SMALL-SCALE MAGNETIC ISLANDS IN THE SOLAR WIND AND THEIR ROLE IN PARTICLE ACCELERATION. I. DYNAMICS OF MAGNETIC ISLANDS NEAR THE HELIOSPHERIC CURRENT SHEET

So far, the problem of a short-term forecast of geomagnetic storms can be considered as solved. Meanwhile, mid-term prognoses of geomagnetic storms with an advance time from 3 hours to 3 days are still unsuccessful (see &#160;https://www.swpc.noaa.gov/sites/default/files/images/u30/Max%20Kp%20and%20GPRA.pdf).&#160;This fact suggests a necessity of looking for specific&#160;processes&#160;in the solar wind preceding geomagnetic storms. Knowing that magnetic cavities filled with magnetic islands and current sheets are formed in front of high-speed streams of any type (Khabarova et al., 2015, 2016, 2018; Adhikari et al., 2019), we have performed an analysis of the corresponding ULF variations in the solar wind density observed at the Earth's orbit from hours to days before the arrival of a geoeffective stream or flow. The fact of the occurrence of ULF-precursors of geomagnetic storms was noticed a long time ago (Khabarova 2007; Khabarova & Yermolaev, 2007) and related&#160;prognostic methods were recently developed (Kogai et al. 2019), while the problem of automatization of the prognosis remained unsolved.&#160;A new geomagnetic storm forecast method, which employs a Recurrent Neural Network (RNN) for an automatic pattern search, is proposed. An ability of self-teaching and extracting deeply hidden non-linear patterns is the main advantage of Deep Neural Networks (DNNs) with multiple layers over traditional Machine Learning methods. We show a success of the&#160;RNN&#160;method, using either the unprocessed solar wind density data or Wavelet analysis coefficients as the input parameter for a DNN to perform an automatic mid-term prognosis of geomagnetic storms.&#160;&#160;Adhikari, L., et al. 2019, The Role of Magnetic Reconnection&#8211;associated Processes in Local Particle Acceleration in the Solar Wind, ApJ, 873, 1, 72,&#160;https://doi.org/10.3847/1538-4357/ab05c6 Kogai T.G. et al., Pre-storm ULF variations in the solar wind density and interplanetary magnetic field as key parameters to build a mid-term prognosis of geomagnetic storms. &#8220;GRINGAUZ 100: PLASMA IN THE SOLAR SYSTEM&#8221;, IKI RAS, Moscow, June 13&#8211;15, 2018, 140-143, ISBN 978-5-00015-043-6.&#160;https://www.researchgate.net/publication/327781146_Pre-storm_ULF_variations_in_the_solar_wind_density_and_interplanetary_magnetic_field_as_key_parameters_to_build_a_mid-term_prognosis_of_geomagnetic_storms &#160;Khabarova O. V., et al. 2018, &#160;Re-acceleration of energetic particles in large-scale heliospheric magnetic cavities, Proceedings of the IAU, 76-82,&#160;https://doi.org/10.1017/S1743921318000285&#160; Khabarova O.V., et al. Small-scale magnetic islands in the solar wind and their role in particle acceleration. II. Particle energization inside magnetically confined cavities. 2016, ApJ, 827,&#160;122,&#160;http://iopscience.iop.org/article/10.3847/0004-637X/827/2/122 Khabarova O., et al. Small-scale magnetic islands in the solar wind and their role in particle acceleration. 1. Dynamics of magnetic islands near the heliospheric current sheet. 2015, ApJ, 808, 181,&#160;https://doi.org/10.1088/0004-637X/808/2/181Khabarova O.V., Current Problems of Magnetic Storm Prediction and Possible Ways of Their Solving. Sun&Geosphere, &#160;http://sg.shao.az/v2n1/SG_v2_No1_2007-pp-33-38.pdf&#160;, 2(1), 33-38, 2007Khabarova O.V. & Yu.I.Yermolaev, Solar wind parameters' behavior before and after magnetic storms, JASTP, 70, 2-4, 2008, 384-390,&#160;http://dx.doi.org/10.1016/j.jastp.2007.08.024

Download Full-text

Fully kinetic PIC simulations of particle acceleration and non-Maxwellian distribution functions due to current sheets in solar wind turbulence

10.5194/egusphere-egu21-12010 ◽

2021 ◽

Author(s):

Patricio A. Munoz ◽

Jörg Büchner ◽

Neeraj Jain

Keyword(s):

