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We have studied the ionospheric response to a coronal hole event of August 2020 using the data from global ionospheric maps, ground magnetometers and parameters from the instruments onboard SWARM and thermosphere, ionosphere, mesosphere energetics and dynamics satellites. The role of different physical drivers, responsible for observed ionospheric disturbance, has been identified. On the storm day (2 August), a steady southward directed interplanetary magnetic field Bz caused the penetration of magnetospheric convection electric field and positive storm effect in daytime sectors. As a result of prompt penetration electric field (PPEF) on 2 August, a polar‐ward expansion of day‐side ionospheric plasma. On 3 August, the signatures of both disturbance dynamo electric field (DDEF) caused by disturbed thermospheric winds and PPEF have been observed. The westward PPEF and eastward disturbance DDEF on the night‐side caused a strong enhancement in ionospheric plasma parameters at the corresponding sectors. The effect of disturbed thermospheric winds and resultant electric field persisted till the end of 7 August. The large decrease in O/N2 ratio at northern mid‐latitudes which is a consequence of the seasonal impact resulted in the negative storm effects at corresponding latitudinal regions. This study has shown that although the storm of August 2020 was a minor one, the associated high speed solar wind streams emanating from a coronal hole resulted in drastic changes in ionospheric parameters including global electron content, in situ electron density and total electron content at the equatorial ionization anomaly and equatorial electric field. Plain Language Summary: Coronal holes are regions on the surface of sun with open magnetic fields. Such regions are associated with high speed solar winds which can significantly disturb the quiet‐time ionosphere. In this work we studied the ionospheric disturbance as generated by coronal hole activity of August 2020 using the data from ground and space borne instruments. We have identified the role of various physical drivers which have been active during this particular space weather events. The signatures of penetrating electric fields, disturbance dynamo, and composition changes are found in this activity. This study highlight that such coronal hole activities can also perturb the ionospheric parameters. Hence, the modeling community should also take into account the role of such drivers while forecasting the ionospheric parameters. Key Points: Ionospheric response to coronal hole event of August 2020 is analyzed using ground and space borne instrumentsThe prompt penetrating electric field has seen to cause a pole‐ward expansion of the day‐side ionospheric plasma on 2 AugustSeasonal effects in disturbed winds have caused negative storm effect in the summer hemisphere [ABSTRACT FROM AUTHOR]

We present here the kinematics of the EUV wave associated with a GOES M1.0-class solar flare, which originates in NOAA AR 12740. The event is thoroughly observed with Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO) with high spatio-temporal resolutions. This event displays many features of EUV waves, which are very decisive for the understanding of the nature of EUV waves. These features include: a fast-mode wave, a pseudo wave, a slow-mode wave and stationary fronts, probably due to mode conversion. One fast-mode wave also propagates towards the coronal hole situated close to the north pole and the wave speed does not change when it encounters the coronal hole. We intend to provide self-consistent interpretations for all these different features. [ABSTRACT FROM AUTHOR]

The properties of the coronal plasma at heliocentric distances of 1.5 – 8.9 R ⊙ (solar radii) were studied with radio-occultation observations using JAXA's Akatsuki spacecraft in 2016. Physical parameters that characterize the solar wind were retrieved from the intensity-scintillation time series by fitting a theoretical spectrum to the observed power spectra. The derived solar-wind velocity clearly shows a difference between the fast wind and the slow wind, which was identified based on IPS observations. The inner scale, at which fluid motions dissipate and kinetic energy is converted to heat, increases with the heliocentric distance, and the fast wind has larger inner scales than the slow wind. By applying wavelet analysis to the frequency time series, we detected quasi-periodic fluctuations in the electron density. The density oscillations are considered to be manifestations of acoustic waves, which were generated from Alfvén waves originating from the photosphere, and the energy fluxes of those acoustic waves were estimated. The relative density-amplitude peaks around 4 – 6 R ⊙ and the wave-energy flux decreases beyond ≈ 6 R ⊙ , implying that the acoustic waves dissipate to heat the corona. The phase-scintillation spectrum that we obtained cannot be expressed by a single power law. A break is seen around the frequency of 0.5 – 2 Hz beyond ≈ 6 R ⊙ , suggesting an excess power other than turbulence at lower frequencies. The enhancement of the relative density amplitude around 6 R ⊙ found by the wavelet analysis might explain this excess power. The acoustic wave-energy flux in the fast solar wind tends to exceed that in the slow wind, suggesting that the fast wind is powered by a larger injection of Alfvén-wave energy than the slow wind. [ABSTRACT FROM AUTHOR]

