Solar wind fluctuations at large scale: A comparison between low and high solar activity conditions

Abstract. The 3-D structure of the solar wind varies dramatically along the Sun's activity cycle. In the present paper we focus on some properties of the polar solar wind. This is a fast, teneous, and steady flow (as compared to low-latitude conditions) that fills the high-latitude heliosphere at low solar activity. The polar wind has been extensively investigated by Ulysses, the first spacecraft to perform in-situ measurements in the high-latitude heliosphere. Though the polar wind is quite a uniform flow, fluctuations in its velocity do not appear negligible. A simple way to characterize the solar wind structure is that of performing a multi-scale statistical analysis of the wind velocity differences. The occurrence frequency distributions of velocity differences at time lags from 1 to 1024h and the corresponding values of mean, standard deviation, skewness, and kurtosis have been obtained. A comparison with previous results in ecliptic wind at both low and high solar activity has been performed. It comes out that the kind of trend observed in the distributions for changing scale is the same for the different solar wind regimes. Differences between different flows just have an effect on the values of the distribution moments and the scales at which the transition from non-Gaussian to Gaussian-like behaviours occurs. This is typical of systems in which random fluctuations are mixed to coherent structures of some characteristic size, in other words, systems where long-range correlations cannot be neglected.

Download Full-text

Implementing the MULTI-VP coronal model in EUHFORIA: Test case results and comparisons with the WSA coronal model

Astronomy and Astrophysics ◽

10.1051/0004-6361/202039325 ◽

2021 ◽

Vol 648 ◽

pp. A35

Author(s):

E. Samara ◽

R. F. Pinto ◽

J. Magdalenić ◽

N. Wijsen ◽

V. Jerčić ◽

...

Keyword(s):

Solar Wind ◽

Solar Activity ◽

High Speed ◽

Weather Prediction ◽

Solar Wind Plasma ◽

High Solar Activity ◽

Source Surface ◽

Field Source ◽

Magnetic Signatures

Context. In this study, we focus on improving EUHFORIA (European Heliospheric Forecasting Information Asset), a recently developed 3D magnetohydrodynamics space weather prediction tool. The EUHFORIA model consists of two parts covering two spatial domains: the solar corona and the inner heliosphere. For the first part, the semiempirical Wang-Sheeley-Arge (WSA) model is used by default; this model employs the potential field source surface and Schatten current sheet models to provide the necessary solar wind plasma and magnetic conditions above the solar surface, at 0.1 AU, which serve as boundary conditions for the inner heliospheric part. Herein, we present the first results of the implementation of an alternative coronal model in EUHFORIA, the so-called MULTI-VP model. Aims. After we replace the default EUHFORIA coronal setup with the MULTI-VP model, we compare their outputs both at 0.1 AU and 1 AU, for test cases involving high speed wind streams (HSSs). We select two distinct cases in which the standard EUHFORIA setup failed to reproduce the HSS plasma and magnetic signatures at Earth to test the performance of MULTI-VP coupled with EUHFORIA-heliosphere. Methods. To understand the quality of modeling with MULTI-VP in comparison with the default coronal model in EUHFORIA, we considered one HSS case during a period of low solar activity and another one during a period of high solar activity. Moreover, the modeling of the two HSSs was performed by employing magnetograms from different providers: one from the Global Oscillation Network Group (GONG) and the second from the Wilcox Space Observatory (WSO). This way, we were able to distinguish differences arising not only because of the different models but also because of different magnetograms. Results. The results indicate that when employing a GONG magnetogram, the combination MULTI-VP+EUHFORIA-heliosphere reproduces the majority of HSS plasma and magnetic signatures measured at L1. On the contrary, the standard WSA+EUHFORIA-heliosphere combination does not capture the arrival of the HSS cases at L1. When employing WSO magnetograms, MULTI-VP+EUHFORIA-heliosphere reproduces the HSS that occurred during the period of high solar activity. However, it is unclear if it models the HSS during the period of low solar activity. For the same magnetogram and periods of time, WSA+EUHFORIA-heliosphere is not able to capture the HSSs of interest. Conclusions. The results show that the accuracy of the simulation output at Earth depends on the choice of both the coronal model and input magnetogram. Nevertheless, a more extensive statistical analysis is necessary to determine how precisely these choices affect the quality of the solar wind predictions.

Download Full-text

THE STRUCTURE OF THE SOLAR WIND IN THE HELIOSPHERE DEPENDING ON THE PHASE OF THE SOLAR CYCLE: LARGE-SCALE DYNAMICS OF THE HELIOSPHERIC CURRENT SHEET

Journal of Oceanological Research ◽

10.29006/1564-2291.jor-2019.47(1).25 ◽

2019 ◽

Vol 47 (1) ◽

pp. 85-87

Author(s):

E.V. Maiewski ◽

R.A. Kislov ◽

H.V. Malova ◽

O.V. Khabarova ◽

V.Yu. Popov ◽

...

