arXiv subjects

K. Fujiki

Publications and source records attributed to K. Fujiki.

16 recordsLinked to original sources

arXiv2026

Measurement of dijet transverse momentum imbalance and azimuthal acoplanarity in $p$+$p$ collisions at $\sqrt{s} = 200$ GeV with the sPHENIX detector

This Letter reports on measurements of dijet transverse momentum ($p_\mathrm{T}$) imbalance and azimuthal acoplanarity in proton-proton collisions at $\sqrt{s} = 200$~GeV, using data recorded by the sPHENIX detector at the Relativistic Heavy Ion Collider corresponding to an integrated luminosity of $41$~pb$^{-1}$. Jets are reconstructed using the anti-$k_t$ algorithm with radius parameters $R = 0.3$ to $0.8$ from electromagnetic and hadronic calorimeter energy deposits. The jet $p_\mathrm{T}$ resolution is determined directly in data using two independent methods. The dijet $p_\mathrm{T}$ imbalance is characterized by the ratio $x_\mathrm{J} = p_\mathrm{T,2}/p_\mathrm{T,1}$ where $p_\mathrm{T,1(2)}$ is the highest (second-highest) jet $p_\mathrm{T}$ in the event. The dijet azimuthal acoplanarity $\Delta\phi = |\phi_1 - \phi_2|$ is also reported. Results are reported for different $p_\mathrm{T,1}$ selections and jet radius parameters, normalized per dijet pair, and compared to the results of \textsc{Pythia} and \textsc{Herwig} Monte Carlo event generators. These measurements provide a stringent quantitative test of the modeling of QCD parton shower and hadronization dynamics, place important constraints on event-generator descriptions at RHIC energies, and establish a comprehensive proton-proton baseline for forthcoming measurements of jet modification in heavy ion collisions.

nucl-ex↗

arXiv2025

Measurement of charged hadron multiplicity in Au+Au collisions at $\sqrt{\text{s}_{\text{NN}}} = 200$ GeV with the sPHENIX detector

The pseudorapidity distribution of charged hadrons produced in Au+Au collisions at a center-of-mass energy of $\sqrt{s_\mathrm{NN}} = 200$ GeV is measured using data collected by the sPHENIX detector. Charged hadron yields are extracted by counting cluster pairs in the inner and outer layers of the Intermediate Silicon Tracker, with corrections applied for detector acceptance, reconstruction efficiency, combinatorial pairs, and contributions from secondary decays. The measured distributions cover $|\eta| < 1.1$ across various centralities, and the average pseudorapidity density of charged hadrons at mid-rapidity is compared to predictions from Monte Carlo heavy-ion event generators. This result, featuring full azimuthal coverage at mid-rapidity, is consistent with previous experimental measurements at the Relativistic Heavy Ion Collider, thereby supporting the broader sPHENIX physics program.

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arXiv2025

Measurement of the transverse energy density in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV with the sPHENIX detector

This paper reports measurements of the transverse energy per unit pseudorapidity ($dE_{T}/d\eta$) produced in Au+Au collisions at $\sqrt{s_{NN}} = 200$ GeV, performed with the sPHENIX detector at the Relativistic Heavy Ion Collider (RHIC). The results cover the pseudorapidity range $\left|\eta\right| < 1.1$ and constitute the first such measurement performed using a hadronic calorimeter at RHIC. Measurements of $dE_{T}/d\eta$ are presented for a range of centrality intervals and the average $dE_{T}/d\eta$ as a function of the number of participating nucleons, $N_{\mathrm{part}}$, is compared to a variety of Monte Carlo heavy-ion event generators. The results are in agreement with previous measurements at RHIC, and feature an improved granularity in $\eta$ and improved precision in low-$N_{\mathrm{part}}$ events.

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arXiv2025

The Ladder and Readout Cables of Intermediate Silicon Strip Detector for sPHENIX

A new silicon-strip-type detector was developed for precise charged-particle tracking in the central rapidity region of heavy ion collisions. A new detector and collaboration at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory is sPHENIX, which is a major upgrade of the PHENIX detector. The intermediate tracker (INTT) is part of the advanced tracking system of the sPHENIX detector complex together with a CMOS monolithic-active-pixel-sensor based silicon-pixel vertex detector, a time-projection chamber, and a micromegas-based detector. The INTT detector is barrel shaped and comprises 56 silicon ladders. Two different types of strip sensors of 78~$\mu m$ pitch and 320~$\mu m$ thick are mounted on each half of a silicon ladder. Each strip sensor is segmented into 8$\times$2 and 5$\times$2 blocks with lengths of 16 and 20 mm. Strips are read out with a silicon strip-readout (FPHX) chip. In order to transmit massive data from the FPHX to the down stream readout electronics card (ROC), a series of long and high speed readout cables were developed. This document focuses on the silicon ladder, the readout cables, and the ROC of the INTT. The radiation hardness is studied for some parts of the INTT devices in the last part of this document, since the INTT employed some materials from the technology frontier of the industry whose radiation hardness is not necessarily well known.

