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M. Derouich

Publications and source records attributed to M. Derouich.

17 recordsLinked to original sources
arXiv2025

Forbush Decreases during strong Geomagnetic Storms: Time Delays, Rigidity Effects, and ICME-Driven Modulation

We investigate the relationship between Forbush decreases (FDs) and associated geomagnetic storms, and their links to interplanetary solar wind parameters, using high-resolution minute data. FDs are classified by main-phase decrease steps and analyzed with superposed epoch analysis. Fast, turbulent, high-field sheath structures occur before and during coronal mass ejection (CME)-driven FDs, whereas corotating interaction region events show delayed amplification and more perturbed dynamics. Time lags between FD and storm onsets are examined for space weather forecasting. FD amplitude correlates more strongly with moderate and strong CME-driven storms than with extreme storms, likely due to complex magnetospheric responses from successive events and prolonged southward IMF Bz. Events with fast shocks and sheath regions show stronger correlations than those without shocks. Energy dependence, derived from twelve neutron monitor stations worldwide, reveals a two-step linear rigidity spectrum: sharp FD amplitude decrease at low rigidity and a more gradual drop at higher rigidity.

astro-ph.SR↗
arXiv2025

Depolarization and polarization transfer rates for the C$_2$ $(X ^1\Sigma^+_g, a ^3\Pi_u)$ + H$(^2S_{1/2})$ collisions in the solar photosphere

This paper is a continuation of a series of studies investigating collisional depolarization of solar molecular lines like those of MgH, CN and C$_2$. It is focused on the case of the solar molecule C$_2$ which exhibits striking scattering polarization profiles although its intensity profiles are inconspicuous and barely visible. In fact, interpretation of the C$_2$ polarization in terms of magnetic fields is incomplete due to the almost complete lack of collisional data. This work aims at accurately computing the collisional depolarization and polarization transfer rates for the C$_2$~$(X ^1\Sigma^+_g, a ^3\Pi_u)$ by isotropic collisions with hydrogen atoms H~$(^2S_{1/2})$. We also investigate the solar implications of our findings. We utilize the MOLPRO package to obtain potential energy surfaces (PESs) for the electronic states $X ^1\Sigma^+_g$ and $a^3\Pi_u$ of C$_2$, and the MOLSCAT code to study the quantum dynamics of the C$_2$~$(X ^1\Sigma^+_g, a ^3\Pi_u)$ + H$(^2S_{1/2})$ systems. We use the tensorial irreducible basis to express the resulting collisional cross-sections and rates. Furthermore, sophisticated genetic programming techniques are employed to determine analytical expressions for the temperature and total molecular angular momentum dependence of these collisional rates. We obtain quantum depolarization and polarization transfer rates for the C$_2$ $(X ^1\Sigma^+_g, a ^3\Pi_u)$ + H$(^2S_{1/2})$ collisions in the temperature range T=2,000--15,000~K. We also determine analytical expressions giving these rates as functions of the temperature and total molecular angular momentum. In addition, we show that isotropic collisions with neutral hydrogen can only partially depolarize the lower state of C$_2$ lines, rather than completely. This highlights the limitations of the approximation of neglecting lower-level polarization while modeling the polarization of C$_2$ lines.

astro-ph.SR↗
arXiv2024

On the collisional sensitivity of polarized Mg II solar lines

Neutral and singly ionized states of the Magnesium (Mg) are the origin of several spectral lines that are useful for solar diagnostic purposes. An important element in modeling such solar lines is collisional data of the Mg with different perturbers abundant in the Sun, specially with neutral hydrogen. This work aims at providing complete depolarization and polarization and population transfer data for Mg II due to collisions with hydrogen atoms. For this purpose, a general formalism is employed to calculate the needed rates of MgII due to collisions with hydrogen atoms. The resulting collisional rates are then employed to investigate the impact of collisions on the polarization of 25 Mg II lines relevant to solar applications by solving the governing statistical equilibrium equations within multi-level and multi-term atomic models. We find that the polarization of some Mg II lines starts to be sensitive to collisions for hydrogen density $n_H \!\gtrsim\!$ 10$^{14}$ cm$^{-3}$.