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Solar Activity ◽

Solar Cycle ◽

Current Sheet ◽

Heliospheric Current Sheet ◽

High Solar Activity ◽

The Sun ◽

High Latitudes ◽

The Magnetic Field

A stationary axisymmetric MHD model of the solar wind has been constructed, which allows us to study the spatial distribution of the magnetic field and plasma characteristics at radial distances from 20 to 400 radii of the Sun at almost all heliolatitudes. The model takes into account the changes in the magnetic field of the Sun during a quarter of the solar cycle, when the dominant dipole magnetic field is replaced by a quadrupole. Selfconsistent solutions for the magnetic and velocity fields, plasma concentration and current density of the solar wind depending on the phase of the solar cycle are obtained. It is shown that during the domination of the dipole magnetic component in the solar wind heliospheric current sheet (HCS) is located in the equatorial plane, which is a part of the system of radial and transverse currents, symmetrical in the northern and southern hemispheres. As the relative contribution of the quadrupole component to the total magnetic field increases, the shape of the HCS becomes conical; the angle of the cone gradually decreases, so that the current sheet moves entirely to one of the hemispheres. At the same time, at high latitudes of the opposite hemisphere, a second conical HCS arises, the angle of which increases. When the quadrupole field becomes dominant (at maximum solar activity), both HCS lie on conical surfaces inclined at an angle of 35 degrees to the equator. The model describes the transition from the fast solar wind at high latitudes to the slow solar wind at low latitudes: a relatively gentle transition in the period of low solar activity gives way to more drastic when high solar activity. The model also predicts an increase in the steepness of the profiles of the main characteristics of the solar wind with an increase in the radial distance from the Sun. Comparison of the obtained dependences with the available observational data is discussed.

Download Full-text

Assessing the Performance of EUHFORIA Modeling the Background Solar Wind

Solar Physics ◽

10.1007/s11207-019-1558-8 ◽

2019 ◽

Vol 294 (12) ◽

Cited By ~ 5

Author(s):

Jürgen Hinterreiter ◽

Jasmina Magdalenic ◽

Manuela Temmer ◽

Christine Verbeke ◽

Immanuel Christopher Jebaraj ◽

...

Keyword(s):

Time Series ◽

Solar Wind ◽

Solar Activity ◽

Coronal Holes ◽

High Solar Activity ◽

Time Interval ◽

Input Parameters ◽

First Results ◽

Combined Time Series

AbstractIn order to address the growing need for more accurate space-weather predictions, a new model named (EUropean Heliospheric FORecasting Information Asset) was recently developed. We present the first results of the performance assessment for the solar-wind modeling with and identify possible limitations of its present setup. Using the basic 1.0.4 model setup with the default input parameters, we modeled background solar wind (no coronal mass ejections) and compared the obtained results with Advanced Composition Explorer (ACE) in-situ measurements. For the purposes of statistical study we developed a technique of combining daily runs into continuous time series. The combined time series were derived for the years 2008 (low solar activity) and 2012 (high solar activity), from which in-situ speed and density profiles were extracted. We find for the low-activity phase a better match between model results and observations compared to the high-activity time interval considered. The quality of the modeled solar-wind parameters is found to be rather variable. Therefore, to better understand the results obtained we also qualitatively inspected characteristics of coronal holes, i.e. the sources of the studied fast streams. We discuss how different characteristics of the coronal holes and input parameters to influence the modeled fast solar wind, and suggest possibilities for the improvement of the model.

Download Full-text

Investigation of the coronal heating through phase relation of solar activity indexes

Publications of the Astronomical Society of Australia ◽

10.1017/pasa.2019.43 ◽

2020 ◽

Vol 37 ◽

Cited By ~ 1

Author(s):

K. J. Li ◽

J. C. Xu ◽

Z. Q. Yin ◽

J. L. Xie ◽

W. Feng

Keyword(s):

