The present paper extends the method of Anstee, Barklem and O'Mara (Anstee [CITE]; Anstee & O'Mara [CITE], [CITE]; Anstee et al. [CITE]; Barklem [CITE]; Barklem & O'Mara [CITE]; Barklem et al. [CITE]), developed during the 1990s 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 () atomic levels. The depolarization cross sections and depolarization rates are computed. In Table [see full text] 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.

Features and peculiarities of the cosmic ray intensity (CRI) and the geomagnetic activity, along with several solar plasma and interplanetary magnetic field, during the period 4–10 September 2017 are studied. The period was characterized by strong solar activity: Several solar flares occurred; several halo coronal mass ejections (CMEs) were ejected in space. In the near‐Earth interplanetary space, the CMEs driving shock(s) and sheath(s) were identified. At the Earth, strong Forbush decreases in CRI and geomagnetic storms were observed. Several large solar flares, one of them of very high X‐ray importance (X9.3) and three halo CMEs were detected in the solar atmosphere. Two shock‐associated interplanetary CMEs were observed during that interval in near‐Earth space; the latter and faster one arrived even as the ejecta of the earlier one was still crossing. Variations in interplanetary plasma and field parameters during, before, and after the Forbush decreases and geomagnetic storms that occurred during the considered period were examined. A detailed time‐lagged correlation analysis using data at three different temporal resolutions (hourly, 5‐min, and 1‐min) was also performed. Cross correlations of time series of CRI with geomagnetic activity during the period 4–10 September 2017 are computed. This cross‐correlation analysis between CRI variability (defined as the difference of the CRI count rate between the current and the previous time step) and the Dst indicates a delay of Dst by 3–4 hr.

This paper presents and interprets some observations of the limb polarization of 4607 Å obtained with the spectropolarimeter of the French-Italian telescope THEMIS in quiet regions close to the solar North Pole on 2002 December 7–9. The linear polarization was measured for a series of limb distances ranging from 4 to 160 arcsec, corresponding to heights of optical depth unity in the line core ranging from about 330 to 220 km, respectively, above the level. To increase the polarimetric sensitivity, the data were averaged along the spectrograph slit (one arcmin long) set parallel to the solar limb. Since the data show no rotation of the linear polarization direction with respect to the limb direction, the observed depolarization is ascribed to the Hanle effect of a turbulent weak magnetic field, the zero-field polarization being derived from a model. The interpretation is performed by means of an algorithm which describes the process of line formation in terms of the atomic density matrix formalism, the solar atmosphere being described by an empirical, plane-parallel model. The collisional rates entering the model (inelastic collisions with electrons, elastic depolarizing collisions with neutral hydrogen), have been computed by applying fast semi-classical methods having a typical accuracy of the order of 20% or better (see Derouich [CITE]), leading to 6% inaccuracy on the magnetic field strength determination. We assume a unimodal distribution for the intensity of the turbulent field. The computed intensity profile has been adjusted to the observed one in both depth and width, by varying both microturbulent and macroturbulent velocities. The best adjustment is obtained for respectively 1.87 km s-1 (micro) and 1.78 km s-1 (macro). The evaluation of the magnetic depolarization leads then to the average value of 46 Gauss for the turbulent magnetic field strength, with a gradient of -0.12 Gauss/km. Our results are in very good agreement with the value of 60 Gauss determined at large μ, in the volume-filling field case, by Trujillo Bueno et al. ([CITE], Nature, 430, 326), using a 3D magneto-convective simulation. This validates our method.

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics

Astronomy & Astrophysics (A&A) is an international journal which publishes papers on all aspects of astronomy and astrophysics