astro-ph.SR↗
arXiv2024

Dynamics and solar wind control of the recovery of strong geomagnetic storms

In this work, we studied the characteristics and dynamical changes during the recovery time of moderate and strong geomagnetic storms (Dst $<-50$ nT). Investigating 57 storms triggered by CMEs/CIRs, we focused on the solar wind's influence on their decay phases. Selected storms were classified into distinct groups based on their recovery characteristics. Using superposed epoch analysis and best fit methods, we scrutinized several interplanetary solar wind plasma and field parameters/functions. The analysis included single, dual, and multiple interplanetary plasma and field parameters/functions. We determined the most representative characteristic time for the storm's recovery profile by fitting an exponential curve. A correlation analysis between Dst and solar wind parameters/functions isolated a coupling function ($\rho^{1/2}$Ey) best describing the decay rate of the ring current. This shows that the electric field term (Ey) coupled with a viscous term ($\rho^{1/2}$) plays a pivotal role in determining the recovery rate of geomagnetic storms. Additionally, we modeled the complex patterns of Dst recovery in relation to solar wind parameters/functions using a second-order polynomial. During the recovery phase, a dynamic correlation between Dst and solar wind parameters/functions was revealed. The three-parameter solar wind-magnetosphere electrodynamical coupling function, combining the viscous term ($\rho^{1/2}$) and the electric field-related function (v$^{4/3}$B) ($\rho^{1/2}$v$^{4/3}$B), significantly impacts the recovery phase of geomagnetic disturbances. Our investigation extended to the relationship between main and recovery phase durations, providing valuable insights into the solar wind's control over the decay of geomagnetic disturbances. These findings advance our comprehension of the complex relationship between solar wind dynamics and the evolution of geomagnetic disturbances.

physics.space-ph↗
arXiv2019

Effect of the isotropic collisions with neutral hydrogen on the polarization of the CN solar molecule

Our work is concerned with the case of the solar molecule CN which presents conspicuous profiles of scattering polarization. We start by calculating accurate PES for the singlet and triplet electronic ground states in order to characterize the collisions between the CN molecule in its $X \; ^2\Sigma$ state and the hydrogen in its ground state $^2S$. The PES are included in the Schr\"oodinger equation to obtain the scattering matrix and the probabilities of collisions. Depolarizing collisional rate coefficients are computed in the framework of the infinite order sudden approximation for temperatures ranging from $T= 2000$ K to $T= 15000$ K. Interpretation of the results and comparison between singlet and triplet collisional rate coefficients are detailed. We show that, for typical photospheric hydrogen density ($n_{H} = 10^{15}-10^{16}$ cm$^{-3}$), the $X \; ^2\Sigma$ state of CN is partially or completely depolarized by isotropic collisions.

astro-ph.SR↗
arXiv2017

Scattering polarization of the $d$-states of ions and solar magnetic field: Effects of isotropic collisions

Analysis of solar magnetic fields using observations as well as theoretical interpretations of the scattering polarization is commonly designated as a high priority area of the solar research. The interpretation of the observed polarization raises a serious theoretical challenge to the researchers involved in this field. In fact, realistic interpretations need detailed investigations of the depolarizing role of isotropic collisions with neutral hydrogen. The goal of this paper is to determine new relationships which allow the calculation of any collisional rates of the d-levels of ions by simply determining the value of n^* and $E_p$ without the need of determining the interaction potentials and treating the dynamics of collisions. The determination of n^* and E_p is easy and based on atomic data usually available online. Accurate collisional rates allow a reliable diagnostics of solar magnetic fields. In this work we applied our collisional FORTRAN code to a large number of cases involving complex and simple ions. After that, the results are utilized and injected in a genetic programming code developed with C-langugae in order to infer original relationships which will be of great help to solar applications. We discussed the accurarcy of our collisional rates in the cases of polarized complex atoms and atoms with hyperfine structure. The relationships are expressed on the tensorial basis and we explain how to include their contributions in the master equation giving the variation of the density matrix elements. As a test, we compared the results obtained through the general relationships provided in this work with the results obtained directly by running our code of collisions. These comparisons show a percentage of error of about 10% in the average value.