In some polar coronal holes, previous studies have identified a coronal hole boundary layer (CHBL), which is different in composition from the coronal hole core region. However, compositionally distinct CHBL structure at low heliographic latitudes remains underexplored. Using solar wind composition measurements from the Advanced Composition Explorer (ACE), we study two particular low‐latitude coronal holes of 2003: a large polar hole extension with outward magnetic polarity and a smaller equatorial hole with inward magnetic polarity. For both of these coronal holes, we examine the distribution of two composition parameters, the O7+ to O6+ charge state density ratio (n(O7+)/n(O6+)) and the Fe to O abundance ratio (n(Fe)/n(O)), and find that each hole has an identifiable CHBL based on an anticorrelation of solar wind speed and n(O7+)/n(O6+). We also examine the FIP (first ionization potential) effect in the CHBL using n(Fe)/n(O), and find that there is no significant change from the CHBL to the core region for one of the coronal holes we study, while the other coronal hole has a noticeable decrease in n(Fe)/n(O) values in its core. Finally, we discuss our results in the context of previous work studying CHBL structure. Key Points: Advanced Composition Explorer composition measurements indicate that the largest coronal hole extension of 2003 has a thick coronal hole boundary layer (CHBL) distinct from its coreA CHBL is also observed in a small, inward‐polarity coronal hole in 2003; its speed range is not as broad as that of the largest extensionIn both coronal holes examined in this work, no change in the FIP effect is observed from the core to the CHBL [ABSTRACT FROM AUTHOR]

The cause of the solar wind is sought on a large‐scale (J × B) force resulting from the solar unipolar induction current, rather than coronal heating and small‐scale processes in the corona. In this paper, we consider the unipolar induction system considered by Alfven (1981, https://doi.org/10.1007/978-94-009-8374-8) the polar current (1.5 × 109 A) flowing out from the polar region of the sun spreads down longitudinally, which produces azimuthal magnetic fields. We considered two models, A and B. In Model A, the current spreads down fairly uniform in five divided regions of the heliosphere (rather than flowing only along the outer surface of the heliosphere). The resulting (J × B) force is mainly directed outward, causing an outward flow of the heliospheric plasmas. However, in this case, the speed at a distance of 1 AU is only 1 km/s, although the acceleration in each divided region is much greater than the solar gravity. In Model B, one third of the dynamo current is assumed to be concentrated in the area between 9.5 R☉ and 10.5 R☉ (where R☉denotes the solar radius) and B = 10−6 T (103 nT) at 10 R☉. The acceleration is estimated to be 1.4 × 102 m/s2 and the resulting speed at 1 AU is 200 km/s. Therefore, there is a possibility that the solar unipolar induction provides partially a new possibility for the cause of a slow, bulk flow of the solar wind. Plain Language Summary: Based on Alfven's solar unipolar induction system, it is shown that the resulting (J × B) force could generate solar wind speed of 200 km/s at 1 AU under several reasonable assumptions. Key Points: The cause of the solar wind is sought on the basis of Alfven's solar unipolar induction current system, rather than an initial heatingThe resulting (J × B) force is applied to the corona between 9.5 and 10.5 solar radii, accelerating the solar wind to 200 km/sThe result is applicable for slow and uniform solar wind with polar angle of 0–70° during the solar cycle minimum phase [ABSTRACT FROM AUTHOR]