physics.ins-det↗

arXiv2024

Prolonged and Extremely Non-radial Solar Wind Flows

We present a study of three highly non-radial solar wind events when the azimuthal solar wind flow angle exceeds > 6 degrees for one day or more. None of the events are associated with coronal mass ejections and co-rotating interaction regions observed at 1 AU. For all events, the solar wind outflows at 1 AU have low solar wind velocity and solar wind density. Based on the significant increase in the Oxygen charge state ratio of O7+/O6+ at 1 AU for all of the events, we have traced them back to the Sun and found that their source regions originated in an active region and coronal hole (AR-CH) pairs mainly located at the central meridian. Further, examining the dynamical evolutions in their source regions using both the Extreme ultra-violet Imaging Telescope and Michelson Doppler Imager, it is found that the changes taking place in AR-CH boundaries eventually disturbed the stable CH configurations, resulting in a reduction of the CH area and finally its disappearance, leaving only with the AR. Our study provides a possible explanation to discuss the origin of the prolonged and highly non-radial solar wind flows.

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arXiv2020

Global Solar Magnetic-field and Interplanetary Scintillations During the Past Four Solar Cycles

The extended minimum of Solar Cycle 23, the extremely quiet solar-wind conditions prevailing, and the mini-maximum of Solar Cycle 24 drew global attention and many authors have since attempted to predict the amplitude of the upcoming Solar Cycle 25, which is predicted to be the third successive weak cycle; it is a unique opportunity to probe the Sun during such quiet periods. Earlier work has established a steady decline, over two decades, in solar photospheric fields at latitudes above $45^{\circ}$ and a similar decline in solar-wind micro-turbulence levels as measured by interplanetary scintillation (IPS) observations. However, the relation between the photospheric magnetic fields and those in the low corona/solar-wind are not straightforward. Therefore, in the present article, we have used potential-field source-surface (PFSS) extrapolations to deduce global magnetic-fields using synoptic magnetograms observed with National Solar Observatory (NSO), Kitt Peak, USA (NSO/KP) and Solar Optical Long-term Investigation of the Sun (NSO/SOLIS) instruments during 1975-2018. Furthermore, we have measured the normalized scintillation index [m] using the IPS observations carried out at the Institute of Space Earth Environment Research (ISEE), Japan during 1983-2017. From these observations, we have found that, since the mid-1990s, the magnetic-field over different latitudes at 2.5 $\rm R_{\odot}$ and 10 $\rm R_{\odot}$(extrapolated using PFSS method) has decreased by $\approx 11.3-22.2 \%$. In phase with the declining magnetic-fields, the quantity m also declined by $\approx 23.6 \%$. These observations emphasize the inter-relationship between the global magnetic-field and various turbulence parameters in the solar corona and solar wind.

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arXiv2018

Solar cycle 24: an unusual polar field reversal

Aims: To investigate solar polar fields during cycle 24, using measurements of solar magnetic fields in the latitude range 55 - 90 degree and 78 - 90 degree, to report a prolonged and unusual hemispheric asymmetry in the polar field reversal pattern in solar cycle 24. Methods: This study was carried out using medium resolution line-of-sight synoptic magnetograms from the magnetic database of the National Solar Observatory at Kitt Peak (NSO/KP), USA for the period between February 1975 and October 2017, covering solar cycles 21-24 and high-resolution line-of-sight synoptic magnetograms from the Michaelson Doppler Imager instrument onboard the Solar Heliospheric Observatory. Synoptic magnetograms using radial measurements from the Heliospheric Magnetic Imager instrument onboard the Solar Dynamics Observatory, covering solar cycle 23 and 24, were also used. Results: We show that the Southern solar hemisphere unambiguously reversed polarity in mid-2013 while the reversal in the field in the Northern solar hemisphere started as early as June 2012, was followed by a sustained period of near-zero field strength lasting until the end of 2014, after which the field began to show a clear rise from its near-zero value. While this study compliments a similar study carried out using microwave brightness measurements (Gopalswamy et al. 2016) which claimed that the field reversal process in cycle 24 was completed by the end of 2015, our results show that the field reversal in cycle 24 was completed earlier i.e. in late 2014. Signatures of this unusual field reversal pattern were also clearly identifiable in the solar wind, using our observations of interplanetary scintillation at 327 MHz which supported our magnetic field observations and confirmed that the field reversal process was completed at the end of 2014.

