphere in to 1 AU where IBEX observes them. In addition to showing all six six-month maps, we introduce new annual ram and anti-ram maps, which can be produced without the need for algorithm-dependent Compton-Getting corrections. Together, the ENA maps, data, and supporting documentation presented here support the full release of these data to the broader scientific community and provide the citable reference for them. In addition, we show that heliospheric ENA emissions have been decreasing over the epoch from 2009 to 2011 with the IBEX Ribbon decreasing by the largest fraction and only the heliotail (which is offset from the down wind direction by the interstellar magnetic field) showing essentially no reduction and actually some increase. Finally, we show how the much more complete observations provided here strongly indicate a quite direct and latitude-dependent solar wind source of the Ribbon.

Low‐speed solar wind from the vicinity of solar active regions

We have investigated the origin of low-speed winds observed in association with active regions near the equator at times of solar activity minimum. The solar wind velocity distribution on a source surface at 2.5 Rs is derived by interplanetary scintillation tomographic analysis, and compact low-speed regions in it are investigated in relation to active regions and large-flux-expansion regions. We show that although the low-speed regions tend to be located near active regions, they are more closely associated with large flux expansion from the vicinity of active regions. We find that slow solar wind does not arise from closed magnetic loops above an active region, but instead the low-speed stream originates from the vicinity of one polarity side of the active region. Therefore the low-speed stream, unlike the helmet streamer, has a single magnetic polarity. This can explain why compact low-speed streams are often not associated with a heliospheric current sheet.

IS THE POLAR REGION DIFFERENT FROM THE QUIET REGION OF THE SUN

Observations of the polar region of the Sun are critically important for understanding the solar dynamo and the acceleration of solar wind. We carried out precise magnetic observations on both the north polar region and the quiet Sun at the east limb with the spectropolarimeter of the Solar Optical Telescope aboard Hinode to characterize the polar region with respect to the quiet Sun. The average area and the total magnetic flux of the kilo-Gauss magnetic concentrations in the polar region appear to be larger than those of the quiet Sun. The magnetic field vectors classified as vertical in the quiet Sun have symmetric histograms around zero in the strengths, showing balanced positive and negative fluxes, while the histogram in the north polar region is clearly asymmetric, showing a predominance of the negative polarity. The total magnetic flux of the polar region is larger than that of the quiet Sun. In contrast, the histogram of the horizontal magnetic fields is exactly the same for both the polar region and the quiet Sun. This is consistent with the idea that a local dynamo process is responsible for the horizontal magnetic fields. A high-resolution potential field extrapolation shows that the majority of magnetic field lines from the kG-patches in the polar region are open with a fanning-out structure very low in the atmosphere, while in the quiet Sun, almost all the field lines are closed.

Three-dimensional propagation of interplanetary disturbances detected with radio scintillation measurements at 327 MHz

Interplanetary scintillation (IPS) measurements at 327 MHz have been used to study the three-dimensional propagation of interplanetary (IP) disturbances between 0.2 AU and near-Earth distance (∼ 1 AU). IPS data of four events, September 23 and August 24, 1998, and November 4 and 6, 1997, events, in which marked IP disturbances were observed in association with energetic solar flares, have been analyzed in this study. From the analysis of these events it is found that the propagation speed of IP disturbances varies with longitude and latitude. We also found that the fast speed direction does not necessarily agree with the flare normal direction. Slow propagation speeds of IP disturbances appear to be closely associated with the slow speed region of ambient solar wind. This suggests that the large-scale structure of the solar wind speed may play an important role in determining the propagation speed of IP disturbances, although further study is necessary.

Geometry of an interplanetary CME on October 29, 2003 deduced from cosmic rays

A coronal mass ejection (CME) associated with an X17 solar flare reached Earth on October 29, 2003, causing an ∼11% decrease in the intensity of high-energy Galactic cosmic rays recorded by muon detectors. The CME also produced a strong enhancement of the cosmic ray directional anisotropy. Based upon a simple inclined cylinder model, we use the anisotropy data to derive for the first rime the three-dimensional geometry of the cosmic ray depleted region formed behind the shock in this event. We also compare the geometry derived from cosmic rays with that derived from in situ interplanetary magnetic field (IMF) observations using a Magnetic Flux Rope model. Copyright 2004 by the American Geophysical Union.

Solar Parameters for Modeling the Interplanetary Background

The goal of the working group on cross-calibration of past and present ultraviolet (UV) datasets of the International Space Science Institute (ISSI) in Bern, Switzerland was to establish a photometric cross-calibration of various UV and extreme ultraviolet (EUV) heliospheric observations. Realization of this goal required a credible and up-to-date model of the spatial distribution of neutral interstellar hydrogen in the heliosphere, and to that end, a credible model of the radiation pressure and ionization processes was needed. This chapter describes the latter part of the project: the solar factors responsible for shaping the distribution of neutral interstellar H in the heliosphere. In this paper we present the solar Lyman-α flux and the topics of solar Lyman-α resonant radiation pressure force acting on neutral H atoms in the heliosphere. We will also discuss solar EUV radiation and resulting photoionization of heliospheric hydrogen along with their evolution in time and the still hypothetical variation with heliolatitude. Furthermore, solar wind and its evolution with solar activity is presented, mostly in the context of charge exchange ionization of heliospheric neutral hydrogen, and dynamic pressure variations. Also electron-impact ionization of neutral heliospheric hydrogen and its variation with time, heliolatitude, and solar distance is discussed. After a review of the state of the art in all of those topics, we proceed to present an interim model of the solar wind and the other solar factors based on up-to-date in situ and remote sensing observations. This model was used by Izmodenov et al. (2013, this volume) to calculate the distribution of heliospheric hydrogen, which in turn was the basis for intercalibrating the heliospheric UV and EUV measurements discussed in Quemerais et al. (2013, this volume). Results of this joint effort will also be used to improve the model of the solar wind evolution, which will be an invaluable asset in interpretation of all heliospheric measurements, including, among others, the observations of Energetic Neutral Atoms by the Interstellar Boundary Explorer (IBEX).

A study of density fluctuations in the solar wind acceleration region

Interplanetary scintillation (IPS) measurements of the density fluctuations in the fast and slow solar wind were made at 2 GHz and 8 GHz in 1994 at the Kashima Space Research Center, Communications Research Laboratory. Using the observations which cover the distance range from 5 to 76 solar radii (R s ), we investigate the radial evolution of the dissipation scale length of the density fluctuations, the so-called inner scale. Our IPS observations reveal that the inner scale shows different radial profiles between the inside and outside of the acceleration region. The size of the inner scale outside 25 R s increases linearly with radial distance, showing good agreement with previous observations made at r ≥ 50 R s . The inner scale inside 25 R s , on the other hand. deviates from the linear relation. We simulate the radial variation of the inner scale using a solar wind acceleration model and then compare the results to that of the observed inner scale. We find that, in the low-speed solar wind. the radial profile of the computed inner scale is in good agreement with that of the observed inner scale and that the solar wind acceleration causes the deviation of the inner scale from a linear relation. However, in the high-speed wind, we cannot reproduce the radial profile of the observed inner scale with the acceleration model, even though the line-of-sight integration effect on IPS measurements is taken into account. We propose that this disagreement is due to the density and magnetic field fluctuations not being correlated in the high-speed solar wind, as shown at greater heliocentric distances by Helios measurements.