Modeling the Lunar Wake Response to a CME Using a Hybrid PIC Model

In the solar wind, a low-density wake region forms downstream of the nightside lunar surface. In this study, we use a series of 3D hybrid particle-in-cell simulations to model the response of the lunar wake to a passing coronal mass ejection (CME). Average plasma parameters are derived from the Wind spacecraft located at 1 au during three distinct phases of a passing halo (Earth-directed) CME on 2015 June 22. Each set of plasma parameters, representing the shock/plasma sheath, a magnetic cloud, and plasma conditions we call the mid-CME phase, are used as the time-static upstream boundary conditions for three separate simulations. These simulation results are then compared with results that use nominal solar wind conditions. Results show a shortened plasma void compared to nominal conditions and a distinctive rarefaction cone originating from the terminator during the CME’s plasma sheath phase, while a highly elongated plasma void reforms during the magnetic cloud and mid-CME phases. Developments of electric and magnetic field intensification are also observed during the plasma sheath phase along the central wake, while electrostatic turbulence dominates along the plasma void boundaries and 2–3 lunar radii R M downstream in the central wake during the magnetic cloud and mid-CME phases. The simulations demonstrate that the lunar wake responds in a dynamic way with the changes in the upstream solar wind during a CME.

Analysis of Deformation and Erosion during CME Evolution

Magnetised coronal mass ejections (CMEs) are quite substantially deformed during their journey form the Sun to the Earth. Moreover, the interaction of their internal magnetic field with the magnetic field of the ambient solar wind can cause deflection and erosion of their mass and magnetic flux. We here analyse axisymmetric (2.5D) MHD simulations of normal and inverse CME, i.e., with the opposite or same polarity as the background solar wind, and attempt to quantify the erosion and the different forces that operate on the CMEs during their evolution. By analysing the forces, it was found that an increase of the background wind density results in a stronger plasma pressure gradient in the sheath that decelerates the magnetic cloud more. This in turn leads to an increase of the magnetic pressure gradient between the centre of the magnetic cloud and the separatrix, causing a further deceleration. Regardless of polarity, the current sheet that forms in our model between the rear of the CME and the closed field lines of the helmet streamer, results in magnetic field lines being stripped from the magnetic cloud. It is also found that slow normal CMEs experience the same amount of erosion, regardless of the background wind density. Moreover, as the initial velocity increases, so does the influence of the wind density on the erosion. We found that increasing the CME speed leads to a higher overall erosion due to stronger magnetic reconnection. For inverse CMEs, field lines are not stripped away but added to the magnetic cloud, leading to about twice as much magnetic flux at 1 AU than normal CMEs with the same initial flux.

Dynamics of helium abundance in magnetic clouds

<p>Helium is the second most abundant ion component of the solar wind. The relative abundance of helium can differ significantly in various large-scale structures of the solar wind generated by the nonstationarity and inhomogeneity of the solar corona. For example the helium abundance is ~3% in slow streams and ~4% in fast streams. The maximum helium abundance is usually observed inside magnetic clouds and can reach >10%. The relative abundance of helium can also dynamically vary inside large-scale structures, which can be the result of local processes in plasma.</p><p> In magnetic clouds, the distribution of the helium abundance has an axisymmetric peak with a maximum in the central region of the magnetic cloud, where the ion current flows [Yermolaev et al., 2020]. This research examines the different-scale dynamics of the relative abundance of helium in magnetic clouds. For this purpose, the dependences of the helium abundance on some plasma parameters were studied on different datasets of the OMNI database from 1976 to 2018. It is shown that the helium abundance increases with an increase in the modulus of the interplanetary magnetic field B and with a decrease in the proton plasma parameter β in the center of the magnetic cloud. The scale of this region is ~1 million kilometers. Similar relations of the helium abundance to interplanetary magnetic field direction angles and other solar wind parameters were studied.</p><p>In addition, the work studied intermediate-scale changes (at scale <1 hour) in helium abundance inside magnetic clouds and compression regions in front of them in comparison with other large-scale wind types. For this aim, a correlation analysis of the time series of density and relative abundance of helium was carried out on base of measurements on SPEKTR-R and WIND spacecraft located at a considerable distance from each other. The dependences of the local correlation coefficients (at scale ~1 hour or less) between measurements at two points on the solar wind plasma parameters are considered. Meanwhile these dependencies are compared with the same for other types of solar wind. It is shown that the median values of the local correlation coefficient in the regions of compressed plasma ahead of magnetic clouds exceed the values in other types of wind by about 15%. In addition, the local correlation coefficient increases with an increase in the amplitude of fluctuations of the investigated parameter and the proton velocity. Thus, intermediate-scale fluctuations in the relative helium abundance observed in these structures are quite stable and apparently are formed in the corona acceleration region and then propagate without changes.<br>The work is supported by <span>RFBR grant № <span>19-02-00177</span>a.</span></p><p>References.<br>Yermolaev, Y.I. et al., Dynamics of large-scale solar-wind streams obtained by the double superposed epoch analysis. 4. Helium abundance, Journal of Geophysical Research, 125 (7) DOI: 10.1029/2020JA027878</p>

SUPERSUBSTORM ON 28 MAY 2011 – GEOMAGNETIC EFFECTS IN THE GLOBAL SCALE

For this analysis, we selected the supersubstorm (SSS) occurred during the strong magnetic storm on 28 May 2011 (SYM/H~100 nT). The ground-based magnetic effects of SSS have been studied basing on the data from the global SuperMAG, INTERMAGNET and IMAGE magnetometer networks, as well as on the magnetic measurements by the ionospheric satellite AMPERE system. According to the SML- index behavior, the SSS event maximum was identified at ~09:00 UT on 28 May 2011 (SML= ~-2600 nT). The SSS occurred during the passage of the magnetic cloud in the solar wind. Before the SSS, the BZ component of the Interplanetary Magnetic Field (IMF) was negative, the IMF BY component was positive, and the local jump in the solar wind dynamic pressure was registered. We found that the SSS developed in the magnetosphere in the global scale. A strong westward electrojet was observed at auroral latitudes from the evening side to the dayside. In contrast to the typical scenario of a classical substorm, a very intense eastward electrojet was detected in the afternoon-evening sector. That may be a result of the formation of an additional partial ring current during the supersubstorm.

Связь временных вариаций корональных выбросов массы и крупномасштабных событий в солнечном ветре с появлением SCв 23 и 24 циклах солнечной активности.

В рамках нашего исследования было проведено сравнение вариаций числа корональных выбросов массы (КВМ) и крупномасштабных событий в солнечном ветре с суммарным числом внезапных импульсов в геомагнитном поле SI и внезапных начал геомагнитных бурь SSC в 23 и 24 циклах солнечной активности (CA). Сопоставлены вариации чисел солнечных пятен (чисел Вольфа) с циклическими вариациями появления КВМ за каждый месяц наблюдений коронографов LASCO в 23 и 24 циклах солнечной активности. Так же было исследовано соотношение изменений суммарного числа КВМ различного углового размера с суммарным числом зарегистрированных приходов к Земле межпланетных ударных волн, вызвавших внезапные импульсы в геомагнитном поле (SI) или внезапные начала геомагнитных бурь (SSC), и с суммарным числом крупномасштабных событий в солнечном ветре таких, как EJECTA и МС (Magnetic cloud) согласно каталогу ИКИ.