Solar Wind ◽

Particle Acceleration ◽

Current Sheet ◽

Plasma Turbulence ◽

Distribution Functions ◽

Heat Fluxes ◽

Magnetic Islands ◽

Ion Distribution ◽

Current Sheets ◽

Particle In Cell

Turbulence is ubiquitous in solar system plasmas like those of the solar wind and Earth's magnetosheath. Current sheets can be formed out of this turbulence, and eventually magnetic reconnection can take place in them, a process that converts magnetic into particle kinetic energy. This interplay between turbulence and current sheet formation has been extensively analyzed with MHD and hybrid-kinetic models. Those models cover all the range between large Alfv&#233;nic scales down to ion-kinetic scales. The consequences of current sheet formation in plasma turbulence that includes electron dynamics has, however, received comparatively less attention. For this sake we carry out 2.5D fully kinetic Particle-in-Cell simulations of kinetic plasma turbulence including both ion and electron spectral ranges. In order to further assess the electron kinetic effects, we also compare our results with hybrid-kinetic simulations including electron inertia in the generalized Ohm's law. We analyze and discuss the electron and ion energization processes in the current sheets and magnetic islands formed in the turbulence. We focus on the electron and ion distribution functions formed in and around those current sheets and their stability properties that are relevant for the micro-instabilities feeding back into the turbulence cascade. We also compare pitch angle distributions and non-Maxwellian features such as heat fluxes with recent in-situ solar wind observations, which demonstrated local particle acceleration processes in reconnecting solar wind current sheets [Khabarova et al., ApJ, 2020].

Download Full-text

SMALL-SCALE MAGNETIC ISLANDS IN THE SOLAR WIND AND THEIR ROLE IN PARTICLE ACCELERATION. II. PARTICLE ENERGIZATION INSIDE MAGNETICALLY CONFINED CAVITIES

The Astrophysical Journal ◽

10.3847/0004-637x/827/2/122 ◽

2016 ◽

Vol 827 (2) ◽

pp. 122 ◽

Cited By ~ 48

Author(s):

Olga V. Khabarova ◽

Gary P. Zank ◽

Gang Li ◽

Olga E. Malandraki ◽

Jakobus A. le Roux ◽

...

Keyword(s):

Solar Wind ◽

Particle Acceleration ◽

Small Scale ◽

Magnetic Islands ◽

Magnetically Confined ◽

Particle Energization

Download Full-text

Effects of local particle acceleration in the solar wind

10.5194/egusphere-egu2020-3711 ◽

2020 ◽

Author(s):

Olga Khabarova ◽

Valentina Zharkova ◽

Qian Xia ◽

Olga Malandraki

Keyword(s):

Solar Wind ◽

Magnetic Reconnection ◽

Heliospheric Current Sheet ◽

Small Scale ◽

Magnetic Islands ◽

Solar Origin ◽

Suprathermal Electrons ◽

Long Time ◽

The Impact ◽

Accelerated Particles

Recent observational&#160;and theoretical&#160;studies have shown that there is an unaccounted population of electrons and protons accelerated locally to suprathermal energies at reconnecting current sheets (RCSs) and 3-D dynamical plasmoids or 2-D magnetic islands (MIs) in the solar wind.&#160;The findings can be summarized as following: (i) RCSs are often subject to instabilities breaking those into 3D small-scale plasmoids/blobs or 2D magnetic islands (MIs) with multiple X-&#160;and O-nullpoints; (ii) RCSs and dynamical MIs can accelerate particles up to the MeV/nuc energies; (iii) accelerated particles may form clouds expanding far from a reconnecting region; and (iv) bi-directional(or counterstreaming) strahls observed in pitch-angle distributions (PADs) of suprathermal electrons may simply represent a signature of magnetic reconnection occurring at closed IMF structures (e.g., MIs), not necessarily connected to the Sun (Zharkova & Khabarova, 2012, 2015;&#160;Zank et al. 2014, 2015;&#160;Khabarova et al. 2015, 2016, 2017; 2018;&#160;le Roux 2016, 2017, 2018, 2019;&#160;Khabarova & Zank, 2017; Adhikari et al. 2019;&#160;Xia & Zharkova, 2018, 2020; Malandraki et al. 2019; Mingalev et al. 2019).&#160;We will briefly present an overview of the effects of local ion acceleration as observed at different heliocentric distances and focus on the impact of the locally-borne population of suprathermal electrons on typical patterns of PADs.&#160;Suprathermal electrons with energies of ~70eV and above are observed at 1 AU as dispersionless halo and magnetic field-aligned beams of strahls. For a long time, it has been thought that both populations originate only from the solar corona. This view has consequently impacted interpretation of typical patterns of suprathermal electron PADs observed in the solar wind. We present multi-spacecraft observations of counterstreaming strahls and dropouts in PADs within a previously reported region filled with plasmoids and RCSs, comparing observed PAD features with those predicted by PIC simulations extended to heliospheric conditions. We show typical features of PADs determined by acceleration of the ambient thermal electrons up to suprathermal energies in single RCSs and dynamical plasmoids. Our study suggests that locally-accelerated suprathermal electrons co-exist with those of solar origin. Therefore, some heat flux dropout and bi-directional strahl events observed in the heliosphere can be explained by local dynamical processes involving magnetic reconnection. Possible implications of the results for the interpretation of the strahl/halo relative density change with heliocentric distance and puzzling features of suprathermal electrons observed at crossings of the heliospheric current sheet and cometary comas are also discussed.