Solar Wind ◽

Solar Activity ◽

Phase Relation ◽

Large Scale ◽

Solar Rotation ◽

Rotation Period ◽

Magnetic Activity ◽

Coronal Heating ◽

Small Scale

Abstract The coronal heating problem is a long-standing perplexing issue. In this study, 13 solar activity indexes are used to investigate their phase relation with the sunspot number (SSN). Only three of them are found to statistically significantly lag the SSN (large-scale magnetic activity) by about one solar rotation period; the three indexes are total solar irradiance (TSI), the modified coronal index, and the solar wind velocity; the former two indexes may represent the long-term variation of energy quantity of the heated photosphere and corona, respectively. The Mount Wilson Sunspot Index (MWSI) and the Magnetic Plage Strength Index (MPSI), which reflect the large- and small-scale magnetic field activities, respectively, are also utilised to investigate their phase relations with the three indexes. The three indexes are found to be much more intimately related to MPSI than to MWSI, and MWSI statistically significantly leads TSI by about one rotation period. The heated corona is found to pulse perfectly in step with the small-scale magnetic activity rather than the large-scale magnetic activity; furthermore, combined with observations, our statistical evidence should thus attribute coronal heating firmly to small-scale magnetic activity phenomena, such as spicules, micro-flares, nano-flares, and others. The photosphere and the corona are synchronously heated, which should seemingly prefer magnetic reconnection heating to wave heating. In the long term, such a coronal heating way is inferred effective. Statistically, it is also small-scale magnetic activity phenomena that produce TSI enhancement. Coronal heating and solar wind acceleration are found to be synchronous, as standard models require.

Download Full-text

Large-scale solar wind fluctuations in the inner heliosphere at low solar activity

Journal of Geophysical Research Atmospheres ◽

10.1029/ja094ia01p00168 ◽

1989 ◽

Vol 94 (A1) ◽

pp. 168 ◽

Cited By ~ 33

Author(s):

B. Bavassano ◽

R. Bruno

Keyword(s):

Solar Wind ◽

Solar Activity ◽

Large Scale ◽

Inner Heliosphere

Download Full-text

On the direction of the velocity and magnetic field fluctuations in undisturbed solar wind

Open Astronomy ◽

10.1515/astro-2021-0024 ◽

2021 ◽

Vol 30 (1) ◽

pp. 184-190

Author(s):

Dmitry V. Erofeev

Keyword(s):

Magnetic Field ◽

Solar Wind ◽

Solar Activity ◽

Alfven Waves ◽

High Solar Activity ◽

Slow Solar Wind ◽

Alfvén Waves ◽

Velocity Fluctuations ◽

Field Fluctuations

Abstract Measurements of velocity and magnetic field in near-Earth heliosphere is analized in order to investigate systematical deflection from transversality of the velocity and magnetic field fluctuations in undisturbed solar wind. Fluctuations occurred in the meridional plain of heliosphere (RN plain of the RTN reference system) are transversal with respect to mean magnetic field during periods of high solar activity, but they become non-transversal close to solar cycle minima. This phenomenon is investigated focusing on a role of Alfvén waves. It is shown that deflections from transversality is mostly expressed by fluctuations in slow solar wind streams with low contribution of Alfvén waves, whereas strongly Alfvénic turbulence undergo such deflection in a less degree. In addition, we consider orientation of velocity fluctuations in the azimuthal (RT) plain of heliosphere, which also indicates some interesting features.

Download Full-text

Statistical investigations of polar patch occurrence during high solar activity

10.5194/egusphere-egu21-7201 ◽

2021 ◽

Author(s):

Rafal Sieradzki ◽

Jacek Paziewski

Keyword(s):

Solar Activity ◽

Large Scale ◽

Density Ratio ◽

Coupled System ◽

High Solar Activity ◽

Seasonal Effect ◽

Polar Regions ◽

The North ◽

Background Density ◽

A Tec

The circumpolar ionosphere is recognised as one of the most disturbed region of the ionized part of the atmosphere. The reasons for that are mainly dynamic conditions in the coupled system of the magnetosphere and the ionosphere as well as feeding of the polar plasma from the mid-latitude reservoir. One of the consequences of these phenomenon is the occurrence of large-scale ionospheric structures called polar patches. These are commonly defined as the enhancement of the F-region plasma characterized with a foreground-to-background density ratio larger than 2 and a size up to several hundred kilometres.In this work we present GNSS-based characteristics of a patch occurrence in the northern hemisphere. The study covers a period of January&#8211;May 2014 corresponding to the maximum of the solar activity. The detection of structures was performed with a relative STEC value that is defined as a difference between epoch-wise L4 data and 4th order polynomial corresponding to background variations of the ionosphere. In order to ensure a continuous monitoring of the ionosphere over the north pole, we used data from ~45 permanent stations. The results prove that ground-based GNSS data can be successfully used in the climatological investigations of polar patches. We found a strong seasonal effect in the occurrence of these structures with the maximum at the turn of February and March and the minimum in May. Such outcomes correspond to variations of a TEC gradient between subauroral and polar regions. This parameter seems to be also responsible for a subdaily pattern of patches observed for particular months. The comparison of GNSS-based results with in-situ SWARM data revealed some differences, which are probably related to different characteristics of the ionosphere provided by both techniques. Furthermore, the study confirms that most of the patches are observed for the negative values of IMF Bz, &#160;whereas IMF By component has no significant impact on the number of analysed structures.&#160;

Download Full-text