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arXiv2017

The response of the terrestrial bow shock and magnetopause of the long term decline in solar polar fields

The location of the terrestrial magnetopause (MP) and it's subsolar stand-off distance depends not only on the solar wind dynamic pressure and the interplanetary magnetic field (IMF), both of which play a crucial role in determining it's shape, but also on the nature of the processes involved in the interaction between the solar wind and the magnetosphere. The stand-off distance of the earth's MP and bow shock (BS) also define the extent of terrestrial magnetic fields into near-earth space on the sunward side and have important consequences for space weather. However, asymmetries due to the direction of the IMF are hard to account for, making it nearly impossible to favour any specific model over the other in estimating the extent of the MP or BS. Thus, both numerical and empirical models have been used and compared to estimate the BS and MP stand-off distances as well as the MP shape, in the period Jan. 1975-Dec. 2016, covering solar cycles 21-24. The computed MP and BS stand-off distances have been found to be increasing steadily over the past two decades, since ~1995, spanning solar cycles 23 and 24. The increasing trend is consistent with earlier reported studies of a long term and steady decline in solar polar magnetic fields and solar wind micro-turbulence levels. The present study, thus, highlights the response of the terrestrial magnetosphere to the long term global changes in both solar and solar wind activity, through a detailed study of the extent and shape of the terrestrial MP and BS over the past four solar cycles, a period spanning the last four decades.

physics.space-ph↗

arXiv2016

A Prolonged Southward IMF-Bz Event of May 02--04, 1998: Solar, Interplanetary Causes and Geomagnetic Consequences

A detailed investigation was carried out to understand a prolonged (~44 hours) weakly southward interplanetary magnetic field (IMF-Bz) condition during May 02--04, 1998. In-situ observations, during the period, showed the passage of an expanding magnetic cloud embedded in an interplanetary coronal mass ejection (ICME), followed up by a shock and an interplanetary discontinuity driven by another ICME. It is the arrival of the ICMEs and the upfront shocks that cause the prolonged southward IMF-Bz condition. The magnetic configuration of the source regions of the IMF associated with the ICME interval were also examined, which showed open magnetic field structures, emanating from a small active region on the north of the heliospheric current sheet (HCS). The structures remained constantly to the north of the HCS, both on April 29 and May 01, suggesting no change in their polarity. The draping of these outward directed radial field lines around the propagating CMEs in the shocked plasma explained the observed polarity changes of the IMF-Bz at 1 AU. In addition, multiple enhancements were also detected in the geomagnetic field variations, which showed a distinct one-to-one correspondence with the density pulses observed at 1 AU, during 0700--1700 UT on May 03. The spectral analysis of both the variations showed the same discrete frequencies of 0.48, 0.65 and 0.75 mHz, demonstrating that the solar wind density enhancements can cause detectable global geomagnetic disturbances. The observations, thus, provide a deeper insight into the possible causes and geomagnetic consequences of a prolonged weakly southward IMF-Bz condition.

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arXiv2015

A Twenty Year Decline in Solar Photospheric Magnetic Fields: Inner-Heliospheric Signatures and Possible Implications?

We report observations of a steady 20 year decline of solar photospheric fields at latitudes $\geq$45${^{o}}$ starting from $\sim$1995. This prolonged and continuing decline, combined with the fact that Cycle 24 is already past its peak, implies that magnetic fields are likely to continue to decline until $\sim$2020, the expected minimum of the ongoing solar Cycle 24. In addition, interplanetary scintillation (IPS) observations of the inner heliosphere for the period 1983--2013 and in the distance range 0.2--0.8 AU, have also shown a similar and steady decline in solar wind micro-turbulence levels, in sync with the declining photospheric fields. Using the correlation between the polar field and heliospheric magnetic field (HMF) at solar minimum, we have estimated the value of the HMF in 2020 to be 3.9 ($\pm$0.6) and a floor value of the HMF of $\sim$3.2 ($\pm$0.4) nT. Given this floor value for the HMF, our analysis suggests that the estimated peak sunspot number for solar Cycle 25 is likely to be $\sim$ 62 ($\pm$12).

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arXiv2014

A study of density modulation index in the inner heliospheric solar wind during solar cycle 23

The ratio of the rms electron density fluctuations to the background density in the solar wind (density modulation index, $ε_{N} \equiv Δ{N}/N$) is of vital importance in understanding several problems in heliospheric physics related to solar wind turbulence. In this paper, we have investigated the behavior of $ε_{N}$ in the inner-heliosphere from 0.26 to 0.82 AU. The density fluctuations $Δ{N}$ have been deduced using extensive ground-based observations of interplanetary scintillation (IPS) at 327 MHz, which probe spatial scales of a few hundred km. The background densities ($N$) have been derived using near-Earth observations from the Advanced Composition Explorer ($\it{ACE}$). Our analysis reveals that $0.001 \lesssim ε_{N} \lesssim 0.02$ and does not vary appreciably with heliocentric distance. We also find that $ε_{N}$ declines by 8% from 1998 to 2008. We discuss the impact of these findings on problems ranging from our understanding of Forbush decreases to the behavior of the solar wind dynamic pressure over the recent peculiar solar minimum at the end of cycle 23..