astro-ph.SR↗
arXiv2016

Inversion of Zeeman polarization for solar magnetic field diagnostics

The topic of magnetic field diagnostics with the Zeeman effect is currently vividly discussed. There are some testable inversion codes available to the spectropolarimetry community and their application allowed for a better understanding of the magnetism of the solar atmosphere. In this context, we propose an inversion technique associated with a new numerical code. The inversion procedure is promising and particularly successful for interpreting the Stokes profiles in quick and sufficiently precise way. In our inversion, we fit a part of each Stokes profile around a target wavelength, and then determine the magnetic field as a function of the wavelength which is equivalent to get the magnetic field as a function of the height of line formation. To test the performance of the new numerical code, we employed "hare and hound" approach by comparing an exact solution (called input) with the solution obtained by the code (called output). The precision of the code is also checked by comparing our results to the ones obtained with the HAO MERLIN code. The inversion code has been applied to synthetic Stokes profiles of the Na D$_{1}$ line available in the literature. We investigated the limitations in recovering the input field in case of noisy data. As an application, we applied our inversion code to the polarization profiles of the Fe {\sc i} $λ$ 6302.5 Å observed at IRSOL in Locarno.

astro-ph.SR↗
arXiv2015

A unified numerical model of collisional depolarization and broadening rates due to hydrogen atom collisions

Interpretation of solar polarization spectra accounting for partial or complete frequency redistribution requires data on various collisional processes. Data for depolarization and polarization transfer are needed but often missing, while data for collisional broadening are usually more readily available. Recent work by Sahal-Bréchot and Bommier concluded that despite underlying similarities in the physics of collisional broadening and depolarization processes, relationships between them are not possible to derive purely analytically. We aim to derive accurate numerical relationships between the collisional broadening rates and the collisional depolarization and polarization transfer rates due to hydrogen atom collisions. Such relationships would enable accurate and efficient estimation of collisional data for solar applications. Using earlier results for broadening and depolarization processes based on general (i.e. not specific to a given atom), semi-classical calculations employing interaction potentials from perturbation theory, genetic programming (GP) has been used to fit the available data and generate analytical functions describing the relationships between them. The predicted relationships from the GP-based model are compared with the original data to estimate the accuracy of the method.

astro-ph.SR↗
arXiv2014

Collisional effects on the formation of the second solar spectrum of the Sr {\sc ii} $λ$4078 line

This work aims to provide key elements to gain better understanding of the formation of the second solar spectrum of the Sr II $λ$4078 line. We show that in a certain range of hydrogen density, the effect of isotropic collisions between Sr II ions with hydrogen atoms is important in determining the polarization of the Sr II $λ$4078 line. The use of an atomic model that neglects the metastable level 4d of Sr II can induce errors of up to 25 \% in the value of the scattering polarization of the solar Sr II $λ$4078 line.

astro-ph.SR↗
arXiv2008

Evidence for collisional depolarization of the \ion{Ba}{ii} $λ4554$ line in the low chromosphere

Context. Rigorous modeling of the \ion{Ba}{ii} $λ4554$ formation is potentially interesting since this strongly polarized line forms in the solar chromosphere where the magnetic field is rather poorly known. Aims. To investigate the role of isotropic collisions with neutral hydrogen in the formation of the polarized \ion{Ba}{ii} $λ4554$ line and, thus, in the determination of the magnetic field. Methods. Multipole relaxation and transfer rates of the $d$ and p-states of \ion{Ba}{ii} by isotropic collisions with neutral hydrogen are calculated. We consider a plane parallel layer of \ion{Ba}{ii} situated at the low chromosphere and anisotropically illuminated from below which produces linear polarization in the $λ4554$ line by scattering processes. To compute that polarization, we solve the statistical equilibrium equations for \ion{Ba}{ii} levels including collisions, radiation and magnetic field effects. Results. Variation laws of the relaxation and transfer rates with hydrogen number density $n_{\textrm {\scriptsize H}}$ and temperature are deduced. The polarization of the $λ4554$ line is clearly affected due to isotropic collisions with neutral hydrogen although the collisional depolarization of its upper level $^2P_{3/2}$ is negligible. This is because the alignment of the metastable levels $^2D_{3/2}$ and $^2D_{5/2}$ of the \ion{Ba}{ii} are vulnerable to collisions. At the height of formation of the $λ4554$ line where $n_{\textrm {\scriptsize H}} \sim 2 \times 10^{14}$ cm$^{-3}$, we find that the neglecting of the collisions induces inaccuracy of $\sim$ 25% on the calculation of the polarization and $\sim$ 35 % inaccuracy on microturbulent magnetic field determination.

astro-ph↗
arXiv2007

Are collisions with neutral hydrogen important for modelling the Second Solar Spectrum of Ti I and Ca II ?