Abstract In order to statistically investigate the relationship between the low-latitude coronal holes (CHs), the solar wind speed, and the geomagnetic activity in solar cycles 23 (1996–2008) and 24 (2009–2016), we conducted a superposed epoch analysis of the variations in CH area, solar winds, the interplanetary magnetic field (IMF), and geomagnetic indices (AL, AU, and SYM-H) for the period from 1996 to 2016. We further divided the temporal variations of the IMF into four types and then investigated the variations in solar winds, the IMF, and the geomagnetic indices before and after the corotating interaction region (CIR) reached Earth’s magnetosphere in each case. As a result, we observed a north–south asymmetry in the CH area, which shows that the CH area was much larger in the southern hemisphere than in the northern hemisphere during solar cycles 23 and 24. In addition, the CH area for solar cycle 24 tended to appear in a wider latitude region compared with that for solar cycle 23. The maximum values of the CH area and the solar wind speed in solar cycle 24 tended to be smaller than those in solar cycle 23. The relationship between these maximum values showed a positive correlation for both solar cycles. The distribution was larger for solar cycle 23 than for solar cycle 24. The variations in solar wind speed and the geomagnetic indices (AE and SYM-H) associated with CIRs in solar cycle 24 tended to be smaller than those in solar cycle 23. We conclude that the geomagnetic activity for solar cycle 24 associated with CIRs was slightly lower compared with that for solar cycle 23. This decrease in geomagnetic activity was due to a decrease in the dawn-to-dusk solar wind electric field intensity, which is obtained as the product of the solar wind speed and the north–south component of the solar wind magnetic field.

We present here the kinematics of the EUV wave associated with a GOES M1.0-class solar flare, which originates in NOAA AR 12740. The event is thoroughly observed with Atmospheric Imaging Assembly (AIA) onboard Solar Dynamics Observatory (SDO) with high spatio-temporal resolutions. This event displays many features of EUV waves, which are very decisive for the understanding of the nature of EUV waves. These features include: a fast-mode wave, a pseudo wave, a slow-mode wave and stationary fronts, probably due to mode conversion. One fast-mode wave also propagates towards the coronal hole situated close to the north pole and the wave speed does not change when it encounters the coronal hole. We intend to provide self-consistent interpretations for all these different features.

We study the geo-effectiveness of high-speed streams (HSS) detected during the consecutive eight years (2009 - 2016) period of solar cycle 24. These HSS are of different speeds and durations. During their passage, solar wind plasma and magnetic field parameters are also not similar. We utilize solar wind plasma and magnetic field parameters during the HSS passage in addition to two geomagnetic indices. Using the superposed-epoch analysis method, we study the time-evolution of geomagnetic disturbances concerning various solar wind parameters during the passage of HSS of different speeds and durations. We identify the solar wind parameter, which best follows the variations in geomagnetic parameters during the evolution of geomagnetic storms, from the start till recovery. Further, utilizing the level of geomagnetic activity due to individual HSS, we also search for solar wind parameter/function, which best correlates with the strength of geomagnetic storms due to these HSS. [ABSTRACT FROM AUTHOR]

We analyze the eruption of a coronal plasma channel (CPC) and an overlying flux rope using Atmospheric Imaging Assembly/Solar Dynamic Observatory (AIA/SDO) and Solar TErrestrial RElations Observatory (STEREO)-A spacecraft data. The CPC erupted first with its low and very faint coronal signature. Later, above the CPC, a diffuse and thin flux rope also developed and erupted. The spreading CPC further triggered a rotating jet-like structure from the coronal hole lying to its northward end. This jet-like eruption may have evolved due to the interaction between spreading CPC and the open field lines of the coronal hole lying towards its northward foot-point. The CPC connected two small trans-equatorial coronal holes lying, respectively, in the northern and southern hemisphere on either side of the Equator. These eruptions were collectively associated with the stealth-type CMEs and CME associated with a jet-like eruption. The source region of the stealth CMEs lay between two coronal holes connected by a coronal plasma channel. Another CME was also associated with a jet-like eruption that occurred from the coronal hole in the northern hemisphere. These CMEs evolved without any low coronal signature and yet were responsible for the third strongest geomagnetic storm of Solar Cycle 24. These stealth CMEs further merged and collectively passed through the interplanetary space. The compound CME further produced an intense geomagnetic storm (GMS) with Dst index = − 176 nT . The z -component of the interplanetary magnetic field [ B z ] switched to negative (− 18 nT ) during this interaction, and simultaneous measurement of the disturbance in the Earth's magnetic field (Kp = 7 ) indicates the onset of the geomagnetic storm. [ABSTRACT FROM AUTHOR]

Abstract Solar activity of cycle 24 following the deep minimum between cycle 23 and cycle 24 is the weakest one since cycle 14 (1902–1913). Geomagnetic activity is also low in cycle 24. We show that this low geomagnetic activity is caused by the weak dawn-to-dusk solar wind electric field (E d–d) and that the occurrence rate of E d–d > 5 mV/m decreased in the interval from 2013 to 2014. We picked up seventeen geomagnetic storms with the minimum Dst index of less than −100 nT and identified their solar sources in cycle 24 (2009–2015). It is shown that the relatively slow coronal mass ejections contributed to the geomagnetic storms in cycle 24.