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arXiv2014

Kinematic Properties of Slow ICMEs and an Interpretation of a Modified Drag Equation for Fast and Moderate ICMEs

We report kinematic properties of slow interplanetary coronal mass ejections (ICMEs) identified by SOHO/LASCO, interplanetary scintillation, and in-situ observations, and propose a modified equation for the ICME motion. We identify seven ICMEs between 2010 and 2011, and examine them with 39 events reported in our previous work. We examine 15 fast (V_SOHO - V_bg > 500 km/s), 25 moderate (0 km/s <= V_SOHO - V_bg <= 500 km/s), and 6 slow (V_SOHO - V_bg < 0 km/s) ICMEs, where V_SOHO and V_bg are the initial speed of ICMEs and the speed of the background solar wind, respectively. For slow ICMEs, we found the following results: i) They accelerate toward the speed of the background solar wind during their propagation, and reach their final speed by 0.34 (+-) 0.03 AU. ii) The acceleration ends when they reach 479 (+-) 126 km/s; this is close to the typical speed of the solar wind during the period of this study. iii) When k_1 and k_2 are assumed to be constants, a quadratic equation for the acceleration a = -k_2(V - V_bg)|V - V_bg| is more appropriate than a linear one a = -k_1(V - V_bg), where V is the propagation speed of ICMEs, while the latter gives a smaller chi-square value than the former. For the motion of the fast and moderate ICMEs, we found a modified drag equation. From the viewpoint of fluid dynamics, we interpret this equation as indicating that ICMEs with 0 km/s <= V - V_bg <= 2300 km/s are controlled mainly by the hydrodynamic Stokes drag force, while the aerodynamic drag force is a predominant factor for the propagation of ICME with V - V_bg > 2300 km/s.

physics.space-ph↗

arXiv2012

Heliolatitude and time variations of solar wind structure from in situ measurements and interplanetary scintillation observations

The 3D structure of solar wind and its evolution in time is 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 the in situ information on the solar wind density structure out of ecliptic is not available apart from the Ulysses data, we derive correlation formulae between solar wind speed and density and use the information on the solar wind speed from interplanetary scintillation observations to retrieve the 3D structure of 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.

astro-ph.SR↗

arXiv2011

Solar parameters for modeling interplanetary background

The goal of the Fully Online Datacenter of Ultraviolet Emissions (FONDUE) Working Team of the International Space Science Institute in Bern, Switzerland, was to establish a common calibration of various UV and EUV heliospheric observations, both spectroscopic and photometric. Realization of this goal required an up-to-date model of 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 solar factors shaping the distribution of neutral interstellar H in the heliosphere. Presented are the solar Lyman-alpha flux and the solar Lyman-alpha resonant radiation pressure force acting on neutral H atoms in the heliosphere, solar EUV radiation and the photoionization of heliospheric hydrogen, and their evolution in time and the still hypothetical variation with heliolatitude. Further, solar wind and its evolution with solar activity is presented in the context of the charge exchange ionization of heliospheric hydrogen, and in the context of dynamic pressure variations. Also the electron ionization and its variation with time, heliolatitude, and solar distance is presented. After a review of all of those topics, we present an interim model of solar wind and the other solar factors based on up-to-date in situ and remote sensing observations of solar wind. Results of this effort will further be utilised to improve on the model of solar wind evolution, which will be an invaluable asset in all heliospheric measurements, including, among others, the observations of Energetic Neutral Atoms by the Interstellar Boundary Explorer (IBEX).

astro-ph.SR↗

arXiv2011

The Prelude to the Deep Minimum between Solar Cycles 23 and 24: Interplanetary Scintillation Signatures in the Inner Heliosphere

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.

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arXiv2006

International Colloquium "Scattering and Scintillation in Radio Astronomy" was held on June 19-23, 2006 in Pushchino, Moscow region, Russia

Topics of the Colloquium: a) Interplanetary scintillation b) Interstellar scintillation c) Modeling and physical origin of the interplanetary and the interstellar plasma turbulence d) Scintillation as a tool for investigation of radio sources e) Seeing through interplanetary and interstellar turbulent media Ppt-presentations are available on the Web-site: http://www.prao.ru/conf/Colloquium/main.html

astro-ph↗