The physical interpretation of scattering line polarization offers a novel diagnostic window for exploring the thermal and magnetic structure of the quiet regions of the solar atmosphere. Here we evaluate the impact of isotropic collisions with neutral hydrogen atoms on the scattering polarization signals of the 13 lines of multiplet 42 of Ti I and on those of the K line and of the IR triplet of Ca II, with emphasis on the collisional transfer rates between nearby J-levels. To this end, we calculate the linear polarization produced by scattering processes considering realistic multilevel models and solving the statistical equilibrium equations for the multipolar components of the atomic density matrix. We confirm that the lower levels of the 13 lines of multiplet 42 of Ti I are completely depolarized by elastic collisions. We find that upper-level collisional depolarization turns out to have an unnoticeable impact on the emergent linear polarization amplitudes, except for the ${λ4536$ line for which it is possible to notice a rather small depolarization caused by the collisional transfer rates. Concerning the Ca II lines, we show that the collisional rates play no role on the polarization of the upper level of the K line, while they have a rather small depolarizing effect on the atomic polarization of the metastable lower levels of the Ca II IR triplet.

astro-ph↗
arXiv2004

Collisional depolarization and transfer rates of spectral lines by atomic hydrogen. IV: application to ionised atoms

The semi-classical theory of collisional depolarization of spectral lines of neutral atoms by atomic hydrogen (Derouich et al. 2003a; Derouich et al. 2003b; Derouich et al. 2004 and references therein) is extended to spectral lines of singly ionised atoms. In this paper we apply our general method to the particular cases of the 3d $^2D$ and $4p$ $^2P$ states of the CaII ion and to the $5p$ $^2P$ state of the SrII ion. Analytical expressions of all rates as a function of local temperature are given. Our results for the CaII ion are compared to recent quantum chemistry calculations. A discussion of our results is presented.

astro-ph↗
arXiv2003

On the collisional depolarization and transfer rates of spectral lines by atomic hydrogen. III: application to $f$-states of neutral atoms

The theory of collisional depolarization of spectral lines by atomic hydrogen (Derouich et al. 2003a; Derouich et al. 2003b) is extended to $f$-atomic levels $(l$=3). Depolarization rates, polarization and population transfer rates are calculated and results are given. Each cross section as a function of the effective quantum number for a relative velocity of 10 $\textrm{km s}^{-1}$ is given together with an exponent $λ$, if it exists, on the assumption that the cross section varies with velocity as $v^{-λ}$. A general trends of depolarization rates, of polarization transfer rates and of population transfer rates are given. A discussion of our results is achieved.

astro-ph↗
arXiv2003

Collisional depolarization and transfer rates of spectral lines by atomic hydrogen. II: application to d states of neutral atoms

The theory of collisional depolarization of spectral lines by atomic hydrogen (Derouich et al. \cite{derouich1}) is extended to $d$ $(l$=2) atomic levels. Depolarization rates, polarization and population transfer rates are calculated and results are given as a function of the temperature. Cross sections as a function of the effective quantum number for a relative velocity of 10 $\textrm{km s}^{-1}$ are also given together with velocity exponents $λ$, if \textbf{they exist}, on the assumption that the cross section varies with velocity as $v^{-λ}$. A discussion of our results is presented.

astro-ph↗
arXiv2003

Semi-classical theory of collisional depolarization of spectral lines by atomic hydrogen. I: application to p states of neutral atoms

The present paper extends the method of Anstee, Barklem and O'Mara (Anstee 1992; Anstee & O'Mara 1991, 1995; Anstee, O'Mara & Ross 1997; Barklem 1998; Barklem & O'Mara 1997; Barklem, O'Mara & Ross 1998), developed during the 1990's for collisional line broadening by atomic hydrogen, to the depolarization of spectral lines of neutral atoms by collisions with atomic hydrogen. In the present paper, we will limit the calculations to p (l=1) atomic levels. The depolarization cross sections and depolarization rates are computed. In Table 2 cross sections as functions of the relative velocity and effective quantum number are given, allowing for the computation for any p atomic level. Our results are compared to quantum chemistry calculations where possible. The sensitivity of depolarization cross sections to regions of the potential is examined. We conclude that the accuracy obtained with our method (< 20 % for the depolarization rates) is promising for its extension to higher l-values for the interpretation of the ``second solar spectrum''. This will be the object of further papers.

astro-ph↗