High‐speed solar wind streams that originate from coronal holes play an important role in space weather disturbances, especially during the declining phase of the solar cycle. Space weather forecasters attempt to find good coronal hole indices that can be used to predict high‐speed streams days in advance. Several indices related to the coronal hole area, brightness, or magnetic field expansion factor have been reported in the literature. Empirical solar wind forecast models have been developed and used in operational service by several organizations by constructing prediction functions that relate the coronal hole index to the solar wind speed. In this paper, we present a new empirical modeling method and test its validity by comparing it with a previously reported method when applied to different coronal hole indices. In total, six empirical models are tested for a long period of time (2011–2018), with a 27‐day persistence model as a comparison benchmark. The results show that while all these empirical models can capture the temporal patterns of the solar wind observations well, the new modeling method and utilization of a composite coronal hole index PCH as an input parameter indeed improves the forecast accuracy. The high‐speed streams can be predicted approximately 3 days in advance, with a probability of detection of 0.78, a positive predictive value of 0.73, and a threat score of 0.61. The uncertainty of the high‐speed stream arrival time is approximately 1 day and the uncertainty of the peak speed is approximately 80 km/s. Key Points: A new empirical modeling method of solar wind speeds based on solar EUV images is presented, showing better forecast than other modelsSix empirical models with different modeling methods and inputs are validated from continuous time series and event‐based perspectivesThe new model predicts the 2011–2018 HSEs with a POD of 0.78, a PPV of 0.73, and a TS of 0.61 [ABSTRACT FROM AUTHOR]

Highlights • Interplanetary mass ejections and high speed streams differ by modulation abilities. • Parameters distributions are more compact for Forbush decreases from coronal holes. • Partial correlation of geomagnetic and Forbush decrease parameters is insignificant. • Correlation of solar wind and recurrent Forbush decrease parameters is moderate. • Correlation of solar wind and sporadic Forbush decrease parameters is high. Abstract In this paper, we compare Forbush decreases associated with solar wind disturbances from two types of solar sources: coronal mass ejections (sporadic Forbush decreases) and coronal holes (recurrent Forbush decreases). We used Forbush Effects and Interplanetary Disturbances database created and maintained by IZMIRAN and associated a certain amount of Forbush decreases in this database with interplanetary disturbances from coronal mass ejections (207 cases) as well as high speed streams from coronal holes (350 cases). Large number of events allows to apply statistical methods (distribution analysis, correlation analysis, multiple linear regression analysis) to compare those two groups of events. The results reveal that Forbush decreases associated with solar wind disturbances from the two types of solar sources differ significantly in their statistical characteristics: distributions, descriptive statistics, correlation and multiple regression coefficients. [ABSTRACT FROM AUTHOR]

We investigated two flares in the solar atmosphere that occurred on June 3, 2012 and July 6, 2012 and caused propagation of Moreton and EIT waves. In the June 3 event, we noticed a filament winking which presumably was caused by the wave propagation from the flare. An interesting feature of this event is that there was a reflection of this wave by a coronal hole located alongside the wave propagation, but not all of this wave was transmitted by the coronal hole. Using the running difference method, we calculated the speed of Moreton and EIT waves and we found values of 926 km/s before the reflection and 276 km/s after the reflection (Moreton wave) and 1,127 km/s before the reflection and 46 km/s after the reflection (EIT wave). In the July 6 event, this phenomenon was accompanied by type II and type III solar radio bursts, and we also performed a running difference analysis to find the speed of the Moreton wave, obtaining a value of 988 km/s. The speed derived from the analysis of the solar radio burst was 1,200 km/s, and we assume that this difference was caused by the different nature of the motions in these phenomena, where the solar radio burst was caused by the propagating particles, not waves.