Download Full-text

The Role of Magnetic Reconnection–associated Processes in Local Particle Acceleration in the Solar Wind

10.5194/egusphere-egu21-9328 ◽

2021 ◽

Author(s):

Laxman Adhikari ◽

Gary Zank ◽

Lingling Zhao

Keyword(s):

Solar Wind ◽

Particle Acceleration ◽

Magnetic Reconnection ◽

Electric Fields ◽

Charged Particles ◽

Heliospheric Current Sheet ◽

Shock Acceleration ◽

Small Scale ◽

Diffusive Shock Acceleration ◽

Theoretical Predictions

Recent studies of unusual or atypical energetic particle flux events (AEPEs) observed at 1 au show that another mechanism, different from diffusive shock acceleration, can energize particles locally in the solar wind. The mechanism proposed by Zank et al. is based on the stochastic energization of charged particles in regions filled with numerous small-scale magnetic islands (SMIs) dynamically contracting or merging and experiencing multiple magnetic reconnection in the super-Alfv&#233;nic solar wind flow. A first- and second-order Fermi mechanism results from compression-induced changes in the shape of SMIs and their developing dynamics. Charged particles can also be accelerated by the formation of antireconnection electric fields. Observations show that both processes often coexist in the solar wind. The occurrence of SMIs depends on the presence of strong current sheets like the heliospheric current sheet (HCS), and related AEPEs are found to occur within magnetic cavities formed by stream&#8211;stream, stream&#8211;HCS, or HCS&#8211;shock interactions that are filled with SMIs. Previous case studies comparing observations with theoretical predictions were qualitative. Here we present quantitative theoretical predictions of AEPEs based on several events, including a detailed analysis of the corresponding observations. The study illustrates the necessity of accounting for local processes of particle acceleration in the solar wind.

Download Full-text

Heliospheric current sheet inclinations at Venus and Earth

Annales Geophysicae ◽

10.1007/s00585-999-0642-z ◽

1999 ◽

Vol 17 (5) ◽

pp. 642-649 ◽

Cited By ~ 1

Author(s):

G. Ma ◽

K. Marubashi ◽

T. Maruyama

Keyword(s):

Solar Wind ◽

Magnetic Fields ◽

Current Sheet ◽

Solar Maximum ◽

Heliospheric Current Sheet ◽

Small Scale ◽

Sector Boundary ◽

Scale Structures ◽

Transient Structures ◽

Small Scale Structures

Abstract. We investigate the inclinations of heliospheric current sheet at two sites in interplanetary space, which are generated from the same solar source. From the data of solar wind magnetic fields observed at Venus (0.72 AU) and Earth (1 AU) during December 1978-May 1982 including the solar maximum of 1981, 54 pairs of candidate sector boundary crossings are picked out, of which 16 pairs are identified as sector boundaries. Of the remainder, 12 pairs are transient structures both at Venus and Earth, and 14 pairs are sector boundaries at one site and have transient structures at the other site. It implies that transient structures were often ejected from the coronal streamer belt around the solar maximum. For the 16 pairs of selected sector boundaries, we determine their normals by using minimum variance analysis. It is found that most of the normal azimuthal angles are distributed between the radial direction and the direction perpendicular to the spiral direction both at Venus and Earth. The normal elevations tend to be smaller than ~ 45° with respect to the solar equatorial plane, indicating high inclinations of the heliospheric current sheet, in particular at Earth. The larger scatter in the azimuth and elevation of normals at Venus than at Earth suggests stronger effects of the small-scale structures on the current sheet at 0.72 AU than at 1 AU. When the longitude difference between Venus and Earth is small (<40° longitudinally), similar or the same inclinations are generally observed, especially for the sector boundaries without small-scale structures. This implies that the heliospheric current sheet inclination tends to be maintained during propagation of the solar wind from 0.72 AU to 1 AU. Detailed case studies reveal that the dynamic nature of helmet streamers causes variations of the sector boundary structure.Key words. Interplanetary physics (interplanetary magnetic fields; sources of solar wind)

Download Full-text